arXiv subjects

P. Petit

Publications and source records attributed to P. Petit.

At least 109 records · Page 6Linked to original sources

arXiv2014

Variable magnetic field geometry of the young sun HN Peg (HD 206860)

The large-scale magnetic field of solar-type stars reconstructed from their spectropolarimetric observations provide important insight into their underlying dynamo processes.We aim to investigate the temporal variability of the large-scale surface magnetic field and chromospheric activity of a young solar analogue, the G0 dwarf HN Peg.The large-scale surface magnetic field topology is reconstructed using Zeeman Doppler Imaging at six observational epochs covering seven years.We also investigated the chromospheric activity variations by measuring the flux in the line cores of the three chromospheric activity indicators: Ca II H&K, H alpha, and the Ca II IRT lines.The magnetic topology of HN Peg shows a complex and variable geometry. While the radial field exhibits a stable positive polarity magnetic region at the poles at each observational epoch, the azimuthal field is strongly variable in strength, where a strong band of positive polarity magnetic field is present at equatorial latitudes. This field disappears during the middle of our time span, reappearing again during the last two epochs of observations. The mean magnetic field derived from the magnetic maps also follow a similar trend to the toroidal field, with the field strength at a minimum in epoch 2009.54. Summing the line of sight magnetic field over the visible surface at each observation, HN Peg exhibits a weak longitudinal magnetic field ranging from -14 G to 13 G, with no significant long-term trend, although there is significant rotational variability within each epoch. Those chromospheric activity indicators exhibit more long-term variations over the time span of observations, where the minimal is observed in Epoch 2008.71.

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arXiv2014

Spectral analysis and abundances of the post-HB star HD 76431

HD76431 is a slow rotating post-HB star that shows an underabundance of helium by 0.5 dex relative to the solar value. These observational facts suggest that atomic diffusion could be active in its atmosphere. We have used the MMT and Bok spectra to estimate the atmospheric parameters of the target star using the model atmospheres and synthetic spectra calculated with TLUSTY and SYNSPEC. The derived values of the effective temperature, surface gravity, helium abundance are consistent with those obtained by Ramspeck et al. (2001b). It appears that NLTE effect are not important for HD76431. We have used Stokes I spectra from ESPaDOnS at CFHT to perform an abundance analysis and a search for observational evidence of vertical stratification of the abundance of certain elements. The results of our abundance analysis are in good agreement with previously published data with respect to average abundances. Our numerical simulations show that carbon and nitrogen reveal signatures of vertical abundance stratification in the atmosphere of HD76431. It appears that the carbon abundance increases toward the deeper atmospheric layers. Nitrogen also shows a similar behaviour, but in deeper atmospheric layers we obtain a significant dispersion for the estimates of its abundance. To our knowledge, this is the first demonstration of vertical abundance stratification of metals in a post-HB star and up to now it is the hottest star to show such stratification features. We also report the detection of two SiIII and one TiIII emission lines in the spectra of HD76431 that were not detected in previous studies.

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arXiv2014

A search for weak or complex magnetic fields in the the B3V star iota Herculis

We obtained 128 high signal-to-noise ratio Stokes $V$ spectra of the B3V star $ι$~Her on 5 consecutive nights in 2012 with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope, with the aim of searching for the presence of weak and/or complex magnetic fields. Least-Squares Deconvolution mean profiles were computed from individual spectra, averaged over individual nights and over the entire run. No Zeeman signatures are detected in any of the profiles. The longitudinal magnetic field in the grand average profile was measured to be $-0.24 \pm 0.32 $ G, as compared to $-0.22 \pm 0.32$ G in the null profile. Our observations therefore provide no evidence for the presence of Zeeman signatures analogous to those observed in the A0V star Vega by Lignieres et al. (2009). We interpret these observations in three ways. First, we compare the LSD profiles with synthetic Stokes $V$ profiles corresponding to organised (dipolar) magnetic fields, for which we find an upper limit of about 8~G on the polar strength of any surface dipole present. Secondly, we compare the grand average profile with calculations corresponding to the random magnetic spot topologies of Kochukhov & Sudnik (2013), inferring that spots, if present, of 2 degrees radius with strengths of 2-4 G and a filling factor of 50% should have been detected in our data. Finally, we compare the observations with synthetic $V$ profiles corresponding to the surface magnetic maps of Vega (Petit et al. 2010) computed for the spectral characteristics of $ι$ Her. We conclude that while it is unlikely we would have detected a magnetic field identical to Vega's, we would have likely detected one with a peak strength of about 30 G, i.e. approximately four times as strong as that of Vega.

