arXiv subjects

C. Neiner

Publications and source records attributed to C. Neiner.

At least 55 records · Page 3Linked to original sources

arXiv2019

Revisiting the pulsational characteristics of the exoplanet host star $β$ Pictoris

Exoplanet properties crucially depend on their host stars' parameters. In case the exoplanet host star shows pulsations, asteroseismology can be used for an improved description of the stellar parameters. We aim to revisit the pulsational properties of beta Pic and identify its pulsation modes from normalised amplitudes in five different passbands. We also investigate the potential presence of a magnetic field. We conduct a frequency analysis using three seasons of BRITE-Constellation observations in the BRITE blue and red filters, the ~620-day long bRing light curve and the nearly 8-year long SMEI photometric time series. We calculate normalised amplitudes using all passbands including previously published values obtained from ASTEP observations. We investigate the magnetic properties of beta Pic using spectropolarimetric observations conducted with the HARPSpol instrument. Using 2D rotating models, we fit the normalised amplitudes and frequencies through Monte Carlo Markov Chains. We identify 15 pulsation frequencies in the range from 34 to 55c/d, where two display clear amplitude variability. We use the normalised amplitudes in up to five passbands to identify the modes as three l = 1, six l = 2 and six l = 3 modes. beta Pic is shown to be non-magnetic with an upper limit of the possible undetected dipolar field of 300G. Multiple fits to the frequencies and normalised amplitudes are obtained including one with a near equator-on inclination for beta Pic, which corresponds to our expectations based on the orbital inclination of beta Pic b and the orientation of the circumstellar disk. This solution leads to a rotation rate of 27% of the Keplerian break-up velocity, a radius of 1.497+-0.025Rsun, and a mass of 1.797+-0.035Msun. The ~2% errors in radius and mass do not account for uncertainties in the models and a potentially erroneous mode-identification.

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arXiv2019

MOBSTER - II. Identification of rotationally variable A stars observed with \emph{TESS} in Sectors 1 to 4

Recently, high-precision optical 2~min cadence light curves obtained with \emph{TESS} for targets located in the mission's defined first four sectors have been released. The majority of these high-cadence and high-precision measurements currently span $\sim28\,{\rm d}$, thereby allowing periodic variability occurring on timescales $\lesssim14\,{\rm d}$ to potentially be detected. Magnetic chemically peculiar (mCP) A-type stars are well known to exhibit rotationally modulated photometric variability that is produced by inhomogeneous chemical abundance distributions in their atmospheres. While mCP stars typically exhibit rotation periods that are significantly longer than those of non-mCP stars, both populations exhibit typical periods $\lesssim10\,{\rm d}$; therefore, the early \emph{TESS} releases are suitable for searching for rotational modulation of the light curves of both mCP and non-mCP stars. We present the results of our search for A-type stars that exhibit variability in their \emph{TESS} light curves that is consistent with rotational modulation based on the first two data releases obtained from sectors 1 to 4. Our search yielded $134$ high-probability candidate rotational variables -- $60$ of which have not been previously reported. Approximately half of these stars are identified in the literature as Ap (mCP) stars. Comparisons between the subsample of high-probability candidate rotationally variable Ap stars and the subsample of stars that are not identified as Ap reveal that the latter subsample exhibits statistically (i) shorter rotation periods and (ii) significantly lower photometric amplitudes.

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arXiv2019

Magnetic OB[A] Stars with TESS: probing their Evolutionary and Rotational properties (MOBSTER) - I. First-light observations of known magnetic B and A stars

In this paper we introduce the MOBSTER collaboration and lay out its scientific goals. We present first results based on the analysis of nineteen previously known magnetic O, B and A stars observed in 2-minute cadence in sectors 1 and 2 of the Transiting Exoplanet Survey Satellite (TESS) mission. We derive precise rotational periods from the newly obtained light curves and compare them to previously published values. We also discuss the overall photometric phenomenology of the known magnetic massive and intermediate-mass stars and propose an observational strategy to augment this population by taking advantage of the high-quality observations produced by TESS.

