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|Discovery of magnetic fields in the βCephei star ξ1 CMa and in several slowly pulsating B stars*|
We present the results of a magnetic survey of a sample of eightβCephei stars and 26 slowly pulsating B (SPBs) stars with the FOcalReducer low dispersion Spectrograph at the Very Large Telescope. A weakmean longitudinal magnetic field of the order of a few hundred Gauss isdetected in the βCephei star ξ1CMa and in 13 SPBstars. The star ξ1CMa becomes the third magnetic staramong the βCephei stars. Before our study, the star ζCas wasthe only known magnetic SPB star. All magnetic SPB stars for which wegathered several magnetic field measurements show a field that varies intime. We do not find a relation between the evolution of the magneticfield with stellar age in our small sample. Our observations imply thatβCephei and SPB stars can no longer be considered as classes ofnon-magnetic pulsators, but the effect of the fields on the oscillationproperties remains to be studied.
|Testing Rotational Mixing Predictions with New Boron Abundances in Main-Sequence B-Type Stars|
New boron abundances for seven main-sequence B-type stars are determinedfrom HST STIS spectroscopy around the B III 2066 Å line. Boronabundances provide a unique and critical test of stellar evolutionmodels that include rotational mixing, since boron is destroyed in thesurface layers of stars through shallow mixing long before otherelements are mixed from the stellar interior through deep mixing. Thestars in this study are all on or near the main sequence and are membersof young Galactic clusters. They show no evidence of mixing with gasfrom H-burning layers from their CNO abundances. Boron abundances rangefrom 12+log(B/H)<=1.0 to 2.2. The boron abundances are compared tothe published values of the stellar nitrogen abundances [all have12+log(N/H)<=7.8] and to their host cluster ages (4-16 Myr) toinvestigate the predictions from models of massive star evolution withrotational mixing effects. We find that the variations in boron andnitrogen are generally within the range of the predictions from thestellar evolution models with rotation (where predictions for modelswith rotation rates from 0 to 450 km s-1 and μ-barriersare examined), especially given their age and mass ranges. Three stars(of 34 B-type stars with detailed boron abundance determinations)deviate from the model predictions, showing either much larger borondepletions than can be explained by the rotating model predictions or aspectroscopic mass that is lower than expected, given the rotating modelpredictions for its age and abundances. The results from these threestars suggest that rotational mixing could be more efficient than thatcurrently modeled at the highest rotation rates.
|Abundance analysis of prime B-type targets for asteroseismology. I. Nitrogen excess in slowly-rotating β Cephei stars|
Seismic modelling of the β Cephei stars promises major advances inour understanding of the physics of early B-type stars on (or close to)the main sequence. However, a precise knowledge of their physicalparameters and metallicity is a prerequisite for correct modeidentification and inferences regarding their internal structure. Herewe present the results of a detailed NLTE abundance study of nine primetargets for theoretical modelling: γ Peg,δ Cet, ν Eri,β CMa, ξ1 CMa,V836 Cen, V2052 Oph,β Cep and DD (12) Lac (hereafter 12Lac). The following chemical elements are considered: He, C,N, O, Mg, Al, Si, S and Fe. Our curve-of-growth abundance analysis isbased on a large number of time-resolved, high-resolution opticalspectra covering in most cases the entire oscillation cycle of thestars. Nitrogen is found to be enhanced by up to 0.6 dex in four stars,three of which have severe constraints on their equatorial rotationalvelocity, Ω R, from seismic or line-profile variation studies:β Cep (Ω R ~ 26 km s-1),V2052 Oph (Ω R ~ 56 km s-1),δ Cet (Ω R < 28 km s-1) andξ1 CMa (Ω R sin i 10 kms-1). The existence of core-processed material at the surfaceof such largely unevolved, slowly-rotating objects is not predicted bycurrent evolutionary models including rotation. We draw attention to thefact that three stars in this subsample have a detected magnetic fieldand briefly discuss recent theoretical work pointing to the occurrenceof diffusion effects in β Cephei stars possibly capable of alteringthe nitrogen surface abundance. On the other hand, the abundances of allthe other chemical elements considered are, within the errors,indistinguishable from the values found for OB dwarfs in the solarneighbourhood. Despite the mild nitrogen excess observed in someobjects, we thus find no evidence for a significantly higherphotospheric metal content in the studied β Cephei stars comparedto non-pulsating B-type stars of similar characteristics.
