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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.
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| Reanalysis of Copernicus Measurements of Interstellar Carbon Monoxide
We used archival data acquired with Copernicus to reexamine CO columndensities, as self-consistent oscillator strengths are now available.Our focus is on lines of sight containing modest amounts of molecularspecies. Our resulting column densities are small enough thatself-shielding from photodissociation does not occur in the cloudsprobed by the observations. While our sample shows that the columndensities of CO and H2 are related, no correspondence withthe CH column density is evident. The case for the CH+ columndensity is less clear. Recent chemical models for these sight linessuggest that CH is mainly a by-product of CH+ synthesis inlow-density gas. The models are most successful in reproducing theamounts of CO in the densest sight lines. Thus, much of the COabsorption must arise from denser clumps along the line of sight toaccount for the trend with H2.
| On the relation between diffuse bands and column densities of H2, CH and CO molecules
Mutual relations between column densities of H2, CH and COmolecules as well as between the latter and strengths of the major 5780and 5797 diffuse bands are presented and discussed. The CH radical seemsto be a good H2 tracer, possibly better than CO. It is alsodemonstrated that the molecular fraction of the H2 moleculeis correlated with an intensity ratio of 5797 and 5780 DIBs, suggestingthe possible formation of narrow DIB carriers in denser clouds,dominated by molecular hydrogen and reasonably shielded from ionizing UVradiation by small dust grains.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/414/949
| Toward an adequate method to isolate spectroscopic families of diffuse interstellar bands
We divide some of the observed diffuse interstellar bands (DIBs) intofamilies that appear to have the spectral structure of single species.Three different methods are applied to separate such families, exploringthe best approach for future investigations of this type. Starting witha statistical treatment of the data, we found that statistical methodsby themselves give insufficient results. Two other methods of dataanalysis (`averaging equivalent widths' and `investigating the figureswith arranged spectrograms') were found to be more useful as tools forfinding the spectroscopic families of DIBs. On the basis of thesemethods, we suggest some candidates as `relatives' of 5780- and5797-Å bands.
| 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 Homogeneity of Interstellar Krypton in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope SpaceTelescope Imaging Spectrograph (STIS) observations of Kr I λ1236absorption in seven sight lines that probe a variety of interstellarenvironments. In combination with krypton and hydrogen column densitiesderived from current and archival STIS and Far-Ultraviolet SpectroscopicExplorer data, the number of sight lines with reliable Kr/H ISMabundance ratios has been increased by 50% to 26-including paths thatsample a range of nearly 5 orders of magnitude in f(H2) andover 2 orders of magnitude in , and extend up to4.8 kpc in length. For sight lines contained entirely within the localspiral arm (the Orion spur), the spread of Kr/H ratios about the mean oflog10[N(Kr)/N(H)]ISM=-9.02+/-0.02is remarkably tight (0.06 dex), less than the typical data-pointuncertainty. Intriguingly, the only two sight lines that extend throughneighboring structures, in particular gas associated with theCarina/Sagittarius arm, exhibit relatively large, near-solar kryptonabundances (log10[N(Kr)/N(H)]combined=-8.75+0.09-0.11).Although these deviations are only measured at the 2 σ level, theysuggest the possibility that krypton abundances beyond the Orion spurmay differ from the local value.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far-Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc. under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS-32985.
| Transitions from Autoionized Single-ionized Tin States: A Theoretical Study of the 5s5p (3Po) nl (nl = 5d, 6s) Levels of Sn II
Lines corresponding to several transitions from autoionized states ofsingle-ionized tin were identified in a laser-produced plasma generatedby 10640 Å irradiation of an Sn target at a flux of2×1010 W cm-2. Spectra were recorded andanalyzed between 2000 and 7000 Å. Theoretical analysis of Sn IIwas extended using relativistic Hartree-Fock calculations andconfiguration interactions in an intermediate-coupling scheme with thesupport of the Cowan code. Our calculations support the experimentalvalue obtained by Schectman et al. in 2000 of the lifetime of5s25d2D3/2 and the absorptionoscillator strength of the resonance transition to this level at 1400.45Å. The parametric description of 5s5p (3Po)nl (nl=5d, 6s) of Sn II levels is improved by taking into account thefar 5p3 configuration mixing effects. The results obtained inthis study will allow a substantial improvement in the interpretation ofthe data of the ultraviolet spectrum of Sn II observed by the GoddardHigh Resolution Spectrograph aboard the Hubble Space Telescope.
