arXiv subjects

C. Aerts

Publications and source records attributed to C. Aerts.

At least 199 records · Page 11Linked to original sources

arXiv2006

A line profile analysis of the pulsating red giant star epsilon Ophiuchi (G9.5III)

So far, solar-like oscillations have been studied using radial velocity and/or light curve variations, which reveal frequencies of the oscillation modes. Line-profile variations, however, are also a valuable diagnostic to characterise radial and non-radial oscillations, including frequencies, amplitudes, the spherical mode wavenumbers (l,m) and the stellar inclination angle. Here we present a line profile analysis of epsilon Ophiuchi, which is a pulsating red giant. The main differences compared to previous line profile analyses done for heat-driven oscillations are the small amplitudes and the predicted short damping and re-excitation times in red giants. Two line diagnostics have been tested to see whether these are sensitive to the small line profile variations present in red giants. In addition, line profiles have been simulated with short damping and re-excitation times and are compared with the observations. This comparison reveals that non-radial modes are detected in the observed line profile variations of epsilon Ophiuchi. This is rather surprising, as theoretical predictions favours the occurrence of radial modes.

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arXiv2006

Pulsations detected in the line profile variations of red giants: Modelling of line moments, line bisector and line shape

Contents: So far, red giant oscillations have been studied from radial velocity and/or light curve variations, which reveal frequencies of the oscillation modes. To characterise radial and non-radial oscillations, line profile variations are a valuable diagnostic. Here we present for the first time a line profile analysis of pulsating red giants, taking into account the small line profile variations and the predicted short damping and re-excitation times. We do so by modelling the time variations in the cross correlation profiles in terms of oscillation theory. Aims: The performance of existing diagnostics for mode identification is investigated for known oscillating giants which have very small line profile variations. We modify these diagnostics, perform simulations, and characterise the radial and non-radial modes detected in the cross correlation profiles. Methods: Moments and line bisectors are computed and analysed for four giants. The robustness of the discriminant of the moments against small oscillations with finite lifetimes is investigated. In addition, line profiles are simulated with short damping and re-excitation times and their line shapes are compared with the observations. Results: For three stars, we find evidence for the presence of non-radial pulsation modes, while for $ξ$ Hydrae perhaps only radial modes are present. Furthermore the line bisectors are not able to distinguish between different pulsation modes and are an insufficient diagnostic to discriminate small line profile variations due to oscillations from exoplanet motion.

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arXiv2006

Abundance analysis of prime B-type targets for asteroseismology I. Nitrogen excess in slowly-rotating beta Cephei stars

We present the results of a detailed NLTE abundance study of nine beta Cephei stars, all of them being prime targets for theoretical modelling: gamma Peg, delta Cet, nu Eri, beta CMa, xi1 CMa, V836 Cen, V2052 Oph, beta Cep and DD (12) Lac. The following chemical elements are considered: He, C, N, O, Mg, Al, Si, S and Fe. Our abundance analysis is based on a large number of time-resolved, high-resolution optical spectra covering in most cases the entire oscillation cycle of the stars. Nitrogen is found to be enhanced by up to 0.6 dex in four stars, three of which have severe constraints on their equatorial rotational velocity, ΩR, from seismic or line-profile variation studies: beta Cep (ΩR~26 km/s), V2052 Oph (ΩR~56 km/s), delta Cet (ΩR < 28 km/s) and xi1 CMa (ΩR sin i < 10 km/s). The existence of core-processed material at the surface of such largely unevolved, slowly-rotating objects is not predicted by current evolutionary models including rotation. We draw attention to the fact that three stars in this subsample have a detected magnetic field and briefly discuss recent theoretical work pointing to the occurrence of diffusion effects in beta Cephei stars possibly capable of altering the nitrogen surface abundance. On the other hand, the abundances of all the other chemical elements considered are, within the errors, indistinguishable from the values found for OB dwarfs in the solar neighbourhood. Despite the mild nitrogen excess observed in some objects, we thus find no evidence for a significantly higher photospheric metal content in the studied beta Cephei stars compared to non-pulsating B-type stars of similar characteristics.

