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XMM-Newton observations of the massive colliding wind binary and non-thermal radio emitter CygOB2#8A [O6If + O5.5III(f)]
We report on the results of four XMM-Newton observations separated byabout ten days from each other of CygOB2#8A [O6If + O5.5III(f)]. Thismassive colliding wind binary is a very bright X-ray emitter - one ofthe first X-ray emitting O-stars discovered by the Einstein satellite -as well as a confirmed non-thermal radio emitter whose binarity wasdiscovered quite recently. The X-ray spectrum between 0.5 and 10.0keV isessentially thermal, and is best fitted with a three-component modelwith temperatures of about 3, 9 and 20MK. The X-ray luminosity correctedfor the interstellar absorption is rather large, i.e. about1034ergs-1. Compared to the `canonical'LX/Lbol ratio of O-type stars, CygOB2#8A was afactor of 19-28 overluminous in X-rays during our observations. The EPICspectra did not reveal any evidence for the presence of a non-thermalcontribution in X-rays. This is not unexpected considering that thesimultaneous detections of non-thermal radiation in the radio and softX-ray (below 10.0keV) domains is unlikely. Our data reveal a significantdecrease in the X-ray flux from apastron to periastron with an amplitudeof about 20 per cent. Combining our XMM-Newton results with those fromprevious ROSAT-PSPC and ASCA-SIS observations, we obtain a light curvesuggesting a phase-locked X-ray variability. The maximum emission leveloccurs around phase 0.75, and the minimum is probably seen shortly afterthe periastron passage. Using hydrodynamic simulations of the wind-windcollision, we find a high X-ray emission level close to phase 0.75, anda minimum at periastron as well. The high X-ray luminosity, the strongphase-locked variability and the spectral shape of the X-ray emission ofCygOB2#8A revealed by our investigation point undoubtedly to X-rayemission dominated by colliding winds.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member states andthe USA (NASA).E-mail: debecker@astro.ulg.ac.be ‡Research Associate FNRS (Belgium).
| Can single O stars produce non-thermal radio emission?
We present a model for the non-thermal radio emission from presumablysingle O stars, in terms of synchrotron emission from relativisticelectrons accelerated in wind-embedded shocks. These shocks areassociated with an unstable, chaotic wind. The main improvement withrespect to earlier models is the inclusion of the radial dependence ofthe shock velocity jump and compression ratio, based on one-dimensionaltime-dependent hydrodynamical simulations. The decrease of the velocityjump and the compression ratio as a function of radius produces arapidly decreasing synchrotron emissivity. This effectively prohibitsthe models from reproducing the spectral shape of the observednon-thermal radio emission. We investigate a number of “escaperoutes” by which the hydrodynamical predictions might bereconciled with the radio observations. We find that the observedspectral shape can be reproduced by a slower decline of the compressionratio and the velocity jump, by the re-acceleration of electrons in manyshocks or by adopting a lower mass-loss rate. However, none of theseescape routes are physically plausible. In particular, re-accelerationby feeding an electron distribution through a number of shocks, is incontradiction with current hydrodynamical simulations. Thesehydrodynamical simulations have their limitations, most notably the useof one-dimensionality. At present, it is not feasible to performtwo-dimensional simulations of the wind out to the distances requiredfor synchrotron-emission models. Based on the current hydrodynamicmodels, we suspect that the observed non-thermal radio emission from Ostars cannot be explained by wind-embedded shocks associated with theinstability of the line-driving mechanism. The most likely alternativemechanism is synchrotron emission from colliding winds. That would implythat all O stars with non-thermal radio emission should be members ofbinary or multiple systems.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Correlation patterns between 11 diffuse interstellar bands and ultraviolet extinction
We relate the equivalent widths of 11 diffuse interstellar bands,measured in the spectra of 49 stars, to different colour excesses in theultraviolet. We find that most of the observed bands correlatepositively with the extinction in the neighbourhood of the2175-Åbump. Correlation with colour excesses in other parts of theextinction curve is more variable from one diffuse interstellar band toanother; we find that some diffuse bands (5797, 5850 and 6376 Å)correlate positively with the overall slope of the extinction curve,while others (5780 and 6284 Å) exhibit negative correlation. Wediscuss the implications of these results on the links between thediffuse interstellar band carriers and the properties of theinterstellar grains.
