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Modelling space-based high-precision photometry for asteroseismic applications
We present a new formalism to model and simulate space-basedhigh-precision photometric time-series of stochastically oscillatingstars. The first component of the formalism models time-series of CCDimages with all important instrumental effects incorporated, while thesecond component models stellar light curves as if there was noinstrumental deterioration. The formalism can and is being used toanalyse ground-based asteroseismic time-series, as well as to predictthe performance of a space-based instrument during its design phase.Next to the description of the formalism, we demonstrate its usefulnessby including two short but real-life applications.
| A new multiperiodic gamma Doradus variable in Andromedae.
gamma Dor-type oscillations have been discovered in the star HD 218427through simultaneous uvby photometric observations carried out in theyear 2003. A few Hbeta -Crawford measurements were alsocollected for calibration purposes which place this star well-locatedinside the gamma Dor instability region. Frecuency analysis was carriedout for different filters, the combined ``vby'' filter was also used andfive frequencies were found as significant with periods ranging between0.3 and 0.8 days. The recently developed Frequency Ratio Method (FRM,Moya et al. 2005) is used in order to perform a modal identification ofthe peaks.
| Asteroseismology of gamma Doradus stars and the COROT mission .
We present results of multisite ground-based observations of a gammaDoradus, and the potential of the COROT EXO-field for these stars.
| First unambiguous asteroseismologic modelling of a gamma Doradus star .
An asteroseismologic model of the gamma Dor star 9 Aurigae is discussedin this work. This is the first time a complete asteroseismologic schemefor analyzing gamma Dor stars is applied. From standard photometricobservations (at least three oscillation frequencies, a photometricerror box in the HR diagram and multicolor photometric observations, inthis case in the Strömgren system) we can reduce the possibletheoretical models for this star, providing constraints to the stellarparameters as mass, overshooting, metallicity, MLT parameter alpha ,Brunt-Väiälä integral (Ith), etc.Simultaneously, an estimate of the modal identification of the observedfrequencies is also obtained.This can be possible by the application, for the first time, of acomplete procedure where different theoretical and computationaltechniques, recently developed, are linked and compared with photometricobservations. The Frequency Ratio Method (FRM) and the Time DependentConvection (TDC) theory are the basis of this complete procedure.
| Fourier-Doppler imaging of non-radial pulsations in gamma Doradus stars .
This paper deals with a class of non-radial pulsators along the mainsequence, namely the gamma Doradus stars for which much effort iscurrently made in order to constrain their pulsation characteristics.However, because of their relatively low amplitude (few tens of mmag inphotometry) and due to the long time scales of the variation (between0.3 and 3 days), the detection and identification of their pulsations israther difficult, using the photometric data only. Consequently, thespectroscopic studies of the stars having well-known photometricproperties are very valuable and we study in detail the line profilevariability (LPV) in gamma Doradus candidates observed at Observatoirede Haute-Provence during a two-year high-resolution spectroscopycampaign. The non-radial behavior of selected stars is revealed with theadvent of Doppler Mapping and two-dimensional Fourier-Doppler Imagingmethods for line-profile analysis, which allowed us to detect andidentify the pulsation modes described below.
| Theoretical Aspects of g-mode Pulsations in gamma Doradus Stars
gamma Dor stars are main sequence variable A-F stars whose long periods(between 0.35 and 3 days) correspond to high-order gravity modespulsation. Most of them are multiperiodic. We will concentrate here ontwo theoretical aspects of these stars. First, an analysis of thedriving mechanism of the gamma Dor g-modes is presented, using thelinear Time-Dependent Convection (TDC) treatment of Gabriel\cite{Gabriel1996} and Grigahcène et al. \cite{Grigahcene}. Thisdriving is due to a periodic flux blocking mechanism at the base oftheir convective envelope. The location of the blue and red edges oftheir instability strip as well as the periods range of their observedmodes is explained by the balance between this driving mechanism andradiative damping in the g-mode cavity. Secondly, the multi-colorphotometric amplitude ratios and the phase differences between the lightand velocity curves are considered. It is shown that the agreementbetween theory and observations obtained with TDC models is much betterthan with Frozen Convection (FC) models. The theoretical analysis ofthese observables makes the photometric identification of the degreeℓ of the modes possible and gives constraints on the characteristicsof the convective envelope of these stars. Finally, the attractivepotential of gamma Dor stars as targets for asteroseismology isconsidered.
| Observational results for northern and southern (candidate) gamma Doradus stars .