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arXiv2006

Orbital elements, masses and distance of lambda Scorpii A and B determined with the Sydney University Stellar Interferometer and high resolution spectroscopy

The triple system HD158926 (lambda Sco) has been observed interferometrically with the Sydney University Stellar Interferometer and the elements of the wide orbit have been determined. These are significantly more accurate than the previous elements found spectroscopically. The inclination of the wide orbit is consistent with the inclination previously found for the orbit of the close companion. The wide orbit also has low eccentricity, suggesting that the three stars were formed at the same time. The brightness ratio between the two B stars was also measured at lambda = 442nm and 700nm. The brightness ratio and colour index are consistent with the previous classification of lambda Sco A as B1.5 and lambda Sco B as B2. Evolutionary models show that the two stars lie on the main sequence. Since they have have the same age and luminosity class (IV) the mass-luminosity relation can be used to determine the mass ratio of the two stars: M_B/M_A = 0.76+/-0.04. The spectroscopic data have been reanalyzed using the interferometric values for P, T, e and omega, leading to revised values for a_1sin i and the mass function. The individual masses can be found from the mass ratio, the mass function, spectrum synthesis and the requirement that the age of both components must be the same: M_A = 10.4+/-1.3 Msun and M_B = 8.1+/-1.0 Msun. The masses, angular semimajor axis and the period of the system can be used to determine the dynamical parallax. We find the distance to lambda Sco to be 112+/-5 pc, which is approximately a factor of two closer than the HIPPARCOS value of 216+/-42 pc.

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arXiv2006

High-precision elements of double-lined spectroscopic binaries from combined interferometry and spectroscopy. Application to the beta Cephei star beta Centauri

We present methodology to derive high-precision estimates of the fundamental parameters of double-lined spectroscopic binaries. We apply the methods to the case study of the double-lined beta Cephei star beta Centauri. We also present a detailed analysis of beta Centauri's line-profile variations caused by its oscillations. We point out that a systematic error in the orbital amplitudes, and any quantities derived from them, occurs if the radial velocities of blended component lines are computed without spectral disentangling. This technique is an essential ingredient in the derivation of the physical parameters if the goal is to obtain a precision of only a few percent. We have devised iteration schemes to obtain the orbital elements for systems whose lines are blended throughout the orbital cycle. We find the following parameters for beta Cen: $M_1=10.7\pm 0.1 M_\odot$ and $M_2=10.3\pm 0.1 M_\odot$, an age of $(14.1\pm 0.6)\times 10^6$ years. We deduce two oscillation frequencies for the broad-lined primary of beta Centauri with degrees higher than 2. We propose that our iteration schemes be used in any future derivations of the spectroscopic orbital parameters of double-lined binaries with blended component lines to which disentangling can be successfully applied.

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arXiv2006

Discovery of magnetic fields in the beta Cephei star xi^1 CMa and in several Slowly Pulsating B stars

We present the results of a magnetic survey of a sample of eight beta Cephei stars and 26 Slowly Pulsating B stars with FORS1 at the VLT. A weak mean longitudinal magnetic field of the order of a few hundred Gauss is detected in the beta Cephei star xi^1 CMa and in 13 SPB stars. The star xi^1 CMa becomes the third magnetic star among the beta Cephei stars. Before our study, the star zeta Cas was the only known magnetic SPB star. All magnetic SPB stars for which we gathered several magnetic field measurements show a field that varies in time. We do not find a relation between the evolution of the magnetic field with stellar age in our small sample. Our observations imply that beta Cephei stars and SPBs can no longer be considered as classes of non-magnetic pulsators, but the effect of the fields on the oscillation properties remains to be studied.

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arXiv2006

Discovery of the new slowly pulsating B star HD 163830 (B5II/III) from MOST spacebased photometry

We report the discovery of a new slowly pulsating B (SPB) star, with the largest number of detected frequencies to date by more than a factor of three, based on 37 days of MOST (Microvariability & Oscillations of STars) satellite guide star photometry. The star HD 163830 (V = 9.3, B5II/III) varies in twenty detected frequencies in the range $0.035 - 1.06$ d$^{-1}$ ($0.4 - 12.3 ~ μ$Hz) with amplitudes from 0.7 to 7.6 mmag ($S/N$ from {4 to 43}). Eighteen of these frequencies are consistent with low-degree, high-order nonradial g-modes {of seismic models of an evolved 4.5$M_{\odot}$ star. We are unable to identify one unique model due to lack of mode identifications. The lowest two frequencies may be associated with the rotation of HD 163830, but firm proof of this must await future spectroscopic data.

