Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
The Remarkable Be Star HD 110432 (BZ Crucis)
HD 110432 (B1e) has gained considerable recent attention because it is ahard, variable X-ray source with local absorption and also because itsoptical spectrum is affected by an extensive Be disk. From time-serialechelle data obtained over 2 weeks during 2005 January and February, wehave discovered several remarkable characteristics in the star's opticalspectrum. The line profiles show rapid variations on some nights, whichcan most likely be attributed to irregularly occurring and short-livedmigrating subfeatures. Such features have been found in spectra ofγ Cas and AB Dor, two stars for which it is believed magneticfields force circumstellar clouds to corotate over the star's surface.The star's optical spectrum also exhibits a number of mainly Fe II andHe I emission features with double-lobed profiles typical of anoptically thin circumstellar disk viewed nearly edge-on. Using spectralsynthesis techniques for the January data, we find that its temperatureand column density are close to 9800 K and roughly3×1022 cm-2, respectively. Its projecteddisk size covers remarkably large 100 stellar areas, and the emittingvolume resides at a surprisingly large distance of 1 AU from the star.Surprisingly, we also find that the absorption wings of the strongestoptical and UV lines in the spectrum extend to at least +/-1000 kms-1, even though the rotational velocity is 300-400 kms-1. We are unable to find a satisfactory explanation forthese extreme line broadenings. Otherwise, HD 110432 and γ Casshare similarly peculiar X-ray and optical characteristics. Theseinclude a high X-ray temperature, erratic X-ray variability ontimescales of a few hours, optical metallic emission lines, andsubmigrating features in optical line profiles. Because of thesesimilarities, we suggest that HD 110432 is a member of a select newclass of ``γ Cas analogs.''
| New γ Cassiopeiae-like objects: X-ray and optical observations of SAO 49725 and HD 161103
A growing number of early Be stars exhibit X-ray luminosities that areintermediate between those typical of early type stars and those emittedby most Be/X-ray binaries in quiescence. We report on XMM-Newtonobservations of two such Be stars, SAO 49725 andHD 161103, which were originally discovered in asystematic cross-correlation between the ROSAT all-sky survey andSIMBAD. The new observations confirm the X-ray luminosity excess(LX ~ 1032-33 erg s-1) and the hardnessof their X-ray spectra, which are both unusual for normal early typestars. An iron Kα complex is clearly detected in HD161103 in which the H-like, He-like, and fluorescentcomponents are resolved, while strong evidences also exist for thepresence of similar features in SAO 49725. X-rayspectra can be equally well-fitted by a thermal plasma (mekal) with T ~108 K and solar abundances or by a power law + iron linemodel with photon index ~1.5-1.8, both with a soft thermal componentwith T ~ 107 K. The intensity of the fluorescence 6.4 keVline reflects the presence of large amounts of cold material close tothe X-ray sources and strongly argues against accretion onto a companionneutron star in a large orbit. On the other hand, the probable thermalorigin of the X-ray emission, as supported by the ionised iron lines,disagrees with those observed in all known Be/X-ray binaries, in which anon-thermal component is always required. Remarkably, the X-ray featuresare similar to those of white dwarves in several cataclysmic variables.There is no evidence of high frequency pulsations in neither of the twosystems. However, a large oscillation in the light curve of HD161103 with P ~ 3200 s is readily detected. The X-ray lightcurve of SAO 49725 exhibits clear variability by ~80%on time scales as short as ~1000 s. New optical observations provideupdated spectral types (B0.5 III-Ve) and disclose a dense, large, andapparently stable circumstellar disc for both stars. The nature of theexcess X-ray emission is discussed in light of the models proposed forγ Cas, magnetic disc-star interaction, oraccretion onto a compact companion object - whether neutron star orwhite dwarf. These two new objects, added to similar cases discovered inXMM-Newton surveys, point to the emergence of a new class ofγ Cas analogs.
| 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.
