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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.
| 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.
| Applications of pulsation amplitudes and phases for B-type main sequence pulsators
Combined data on pulsation amplitudes and phases from multicolourphotometry and spectroscopy yield constraints on mode identification aswell as on mean stellar parameters. The data allow for determination ofa certain complex parameter which may be compared with the model valueand thus constitute a new seismic probe of stellar interior,specifically of its outer layers. We present here some results obtainedfor the two beta Cephei stars delta Ceti and nu Eridani. We foundsignificant differences between models calculated with opacities fromthe OPAL and OP projects.
| Progress in understanding and exploiting stellar oscillation spectra .
Rich oscillation spectra of dwarf-like pulsators contain a wealth ofinformation about the object interiors and, in particular, aboutmacroscopic transport processes, which is the most difficult aspect ofstellar physics. Examples of extracting such information from data onsolar-like and opacity driven pulsators are given. Problems inunderstanding new oscillation spectra are discussed. Importance ofemploying various data on excited mode is emphasized.
| Asteroseismological Studies of Long-Period Variable Subdwarf B Stars. II. Two-Color Photometry of PG 1338+481
We present the results of an observational campaign for the long-periodvariable subdwarf B star PG 1338+481. Seven continuous weeks ofobserving time at the Steward Observatory 1.55 m Kuiper telescope onMount Bigelow, Arizona, and the 1.3 m MDM telescope at Kitt Peakrendered ~250 hr of simultaneous U/R time series photometry, as well asan extra ~70 hr of R-band-only data. The analysis of the combined lightcurves resulted in the extraction of 13 convincing periodicities in the2100-7200 s range, with amplitudes up to ~0.3% and ~0.2% in the U and R,respectively. Comparing the ratios of amplitudes in the two wave bandsto those predicted from theory suggests the presence of dipole modes, anotion that is further supported by the period spacing between thehighest amplitude peaks. If confirmed, this poses a challenge to currentnonadiabatic theory. At the quantitative level, we find that thedistribution of the observed period spectrum is highly nonuniform andmuch sparser than that predicted from a representative model. We providea possible interpretation in the text. The asteroseismological analysisattempted for PG 1338+481 on the basis of six observed periodicitiesbelieved to constitute consecutive dipole modes renders encouragingresults. Fixing the effective temperature and surface gravity to thespectroscopic estimates, we successfully isolate just one family ofoptimal models that can reproduce the measured periods to better than1%. While the stellar parameters thus inferred must be regarded aspreliminary, the achieved fit bodes well for future asteroseismicanalyses of long-period variable subdwarf B stars.Some of the observations reported here were obtained at the MMTObservatory, a joint facility of the University of Arizona and theSmithsonian Institution.
| 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.
| 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.
| Effects of moderately fast shellular rotation on adiabatic oscillations
We investigate adiabatic oscillations for δ Scuti star models,taking a moderate rotation velocity (around 100 {km s-1})into account. The resulting oscillation frequencies include correctionsfor rotation up to second order in the rotation rate including those ofnear degeneracy. Effects of either a uniform rotation or a rotationprofile assuming local angular momentum conservation of the formΩ=Ω(r) on oscillation frequencies are compared. As expected,important differences (around 3 {μ Hz}) are obtained in the g andmixed-mode regions. For higher-frequency p modes, differences rangebetween 1 {μ Hz} and 3 {μ Hz}. Such differences are likely to bedetectable with future space missions such as COROT, where precisions infrequency around 0.5 {μ Hz} are expected to be reached.
| 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%).
| A hybrid β Cephei-SPB star in a binary system: γ Pegasi
High resolution and high signal to noise ratio spectroscopicobservations of the classical β Cephei star γ Peg wereobtained between 1991 and 2005. The analysis of these data combined withpreviously published results shows that γ Peg is a spectroscopicbinary with an orbital period of 370.5 d. We discovered three newfrequencies in addition to the well-known 6.5897 d-1 (0.15175d) one. That at 6.01 d-1 is a typical β Cepheifrequency. The two others at 0.68 and 0.87 d-1 are similar tothe high degree g-mode frequencies found in SPB stars. Thus, γ Pegis a hybrid β Cephei-SPB star. Its position in the HR diagram iscompatible with such a status. In addition, a small increase of the mainperiod has been detected between the 1995 and 2005 observations.