We report on observational results obtained for 78 objects originallyclassified as bona-fide or candidate gamma Doradus stars. For thesouthern objects, we gathered echelle spectra with the CORALIEspectrograph attached to the Euler telescope in 1998-2003 and/orJohnson-Cousins B,V,I_c observations with the MODULAR photometerattached to the 0.5-m SAAO telescope in 1999-2000. For the northernobjects, we obtained Geneva U,B,B_1,B_2,V,V_1,G observations with the P7photometer attached to the 1.2-m Mercator telescope in 2001-2004. Atleast 15 of our objects are binaries, of which 7 are new. For 6binaries, we determined the orbit for the first time. At least 17objects show profile variations and at least 12 objects aremultiperiodic photometric variables. Our results allow us to upgrade 11objects to bona-fide gamma Doradus stars and to downgrade 8 objects toconstant up to the current detection limits. Mode identification isstill ongoing, but so far, only ℓ = 1 and 2 modes have beenidentified.
| Direct imaging photometry with the MOST satellite
Canada's first space telescope, MOST (Microvariablity and Oscillationsof Stars) was successfully launched on June 30, 2003 with a primarymission to perform ultra-high-precision photometry to detect acousticoscillations in solar-like stars. MOST has the ability to observe singlefields for uninterrupted periods of up to two months and targets can beobserved either through Fabry lens imaging or Direct imaging. Thisreport reviews the Direct imaging capabilities of the MOST satellite andthe extraction of accurate stellar photometry.MOST is a Canadian Space Agency mission, operated jointly by Dynacon, Inc., and the Universities of Toronto and British Columbia, with assistance from the University of Vienna.
| Analysis of main-sequence A-type stars showing radial velocity variability.
We obtained high-resolution spectroscopic data for 33 bright (V < 8)A-type HIPPARCOS programme stars at the ``Observatoire deHaute-Provence'' (OHP) in December 2004. All our targets show someindication of radial velocity variability (Grenier et al, 1999), arelocated in or near the lower part of the Cepheid instability strip, andare poorly studied objects. In this contribution, we present the newdata of the most interesting programme stars and we derive a physicalinterpretation for their variable nature.
| Influence of overshooting and metallicity on the delta Scuti and gamma Doradus instability strips
Computations of theoretical instability strips (IS) for delta Scuti(Sct) and gamma Dor (Dor) stars are presented. The Time-DependentConvection (TDC) theory of Gabriel (1996) and Grigahcène etal. (2005) is adopted in our models. We are able to obtain the delta Sctand gamma Dor IS. We present a prospective study on the influence ofthe overshooting and metallicity on the location of these IS in the HRdiagram.
| Application of time-dependent convection models to the photometric mode identification in gamma Doradus stars
We apply the Time-Dependent Convection (TDC) treatment of Gabriel\cite{Gabriel1996} and Grigahcène et al. \cite{Grigahcene} to thephotometric mode identification in gamma Dor stars. Comparison of ourtheoretical results with the observed amplitudes and phases of the stargamma Dor is presented. This comparison makes the identification of thedegree ℓ of its pulsation modes possible and shows that our TDCmodels better agree with observations than Frozen Convection (FC)models.
| Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks
We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS)5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60μm excess. The spectra of five objects possess spectral features thatare well-modeled using micron-sized grains and silicates withcrystalline mass fractions 0%-80%, consistent with T Tauri and HerbigAeBe stars. With the exception of η Crv, these objects are youngwith ages <=50 Myr. Our fits require the presence of a cool blackbodycontinuum, Tgr=80-200 K, in addition to hot, amorphous, andcrystalline silicates, Tgr=290-600 K, suggesting thatmultiple parent body belts are present in some debris disks, analogousto the asteroid and Kuiper belts in our solar system. The spectra forthe majority of objects are featureless, suggesting that the emittinggrains probably have radii a>10 μm. We have modeled the excesscontinua using a continuous disk with a uniform surface densitydistribution, expected if Poynting-Robertson and stellar wind drag arethe dominant grain removal processes, and using a single-temperatureblackbody, expected if the dust is located in a narrow ring around thestar. The IRS spectra of many objects are better modeled with asingle-temperature blackbody, suggesting that the disks possess innerholes. The distribution of grain temperatures, based on our blackbodyfits, peaks at Tgr=110-120 K. Since the timescale for icesublimation of micron-sized grains with Tgr>110 K is afraction of a Myr, the lack of warmer material may be explained if thegrains are icy. If planets dynamically clear the central portions ofdebris disks, then the frequency of planets around other stars isprobably high. We estimate that the majority of debris disk systemspossess parent body masses, MPB<1 M⊕. Thelow inferred parent body masses suggest that planet formation is anefficient process.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| An Atlas of K-Line Spectra for Cool Magnetic CP Stars: The Wing-Nib Anomaly (WNA)
We present a short atlas illustrating the unusual Ca II K-line profilesin upper main-sequence stars with anomalous abundances. Slopes of theprofiles for 10 cool, magnetic chemically peculiar (CP) stars changeabruptly at the very core, forming a deep ``nib.'' The nibs show thesame or nearly the same radial velocity as the other atomic lines. Thenear wings are generally more shallow than in normal stars. In threemagnetic CP stars, the K lines are too weak to show this shape, althoughthe nibs themselves are arguably present. The Ca II H lines also showdeep nibs, but the profiles are complicated by the nearby, strongHɛ absorption. The K-line structure is nearly unchanged withphase in β CrB and α Cir. Calculations, including NLTE, showthat other possibilities in addition to chemical stratification mayyield niblike cores.
| Non-radial pulsations in the γ Doradus star HD 195068
We present high resolution spectroscopic observations of the γDoradus star HD 195068. About 230 spectra werecollected over 2 years. Time series analysis performed on radialvelocity data shows a main peak at 1.61 d-1 , a frequency notyet detected in photometry. The Hipparcos photometric 1.25d-1 frequency is easily recovered as is 1.30 d-1while the third photometric frequency, 0.97 d-1 , is onlymarginally present. The good quality of our data, which includes 196spectra collected over seven consecutive nights, shows that both the1.61 d-1 and intermediate 1.27 d-1 (mixture of1.25 and 1.30 d-1 ) frequencies are present in the lineprofile variations. Using the Fourier-Doppler Imaging (FDI) method, thevariability associated with 1.61 d-1 can be successfullymodeled by a non-radial pulsation mode ℓ=5± 1, |m|=4±1. For the intermediate frequency 1.27 d-1 we deduceℓ=4± 1, |m|=3± 1. Evidence that the star is notpulsating in the radial mode (ℓ=0) rules out a previousclassification as an RR Lyrae type star. We investigate the timevariability of FDI power spectra concluding that the observed temporalvariability of modes can be explained by a beating phenomenon betweenclosely spaced frequencies of two non-radial modes. The distribution ofthe oscillation power within the line profile indicates that there is asignificant tangential velocity component of oscillations characteristicof high radial order gravity modes which are predicted to be observed inγ Doradus type stars.
| The frequency ratio method and the new multiperiodic γ Doradus star HD 218427
Oscillations of γ Dor-type were discovered in the star HD 218427through simultaneous uvby photometric observations carried out in theyear 2003. A few Hβ-Crawford measurements were also collected forcalibration purposes and they locate this star well inside the γDor instability region. We find HD 218427 to be deficient in metals,similar to other well-defined γ Dor stars, and discuss thepossibility that it has a λ Boo nature. We carried out frequencyanalysis for different filters, including the combined "vby" filter, andfive frequencies were found as significant with periods ranging between0.3 and 0.8 days. The recently-developed frequency ratio method is usedin order to identify the excited modes. The results are consistent withan l=2 identification for all the modes and with high radial quantumnumbers (n40) for the three main observed periodicities. Thepossibility of multiplet structures is also discussed. However, noconsistency is found when using the time-dependent convection treatmentto discriminate modes. This disagreement can be due to the largerotation velocity taking place in HD 218427 and, consequently, thesignificant coupling between the modes.
