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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.
| 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.
| The Vertical Stellar Kinematics in Face-On Barred Galaxies: Estimating the Ages of Bars
In order to perform a detailed study of the stellar kinematics in thevertical axis of bars, we obtained high signal-to-noise spectra alongthe major and minor axes of the bars in a sample of 14 face-on galaxiesand used them to determine the line-of-sight stellar velocitydistribution, parameterized as a Gauss-Hermite series. With these data,we developed a diagnostic tool that allows one to distinguish betweenrecently formed and evolved bars, as well as to estimate their ages,assuming that bars form in vertically thin disks that are recognizableby low values for the vertical velocity dispersion σz.Through N-body realizations of bar unstable disk galaxies we were alsoable to check the timescales involved in the processes that give bars animportant vertical structure. We show that σz inevolved bars is roughly 100 km s-1, which translates to aheight scale of about 1.4 kpc, giving support to scenarios in whichbulges form through disk material. Furthermore, the bars in ournumerical simulations have values for σz generallysmaller than 50 km s-1, even after evolving for 2 Gyr,suggesting that a slow process is responsible for making bars asvertically thick as we observe. We verify theoretically that theSpitzer-Schwarzschild mechanism is quantitatively able to explain theseobservations if we assume that giant molecular clouds are twice asconcentrated along the bar as in the rest of the disk.
| Low-Mass Runaway Stars from the Orion Trapezium Cluster
In the course of a search for common proper-motion binaries in the Jones& Walker (JW) catalog of proper motions in the Orion Nebula Cluster,we came across several faint stars with proper motions larger than 1"per century and probabilities of membership P larger than 0.90. Suchstars are interesting because they could be low-mass runaway starsrecently accelerated by n-body interactions in compact multiple systems.Of particular interest among these stars is JW 451, which has a P =0.98, the largest transverse velocity among all the stars with P>=0.5( 69+/-38 km s-1), and a proper-motion vector suggesting thatit was accelerated by the θ1 Orionis C triple systemsome 1000 years ago. A closer examination of those JW stars withμ>1'' per century revealed that two other stars, JW 349and JW 355 (with transverse velocities of 38+/-9 and 90+/-9 kms-1, respectively), in spite of being listed with P = 0 byJW, should also be considered part of the cluster, because these objectsare also externally ionized proplyds. In fact, Hillenbrand assigns tothem probabilities of membership of 0.99. Moreover, the proper-motionerrors of these two stars are relatively small, and so they are goodcandidates to be runaway stars recently accelerated in the Orion NebulaCluster.
| UVBLUE: A New High-Resolution Theoretical Library of Ultraviolet Stellar Spectra
We present an extended ultraviolet-blue (850-4700 Å) library oftheoretical stellar spectral energy distributions computed at highresolution, λ/Δλ=50,000. The UVBLUE grid, as wenamed the library, is based on LTE calculations carried out with ATLAS9and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800entries that cover a large volume of the parameter space. It spans arange in Teff from 3000 to 50,000 K, the surface gravityranges from logg=0.0 to 5.0 with Δlogg=0.5 dex, while sevenchemical compositions are considered:[M/H]=-2.0,-1.5,-1.0,-0.5,+0.0,+0.3, and +0.5 dex. For its coverageacross the Hertzsprung-Russell diagram, this library is the mostcomprehensive one ever computed at high resolution in theshort-wavelength spectral range, and useful application can be foreseenfor both the study of single stars and in population synthesis models ofgalaxies and other stellar systems. We briefly discuss some relevantissues for a safe application of the theoretical output to ultravioletobservations, and a comparison of our LTE models with the non-LTE (NLTE)ones from the TLUSTY code is also carried out. NLTE spectra are found,on average, to be slightly ``redder'' compared to the LTE ones for thesame value of Teff, while a larger difference could bedetected for weak lines, which are nearly wiped out by the enhanced coreemission component in case of NLTE atmospheres. These effects seem to bemagnified at low metallicity (typically [M/H]<~-1). A match with aworking sample of 111 stars from the IUE atlas, with availableatmosphere parameters from the literature, shows that UVBLUE modelsprovide an accurate description of the main mid- and low-resolutionspectral features for stars along the whole sequence from the B to ~G5type. The comparison sensibly degrades for later spectral types, withsupergiant stars that are in general more poorly reproduced than dwarfs.As a possible explanation of this overall trend, we partly invoke theuncertainty in the input atmosphere parameters to compute thetheoretical spectra. In addition, one should also consider the importantcontamination of the IUE stellar sample, where the presence of binaryand variable stars certainly works in the sense of artificiallyworsening the match between theory and observations.
