Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking intoaccount the effects induced by the fast rotation on their fundamentalparameters. All program stars were observed in the BCDspectrophotometric system in order to minimize the perturbationsproduced by the circumstellar environment on the spectral photosphericsignatures. This is one of the first attempts at determining stellarmasses and ages by simultaneously using model atmospheres andevolutionary tracks, both calculated for rotating objects. The stellarages (τ) normalized to the respective inferred time that eachrotating star can spend in the main sequence phase (τ_MS) reveal amass-dependent trend. This trend shows that: a) there are Be starsspread over the whole interval 0 τ/τ_MS 1 of themain sequence evolutionary phase; b) the distribution of points in the(τ/τMS,M/Mȯ) diagram indicates thatin massive stars (M 12~Mȯ) the Be phenomenon ispresent at smaller τ/τ_MS age ratios than for less massive stars(M 12~Mȯ). This distribution can be due to: i)higher mass-loss rates in massive objets, which can act to reduce thesurface fast rotation; ii) circulation time scales to transport angularmomentum from the core to the surface, which are longer the lower thestellar mass.
| 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.
| The Complete Far Ultraviolet Spectroscopic Explorer Spectrum of the Small Magellanic Cloud Star Sk 108
The complete FUSE spectrum of the SMC Wolf-Rayet binary star Sk 108 ispresented. The spectrum covers the 905-1186 Å wavelength range ata resolution of about 12,500 and a S/N of 25-50. Detailed componentinformation from higher resolution, UV and optical spectra is used tomodel the far-UV absorption lines of the ISM in the Galaxy and SMC, andspecial attention is given to the accuracy of the wavelength scale.Improved FUSE measurements, compared to those previously published, ofcolumn densities of Ar, N, P, and H2 and new measurements ofO are presented. Relative abundances for P and the four α-particleelements-Ar, O, S, and Si-appear to be at solar values, relative to Zn,in the SMC, even though they may be depleted in the Galaxy. Fe isdepleted in both the SMC and the Galaxy, but at higher levels in theGalaxy. This implies that, compared to the Galactic environment, the SMCinterstellar clouds either exhibit a lower dust-to-metal ratio or thatthe dust grains are destroyed (with some remaining dust grains whichcontain Fe) or that there is an enhancement of α-elements.Furthermore, the overall N abundance appears to be truly deficient inthe SMC. In addition, empirical oscillator strengths for a series of FeII lines are derived, using the predetermined column densities for theFe II components from GHRS data.
| Interstellar Silicon Abundance
We present 34 measurements of silicon gas phase column densities in theinterstellar medium. We have used spectra containing the SiII 1808 Angline which were obtained with the Goddard High Resolution Spectrograph(GHRS) aboard the Hubble Space Telescope (HST). Extinction curveparameters are determined for analyzed lines of sight and relationbetween Si/H ratio and extinction parameters is discussed. We find theabundance of gas phase silicon in diffuse clouds to be lower than thesolar value by a factor of four.
| Non-radially pulsating Be stars
Based on more than 3000 high-resolution echelle spectra of 27 early-typeBe stars, taken over six years, it is shown that the short-term periodicline profile variability of these objects is due to non-radialpulsation. The appearance of the line profile variability depends mostlyon the projected rotational velocity v sin i and thus, since all Bestars rotate rapidly, on the inclination i. The observed variability ofthe investigated stars is described, and for some of them line profilevariability periods are given for the first time. For two of theinvestigated stars the line profile variability was successfully modeledas non-radial pulsation with l=m=+2 already in previous works. Since Bestars with similarly low v sin i share the same variability properties,these are in general explainable under the same model assumptions. Theline profile variability of stars with higher v sin i is different fromthe one observed in low v sin i stars, but can be reproduced by thesame model, if only the model inclination is modified to more equatorialvalues. Only for a few stars with periodic line profile variability thel=m=2 non-radial pulsation mode is not able to provide a satisfyingexplanation. These objects might pulsate in different modes (e.g.tesseral ones, l != |m|). Almost all stars in the sample show traces ofoutburst-like variability, pointing to an ephemeral nature of themass-loss phenomenon responsible for the formation of the circumstellardisk of early-type Be stars, rather than a steady star-to-disk masstransfer. In addition to the variability due to non-radial pulsationpresent in most stars, several objects were found to show other periodsresiding in the immediate circumstellar environment. The presence ofthese secondary periods is enhanced in the outburst phases. Short-livedaperiodic phenomena were clearly seen in two stars. But, given theunfavourable sampling of our database to follow rapid variability oftransient nature, they might be more common. Only in two out of 27 starsshort-term spectroscopic variability was not detected at all.Based on observations collected at the European Southern Observatory atLa Silla, Chile, 55.D-0502, 56.D-0381, 58.D-0697, 62.H-0319, 63.H-0080,64.H-0548, and 267.D-5702, the German-Spanish Astronomical Centre, CalarAlto, operated by the Max-Planck-Institute for Astronomy, Heidelberg,jointly with the Spanish National Commission for Astronomy, and onobservations with the Wendelstein 80-cm and the Ondřejov 2-mtelescopes, both equipped with the HEROS spectrograph provided by theLandessternwarte Heidelberg.
