Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
A FUSE Survey of the Rotation Rates of Very Massive Stars in the Small and Large Magellanic Clouds
We present projected rotational velocity values for 97 Galactic, 55 SMC,and 106 LMC O-B type stars from archival FUSE observations. The evolvedand unevolved samples from each environment are compared through theKolmogorov-Smirnov test to determine if the distribution of equatorialrotational velocities is metallicity dependent for these massiveobjects. Stellar interior models predict that massive stars with SMCmetallicity will have significantly reduced angular momentum loss on themain sequence compared to their Galactic counterparts. Our results findsome support for this prediction but also show that even at Galacticmetallicity, evolved and unevolved massive stars have fairly similarfractions of stars with large Vsin i values. Macroturbulent broadeningthat is present in the spectral features of Galactic evolved massivestars is lower in the LMC and SMC samples. This suggests the processesthat lead to macroturbulence are dependent upon metallicity.
| Protoplanetary Disk Evolution Around the Triggered Star-Forming Region Cepheus B
The Cepheus B (Cep B) molecular cloud and a portion of the nearby CepOB3b OB association, one of the most active regions of star formationwithin 1 kpc, have been observed with the Infrared Array Camera detectoron board the Spitzer Space Telescope. The goals are to studyprotoplanetary disk evolution and processes of sequential triggered starformation in the region. Out of ~400 pre-main-sequence (PMS) starsselected with an earlier Chandra X-ray Observatory observation, ~95% areidentified with mid-infrared sources and most of these are classified asdiskless or disk-bearing stars. The discovery of the additional >200IR-excess low-mass members gives a combined Chandra+Spitzer PMS samplethat is almost complete down to 0.5 M sun outside of thecloud, and somewhat above 1 M sun in the cloud. Analyses ofthe nearly disk-unbiased combined Chandra and Spitzer-selected stellarsample give several results. Our major finding is a spatio-temporalgradient of young stars from the hot molecular core toward the primaryionizing O star HD 217086. This strongly supports the radiation-drivenimplosion (RDI) model of triggered star formation in the region. Theempirical estimate for the shock velocity of ~1 km s-1is very similar to theoretical models of RDI in shocked molecularclouds. The initial mass function (IMF) of the lightly obscuredtriggered population exhibits a standard Galactic field IMF shape. Theunusually high apparent value of gsim70% star formation efficiencyinferred from the ratio of star mass to current molecular gas massindicates that most of the Cep B molecular cloud has been alreadyablated or transformed to stars. Contrary to the current RDIsimulations, our findings indicate that star formation triggering by HII region shocks is not restricted to a single episode but can continuefor millions of years. Other results include: (1) agreement of the diskfractions, their mass dependency, and fractions of transition disks withother clusters; (2) confirmation of the youthfulness of the embedded CepB cluster; (3) confirmation of the effect of suppression oftime-integrated X-ray emission in disk-bearing versus diskless systems.
| Analysis of Galactic late-type O dwarfs: more constraints on the weak wind problem
Aims. We investigate the stellar and wind properties of a sample oflate-type O dwarfs. Previous analyses of such stars have found very lowmass-loss rates; rates much lower than predicted by theory (the weakwind problem). Methods: Far-UV to optical spectra of five Galactic Ostars were analyzed: HD 216898 (O9IV/O8.5V), HD 326329 (O9V), HD 66788(O8V/O9V), ζ Oph (O9.5Vnn), and HD 216532 (O8.5V((n))). We used agrid of TLUSTY models to obtain effective temperatures, gravities,rotational velocities, and to identify wind lines. Wind parameters foreach object were obtained using expanding atmosphere models calculatedwith the CMFGEN code. Results: The spectra of our sample have primarilya photospheric origin. A weak wind signature is seen in C ivλλ1548, 1551, from which mass-loss rates consistent withprevious CMFGEN results for O8-O9V stars were derived(˜10-10-10-9 M_ȯ yr-1). Adiscrepancy of roughly