תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
A multiwavelength investigation of the temperature of the cold neutral medium
We present measurements of the HI spin temperatures (Ts) ofthe cold neutral medium (CNM) towards radio sources that are closelyaligned with stars for which published H2 ortho-paratemperatures (T01) are available from ultraviolet (UV)observations. Our sample consists of 18 radio sources close to 16 nearbystars. The transverse separation of the lines of sight of thecorresponding UV and radio observations varies from 0.1 to 12.0 pc atthe distance of the star. The UV measurements do not have velocityinformation, so we use the velocities of low ionization species (e.g.NaI/KI/CI) observed towards these same stars to make a plausibleidentification of the CNM corresponding to the H2 absorption.We then find that T01 and Ts match withinobservational uncertainties for lines of sight with H2 columndensity above 1015.8cm-2, but deviate from eachother below this threshold. This is consistent with the expectation thatin the CNM Ts tracks the kinetic temperature due tocollisions and that T01 is driven towards the kinetictemperature by proton exchange reactions.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| Near-infrared identification of the counterpart to X1908+075: a new OB-supergiant X-ray binary
We report the near-infrared (near-IR) identification of the likelycounterpart to X1908+075, a highly absorbed Galactic X-ray sourcerecently suspected to belong to the rare class of OB supergiant-neutronstar binary systems. Our JHKs-band imaging of the fieldreveals the existence within the X-ray error boxes of a near-IR sourceconsistent with an early-type star lying at d~ 7 kpc and sufferingAV~ 16 mag of extinction, the latter value being in goodagreement with the hydrogen column density derived from modelling of theX-ray spectrum. Our follow-up, near-IR spectroscopic observationsconfirm the nature of this candidate and lead to a late O-typesupergiant classification, thereby supporting the identification of anew Galactic OB-supergiant X-ray binary.
| The Young Cluster NGC 2362
An Hα emission survey of the young cluster NGC 2362 resulted inthe detection of 130 Hα emission stars in an11'×11' field approximately centered on thefourth magnitude O9 Ib multiple star τ CMa. The survey was carriedout using the wide-field grism spectrograph on the University of Hawaii2.2 m telescope and the Gemini Multi-Object Spectrograph (GMOS) onGemini North. Deep optical VRCIC (to V~23.0) andnear-infrared (NIR) photometry (JHK) to K~16 were obtained for severalfields within the cluster. Spectra covering the 6000-8000 Å regionat a resolution of R~3000 (adequate for the determination of Li Iλ6708 line strengths) were also acquired for ~200pre-main-sequence (PMS) candidates with GMOS. Ages and masses for theHα emitters in NGC 2362 were inferred from the isochrones andevolutionary tracks of D'Antona & Mazzitelli, as well as those ofBaraffe et al. An estimated cluster age of ~1.8 Myr follows from themodels of D'Antona & Mazzitelli and 3.5-5.0 Myr from those ofBaraffe et al. The fraction of the T Tauri star (TTS) population that iscomposed of weak-line emitters, f(WTTS), is 0.91, compared with 0.43 forthe TTS population of NGC 2264. On the basis of W(Hα) alone, thefraction of TTSs still undergoing accretion is 5%-9%, comparable to theinner disk fraction determined from JHKL-band excesses by Haisch andcoworkers (12%). Approximately 15% of the PMS sample in this studyexhibits possible NIR excess, having EH-K>0.1 mag. Giventhe lack of NIR excess and strong Hα emission from the majority ofcluster members, it is inferred that the inner disk regions of the TTSpopulation have dissipated significantly. The mean level ofchromospheric activity among the WTTS population of NGC 2362 islog(LHα/Lbol)=-3.65, significantly greaterthan that of the low-mass population of the 600 Myr old Hyades cluster,log(LHα/Lbol)=-3.90. The total mass of theHα emitters and the OB stellar population of NGC 2362 defines alower limit for the cluster mass of ~300 Msolar. Allowancefor A- and F-type stars still on the radiative track, multiplicity,outlying members, and the low-mass population lying below thecompleteness limit of the Hα emission survey increases this lowerlimit to well over 500 Msolar. The derived relaxation,disruption, and evaporation timescales for the cluster imply that NGC2362 will likely survive beyond the age of the Pleiades, but statisticsof galactic cluster lifetimes favor its disruption well before the ageof the Hyades.
