We show evidence that the red giant star xi Hya has an oscillation modelifetime, tau , of about 2 days significantly shorter than predicted bytheory (tau =17 days, \citealt{HoudekGough02}). If this is a generaltrend of red giants it would limit the prospects of asteroseismology onthese stars because of poor coherence of the oscillations.
We review recent progress in observations of ground-based oscillations.Excellent observations now exist for a few stars (alpha Cen A{} and B,mu Ara), while there is some controversy over others (Procyon, etaBoo). We have reached the stage where single-site observations are oflimited value and where careful planning is needed to ensure the futureof asteroseismology.
We present here the detection and characterization of solar-likeoscillations in several targets such as beta Vir, eta Boo, delta Eri,chi Eri and the Am star HD 209625 obtained with the CORALIE and HARPSspectrographs based at the ESO La Silla Observatory. The measurement ofthe frequencies of p-mode oscillations provides an insight into theinternal structure and is nowadays the most powerful constraint on thetheory of stellar evolution.
Since the success of helioseismology, numerous efforts have been made todetect solar-like oscillations on other stars. Thanks to newspectrographs developed for extra-solar planet searches, the accuracyneeded to detect such oscillations has recently been achieved. In thispaper, we present new asteroseismic measurements obtained with theCORALIE and HARPS spectrographs as well as new theoretical analysesbased on these observations. In particular, we focus on the effects ofrotation on the modelling of solar-type stars and on its influence onthe determination of fundamental stellar parameters.
We present the first results of a new abundance survey of the Milky Waybulge based on Keck HIRES spectra of 27 K giants in the Baade's Window(l=1deg, b=-4deg) field. The spectral data used inthis study are of much higher resolution and signal-to-noise ratio thanprevious optical studies of Galactic bulge stars. The [Fe/H] values ofour stars, which range between -1.29 and +0.51, were used to recalibratelarge low-resolution surveys of bulge stars. Our best value for the mean[Fe/H] of the bulge is -0.10+/-0.04. This mean value is similar to themean metallicity of the local disk and indicates that there cannot be astrong metallicity gradient inside the solar circle. The metallicitydistribution of stars confirms that the bulge does not suffer from theso-called G dwarf problem. This paper also details the new abundancetechniques necessary to analyze very metal-rich K giants, including anew Fe line list and regions of low blanketing for continuumidentification.Based on data obtained at the W. M. Keck Observatory, which is operatedas a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation.
This paper presents the modeling of the F9 V star βVirginis based on seismological measurements. Using the Genevaevolution code including rotation and atomic diffusion, we find that twodistinct solutions reproduce all existing asteroseismic andnon-asteroseismic observational constraints well: a main-sequence modelwith a mass of 1.28 ± 0.03~Mȯ and an age t=3.24± 0.20 Gyr, or a model in the post-main sequence phase ofevolution with a lower mass of 1.21 ± 0.02~Mȯ andan age t=4.01 ± 0.30 Gyr. The small spacings δν02 and the ratio r02 between small and largespacings are sensitive to the differences in the structure of thecentral layers between these two solutions and are also sensitive to thestructural changes due to the rotational mixing. They can therefore beused to unambiguously determine the evolutionary state ofβ Vir and to study the effects of rotation onthe inner structure of the star. Unfortunately, existing asteroseismicdata do not enable such precise determination. We also show that thescatter in frequencies introduced by the rotational splittings canaccount for the larger dispersion of the observed large spacings for thenon-radial modes than for the radial modes.
We introduce a new method to measure frequency separations and modelifetimes of stochastically excited and damped oscillations, so-calledsolar-like oscillations. Our method shows that velocity data of the redgiant star ξ Hya (Frandsen et al. 2002) support a large frequencyseparation between modes of roughly 7~μHz. We also conclude that thedata are consistent with a mode lifetime of 2 days, which is so shortrelative to its pulsation period that none of the observed frequenciesare unambiguous. Hence, we argue that the maximum asteroseismic outputthat can be obtained from these data is an average large frequencyseparation, the oscillation amplitude and the average mode lifetime.However, the significant discrepancy between the theoreticalcalculations of the mode lifetime (Houdek & Gough 2002) and ourresult based on the observations of ξ Hya, implies that red giantstars can help us better understand the damping and driving mechanismsof solar-like p-modes by convection.
We analyse visual observations of the pulsations of the red giantvariable L2 Puppis (L2 Pup). The data cover 77 yrbetween 1927 and 2005, thus providing an extensive empirical base forcharacterizing properties of the oscillations. The power spectrum of thelight curve shows a single mode resolved into multiple peaks under anarrow envelope. We argue that this results from stochastic excitation,as seen in solar oscillations, with a mode lifetime of about 5 yr. Therandom fluctuations in phase also support this idea. A comparison with XCam, a true Mira star with the same pulsation period, and W Cyg, a truesemiregular star, illustrates the basic differences in phase behaviours.The Mira shows very stable phase, consistent with excitation by theκ-mechanism, whereas W Cyg shows large phase fluctuations thatimply stochastic excitation. We find L2 Pup to beintermediate, implying that both mechanisms play a role in itspulsation. Finally, we also checked the presence of low-dimensionalchaos and could safely exclude it.