two orders of magnitude is found between thesemass-loss rates and the values predicted by theory (dot{M}_Vink),confirming a breakdown or a steepening of the modified windmomentum-luminosity relation at log L_star/L_ȯ ⪉ 5.2. We haveestimated the carbon abundance for the stars of our sample and concludedthat its value cannot be reduced to sufficiently small values to solvethe weak wind problem. Upper limits on dot{M} were established for allobjects using lines of different ions: P v λλ1118, 1128, Ciii λ 1176, N v λλ1239, 1243, Si ivλλ1394, 1403, and N iv λ1718. All the valuesobtained are in disagreement with theoretical predictions, bringingsupport to the reality of weak winds. Together with C ivλλ1548, 1551, the use of N v λλ1239, 1243results in the lowest mass-loss rates: the upper limits indicate thatdot{M} must be less than about -1.0 dex dot{M}_Vink. Upper mass-lossrate limits obtained for other transitions are also low: they indicatethat dot{M} must be less than about (-0.5 ± 0.2) dex dot{M}_Vink.We studied the behavior of the Hα line with different mass-lossrates. For two stars, only models with very low dot{M}'s provide thebest fit to the UV and optical spectra. We also explored ways to fit theobserved spectra with the theoretical mass-loss rates. By using largeamounts of X-rays, we could reduce the predicted wind emission to theobserved levels. However, unrealistic X-ray luminosities had to be used(log L_X/L_Bol ⪆ -3.5). The validity of the models used in ouranalyses is discussed.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer and by the NASA-ESA-SERC InternationalUltraviolet Explorer , and retrieved from the Multimission Archive atthe Space Telescope Science Institute (MAST). Based on observationscollected with the ELODIE spectrograph on the 1.93-m telescope(Observatoire de Haute-Provence, France). Based on observationscollected with the FEROS instrument on the ESO 2.2 m telescope, program074.D-0300 and 075.D-0061. Appendices A and B are only available inelectronic form at http://www.aanda.org
| Adaptive Optics Photometry and Astrometry of Binary Stars. III. a Faint Companion Search of O-Star Systems
We present the results of an adaptive optics survey for faint companionsamong Galactic O-type star systems (with V lsim 8) using the AdvancedElectro-Optical System (AEOS) 3.6 m telescope on Haleakala. We surveyedthese O-star systems in the I-band, typically being able to detect acompanion with a magnitude difference of utrimI lsim 6 in theprojected separation range 0farcs5 < ρ < 1farcs0, andutrimI lsim 9.5 in the range 1farcs0 < ρ < 5farcs0.In the course of the survey, we discovered 40 new companions among 31 ofthe 116 objects examined and made astrometric and differential magnitudemeasurements of 24 additional known pairs, several of them beingconfirmation detections. We present new astrometric orbits for twobinaries, BU 1032AB (WDS 05387–0236 σ Ori AB) and SEE 322(WDS 17158–3344 HD 155889AB). We lack magnitude differences forother filter bands, so it is difficult to determine physical fromline-of-sight companions, but we present empirical arguments for thelimiting magnitude difference where field contamination is significant.Based on observations made at the Maui Space Surveillance Systemoperated by Detachment 15 of the US Air Force Research Laboratory'sDirected Energy Directorate.
| Optical Spectroscopy of Type ia Supernovae
We present 432 low-dispersion optical spectra of 32 Type Ia supernovae(SNe Ia) that also have well-calibrated light curves. The coverageranges from 6 epochs to 36 epochs of spectroscopy. Most of the data wereobtained with the 1.5 m Tillinghast telescope at the F. L. WhippleObservatory with typical wavelength coverage of 3700-7400 Å and aresolution of ~7 Å. The earliest spectra are 13 days before B-bandmaximum; two-thirds of the SNe were observed before maximum brightness.Coverage for some SNe continues almost to the nebular phase. Theconsistency of the method of observation and the technique of reductionmakes this an ideal data set for studying the spectroscopic diversity ofSNe Ia.Based in part on observations obtained at the F. L. Whipple Observatory,which is operated by the Smithsonian Astrophysical Observatory, and theMMT Observatory, a joint facility of the Smithsonian Institution and theUniversity of Arizona.
