Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Photometric studies of three multiperiodic β Cephei stars: β CMa, 15 CMa and KZ Mus
We have carried out single and multisite photometry of the three βCephei stars β and 15 CMa as well as KZ Mus. For the two stars inCMa, we obtained 270h of measurement in the Strömgren uvy andJohnson V filters, while 150h of time-resolved Strömgren uvyphotometry was acquired for KZ Mus. All three stars are multiperiodicvariables, with three (β CMa) and four (15 CMa, KZ Mus) independentpulsation modes. Two of the mode frequencies of 15 CMa are newdiscoveries and one of the known modes showed amplitude variations overthe last 33yr. Taken together, this fully explains the diverse behaviourof the star reported in the literature.Mode identification by means of the amplitude ratios in the differentpassbands suggests one radial mode for each star. In addition, βCMa has a dominant l = 2 mode while its third mode is non-radial withunknown l. The non-radial modes of 15 CMa, which are l <= 3, form analmost equally split triplet that, if physical, would imply that we seethe star under an inclination angle larger than 55°. The strongestnon-radial mode of KZ Mus is l = 2, followed by the radial mode and adipole mode. Its weakest known mode is non-radial with unknown l,confirming previous mode identifications for the pulsations of the star.The phased light curve for the strongest mode of 15 CMa has a descendingbranch steeper than the rising branch. A stillstand phenomenon duringthe rise to maximum light is indicated. Given the low photometricamplitude of this non-radial mode this is at first sight surprising, butit can be explained by the aspect angle of the mode.
| Asteroseismology of the β Cephei star 12 (DD) Lacertae: photometric observations, pulsational frequency analysis and mode identification
We report a multisite photometric campaign for the β Cephei star 12Lacertae. 750 h of high-quality differential photoelectricStrömgren, Johnson and Geneva time-series photometry were obtainedwith nine telescopes during 190 nights. Our frequency analysis resultsin the detection of 23 sinusoidal signals in the light curves. Ten ofthose correspond to independent pulsation modes, and the remainder arecombination frequencies. We find some slow aperiodic variability such asthat seemingly present in several β Cephei stars. We perform modeidentification from our colour photometry, derive the spherical degree lfor the five strongest modes unambiguously and provide constraints on lfor the weaker modes. We find a mixture of modes of 0 <=l<= 4. Inparticular, we prove that the previously suspected rotationally splittriplet within the modes of 12 Lac consists of modes of different ltheir equal frequency splitting must thus be accidental.One of the periodic signals we detected in the light curves is argued tobe a linearly stable mode excited to visible amplitude by non-linearmode coupling via a 2:1 resonance. We also find a low-frequency signalin the light variations whose physical nature is unclear; it could be aparent or daughter mode resonantly coupled. The remaining combinationfrequencies are consistent with simple light-curve distortions.The range of excited pulsation frequencies of 12 Lac may be sufficientlylarge that it cannot be reproduced by standard models. We suspect thatthe star has a larger metal abundance in the pulsational driving zone, ahypothesis also capable of explaining the presence of β Cepheistars in the Large Magellanic Cloud.
| δ Ceti Is Not Monoperiodic: Seismic Modeling of a β Cephei Star from MOST Space-based Photometry
The β Cephei star δ Ceti was considered one of the fewmonoperiodic variables in its class. Despite (or perhaps because of) itsapparently simple oscillation spectrum, it has been challenging andcontroversial to identify this star's pulsation mode and constrain itsphysical parameters seismically. Broadband time-resolved photometry ofδ Ceti spanning 18.7 days with a duty cycle of about 65% obtainedby the Microvariability and Oscillations of Stars (MOST) satellite-thefirst scientific observations ever obtained by MOST-reveals that thestar is actually multiperiodic. Besides the well-known dominantfrequency of f1=6.205886 day-1, we have discoveredin the MOST data its first harmonic 2f1 and three otherfrequencies (f2=3.737, f3=3.673, andf4=0.318 day-1), all detected with asignal-to-noise ratio (S/N)>4. In retrospect, f2 was alsopresent in archival spectral line-profile data but at lower S/N. Wepresent seismic models whose modes match exactly the frequenciesf1 and f2. Only one model falls within the commonpart of the error boxes of the star's observed surface gravity andeffective temperature from photometry and spectroscopy. In this model,f1 is the radial (l=0) first overtone, and f2 isthe g2 (l=2, m=0) mode. This model has a mass of 10.2+/-0.2Msolar and an age of 17.9+/-0.3 Myr, making δ Ceti anevolved β Cephei star. If f2 and f3 arerotationally split components of the same g2 mode, then thestar's equatorial rotation velocity is either 27.6 km s-1 orhalf this value. Given its vsini of about 1 km s-1, thisimplies that we are seeing δ Ceti nearly pole-on.Based on data from the MOST satellite, a Canadian Space Agency mission,jointly operated by Dynacon Inc., the University of Toronto Institutefor Aerospace Studies, and the University of British Columbia, with theassistance of the University of Vienna.
