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A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
A study of gas-phase element abundances reported in the literature for17 different elements sampled over 243 sight lines in the local part ofour Galaxy reveals that the depletions into solid form (dust grains) areextremely well characterized by trends that employ only three kinds ofparameters. One is an index that describes the overall level ofdepletion applicable to the gas in any particular sight line, and theother two represent linear coefficients that describe how to derive eachelement's depletion from this sight-line parameter. The information fromthis study reveals the relative proportions of different elements thatare incorporated into dust at different stages of grain growth. Anextremely simple scheme is proposed for deriving the dust contents andmetallicities of absorption-line systems that are seen in the spectra ofdistant quasars or the optical afterglows of gamma-ray bursts. Contraryto presently accepted thinking, the elements sulfur and krypton appearto show measurable changes in their depletions as the general levels ofdepletions of other elements increase, although more data are needed toascertain whether or not these findings are truly compelling. Nitrogenappears to show no such increase. The incorporation of oxygen into solidform in the densest gas regions far exceeds the amounts that can takethe form of silicates or metallic oxides; this conclusion is based ondifferential measurements of depletion and thus is unaffected byuncertainties in the solar abundance reference scale.Based in large part on published observations from (1) the NASA/ESAHubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555, (2) theFar Ultraviolet Spectroscopic Explorer (FUSE) mission operated by JohnsHopkins University, supported by NASA contract NAS5-32985, and (3) TheCopernicus satellite, supported by NASA grant NAGW-77 to PrincetonUniversity.
| Ultraviolet Survey of CO and H2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
We carried out a comprehensive far-UV survey of 12CO andH2 column densities along diffuse molecular Galactic sightlines. This sample includes new measurements of CO from HST spectraalong 62 sight lines and new measurements of H2 from FUSEdata along 58 sight lines. In addition, high-resolution optical datawere obtained at the McDonald and European Southern Observatories,yielding new abundances for CH, CH+, and CN along 42 sightlines to aid in interpreting the CO results. These new sight lines wereselected according to detectable amounts of CO in their spectra andprovide information on both lower density (<=100 cm-3) andhigher density diffuse clouds. A plot of logN(CO) versuslogN(H2) shows that two power-law relationships are neededfor a good fit of the entire sample, with a break located atlogN(CO,cm-2)=14.1 and logN(H2)=20.4,corresponding to a change in production route for CO in higher densitygas. Similar logarithmic plots among all five diatomic molecules revealadditional examples of dual slopes in the cases of CO versus CH (breakat logN=14.1, 13.0), CH+ versus H2 (13.1, 20.3),and CH+ versus CO (13.2, 14.1). We employ both analytical andnumerical chemical schemes in order to derive details of the molecularenvironments. In the denser gas, where C2 and CN moleculesalso reside, reactions involving C+ and OH are the dominantfactor leading to CO formation via equilibrium chemistry. In thelow-density gas, where equilibrium chemistry studies have failed toreproduce the abundance of CH+, our numerical analysis showsthat nonequilibrium chemistry must be employed for correctly predictingthe abundances of both CH+ and CO.
| Interstellar Krypton Abundances: The Detection of Kiloparsec-scale Differences in Galactic Nucleosynthetic History
We present an analysis of Kr I λ1236 line measurements from 50sight lines in the Hubble Space Telescope Space Telescope ImagingSpectrograph and Goddard High Resolution Spectrograph data archives thathave sufficiently high resolution and signal-to-noise ratio to permitreliable krypton-to-hydrogen abundance ratio determinations. Thedistribution of Kr/H ratios in this sample is consistent with a singlevalue for the ISM within 5900 pc of the Sun, log10(Kr/H)=-9.02+/-0.02,apart from a rough annulus from between ~600 and 2500 pc distant. TheKr/H ratio toward stars within this annulus is elevated by approximately0.11 dex, similar to previously noted elevations of O/H and Cu/Hgas-phase abundances beyond ~800 pc. A significant drop in the gas-phaseN/O ratio in the same region suggests that this is an artifact ofnucleosynthetic history. Since the physical scale of the annulus' inneredge is comparable to the radius of the Gould Belt and the outer limitof heliocentric distances where the D/H abundance ratio is highlyvariable, these phenomena may be related to the Gould Belt's origins.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 NAS 5-26555 FUSE is operated for NASA by theJohn Hopkins University under NASA contract NAS-32985.