| The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars
We present a comprehensive study of the observational dependence of themass-loss rate in stationary stellar winds of hot massive stars on themetal content of their atmospheres. The metal content of stars in theMagellanic Clouds is discussed, and a critical assessment is given ofstate-of-the-art mass-loss determinations of OB stars in these twosatellite systems and the Milky-Way. Assuming a power-law dependence ofmass loss on metal content, dot{M} ∝ Zm, and adopting atheoretical relation between the terminal flow velocity and metalcontent, v_∞ ∝ Z0.13 (Leitherer et al. 1992, ApJ,401, 596), we find m = 0.83 ± 0.16 for non-clumped outflows froman analysis of the wind momentum luminosity relation (WLR) for starsmore luminous than 105.2 {Lȯ}. Within theerrors, this result is in agreement with the prediction m = 0.69± 0.10 by Vink et al. (2001, A&A, 369, 574). Absoluteempirical values for the mass loss, based on Hα and ultraviolet(UV) wind lines, are found to be a factor of two higher than predictionsin this high luminosity regime. If this difference is attributed toinhomogeneities in the wind, and this clumping does not impact thepredictions, this would imply that luminous O and early-B stars haveclumping factors in their Hα and UV line forming regions of abouta factor of four. For lower luminosity stars, the winds are so weak thattheir strengths can generally no longer be derived from optical spectrallines (essentially Hα) and one must currently rely on the analysisof UV lines. We confirm that in this low-luminosity domain the observedGalactic WLR is found to be much steeper than expected from theory(although the specific sample is rather small), leading to a discrepancybetween UV mass-loss rates and the predictions by a factor 100 atluminosities of L 104.75 {Lȯ}, the originof which is unknown. We emphasize that even if the current mass-lossrates of hot luminous stars would turn out to be overestimated as aresult of wind clumping, but the degree of clumping would be ratherindependent of metallicity, the scalings derived in this study areexpected to remain correct.Appendix A is only available in electronic form at http://www.aanda.org
| V733 Cep (Persson's Star): A New FU Orionis Object in Cepheus
Persson recently found that a faint star had appeared in a cloud inCepheus. A CCD image shows a R~17.3 nebulous star, now known as V733Cep, located in the L1216 = Cep F cloud at the apex of a cavity in thecloud. Infrared photometry indicates a modest infrared excess. Opticalspectroscopy shows a well-defined Li I λ6707 line, andblueshifted absorption troughs at the Hα and Na I D linesextending to at least 200 km s-1, indicative of a massivefast wind. An infrared 1-4 μm spectrum of V733 Cep shows the presenceof strong water vapor features, and is almost identical to a similar IRspectrum of FU Ori that is reddened by AV=8 mag. Assuming anintrinsic energy distribution similar to that of FU Ori, V733 Cep has aluminosity of about 135 Lsolar at the assumed distance of 800pc. The star was detected by the Midcourse Space Experiment satellite at8.3 μm, but not by IRAS. Nor is it detected at 850 μm, indicatingthat while the star possesses circumstellar material it is notsurrounded by a significant cool envelope. A 12CO(3-2) mapshows what appears to be a small molecular outflow along the same axisas the cavity seen in optical images. There is evidence for a limitedamount of other low- and medium-mass star formation in the Cep F cloud.The totality of the evidence strongly indicates that V733 Cep is a newFU Ori-type object that must have erupted sometime between 1953 and1984.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Study of photon dominated regions in Cepheus B
Aims. The aim of the paper is to understand the emission from the photondominated regions in Cepheus B, estimate the column densities of neutralcarbon in bulk of the gas in Cepheus B and to derive constraints on thefactors which determine the abundance of neutral carbon relative toCO. Methods: .This paper presents 15 arcmin ×15 arcmin fullysampled maps of [C I] at 492 GHz and 12CO 4-3 observed withKOSMA at 1´ resolution. The new observations have been combinedwith the FCRAO 12CO 1-0, IRAM-30 m 13CO 2-1 andC18O 1-0 data, and far-infrared continuum data fromHIRES/IRAS. The KOSMA-τ spherical PDR model has been used tounderstand the [C I] and CO emission from the PDRs in Cepheus B and toexplain the observed variation of the relative abundances of bothC0 and CO. Results: .The emission from the PDRassociated with Cepheus B is primarily at V_LSR between -14 and -11 kms-1. We estimate about 23% of the observed [C II] emissionfrom the molecular hotspot is due to the ionized gas in the H II region.Over bulk of the material the C0 column density does notchange significantly, (2.0±1.4)×1017cm-2, although the CO column density changes by an order ofmagnitude. The observed C/CO abundance ratio varies between 0.06 and 4in Cepheus B. We find an anti-correlation of the observed C/CO abundanceratio with the observed hydrogen column density, which holds even whenall previous observations providing C/CO ratios are included. Here weshow that this observed variation of C/CO abundance with total columndensity can be explained only by clumpy PDRs consisting of an ensembleof clumps. At high H2 column densities high mass clumps, which exhibitlow C/CO abundance, dominate, while at low column densities, low massclumps with high C/CO abundance dominate.
| An outbursting protostar of the FU Orionis type in the Cygnus OB7 molecular cloud
Context: .To follow the early evolution of stars we need to understandhow young stars accrete and eject mass. It is generally assumed that theFU Orionis phenomenon is related to the variations in the diskaccretion, but many questions remain still open, in particular becauseof the rarity of FU Ori type stars. Aims: .We explore here thecharacteristics of the outburst and of the environment of one newobject, discovered recently in the active star formation regioncontaining RNO 127, within the Cygnus OB7 dark cloud complex.Methods: .We present an extensive optical and near-infrared study of anew candidate of FU Orionis object, including its direct imaging,spectroscopy and scanning Fabry-Pérot interferometry.Results: .The source, associated with the variable reflection nebula,underwent prodigious outburst. The "Braid" nebula, which appeared in2000, as is indicated by its name, consists of two intertwined features,illuminated by the outburst. Subsequent NIR observations revealed thebright source, which was not visible on 2MASS images, and its estimatedbrightening was more than 4 mag. Optical and infrared spectral data showfeatures, which are necessary for the system to be referred to as a FUorobject. The bipolar optical flow directed by the axis of nebula also wasfound. Various estimates give the November/December 1999 as the mostprobable date for the eruption.
