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Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| Variations in D/H and D/O from New Far Ultraviolet Spectroscopic Explorer Observations
We use data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to determine the interstellar abundances of D I, N I, O I, Fe II,and H2 along the sight lines to WD 1034+001, BD +39 3226, andTD1 32709. Our main focus is on determining the D/H, N/H, O/H, and D/Oratios along these sight lines, with logN(H)>20.0, that probe gaswell outside of the Local Bubble. Hubble Space Telescope (HST) andInternational Ultraviolet Explorer (IUE) archival data are used todetermine the H I column densities along the WD 1034+001 and TD1 32709sight lines, respectively. For BD +39 3226, a previously published N(HI) is used. We find(D/H)×105=2.14+0.53-0.45,1.17+0.31-0.25, and1.86+0.53-0.43 and(D/O)×102=6.31+1.79-1.38,5.62+1.61-1.31, and7.59+2.17-1.76 for the WD 1034+001, BD +39 3226,and TD1 32709 sight lines, respectively (all 1 σ). The scatter inthese three D/H ratios exemplifies the scatter that has been found byother authors for sight lines with column densities in the range19.2
| 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.
| Bright OB stars in the Galaxy. III. Constraints on the radial stratification of the clumping factor in hot star winds from a combined Hα, IR and radio analysis
Context: .Recent results strongly challenge the canonical picture ofmassive star winds: various evidence indicates that currently acceptedmass-loss rates, {dot M}, may need to be revised downwards, by factorsextending to one magnitude or even more. This is because the mostcommonly used mass-loss diagnostics are affected by "clumping"(small-scale density inhomogeneities), influencing our interpretation ofobserved spectra and fluxes. Aims: .Such downward revisions wouldhave dramatic consequences for the evolution of, and feedback from,massive stars, and thus robust determinations of the clumping propertiesand mass-loss rates are urgently needed. We present a first attemptconcerning this objective, by means of constraining the radialstratification of the so-called clumping factor. Methods: .To thisend, we have analyzed a sample of 19 Galactic O-type supergiants/giants,by combining our own and archival data for Hα, IR, mm and radiofluxes, and using approximate methods, calibrated to more sophisticatedmodels. Clumping has been included into our analysis in the"conventional" way, by assuming the inter-clump matter to be void.Because (almost) all our diagnostics depends on the square of density,we cannot derive absolute clumping factors, but only factors normalizedto a certain minimum. Results: .This minimum was usually found tobe located in the outermost, radio-emitting region, i.e., the radiomass-loss rates are the lowest ones, compared to {dot M} derived fromHα and the IR. The radio rates agree well with those predicted bytheory, but are only upper limits, due to unknown clumping in the outerwind. Hα turned out to be a useful tool to derive the clumpingproperties inside r < 3{ldots}5 Rstar. Our most importantresult concerns a (physical) difference between denser and thinnerwinds: for denser winds, the innermost region is more strongly clumpedthan the outermost one (with a normalized clumping factor of 4.1± 1.4), whereas thinner winds have similar clumping properties inthe inner and outer regions. Conclusions: .Our findings arecompared with theoretical predictions, and the implications arediscussed in detail, by assuming different scenarios regarding the stillunknown clumping properties of the outer wind.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| Wolf-Rayet and O star runaway populations from supernovae
We present numerical simulations of the runaway fractions expectedamongst O and Wolf-Rayet star populations resulting from stars ejectedfrom binaries by the supernova of the companion. Observationally, therunaway fraction for both types of star is similar, prompting theexplanation that close dynamical interactions are the main cause ofthese high-velocity stars. We show that, provided that the initialbinary fraction is high, a scenario in which two-thirds of massiverunaways are from supernovae is consistent with these observations. Ourmodels also predict a low frequency of runaways with neutron starcompanions and a very low fraction of observable Wolf-Rayet-compactcompanion systems.
| 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.
| 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.
