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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.
| The Homogeneity of Interstellar Elemental Abundances in the Galactic Disk
We present interstellar elemental abundance measurements derived fromSpace Telescope Imaging Spectrograph echelle observations of 47 sightlines extending up to 6.5 kpc through the Galactic disk. These pathsprobe a variety of interstellar environments, covering ranges of nearly4 orders of magnitude in molecular hydrogen fraction f(H2)and more than 2 in mean hydrogen sight-line density. Coupling the current data with Goddard HighResolution Spectrograph data from 17 additional sight lines and thecorresponding Far Ultraviolet Spectroscopic Explorer and Copernicusobservations of H2 absorption features, we explore magnesium,phosphorus, manganese, nickel, copper, and germanium gas-phase abundancevariations as a function of : density-dependentdepletion is noted for each element, consistent with a smooth transitionbetween two abundance plateaus identified with warm and cold neutralinterstellar medium depletion levels. The observed scatter with respectto an analytic description of these transitions implies that totalelemental abundances are homogeneous on length scales of hundreds ofparsecs, to the limits of abundance measurement uncertainty. Theprobable upper limit we determine for intrinsic variability at any is 0.04 dex, aside from an apparent 0.10 dexdeficit in copper (and oxygen) abundances within 800 pc of the Sun.Magnesium dust abundances are shown to scale with the amount of siliconin dust, and in combination with a similar relationship between iron andsilicon, these data appear to favor the young F and G star values ofSofia & Meyer as an elemental abundance standard for the Galaxy.Based on observations with the NASA/ESA.
| 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.
| 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.
| Caroline Herschel as observer
Not Available
| Ion-by-Ion Differential Emission Measure Determination of Collisionally Ionized Plasma. II. Application to Hot Stars
In a previous paper we have described a technique to derive constraintson the differential emission measure (DEM) distribution, a measure ofthe temperature distribution, of collisionally ionized hot plasmas fromtheir X-ray emission line spectra. We apply this technique to theChandra HETGS spectra of all of the nine hot stars available to us atthe time that this project was initiated. We find that DEM distributionsof six of the seven O stars in our sample are very similar, but thatθ1 Ori C has an X-ray spectrum characterized by highertemperatures. The DEM distributions of both of the B stars in our samplehave lower magnitudes than those of the O stars, and one, τ Sco, ischaracterized by higher temperatures than the other, β Cru. Theseresults confirm previous work in which high temperatures have been foundfor θ1 Ori C and τ Sco and taken as evidence forchanneling of the wind in magnetic fields, the existence of which isrelated to the stars' youth. Our results demonstrate the utility of ourmethod for deriving temperature information for large samples of X-rayemission-line spectra.
| Molecular Clouds Toward a New OB Association in Pup-CMa
We have mapped 16 molecular clouds toward a new OB association in thePup-CMa region to derive their physical properties. The observationswere carried out in the 12CO (J = 1 0) line with the Southernmillimetre-wave Telescope at Cerro Tololo, Chile. Distances have beendetermined kinematically using the rotation curve of Brand with Rȯ= 8.5 kpc and Vȯ = 220 km/s. Masses have been derived adopting a COluminosity to H2 conversion factor X = 3.8 . 1020 molecules cm-2 (Kkm/s)-1. The observed mean radial velocity of the clouds is comparablewith the mean radial velocity of stars composing an OB association inPup-CMa; it is in favor of the close connection of clouds with thesestars.
| 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.
| 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.
| 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.
| The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope (HST)Space Telescope Imaging Spectrograph (STIS) observations of O Iλ1356 and H I Lyα absorption in 36 sight lines that probe avariety of Galactic disk environments and include paths that range overnearly 4 orders of magnitude in f(H2), over 2 orders ofmagnitude in , and that extend up to 6.5 kpc inlength. Since the majority of these sight lines have also been observedby the Far Ultraviolet Spectroscopic Explorer (FUSE), we have undertakenthe study of gas-phase O/H abundance ratio homogeneity using the currentsample and previously published Goddard High Resolution Spectrograph(GHRS) results. Two distinct trends are identified in the 56 sight linesample: an apparent decrease in gas-phase oxygen abundance withincreasing mean sight-line density () and a gapbetween the mean O/H ratio for sight lines shorter and longer than about800 pc. The first effect is a smooth transition between two depletionlevels associated with large mean density intervals; it is centered near=1.5cm-3 and is similar to trendsevident in gas-phase abundances of other elements. Paths less dense thanthe central value exhibit a mean O/H ratio of log10(O/H)=-3.41+/-0.01 (or 390+/-10ppm), which is consistent with averages determined for several longlow-density paths observed by STIS (André et al. 2003) and shortlow-density paths observed by FUSE (Moos et al. 2002). Sight lines ofhigher mean density exhibit an average O/H value of log10(O/H)=-3.55+/-0.02 (284+/-12ppm). The data points for low- paths are scatteredmore widely than those for denser sight lines, because O/H ratios forsuch paths shorter than 800 pc are generally about 0.10 dex lower thanthe values for longer ones. Scenarios that would be consistent withthese results include a recent infall of metal-poor gas onto the localGalactic disk and an interstellar environment toward Orion that isconducive to reducing the apparent gas-phase oxygen abundance.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985.
