Author name code: seaton
ADS astronomy entries on 2022-09-14
author:"Seaton, Mike J."
------------------------------------------------------------------------
Title: OPserver: opacities and radiative accelerations on demand
Authors: Mendoza, C.; González, J.; Seaton, M. J.; Buerger, P.;
Bellorín, A.; Meléndez, M.; Rodríguez, L. S.; Delahaye, F.; Zeippen,
C. J.; Palacios, E.; Pradhan, A. K.
Bibcode: 2009RMxAC..35..292M
Altcode:
We report on developments carried out within the Opacity Project (OP)
to upgrade atomic database services to comply with e-infrastructure
requirements. We give a detailed description of an interactive,
online server for astrophysical opacities, referred to as OPserver,
to be used in sophisticated stellar modelling where Rosseland mean
opacities and radiative accelerations are computed at every depth point
and each evolution cycle. This is crucial, for instance, in chemically
peculiar stars and in the exploitation of the new asteroseismological
data. OPserver, downloadable with the new OPCD_3.0 release from the
Centre de Données Astronomiques de Strasbourg, France, computes mean
opacities and radiative data for arbitrary chemical mixtures from the
OP monochromatic opacities. It is essentially a client-server network
restructuring and optimization of the suite of codes included in the
earlier OPCD_2.0 release. The server can be installed locally or,
alternatively, accessed remotely from the Ohio Supercomputer Center,
Columbus, Ohio, USA. The client is an interactive web page or a
subroutine library that can be linked to the user code. The suitability
of this scheme in grid computing environments is emphasized, and its
extension to other atomic database services for astrophysical purposes
is discussed.
Title: Updated Opacity Project radiative accelerations
Authors: Seaton, M. J.
Bibcode: 2007MNRAS.382..245S
Altcode: 2007MNRAS.tmp..908S
Updated data for the calculation of radiative accelerations,
grad, referred to as OP 2005, have been made generally
available on the Web. They differ from the earlier 1997 OP data mainly
in the inclusion of contributions from inner-shell processes. The
frequency resolution used for OP 2005 is shown to be satisfactory except
for extreme cases of low concentrations and low densities. The 2005
data are in reasonably good agreement with results from OPAL, given
by Richer et al. in graphical form, for a model with Teff =
104 and log (R) = -3. They are in less good agreement with
OPAL results of Turcotte et al. for the solar radiative interior.
Title: OPserver: interactive online computations of opacities and
radiative accelerations
Authors: Mendoza, C.; Seaton, M. J.; Buerger, P.; Bellorín, A.;
Meléndez, M.; González, J.; Rodríguez, L. S.; Delahaye, F.;
Palacios, E.; Pradhan, A. K.; Zeippen, C. J.
Bibcode: 2007MNRAS.378.1031M
Altcode: 2007arXiv0704.1583M; 2007MNRAS.tmp..440M
Codes to compute mean opacities and radiative accelerations for
arbitrary chemical mixtures using the Opacity Project recently
revised data have been restructured in a client-server architecture
and transcribed as a subroutine library. This implementation increases
efficiency in stellar modelling where element stratification due to
diffusion processes is depth dependent, and thus requires repeated fast
opacity re-estimates. Three user modes are provided to fit different
computing environments, namely, a web browser, a local workstation
and a distributed grid.
Title: Opacity Project data on CD for mean opacities and radiative
accelerations
Authors: Seaton, M. J.
Bibcode: 2005MNRAS.362L...1S
Altcode: 2005MNRAS.tmpL..61S; 2004astro.ph.11010S
All monochromatic opacity data from the Opacity Project (OP), together
with all codes required for the calculation of mean opacities and
radiative accelerations for any required chemical mixture, temperature
and mass density, are being put on a 700-MB CD which will be made
generally available. The present letter gives a concise summary of
the contents of the CD. More complete documentation will be provided
on the CD itself.
Title: Dielectronic Recombination, Photoionization and Opacities
Authors: Badnell, N. R.; Seaton, M. J.; Summers, H. P.
Bibcode: 2005AIPC..774..305B
Altcode:
We review methodologies for describing dielectronic recombination
(DR). We look briefly at the state-of-play of theory vs experiment. We
then discuss how to deliver DR data in a form suitable for modelling
and we review the status of the partial and total DR database being
assembled for the modelling of dynamic finite-density plasmas. We
comment on the role of fields and how modelling l-changing collisions
approximately models the dynamic part of the plasma microfield. We
also consider the inverse process of photoionization and discuss how the
same approach to DR is being used for inner-shell photoionization and
the determination of opacities in stellar interiors. In particular, we
pay attention to the role of opacity in connection with the discrepancy
between the helioseismology observations and stellar evolution models
for the position of the base of the solar convective zone.
Title: Updated opacities from the Opacity Project
Authors: Badnell, N. R.; Bautista, M. A.; Butler, K.; Delahaye, F.;
Mendoza, C.; Palmeri, P.; Zeippen, C. J.; Seaton, M. J.
Bibcode: 2005MNRAS.360..458B
Altcode: 2004astro.ph.10744B; 2005MNRAS.tmp..442B
Using the code AUTOSTRUCTURE, extensive calculations of inner-shell
atomic data have been made for the chemical elements He, C, N, O, Ne,
Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni. The results are used
to obtain updated opacities from the Opacity Project (OP). A number
of other improvements on earlier work have also been included. Rosseland-mean opacities from the OP are compared with those from
OPAL. Differences of 5-10 per cent occur. The OP gives the `Z-bump',
at log(T) ~= 5.2, to be shifted to slightly higher temperatures. The
opacities from the OP, as functions of temperature and density, are
smoother than those from OPAL. The accuracy of the integrations
used to obtain mean opacities can depend on the frequency mesh
used. Tests involving variation of the numbers of frequency points show
that for typical chemical mixtures the OP integrations are numerically
correct to within 0.1 per cent. The accuracy of the interpolations
used to obtain mean opacities for any required values of temperature and
density depends on the temperature-density meshes used. Extensive tests
show that, for all cases of practical interest, the OP interpolations
give results correct to better than 1 per cent. Prior to a number
of recent investigations which have indicated a need for downward
revisions in the solar abundances of oxygen and other elements,
there was good agreement between properties of the Sun deduced from
helioseismology and from stellar evolution models calculated using
OPAL opacities. The revisions destroy that agreement. In a recent
paper, Bahcall et al. argue that the agreement would be restored if
opacities for the regions of the Sun with 2 × 106<~T<~
5 × 106 K (0.7-0.4 Rsolar) were larger than
those given by OPAL by about 10 per cent. In the region concerned,
the present results from the OP do not differ from those of OPAL by
more than 2.5 per cent.
Title: Up-dated Opacities from the Opacity Project.
Authors: Badnell, N. R.; Seaton, M. J.; Bautista, M. A.; Butler, K.;
Delahaye, F.; Mendoza, C.; Palmeri, P.; Zeippen, C. J.
Bibcode: 2004AAS...205.5214B
Altcode: 2004BAAS...36.1425B
Using the code AUTOSTRUCTURE, extensive calculations of inner-shell
atomic data have been made for the chemical elements He, C, N, O, Ne,
Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni. The results are used
to obtain up-dated opacities from the Opacity Project, OP. A number
of other improvements on earlier work have also been included. Rosseland-mean opacities from OP are compared with those from
OPAL. Differences of 5 to 10% occur. OP gives the `Z-bump', at log (T)
∼ 5.2, to be shifted to slightly higher temperatures. The opacities
from OP, as functions of temperature and density, are smoother than
those from OPAL. Extensive tests show that the numerical accuracy
of the OP opacities following integration over frequency mesh and
interpolation on temperature--density mesh is better than 1%. Prior to a number of recent investigations which have indicated a
need for a downward revision in the solar abundances of oxygen and
other elements, there was good agreement between properties of the
sun deduced from helioseismology and from stellar evolution models
calculated using OPAL opacities. The revisions in abundances destroy
that agreement. Results from up-dated OP opacities will be presented
for the region of the solar radiative interior.
Title: A comparison of Rosseland-mean opacities from OP and OPAL
Authors: Seaton, M. J.; Badnell, N. R.
Bibcode: 2004MNRAS.354..457S
Altcode: 2004astro.ph..4437S
Monochromatic opacities from the Opacity Project (OP) have
been augmented by hitherto missing inner-shell contributions. OP
Rosseland-mean opacities, κR, are compared with results
from OPAL for the six elements H, He, C, O, S and Fe. The OPAL data
are obtained from the project's website. Agreement for H is
close everywhere except for the region of log(T) ~= 6 and log(R)
~=-1 (R=ρ/T36 where ρ is mass density in g
cm-3 and T6= 10-6×T with T in K). In
that region κR(OPAL) is larger than κR(OP) by
up to 13 per cent. The differences are caused by different equations
of state (EOS). In the region concerned, OP has the H ground state
undergoing dissolution, leading to a small H-neutral ionization
fraction, while OPAL has larger values for that fraction. A similar
difference occurs for He at log(R) ~=-1 and log(T) ~= 6.4, where OP
has the He+ ground state undergoing dissolution. The
OPAL website does not provide single-element Rosseland means for
elements other than H and He. Comparisons between OP and OPAL are
made for mixtures with X= 0.9, Z= 0.1 and Z containing pure C, O or
S. There are some differences: at the lower temperatures, say log(T)
<= 5.5, owing to differences in atomic data, with the OP R-matrix
data probably being the more accurate; and at higher temperatures
mainly owing to differences in level populations resulting from
the use of different EOS theories. In the original OP work,
R-matrix data for iron were supplemented by data obtained using the
configuration-interaction (CI) code SUPERSTRUCTURE. The experiment
is made of replacing much of the original iron data with new data
from the CI code AUTOSTRUCTURE. Inclusion of intercombination lines
gives an increase in κR of up to 18 per cent. The
OPAL website does not allow for Z containing pure iron. Comparisons
are made for an iron-rich mixture, X= 0.9, Z= 0.1 and Z containing C
and Fe with C:Fe = 2:1 by number fraction. There are some differences
between OP and OPAL for that case: the OP `Z-bump' in κR
is shifted to slightly higher temperatures, compared with OPAL. Overall, there is good agreement between OP and OPAL Rosseland-mean
opacities for the six elements, but there are some differences. Recent
work has shown that helioseismology measurements give a very accurate
value for the depth of the solar convective zone, RCZ, and
that, taking account of recent revisions in abundances, solar models
give agreement with that value only if opacities at RCZ
are about 20 per cent larger than OPAL values. For the six-element
mix at RCZ we obtain κR(OP) to be larger than
κR(OPAL) by 5 per cent.
Title: On the importance of inner-shell transitions for opacity
calculations
Authors: Badnell, N. R.; Seaton, M. J.
Bibcode: 2003JPhB...36.4367B
Altcode: 2003astro.ph..8393B
For high temperatures and densities, stellar opacities obtained from
the Opacity Project (OP) were smaller than those obtained from the OPAL
project. Iglesias and Rogers (1995 Astrophys. J. 443 469) suggested
that the discrepancy was due to the omission by OP of important atomic
inner-shell processes, and considered in detail results for a mixture
of six elements: H, He, C, O, S and Fe. Extensive new inner-shell data
have now been computed using the code AUTOSTRUCTURE. It is shown that
the inclusion of these data in the OP work gives opacities for the
six-element mix which are in much closer agreement with those from
OPAL. We also discuss a number of problems relating to the calculation
of opacities and of equations of state for dense plasmas.
Title: Tiptopbase
Authors: Palmeri, Patrick; Mendoza, Claudio; Seaton, Mike J.; Nahar,
Sultana N.; Pradhan, Anil K.; Kallman, Timothy R.; Zeippen, Claude J.
Bibcode: 2003IAUJD..17E..28P
Altcode:
The aims of the international collaborations known as the Opacity
Project (OP) [1] the IRON Project (IP) [2] and the RmaX Network
are to compute accurate atomic data for radiative and collisional
processes and opacities of astrophysical interest. A new database
to be referred to as TIPTOPbase is accessible from the NASA Goddard
Space Flight Center [3]. It currently provides the complete atomic and
opacity data from the OP and IP. Efforts have been made to structure
and present data sets so as to facilitate astrophysical modeling. In
the present contribution a general description of the TIPTOPbase is
given. [1] The Opacity Project Team Vols. (12) IOPP Bristol (1995
1996). [2] D.G. Hummer et al. Astron. Astrophys. 279 298 (1993). [3]
http://heasarc.gsfc.nasa.gov/topbase
Title: Tiptopbase: the Iron Project and the Opacity Project Atomic
Database
Authors: Mendoza, Claudio; Seaton, Mike; Nahar, Sultana; Pradhan,
Anil; Kallman, Tim; Zeippen, Claude
Bibcode: 2002APS..DMP.J6054M
Altcode:
The aims of the international collaborations, Opacity Project
(OP), the IRON Project (IP), and the RmaX Network (The Opacity
Project Team, Vol.1,2), IOPP, Bristol (1995,1996); Hummer et al.,
Astron. Astrophys. 279, 298 (1993) are to obtain accurate atomic data
for radiative and collisional processes, and plasma opacities for
astrophysical applications. A new, considerably enlarged, version of
the existing electronic database TOPbase (http://heasarc.gsfc.nasa.gov
OR http://vizier.u-strasbg.fr/OP.html) is under development. It
is called TIPTOPBASE and will contain (i) photoionization cross
sections (σ_PI), energy levels and oscillator strengths (f-values),
and monochromatic and Rosseland mean opacities of atoms and ions
with Z = 1 - 14, 16, 18, 20, 26, as well as new data for P, Cl, K,
Ni II-III, (ii) collisional data for iron and iron peak elements,
(iv) σ_PI at high energies including inner-shell ionization, (v)
total and level specific unified recombination rate coefficients,
(vi) f-values for inner-shell excitations in Fe VIII - XIII, (vi)
relativistic fine structure σ_PI and f-values for selected ions,
(vii) on-line computational facilities for 'customized' opacities
and radiative forces for user-specified mixture of elements. Partial
support from NASA is acknowledged.
Title: VizieR Online Data Catalog: Radiative forces for stellar
envelopes (Seaton, 1997)
Authors: Seaton, M. J.; Yan, Y.; Mihalas, D.; Pradhan, A. K.
