arXiv subjects

C. Aerts

Publications and source records attributed to C. Aerts.

209 records · Page 12Linked to original sources

arXiv2004

Multi-site, multi-technique survey of Gamma Doradus candidates

We present the first results of a 2-year high-resolution spectroscopy campaign of 59 candidate Gamma Doradus stars which were mainly discovered from the HIPPARCOS astrometric mission. More than 60 % of the stars present line profile variations which can be interpreted as due to pulsation related to Gamma Doradus stars. For all stars we also derived the projected rotation velocity (up to more than 200 km\s). The amplitude ratios 2K/Delta m for the main HIPPARCOS frequency are in the range 35 - 96 (km\s)\mag. Less than 50 % of the candidates are possible members of binary systems, with 20 stars being confirmed Gamma Doradus. At least 6 stars present composite spectra, and in all but one case (for which only one spectrum could be obtained), the narrow component shows line profile variations, pointing towards an uncomfortable situation if this narrow component originates from a shell surrounding the star. This paper is the first of a series concerning mode identification using both photometric and spectroscopic methods for the confirmed Gamma Doradus stars of the present sample.

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arXiv2004

Simulating stochastically excited oscillations. The mode lifetime of xi Hya

The discovery of solar-like oscillations in the giant star xy Hya (G7III) was reported by Frandsen et al. (2002). Their frequency analysis was very limited due to alias problems in the data set (caused by single-site observations). The extent to which the aliasing affected their analysis was unclear due to the unknown damping time of the stellar oscillation modes. In this paper we describe a simulator created to generate time series of stochastically excited oscillations, which takes as input an arbitrary window function and includes both white and non-white noise. We also outline a new method to compare a large number of simulated time series with an observed time series to determine the damping time, amplitude, and limited information on the degree of the stochastically excited modes. For xi Hya we find the most likely amplitude to be ~ 2 m/s, in good agreement with theory (Houdek & Gough, 2002), and the most likely damping time to be ~ 2 days, which is much shorter than the theoretical value of 15-20 days calculated by Houdek & Gough (2002).

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arXiv2003

Preparing the COROT space mission: incidence and characterisation of pulsation in the Lower Instability Strip

By pursuing the goal to find new variables in the COROT field-of-view we characterised a sample of stars located in the lower part of the instability strip. Our sample is composed of stars belonging to the disk population in the solar neighbourhood. We found that 23% of the stars display multiperiodic light variability up to few mmag of amplitude. uvbyBeta photometry fixed most of the variables in the middle of the instability strip and high-resolution spectroscopy established that they have vsin i>100 km/s. The comparison with delta Sct stars in the whole Galaxy shows slightly different features, i.e., most delta Sct stars have a 0.05-mag redder (b-y)_0 index and lower vsin i values. Additional investigation in the open cluster NGC 6633 confirms the same incidence of variability, i.e., around 20%. The wide variety of pulsational behaviours of delta Sct stars (including unusual objects such as a variable beyond the blue edge or a rapidly rotating high-amplitude pulsator) makes them very powerful asteroseismic tools to be used by COROT. Being quite common among bright stars, delta Sct stars are suitable targets for optical observations from space.

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arXiv2002

A photometric mode identification method, including an improved non-adiabatic treatment of the atmosphere

We present an improved version of the method of photometric mode identification of Heynderickx et al. (1994). Our new version is based on the inclusion of precise non-adiabatic eigenfunctions determined in the outer stellar atmosphere according to the formalism recently proposed by Dupret et al.(2002). Our improved photometric mode identification technique is therefore no longer dependent on ad hoc parameters for the non-adiabatic effects. It contains the complete physical conditions of the outer atmosphere of the star, provided that rotation does not play a key role. We apply our improved method to the two slowly pulsating B stars HD 74560 and HD 138764 and to the beta Cephei star EN (16) Lac. Besides identifying the degree l of the pulsating stars, our method is also a tool for improving the knowledge of stellar interiors and atmospheres, by imposing constraints on parameters such as the metallicity and the mixing-length parameter alpha (a procedure we label non-adiabatic asteroseismology).

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arXiv2002

Searching for Line Profile Variability in HgMn Stars

Spectra of four non-magnetic chemically peculiar late B type stars (HgMn) stars are analysed to detect periodic spectral line variations (LPVs). A procedure developed to study LPVs in Slowly Pulsating B stars has been adopted as pulsational properties of HgMn stars should be expected to be similar. In the preliminary results discussed here no conclusive evidence for periodic LPVs was uncovered. A more sensitive re-analysis of the data is under way.