|Attempts to measure the magnetic field of the pulsating B star ν Eridani|
We report on attempts to measure the magnetic field of the pulsating Bstar ν Eridani with the Musicos spectropolarimeter attached to the 2m telescope at the Pic du Midi, France. This object is one of the mostextensively studied stars for pulsation modes, and the existence of amagnetic field was suggested from the inequality of the frequencyseparations of a triplet in the stars' oscillation spectrum. We showthat the inferred 5-10 kG field was not present during our observations,which cover about one year. We discuss the influence of the strongpulsations on the analysis of the magnetic field strength and set anupper limit to the effective longitudinal field strength and to thefield strength for a dipolar configuration. We also find that theobserved wind line variability is caused by the pulsations.
|A high-resolution spectroscopy survey of β Cephei pulsations in bright stars|
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
|The magnetic field and confined wind of the O star θ1 Orionis C|
Aims.In this paper we confirm the presence of a globally-ordered,kG-strength magnetic field in the photosphere of the young O starθ1 Orionis C, and examine the properties of itsoptical line profile variations. Methods: .A new series ofhigh-resolution MuSiCoS Stokes V and I spectra has been acquired whichsamples approximately uniformly the rotational cycle ofθ1 Orionis C. Using the Least-Squares Deconvolution(LSD) multiline technique, we have succeeded in detecting variableStokes V Zeeman signatures associated with the LSD mean line profile.These signatures have been modeled to determine the magnetic fieldgeometry. We have furthermore examined the profile variations of linesformed in both the wind and photosphere using dynamic spectra.Results: .Based on spectrum synthesis fitting of the LSD profiles, wedetermine that the polar strength of the magnetic dipole component is1150 B_d 1800 G and that the magnetic obliquity is 27° β 68°, assuming i=45± 20°. The best-fitvalues for i=45° are Bd = 1300 ± 150~G~ (1σ)and β = 50° ± 6° (1σ). Our data confirm theprevious detection of a magnetic field in this star, and furthermoredemonstrate the sinusoidal variability of the longitudinal field andaccurately determine the phases and intensities of the magnetic extrema.The analysis of "photospheric" and "wind" line profile variationssupports previous reports of the optical spectroscopic characteristics,and provides evidence for infall of material within the magneticequatorial plane.
|Observations of Hα, iron, and oxygen lines in B, Be, and shell stars|
We carried out a spectroscopic survey of several B, Be, and shell starsin optical and near-infrared regions. Line profiles of the Hα lineand of selected Fe II and O I lines are presented.
|Evolution of interacting binaries with a B type primary at birth|
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
|The chemical composition of δ Scuti*|
We present chemical abundances in the photosphere of δ Scuti(δ Sct) - a prototype of the class of pulsating variables -determined from the analysis of a spectrum obtained by using the 2-mtelescope at the Peak Terskol Observatory and a high-resolutionspectrometer with R= 52000, a signal-to-noise ratio 250 and fromInternational Ultraviolet Explorer spectra. The abundance pattern ofδ Sct consists of 49 chemical elements. The abundances of 19elements have not been investigated previously. The abundances of Pr andNd obtained from the lines of the second and third spectra are equal.The abundances of heavy elements indicate the overabundances withrespect to the solar system values up to 1 dex. The abundance pattern ofδ Sct is similar to the abundance patterns of Am-Fm or δ Deltype stars.A splitting of the cores of all clean lines is observed for the spectraof δ Sct and HD 57749. This can signify evidence of non-radialpulsations in these stars.