| Modeling the Neutral Hydrogen Interstellar Medium: A Better Kinematic Distance Tool
An advanced approach to the kinematic distance method is developed. Themethod is applicable to second- and third-quadrant Galactic objects withknown velocities. It is based on fitting a model of the density andvelocity features in an isothermal H I disk to observed H I data. Thevelocity field of the gas is modeled with a power law for basic circularrotation, underlying noncircular motions from a two-armed density wavepattern. With a reasonable number of adjustable parameters andconstraints the model reproduces observations toward many Galacticobjects, and accurate distances are found from the modeled velocityfield. High-resolution H I spectral line data from the Canadian GalacticPlane Survey (CGPS; Taylor et al.) are used to discriminate clouds fromthe intercloud medium (the ``stratum'') for which the model is intended.The ability of the model to reproduce these data is demonstrated in one-[Tb(v)] and two- [Tb(l, v)] dimensional fits.Distances to 22 H II regions and SNRs calculated by the fitted velocityfield compare extremely well with other kinematically independentdistances.
| Chandra Study of the Cepheus B Star-forming Region: Stellar Populations and the Initial Mass Function
The Cepheus B (Cep B) molecular cloud and a portion of the nearby CepOB3b OB association, one of the most active regions of star formationwithin 1 kpc, have been observed with the ACIS detector on board theChandra X-Ray Observatory. We detect 431 X-ray sources, of which 89% areconfidently identified as clustered pre-main-sequence (PMS) stars. Twomain results are obtained. First, we provide the best census to date forthe stellar population of the region. We identify many members of tworich stellar clusters: the lightly obscured Cep OB3b association and thedeeply embedded cluster in Cep B, whose existence was previously tracedonly by a handful of radio sources and T Tauri stars. Second, we find adiscrepancy between the X-ray luminosity functions of the Cep OB3b andthe Orion Nebula cluster. This may be due to the different initial massfunctions of the two regions (an excess of ~=0.3 Msolarstars) or different age distributions. Several other results areobtained. A diffuse X-ray component seen in the field is attributed tothe integrated emission of unresolved low-mass PMS stars. The X-rayemission from HD 217086 (O7n), the principle ionizing source of theregion, follows the standard model, involving many small shocks in anunmagnetized radiatively accelerated wind. X-ray source 294 joins anumber of similar superflare PMS stars for which long magneticstructures may connect the protoplanetary disk to the stellar surface.
| The Discordance of Mass-Loss Estimates for Galactic O-Type Stars
We have determined accurate values of the product of the mass-loss rateand the ion fraction of P+4, M˙q(P+4), for asample of 40 Galactic O-type stars by fitting stellar wind profiles toobservations of the P V resonance doublet obtained with FUSE, ORFEUSBEFS, and Copernicus. When P+4 is the dominant ion in thewind [i.e., 0.5<~q(P+4)<=1], M˙q(P+4)approximates the mass-loss rate to within a factor of <~2. Theorypredicts that P+4 is the dominant ion in the winds of O7-O9.7stars, although an empirical estimator suggests that the range O4-O7 maybe more appropriate. However, we find that the mass-loss rates obtainedfrom P V wind profiles are systematically smaller than those obtainedfrom fits to Hα emission profiles or radio free-free emission bymedian factors of ~130 (if P+4 is dominant between O7 andO9.7) or ~20 (if P+4 is dominant between O4 and O7). Thesediscordant measurements can be reconciled if the winds of O stars in therelevant temperature range are strongly clumped on small spatial scales.We use a simplified two-component model to investigate the volumefilling factors of the denser regions. This clumping implies thatmass-loss rates determined from ``ρ2'' diagnostics havebeen systematically overestimated by factors of 10 or more, at least fora subset of O stars. Reductions in the mass-loss rates of this size haveimportant implications for the evolution of massive stars andquantitative estimates of the feedback that hot-star winds provide totheir interstellar environments.
| A Medium Resolution Near-Infrared Spectral Atlas of O and Early-B Stars
We present intermediate-resolution (R~8000-12,000) high signal-to-noise(S/N) H- and K-band spectroscopy of a sample of 37 optically visiblestars, ranging in spectral type from O3 to B3 and representing mostluminosity classes. Spectra of this quality can be used to constrain thetemperature, luminosity, and general wind properties of OB stars, whenused in conjunction with sophisticated atmospheric model codes. Mostimportant is the need for moderately high resolutions (R>=5000) andvery high signal-to-noise (S/N>=150) spectra for a meaningful profileanalysis. When using near-infrared spectra for a classification system,moderately high signal-to-noise (S/N~100) is still required, though theresolution can be relaxed to just a thousand or two. In the Appendix weprovide a set of very high-quality near-infrared spectra of Brackettlines in six early-A dwarfs. These can be used to aid in the modelingand removal of such lines when early-A dwarfs are used for telluricspectroscopic standards.