| Astrometric orbits of SB^9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to deriveastrometric orbital elements for spectroscopic binaries from the newlyreleased Ninth Catalogue of Spectroscopic Binary Orbits(SB^9). This endeavour is justified by the fact that (i) theastrometric orbital motion is often difficult to detect without theprior knowledge of the spectroscopic orbital elements, and (ii) suchknowledge was not available at the time of the construction of theHipparcos Catalogue for the spectroscopic binaries which were recentlyadded to the SB^9 catalogue. Among the 1374 binaries fromSB^9 which have an HIP entry (excluding binaries with visualcompanions, or DMSA/C in the Double and Multiple Stars Annex), 282 havedetectable orbital astrometric motion (at the 5% significance level).Among those, only 70 have astrometric orbital elements that are reliablydetermined (according to specific statistical tests), and for the firsttime for 20 systems. This represents a 8.5% increase of the number ofastrometric systems with known orbital elements (The Double and MultipleSystems Annex contains 235 of those DMSA/O systems). The detection ofthe astrometric orbital motion when the Hipparcos IAD are supplementedby the spectroscopic orbital elements is close to 100% for binaries withonly one visible component, provided that the period is in the 50-1000 drange and the parallax is >5 mas. This result is an interestingtestbed to guide the choice of algorithms and statistical tests to beused in the search for astrometric binaries during the forthcoming ESAGaia mission. Finally, orbital inclinations provided by the presentanalysis have been used to derive several astrophysical quantities. Forinstance, 29 among the 70 systems with reliable astrometric orbitalelements involve main sequence stars for which the companion mass couldbe derived. Some interesting conclusions may be drawn from this new setof stellar masses, like the enigmatic nature of the companion to theHyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but thecompanion remains elusive.
| To see or not to see a bow shock. Identifying bow shocks with Hα allsky surveys
OB-stars have the highest luminosities and strongest stellar winds ofall stars, which enables them to interact strongly with theirsurrounding ISM, thus creating bow shocks. These offer us an idealopportunity to learn more about the ISM. They were first detected andanalysed around runaway OB-stars using the IRAS allsky survey by vanBuren et al. (1995, AJ, 110, 2614). Using the geometry of such bowshocks information concerning the ISM density and its fluctuations canbe gained from such infrared observations. As to help to improve the bowshock models, additional observations at other wavelengths, e.g.Hα, are most welcome. However due to their low velocity these bowshocks have a size of 1°, and could only be observed as awhole with great difficulties. In the light of the new Hα allskysurveys (SHASSA/VTSS) this is no problem any more. We developeddifferent methods to detect bow shocks, e.g. the improved determinationof their symmetry axis with radial distance profiles. Using twoHα-allsky surveys (SHASSA/VTSS), we searched for bow shocks andcompared the different methods. From our sample we conclude, that thecorrelation between the direction of both proper motion and the symmetryaxis determined with radial distance profile is the most promisingdetection method. We found eight bow shocks around HD17505, HD 24430, HD48099, HD 57061, HD92206, HD 135240, HD149757, and HD 158186 from 37 candidatestaken from van Buren et al. (1995, AJ, 110, 2614). Additionally to thetraditional determination of ISM parameters using the standoff distanceof the bow shock, another approach was chosen, using the thickness ofthe bow-shock layer. Both methods lead to the same results, yieldingdensities ( 1 cm-3) and the maximal temperatures (104 K), that fit well to the up-to-date picture of the WarmIonised Medium.
