arXiv subjects

C. Aerts

Publications and source records attributed to C. Aerts.

At least 181 records · Page 10Linked to original sources

arXiv2008

Spectroscopic and photometric variability of the O9.5Vp star HD93521

The line profile variability and photometric variability of the O9.5 Vp star HD93521 are examined in order to establish the properties of the non-radial pulsations in this star. Fourier techniques are used to characterize the modulations of the He I 5876, 6678 and H-alpha lines in several spectroscopic time series and to search for variations in a photometric time series. Our spectroscopic data confirm the existence of two periods of 1.75 and 2.89 hr. The line profiles, especially those affected by emission wings, exhibit also modulations on longer time scales, but these are epoch-dependent and change from line to line. Unlike previous claims, we find no unambiguous signature of the rotational period in our data, nor of a third pulsation period (corresponding to a frequency of 2.66 day$^{-1}$). HD 93521 very likely exhibits non-radial pulsations with periods of 1.75 and 2.89 hr with $l \simeq 8 \pm 1$ and $l \simeq 4 \pm 1$ respectively. No significant signal is found in the first harmonics of these two periods. The 2.89 hr mode is seen at all epochs and in all lines investigated, while the visibility of the 1.75 hr mode is clearly epoch dependent. Whilst light variations are detected, their connection to these periodicities is not straightforward.

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arXiv2008

Automated supervised classification of variable stars II. Application to the OGLE database

We aim to extend and test the classifiers presented in a previous work against an independent dataset. We complement the assessment of the validity of the classifiers by applying them to the set of OGLE light curves treated as variable objects of unknown class. The results are compared to published classification results based on the so-called extractor methods.Two complementary analyses are carried out in parallel. In both cases, the original time series of OGLE observations of the Galactic bulge and Magellanic Clouds are processed in order to identify and characterize the frequency components. In the first approach, the classifiers are applied to the data and the results analyzed in terms of systematic errors and differences between the definition samples in the training set and in the extractor rules. In the second approach, the original classifiers are extended with colour information and, again, applied to OGLE light curves. We have constructed a classification system that can process huge amounts of time series in negligible time and provide reliable samples of the main variability classes. We have evaluated its strengths and weaknesses and provide potential users of the classifier with a detailed description of its characteristics to aid in the interpretation of classification results. Finally, we apply the classifiers to obtain object samples of classes not previously studied in the OGLE database and analyse the results. We pay specific attention to the B-stars in the samples, as their pulsations are strongly dependent on metallicity.

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arXiv2008

Asteroseismology and Interferometry

Asteroseismology provides us with a unique opportunity to improve our understanding of stellar structure and evolution. Recent developments, including the first systematic studies of solar-like pulsators, have boosted the impact of this field of research within Astrophysics and have led to a significant increase in the size of the research community. In the present paper we start by reviewing the basic observational and theoretical properties of classical and solar-like pulsators and present results from some of the most recent and outstanding studies of these stars. We centre our review on those classes of pulsators for which interferometric studies are expected to provide a significant input. We discuss current limitations to asteroseismic studies, including difficulties in mode identification and in the accurate determination of global parameters of pulsating stars, and, after a brief review of those aspects of interferometry that are most relevant in this context, anticipate how interferometric observations may contribute to overcome these limitations. Moreover, we present results of recent pilot studies of pulsating stars involving both asteroseismic and interferometric constraints and look into the future, summarizing ongoing efforts concerning the development of future instruments and satellite missions which are expected to have an impact in this field of research.

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arXiv2008

The Current Status of Asteroseismology

Stellar evolution, a fundamental bedrock of modern astrophysics, is driven by the physical processes in stellar interiors. While we understand these processes in general terms, we lack some important ingredients. Seemingly small uncertainties in the input physics of the models, e.g., the opacities or the amount of mixing and of interior rotation, have large consequences for the evolution of stars. The goal of asteroseismology is to improve the description of the interior physics of stars by means of their oscillations, just as global helioseismology led to a huge step forward in our knowledge about the internal structure of the Sun. In this paper we present the current status of asteroseismology by considering case studies of stars with a variety of masses and evolutionary stages. In particular, we outline how the confrontation between the observed oscillation frequencies and those predicted by the models allows us to pinpoint limitations of the input physics of current models and improve them to a level that cannot be reached with any other current method.