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arXiv2006

delta Ceti is not monoperiodic: seismic modeling of a beta Cephei star from MOST spacebased photometry

The beta Cephei star delta Ceti was considered one of the few monoperiodic variables in the class. Despite (or perhaps because of) its apparently simple oscillation spectrum, it has been challenging and controversial to identify this star's pulsation mode and constrain its physical parameters seismically. Broadband time-resolved photometry of delta Ceti spanning 18.7 days with a duty cycle of about 65% obtained by the MOST (Microvariability & Oscillations of STars) satellite -- the first scientific observations ever obtained by MOST -- reveals that the star is actually multiperiodic. Besides the well-known dominant frequency of f1 = 6.205886/d, we have discovered in the MOST data its first harmonic 2f1 and three other frequencies (f2 = 3.737/d, f3 = 3.673/d and f4 = 0.318/d), all detected with S/N > 4. In retrospect, f2 was also present in archival spectral line profile data but at lower S/N. We present seismic models whose modes match exactly the frequencies f1 and f2. Only one model falls within the common part of the error boxes of the star's observed surface gravity and effective temperature from photometry and spectroscopy. In this model, f1 is the radial (l = 0) first overtone and f2 is the g2 (l = 2, m = 0) mode. This model has a mass of 10.2+/-0.2 Msun and an age of 17.9+/-0.3 million years, making delta Ceti an evolved beta Cephei star. If f2 and f3 are rotationally split components of the same g2 mode, then the star's equatorial rotation velocity is either 27.6 km/s or half this value. Given its vsini of about 1 km/s, this implies we are seeing delta Ceti nearly pole-on.

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arXiv2005

Multiperiodicity in the large-amplitude rapidly-rotating $β$Ceph ei star HD 203664

We perform a seismic study of the young massive $β$Cephei star HD 203664 with the goal to constrain its interior structure. Our study is based on a time series of 328 new Geneva 7-colour photometric data of the star spread over 496.8 days. The data confirm the frequency of the dominant mode of the star which we refine to $f_1=6.02885 $c d$^{-1}$. The mode has a large amplitude of 37 mmag in V and is unambiguously identified as a dipole mode ($\ell=2$) from its amplitude ratios and non-adiabatic computations. Besides $f_1$, we discover two additional new frequencies in the star with amplitudes above $4σ$: $f_2=6.82902 $c d$^{-1}$ and $f_3=4.81543 $c d$^{-1}$ or one of their daily aliases. The amplitudes of these two modes are only between 3 and 4 mmag which explains why they were not detected before. Their amplitude ratios are too uncertain for mode identification. We show that the observed oscillation spectrum of HD 203664 is compatible with standard stellar models but that we have insufficient information for asteroseismic inferences. Among the large-amplitude $β$Cephei stars, HD 203664 stands out as the only one rotating at a significant fraction of its critical rotation velocity ($\sim 40%$).

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arXiv2005

Observational results for northern and southern (candidate) gamma Doradus stars

We report on observational results obtained for 78 objects originally classified as bona-fide or candidate gamma Doradus stars. For the southern objects, we gathered echelle spectra with the CORALIE spectrograph attached to the Euler telescope in 1998-2003 and/or Johnson-Cousins B,V,Ic observations with the MODULAR photometer attached to the 0.5-m SAAO telescope in 1999-2000. For the northern objects, we obtained Geneva U,B,B1,B2,V,V1,G observations with the P7 photometer attached to the 1.2-m MERCATOR telescope in 2001-2004. At least 15 of our objects are binaries, of which 7 are new. For 6 binaries, we determined the orbit for the first time. At least 17 objects show profile variations and at least 12 objects are multiperiodic photometric variables. Our results allow us to upgrade 11 objects to bona-fide gamma Doradus stars and to downgrade 8 objects to constant up to the current detection limits. Mode identification is still ongoing, but so far, only l = 1 and 2 modes have been identified.