| Autocorrelation Analysis of Hipparcos Photometry of Short-Period Be Stars
We have used Hipparcos epoch photometry and a form of autocorrelationanalysis to investigate the amplitude and timescale of the short-periodvariability of 82 Be stars, including 46 Be stars that were analyzed byHubert & Floquet using Fourier and CLEAN analysis and 36 other Bestars that were suspected of short-period variability. Our method hasgiven useful information for about 84% of these stars; for the rest, thetime distribution of the Hipparcos epoch photometry limits thecapability of our technique.
| 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.
| 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.
| Far Ultraviolet Spectroscopic Explorer Observations of Interstellar Gas toward the Small Magellanic Cloud Star Sk 108
We discuss the interstellar absorption lines found in Far UltravioletSpectroscopic Explorer spectra of the Wolf-Rayet binary Sk 108, which islocated in the northeastern part of the main ``bar'' of the SmallMagellanic Cloud. The spectra cover the 988-1187 Å wavelengthrange at a resolution of about 12,000 and a signal-to-noise ratio of20-40. We use detailed component information from higher resolutionnear-UV and optical spectra to model the far-UV lines of similarlydistributed species. Both the Galactic and SMC gas toward Sk 108 seem tobe predominantly neutral, although a significant fraction of the SMC gasis ionized. The column densities of P II, S II, and Ar I are consistentwith essentially solar ratios, relative to N(Zn II), in both theGalactic and SMC gas; the column density of N I remains somewhatuncertain. Molecular hydrogen is present in the Galactic gas, withproperties similar to those found in low mean density Galactic lines ofsight and in the Galactic gas toward several other LMC and SMC stars. Wereport a tentative detection of H2 in the SMC gas for J=1 and3, with rotational level populations consistent with an excitationtemperature on the order of 1000 K-similar to the H2 found indiffuse Galactic gas toward ζ Puppis. Strong absorption from N III,S III, and Fe III has revealed a significant ionized component,particularly in the SMC; O VI is present, but relatively weak,especially in the Galactic gas. The N(C IV)/N(O VI) ratio variessomewhat within the SMC, suggesting that several processes maycontribute to the observed high ion abundances.
| A Search for High-Velocity Be Stars
We present an analysis of the kinematics of Be stars based uponHipparcos proper motions and published radial velocities. We findapproximately 23 of the 344 stars in our sample have peculiar spacemotions greater than 40 km s-1 and up to 102 kms-1. We argue that these high-velocity stars are the resultof either a supernova that disrupted a binary or ejection by closeencounters of binaries in young clusters. Be stars spun up by binarymass transfer will appear as high-velocity objects if there wassignificant mass loss during the supernova explosion of the initiallymore massive star, but the generally moderate peculiar velocities of BeX-ray binaries indicate that the progenitors lose most of their massprior to the supernova (in accordance with model predictions). Binaryformation models for Be stars predict that most systems bypass thesupernova stage (and do not receive runaway velocities) to createultimately Be+white dwarf binaries. The fraction of Be stars spun up bybinary mass transfer remains unknown, since the post-mass transfercompanions are difficult to detect.
| Evolution in circumstellar envelopes of Be stars: From disks to rings?