| An asteroseismic study of the β Cephei star θ Ophiuchi: spectroscopic results
We present the results of a detailed analysis of 121 ground-basedhigh-resolution, high signal-to-noise ratio spectroscopic measurementsspread over 3 yr for the β Cephei star θ Ophiuchi. Wediscovered θ Oph to be a triple system. In addition to thealready known speckle B5 companion of the B2 primary, we showed thepresence of a low-mass spectroscopic companion and we derived an orbitalperiod of 56.71 d with an eccentricity of 0.1670. After removing theorbit we determined two frequencies for the primary in the residualradial velocities: f1= 7.1160 cd-1 andf2= 7.4676 cd-1. We also found the presence off3= 7.3696 cd-1 by means of a two-dimensionalfrequency search across the SiIII 4567-Åprofiles. We identifiedthe m-value of the main mode with frequency f1 by taking intoaccount the photometric identifications of the degrees l. By means ofthe moment method and the amplitude and phase variations across the lineprofile, we derived (l1, m1) = (2, -1). Thisresult allows us to fix the mode identifications of the whole quintupletfor which three components were detected in photometry. This is ofparticular use for our forthcoming seismic modelling of the primary. Wealso determined stellar parameters of the primary by non-localthermodynamic equilibrium hydrogen, helium and silicon line profilefitting and we obtained Teff= 24000 K and logg= 4.1, which isconsistent with photometrically determined values.
| An asteroseismic study of the β Cephei star θ Ophiuchi: photometric results
We have carried out a three-site photometric campaign for the βCephei star θ Oph from 2003 April to August. 245 h ofdifferential photoelectric uvy photometry were obtained during 77 clearnights. The frequency analysis of our measurements has resulted in thedetection of seven pulsation modes within a narrow frequency intervalbetween 7.116 and 7.973 c d-1. No combination or harmonicfrequencies have been found. We have performed a mode identification ofthe individual pulsations from our colour photometry that shows thepresence of one radial mode, one rotationally split l= 1 triplet andpossibly three components of a rotationally split l= 2 quintuplet. Wediscuss the implications of our findings and point out the similarity ofthe pulsation spectrum of θ Oph to that of another β Cepheistar, V836 Cen.
| Asteroseismology of the β Cephei star ν Eridani - IV. The 2003-2004 multisite photometric campaign and the combined 2002-2004 data
We report on the second multisite photometric campaign devoted to νEridani (ν Eri). The campaign, carried out from 2003 September 11 to2004 February 16, was very nearly a replica of the first campaign,2002-2003: the five telescopes and photometers we used were the same asthose in the first campaign, the comparison stars and observingprocedure were identical, and the numbers and time baselines of the datawere comparable.For ν Eri, analysis of the new data adds four independent frequenciesto the nine derived previously from the 2002-2003 data: three in therange 7.20-7.93 d-1 and a low one, equal to 0.614d-1. Combining the new and the old data results in twofurther independent frequencies, equal to 6.7322 and 6.2236d-1. Altogether, the oscillation spectrum is shown to consistof 12 high and two low frequencies. The latter have u amplitudes abouttwice as large as the v and y amplitudes, a signature of highradial-order g modes. Thus, we confirm the suggestion, put forward onthe basis of the data of the first campaign, that ν Eri is both aβ Cephei and a slowly pulsating B (SPB) star.Nine of the 12 high frequencies form three triplets, of which two arenew. The triplets represent rotationally split l= 1 modes, although incase of the smallest-amplitude one this may be questioned. Meanseparations and asymmetries of the triplets are derived with accuracysufficient for meaningful comparison with models.The first comparison star, μ Eri, is shown to be an SPB variable withan oscillation spectrum consisting of six frequencies, three of whichare equidistant in period. The star is also found to be an eclipsingvariable. The eclipse is a transit, probably total, the secondary isfainter than the primary by several magnitudes, and the system is widelydetached.The second comparison star, ξ Eri, is confirmed to be a δ Scutivariable. To the frequency of 10.8742 d-1 seen already in thedata of the first campaign, another, equal to 17.2524 d-1, isadded.
| Orbital parameters, masses and distance to β Centauri determined with the Sydney University Stellar Interferometer and high-resolution spectroscopy
The bright southern binary star β Centauri (HR5267) has beenobserved with the Sydney University Stellar Interferometer (SUSI) andspectroscopically with the European Southern Observatory Coude AuxiliaryTelescope and Swiss Euler telescope at La Silla. The interferometricobservations have confirmed the binary nature of the primary componentand have enabled the determination of the orbital parameters of thesystem. At the observing wavelength of 442nm the two components of theprimary system have a magnitude difference of 0.15 +/- 0.02. Thecombination of interferometric and spectroscopic data gives thefollowing results: orbital period 357.00 +/- 0.07d, semimajor axis 25.30+/- 0.19mas, inclination 67.4 +/-0.3, eccentricity0.821 +/- 0.003, distance 102.3 +/- 1.7pc, primary and secondary massesM1=M2= 9.1 +/- 0.3 Msolar and absolutevisual magnitudes of the primary and secondary M1V=-3.85 +/-0.05 and M2V=-3.70 +/- 0.05, respectively. The high degree ofaccuracy of the results offers a fruitful starting point for futureasteroseismic modelling of the pulsating binary components.
