Home     To Survive in the Universe    
Services
    Why to Inhabit     Top Contributors     Astro Photo     The Collection     Forum     Blog New!     FAQ     Login  

HD 151804


Contents

Images

Upload your image

DSS Images   Other Images


Related articles

The Optical Counterpart to the Peculiar X-Ray Transient XTE J1739-302
The weak X-ray transient XTE J1739-302, characterized by extremely shortoutbursts, has recently been identified with a reddened star. Here wepresent spectroscopy and photometry of the counterpart, identifying itas a O8 Iab(f) supergiant at a distance of ~2.3 kpc. XTE J1739-302 thusbecomes the prototype of the new class of supergiant fast X-raytransients (SFXTs). The optical and infrared spectra of the counterpartto XTE J1739-302 do not reveal any obvious characteristics setting itapart from other X-ray binaries with supergiant companions, whichdisplay a very different type of X-ray light curve.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO 73.D-0081).

Isolated, Massive Supergiants near the Galactic Center
We have carried out a pilot project to assess the feasibility of usingradio, infrared, and X-ray emission to identify young, massive starslocated between 1 and 25 pc from the Galactic center. We first comparedcatalogs compiled from the VLA, Chandra, and 2MASS. We identified twomassive, young stars: the previously identified star that is associatedwith the radio H II region H2 and a newly identified star that we referto as CXOGC J174516.1-290315. The infrared spectra of both stars exhibitvery strong Brγ and He I lines and resemble those of massivesupergiants that have evolved off of the main sequence but not yetreached the Wolf-Rayet phase. We estimate that each star has abolometric luminosity >~106 Lsolar. These twostars are also associated with bright mid-infrared sources from the MSXsurvey, although the origin of this emission is uncertain. Likewise, thedetection of these two sources in X-rays is surprising because stars atsimilar evolutionary states are not uniformly bright X-ray sources.Therefore, we suggest that both stars are in binary systems that containeither OB stars whose winds collide with those of the luminoussupergiants or compact objects that are accreting from the winds of thesupergiants. We also identify X-ray emission from a nitrogen-typeWolf-Rayet star and place upper limits on the X-ray luminosities ofthree more evolved, massive stars that previously have been identifiedbetween 1 and 25 pc from Sgr A*. Finally, we briefly discuss theimplications that future searches for young stars will have for ourunderstanding of the recent history of star formation near the Galacticcenter.

The Discordance of Mass-Loss Estimates for Galactic O-Type Stars
We have determined accurate values of the product of the mass-loss rateand the ion fraction of P+4, M˙q(P+4), for asample of 40 Galactic O-type stars by fitting stellar wind profiles toobservations of the P V resonance doublet obtained with FUSE, ORFEUSBEFS, and Copernicus. When P+4 is the dominant ion in thewind [i.e., 0.5<~q(P+4)<=1], M˙q(P+4)approximates the mass-loss rate to within a factor of <~2. Theorypredicts that P+4 is the dominant ion in the winds of O7-O9.7stars, although an empirical estimator suggests that the range O4-O7 maybe more appropriate. However, we find that the mass-loss rates obtainedfrom P V wind profiles are systematically smaller than those obtainedfrom fits to Hα emission profiles or radio free-free emission bymedian factors of ~130 (if P+4 is dominant between O7 andO9.7) or ~20 (if P+4 is dominant between O4 and O7). Thesediscordant measurements can be reconciled if the winds of O stars in therelevant temperature range are strongly clumped on small spatial scales.We use a simplified two-component model to investigate the volumefilling factors of the denser regions. This clumping implies thatmass-loss rates determined from ``ρ2'' diagnostics havebeen systematically overestimated by factors of 10 or more, at least fora subset of O stars. Reductions in the mass-loss rates of this size haveimportant implications for the evolution of massive stars andquantitative estimates of the feedback that hot-star winds provide totheir interstellar environments.

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.

Near-infrared identification of the counterpart to X1908+075: a new OB-supergiant X-ray binary
We report the near-infrared (near-IR) identification of the likelycounterpart to X1908+075, a highly absorbed Galactic X-ray sourcerecently suspected to belong to the rare class of OB supergiant-neutronstar binary systems. Our JHKs-band imaging of the fieldreveals the existence within the X-ray error boxes of a near-IR sourceconsistent with an early-type star lying at d~ 7 kpc and sufferingAV~ 16 mag of extinction, the latter value being in goodagreement with the hydrogen column density derived from modelling of theX-ray spectrum. Our follow-up, near-IR spectroscopic observationsconfirm the nature of this candidate and lead to a late O-typesupergiant classification, thereby supporting the identification of anew Galactic OB-supergiant X-ray binary.