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arXiv2008

Precise radial velocities of giant stars. IV. A correlation between surface gravity and radial velocity variation and a statistical investigation of companion properties

Since 1999, we have been conducting a radial velocity survey of 179 K giants using the CAT at UCO/Lick observatory. At present ~20-100 measurements have been collected per star with a precision of 5 to 8 m/s. Of the stars monitored, 145 (80%) show radial velocity (RV) variations at a level >20 m/s, of which 43 exhibit significant periodicities. Our aim is to investigate possible mechanism(s) that cause these observed RV variations. We intend to test whether these variations are intrinsic in nature, or possibly induced by companions, or both. In addition, we aim to characterise the parameters of these companions. A relation between log g and the amplitude of the RV variations is investigated for all stars in the sample. Furthermore, the hypothesis that all periodic RV variations are caused by companions is investigated by comparing their inferred orbital statistics with the statistics of companions around main sequence stars. A strong relation is found between the amplitude of the RV variations and log g in K giant stars, as suggested earlier by Hatzes & Cochran (1998). However, most of the stars exhibiting periodic variations are located above this relation. These RV variations can be split in a periodic component which is not correlated with log g and a random residual part which does correlate with log g. Compared to main-sequence stars, K giants frequently exhibit periodic RV variations. Interpreting these RV variations as being caused by companions, the orbital param eters are different from the companions orbiting dwarfs. Intrinsic mechanisms play an important role in producing RV variations in K giants stars, as suggested by their dependence on log g. However, it appears that periodic RV variations are additional to these intrinsic variations, consistent with them being caused by companions.

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arXiv2007

A spectroscopic study of southern (candidate) gamma Doradus stars. II. Detailed abundance analysis and fundamental parameters

The gamma Doradus stars are a recent class of variable main sequence F-type stars located on the red edge of the Cepheid instability strip. They pulsate in gravity modes, and this makes them particularly interesting for detailed asteroseismic analysis, which can provide fundamental knowledge of properties near the convective cores of intermediate-mass main sequence stars. To improve current understanding of gamma Dor stars through theoretical modelling, additional constraints are needed. Our aim is to estimate the fundamental atmospheric parameters and determine the chemical composition of these stars. Detailed analyses of single stars have previously suggested links to Am and lambda Bootis stars, so we wish to explore this interesting connection between chemical peculiarity and pulsation. We have analysed a sample of gamma Dor stars for the first time, including nine bona fide and three candidate members of the class. We determined the fundamental atmospheric parameters and compared the abundance pattern with other A-type stars. We used the semi-automatic software package VWA for the analysis. This code relies on the calculation of synthetic spectra and thus takes line-blending into account. This is important because of the fast rotation in some of the sample stars, and we made a thorough analysis of how VWA performs when increasing vsini. We obtained good results in agreement with previously derived fundamental parameters and abundances in a few selected reference stars with properties similar to the gamma Dor stars. We find that the abundance pattern in the gamma Dor stars is not distinct from the constant A- and F-type stars we analysed.

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arXiv2007

Automated supervised classification of variable stars I. Methodology

The fast classification of new variable stars is an important step in making them available for further research. Selection of science targets from large databases is much more efficient if they have been classified first. Defining the classes in terms of physical parameters is also important to get an unbiased statistical view on the variability mechanisms and the borders of instability strips. Our goal is twofold: provide an overview of the stellar variability classes that are presently known, in terms of some relevant stellar parameters; use the class descriptions obtained as the basis for an automated `supervised classification' of large databases. Such automated classification will compare and assign new objects to a set of pre-defined variability training classes. For every variability class, a literature search was performed to find as many well-known member stars as possible, or a considerable subset if too many were present. Next, we searched on-line and private databases for their light curves in the visible band and performed period analysis and harmonic fitting. The derived light curve parameters are used to describe the classes and define the training classifiers. We compared the performance of different classifiers in terms of percentage of correct identification, of confusion among classes and of computation time. We describe how well the classes can be separated using the proposed set of parameters and how future improvements can be made, based on new large databases such as the light curves to be assembled by the CoRoT and Kepler space missions.