|A rigidly rotating magnetosphere model for circumstellar emission from magnetic OB stars|
We present a semi-analytical approach for modelling circumstellaremission from rotating hot stars with a strong dipole magnetic fieldtilted at an arbitrary angle to the rotation axis. By assuming therigid-field limit in which material driven (e.g. in a wind outflow) fromthe star is forced to remain in strict rigid-body corotation, we areable to solve for the effective centrifugal-plus-gravitational potentialalong each field line, and thereby identify the location of potentialminima where material is prone to accumulate. Applying basic scalingsfor the surface mass flux of a radiatively driven stellar wind, wecalculate the circumstellar density distribution that obtains onceejected plasma settles into hydrostatic stratification along fieldlines. The resulting accumulation surface resembles a rigidly rotating,warped disc, tilted such that its average surface normal lies betweenthe rotation and magnetic axes. Using a simple model of the plasmaemissivity, we calculate time-resolved synthetic line spectra for thedisc. Initial comparisons show an encouraging level of correspondencewith the observed rotational phase variations of Balmer-line emissionprofiles from magnetic Bp stars such as σ Ori E.
|Dynamo-generated magnetic fields at the surface of a massive star|
Spruit has shown that an astrophysical dynamo can operate in thenon-convective material of a differentially rotating star as a result ofa particular instability in the magnetic field (the Tayler instability).By assuming that the dynamo operates in a state of marginal instability,Spruit has obtained formulae which predict the equilibrium strengths ofazimuthal and radial field components in terms of local physicalquantities. Here, we apply Spruit's formulae to our previously publishedmodels of rotating massive stars in order to estimate Tayler dynamofield strengths. There are no free parameters in Spruit's formulae. Inour models of 10- and 50-Msolar stars on the zero-age mainsequence, we find internal azimuthal fields of up to 1 MG, and internalradial components of a few kG. Evolved models contain weaker fields. Inorder to obtain estimates of the field strength at the stellar surface,we examine the conditions under which the Tayler dynamo fields aresubject to magnetic buoyancy. We find that conditions for Taylerinstability overlap with those for buoyancy at intermediate to highmagnetic latitudes. This suggests that fields emerge at the surface of amassive star between magnetic latitudes of about 45° and the poles.We attempt to estimate the strength of the field which emerges at thesurface of a massive star. Although these estimates are very rough, wefind that the surface field strengths overlap with values which havebeen reported recently for line-of-sight fields in several O and Bstars.
|Meridional Circulation in Young Massive Stars with Laminar Rotation|
We study the rotation of a chemically homogeneous star with a mass of16M ȯ, assuming that the angular-momentum distribution in itsradiative envelope is determined by hydrodynamical processesflowsand turbulent diffusion. Meridional circulation and horizontal shearturbulence are the main hydrodynamical processes forming the radialdistribution of the angular momentum in young massive stars in theabsence of magnetic fields. The rotation of such stars is close tosteady-state. The angular velocity of rotation of the convective corecan be 5 20% higher than the surface value. Under these conditions,the characteristic time for the radial transport of angular momentum bymeridional flows and shear turbulence is comparable to the nuclear timescale.
|Observed Orbital Eccentricities|
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
|B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?|
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
|The orbit of the close spectroscopic binary \varepsilon Lup and the intrinsic variability of its early B-type components|
We subjected 106 new high-resolution spectra of the double-linedspectroscopic close binary \varepsilon Lup, obtained in a time-span of17 days from two different observatories, to a detailed study of orbitaland intrinsic variations. We derived accurate values of the orbitalparameters. We refined the sidereal orbital period to 4.55970d days andthe eccentricity to e=0.277. By adding old radial velocities, wediscovered the presence of apsidal motion with a period of the rotationof apses of about 430 years. Such a value agrees with theoreticalexpectations. Additional data is needed to confirm and refine thisvalue. Our dataset did not allow us to derive the orbit of the thirdbody, which is known to orbit the close system in 64 years. We presentthe secondary of \varepsilon Lup as a new β Cephei variable, whilethe primary is a β Cephei suspect. A first detailed analysis ofline-profile variations of both primary and secondary led to detectionof one pulsation frequency near 10.36 c d-1 in thevariability of the secondary, while no clear periodicity was found inthe primary, although low-amplitude periodicities are still suspected.The limited accuracy and extent of our dataset did not allow any furtheranalysis, such as mode-identification.