| The Search for H- in Astrophysical Environments
The negative ion H- is widely understood to be important inmany astrophysical environments, including the atmospheres of late-typestars like the Sun. However, the ion has never been detectedspectroscopically outside the laboratory. A search for thefar-ultraviolet autodetaching transitions of H- ininterstellar and circumstellar matter seems to be the best hope fordirectly detecting this ion. We undertook a highly sensitive searchusing data from the FUSE instrument. We concentrated on two types ofsight lines: planetary nebulae, where model calculations suggest asufficient abundance of H- to be determined, and translucentclouds, where H- might form on dust grains as an intermediatestep in molecular hydrogen formation. Upper limits on H-abundances were set.
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| A VLT/NACO survey for triple and quadruple systems among visual pre-main sequence binaries
Aims.This paper describes a systematic search for high-ordermultiplicity among wide visual Pre-Main Sequence (PMS) binaries. Methods: .We conducted an Adaptive Optics survey of a sample of 58 PMSwide binaries from various star-forming regions, which include 52 TTauri systems with mostly K- and M-type primaries, with the NIRinstrument NACO at the VLT. Results: .Of these 52 systems, 7 arefound to be triple (2 new) and 7 quadruple (1 new). The new closecompanions are most likely physically bound based on their probabilityof chance projection and, for some of them, on their position on acolor-color diagram. The corresponding degree of multiplicity among widebinaries (number of triples and quadruples divided by the number ofsystems) is 26.9 ± 7.2% in the projected separation range ~0.07arcsec -12'', with the largest contribution from the Taurus-Aurigacloud. We also found that this degree of multiplicity is twice in Tauruscompared to Ophiuchus and Chamaeleon for which the same number ofsources are present in our sample. Considering a restricted samplecomposed of systems at distance 140-190 pc, the degree of multiplicityis 26.8 ± 8.1%, in the separation range 10/14 AU-1700/2300 AU (30binaries, 5 triples, 6 quadruples). The observed frequency agrees withresults from previous multiplicity surveys within the uncertainties,although a significant overabundance of quadruple systems compared totriple systems is apparent. Tentatively including the spectroscopicpairs in our restricted sample and comparing the multiplicity fractionsto those measured for solar-type main-sequence stars in the solarneighborhood leads to the conclusion that both the ratio of triples tobinaries and the ratio of quadruples to triples seems to be in excessamong young stars. Most of the current numerical simulations of multiplestar formation, and especially smoothed particles hydrodynamicssimulations, over-predict the fraction of high-order multiplicity whencompared to our results. The circumstellar properties around theindividual components of our high-order multiple systems tend to favormixed systems (i.e. systems including components of wTTS and cTTS type),which is in general agreement with previous studies of disks inbinaries, with the exception of Taurus, where we find a preponderance ofsimilar type of components among the multiples studied.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| The short period multiplicity among T Tauri stars
We present the results of high-resolution spectroscopic observationscarried out over three years aimed at estimating the short-period(P_orb<100 days) binary frequency of a sample of T Tauri stars inOph-Sco, Cha, Lup, CrA star forming regions (SFRs), already observedwith high angular resolution techniques by Ghez et al.(\cite{Ghezetal93}) and by Ghez et al. (\cite{Ghezetal97}) to detectwider components. When combining all four SFRs, the short-period binaryfrequency is indistinguishable from that found by Duquennoy & Mayor(\cite{DuqMay91}) for the solar-type field stars which is alsoconsistent with the previous result obtained by Mathieu(\cite{Mathieu92}, \cite{Mathieu94}). When Oph-Sco is analyzedseparately, it seems that there is an excess of short-period binaries ofa factor 2-2.5. On the contrary, short-period binary systems seem to beabsent in the sample containing stars in Cha/Lup/CrA. Such a trend wasequally found by Mathieu (\cite{Mathieu92}) in Taurus. An excess ofspectroscopic systems among the components of visual multiple systems isalso observed.Based on observations collected with the Swiss Euler Telescope and 1.5-mESO, proposal 63.I-0112.