| The nearest star of spectral type O3: a component of the multiple system HD 150136
From radial velocities determined in high signal-to-noise digitalspectra, we report the discovery that the brightest component of thebinary system HD 150136 is of spectral type O3. We also present thefirst double-lined orbital solution for this binary. Our radialvelocities confirm the previously published spectroscopic orbital periodof 2.6 d. HeII absorptions appear double at quadratures, but singlelines of NV and NIV visible in our spectra define a radial velocityorbit of higher semi-amplitude for the primary component than do theHeII lines. From our orbital analysis, we obtain minimum masses for thebinary components of 27 and 18 Msolar. The neutral Heabsorptions apparently do not follow the orbital motion of any of thebinary components, thus they most probably arise in a third star in thesystem.
| Interstellar Ca II Line Intensities and the Distances of the OB stars
We show that the equivalent widths of the well-known interstellar Ca IIH and K lines can be used to determine the distances to OB stars in ourGalaxy. The equivalent widths, measured in the spectra of 147 early-typestars, are strongly related to the Hipparcos parallaxes of thoseobjects. The lines fitted to the parallax-equivalent width data aregiven by the formulae π=1/[2.78EW(K)+95] and π=1/[4.58EW(H)+102],where π is in arcseconds and EW is in milliangstroms. The form of theformulae, yielding a finite parallax even for zero absorption, showsthat space within ~100 pc of the Sun contains very little Ca II, whichis in agreement with the known dimensions of the Local Bubble. Using CaII lines for distance determination does not require the knowledge ofthe absolute magnitude of the object; it is thus well suited for targetsfor which the absolute calibration is either not precise (OBsupergiants) or not available at all (peculiar objects). We alsodemonstrate that neither the reddening E(B-V) nor the equivalent widthsof interstellar K I and CH lines are suitable candidates for distanceestimation, their relation with parallaxes being far less tight than forCa II.
| FUSE Measurements of Far-Ultraviolet Extinction. I. Galactic Sight Lines
We present extinction curves that include data down to far-ultravioletwavelengths (FUV; 1050-1200 Å) for nine Galactic sight lines. TheFUV extinction was measured using data from the Far UltravioletSpectroscopic Explorer. The sight lines were chosen for their unusualextinction properties in the infrared through the ultraviolet; that theyprobe a wide range of dust environments is evidenced by the large spreadin their measured ratios of total to selective extinction,RV=2.43-3.81. We find that extrapolation of the Fitzpatrick& Massa relationship from the ultraviolet appears to be a goodpredictor of the FUV extinction behavior. We find that predictions ofthe FUV extinction based on the Cardelli, Clayton, & Mathis (CCM)dependence on RV give mixed results. For the seven extinctioncurves well represented by CCM in the infrared through ultraviolet(x<8 μm-1), the FUV extinction is well predicted inthree sight lines, overpredicted in two sight lines, and underpredictedin two sight lines. A maximum entropy method analysis using a simplethree-component grain model shows that seven of the nine sight lines inthe study require a larger fraction of grain materials to be in dustwhen FUV extinction is included in the models. Most of the added grainmaterial is in the form of small (radii <~ 200 Å) grains.Based on observations with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| Abundances and Depletions of Interstellar Oxygen
We report on the abundance of interstellar neutral oxygen (O I) for 26sight lines, using data from the Far Ultraviolet Spectroscopic Explorer,the International Spectroscopic Explorer, and the Hubble SpaceTelescope. O I column densities are derived by measuring the equivalentwidths of several ultraviolet absorption lines and subsequently fittingthose to a curve of growth. We consider both our general sample of 26sight lines and a more restrictive sample of 10 sight lines that utilizeHST data for a measurement of the weak 1355 Å line of oxygen andare thus better constrained owing to our sampling of all three sectionsof the curve of growth. The column densities of our HST sample showratios of O/H that agree with the current best solar value if dust isconsidered, with the possible exception of one sight line (HD 37903). Wenote some very limited evidence in the HST sample for trends ofincreasing depletion with respect to RV and f(H2),but the trends are not conclusive. Unlike a recent result from Cartledgeet al., we do not see evidence for increasing depletion with respect to, but our HST sample contains only two points moredense than the critical density determined in that paper. The columndensities of our more general sample show some scatter in O/H, but mostagree with the solar value to within errors. We discuss these results inthe context of establishing the best method for determining interstellarabundances, the unresolved question of the best value for O/H in theinterstellar medium, the O/H ratios observed in Galactic stars, and thedepletion of gas-phase oxygen onto dust grains.