| On the massive stellar population of the super star cluster Westerlund 1
We present new spectroscopic and photometric observations of the youngGalactic open cluster Westerlund 1 (Wd 1) that reveala unique population of massive evolved stars. We identify ~200 clustermembers and present spectroscopic classifications for ~25% of these. Wefind that all stars so classified are unambiguously post-Main Sequenceobjects, consistent with an apparent lack of an identifiable MainSequence in our photometric data to V 20. We are able to identifyrich populations of Wolf Rayet stars, OB supergiants and short livedtransitional objects. Of these, the latter group consists of both hot(Luminous Blue Variable and extreme B supergiant) and cool (YellowHypergiant and Red Supergiant) objects - we find that half the knownGalactic population of YHGs resides within Wd 1. We obtain a meanV-MV ~ 25 mag from the cluster Yellow Hypergiants, implying aMain Sequence turnoff at or below MV =-5 (O7 V or later).Based solely on the masses inferred for the 53 spectroscopicallyclassified stars, we determine an absolute minimum mass of ~1.5 ×10^3~Mȯ for Wd 1. However, considering the completephotometrically and spectroscopically selected cluster population andadopting a Kroupa IMF we infer a likely mass for Wd 1 of~10^5~Mȯ, noting that inevitable source confusion andincompleteness are likely to render this an underestimate. As such, Wd 1is the most massive compact young cluster yet identified in the LocalGroup, with a mass exceeding that of Galactic Centre clusters such asthe Arches and Quintuplet. Indeed, the luminosity, inferred mass andcompact nature of Wd 1 are comparable with those of Super Star Clusters- previously identified only in external galaxies - and is consistentwith expectations for a Globular Cluster progenitor.
| Close binary stars in ob-association regions i. preliminary investigation
We performed a sample of O- and B-eclipsing binary stars inOB-association regions and obtained the preliminary list of 147 binariesin 45 OB-association regions. We tried to elucidate the question whether(or not) the close binaries belong to corresponding OB-associations,from the commonness of their proper motions, radial velocities anddistances. Based on the completeness of the data,the binaries aredevided into three groups and the scheme for calculation of degree ofbelonging of stars to OB-associations is developed. Necessary data arenot available for nine systems and they are given in a specific table.For 12 cases, the binaries project onto the regions of two associations.We show that 33 (22.3%) close binary stars are members, 65 (43.9%) areprobable members and 39 (26.4%) are less probable members of theOB-associations. We find that 11 binaries belong to the Galaxybackground. The comparison of the distributions of orbital periods forthe binaries in OB-associations and for O-, B-binaries of the Galaxybackground shows their considerable differences in the vicinity of thetwo-day period.
| 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.
| 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.
| Masses and other parameters of massive binaries
Binary stars provide us with the means to measure stellar mass. Here Ipresent several lists of known O-type stars with reliable mass estimatesthat are members of eclipsing, double-lined spectroscopic binaries. Themasses of young, unevolved stars in binaries are suitable for testingthe predictions of evolutionary codes, and there is good agreementbetween the observed and predicted masses (based upon temperature andluminosity) if the lower temperature scale from line-blanketed modelatmospheres is adopted. A final table lists masses for systems in awide variety of advanced evolutionary stages.
| Evolution of Wolf-Rayet Stars in Binary Systems: An Analysis of the Mass and Orbital-Eccentricity Distributions
We have undertaken a statistical study of the component mass ratios andthe orbital eccentricities of WR + O close binary, detachedmain-sequence (DMS), contact early-type (CE), and semidetached (SD)systems. A comparison of the characteristics of WR + O systems and ofDMS, CE, and SD systems has enabled us to draw certain conclusions aboutthe evolutionary paths of WR + O binaries and to demonstrate that up to90% of all known WR + O binaries formed as a result of mass transfer inmassive close O + O binary systems. Since there is a clear correlationbetween the component masses in SD systems with subgiants, the absenceof an anticorrelation between the masses of the WR stars and O stars inWR + O binaries cannot be considered evidence against the formation ofWR + O binaries via mass transfer. The spectroscopic transitionalorbital period P tr sp corresponding to the transition from nearlycircular orbits (e sp<0.1) to elliptical orbits (e sp≥0.1) is14d for WR + O systems and 2d 3d for OB + OB systems. Theperiod range in which all WR + O orbits are circular &$(1mathop dlimits_. 6 ≤slant P ≤slant 14(d) ); is close to the range for SD systems with subgiants, &0mathop dlimits_. 7 ≤slant P ≤slant 15(d); . The large difference between the P tr sp values for WR + O and OB +OB systems suggests that a mechanism of orbit circularization additionalto that for OB + OB systems at the DMS stage (tidal dissipation of theorbital energy due to radiative damping of the dynamical tides) acts inWR + O binaries. It is natural to suggest mass transfer in the parent O+ O binaries as this supplementary orbit-circularization mechanism.Since the transitional period between circular and elliptical orbits forclose binaries with convective envelopes and ages of 5×109 yearsis &P_{tr} = 12mathop dlimits_. 