arXiv subjects

D. M. Bowman

Publications and source records attributed to D. M. Bowman.

At least 73 records · Page 4Linked to original sources

arXiv2019

Magnetic OB[A] Stars with TESS: probing their Evolutionary and Rotational properties -- The MOBSTER Collaboration

In this contribution, we present the MOBSTER Collaboration, a large community effort to leverage high-precision photometry from the Transiting Exoplanet Survey Satellite (\textit{TESS}) in order to characterize the variability of magnetic massive and intermediate-mass stars. These data can be used to probe the varying column density of magnetospheric plasma along the line of sight for OB stars, thus improving our understanding of the interaction between surface magnetic fields and massive star winds. They can also be used to map out the brightness inhomogeneities present on the surfaces of Ap/Bp stars, informing present models of atomic diffusion in their atmospheres. Finally, we review our current and ongoing studies, which lead to new insights on this topic.

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arXiv2019

Rotational and pulsational variability in the TESS light curve of HD27463

The new photometric data on pulsating Ap star HD~27463 obtained recently with the Transiting Exoplanet Survey Satellite (\textit{TESS}) are analysed to search for variability. Our analysis shows that HD~27463 exhibits two types of photometric variability. The low frequency variability with the period $P$ =~2.834274 $\pm$ 0.000008 d can be explained in terms of axial stellar rotation assuming the oblique magnetic rotator model and presence of surface abundance/brightness spots, while the detected high-frequency variations are characteristics of $δ$~Scuti pulsations. From the analysis of Balmer line profiles visible in two FEROS spectra of HD~27463 we have derived its effective temperature and surface gravity, finding values that are close to those published for this star in the \textit{TESS} Input Catalogue (TIC). Knowing the rotation period and the v$\sin{i}$ value estimated from the fitting of Balmer line profiles we found that the rotational axis is inclined to the line of sight with an angle of $i=33\pm8°$. Our best-fitting model of the observed pulsation modes results in an overshoot parameter value $f_{ov} = 0.014$ and values of global stellar parameters that are in good agreement with the data reported in the TIC and with the data derived from fitting Balmer line profiles. This model indicates an age of 5.0 $\pm$~0.4 $\times 10^8$~yrs, which corresponds to a core hydrogen fraction of 0.33.

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arXiv2019

The first view of $δ$ Scuti and $γ$ Doradus stars with the TESS mission

We present the first asteroseismic results for $δ$ Scuti and $γ$ Doradus stars observed in Sectors 1 and 2 of the TESS mission. We utilise the 2-min cadence TESS data for a sample of 117 stars to classify their behaviour regarding variability and place them in the Hertzsprung-Russell diagram using Gaia DR2 data. Included within our sample are the eponymous members of two pulsator classes, $γ$ Doradus and SX Phoenicis. Our sample of pulsating intermediate-mass stars observed by TESS also allows us to confront theoretical models of pulsation driving in the classical instability strip for the first time and show that mixing processes in the outer envelope play an important role. We derive an empirical estimate of 74% for the relative amplitude suppression factor as a result of the redder TESS passband compared to the Kepler mission using a pulsating eclipsing binary system. Furthermore, our sample contains many high-frequency pulsators, allowing us to probe the frequency variability of hot young $δ$ Scuti stars, which were lacking in the Kepler mission data set, and identify promising targets for future asteroseismic modelling. The TESS data also allow us to refine the stellar parameters of SX Phoenicis, which is believed to be a blue straggler.

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arXiv2019

New $β$ Cep pulsators discovered with K2 space photometry

We present the discovery of three new $β$ Cep pulsators, three new pulsators with frequency groupings, and frequency patterns in a B3Ib star, all of which show pulsations with frequencies as high as about 17 d$^{-1}$, with K2 space mission photometry. Based on a Fourier analysis and iterative pre-whitening we present a classification and evaluate the potential for asteroseismic modelling. We include the lists of pulsation frequencies for three new $β$ Cep pulsators, CD-28 12286, CD-27 10876, LS 3978, and additional pulsation mode frequencies for the known $β$ Cep pulsator HD 164741. In addition we characterise the regular frequency spacing found in the new pulsator HD 169173, and discuss its origin. We place the newly discovered variables in a colour-magnitude diagram using parallaxes from Gaia-DR2, showcasing their approximate location in the massive star domain. The identified frequency lists of these multiperiodic pulsators are a good starting point for future forward seismic modelling, after identification of at least one pulsation frequency from high-resolution time series spectroscopy and/or multi-colour photometry.

