Asteroseismic Observations Research Articles

Accurate and versatile equations of state for the Sun and Sun-like stars

For models of the Sun and Sun-like stars, a high-quality equation of state is crucial. In addition, helioseismic and asteroseismic observations also put constraints on the physical formalisms. Thus, they effectively turn the Sun and the stars into laboratories for dense plasmas. Currently, the main astrophysical beneficiary of a good equation of state is the determination of the chemical composition. Here, seismic data have supplemented spectroscopic information. Recently, there has been theoretical progress in the equation of state, thanks to renewed rigorous and phenomenological approaches.

SIAMOIS, asteroseismolgy at Dome C after CoRoT

SIAMOIS is a ground-based asteroseismology project, to pursue velocity measurements from the Dome C Concordia station in Antarctica. Dome C appears to be the ideal place for ground-based asteroseismic observations as it is capable of delivering a duty cycle as high as 87% during the three-month long polar night. This high duty cycle, a crucial point for asteroseismology, is comparable to the best space-borne observations. We present the quality of the SIAMOIS project, that yields to its selection as top priority by the working group of ARENA dedicated to “Time series observations from Dome C”. We show that, after CoRoT, SIAMOIS proposes a large scientific case, that extends the pioneering work and great discoveries of CoRoT. We depict a few important ingredients of the project: full automation, space-like philosophy, and compatibility with the current facilities at the Concordia station. SIAMOIS has no real competitor before at least 2016/2017, and can be considered as a pathfinder for many important issues concerning astronomical observations at Dome C.

Using p-mode excitation rates for probing convection in solar-like stars

Using p-mode excitation rates for probing convection in solar-like stars

Asteroseismology of solar–type stars with SARG@TNG

Die Serie Communications in Asteroseismology (CoAst) stellt eine international gefragte Plattform für wissenschaftliche Publikationen der neuesten Forschungsergebnisse auf dem Gebiet der Asteroseismologie dar. Forschungsgegenstand der Asteroseismologie sind veränderliche und pulsierende Sterne. Neben den halbjährlich erscheinenden regulären Ausgaben mit Beiträgen aus der laufenden Forschung werden auch Sonderausgaben für Konferenzberichte bzw. Programmhandbücher herausgegeben. JENAM (Joint European and National Astronomy Meeting) ist die größte jährliche Zusammenkunft der europäischen Berufsastronomen, die diesmal im September 2008 in Wien stattfand. Die Konferenz beherbergte neun Minisymposien, wobei eines der Asteroseismologie gewidmet war. Die neuesten Entwicklungen auf diesem Gebiet sind im vorliegenden Band im Rahmen von Übersichts- und Detailartikeln zusammengefasst.

Asteroseismic observations and modelling of 70 Ophiuchi AB

The analysis of solar-like oscillations for stars belonging to a binary system provides an opportunity to probe the internal stellar structure and to test our knowledge of stellar physics. We present asteroseismic observations of 70 Oph A performed with the HARPS spectrograph together with a comprehensive theoretical calibration of the 70 Ophiuchi system.

AsteroFLAG — from the Sun to the stars

We stand on the threshold of a critical expansion of asteroseismology of Sun-like stars, the study of stellar interiors by observation and analysis of their global acoustic modes of oscillation. The Sun-like oscillations give a very rich spectrum allowing the internal structure and dynamics to be probed down into the stellar cores to very high precision. Asteroseismic observations of many stars will allow multiple-point tests of crucial aspects of stellar evolution and dynamo theory. The aims of the asteroFLAG collaboration are to help the community to refine existing, and to develop new, methods for analysis of the asteroseismic data on the Sun-like oscillators.

Asteroseismology of solar-type stars: particular physical effects

Since the success of helioseismology, numerous efforts have been made to detect solar-like oscillations on other stars. The measurement of the frequencies of p-mode oscillations provides an insight into the internal structure and is nowadays the most powerful constraint on the theory of stellar evolution. The existing asteroseismic observations were mainly motivated by the need to explore the seismological properties of stars with various global parameters, i.e. various locations in the HR diagram. With the aim of testing different physical effects on solar-like oscillations, we report here detection of acoustic modes on solar-like targets achieved with the spectrograph HARPS installed on the 3.6-m telescope at ESO La Silla Observatory.