4$; , the orbits of most known SD systems with subgiants had enough timeto circularize during the DMS stage, prior to the mass transfer. Thus,for most SD systems, mass transfer plays a secondary role incircularization of their orbits. In many cases, the initial orbitaleccentricities of the O + O binary progenitors of WR + O systems arepreserved, due to the low viscosity of the O-star envelopes and theshort timescale for their nuclear evolution until the primary O starfills its Roche lobe and the mass transfer begins. The mass transfer inthe parent O + O systems is short-lived, and the number of orbitalcycles during the early mass-transfer stage is relatively low (lowerthan for the progenitors of SD systems by three or four orders ofmagnitude). The continued transfer of mass from the less massive to themore massive star after the component masses have become equal leads tothe formation of a WR + O system, and the orbit's residual eccentricityincreases to the observed value. The increase of the orbitaleccentricity is also facilitated by variable radial mass loss via thewind from the WR star in the WR + O system during its motion in theelliptical orbit. The result is that WR + O binaries can haveconsiderable orbital eccentricities, despite their intense masstransfer. For this reason, the presence of appreciable eccentricitiesamong WR + O binaries with large orbital periods cannot be consideredfirm evidence against mass transfer in the parent O + O binary systems.Only for the WR + O binaries with the longest orbital periods (4 of 35known systems, or 11 %) can the evolution of the parent O + O binariesoccur without filling of the Roche lobe by the primary O star, beinggoverned by radial outflow in the form of the stellar wind and possiblyby the LBV phenomenon, as in the case of HD 5980.
| Chandra X-Ray Spectroscopic Imaging of Sagittarius A* and the Central Parsec of the Galaxy
We report the results of the first-epoch observation with the ACIS-Iinstrument on the Chandra X-Ray Observatory of Sagittarius A* (Sgr A*),the compact radio source associated with the supermassive black hole(SMBH) at the dynamical center of the Milky Way. This observationproduced the first X-ray (0.5-7 keV) spectroscopic image with arcsecondresolution of the central 17'×17'(40pc×40pc) of the Galaxy. We report the discovery of an X-raysource, CXOGC J174540.0-290027, coincident with Sgr A* within0.27"+/-0.18". The probability of a false match is estimated to be<~0.5%. The spectrum is well fitted either by an absorbed power lawwith photon index Γ~2.7 or by an absorbed optically thin thermalplasma with kT~1.9 keV and column densityNH~1×1023 cm-2. The observed fluxin the 2-10 keV band is ~1.3×10-13 ergs cm-2s-1, and the absorption-corrected luminosity is~2.4×1033 ergs s-1. The X-ray emission atthe position of Sgr A* is extended, with an intrinsic size of ~1.4"(FWHM), consistent with the Bondi accretion radius for a2.6×106 Msolar black hole. A compactcomponent within the source flared by up to a factor of 3 over a periodof ~1 hr at the start of the observation. The search for Kα lineemission from iron was inconclusive, yielding an upper limit on theequivalent width of 2.2 keV. Several potential stellar origins for theX-ray emission at Sgr A* are considered, but we conclude that thevarious properties of the source favor accretion onto the SMBH as theorigin for the bulk of the emission. These data are inconsistent with``standard'' advection-dominated accretion flow (ADAF) models or Bondimodels, unless the accretion rate from stellar winds is much lower thananticipated. The central parsec of the Galaxy contains an ~1.3 keVplasma with electron densityne~26η-1/2fcm-3, whereηf is the filling factor. This plasma should supply~10-6 Msolar yr-1 of material to theaccretion flow at the Bondi radius, whereas measurements of linearpolarization at 150 GHz and above limit the accretion rate near theevent horizon to <~10-8 Msolar yr-1,assuming an equipartition magnetic field. Taken together, the X-ray andradio results imply that outflows or convection are playing a role inADAF models and subequipartition magnetic fields in Bondi models, orelse the X-ray emission must be generated predominantly via thesynchrotron self-Compton (SSC) process. The measured extent of thesource and the detection of short timescale variability are evidencethat the emission from Sgr A* contains both thermal and nonthermalemission components at comparable levels. We also discuss the complexstructure of the X-ray emission from the Sgr A radio complex and alongthe Galactic plane. Morphological evidence is presented that Sgr A* andthe H II region Sgr A West lie within the hot plasma in the centralcavity of Sgr A East, which we interpret as a supernova remnant that mayhave passed through the position of the SMBH, leading to a period ofincreased activity that ended within the past ~300 yr. Similarly, wehave discovered bright clumps of X-ray emission located on oppositesides of the Galactic plane, along a line passing through the centralparsec of the Galaxy. The arrangement of these lobes suggests that SgrA* may have experienced an earlier period of increased activity lastingseveral thousand years during which it expelled hot gas in a bipolaroutflow oriented roughly perpendicular to the Galactic plane.Additionally, we present an analysis of stellar emission within thecentral parsec of the Galaxy.