| An XMM-Newton observation of the multiple system HD 167971 (O5-8V + O5-8V + (O8I)) and the young open cluster NGC 6604
We discuss the results of two XMM-Newton observations of the opencluster NGC 6604 obtained in April and September 2002. We concentratemainly on the multiple system HD 167971 (O5-8V + O5-8V + (O8I)). Thesoft part of the EPIC spectrum of this system is thermal with typicaltemperatures of about 2 × 106 to 9 ×106 K. The nature (thermal vs. non-thermal) of the hard partof the spectrum is not unambiguously revealed by our data. If theemission is thermal, the high temperature of the plasma (~2.3 ×107 to 4.6 × 107 K) would be typical of whatshould be expected from a wind-wind interaction zone within a longperiod binary system. This emission could arise from an interactionbetween the combined winds of the O5-8V + O5-8V close binary system andthat of the more distant O8I companion. Assuming instead that the hardpart of the spectrum is non-thermal, the photon index would be rathersteep (~3). Moreover, a marginal variability between our two XMM-Newtonpointings could be attributed to an eclipse of the O5-8V + O5-8V system.The overall X-ray luminosity points to a significant X-ray luminosityexcess of about a factor 4 possibly due to colliding winds. ConsideringHD 167971 along with several recent X-ray and radio observations, wepropose that the simultaneous observation of non-thermal radiation inthe X-ray (below 10.0 keV) and radio domains appears rather unlikely.Our investigation of our XMM-Newton data of NGC 6604 reveals a rathersparse distribution of X-ray emitters. Including the two brightnon-thermal radio emitters HD 168112 and HD 167971, we present a list of31 X-ray sources along with the results of the cross-correlation withoptical and infrared catalogues. A more complete spectral analysis ispresented for the brightest X-ray sources. Some of the members of NGC6604 present some characteristics suggesting they may be pre-mainsequence star candidates.
| Non-thermal radio emission from O-type stars. I. HD168112
We present a radio lightcurve of the O5.5 III(f^+) star HD 168112, basedon archive data from the Very Large Array (VLA) and the AustraliaTelescope Compact Array (ATCA). The fluxes show considerable variabilityand a negative spectral index, thereby confirming that HD 168112 is anon-thermal radio emitter. The non-thermal radio emission is believed tobe due to synchrotron radiation from relativistic electrons that havebeen Fermi accelerated in shocks. For HD 168112, it is not known whetherthese shocks are due to a wind-wind collision in a binary system or tothe intrinsic instability of the stellar wind driving mechanism.Assuming HD 168112 to be a single star, our synchrotron model shows thatthe velocity jump of the shocks should be very high, or there should bea very large number of shocks in the wind. Neither of these iscompatible with time-dependent hydrodynamical calculations of O starwinds. If, on the other hand, we assume that HD 168112 is a binary, thehigh velocity jump is easily explained by ascribing it to the wind-windcollision. By further assuming the star to be an eccentric binary, wecan explain the observed radio variability by the colliding-wind regionmoving in and out of the region where free-free absorption is important.The radio data presented here show that the binary has a period ofbetween one and two years. By combining the radio data with X-ray data,we find that the most likely period is ~1.4 yr.