Stellar ages from asteroseismology

AbstractAsteroseismology has been recognized for a long time as a very powerful mean to probe stellar interiors. The oscillations frequencies are closely related to stellar internal structure properties via the density and the sound speed profiles. Since these properties are in turn tightly linked with the mass and evolutionary state, we can expect to determine the age and mass of a star from the comparison of its oscillation spectrum with the predictions of stellar models. Such a comparison will of course suffer both from the problems we face when modeling a particular star (for instance the uncertainties on its global parameters and chemical composition) and from our general misunderstanding of the physical processes at work in stellar interiors (for instance the various transport processes that may lead to core mixing and affect the ages predicted by models). However for stars where observations have provided very precise and numerous oscillation frequencies together with accurate global parameters and additional information (as the radius or the mass of the star if it is member of a binary system, the radius if it observable in interferometry or the mean density if the star is an exoplanet host), we can also expect to better constrain the physical description of the stellar structure and transport processes and to finally get a more reliable age estimation.After a brief survey of stellar pulsations, we present some general seismic diagnostics that can be used to infer the age of a pulsating star as well as their limitations. We then illustrate the ability of asteroseismology to scrutinize stellar interiors on the basis of a few examples. In the years to come, extended very precise asteroseismic observations are expected, either in photometry or in spectroscopy, from present and future ground-based (HARPS, CORALIE, ELODIE, UVES, UCLES, SIAMOIS, SONG) or spatial devices (MOST, CoRoT, WIRE, Kepler, PLATO). This will considerably enlarge the sample of stars eligible to asteroseismic age determination and should allow to estimate the age of individual stars with a 10-20% accuracy.

SIAMOIS: Seismic Interferometer to Measure Oscillations in the Interior of Stars

SIAMOIS is a project devoted to ground-based asteroseismology, involving an instrument to be installed at the Dome C Concordia station in Antarctica. SIAMOIS provides an asteroseismic programme that can follow the way currently opened by the space project CoRoT, with unique information on G and K type bright stars on the main sequence. In addition, spectrometric observations with SIAMOIS will be able to detect oscillation modes that cannot be analyzed in photometry: the Doppler data, less affected by the stellar activity noise, yield a more precise mode structure inversion. The SIAMOIS concept is based on Fourier Transform interferometry. Such a principle leads to a small instrument designed and developed for the harsh conditions in Antarctic. The instrument will be fully automatic, with no moving parts, and a very simple initial set up in Antarctic. The dedicated scientific programme will avoid the complications related to a versatile instrument. Data reduction will be performed in real time, and the transfer of the asteroseismic data to Europe will require only a modest bandwidth. SIAMOIS will observe with a dedicated small 40-cm telescope. Dome C appears to be the ideal place for ground-based asteroseismic observations. The unequalled weather conditions yield a duty cycle as high as 90% over 3 months, as was observed during the 2005 wintering. This high duty cycle, a crucial point for asteroseismology, is comparable to the best space-based observations. Long time series (up to 3 months) will be possible, thanks to the long duration of the polar night. SIAMOIS can be seen as one of the very first observational projects in astronomy at Dome C. Its scientific programme will take full advantage of the unique quality of this site, and will constitute a necessary first step in preparation of future more ambitious programmes requiring more sophisticated instrumentation and larger collectors.

Effect of the stellar inclination angle upon theoretical errors of l = 1 p‐mode parameters

AbstractThe asteroseismic observations provided by current and future missions like CoRoT or Kepler will have a quality closer to those obtained for the Sun. In this context, tools and methods developed for helioseismology can be applied to other stars. In this paper, we focus on solar‐like oscillations of stars with an unknown rotation axis inclination and study, by means of maximum‐likelihood estimation, the errors on the determination of l = 1 p‐mode parameters. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On deriving p-mode parameters for inclined solar-like stars

Context. Thanks to their high quality, new and upcoming asteroseismic observations ‐ with CoRoT, Kepler, and from the ground... ‐ can benefit from the experience gained with helioseismology . Aims. We focus in this paper on solar-like oscillations, for which the inclination of the rotation axis is unknown. We present a theoretical study of the errors of p-mode parameters determined by means of a maximum-likelihood estimator, and we also analyze correlations and biases. Methods. We have used different, complementary approaches: we have performed either semi-analytical computation of the Hessian matrix, fitting of single mean profiles, or Monte Carlo simula tions. Results. We give first analytical approximations for the errors of fre quency, inclination and rotational splitting. The determi nation of the inclination is very challenging for the common case of slow rotators (like the Sun), making diffi cult the determination of a reliable rotational splitting. Moreover, due to the numerous correl ations, biases ‐ more or less significant ‐ can appear in the de termination of various parameters in the case of bad inclination fittings, e specially when a locking at 90 ◦ occurs. This issue concerning inclination locking is also discussed. Nevertheless, the central frequ ency and some derived parameters such as the total power of the mode are free of such biases.

