Author name code: espagnet ADS astronomy entries on 2022-09-14 author:"Espagnet, O." ------------------------------------------------------------------------ Title: Pollux: A weak dynamo-driven dipolar magnetic field and implications for its probable planet Authors: Aurière, M.; Petit, P.; Mathias, P.; Konstantinova-Antova, R.; Charbonnel, C.; Donati, J. -F.; Espagnet, O.; Folsom, C. P.; Roudier, T.; Wade, G. A. Bibcode: 2021A&A...646A.130A Altcode: 2021arXiv210102016A Context. Pollux is considered as an archetype of a giant star hosting a planet since its radial velocity (RV) presents very stable sinusoidal variations with a period of about 590 d. We then discovered a weak magnetic field at its surface using spectropolarimetry, questioning the planetary hypothesis.
Aims: We followed up our investigations on Pollux to characterize its magnetic field and to infer the effects of magnetic activity on the RV variations.
Methods: We first used ESPaDOnS at CFHT and then Narval at TBL to obtain Stokes I and Stokes V spectra of Pollux to study their variations for a duration of 4.25 years, that is, for more than two periods of the RV variations. We used the least-squares deconvolution profiles to measure the longitudinal magnetic field and to perform a Zeeman Doppler imaging (ZDI) investigation.
Results: The longitudinal magnetic field of Pollux is found to vary with a sinusoidal behavior and a period similar to that of the RV variations. From the ZDI investigation a rotation period of Pollux is determined to be equal to 660 ± 15 days and possibly different than the period of variations of the RV. As to the magnetic topology, the poloidal component is dominant and almost purely dipolar with an inclination of 10.5° of the dipole with respect to the rotation axis. The mean strength of the surface magnetic field is 0.44 G. Pollux is found approximately as active as the Sun observed as a star and this activity could induce moderate RV variations.
Conclusions: As to the origin of the magnetic field of Pollux, we favor the hypothesis that it is maintained through contemporaneous dynamo action. Pollux appears as the representative of a class of slowly rotating and weakly magnetic G-K red giants. To explain the sinusoidal RV variations of Pollux, two scenarios are proposed. If the RV period is different from the rotation period, the observed periodic RV variations are due to the hosted planet and the contribution of Pollux magnetic activity is not significantly detected. In the peculiar case in which the two periods are equal, we cannot discard the possibility that the activity of Pollux could explain the total RV variations and that the planet hypothesis would appear unnecessary. In any case magnetic activity could contribute significantly to RV variations in some intermediate mass G-K red giants hosting planets, particularly those with small amplitude RV variations.

Based on observations obtained at the Télescope Bernard Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse, France, and the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council of Canada, CNRS/INSU and the University of Hawaii. Title: Pollux: a stable weak dipolar magnetic field but no planet? Authors: Aurière, Michel; Konstantinova-Antova, Renada; Espagnet, Olivier; Petit, Pascal; Roudier, Thierry; Charbonnel, Corinne; Donati, Jean-François; Wade, Gregg A. Bibcode: 2014IAUS..302..359A Altcode: 2013arXiv1310.6907A Pollux is considered as an archetype of a giant star hosting a planet: its radial velocity (RV) presents sinusoidal variations with a period of about 590 d, which have been stable for more than 25 years. Using ESPaDOnS and Narval we have detected a weak (sub-gauss) magnetic field at the surface of Pollux and followed up its variations with Narval during 4.25 years, i.e. more than for two periods of the RV variations. The longitudinal magnetic field is found to vary with a sinusoidal behaviour with a period close to that of the RV variations and with a small shift in phase. We then performed a Zeeman Doppler imaging (ZDI) investigation from the Stokes V and Stokes I least-squares deconvolution (LSD) profiles. A rotational period is determined, which is consistent with the period of variations of the RV. The magnetic topology is found to be mainly poloidal and this component almost purely dipolar. The mean strength of the surface magnetic field is about 0.7 G. As an alternative to the scenario in which Pollux hosts a close-in exoplanet, we suggest that the magnetic dipole of Pollux can be associated with two temperature and macroturbulent velocity spots which could be sufficient to produce the RV variations. We finally investigate the scenarii of the origin of the magnetic field which could explain the observed properties of Pollux. Title: Spatial relation between the 5-minute oscillations and granulation patterns. Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, P.; Mein, N.; Malherbe, J. M. Bibcode: 1996A&A...313..297E Altcode: A white light, high resolution movie of the quiet photosphere has been used to study the spatial relation between the solar granulation and the 5-min oscillations of high degree mode (small spatial scale). The granular and oscillatory brightness components have been separated with the help of a specific filter, in the k-ω diagram. The surface of the Sun appears to be covered with adjacent oscillating cells of apparent size 2" (1500km) to 3" (2000km). Some cells are noticeable by their amplitude which is much larger than average. These strong oscillations usually appear in expanding intergranular spaces. Our observations suggest that the 5-min oscillations are stochastically generated by turbulent convection. However, the most energetic oscillations occur in downflows only, well separated of each other in time and space. Title: Penetration of the solar granulation into the photosphere: height dependence of intensity and velocity fluctuations. Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N.; Mein, P. Bibcode: 1995A&AS..109...79E Altcode: A 16-min time series of two-dimensional "Multichanel Subtractive Double Pass" spectrograms, recorded in the NaD_2_ 5690 line, has been used to study the vertical structure of the photosphere and, in particular, the penetration of granules. A statistical and a morphological investigations have been made. The 5-min oscillations have been carefully filtered in the k-ω space. Intensity fluctuations associated to granules disappear in a very short distance, about 60 to 90km above the continuum level. The intensity features in the upper photosphere are not related to the granules. The velocity fluctuations associated to the granulation, cross the whole thickness of the photosphere. The photosphere can thus be separated into two distinct layers, but only for the intensity features, not for the velocity features which are the same from the bottom to the top of the stable atmosphere. In the lower photosphere, below 90km, only granules are detected. In the upper photosphere, our observations suggest that the weak pattern visible there, is turbulent in origin. In addition to this weak pattern, bright sinking plumes have been discovered; they are associated with strong downflows, often adjacent to strong upflows. The size of most plumes is 1" to 2"; however the most outstanding of them may be as large as 4". Their origin, which may be photospheric or chromospheric, is still unknown. Title: Peculiar interactions between granules and network bright points in the solar photosphere Authors: Roudier, T.; Espagnet, O.; Muller, R.; Vigneau, J. Bibcode: 1994A&A...287..982R Altcode: The magnetic elements which form the photospheric network, outside active regions, are visible as tiny bright features, smaller than 0.5", called network bright points (NBPs). 15 to 20% of them elongate during their life, when they are squeezed between two expanding granules, or simply pushed by an expanding or a moving granule. Some statistical properties are reported. The most interesting property is that their size and brightness increases simultaneously indicating a strong increase of radiative flux as they grow. They can become as large as 1000 km x 400 km and as bright as 1.25 the brightness of the average photosphere. As theoretical models predict that magnetic flux tubes of size larger than 300 km should be darker than the mean photosphere, our observations suggest that during the elongation, some mechanical energy should contribute in the heating of the tubes. This additional energy may heat some features in the upper photosphere like chromospheric spicules and transition zone jets or microflares, whose production rates and lifetimes are about the same as those of elongation of NBPS. Elongated NBPs are shaked by the turbulent granules, similarly to the normal NBPs and thus should contribute to the heating of the solar corona too. Title: La granulation solaire: origine, pénétration dans la photosphère et interactions avec les oscillations de 5 minutes Authors: Espagnet, O. Bibcode: 1994PhDT........47E Altcode: No abstract at ADS Title: Turbulent power spectra of solar granulation. Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N. Bibcode: 1993A&A...271..589E Altcode: High resolution "Multichannel Subtractive Double Pass" spectrograms and white light photographs have been used to compute power spectra of the solar granulation. The 5 min oscillations have been carefully filtered, and the influence of the variation of the mean size of granules at the mesogranulation scale has been taken into account.