| 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.
| 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.
| Deuterium Depletion and Magnesium Enhancement in the Local Disk
The local disk deuterium is known to be depleted in comparison to thelocal bubble. We used Hubble Space Telescope (HST) spectra to obtaincolumn densities of Si, Mg and Fe. We compared normalized columndensities of these elements in the directions with high and lowdeuterium abundances.We show, that the lines of sight that are depleted in deuterium, areenhanced in magnesium. This observation implicates that astration isresponsible for both deuterium depletion and magnesium enhancement.
| Early and late time VLT spectroscopy of SN 2001el - progenitor constraints for a type Ia supernova
We present early time high-resolution (VLT/UVES) and late timelow-resolution (VLT/FORS) optical spectra of the normal type Iasupernova, SN 2001el. The high-resolution spectra were obtained 9 and 2days before (B-band) maximum light. This was in order to allow thedetection of narrow hydrogen and/or helium emission lines from thecircumstellar medium of the supernova. No such lines were detected inour data. We therefore use these spectra together with photoionisationmodels to derive upper limits of 9×10-6 {M}_ȯyr-1 and 5×10-5 {M}_ȯ yr-1for the mass loss rate from the progenitor system of SN 2001el assumingvelocities of 10 km s-1 and 50 km s-1,respectively, for a wind extending to outside at least a few ×1015 cm away from the supernova explosion site. So far, theseare the best Hα based upper limits obtained for a type Iasupernova, and exclude a symbiotic star in the upper mass loss rateregime (so called Mira type stars) from being the progenitor of SN2001el. The low-resolution spectrum was obtained in the nebular phase ofthe supernova, 400 days after the maximum light, to search for anyhydrogen rich gas originating from the supernova progenitor system.However, we see no signs of Balmer lines in our spectrum. Therefore, wemodel the late time spectra to derive an upper limit of 0.03Mȯ for solar abundance material present at velocitieslower than 1000 km s-1 within the supernova explosion site.According to numerical simulations of Marietta et al. (2000) this isless than the expected mass lost by a subgiant, red giant or amain-sequence secondary star at a small binary separation as a result ofthe SN explosion. Our data therefore exclude these scenarios as theprogenitor of SN 2001el. Finally, we discuss the origin of high velocityCa II lines previously observed in a few type Ia supernovae before themaximum light. We see both the Ca II IR triplet and the H&K lines inour earliest (-9 days) spectrum at a very high velocity of up to 34 000km s-1. The spectrum also shows a flat-bottomed Si II "6150Å" feature similar to the one previously observed in SN 1990N(Leibundgut et al. 1991, ApJ, 371, L23) at 14 days before maximum light.We compare these spectral features in SN 2001el to those observed in SN1984A and SN 1990N at even higher velocities.