| Interstellar Carbon in Translucent Sight Lines
We report interstellar C II column densities or upper limits determinedfrom weak absorption of the 2325.4029 Å intersystem transitionobserved in six translucent sight lines (AV>~1) with theSpace Telescope Imaging Spectrograph (STIS). The sight lines sample awide range of interstellar characteristics, including total to selectiveextinction, RV=2.6-5.1 average hydrogen density along thesight line, =3-14 cm-3 and fraction of H inmolecular form, 0-~40%. Four of the sight lines, those toward HD 37021,HD 37061, HD 147888, and HD 207198, have interstellar gas-phaseabundances that are consistent with the diffuse sight line ratio of161+/-17 carbon atoms in the gas per million hydrogen nuclei. We notethat while it has a gas-phase carbon abundance that is consistent withthe other sight lines, a large fraction of the C II toward HD 37061 isin an excited state. The sight line toward HD 152590 has a measuredinterstellar gas-phase carbon abundance that is well above the diffusesight line average; the column density of C in this sight line may beoverestimated because of noise structure in the data. Toward HD 27778 wefind a 3 σ abundance upper limit of <=108 C atoms in the gasper million H, a substantially enhanced depletion of C as compared tothe diffuse sight line value. The interstellar characteristics toward HD27778 are otherwise not extreme among the sample, except for anunusually large abundance of CO molecules in the gas.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| High Resolution Spectra of Stellar Winds in Very Young Massive Stars
We present a series of spectra from hot stars obtained with the highenergy transmission grating spectrometer (HETGS) onboard the ChandraX-ray Observatory. Most of the selected stars are located at or near thecenters of young star-forming regions, which include IC 1396, NGC 2362,and Orion. The median ages of these clusters range from 0.3 to 12 Myr.The X-ray properties of these young, hot stars are then compared to ahighly resolved X-ray spectrum of the more evolved field star zeta Pup.Cluster stars with ages larger than 3 Myr show moderate temperatures andbroad asymmetric lines. These properties are quite similar to what hasbeen observed in zeta Pup and are consistent with X-rays produced byshock instabilities in a radiation-driven wind. Stars with agessignificantly less than 1 Myr, as found in the Orion Trapezium, showcompletely different characteristics. Here most of the X-ray flux showsvery high temperatures and the X-ray lines are narrow and unresolved.This strongly indicates that the X-ray properties are dominated bymagnetic field effects.
| Infrared study of the η Chamaeleontis cluster and the longevity of circumstellar discs
We have analysed JHKL observations of the stellar population of the ~9Myr-old η Chamaeleontis cluster. Using infrared (IR) colour-colourand colour-excess diagrams, we find that the fraction of stellar systemswith near-IR excess emission is 0.60 +/- 0.13 (2σ). This resultimplies considerably longer disc lifetimes than found in some recentstudies of other young stellar clusters. For the classical T Tauri (CTT)and weak-lined T Tauri (WTT) star population, we also find a strongcorrelation between the IR excess and Hα emission. The IR excessesof these stars indicate a wide range of star-disc activity: from a CTTstar showing high levels of accretion to CTT-WTT transition objects withevidence for some on-going accretion and WTT stars with weak or absentIR excesses. Of the 15 known cluster members, four stars with IRexcesses Δ(K-L) > 0.4 mag are likely experiencing ongoingaccretion owing to strong or variable optical emission. The resultingaccretion fraction (0.27 +/- 0.13; 2σ) shows that the accretionphase, in addition to the discs themselves, can endure for at least ~10Myr.