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arXiv2019

Period spacings of gravity modes in rapidly rotating magnetic stars I. Axisymmetric fossil field with poloidal and toroidal components

Context. Stellar magnetic fields are one of the candidates often invoked to explain the missing transport of angular momentum observed in the models of stellar interiors. However, the properties of an internal magnetic field and the consequences of its presence on stellar evolution are largely unknown. Aims. We study the effect of an axisymmetric internal magnetic field on the frequency of gravity modes in rapidly rotating stars to check whether gravity modes can be used to detect and probe such a field. Methods. Rotation is taken into account using the traditional approximation of rotation and the effect of the magnetic field is computed using a perturbative approach. As a proof of concept, we compute frequency shifts due to a mixed (i.e. with both poloidal and toroidal components) fossil magnetic field for a representative model of a known magnetic, rapidly rotating, slowly pulsating B-type star, namely HD 43317. Results. We find that frequency shifts induced by the magnetic field scale with the square of its amplitude. A magnetic field with a near-core strength of order 150 kG (which is consistent with the observed surface field strength of order 1 kG) leads to signatures that are detectable in period spacings for high-radial-order gravity modes. Conclusions. The predicted frequency shifts can be used to constrain internal magnetic fields and offers the potential for a significant step forward in our interpretation of the observed structure of gravity-mode period spacing patterns in rapidly rotating stars.

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arXiv2019

Rotation and pulsation in Ap stars: first light results from TESS sectors 1 and 2

We present the first results from the Transiting Exoplanet Survey Satellite (TESS) on the rotational and pulsational variability of magnetic chemically peculiar A-type stars. We analyse TESS 2-min cadence data from sectors 1 and 2 on a sample of 83 stars. Five new rapidly oscillating Ap (roAp) stars are announced. One of these pulsates with periods around 4.7 min, making it the shortest period roAp star known to date. Four out of the five new roAp stars are multiperiodic. Three of these, and the singly-periodic one show the presence of rotational mode splitting. Individual frequencies are provided in all cases. In addition, seven previously known roAp stars are analysed. Additional modes of oscillation are found in some stars, while in others we are able to distinguish the true pulsations from possible aliases present in the ground-based data. We find that the pulsation amplitude in the TESS filter is typically a factor 6 smaller than that in the $B$ filter which is usually used for ground-based observations. For four roAp stars we set constraints on the inclination angle and magnetic obliquity, through the application of the oblique pulsator model. We also confirm the absence of roAp-type pulsations down to amplitude limits of 6 and 13 micromag, respectively, in two of the best characterised non-oscillating Ap (noAp) stars. We announce 27 new rotational variables along with their rotation periods, and provide different rotation periods for seven other stars. Finally, we discuss how these results challenge state-of-the-art pulsation models for roAp stars.

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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

Three-Dimensional Simulations of Massive Stars: I. Wave Generation and Propagation

We present the first three-dimensional (3D), hydrodynamic simulations of the core convection zone (CZ) and extended radiative zone spanning from 1% to 90% of the stellar radius of an intermediate mass (3 $\mathrm{M}_\odot$) star. This allows us to self-consistently follow the generation of internal gravity waves (IGWs) at the convective boundary and their propagation to the surface. We find that convection in the core is dominated by plumes. The frequency spectrum in the CZ and that of IGW generation is a double power law as seen in previous two-dimensional (2D) simulations. The spectrum is significantly flatter than theoretical predictions using excitation through Reynolds stresses induced by convective eddies alone. It is compatible with excitation through plume penetration. An empirically determined distribution of plume frequencies generally matches the one necessary to explain a large part of the observed spectrum. We observe waves propagating in the radiation zone and excited standing modes, which can be identified as gravity and fundamental modes. They show similar frequencies and node patterns to those predicted by the stellar oscillation code GYRE. The continuous part of the spectrum fulfills the IGW dispersion relation. A spectrum of tangential velocity and temperature fluctuations close to the surface is extracted, which are directly related to observable brightness variations in stars. Unlike 2D simulations we do not see the high frequencies associated with wave breaking, likely because these 3D simulations are more heavily damped.