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arXiv2014

UVMag: stellar formation, evolution, structure and environment with space UV and visible spectropolarimetry

Important insights into the formation, structure, evolution and environment of all types of stars can be obtained through the measurement of their winds and possible magnetospheres. However, this has hardly been done up to now mainly because of the lack of UV instrumentation available for long periods of time. To reach this aim, we have designed UVMag, an M-size space mission equipped with a high-resolution spectropolarimeter working in the UV and visible spectral range. The UV domain is crucial in stellar physics as it is very rich in atomic and molecular lines and contains most of the flux of hot stars. Moreover, covering the UV and visible spectral domains at the same time will allow us to study the star and its environment simultaneously. Adding polarimetric power to the spectrograph will multiply tenfold the capabilities of extracting information on stellar magnetospheres, winds, disks, and magnetic fields. Examples of science objectives that can be reached with UVMag are presented for pre-main sequence, main sequence and evolved stars. They will cast new light onto stellar physics by addressing many exciting and important questions. UVMag is currently undergoing a Research and Technology study and will be proposed at the forthcoming ESA call for M-size missions. This spectropolarimeter could also be installed on a large UV and visible observatory (e.g. NASA's LUVOIR project) within a suite of instruments.

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arXiv2014

PolarBase: a data base of high resolution spectropolarimetric stellar observations

PolarBase is an evolving data base that contains all stellar data collected with the ESPaDOnS and NARVAL high-resolution spectropolarimeters, in their reduced form, as soon as they become public. As of early 2014, observations of 2,000 stellar objects throughout the Hertzsprung-Russell diagram are available. Intensity spectra are available for all targets, and the majority of the observations also include simultaneous spectra in circular or linear polarization, with the majority of the polarimetric measurements being performed only in circularly polarized light (Stokes V). Observations are associated with a cross-correlation pseudo-line profile in all available Stokes parameters, greatly increasing the detectability of weak polarized signatures. Stokes V signatures are detected for more than 300 stars of all masses and evolutionary stages, and linear polarization is detected in 35 targets. The detection rate in Stokes V is found to be anti-correlated with the stellar effective temperature. This unique set of Zeeman detections offers the first opportunity to run homogeneous magnetometry studies throughout the H-R diagram. The web interface of PolarBase is available at http://polarbase.irap.omp.eu.

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arXiv2014

Stellar magnetism: empirical trends with age and rotation

We investigate how the observed large-scale surface magnetic fields of low-mass stars (~0.1 -- 2 Msun), reconstructed through Zeeman-Doppler imaging (ZDI), vary with age t, rotation and X-ray emission. Our sample consists of 104 magnetic maps of 73 stars, from accreting pre-main sequence to main-sequence objects (1 Myr < t < 10 Gyr). For non-accreting dwarfs we empirically find that the unsigned average large-scale surface field <|Bv|> is related to age as $t^{-0.655 \pm 0.045}$. This relation has a similar dependence to that identified by Skumanich (1972), used as the basis for gyrochronology. Likewise, our relation could be used as an age-dating method ("magnetochronology"). The trends with rotation we find for the large-scale stellar magnetism are consistent with the trends found from Zeeman broadening measurements (sensitive to large- and small-scale fields). These similarities indicate that the fields recovered from both techniques are coupled to each other, suggesting that small- and large-scale fields could share the same dynamo field generation processes. For the accreting objects, fewer statistically significant relations are found, with one being a correlation between the unsigned magnetic flux and rotation period. We attribute this to a signature of star-disc interaction, rather than being driven by the dynamo.