| 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.
| 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.
| Multiperiodicity in the large-amplitude rapidly-rotating β Cephei star HD 203664
Aims.We made a seismic study of the young massive β Cephei star HD203664 with the goal of constraining its interior structure.Methods.Ourstudy is based on a time series of 328 new Geneva 7-colour photometricdata of the star spread over 496.8 days.Results.The data confirm thefrequency of the dominant mode of the star, which we refined tof_1=6.02885 c d-1. The mode has a large amplitude of 37 mmagin V and is unambiguously identified as a dipole mode (&ea;=2) from itsamplitude ratios and non-adiabatic computations. Besides f_1, wediscovered two additional new frequencies in the star with amplitudesabove 4σ: f_2=6.82902 c d-1 and f_3=4.81543 cd-1, or one of their daily aliases. The amplitudes of thesetwo modes are only between 3 and 4 mmag, which explains why they werenot detected before. Their amplitude ratios are too uncertain for modeidentification. Conclusions.We show that the observed oscillationspectrum of HD 203664 is compatible with standard stellar models butthat we have insufficient information for asteroseismic inferences.Among the large-amplitude β Cephei stars, HD 203664 stands out asthe only one rotating at a significant fraction of its critical rotationvelocity (~ 40%).
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| 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.
| 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
| 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.
| Asteroseismology of the β Cephei star ν Eridani - I. Photometric observations and pulsational frequency analysis
We undertook a multisite photometric campaign for the β Cephei starν Eridani. More than 600 h of differential photoelectric uvyVphotometry were obtained with 11 telescopes during 148 clear nights.The frequency analysis of our measurements shows that the variability ofν Eri can be decomposed into 23 sinusoidal components, eight of whichcorrespond to independent pulsation frequencies between 5 and 8cd-1. Some of these are arranged in multiplets, whichsuggests rotational m-mode splitting of non-radial pulsation modes asthe cause. If so, the rotation period of the star must be between 30 and60 d.One of the signals in the light curves of ν Eri has a very lowfrequency of 0.432 cd-1. It can be a high-order combinationfrequency or, more likely, an independent pulsation mode. In the lattercase, ν Eri would be both a β Cephei star and a slowly pulsatingB (SPB) star.The photometric amplitudes of the individual pulsation modes of ν Eriappear to have increased by about 20 per cent over the last 40 years. Sohave the amplitudes of the dominant combination frequencies of the star.Among the latter, we could only identify sum frequencies with certainty,not difference frequencies, which suggests that neither light-curvedistortion in its simplest form nor resonant mode coupling is theirsingle cause.One of our comparison stars, μ Eridani, turned out to be variablewith a dominant time-scale of 1.62 d. We believe either that it is anSPB star just leaving its instability strip or that its variations areof rotational origin.
| 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.
| Asteroseismology of the β Cep star HD 129929. II. Seismic constraints on core overshooting, internal rotation and stellar parameters
We perform a detailed seismic study of the β Cep star HD 129929.Our analysis is based on the recent derivation of six pulsationfrequencies. These frequencies are unambiguously identified from theseismic modelling and the photometric amplitudes to be the radialfundamental, the ℓ=1, p1 triplet, two consecutivecomponents of the ℓ=2, g1 quintuplet. A non-adiabaticanalysis allows us to constrain the metallicity of the star to Z \in[0.016, 0.022]. In addition, the fitting of three independentfrequencies, two of which correspond to axisymmetric (m=0) modes, allowsus to constrain the core overshooting parameter toαov=0.10 ± 0.05, as well as the other globalparameters of the star. Finally, from the observation of the ℓ=1triplet and part of the ℓ=2 quintuplet, we derive constraints on theinternal rotation of this star.
| β Cep stars from a spectroscopic point of view
In this review we present the current status of line-profile-variationstudies of β Cep stars. Such studies have been performed for 26bright members of this class of pulsating stars in the past 25 years. Wedescribe all these currently available data and summarize theinterpretations based on them in terms of the excited pulsation modes.We emphasize that line-profile variations offer a much more detailedpicture of the pulsational behaviour of pulsating stars compared toground-based photometric data. The latter, however, remain necessary tounravel the often complex frequency pattern and to achieve unambiguousmode identification for multiperiodic β Cep stars and also toderive the pulsational properties of the faint members of the class. Wehighlight the statistical properties of the sample of 26 stars for whichaccurate spectroscopic studies are available and point out some futureprospects.