| X-Ray Counterparts of Runaway Stars
An X-ray search for possible compact companions of runaway OB stars hasbeen conducted using pointed ROSAT observations. Of a list of 71 runawaystars, ROSAT exposures were available for 24, of which 13 are detected.These numbers are nearly 3 times larger than for a previously studiedEinstein sample, and spectral information is exploited as well.Luminosities, hardness ratios, and long-term variability are as fornormal OB stars and do not suggest the presence of collapsed companions.A result like this is often interpreted as support for dynamicalejection from a dense group rather than a supernova event in a binary asa production process for runaway stars. There are, however, severalcircumstances that may adversely affect the observability of a compactcompanion, or after a supernova explosion systems may be disruptedbecause of the large natal kick velocity imparted to the neutron star asa result of asymmetries in the explosions. It is noted that there isactually evidence for both of these production routes and that they maybe expected to occur sequentially in the evolution of OB associations.
| Evidence of Correlated Titanium and Deuterium Depletion in the Galactic Interstellar Medium
Current measurements indicate that the deuterium abundance in diffuseinterstellar gas varies spatially by a factor of ~4 among sight linesextending beyond the Local Bubble. One plausible explanation for thescatter is the variable depletion of D onto dust grains. To test thisscenario, we have obtained high signal-to-noise, high- resolutionprofiles of the refractory ion Ti II along seven Galactic sight lineswith D/H ranging from 0.65 to 2.1×10-5. Thesemeasurements, acquired with the recently upgraded Keck/HIRESspectrometer, indicate a correlation between Ti/H and D/H at the betterthan 95% confidence level Therefore, our observations support theinterpretation that D/H scatter is associated with differentialdepletion. We note, however, that Ti/H values taken from the literaturedo not uniformly show the correlation. Finally, we identify significantcomponent-to-component variations in the depletion levels amongindividual sight lines and discuss complications arising from thisbehavior.
| The Birth of High-Mass Stars: Accretion and/or Mergers?
The observational consequences of the merger scenario for massive starformation are explored and contrasted with the gradual accumulation ofmass by accretion. In high-density protostar clusters, envelopes anddisks provide a viscous medium that can dissipate the kinetic energy ofpassing stars, greatly enhancing the probability of capture.Protostellar mergers may produce high-luminosity infrared flares lastingyears to centuries followed by a luminosity decline on theKelvin-Helmholtz timescale of the merger product. Mergers may besurrounded by thick tori of expanding debris, impulsive wide-angleoutflows, and shock-induced maser and radio continuum emission.Collision products are expected to have fast stellar rotation and alarge multiplicity fraction. Close encounters or mergers will producecircumstellar debris disks with an orientation that differs from that ofa preexisting disk. Thus, massive stars growing by a series of mergersmay produce eruptive outflows with random orientations; the walls of theresulting outflow cavities may be observable as filaments of dense gasand dust pointing away from the massive star. The extremely rare mergerof two stars close to the upper-mass end of the initial mass functionmay be a possible pathway to hypernova-generated gamma-ray bursts. Incontrast with the violence of merging, the gradual growth of massivestars by accretion is likely to produce less infrared variability,relatively thin circumstellar accretion disks that maintain theirorientation, and collimated bipolar outflows that are scaled-up versionsof those produced by low-mass young stellar objects. While suchaccretional growth can lead to the formation of massive stars inisolation or in loose clusters, mergers can only occur in high-densitycluster environments. It is proposed that the outflow emerging from theOMC-1 core in the Orion molecular cloud was produced by a protostellarmerger that released between 1048 and 1049 ergsless than a thousand years ago.