Bibcode: 2000yCat.6089....0S
Altcode:
(1) Primary data files, stages.zz These files give data for the
calculation of radiative accelerations, GRAD, for elements with nuclear
charge zz. Data are available for zz=06, 07, 08, 10, 11, 12, 13, 14,
16, 18, 20, 24, 25, 26 and 28. Calculations are made using data from
the Opacity Project (see papers SYMP and IXZ). The data are given
for each ionisation stage, j. They are tabulated on a mesh of (T, Ne,
CHI) where T is temperature, Ne electron density and CHI is abundance
multiplier. The files include data for ionisation fractions, for each
(T, Ne). The file contents are described in the paper ACC and as
comments in the code add.f (2) Code add.f This reads a file stages.zz
and creates a file acc.zz giving radiative accelerations averaged
over ionisation stages. The code prompts for names of input and output
files. The code, as provided, gives equal weights (as defined in the
paper ACC) to all stages. Th weights are set in SUBROUTINE WEIGHTS,
which could be changed to give any weights preferred by the user. The
dependence of diffusion coefficients on ionisation stage is given by a
function ZET, which is defined in SUBROUTINE ZETA. The expressions used
for ZET are as given in the paper. The user can change that subroutine
if other expressions are preferred. The output file contains values,
ZETBAR, of ZET, averaged over ionisation stages. (3) Files acc.zz
Radiative accelerations computed using add.f as provided. The user
will need to run the code add.f only if it is required to change
the subroutines WEIGHTS or ZETA. The contents of the files acc.zz
are described in the paper ACC and in comments contained in the code
add.f. (4) Code accfit.f This code gives gives radiative accelerations,
and some related data, for a stellar model. Methods used to interpolate
data to the values of (T, RHO) for the stellar model are based on
those used in the code opfit.for (see the paper OPF). The executable
file accfit.com runs accfit.f. It uses a list of files given in
accfit.files (see that file for further description). The mesh used
for the abundance-multiplier CHI on the output file will generally be
finer than that used in the input files acc.zz. The mesh to be used
is specified on a file chi.dat. For a test run, the stellar model
used is given in the file 10000_4.2 (Teff=10000 K, LOG10(g)=4.2) The
output file from that test run is acc100004.2. The contents
of the output file are described in the paper ACC and as comments in
the code accfit.f. (5) The code diff.f This code reads the output file
(e.g. acc1000004.2) created by accfit.f. For any specified
depth point in the model and value of CHI, it gives values of radiative
accelerations, the quantity ZETBAR required for calculation of diffusion
coefficients, and Rosseland-mean opacities. The code prompts for input
data. It creates a file recording all data calculated. The code diff.f
is intended for incorporation, as a set of subroutines, in codes for
diffusion calculations. (1 data file).
Title: Diffusion of iron-group elements in the envelopes of HgMn stars
Authors: Seaton, M. J.
Bibcode: 1999MNRAS.307.1008S
Altcode:
The observed abundance anomalies for iron-group elements in atmospheres
of HgMn stars are due to diffusive movements which are driven by
radiation-pressure forces and which persist in the stellar envelopes,
going down to regions with temperatures of about 10^6K. Studies of
diffusion in the envelopes are required both in order to understand
the observed atmospheric abundances and in order to calculate the
changes in opacities that result from changes in abundances. Let tau
be the Rosseland-mean optical depth. It is shown that one can define
an upper boundary, tautau_u, such that one can obtain solutions for
the diffusive movements in the region of tau>=tau_u without any
knowledge of what happens in the higher layers of tau<tau_u. The
paper is concerned with a description of the numerical methods that can
be used to obtain such solutions. For Cr and Mn we are able to follow
the diffusion for times of order 10^8yr with tau_u=1. For Fe we are
also able to obtain some estimates of abundances at tau=1 allowing for
diffusion processes. For Mn, Cr and Fe we attempt some comparisons of
abundances computed for tau_u=1 with observed atmospheric abundances
and obtain results that are not discouraging. For Fe and Ni, larger
values of tau_u are required as the diffusion proceeds (after 10^8yr
we require tau_u~=8 for Fe and tau_u~=70 for Ni). For the outer regions
with tau<tau_u it will be necessary to obtain solutions allowing for
outflows of iron-group elements at the stellar surfaces. In such work it
should be possible to match the outer-region solutions to the envelope
solutions obtained using the methods described in this paper. The
diffusive movements lead to changes in Rosseland-mean opacities by
factors of up to 4. For Fe there is a build-up in concentrations in
the region of log(T)~=5.1, where the dominant ionization stages are
near Ar-like. This leads to the Z-bump in opacities being shifted
from its normal position at log(T)~=5.3 to lower values of log(T)
and becoming more sharply peaked. There is also a large build-up in
Ni concentrations in the outer parts of the envelopes, leading to
opacity enhancements. In the present work we allow neither for the
normal main-sequence evolution of the stars nor for the modifications
in that evolution which will result from changes in opacities. Solution
for both envelopes and outer regions will eventually be required.
Title: Photoionization from excited states of NeI
Authors: Seaton, M. J.
Bibcode: 1998MNRAS.300L...1S
Altcode:
As a result of fine-structure splitting of the NeII core, excited
states of NeI show large departures from SL coupling. It is shown
that the photoionization cross-sections for the physical states can
be expressed as linear combinations of the calculated cross-sections
for the SL states. The coefficients in the expansions are given for
all states with n=3 and 4.
Title: Diffusion in Stellar Envelopes
Authors: Seaton, M. J.
Bibcode: 1998HiA....11..664S
Altcode:
No abstract at ADS
Title: Radiative accelerations in stellar envelopes
Authors: Seaton, M. J.
Bibcode: 1997MNRAS.289..700S
Altcode:
In stars which are sufficiently quiescent, changes in the relative
abundances of the chemical elements can result from gravitational
settling and from levitation produced by radiation pressure forces,
usually expressed as radiative accelerations g_rad. Those changes
can affect the structure of such stars, due to modifications in
opacities, and can lead to marked peculiarities in observed atmospheric
abundances. It is necessary to consider diffusive movements both in the
atmospheres and in much deeper layers of the stellar envelopes. For
the envelopes the equation of radiative transfer can be solved in
a diffusion approximation and, for an element k in ionization stage
j, one obtains expressions for g_rad(j, k) proportional to the total
radiative flux, to the Rosseland-mean opacity kappa_R (which may depend
on the abundance of k), and to a dimensionless quantity gamma(j, k)
which, due to saturation effects, can be sensitive to the abundance
of k. The radiative accelerations are required for each ionization
stage, because the diffusion coefficients depend on j. Using atomic
data obtained in the course of the work of the Opacity Project (OP),
we calculate kappa_R and gamma(j, k) for the chemical elements C, N, O,
Ne, Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni. We start from standard
Solar system abundances, and then vary the abundance of one element at
a time (element k) by a factor chi. The following results are obtained
and are available at the Centre de Donnees astronomiques de Strasbourg
(CDS). (1) Files stages.zz (where zz specifies the nuclear charge of
the selected element k) containing values of kappa_R and gamma(j, k)
on a mesh of values of (T, N_e, chi), where T is temperature, and N_e
is electron density. We include derivatives of kappa_R and gamma(j,
k) with respect to chi, which are used for making interpolations. (2)
A code add.f which reads a file stages.zz and writes a file acc.zz
containing values of gamma(k) obtained on summing the gamma(j, k),
weighted by diffusion coefficients. The diffusion coefficients to be
employed can be selected by the user. (3) A code acc.f which reads
a file acc.zz and provides facilities for interpolations of kappa_R
and g_rad(k) to values of (T, rho, chi) for a stellar model, where rho
is mass density. The mesh to be used for log(chi) is specified by the
user. (4) A code diff.f intended for use in diffusion calculations. It
reads a file created by acc.f and provides function subroutines for
the calculation of kappa_R and g_rad(k) for any specified depth-point
and any value of chi. Results are compared with those from other recent
work for C, N, O, Ca and Fe.
Title: VizieR Online Data Catalog: Opacities from the Opacity Project
(Seaton+, 1995)
Authors: Seaton, M. J.; Yan, Y.; Mihalas, D.; Pradhan, A. K.
Bibcode: 1997yCat.6080....0S
Altcode:
1 CODES. ***** 1.1 Code rop.for ************ This code reads
opacity files written in standard OP format. Its main purpose is to
provide documentation on the contents of the files. This code,
like the other codes provided, prompts for the name of the file (or
files) to be read. The file names read in response to the prompt may
have up to 128 characters. 1.2 Code opfit.for ************** This code reads opacity files in standard OP format, and provides for
interpolation of opacities to any required values of temperature and
mass-density. The method used is described in OPF. The code prompts
for the name of a file giving all required control parameters. As
an example, the file opfit.dat is provided (users will need to
change directory names and file names). The use of opfit.for is
illustrated using opfit.dat. Most users will probably want to adapt
opfit.for for use as a subroutine in other codes. Timings for DEC
7000 ALPHA: 0.3 sec for data read and initialisations; then 0.0007 sec
for each temperature-density point. Users who like OPAL formats
should note that opfit.for has a facility to produce files of OP data
in OPAL-type formats. 1.3 Code ixz.for ************ This code
provides for interpolations to any required values of X and Z. See
IXZ. It prompts for the name of a file giving all required control
parameters. An example of such a file if provided, ixz.dat (the user
will need to change directory and file names). The output files
have names s92INT.'nnn'. The user specifies the first value of nnn,
and the number of files to be produced. 2. DATA FILES **********
2.1 Data files for solar metal-mix ******************************
Data for solar metal-mix s92 as defined in SYMP. These files are
from version 2 runs of December 1994 (see IXZ for details on Version
2). There are 213 files with names s92.'nnn', 'nnn'=201 to 413. Each
file occupies 83762 bytes. The file s92.version2 gives values of X
(hydrogen mass-faction) and Z (metals mass-fraction) for each value
of 'nnn'. The user can get s92.version2, select the values of 'nnn'
required, then get the required files s92.'nnn'. The user can see the
file in ftp, displayed on the screen, by typing "get s92.version2
-". The files s92.'nnn' can be used with opfit.for to obtain
opacities for any requires value of temperature and mass density. Files for other metal-mixtures will be added in due course. Send
requests to mjs@star.ucl.ac.uk. 2.2 Files for interpolation
in X and Z ********************************** The data files
have names s92xz.'mmm', where 'mmm'=001 to 096. They differ from
the standard OP files (such as s92.'nnn' --- section 2.1 above) in
that they contain information giving derivatives of opacities with
respect to X and Z. Each file s92xz.'mmm' occupies 148241 bytes. The interpolations to any required values of X and Z are made
using ixz.for. Timings: on DEC 7000 ALPHA, 2.16 sec for each
new-mixture file. For interpolations to some specified values of
X and Z, one requires just 4 files s92xz.'mmm'. Most users will not
require the complete set of files s92xz.'mmm'. The file s92xz.index
includes a table (starting on line 3) giving values, for each 'mmm'
file, of x,y,z (abundances by number-factions) and X,Y,Z (abundances
by mass-fractions). Users are advised to get the file s92.index, and
select values of 'mmm' for files required, then get those files. The files produced by ixz.for are in standard OP format and can
be used with opfit.for to obtain opacities for any required values
of temperature and mass density. 3 RECOMMENDED PROCEDURE FOR
USE OF OPACITY FILES **********************************************
(1) Get the file s92.version2. (2) If the values of X and
Z you require are available in the files s92.'nnn' then get those
files. (3) If not, get the file s92xz.index. (4) Select
from s92xz.index the values of 'mmm' which cover the range of X and
Z in which your are interested. Get those files and use ixz.for to
generate files for your exact required values of X and Z. (5) Note
that the exact abundance mixtures used are specified in each file (see
rop.for). Also each run of opfit.for produces a table of abundances. (6) If you want a metal-mix different from that of s92, contact
mjs@star.ucl.ac.uk. 4 FUTURE DEVELOPMENTS *******************
(1) Data for the calculation of radiative forces are provided as
the CDS catalog <VI/89> (added August 1997) (2) Facilities
will be added later which will enable the user to make calculations
giving files for any required mixtures. (9 data files).
Title: Albrecht Unsöld, 20 April 1905 - 23 September 1995.
Authors: Seaton, M.
Bibcode: 1997A&G....38...37S
Altcode:
No abstract at ADS
Title: Diffusion in Stellar Envelopes
Authors: Seaton, M. J.
Bibcode: 1997IAUJD..16E.104S
Altcode:
Abundances in stellar atmospheres can depend on diffusive movements
in much deeper layers of stellar envelopes. Diffusion in envelopes is
also of interest in that it can lead to changes in opacities and hence
to the structures of stars. For envelopes the radiative accelerations
grad can be expressed in terms of quantities which depend
only on temperatures, densities and chemical compositions. Computations
have been made for the elements C, N, O, Ne, Na, Mg, Al, Si, S, Ar,
Ca, Cr, Mn, Fe and Ni and tables are being made generally available
through CDS (Strasbourg). Some results from those computations will
be presented. The computed values of grad are used to study
diffusion of iron-group elements in envelopes of HgMn stars. It is shown
that one can define a value tau_0 of the Rosseland-mean optical depth
tau such that diffusive movements for tau >= tau_0 do not depend on
those for tau <= tau_0. For Cr and Mn we obtain solutions with tau_0
= 1 and are able to make some meaningful comparisons of abundances, as
computed and as observed in atmospheres. For Fe we find that diffusive
movements are slowed down in regions of T ~= 10^5 K where the dominant
ionisation stages are near argon-like. Diffusion of Fe-group elements
can produce substantial changes in opacities.
Title: The Opacity Project
Authors: Seaton, M.
Bibcode: 1996Obs...116..177S
Altcode:
No abstract at ADS
Title: Interpolations of Rosseland-mean opacities for variable X and Z
Authors: Seaton, M. J.
Bibcode: 1996MNRAS.279...95S
Altcode:
For fixed relative abundances of `metal' atoms, stellar compositions
are specified by mass fractions X for hydrogen and Z for metals (the
helium mass fraction being Y=1-X-Z). Rosseland-mean opacities depend on
temperature, T, density, rho, and chemical compositions. Interpolations
in T and rho were discussed in a previous paper. The present paper is
concerned with interpolations in X and Z. The opacities are calculated
on a grid of (X, Z) values together with information giving derivatives
of opacities with respect to X and Z. Opacities for any required value
of X and Z can then be obtained using bicubic interpolations. The
opacity tables, and codes for making interpolations, are available at
the Centre de Donnees de Strasbourg (CDS).*
Title: Levitation
Authors: Seaton, M. J.