|A Be star with a low nitrogen abundance in the SMC cluster NGC 330|
High-resolution UVES/VLT spectra of B 12, an extreme pole-on Be star inthe SMC cluster NGC 330, have been analysed using non-LTE modelatmospheres to obtain its chemical composition relative to the SMCstandard star AV 304. We find a general underabundance of metals whichcan be understood in terms of an extra contribution to the stellarcontinuum due to emission from a disk which we estimate to be at the~25% level. When this is corrected for, the nitrogen abundance for B 12shows no evidence of enhancement by rotational mixing as has been foundin other non-Be B-type stars in NGC 330, and is inconsistent withevolutionary models which include the effects of rotational mixing. Asecond Be star, NGC 330-B 17, is also shown to have no detectablenitrogen lines. Possible explanations for the lack of rotational mixingin these rapidly rotating stars are discussed, one promising solutionbeing the possibility that magnetic fields might inhibit rotationalmixing.
|XMM-Newton observations of β Centauri (B1 III): The temperature structure in the hot plasma and the photosphere-wind connection|
We present XMM-Newton observations of the B giant β Centauri (B1III). The spectra are rich of spectral lines from a wide range ofionization stages which indicate temperatures in the range ~0.1-0.6 keV.Simultaneous fits to the rgs, epic-mos, and epic-pn spectra yield threeplasma temperatures (0.1, 0.2, and 0.6 keV), emission measures, andelemental abundances which are quite close to solar values. Thesetemperatures are confirmed by DEM modeling. According to the derivedmodels the intrinsic source X-ray luminosity in the energy range 0.3-10keV is Lx = 10 × 1030 erg s-1 ata distance of 161 pc. An analysis of the X-ray light curve suggests thatthe photospheric variability does not have much of an effect on theproperties of the X-ray luminosity. The sensitivity of the He-likeforbidden and intercombination lines to a strong ultraviolet stellarradiation field is used to constrain the radial distances at which thelines of Ne ix, O vii, and N vi originate.
|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.
|Metallicity of mono- and multiperiodic β Cephei stars|
Analyzing IUE ultraviolet spectra of β Cep pulsating stars wenoticed that multiperiodic variables have a larger mean metal abundancein the photosphere, [ m/H] , than monoperiodic ones. We applystatistical tests to verify this dichotomy. We obtain that, with a largeprobability, the multiperiodic β Cep stars have greater values of [m/H] . This result is consistent with the linear non-adiabatic theory ofpulsation of early B-type stars.
|Metallicities of the β Cephei stars from low-resolution ultraviolet spectra|
We derive basic stellar parameters (angular diameters, effectivetemperatures, metallicities) and interstellar reddening for all βCephei stars observed during the IUE satellite mission, including thosebelonging to three open clusters. The parameters are derived by means ofan algorithmic procedure of fitting theoretical flux distributions tothe low-resolution IUE spectra and ground-based spectrophotometricobservations. Since the metallicity has a special importance forpulsating B-type stars, we focus our attention in particular on thisparameter.Tables 1, 2, 4 and 5 are only available in electronic form athttp://www.edpsciences.org
|PSST: The Planet Search Survey Telescope|
The Planet Search Survey Telescope is an automated small-aperture CCDimaging photometer designed to search for transits by extrasolar planetsacross the disks of their parent stars. It simultaneously observesthousands of stars with apparent R magnitudes between 10 and 13 in afield approximately 6deg×6deg. Stars in thisbrightness range are well within the capability of the high-precisionradial velocity systems that have successfully detected over 100extrasolar planets to date. The combination of the photometric transitdepth and radial velocity amplitude can provide both the radius of theplanet and a good estimate of its mass, since the orbit is nearlyedge-on. As a result, estimates of the planet's density and otherparameters can be obtained.