| High-mass X-ray binaries and OB runaway stars
High-mass X-ray binaries (HMXBs) represent an important phase in theevolution of massive binary systems and provide fundamental informationon the properties of the OB-star primaries and their compact secondaries(neutron star, black hole). Recent observations indicate that theneutron stars in some of these systems (Vela X-1, 4U 1700-37) are moremassive than the canonical mass of 1.35 MMȯ. Theseobservations have important consequences for the equation of state atsupranuclear densities and the formation mechanism(s) of neutron starsand black holes: supernovae and gamma-ray bursts. As a consequence ofthe supernova explosion that produced the compact star in these systems,HMXBs have high space velocities and thus are runaways. Alternatively,OB-runaway stars can be ejected from a cluster through dynamicalinteractions. Observations obtained with the Hipparcos satelliteindicate that both scenarios are at work.
| N-body simulations of stars escaping from the Orion nebula
We study the dynamical interaction in which the two single runawaystars, AE Aurigæ and μ Columbæ, and the binary ιOrionis acquired their unusually high space velocity. The two singlerunaways move in almost opposite directions with a velocity greater than100 km s-1 away from the Trapezium cluster. The star ιOrionis is an eccentric (e~= 0.8) binary moving with a velocity of about10 km s-1 at almost right angles with respect to the twosingle stars. The kinematic properties of the system suggest that astrong dynamical encounter occurred in the Trapezium cluster about 2.5Myr ago. Curiously enough, the two binary components have similarspectral type but very different masses, indicating that their ages mustbe quite different. This observation leads to the hypothesis that anexchange interaction occurred in which an older star was swapped intothe original ι Orionis binary. We test this hypothesis by acombination of numerical and theoretical techniques, using N-bodysimulations to constrain the dynamical encounter, binary evolutioncalculations to constrain the high orbital eccentricity of ιOrionis and stellar evolution calculations to constrain the agediscrepancy of the two binary components. We find that an encounterbetween two low eccentricity (0.4 <~e<~ 0.6) binaries withcomparable binding energy, leading to an exchange and the ionization ofthe wider binary, provides a reasonable solution to this problem.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| 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.
| Rocket and Far Ultraviolet Spectroscopic Explorer Observations of IC 405: Differential Extinction and Fluorescent Molecular Hydrogen
We present far-ultraviolet spectroscopy of the emission/reflectionnebula IC 405 obtained by a rocket-borne long-slit spectrograph and theFar Ultraviolet Spectroscopic Explorer (FUSE). Both data sets show arise in the ratio of the nebular surface brightness to stellar flux(S/F*) of approximately 2 orders of magnitude toward the blueend of the far-UV bandpass. Scattering models using simple dustgeometries fail to reproduce the observed S/F* for realisticgrain properties. The high spectral resolution of the FUSE data revealsa rich fluorescent molecular hydrogen spectrum ~1000" north of the starthat is clearly distinguished from the steady blue continuum. TheS/F* remains roughly constant at all nebular pointings,showing that fluorescent molecular hydrogen is not the dominant cause ofthe blue rise. We discuss three possible mechanisms for the ``bluedust'': differential extinction of the dominant star (HD 34078), unusualdust-grain properties, and emission from nebular dust. We conclude thatuncertainties in the nebular geometry and the degree of dust clumpingare most likely responsible for the blue rise. As an interestingconsequence of this result, we consider how IC 405 would appear in aspatially unresolved observation. If IC 405 were observed with a spatialresolution of less than 0.4 pc, for example, an observer would infer afar-UV flux that was 2.5 times the true value, giving the appearance ofa stellar continuum that was less extinguished than radiation from thesurrounding nebula, an effect that is reminiscent of the observedultraviolet properties of starburst galaxies.
| 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.