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arXiv2018

Photometric detection of internal gravity waves in early-type stars observed by CoRoT

Early-type stars are predicted to excite an entire spectrum of internal gravity waves (IGWs) at the interface of their convective cores and radiative envelopes. Numerical simulations of IGWs predict stochastic low-frequency variability in photometric observations, yet the detection of IGWs in early-type stars has been limited by a dearth of high-quality photometric time series. We present observational evidence of stochastic low-frequency variability in the CoRoT photometry of a sample of O, B, A and F stars. The presence of this stochastic low-frequency variability in stars across the upper main-sequence cannot be universally explained as granulation or stellar winds, but its morphology is found to be consistent with predictions from IGW simulations.

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arXiv2018

On the photometric detection of Internal Gravity Waves in upper main-sequence stars I. Methodology and application to CoRoT targets

Context. Main sequence stars with a convective core are predicted to stochastically excite Internal Gravity Waves (IGWs), which effectively transport angular momentum throughout the stellar interior and explain the observed near-uniform interior rotation rates of intermediate-mass stars. However, there are few detections of IGWs, and fewer still made using photometry, with more detections needed to constrain numerical simulations. Aims. We aim to formalise the detection and characterisation of IGWs in photometric observations of stars born with convective cores (M > 1.5 M$_{\odot}$) and parameterise the low-frequency power excess caused by IGWs. Methods. Using the most recent CoRoT light curves for a sample of O, B, A and F stars, we parameterise the morphology of the flux contribution of IGWs in Fourier space using an MCMC numerical scheme within a Bayesian framework. We compare this to predictions from IGW numerical simulations and investigate how the observed morphology changes as a function of stellar parameters. Results. We demonstrate that a common morphology for the low-frequency power excess is observed in early-type stars observed by CoRoT. Our study shows that a background frequency-dependent source of astrophysical signal is common, which we interpret as IGWs. We provide constraints on the amplitudes of IGWs and the shape of their detected frequency spectrum across a range of mass, which is the first ensemble study of stochastic variability in such a diverse sample of stars. Conclusions. The evidence of a low-frequency power excess across a wide mass range supports the interpretation of IGWs in photometry of O, B, A and F stars. We also discuss the prospects of observing hundreds of massive stars with the Transiting Exoplanet Survey Satellite (TESS) in the near future.

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arXiv2018

Binary Asteroseismic Modelling: isochrone-cloud methodology and application to Kepler gravity-mode pulsators

The simultaneous presence of variability due to both pulsations and binarity is no rare phenomenon. Unfortunately, the complexities of dealing with even one of these sources of variability individually means that the other signal is often treated as a nuisance and discarded. However, both types of variability offer means to probe fundamental stellar properties in robust ways through asteroseismic and binary modelling. We present an efficient methodology that includes both binary and asteroseismic information to estimate fundamental stellar properties based on a grid-based modelling approach. We report parameters for three gravity mode pulsating {\it Kepler} binaries , such as mass, radius, age, as well the mass of the convective core and location of the overshoot region. We discuss the presence of parameter degeneracies and the way our methodology deals with them. We provide asteroseismically calibrated isochrone-clouds to the community; these are a generalisation of isochrones when allowing for different values of the core overshooting in the two components of the binary.