Bibcode: 1996Ap&SS.237..107S
Altcode:
Some stars have outer layers which are sufficiently quiescent for
diffusion to occur and to modify the relative abundances of the chemical
elements. Levitation occurs when the forces due to radiation pressure
are larger than those due to gravitation. The paper describes some
recent work on the calculation of the radiative forces using atomic
data obtained in the course of the work of the Opacity Project. Large
abundance anomalies are observed for the HgMn stars, which lie on or
close to the main sequence and have effective temperatures in the range
11000 to 15000 K. Some results are given for calculated abundances of
elements of the iron group in the HgMn stars.
Title: Sir David Robert Bates (18 November 1916 - 1994).
Authors: Seaton, M. J.
Bibcode: 1996QJRAS..37...81S
Altcode:
No abstract at ADS
Title: Atomic data for the calculation of radiative accelerations
and diffusion of Mn in HgMn stars.
Authors: Seaton, M. J.
Bibcode: 1996PhST...65..129S
Altcode:
Archived monochromatic opacities, obtained in the course of the work
of the Opacity Project, are used to obtain data required for the
calculation of radiative accelerations. These data are being made
generally available through the Centre de Données de Strasbourg
(CDS). The new atomic data are used in an exploratory study of the
abundances of manganese in HgMn stars. In most such stars observations
show enhanced atmospheric Mn abundances, which result from processes of
diffusion. The author defines χ to be the factor by which abundances
are enhanced (or depleted). Due to saturation effects, the radiative
accelerations grad(χ) decrease as χ increases and may
eventually reach values smaller that the gravitational acceleration
ggrav: χstat is defined to be the value of χ for
which the diffusion velocity is zero. Using results from ultra-violet
observations one obtains abundances χobs always a good deal
smaller than χstat. It follows that there must be outflow
at the stellar surfaces. Solutions are obtained of the equations for
time-dependent diffusion.
Title: New Atomic Data for Astronomy: an Introductory Review
Authors: Seaton, M. J.
Bibcode: 1995ASPC...78....1S
Altcode: 1995aapn.conf....1S
No abstract at ADS
Title: The opacity project
Authors: Seaton, M. J.
Bibcode: 1995oppr.book.....S
Altcode: 1995QB809.O63......
No abstract at ADS
Title: The Opacity Project.
Authors: Seaton, M. J.
Bibcode: 1995AIPC..322..117S
Altcode:
The Opacity Project (OP) which has been concerned with the calculation
of opacities for stellar envelopes, has been in progress for 10
years. The author defines stellar envelopes to be outer regions of
stellar interiors for which the mass density ρ is not very high,
say ρ ≤ 0.01 g cm-3. In envelopes opacities are mainly
determined by very large numbers of processes involving absorption
of radiation by complex atomic systems and the availability of
accurate and extensive atomic data is therefore a requirement of prime
importance. For the calculation of level populations (the problem of
the equation of state (EOS)) and for the determination of line profiles
one must allow for perturbations of atoms by the plasma environment. The
author considers the following topics: (1) the definition of Rosseland
mean opacities, the main quantity with which the author is concerned;
(2) the OP atomic-data work; (3) briefly, the OP approach to the EOS
problems; (4) line-profile problems; (5) the OP opacity calculations;
(6) comparisons between the results and those from other calculations.
Title: New atomic data for astronomy: an introductory review
Authors: Seaton, M. J.
Bibcode: 1995HiA....10..570S
Altcode:
No abstract at ADS
Title: Opacities for stellar envelopes
Authors: Seaton, M. J.; Yan, Y.; Mihalas, D.; Pradhan, A. K.
Bibcode: 1994MNRAS.266..805S
Altcode:
We define stellar envelopes to be those regions of stellar interiors
in which atoms exist and are not markedly perturbed by the plasma
environment. Availability of accurate and extensive atomic data is
a prime requirement for the calculation of envelope opacities. For
envelopes we adopt the criterion of mass density p < 0.01 ρ≥g
cm-3. We present radiative Rosseland mean opacities
for envelopes obtained using atomic data calculated in an international
collaboration referred to as the Opacity Project, or OP. Equations of
state are calculated using an occupation-probability formalism. To
a good approximation, ionization equilibria and level populations
in envelopes depend only on the temperature T and electron density
Ne and are insensitive to chemical mixtures. Monochromatic
opacities for all abundant chemical elements are therefore calculated
on a grid of (T, Ne) values and are archived. Rosseland mean
opacities are then readily calculated for any chemical mixture. Tables
of Rosseland means, for any required mixtures and as functions of ρ
and T, are available on request in computer-readable form. The
present, op, results are compared with those from another recent
study, referred to as OPAL, by C. A. Iglesias and F. A. Rogers at
the Lawrence Livermore National Laboratory. The agreement between the
OP and OPAL calculations is generally good, although there are some
differences. Both calculations give results larger than those obtained
in earlier work, by factors of up to 3 or more.
Title: Atomic Opacities
Authors: Seaton, M. J.
Bibcode: 1994LNP...428..265S
Altcode: 1994IAUCo.146..265S; 1994mse..conf..265S
No abstract at ADS
Title: OP versus OPAL opacities: consequences for B star oscillations
Authors: Pamyatnykh, A. A.; Dziembowski, W. A.; Moskalik, P.; Seaton,
M. J.
Bibcode: 1994IAUS..162...70P
Altcode:
No abstract at ADS
Title: Fitting and smoothing of opacity data
Authors: Seaton, M. J.
Bibcode: 1993MNRAS.265L..25S
Altcode:
Techniques are described for the fitting and smoothing of
Rosseland mean opacities, κR(T, ρ), where T is
the temperature and ρ the mass density, and for calculating
the first two derivatives, ∂κR(/∂T|ρ and
∂κR(/∂ρ|T. The codes OPALFIT and OPFIT are
available for the processing of opacities calculated by the projects
OPAL and OP. Both projects use techniques of opacity sampling
(that is to say, a number of frequency points insufficient to resolve
all detailed structures in monochromatic opacities), which lead to some
lack of smoothness in the tabulated values of κR(T,ρ). It
is shown that improved accuracy for the derivatives is obtained by
using a technique of two-dimensional smoothing.
Title: The Opacity Project
Authors: Seaton, M. J.
Bibcode: 1993npsp.conf..231S
Altcode: 1993IAUCo.139..231S
No abstract at ADS
Title: Radiative opacities.
Authors: Seaton, M. J.
Bibcode: 1993ASPC...40..222S
Altcode: 1993ist..proc..222S; 1993IAUCo.137..222S
A review is given of opacity calculations made during the past decade.
Title: The International Opacity Project
Authors: Seaton, M. J.; Mihalas, D.; Yan, Y.
Bibcode: 1992AAS...180.2203S
Altcode: 1992BAAS...24..759S
No abstract at ADS
Title: The Opacity Project - a Post-Script
Authors: Seaton, M. J.
Bibcode: 1992RMxAA..23..180S
Altcode:
New Opacity Project results of January 1992 are presented.
Title: The Opacity Project - Computation of Atomic Data
Authors: Seaton, M. J.; Zeippen, C. J.; Tully, J. A.; Pradhan, A. K.;
Mendoza, C.; Hibbert, A.; Berrington, K. A.
Bibcode: 1992RMxAA..23...19S
Altcode:
A general description is given of the methods used by the participants
in the international C acity Project to produce massive sets of accurate
radiative atomic data, followed by some illustrative examples of
results obtained. Key words: ATOMIC PROCESSES - TRANSITION PROBABILITIES
Title: Stellar Envelope Opacities
Authors: Seaton, M. J.
Bibcode: 1990BAAS...22Q.844S
Altcode:
No abstract at ADS
Title: Mass production of accurate atomic data
Authors: Seaton, M. J.
Bibcode: 1990asos.conf..120S
Altcode:
No abstract at ADS
Title: Excitation of O III by electron collisions and photoionization
of O II
Authors: Burke, V. M.; Lennon, D. J.; Seaton, M. J.
Bibcode: 1989MNRAS.236..353B
Altcode:
The close coupling method is used to study the (O III + e) collision
problem and the bound states of O II, and to calculate the cross-section
for the photoionization of O II 2p3 4S0. Elaborate wave functions are
employed for the O(2+) states. The calculated collision strengths are
consistent with previous data.
Title: Book-Review - Spectroscopy of Astrophysical Plasmas
Authors: Dalgarno, A.; Layzer, D.; Seaton, M. J.
Bibcode: 1988Obs...108..247D
Altcode:
No abstract at ADS
Title: Atomic data for opacity calculations. I. General description
Authors: Seaton, M. J.
Bibcode: 1987JPhB...20.6363S
Altcode:
Extensive calculations of accurate data are being made in
a collaborative effort referred to as the Opacity Project. These
data will be used to obtain improved values for opacities in stellar
envelopes, and should also be of interest for other problems in physics
and astronomy. The present paper, which is the first in a series,
gives some of the formulas from thermodynamics and atomic physics
which are required for opacity calculations.
Title: Nova Aquilae 1982.
Authors: Snijders, M. A. J.; Batt, T. J.; Roche, P. F.; Seaton, M. J.;
Morton, D. C.; Spoelstra, T. A. T.; Blades, J. C.
Bibcode: 1987MNRAS.228..329S
Altcode:
Observations of Nova Aquilae obtained from the IUE, the Anglo-Australian
Telescope, and the Westerbork Synthesis Radio Telescope are
discussed. Light curves for B, V, and continuum fluxes at 2800 and
1300 A are presented. Velocities obtained from line profiles indicate
the presence of three main gaseous components: a high velocity gas
(HVG) component found only in absorption lines on D = 29 and 35, and
a medium-velocity gas (MVG) and low-velocity gas component found in
emission lines and, for the MVG, in the Na I D-line absorption. A model
for thermal emission from the HVG is discussed. It is suggested that
ionization in the MVG was maintained by photoionization. Properties of
the dust grains are studied using data from absorption in the optical
and UV and from emission in the IR.
Title: Atomic data required for opacity calculations
Authors: Seaton, M. J.
Bibcode: 1987sls..conf..583S
Altcode:
The close-coupling approximation here implemented by means of the
R-matrix method furnishes an efficient computational technique for the
calculation of opacities. The line-broadening is noted to be of crucial
importance for opacity calculations; a line with a delta-function
profile yields zero distribution to the Roseland mean opacity. The
line width of importance is that for which the line wing returns to
nearly the level of the background continuum. For nonhydrogenic ions,
the dominant contribution to the broadening is likely to be due to
electron impacts.
Title: Photo-ionisation of Atomic Oxygen
Authors: Seaton, M. J.
Bibcode: 1987ramp.conf...29S
Altcode:
No abstract at ADS
Title: Nova Aquilae 1982
Authors: Morton, D. C.; Snijders, M. A. J.; Batt, T. J.; Roche, P. F.;
Seaton, M. J.; Spoelstra, T. A. T.; Blades, J. C.
Bibcode: 1986BAAS...18..918M
Altcode:
No abstract at ADS
Title: Book-Review - Physics of Gaseous Nebulae
Authors: Aller, L. H.; Seaton, M. J.
Bibcode: 1986Obs...106...78A
Altcode:
No abstract at ADS
Title: Atomic data for astrophysics
Authors: Seaton, M. J.
Bibcode: 1986lect.conf....1S
Altcode:
No abstract at ADS
Title: Collision Strength for the Neii Line at 12.8-MICRONS
Authors: Bayes, F. A.; Saraph, H. E.; Seaton, M. J.
Bibcode: 1985MNRAS.215P..85B
Altcode:
New calculations have been made of the
collision strength for Ne+2p5
2P3/2-2P1/2, using both
semi-empirical and elaborate ab initio methods. The results obtained
are in satisfactory agreement and the recommended thermally averaged
collision strength is Y = 0.293+0.8×10-6Te
with Te in K.
Title: Atomic processes in astronomy.
Authors: Seaton, M. J.
Bibcode: 1985EN.....16...12S
Altcode:
No abstract at ADS
Title: Obituary - Massey, Sir-Harrie
Authors: Seaton, M. J.
Bibcode: 1984QJRAS..25..530S
Altcode: 1984QJRAS..25..530.
No abstract at ADS
Title: Nova Aquilqe 1982 - a short report.
Authors: Snijders, M. A. J.; Batt, T. J.; Seaton, M. J.; Blades,
J. C.; Morton, D. C.
Bibcode: 1984MNRAS.211P...7S
Altcode:
Observations of Nova Aquilae 1982, obtained using the IUE satellite and
the Anglo-Australian telescope, are reported. The nova was unusual in
having very high expansion velocities (up to 10000 km s-1),
high abundances of heavier elements and large fluxes (reported by other
observers) at IR and radio wavelengths. The ejecta had comparable masses
in gas and dust, and grain formation may have led to gas-phase element
depletions. The hydrogen mass fraction for the ejecta is estimated to
be about 0.03. The progenitor may have been a close-binary system.
Title: K 648, the planetary nebula in the globular cluster M 15.
Authors: Adams, S.; Seaton, M. J.; Howarth, I. D.; Auriere, M.; Walsh,
J. R.
Bibcode: 1984MNRAS.207..471A
Altcode:
IUE observations of the planetary nebula K648 in the globular cluster
M15 have been obtained, and the first determinations of the C abundance
in the nebular envelope have been made. Improved measurements of
fluxes in the optical nebular emission lines and in forbidden N II 6583
have been obtained. Semiforbidden C III 1908 and C II 2326 are strong
nebular emission features and C IV 1549 is a broad stellar absorption
feature. The nebular shell abundances include: (C/H) = +0.05; (N/H) =
-1.5; (O/H) = -1.25; and (He/H) = -1.5. The large C abundance indicates
enrichment by material which has been involved in He burning. The C
IV absorption is interpreted as being due to a P Cygni feature. The
stellar mass loss rate is about 2 x 10 to the -10th solar masses/yr. The
ionized hydrogen mass is 0.011 solar mass. The central star has 3000
solar luminosities for an adopted effective temperature of 38,000 +
or - 4000 K.
Title: UV Spectra of Nebulae and Novae (invited Paper, Short Abstract)
Authors: Seaton, M. J.
Bibcode: 1984uxsa.coll...67S
Altcode: 1984IAUCo..86...67S; 1984uxsa.conf...67S
No abstract at ADS
Title: Ultraviolet and optical observations of two stars belonging
to the blue horizontal branch of the globular cluster M 15.
Authors: Auriere, M.; Adams, S.; Seaton, M. J.