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arXiv2019

Stellar physics with high-resolution UV spectropolarimetry

Current burning issues in stellar physics, for both hot and cool stars, concern their magnetism. In hot stars, stable magnetic fields of fossil origin impact their stellar structure and circumstellar environment, with a likely major role in stellar evolution. However, this role is complex and thus poorly understood as of today. It needs to be quantified with high-resolution UV spectropolarimetric measurements. In cool stars, UV spectropolarimetry would provide access to the structure and magnetic field of the very dynamic upper stellar atmosphere, providing key data for new progress to be made on the role of magnetic fields in heating the upper atmospheres, launching stellar winds, and more generally in the interaction of cool stars with their environment (circumstellar disk, planets) along their whole evolution.

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arXiv2019

The role of Active Galactic Nuclei in galaxy evolution: insights from space ultraviolet spectropolarimetry

This Astro2020 white paper summarizes the unknowns of active galactic nuclei (AGN) physics that could be unveiled thanks to a new, space-born, ultraviolet spectropolarimeter. The unique capabilities of high energy polarimetry would help us to understand the precise mechanisms of matter and energy transfer and supermassive black holes growth, together with the impact of AGN feedback on galaxy evolution.

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arXiv2019

The Magnetic Early B-type Stars II: stellar atmospheric parameters in the era of Gaia

Atmospheric parameters determined via spectral modelling are unavailable for many of the known magnetic early B-type stars. We utilized high-resolution spectra together with NLTE models to measure effective temperatures $T_{\rm eff}$ and surface gravities $\log{g}$ of stars for which these measurements are not yet available. We find good agreement between our $T_{\rm eff}$ measurements and previous results obtained both photometrically and spectroscopically. For $\log{g}$, our results are compatible with previous spectroscopic measurements; however, surface gravities of stars previously determined photometrically have been substantially revised. We furthermore find that $\log{g}$ measurements obtained with HARPSpol are typically about 0.1 dex lower than those from comparable instruments. Luminosities were determined using Gaia Data Release 2 parallaxes. We find Gaia parallaxes to be unreliable for bright stars ($V<6$ mag) and for binaries; in these cases we reverted to Hipparcos parallaxes. In general we find luminosities systematically lower than those previously reported. Comparison of $\log{g}$ and $\log{L}$ to available rotational and magnetic measurements shows no correlation between either parameter with magnetic data, but a clear slow-down in rotation with both decreasing $\log{g}$ and increasing $\log{L}$, a result compatible with the expectation that magnetic braking should lead to rapid magnetic spindown that accelerates with increasing mass-loss.

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arXiv2018

A Volume Limited Survey of mCP Stars Within 100pc II: Rotational and Magnetic Properties

Various surveys focusing on the magnetic properties of intermediate-mass main sequence (MS) stars have been previously carried out. One particularly puzzling outcome of these surveys is the identification of a dichotomy between the strong ($\gtrsim100\,{\rm G}$), organized fields hosted by magnetic chemically peculiar (mCP) stars and the ultra-weak ($\lesssim1\,{\rm G}$) fields associated with a small number of non-mCP MS stars. Despite attempts to detect intermediate strength fields (i.e. those with strengths $\gtrsim10\,{\rm G}$ and $\lesssim100\,{\rm G}$), remarkably few examples have been found. Whether this so-called "magnetic desert", separating the stars hosting ultra-weak fields from the mCP stars truly exists has not been definitively answered. In 2007, a volume-limited spectropolarimetric survey of mCP stars using the MuSiCoS spectropolarimeter was initiated to test the existence of the magnetic desert by attempting to reduce the biases inherent in previous surveys. Since then, we have obtained a large number of ESPaDOnS and NARVAL Stokes $V$ measurements allowing this survey to be completed. Here we present the results of our homogeneous analysis of the rotational periods (inferred from photometric and magnetic variability) and magnetic properties (dipole field strengths and obliquity angles) of the 52 confirmed mCP stars located within a heliocentric distance of $100\,{\rm pc}$. No mCP stars exhibiting field strengths $\lesssim300\,{\rm G}$ are found within the sample, which is consistent with the notion that the magnetic desert is a real property and not the result of an observational bias. Additionally, we find evidence of magnetic field decay, which confirms the results of previous studies.