| Statistics of the Instability Strip of β Cephei Stars
We present a study of the β Cephei instability strip based on asample of 49 stars of this type. After deriving their effectivetemperatures and luminosities from their observed (B-V), (U-B) colorsand parallaxes we find their positions in the HR diagram to be mostlyconfined to the main sequence, and their masses to lie between 7Mȯ and 30 Mȯ. Their distribution on theHR diagram matches well with our previous theoretical instability stripwhich has an upper bound in the luminosity and rather tight boundariesin the effective temperature.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Boron Abundances in B-Type Stars: A Test of Rotational Depletion during Main-Sequence Evolution
Boron abundances have been derived for seven main-sequence B-type starsfrom Hubble Space Telescope STIS spectra around the B III λ2066line. In two stars, boron appears to be undepleted with respect to thepresumed initial abundance. In one star, boron is detectable but isclearly depleted. In the other four stars, boron is undetectable,implying depletions of 1-2 dex. Three of these four stars are nitrogenenriched, but the fourth shows no enrichment of nitrogen. Onlyrotationally induced mixing predicts that boron depletions areunaccompanied by nitrogen enrichments. The inferred rate of borondepletion from our observations is in good agreement with thesepredictions. Other boron-depleted nitrogen-normal stars are identifiedfrom the literature. In addition, several boron-depleted nitrogen-richstars are identified, and while all fall on the boron-nitrogen trendpredicted by rotationally induced mixing, a majority have nitrogenenrichments that are not uniquely explained by rotation. The spectrahave also been used to determine iron group (Cr, Mn, Fe, and Ni)abundances. The seven B-type stars have near-solar iron groupabundances, as expected for young stars in the solar neighborhood. Wehave also analyzed the halo B-type star PG 0832+676. We find[Fe/H]=-0.88+/-0.10, and the absence of the B III line gives the upperlimit [B/H]<-2.5. These and other published abundances are used toinfer the star's evolutionary status as a post-asymptotic giant branchstar. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal GO 07400.
| Multiperiodicity in the light variations of the beta Cephei star beta Crucis
High-resolution spectroscopic observations have led recently to thediscovery that the beta Cephei star beta Crucis (Mimosa) ismultiperiodic with at least three non-radial pulsation modes. Dataobtained by the star tracker of the WIRE satellite have now allowed usto confirm this multiperiodicity in the light variations. A total of 5million observations covering 17 days was analysed and the three mainperiods we find in this work are in perfect agreement with the resultsderived from the line profile variations. The photometric amplitudes aresmall (3, 2.7 and 0.6 millimag for the dominant modes), but this is notsurprising in view of the mode identifications derived earlier from theline profiles. Additional periods of low-amplitude modes (between0.2-0.3 mmag) are also derived, including one suggested earlier by theradial velocity data.
| Behaviour of the periods in double mode RR Lyrae, V26 in M15
The changes in the periods of the fundamental and first overtonepulsation of the variable V26 in the globular cluster M15 are studied.The analysis was based on published observations. The method ofinvestigation was outlined in our previous paper (Paparó et al.\cite{pap98}). The Fourier phase diagrams for both frequencies have beenconstructed. General guidelines concerning the cycle counting problemsare discussed. In both periods a sudden increase took place around JD2431500 (~1945) and the changes were the same in sign but slightlydifferent in magnitude. The period change behaviour of the fundamentalmode was noticed for the first time. Before the abrupt change thefundamental and first overtone periods were P0 = 0fd538898and P1 = 0fd402256 days and after it P0 =0fd538979 and P1 = 0fd402273 days. Although the fundamentalcomponent is very weak, there is a remarkable concordance between ourperiod after the abrupt change and the period derived by Clement &Walker (\cite{cle91}). The disagreement between the observed periodchanges and the expectation of stellar evolutionary theory stronglysuggest that period changes are dominated by short term changes governedby the physics of the star. Fast changes in oscillation frequency suchas in V26/M15 cannot take place without sudden changes in the stellarstructure. The possible physical processes are the mixing events in asmall part of the semiconvective zone (Sweigart & Renzini\cite{sr79}) or the dredge up of a small amount of helium by occasionalconvection overshooting (Cox \cite{cox98}).