Bibcode: 1983MNRAS.205..571A
Altcode:
Two stars belonging to the blue horizontal branch of the globular
cluster M 15 have been observed in the ultraviolet with lUE and
measurements have been made of their U B and V magnitudes. Measurements
have also been made of B and V magnitudes for all the other stars
which were in the JUE aperture and which are resolved in optical
observations. Comparison of the observed optical and ultraviolet fluxes
with fluxes computed from models gives effective temperatures of 17
500 ± 1000K for the two stars of the blue horizontal branch.
Title: Analysis of nebulosity in the planetary nebula NGC 40.
Authors: Clegg, R. E. S.; Seaton, M. J.; Peimbert, M.; Torres-Peimbert,
S.
Bibcode: 1983MNRAS.205..417C
Altcode:
Optical and IUE spectra of a bright region of nebulosity in the
planetary nebula NGC 40 have been combined for an analysis of the
physical conditions and abundances in the nebula. Resulting abundances
are typical of planetary nebulae and indicate that the nebulosity
does not share the extreme abundances of the Wolf-Rayet WC8 central
star. The continuum observed in the IUE spectra at an offset position
can be fitted well to a calculated nebular continuum; the nebular
lines are measured relative to the continuum and hence put on the same
scale as optical lines measured relative to H-beta. The absence of
stellar features in the offset spectra show that the optical depth for
scattering by dust is less than 0.06. The emission in CIV wavelength
1549 from the nebula is anomalously strong. It may be explained, in
part, as due to resonance scattering of radiation from the star or
from the inner part of the nebula. New recombination lines of CII are
identified in IUE spectra; their intensities relative to CII wavelengths
4267, 7230 in NGC 40 and IC 418 are analyzed.
Title: REVIEW ARTICLE: Quantum defect theory
Authors: Seaton, M. J.
Bibcode: 1983RPPh...46..167S
Altcode:
No abstract at ADS
Title: Ultraviolet Spectrophotometry of Some Hotter Central Stars
Authors: Clegg, R. E. S.; Seaton, M. J.
Bibcode: 1983IAUS..103..536C
Altcode:
No abstract at ADS
Title: Optical and UV nebular spectra of NGC 40.
Authors: Peimbert, M.; Torres-Peimbert, S.; Clegg, R. E. S.; Seaton,
M. J.
Bibcode: 1983IAUS..103..521P
Altcode:
No abstract at ADS
Title: Nebular Abundances and Central Star Parameters for Eight
Planetary Nebulae in the Magellanic Clouds
Authors: Barlow, M. J.; Adams, S.; Seaton, M. J.; Willis, A. J.;
Walker, A. R.
Bibcode: 1983IAUS..103..538B
Altcode:
No abstract at ADS
Title: Lines From Nebular Envelopes of Importance for Spectroscopic
Diagnostic and Abundance Determinations
Authors: Seaton, M. J.
Bibcode: 1983iue..prop.1631S
Altcode:
No abstract at ADS
Title: Why is IC 4642 of such high-excitation class ?
Authors: Penn, C. J.; Flower, D. R.; Barlow, M. J.; Seaton, M. J.;
Aller, L. H.
Bibcode: 1983IAUS..103..539P
Altcode:
No abstract at ADS
Title: Some recent results from UV observations.
Authors: Seaton, M. J.
Bibcode: 1983IAUS..103..129S
Altcode:
Selected IUE observations of planetary nebulae (PN) are summarized. The
determination of C/O ratios and C species abundances is reviewed, taking
the effects of resonance lines and dust absorption into account. In
five PN with a silicate IR feature, C/O was found to be less than or
equal to 0.5, while C/O was greater than about 1 for four PN with an
SiC feature. Dust-opacity estimations (tauD = about 0.1)
are shown to agree well with observed thermal-IR emission data. IUE
and optical photometric studies of PN central stars reveal Zanstra
temperatures significantly higher than those calculated by Pottasch et
al. (1978). The PN K 648 in the globular cluster M15 is characterized
by C/O 2.4, central-star temperature 30,000 K, and luminosity 1700
solar L (assuming the distance of M15 to be 10 kpc).
Title: Ultraviolet spectra of planetary nebulae. IX. High-dispersion
observations of NGC 7662.
Authors: Flower, D. R.; Penn, C. J.; Seaton, M. J.
Bibcode: 1982MNRAS.201P..39F
Altcode:
High-dispersion IUE spectra of NGC 7662 have been obtained with exposure
times optimized for measurement of the relative strengths of the
components of: CIV lambda-lambda 1548, 1551; the forbidden line CIII
lambda-lambda 1907, 1909 and the forbidden line Ne IV lambda-lambda
2422, 2424. The CIV ratio (lambda 1548)/(lambda 1551) is expected to
be 2.00, the ratio of the statistical weights of the emitting levels,
and the observed value is within 0.15 of 1.92. The forbidden line CIII
and the forbidden line Ne IV ratios depend upon the electron density,
Ne, and from the observations it is concluded that Ne for NGC 7662
is in the range 1300 to 5000/cu cm. The presence of the dielectronic
recombination line CIII lambda 2297 is established beyond doubt. The
absolute calibration of IUE high-dispersion spectra is also discussed.
Title: Ultraviolet spectra of planetary nebulae. VII. The abundance
of carbon in the very low excitation nebula HE 2-131.
Authors: Adams, S.; Seaton, M. J.
Bibcode: 1982MNRAS.200P...7A
Altcode:
Features observed in infrared spectra suggest that certain very low
excitation (VLE) nebulae have low C/O abundance ratios (Cohen &
Barlow 1980; Aitken & Roche 1982). Fluxes in the multiplets [O
II] sun2470 and [O II] λ2326 have been measured for the
VLE nebula He 2-131 = HD 138403 using lUE high-dispersion spectra. An
analysis similar to that of Harrington et al. (1980) for IC 418 gives
C/O = 0.3 for He 2-131, compared with C/O = 1.3 for IC 418 and 0.6
for the Sun.
Title: Ultraviolet observations of Nova Aquila 1982.
Authors: Snijders, M. A. J.; Seaton, M. J.; Blades, J. C.
Bibcode: 1982ESASP.176..177S
Altcode: 1982iue..conf..177S; 1982IUE3r......177S
Observations by IUE of the nova Aquila, announced on 27.85 Jan. 1982 UT,
with visual magnitude of 6 to 7 are described. Observations made during
24 Feb. and 2 Mar. show that the nova has a heavily reddened spectrum,
E(B-V) = 0.55 + or - 0.15, and a complex short wavelength spectrum
with both narrow emission features and broad absorption troughs,
indicating terminal velocities as high as 10,000 km/sec. Both the
continuum flux and the absorption line profiles show complex changes
between 24 Feb. and 2 Mar. On 24 Feb. short period variations were also
present. Complementary optical, infrared and ultraviolet observations
were obtained on 9 April.
Title: Ultraviolet spectra of planetary nebulae. 6 NGC 7662.
Authors: Harrington, J. P.; Seaton, M. J.; Adams, S.; Lutz, J. H.
Bibcode: 1982MNRAS.199..517H
Altcode:
The planetary nebula NGC 7662 considered in the present investigation
has been a popular object for detailed modelling. Two models are
considered. In Model I, which is similar to a model considered by
Harrington et al. (1979), dust absorption is neglected. The model cannot
provide results which are in agreement with two types of observational
data. It is believed that Model II gives a better description of
NGC 7662. Observations are considered along with aspects of data
extraction and reduction, absolute flux measurements, the accuracy of
the measurements made with the aid of the International Ultraviolet
Explorer (IUE) spacecraft, and the continuum fluxes. The temperature of
the central star is discussed, taking into account Zanstra temperatures,
and the color temperature from IUE observations. Attention is also
given to adopted fluxes in nebular lines, the computer program for
models of planetary nebulae, and model results.
Title: Presidential Address - Atoms Astronomy and Aeronomy
Authors: Seaton, M. J.
Bibcode: 1982QJRAS..23....2S
Altcode:
No abstract at ADS
Title: Address of President Wolfendale on the Work of Thackeray,
A.D. 1981JUL10
Authors: Seaton, M. J.
Bibcode: 1982QJRAS..23....1S
Altcode:
No abstract at ADS
Title: Nova Aquilae 1982
Authors: Argyle, R.; Rosino, L.; Iijima, T.; Blades, J. C.; Seaton,
M. J.; Snijders, M. A. J.
Bibcode: 1982IAUC.3673....2A
Altcode:
R. Argyle, Royal Greenwich Observatory, provides the following
precise position derived from a single plate taken on Feb. 22 at mpg
= 11: R.A. = 19h20m50s14, Decl. = +2o23'35".4 (+/- 0".5; equinox
1950.0). L. Rosino, Asiago Astrophysical Observatory, telexes: "A
spectrogram obtained at Asiago by T. Iijima on Feb. 13 (dispersion 6 nm
at H-gamma) shows wide and diffuse Balmer lines (halfwidth ~ 2300 km/s)
with a sharp central core. Narrow emission lines of He I and Fe II are
also represented. No P-Cyg absorption has been observed. The presence
of a weak, broad emission of He II 469 nm is suspected. The spectrum
of this nova, which has had an extremely rapid decline, is therefore
rather peculiar." J. C. Blades, European Space Agency, Villafranca,
reports that he, M. J. Seaton and M. A. J. Snijders observed the
nova with the IUE satellite. The fes nagnitude (520 nm) averaged 10.8
on Feb. 24.42 UT, but monitoring suggests the optical flux might be
variable on a timescale of hours. The 200-300-nm wavelength region shows
a heavily-reddened continuum with broad H-sigma II 280 nm emission. This
spectrum closely resembles that found for the transitional phase in
V1668 Cyg (= Nova Cyg 1978). However, the 120-200-nm region differs
drastically: instead of a faint continuum with strong low-ionization
lines there is a prominent continuum with P-Cyg-type features at 155
and 139 nm tentatively identified with C IV and Si IV; there is also
strong absorption at 123 nm that could be N V.
Title: Planetary Nebulae and Their Central Stars
Authors: Seaton, M. J.
Bibcode: 1982iue..prop.1343S
Altcode:
No abstract at ADS
Title: Ultraviolet observations of Nova Aquilae 1982.
Authors: Snijders, M. A. J.; Seaton, M. J.; Blades, J. C.
Bibcode: 1982NASCP2238..625S
Altcode: 1982NASCP2338..625S; 1982IUE82......625S; 1982auva.nasa..625S
A bright nova in Aquila was discovered with a visual magnitude of 6
to 7. A number LWR and SWP spectra of the object were obtained with
the IUE. The nova has a heavily reddened spectrum, E(B-V)= 0.55 + or -
0.15, and a complex shot wavelength spectrum with both narrow emission
features and broad absorption troughs, indicating terminal velocities as
high as 10,000 km/s. Both the continuum flux and the absorption line
profiles changed between two observations. Short period variations
were also observed.
Title: Nova Cygni 1978 - I. The nebular phase.
Authors: Stickland, D. J.; Penn, C. J.; Seaton, M. J.; Snijders,
M. A. J.; Storey, P. J.
Bibcode: 1981MNRAS.197..107S
Altcode:
UV observations of Nova Cygni 1978 have been obtained using IUE on
17 dates between 1978 September 12 and 1979 July 8 inclusive. The
discussion includes the interpretation, during the nebular stage,
of the UV observations together with optical and IR data from other
observers. Analysis of the available data indicates that Nova Cygni
1978 was produced by a mechanism involving a thermonuclear runaway.
Title: Ultraviolet spectra of planetary nebulae - the C II 1335
dielectronic recombination lines in IC 418.
Authors: Clavel, J.; Flower, D. R.; Seaton, M. J.
Bibcode: 1981MNRAS.197..301C
Altcode:
High-dispersion observations of IC 418 by the IUE satellite show the C
II lines (1) 1334.53 A, (2) 1335.66 A, and (3) 1335.71 A. The profiles
are of the P Cygni type, a form expected to result from scattering
in the expanding nebular shell, with a mean velocity difference
between nebular emission lines and interstellar absorption lines of 47
km/sec. The interpretation of flux ratios demonstrates that the 1335
A lines are excited by dielectronic recombination, and comparison
with the C II 4267 A recombination line indicates that the flux in
1335 A is attenuated by a factor of 0.54 due to absorption by dust
within the nebula. The observed ratio F(2 + 3)/F(1) of fluxes in the
C II lines is 1.5 + or - 0.4, compared with the ratio of 2.0 expected
for LS coupling. Further research is recommended to determine whether
dielectronic recombination can lead to significant departures from
the LS coupling ratio.
Title: Ultraviolet spectra of planetary nebulae. IV. The CIII 2297
dielectronic recombination line and dust absorption in the CIV 1549
resonance doublet.
Authors: Harrington, J. P.; Lutz, J. H.; Seaton, M. J.
Bibcode: 1981MNRAS.195P..21H
Altcode:
Calculations by Storey confirm, as suggested in Paper I of the present
series, that dielectronic recombination via low-lying auto-ionizing
states is important for the calculation of the ionization equilibria
of carbon ions. These processes also produce various spectrum
lines, of which one is C III 2p2 1D → 2s 2p
1P λ2297. Observations of this line are reported for NGC
7009 and 7662. Using C3+ abundances deduced from λ 2297,
it is shown that the fluxes in the C IV λ 1549 resonance doublet are
attenuated by dust absorption. The C/H abundance ratios deduced from
C III λ2297, C III] λ1908 and C II] λ2326 are 4 × 10-4
for NGC 7009 and NGC 7662.
Title: Gold Medal Awarded to Knopoff, Leon - 1980SEP5
Authors: Seaton, M. J.
Bibcode: 1981QJRAS..22....1S
Altcode:
No abstract at ADS
Title: Address of the President on the Presentation of the Gold
Medal to Lovell, Sir-Bernard on 1981FEB13
Authors: Seaton, M. J.
Bibcode: 1981QJRAS..22..225S
Altcode:
No abstract at ADS
Title: Ultraviolet spectra of planetary nebulae. III. Mass loss from
the central star of NGC 6543.
Authors: Castor, J. I.; Lutz, J. H.; Seaton, M. J.