The systematic patterns of separations between frequencies of modes ofdifferent degree and order are a characteristic of p-mode oscillationsof stars. The frequency separations depend on the internal structure ofthe star and so measuring them in the observed oscillation spectra ofvariable stars gives valuable diagnostics of the interior of a star.Roxburgh & Vorontsov proposed using the ratio of the so-called smallfrequency separation to the large frequency separation as a diagnosticof the stellar interior, and demonstrated that this ratio was lesssensitive than the individual frequency separations themselves touncertain details of the near-surface structure. Here we derive kernelsrelating the frequency separation ratio to structure, and show why theratio is relatively insensitive to the near-surface structure in termsof the very small amplitude of the kernels in the near-surface layers.We also investigate the behaviour of the separation ratio for stars ofdifferent masses and ages, and demonstrate the usefulness of the ratioin the so-called asteroseismic Hertzsprung-Russell diagram.
The amplitude of solar-like oscillations results from a balance betweenexcitation and damping. As in the sun, the excitation is attributed toturbulent motions that stochastically excite the p modes in theupper-most part of the convective zone. We present here a model for theexcitation mechanism. Comparisons between modeled amplitudes and helioand stellar seismic constraints are presented and the discrepanciesdiscussed. Finally the possibility and the interest of detecting suchstochastically excited modes in pre-main sequence stars are alsodiscussed.
Using VLTI/VINCI angular diameter measurements, we constrain theevolutionary status of three asteroseismic targets: the stars δEri, ξ Hya, η Boo. Our predictions of the mean large frequencyspacing of these stars are in agreement with published observationalestimations. Looking without success for a companion of δ Eri, wedoubt its classification as an RS CVn star.
The ESO Very Large Telescope Interferometer (VLTI) is one of the leadinginterferometric facilities. It is equipped with several 8.2 and 1.8 mtelescopes, a large number of baselines up to 200 m, and with severalsubsystems designed to enable high quality measurements and to improvesignificantly the limits of sensitivities currently available tolong-baseline interferometry. The full scientific potential of the VLTIcan be exploited only if a consistent set of good quality calibrators isavailable. For this, a large number of observations of potentialcalibrators have been obtained during the commissioning phase of theVLTI. These data are publicly available. We briefly describe theinterferometer, the VINCI instrument used for the observations, the dataflow from acquisition to processed results, and we present and commenton the volume of observations gathered and scrutinized. The result is alist of 191 calibrator candidates, for which a total of 12 066observations can be deemed of satisfactory quality. We present a generalstatistical analysis of this sample, using as a starting point theangular diameters previously available in the literature. We derive thegeneral characteristics of the VLTI transfer function, and its trendwith time in the period 2001 through mid-2004. A second paper will bedevoted to a detailed investigation of a selected sample, aimed atestablishing a VLTI-based homogeneous system of calibrators.
In July, 1999, the Hubble Space Telescope (HST) pointed for more than 8days continuously at the globular cluster 47 Tucanae. Two guide starswere used during this campaign by the Fine Guidance Sensors (FGS). Usingmore than 19 million photometric FGS data for the guide star GSC09137-03505 we found brightness variability corresponding to threeapproximately equidistant frequencies. The detected frequencies rangefrom 21 to 71 μHz with amplitudes between 341 and 291 ppm. MulticolorCCD photometry was obtained at the Cerro Tololo Inter-AmericanObservatory to derive color information for the guide stars. Inaddition, flux calibrated spectra were observed at the South AfricanAstronomical Observatory. Temperature and surface gravity weredetermined from a comparison of observed and synthetic spectral fluxesusing the nemo (Vienna New Model Grid of Stellar Atmospheres) modelatmosphere grid developed by Heiter et al. (2002, A&A, 392, 619) andNendwich et al. (2004, CoAst, 144, 43). We also performed linearnonadiabatic analyses of various stellar models in an attempt tointerprete the frequencies.