|Progress in study on the evolution of massive stars|
Due to their high luminosity and high mass-loss rates, massive stars areplaying an important role in the studies of modern astrophysics, such asthe formation and the evolution of galaxies. Since the middle of thetwentieth century, we have been cognizant of the main characters of thestructure and evolution of massive stars approximately and someevolution models have been constructed. However, in recent years, someobservations on massive stars indicate that there are seriousdiscrepancies between observations and theoretical models. Thesediscrepancies show us many defects in present theoretical models dealingsome leading factors in the evolution of massive stars, such as massloss and convection. Current research o these problems is reviewed andfuture perspectives are proposed in this paper.
|Large-scale wind structures in OB supergiants: a search for rotationally modulated Hα variability|
We present the results of a long-term monitoring campaign of theHα line in a sample of bright OB supergiants (O7.5-B9) which aimsat detecting rotationally modulated changes potentially related to theexistence of large-scale wind structures. A total of 22 objects weremonitored during 36 nights spread over six months in 2001-2002.Coordinated broad-band photometric observations were also obtained forsome targets. Conspicuous evidence for variability in Hα is foundfor the stars displaying a feature contaminated by wind emission. Mostchanges take place on a daily time-scale, although hourly variations arealso occasionally detected. Convincing evidence for a cyclical patternof variability in Hα has been found in two stars: HD 14134 and HD42087. Periodic signals are also detected in other stars, butindependent confirmation is required. Rotational modulation is suggestedfrom the similarity between the observed recurrence time-scales (in therange 13-25 d) and estimated periods of stellar rotation. We callattention to the atypical case of HD 14134, which exhibits a clear12.8-d periodicity, both in the photometric and in the spectroscopicdata sets. This places this object among a handful of early-type starswhere one may observe a clear link between extended wind structures andphotospheric disturbances. Further modelling may test the hypothesisthat azimuthally-extended wind streams are responsible for the patternsof spectral variability in our target stars.
|Magnetic fields in massive stars: dynamics and origin|
Evidence continues to accumulate in favour of the presence of magneticfields on the surfaces of massive stars. Some authors hypothesize thatsuch fields originate in a dynamo in the convective core, with buoyancybringing flux tubes to the surface. Here we show that, when realisticstellar models are used, this `core dynamo hypothesis' encounters aserious difficulty: in order for surface fields to originate in a coredynamo, the core must create magnetic fields that are much stronger thanequipartition values.As an alternative hypothesis for the origin of magnetic fields in OBstars, we suggest that a dynamo is operating in shear-unstable gas inthe radiative stellar envelope. Using a recently developed code for theevolution of rotating stars, we find that in a 10-Msolarzero-age main sequence (ZAMS) star with a typical rotation period, morethan 90 per cent of the volume of the star is subject to shearinstability. We have recently proposed that dynamo operation inshear-unstable gas helps to explain certain properties of mass loss incool giants. Here, in the context of hot stars, we show that fieldswhich originate in the shear-unstable regions of the envelope can reachthe surface without violating the constraints of equipartition. Suchfields rise to the surface on time-scales which are short compared tomain-sequence lifetimes.
|δ Sct-type pulsations in eclipsing binary systems: RZ Cas|
We present the results of a three-continent multisite photometriccampaign carried out on the Algol-type eclipsing binary system RZ Cas,in which the primary component has recently been discovered to be aδ Sct-type pulsator. The present observations include, for thefirst time, complete simultaneous Strömgren uvby light curvestogether with a few Crawford Hβ data collected around the orbitalphase of the first quadrature. The new observations confirm thepulsational behaviour of the primary component. A detailed photometricanalysis, based on these observations, is presented for both binarityand pulsation. The results indicate a semidetached system where thesecondary fills its Roche lobe. The appearance of the light curvesreveals the presence of the mass stream from the secondary component anda hotspot where this stream impacts on the surface of the primary star.There are also some indications of chromospheric activity in thesecondary. On the other hand, the pulsational behaviour out-of-primaryeclipse can be well described with only one frequency at 64.1935cd-1 similar to the main peak found by Ohshima et al. Theexistence of multiperiodicity is not confirmed in our data. Concerningthe mode identification, our results indicate non-radial pulsation in ahigh radial order (n= 6), with l= 2, |m|= 1, 2 as the most suitable.However, additional effects must be taken into account in thepredictions. Moreover, the pulsation amplitude in the u band is largerthan in b and v, which is unusual among the δ Sct-type variables.This can be explained as due to pulsation in a high n value and close tothe blue edge of the δ Sct region. On the other hand, the earlydata of Ohshima et al. have also been analysed and similar results arefound concerning the frequency content and pulsational amplitude.Finally, a revision of all the photometric out-of-primary-eclipse datasets available in the literature is made together with some additionalunpublished data leading to interesting findings relative to changestaking place in the pulsation amplitudes and frequencies from season toseason. Furthermore, multiperiodicity is probably present in someepochs.