| A spectroscopic study of southern (candidate) γ Doradus stars. I. Time series analysis
We present the results of a spectroscopic study of 37 southern(candidate) γ Doradus stars based on échelle spectra. Theobserved spectra were cross-correlated with the standard templatespectrum of an F0-type star for easier detection of binary and intrinsicvariations. We identified 15 objects as spectroscopic binaries,including 7 new ones, while another 3 objects are binary suspects. Atleast 12 objects show composite spectra. We could determine the orbitalparameters for 9 binaries, of which 4 turned out to be ellipsoidalvariables. For 6 binaries, we estimated the expected time-base of theorbital variations. Clear profile variations were observed for 17objects, pointing towards stellar pulsation. For 8 of them, we haveevidence 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 γDoradus stars, 1 object as a new bona fide δ Scuti star, and 8objects as constant stars. Finally, we determined the projectedrotational velocity by two independent methods. The resulting v sin ivalues range from 3 to 135 {km s-1}. For the bona fideγ Doradus stars, the majority has v sin i below 60 {kms-1}.
| Time-dependent convection seismic study of five γ Doradus stars
We apply for the first time the time-dependent convection (TDC)treatment of Gabriel and Grigahcène et al. to the photometricmode identification in γ Doradus (γ Dor) stars. We considerthe influence of this treatment on the theoretical amplitude ratios andphase differences. Comparison with the observed amplitudes and phases ofthe stars γ Dor, 9 Aurigae, HD 207223 = HR 8330, HD 12901 and48501 is presented and enables us to identify the degree l of thepulsation modes for four of them. We also determine the mode stabilityfor different models of these stars. We show that our TDC models agreebetter with observations than with frozen convection models. Finally, wecompare the results obtained with different values of the mixing-lengthparameter α.
| Can Life Develop in the Expanded Habitable Zones around Red Giant Stars?
We present some new ideas about the possibility of life developingaround subgiant and red giant stars. Our study concerns the temporalevolution of the habitable zone. The distance between the star and thehabitable zone, as well as its width, increases with time as aconsequence of stellar evolution. The habitable zone moves outward afterthe star leaves the main sequence, sweeping a wider range of distancesfrom the star until the star reaches the tip of the asymptotic giantbranch. Currently there is no clear evidence as to when life actuallyformed on the Earth, but recent isotopic data suggest life existed atleast as early as 7×108 yr after the Earth was formed.Thus, if life could form and evolve over time intervals from5×108 to 109 yr, then there could behabitable planets with life around red giant stars. For a 1Msolar star at the first stages of its post-main-sequenceevolution, the temporal transit of the habitable zone is estimated to beseveral times 109 yr at 2 AU and around 108 yr at9 AU. Under these circumstances life could develop at distances in therange 2-9 AU in the environment of subgiant or giant stars, and in thefar distant future in the environment of our own solar system. After astar completes its first ascent along the red giant branch and the Heflash takes place, there is an additional stable period of quiescent Hecore burning during which there is another opportunity for life todevelop. For a 1 Msolar star there is an additional109 yr with a stable habitable zone in the region from 7 to22 AU. Space astronomy missions, such as proposed for the TerrestrialPlanet Finder (TPF) and Darwin, that focus on searches for signatures oflife on extrasolar planets, should also consider the environments ofsubgiants and red giant stars as potentially interesting sites forunderstanding the development of life. We performed a preliminaryevaluation of the difficulty of interferometric observations of planetsaround red giant stars compared to a main-sequence star environment. Weshow that pathfinder missions for TPF and Darwin, such as Eclipse andFKSI, have sufficient angular resolution and sensitivity to search forhabitable planets around some of the closest evolved stars of thesubgiant and red giant class.