| Interstellar Ca II Line Intensities and the Distances of the OB stars
We show that the equivalent widths of the well-known interstellar Ca IIH and K lines can be used to determine the distances to OB stars in ourGalaxy. The equivalent widths, measured in the spectra of 147 early-typestars, are strongly related to the Hipparcos parallaxes of thoseobjects. The lines fitted to the parallax-equivalent width data aregiven by the formulae π=1/[2.78EW(K)+95] and π=1/[4.58EW(H)+102],where π is in arcseconds and EW is in milliangstroms. The form of theformulae, yielding a finite parallax even for zero absorption, showsthat space within ~100 pc of the Sun contains very little Ca II, whichis in agreement with the known dimensions of the Local Bubble. Using CaII lines for distance determination does not require the knowledge ofthe absolute magnitude of the object; it is thus well suited for targetsfor which the absolute calibration is either not precise (OBsupergiants) or not available at all (peculiar objects). We alsodemonstrate that neither the reddening E(B-V) nor the equivalent widthsof interstellar K I and CH lines are suitable candidates for distanceestimation, their relation with parallaxes being far less tight than forCa II.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Spectral analysis of early-type stars using a genetic algorithm based fitting method
We present the first automated fitting method for the quantitativespectroscopy of O- and early B-type stars with stellar winds. The methodcombines the non-LTE stellar atmosphere code fastwind from Puls et al.(2005, A&A, 435, 669) with the genetic algorithm based optimizationroutine pikaia from Charbonneau (1995, ApJS, 101, 309), allowing for ahomogeneous analysis of upcoming large samples of early-type stars (e.g.Evans et al. 2005, A&A, 437, 467). In this first implementation weuse continuum normalized optical hydrogen and helium lines to determinephotospheric and wind parameters. We have assigned weights to theselines accounting for line blends with species not taken into account,lacking physics, and/or possible or potential problems in the modelatmosphere code. We find the method to be robust, fast, and accurate.Using our method we analysed seven O-type stars in the young cluster CygOB2 and five other Galactic stars with high rotational velocities and/orlow mass loss rates (including 10 Lac, ζ Oph, and τ Sco) thathave been studied in detail with a previous version of fastwind. Thefits are found to have a quality that is comparable or even better thanproduced by the classical “by eye” method. We defineerrorbars on the model parameters based on the maximum variations ofthese parameters in the models that cluster around the global optimum.Using this concept, for the investigated dataset we are able to recovermass-loss rates down to ~6 × 10-8~Mȯyr-1 to within an error of a factor of two, ignoringpossible systematic errors due to uncertainties in the continuumnormalization. Comparison of our derived spectroscopic masses with thosederived from stellar evolutionary models are in very good agreement,i.e. based on the limited sample that we have studied we do not findindications for a mass discrepancy. For three stars we findsignificantly higher surface gravities than previously reported. Weidentify this to be due to differences in the weighting of Balmer linewings between our automated method and “by eye” fittingand/or an improved multidimensional optimization of the parameters. Theempirical modified wind momentum relation constructed on the basis ofthe stars analysed here agrees to within the error bars with thetheoretical relation predicted by Vink et al. (2000, A&A, 362, 295),including those cases for which the winds are weak (i.e. less than a fewtimes 10-7 Mȯ yr-1).