| On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking intoaccount the effects induced by the fast rotation on their fundamentalparameters. All program stars were observed in the BCDspectrophotometric system in order to minimize the perturbationsproduced by the circumstellar environment on the spectral photosphericsignatures. This is one of the first attempts at determining stellarmasses and ages by simultaneously using model atmospheres andevolutionary tracks, both calculated for rotating objects. The stellarages (τ) normalized to the respective inferred time that eachrotating star can spend in the main sequence phase (τ_MS) reveal amass-dependent trend. This trend shows that: a) there are Be starsspread over the whole interval 0 τ/τ_MS 1 of themain sequence evolutionary phase; b) the distribution of points in the(τ/τMS,M/Mȯ) diagram indicates thatin massive stars (M 12~Mȯ) the Be phenomenon ispresent at smaller τ/τ_MS age ratios than for less massive stars(M 12~Mȯ). This distribution can be due to: i)higher mass-loss rates in massive objets, which can act to reduce thesurface fast rotation; ii) circulation time scales to transport angularmomentum from the core to the surface, which are longer the lower thestellar mass.
| Interstellar 12C/13C ratios through CH^+λλ 3957,4232 absorption in local clouds: incomplete mixing in the ISM
The 12C/13C isotope ratio is a tracer of stellaryields and the efficiency of mixing in the ISM.12CH+/13CH+ is not affectedby interstellar chemistry, and is the most secure way of measuring12C/13C in the diffuse ISM.R=12C/13C is 90 in the solar system. Previousmeasurements of 12CH+λλ3957.7,4232.3and 13CH+λλ3958.2,4232.0 absorptiontoward nearby stars indicate some variations in12C/13C, with values ranging from 40 to 90suggesting inefficient mixing. Except for the cloud toward ζOph,these R values are strongly affected by noise. With UVES on the VLT wehave improved on the previous interstellar 12C/13Cmeasurements. The weighted 12C/13C ratio in thelocal ISM is 78.27 ± 1.83, while the weighted dispersion of ourmeasurements is 12.7, giving a 6.9σ scatter. Thus we report on a6.9σ detection of 16.2% root-mean-square variations in the carbonisotopic ratio on scales of ~100 pc: R= 74.7 ± 2.3 in theζOph cloud, while R = 88.6 ± 3.0 toward HD 152235 in theLupus clouds, R = 62.2 ± 5.3 towards HD 110432 in the Coalsack,and R = 98.9 ± 10.1 toward HD 170740. The observed variations in13C/12C are the first significant detection ofchemical heterogeneity in the local ISM.
| Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines
We present column density measurements of the HD molecule in theinterstellar gas toward 17 Galactic stars. The values for the seven mostheavily reddened sightlines, with E(B-V) = 0.38-0.72, are derived fromobservations with the Far Ultraviolet Spectroscopic Explorer (FUSE). Theother ten values are from a reanalysis of spectra obtained withCopernicus. In all cases, high-resolution ground-based observations of KI and/or the CH molecule were used to constrain the gas velocitystructure and to correct for saturation effects. Comparisons of thecolumn densities HD, CH, CN, and K I in these 17 sightlines indicatethat HD is most tightly correlated with CH. Stringent lower limits tothe interstellar D/H ratio, derived from the HD/2H2 ratio,range from 3.7 × 10-7 to 4.3 × 10-6.Our results also suggest that the HD/H2 ratio increases withthe molecular fraction f(H2) and that the interstellar D/Hratio might be obtained from HD by probing clouds with f(H2) 1. Finally, we note an apparent relationship between the molecularfractions of hydrogen and deuterium.
| A Hot Envelope around the Southern Coalsack: X-Ray and Far-Ultraviolet Observations
We present Far Ultraviolet Spectroscopic Explorer and ROSAT X-rayobservations toward the Southern Coalsack. An almost complete X-ray halocan be seen around the cloud in the 0.75 and 1.5 keV images, and most ofthe observed stars show O VI absorption. Both the cloud and the starshave highly accurate distance determinations, allowing us to reliablyplace the stars and the cloud relative to each other. Using thesedistance determinations, we find no O VI-bearing gas in the foregroundof the Coalsack, while for stars in the background of the cloud, O VIabsorption is the norm. The column density of O VI correlates with the0.75 and 1.5 keV intensities. These results suggest that theX-ray-emitting hot plasma is associated with the dense cloud. We proposethat the heating of the Coalsack envelope is due to the hot gas in theinterior of the Upper Cen-Lup superbubble. The Coalsack interactionregion provides a nearby example of the hot-cold gas interfaces thoughtto be responsible for the O VI absorptions seen on many sight linesthroughout the Galaxy.
| Deuterium in Molecules of the Interstellar Medium
Deuterated molecules are detected both in interstellar translucentclouds and in cold dark clouds, as well as in star forming regions. Wereview the recent observational studies of deuterated molecules rangingfrom the VUV to the millimeter wavelength range. We outline some sourcesof uncertainties on the deuterium fractionation and on the subsequentderivation of the elemental deuterium to hydrogen ratio. Steady stateversus time dependent models are discussed and the role of initialconditions is emphasized.