|Asteroseismology of the β Cephei star ν Eridani - II. Spectroscopic observations and pulsational frequency analysis|
We undertook a multisite spectroscopic campaign for the β Cepheistar ν Eridani. A total of 2294 high-resolution spectra were obtainedfrom telescopes at 11 different observatories around the world. The timebase of dedicated multisite observations is 88 d. To this data set wehave added 148 older, previously unpublished spectra, such that theoverall time-span of the 2442 spectra is 430 d. The analysis of theradial velocity variations derived from the SiIII triplet centred on4560Å leads to 19 significant frequencies, of which sevencorrespond to independent pulsation frequencies. Five of these aremembers of multiplets with an average spacing of 0.018 +/- 0.002cd-1. Our spectroscopic results agree well with those derivedfrom a simultaneous multisite photometric campaign of the star, albeitthat we do not recover their low frequency at 0.43218 cd-1.We find three different candidate frequencies below 1 cd-1instead. We also find that the radial velocity amplitude of the mainmode has increased by some 30 per cent over the last 15 years, which isconsistent with the photometry data. We derive a relative equivalentwidth variation of 6.5 per cent, which is completely dominated by themain radial mode. The phase difference between the radial velocity andlight variations for the main frequency is , which is clearly deviantfrom the adiabatic value and confirms the radial nature of the dominantmode. The spectral line broadening leads to an upper limit of 20 kms-1 for vsini, which is consistent with the long rotationperiod derived from the frequency splittings.
|The Indo-US Library of Coudé Feed Stellar Spectra|
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
|Tidal Effects in Binaries of Various Periods|
We found in the published literature the rotational velocities for 162B0-B9.5, 152 A0-A5, and 86 A6-F0 stars, all of luminosity classes V orIV, that are in spectroscopic or visual binaries with known orbitalelements. The data show that stars in binaries with periods of less thanabout 4 days have synchronized rotational and orbital motions. Stars inbinaries with periods of more than about 500 days have the samerotational velocities as single stars. However, the primaries inbinaries with periods of between 4 and 500 days have substantiallysmaller rotational velocities than single stars, implying that they havelost one-third to two-thirds of their angular momentum, presumablybecause of tidal interactions. The angular momentum losses increase withdecreasing binary separations or periods and increase with increasingage or decreasing mass.
|Synthetic photometry for non-radial pulsations in subdwarf B stars|
We describe a method for computing theoretical photometric amplituderatios for a number of modes of nonradially pulsating subdwarf B starsin both SDSS and UBVR systems. In order to avoid costly solutions of thenon-adiabatic non-radial pulsation equations, we have adopted theadiabatic approximation. We argue that this is a valid approach, atleast for the V361 Hya stars, because observations show that thetemperature perturbations dominate the radius perturbations in the fluxvariation. We find that for V361 Hya stars, low-degree (ℓ=0,1,2)modes may be difficult to distinguish using optical photometry. However,the high degree modes (ℓ= 3,4) are relatively well separated and maybe distinguished more easily. We have also computed the amplitude ratiosfor a number of modes in PG 1716+426 stars. For these stars, theamplitude ratios for low degree modes (ℓ=0,1) are well resolved. Foroscillations with periods 40 min, higher-degree modes (ℓ=2-4)may also be identified easily from their amplitude ratios. However forlonger period oscillations, the ℓ=3 and the ℓ=2,4 modes approachthe ℓ=0 and ℓ=1 modes respectively.
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