| Quantitative H and K band spectroscopy of Galactic OB-stars at medium resolution
In this paper we have analyzed 25 Galactic O and early B-stars by meansof H and K band spectroscopy, with the primary goal to investigate towhat extent a lone near-IR spectroscopy is able to recover stellar andwind parameters derived in the optical. Most of the spectra have beentaken with subaru-ircs, at an intermediate resolution of 12 000, andwith a very high S/N, mostly on the order of 200 or better. In order tosynthesize the strategic H/He lines, we have used our recent,line-blanketed version of fastwind (Puls et al. 2005, A&A, 435,669). In total, seven lines have been investigated, where for two starswe could make additional use of the Hei2.05 singlet which has beenobserved with irtf-cshell. Apart from Brγ and Heii2.18, the otherlines are predominately formed in the stellar photosphere, and thusremain fairly uncontaminated from more complex physical processes,particularly clumping. First we investigated the predicted behaviour ofthe strategic lines. In contradiction to what one expects from theoptical in the O-star regime, almost all photospheric H/Hei/Heii H/Kband lines become stronger if the gravity decreases. Concerning H andHeii, this finding is related to the behaviour of Stark broadening as afunction of electron density, which in the line cores is different formembers of lower (optical) and higher (IR) series. Regarding Hei, thepredicted behaviour is due to some subtle NLTE effects resulting in astronger overpopulation of the lower level when the gravity decreases.We have compared our calculations with results from the alternative NLTEmodel atmosphere code cmfgen (Hillier & Miller 1998, ApJ, 496, 407).In most cases, we found reasonable or nearly perfect agreement. Only theHei2.05 singlet for mid O-types suffers from some discrepancy, analogouswith findings for the optical Hei singlets. For most of our objects, weobtained good fits, except for the line cores of Brγ in earlyO-stars with significant mass-loss. Whereas the observations showBrγ mostly as rather symmetric emission lines, the models predicta P Cygni type profile with strong absorption. This discrepancy (whichalso appears in lines synthesized by cmfgen) might be an indirect effectof clumping. After having derived the stellar and wind parameters fromthe IR, we have compared them to results from previous optical analyses.Overall, the IR results coincide in most cases with the optical oneswithin the typical errors usually quoted for the correspondingparameters, i.e., an uncertainty in T_eff of 5%, in log g of 0.1 dex andin {dot M} of 0.2 dex, with lower errors at higher wind densities.Outliers above the 1-σ level where found in four cases withrespect to log g and in two cases for {dot M}.
| Spectroscopy of six highly evolved Abell planetary nebulae
We have undertaken visual spectroscopy of the highly evolved planetarynebulae (PNe) A8, A13, A62, A72, A78 and A83 over a wavelength range4330 < λ < 6830Å. This permits us to specify relativeline intensities in various sectors of the nebular shells, and toinvestigate the variation of emission as a function of radius. Wedetermine that the spectrum of the central star of A78 has variedappreciably over a period of 25 yr. There is now evidence for strong PCygni absorption in the λ4589 and λ5412 transitions ofHeII, implying terminal velocities of the order ofV∞≅ 3.83 × 103 km s-1.We also note that the emission-line profiles of the sources can be usedto investigate their intrinsic emission structures. We find that mostPNe show appreciable levels of emission throughout their volumes; onlyone source (A13) possesses a thin-shell structure. Such results are inconformity with evolutionary theory, and probably reflect theconsequences of adiabatic cooling in highly evolved outflows.
| Chandra Observations of Triggered Star Formation in Cepheus
We present the first high-spatial-resolution X-ray images and spectra ofthe Cepheus B/Sharpless 155 interface region, obtained in a single30-ksec Chandra/ACIS observation in March 2003. This rich high-massstar-forming region reveals a complex mix of point source and diffuseX-ray emission. The OB association Cep OB3b is resolved at the arcsecondlevel into more than 400 sources. An embedded stellar cluster,representing the most recent generation of star formation at theS155/CepB interface, is resolved into several components. Soft diffuseX-ray emission (kT=0.8 keV) centered on the O7 star HD 217086 pervadesthe complex. A remarkably strong X-ray flare is seen in one of thepre-main-sequence cluster members. These and other highlights of theChandra data will be described.This work is supported by NASA contract NAS8-38252 (G.P. Garmire, PI).
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| Correlations between diffuse interstellar bands and atomic lines
We present and discuss correlations between strengths of the well-known,strong interstellar atomic lines of KI and CaII, and four selected,strong unidentified diffuse interstellar bands (DIBs): 5780, 5797, 5850and 6614. In order to analyse a homogeneous sample of echellehigh-resolution spectra it has been chosen to use measurements fromTerskol Observatory in Northern Caucasus plus a selected number ofhigher resolution observations performed using other instruments. Wedemonstrate that the strength of certain DIBs correlate well withneutral potassium lines and to a much lower degree with ionized calciumlines. This fact suggests that the degree of irradiation of a cloud withUV photons, capable to ionize interstellar atoms, plays a crucial rolein the formation/maintenance of certain molecular species: possiblecarriers of DIBs.