| 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.
| Potential Variations in the Interstellar N I Abundance
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and SpaceTelescope Imaging Spectrograph observations of the weak interstellar N Iλ1160 doublet toward 17 high-density sight lines[N(Htot)>=1021 cm-2]. When combinedwith published data, our results reveal variations in the fractional N Iabundance showing a systematic deficiency at large N(Htot).At the FUSE resolution (~20 km s-1), the effects ofunresolved saturation cannot be conclusively ruled out, although O Iλ1356 shows little evidence of saturation. We investigated thepossibility that the N I variability is due to the formation ofN2 in our mostly dense regions. The 0-0 band of thec'41Σ+u-X1Σ+gtransition of N2 at 958 Å should be easily detected inour FUSE data; for 10 of the denser sight lines, N2 is notobserved at a sensitivity level of a few times 1014cm-2. The observed N I variations are suggestive of anincomplete understanding of nitrogen chemistry.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985, and the NASA/ESA HubbleSpace Telescope, obtained from the Multimission Archive at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under the NASA contractNAS 5-26555.
| Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
Ultraviolet absorption from interstellar 12CO and13CO was detected toward ρ Oph A and χ Oph. Themeasurements were obtained at medium resolution with the Goddard HighResolution Spectrograph on the Hubble Space Telescope. Column densityratios, N(12CO)/N(13CO), of 125+/-23 and 117+/-35were derived for the sight lines toward ρ Oph A and χ Oph,respectively. A value of 1100+/-600 for the ratioN(12C16O)/N(12C18O) towardρ Oph A was also obtained. Absorption from vibrationally excitedH2 (v''=3) was clearly seen toward this star aswell. The ratios are larger than the isotopic ratios for carbon andoxygen appropriate for ambient interstellar material. Since for bothcarbon and oxygen the more abundant isotopomer is enhanced, selectiveisotopic photodissociation plays the key role in the fractionationprocess for these directions. The enhancement arises because the moreabundant isotopomer has lines that are more optically thick, resultingin more self-shielding from dissociating radiation. A simple argumentinvolving the amount of self-shielding [from N(12CO)] and thestrength of the ultraviolet radiation field permeating the gas (from theamount of vibrationally excited H2) shows that selectiveisotopic photodissociation controls the fractionation seen in these twosight lines, as well as the sight line to ζ Oph.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 NAS5-26555.
| Inferring Physical Conditions in Interstellar Clouds of H2
We have developed a code that models the formation, destruction,radiative transfer, and vibrational/rotational excitation ofH2 in a detailed fashion. We discuss generally how suchcodes, together with Far Ultraviolet Spectrographic Explorer (FUSE)observations of H2 in diffuse and translucent lines of sight,may be used to infer various physical parameters. We illustrate theeffects of changes in the major physical parameters (UV radiation field,gas density, metallicity), and we point out the extent to which changesin one parameter may be mirrored by changes in another. We provide ananalytic formula for the molecular fraction, fH2,as a function of cloud column density, radiation fields, and grainformation rate of H2. Some diffuse and translucent lines ofsight may be concatenations of multiple distinct clouds viewed together.Such situations can give rise to observables that agree with the data,complicating the problem of uniquely identifying one set of physicalparameters with a line of sight. Finally, we illustrate the applicationof our code to an ensemble of data, such as our FUSE survey ofH2 in the Large and Small Magellanic Clouds, in order toconstrain the elevated UV radiation field intensity and reduced grainformation rate of H2 in those low-metallicity environments.
| High-energy sources before INTEGRAL. INTEGRAL reference catalog
We describe the INTEGRAL reference catalog which classifies previouslyknown bright X-ray and gamma-ray sources before the launch of INTEGRAL.These sources are, or have been at least once, brighter than ~ 1 mCrababove 3 keV, and are expected to be detected by INTEGRAL. This catalogis being used in the INTEGRAL Quick Look Analysis to discover newsources or significantly variable sources. We compiled several publishedX-ray and gamma-ray catalogs, and surveyed recent publications for newsources. Consequently, there are 1122 sources in our INTEGRAL referencecatalog. In addition to the source positions, we show an approximatespectral model and expected flux for each source, based on which wederive expected INTEGRAL counting rates. Assuming the default instrumentperformances and at least ~ 105 s exposure time for anypart of the sky, we expect that INTEGRAL will detect at least ~ 700sources below 10 keV and ~ 400 sources above 20 keV over the missionlife.The Catalog is available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/411/L59
| 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.''