| Single-Visit Photometric and Obscurational Completeness
We report a method that uses ``completeness'' to estimate the number ofextrasolar planets discovered by an observing program with adirect-imaging instrument. We develop a completeness function forEarth-like planets on ``habitable'' orbits for an instrument with acentral field obscuration, uniform sensitivity in an annular detectionzone, and limiting sensitivity that is expressed as a ``deltamagnitude'' with respect to the star, determined by systematic effects(given adequate exposure time). We demonstrate our method of estimationby applying it to our understanding of the coronagraphic version of theTerrestrial Planet Finder (TPF-C) mission as of 2004 October. Weestablish an initial relationship between the size, quality, andstability of the instrument's optics and its ability to meet missionscience requirements. We provide options for increasing the fidelity andversatility of the models on which our method is based, and we discusshow the method could be extended to model the TPF-C mission as a wholeto verify that its design can meet the science requirements.
| Convection-pulsation coupling. II. Excitation and stabilization mechanisms in δ Sct and γ Dor stars
We apply here the Time Dependent Convection (TDC) treatment presented inour earlier paper in this series to the study of δ Sct and γDor pulsating stars. Stabilization of the δ Sct p-modes at the rededge of the Instability Strip (IS) and the driving of the γ Dorg-modes are explained by our models. Theoretical IS obtained withdifferent values of the Mixing Length (ML) parameter α arecompared to observations and a good agreement is obtained for αbetween 1.8 and 2. The influence of each term of our TDC treatment(perturbation of convective flux, turbulent pressure, and turbulentkinetic energy dissipation) on the eigenfrequencies and on the drivingand damping mechanisms is investigated. Finally, we show that our TDCmodels predict the likely existence of hybrid stars with both δSct p-modes and γ Dor g-modes oscillations.
| Convection-pulsation coupling. I. A mixing-length perturbative theory
We present in details a time-dependent convection treatment in the frameof the Mixing-Length Theory (MLT). Following the original ideas by Unno(1967, PASJ, 19, 140), this theory has been developed by Gabriel et al.(1974, Bull. Ac. Roy. Belgique, Classe des Sciences, 60, 866) andGabriel (1996, Bull. Astron. Soc. India, 24, 233). In this paper, wepresent it in a united form, we detail the basic derivations andapproximations and give final improvements. A new perturbation of theenergy closure equation is proposed for the first time, making itpossible to avoid the occurrence of short wavelength spatialoscillations of the thermal eigenfunctions. This theory accounts for theperturbation of the convective flux, the turbulent Reynolds stress andthe turbulent kinetic energy dissipation. It has been numericallyimplemented in a non-radial non-adiabatic pulsation code and the firstresults published in a Letter by Dupret et al. (2004a, A&A, 414,L17) indicate that the theory predicts the observed red border of thelower end of the instability strip and the driving mechanism of therecently discovered γ Dor stars.
| Mid-Infrared Spectra of Dust Debris around Main-Sequence Stars
We report spectra obtained with the Spitzer Space Telescope in theλ=14-35μm range of 19 nearby main-sequence stars with infraredexcesses. The six stars with strong dust emission show no recognizablespectral features, suggesting that the bulk of the emitting particleshave diameters larger than 10 μm. If the observed dust results fromcollisional grinding of larger solids, we infer minimum masses of theparent body population between 0.004 and 0.06 M⊕. Weestimate grain production rates of ~1010 g s-1around λ Boo and HR 1570; selective accretion of this matter mayhelp explain their peculiar surface abundances. There appear to be innertruncations in the dust clouds at 48, 11, 52, and 54 AU around HR 333,HR 506, HR 1082, and HR 3927, respectively.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| On the Flux of Extrasolar Dust in Earth's Atmosphere
Micron-size extrasolar dust particles have been convincingly detected bysatellites. Larger extrasolar meteoroids (5-35 μm) have most likelybeen detected by ground-based radar at Arecibo and New Zealand. Wepresent estimates of the minimum detectable particle sizes and thecollecting areas for both radar systems. We show that particles largerthan ~10 μm can propagate for tens of parsecs through theinterstellar medium, opening up the possibility that ground-based radarsystems can detect AGB stars, young stellar objects such as T Tauristars, and debris disks around Vega-like stars. We provide analyticaland numerical estimates of the ejection velocity in the case of a debrisdisk interacting with a Jupiter-mass planet. We give rough estimates ofthe flux of large micrometeoroids from all three classes of sources.Current radar systems are unlikely to detect significant numbers ofmeteors from debris disks such as β Pictoris. However, we suggestimprovements to radar systems that should allow for the detection ofmultiple examples of all three classes.