| Puzzling wind properties of young massive stars in SMC-N81
We present a quantitative study of massive stars in the High ExcitationBlob N81, a compact star forming region in the SMC. The stellar contentwas resolved by HST, and STIS was used to obtain medium resolutionspectra. The qualitative analysis of the stellar properties presented inHeydari-Malayeri et al. (\cite{papI}) is extended using non-LTEspherically extended atmosphere models including line-blanketingcomputed with the code CMFGEN (Hillier & Miller \cite{hm98}), andthe wind properties are investigated. The main results are thefollowing:The SMC-N81 components are young (˜0-4 Myrs) O stars witheffective temperatures compatible with medium to late subtypes and withluminosities lower than those of average Galactic O dwarfs, renderingthem possible ZAMS candidates.The winds are extremely weak: with values of the order of10-8/10-9 Mȯ yr-1, themass loss rates are lower than observed so far for Galactic dwarfs. Onlythe recent study of SMC stars by Bouret et al. (\cite{jc03}) show thesame trend. The modified wind momenta (\dot{M}v∞\sqrt{R}) are also 1 to 2 orders of magnitude lowerthan observed for Galactic stars. Both the mass loss rates and themodified wind momenta are lower than the predictions of the most recenthydrodynamical models.The accuracy of the UV based mass loss rate determination, relying inparticular on the predicted ionisation fractions, are carefullyexamined. We find that \dot{M} could be underestimated by a factor of upto 10. Even in this unlikely case, the above conclusions remain validqualitatively.The reasons for such weak winds are investigated with special emphasison the modified wind momenta:There may be a break-down of the wind momentum-luminosity relation (WLR)for dwarf stars at low luminosity (log L/Lȯ⪉ 5.5).However, reasons for such a breakdown remain unknown.The slope of the WLR may be steeper at low metallicity. This ispredicted by the radiation driven wind theory, but the currenthydrodynamical simulations do not show any change of the slope at SMCmetallicity. Moreover, there are indications that some Galactic objectshave wind momenta similar to those of the SMC stars.Decoupling may take place in the atmosphere of the SMC-N81 stars,leading to multicomponent winds. However, various tests indicate thatthis is not likely to be the case.The origin of the weakness of the wind observed in the SMC-N81 starsremains unknown. We suggest that this weakness may be linked with theyouth of these stars and represents possibly the onset of stellar windsin recently formed massive stars.Appendix A is only available in electronic form athttp://www.edpsciences.org
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| Stellar and wind parameters of Galactic O-stars. The influence of line-blocking/blanketing
We have re-analyzed the Galactic O-star sample from \citet{puls96} bymeans of line-blanketed NLTE model atmospheres in order to investigatethe influence of line-blocking/blanketing on the derived parameters. Theanalysis has been carried out by fitting the photospheric and wind linesfrom H and He. In most cases we obtained a good fit, but we have alsofound certain inconsistencies which are probably related to a stillinadequate treatment of the wind structure. These inconsistenciescomprise the line cores of Hγ and Hβ insupergiants (the synthetic profiles are too weak when the mass-loss rateis determined by matching Hα) and the ``generalizeddilution effect'' (cf. \citealt{vo89}) which is still present in He I4471 of cooler supergiants and giants.Compared to pure H/He plane-parallel models we found a decrease ineffective temperatures which is largest at earliest spectral types andfor supergiants (with a maximum shift of roughly 8000 K). This findingis explained by the fact that line-blanketed models of hot stars havephotospheric He ionization fractions similar to those from unblanketedmodels at higher Teff and higher log g. Consequently, anyline-blanketed analysis based on the He ionization equilibrium resultsin lower Teff-values along with a reduction of either log gor helium abundance (if the reduction of log g is prohibited by theBalmer line wings). Stellar radii and mass-loss rates, on the otherhand, remain more or less unaffected by line-blanketing.We have calculated ``new'' spectroscopic masses and compared them withprevious results. Although the former mass discrepancy \citep{h92}becomes significantly reduced, a systematic trend for masses below 50Msun seems to remain: The spectroscopically derived valuesare smaller than the ``evolutionary masses'' by roughly 10Msun. Additionally, a significant fraction of our samplestars stays over-abundant in He, although the actual values were foundto be lower than previously determined.Also the wind-momentum luminosity relation (WLR) changes because oflower luminosities and almost unmodified wind-momentum rates. Comparedto previous results, the separation of the WLR as a function ofluminosity class is still present but now the WLR for giants/dwarfs isconsistent with theoretical predictions.We argue that the derived mass-loss rates of stars withHα in emission are affected by clumping in the lowerwind region. If the predictions from different and independenttheoretical simulations (\citealt {Vink00, Paul03, puls03a}) that theWLR should be independent of luminosity class were correct, a typicalclumping factor <ρ2>/<ρ>2 ≈5 should be derived by ``unifying'' the different WLRs.Based upon observations obtained at the INT and the European SouthernObservatory, La Silla, Chile. The INT is operated on the island of LaPalma by the ING in the Spanish Observatorio de El Roque de losMuchachos of the Instituto de Astrofísica de Canarias.Appendix A in only available in electronic form athttp://www.edpsciences.org