Seismic analysis of 70 Ophiuchi A: A new quantity proposed

The basic intent of this paper is to model 70 Ophiuchi A using the latest asteroseismic observations as complementary constraints and to determine the fundamental parameters of the star. Additionally, we propose a new quantity to lift the degeneracy between the initial chemical composition and stellar age. Using the Yale stellar evolution code (YREC7), we construct a series of stellar evolutionary tracks for the mass range M = 0.85–0.93 M ⊙ with different composition Y i (0.26–0.30) and Z i (0.017–0.023). Along these tracks, we select a grid of stellar model candidates that fall within the error box in the HR diagram to calculate the theoretical frequencies, the large- and small-frequency separations using Guenther’s stellar pulsation code. Following the asymptotic formula of stellar p-modes, we define a quantity r 01 which is correlated with stellar age. Also, we test it by theoretical adiabatic frequencies of many models. Many detailed models of 70 Ophiuchi A are listed in Table 3. By combining all non-asteroseismic observations available for 70 Ophiuchi A with these seismological data, we think that Model 60, Model 125 and Model 126, listed in Table 3, are the optimum models presently. Meanwhile, we predict that the radius of this star is about 0.860–0.865 R ⊙ and the age is about 6.8–7.0 Gyr with mass 0.89–0.90 M ⊙. Additionally, we prove that the new quantity r 01 can be a useful indicator of stellar age.

Analysis of 70 Ophiuchi AB including seismic constraints

The analysis of solar-like oscillations for stars belonging to a binary system provides a unique opportunity to probe the internal stellar structure and to test our knowledge of stellar physics. Such oscillations have been recently observed and characterized for the A component of the 70 Ophiuchi system. A model of 70 Ophiuchi AB that correctly reproduces all observational constraints available for both stars is determined. An age of 6.2 +- 1.0 Gyr is found with an initial helium mass fraction Y_i=0.266 +- 0.015 and an initial metallicity (Z/X)_i=0.0300 +- 0.0025 when atomic diffusion is included and a solar value of the mixing-length parameter assumed. A precise and independent determination of the value of the mixing-length parameter needed to model 70 Oph A requires accurate measurement of the mean small separation, which is not available yet. Current asteroseismic observations, however, suggest that the value of the mixing-length parameter of 70 Oph A is lower or equal to the solar calibrated value. The effects of atomic diffusion and of the choice of the adopted solar mixture were also studied. We also tested and compared the theoretical tools used for the modeling of stars for which p-modes frequencies are detected by performing this analysis with three different stellar evolution codes and two different calibration methods. We found that the different evolution codes and calibration methods we used led to perfectly coherent results.

Seismological Analysis of the Stars γ Serpentis and ι Leonis: Stellar Parameters and Evolution

In order to assess the information that will be available from targets of the Microvariability and Oscillations of Stars (MOST) satellite, we explore the possible evolutionary status and perform preliminary seismological analysis of two targets: the stars gamma Ser and iota Leo, which show solar-like oscillations. Taking into account the astrometric observational constraints on the stars, we find that gamma Ser is in the main-sequence phase of evolution, while iota Leo has two distinct solutions that match well with observations: main-sequence models and post-main-sequence models. The evolutionary tracks cover the following ranges of mass and initial metallicity: gamma Ser, 1.15-1.20 M-circle dot and Z = 0.0135 +/- 0.003; iota Leo, 1.62-1.70 M-circle dot and Z = 0.024 +/- 0.002 (for values of the Galactic helium enrichment ratio Delta Y/Delta Z in the range 0 - 2). By combining stellar models with observational constraints, we determine age ranges of 3.2 +/- 0.6 Gyr for gamma Ser and 1.7 +/- 0.2 Gyr for iota Leo. The results show that the masses and ages of the stars are very sensitive to metallicity and evolutionary phase. Furthermore, we find that models with overshooting from a convective core predict a larger age for a given mass. Finally, we discuss future prospects for constraining stellar models with the help of asteroseismic observations.

Future projects in asteroseismology: the unique role of Antarctica

Asteroseismology requires observables registered in stringent conditions: very high sensitivity, uninterrupted time series, long duration. These specifications then allow to study the details of the stellar interior structure. Space-borne and ground-based asteroseismic projects are presented and compared. With CoRoT as a precursor, then Kepler and maybe Plato, the roadmap in space appears to be precisely designed. In parallel, ground-based projects are necessary to provide different and unique information on bright stars with Doppler measurements. Dome C appears to be the ideal place for ground-based asteroseismic observations. The unequalled weather conditions yield a duty cycle comparable to space. Long time series (up to 3 months) will be possible, thanks to the long duration of the polar night.