| O stars with weak winds: the Galactic case
We study the stellar and wind properties of a sample of Galactic Odwarfs to track the conditions under which weak winds (i.e. mass lossrates lower than 10-8 Mȯ yr-1)appear. The sample is composed of low and high luminosity dwarfsincluding Vz stars and stars known to display qualitatively weak winds.Atmosphere models including non-LTE treatment, spherical expansion andline blanketing are computed with the code CMFGEN (Hillier & Miller1998, ApJ, 496, 407). Both UV and Hα lines are used to derive windproperties while optical H and He lines give the stellar parameters. Wefind that the stars of our sample are usually 1 to 4 Myr old. Mass lossrates of all stars are found to be lower than expected from thehydrodynamical predictions of Vink et al. (2001, A&A, 369, 574). Forstars with log {L}/{Lȯ} 5.2, the reduction is byless than a factor 5 and is mainly due to the inclusion of clumping inthe models. For stars with log {L}/{Lȯ} 5.2 thereduction can be as high as a factor 100. The inclusion of X-rayemission (possibly due to magnetic mechanisms) in models with lowdensity is crucial to derive accurate mass loss rates from UV lines,while it is found to be unimportant for high density winds. The modifiedwind momentum - luminosity relation shows a significant change of slopearound this transition luminosity. Terminal velocities of low luminositystars are also found to be low. Both mass loss rates and terminalvelocities of low L stars are consistent with a reduced line forceparameter α. However, the physical reason for such a reduction isstill not clear although the finding of weak winds in Galactic starsexcludes the role of a reduced metallicity. There may be a link betweenan early evolutionary state and a weak wind, but this has to beconfirmed by further studies of Vz stars. X-rays, through the change inthe ionisation structure they imply, may be at the origin of a reductionof the radiative acceleration, leading to lower mass loss rates. Abetter understanding of the origin of X-rays is of crucial importancefor the study of the physics of weak winds.
| New runaway OB stars with HIPPARCOS
A Monte Carlo method for detection of runaway OB stars fromobservational data is proposed. 61 runaway OB star candidates have beendetected by an analysis of Hipparcos proper motions. The peculiartangential and total transverse velocities have been determined forthese stars. A list of the detected runaway star candidates ispresented. The evidence of a discrepancy between photometric andparallactic distances of runaway OB star candidates is presented.
| 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.
| CaII K interstellar observations towards early-type disc and halo stars - distances to intermediate- and high-velocity clouds
We compare existing high spectral resolution(R=λ/Δλ~ 40000) CaII K observations(λair= 3933.66 Å) towards 88 mainly B-typestars, and new observations taken using the Intermediate dispersionSpectrograph and Imaging System (ISIS) on the William Herschel Telescopeat R~ 10000 towards three stars taken from the Palomar-Green Survey,with 21-cm HI emission-line profiles, in order to search for opticalabsorption towards known intermediate- and high-velocity cloudcomplexes. Given certain assumptions, limits to the gas phase abundanceof CaII are estimated for the cloud components. We use the data toderive the following distances from the Galactic plane (z). (i)Tentative lower z-height limits of 2800 and 4100 pc towards complex Cusing lack of absorption in the spectra of HD341617 and PG0855+294,respectively. (ii) A weak lower z-height of 1400 pc towards complexWA-WB using lack of absorption in EC09470-1433 and a weak lower limit of2470 pc using lack of absorption in EC09452-1403. (iii) An upperz-height of 2470 pc towards a southern intermediate-velocity cloud (IVC)with vLSR=-55 km s-1 using PG2351+198. (iv)Detection of a possible IVC in CaII absorption at vLSR=+52 kms-1 using EC20104-2944. No associated HI in emission isdetected. At this position, normal Galactic rotation predicts velocitiesof up to ~+25 km s-1. The detection puts an upper z-height of1860 pc to the cloud. (v) Tentative HI and CaII K detections towards anIVC at ~+70 km s-1 in the direction of high-velocity cloud(HVC) complex WE, sightline EC06387-8045, indicating that the IVC may beat a z-height lower than 1770 pc. (vi) Detection of CaII K absorption inthe spectrum of PG0855+294 in the direction of IV20, indicating thatthis IVC has a z-height smaller than 4100 pc. (vii) A weak lowerz-height of 4300 pc towards a small HVC with vLSR=+115 kms-1 at l, b= 200°, + 52°, using lack of absorption inthe CaII K spectrum of PG0955+291.
| 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.
| 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.
| 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.
| 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.