| Asteroseismology: Past, Present and Future
Asteroseismology studies stars with a wide variety of interior andsurface conditions. For two decades asteroseismic techniques have beenapplied to many pulsating stars across the HR diagram. Asteroseismologyis now a booming field of research with stunning new discoveries; Ihighlight a personal selection of these in this review, many of whichare discussed in more detail elsewhere in these proceedings. For manyyears the Nainital-Cape Survey for northern roAp stars has been runningat ARIES, so I emphasise new spectroscopic results for roAp stars andpoint out the outstanding prospects for the planned ARIES 3-m telescopeat Devastal. High precision spectroscopy has revolutionised theasteroseismic study of some types of stars - particularly solar-likeoscillators and roAp stars - while photometry is still the best way tostudy the frequency spectra that are the basic data of asteroseismology.New telescopes, new photo-meters and space missions are revolutionisingasteroseismic photometry. In addition to the ground-based potential ofasteroseismic spectroscopy, India has the knowledge and capability forspace-based asteroseismic photometry. The future for asteroseismology isbright indeed, especially for Indian astronomers.
| 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.
| Constraints on parameters of B-type pulsators from combined multicolour photometry and radial velocity data. I. β Cephei stars
We analyze data on pulsation amplitudes and phases for two β Cepheistars, δ Cet and ν Eri. Strömgren photometry and radialvelocity measurements are used simultaneously to obtain constraints onmean parameters of the stars and identification of the excited modes.The inference about the radial mode order and mean star parameters isbased on comparison of certain complex parameter, f, determined fromdata, with its counterpart derived from linear nonadiabatic modelling ofstellar oscillations. The theoretical f values are very sensitive to theadopted opacity data. In our modelling we rely on the data from OPAL andOP projects. Significant differences were found. New seismic models ofν Eri were constructed with both the OPAL and OP opacities.
| NaI and CaII absorption components observed towards the Orion-Eridanus Superbubble
We present medium-resolution spectra (R ~ 7.5 km s-1) of theinterstellar NaI and CaII interstellar absorption lines observed towards16 early-type stars with distances of 160-1 kpc in the line-of-sighttowards the Orion-Eridanus Superbubble (OE-S). These data have beensupplemented with measurements of NaI absorption towards a further 13stars with similar sight-lines taken from the literature. We detect twomajor absorption components with velocities of Vlsr ~ +7.0and -8.0 km s-1. The former component, seen in 70% of thesight-lines is associated with the boundary to the Local Bubble cavitylocated at a distance of 140-150 pc. The other absorption component isonly detected towards a limited region of the sky bounded by (190°< l < 215°) and (-50° < b < -30°). If gas withthis velocity is associated with an outer expansion shell of the OE-S,then we can place its distance at 163-180 pc in agreement with theestimate by Guo et al. (1995, ApJ, 453, 256). Several other negativevelocity components at Vlsr ~ -20.4, -28.5 and -33.5 kms-1 have also been detected for sight-line distances > 220pc within an area coincident with that of the 0.75 keV X-ray enhancementof the OE-S. Column density ratios, N(NaI)/N(CaII), for the mostnegative velocity components have values < 1.0, suggesting that thisgas has been disrupted by a possible shock event. Our data do notsupport a simple model for the OE-S that involves a single stellarbubble cavity that stretches from the Orion Nebula to high galacticlatitudes. Instead, our detection of multiple positive and negativevelocity components suggests the presence of several gas shells producedby supernovae and/or stellar wind-driven shocks. We also confirm thatthe prominent "hook-like" feature of H-α emission thatcharacterizes the OE-S, in in fact composed of two physically separateemission arcs, with the brighter Arc A being at a distance > 500 pc.Finally, we place a similar distance limit for any coherently structuredrear shell of neutral gas associated with expansion of the OE-S towardsthe galactic halo.