Bibcode: 1981MNRAS.194..547C
Altcode:
An ultraviolet spectrum of the central star of NGC 6543 has been
obtained at low dispersion with the short wave camera of the IUE
satellite, and the terminal velocity of the stellar wind has been
estimated at 2150 km/s. By assuming the effective temperature to be the
Zanstra temperature of 43,000 K and the luminosity to be that derived
from the evolutionary tracks of Paczynski (1971), the stellar radius
is found to be 0.8 solar radius. A mass loss rate of the order of 10
to the -7th solar masses per year is deduced from the optically thin
UV P Cygni lines and found to be reasonably consistent with the lambda
4686 He II emission. No evidence of marked abundance anomalies in the
central star has been found.
Title: Planetary Nebulae and Their Central Stars
Authors: Seaton, M. J.
Bibcode: 1981iue..prop..950S
Altcode:
No abstract at ADS
Title: Where exactly is the planetary nebula in M 15?
Authors: Adams, S.; Penn, C. J.; Seaton, M. J.
Bibcode: 1980Obs...100..209A
Altcode:
No abstract at ADS
Title: Oscillator strengths for O III and the Bowen fluorescent
mechanism
Authors: Saraph, H. E.; Seaton, M. J.
Bibcode: 1980MNRAS.193..617S
Altcode:
Transition probabilities and relative intensities are calculated for
transitions between O III triplets excited by the Bowen fluorescence
mechanism, which have been observed in the optical spectra of nebulae. A
frozen cores approximation is used to obtain wavelengths and oscillator
strengths for the dipole transitions and the probabilities of a Bowen
cascade and the various transitions within the cascade following
excitation of the 2p 3d 3P(0)2 state of O III. Intensities of the O
III lines and multiples calculated relative to those for the He II
line at 4686 A and cascade proportions are shown to agree with the
values observed in the optical and UV of the planetary nebula NGC
7662. The observed relative intensities of the O III lines, in the
solar spectrum at wavelengths of 644 and 374 A, however, are found to
be inconsistent with the Bowen mechanism.
Title: Presidential address : spectra of gaseous nebulae.
Authors: Seaton, M. J.
Bibcode: 1980QJRAS..21..229S
Altcode: 1980QJRAS..21..229.
A brief historical review of observations of planetary nebulae are
presented with attention given to Herschel's observation of NGC
1514 (1790) and Huggins' observations (1864). The current status of
observations of gaseous nebulae is also presented; consideration is
given to the identification of nebular lines, physical processes,
the radio continuum, recombination lines, infrared observations,
optical spectra, and ultraviolet spectra.
Title: Ultraviolet spectra of planetary nebulae. I. The abundance
of carbon in IC 418.
Authors: Harrington, J. P.; Lutz, J. H.; Seaton, M. J.; Stickland,
D. J.
Bibcode: 1980MNRAS.191...13H
Altcode:
IUE observations of IC 418 give fluxes in the lines C III λ1908,
C II] λ2326, [O II] λ2470 and Mg II λ2800. Fluxes on the scale
of F(Hβ) = 100 are obtained using a computed [O II] branching
ratio F(λλ7320,30)/F(λ2470) and an observed ratio F(λλ7320,
30)/F(Hβ). The deduced carbon abundance is log {C} = 8.85 ± 0.2
compared with log {C} = 8.67 ± 0.1 for the Sun. A critical discussion
is given on methods used previously to determine carbon abundances in
planetary nebulae.
Title: Presidential Address on the Award of the Chapman Medal to
Parker, Eugene
Authors: Seaton, M. J.
Bibcode: 1980QJRAS..21...73S
Altcode:
No abstract at ADS
Title: Obituary - Hume, Alan
Authors: Tayler, R. J.; Seaton, M. J.
Bibcode: 1980QJRAS..21...71T
Altcode:
No abstract at ADS
Title: Theory of recombination lines
Authors: Seaton, M. J.
Bibcode: 1980ASSL...80....3S
Altcode: 1980rrl..conf....3S
A historical perspective on the theory of recombination lines is
presented, and a simple conceptual approach to the basic ideas of this
theory is given with reference to astronomical observations. Attention
is given to recombination lines observed in radio, IR, optical, and UV
spectral regions (up to 1.83 MGHz or 1640 A). Some recombination-line
results for Orion are discussed.
Title: Observations of Selected Planetary Nebulae
Authors: Seaton, M. J.
Bibcode: 1980iue..prop..606S
Altcode:
No abstract at ADS
Title: Ultraviolet spectra of planetary nebulae and their central
stars.
Authors: Seaton, M. J.
Bibcode: 1980HiA.....5..247S
Altcode:
No abstract at ADS
Title: Carbon abundance in IC 418.
Authors: Harrington, J. P.; Lutz, J. H.; Seaton, M. J.; Stickland,
D. J.
Bibcode: 1979BAAS...11..628H
Altcode:
No abstract at ADS
Title: IUE observations of the planetary nebula NGC 7662.
Authors: Harrington, J. P.; Lutz, J. H.; Seaton, M. J.
Bibcode: 1979JRASC..73..305H
Altcode:
No abstract at ADS
Title: Extinction of NGC 7027.
Authors: Seaton, M. J.
Bibcode: 1979MNRAS.187..785S
Altcode:
Emission intensities of recombination lines in hydrogenic spectra
are known accurately relative to intensities in the free-free radio
continuum. For NGC 7027 intensities have been measured for the
radio continuum and for H I and He II lines in the wavelength range
from 2.17 microns to 1640 A; comparison with the calculated emission
intensities gives the wavelength-dependent extinction. Determinations
of the standard interstellar extinction function are critically
discussed. The extinction deduced for the total radiation from NGC 7027
has a dependence on wavelength for wavelengths between 6563 A and 1640
A which is in excellent agreement with the adopted standard results,
but there are some anomalies for longer wavelengths and for the ratio
of total to selective extinction. These can be explained by using a
model that allows for a local contribution to the extinction which is
variable over the surface of the nebula.
Title: The [Ne IV] 2D -> 4S lines in the
planetary nebula NGC 7662.
Authors: Lutz, J. H.; Seaton, M. J.
Bibcode: 1979MNRAS.187P...1L
Altcode:
Results are reported for high-resolution IUE satellite observations of
the 2p 3 2D 3/2 - 4S, 2D 5/2 - 4S UV forbidden lines of Ne IV in the
spectrum of NGC 7662. Precision measurements of the wavelengths are
obtained, along with an improved determination of Ne IV doublet-quartet
separations, a measurement of the intensity ratio for the forbidden
Ne IV lines, and velocity profiles for the 2D - 4S lines. A nebular
expansion velocity of 20 + or - 3 km/s is measured for the 2D 3/2 -
4S forbidden line of Ne IV.
Title: Interstellar extinction in the UV.
Authors: Seaton, M. J.
Bibcode: 1979MNRAS.187P..73S
Altcode:
The UV interstellar extinction obtained from observations with the
OAO 2, Copernicus, and TD 1 satellites is fitted to simple analytical
expressions in the variable x = 1/lambda. The feature centered at a
wavelength of about 2200 A can be fitted to a Lorentz profile; this
result may be of physical significance.
Title: Ultra-violet observations of Nova Cygni 1978
Authors: Stickland, D. J.; Penn, C. J.; Seaton, M. J.; Snijders,
M. A. J.; Storey, P. J.; Kitchin, C. R.
Bibcode: 1979IUE1.symp...63S
Altcode:
Ultra-violet observations of Nova Cygni 1978 made with IUE are
described. The time development of the spectra is discussed and the
main spectral features characteristic of the maximum, early decline,
transitional and nebular stages are outlined. An estimate of the
colour excess is obtained. Total observed fluxes from the nova have
been calculated for the first 50 days, including estimates of optical
and infra-red contributions. The flux is found to decline by a factor
of two to three over the first 8 days after maximum light, and by a
further factor two between 8 and 50 days after maximum. Temperatures
are deduced from line ratios in C II and C III, and the abundance ratio
of nitrogen to carbon is derived. Nitrogen is found to be overabundant
relative to carbon by a factor of 5 to 8 compared to the cosmic value
Title: Planetary Nebulae
Authors: Seaton, M. J.
Bibcode: 1979iue..prop..380S
Altcode:
No abstract at ADS
Title: IUE observations of the planetary nebula NGC 7662
Authors: Lutz, J. H.; Harrington, J. P.; Seaton, M. J.
Bibcode: 1979IUE1.symp..199L
Altcode:
IUE spectra of planetary nebulae and their central stars have been
obtained by a number of observers and should provide important new
information about these objects. The paper is concerned with detailed
observations of NGC 7662, for which a rocket UV spectrum has been
previously obtained by Bohlin et al. (1978). The observations were
made for three regions: (1) region A, large slot centered on central
star; (2) region B, small slot centered on star; and (3) region C,
small slot off-set approximately 5 in. from star. For all regions a
number of exposures were obtained, longer exposures to give good SNRs
for weak lines and shorter exposures to obtain strong unsaturated
lines. The results are compared with predictions from models of the
ionization and thermal balance in the nebula. The results should be
useful in establishing reliable methods of analysis which can be used
for studying other planetaries.
Title: Calculated intensities of He II recombination lines in the
ultraviolet
Authors: Seaton, M. J.
Bibcode: 1978MNRAS.185P...5S
Altcode:
Summary. Scaling laws are used to calculate the intensities of He ii
recombination lines observed in the ultraviolet.
Title: IUE observations of the interstellar medium.
Authors: Grewing, M.; Boksenberg, A.; Seaton, M. J.; Snijders,
M. A. J.; Wilson, R.; Boggess, A.; Bohlin, R. C.; Perry, P. M.;
Schiffer, I. H., III; Gondhalekar, P. M.; Macchetto, F.; Savage,
B. D.; Jenkins, E. B.; Johnson, H. M.; Perinotto, M.; Whittet, D. C. B.
Bibcode: 1978Natur.275..394G
Altcode:
Results from a preliminary study of interstellar absorption lines in the
spectra of the three stars HD149757, HD93521 and HD153919, observed by
the IUE during its commissioning phase are presented. They demonstrate
the capabilities of the IUE for interstellar research. Results from a
preliminary study of the emission line spectrum of the planetary nebula
NGC 7027 are outlined and used to study its interstellar reddening.
Title: A Study of Novae and Supernovae
Authors: Seaton, M. J.
Bibcode: 1978iue..prop..101S
Altcode:
No abstract at ADS
Title: The Structure of Planetary Nebulae
Authors: Seaton, M. J.
Bibcode: 1978iue..prop..103S
Altcode:
No abstract at ADS
Title: Advances in Studies of Physical Processes
Authors: Seaton, M. J.
Bibcode: 1978IAUS...76..131S
Altcode:
Advances in calculations of physical processes in planetary nebulae have
been attained with regard to (1) excitation of forbidden lines, (2)
transition probabilities np3 2D5/2 to 4S, 2D3/2 to 4S; (3) hydrogenic
recombination spectra; and (4) permitted lines other than those of
H I, He I and He II. Problems in these calculations are mentioned,
and accuracies are judged.
Title: Some O I oscillator strengths and the interstellar abundance
of oxygen.
Authors: Zeippen, C. J.; Seaton, M. J.; Morton, D. C.
Bibcode: 1977MNRAS.181..527Z
Altcode:
Calculated and experimental oscillator strengths for the O I
intersystem line at 1356 A and for other O I lines of interest in
interstellar absorption-line studies are discussed. Attention is
given to experimental f-values for the lines at 1302, 1305, and 1306
A, previous work on the f-values for the lines at 1356 and 1359 A,
wave-function expansion, and calculations for permitted as well as
intercombination lines. Copernicus observations of several interstellar
absorption lines due to O I, C II, P II, and Ni II toward Zeta Oph are
reported, equivalent widths are determined, and a curve-of-growth
analysis is performed for the O I absorption lines. Oscillator
strengths are recommended for the far-UV resonance lines of O I,
and it is concluded that the oxygen in the interstellar H I regions
toward Zeta Oph is depleted by 45% to 69%.
Title: Applications of Quantum-Defect Theory.
Authors: Seaton, M. J.
Bibcode: 1977uxsa.collE..12S
Altcode: 1977IAUCo..43.....S
No abstract at ADS
Title: Di-electronic recombination.
Authors: Seaton, M. J.; Storey, P. J.
Bibcode: 1976apa..conf..133S
Altcode:
No abstract at ADS
Title: Collision strengths for [N II], [O III], [Ne II] and [Ne III].
Authors: Seaton, M. J.
Bibcode: 1975MNRAS.170..475S
Altcode:
The results of some recent collision strength calculations are given
in a form convenient for the interpretation of observations. The
calculations are estimated to be accurate within 10 percent based on:
(1) comparison of observed and calculated energies for the bound states
of the (electron + ion) systems. Semi-empirical corrections are made for
forbidden Ne II; (2) comparisons of calculated positions obtained from
iso-electronic sequence extrapolations. This of particular importance
for forbidden O III; and (3) the convergence of the expansions used
for the wave functions, in particular for forbidden N II and Ne III,
where inclusion of additional states produces only small changes in
the near-threshold results.
Title: Radio continuum spectra
Authors: Seaton, M. J.
Bibcode: 1975LNP....42..260S
Altcode: 1975hrrt.conf..260S
Radio continuum flux spectra S, depend on electron temperatures
Te and on angular areas Ω (E) which enclose contours of
constant emission measure E. Different methods of analyzing observations
are discussed.
Title: Temperatures of Gaseous Nebulae-a Decade of Depression
Authors: Seaton, M. J.
Bibcode: 1974QJRAS..15..370S
Altcode:
Prior to 1963, electron temperatures in nebulae of about 10,000 K,
deduced from relative intensities of forbidden O III lines, were
generally accepted. In 1963, Burbidge, Gould and Pottasch argued that
temperatures should be much lower; thus for a nebula excited by a star
of temperature 30,000 K, they computed an electron temperature of 3,000
K. Subsequent observational work on radio surface brightnesses, radio
recombination lines, Balmer decrements and Balmer line-to-continuum
ratios appeared to confirm their results. However, all of the most
recent observational and theoretical work shows that these low electron
temperatures are not correct.
Title: On the interpretation of continuum flux observations from
thermal radio sources-1. Continuum spectra and brightness contours
Authors: Salem, M.; Seaton, M. J.