| A Steady, Radiative-Shock Method for Computing X-Ray Emission from Colliding Stellar Winds in Close, Massive-Star Binaries
We present a practical, efficient, semianalytic formalism for computingsteady state X-ray emission from radiative shocks between collidingstellar winds in relatively close (orbital period up to order tens ofdays) massive-star, binary systems. Our simplified approach idealizesthe individual wind flows as smooth and steady, ignoring the intrinsicinstabilities and associated structure thought to occur in such flows.By also suppressing thin-shell instabilities for wind-collisionradiative shocks, our steady state approach avoids the extensivestructure and mixing that has thus far precluded reliable computation ofX-ray emission spectra from time-dependent hydrodynamical simulations ofclose-binary, wind-collision systems; but in ignoring the unknownphysical level of such mixing, the luminosity and hardness of X-rayspectra derived here represent upper limits to what is possible for agiven set of wind and binary parameters. A key feature of our approachis the separation of calculations for the small-scale shock-emissionfrom the ram-pressure-balance model for determining the large-scale,geometric form of the wind-wind interaction front. Integrating thelocalized shock emission over the full interaction surface and using awarm-absorber opacity to take account of attenuation by both the smoothwind and the compressed, cooled material in the interaction front, themethod can predict spectra for a distant observer at any arbitraryorbital inclination and phase. We illustrate results for a sampleselection of wind, stellar, and binary parameters, providing both fullX-ray light curves and detailed spectra at selected orbital phases. Thederived spectra typically have a broad characteristic form, and bysynthetic processing with the standard XSPEC package, we demonstratethat they simply cannot be satisfactorily fitted with the usualattenuated single- or two-temperature thermal-emission models. Weconclude with a summary of the advantages and limitations of ourapproach and outline its potential application for interpreting detailedX-ray observations from close, massive-star binary systems.
| Two New Low Galactic D/H Measurements from the Far Ultraviolet Spectroscopic Explorer
We analyze interstellar absorption observed toward two subdwarf O stars,JL 9 and LS 1274, using spectra taken by the Far UltravioletSpectroscopic Explorer (FUSE). Column densities are measured for manyatomic and molecular species (H I, D I, C I, N I, O I, P II, Ar I, FeII, and H2), but our main focus is on measuring the D/Hratios for these extended lines of sight, as D/H is an importantdiagnostic for both cosmology and Galactic chemical evolution. We findD/H=(1.00+/-0.37)×10-5 toward JL 9 andD/H=(0.76+/-0.36)×10-5 toward LS 1274 (2 σuncertainties). With distances of 590+/-160 and 580+/-100 pc,respectively, these two lines of sight are currently among the longestGalactic lines of sight with measured D/H. With the addition of thesemeasurements, we see a significant tendency for longer Galactic lines ofsight to yield low D/H values, consistent with previous inferences aboutthe deuterium abundance from D/O and D/N measurements. Short lines ofsight with H I column densities of logN(HI)<19.2 suggest that thegas-phase D/H value within the Local Bubble is(D/H)LBg=(1.56+/-0.04)×10-5. However, thefour longest Galactic lines of sight with measured D/H, which haved>500 pc and logN(HI)>20.5, suggest a significantly lower valuefor the true local disk gas-phase D/H value,(D/H)LDg=(0.85+/-0.09)×10-5. Oneinterpretation of these results is that D is preferentially depletedonto dust grains relative to H and that longer lines of sight thatextend beyond the Local Bubble sample more depleted material. In thisscenario, the higher Local Bubble D/H ratio is actually a betterestimate than (D/H)LDg for the true local disk D/H,(D/H)LD. However, if (D/H)LDg is different from(D/H)LBg simply because of variable astration and incompleteinterstellar medium mixing, then (D/H)LD=(D/H)LDg.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE), which is operated for NASA by JohnsHopkins University under NASA contract NAS5-32985.