| Nonthermal Chemistry in Diffuse Clouds with Low Molecular Abundances
High-quality archival spectra of interstellar absorption from C I towardnine stars, taken with the Goddard High Resolution Spectrograph on theHubble Space Telescope, were analyzed. Our sample was supplemented bytwo sight lines, 23 Ori and β1 Sco, for which the C Imeasurements of Federman, Welty, & Cardelli were used. Directionswith known CH+ absorption, but only upper limits onabsorption from C2 and CN, were considered for our study.This restriction allows us to focus on regions where CH+chemistry dominates the production of carbon-bearing molecules. Profilesynthesis of several multiplets yielded column densities and Dopplerparameters for the C I fine-structure levels. Equilibrium excitationanalyses, using the measured column densities as well as the temperaturefrom H2 excitation, led to values for gas density. Thesedensities, in conjunction with measurements of CH, CH+,C2, and CN column densities, provided estimates for theamount of CH associated with CH+ production, which in turnset up constraints on the present theories for CH+ formationin this environment. We found for our sample of interstellar clouds thaton average 30%-40% of the CH originates from CH+ chemistry,and in some cases it can be as high as 90%. A simple chemical model forgas containing nonequilibrium production of CH+ was developedfor the purpose of predicting column densities for CH, CO,HCO+, CH+2, andCH+3 generated from large abundances ofCH+. Again, our results suggest that nonthermal chemistry isnecessary to account for the observed abundance of CH and probably thatof CO in these clouds.Based on observations obtained with the NASA/ESA Hubble Space Telescopethrough the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Observational Studies of Early-Type Overcontact Binaries: TU Muscae
We present new spectroscopic and photometric data on the early-typeovercontact binary TU Muscae. The analysis of the spectroscopic datashows that the line of sight to the system crosses three kinematicallysharp and well-separated interstellar reddening sources and that thestars rotate synchronously. We present new radial velocities that are ingood agreement with earlier optical velocities and, thus, do not confirmthe systematically smaller velocities obtained from IUE spectra. Theoptical velocities are analyzed simultaneously with the photometric datato derive accurate absolute dimensions for the binary components. Theresults show that TU Mus consists of an O7.5 primary withM1=23.5+/-0.8 Msolar and R1=7.48+/-0.08Rsolar and an O9.5 secondary with M2=15.3+/-0.4Msolar and R2=6.15+/-0.07 Rsolar in anovercontact configuration, and that the orbital period has remainedconstant over the three decades covered by the observations. Theseresults might imply that the mass transfer seen in late-type overcontactbinaries does not occur in their early-type counterparts.