Bibcode: 1974MNRAS.167..493S
Altcode:
From maps of the brightness temperatures Tb of a thermal radio source,
at frequency vo, one obtains angular areas enclosed by contours,
w(T (vo)); this function is such that Tb> Tb' within the area
w(Tb'). The spectrum of the total flux density, Sp, observed at many
frequencies V, may be fitted to an analytic function of a frequency
variable x = f(v). It is shown that, using an inverse Laplace
transform, one obtains Te (E) as a function of V = E/ Te312, where
Te is the electron temperature, E is the emission measure, and (E)
is such that E> E' within the angular area Assuming a value for Te,
one may deduce (Tb(Vo)) from the spectrum observations. Adjustment of
Te can be made so as to obtain a best fit to a)(Tb(vo)) deduced from
observations with high angular resolution. If significant structural
detail has not been resolved, the results obtained for Te will tend
to be too small. Information concerning the extent to which structural
detail is resolved can be deduced if an independent estimate of Te is
available. For the Orion nebula, line observations at optical and radio
wavelengths indicate that Te = 9000+ 1000 K. It is shown that the radio
continuum data is consistent with this result, but that full resolution
of structural detail has not been achieved in some of the published
work on brightness contours. For the compact H+ region DR2I the best
fit is obtained with Te = 8ooo K, in agreement with the work of Harris,
but the available data does not exclude a higher value of Te, say Te =
9000 K. Various sources of error in the processes of observation and
analysis are discussed.
Title: Book reviews
Authors: Tauber, G. E.; Ceplecha, Zdenek; Pace, Nello; Peters, Rudolph
A.; de Graaff, W.; Vishniac, Wolf V.; Rees, M. J.; Kwee, K. K.; Rawer,
K.; de Jager, C.; Brandt, John C.; Reeves, H.; Kleczek, J.; Fokker,
A. D.; Seaton, M. J.; Öpik, E. J.
Bibcode: 1973SSRv...15..147T
Altcode:
No abstract at ADS
Title: Thermal conduction in planetary nebulae.
Authors: Sage, G.; Seaton, M. J.
Bibcode: 1973LIACo..18..241S
Altcode: 1973MSRSL...5..241S
No abstract at ADS
Title: Some recent calculations on excitation of [O III] lines.
Authors: Eissner, W.; Seaton, M. J.
Bibcode: 1973LIACo..18..203E
Altcode: 1973MSRSL...5..203E
No abstract at ADS
Title: Interpretation of the spectra of planetary nebulae.
Authors: Hummer, D. G.; Seaton, M. J.
Bibcode: 1973LIACo..18..225H
Altcode: 1973MSRSL...5..225H
No abstract at ADS
Title: Low Energy Cross Sections for Transitions between Highly
Excited States of Atoms
Authors: Percival, I. C.; Seaton, M. J.
Bibcode: 1972ApL....11...31P
Altcode:
No abstract at ADS
Title: University College London: Theoretical Atomic Physics and
Astrophysics (Department of Physics). Report 1970 January 1 to 1971
July 31.
Authors: Seaton, M. J.
Bibcode: 1972QJRAS..13..106S
Altcode:
No abstract at ADS
Title: Pressure broadening of radio recombination lines.
Authors: Peach, G.; Seaton, M. J.
Bibcode: 1972CoAMP...3..107P
Altcode:
No abstract at ADS
Title: Observations of radio recombination lines.
Authors: Brocklehurst, M.; Seaton, M. J.
Bibcode: 1972CoAMP...3..113B
Altcode:
No abstract at ADS
Title: On the interpretation of radio recombination line observations
Authors: Brocklehurst, M.; Seaton, M. J.
Bibcode: 1972MNRAS.157..179B
Altcode:
A substantial amount of data from atomic physics is required for the
interpretation of radio recombination line observations. A critical
review of the available data is presented, and it is concluded that
possible errors in this data are no larger than the errors which
typically occur in the observational results. The equation of transfer
for the line radiation is discussed and a convenient linearized form is
obtained. The use of this form is shown to be entirely adequate. The use
of constant density models is discussed and it is shown that such models
cannot explain all of the observational results for line-to-continuum
ratios and line profiles. In particular, observations of high n lines
can be explained only in terms of models which contain extensive outer
regions of low density. A spherically symmetric model is constructed for
the Orion nebula. The electron density Ne is tabulated as a function of
the distance r from the centre, and the electron temperature is taken
to be >c io30K. This model gives agreement with the following radio
observations, to within observational errors: the total continuum flux
as a function of frequency; all observed line-to-continuum ratios;
all observed line profiles. The electron temperature is in good
agreement with temperatures deduced from the relative intensities
of forbidden lines. From the success achieved with this model it is
concluded that the basic theory used for the interpretation of the
recombination lines is correct. An essential feature of the theory
is the assumption that Ne decreases as r increases. In order to take
account of other observations, having higher angular resolution, it
will be necessary to consider more complicated models, which allow
for local fluctuations in Ne.
Title: The Spectra of Gaseous Nebulae
Authors: Seaton, M. J.
Bibcode: 1972ama..conf..121S
Altcode:
No abstract at ADS
Title: The Profiles of Radio Recombination Lines
Authors: Brocklehurst, M.; Seaton, M. J.
Bibcode: 1971ApL.....9..139B
Altcode:
No abstract at ADS
Title: Recombiantion spectra of gaseous nebulae.
Authors: Seaton, M. J.
Bibcode: 1971CoAMP...3...46S
Altcode:
No abstract at ADS
Title: Abundances of helium in gaseous nebulae.
Authors: Seaton, M. J.
Bibcode: 1971heun.conf..288S
Altcode:
No abstract at ADS
Title: Abundances of Helium in Gaseous Nebulae
Authors: Seaton, M. J.
Bibcode: 1971HiA.....2..288S
Altcode:
No abstract at ADS
Title: Atomic Data of Importance for Ultraviolet and X-Ray Astronomy:
a Review of Theory
Authors: Seaton, M. J.
Bibcode: 1971HiA.....2..503S
Altcode:
No abstract at ADS
Title: Atomic data of importance for ultra-violet and X-ray astronomy:
A review of theory.
Authors: Seaton, M. J.
Bibcode: 1971adiu.conf..503S
Altcode:
No abstract at ADS
Title: Collision strengths for transitions in ions with configurations
3p3
Authors: Czyzak, S. J.; Krueger, T. K.; de A. P. Martins, P.; Saraph,
H. E.; Seaton, M. J.
Bibcode: 1970MNRAS.148..361C
Altcode:
Collision strengths are given for electron-impact transitions between
the levels 4S312, 2D312, 2D512, p112 and p312 in ions with configuration
3p3. At energies below the threshold for excitation of 2 the collision
strengths are averaged over resonances in series converging onto p.
Title: Electron densities in planetary nebulae.
Authors: Saraph, H. E.; Seaton, M. J.
Bibcode: 1970MNRAS.148..367S
Altcode:
Emissivities are tabulated for lines of [0 ii] (configuration 2p3)
and [S ii], [Cl iii], [Ar iv] and [K v] (configuration 3p3), using the
most accurate atomic data available. The intensity ratios R = I(2Ds/
)/I(2Ds/ ) are sensitive to electron density. Densities are obtained
from observed ratios in eleven planetary nebulae. The results obtained
from [0 ii], [S ii] and [Cl iii] are in good agreement. Larger densities
are obtained from [Ar iv] and [K v] in high excitation planetaries;
this may be evidence for large-scale density variations.
Title: A program to calculate radiative recombination coefficients
of hydrogenic ions
Authors: Flower, D. R.; Seaton, M. J.
Bibcode: 1969CoPhC...1...31F
Altcode:
No abstract at ADS
Title: Excitation of Forbidden Lines in Gaseous Nebulae I. Formulation
and Calculations for 2pq Ions
Authors: Saraph, Hannelore E.; Seaton, M. J.; Shemming, Jillian
Bibcode: 1969RSPTA.264...77S
Altcode:
A formulation is given for electron collisions with ions in
configurations 1s22s22pq and
1s22s22p63s23pq.
The main approximation is neglect of coupling to other
configurations. Hartree-Fock functions are used for the ion states and
the complete wave functions are expressed as sums of vector-coupled
anti-symmetrized products of ion functions and orbitals for the
colliding electron. Variational principles are used to obtain coupled
integro-differential equations for the radial functions for the
colliding electron, and to correct results obtained from approximate
solutions of these equations. All algebraic reductions are carried out
without the introduction of subsidiary approximations, and conservation
and reciprocity theorems are therefore satisfied exactly. Expressions
are tabulated for all algebraic coefficients. Numerical calculations
are made in two approximations: in the exact resonance approximation,
used only for p-waves, the wave functions are calculated with quadrupole
interactions neglected; and in the distorted wave approximation the wave
functions are calculated from static central potentials. Variational
corrections are calculated and are found to be reasonably small. It
is concluded that the final corrected results should agree closely
with results which would be obtained from exact solutions of the
coupled equations. Collision strengths are calculated for all inelastic
collisions in configurations 1s22s22pq,
q = 1 to 5, for at least three different energies, and for values of
the residual charge z = 1, 2, 3, 4, 5 and the limit of z --> ∞
. Results may be interpolated for all members of the iso-electronic
sequences. Results for energies such that some channels are open and
others closed are obtained by means of extrapolation techniques.
Title: Excitation of high Balmer lines case C of Aller, Baker
and Menzel
Authors: Seaton, M. J.
Bibcode: 1969MNRAS.145...91S
Altcode:
In case C of Aller, Baker & Menzel (1939) it is supposed that
absorption of stellar radiation in spectrum lines contributes to the
excitation of Balmer lines observed in gaseous nebulae. Calculations
are made of the ratio, for various levels n of the H atom, Rn =
(number of excitations by line absorption)! (number of excitations
by recombination). It is found that these ratios are small and it
is concluded that the case C mechanism cannot explain the anomalous
intensities which have been reported for the high Balmer lines.
Title: Electron collisions with positive ions.
Authors: Seaton, M. J.
Bibcode: 1969atph.conf..295S
Altcode: 1969agph.conf..295S
No abstract at ADS
Title: Excitation of Forbidden Lines by Electron Impact, Introductory
Report
Authors: Seaton, M. J.
Bibcode: 1969LIACo..15...45S
Altcode: 1969MSRSL..17...45S
No abstract at ADS
Title: Resonances in collision strengths for excitation of [O II]
and [O III]
Authors: Eissner, W.; de A. P. Martins, P.; Nussbaumer, H.; Saraph,
Hannelore E.; Seaton, M. J.
Bibcode: 1969MNRAS.146...63E
Altcode:
The collision strength for the transition 0+ 2S22P3 2D312-2D512 has
a complicated resonance structure at energies below the threshold
for excitation of 0+ p. The collision strengths for the transitions
Q2+ s22p2 3P-'D, 3P-1S and 1D-'S contain near-threshold resonances
due to collisional coupling between the configurations 2s22p2 and
2s2p3. The results obtained in recent calculations are presented in a
form convenient for astrophysical applications. The resonances do not
produce any large changes in the intensity ratios, [0 ii] (A 3729)I(A
37z6) and [0 iii] (A 495 + A 5oo7)/(A 4363), as functions of electron
temperature and electron density.
Title: Low Density Astrophysics: H+ Regions and the
Solar Corona
Authors: Seaton, M. J.
Bibcode: 1969atph.conf..501S
Altcode: 1969agph.conf..501S
No abstract at ADS
Title: Excitation of forbidden lines by electron impact.
Authors: Seaton, M. J.
Bibcode: 1969MSRSL..25...45S
Altcode: 1969tisa.conf...45S
No abstract at ADS
Title: Forbidden line radiation from gaseous nebulae.
Authors: Flower, D. R.; Seaton, M. J.
Bibcode: 1969MSRSL..25..251F
Altcode: 1969tisa.conf..251F
No abstract at ADS
Title: The excitation of spectrum lines in nebulae by resonant
scattering of radiation from the central stars
Authors: Seaton, M. J.
Bibcode: 1968MNRAS.139..129S
Altcode:
The spectra of planetary nebulae contain weak permitted lines of
ions of oxygen, carbon and various other elements. It is shown that
ion abundances, obtained on assuming these lines to be excited by
recombination, are an order of magnitude too large. An alternative
mechanism is therefore proposed: that excitation is by absorption of
stellar radiation in spectrum lines. Approximate calculations show
that this mechanism should be capable of explaining the observed line
intensities. An improved estimate is obtained for the oxygen abundance
in NGC 7027, N(O)!N(H) = X I0- .
Title: Distances of Planetary Nebulae
Authors: Seaton, M. J.
Bibcode: 1968ApL.....2...55S
Altcode:
No abstract at ADS
Title: Review of Atomic Collision Processes
Authors: Seaton, M. J.
Bibcode: 1968IAUS...34..129S
Altcode:
No abstract at ADS
Title: An Introductory Review
Authors: Seaton, M. J.
Bibcode: 1968IAUS...34....1S
Altcode:
No abstract at ADS
Title: Collision Strengths for Excitation of Forbidden Lines
Authors: Czyzak, S. J.; Krueger, T. K.; de A. P. Martins, P.; Saraph,
H. E.; Seaton, M. J.; Shemming, J.
Bibcode: 1968IAUS...34..138C
Altcode:
No abstract at ADS
Title: Excitation of forbidden lines in gaseous
nebulae. I. Formulation and calculations for 2pq ions.
Authors: Saraph, H. E.; Seaton, M. J.; Shemming, J.
Bibcode: 1968RSPTA.264...78S
Altcode:
No abstract at ADS
Title: Book Review: A guide to the solar corona. D.E. BILLINGS:
Academic Press, New York, 1966. 10+323. 112s
Authors: Seaton, M. J.
Bibcode: 1967P&SS...15.1958S
Altcode:
No abstract at ADS
Title: Book Review: Annual Review of Astronomy and
Astrophysics. L. GOLDBERG, A. J. DEUTSCH and D. LAYZER (Ed.):
Vol. 3. Annual Reviews 1965. vii + 438 pp. $9.
Authors: Seaton, M. J.
Bibcode: 1966P&SS...14.1039S
Altcode:
No abstract at ADS
Title: The ionization structure of planetary nebulae. V. Radii,
luminosities and problems of evolution
Authors: Seaton, M. J.