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arXiv2014

The dichotomy between strong and ultra-weak magnetic fields among intermediate-mass stars

Until recently, the detection of magnetic fields at the surface of intermediate-mass main-sequence stars has been limited to Ap/Bp stars, a class of chemically peculiar stars. This class represents no more than 5-10% of the stars in this mass range. This small fraction is not explained by the fossil field paradigm that describes the Ap/Bp type magnetism as a remnant of an early phase of the star-life. Also, the limitation of the field measurements to a small and special group of stars is obviously a problem to study the effect of the magnetic fields on the stellar evolution of a typical intermediate-mass star. Thanks to the improved sensitivity of a new generation of spectropolarimeters, a lower bound to the magnetic fields of Ap/Bp stars, a two orders of magnitude desert in the longitudinal magnetic field and a new type of sub-gauss magnetism first discovered on Vega have been identified. These advances provide new clues to understand the origin of intermediate-mass magnetism as well as its influence on stellar evolution. In particular, a scenario has been proposed whereby the magnetic dichotomy between Ap/Bp and Vega-like magnetism originate from the bifurcation between stable and unstable large scale magnetic configurations in differentially rotating stars. In this paper, we review these recent observational findings and discuss this scenario.

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arXiv2013

Search for surface magnetic fields in Mira stars. First detection in chi Cyg

In order to complete the knowledge of the magnetic field and of its influence during the transition from Asymptotic Giant Branch to Planetary Nebulae stages, we have undertaken a search for magnetic fields at the surface of Mira stars. We used spectropolarimetric observations, collected with the Narval instrument at TBL, in order to detect - with Least Squares Deconvolution method - a Zeeman signature in the visible part of the spectrum. We present the first spectropolarimetric observations of the S-type Mira star chi Cyg, performed around its maximum light. We have detected a polarimetric signal in the Stokes V spectra and we have established its Zeeman origin. We claim that it is likely to be related to a weak magnetic field present at the photospheric level and in the lower part of the stellar atmosphere. We have estimated the strength of its longitudinal component to about 2-3 Gauss. This result favors a 1/r law for the variation of the magnetic field strength across the circumstellar envelope of chi Cyg. This is the first detection of a weak magnetic field at the stellar surface of a Mira star and we discuss its origin in the framework of shock waves periodically propagating throughout the atmosphere of these radially pulsating stars. At the date of our observations of chi Cyg, the shock wave reaches its maximum intensity, and it is likely that the shock amplifies a weak stellar magnetic field during its passage through the atmosphere. Without such an amplification by the shock, the magnetic field strength would have been too low to be detected. For the first time, we also report strong Stokes Q and U signatures (linear polarization) centered onto the zero velocity (i.e., at the shock front position). They seem to indicate that the radial direction would be favored by the shock during its propagation throughout the atmosphere.

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arXiv2013

Evidence for dynamo bistability among very low mass stars

Dynamo action in fully convective stars is a debated issue that also questions our understanding of magnetic field generation in partly convective Sun-like stars. During the past few years, spectropolari- metric observations have demonstrated that fully convective objects are able to trigger strong large-scale and long-lived magnetic fields. We present here the first spectropolarimetric study of a sample of active late M dwarfs (M5-M8) carried out with ESPaDOnS@CFHT. It reveals the co-existence of two distinct types of magnetism among stars having similar masses and rotation rates. A possible explanation for this unexpected discovery is the existence of two dynamo branches in this parameter regime, we discuss here the possible identification with the weak vs strong field bistability predicted for the geodynamo.

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arXiv2013

A Bcool magnetic snapshot survey of solar-type stars

Stellar magnetic field measurements obtained from spectropolarimetry offer key data for activity and dynamo studies, and we present the results of a major high-resolution spectropolarimetric Bcool project magnetic snapshot survey of 170 solar-type stars from observations with the Telescope Bernard Lyot and the Canada-France-Hawaii Telescope. For each target star a high signal-to-noise circularly polarised Stokes V profile has been obtained using Least-Squares Deconvolution, and used to detect surface magnetic fields and measure the corresponding mean surface longitudinal magnetic field ($B_{l}$). Chromospheric activity indicators were also measured. Surface magnetic fields were detected for 67 stars, with 21 of these stars classified as mature solar-type stars, a result that increases by a factor of four the number of mature solar-type stars on which magnetic fields have been observed. In addition, a magnetic field was detected for 3 out of 18 of the subgiant stars surveyed. For the population of K-dwarfs the mean value of $B_{l}$ ($|B_{l}|_{mean}$) was also found to be higher (5.7 G) than $|B_{l}|_{mean}$ measured for the G-dwarfs (3.2 G) and the F-dwarfs (3.3 G). For the sample as a whole $|B_{l}|_{mean}$ increases with rotation rate and decreases with age, and the upper envelope for $|B_{l}|$ correlates well with the observed chromospheric emission. Stars with a chromospheric S-index greater than about 0.2 show a high magnetic field detection rate and so offer optimal targets for future studies. This survey constitutes the most extensive spectropolarimetric survey of cool stars undertaken to date, and suggests that it is feasible to pursue magnetic mapping of a wide range of moderately active solar-type stars to improve understanding of their surface fields and dynamos.