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arXiv2002

Detection of Solar-like Oscillations in the G7 Giant Star xi Hya

We report the firm discovery of solar-like oscillations in a giant star. We monitored the star xi Hya (G7III) continuously during one month with the CORALIE spectrograph attached to the 1.2m Swiss Euler telescope. The 433 high-precision radial-velocity measurements clearly reveal multiple oscillation frequencies in the range 50 - 130 uHz, corresponding to periods between 2.0 and 5.5 hours. The amplitudes of the strongest modes are slightly smaller than 2 m/s. Current model calculations are compatible with the detected modes.

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arXiv2002

Tidally induced radial-velocity variations in close binaries

A theoretical framework for the determination of tidally induced radial-velocity variations in a component of a close binary is presented. Both the free and the forced oscillations of the component are treated as linear, isentropic perturbations of a spherically symmetric star. Resonances between dynamic tides and free oscillation modes are taken into account by means of the formalism developed by Smeyers et al. (1998). The amplitude of the tidally induced radial-velocity variations seen by the observer depends on the orbital eccentricity and on the orbital inclination. The amplitude increases with increasing orbital eccentricity and is most sensitive to the value of the orbital inclination when 20 < i < 70. In the case of a 5 solar mass ZAMS star with a 1.4 solar mass compact companion, it is shown that resonant dynamic tides can lead to radial-velocity variations with amplitudes large enough to be detected in observations. The shape of the tidally induced radial-velocity curves varies from very irregular for orbital periods away from any resonances with free oscillation modes to sinusoidal for orbital periods close to a resonance with a free oscillation mode. Our investigation is concluded with an application to the slowly pulsating B star HD 177863 showing the possibility of resonant excitation of a high-order second-degree g-mode in this star.

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arXiv2001

A gamma Doradus star campaign

We report on the results from a large photometric campaign on thirty five gamma Dor star candidates undertaken at the Institute of Astronomy of the University of Leuven in the framework of the Flanders -- South-Africa grant. An overview of the data, as well as the results of the analysis of the obtained time series are presented, the main conclusion being that nine stars are thought to be multiperiodic gamma Dor stars and eight monoperiodic. We also performed a photometric mode identification for two stars of the sample by comparing the amplitude ratios in the different passbands of the Geneva photometric system. Both stars seem to pulsate in non-radial modes of degree l=1.

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arXiv2000

The observation of Delta Sct and Gamma Dor stars by using MONS satellite

The MONS space experiment will be able to verify some of the most important topics in the asteroseismologic studies of Delta Sct and Gamma Dor stars. We discuss how the $γ$ Doradus can be considered possible "bridges" between the opacity driven overstable pulsation and the stochastically excited solar like oscillation. For this reason the possible inclusion of a Gamma Dor representative in the MONS main target list is presented and discussed. To study Delta Sct stars, an appropriate frequency resolution, comparable to or better than what is currently achieved by ground-based observations, is necessary and hence two runs are recommended. To avoid over-scheduling of the Main Camera, a possible combination of observations from the Main Camera and from the Star Trackers is proposed. As a result, key-objects could be adequately monitored, affording the possibility to enter into details of amplitude variations. Simultaneously, the same results could be obtained on two Gamma Dor stars.

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arXiv2000

Asteroseismology and Oblique Pulsator Model of Beta Cephei

We discuss the oscillation features of Beta Cephei, which is a magnetic star in which the magnetic axis seems to be oblique to the rotation axis. We interpret the observed equi-distant fine structures of the frequency spectrum as a manifestation of a magnetic perturbation of an eigenmode, which would be a radial mode in the absence of the magnetic field. Besides these frequency components, we interpret another peak in the frequency spectrum as an independent quadrupole mode. By this mode identification, we deduce the mass, the evolutionary stage, the rotational frequency, the magnetic field strength, and the geometrical configuration of Beta Cephei.

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arXiv1998

Interferometry and spectroscopy of Beta Cen: a Beta Cephei star in a binary system

Beta Cen is a bright Beta Cephei variable and has long been suspected to be a binary. Here we report interferometric observations with the 3.9-m Anglo-Australian Telescope at a single epoch which show the star to be a binary with components separated by 15 milliarcsec and having approximately equal luminosities at 486 nm. We also present high-resolution spectra taken over five nights with the ESO CAT which show Beta Cen to be a double-lined spectroscopic binary. We identify two pulsation frequencies in the primary. Further spectroscopic and interferometric studies of this double star should allow determination of its orbital parameters and component masses.

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