| 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
| Ultra-high-resolution observations of CH in Southern Molecular Cloud envelopes
We present a mini-survey of ultrahigh-resolution spectroscopy (UHRS) ofCH towards three southern molecular cloud envelopes. The sightlines areselected to probe physically similar gas in different Galacticenvironments. With a velocity resolution of ~0.5kms-1(R=575000) these observations resolve most kinematic components of theabsorption lines. We do, however, detect one line component in the Lupusregion, which is not resolved and for which an upper limit ofb<0.3kms-1 is found. We find a correlation betweendistance of the absorbing gas from the Galactic mid-plane and thefractional abundance of CH. We show that this correlation can beexplained as being a result of a fall-off in the ultraviolet radiationfield intensity and propose that CH observations in carefully selectedsightlines might allow a mapping of the variations in the interstellarradiation field.
| Imaging the Circumstellar Environments of Young Binaries in Southern Star-Forming Regions
A sample of 14 pre-main-sequence visual binary systems in southernstar-forming regions were imaged at 2 ?m with the Keck I telescope tosearch for tertiary companions, scattered-light disks, and compactnebulae at linear scales of 5-100 AU. Speckle holography was used toproduce images with diffraction-limited resolution and high dynamicrange, and photometry in four filters between J (1.25 ?m) and L' (3.8?m) was used to provide a diagnostic of the infrared excess. Of the14 visual binary systems studied, nine contain components that showevidence for resolved structure. Two of them (WSB 18 and B59-1) havewell-resolved tertiary companions separated from the primary stars by~100 mas. The remainder are only marginally resolved at the diffractionlimit of the telescope. Sz 116, AS 205, Elias 2-22, ESO H? 282,and B59-2 each have one component that is marginally resolved along oneaxis and unresolved on the perpendicular axis, consistent withtertiaries at separations between ~6 and 40 mas. If all these systemsare hierarchical triples, then fully half the nominal binaries in thesample are triple. The primaries in Elias 2-49 and WSB 71 are marginallyresolved along all position angles, suggesting the presence of dusthalos. No disks were unambiguously detected in the sample.
| 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
| New "weak-line"--T Tauri stars in Lupus
We present first results obtained by a survey of the Lupus star formingregion in search of new T Tauri stars. This study has been performed onthe basis of deep pointed ROSAT observations in the Lupus dark clouds aswell as data from the ROSAT All-Sky-Survey in the surrounding, lessobscured regions. Our survey covers an area of about 230 square degrees,located between 15^h,6^m and 16^h,24^m$ in right ascension and between-47^\circ and -32^\circ in declination. Identification of ROSATAll-Sky-Survey sources in this area by means of optical spectroscopyrevealed 89 T Tauri stars, 86 of them "weak-line" T Tauri stars (WTTS)not known from previous studies of this region. Our pointed ROSATobservations led to the identification of 47 more T Tauri stars, givinga total of 136 new T Tauri stars. The large area of our study, ascompared with previous works, allows us to study the spatialdistribution of WTTS in this star forming region on a large scale. Wefind the new WTTS to be distributed over the whole area of our survey,indicating that their spatial distribution might extend well beyond ourstudy area. Contrary to the Lupus T Tauri stars known prior to thisstudy, the WTTS discovered by the ROSAT All-Sky-Survey are not clusteredin the regions of highest extinction, i.e. the dark clouds. Based onobservations collected at European Southern Observatory, La Silla, Chile(observing proposals ESO Nos. 49.7-0010, 50.7-0109, 51.7-0106,51.7-0029). Tables 5--12 are only available in electronic form at theCDS via anonymous ftp 130.79.128.5 or on www athttp://cdsweb.u-strasbg.fr/abstract.html.