| The Homogeneity of Interstellar Krypton in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope SpaceTelescope Imaging Spectrograph (STIS) observations of Kr I λ1236absorption in seven sight lines that probe a variety of interstellarenvironments. In combination with krypton and hydrogen column densitiesderived from current and archival STIS and Far-Ultraviolet SpectroscopicExplorer data, the number of sight lines with reliable Kr/H ISMabundance ratios has been increased by 50% to 26-including paths thatsample a range of nearly 5 orders of magnitude in f(H2) andover 2 orders of magnitude in , and extend up to4.8 kpc in length. For sight lines contained entirely within the localspiral arm (the Orion spur), the spread of Kr/H ratios about the mean oflog10[N(Kr)/N(H)]ISM=-9.02+/-0.02is remarkably tight (0.06 dex), less than the typical data-pointuncertainty. Intriguingly, the only two sight lines that extend throughneighboring structures, in particular gas associated with theCarina/Sagittarius arm, exhibit relatively large, near-solar kryptonabundances (log10[N(Kr)/N(H)]combined=-8.75+0.09-0.11).Although these deviations are only measured at the 2 σ level, theysuggest the possibility that krypton abundances beyond the Orion spurmay differ from the local value.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far-Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc. under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS-32985.
| X-Ray Modeling of Very Young Early-Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary Implications
The Orion Trapezium is one of the youngest and closest star-formingregions within our Galaxy. With a dynamic age of ~3×105yr, it harbors a number of very young hot stars, which likely are on thezero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectrain the wavelength range of 1.5-25 Å of three of itsX-ray-brightest members (Θ1 Ori A, C, and E) obtainedwith the High Energy Transmission Grating Spectrometer (HETGS) on boardthe Chandra X-Ray Observatory. We measured X-ray emission lines,calculated differential emission measure distributions (DEMs), andfitted broadband models to the spectra. The spectra from all three starsare very rich in emission lines, specifically from highly ionized Fe,which includes emission from Fe XVII to Fe XXV ions. A complete linelist is included. This is a mere effect of high temperatures rather thanan overabundance of Fe, which in fact turns out to be underabundant inall three Trapezium members. Similarly there is a significantunderabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appearclose to solar. The DEM derived from over 80 emission lines in thespectrum of Θ1 Ori C indicates three peaks located at7.9, 25, and 66 MK. The emission measure varies over the 15.4 day windperiod of the star. For the two phases observed, the low-temperatureemission remains stable, while the high-temperature emission showssignificant differences. The line widths seem to show a similarbifurcation, where we resolve some of the soft X-ray lines withvelocities up to 850 km s-1 (all widths are stated ashalf-width at half-maximum), whereas the bulk of the lines remainunresolved with a confidence limit of 110 km s-1. Thebroadband spectra of the other two stars can be fitted with severalcollisionally ionized plasma model components within a temperature rangeof 4.3-46.8 MK for Θ1 Ori E and 4.8-42.7 MK forΘ1 Ori A. The high-temperature emissivity contributesover 70% to the total X-ray flux. None of the lines are resolved forΘ1 Ori A and E with a confidence limit of 160 kms-1. The influence of the strong UV radiation field on theforbidden line in the He-like triplets allows us to set an upper limiton distance of the line-emitting region from the photosphere. The bulkof the X-ray emission cannot be produced by shock instabilities in aradiation-driven wind and are likely the result of magnetic confinementin all three stars. Although confinement models cannot explain all theresults, the resemblance of the unresolved lines and of the DEM withrecent observations of active coronae in II Peg and AR Lac during flaresis quite obvious. Thus we speculate that the X-ray production mechanismin these stars is similar, with the difference that the Orion stars maybe in a state of almost continuous flaring driven by the wind. Weclearly rule out major effects due to X-rays from a possible companion.The fact that all three stars appear to be magnetic and are near zeroage on the main sequence also raises the issue of whether the Orionstars are simply different or whether young massive stars enter the mainsequence carrying significant magnetic fields. The ratiologLX/Lbol using the ``wind'' component of thespectrum is -7 for the Trapezium stars, consistent with the expectationfrom O stars. This suggests that massive ZAMS stars generate their X-rayluminosities like normal O stars and magnetic confinement provides anadditional source of X-rays.
| 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.
| 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.