Anomalous dust-to-gas ratios in the Galaxy
Lines of sight with E(B-V)/N(HI) considerably smaller than the averagevalue for the solar neighbourhood have been selected from the catalogueof Diplas & Savage. In order to develop quantitative considerations,estimates of the molecular hydrogen column density were obtained usingthe relation of Savage et al. extended at E(B-V) > 0.4 with therecent data of Rachford et al. Contrary to the prevailing opinion in theliterature for sightlines with similar behaviour, we found that only 22per cent of our sample was characterized by both an average gas densitylarger than 1 cm-3 and a value of RV larger thanthat in the diffuse interstellar medium. By computing extinction models,we were able to fit the E(B-V)/N(HI) by changing the value ofRV only for some sightlines. For the remaining ones, aρd/ρH ratio different from the averageGalactic value must be invoked. The application of the Kramers-Kronigrelation to the observed extinction curves confirmed this possibility.Moreover, attempts to fit such curves with models having grain volumescorresponding to the standard ρd/ρH ratiofailed.We find a linear relation between ρd/ρHand E(B-V)/N(H) for our sightlines. The average Galactic value marks theseparation into two groups characterized by lower abundances of C and Sitrapped into the grains when E(B-V)/N(H) is smaller than the Galacticvalue, and by larger abundances when E(B-V)/N(H) is greater.

Large-scale wind structures in OB supergiants: a search for rotationally modulated Hα variability
We present the results of a long-term monitoring campaign of theHα line in a sample of bright OB supergiants (O7.5-B9) which aimsat detecting rotationally modulated changes potentially related to theexistence of large-scale wind structures. A total of 22 objects weremonitored during 36 nights spread over six months in 2001-2002.Coordinated broad-band photometric observations were also obtained forsome targets. Conspicuous evidence for variability in Hα is foundfor the stars displaying a feature contaminated by wind emission. Mostchanges take place on a daily time-scale, although hourly variations arealso occasionally detected. Convincing evidence for a cyclical patternof variability in Hα has been found in two stars: HD 14134 and HD42087. Periodic signals are also detected in other stars, butindependent confirmation is required. Rotational modulation is suggestedfrom the similarity between the observed recurrence time-scales (in therange 13-25 d) and estimated periods of stellar rotation. We callattention to the atypical case of HD 14134, which exhibits a clear12.8-d periodicity, both in the photometric and in the spectroscopicdata sets. This places this object among a handful of early-type starswhere one may observe a clear link between extended wind structures andphotospheric disturbances. Further modelling may test the hypothesisthat azimuthally-extended wind streams are responsible for the patternsof spectral variability in our target stars.

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.

Metallicity in the Galactic Center: The Arches Cluster
We present a quantitative spectral analysis of five very massive starsin the Arches cluster, located near the Galactic center, to determinestellar parameters, stellar wind properties, and, most importantly,metallicity content. The analysis uses a new technique, presented herefor the first time, and uses line-blanketed non-LTE wind/atmospheremodels fitted to high-resolution near-infrared spectra of late-typenitrogen-rich Wolf-Rayet stars and OfI+ stars in the cluster.It relies on the fact that massive stars reach a maximum nitrogenabundance that is related to initial metallicity when they are in theWNL phase. We determine the present-day nitrogen abundance of the WNLstars in the Arches cluster to be 1.6% (mass fraction) and constrain thestellar metallicity in the cluster to be solar. This result is invariantto assumptions about the mass-luminosity relationship, the mass-lossrates, and rotation speeds. In addition, from this analysis, we find theage of the Arches cluster to be 2-2.5 Myr, assuming coeval formation.

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.

NLTE models of line-driven stellar winds. I. Method of calculation and first results for O stars
New numerical models of line-driven stellar winds of late O stars arepresented. Statistical equilibrium (NLTE) equations of the most abundantelements are solved. Properly obtained occupation numbers are used tocalculate consistent radiative force and radiative heating terms. Winddensity, velocity and temperature are calculated as a solution of modelhydrodynamical equations. Contrary to other published models we accountfor a multicomponent wind nature and do not simplify the calculation ofthe radiative force (e.g. using force multipliers). We discuss theconvergence behaviour of our models. The ability of our models topredict correct values of mass-loss rates and terminal velocities ofselected late O stars (mainly giants and supergiants) is demonstrated.The systematic difference between predicted and observed terminalvelocities reported in the literature has been removed. Moreover, wefound good agreement between the theoretical wind momentum-luminosityrelationship and the observed one for Cyg OB2 supergiants.Appendices A, B and C are only available in electronic form athttp://www.edpsciences.org

Observational studies of wind and photospheric variability in three early-type stars
Not Available

Toward an adequate method to isolate spectroscopic families of diffuse interstellar bands
We divide some of the observed diffuse interstellar bands (DIBs) intofamilies that appear to have the spectral structure of single species.Three different methods are applied to separate such families, exploringthe best approach for future investigations of this type. Starting witha statistical treatment of the data, we found that statistical methodsby themselves give insufficient results. Two other methods of dataanalysis (`averaging equivalent widths' and `investigating the figureswith arranged spectrograms') were found to be more useful as tools forfinding the spectroscopic families of DIBs. On the basis of thesemethods, we suggest some candidates as `relatives' of 5780- and5797-Å bands.