| Library of flux-calibrated echelle spectra of southern late-type dwarfs with different activity levels
We present Echelle spectra of 91 late-type dwarfs, of spectral typesfrom F to M and of different levels of chromospheric activity, obtainedwith the 2.15 m telescope of the CASLEO Observatory located in theArgentinean Andes. Our observations range from 3890 to 6690 Å, ata spectral resolution from 0.141 to 0.249 Å per pixel(R=λ/δ λ ≈ 26 400). The observations were fluxcalibrated with the aid of long slit spectra. A version of thecalibrated spectra is available via the World Wide Web.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/699The spectra are available as FITS and ascii-files at the URL:http://www.iafe.uba.ar/cincunegui/spectra/Table2.html. They are alsoavailable 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/qcat?J/A+A/414/699. When convertingthe fits to ascii, the spectra were oversampled to a constant δλ ≈ 0.15 Å.Table 2 is also available in electronic form at the CDS via anonymous
| CaII K interstellar observations towards early-type disc and halo stars
We present high-resolution (R=λ/Δλ~ 40000) CaII Kinterstellar observations (λair= 3933.66Å)towards 88 mainly B-type stars, of which 74 are taken from theEdinburgh-Cape or Palomar-Green surveys, and 81 have |b| > 25°.The majority of the data come from previously existing spectroscopy,although also included are 18 new observations of stars with echellespectra taken with UVES on the Very Large Telescope UT2 (Kueyen). Some49 of the sample stars have distance estimates above the Galactic plane(|z|) >= 1 kpc, and are thus good probes of the halo interstellarmedium. Of the 362 interstellar Ca K components that we detect, 75 (21per cent) have absolute values of their LSR velocity values exceeding 40km s-1. In terms of the deviation velocity for the sightlineswith distance estimates, 46/273 (17 per cent) of components havevelocity values exceeding those predicted by standard Galactic rotationby more than 40 km s-1. Combining this data set with previousobservations, we find that the median value of the reduced equivalentwidth (REW) of stars with |z| >= 1 kpc (EW×sin|b|) is ~115mÅ (n= 80), similar to that observed in extragalactic sightlinesby Bowen. Using data of all z distances, the REW at infinity is found tobe ~130 mÅ, with the scaleheight (l) of the CaII K column densitydistribution being ~800 pc (n= 196) and reduced column density atinfinity of log[N(CaII K) cm-2]~12.24. This implies that ~30per cent of CaII K absorption occurs at distances exceeding ~1 kpc. Fornine sightlines with distance exceeding 1 kpc and with a companionobject within 5°, we find that all but two have values of CaIIreduced equivalent width the same to within ~20 per cent, when the REWof the nearest object is extrapolated to the distance of the further ofthe pair, and assuming l= 800 pc. For 29 of our sightlines with |z|>= 1 kpc and a HI detection from the Leiden-Dwingeloo survey(beamsize of 0.5°), we find log(N(CaII K)/N(HI)) ranging from -7.4to -8.4. Values of the CaII K abundance relative to neutral hydrogen(log[N(CaIIK)cm-2]-log[N(HI)cm-2]) are found to bemore than ~0.5dex higher in stars with distances exceeding ~100 pc, whencompared with the (log[N(CaII K) cm-2]-log[N(Htot) cm-2]) values found in nearbysightlines such as those in Wakker & Mathis (2000). Finally, stellarCaII K equivalent widths of the sample are determined for 26 objects.
| The Chemical Composition and Gas-to-Dust Mass Ratio of Nearby Interstellar Matter
We use recent results on interstellar gas toward nearby stars andinterstellar by-products within the solar system to select among theequilibrium radiative transfer models of the nearest interstellarmaterial presented in Slavin & Frisch. For the assumption thatO/H~400 parts per million, models 2 and 8 are found to yield good fitsto available data on interstellar material inside and outside of theheliosphere, with the exception of the Ne abundance in the pickup ionand anomalous cosmic-ray populations. For these models, the interstellarmedium (ISM) at the entry point to the heliosphere hasn(H0)=0.202-0.208 cm-3,n(He0)=0.0137-0.0152 cm-3, and ionizationsχ(H)=0.29-0.30, χ(He)=0.47-0.51. These best models suggest thatthe chemical composition of the nearby ISM is ~60%-70% subsolar if S isundepleted. Both H0 and H+ need to be includedwhen evaluating abundances of ions found in warm diffuse clouds. Models2 and 8 yield an H filtration factor of ~0.46. Gas-to-dust mass ratiosfor the ISM toward ɛ CMa are Rgd=178-183 for solarabundances of Holweger or Rgd=611-657 for an interstellarabundance standard 70% solar. Direct observations of dust grains in thesolar system by Ulysses and Galileo yield Rgd~=115 for models2 and 8, supporting earlier results (Frisch and coworkers). If the localISM abundances are subsolar, then gas and dust are decoupled over smallspatial scales. The inferred variation in Rgd over parseclength scales is consistent with the fact that the ISM near the Sun ispart of a dynamically active cluster of cloudlets flowing away from theSco-Cen association. Observations toward stars within ~500 pc show thatRgd correlates with the percentage of the dust mass that iscarried by iron, suggesting that an Fe-rich grain core (by mass) remainsafter grain destruction. Evidently large dust grains (>10-13g) and small dust grains (<10-13 g) are not well mixedover parsec length spatial scales in the ISM. It also appears that verysmall C-dominated dust grains have been destroyed in the ISM withinseveral parsecs of the Sun, since C appears to be essentiallyundepleted. However, if gas-dust coupling breaks down over the cloudlifetime, the missing mass arguments applied here to determineRgd and dust grain mineralogy are not appropriate.