| Abundances and Physical Conditions in the Interstellar Gas toward HD 192639
We present a study of the abundances and physical conditions in theinterstellar gas toward the heavily reddened star HD 192639[E(B-V)=0.64], based on analysis of Far UltravioletSpectroscopic Explorer (FUSE) and Hubble Space Telescope/Space TelescopeImaging Spectrograph spectra covering the range from 912 to 1361Å. This work constitutes a survey of the analyses that can beperformed to study the interstellar gas by combining data from differentinstruments. Low-velocity (-18 to -8 km s-1) components areseen primarily for various neutral and singly ionized species such as CI, O I, S I, Mg II, Cl I, Cl II, Mn II, Fe II, and Cu II. Numerous linesof H2 are present in the FUSE spectra, with a kinetictemperature for the lowest rotational levels T01=90+/-10 K.Analysis of the C I fine-structure excitation implies an average localdensity of hydrogen nH=16+/-3 cm-3. The averageelectron density, derived from five neutral/first ion pairs under theassumption of photoionization equilibrium, is ne=0.11+/-0.02cm-3. The relatively complex component structure seen inhigh-resolution spectra of K I and Na I, the relatively low averagedensity, and the measured depletions all suggest that the line of sightcontains a number of diffuse clouds, rather than a single dense,translucent cloud. Comparisons of the fractions of Cl in Cl I and ofhydrogen in molecular form suggest a higher molecular fraction, in theregion(s) where H2 is present, than that derived consideringthe average line of sight. In general, such comparisons may allow theidentification and characterization of translucent portions of suchcomplex lines of sight. The combined data also show high-velocitycomponents near -80 km s-1 for various species that appear tobe predominantly ionized and may be due to a radiative shock. A briefoverview of the conditions in this gas will be given. Based onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the Data Archive at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal 8241.
| X-Ray and Optical Variations in the Classical Be Star γ Cassiopeia: The Discovery of a Possible Magnetic Dynamo
The classical B0.5e star γ Cassiopeia is known to be a uniqueX-ray source by virtue of its moderate LX (1033ergs s-1), hard X-ray spectrum, and light curve punctuated byubiquitous flares and slow undulations. The peculiarities of this starhave led to a controversy concerning the origin of these emissions:whether they are from wind infall onto a putative degenerate companion,as in the case of normal Be/X-ray binaries, or from the Be star itself.Recently, much progress has been made to resolve this question: (1) thediscovery that γ Cas is a moderately eccentric binary system(P=203.59 days) with unknown secondary type, (2) the addition of RXTEobservations at six epochs in 2000, adding to three others in 1996-1998,and (3) the collation of robotic telescope (Automated PhotometricTelescope) B- and V-band photometric observations over four seasons thatshow a 3%, cyclical flux variation with cycle lengths of 55-93 days. Wefind that X-ray fluxes at all nine epochs show random variations withorbital phase, thereby contradicting the binary accretion model, whichpredicts a substantial modulation. However, these fluxes correlate wellwith the cyclical optical variations. In particular, the six fluxmeasurements in 2000, which vary by a factor of 3, closely track theinterpolated optical variations between the 2000 and 2001 observingseasons. The energy associated with the optical variations greatlyexceeds the energy in the X-rays, so that the optical variability cannotsimply be due to reprocessing of X-ray flux. However, the strongcorrelation between the two suggests that they are driven by a commonmechanism. We propose that this mechanism is a cyclical magnetic dynamoexcited by a Balbus-Hawley instability located within the inner part ofthe circumstellar disk. According to our model, variations in the fieldstrength directly produce the changes in the magnetically related X-rayactivity. Turbulence associated with the dynamo results in changes tothe density (and therefore the emission measure) distribution within thedisk and creates the observed optical variations.
| 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.