| On the massive stellar population of the super star cluster Westerlund 1
We present new spectroscopic and photometric observations of the youngGalactic open cluster Westerlund 1 (Wd 1) that reveala unique population of massive evolved stars. We identify ~200 clustermembers and present spectroscopic classifications for ~25% of these. Wefind that all stars so classified are unambiguously post-Main Sequenceobjects, consistent with an apparent lack of an identifiable MainSequence in our photometric data to V 20. We are able to identifyrich populations of Wolf Rayet stars, OB supergiants and short livedtransitional objects. Of these, the latter group consists of both hot(Luminous Blue Variable and extreme B supergiant) and cool (YellowHypergiant and Red Supergiant) objects - we find that half the knownGalactic population of YHGs resides within Wd 1. We obtain a meanV-MV ~ 25 mag from the cluster Yellow Hypergiants, implying aMain Sequence turnoff at or below MV =-5 (O7 V or later).Based solely on the masses inferred for the 53 spectroscopicallyclassified stars, we determine an absolute minimum mass of ~1.5 ×10^3~Mȯ for Wd 1. However, considering the completephotometrically and spectroscopically selected cluster population andadopting a Kroupa IMF we infer a likely mass for Wd 1 of~10^5~Mȯ, noting that inevitable source confusion andincompleteness are likely to render this an underestimate. As such, Wd 1is the most massive compact young cluster yet identified in the LocalGroup, with a mass exceeding that of Galactic Centre clusters such asthe Arches and Quintuplet. Indeed, the luminosity, inferred mass andcompact nature of Wd 1 are comparable with those of Super Star Clusters- previously identified only in external galaxies - and is consistentwith expectations for a Globular Cluster progenitor.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| 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.
| Quasi-simultaneous XMM-Newton and VLA observation of the non-thermal radio emitter HD 168112 (O5.5III(f+))
We report the results of a multiwavelength study of the non-thermalradio emitter HD 168112 (O5.5III(f+)). The detailed analysisof two quasi-simultaneous XMM-Newton and VLA observations reveals strongvariability of this star both in the X-ray and radio ranges. The X-rayobservations separated by five months reveal a decrease of the X-rayflux of 30%. The radio emission on the other hand increases by afactor 5-7 between the two observations obtained roughly simultaneouslywith the XMM-Newton pointings. The X-ray data reveal a hard emissionthat is most likely produced by a thermal plasma at kT 2-3 keVwhile the VLA data confirm the non-thermal status of this star in theradio waveband. Comparison with archive X-ray and radio data confirmsthe variability of this source in both wavelength ranges over a yet illdefined time scale. The properties of HD 168112 in the X-ray and radiodomain point towards a binary system with a significant eccentricity andan orbital period of a few years. However, our optical spectra reveal nosignificant changes of the star's radial velocity suggesting that if HD168112 is indeed a binary, it must be seen under a fairly lowinclination.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member states andthe USA (NASA). Also based on observations collected with the VLA, aninstrument of the National Radio Astronomy Observatory, which is afacility of the National Science Foundation operated by AssociatedUniversities, Inc. Optical data were collected at the European SouthernObservatory (La Silla, Chile), and at the ObservatorioAstronómico Nacional of San Pedro Mártir (Mexico).