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arXiv2007

Radial velocities of giant stars: an investigation of line profile variations

Since 1999, a radial velocity survey of 179 red giant stars is ongoing at Lick Observatory with a one month cadence. At present ~20-100 measurements have been collected per star with an accuracy of 5 to 8 m/s. Of the stars monitored, 145 (80%) show radial velocity (RV) variations at a level >20 m/s, of which 43 exhibit significant periodicities. Here, we investigate the mechanism causing the observed radial velocity variations. Firstly, we search for a correlation between the radial velocity amplitude and an intrinsic parameter of the star, in this case surface gravity (log g). Secondly, we investigate line profile variations and compare these with theoretical predictions.

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arXiv2007

An evolutionary study of the pulsating subdwarf B eclipsing binary PG1336-018 (NY Vir)

The formation of subdwarf B (sdB) stars is not well understood within the current framework of stellar single and binary evolution. In this study, we focus on the formation and evolution of the pulsating sdB star in the very short-period eclipsing binary PG1336-018. We aim at refining the formation scenario of this unique system, so that it can be confronted with observations. We probe the stellar structure of the progenitors of sdB stars in short-period binaries using detailed stellar evolution calculations. Applying this to PG1336-018 we reconstruct the common-envelope phase during which the sdB star was formed. The results are interpreted in terms of the standard common-envelope formalism (the alpha-formalism) based on the energy equation, and an alternative description (the gamma-formalism) using the angular momentum equation. We find that if the common-envelope evolution is described by the alpha-formalism, the sdB progenitor most likely experienced a helium flash. We then expect the sdB mass to be between 0.39 and 0.48 Msun, and the sdB progenitor initial mass to be below ~2 Msun. However, the results for the gamma-formalism are less restrictive, and a broader sdB mass range (0.3 - 0.8 Msun) is possible in this case. Future seismic mass determination will give strong constraints on the formation of PG1336-018 and, in particular, on the CE phase.

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arXiv2007

The binary properties of the pulsating subdwarf B eclipsing binary PG 1336-018 (NY Vir)

We present an unbiased orbit solution and mass determination of the components of the eclipsing binary NY Vir as a critical test for the formation scenarios of subdwarf B stars. We obtained high-resolution time series VLT/UVES spectra and high-speed multicolour VLT/ULTRACAM photometric observations of NY Vir, a rapidly pulsating subdwarf B star in a short period eclipsing binary. Combining the radial velocity curve obtained from the VLT/UVES spectra with the VLT/ULTRACAM multicolour lightcurves, we determined numerical orbital solutions for this eclipsing binary. Due to the large number of free parameters and their strong correlations, no unique solution could be found, only families of solutions. We present three solutions of equal statistical significance, two of which are compatible with the primary having gone through a core He-flash and a common-envelope phase described by the $α$-formalism. These two models have an sdB primary of 0.466 \msol and 0.389 \msol, respectively. Finally, we report the detection of the Rossiter-McLaughlin effect for NY Vir.

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arXiv2007

An asteroseismic study of the beta Cephei star theta Ophiuchi: constraints on global stellar parameters and core overshooting

We present a seismic study of the beta Cephei star theta Ophiuchi. Our analysis is based on the observation of one radial mode, one rotationally split l = 1 triplet and three components of a rotationally split l = 2 quintuplet for which the m-values were well identified by spectroscopy. We identify the radial mode as fundamental, the triplet as p_1 and the quintuplet as g_1. Our NLTE abundance analysis results in a metallicity and CNO abundances in full agreement with the most recent updated solar values. With X \in [0.71,0.7211] and Z \in [0.009,0.015], and using the Asplund et al. (2005) mixture but with a Ne abundance about 0.3 dex larger (Cunha et al. 2006), the matching of the three independent modes, enables us to deduce constrained ranges for the mass (M = 8.2 +/- 0.3 Msun) and central hydrogen abundance (X_c = 0.38 +/- 0.02) of theta Oph and to prove the occurrence of core overshooting (alpha_ov = 0.44 +/- 0.07). We also derive an equatorial rotation velocity of 29 +/- 7 km/s. Moreover, we show that the observed non-equidistance of the l=1 triplet can be reproduced by second order effects of rotation. Finally, we show that the observed rotational splitting of two modes cannot rule out a rigid rotation model.