This paper presents the H band near-infrared (NIR) spectral library of135 solar type stars covering spectral types O5-M3 and luminosityclasses I-V as per MK classification. The observations were carried outwith 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, Indiausing a NICMOS3 HgCdTe 256 x 256 NIR array based spectrometer. Thespectra have a moderate resolution of 1000 (about 16 A) at the H bandand have been continuum shape corrected to their respective effectivetemperatures. This library and the remaining ones in J and K bands oncereleased will serve as an important database for stellar populationsynthesis and other applications in conjunction with the newly formedlarge optical coude feed stellar spectral library of Valdes et al.(2004). The complete H-Band library is available online at: http://vo.iucaa.ernet.in/~voi/NIR_Header.html
As a part of an on-going program to explore the signature of p-modes insolar-like stars by means of high-resolution absorption linespectroscopy, we have studied four stars (α CMi, η Cas A,ζ Her A and β Vir). We present here new results from two-siteobservations of Procyon A acquired over twelve nights in 1999.Oscillation frequencies for l=1 and 0 (or 2) p-modes are detected in thepower spectra of these Doppler shift measurements. A frequency analysispoints out the difficulties of the classical asymptotic theory inrepresenting the p-mode spectrum of Procyon A.Based on observations obtained at the Observatoire de Haute-Provence(CNRS, France) and at the Whipple Observatory (Arizona, USA).
The five-minute oscillations in the Sun have provided a wealth ofinformation about the solar interior. After many attempts, positivedetections of similar oscillations in solar-type stars have now beenmade. This review discusses the properties of solar-like oscillations,the methods used to observe them, and the results on individual stars.We conclude that the study of solar-like oscillations from the groundand space has an exciting future.
The low scintillation noise expected for night time observations at theConcordia station leads to the expectation of scintillation-limitedphotometric noise as much as 10 times lower than can be obtained fromany other site in the world. This is of particular interest for brighthigh frequency pulsators - solar-like pulsators, roAp stars - wherethe noise is scintillation limited. For pulsating white dwarfs stars,sub-dwarf B pulsators, and many other stars of intense asteroseismicinterest, photometric noise will also be less than for other observingsites. A single 2-m photometric telescope at Concordia will be able toproduce higher precision asteroseismic data sets than the best yetobtained by the Whole Earth Telescope (WET) - data obtained by the vasteffort of dozens of astronomers observing at sites all over the world.In just a few years of observations, the cost of such a telescope willbe less than the cost of running WET for the same length of time. The35-cm test telescope will immediately be capable of producingphotometric data sets superior to any obtained heretofore on brightstars such as the roAp stars and solar-like oscillators. Examples usingdata on the roAp stars HR 3831 from one and two sites, and on the roApstar HR 1217 using WET indicate that the level of the highest noisepeaks in the amplitude spectra of light curves obtained with the 35-cmtest telescope should be as low as 6 mu mag for a three-week run on abright roAp star. With a 2-m telescope could come down to 1 mu mag forthe brightest stars.
We show for the first time, for any star other than the Sun, thevertical resolution of pulsation modes into standing waves in theatmosphere and running waves in the magnetoacoustic reflective boundarylayer of the upper atmosphere. This has only become possible with thenovel use of the Very Large Telescope (VLT) on a bright, exceedinglypeculiar magnetic star, HD 166473. These are the first observations thatmay directly test recent theoretical developments for magnetoacousticstellar pulsation modes in rapidly oscillating Ap stars.
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
Two sets of archive time series observations of an old open cluster NGC6791 were reanalyzed using an image subtraction technique. We reportidentification of four new variable stars. Proper motion data areavailable for three of them and indicate that they are likely clustermembers. Photometry of these stars is discussed along with the data fortwo earlier identified variables. The sample analyzed includes oneyellow and three red stragglers. An eclipsing red straggler V9 is ofparticular interest. Cluster membership of this RS Cvn type binary wouldimply that its cooler component is in a stage of thermal inequilibrium.One of variables is a K type red giant showing low-amplitude variabilitywith a period of about 0.33d.
Amplitudes of stochastically excited oscillations of models of ξHydrae (HR4450) are presented. The theoretical results are compared withthe recent measurements by an international group of astronomersannounced in the ESO press release 10/02. Using a stochastic excitationmodel we find fair agreement between estimated velocity amplitudes andthe values quoted in the ESO press release.
Thirteen red clump stars from Baade's window were observed with highresolution in the red part of the optical spectrum with the UVES echellespectrograph at the Mount Paranal ESO Observatory. The model atmosphereabundance analysis placed their [Fe/H] values in a range from 0.0 to-1.52 dex. Present results, based on direct measurements of ironabundance, confirm former suggestions that the I-band brightness of thered clump giants only weakly depends on [Fe/H]. The determined values of[Fe/H] may contain a slight systematic error connected with still nowunexplained difference in colors between stars in the Galactic bulge andin the solar vicinity.
We report the firm discovery of solar-like oscillations in a giant star.We monitored the star xi Hya (G7III) continuously during one month withthe CORALIE spectrograph attached to the 1.2 m Swiss Euler telescope.The 433 high-precision radial-velocity measurements clearly revealmultiple oscillation frequencies in the range 50-130 muHz, correspondingto periods between 2.0 and 5.5 hours. The amplitudes of the strongestmodes are slightly smaller than 2 ms. Current model calculations arecompatible with the detected modes. Based on observations obtained withthe CORALIE spectrograph on the 1.2-m Swiss Euler telescope at La Silla,Chile.