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arXiv2018

On the sensitivity of gravito-inertial modes to differential rotation in intermediate-mass main-sequence stars

Context. While rotation has a major impact on stellar structure and evolution, its effects are not well understood. Thanks to high- quality and long timebase photometric observations obtained with recent space missions, we are now able to study stellar rotation more precisely. Aims. We aim to constrain radial differential rotation profiles in gamma Doradus (gamma Dor) stars, and to develop new theoretical seismic diagnosis for such stars with rapid and potentially non-uniform rotation. Methods. We derive a new asymptotic description which accounts for the impact of weak differential near-core rotation on gravity- mode period spacings. The theoretical predictions are illustrated from pulsation computations with the code GYRE and compared with observations of gamma Dor stars. When possible, we also derive the surface rotation rates in these stars by detecting and analysing signatures of rotational modulation, and compute the core-to-surface rotation ratios. Results. Stellar rotation has to be strongly differential before its effects on period spacing patterns can be detected, unless multiple period spacing patterns can be compared. Six stars in our sample exhibit a single unexplained period spacing pattern of retrograde modes. We hypothesise that these are Yanai modes. Finally, we find signatures of rotational spot modulation in the photometric data of eight targets. Conclusions. If only one period spacing pattern is detected and analysed for a star, it is difficult to detect differential rotation. A rigidly rotating model will often provide the best solution. Differential rotation can only be detected when multiple period spacing patterns have been found for a single star or its surface rotation rate is known as well. This is the case for eight stars in our sample, revealing surface-to-core rotation ratios between 0.95 and 1.05.

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arXiv2018

Forward asteroseismic modeling of stars with a convective core from gravity-mode oscillations: parameter estimation and stellar model selection

We propose a methodological framework to perform forward asteroseismic modeling of stars with a convective core, based on gravity-mode oscillations. These probe the near-core region in the deep stellar interior. The modeling relies on a set of observed high-precision oscillation frequencies of low-degree coherent gravity modes with long lifetimes and their observational uncertainties. Identification of the mode degree and azimuthal order is assumed to be achieved from rotational splitting and/or from period spacing patterns. This paper has two major outcomes. The first is a comprehensive list and discussion of the major uncertainties of theoretically predicted gravity-mode oscillation frequencies based on linear pulsation theory, caused by fixing choices of the input physics for evolutionary models. Guided by a hierarchy among these uncertainties of theoretical frequencies, we subsequently provide a global methodological scheme to achieve forward asteroseismic modeling. We properly take into account correlations amongst the free parameters included in stellar models. Aside from the stellar mass, metalicity and age, the major parameters to be estimated are the near-core rotation rate, the amount of convective core overshooting, and the level of chemical mixing in the radiative zones. This modeling scheme allows for maximum likelihood estimation of the stellar parameters for fixed input physics of the equilibrium models, followed by stellar model selection considering various choices of the input physics. Our approach uses the Mahalanobis distance instead of the often used $χ^2$ statistic and includes heteroscedasticity. It provides estimation of the unknown variance of the theoretically predicted oscillation frequencies.

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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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arXiv2018

K2 photometry and HERMES spectroscopy of the blue supergiant rho Leo: rotational wind modulation and low-frequency waves

We present an 80-d long uninterrupted high-cadence K2 light curve of the B1Iab supergiant rho Leo (HD 91316), deduced with the method of halo photometry. This light curve reveals a dominant frequency of $f_{\rmrot}=0.0373$d$^{-1}$ and its harmonics. This dominant frequency corresponds with a rotation period of 26.8d and is subject to amplitude and phase modulation. The K2 photometry additionally reveals multiperiodic low-frequency variability ($<1.5 $d$^{-1}$) and is in full agreement with low-cadence high-resolution spectroscopy assembled during 1800 days. The spectroscopy reveals rotational modulation by a dynamic aspherical wind with an amplitude of about 20km s$^{-1}$ in the H$α$ line, as well as photospheric velocity variations of a few km s$^{-1}$ at frequencies in the range 0.2 to 0.6 d$^{-1}$ in the SiIII 4567Å line. Given the large macroturbulence needed to explain the spectral line broadening of the star, we interpret the detected photospheric velocity as due to travelling super-inertial low-degree large-scale gravity waves with dominant tangential amplitudes and discuss why $ρ$~Leo is an excellent target to study how the observed photospheric variability propagates into the wind.

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