| 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
| 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.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| Some Diffuse Interstellar Bands Related to Interstellar C2 Molecules
We have investigated the correlations between the equivalent widths of21 selected diffuse interstellar bands (DIBs) and the correspondinginterstellar column densities N(C2), N(CN), and N(CH), toward53 stars with color excesses 0.11<=E(B-V)<=1.99. The observationaldata were derived primarily from echelle spectra acquired at R=38,000 aspart of our extensive, continuing survey of the bands. All but six ofthe 53 final spectra show signal-to-noise ratios >=800 at 5780Å. The principal result presented here is that seven of the 21bands prove to be examples of ``the C2 DIBs,'' a class ofweak, narrow bands whose normalized equivalent widthsWλ(X)/Wλ (λ6196) are wellcorrelated specifically with N(C2)/E(B-V) via power laws. Incontrast, the similarly normalized equivalent widths of the 14 other,well-known DIBs analyzed here are uncorrelated, or weaklyanticorrelated, with N(C2)/E(B-V), to within theobservational uncertainties. Thus, the polyatomic molecule(s) presumedto cause these seven C2 DIBs may bear a direct chemicalrelation to C2 that is not shared by the polyatomic moleculesputatively responsible for the other 14 bands. The C2 DIBsalso show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) inour particular sample of light paths, although generally with shallowerslopes in the case of N(CN) and with greater scatter in the case ofN(CH). Eleven additional C2 DIBs are also identified but arenot analyzed here. Among the 18 C2 DIBs identified, fourapparently have not been previously detected. The λ4963 band isgenerally the strongest of the 18 C2 DIBs, while theλ4734 band shows the most sensitive correlation withN(C2).Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Spectropolarimetry of O supergiants
We present medium-resolution spectropolarimetry at high signal-to-noiseratio of the Hα emission line of 20 O-type supergiants. Five stars(25 per cent) of the sample show a statistically significant change inpolarization through the line. We combine our Hα data with newK-band spectropolarimetry and archival low-resolution opticalspectropolarimetry to determine the polarigenic mechanism in the starsthat show a line effect. We show that the line polarization change inthe binary systems is caused by the classical `dilution' mechanism, inwhich the Hα emission is essentially unpolarized and the continuumpolarization is caused by intrabinary scattering. We find that the lineeffect in HD 108 is also well modelled by pure dilution, but suggestthat the continuum polarization is the result of stochastic windclumping. A similar description applies to the continuum polarization ofHD 188001, although the line effect cannot be reproduced by puredilution. We use low-resolution spectropolarimetry to determine theinterstellar polarization vector to λ Cephei, and confirm thatthe intrinsic polarization of the object is very low (<0.1 per cent,corresponding to an equator:pole density ratio of <1.25). The linepolarization of this star is modelled using the TORUS three-dimensionalradiative-transfer code. We show that the line effect is a consequenceof symmetry breaking caused by the rapid rotation of the system (>200km s-1), and that the system is similar polarimetrically tothe O4 supergiant ζ Puppis. Finally, we note that the precision ofcurrent photo- and spectro-polarimetric observations is insufficient totest structured wind models, which predict a continuum polarization of~0.1 per cent.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| Gas-Phase Iron Abundances and Depletions in Translucent Interstellar Lines of Sight from Far Ultraviolet Spectroscopic Explorer Observations of Fe II Lines
Far Ultraviolet Spectroscopic Explorer (FUSE) wavelength coverageincludes several weak- to moderate-strength lines of Fe II, allowing thedetermination, through curve-of-growth analysis, of accurate Fe IIabundances and hence iron depletions. We have analyzed Fe II absorptionlines toward 18 of the reddened stars included in the FUSE survey ofmolecular hydrogen abundances in translucent clouds. Our analysis isbased on equivalent width measurements and curves of growth, aided bythe fact that some of the observed lines are weak enough to be on thelinear part of the curve of growth. In interpreting our abundance anddepletion results, we have combined our data with those of an earliersurvey of interstellar iron abundances and depletions in diffuse clouds,based on Copernicus data. The principal result of our survey is thatiron depletions, known from earlier work to increase with averageline-of-sight density for diffuse clouds, do not continue to increasewith either density or extinction in translucent clouds; i.e., there isno significant trend of increasing depletion with increasing extinctionor molecular fraction. This may be due to the fact that our data setdoes not probe lines of sight with greater average volume densities thanthose that were covered by the previous Copernicus-based survey of irondepletions. We conclude by reevaluating the definition of translucentclouds, based on the lack of enhanced iron depletions in our sample.