| Multiperiodicities from the Hipparcos epoch photometry and possible pulsation in early A-type stars
A selection criterion based on the relative strength of the largestpeaks in the amplitude spectra, and an information criterion are used incombination to search for multiperiodicities in Hipparcos epochphotometry. The method is applied to all stars which have beenclassified as variable in the Hipparcos catalogue: periodic, unsolvedand microvariables. Results are assessed critically: although there aremany problems arising from aliasing, there are also a number ofinteresting frequency combinations which deserve further investigation.One such result is the possible occurrence of multiple periods of theorder of a day in a few early A-type stars. The Hipparcos catalogue alsocontains a number of these stars with single periodicities: such starswith no obvious variability classifications are listed, and informationabout their properties (e.g., radial velocity variations) discussed.These stars may constitute a new class of pulsators.
| Ultraviolet Spectrophotometry of Variable Early-Type Be and B Stars Derived from High-Resolution IUE Data
High-dispersion IUE data encode significant information about aggregateline absorptions that cannot be conveniently extracted from individualstellar spectra. Here we apply a new technique in which fluxes from eachechelle order of a short-wavelength IUE spectrum are binned together toconstruct low-resolution spectra of a rapidly varying B or Be star. Thedivision of binned spectra obtained during a ``bright-star'' phase byspectra from a ``faint-star'' phase leads to a ratioed spectrum thatcontains information about the mechanism responsible for a star'svariability. The most likely candidate mechanisms are either theperiodic or episodic occultations of the star by ejected matter or achange in photospheric structure, e.g., from pulsation. We model thevariations caused by these mechanism by means of model atmosphere andabsorbing-slab codes. Line absorptions strength changes are rathersensitive to physical conditions in circumstellar shells and ``clouds''at temperatures of 8000-13,000 K, which is the regime expected forcircumstellar structures of early B stars. To demonstrate proofs of thisconcept, we construct spectral ratios for circumstellar structuresassociated with flux variability in various Be stars: (1) Vela X-1 has abow-shock wind trailing its neutron star companion; at successive phasesand hence in different sectors, the wind exhibits spectrophotometricsignatures of a 13,000 or 26,000 K medium; (2) 88 Her undergoes episodic``outbursts'' during which its UV flux fades, followed a year later by adimming at visible wavelengths as well; the ratioed spectrum indicatesthe ``phase lag'' is a result of a nearly gray opacity that dominatesall wavelengths as the shell expands from the star and cools, permittingthe absorptions in the visible to ``catch up'' to those in the UV; and(3) ζ Tau and 60 Cyg exhibit periodic spectrum and flux changes,which match model absorptions for occulting clouds but are actually mosteasily seen from selective variations of various resonance lines. Inaddition, ratioed UV spectra of radial and large-amplitude nonradialpulsating stars show unique spectrophotometric signatures, which can besimulated with model atmospheres. An analysis of ratioed spectraobtained for a representative sample of 18 classical Be stars known tohave rapid periodic flux variations indicates that 13 of them haveratioed spectra that are relatively featureless or have signatures ofpulsation. Ratioed spectra of three others in the sample exhibitsignatures that are consistent with the presence of corotating clouds.
| Boron Abundances in Early B Stars: Results from the B III Resonance Line in IUE Data
We have used archival International Ultraviolet Explorer high-dispersionSWP spectra to study the B III resonance line at 2065.8 Å in 44early B stars. We find a median boron abundance about half that of solarsystem meteoritic material, consistent with the values found fromprevious studies of boron in early B stars. About one-third of the starsstudied appear to have boron abundances that are a factor of 4 or morelower than this median. Many of these are stars with enhanced nitrogenabundances, confirming the belief that deep envelope mixing can occur inmain-sequence B stars. A few stars with low boron abundances have normalN/C ratios. It is unclear whether all of these can be explained as starswhere mixing depleted the boron but did not go deep enough to bringCN-processed material to the surface or if some stars were actuallyformed with an unusually low boron abundance.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| On the Variability of O4-B5 Luminosity Class III-V Stars
We investigate the Hipparcos Satellite photometry of O4-B5 luminosityclass III-V stars. Some for which further study is desirable areidentified. These stars in general are more variable than cooler stars
| Long-term period and amplitude variations in beta Cephei stars.
Not Available
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Lacerta |
| Ascensió Recta: | 22h41m28.60s |
| Declinació: | +40°13'32.0" |
| Magnitud Aparent: | 5.25 |
| Distancia: | 427.35 parsecs |
| Moviment propi RA: | -1.8 |
| Moviment propi Dec: | -4.3 |
| B-T magnitude: | 5.042 |
| V-T magnitude: | 5.205 |
Catàlegs i designacions:
|