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arXiv2018

A Volume-Limited Survey of mCP Stars Within 100pc I: Fundamental Parameters and Chemical Abundances

We present the first results of a volume-limited survey of main sequence (MS) magnetic chemically peculiar (mCP) stars. The sample consists of all identified intermediate-mass MS stars (mCP and non-mCP) within a heliocentric distance of $100\,{\rm pc}$ as determined using Hipparcos parallaxes. The two populations are compared in order to determine the unique properties that allow a small fraction of MS stars with masses $\gtrsim1.4\,M_\odot$ to host strong, large scale magnetic fields. A total of 52 confirmed mCP stars are identified using published magnetic, spectroscopic, and photometric observations along with archived and newly obtained spectropolarimetric (Stokes $V$) observations. We derive the fundamental parameters (effective temperatures, luminosities, masses, and evolutionary states) of the mCP and non-mCP populations using homogeneous analyses. A detailed analysis of the mCP stars is performed using the {\sc llmodels} code, which allows observed spectral energy distributions to be modeled while incorporating chemical peculiarities and magnetic fields. The surface gravities and mean chemical abundances are derived by modelling averaged spectra using the {\sc gssp} and {\sc zeeman} spectral synthesis codes. Masses and stellar ages are derived using modern, densely calculated evolutionary model grids. We confirm a number of previously reported evolutionary properties associated with mCP stars including a conspicuously high incidence of middle-aged MS stars with respect to the non-mCP subsample; the incidence of mCP stars is found to sharply increase with mass from $0.3$~per~cent at $1.5\,M_\odot$ to $\approx11$~per~cent at $3.8\,M_\odot$. Finally, we identify clear trends in the mean photospheric chemical abundances with stellar age.

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arXiv2018

The magnetic field of evolved hot stars

About 10% of hot stars host a fossil magnetic field on the pre-main sequence and main sequence. However, the first magnetic evolved hot stars have been discovered only recently. An observing program has been set up to find more such objects. This will allow us to test how fossil fields evolve, and the impact of magnetism on stellar evolution. Already 7 evolved magnetic hot stars are now known and the rate of magnetic discoveries in the survey suggests that they host dynamo fields in addition to fossil fields. Finally, the weakness of the measured fields is compatible at first order with simple magnetic flux conservation, although the current statistics cannot exclude intrinsic decay or enhancement during stellar evolution.

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arXiv2018

The BeSS database: a fruitful professional-amateur collaboration

BeSS is a database containing a catalogue of Be stars and their spectra, set up more than 10 years ago as a collaboration between professional and amateur astronomers. It currently contains over 177000 spectra of 2340 stars, provided by ~150 different observers. Its continuous success has already led to the use of BeSS data in more than 70 scientific papers.

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arXiv2018

Massive stars with Pollux on LUVOIR

Many open questions remain about massive stars, for example about their evolution, their wind, and their maximum mass at formation. These issues could be ideally adressed by the Pollux UV spectropolarimeter onboard LUVOIR. Here we present examples of the science themes that one could study with Pollux regarding massive stars.

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arXiv2018

Magnetic field topologies of the bright, weak-field Ap stars theta Aurigae and epsilon Ursae Majoris

The brightest magnetic chemically peculiar stars theta Aur and eps UMa were targeted by numerous studies of their photometric and spectroscopic variability. Detailed maps of chemical abundance spots were repeatedly derived for both stars. However, very little information on the magnetic field geometries of these stars is available. In this study we aim to determine detailed magnetic field topologies of theta Aur and eps UMa based on modern, high-resolution spectropolarimetric observations. Both targets were observed in all four Stokes parameters using the Narval and ESPaDOnS spectropolarimeters. A multi-line technique of least-squares deconvolution was employed to detect polarisation signatures in spectral lines. These signatures were modelled with a Zeeman-Doppler imaging code. We succeeded in detecting variable circular and linear polarisation signatures for theta Aur. Only circular polarisation was detected for eps UMa. We obtained new sets of high-precision longitudinal magnetic field measurements using mean circular polarisation metal line profiles as well as hydrogen line cores, which are consistent with historical data. Magnetic inversions revealed distorted dipolar geometries in both stars. The Fe and Cr abundance distributions, reconstructed simultaneously with magnetic mapping, do not show a clear correlation with the local magnetic field properties, with the exception of a relative element underabundance in the horizontal field regions along the magnetic equators. Our study provides the first ever detailed surface magnetic field maps for broad-line, weak-field chemically peculiar stars, showing that their field topologies are qualitatively similar to those found in stronger-field stars. The Fe and Cr chemical abundance maps reconstructed for theta Aur and eps UMa are at odds with the predictions of current theoretical atomic diffusion calculations.