| 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
| Disentangling component spectra of κ Scorpii, a spectroscopic binary with a pulsating primary. II. Interpretation of the line-profile variability
We analyse the complex short-term SiIII line-profile variability of the spectroscopic binary β Cep star κ Scorpii after orbit subtraction, before and after spectral disentangling. We refine the known oscillation frequency of the star: f1=4.99922 c d-1 and detect2f1. Variability is also found at frequencies nearf2≃ 4.85 c d-1 and f3≃ 5.69c d-1 or their aliases. These frequencies are not significantif we consider the spectra alone, but they survive our selection afterthe consideration that they were derived previously from independentground-based and space photometry by different teams. Moreover, we finddominant variability in the equivalent width with a frequency in theinterval [0.22,0.30] c d-1 which we interpret as therotational frequency frot of the star. The complex windowfunction does not allow us to determine definite values forf2, f3, frot. The variability withf1 is interpreted as a prograde non-radial oscillation modewith spherical wavenumbers (ℓ,m)=(2,-1) or (1,-1). The additionalfrequencies are explained in terms of rotational modulation superposedto the main oscillation. We also point out that we cannot disprove thevariability in κ Scorpii to originate from co-rotating structures. KOREL disentangling preserves the large-amplitude line-profile variability but its performance for complex low-amplitude variability remains to bestudied in detail.Based on observations obtained with the Coudé ÉchelleSpectrograph on the ESO CAT telescope and with the CORALIEéchelle spectrograph on the 1.2-m Euler Swiss telescope, bothsituated at La Silla, Chile.
| Asteroseismology of the β Cephei star ν Eridani: massive exploration of standard and non-standard stellar models to fit the oscillation data
We present the results of a detailed seismic modelling of the βCephei star ν Eridani with the Liège evolution and pulsationcodes. We selected four clearly detected, well-identified andindependent pulsation modes from the frequency spectrum obtained from arecent five-month multisite, multitechnique campaign, while previousmodelling work only took into account three frequencies. We show bymeans of a massive exploration of the parameter space that no standardstellar model both matches and excites these four observed modes, incontrast to the conclusion reached when considering only threefrequencies. Therefore, we have considered stellar models with differentmetal mixtures and different initial hydrogen abundance values. We showthat an increase in the relative number fraction of iron throughout thewhole star or a large decrease in the initial hydrogen abundance makethe stellar models matching the four selected modes satisfy allobservational constraints and we provide the general properties of thebest such physical models.
| Asteroseismology of the β Cephei star ν Eridani - III. Extended frequency analysis and mode identification
Using the large photometric and spectroscopic data sets of the νEridani multisite campaign given in our two recent papers (Aerts et al.and Handler et al.), we present an extended frequency analysis and aphotometric mode identification. For the extended frequency analysis, weused an improved radial velocity time series, the second-moment timeseries and the line profiles themselves. In the radial velocity timeseries, we can now detect an additional pulsation frequency that waspreviously only found in photometric time series. We also report severalnew candidate pulsation frequencies. For seven frequencies, thephotometric mode identification indicates that they belong to a radialmode and six dipole modes, and for three frequencies the degree l couldnot be unambiguously determined. We also placed ν Eri in theHertzsprung-Russell diagram by determining Teff using Genevaplus Strömgren photometric calibrations, spectral energydistribution fitting, by non-local thermodynamic equilibrium hydrogen,helium and silicon line profile fitting, and by determininglog(L/Lsolar) using the Hipparcos parallax and an Hβcalibration.
| Asteroseismology of the β Cephei star ν Eridani: interpretation and applications of the oscillation spectrum
The oscillation spectrum of ν Eri is the richest known for anyvariable of the β Cephei type. We interpret the spectrum in termsof normal mode excitation and construct seismic models of the star. Thefrequency data combined with data on mean colours set the upper limit onthe extent of overshooting from the convective core. We use data onrotational splitting of two dipole (l= 1) modes (g1 andp1) to infer properties of the internal rotation rate.Adopting a plausible hypothesis of nearly uniform rotation in theenvelope and increasing rotation rate in the μ-gradient zone, we findthat the mean rotation rate in this zone is about three times fasterthan in the envelope. In our standard model only the modes in the middlepart of the oscillation spectrum are unstable. To account for excitationof a possible high-order g mode at ν= 0.43 cd-1 as well asp modes at ν > 6 cd-1 we have to invoke anoverabundance of Fe in the driving zone.
| 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.
| 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.
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Membro dei seguenti gruppi:
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Osservazione e dati astrometrici
| Costellazione: | Eridano |
| Ascensione retta: | 04h36m19.10s |
| Declinazione: | -03°21'09.0" |
| Magnitudine apparente: | 3.93 |
| Distanza: | 179.856 parsec |
| Moto proprio RA: | 0 |
| Moto proprio Dec: | 0 |
| B-T magnitude: | 3.681 |
| V-T magnitude: | 3.896 |
Cataloghi e designazioni:
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INHABITED SKY