New series of echelle spectra were obtained to study the medium- andlong-term evolution of the disks of several Be stars. Subtle variationsin the wings of optically thin and thick emission lines suggest that theconventional, static picture of the disk being in quasi-contact with thecentral star is justified primarily (or perhaps only) after an outburstevent. Some weeks to months later, a low-density region seems to developabove the star and slowly grows outwards. A subsequent outburst maylater replenish this cavity. In fact, in two stars this more ring-likestructure is apparently at times detached far enough from the star toallow for the formation of a secondary inner disk from the ejecta of alater outburst. This behaviour is not necessarily representative of Bestars in general because in the later spectral sub-types, discrete massloss events have not so far been observed to play a major role. In thelight of the apparent life cycle of such disks, a brief discussion isgiven of the differences in strength and variability between the windsof Be and normal B stars. It seems possible to attribute thesedifferences to matter that was initially in the disk and thereforelargely shielded from the stellar radiation, but that during the courseof the inner excavation (or even complete distruction) of the diskbecomes exposed. Based on observations collected at the EuropeanSouthern Observatory at La Silla, Chile, ESO proposal No. 64.H-0548 andon observations with the Wendelstein 80-cm and the Ondřejov 2-mtelescopes, both equipped with the HEROS spectrograph provided by theLandessternwarte Heidelberg.
| High and intermediate-resolution spectroscopy of Be stars 4481 lines
We present an atlas of Hγ , He i lambda 4471 and Mg ii lambda 4481line profiles obtained in a 10 year observation period of 116 Be stars,which enabled many of them to be observed at quite different emissionepochs. From the best fit of the observed He i lambda 4471 line profileswith non-LTE, uniform (Teff,log g) and full limb-darkenedmodel line profiles, we determined the V sin i of the program stars. Toaccount, to some degree, for the line formation peculiarities related tothe rapid rotation-induced non-uniform distributions of temperature andgravity on the stellar surface, the fit was achieved by considering(Teff,log g) as free parameters. This method produced V sin iestimations that correlate with the rotational velocities determined bySlettebak (1982) within a dispersion sigma <= 30 km s-1and without any systematic deviation. They can be considered as given inthe new Slettebak's et al. (1975) system. Only 13 program stars havediscrepant V sin i values. In some objects, this discrepancy could beattributed to binary effects. Using the newly determined V sin iparameters, we found that the ratio of true rotational velocitiesV/Vc of the program Be stars has a very low dispersion aroundthe mean value. Assuming then that all the stars are rigid rotators withthe same ratio V(/lineω)/Vc, we looked for the value of/line ω that better represents the distribution of V sini/Vc for randomly oriented rotational axes. We obtained/lineω = 0.795. This value enabled us to determine the probableinclination angle of the stellar rotation axis of the program stars. Inthe observed line profiles of Hγ , He i lambda 4471, Mg ii lambda4481 and Fe ii lambda 4351 we measured several parameters related to theabsorption and/or emission components, such as: equivalent width,residual emission and/or absorption intensity, FWHM, emission peakseparations, etc. The parameters related to the Hγ line emissionprofiles were used to investigate the structure of the nearbyenvironment of the central star. From the characteristics of thecorrelations between these quantities and the inferred inclinationangle, we concluded that in most of cases the Hγ line emissionforming regions may not be strongly flattened. Using a simplerepresentation of the radiation flux emitted by the star+envelopesystem, we derived first order estimates of physical parameterscharacterizing the Hγ line emission formation region. Thus, weobtained that the total extent of the Hγ region is Rf=~ 2.5 +/- 1.0 R* and that the density distribution in theselayers can be mimicked with a power law rho ~ R-alpha , wherealpha =2.5+2.2-0.6. The same approach enabled usto estimate the optical depth of the Hγ line emission formationregion. From its dependence with the aspect angle, we concluded thatthese regions are caracterized by a modest flattening and that the rho(equator)/rho (pole) density contrast of the circumstellar envelope nearthe star should be two orders of magnitude lower than predicted bymodels based on a priori disc-shaped circumstellar envelopes. We foundthat the separation between the emission peaks, Deltap, andthe full width at half maximum, Delta 1/2, of the Hγline emission are not only sensitive to kinematic effects, but to lineoptical depth as well. This finding agrees with previous theoreticalpredictions and confirms that Huang's (1972) relation overestimates theextent of the Hγ line emission formation region. Data obtained atCASLEO operated under agreement between the CONICET and the NationalUniversities of La Plata, Córdoba and San Juan, Argentina, at ESOLa Silla, Chile and at OHP, France.}\fnmsep\thanks{Tables 2 to 7 andFigs. 1 and 2 are only available in full in electronic form at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/861}} \subtitle{Anatlas of Hγ , He {\fontsize {10pt}{12pt}\selectfont I} 4471 and Mg{\fontsize {10pt}{12pt}\selectfont II