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arXiv2005

A spectroscopic study of southern (candidate) gamma Doradus stars. I. Time series analysis

We present results of a spectroscopic study of 37 southern (candidate) gamma Doradus stars based on echelle spectra. The observed spectra were cross-correlated with the standard template spectrum of an F0-type star for an easier detection of binary and intrinsic variations. We identified 15 objects as spectroscopic binaries, including 7 new ones, and another 3 objects are binary suspects. At least 12 objects show composite spectra. We could determine the orbital parameters for 9 binaries, of which 4 turn out to be ellipsoidal variables. For 6 binaries, we estimated the expected time-base of the orbital variations. Clear profile variations are observed for 17 objects, pointing towards stellar pulsation. For 8 of them, we have evidence that the main spectroscopic and photometric periods coincide. Our results, in combination with prior knowledge from the literature, lead to the classification of 10 objects as new bona-fide gamma Doradus stars, 1 object as new bona-fide delta Scuti star, and 8 objects as constant stars. Finally, we determined the projected rotational velocity with two independent methods. The resulting vsini values range from 3 to 135 km/s. For the bona-fide gamma Doradus stars, the majority has vsini below 60 km/s.

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arXiv2005

Analysis of MERCATOR data Part I: variable B stars

We re-classified 31 variable B stars which were observed more than 50 times in the Geneva photometric system with the P7 photometer attached to the MERCATOR telescope (La Palma) during its first 3 years of scientific observations. HD89688 is a possible beta Cephei/slowly pulsating B star hybrid and the main mode of the COROT target HD180642 shows non-linear effects. The Maia candidates are re-classified as either ellipsoidal variables or spotted stars. Although the mode identification is still ongoing, all the well-identified modes so far have a degree l = 0, 1 or 2.

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arXiv2005

Asteroseismology of the Beta Cephei star 12 (DD) Lacertae: photometric observations, pulsational frequency analysis and mode identification

We report a multisite photometric campaign for the Beta Cephei star 12 Lacertae. 750 hours of high-quality differential photoelectric Stromgren, Johnson and Geneva time-series photometry were obtained with 9 telescopes during 190 nights. Our frequency analysis results in the detection of 23 sinusoidal signals in the light curves. Eleven of those correspond to independent pulsation modes, and the remainder are combination frequencies. We find some slow aperiodic variability such as that seemingly present in several Beta Cephei stars. We perform mode identification from our colour photometry, derive the spherical degree l for the five strongest modes unambiguously and provide constraints on l for the weaker modes. We find a mixture of modes of 0 <= l <= 4. In particular, we prove that the previously suspected rotationally split triplet within the modes of 12 Lac consists of modes of different l; their equal frequency splitting must thus be accidental. One of the periodic signals we detected in the light curves is argued to be a linearly stable mode excited to visible amplitude by nonlinear mode coupling via a 2:1 resonance. We also find a low-frequency signal in the light variations whose physical nature is unclear; it could be a parent or daughter mode resonantly coupled. The remaining combination frequencies are consistent with simple light-curve distortions. The range of excited pulsation frequencies of 12 Lac may be sufficiently large that it cannot be reproduced by standard models. We suspect that the star has a larger metal abundance in the pulsational driving zone, a hypothesis also capable of explaining the presence of Beta Cephei stars in the LMC.

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arXiv2005

The orbit of the close spectroscopic binary epsilon Lupi and the intrinsic variability of its early B-type components

We subjected 106 new high-resolution spectra of the double-lined spectroscopic close binary epsilon Lupi, obtained in a time-span of 17 days from two different observatories, to a detailed study of orbital and intrinsic variations. We derived accurate values of the orbital parameters. We refined the sidereal orbital period to 4.55970 days and the eccentricity to e=0.277. By adding old radial velocities, we discovered the presence of apsidal motion with a period of the rotation of apses of about 430 years. Such a value agrees with theoretical expectations. Additional data is needed to confirm and refine this value. Our dataset did not allow us to derive the orbit of the third body, which is known to orbit the close system in approximately 64 years. We present the secondary of epsilon Lupi as a new beta Cephei variable, while the primary is a beta Cephei suspect. A first detailed analysis of line-profile variations of both primary and secondary led to detection of one pulsation frequency near 10.36 c/d in the variability of the secondary, while no clear periodicity was found in the primary, although low-amplitude periodicities are still suspected. The limited accuracy and extent of our dataset did not allow any further analysis, such as mode-identification.