Radio continuum observations of massive stars in open cluster NGC 6231 and the Sco OB1 association
We present results of the Australia Telescope Compact Array (ATCA) radiocontinuum observations of massive stars in the Sco OB1 association. Most stars detected in the program show spectral indices lower thanvalue expected from thermal free-free emission.

High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.

The 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.

Massive Stars in the Arches Cluster
We present and use new spectra and narrowband images, along withpreviously published broadband images, of stars in the Arches cluster toextract photometry, astrometry, equivalent width, and velocityinformation. The data are interpreted with a wind/atmosphere code todetermine stellar temperatures, luminosities, mass-loss rates, andabundances. We have doubled the number of known emission-line stars, andwe have also made the first spectroscopic identification of the mainsequence for any population in the Galactic center. We conclude that themost massive stars are bona fide Wolf-Rayet (W-R) stars; are some of themost massive stars known, havingMinit>100Msolar and have prodigious winds,M>10-5 Msolar yr-1, that areenriched with helium and nitrogen; with these identifications, theArches cluster contains about 5% of all known W-R stars in the Galaxy.We find an upper limit to the velocity dispersion of 22 kms-1, implying an upper limit to the cluster mass of7×104 Msolar within a radius of 0.23 pc; wealso estimate the bulk heliocentric velocity of the cluster to bevcluster,solar~+95 km s-1. Taken together, theseresults suggest that the Arches cluster was formed in a short, butmassive, burst of star formation about 2.5+/-0.5 Myr ago, from amolecular cloud that is no longer present. The cluster happens to beapproaching and ionizing the surface of a background molecular cloud,thus producing the thermal arched filaments. We estimate that thecluster produces 4×1051 ionizing photonss-1, more than enough to account for the observed thermalradio flux from the nearby cloud, 3×1049 ionizingphotons s-1. Commensurately, it produces 107.8Lsolar in total luminosity, providing the heating source forthe nearby molecular cloud, Lcloud~107Lsolar. These interactions between a cluster of hot stars anda wayward molecular cloud are similar to those seen in the``Quintuplet/Sickle'' region. The small spread of formation times forthe known young clusters in the Galactic center and the relative lack ofintermediate-age stars(τage=107.0-107.3 yr) suggest thatthe Galactic center has recently been host to a burst of star formation.Finally, we have made new identifications of near-infrared sources thatare counterparts to recently identified X-ray and radio sources. Basedon observations with the NASA/ESA Hubble Space Telescope, obtained atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.

An explanation for the curious mass loss history of massive stars: From OB stars, through Luminous Blue Variables to Wolf-Rayet stars
The stellar winds of massive stars show large changes in mass-loss ratesand terminal velocities during their evolution from O-star through theLuminous Blue Variable phase to the Wolf-Rayet phase. The luminosityremains approximately unchanged during these phases. These large changesin wind properties are explained in the context of the radiation drivenwind theory, of which we consider four different models. They are due tothe evolutionary changes in radius, gravity and surface composition andto the change from optically thin (in continuum) line driven winds tooptically thick radiation driven winds.

In hot pursuit of the hidden companion of eta Carinae: An X-ray determination of the wind parameters
We present X-ray spectral fits to a recently obtained Chandra gratingspectrum of eta Carinae, one of the most massive and powerful stars inthe Galaxy and which is strongly suspected to be a colliding wind binarysystem. Hydrodynamic models of colliding winds are used to generatesynthetic X-ray spectra for a range of mass-loss rates and windvelocities. They are then fitted against newly acquired Chandra gratingdata. We find that due to the low velocity of the primary wind (~500kmps), most of the observed X-ray emission appears to arise from theshocked wind of the companion star. We use the duration of thelightcurve minimum to fix the wind momentum ratio at eta = 0.2. We arethen able to obtain a good fit to the data by varying the mass-loss rateof the companion and the terminal velocity of its wind. We find that{dot M}2 ~ 10-5 Msunyr-1 andvinfty_2 ~ 3000 kmps. With observationally determined valuesof ~500-700 kmps for the velocity of the primary wind, our fit implies aprimary mass-loss rate of {dot M}1 ~ 2.5 x 10-4Msunyr-1. This value is smaller than commonlyinferred, although we note that a lower mass-loss rate can reduce someof the problems noted by Hillier et al. (\cite{HDIG2001}) when a valueas high as 10-3 Msunyr-1 is used. Thewind parameters of the companion are indicative of a massive star whichmay or may not be evolved. The line strengths appear to show slightlysub-solar abundances, although this needs further confirmation. Based onthe over-estimation of the X-ray line strengths in our model, andre-interpretation of the HST/FOS results, it appears that the Homunculusnebula was produced by the primary star.