| Intermediate- and High-Velocity Ionized Gas toward ζ Orionis
We combine near-UV spectra obtained with the Hubble Space Telescope GHRSechelle with far-UV spectra obtained with the Interstellar MediumAbsorption Profile Spectrograph and Copernicus to study the abundancesand physical conditions in the predominantly ionized gas seen at highvelocity (-105 km s-1<~vsolar<~-65 kms-1) and at intermediate velocity (-60 kms-1<~vsolar<~-10 km s-1) alongthe line of sight to the star ζ Ori. We have high-resolution(FWHM~3.3-4.5 km s-1) and/or high signal-to-noise ratiospectra for at least two significant ions of C, N, Al, Si, S, andFe-enabling accurate estimates for both the total N(H II) and theelemental depletions. C, N, and S have essentially solar relativeabundances; Al, Si, and Fe appear to be depleted by about 0.8, 0.3-0.4,and 0.95 dex, respectively, relative to C, N, and S. While various ionratios would be consistent with collisional ionization equilibrium (CIE)at temperatures of 25,000-80,000 K, the widths of individualhigh-velocity absorption components indicate that T~9000+/-2000 K-so thegas is not in CIE. Analysis of the C II fine-structure excitationequilibrium, at that temperature, yields estimates for the densities(ne~nH~0.1-0.2 cm-3), thermal pressures(2nHT~2000-4000 cm-3 K), and thicknesses (0.5-2.7pc) characterizing the individual clouds. We compare the abundances andphysical properties derived for these clouds with those found for gas atsimilar velocities toward 23 Ori and τ CMa and also with severaldifferent models for shocked gas. While the shock models can reproducesome features of the observed line profiles and some of the observed ionratios, there are also significant differences between the models andthe data. The measured depletions suggest that roughly 10% of the Al,Si, and Fe originally locked in dust in the preshock medium may havebeen returned to the gas phase, consistent with recent predictions forthe destruction of silicate dust in a 100 km s-1 shock. Theobserved near-solar gas-phase abundance of carbon, however, appears tobe inconsistent with the predicted longer timescales for the destructionof graphite grains. Based in part on observations with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Velocity Distribution of Stars in the Pup-CMa Association
The distribution of proper motions of stars in the Pup-CMa associationis presented. The stars' velocities are approximately parallel to eachother, which indicates that the stars are close together in space. Themutual distribution of stars and molecular clouds in the association isinterpreted as proof that the stars emerged from a single giganticprimordial molecular cloud (or several large clouds), destroyed byradiation and/or stellar wind coming from those stars. It is assumedthat part of that cloud is being dissipated, while part is being brokeninto several small clouds, which we are observing at present.
| The accretion/diffusion theory for lambda Bootis stars in the light of spectroscopic data
Most of the current theories suggest the lambda Bootis phenomenon tooriginate from an interaction between the stellar surface and its localenvironment. In this paper, we compare the abundance pattern of thelambda Bootis stars to that of the interstellar medium and find largerdeficiencies for Mg, Si, Mn and Zn than in the interstellar medium. Acomparison with metal poor post-AGB stars showing evidence forcircumstellar material indicates a similar physical process possiblybeing at work for some of the lambda Bootis stars, but not for all ofthem. Despite the fact that the number of spectroscopically analysedlambda Bootis stars has considerably increased in the past, a test ofpredicted effects with observations shows current abundance andtemperature data to be still controversial.
| 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.
| Optical spectroscopy of XMEGA targets in the Carina Nebula - III. The multiple system Tr 16-104 (≡CPD -59° 2603)
We discuss the orbital elements of the multiple system Tr 16-104 whichis usually believed to be a member of the open cluster Trumpler 16 inthe Carina complex. We show that Tr 16-104 could be a hierarchicaltriple system consisting of a short-period (2.15d) eclipsing O7V+O9.5Vbinary bound to a B0.2 IV star. Our preliminary orbital solution of thethird body indicates that the B star most probably describes aneccentric orbit with a period of ~285 or ~1341d around the close binary.Folding photometric data from the literature with our new ephemerides,we find that the light curve of the close binary exhibits rather narroweclipses indicating that the two O stars must be well inside their Rochelobes. Our analysis of the photometric data yields a lower limit on theinclination of the orbit of the close binary of i>=77°. Thestellar radii and luminosities of the O7 V and O9.5 V stars aresignificantly smaller than expected for stars of this spectral type. Ourresults suggest that Tr 16-104 lies at a distance of the order of 2.5kpcand support a fainter absolute magnitude for zero-age main-sequence Ostars than usually adopted. We find that the dynamical configuration ofTr 16-104 corresponds to a hierarchical system that should remain stableprovided that it suffers no strong perturbation. Finally, we also reportlong-term temporal variations of high-velocity interstellar Caiiabsorptions in the line of sight towards Tr 16-104.