| The Angular Momentum Evolution of 0.1-10 Msolar Stars from the Birth Line to the Main Sequence
Projected rotational velocities (vsini) have been measured for a sampleof 145 stars with masses between 0.4 and greater than 10Msolar (median mass 2.1 Msolar) located in theOrion star-forming complex. These measurements have been supplementedwith data from the literature for Orion stars with masses as low as 0.1Msolar. The primary finding from analysis of these data isthat the upper envelope of the observed values of angular momentum perunit mass (J/M) varies as M0.25 for stars on convectivetracks having masses in the range ~0.1 to ~3 Msolar. Thispower law extends smoothly into the domain of more massive stars (3-10Msolar), which in Orion are already on the zero-age mainsequence. This result stands in sharp contrast to the properties ofmain-sequence stars, which show a break in the power law and a sharpdecline in J/M with decreasing mass for stars with M<2Msolar. A second result of our study is that this break isseen already among the pre-main-sequence stars in our Orion sample thatare on radiative tracks, even though these stars are only a few millionyears old. A comparison of rotation rates seen for stars on either sideof the convective-radiative boundary shows that stars do not rotate assolid bodies during the transition from convective to radiative tracks.As a preliminary demonstration of how observations can be used toconstrain the processes that control early stellar angular momentum, weshow that the broad trends in the data can be accounted for by simplemodels that posit that stars (1) lose angular momentum before they aredeposited on the birth line, plausibly through star-disk interactions;(2) undergo additional braking as they evolve down their convectivetracks; and (3) are subject to core-envelope decoupling during theconvective-radiative transition.
| 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.
| A radio and mid-infrared survey of northern bright-rimmed clouds
We have carried out an archival radio, optical and infrared wavelengthimaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA SkySurvey (NVSS) archive, images from the Digitised Sky Survey (DSS) andthe Midcourse Space eXperiment (MSX). The data characterise the physicalproperties of the Ionised Boundary Layer (IBL) of the BRCs. We haveclassified the radio detections as: that associated with the ionisedcloud rims; that associated with possible embedded Young Stellar Objects(YSOs); and that unlikely to be associated with the clouds at all. Thestars responsible for ionising each cloud are identified and acomparison of the expected ionising flux to that measured at the cloudrims is presented. A total of 25 clouds display 20 cm radio continuumemission that is associated with their bright optical rims. The ionisingphoton flux illuminating these clouds, the ionised gas pressure and theelectron density of the IBL are determined. We derive internal molecularpressures for 9 clouds using molecular line data from the literature andcompare these pressures to the IBL pressures to determine the pressurebalance of the clouds. We find three clouds in which the pressureexerted by their IBLs is much greater than that measured in the internalmolecular material. A comparison of external pressures around theremaining clouds to a global mean internal pressure shows that themajority of clouds can be expected to be in pressure equilibrium withtheir IBLs and hence are likely to be currently shocked byphotoionisation shocks. We identify one source which shows 20 cmemission consistent with that of an embedded high-mass YSO and confirmits association with a known infrared stellar cluster. This embeddedcluster is shown to contain early-type B stars, implying that at leastsome BRCs are intimately involved in intermediate to high mass starformation.Figure \ref{fig:images} and Table \ref{tbl:istars1} are only availablein electronic form at http://www.edpsciences.org
| Bright OB stars in the Galaxy. I. Mass-loss and wind-momentum rates of O-type stars: A pure H\alpha analysis accounting for line-blanketing
We study mass-loss and wind momentum rates of 29 Galactic O-type starswith luminosity classes I, III and V by means of a pure H\alpha profileanalysis and investigate to what extent the results compare to thoseoriginating from a state-of-the-art, complete spectral analysis. Ourinvestigation relies on the approximate method developed by\citet{Puls96} which we have modified to account for the effects ofline-blanketing. Effective temperatures and gravities needed to obtainquantitative results from such a simplified approach have been derivedby means of calibrations based on most recent spectroscopic NLTEanalyses and models of Galactic stars by \citet{Repo03} and\citet{Martins02}. Comparing (i) the derived wind-densities to thosedetermined by \citet{Repo03} for eleven stars in common and (ii) theWind-momentum Luminosity Relationship (WLR) for our sample stars tothose derived by other investigations, we conclude that our approximateapproach is actually able to provide consistent results. Additionally,we studied the consequences of ``fine tuning'' some of the direct andindirect parameters entering the WLR, especially by accounting fordifferent possible values of stellar reddening and distances. Combiningour data set with the corresponding data provided by \citet{Herrero02}and \citet{Repo03} we finally study the WLR for the largest sample ofGalactic O-type stars gathered so far, including an elaborate errortreatment. The established disagreement between the theoreticalpredictions and the ``observed'' WLRs being a function of luminosityclass is suggested to be a result of wind clumping. Different strategiesto check this hypothesis are discussed, particularly by comparing theH\alpha mass-loss rates with the ones derived from radio observations.