Bibcode: 1966MNRAS.132..113S
Altcode: 1966MNRAS.132..113R
The forty-seven planetaries of Paper IV are divided into two Classes:
(a) with He II lines and (b)without He II lines. From results obtained
in IV, the ionized hydrogen mass M of Class (a) nebulae is expected
to be equal, or nearly equal, to the total nebular hydrogen mass M
. For fourteen of the nebulae considered electron densities Ne can be
deduced from relative intensities of forbidden lines, and nebular radii
R(Ne) and values of M can then be obtained using measured Hfl surface
brightuesses. Of the eight Class (a) nebulae for which M can be deduced
in this way, six have M close to . Assuming this value of M, and using
measured surface brightuesses, radii R(M) are obtained for thirty-seven
Class (a) nebulae. Most Class (b) nebulae have values of M less than ;
this is a consequence of their being optically thick. Radii R(Ne) are
obtained for six Class (b) objects. Deduced distances, r = R/O where 6
is the angular radius, are compared with distance estimates obtained by
other methods. Satisfactory agreement is obtained. Using the results of
Paper IV, central star temperatures T and luminosities L are obtained
for forty planetaries and are plotted on the [log T, log(L/L0)] H-R
diagram. An evolutionary track is obtained: starting at [4.5, 18], T
and L increase to , 4.4], then T increases at approximately constant
L to [ o, 4.4] and finally L decreases to [ o, ]. This whole process
takes place in a time of 5 X I0 years. The evolution of the nebulae
is discussed. They are optically thick for radii R < parsecs,
optically thin for <R < and again optically thick for R>
parsecs. The final optically thick stage is a consequence of the final
luminosity drop. These results provide a satisfactory explanation of
the apparent size distribution obtained by O'Dell, assuming all nebulae
to be optically thin. The evolutionary track shows that the central
stars are evolving towards the white dwarfs, the final luminosity drop
being a consequence of the onset of degeneracy. The average central
star mass M5 is equated to the average white dwarf mass, giving M5 =
. The average nebular mass, including helium, is and the average mass
of the stars from which planetaries are formed is therefore . These
stars belong to the Disk Population, which indicates an age of 5 X I0
years. This is in satisfactory agreement with the age to be expected
for a highiy evolved star of mass I 2M0. The central star radii R5 are
calculated and are found first to increase and then to decrease. During
the later stages of the evolution of the central stars the luminosities
to be expected from the rate of change of the total stellar energies
are found to be greater than the observed photon luminosities by factors
of about 30. Processes of neutrino emission are probably important.
Title: The ionization structure of planetary nebulae, IV. Optical
thickness of the nebulae and temperatures of the central stars
Authors: Harman, R. F.; Seaton, M. J.
Bibcode: 1966MNRAS.132...15H
Altcode:
Of forty-seven planetaries for which measurements have been made of
angnlar radii, H fluxes, relative line intensities and central star
magnitudes, forty-two are shown to satisfy criteria for complete
absorption in the continua of H I, of He I or of He ii. For these
forty-two nebulae values are obtained for the central star temperatures
T, for the luminosity parameter A = L/r2L0 where r is the distance,
and for a parameter TI which provides a measure of the completeness of
absorption in the H I Lyman continuum. It is found that for thirteen
of them absorption in the Lyman continuum is complete, or practically
complete. The star temperatures range from x I0 0K to x I0 0K.
Title: Calculations of Collisional-Radiative Decay
Authors: Seaton, M. J.
Bibcode: 1965SAOSR.174..215S
Altcode:
No abstract at ADS
Title: Collision Cross Sections
Authors: Seaton, M. J.
Bibcode: 1965SAOSR.174...33S
Altcode:
No abstract at ADS
Title: Review paper : Far ultra-violet radiation : stars, nebulae
and insterstellar matter
Authors: Seaton, M. J.
Bibcode: 1965IAUS...23..133S
Altcode:
No abstract at ADS
Title: The Theory of the Coronal Spectrum
Authors: Seaton, M. J.
Bibcode: 1965ASSL....1..273S
Altcode: 1965sosp.conf..273S
No abstract at ADS
Title: Book Review: Advances in astronomy and astrophysics. Z. KOPAL
(Ed): Volume 2. 314 + ix pp. Academic Press, New York, 1963. $11.50
Authors: Seaton, M. J.
Bibcode: 1964P&SS...12.1213S
Altcode:
No abstract at ADS
Title: The Central Stars of Planetary Nebulae.
Authors: Harman, R. J.; Seaton, M. J.
Bibcode: 1964ApJ...140..824H
Altcode:
No abstract at ADS
Title: Book Review: Atmospheric processes. PAUL J. NAWROCKI and
ROBERT PAPA: Prentice Hall, 1963. xi + 699pp
Authors: Seaton, M. J.
Bibcode: 1964P&SS...12..185S
Altcode:
No abstract at ADS
Title: The ionization structure of planetary nebulae, III
Authors: Hummer, D. G.; Seaton, M. J.
Bibcode: 1964MNRAS.127..217H
Altcode:
In planetary nebulae the helium to hydrogen abundance ratio, by numbers
of atoms, is Y=o I to C 2 the abundance of heavier elements, by numbers
of atoms relative to hydrogen, is Z IC- . The heavier elements may be
neglected when considering the ionization of hydrogen and helium, but
are of importance in considering the thermal balance. Three main regions
are considered In region (i), that is, the innermost region, the most
abundant ions are He+2 and H+. The He+2 - He+ ionization equilibrium
is calculated by the method of Paper I. Recombinations of He+2 to He+
produce sufficient quanta to keep the hydrogen ionized. With Z =0,
the electron temperature in region (i) would be very high (T8 >7.5
x io4 K with no collisional excitation), but with Z io an efficient
cooling process is shown to be excitation of allowed transitions in
lithium-like ions such as C+3, giving Te 2 X i04 K. In region (ii)
the most abundant ions are He+ and H+. The He+-He0 and H+-H0 ionization
equilibria are calculated on solving coupled equations. At lower star
temperatures (Ts <5 X I0 K for Y= ), the inner part of region (ii)
contains He+ and H+ and the outer part contains He0 and H+. At higher
star temperatures the transitions from He + to He0 and from H+ to H0
occur at approximately the same point. In region (iii) none of the
gas is ionized. This region exists only if the total amount of gas is
sufficient to produce absorption of all ionizing stellar quanta.
Title: Excitation of coronal lines by proton impact
Authors: Seaton, M. J.
Bibcode: 1964MNRAS.127..191S
Altcode:
Cross sections are calculated for the transition Fe+13 3p 3p produced
by proton impact. The excitation rate increases rapidiy with increasing
temperature. At coronal temperatures the number of 3p 3p transitions
produced by proton impact is comparable with the number directly
produced by electron impact but is smaller than the rate of population
of 3p due to electron impact excitation of allowed transitions from 3pW,
followed by ascade to 3p .
Title: The spectrum of the solar corona
Authors: Seaton, M. J.
Bibcode: 1964P&SS...12...55S
Altcode:
The paper reviews the interpretation of coronal observations in X-ray,
ultra-violet visible and radio wavelengths.
Title: The ionization equilibrium for iron in the solar corona
Authors: Burgess, A.; Seaton, M. J.
Bibcode: 1964MNRAS.127..355B
Altcode:
It has been shown by Burgess that, when allowance is made for
dielectronic processes, recombination rates for coronal ions are much
larger than have hitherto been supposed. New calculations are made of
the ionization equilibrium for iron. Although allowance for dielectronic
processes results in the discrepancies being much reduced, it still
appears that there may be some tendency for temperatures obtained from
line-widths to be larger than those deduced from ionization theory.
Title: Recombination spectra, II
Authors: Pengelly, R. M.; Seaton, M. J.
Bibcode: 1964MNRAS.127..165P
Altcode:
Previous work has shown that calculated recombination spectra may
be sensitive to coffisional transitions of the type ' (1=11 1). With
equal energies for nI, nI', the usual collision theory expressions give
infinite cross-sections, but finite results are obtained on introducing
a cut-off at large impact parameters. At very low densities the cut-off
is determined by the radiative lifetimes of the excited states and at
higher densities it is determined by the Debye radius. For conditions
in planetary nebulae (Ne IO cm , Te IO 0K), collisional transitions,
', are faster than radiative transitions, '1' (n' <n), for n is
for H I and for n 2Z for He Ir.
Title: Recombination spectra, III
Authors: Seaton, M. J.
Bibcode: 1964MNRAS.127..177S
Altcode:
Assuming an electron temperature T= io 0K and an electron density
Ne = -3, which are typical values for planetary nebulae, the level
populations of hydrogen are calculated allowing for: radiative capture
and cascade; collisional ionization and three-body recombination;
collisional I I transitions. It is only for large values of n, n 4O,
that the level populations differ significantly from those obtained
in the usual radiative theory. Whereas the radiative theory predicts a
sharp drop in intensity on the long wavelength side of the Balmer limit,
present theory predicts a more gradual decrease, extending over about 5
A. This effect should be clearly observable on high dispersion spectra.
Title: Model Atmospheres for Central Stars of Planetary Nebulæ
Authors: Gebbie, K. B.; Seaton, M. J.
Bibcode: 1963Natur.199..580G
Altcode:
THE central stars of planetary nebulæ have effective
temperatures1-3, Ts, ranging from 3 ×
104 ° K to about 2.5 × 105 ° K. Such stars
may therefore be much hotter than the hottest main sequence stars
which have temperatures of about 4 × 104 ° K. To obtain
a better understanding of the nature of very hot stars, a number of
non-grey model atmospheres have been computed.
Title: Book Review: Advances in astronomy and astrophysics, vol.1
. Z. KOPAL (ed.). Academic Press, New York, 1962. x + 366 pp. 80s
Authors: Seaton, M. J.
Bibcode: 1963P&SS...11..217S
Altcode:
No abstract at ADS
Title: The ionization structure of planetary nebulae, I. Pure
hydrogen nebulae
Authors: Hummer, D. G.; Seaton, M. J.
Bibcode: 1963MNRAS.125..437H
Altcode:
The problem is formulated in terms of the equations of radiative
transfer, of ionization equilibrium and of the thermal balance. The
intensity of ionizing radiation is I = lyS + 1yd where lyS is the
attenuated intensity of stellar radiation and lyd the intensity of
diffuse radiation produced in the nebula. In Approximation I it is
assumed that there is no transfer problem for the diffuse radiation,
emission and absorption occurning at the same place. The problem is
then solved using a method due to Zanstra and de Jong. Assuming the
star to radiate as a black body, numerical results are obtained for
a wide range of star temperatures. In Approximation II the transfer
equation for 1yd is solved using the source function from Approximation
I. For an isothermal plane parallel model it is found that the mean
intensities Jy as given by Approximations I and II, never differ by
more than a few per cent.
Title: The temperature of the solar corona
Authors: Seaton, M. J.
Bibcode: 1962Obs....82..111S
Altcode:
No abstract at ADS
Title: Book Review: The abundance of the elements. LAWRENCE
H. ALLER: Vol. VII, Interscience Monographs and Texts in Physics
and Astronomy. Interscience, New York and London, 1961. 283 + xi
pp., $10.00
Authors: Seaton, M. J.
Bibcode: 1962P&SS....9..129S
Altcode:
No abstract at ADS
Title: The Ultra-Violet Radiation of the Central Stars of Planetary
Nebulae
Authors: Hummer, D. G.; Seaton, M. J.
Bibcode: 1961LIACo..10..539H
Altcode: 1961MSRSL...4..538H; 1961LIACo..10..538H
No abstract at ADS
Title: Problems of nebular photometry
Authors: Seaton, M. J.
Bibcode: 1960AnAp...23..360S
Altcode:
No abstract at ADS
Title: Radiative recombination of He
Authors: Burgess, A.; Seaton, M. J.
Bibcode: 1960MNRAS.121..471B
Altcode:
Re combination coefficients for
Title: HI, HeII, and HeII intensities in planetary nebulae
Authors: Seaton, M. J.
Bibcode: 1960MNRAS.120..326S
Altcode:
Recombination spectra have been calculated for H I, He ii and the 2p-nd
series of He I. Expressions are given for the calculation of He+2/H+
and He+/H+ abundance ratios. It is shown that the ionized region
of a nebula will contain a negligible amount of neutral helium if I
(He ii, A 4686)/I (He I, A 5876) exceeds I'2. For NGC 7662 and 2392
the measured relative intensities within each spectrum are in good
agreement with theory but for NGC 7027 the agreement is poor for the
weak lines and for the infra-red. The suggestion that the intensity
anomalies for NGC 7027 may be due to collisional processes is found
to be difficult to accept. The possibility is considered that there
may be systematic errors in the intensity measurements. The He/H
abundance ratios, by numbers of atoms, are found to be for NGC 7662,
for NGC 2392 and for NGC 7027.
Title: Planetary nebulae
Authors: Seaton, M. J.
Bibcode: 1960RPPh...23..313S
Altcode:
No abstract at ADS
Title: A general formula for the calculation of atomic
photo-ionization cross-sections
Authors: Burgess, A.; Seaton, M. J.
Bibcode: 1960MNRAS.120..121B
Altcode:
The general formula is derived by considering the model of a single
electron moving in a central field. Approximate bound-state radial
functions, accurate for large radial distances, may be obtained once
the effective quantum numbers v( = n *) are known. Bates and Damgaard
have shown that such functions may be used to obtain good estimates
for bound-bound transition integrals. For bound-free transitions
use is made of approximate free-state radial functions having exact
asy1nptotic forms, their phases being given by 5= where M is the
extrapolated quantum defect ( =n - v). The results of extensive
numerical calculations are summarized in tables which permit the
rapid calculation of transition integrals once the energy levels are
known. Both bound-free and bound-bound transition integrals may be
obtained. For bound-bound transitions ' good agreement is obtained
with the Bates and Damgaard tables for v - V' > . Comparisons
are made with other results for bound-free transitions, as obtained
both from theory and from experiment. in nearly all cases the general
formula gives results at least as accurate as those obtained in the
best alternative methods of calculation.
Title: The abundance of oxygen in the planetary nebula NGC 7027.
Authors: Burgess, A.; Seaton, M. J.
Bibcode: 1960MNRAS.121...76B
Altcode:
No abstract at ADS
Title: Radiative recombination of hydrogenic ions
Authors: Seaton, M. J.
Bibcode: 1959MNRAS.119...81S
Altcode:
Using the first three terms in the asymptotic expansion of the
KramersGaunt factor, calculations are made for the rate of recombination
and for the mean kinetic energy of the recombining electrons.
Title: The solution of capture-cascade equations for hydrogen
Authors: Seaton, M. J.
Bibcode: 1959MNRAS.119...90S
Altcode:
Calculations are made for T= x I0 , 5 x io , I X i and 2 X io 0K taking
an infinite number of levels into account but assuming that b = [(21+
i)/n2] b The results obtained by Baker and Menzel are shown to be in
error by about 5 per cent for Case A and 20 per cent for Case B. An
improved method of calculation results from the introduction of the
cascade matrix, the elements C giving the total probability of the n'
n transition when all cascade routes are taken into account.