| The Multiplicity of Pre-Main-Sequence Stars in Southern Star-forming Regions
High-resolution studies of young stars in the star-forming regions ofTaurus and Ophiuchus have revealed a large population of multiple starsystems. To test how applicable this earlier result is for otherstar-forming regions, we have carried out a K-band (2.2 mu m)multiplicity survey of pre--main-sequence stars located in the darkcloud complexes Chameleon, Lupus, and Corona Australis. This survey,which was conducted with both speckle- and direct-imaging techniques,covers a binary star separation range of 0."1--12" (15--1800 AU) andidentifies 25 companion stars of which nine are new detections. Thecompanion star fraction over the separation range covered by this surveyis estimated to be 0.52 +/- 0.11, in agreement with Taurus (0.58 +/-0.08) and Ophiuchus (0.50 +/- 0.12). A comparison of the direct-imagingportion of this survey with Reipurth & Zinnecker's opticalmultiplicity study reveals that 4% of the overlap sample have "infraredcompanions," companions too red to be detected at optical wavelengths.This suggests that infrared surveys will systematically measure aslightly higher companion star fraction compared with optical surveys.The result of combining all K-band surveys of dark cloud complexes,which cover the separation range 15--1800 AU, shows a factor of 2 excessof the companion star fraction for young stars compared with that forthe solar-type stars in the solar neighborhood (0.54 +/- 0.06 vs. 0.26+/- 0.04).
| Comparison of Star Formation in Five Nearby Molecular Clouds
We have calculated bolometric temperature (Tbol) and bolometricluminosity (Lbol) for 383 young stellar objects (YSOs) in five molecularclouds within 200 pc in Corona Australis (CrA), Ophiuchus (Oph), Taurus(Tau), Chamaeleon (Cha), and Lupus (Lup). We used Tbol, Lbol, andbolometric luminosity-temperature (BLT) diagrams to characterize andcompare the overall-star-formation activity of the clouds on aself-consistent basis. The main results are the following: (1) the YSOpopulations in the clouds can be differentiated by the fraction of theirlow-Tbol sources, which increases systematically from Lup and Cha to Tauand to Oph and CrA. This trend is interpreted as increasing currentstar-forming activity in the same order; (2) the clouds with higher coldsource fractions also seem to have higher bright source fractions; (3)In the BLT diagram, the CrA and Oph sources are more uniformlydistributed while the Cha and Lup sources are aggregated near thezero-age mainsequence (ZAMS). Tau sources appear to be an intermediatecase. Taurus also seems to contain more cold (Tbol < 1000 K) andlow-luminosity (Lbol < 1 Lȯ) sources than the other complexes;(4) the YSOs show a characteristic distribution in the median BLTdiagram. This distribution is qualitatively consistent with the earlyYSO evolution from a protostar to a pre--main-sequence star and providesa unique observational test to star-formation models; (5) for Luppre--main-sequence stars, the ratio of their Tbol to Teff increasesduring their approach to the ZAMS. This increase can be explained by thedisk and envelope dissipation during the pre--main-sequence evolution;(6) the most active star-forming clouds (Oph and CrA) also have densermolecular cores as measured by C18O J = 1--0 line emission, suggestingthat the star formation occurs in the densest parts of the molecularclouds; and (7) we find an anti-correlation between Tbol and C18Oemission for the class 0 and I Tau sources (Tbol < 650 K). This showsthat Tbol measures the intrinsic redness of YSOs, rather than theirdisk-envelope orientation. The disk orientation may have a moreimportant effect on Tbol of the pre--main-sequence stars.