| 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.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp:cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/411/447
| Astrophysics in 2001
During the year, astronomers provided explanations for solar topicsranging from the multiple personality disorder of neutrinos tocannibalism of CMEs (coronal mass ejections) and extra-solar topicsincluding quivering stars, out-of-phase gaseous media, black holes ofall sizes (too large, too small, and too medium), and the existence ofthe universe. Some of these explanations are probably possibly true,though the authors are not betting large sums on any one. The data oughtto remain true forever, though this requires a careful definition of``data'' (think of the Martian canals).
| Echelle spectroscopy of Caii HK activity in Southern Hemisphere planet search targets
We present the results of ultraviolet echelle spectroscopy of a sampleof 59 F, G, K and M stars from the Anglo-Australian Planet Search targetlist. Caii activity indices, which are essential in the interpretationof planet detection claims, have been determined for these stars andplaced on the Mount Wilson R 'HK system.
| The Heavy-Element Enrichment of Lyα Clouds in the Virgo Supercluster
Using high signal-to-noise ratio echelle spectra of 3C 273 obtained withthe Space Telescope Imaging Spectrograph (resolution of 7 kms-1 FWHM), we constrain the metallicities of two Lyαclouds in the vicinity of the Virgo Cluster. We detect C II, Si II, andSi III absorption lines in the Lyα absorber atzabs=0.00530. Previous observations with the Far UltravioletSpectroscopic Explorer have revealed Lyβ-Lyθ absorptionlines at the same redshift, thereby accurately constraining the H Icolumn density. We model the ionization of the gas and derive[C/H]=-1.2+0.3-0.2, [Si/C]=0.2+/-0.1, andlognH=-2.8+/-0.3. The model implies a small absorberthickness, ~70 pc, and thermal pressure p/k~40 cm-3 K. It ismost likely that the absorber is pressure confined by an external mediumbecause gravitational confinement would require a very high ratio ofdark matter to baryonic matter. Based on a sample of Milky Way sightlines in which carbon and silicon abundances have been reliably measuredin the same interstellar cloud (including new measurements presentedherein), we argue that it is unlikely that the overabundance of Sirelative to C is due to depletion onto dust grains. Instead, thisprobably indicates that the gas has been predominately enriched byejecta from Type II supernovae. Such enrichment is most plausiblyprovided by an unbound galactic wind, given the absence of knowngalaxies within a projected distance of 100 kpc and the presence ofgalaxies capable of driving a wind at larger distances (e.g., H I1225+01). Such processes have been invoked to explain the observedabundances in the hot, X-ray-emitting gas in Virgo. However, the sightline to 3C 273 is more than 10° away from the X-ray emission region.We also constrain the metallicity and physical conditions of the Virgoabsorber at zabs=0.00337 in the spectrum of 3C 273 based ondetections of O VI and H I and an upper limit on C IV. If this absorberis collisionally ionized, the OVI/CIV limit requiresT>~105.3 K in the O VI-bearing gas. For either collisionalionization or photoionization, we find that [O/H]>~-2.0 atzabs=0.00337. Based on observations with the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| Stellar and wind properties of LMC WC4 stars. A metallicity dependence for Wolf-Rayet mass-loss rates