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arXiv2013

UVMag: a UV and optical spectropolarimeter for stellar physics

UVMag is a space project currently under R&D study. It consists in a medium-size telescope equipped with a spectropolarimeter to observe in the UV and optical wavelength domains simultaneously. Its first goal is to obtain time series of selected magnetic stars over their rotation period, to study them from their surface to their environment, in particular their wind and magnetospheres. As the star rotates it will be possible to reconstruct 3D maps of the star and its surroundings. The second goal of UVMag is to obtain two observations of a large sample of stars to construct a new database of UV and optical spectropolarimetric measurements.

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arXiv2013

Magnetic fields in single late-type giants in the Solar vicinity: How common is magnetic activity on the giant branches?

We present our first results on a new sample containing all single G,K and M giants down to V = 4 mag in the Solar vicinity, suitable for spectropolarimetric (Stokes V) observations with Narval at TBL, France. For detection and measurement of the magnetic field (MF), the Least Squares Deconvolution (LSD) method was applied (Donati et al. 1997) that in the present case enables detection of large-scale MFs even weaker than the solar one (the typical precision of our longitudinal MF measurements is 0.1-0.2 G). The evolutionary status of the stars is determined on the basis of the evolutionary models with rotation (Lagarde et al. 2012; Charbonnel et al., in prep.) and fundamental parameters given by Massarotti et al. (1998). The stars appear to be in the mass range 1-4 M_sun, situated at different evolutionary stages after the Main Sequence (MS), up to the Asymptotic Giant Branch (AGB). The sample contains 45 stars. Up to now, 29 stars are observed (that is about 64 % of the sample), each observed at least twice. For 2 stars in the Hertzsprung gap, one is definitely Zeeman detected. Only 5 G and K giants, situated mainly at the base of the Red Giant Branch (RGB) and in the He-burning phase are detected. Surprisingly, a lot of stars ascending towards the RGB tip and in early AGB phase are detected (8 of 13 observed stars). For all Zeeman detected stars v sin i is redetermined and appears in the interval 2-3 km/s, but few giants with MF possess larger v sin i.

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arXiv2013

Magnetic Field Structure and Activity of the He-burning Giant 37 Comae

We present the first magnetic map of the late-type giant 37 Com. The Least Squares Deconvolution (LSD) method and Zeeman Doppler Imaging (ZDI) inversion technique were applied. The chromospheric activity indicators H?_alpha, S-index, Ca ii IRT and the radial velocity were also measured. The evolutionary status of the star has been studied on the basis of state-of-the-art stellar evolutionary models and chemical abundance analysis. 37 Com appears to be in the core Helium-burning phase.

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arXiv2013

High-resolution spectropolarimetric of k Cet: A proxy for the young Sun

Among the solar proxies studied in the Sun in Time k Cet (HD 20630) stands out as potentially having a mass very close to solar and a young age. On this study, we monitored the magnetic field and the chromospheric activity from the Ca II H & K lines of k Cet. We used the Least-Square-Deconvolution (LSD, Donati et al. 1997) by simultaneously extracting the information contained in all 8,000 photospheric lines of the echelogram (K1 type star). To reconstruct a reliable magnetic map and characterize the surface differential rotation of k Cet we used 14 exposures spread over 2 months, in order to cover at least two rotational cycles (Prot ~ 9.2 days). The LSD technique was applied to detect the Zeeman signature of the magnetic field in each of our 14 observations and to measure its longitudinal component. In order to reconstruct the magnetic field geometry of k Cet, we applied the Zeeman Doppler Imaging (ZDI) inversion method. ZDI revealed a structure in the radial magnetic field consisting of a polar magnetic spot. On this study, we present the fisrt look results of a high-resolution spectropolarimetric campaign to characterize the activity and the magnetic fields of this young solar proxy.

