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Asteroseismology of Delta Scuti and Gamma Doradus Stars
We give an overview of past and present efforts to make seismology ofδ Scuti and γ Doradus stars possible. Previous work has notled to the observational detection and identification of a sufficientnumber of pulsation modes for these pulsators for the construction ofunique seis-mic models. However, recent efforts including largeground-based obser-vational campaigns, work on pre-main sequencepulsators, asteroseismic satellite missions, theoretical advances onmode identification methods, and the discovery of a star showingsimultaneous self-excited δ Scuti and γ Doradus oscillationssuggest that we may be able to explore the interiors of these pulsatorsin the very near future.
| 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.
| On the Spectroscopic Nature of HD 221866
On the basis of a new classification-resolution spectrum, we find thatHD 221866, a member of the newly discovered γ Doradusvariable-star class, is an Am (metallic-line A-type) star. Observationswith the Cambridge radial-velocity spectrometer reveal that the star isa double-lined spectroscopic binary with a period of 135 days and a massratio of 1.11+/-0.03. We have determined the basic physical parametersof both components through spectral analysis of theclassification-resolution spectrum and a medium-resolution spectrum fromthe KPNO coudé-feed spectrograph in conjunction with fluxes fromvisible spectrophotometry and the TD-1 satellite. We confirm the resultsof Fekel et al. that the primary is the Am star, whereas the secondaryappears to be a normal early F-type dwarf. We reanalyze the time-seriesphotometric data for HD 221866 and confirm the existence of two periodsfound by Henry & Fekel.
| Multi-site, multi-technique survey of γ Doradus candidates. I. Spectroscopic results for 59 stars
We present the first results of a 2-year high-resolution spectroscopycampaign of 59 candidate γ Doradus stars which were mainlydiscovered from the HIPPARCOS astrometric mission. More than 60% of thestars present line profile variations which can be interpreted as due topulsation related to γ Doradus stars. For all stars we alsoderived the projected rotation velocity (up to more than 200 kms-1). The amplitude ratios 2K/Δ m for the mainHIPPARCOS frequency are in the range 35-96 kms-1,mag-1. About 50% of the candidates arepossible members of binary systems, with 20 stars being confirmedγ Doradus. At least 6 stars present composite spectra, and in allbut one case (for which only one spectrum could be obtained), the narrowcomponent shows line profile variations, pointing towards anuncomfortable situation if this narrow component originates from a shellsurrounding the star. This paper is the first of a series concerningmode identification using both photometric and spectroscopic methods forthe confirmed γ Doradus stars of the present sample.Partially based on observations obtained at the Observatoire deHaute-Provence.
| 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.
| 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.
| Spectroscopy of Early F Stars: γ Doradus Candidates and Possible Metallic Shell Stars
We obtained high-resolution spectroscopic observations of 34 γDoradus candidates. From the red-wavelength spectra, we determinedspectral classes, radial velocities, and projected rotationalvelocities. The spectra of seven late A or early F stars show metalliclines that have composite profiles consisting of a narrow component nearthe center of a broad line, indicating that they may be shell stars orbinaries. Several stars, including HD 152896, HD 173977, HD 175337, andHD 195068/9, show large line profile asymmetries. Two stars, HD 11443(=α Trianguli) and HD 149420, are ellipsoidal variables and notγ Doradus stars. The percentage of binary systems in our samplemay be as high as 74%.
| 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.
| 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.
| On the relationship between the δ Scuti and γ Doradus pulsators
We searched for δ Scuti-type pulsations amongst known andcandidate γ Doradus stars. The motivations for such a project comefrom the need to understand the relationship of these two classes ofpulsator better, from the present poor knowledge of the hot border ofthe γ Doradus phenomenon, and from the exciting prospects forasteroseismology should stars be found which have both types ofpulsation excited. We acquired 270h of observations and monitored atotal of 26 stars. One target, HD 209295, turned out to be a member ofboth classes of pulsating star, but this object is peculiar in the sensethat it is a close binary. We classify six of our targets as new bonafide γ Doradus stars, whereas nine more are good γ Doraduscandidates, and three turned out to be ellipsoidal variables. One of ourprogramme stars was found to be a δ Scuti star, with no additionalγ Doradus variations. Furthermore, one star was already known tobe a bona fide γ Doradus star, and we could not find anunambiguous explanation for the variability of five more stars. Theanalysis of our data together with improved knowledge of stars from theliterature enabled us to revise the blue border of the γ Doradusphenomenon towards cooler temperatures. This new blue edge is muchbetter defined than the previous one and extends from a temperature ofabout 7550K on the ZAMS to 7400K one magnitude above it. Five bona fideγ Doradus stars we observed are located inside the δ Scutiinstability strip, but none of them exhibited observable δ Scutipulsations. We therefore suggest that γ Doradus stars are lesslikely to be δ Scuti pulsators compared with other normal stars inthe same region of the lower instability strip. In addition, we showthat there is a clear separation between the pulsation constants Q ofδ Scuti and γ Doradus stars. The γ Doradus stars knownto date all have Q>0.23d.
| Driving the Gravity-Mode Pulsations in γ Doradus Variables
The γ Doradus stars are a newly discovered class of gravity-modepulsators that lie just at or beyond the red edge of the δ Scutiinstability strip. We present the results of calculations that thepredict pulsation instability of high-order g-modes with periods between0.4 and 3 days, as observed in these stars. The pulsations are driven bythe modulation of the radiative flux by convection at the base of a deepenvelope convection zone. Pulsation instability is predicted only formodels with temperatures at the convection zone base between ~200,000and ~480,000 K. The estimated shear dissipation that is due to turbulentviscosity within the convection zone or in an overshoot region below theconvection zone can be comparable to or even exceed the predicteddriving and is likely to reduce the number of unstable modes or possiblyquench the instability. Additional refinements in the pulsation modelingare required to determine the outcome. At least one γ Doradus starhas been observed that also pulsates in δ Scuti-type p-modes, andothers have been identified as chemically peculiar. Since our calculateddriving region is relatively deep, γ Doradus pulsations are notnecessarily incompatible with surface abundance peculiarities or withδ Scuti p-mode pulsations driven by the H and He ionizationκ-effect. Such stars will provide useful observational constraintson the proposed γ Doradus pulsation mechanism.
| New uvbybeta photometry of stars of ``astrophysical interest''
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| The domain of γ Doradus variables in the Hertzsprung-Russell diagram
70 new γ Doradus candidates were identified from Hipparcosphotometry, which represents more than a doubling of the presently knownnumber. Selecting the objects with good evidence for multiperiodicity,it is found that these stars, together with bona fide members of theclass, occupy a well-defined region in a colour-magnitude diagram. Thisdomain corresponds to a range of 7200-7700K on the zero-age mainsequence (ZAMS) and 6900-7500K one magnitude above it, which partlyoverlaps with the instability strip of δ Scuti stars. For thefirst time, γ Doradus stars can be discussed as a group. They canbe found over a significant fraction of the main sequence lifetimes forobjects in the relevant temperature range. An upper limit on the surfacemetallicity of γ Doradus stars is apparent, which may guide thesearch for their pulsation driving mechanism. The importance of possibleobjects exhibiting both γ Doradus and δ Scuti-typepulsations is discussed.
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