| 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.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe 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/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 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/404/913
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is 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/399/141
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| The mass ratio distribution of B-type visual binaries in the Sco OB2 association
A sample of 115 B-type stars in the Sco OB2 association is examined forexistence of visual companions in the J and K_s bands, using the ADONISnear-infrared adaptive optics system and coronograph. Practically allthe components in the separation range 0farcs3 -6farcs4 (45-900 AU) andmagnitudes down to K = 16 were detected. The K and J - K photometry ofthe primaries and differential photometry and astrometry of the 96secondaries are presented. Ten secondaries are new physical components,as inferred from the photometric and statistical criteria, while therest of the newly detected objects are faint background stars. After asmall correction for detection incompleteness and a conversion of thefluxes into masses, an unbiased distribution of the components massratio q was derived. The power law f(q)~ q-0.5 fits theobservations well, whereas a q-1.8 distribution, whichcorresponds to a random pairing of stars, is rejected. The companionstar fraction is 0.20+/-0.04 per decade of separation which iscomparable to the highest measured binary fraction among low-mass PMSstars and ~ 1.6 times higher than the binary fraction of low-mass dwarfsin the solar neighborhood and in open clusters in the same separationrange. Based on observations collected at the European SouthernObservatory, La Silla, Chile (ESO programme 65.H-0179). Tables 1, 3 andthe full version of Table 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/382/92
| The accretion/diffusion theory for lambda Bootis stars in the light of spectroscopic data
Most of the current theories suggest the lambda Bootis phenomenon tooriginate from an interaction between the stellar surface and its localenvironment. In this paper, we compare the abundance pattern of thelambda Bootis stars to that of the interstellar medium and find largerdeficiencies for Mg, Si, Mn and Zn than in the interstellar medium. Acomparison with metal poor post-AGB stars showing evidence forcircumstellar material indicates a similar physical process possiblybeing at work for some of the lambda Bootis stars, but not for all ofthem. Despite the fact that the number of spectroscopically analysedlambda Bootis stars has considerably increased in the past, a test ofpredicted effects with observations shows current abundance andtemperature data to be still controversial.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| A High-Resolution Survey of Interstellar K I Absorption
We present high-resolution (FWHM ~0.4-1.8 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar K Iabsorption toward 54 Galactic stars. These new K I spectra revealcomplex structure and narrow, closely blended components in many linesof sight. Multicomponent fits to the line profiles yield estimates forthe column densities, line widths, and velocities for 319 individualinterstellar cloud components. The median component width (FWHM) and thetrue median separation between adjacent components are both <~1.2 kms-1. The median and maximum individual component K I columndensities, about 4×1010 and 1012cm-2, correspond to individual component hydrogen columndensities of about 2×1020 and 1021cm-2 and E(B-V)~0.03 and 0.17, respectively. If T istypically ~100 K, then at least half the individual components havesubsonic internal turbulent velocities. We also reexamine therelationships between the column densities of K I, Na I, C I, Li I,Htot, H2, and CH. The four trace neutral speciesexhibit essentially linear relationships with each other over wideranges in overall column density. If C is uniformly depleted by 0.4 dex,then Li, Na, and K are each typically depleted by 0.6-0.7 dex. The totalline of sight values for N(K I) and N(Na I) show roughly quadraticdependences on N(Htot), but the relationships for theensemble of individual clouds could be significantly steeper. Thesequadratic (or steeper) dependences appear to rule out significantcontributions to the ionization from cosmic rays, X-rays, and/or chargeexchange with C II in most cases. Charge exchange with negativelycharged large molecules may often be more important than radiativerecombination in neutralizing most singly ionized atomic species in coolH I clouds, however-suggesting that the true ne,nH, and thermal pressures may be significantly smaller thanthe values estimated by considering only radiative recombination. BothN(CH) and N(H2) are nearly linearly proportional to N(K I)and N(Na I) [except for 1015cm-2<~N(H2)<~1019cm-2, over which H2 makes the transition to theself-shielded regime]. Those relationships appear also to hold for manyindividual components and component groups, suggesting thathigh-resolution spectra of K I and Na I can be very useful forinterpreting lower resolution molecular data. The scatter about allthese mean relationships is generally small (<~0.1-0.2 dex), ifcertain consistently ``discrepant'' sight lines are excluded-suggestingthat both the relative depletions and the relative ionization of Li, C,Na, and K are generally within factors of 2 of their mean values.Differences noted for sight lines in Sco-Oph, in the Pleiades, near theOrion Trapezium, and in the LMC and SMC may be due to differences in thestrength and/or shape of the ambient radiation fields, perhaps amplifiedby the effects of charge transfer with large molecules.
| Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars
We present the results of statistical analyses of a sample of 627 Bestars. The parameters of intrinsic polarization (p*),projected rotational velocity (v sin i), and near IR excesses have beeninvestigated. The values of p* have been estimated for a muchlarger and more representative sample of Be stars (~490 objects) thanpreviously. We have confirmed that most Be stars of early spectral typehave statistically larger values of polarization and IR excesses incomparison with the late spectral type stars. It is found that thedistributions of p* diverge considerably for the differentspectral subgroups. In contrast to late spectral types (B5-B9.5), thedistribution of p* for B0-B2 stars does not peak at the valuep*=0%. Statistically significant differences in the meanprojected rotational velocities (/line{vsin i}) are found for differentspectral subgroups of Be stars in the sense that late spectral typestars (V luminosity class) generally rotate faster than early types, inagreement with previously published results. This behaviour is, however,not obvious for the III-IV luminosity class stars. Nevertheless, thecalculated values of the ratio vt/vc of the truerotational velocity, vt, to the critical velocity forbreak-up, vc, is larger for late spectral type stars of allluminosity classes. Thus, late spectral type stars appear to rotatecloser to their break-up rotational velocity. The distribution of nearIR excesses for early spectral subgroups is bi-modal, the position ofthe second peak displaying a maximum value E(V-L)~ 1 . m 3for O-B1.5 stars, decreasing to E(V-L)~0. m8 for intermediatespectral types (B3-B5). It is shown that bi-modality disappears for latespectral types (B6-B9.5). No correlations were found betweenp* and near IR excesses and between E(V-L) and vsin i for thedifferent subgroups of Be stars. In contrast to near IR excesses, arelation between p* and far IR excesses at 12 mu m is clearlyseen. A clear relation between p* and vsin i (as well asbetween p* and /line{vsin i}/vc) is found by thefact that plots of these parameters are bounded by a ``triangular"distribution of p*: vsin i, with a decrease of p*towards very small and very large vsin i (and /line{vsini}/vc) values. The latter behaviour can be understood in thecontext of a larger oblateness of circumstellar disks for the stars witha rapid rotation. From the analysis of correlations between differentobservational parameters we conclude that circumstellar envelopes forthe majority of Be stars are optically thin disks with the range of thehalf-opening angle of 10degr
| NaI and HI 3-D density distribution in the solar neighbourhood
A study of the local interstellar medium (LISM) using a robust inversionmethod, similar to current tomography techniques, is applied to compileddata on neutral interstellar absorbers and Hipparcos parallaxes. Wepresent here the 3-D distribution of two neutral species, NaI and HI.Our analysis enables us to obtain a reliable 3-D density distribution ofthe IS matter in the solar neighbourhood, providing a new basis for thediscussion of origin, present state and evolution of the LISM. We showthat neutral IS matter is distributed in compact clouds or in cloudcomplexes with cavities between them. It is now easy to distinguish theso-called Local Bubble and the Loop I cavities and also two tunnelslinking the Local Bubble to the outer regions of the galaxy, away fromthe galactic plane. Better accuracy is achieved for NaI, as to a largernumber of lines-of-sight and target stars than are available for HI. Arather detailed NaI 3-D density distribution is obtained with a 40 pcsmoothing length. The extended high-density regions in the NaI and HImaps are correlated which is not the case for the diffuse regions. Thedensity ratio rho_HI /rho_NaI is lower or equal to 9.1.108+/- 3.108 for extended high density clouds. Usingobservations from the ESA Hipparcos space astrometry mission.
| A study of interstellar Nai D absorption lines towards the Lupus molecular clouds
Intermediate-resolution (60000<=R<=120000) observations ofinterstellar Nai lines towards 29 stars in the general direction of theLupus molecular clouds (330°<~l<~350°0°<~b<~25°) are presented. Previously published spectratowards an additional seven stars are also included. Based on theHipparcos distances to these stars, and the minimum distance at whichstrong interstellar Nai lines appear in the spectra, I obtain a distanceof ~150+/-10pc to the Lupus molecular complex. While in agreement with anumber of other independent estimates, this result is at odds with thevalue of 100pc recently obtained by Knude & Høg from aHipparcos-based study of interstellar extinction. A possible explanationfor this discrepancy is discussed, and it is concluded that the value of150+/-10pc obtained here is to be preferred. In addition, theseobservations have some other implications for the structure of theinterstellar medium in this direction, and these are briefly considered.