It results that both the power spectra of velocity and intensity fluctuations, displayed in a log P - log k scale, are characteristic of a turbulent atmosphere: they present a discontinuity at 3" = 2000 km (which corresponds to the size of a large granules) and an energy which decreases according to the Kolmogorov -5/3 power law in the granulation range; another discontinuity, at 1".4 = 1000 km (which corresponds approximately to a Peclet number Pe = 1, for which thermal diffusion and advection are of equal importance) is present in the intensity power spectrum, which decreases with a -17/3 power law, in the range of sizes smaller than 1".4.

These results indicate that granules are turbulent eddies. They have some convective characters (like the correlation between velocities and brightness) because they are formed in a strongly superadiabatic atmosphere. Title: Turbulence et granulation solaire. Authors: Espagnet, O.; Muller, R.; Roudier, T. Bibcode: 1992JAF....43...35E Altcode: No abstract at ADS Title: Dynamique de la photosphère solaire: granulation, mésogranulation, oscillations. Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N.; Mein, P. Bibcode: 1992JAF....43...36E Altcode: No abstract at ADS Title: Dynamics of the solar granulation. II - Statistical analysis: Power spectra, coherence, phase Authors: Roudier, T.; Vigneau, J.; Espagnet, O.; Muller, R.; Mein, P.; Malherbe, J. M. Bibcode: 1991A&A...248..245R Altcode: The solar granulation is analyzed statistically by the computation of coherence, phase, and power spectra using data provided by the Multichannel Subtractive Double Pass spectrograph of the Pic du Midi Observatory. The main result of the analysis is that the velocity power spectrum of the granulation changes in shape at 3 arcsec: the raw spectrum features a slope discontinuity; the corrected spectra have a power maximum at 3 arcsec. Between 3 arcsec and the resolution limit (0.8 arcsec), the power decreases almost linearly, with a slope close to -5/3, consistent with the Kolmogorov power law for a turbulent energy cascade. Title: Results from high resolution solar images and spectra obtained at the Pic du Midi Observatory (1986-1990) Authors: Roudier, Th.; Muller, R.; Vigneau, J.; Auffret, H.; Espagnet, O.; Simon, G. W.; Title, A. M.; Frank, Z.; Shine, R. A.; Tarbell, T. D.; Mein, P.; Malherbe, J. M. Bibcode: 1991AdSpR..11e.205R Altcode: 1991AdSpR..11..205R We present an overview of our recent results about solar granulation and mesogranulation, obtained with Pic du Midi observations. These results were obtained during 1986-1990 using image and spectrographic analysis of high spatial resolution data. The study of the solar granulation, with 2 Dim. ``Multichannel Subtractive Double Pass'' (M.S.D.P.) spectra, shows a clear change of the dynamical regime at 3'' (⋍ 2200 km) of the photospheric velocity field when oscillatory components are filtered out.

A three hour movie obtained on film at Pic du Midi Observatory and analyzed at the Lockheed Research Laboratory and the National Solar Observatory (Sacramento Peak) was used to calculate the horizontal flow pattern. The mean lifetime of the diverging areas related to mesogranulation is estimated at 3 hours; these diverging areas are swept by the supergranulation flow towards the supergranule boundary with a mean speed of 0.4 km/s.