| Asteroseismology with robotic telescopes
Asteroseismology explores the interior of pulsating stars by analysingtheir normal mode spectrum. The detection of a sufficient number ofpulsation modes for seismic modelling of main sequence variablesrequires large quantities of high-precision time resolved photometry.Robotic telescopes have become an asset for asteroseismology because oftheir stable instrumentation, cost- and time-efficient operation and thepotentially large amounts of observing time available. We illustratethese points by presenting selected results on several types ofpulsating variables, such as δ Scuti stars (main sequence andpre-main sequence), γ Doradus stars, rapidly oscillating Ap starsand β Cephei stars, thereby briefly reviewing recent successstories of asteroseismic studies of main sequence stars.
| A Theoretical γ Doradus Instability Strip
In this paper, we present the first theoretical γ Doradusinstability strip. We find that our model instability strip agrees verywell with the previously established, observationally based, instabilitystrip of Handler & Shobbrook. We stress, as do Guzik et al., thatthe convection zone depth plays the major role in the determination ofour instability strip. Once this depth becomes too deep or too shallow,the convection zone no longer allows for pulsational instability. Ourtheoretical γ Dor instability strip is bounded by ~6850 and 7360 Kat the red and blue edge, respectively, on the zero-age main sequenceand by ~6560 and 7000 K at the red and blue edge, respectively,approximately 2 mag more luminous. This theoretical strip, transformedto the observer's color-magnitude diagram, overlays the region wheremost of the 30 bona fide γ Dor stars are found.
| A Dozen New γ Doradus Stars
We use new high-dispersion spectroscopic and precise photometricobservations to identify 12 new γ Doradus stars. Two of the 12systems are double-lined binaries that show obvious velocityvariability. Five other stars have metallic lines with compositeprofiles characterized by a narrow feature near the center of each broadcomponent. Spectrograms of the Hα line indicate that all fivestars are binaries rather than shell stars. The remaining five stars inour sample are probably single. All 12 stars are photometricallyvariable with amplitudes between 6 and 87 mmag in Johnson B and periodsbetween 0.3 and 1.2 days. Four stars are monoperiodic; the rest havebetween two and five independent periods. The variability at all periodsapproximates a sinusoid. Although many of the stars lie within theδ Scuti instability strip, none exhibit the higher frequencyvariability seen in δ Scuti stars. We have increased the sample ofknown γ Doradus stars by 40% and revised the positions of a numberof variables in the H-R diagram by accounting for duplicity. Our list of42 confirmed γ Doradus variables gives some of their properties.All are dwarfs or subgiants and lie within a well-defined region of theH-R diagram that overlaps the cool edge of the δ Scuti instabilitystrip. We compare the observed location of the γ Doradus variableswith a recently published theoretical γ Doradus instability stripand find good agreement.
| The Orbit and Pulsation Periods of the γ Doradus Variable HR 6844 (V2502 Ophiuchi)
We obtained spectroscopic and photometric observations of the γDoradus variable HR 6844 (=V2502 Ophiuchi). Radial velocities show thatthis star is a single-lined binary with a period of 4.4852 days. Theprimary is an F1 V star, while the secondary is likely an M dwarf.Velocity residuals to a circular orbit have a period of 1.3071 days andan amplitude of ~3 km s-1. Three periods of light variationwere detected, 1.30702, 1.4350, and 0.62286 days. The first period isessentially identical to that found in the radial velocities and has thelargest amplitude, a peak-to-peak value of 0.067 mag in B. Thephotometric check star, 73 Oph (=HR 6795), has light variations with aperiod of 0.61439 in B. Although the star is a close visual binary, thelight variations are ascribed to the primary, making it most likely anewly discovered γ Doradus variable.