| C18O abundance in the nearby globule Barnard 68
We have studied the radial variation of the CO abundance in the nearbyisolated globule Barnard 68 (B68). For this purpose, B68 was mapped inthe three rotational lines 13CO(J=1-0),C18O(J=1-0) and C18O(J=2-1). Using the recentdiscovery of Alves et al. (\cite{alves_01}) that the density structureof B68 agrees with the prediction for a pressure bound distribution ofisothermal gas in hydrostatic equilibrium (Bonnor-Ebert sphere), we showthat the flat CO column density distribution can be explained bymolecular depletion. By combining the physical model with the observedCO column density profile, it was found that the density dependence ofthe CO depletion factor fd can be well fitted with the lawfd=1+const. n(H2), which is consistent with anequilibrium between the accretion and the desorption processes. In thecloud centre, between 0.5% and 5% of all CO molecules are in the gasphase. Our observations suggest a kinetic temperature of ~8 K. Incombination with the assumption that B68 is a Bonnor-Ebert sphere, thisleads to a distance of 80 pc. The cloud mass consistent with thesevalues is 0.7 Msun, considerably less than previouslyestimated. We find in B68 no clear deviance of the near-infraredreddening efficiency of dust grains per unit H2 columndensity with respect to values derived in diffuse clouds. Based onobservations made with the Swedish-ESO Submillimetre Telescope (SEST),Chile.
| Far Ultraviolet Spectroscopic Explorer Observations of Molecular Hydrogen in Translucent Interstellar Clouds. II. The Line of Sight toward HD 110432
We report the second study from the FUSE survey of molecular hydrogen intranslucent clouds for the line of sight toward HD 110432. This starlies beyond the Coalsack dark nebula, and, with E(B-V)=0.40 andAV=1.32, this line of sight bridges the gap betweenless-extinguished diffuse-cloud lines of sight with AV~1,such as ζ Ophiuchi, and translucent-cloud lines of sight withAV>~2, such as HD 73882. Through profile fitting and acurve-of-growth analysis, we have derived rotational populations forH2 for J=0-7. The line of sight has a total molecularhydrogen column density, logN(H2)=20.68+/-0.05cm-2, nearly identical to that toward ζ Oph but a factorof 3 less than that toward HD 73882. The ratio of N(J=1) to N(J=0)yields a kinetic temperature Tkin=63+/-7 K, which is similarto other lines of sight with AV>~1. The high-J lines showconsiderable excitation above this temperature. However, the high-Jlines cannot be well fitted to a single excitation temperature, and theeven-J lines of para-hydrogen appear slightly enhanced relative to theodd-J lines of ortho-hydrogen. The high-J excitation is similar to thattoward ζ Oph but is much smaller than that toward HD 73882.Chemical modeling indicates that the physical conditions in the cloud(s)are very similar to those in the cloud(s) toward ζ Oph. An analysisof IUE spectra of the Lyα line gives N(H I)=20.85+/-0.15cm-2. Combined with N(H2), we derive a hydrogenmolecular fraction, fH2=0.58+/-0.12, statisticallyidentical to that found for the lines of sight toward ζ Oph and HD73882. From the FUSE data and a curve-of-growth analysis using the samecomponent structure as H2, we findlogN(HD)=15.2+0.7-0.4 cm-2, which isbetween the values found for ζ Oph and HD 73882. Profile fittingsuggests smaller b values for HD than for H2, andlogN(HD)=16.0+0.2-0.3 cm-2. From FUSEand IUE data, we derive logN(CO)~14.3 cm-2, assuming the samecomponent structure as CH or logN(CO)~14.8 cm-2, if allobserved CO is cospatial with the strongest CH component. From thecombined measurements of hydrogen- and carbon-containing molecules, theline of sight toward HD 110432 appears quite similar to thediffuse-cloud line of sight toward ζ Oph and quite dissimilar tothe translucent-cloud line of sight toward HD 73882. Upcoming FUSEobservations will further explore the transition between diffuse andtranslucent clouds.
| The Hard X-Ray Luminosity of OB Star Populations: Implications for The Contribution of Star Formation to the Cosmic X-Ray Background
We present an empirical analysis of the integrated X-ray luminosityarising from populations of OB stars. In particular, we utilize resultsfrom the All-Sky Monitor on RXTE, along with archival data from previousmissions, to assess the mean integrated output of X-rays in the 2-10 keVband from accreting early-type binaries within 3 kpc of the Sun. Using arecent OB star census of the solar neighborhood, we then calculate thespecific X-ray luminosity per O star from accretion-powered systems. Wealso assess the contribution to the total X-ray luminosity of an OBpopulation from associated T Tauri stars, stellar winds, and supernovae.We repeat this exercise for the major Local Group galaxies, concludingthat the total X-ray luminosity per O star spans a broad range from 2 to20×1034 ergs s-1. Contrary to previousresults, we do not find a consistent trend with metallicity; in fact,the specific luminosities for M31 and the SMC are equal, despite havingmetallicities that differ by an order of magnitude. In light of theseresults, we assess the fraction of the observed 2-10 keV emission fromstarburst galaxies that arises directly from their OB star populations,concluding that, while binaries can explain most of the hard X-rayemission in many local starbursts, a significant additional component orcomponents must be present in some systems. A discussion of the natureof this additional emission, along with its implications for thecontribution of starbursts to the cosmic X-ray background, concludes ourreport.