| A representative sample of Be stars III: H band spectroscopy
We present H band (1.53 mu m-1.69 mu m) spectra of 57 isolated Be starsof spectral types O9-B9 and luminosity classes III, IV & V. The H iBrackett (n-4) series is seen in emission from Br-11-18, and Fe iiemission is also apparent for a subset of those stars with H i emission.No emission from species with a higher excitation temperature, such asHe ii or C iii is seen, and no forbidden line emission is present. Asubset of 12 stars show no evidence for emission from any species; thesestars appear indistinguishable from normal B stars of a comparablespectral type. In general the line ratios constructed from thetransitions in the range Br-11-18 do not fit case B recombination theoryparticularly well. Strong correlations between the line ratios withBr-gamma and spectral type are found. These results most likelyrepresent systematic variations in the temperature and ionization of thecircumstellar disc with spectral type. Weak correlations between theline widths and projected rotational velocity of the stars are observed;however no systematic trend for increasing line width through theBrackett series is observed.
| Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars
We present the results of statistical analyses of a sample of 627 Bestars. The parameters of intrinsic polarization (p*),projected rotational velocity (v sin i), and near IR excesses have beeninvestigated. The values of p* have been estimated for a muchlarger and more representative sample of Be stars (~490 objects) thanpreviously. We have confirmed that most Be stars of early spectral typehave statistically larger values of polarization and IR excesses incomparison with the late spectral type stars. It is found that thedistributions of p* diverge considerably for the differentspectral subgroups. In contrast to late spectral types (B5-B9.5), thedistribution of p* for B0-B2 stars does not peak at the valuep*=0%. Statistically significant differences in the meanprojected rotational velocities (/line{vsin i}) are found for differentspectral subgroups of Be stars in the sense that late spectral typestars (V luminosity class) generally rotate faster than early types, inagreement with previously published results. This behaviour is, however,not obvious for the III-IV luminosity class stars. Nevertheless, thecalculated values of the ratio vt/vc of the truerotational velocity, vt, to the critical velocity forbreak-up, vc, is larger for late spectral type stars of allluminosity classes. Thus, late spectral type stars appear to rotatecloser to their break-up rotational velocity. The distribution of nearIR excesses for early spectral subgroups is bi-modal, the position ofthe second peak displaying a maximum value E(V-L)~ 1 . m 3for O-B1.5 stars, decreasing to E(V-L)~0. m8 for intermediatespectral types (B3-B5). It is shown that bi-modality disappears for latespectral types (B6-B9.5). No correlations were found betweenp* and near IR excesses and between E(V-L) and vsin i for thedifferent subgroups of Be stars. In contrast to near IR excesses, arelation between p* and far IR excesses at 12 mu m is clearlyseen. A clear relation between p* and vsin i (as well asbetween p* and /line{vsin i}/vc) is found by thefact that plots of these parameters are bounded by a ``triangular"distribution of p*: vsin i, with a decrease of p*towards very small and very large vsin i (and /line{vsini}/vc) values. The latter behaviour can be understood in thecontext of a larger oblateness of circumstellar disks for the stars witha rapid rotation. From the analysis of correlations between differentobservational parameters we conclude that circumstellar envelopes forthe majority of Be stars are optically thin disks with the range of thehalf-opening angle of 10degr
| 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
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,354 of the stars in Part III are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives, in addition to the SI mode, the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| Detection of a variable interstellar absorption component towards δ Orionis A
Observations of δ Ori A made with the UHRF in its highestresolution mode (R~900000) have revealed the presence of a cool(Tk<=350K) variable absorption component at a heliocentricvelocity of +21.3kms-1. The component is detected in NaiD1, where clear hyperfine splitting is seen, and Caii K.Comparison of our data with existing spectra suggests that the componenthas consistently increased in strength from 1966 to 1994, andsubsequently reduced in intensity by 1999. Following a discussion of thepossible origins of this component it is concluded that an interstellar,rather than circumstellar, origin is most likely. This is one of veryfew detections of variable interstellar absorption reported in theliterature, and we suggest an origin within filamentary materialassociated with the expanding Hi shell surrounding the Orion-Eridanussuperbubble.