| The Remarkable Alternating Spectra of the Of?p Star HD 191612
The spectrum of HD 191612 has been found to display large, recurrentvariations between two highly reproducible, peculiar states; at leastfour transformations have occurred since 1950. In one state, thespectral type is O6-O7, with C III λ4650 emission comparable to NIII λ4640 (the definition of the Of?p category) and P Cygniprofiles at He II λ4686 and Hα. In the other state, thespectral type is O8, with the C III emission absent, very strong N IIIλ4097 absorption, broad He II λ4686 absorption with narrowcentral emission (a profile that may be unprecedented in this line amongknown O-type spectra), and a broad asymmetrical absorption at Hα.One observing sequence over several consecutive nights shows no spectralvariations, practically ruling out a short-period, interacting binary asthe origin of the phenomenon; moreover, no significant radial velocityvariations have been found. Although the sporadic observational recordprior to the discovery of the variations in early 2001 precludesdefinite conclusions, it is possible that a given state is maintainedfor a decade or longer, but one transformation occurred within 13months, and the data obtained during 2002 suggest an event with ashorter timescale.The Of?p category currently contains only five members: three in theGalaxy and two in the Small Magellanic Cloud. The other two Galacticmembers also display bizarre and unexplained phenomena; in the case ofHD 108, they are strikingly similar to those described here. Because oftheir relatively high X-ray luminosities, all three Galactic objectshave been suggested to have collapsed companions. If the spectralvariations of HD 108 and HD 191612 are due to binary interactions, theyare likely multiyear, eccentric systems like WR 140 and η Carinae.The axisymmetric shell ejections of HD 148937 could have a similarorigin. Alternatively, these stars may be rapid rotators or in anunstable evolutionary transitional stage. Further intensivespectroscopic monitoring is required to reveal their nature.
| An Improved, ``Newtonian'' Version of the Ebert-Fastie Spectrograph
A new variation of the off-plane Ebert-Fastie spectrograph is describedin which a Newtonian folding arrangement is mounted between thecollimator/camera mirror and the grating. This arrangement removes thesagittal and tangential astigmatism and coma of the classicalEbert-Fastie, allowing the design to be extended from a monochromator toCCD spectroscopy. The Newtonian modification (which includes a sphericalcollimator/camera) results in a point-spread function that is nearlyisotropic across the field of view, well suited to spectroscopic studiesof stellar objects. This spectrograph is the dedicated scienceinstrument of the Georgia State University 1 m Multiple TelescopeTelescope.
| Analysis of Brightness Variations in the Massive Close Binary System UU Cassiopeia
We derive the parameters of the hot, massive close binary system UU Casfrom earlier and recent photoelectric UBVR observations: M 1 = 34.5Mȯ, M 2 = 25.7M ȯ, A = 69R ȯ, and i=68°; thecomponents' limb darkening coefficients are close to 1. Our fitting ofthe light curves provides evidence for a high degree of over-contact:the components fill their inner critical surfaces by 132% and 100%. Itis possible that the appearance of the light curve is determined notonly by the distorted shapes of the components and their eclipses but,to a considerable extent, by screening by a complex envelope and theenvelope's contribution to the system's total luminosity.