| High and intermediate-resolution spectroscopy of Be stars 4481 lines
We present an atlas of Hγ , He i lambda 4471 and Mg ii lambda 4481line profiles obtained in a 10 year observation period of 116 Be stars,which enabled many of them to be observed at quite different emissionepochs. From the best fit of the observed He i lambda 4471 line profileswith non-LTE, uniform (Teff,log g) and full limb-darkenedmodel line profiles, we determined the V sin i of the program stars. Toaccount, to some degree, for the line formation peculiarities related tothe rapid rotation-induced non-uniform distributions of temperature andgravity on the stellar surface, the fit was achieved by considering(Teff,log g) as free parameters. This method produced V sin iestimations that correlate with the rotational velocities determined bySlettebak (1982) within a dispersion sigma <= 30 km s-1and without any systematic deviation. They can be considered as given inthe new Slettebak's et al. (1975) system. Only 13 program stars havediscrepant V sin i values. In some objects, this discrepancy could beattributed to binary effects. Using the newly determined V sin iparameters, we found that the ratio of true rotational velocitiesV/Vc of the program Be stars has a very low dispersion aroundthe mean value. Assuming then that all the stars are rigid rotators withthe same ratio V(/lineω)/Vc, we looked for the value of/line ω that better represents the distribution of V sini/Vc for randomly oriented rotational axes. We obtained/lineω = 0.795. This value enabled us to determine the probableinclination angle of the stellar rotation axis of the program stars. Inthe observed line profiles of Hγ , He i lambda 4471, Mg ii lambda4481 and Fe ii lambda 4351 we measured several parameters related to theabsorption and/or emission components, such as: equivalent width,residual emission and/or absorption intensity, FWHM, emission peakseparations, etc. The parameters related to the Hγ line emissionprofiles were used to investigate the structure of the nearbyenvironment of the central star. From the characteristics of thecorrelations between these quantities and the inferred inclinationangle, we concluded that in most of cases the Hγ line emissionforming regions may not be strongly flattened. Using a simplerepresentation of the radiation flux emitted by the star+envelopesystem, we derived first order estimates of physical parameterscharacterizing the Hγ line emission formation region. Thus, weobtained that the total extent of the Hγ region is Rf=~ 2.5 +/- 1.0 R* and that the density distribution in theselayers can be mimicked with a power law rho ~ R-alpha , wherealpha =2.5+2.2-0.6. The same approach enabled usto estimate the optical depth of the Hγ line emission formationregion. From its dependence with the aspect angle, we concluded thatthese regions are caracterized by a modest flattening and that the rho(equator)/rho (pole) density contrast of the circumstellar envelope nearthe star should be two orders of magnitude lower than predicted bymodels based on a priori disc-shaped circumstellar envelopes. We foundthat the separation between the emission peaks, Deltap, andthe full width at half maximum, Delta 1/2, of the Hγline emission are not only sensitive to kinematic effects, but to lineoptical depth as well. This finding agrees with previous theoreticalpredictions and confirms that Huang's (1972) relation overestimates theextent of the Hγ line emission formation region. Data obtained atCASLEO operated under agreement between the CONICET and the NationalUniversities of La Plata, Córdoba and San Juan, Argentina, at ESOLa Silla, Chile and at OHP, France.}\fnmsep\thanks{Tables 2 to 7 andFigs. 1 and 2 are only available in full in electronic form at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/861}} \subtitle{Anatlas of Hγ , He {\fontsize {10pt}{12pt}\selectfont I} 4471 and Mg{\fontsize {10pt}{12pt}\selectfont II
| BeppoSAX survey of Be/X-ray binary candidates
We present a BeppoSAX survey of five Be/X-ray binary candidates. Wereport on the identification of two of them, HD 110432 and HD 141926, aslow luminosity Be/X-ray binaries. For HD 110432 we report on thedetection of a pulsation period of ~ 14 ks. Because the luminosity ofthese sources is low and their spectra do not require non-thermalemission models, these systems are good Be+White Dwarf candidates. Ifthe pulsation period for HD 110432 is confirmed, this system would bethe most firm Be+WD candidate found up to date. The other three objectsHD 65663, HD 249179 and BD+53 2262 did not show detectable X-rayemission. We argue that, while the properties of BD+53 2262 are stillconsistent with a quiescent Be+Neutron Star scenario, the lack ofdetection for the other two objects implies that they are most probablynot X-ray binaries.