| Interstellar Carbon Abundance
We present 10 new measurements of carbon gas phase column density in theinterstellar medium. We have used spectra made with the Goddard HighResolution Spectrograph aboard the Hubble Space Telescope containing theCII 1334.5 Ang and CII* 1335.7 Ang lines. The continuum reconstructionmethod has been used to obtain the carbon column density from theLorentzian damped lines. Extinction curve parameters are determined inselected directions and relation between C/H ratio and extinctionparameters is discussed. A correlation has been found between C/H andthe strength of the 2175 Ang bump. Unlike previous results, we noticethat C/H changes with fractional abundance of molecular hydrogen,f(H_2). The average value of C/H=3.55*10^{-4} for lines of sight withf(H_2)<1*10^{-3} is the same as solar photospheric abundance fromGrevese and Noels (1993) and may represent the real cosmic carbonabundance.
| A representative sample of Be stars. II. K band spectroscopy
We present K band (2.05 mu m - 2.22 mu m) spectra of 66 isolated Bestars of spectral types O9-B9 and luminosity classes III, IV & V. Wefind that objects with He i features either in emission or absorptionare B3 or earlier. Objects with Mg ii emission but no He i are B2 to B4,while objects with Brgamma emission but no evidence of He i or Mg ii areB5 or later. Na i emission in the spectra of 4 objects appears toindicate that regions of the circumstellar envelopes of these stars mustbe shielded from direct stellar radiation. Systematic trends in the linestrength and profile of Brgamma are seen from early to late spectraltypes which can be understood in terms of differences in the disctemperature and density. 30 percent of the stars do not currently showevidence for line emission. Compared to the emission line stars theseobjects have a significantly lower mean rotational velocity and adistribution of spectral types that is significantly earlier. This canbe explained either as the original misidentification of these objectsas Be stars (i.e. they never had line emission), or as evidence thatstars with lower rotational velocities may be more prone to changesbetween the Be and B phases.
| The distance to the nearest star-forming clouds: MBM12 and MBM20
We present high-resolution spectra (R ~ 49,000) of stars that haveparallax measurements from the Hipparcos satellite and are projectedalong the line of sight to the two nearest known star forming clouds tothe Sun: MBM12 and MBM20. The spectra were obtained with the FOCESEchelle Spectrograph at the 2.2 meter telescope in Calar Alto, Spain andthe wavelength range was chosen to include the interstellar Na I D linesat lambda 5889.950 Ä and lambda 5895.924 Ä. Since the starsare at a range of distances, we use their spectra along with theirparallaxes from Hipparcos to determine the distance to the moleculargas. The stars in front of the cloud do not show interstellar Na I Dabsorption features while the stars behind the cloud do showinterstellar absorption features. We find that both clouds are somewhatmore distant than previously estimated. The revised distance to MBM12 is58+/-5 pc < d < 90+/-12 pc and the distance to MBM20 is 112+/-15pc < d < 161+/-21 pc.
| 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.