| A Critical Examination of the l-C3H-2 Spectrum and the Diffuse Interstellar Bands
It has recently been suggested by J. P. Maier's group that the originband and three vibronic bands of the linear propadienylidene anionl-C3H-2 match the diffuse interstellarbands (DIBs). We have examined the wavelength ranges in question usingdata from our ongoing DIB survey at the Apache Point Observatory. Wefind that the strongest DIB (λ6993) is not an acceptablewavelength match to the origin band ofl-C3H-2, based on high-resolutionlaboratory data. The nondetection of interstellar features correspondingto the K=2<--1 and K=0<--1 branches of paral-C3H-2 also argues against theassignment of λ6993 to the K=1<--0 branch of orthol-C3H-2. Two of the three DIBs thathave been attributed to vibronic bands do not correlate in intensitywith λ6993, providing further evidence against the assignment ofthis set of DIBs to l-C3H-2.
| A multi-wavelength investigation of the non-thermal radio emitting O-star 9 Sgr
We report the results of a multi-wavelength investigation of the O4 Vstar 9 Sgr (= HD 164794). Our data include observations in the X-raydomain with XMM-Newton, in the radio domain with the VLA as well asoptical spectroscopy. 9 Sgr is one of a few presumably single OB starsthat display non-thermal radio emission. This phenomenon is attributedto synchrotron emission by relativistic electrons accelerated in stronghydrodynamic shocks in the stellar wind. Given the enormous supply ofphotospheric UV photons in the wind of 9 Sgr, inverse Compton scatteringby these relativistic electrons is a priori expected to generate anon-thermal power law tail in the X-ray spectrum. Our EPIC and RGSspectra of 9 Sgr reveal a more complex situation than expected from thissimple theoretical picture. While the bulk of the thermal X-ray emissionfrom 9 Sgr arises most probably in a plasma at temperature ~ 3 x106 K distributed throughout the wind, the nature of the hardemission in the X-ray spectrum is less clear. Assuming a non-thermalorigin, our best fitting model yields a photon index of >=2.9 for thepower law component which would imply a low compression ratio of<=1.79 for the shocks responsible for the electron acceleration.However, the hard emission can also be explained by a thermal plasma ata temperature >=2 x 107 K. Our VLA data indicate that theradio emission of 9 Sgr was clearly non-thermal at the time of theXMM-Newton observation. Again, we derive a low compression ratio (1.7)for the shocks that accelerate the electrons responsible for thesynchrotron radio emission. Finally, our optical spectra reveallong-term radial velocity variations suggesting that 9 Sgr could be along-period spectroscopic binary. Based on observations with XMM-Newton,an ESA Science Mission with instruments and contributions directlyfunded by ESA Member states and the USA (NASA). Also based onobservations collected at the European Southern Observatory (La Silla,Chile) and with the Very Large Array. The VLA is a facility of theNational Radio Astronomy Observatory which is operated by the AssociatedUniversities Inc. under cooperative agreement with the National ScienceFoundation.
| Optical spectroscopy of XMEGA targets in the Carina Nebula - III. The multiple system Tr 16-104 (≡CPD -59° 2603)
We discuss the orbital elements of the multiple system Tr 16-104 whichis usually believed to be a member of the open cluster Trumpler 16 inthe Carina complex. We show that Tr 16-104 could be a hierarchicaltriple system consisting of a short-period (2.15d) eclipsing O7V+O9.5Vbinary bound to a B0.2 IV star. Our preliminary orbital solution of thethird body indicates that the B star most probably describes aneccentric orbit with a period of ~285 or ~1341d around the close binary.Folding photometric data from the literature with our new ephemerides,we find that the light curve of the close binary exhibits rather narroweclipses indicating that the two O stars must be well inside their Rochelobes. Our analysis of the photometric data yields a lower limit on theinclination of the orbit of the close binary of i>=77°. Thestellar radii and luminosities of the O7 V and O9.5 V stars aresignificantly smaller than expected for stars of this spectral type. Ourresults suggest that Tr 16-104 lies at a distance of the order of 2.5kpcand support a fainter absolute magnitude for zero-age main-sequence Ostars than usually adopted. We find that the dynamical configuration ofTr 16-104 corresponds to a hierarchical system that should remain stableprovided that it suffers no strong perturbation. Finally, we also reportlong-term temporal variations of high-velocity interstellar Caiiabsorptions in the line of sight towards Tr 16-104.