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arXiv2007

On the co-existence of chemically peculiar Bp stars, slowly pulsating B stars and constant B stars in the same part of the H-R diagram

Aims. In order to better model massive B-type stars, we need to understand the physical processes taking place in slowly pulsating B (SPB) stars, chemically peculiar Bp stars, and non-pulsating normal B stars co-existing in the same part of the H-R diagram. Methods. We carry out a comparative study between samples of confirmed and well-studied SPB stars and a sample of well-studied Bp stars with known periods and magnetic field strengths. We determine their evolutionary state using accurate HIPPARCOS parallaxes and Geneva photometry. We discuss the occurrence and strengths of magnetic fields as well as the occurrence of stellar pulsation among both groups. Further, we make a comparison of Geneva photometric variability for both kinds of stars. Results. The group of Bp stars is significantly younger than the group of SPB stars. Longitudinal magnetic fields in SPB stars are weaker than those of Bp stars, suggesting that the magnetic field strength is an important factor for B type stars to become chemically peculiar. The strongest magnetic fields appear in young Bp stars, indicating a magnetic field decay in stars at advanced ages. Rotation periods of Bp and pulsation periods of SPB stars are of the same order and the behaviour of Geneva photometric variability of some Bp stars cannot be distinguished from the variability of SPB stars, illustrating the difficulty to interpret the observed variability of the order of days for B-type stars. We consider the possibility that pulsation could be responsible for the variability among chemically peculiar stars. In particular, we show that a non-linear pulsation model is not excluded by photometry for the Bp star HD175362.

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arXiv2007

The early-type close binary CV Velorum revisited

Our goal was to improve the fundamental parameters of the massive close double-lined eclipsing B2.5V+B2.5V binary CV Velorum.We gathered new high-resolution echelle spectroscopy on 13 almost consecutive nights covering essentially two orbits. We computed a simultaneous solution to all the available high-quality radial-velocity and light data with the latest version of the Wilson-Deviney code. We obtained the following values for the physical parameters: $M_1 = 6.066(74) M_\odot$, $M_2 = 5.972(70) M_\odot$, $R_1 = 4.126(24) R_\odot$, $R_2 = 3.908(27) R_\odot$, $\log L_1 = 3.20(5) L_\odot$, and $\log L_2 = 3.14(5) L_\odot$. The quoted errors contain a realistic estimate of systematic uncertainties mainly stemming from the effective temperature estimation. We derived abundances for both components and found them to be compatible with those of B stars in the solar neighbourhood. We discovered low-amplitude periodic line-profile variations with the orbital frequency for both components. Their interpretation requires new data with a longer time span. The primary rotates subsynchronously while the secondary's $v\sin i$ and radius are compatible with synchronous rotation. Finally, we provide an update of the empirical mass-luminosity relation for main-sequence B stars which can be used for statistical predictions of masses or luminosities.

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arXiv2007

High-Precision Spectroscopy of Pulsating Stars

We review methodologies currently available to interprete time series of high-resolution high-S/N spectroscopic data of pulsating stars in terms of the kind of (non-radial) modes that are excited. We illustrate the drastic improvement of the detection treshold of line-profile variability thanks to the advancement of the instrumentation over the past two decades. This has led to the opportunity to interprete line-profile variations with amplitudes of order m/s, which is a factor 1000 lower than the earliest line-profile time series studies allowed for.