| Near-simultaneous Spectroscopic and Broadband Polarimetric Observations ofBE Stars
Near simultaneous optical spectroscopic (on four nights) and broadbandlinear continuum (B, V, R,and I bands) polarimetric (on seven nights)observations of 29 Be stars were carried out during1993November-December. The program Be stars displayed wavelengthdependence of intrinsic polarizations with no frequency dependence ofpolarimetric position angles. Some of the Be stars displayed long-termpolarization variability. The Be and Be-shell stars could not bedistinguished from one another solely on the basis of their polarizationvalues. Full widths at half-maximum of the Hα profiles and theintrinsic linear continuum polarizations are closely correlated with theprojected rotational velocities of the program stars.Photospheric-absorption-corrected equivalent widths of Hα profiles[W(alpha)] and the radii of Hα-emitting or -absorbing envelopes(R_e or R_a) are nonlinearly correlated with the intrinsic continuumpolarizations of these stars. However, W(alpha) and R_e are linearlycorrelated. With large uncertainties, there is a trend of spectraldependence of polarization. Detailed discussion of these results ispresented in this paper.
| ROSAT High Resolution Imager Identifications of Suspected Stellar Sources from the Einstein Slew Survey
We present analysis of a series of ROSAT High Resolution Imager (HRI)observations of 16 X-ray detections from the Einstein Slew Survey (ESS).The data were taken to resolve ambiguous ESS identifications based onoptical spectroscopy. For 10 of the 16 detections, the previousidentification has been confirmed-six with solar-type stars (spectraltypes F-K), one each with a cataclysmic variable, a B2 V, a dMe star,and one with a BL Lac object. For another detection, two potentialcounterparts are unresolvable even with the HRI. The HRI does resolvetwo other ESS detections into two distinct X-ray sources, but no HRIsource is found for the remaining three X-ray detections. HRI-derivedfluxes are consistent with those measured by the ROSAT All-Sky Surveyfor their 12 common detections, and improved positions are obtained forthese sources. We calculate values of f_X/f_V and L_X for these starswhere possible and find values consistent with those expected forcorresponding spectral types.
| Long-term visual spectrophotometric behaviour of Be stars. II. Correlations with fundamental stellar parameters and interpretation
The long-term visual spectrophotometric (SPh) behaviour of Be stars as afunction of fundamental stellar parameters is studied. Some previous SPhresults obtained by other authors are confirmed. Moreover, a tendencyfor temperature and aspect angle dependency of SPh variations is found.From the characteristics of visual SPh behaviour in Be stars we deriveconstraints for models of regions in circumstellar envelopes where thevisual continuum spectrum is formed: (i) The SPh emission and absorptionphases should not imply preferential aspect angles, as they can bothappear whatever the stellar inclination. This phenomenon cannot bealways accounted for by strongly flattened circumstellar envelopes; (ii)Radii of the visible continuum forming regions cannot be larger than afew R_*; (iii) Electron densities of these regions should not exceed N_e~ 10(13) cm(-3) ; (iv) Electron temperature of circumstellar layersproducing the SPh emission phases compare with the stellar Balmercontinuum radiation temperature and it is much lower in those producingthe SPh absorption phases. Three scenarios were studied to produce theobserved characteristics of emissions in the V magnitude and in thesecond component of Balmer discontinuity (Delta D) during the SPhemission phases: (a) expansion of a massive circumstellar shell thatpreserves circumstellar envelope flattening; (b) expansion of acircumstellar shell which increases the global flattening, so that adisc-like structure is formed; (c) continuous mass ejection thatincreases the storage of mass in a constant volume with a givenflattening. Mechanisms (a) and (b) produce a double valued (DeltaV,Delta D) SPh relation, while (c) produces a single valued relation.Only mechanisms (a) and (c) can easily produce the observed amounts ofemission Delta V and Delta D without violating the modeling constraintsfrom (i) to (iv) imposed by observations. The model SPh slopes of (DeltaV,Delta D) show the global sin i and T_eff observed dependencies. Thescenarios used to describe the double valued (Delta V,Delta D) suggestanother possible way how to build up circumstellar envelopes around Bestars.