| OB association members in the ACT and TRC catalogues
The Hipparcos Catalogue contains members of nearby OB associationsbrighter than 12th magnitude in V. However, membership lists arecomplete only to magnitude V=7.3. In this paper we discuss whetherproper motions listed in the `Astrographic Catalogue+Tycho' referencecatalogue (ACT) and the Tycho Reference Catalogue (TRC), which arecomplete to V~10.5mag, can be used to find additional associationmembers. Proper motions in the ACT/TRC have an average accuracy of~3masyr-1. We search for ACT/TRC stars which have propermotions consistent with the spatial velocity of the Hipparcos members ofthe nearby OB associations already identified by de Zeeuw et al. Thesestars are first selected using a convergent-point method, and thensubjected to further constraints on the proper-motion distribution,magnitude and colour to narrow down the final number of candidatemembers. Monte Carlo simulations show that the proper-motiondistribution, magnitude, and colour constraints remove ~97per cent ofthe field stars, while at the same time retain more than 90per cent ofthe cluster stars. The procedure has been applied to five nearbyassociations: the three subgroups of Sco OB2, plus Per OB3 and Cep OB6.In all cases except Cep OB6, we find evidence for new associationmembers fainter than the completeness limit of the Hipparcos Catalogue.However, narrow-band photometry and/or radial velocities are needed topinpoint the cluster members, and to study their physicalcharacteristics.
| Interstellar Carbon Abundance
We present 10 new measurements of carbon gas phase column density in theinterstellar medium. We have used spectra made with the Goddard HighResolution Spectrograph aboard the Hubble Space Telescope containing theCII 1334.5 Ang and CII* 1335.7 Ang lines. The continuum reconstructionmethod has been used to obtain the carbon column density from theLorentzian damped lines. Extinction curve parameters are determined inselected directions and relation between C/H ratio and extinctionparameters is discussed. A correlation has been found between C/H andthe strength of the 2175 Ang bump. Unlike previous results, we noticethat C/H changes with fractional abundance of molecular hydrogen,f(H_2). The average value of C/H=3.55*10^{-4} for lines of sight withf(H_2)<1*10^{-3} is the same as solar photospheric abundance fromGrevese and Noels (1993) and may represent the real cosmic carbonabundance.
| A new survey of diffuse interstellar bands (5650 - 6865 Å)
This paper presents a new systematic survey of diffuse interstellarbands in the optical wavelength range from 5650 to 6865 Ä based onechelle spectra of medium-reddened early-type stars acquired at McDonaldObservatory. Adding up interstellar spectra of the same type (as it wasdone by Krełowski et al. \cite{K97}) we reached a very highsignal-to-noise ratio. In this spectral range 89 new features have beendiscovered, 62 of them certain. In the whole spectral range of thissurvey we have found 240 features, 213 of them certain. Figure~4 is onlyavailable in electronic form at http://www.edpsciences.org
| Radiative lifetimes, branching fractions and transition probabilities in GeI - solar implications
New radiative lifetimes have been measured for the 5s ^3P^o_0,1,2 and 5s^1P^o_1 levels of GeI using a laser-produced plasma subject to selectivelaser excitation. A diffraction grating based emission spectrometer ofmoderately high resolution has been used to remeasure the branchingratios of the 4p-5s depopulating transitions. The oscillator strengthsof astrophysical interest, which have been derived from these new data,are compared with theoretical values calculated within apseudo-relativistic Hartree-Fock approach. The new set of f-values hasbeen compared with the few results available in the literature, whichare generally characterized by a large scatter and a low accuracy. Thesolar photospheric abundance of germanium deduced from the new atomicdata of the present work is A_Ge = 3.58 +/- 0.05, on the usuallogarithmic scale, in agreement with the meteoritic value.
| The Abundance of Interstellar Tin and Cadmium
We have determined the gas-phase interstellar abundance of the dominantions of tin and cadmium (Sn II and Cd II) in the diffuse clouds toward14 and 5 stars, respectively. Our measurements show that tin isexchanged between the gas and dust phases of the diffuse interstellarmedium, while cadmium is not. The sight lines showing the highestgas-phase tin-to-hydrogen ratios (the least amount of Sn depletion) havevalues of Sn/H that are supersolar. Tin is the first element to show awell-determined interstellar gas-phase abundance that appears to beenriched with respect to the Sun. This finding is direct evidence ofs-process enrichment of the ISM by low- to intermediate-mass asymptoticgiant branch stars. The average Cd/H abundance ratio in our sample is0.09 dex below the Sun's. This ratio does not preclude an enhancement ofcadmium with respect to the Sun if Cd is incorporated into dust and/orif the adopted f-value for the cadmium transition has a large error.