| Far-Ultraviolet Stellar Photometry: Fields Centered on rho Ophiuchi and the Galactic Center
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..104..101S&db_key=AST
| Far-ultraviolet stellar photometry: Fields in Sagittarius and Scorpius
Far-ultraviolet photometry for 741 objects in a field in Sagittariuscentered near M8 and 541 objects in a field centered near sigma Scorpiiis presented. These data were extracted from electographic imagesobtained with two cameras during a shuttle flight in 1991 April/May. Thecameras provided band passes with lambdaeff = 1375 A andlambdaeff = 1781 A. Synthetic colors show that these bandsare sensitive to effective temperature for hot stars. Our measurementswere placed on a quantitative far-ultraviolet magnitude scale byconvolving the spectra of stars observed by IUE with our cameras'spectral response functions. Fifty-eight percent of the ultravioletobjects were identified with visible stars using the SIMBAD databasewhile another 40% of the objects are blends of early type stars tooclose together to separate with our resolution. Our photometry iscompared with that from the TD-1, OAO 2, and ANS satellites and the S201(Apollo 16) far-ultraviolet camera and found to agree at the level of afew tenths of a magnitude. Unlike previous studies, almost half of theidentified visual counterparts to the ultraviolet objects are early Bstars. A plot of distance modulus against ultraviolet color excessreveals a significant population of stars with strong ultravioletexcess.
| The stellar population of the Lupus clouds
We present photometric and spectroscopic observations of the H alphaemission stars in the Lupus dark cloud complex. We estimate theeffective temperatures of the stars from their spectral types andcalculate the reddening towards each object from the (R-I) colors. Fromthese data, we derive mass and age distributions for the Lupus starsusing a new set of pre-main sequence evolutionar tracks. We compare theresults for the Lupus stars with those for a similar population of youngstellar objects in Taurus-Auriga and Chamaeleon and with the initialmass function for field stars in the solar neighborhood. From the H-Rdiagrams, Lupus appears to contain older stars than Taurus. The Lupusdark clouds form a greater proportion of low mass stars than the Tauruscomplex. Also, the proportion of low mass stars in Lupus is higher thanthat predicted by the Miller-Scalo initial mass function, and the lowestmass stars in Lupus are less active than similar T Tauri stars in otherregions.
| Visual binaries among pre-main sequence stars
In an effort to determine statistical properties of visual pre-mainsequence (PMS) binaries, we have carried out an extensive systematic CCDimaging survey of young low-mass stars in nearby southern dark clouds,all at about 150 pc distance. The observations were made with theEuropean Southern Observatory (ESO) New Technology Telescope at awavelength of 0.9 micrometers under sub-arcsecond seeing conditions.Among the 238 young stars systematically observed, we detect 37 binaries(22 new discoveries) and 1 triple system (Sz 30, also new) in the rangeof separations between 1 arcseconds and 12 arcseconds. Further binarieswere identified outside these limits, including 6 sub-arcsec binaries.The lower limit of 1 arcseconds was imposed to ensure completeness ofour sample, which we have tested through numerical simulations. Thus, wefind a frequency of 16% for PMS binaries with projected separationsbetween about 150 AU and 1800 AU. We compare these observations with theknown main sequence (MS) distributions of G and K dwarfs in the sameinterval of separations, and find an apparent excess of PMS binaries.Under the assumption that total PMS and MS binary frequencies should bethe same, the excess may imply that binary orbits undergo secularevolution towards the main sequence. We briefly discuss implications ofour results for the star formation efficiency and the initial massfunction. Our data suggest that in T associations binary star formationappears to be the rule, while the formation of single stars is probablythe exception. We have in the same way searched for multiplicity in thisextended sample. As a result, we have additionally discovered another 18new PMS binaries and have reobserved another 32 already known PMSbinaries. Therefore, in total we here provide information (componentseparations, brightness-ratios, and position angles) on 87 PMS binariesand 1 triple, approximately half of which are newly discovered.