| A Dozen Colliding-Wind X-Ray Binaries in the Star Cluster R136 in the 30 Doradus Region
We analyzed archival Chandra X-ray observations of the central portionof the 30 Doradus region in the Large Magellanic Cloud. The imagecontains 20 X-ray point sources with luminosities between5×1032 and 2×1035 ergs s-1(0.2-3.5 keV). A dozen sources have bright WN Wolf-Rayet or spectraltype O stars as optical counterparts. Nine of these are within ~3.4 pcof R136, the central star cluster of NGC 2070. We derive an empiricalrelation between the X-ray luminosity and the parameters for the stellarwind of the optical counterpart. The relation gives good agreement forknown colliding-wind binaries in the Milky Way Galaxy and for theidentified X-ray sources in NGC 2070. We conclude that probably allidentified X-ray sources in NGC 2070 are colliding-wind binaries andthat they are not associated with compact objects. This conclusioncontradicts earlier studies where it was argued, using ROSAT data, thattwo earlier discovered X-ray sources are accreting black hole binaries.Five of the 18 brightest stars in R136 are not visible in our X-rayobservations. These stars either are single, have low-mass companions,or have very wide orbits. The resulting binary fraction among early-typestars is then unusually high (at least 70%).
| Spectroscopic Classification of 42 Large Magellanic Cloud OB Stars: Selection of Probes for the Hot Gaseous Halo of the Large Magellanic Cloud
Interstellar C IV absorption-line studies of the hot gaseous halo of theLarge Magellanic Cloud (LMC) have been hindered by nonideal selectionsof early-type probe stars in regions where C+3 can beproduced locally via photoionization, fast stellar winds, or supernovae.To observe stars outside such regions, precise spectral classificationsof OB stars in the field are needed. Therefore, we have obtainedmedium-dispersion spectra of 42 early-type stars in the LMC that aredistributed outside superbubbles or supergiant shells. The spectralclassification of these stars is presented in this paper. Nineteen ofthese program stars have spectral types between B1 and O7 and are thussuitable probes for interstellar C IV absorption-line studies of the hotgaseous halo of the LMC.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| Tomographic Separation of Composite Spectra. VII. The Physical Properties of the Massive Triple System HD 135240 (δ Circini)
We present the results of a radial velocity study of the massive,double-lined, O binary HD 135240 based primarily on UV spectroscopy fromthe International Ultraviolet Explorer. Cross-correlation methodsindicate the presence of a third stationary spectral line componentwhich indicates that the system is a triple consisting of a central 3.9day close binary with a distant companion. We measured radial velocitiesfrom the cross-correlation functions after removal of the thirdcomponent, and we combined these with velocities obtained from Hαspectroscopy to reassess the orbital elements. We applied a Dopplertomography algorithm to reconstruct the individual UV spectra of allthree stars, and we determine spectral classifications of O7 III-V, O9.5V, and B0.5 V for the primary, secondary, and tertiary, respectively,using UV criteria defined by Penny, Gies, & Bagnuolo. We comparethese reconstructed spectra to standard single-star spectra to find theUV flux ratios of the components(F2/F1=0.239+/-0.022, andF3/F1=0.179+/-0.021). Hipparcos photometry revealsthat the central pair is an eclipsing binary, and we present the firstmodel fit of the light curve from which we derive an orbitalinclination, i=74deg+/-3deg. This analysisindicates that neither star is currently experiencing Roche lobeoverflow. We place the individual components in the theoretical H-Rdiagram, and we show that the masses derived from the combinedspectroscopic and photometric analysis(Mp/Msolar=21.6+/-2.0 andMs/Msolar=12.4+/-1.0) are significantly lower thanthose computed from evolutionary tracks for single stars.
| Mass loss rate determination of southern OB stars
A sample of OB stars (eleven Of, one O and one B supergiant) has beensurveyed with the Australia Telescope Compact Array at 4.8 and 8.64 GHzwith a resolution of ~ 2 arcsec-4 arcsec. Five stars were detected;three of them have negative spectral indices, consistent withnon-thermal emission, and two have positive indices. The thermalradiation from HD 150135 and HD 163181 can be explained as coming froman optically thick ionized stellar wind. The non-thermal radiation fromCD-47deg 4551, HD 124314 and HD 150136 possibly comes fromstrong shocks in the wind itself and/or in the wind colliding region ifthe stars have a massive early-type companion. The percentage ofnon-thermal emitters among detected O stars has increased up to ~ 50%.The Of star HD 124314 clearly shows flux density variations. Mass lossrates (or upper limits) were derived for all the observed stars and theresults compared with non-radio measurements and theoreticalpredictions.