We use ultraviolet space-based (FUSE, HST) and optical/IR ground-based(2.3 m MSSSO, NTT) spectroscopy to determine the physical parameters ofsix WC4-type Wolf-Rayet stars in the Large Magellanic Cloud. Stellarparameters are revised significantly relative to Gräfener et al.(\cite{Grafener1998}) based on improved observations and moresophisticated model atmosphere codes, which account for line blanketingand clumping. We find that stellar luminosities are revised upwards byup to 0.4 dex, with surface abundances spanning a lower range of 0.1 leC/He le 0.35 (20-45% carbon by mass) and O/He le 0.06 (<=10% oxygenby mass). Relative to Galactic WC5-8 stars at known distance, andanalysed in a similar manner, LMC WC4 stars possess systematicallyhigher stellar luminosities, ~ 0.2 dex lower wind densities, yet asimilar range of surface chemistries. We illustrate how theclassification C III lambda 5696 line is extremely sensitive to winddensity, such that this is the principal difference between the subtypedistribution of LMC and Galactic early-type WC stars. Temperaturedifferences do play a role, but carbon abundance does not affect WCspectral types. We illustrate the effect of varying temperature andmass-loss rate on the WC spectral type for HD 32257 (WC4, LMC) and HD156385 (WC7, Galaxy) which possess similar abundances and luminosities.Using the latest evolutionary models, pre-supernova stellar masses inthe range 11-19 Msun are anticipated for LMC WC4 stars, with7-14 Msun for Galactic WC stars with known distances. Thesevalues are consistent with pre-cursors of bright type-Ic supernovae suchas SN 1998bw (alias GRB 980425) for which a minimum total mass of C andO of 14 Msun has been independently derived. Based onobservations made with the NASA-CNES-CSA Far Ultraviolet SpectroscopicExplorer, and NASA-ESA Hubble Space Telescope. Also based onobservations collected at the European Southern Observatory in program63.H-0683, and at the Australian National University Siding SpringObservatory.
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/390/1033
| An atlas of 2.4 to 4.1 mu m ISO/SWS spectra of early-type stars
We present an atlas of spectra of O- and B-type stars, obtained with theShort Wavelength Spectrometer (SWS) during the Post-Helium program ofthe Infrared Space Observatory (ISO). This program is aimed at extendingthe Morgan & Keenan classification scheme into the near-infrared.Later type stars will be discussed in a separate publication. Theobservations consist of 57 SWS Post-Helium spectra from 2.4 to 4.1 μm, supplemented with 10 spectra acquired during the nominal mission witha similar observational setting. For B-type stars, this sample providesample spectral coverage in terms of subtype and luminosity class. ForO-type stars, the ISO sample is coarse and therefore is complementedwith 8 UKIRT Larcmin -band observations. In terms of the presence ofdiagnostic lines, the Larcmin -band is likely the most promising of thenear-infrared atmospheric windows for the study of the physicalproperties of B stars. Specifically, this wavelength interval containsthe Bralpha , Pfgamma , and other Pfund lines which are probes ofspectral type, luminosity class and mass loss. Here, we present simpleempirical methods based on the lines present in the 2.4 to 4.1 mu minterval that allow the determination of i) the spectral type of Bdwarfs and giants to within two subtypes; ii) the luminosity class of Bstars to within two classes; iii) the mass-loss rate of O stars and Bsupergiants to within 0.25 dex. Based on observations with ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA. The appendix is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin.qcat?J/A+A/384/473
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Dades d'Observació i Astrometria
| Constel·lació: | Columba |
| Ascensió Recta: | 05h45m59.90s |
| Declinació: | -32°18'23.0" |
| Magnitud Aparent: | 5.17 |
| Distancia: | 396.825 parsecs |
| Moviment propi RA: | 3.4 |
| Moviment propi Dec: | -23.5 |
| B-T magnitude: | 4.814 |
| V-T magnitude: | 5.114 |
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