| A study of RV in Galactic O stars from the 2MASS catalogue
We present new measurements of the interstellar reddening parameterRV=AV/E(B-V) towards 185 O stars, using J, H,Ks photometry from the 2MASS project. The results arecombined with data from the literature of 95 stars where RVhas been derived with the same technique, 22 of which in common with ourpresent sample from the 2MASS project catalogue. The averageRV from these 258 O stars is of 3.19 +/- 0.50. All objectswhose RV departs from this value by more than 2 sigma havebeen recognized. Ten objects have RV higher than this valueand two lower. It is found that anomalous RV can scarcely beassociated with anomalies in the general interstellar medium, e.g. withdifferent behaviour in different spiral arms. They are clearly linked tolocal cloud effect. In the Cygnus region RV values follow thebehaviour of the general interstellar medium, while in the Carina arm,in spite of the relatively larger distance, local cloud effects prevail.An explanation for this is suggested. The relatively few stars of oursample whose Hipparcos parallaxes are reliable, are found to havedistances systematically smaller than the distances derived by thespectroscopic parallaxes. We argue that this effect is consistent withthe recently claimed discovery of grey extinction towards OB stars.This publication makes use of data products from the Two Micron All SkySurvey (2MASS), which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and SpaceAdministration and the National Science Foundation.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/905
| The discovery of low-mass pre-main-sequence stars in Cepheus OB3b
We report the discovery of a low-mass pre-main-sequence (PMS) stellarpopulation in the younger subgroup of the Cepheus OB3 association, CepOB3b, using UBVI CCD photometry and follow-up spectroscopy. The opticalsurvey covers approximately 1300 arcmin2 on the sky and givesa global photometric and astrometric catalogue for more than 7000objects. The location of a PMS population is well defined in a V versus(V-I) colour-magnitude diagram.Multifibre spectroscopic results for optically selected PMS candidatesconfirm the T Tauri nature for 10 objects, with equal numbers ofclassical TTS (CTTS) and weak-line TTS (WTTS). There are six otherobjects that we classify as possible PMS stars. The newly discovered TTSstars have masses in the range ~0.9-3.0 Msolar and ages from<1 to nearly 10 Myr, based on the Siess, Dufour & Forestiniisochrones. Their location close to the O and B stars of the association(especially the O7n star) demonstrates that low-mass star formation isindeed possible in such an apparently hostile environment dominated byearly-type stars and that the latter must have been less effective ineroding the circumstellar discs of their lower-mass siblings comparedwith other OB associations (e.g. λ-Ori). We attribute this to thenature of the local environment, speculating that the bulk of molecularmaterial, which shielded low-mass stars from the ionizing radiation oftheir early-type siblings, has only recently been removed.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| The Extinction and Distance of Maffei 1
We have obtained low- and high-resolution spectra of the core of thehighly reddened elliptical galaxy Maffei 1. From these data, we haveobtained the first measurement of the Mg2 index and havemeasured the velocity dispersion and radial velocity with improvedaccuracy. To evaluate the extinction, a correlation between theMg2 index and effective V-I color has been established forelliptical galaxies. Using a new method for correcting for effectivewavelength shifts, the V-I color excess reveals that the optical depthof Galactic dust at 1 μm is 1.69+/-0.07. Thus,AV=4.67+/-0.19 mag, which is lower by 0.4 mag than previouslythought. To establish the distance, the fundamental plane for ellipticalgalaxies has been constructed in I. The velocity dispersion of Maffei 1,measured to be 186.8+/-7.4 km s-1, in combination with modernwide-field photometry in I, leads to a distance of 2.92+/-0.37 Mpc. TheDn-σ relation, which is independently calibrated, gives3.08+/-0.85 and 3.23+/-0.67 Mpc from photometry in B and K',respectively. The weighted mean of the three estimates is 3.01+/-0.30Mpc, which is lower than distances judged with reference to M32 and thebulge of M31 from the brightest stars seen at K'. Since the luminosityof asymptotic giant branch stars at K' is strongly dependent on age, thelower distance suggests that the last epoch of star formation in Maffei1 occurred farther in the past than in these other systems. The distanceand luminosity make Maffei 1 the nearest giant elliptical galaxy. In theabsence of extinction, the galaxy would be among the brightest in thesky and would have an apparent size 2/3 that of the full Moon. Theradial velocity of Maffei 1 is +66.4+/-5.0 km s-1,significantly higher than the accepted value of -10 km s-1.The Hubble distance corresponding to the mean velocity of Maffei 1,Maffei 2, and IC 342 is 3.5 Mpc. Thus, it is unlikely that Maffei 1 hashad any influence on Local Group dynamics.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Cefeusz |
| Rektaszcenzió: | 22h56m47.19s |
| Deklináció: | +62°43'37.6" |
| Vizuális fényesség: | 7.721 |
| Távolság: | 833.333 parszek |
| RA sajátmozgás: | 3.8 |
| Dec sajátmozgás: | -8.7 |
| B-T magnitude: | 8.343 |
| V-T magnitude: | 7.773 |
Katalógusok és elnevezések:
|