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arXiv2018

BRITE photometry of the massive post-RLOF system HD149404

HD149404 is an evolved non-eclipsing O-star binary that has previously undergone a Roche lobe overflow interaction. Understanding some key properties of the system requires a determination of the orbital inclination and of the dimensions of the components. The BRITE-Heweliusz satellite was used to collect photometric data of HD149404. Additional photometry was retrieved from the SMEI archive. These data were analysed using a suite of period search tools. The orbital part of the lightcurve was modelled with the nightfall binary star code. The Gaia-DR2 parallax of HD149404 was used to provide additional constraints. The periodograms reveal a clear orbital modulation of the lightcurve with a peak-to-peak amplitude near 0.04 mag. The remaining non-orbital part of the variability is consistent with red noise. The lightcurve folded with the orbital period reveals ellipsoidal variations, but no eclipses. The minimum when the secondary star is in inferior conjunction is deeper than the other minimum due to mutual reflection effects between the stars. Combined with the Gaia-DR2 parallaxes, the photometric data indicate an orbital inclination in the range of 23° to 31° and a Roche lobe filling factor of the secondary larger than or equal to 0.96. The luminosity of the primary star is consistent with its present-day mass, whereas the more evolved secondary appears overluminous for its mass. We confirm that the primary's rotation period is about half the orbital period. Both features most probably stem from the past Roche lobe overflow episode.

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arXiv2018

Probing the structure and evolution of active galactic nuclei with the ultraviolet polarimeter POLLUX aboard LUVOIR

The ultraviolet (UV) polarization spectrum of nearby active galactic nuclei (AGN) is poorly known. The Wisconsin Ultraviolet Photo-Polarimeter Experiment and a handful of instruments on board the Hubble Space Telescope were able to probe the near- and mid-UV polarization of nearby AGN, but the far-UV band (from 1200 angs down to the Lyman limit at 912 angs) remains completely uncharted. In addition, the linewidth resolution of previous observations was at best 1.89 angs. Such a resolution is not sufficient to probe in detail quantum mechanical effects, synchrotron and cyclotron processes, scattering by electrons and dust grains, and dichroic extinction by asymmetric dust grains. Exploring those physical processes would require a new, high-resolution, broadband polarimeter with full ultraviolet-band coverage. In this context, we discuss the AGN science case for POLLUX, a high-resolution UV spectropolarimeter, proposed for the 15-meter primary mirror option of LUVOIR (a multi-wavelength space observatory concept being developed by the Goddard Space Flight Center and proposed for the 2020 Decadal Survey Concept Study).

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arXiv2018

Magnetic characterization and variability study of the magnetic SPB star o Lup

Thanks to large dedicated surveys, large-scale magnetic fields have been detected for about 10% of early-type stars. We aim to precisely characterize the large-scale magnetic field of the magnetic component of the wide binary $o$ Lup, by using high-resolution ESPaDOnS and HARPSpol spectropolarimetry to analyse the variability of the measured longitudinal magnetic field. In addition, we investigate the periodic variability using space-based photometry collected with the BRITE-Constellation by means of iterative prewhitening. The rotational variability of the longitudinal magnetic field indicates a rotation period $P_{\mathrm{rot}}=2.95333(2)$d and that the large-scale magnetic field is dipolar, but with a significant quadrupolar contribution. Strong differences in the strength of the measured magnetic field occur for various chemical elements as well as rotational modulation for Fe and Si absorption lines, suggesting a inhomogeneous surface distribution of chemical elements. Estimates of the geometry of the large-scale magnetic field indicate $i=27\pm 10^{\circ}$, $β= 74^{+7}_{-9}\,^{\circ}$, and a polar field strength of at least 5.25 kG. The BRITE photometry reveals the rotation frequency and several of its harmonics, as well as two gravity mode pulsation frequencies. The high-amplitude g-mode pulsation at $f=1.1057\mathrm{d^{-1}}$ dominates the line-profile variability of the majority of the spectroscopic absorption lines. We do not find direct observational evidence of the secondary in the spectroscopy. Therefore, we attribute the pulsations and the large-scale magnetic field to the B5IV primary of the $o$ Lup system, but we discuss the implications should the secondary contribute to or cause the observed variability.