| Atomic and Molecular Interstellar Absorption Lines toward the High Galactic Latitude Stars HD 141569 and HD 157841 at Ultra-High Resolution
We present ultra-high-resolution (0.32 km s-1) spectra obtained with the3.9 m Anglo-Australian Telescope (AAT) and Ultra-High-ResolutionFacility (UHRF) of interstellar Na I D1, Na I D2, Ca II K, K I, and CHabsorption toward two high Galactic latitude stars HD 141569 and HD157841. We have compared our data with 21 cm observations obtained fromthe Leiden/Dwingeloo H I survey. We derive the velocity structure andcolumn densities of the clouds represented by the various components andidentify the clouds with ISM structures seen in the region at otherwavelengths. We further derive abundances, linear depletions, and H2fractional abundances for these clouds wherever possible. Both stars arelocated in regions of IRAS 100 mu m emission associated with highGalactic latitude molecular clouds (HLCs): HD 141569 lies, inprojection, close to MBM 37 and the Lynds dark cloud L134N, whereas HD157841 is in the vicinity of the MBM 151. Toward HD 141569 we detect twocomponents in our UHRF spectra: a weak, broad b = 4.5 km s-1 componentat -15 km s-1, seen only in Ca II K absorption, and another component at0 km s-1, seen in Na I D1, Na I D2, Ca II K, K I, and CH absorption. Thecloud represented by the -15 km s-1 component is warm and may be locatedin a region close to the star. The cloud represented by the 0 km s-1component has a Ca linear depletion delta (Ca) = 1.4 x 10-4 and showsevidence for the presence of dust, consistent with strong 100 mu memission seen in this region. The H2 fractional abundance f(H2) derivedfor this cloud is 0.4, which is typically what is observed toward HLCs.We conclude that this 0 km s-1 cloud is associated with MBM 37 and L134Nbased on the presence of dust and molecular gas (CH) and good velocityagreement with CO emission from these two clouds. This places HD 141569beyond MBM 37 and L134N, which are estimated to be at ~110 pc. In thecase of the HD 157841 sight line, a total of six components are seen onour UHRF spectra in Na I D1, Na I D2, Ca II K, K I, and CH absorption.Two of these six components are seen only in a single species. The cloudrepresented by the components at 1.85 km s-1 has a Ca linear depletiondelta (Ca) = 2.8 x 10-4, indicating the presence of dust. The f(H2)derived for this cloud is 0.45, and there is good velocity agreementwith CO emission from MBM 151. To the best of our knowledge, this 1.85km s-1 component toward HD 157841 is the first one found to haverelative line widths that are consistent with pure thermal broadeningonly. We associate the 1.85 km s-1 cloud seen in our UHRF spectra withMBM 151 and conclude that HD 157841 must lie beyond ~200 pc, theestimated distance to MBM 151.
| Candidate Main-Sequence Stars with Debris Disks: A New Sample of Vega-like Sources
Vega-like sources are main-sequence stars that exhibit IR fluxes inexcess of expectations for stellar photospheres, most likely due toreradiation of stellar emission intercepted by orbiting dust grains. Wehave identified a large sample of main-sequence stars with possibleexcess IR radiation by cross-correlating the Michigan Catalog ofTwo-dimensional Spectral Types for the HD Stars with the IRAS FaintSource Survey Catalog. Some 60 of these Vega-like sources were not foundduring previous surveys of the IRAS database, the majority of whichemployed the lower sensitivity Point Source Catalog. Here, we providedetails of our search strategy, together with a preliminary examinationof the full sample of Vega-like sources.