| Interstellar Abundances in the Magellanic Clouds. II. The Line of Sight to SN 1987A in the Large Magellanic Cloud
We have analyzed both high-resolution optical absorption-line spectraand UV spectra obtained with IUE of the LMC SN 1987A, in order todetermine abundances and physical conditions in the various neutralinterstellar clouds along the line of sight to the supernova (SN). Wehave used a flat-fielding procedure to enhance the signal-to-noiseratios (S/Ns) and the reliability of weak features in the UV spectra andhave modeled the UV line profiles using the component structure derivedfrom the higher resolution, high-S/N optical spectra of Ca II and Na I.Fits to the Ca II, Ca I, and Na I absorption-line profiles reveal (atleast) 46 components, at velocities -24 km s^-1<~v_solar<~296 kms^-1, which can be associated with the 10 component groups discerniblein the lower resolution UV spectra. From the UV spectra, we determinedcomponent-group column densities for C I, Mg I, Mg II, Al II, Si II, PII, Cl I, Ti II, Cr II, Mn II, Fe II, Ni II, and Zn II-with 1 sigmauncertainties less than 0.1 dex in many cases. These are the mostextensive and accurate abundances yet measured for the neutral ISM inthe LMC. The component velocities, the patterns of relative elementalabundances [X/Zn] and [X/Fe], and various diagnostic ratios have beenused to estimate the locations and physical characteristics [N(H), T,n_e] of these component groups. (Systematic differences among thediagnostic ratios make the derived physical properties somewhatuncertain, however.) The components at low velocities (5 kms^-1<~v<~23 km s^-1) have relative abundances and values for thediagnostic ratios very similar to those found for warm, diffuse Galacticdisk clouds and likely are due to a mixture of warm and cool gas in theGalactic disk. The components at velocities 56 km s^-1<~v<~90 kms^-1 are due to a mixture of warm and cool gas, apparently withnegligible depletions, in the Galactic halo. The twointermediate-velocity component groups (109 km s^-1<~v<~140 kms^-1 and 155 km s^-1<~v<~176 km s^-1) both have relativeabundances similar to those found for Galactic halo clouds. These warm(T>~4500 K), partially ionized clouds are probably located in theGalactic halo and in the LMC, respectively. The components at velocities191 km s^-1<~v<~225 km s^-1 also have relative abundances similarto those in the halo clouds but are likely due to gas in the LMC,perhaps very close to the SN. The component groups at 238 kms^-1<~v<~255 km s^-1 and 265 km s^-1<~v<~270 km s^-1 areprobably located on opposite sides of the main LMC component group (atvelocities 275 km s^-1<~v<~296 km s^-1) (using absorption-linedata for several other adjacent lines of sight and the structureinferred from SN light-echo observations). Although the relativeabundances and diagnostic ratios for those three LMC groups are similarto those found for warm, low-density Galactic disk clouds, the widths ofindividual components seen in very high resolution spectra of Na I and KI imply that T is generally less than about 1500 K. Higher N(Na I)/N(CaII) ratios, the presence of CH, and the C I fine structure levelpopulations suggest that the main LMC group contains both cool and warmgas. For the LMC components, the total N(H) estimated from the observedrelative abundances and inferred depletions is consistent with the valueobtained from Lyalpha absorption toward the neighboring star Sk -69deg203, after accounting for differences in reddening and for an overallsubsolar metallicity of 0.2-0.3 dex for the LMC ISM. Since the relativeabundance patterns determined for stars and gaseous nebulae in both theSMC and the LMC appear to be similar to the solar pattern (for theelements whose interstellar abundances we have considered), thesimilarities in relative gas-phase interstellar abundances in our Galaxyand in the Magellanic Clouds suggest that the dust depletions followsimilar patterns as well-despite differences in metallicity anddust-to-gas ratio among the three galaxies. These local relativeabundance/depletion patterns may thus be used to infer total (gas+dust)abundances for QSO absorption-line systems at various redshifts.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
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