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arXiv2018

Forward seismic modelling of the pulsating magnetic B-type star HD 43317

The large-scale magnetic fields detected at the surface of about 10% of hot stars extend into the stellar interior, where they may alter the structure. Deep inner regions of stars are only observable using asteroseismology. Here, we investigated the pulsating magnetic B3.5V star HD43317, inferred its interior properties and assessed whether the dipolar magnetic field with a surface strength of $B_p = 1312 \pm 332$G caused different properties compared to those of non-magnetic stars. We analysed the latest version of the star's 150d CoRoT light curve and extracted 35 significant frequencies, 28 of which were determined to be independent and not related to the known surface rotation period of $P_{\rm rot} = 0.897673$d. We performed forward seismic modelling based on non-magnetic, non-rotating 1D MESA models and the adiabatic module of the pulsation code GYRE, utilizing a grid-based approach. Our aim was to estimate the stellar mass, age, and convective core overshooting. The GYRE calculations were done for uniform rotation with $P_{\rm rot}$. This modelling was able to explain 16 of the 28 frequencies as gravity modes belonging to retrograde modes with $(\ell, m) = (1, -1)$ and $(2, -1)$ period spacing patterns and one distinct prograde $(2,2)$ mode. The modelling resulted in a stellar mass $M_{\star} = 5.8^{+0.1}_{-0.2}$$\mathrm{M_{\odot}}$, a central hydrogen mass fraction $X_c = 0.54^{+0.01}_{-0.02}$, and exponential convective core overshooting parameter $f_{\rm ov} = 0.004^{+0.014}_{-0.002}$. The low value for $f_{\rm ov}$ is compatible with the suppression of near-core mixing due to a magnetic field but the uncertainties are too large to pinpoint such suppression as the sole physical interpretation. $[...]$

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arXiv2018

K2 space photometry reveals rotational modulation and stellar pulsations in chemically peculiar A and B stars

The physics of magnetic hot stars and how a large-scale magnetic field affects their interior properties is largely unknown. Few studies have combined high-quality observations and modelling of magnetic pulsating stars, known as magneto-asteroseismology, primarily because of the dearth of detected pulsations in stars with a confirmed and well-characterised large-scale magnetic field. We aim to characterise observational signatures of rotation and pulsation in chemically peculiar candidate magnetic stars using photometry from the K2 space mission. Thus, we identify the best candidate targets for ground-based, optical spectropolarimetric follow-up observations to confirm the presence of a large-scale magnetic field. We employed customised reduction and detrending tools to process the K2 photometry into optimised light curves for a variability analysis. We searched for the periodic photometric signatures of rotational modulation caused by surface abundance inhomogeneities in 56 chemically peculiar A and B stars. Furthermore, we searched for intrinsic variability caused by pulsations (coherent or otherwise) in the amplitude spectra of these stars. The rotation periods of 38 chemically peculiar stars are determined, 16 of which are the first determination of the rotation period in the literature. We confirm the discovery of high-overtone roAp pulsation modes in HD 177765 and find an additional 3 Ap and Bp stars that show evidence of high-overtone pressure modes found in roAp stars in the form of possible Nyquist alias frequencies in their amplitude spectra. Furthermore, we find 6 chemically peculiar stars that show evidence of intrinsic variability caused by gravity or pressure pulsation modes. The discovery of pulsations in a non-negligible fraction of chemically peculiar stars make these stars high-priority targets for spectropolarimetric campaigns.

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arXiv2018

Detection of magnetic fields in chemically peculiar stars observed with the K2 space mission

We report the results of an observational study aimed at searching for magnetic pulsating hot stars suitable for magneto-asteroseismology. A sample of sixteen chemically peculiar stars was selected and analysed using both high-resolution spectropolarimetry with ESPaDOnS and K2 high-precision space photometry. For all stars, we derive the effective temperature, surface gravity, rotational and non-rotational line broadening from our spectropolarimetric data. High-quality K2 light curves were obtained for thirteen of the sixteen stars and revealed rotational modulation, providing accurate rotation periods. Two stars show evidence for roAp pulsations, and one star shows signatures of internal gravity waves or unresolved g-mode pulsations. We confirm the presence of a large-scale magnetic field for eleven of the studied stars, of which nine are first detections. Further, we report one marginal detection and four non-detections. Two of the stars with a non-detected magnetic field show rotational modulation due to surface abundance inhomogeneities in the K2 light curve, and we confirm that the other two are chemically peculiar. Thus, these five stars likely host a weak (undetected) large-scale magnetic field.

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