| A southern Be star survey - Spectra and envelope radii
We describe the hydrogen line spectra of 63 southern Be stars, obtainedat ESO, Chile, in one observational run in August 1978. The spectra wererecorded on photographic plates. We also provide the outer radii of theline emission forming region for a number of stars, based upon theemission peak separation of the H4 and H5 lines. Average values soobtained are in good agreement with other determinations based upon asmaller number of objects.
| A homogeneous catalog of new UBV and H-beta photometry of B- and A-type stars in and around the Scorpius-Centaurus OB association
B- and A-type stars in and near the Sco-Cen OB association areinvestigated with UBV and H-beta photometry to acquire data relevant tothe luminosity function of Sco-Cen. The measurements generally consistof two 10-s integrations of U, B, V, (W, N) filters, and theobservations are corrected iteratively for atmospheric extinction andinstrumental response. The data presented give the mean V magnitude,mean B-V, mean U-B, and the estimated uncertainties for these values.The catalog provides a homogeneous catalog of data for a large fieldwith stellar objects delineating membership to the association Sco-Cenand that affect the luminosity function of the aggregate.
| Instrumental effects and the Stroemgren photometric system
The extent to which the use of different photometers can affect resultson stellar color indices was investigated by simultaneously observing asample of widely different stars with several uvby photometric systemsavailable at the ESO, La Silla, Chile. In one instance, the ESO 50-cmtelescope and the four-channel photometer at the Danish 50-cm telescopewere used simultaneously. The reductions were performed separately oneach data set, using a linear color transformation procedure. Theresults agree with theoretical investigations which showed thatsubstantial errors can arise from the nonconformity of passbands. It isemphasized that it is necessary to use separate color transformationsfor various stellar types and classes and for different interstellarreddenings.
| Distances, reddenings and distribution of emission B-stars in the galactic centre region /l/ not greater than 45 deg
The distribution of Be stars in the region surrounding the Galacticcenter and their correlation to the spiral structure of the Galaxy hasbeen studied. The results are discussed in terms of reddenings anddistances of these stars. Data are presented on the Galacticcoordinates, colors, interstellar color excesses, reddening-freemagnitudes and colors, adopted absolute magnitudes, distances in kpc,distances from the Galactic plane, and MK spectral type.
| Photometric observations of emission B-stars in the southern Milky Way
In order to study the distribution of Be stars and their correlation tothe local spiral structure of the Galaxy photoelectric UBV photometrywas carried out for a total of 488 Be stars located in the southernMilky Way between galactic longitudes 315 and 45 deg. UBV magnitudes arepresented for these stars.
| A Catalogue of Be-Stars
Not Available
| A survey of southern dark clouds for Herbig-Haro objects and H-alpha emission stars
Abstract image available at:http://adsabs.harvard.edu/abs/1977ApJS...35..161S
| Observations of southern emission-line stars
A catalog of 1929 stars showing H-alpha emission on photographic platesis presented which covers the entire southern sky south of declination-25 deg to a red limiting magnitude of about 11.0. The catalog providesprevious designations of known emission-line stars equatorial (1900) andgalactic coordinates, visual and photographic magnitudes, H-alphaemission parameters, spectral types, and notes on unusual spectralfeatures. The objects listed include 16 M stars, 25 S stars, 37 carbonstars, 20 symbiotic stars, 40 confirmed or suspected T Tauri stars, 16novae, 14 planetary nebulae, 11 P Cygni stars, 9 Bep stars, 87 confirmedor suspected Wolf-Rayet stars, and 26 'peculiar' stars. Two new Tassociations are discovered, one in Lupus and one in Chamaeleon. Objectswith variations in continuum or H-alpha intensity are noted, and thedistribution by spectral type is analyzed. It is found that the skydistribution of these emission-line stars shows significantconcentrations in the region of the small Sagittarius cloud and in theCarina region.
| UBV photometry of southern early-type stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974MNRAS.168..451H&db_key=AST
| UBV Photometry of 500 Southern Stars [erratum: 1973MNSSA..32...48C]
Not Available
| On faint H-alpha emission stars in Lupus and Scorpius.
Not Available
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