| Selection criteria for targets of asteroseismic campaigns
Various dedicated satellite projects are underway or in advanced stagesof planning to perform high-precision, long duration time seriesphotometry of stars, with the purpose of using the frequencies ofstellar oscillations to put new constraints on the internal structure ofstars. It is known (cf. \cite{Bro+94}) that the effectiveness ofoscillation frequencies in constraining stellar model parameters issignificantly higher if classical parameters such as effectivetemperature and luminosity are known with high precision. In order tooptimize asteroseismic campaigns it is therefore useful to selecttargets from among candidates for which good spectroscopic andastrometric data already exists. This paper presents selection criteria,as well as redeterminations of stellar luminosity and reddening forstars satisfying these criteria.
| Six New γ Doradus Stars
We present high-resolution spectroscopy and precision photometry of sixnew γ Doradus stars, one of which was independently discovered byanother group. This brings the total number of confirmed γ Doradusvariables to 30. All six of these variables fall in the spectral classrange F0-F2 all but one are subgiants. The six stars have between oneand five photometric periods in the range 0.3-1.2 days. We find noevidence for higher frequency δ Scuti pulsations in any of thesesix stars. Our spectroscopic observations reveal HD 108100 to be thefirst confirmed γ Doradus variable with composite broad and narrowline profiles suggesting the presence of a circumstellar shell or disk.HD 221866 has the most asymmetric absorption lines of the six stars inthis paper and also the largest photometric amplitude. Most of the 30confirmed γ Doradus variables lie in a fairly tight region of theH-R diagram on or just above the main sequence that partially overlapsthe cool edge of the δ Scuti instability strip. However, threestars, including two of the new variables in this paper, are subgiantsthat lie well within the δ Scuti strip. Among the 30 confirmedγ Doradus variables, we find no correlation between thephotometric periods and intrinsic color, absolute magnitude, orluminosity.
| Discovery and analysis of p-mode and g-mode oscillations in the A-type primary of the eccentric binary HD 209295*
We have discovered both intermediate-order gravity mode and low-orderpressure mode pulsation in the same star, HD 209295. It is thereforeboth a γ Doradus and a δ Scuti star, which makes it thefirst pulsating star to be a member of two classes. The analysis of our128h of multisite spectroscopic observations carried out over twoseasons reveals that the star is a single-lined spectroscopic binarywith an orbital period of 3.10575+/-0.00010d and an eccentricity of0.352+/-0.011. Only weak pulsational signals are found in both theradial velocity and line-profile variations, but we have succeeded inshowing that the two highest-amplitude γ Doradus pulsation modesare consistent with l=1 and |m|=1. These two modes dominated our 280h ofBVIC multisite photometry, also obtained over two seasons. Wedetected altogether ten frequencies in the light variations, one in theδ Scuti regime and nine in the γ Doradus domain. Five of theγ Doradus frequencies are exact integer multiples of the orbitalfrequency. This observation leads us to suspect they are tidallyexcited. Attempts to identify modes from the multicolour photometryfailed. We performed model calculations and a stability analysis of thepulsations. The frequency range in which δ Scuti modes are excitedagrees well with observations. However, our models do not showexcitation of γ Doradus pulsations, although the damping issmaller in the observed range. We also investigated tidal excitation ofγ Doradus modes. Some of the observed harmonics of the orbitalperiod were found to be unstable. The observed orbital harmonics whichare stable in the models can be understood as linear combinations of theunstable modes. We could not detect the secondary component of thesystem in infrared photometry, suggesting that it may not be amain-sequence star. Archival data of this star show that it has a strongultraviolet (UV) excess, the origin of which is not known. The orbit ofthe primary is consistent with a secondary mass ofM>1.04Msolar, which is indicative of a neutron star,although a white dwarf companion is not ruled out.
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| קבוצת-כוכבים: | דג זהב |
| התרוממות ימנית: | 04h16m01.60s |
| סירוב: | -51°29'12.0" |
| גודל גלוי: | 4.25 |
| מרחק: | 20.3 פארסק |
| תנועה נכונה: | 101.5 |
| תנועה נכונה: | 184.7 |
| B-T magnitude: | 4.613 |
| V-T magnitude: | 4.285 |
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