Asteroseismology of Procyon with SOPHIE

Context.This paper reports a 9-night asteroseismic observation program conducted in January 2007 with the new spectrometer sophie at the OHP 193-cm telescope, on the F5 IV-V target Procyon A.

Asteroseismic signatures of stellar magnetic activity cycles

Abstract Observations of stellar activity cycles provide an opportunity to study magnetic dynamos under many different physical conditions. Space-based asteroseismology missions will soon yield useful constraints on the interior conditions that nurture such magnetic cycles, and will be sensitive enough to detect shifts in the oscillation frequencies due to the magnetic variations. We derive a method for predicting these shifts from changes in the Mg ii activity index by scaling from solar data. We demonstrate this technique on the solar-type subgiant β Hyi, using archival International Ultraviolet Explorer spectra and two epochs of ground-based asteroseismic observations. We find qualitative evidence of the expected frequency shifts and predict the optimal timing for future asteroseismic observations of this star.

A Fourier Tachometer at Dome C in Antarctica

Inhalt (u.a.): M. BREGER: Delta Scuti stars: Observational aspects; M.S. Cunha: Theory of rapidly oscillating Ap stars; M. SACHKOV et al.: Vertical structure of pulsations in roAp stars; G. HOUDEK: Theoretical asteroseismology of solar-like oscillations; B.L. POPIELSKI: Core modes as a seismic probe of mixing beyond the convective core; M. GODART: Temperature gradients in the core overshooting region; W.A. DZIEMBOWSKI – J. DASZYŃSKA-DASZKIEWICZ – A.A. PAMYATNYKH: Interpretation of the Be star HD 163868 oscillation spectrum based on the MOST observations; D. KILKENNY: Pulsating Hot Subdwarfs – An Observational Review; M.H. MONTGOMERY: Mapping Convection using Pulsating White Dwarf Stars; M. ROTH: The Network Activities in HELAS; I. KUDZEJ et al.: A New Slovak Observatory 500 km from Vienna; W.W. WEISS: Microsatellites; G. KOPACKI – S. FRANDSEN: RR Lyrae stars in M4

Duty Cycle of Doppler Ground‐based Asteroseismic Observations11Based on observations made by E. Aristidi during the 2006 second overwinter at the French‐Italian Concordia Station, Dome C, Antarctica.

ABSTRACTWe report the observations of the clear‐sky fraction at the Concordia station during winter 2006 and derive from it the duty cycle for the astronomical observations. The duty cycle and observation duration at Dome C allow for efficient asteroseismic observations. This performance is analyzed and compared to network observations. For network observations, simulations were run using the helioseismic Global Oscillation Network Group as a reference. Observations with one site in Antarctica provide performance similar to or better than that with a six‐site network, since the duty cycle limited by meteorology is as high as 92%. On bright targets, a 100 day long time series with a duty cycle of about 87% can be observed, which is not possible for a network observation.

SIAMOIS: Seismic Interferometer to Measure Oscillations in the Interior of Stars

SIAMOIS is a project devoted to ground-based asteroseismology, involving a small collector and an instrument to be installed at the Dome C Concordia station in Antarctica. After the space project CoRoT, SIAMOIS will provide unique Doppler information on G and K type bright stars on the main sequence. Spectrometric observations with SIAMOIS will be able to detect oscillation modes that cannot be analyzed in photometry, and will be less affected by stellar activity noise. The SIAMOIS concept is based on Fourier Transform interferometry. Such a principle leads to a small instrument designed and developed for the harsh conditions in Antarctica. The instrument will be fully automatic, with no moving parts, and a very simple initial set up at Dome C. Dome C appears to be the ideal place for ground-based asteroseismic observations. The unequalled weather conditions yield a duty cycle as high as 90% over 3 months, as was observed during the 2006 wintering. This high duty cycle, a crucial point for asteroseismology, is comparable to the best space-based observations. Long time series (up to 3 months) will be possible, thanks to the long duration of the polar night. SIAMOIS can be seen as one of the very first astronomical projects to be conducted at Dome C. Its scientific programme will take full advantage of the unique quality of this site, and will constitute a necessary first step in preparation of future more ambitious programmes requiring more sophisticated instrumentation and larger collectors.