| An extensive Delta a-photometric survey of southern B and A type bright stars
Photoelectric photometry of 803 southern BS objects in the Deltaa-system as detection tool for magnetic chemically peculiar (=CP2) starshas been carried out and compared to published spectral types. Thestatistical yield of such objects detected by both techniques ispractically the same. We show that there are several factors whichcontaminate the search for these stars, but this contamination is onlyof the order of 10% in both techniques. We find a smooth transition fromnormal to peculiar stars. Our sample exhibits the largest fraction ofCP2 stars at their bluest colour interval, i.e. 10% of all stars in thecolour range -0.19 <= B-V < -0.10 or -0.10 <= b-y < -0.05.No peculiar stars based on the Delta a-criterion were found at bluercolours. Towards the red side the fraction of CP2 stars drops to about3% for positive values of B-V or b-y with red limits roughlycorresponding to normal stars of spectral type A5. The photometricbehaviour of other peculiar stars: Am, HgMn, delta Del, lambda Boo, Heabnormal stars, as well as Be/shell stars and supergiants shows someslight, but definite deviations from normal stars. Spectroscopic andvisual binaries are not distinguished from normal stars in their Delta abehaviour. The results of this work justify larger statistical work(e.g. in open clusters) employing more time-saving photometric methods(CCD). \newpage Based on observations obtained at the European SouthernObservatory, La Silla, Chile. This research has made use of the Simbaddatabase, operated at CDS, Strasbourg, France. Table 2 is only availablein electronic form via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Long-term visual spectrophotometric behaviour of Be stars
The long-term spectrophotometric variations of 49 Be stars are studiedusing the U and V magnitudes of the UBV system, the total Balmerdiscontinuity D and the visible gradient Phi _rb. BCD spectrophotometricand photometric data in five different photometric systems, obtained inmost cases since 1950 and reduced to the BCD system, were used. The(U,D), (V,D), (Phi _rb,D) and (Phi _rb,V) correlations obtained differfrom star to star and they can be single or double-valued. They differclearly for Be phases or Be-shell phases. Be stars with small Vsin ishowing the ``spectrophotometric shell behaviour'': D > D_*, werefound. This finding implies either that strongly flattened models ofcircumstellar envelopes are in doubt for these stars, or that not all Bestars are rapid rotators. Comparison of observed variations with thosepredicted for model Be stars with spherical circumstellar envelopes ofvariable densities and dimensions implies that spectrophotometricpatterns of Be phases are due to circumstellar envelopes in low opacityregimes, while those of spectrophotometric shell phases are due tocircumstellar envelopes in high opacity regimes. In a given star, theenvelope regions responsible for the observed variations of D and Phi_rbin spectrophotometric shell phases seem to be smaller and denser thanthose producing the observed variations of these parameters inspectrophotometric Be phases. The high positive RV found in strong shellphases might favor the formation of compact circumstellar layers nearthe star. Figure 6 is only available in electronic form at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| O VI + Ly beta + C II from Starburst and Poststarburst Galaxies. I. Stellar Library and Evolutionary Synthesis Profiles
Evolutionary synthesis models of a stellar population in thefar-ultraviolet are presented. The spectra include the lines O VI lambdalambda 1032, 1038, Ly beta , and C II lambda lambda 1036, 1037. They arebased on a stellar library built with observations of O and B starscollected with Copernicus and the Hopkins Ultraviolet Telescope (HUT).This library is used as input into an evolutionary synthesis code. Theline profile of O VI + Ly beta + C II is computed for different starformation histories (instantaneous burst and continuous star formation)and different assumptions about the initial mass function (IMF). Themetallicity is near the solar value. O VI lambda lambda 1032, 1038 is avery sensitive indicator of the presence or absence of O stars. O VIdevelops a P Cygni profile when formed in stellar winds of the mostmassive stars. When these stars are absent, no O VI is formed. Incontrast, Ly beta and C II are very sensitive indicators for B stars. Ifthese stars dominate, as is the case in poststarburst galaxies, Ly betaand C II are present as strong absorption features, and they are formedin the photosphere of B stars. An equivalent width of Ly beta + C IIlarger than 1 Angstroms always indicates a population younger than 1Gyr. Because of the universal strength of O VI in O stars, O VI is not agood discriminator between instantaneous versus continuous starformation for ages in the starburst phase, but the absence of O VI andthe presence of stellar Ly beta and C II is a good indicator of a shortburst duration and for the galaxy being in a poststarburst phase.Application of this technique to starburst or poststarburst galaxieswill require careful attention to interstellar absorption.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Eridano |
| Ascensione retta: | 04h29m06.90s |
| Declinazione: | -13°02'54.0" |
| Magnitudine apparente: | 5.6 |
| Distanza: | 483.092 parsec |
| Moto proprio RA: | -1.2 |
| Moto proprio Dec: | -1.6 |
| B-T magnitude: | 5.325 |
| V-T magnitude: | 5.555 |
Cataloghi e designazioni:
|
INHABITED SKY