| 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
| Population synthesis of Be/white dwarf binaries in the Galaxy
Using the ``Scenario Machine'' (a numerical co-de that models theevolution of large ensembles of binary systems) we study the number andphysical properties of binary Be stars with white dwarfs taking accountof the compact object cooling and we discuss the ways of theirformation. In our calculations we take into account the influence oftidal synchronization on the evolution of stars in a close binary. Thesynchronization time scale may be less than the life-time of a Be staron the main sequence after the first mass transfer. It has strongeffects on the resulting number distribution of binary Be stars overorbital periods. In particular, it can explain the lack of short periodBe binaries. According to our calculations the number of binary systemscontaining a Be star paired with a white dwarf in the Galaxy is verylarge - 70% of all Be stars formed as a result of binary evolution musthave a white dwarf as a companion. Based on our calculations we concludethat the compact companion in these systems must have a high surfacetemperature. The number distribution over the surface temperature peaksat 2 104 K for all white dwarfs and at 4 104 K forwhite dwarfs paired with early-type Be stars (between B0 and B2). Theregistration of white dwarfs in such systems is hampered by the factthat the entire orbit of a white dwarf is embedded in the densecircumstellar envelope of the primary star (our calculations show thatthe majority of Be/WD systems have orbital periods less than one year)and all extreme-UV and soft X-ray photons of a compact companion areabsorbed by the Be star envelope. The detection of a white dwarf ispossible during the period when the Be star disc-like envelope islacking by the detection of white dwarf extreme-UV and soft X-rayemission. This method of registration appears to be particularlypromising for ``single'' early-type Be stars because in these systemsthe white dwarfs must have a very high surface temperature. However, theloss of the Be disc-like envelope does not often occur and it is arather rare event for many Be stars. The best possibility of white dwarfdetection is given by the study of helium spectral lines found inemission from several late-type Be stars. The ultraviolet continuumenergy of these Be stars is found to be not enough to produce theobserved helium emission. Besides, we also discuss the orbitalproperties of binary Be star systems with other evolved companions suchas helium stars and neutron stars and give a possible explanation forthe lack of Be/black hole binaries.
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,354 of the stars in Part III are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives, in addition to the SI mode, the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| kappa Velorum: another variable interstellar sightline?.
We present ultra-high-resolution (R = 900,000) observations ofinterstellar Na i and K i absorption lines towards kappa Vel (HD 81188)which show clear evidence for temporal variation between 1994 and 2000.Specifically, the column densities of K0 and Na0in the main velocity component have increased by 40 and 16 per cent,respectively, over this period. Earlier work had suggested that thiscomponent actually consists of two unresolved sub-components; thisresult is confirmed here, and the overall line profile is found to beconsistent with only one of these sub-components having increased instrength since 1994. We argue that this variation is consistent withthe line of sight gradually probing a cold, dense interstellar filamentof the kind recently proposed by Heiles to explain other observations ofsmall-scale structure in the interstellar medium.
| Interstellar Nai D lines towards the Southern Coalsack
The interstellar Nai D absorption-line profiles observed for 15 starswith lines of sight towards the Southern Coalsack are analysed. Themethod of profile fitting was used in an attempt to determine columndensities, linewidths and velocities for the individual interstellarclouds contributing to the observed absorption lines. In common, theobserved spectra show a prominent component which is probably associatedwith the nearest absorbing material composing the Coalsack. The obtainedspatial velocity distribution shows great similarity with earlierresults from CO emission. In addition, the Nai D data reveal evidencefor the existence of two or three other structures with radialvelocities of about -22, -33 and -40kms-1. Such componentsmay be the counterparts of interstellar structures observed in diffuseHα and CO emission. The assumption that at least one of thesecomponents originated in the Carina arm imposes ~0.9-1.0kpc as themaximum distance to the near side of that arm.