| Far-ultraviolet extinction and diffuse interstellar bands
We relate the equivalent widths of the major diffuse interstellar bands(DIBs) near 5797 and 5780Å with different colour excesses,normalized by E(B-V), which characterize the growth of interstellarextinction in different wavelength ranges. It is demonstrated that thetwo DIBs correlate best with different parts of the extinction curve,and the ratio of these diffuse bands is best correlated with thefar-ultraviolet (UV) rise. A number of peculiar lines of sight are alsofound, indicating that the carriers of some DIBs and the far-UVextinction can be separated in certain environments, e.g. towards thePer OB2 association.
| High-mass binaries in the very young open cluster NGC 6231. Implication for cluster and star formation
New radial-velocity observations of 37 O- and B stars in the very youngopen cluster NGC 6231 confirm the high frequency of short-periodspectroscopic binaries on the upper main sequence. Among the 14 O-typestars, covering all luminosity classes from dwarfs to supergiants, 8 aredefinitively double-lined systems and all periods but one are shorterthan 7 days. Several additional binaries have been detected among theearly B-type stars. NGC 6231 is an exceptional cluster to constrain thescenarios of cluster- and binary-star formation over a large range ofstellar masses. We discuss the evidences, based on NGC 6231 and 21 otherclusters, with a total of 120 O-type stars, for a clear dichotomy in themultiplicity rate and structure of very young open clusters containingO-type stars in function of the number of massive stars. However, wecannot answer the question whether the observed characteristics resultfrom the formation processes or from the early dynamical evolution.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| The early-type multiple system QZ Carinae
We present an analysis of the early-type quadruple system QZ Car,consisting of an eclipsing and a non-eclipsing binary. The spectroscopicinvestigation is based on new high dispersion echelle and CAT/CESspectra of H and He lines. The elements for the orbit of thenon-eclipsing pair could be refined. Lines of the brighter component ofthe eclipsing binary were detected in near-quadrature spectra, whilesignatures of the fainter component could be identified in only fewspectra. Lines of the primary component of the non-eclipsing pair and ofboth components of the eclipsing pair were found to be variable inposition and strength; in particular, the He ii 4686 emission line ofthe brighter eclipsing component is strongly variable. An ephemeris forthe eclipsing binary QZ Car valid at present was derived Prim. Min. =hel. JD 2448687.16 + 5fd9991 * E. The relative orbit of the two binaryconstituents of the multiple system is discussed. In contrast to earlierinvestigations we found radial velocity changes of the systemicvelocities of both binaries, which were used - together with an O-Canalysis of the expected light-time effect - to derive approximateparameters of the mutual orbit of the two pairs. It is shown that thisorbit and the distance to QZ Car can be further refined by minima timingand interferometry. Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile.
| Narrow Diffuse Interstellar Bands: A Survey with Precise Wavelengths
We present an atlas of 271 diffuse interstellar bands (DIBs) between4460 and 8800 Å based on echelle spectra (R=45,000), of which morethan 100 are new DIBs discovered in this survey. The atlas is restrictedmostly to narrow features, and we describe the tests for an interstellarorigin. The rest wavelength of each DIB was determined in a high-qualitycomposite spectrum of the star HD 23180 using the interstellar Na I (D1and D2) lines to establish the radial velocity of the single interveningcloud. DIB wavelengths are quoted to 0.01 Å and are probablyaccurate to ~0.1 Å. Other, weak DIBs found in the spectra ofheavily reddened stars are included with a lower wavelength precision.
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