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arXiv2005

An asteroseismic study of the Beta Cephei star Theta Ophiuchi: spectroscopic results

We present the results of a detailed analysis of 121 ground-based high-resolution high S/N spectroscopic measurements spread over 3 years for the Beta Cephei star Theta Ophiuchi. We discovered Theta Oph to be a triple system. In addition to the already known Speckle B5 companion of the B2 primary, we showed the presence of a low-mass spectroscopic companion and we derived an orbital period of 56.71 days with an eccentricity of 0.1670. After removing the orbit we determined two frequencies for the primary in the residual radial velocities: f1 = 7.1160 c/d and f2 = 7.4676 c/d. We also found the presence of f3 = 7.3696 c/d by means of a two dimensional frequency search across the Si III 4567 A profiles. We identified the m-value of the main mode with frequency f1 by taking into account the photometric identifications of the degrees l. By means of the moment method and the amplitude and phase variations across the line profile, we derived (l1,m1) = (2,-1). This result allows us to fix the mode identifications of the whole quintuplet for which three components were detected in photometry. This is of particular use for our forthcoming seismic modelling of the primary. We also determined stellar parameters of the primary by non-local thermodynamic equilibrium hydrogen, helium and silicon line profile fitting and we obtained Teff = 24000 K and log g = 4.1, which is consistent with photometrically determined values.

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arXiv2005

Preparing the COROT space mission: new variable stars in the galactic Anticenter direction

The activities related to the preparation of the asteroseismic, photometric space mission COROT are described. Photoelectric observations, wide--field CCD photometry, uvbyB calibrations and further time--series have been obtained at different observatories and telescopes. They have been planned to complete the COROT programme in the direction of the galactic Anticenter. In addition to suitable asteroseismic targets covering the different evolutionary stages between ZAMS and TAMS, we discovered several other variable stars, both pulsating and geometrical. We compared results on the incidence of variability in the galactic Center and Anticenter directions. Physical parameters have been obtained and evolutionary tracks fitting them have been calculated. The peculiarities of some individual stars alre pointed out. Paper based on observations collected at the San Pedro Martir, Sierra Nevada, Teide, La Silla, Haute-Provence and Roque de Los Muchachos (Telescopio Nazionale Galileo and Mercator telescopes) observatories.

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arXiv2004

Orbital parameters, masses and distance to Beta Centauri determined with the Sydney University Stellar Interferometer and high resolution spectroscopy

The bright southern binary star beta Centauri (HR 5267) has been observed with the Sydney University Stellar Interferometer (SUSI) and spectroscopically with the ESO CAT and Swiss Euler telescopes at La Silla. The interferometric observations have confirmed the binary nature of the primary component and have enabled the determination of the orbital parameters of the system. At the observing wavelength of 442 nm the two components of the binary system have a magnitude difference of 0.15. The combination of interferometric and spectroscopic data gives the following results: orbital period 357 days, semi-major axis 25.30 mas, inclination 67.4 degrees, eccentricity 0.821, distance 102.3 pc, primary and secondary masses M1 = M2 = 9.1 solar masses and absolute visual magnitudes of the primary and secondary M1V = -3.85 and M2V = -3.70. The high accuracy of the results offers a fruitful starting point for future asteroseismic modelling of the pulsating binary components.

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arXiv2004

GAUDI: a preparatory archive for the COROT mission

The GAUDI database (Ground-based Asteroseismology Uniform Database Interface, http://sdc.laeff.esa.es/gaudi/) is a preparatory archive for the COROT (COnvection, ROtation and planetary Transits, http://www.astrsp-mrs.fr/projets/corot/) mission developed at LAEFF (Laboratory for Space Astrophysics and Theoretical Physics, http://www.laeff.esa.es). Its intention is to make the ground-based observations obtained in the preparation of the asteroseismology programme available in a simple and efficient way. It contains spectroscopic and photometric data together with inferred physical parameters for more than 1500 objects gathered since January 1998 in 6 years of observational campaigns. In this paper, the main functionalities and characteristics of the system are described. The observations have been collected at ESO-La Silla, Telescopio Nazionale Galileo, Observatoire de Haute-Provence, South African Astronomical Observatory, Tautenberg Observatory and Sierra Nevada Observatory.

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