Title: Local Density Variations in Planetary Nebulae
Authors: Seaton, M. J.
Bibcode: 1958RvMP...30.1034S
Altcode: 1958IAUS....8.1034S
No abstract at ADS
Title: Cross Sections for Photoionization from Valence-Electron States
Authors: Burgess, A.; Seaton, M. J.
Bibcode: 1958RvMP...30..992B
Altcode: 1958IAUS....8..992B
No abstract at ADS
Title: Part VIII: Summaries and Conclusions
Authors: Burgers, J. M.; Liepmann, H.; van de Hulst, H. C.; Kahn,
F. D.; Seaton, M. J.; Pecker, J. C.; Schatzman, E.; Spitzer, L., Jr.
Bibcode: 1958RvMP...30.1095B
Altcode: 1958IAUS....8.1095B
No abstract at ADS
Title: Thermal Inelastic Collision Processes
Authors: Seaton, M. J.
Bibcode: 1958RvMP...30..979S
Altcode: 1958IAUS....8..979S
No abstract at ADS
Title: The Quantum Defect Method
Authors: Seaton, M. J.
Bibcode: 1958MNRAS.118..504S
Altcode:
The Quantum Defect Method uses interpolated or extrapolated quantum
defects to determine the asymptotic forms of atomic wave functions. The
method may be used in the calculation of atomic transition probabilities
and photo-ionization cross sections, in electron-ion collision
calculations and also in connection with solid state problems. The
paper gives a summary of previous work on the fundamental ideas of the
method and presents some new results for positive energy states and
for the normalization of bound-state wave functions. Some applications
of the method are discussed.
Title: Oxygen Red Lines in the Airglow. II. Collisional Deactivation
Effects.
Authors: Seaton, M. J.
Bibcode: 1958ApJ...127...67S
Altcode:
Deactivation rates for upper-atmosphere metastable atomic oxygen
may be deduced from (1) time variations in auroral emissions; (2)
emission heights much greater than 100 km for the nightglow red lines,
despite the fact that excitation must occur at 100 km; (3) observations
of the [0 1] red-to-green intensity ratio in aurorae. The auroral
observations may be explained in two ways: (1) assuming deactivation
rates proportional to air density and a red-to-green excitation
ratio 32/S3 varying with altitude, as would be expected if discharge
mechanisms are operative, or (2) assuming 52 /53 independent of altitude
and deactivation rates not proportional to gas density, which may be
the case if resonance collisions O(1D) + O3( ) O(3P) + O (' , `3 =
2) are important. In both cases the deduced deactivation rates are
consistent with suppression of the airglow red-line emission at 100 km.
Title: Relative [O II] Intensities in Gaseous Nebulae.
Authors: Seaton, M. J.; Osterbrock, D. E.
Bibcode: 1957ApJ...125...66S
Altcode:
The intensity ratio r = I(3729)/I(3726) tends to a value r(∞) at high
densities determined by radiative transition probabilities and a value
r(0) at low densities determined by collision strengths, proportional to
collision cross-sections. This ratio has been measured in nebulae for
which these two limiting values should be approached. The transition
probability calculations are discussed; the best-calculated value of
r(∞) obtained is 0.43. From observations of IC 4997 it is concluded
that the correct value is 0 35 + 0.04. For future applications, values
of the transition probabilities are adopted consistent with r(∞) =
0.35. Results of improved collision-strength calculations are presented,
including those for transitions between all individual J-levels. The
quantities required in these calculations are used to calculate quantum
defects in various O I spectral series. Comparison with observed quantum
defects shows that the collision strengths should be correct to within
per cent. The calculations give r(0) to be 1.50 at low temperatures and
1.42 at high temperatures. Electron-density estimates of 18 and 10 are
obtained from the surface brightnesses of two nebulae, NGC 281 and NGC
7000, for which the low- density limit should be approached Assuming
T0 = 104° K, these densities would be consistent
with r = 1.47-1.48. The observed ratios are 1.37 and 1.38, respectively,
suggesting densities of the order of 100cm-3. The discrepancy
is probably real and due to an inhomogeneous density distribution,
together with selection effects. The ratio r' = I(7320)/I(7330),
calculated to be between 1.24 and 1.31 for all values of Te and Ne, is
in satisfactory agreement with available measurements Expressions are
obtained for the intensity ratios r and r" = I(3729) + I(3726)/I(7320)
+ I(7330) as functions of T0 and N0. For IC 418
the measured value of r, 0.37 ± 0.03 is consistent with the density
Ne = 2.5 x 104 cm-3 obtained from
the surface brightness, interpreted assuming the hydrogen emission
to be concentrated in a hollow spherical shell. For NGC 7027, r =
0.47, measured by Aller and Minkowski, gives N0 = 0.85 x
104 cm-3 , which is considerably smaller than
values previously obtained from other forbidden-line ratios. The
latter values are confirmed by using r" = 1.38, measured by Aller,
Bowen, and Minkowski, which gives Ne = 3 x 104
cm-3. It is considered that the discrepancy, which is
too large to be attributed to errors in observations or in atomic
parameters, is due to the occurrence of local density fluctuations. The
measured values of r and r" are consistent with a model having a
background density of the order of 5 x 102 cm-3
and in which dense clouds or filaments, with densities of the order of
7 x 104 cm-3, occupy 1 or 2 per cent of the total
volume. There is similar evidence for density fluctuations in a number
of other bright planetaries In many cases direct photographs reveal
the presence of filamentary structure It is pointed out that for IC
418 there is evidence of large-scale density and possibly temperature
variations, but the spectrophotometric evidence does not suggest the
existence of local density fluctuations This is consistent with the
particularly uniform appearance of this object.
Title: Further Calculations on Electron Excitation of Forbidden Lines:
Transitions with Electric Quadrupole Moments
Authors: Seaton, M. J.
Bibcode: 1955RSPSA.231...37S
Altcode:
Collisional excitation of the ground configuration terms of various
ions such as OI, OII, OIII, NI, NII and SII is of importance in
various cosmic sources. The collision strengths defined as Ω (n', n) =
kn2ω nQ (n --> n')/π , have been
calculated previously for a number of transitions dependent on pure
exchange coupling. The present paper is concerned with the potential
coupling terms for those transitions having electric quadrupole
moments. For partial waves other than the p-wave the coupling strength
is sufficiently weak for the distorted wave (D.W.) approximation to be
used. The general theory of the D.W. approximation is discussed for
complex atoms. It is shown that the angular and spin integrations
occurring in the expressions for the collision strengths may be
expressed in terms of the angular and spin integrations occurring in
the calculation of electric multipole moments. Results of detailed
numerical calculations are considered to be correct to within 30 to 40%.
Title: Relative [O III] intensities in the eclipsing variable
AR Pavonis
Authors: Seaton, M. J.
Bibcode: 1955Obs....75...85S
Altcode:
No abstract at ADS
Title: Continuum intensities in planetary nebulae
Authors: Seaton, M. J.
Bibcode: 1955MNRAS.115..279S
Altcode:
Thirty-four planetaries are considered for which continuum intensities
have been measured, Te may be calculated from the [0 `III] intensities
and Ne may be calculated from forbidden line intensities or from the
surface brightness. Values of Be/H8 measured by Aller (Be = Balmer
continuum intensity) are in reasonable agreement with theory. About
half of the nebulae considered appear to come closer to Case A than to
Case B (A = optically thin, B = optically thick in Lyman lines). Total
continuum intensities are calculated for H II regions allowing for:
(I) all H and He II recombination and continua and (2) the H 25 15
two-quantum continuum, collisional deactivation of H 25 being taken into
account. The calculated intensities, relative to H8, are compared with
the measurements of Page for both sides of the Balmer limit. The mean
results are in fair accord but there is a considerable scatter probably
due to observational errors. Recently measured Balmer discontinuities
(A) are in good agreement with theory for 6 nebulae, but for NGC 6826 A
(obs) is significantly smaller than A (calc). This object is unusual in
that it is surrounded by a faint outer envelope. The small value of A
(obs) is probably due to the conversion of Ly into H 25 15 quanta in
an extensive H I region. when such processes are not important A (obs)
may be used to improve the estimates of Te and Ne.
Title: The Forbidden Line Spectra of Gaseous Nebulae
Authors: Seaton, M. J.
Bibcode: 1955IAUS....2...75S
Altcode:
No abstract at ADS
Title: Interplanetary Dust and Physical Processes in the Earth's
Upper Atmosphere
Authors: Kaiser, T. R.; Seaton, M. J.
Bibcode: 1955LIACo...6...48K
Altcode:
No abstract at ADS
Title: Interpretation of the Spectrum of the Orion Nebulae
Authors: Seaton, M. J.
Bibcode: 1955LIACo...6..462S
Altcode:
No abstract at ADS
Title: Electron temperatures and electron densities in planetary
nebulae
Authors: Seaton, M. J.
Bibcode: 1954MNRAS.114..154S
Altcode:
Electron temperatures Te and electron densities Ne are estimated from
a study of the relative intensities of the forbidden lines in the
following nebulae: NGC 7027, 2440, 7662, 6572, IC 418 and Orion. For
certain ions (N ii, 0 iii, 0 ii and S ii) it is possible to measure
the relative intensities of two multiplets originating from different
metastable terlns. Each intensity ratio gives a relation between Te and
Ne. A ftither relation may be obtained if it is assumed that N1(O I)/N1(
I) =N1(O II)/N1(N II), where N1(X) is the number of ions X in the ground
state per cm . Recently calculated electron excitation cross-sections
enable these various relations to be obtained in numerical form. The
observed relative intensities of the forbidden lines are corrected for
absorption and systematic calibration errors by comparing observed and
theoretical relative intensities in the hydrogen spectrlm. Simultaneous
solutions for Te and Ne are then obtained from the forbidden line
intensity ratios. The values of Te range from I 3 X I0 to X I0 deg. K,
and the valuesofNe from8xi o to 5 X I0 per cm . The consistency of
the various Te, Ne relations is particularly satisfactory for NGC
7027, NGC 2440 and IC 8; for the other objects considered the observed
intensities tend to be either less reliable or less complete. With the
exception of IC 418, the densities obtained from the forbidden lines are
greater than those obtained from the surface brightness by a factor of
order 3. These differences are probably a consequence of the densities
being non-uniform The surface brightness method gives essentially a
r.m.s. density for the total geometrical volume, while the forbidden
line methods give average densities for the active emitting regions
(clouds and filaments). Such differences would not be expected for IC
418, which is particularly uniform. In a final section the abundance
ratios N(O)/N(N) are estimated to be I 6, and for NGC 7027, NGC 2440
and IC 418 respectively. It is concluded that the results reveal a real
difference in composition between NGC 2440 and the other two objects.
Title: Relative line intensities for [OII] and [SII]
2D-2S in gaseous nebulae leuses extragalactiques
(2e série de mesures)
Authors: Seaton, M. J.
Bibcode: 1954AnAp...17...74S
Altcode:
No abstract at ADS
Title: Deactivation by collisions of the 2s metastable state of
hydrogen in planetary nebulae
Authors: Seaton, M. J.
Bibcode: 1954AnAp...17..296S
Altcode:
No abstract at ADS
Title: Collisions between protons and metastable atoms or ions having
outer pq shells
Authors: Seaton, M. J.
Bibcode: 1954AnAp...17..550S
Altcode:
No abstract at ADS
Title: Electron Excitation of Forbidden Lines Occurring in Gaseous
Nebulae
Authors: Seaton, M. J.
Bibcode: 1953RSPSA.218..400S
Altcode:
In a previous paper the Hartree-Fock method has been used to calculate
electron excitation cross-sections for the ground configuration terms
of OI. In the present paper the modifications required for similar
calculations on positive ions are discussed, and cross-sections
are calculated for electron excitation of the ground configuration
terms of N II, O II, O III, Ne III and S II. These cross-sections
are of fundamental importance in discussing the physical state and
chemical composition of gaseous nebulae. A first approximation to the
solution of the coupled integro-differential equations is obtained by
neglecting energy differences (the E.R. approximation). A comparison
is made between the cross-sections calculated using exact solutions
of the E.R. equations and using weak coupling approximations to these
solutions. It is found that, although allowance for distortion gives
a considerable improvement, the weak coupling approximations cannot
in general be relied upon to give accurate results. Estimates of the
magnitudes of the terms omitted in the E.R. approximation are made
by perturbation methods, and in all cases these terms are found to
be reasonably small. Final values for the cross-sections are probably
correct to within ± 40% in absolute value and rather more accurate than
this in the relative values. Cross-sections estimated for a number of
other ions of astrophysical interest are considered to be correct to
within a factor of 2. Hartree-Fock wave functions for N II are given
in an appendix.
Title: The chemical composition of the interstellar gas
Authors: Seaton, M. J.
Bibcode: 1951MNRAS.111..368S
Altcode:
Summaries are given of the atomic ionization and recombination data
required in the calculation of interstellar ionization equilibrium. The
chemical composition of the interstellar gas is discussed from the
standpoint of seeking to establish whether there is any evidence for
discrepancies between the interstellar and "mean cosmic" abundance
ratios. The important Ca/Na ratio is the only case for which there is
an apparent discrepancy, but it is shown that this may be removed if
recent work on the ultra-violet radiation field is accepted as correct.
Title: The continuous radiative absorption cross-section of single
ionized potassium
Authors: Seaton, M. J.
Bibcode: 1950MNRAS.110..247S
Altcode:
After obtaining exchange wave functions of K III the continuous
absorption cross-section of K II is calculated over the energy range 0
to 1353 e.V. of the ejected electron, using both the dipole length and
the dipole velocity formulae and allowing for the effect of electron
exchange. The recommended mean value is taken to be = 32 X io-'7v1!v
cm.2, where V1 is the frequency at the spectral head. The cross-section
data are used to evaluate the "modified dilution #actor" introduced
by in considering the ionization balance in ntersteIlar space. The
possibility of captures to excited states introduces a correction
factor of 430 (Te 102), 290 (Te ioi), 160 (Te I0 ), where Te is the
electron temperature (deg. K.). The abundance of interstellar potassium
is briefly discussed.
Title: The Quantal Theory of Continuous Absorption of Radiation by
Various Atoms in their Ground States. II. Further Calculations on
Oxygen, Nitrogen and Carbon
Authors: Bates, D. R.; Seaton, M. J.
Bibcode: 1949MNRAS.109..698B
Altcode:
No abstract at ADS