| A catalogue of high-mass X-ray binaries
We present a catalogue of high-mass X-ray binaries. The catalogue is anupdated version of the catalogue of van Paradijs (1995). This newcatalogue contains 130 sources, 61 new high-mass X-ray binaries inaddition to the 69 sources listed in van Paradijs' catalogue. Most ofthe new sources are identified to be Be/X-ray binaries. Some sources,however, are only tentatively identified as high-mass X-ray binaries onthe basis of a transient character and/or a hard X-ray spectrum. Furtheridentification in other wavelength bands is needed to finally determinethe features of these sources. The aim of this catalogue is to providethe reader with some basic information on the X-ray sources and theircounterparts in other wavelength ranges (UV, optical, IR, radio). Incases where there is some doubt about the high-mass nature of the X-raybinary this is mentioned. Some doubtful cases have not been included inthe catalogue although they had been suggested by some authors to behigh-mass X-ray binaries, for example GRS 1915+105 and 1WGAJ1958.2+3232. The catalogue and references are also available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://vizier.u-strasbg.fr/cgi-bin/qcat?J/A+AS/147/25
| On the origin of the difference between the runaway velocities of the OB-supergiant X-ray binaries and the Be/X-ray binaries
The recent finding by Chevalier & Ilovaisky (1998) from Hipparcosobservations that OB-supergiant X-ray binaries have relatively largerunaway velocities (mean peculiar tangential velocity avvt = 42 +/- 14\kms), whereas Be/X-ray binaries have low runaway velocities (avvt = 15+/- 6 \kms), provides confirmation of the current models for theformation of these two types of systems. These predict a difference inrunaway velocity of this order of magnitude. This difference basicallyresults from the variation of the fractional helium core mass as afunction of stellar mass, in combination with the conservation oforbital angular momentum during the mass transfer phase that precededthe formation of the compact object in the system. This combinationresults into: (i) Systematically narrower pre-supernova orbits in theOB-supergiant systems than in the Be-systems, and (ii) A largerfractional amount of mass ejected in the supernovae in high-mass systemsrelative to systems of lower mass. Regardless of possible kickvelocities imparted to neutron stars at birth, this combination leads toa considerable difference in average runaway velocity between these twogroups. If one includes the possibility for non-conservative masstransfer the predicted difference between the runaway velocity of thetwo groups becomes even more pronounced. The observed low runawayvelocities of the Be/X-ray binaries confirm that in most cases not morethan 1 to 2 \msun\ was ejected in the supernovae that produced theirneutron stars. This, in combinatioThe values given here are not identical (though similar) to those listedin Chevalier & Ilovaisky (1998). The corrections we applied areoutlined below.
| CO and C_2 Absorption toward W40 IRS 1A
The H II region W40 harbors a small group of young, hot stars behindroughly 9 mag of visual extinction. We have detected gaseous carbonmonoxide (CO) and diatomic carbon (C_2) in absorption toward the starW40 IRS 1a. The J=2-0 R0, R1, and R2 lines of ^12CO at 2.3 μm weremeasured using the CSHELL on the NASA Infrared Telescope Facility (withupper limits placed on R3, R4, and R5) yielding an N_CO of(1.1+/-0.2)x10^18 cm^-2. Excitation analysis indicates T_kin>7 K. ThePhillips system of C_2 transitions near 8775 Å was measured usingthe Kitt Peak 4 m telescope and echelle spectrometer. Radiative pumpingmodels indicate a total C_2 column density of (7.0+/-0.4)x10^14 cm^-2,two excitation temperatures (39 and 126 K), and a total gas density ofn~250 cm^-3. The CO ice band at 4.7 μm was not detected, placing anupper limit on the CO depletion of delta<1%. We postulate that thesight line has multiple translucent components and is associated withthe W40 molecular cloud. Our data for W40 IRS 1a, coupled with othersight lines, shows that the ratio of CO/C_2 increases from diffusethrough translucent environs. Finally, we show that the hydrogen-to-dustratio seems to remain constant from diffuse to dense environments,whereas the CO-to-dust ratio apparently does not.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Dél Keresztje |
| Rektaszcenzió: | 12h42m50.30s |
| Deklináció: | -63°03'31.0" |
| Vizuális fényesség: | 5.31 |
| Távolság: | 301.205 parszek |
| RA sajátmozgás: | -12 |
| Dec sajátmozgás: | -3.1 |
| B-T magnitude: | 5.528 |
| V-T magnitude: | 5.332 |
Katalógusok és elnevezések:
|
INHABITED SKY