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arXiv2007

Estimating Stellar Parameters from Spectra using a Hierarchical Bayesian Approach

A method is developed for fitting theoretically predicted astronomical spectra to an observed spectrum. Using a hierarchical Bayesian principle, the method takes both systematic and statistical measurement errors into account, which has not been done before in the astronomical literature. The goal is to estimate fundamental stellar parameters and their associated uncertainties. The non-availability of a convenient deterministic relation between stellar parameters and the observed spectrum, combined with the computational complexities this entails, necessitate the curtailment of the continuous Bayesian model to a reduced model based on a grid of synthetic spectra. A criterion for model selection based on the so-called predictive squared error loss function is proposed, together with a measure for the goodness-of-fit between observed and synthetic spectra. The proposed method is applied to the infrared 2.38--2.60 \mic ISO-SWS data (Infrared Space Observatory - Short Wavelength Spectrometer) of the star $α$ Bootis, yielding estimates for the stellar parameters: effective temperature \Teff = 4230 $\pm$ 83 K, gravity $\log$ g = 1.50 $\pm$ 0.15 dex, and metallicity [Fe/H] = $-0.30 \pm 0.21$ dex.

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arXiv2006

Statistical properties of a sample of periodically variable B-type supergiants - Evidence for opacity-driven gravity-mode oscillations

We have studied a sample of 28 periodically variable B-type supergiants selected from the HIPPARCOS mission and 12 comparison stars covering the whole B-type spectral range. Our goal is to test if their variability is compatible with opacity-driven non-radial oscillations. We have used the NLTE atmosphere code FASTWIND to derive the atmospheric and wind parameters of the complete sample through line profile fitting. We applied the method to selected H, He and Si line profiles, measured with the high resolution CES spectrograph attached to the ESO CAT telescope in La Silla, Chile. From the location of the stars in the (log Teff, log g) diagram, we suggest that variability of our sample supergiants is indeed due to the gravity modes resulting from the opacity mechanism. We find nine of the comparison stars to be periodically variable as well, and suggest them to be new alpha Cyg variables. We find marginal evidence of a correlation between the amplitude of the photometric variability and the wind density. We investigate the Wind Momentum Luminosity Relation for the whole range of B spectral type supergiants, and find that the later types (> B5) perfectly follow the relation for A supergiants. Additionally, we provide a new spectral type - Teff calibration for B supergiants. Our results imply the possibility to probe internal structure models of massive stars of spectral type B through seismic tuning of gravity modes.

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arXiv2006

The pulsations and potential for seismology of B stars

We review the nature of the oscillations of main-sequence and supergiant stars of spectral type B. Seismic tuning of the interior structure parameters of the $β$Cep stars has been achieved since three years. The results are based on frequencies derived from long-term monitoring and progress in this area is rapid. Oscillations in mid-B stars as well as Be stars are well established by now, but we lack good mode identification to achieve seismic modelling. We provide recent evidence of g-mode pulsations in supergiant B stars. The spherical wavenumbers of their modes are yet unidentified, preventing seismic probing of such evolved hot stars at present. Improving the situation for the three groups of g-mode oscillators requires multi-site long-term high-resolution spectroscopy in combination with either space photometry or ground-based multicolour photometry. The CoRoT programme and its ground-based programme will deliver such data in the very near future.

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arXiv2006

A Grid of FASTWIND NLTE Model Atmospheres of Massive Stars

In the last few years our knowledge of the physics of massive stars has improved tremendously. However, further investigations are still needed, especially regarding accurate calibrations of their fundamental parameters. To this end, we have constructed a comprehensive grid of NLTE model atmospheres and corresponding synthetic spectra in the massive star domain. The grid covers the complete B type spectral range, extended to late O on the hot side and early A on the cool side, from supergiants to dwarfs and from weak stellar winds to very strong ones. It has been calculated with the latest version of the FASTWIND code. The analysis of an extensive sample of OB stars in the framework of the COROT space mission will lead to accurate calibrations of effective temperatures, gravities, mass loss rates etc. This paper contains a detailed description of the grid, which has been baptised as BSTAR06 and which will be available for further research in the near future.

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