| O Iiject{HD 152248}: Evidence for a colliding wind interaction
We present the results of a four year spectroscopic campaign on theearly-type binary system HD 152248. Our analysis yields a newclassification as O7.5III(f) + O7III(f), in contradiction with previousclassifications of the components as supergiants. We derive improvedorbital elements and the corresponding masses (M_1=29.6 Msunand M_2=29.9 Msun) are somewhat larger than previouslyreported in the literature, although they are still significantly lowerthan the ones expected from evolutionary tracks. Both components of thesystem are close to filling their Roche lobe at periastron passage. Wealso investigate the equivalent width variations of the lines of the twocomponents and we discuss their relation to the Struve-Sahade effect.Finally, we show that the line profile variability of the He Iil4686 andHα lines is consistent with a strong wind interaction between thetwo stars, quite as predicted by current colliding wind models. Based ondata collected at the European Southern Observatory (La Silla, Chile)and at the Cerro Tololo Inter-American Observatory (Cerro Tololo,Chile).
| 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
| Coordinated monitoring of the eccentric O-star binary Iota Orionis: the X-ray analysis
We analyse two ASCA observations of the highly eccentric O9 III+B1 IIIbinary Iota Orionis obtained at periastron and apastron. Based on theassumption of a strong colliding-wind shock between the stellarcomponents, we expected to see significant variation in the X-rayemission between these phases. The observations proved otherwise: theX-ray luminosities and spectral distributions were remarkably similar.The only noteworthy feature in the X-ray data was the hint of aproximity effect during periastron passage. Although this `flare' is ofrelatively low significance, it is supported by the notable proximityeffects seen in the optical, and the phasing of the X-ray and opticalevents is in very good agreement. However, other interpretations arealso possible. In view of the degradation of the SIS instrument andsource contamination in the GIS data we discuss the accuracy of theseresults, and also analyse archival ROSAT observations. We investigatewhy we do not see a clear colliding-wind signature. A simple model showsthat the wind attenuation to the expected position of the shock apex isnegligible throughout the orbit, which poses the puzzling question ofwhy the expected 1/D variation (i.e. a factor of 7.5) in the intrinsicluminosity is not seen in the data. Two scenarios are proposed: eitherthe colliding-wind emission is unexpectedly weak such that intrinsicshocks in the winds dominate the emission, or, alternatively, theemission observed is colliding-wind emission but in a more complex formthan we would naively expect. Complex hydrodynamical models are thenanalysed. Despite strongly phase-variable emission from the models, bothare consistent with the observations. We find that if the mass-lossrates of the stars are low then intrinsic wind shocks could dominate theemission. However, when we assume higher mass-loss rates of the stars,we find that the observed emission could also be consistent with apurely colliding-wind origin. A summary of the strengths and weaknessesof each interpretation is presented. To distinguish between thedifferent models X-ray observations with improved phase coverage will benecessary.
| Coordinated monitoring of the eccentric O-star binary Iota Orionis: optical spectroscopy and photometry
With the objective of investigating the wind-wind collision phenomenonand supporting contemporaneous X-ray observations, we have organized alarge-scale, coordinated optical monitoring campaign of the massive,highly eccentric O9III+B1III binary Iota Orionis. Successfullyseparating the spectra of the components, we refine the orbital elementsand confirm the rapid apsidal motion in the system. We also see stronginteraction between the components during periastron passage and detectphase-locked variability in the spectrum of the secondary star. However,we find no unambiguous signs of the bow shock crashing on the surface ofthe secondary, despite the predictions of hydrodynamic simulations.Combining all available photometric data, we find rapid, phase-lockedvariations and model them numerically, thus restricting the orbitalinclination to 50°<~i<~70°.