| 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.
| 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.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| The Deuterium-to-Oxygen Ratio in the Interstellar Medium
Because the ionization balances for H I, O I, and D I are lockedtogether by charge exchange, the deuterium-to-oxygen ratio, D/O, is animportant tracer for the value of the D/H ratio and for potentialspatial variations in the ratio. As the D I and O I column densities areof similar orders of magnitude for a given sight line, comparisons ofthe two values will generally be less subject to systematic errors thancomparisons of D I and H I, which differ by about 5 orders of magnitude.Moreover, D/O is additionally sensitive to astration, because as starsdestroy deuterium, they should produce oxygen. We report here theresults of a survey of D/O in the interstellar medium performed with theFar Ultraviolet Spectroscopic Explorer. We also compare these resultswith those for D/N. Together with a few results from previous missions,the sample totals 24 lines of sight. The distances range from a few to~2000 pc and logN(D I) from ~13 to ~16 cm-2. The D/O ratio isconstant in the local interstellar medium out to distances of ~150 pcand N(DI)~=1×1015 cm-2, i.e., within theLocal Bubble. In this region of the interstellar space, we findD/O=(3.84+/-0.16)×10-2 (1 σ in the mean). Thehomogeneity of the local D/O measurements shows that the spatialvariations in the local D/H and O/H must be extremely few, if any. Acomparison of the Local Bubble mean value with the few D/O measurementsavailable for low-metallicity quasar sight lines shows that the D/Oratio decreases with cosmic evolution, as expected. Beyond the LocalBubble, we detected significant spatial variations in the value of D/O.This likely implies a variation in D/H, as O/H is known to not varysignificantly over the distances covered in this study. Our data setsuggests a present-epoch deuterium abundance below1×10-5, i.e., lower than the value usually assumed,around 1.5×10-5.
| Potential Variations in the Interstellar N I Abundance
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and SpaceTelescope Imaging Spectrograph observations of the weak interstellar N Iλ1160 doublet toward 17 high-density sight lines[N(Htot)>=1021 cm-2]. When combinedwith published data, our results reveal variations in the fractional N Iabundance showing a systematic deficiency at large N(Htot).At the FUSE resolution (~20 km s-1), the effects ofunresolved saturation cannot be conclusively ruled out, although O Iλ1356 shows little evidence of saturation. We investigated thepossibility that the N I variability is due to the formation ofN2 in our mostly dense regions. The 0-0 band of thec'41Σ+u-X1Σ+gtransition of N2 at 958 Å should be easily detected inour FUSE data; for 10 of the denser sight lines, N2 is notobserved at a sensitivity level of a few times 1014cm-2. The observed N I variations are suggestive of anincomplete understanding of nitrogen chemistry.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985, and the NASA/ESA HubbleSpace Telescope, obtained from the Multimission Archive at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under the NASA contractNAS 5-26555.
| 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.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Orion |
| Rektaszcenzió: | 05h35m26.00s |
| Deklináció: | -05°54'36.0" |
| Vizuális fényesség: | 2.77 |
| Távolság: | 406.504 parszek |
| RA sajátmozgás: | 0 |
| Dec sajátmozgás: | 0 |
| B-T magnitude: | 2.485 |
| V-T magnitude: | 2.715 |
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INHABITED SKY