| NGC 2362: A Template for Early Stellar Evolution
We present UBVRI photometry for the young open cluster NGC 2362. Fromanalysis of the appropriate color-color and color-magnitude diagrams wederive the fundamental parameters of the NGC 2362 cluster to beage=5+1-2 Myr, distance=1480 pc, and E(B-V)=0.10mag. The cluster age was independently determined for both high-mass(2.1-36 Msolar) and low-mass (0.7-1.2 Msolar)stars with excellent agreement between the ages derived usingpost-main-sequence (Girardi et al. 2000) and pre-main-sequence (Baraffeet al. 1998) evolutionary tracks for the high- and low-mass stars,respectively. Analysis of this cluster's color-magnitude diagram revealsa well-defined pre-main sequence (covering ΔV~9 mag in V andextending from early A stars to near the hydrogen-burning limit), whichmakes this cluster an ideal laboratory for pre-main-sequence evolutionstudies. Based on observations collected at the European SouthernObservatory, La Silla, under project 66.C-0119.
| Space Telescope Imaging Spectrograph Observations of Interstellar Oxygen and Krypton in Translucent Clouds
We have obtained high-resolution Hubble Space Telescope (HST) SpaceTelescope Imaging Spectrograph (STIS) observations of O I λ1356and Kr I λ1236 absorption in 11 sight lines characterized by highextinction, large H I column densities, and/or long path lengths.Previous Goddard High Resolution Spectrograph (GHRS) measurements ofthese weak features in seven relatively nearby diffuse clouds have shownno evidence for density-dependent depletion of either oxygen or kryptonand have yielded a weighted mean gas-phase abundance ratio oflog[N(O)/N(Kr)]GHRS=5.56+/-0.04. Our STIS measurements yielda lower weighted mean of log[N(O)/N(Kr)]STIS=5.48 thedifference is due primarily to several translucent sight lines in theSTIS data set that diverge from the GHRS value. These translucent cloudsight lines pass near dense, star-forming regions, notably the ρOph, Orion, and Taurus molecular clouds. Since Kr, as a noble gas,should not be depleted much into grains, these cases suggest a trendtoward the enhanced oxygen depletion predicted for denser ISM clouds.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc. under NASAcontract NAS 5-26555.
| Disk Frequencies and Lifetimes in Young Clusters
We report the results of the first sensitive L-band survey of theintermediate-age (2.5-30 Myr) clusters NGC 2264, NGC 2362, and NGC 1960.We use JHKL colors to obtain a census of the circumstellar diskfractions in each cluster. We find disk fractions of 52%+/-10%,12%+/-4%, and 3%+/-3% for the three clusters, respectively. Togetherwith our previously published JHKL investigations of the younger NGC2024, Trapezium, and IC 348 clusters, we have completed the firstsystematic and homogeneous survey for circumstellar disks in a sample ofyoung clusters that both span a significant range in age (0.3-30 Myr)and contain statistically significant numbers of stars whose masses spannearly the entire stellar mass spectrum. Analysis of the combined surveyindicates that the cluster disk fraction is initially very high(>=80%) and rapidly decreases with increasing cluster age, such thatone-half the stars within the clusters lose their disks in <~3 Myr.Moreover, these observations yield an overall disk lifetime of ~6 Myr inthe surveyed cluster sample. This is the timescale for essentially allthe stars in a cluster to lose their disks. This should set a meaningfulconstraint for the planet-building timescale in stellar clusters. Theimplications of these results for current theories of planet formationare briefly discussed.
| High-mass binaries in the very young open cluster NGC 6231. Implication for cluster and star formation
New radial-velocity observations of 37 O- and B stars in the very youngopen cluster NGC 6231 confirm the high frequency of short-periodspectroscopic binaries on the upper main sequence. Among the 14 O-typestars, covering all luminosity classes from dwarfs to supergiants, 8 aredefinitively double-lined systems and all periods but one are shorterthan 7 days. Several additional binaries have been detected among theearly B-type stars. NGC 6231 is an exceptional cluster to constrain thescenarios of cluster- and binary-star formation over a large range ofstellar masses. We discuss the evidences, based on NGC 6231 and 21 otherclusters, with a total of 120 O-type stars, for a clear dichotomy in themultiplicity rate and structure of very young open clusters containingO-type stars in function of the number of massive stars. However, wecannot answer the question whether the observed characteristics resultfrom the formation processes or from the early dynamical evolution.
| 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
| 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.
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