| The distribution of bright OB stars in the Canis Major-Puppis-Vela region of the Milky Way
The picture of the young stellar groups in the Canis Major-Puppis-Vela(215 deg
| Measuring the Ionization of O Star Winds
We present an analysis of wind line profiles from Far UltravioletSpectroscopic Explorer (FUSE) spectra of two O7 supergiants in the Largeand Small Magellanic Clouds (Sk -67°111 and AV 232, respectively).Model fits yield the column densities of S IV, S VI, P IV, P V, N III,and N IV, providing the first direct measurement of the ionizationbalance in stellar winds. The ratios of S IV/S VI and P IV/P V areconsistently lower in the LMC star. IUE and Hubble Space Telescopearchival spectra are also used to measure N IV and N V, but the muchhigher optical depth makes the N V measurements inconclusive. Thevelocity and optical depth distributions in the wind are qualitativelysimilar between the two stars, when scaled to their terminal velocities.The terminal velocities are different, with AV 232 being lower (as foundpreviously in SMC stars and linked to lower metallicity). These are thefirst results from a program to investigate wind ionization and velocitystructure among hot stars in local galaxies, and they demonstrate thehigher accuracy in measuring column densities of less abundant ions,such as phosphorus and sulfur, observable in the FUSE range.
| The Struve-Sahade Effect: A Tale of Three Stars
The ``Struve-Sahade effect'' (S-S effect) is the apparent weakness oflines of the secondary in massive binaries when the secondary isreceding. This effect poses problems for the accurate reconstruction ofthe separated primary and secondary spectra. We have reexamined IUEspectra of three classical, hot binaries studied by D. Stickland in1997. From various cross-correlation and tomographic techniques we finddifferent results for each of the three systems. For HD 1337 (AO Cas),we find a slight S-S effect which can be explained by a mechanism oflocalized heating by the colliding stellar winds, as proposed by Gies,Bagnuolo, & Penny in 1997. For HD 47129 (Plaskett's star), noconsistent S-S effect was found, and none is expected based on the windheating model because the secondary has a slightly stronger wind.Finally, for HD 57060 (29 UW CMa), a strong S-S effect was found duringthe receding phases (0.13-0.63). In addition, we find a strong featurenear phase 0.2, similar to the secondary in spectral type but shifted by~-400 km s-1. We explain this by absorption and reemission bystrong winds wrapping around the secondary. Thus, in general, we findthat the S-S effect may arise from several distinct mechanisms.
| The Interacting Early-Type Binary BD +40 deg4220 (V729 Cyg): Modeling the Colliding Winds Region
We present an analysis of an extensive set of spectroscopic observationsof the mysterious early-type binary BD +40 deg4220. A new orbitalsolution is derived from the radial velocities of the absorption lines.We confirm that the secondary star is highly overluminous for its mass.The absorption lines of both components display phase-locked profilevariations, with some of the secondary's lines going into emissionbetween phi=0.20 and phi=0.55. A detailed investigation of the profilevariability of the He II lambda4686 emission line reveals that thepattern of variability of this line is very stable. We show that part ofthe He II lambda4686 emission is produced in the wind interaction regionbetween the stars. Most of the emission lines in the visual spectrum ofBD +40 deg4220 display variations that are reminiscent of those observedon the He II lambda4686 line, pointing toward a similar origin. Wepresent numerical simulations of emission-line profiles in a collidingwinds binary, showing that an important part of the variability observedin BD +40 deg4220 can be explained by a colliding winds phenomenon. Theproperties of the wind interaction region can be accounted for if weassume that the secondary star is an evolved object, most probably somekind of Ofpe/WN9 transition star, with a mass loss rate of ~5.5x10^-6M_solar yr^-1. We finally discuss the fundamental parameters of thebinary, concluding that mass transfer must have played a crucial role inthe evolution of this system.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | כלב גדול |
| התרוממות ימנית: | 07h18m40.30s |
| סירוב: | -24°33'32.0" |
| גודל גלוי: | 4.98 |
| מרחק: | 917.431 פארסק |
| תנועה נכונה: | -4.1 |
| תנועה נכונה: | 2.6 |
| B-T magnitude: | 4.757 |
| V-T magnitude: | 4.927 |
קטלוגים וכינוים:
|