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arXiv2013

Descendants of Magnetic and non-Magnetic A-type Stars

We have studied the magnetic field of about 50 active and non-active single G-K-type red giants by means of spectropolarimetry with Narval and ESPaDOnS. 30 giants have been significantly Zeeman-detected. A close study of the 17 giants with known rotational periods shows that the measured magnetic field strength is correlated to the rotation, in particular to the Rossby number. 4 giants for which the magnetic field is measured to be outstandingly strong with respect to the rotational period or the evolutionary state are identified as probable Ap-star descendants. We detail their magnetic properties and propose criteria to identify Ap-star descendants.

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arXiv2013

The evolution of surface magnetic fields in young solar-type stars

The surface rotation rates of young solar-type stars decrease rapidly with age from the end of the pre-main sequence though the early main sequence. This suggests that there is also an important change in the dynamos operating in these stars, which should be observable in their surface magnetic fields. Here we present early results in a study aimed at observing the evolution of these magnetic fields through this critical time period. We are observing stars in open clusters and stellar associations to provide precise ages, and using Zeeman Doppler Imaging to characterize the complex magnetic fields. Presented here are results for six stars, three in the in the beta Pic association (~10 Myr old) and three in the AB Dor association (~100 Myr old).

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arXiv2013

Magnetic field structure in single late-type giants: Beta Ceti in 2010 - 2012

The data were obtained using two spectropolarimeters - Narval at the Bernard Lyot Telescope, Pic du Midi, France, and ESPaDOnS at CFHT, Hawaii. Thirty-eight circularly-polarized spectra have been collected in the period June 2010 - January 2012. The Least Square Deconvolution method was applied for extracting high signal-to-noise ratio line profiles, from which we measure the surface-averaged longitudinal magnetic field Bl. Chromospheric activity indicators CaII K, H_alpha, CaII IR (854.2 nm) and radial velocity were simultaneously measured and their variability was analysed together with the behavior of Bl. The Zeeman Doppler Imaging (ZDI) inversion technique was employed for reconstruction of the large-scale magnetic field and two magnetic maps of Beta Ceti are presented for two periods (June 2010 - December 2010 and June 2011 - January 2012). Bl remains of positive polarity for the whole observational period. The behavior of the line activity indicators is in good agreement with the Bl variations. The two ZDI maps show a mainly axisymmetric and poloidal magnetic topology and a simple surface magnetic field configuration dominated by a dipole. Little evolution is observed between the two maps, in spite of a 1 yr interval between both subsets. We also use state-of-the-art stellar evolution models to constrain the evolutionary status of Beta Ceti. We derive a mass of 3.5 M_sun and propose that this star is already in the central-helium burning phase. Taking into account all our results and the evolutionary status of the star, we suggest that dynamo action alone may not be eficient enough to account for the high magnetic activity of Beta Ceti. As an alternate option, we propose that it may be an Ap star descendant presently undergoing central helium-burning and still exhibiting a remnant of the Ap star magnetic field.

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arXiv2013

Long-term spectropolarimetric monitoring of the cool supergiant Betelgeuse

We report on a long-term monitoring of the cool supergiant Betelgeuse, using the NARVAL and ESPaDOnS high-resolution spectropolarimeters, respectively installed at Telescope Bernard Lyot (Pic du Midi Observatory, France) and at the Canada-France-Hawaii Telescope (Mauna Kea Observatory, Hawaii). The data set, constituted of circularly polarized (Stokes V) and intensity (Stokes I) spectra, was collected between 2010 and 2012. We investigate here the temporal evolution of magnetic field, convection and temperature at photospheric level, using simultaneous measurements of the longitudinal magnetic field component, the core emission of the Ca II infrared triplet, the line-depth ratio of selected photospheric lines and the radial velocity of the star.

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