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.

Stellar populations in Seyfert 2 galaxies. I. Atlas of near-UV spectra
We have carried out a uniform spectroscopic survey of Seyfert 2 galaxiesto study the stellar populations of the host galaxies. New spectra havebeen obtained for 79 Southern galaxies classified as Seyfert 2 galaxies,7 normal galaxies, and 73 stars at a resolution of 2.2 Å over thewavelength region 3500-5300 Å. Cross-correlation between thestellar spectra is performed to group the individual observations into44 synthesis standard spectra. The standard groups include a solarabundance sequence of spectral types from O5 to M3 for dwarfs, giants,and supergiants. Metal-rich and metal-weak F-K giants and dwarfs arealso included. A comparison of the stellar data with previouslypublished spectra is performed both with the individual spectra and thestandard groups. For each galaxy, two distinct spatial regions areconsidered: the nucleus and the external bulge. Spectroscopic variationsfrom one galaxy to another and from the central to the external regionare briefly discussed. It is found that the central region of a Seyfert2 galaxy, after subtracting the bulge stellar population, always shows anear-UV spectrum similar to one of three representative categories: a)many strong emission lines and only two visible absorption lines (Ca IiK and G band) (Sey2e); b) few emission lines, many absorption lines, anda redder continuum than the previous category (Sey2a); c) an almost flatcontinuum and high-order Balmer lines seen in absorption (Sey2b). Theproportion of Seyfert 2 galaxies belonging to each class is found to be22%, 28%, and 50% respectively. We find no significative differencesbetween morphology distributions of Seyfert 2 galaxies with Balmer linesdetected in absorption and the rest of the sample. This quick lookthrough the atlas indicates that half of Seyfert 2 galaxies harbour ayoung stellar population (about or less than 100 Myr) in their centralregion, clearly unveiled by the high order Balmer series seen inabsorption. Based on observations collected at the European SouthernObservatory, Chile (ESO 65.P-0014(A)). Tables 1-3 and 8 and Fig. A.1(Appendix A) are only available in electronic form athttp://www.edpsciences.org

Multicomponent radiatively driven stellar winds. II. Gayley-Owocki heating in multitemperature winds of OB stars
We show that the so-called Gayley-Owocki (Doppler) heating is importantfor the temperature structure of the wind of main sequence stars coolerthan the spectral type O6. The formula for Gayley-Owocki heating isderived directly from the Boltzmann equation as a direct consequence ofthe dependence of the driving force on the velocity gradient. SinceGayley-Owocki heating deposits heat directly on the absorbing ions, wealso investigated the possibility that individual components of theradiatively driven stellar wind have different temperatures. This effectis negligible in the wind of O stars, whereas a significant temperaturedifference takes place in the winds of main sequence B stars for starscooler than B2. Typical temperature differences between absorbing ionsand other flow components for such stars is of the order 103K. However, in the case when the passive component falls back onto thestar, the absorbing component reaches temperatures of order106 K, which allows for emission of X-rays. Moreover, wecompare our computed terminal velocities with the observed ones. Wefound quite good agreement between predicted and observed terminalvelocities. The systematic difference coming from the using of the socalled ``cooking formula'' has been removed.

What is the real nature of HD 108?
Since the beginning of the past century, the nature of HD 108 has been asubject of intense debate. One after another, astronomers explored itsvariability and attributed it either to binarity, or to changes in thestellar wind of a single star. In this article, we analyse a 30 yearcampaign of spectroscopic observations of this star with specialemphasis on the last 15 years during which photographic plates have beenreplaced by CCD detectors. Our investigation of the radial velocities ofHD 108 yields no significant short- or long-term period and does notconfirm the published periodicities either. Though the radial velocityof HD 108 appears clearly variable, the variations cannot be explainedby the orbital motion in a spectroscopic binary. However, our datareveal spectacular changes in the H I Balmer lines and some He Iprofiles over the years. These lines continuously evolved from P Cygniprofiles to ``pure'' absorption lines. A similar behaviour has alreadybeen observed in the past, suggesting that these changes are recurrent.HD 108 seems to share several characteristics of Oe stars and we discussdifferent hypotheses for the origin of the observed long-termvariations. As we are now in a transition period, a continuousmonitoring of HD 108 should be considered for the next few years. Basedon observations collected at the Observatoire de Haute-Provence(France).

The origin of the runaway high-mass X-ray binary HD 153919/4U1700-37
Based on its Hipparcos proper motion, we propose that the high-massX-ray binary HD 153919/4U1700-37 originates in the OB association ScoOB1. At a distance of 1.9 kpc the space velocity of 4U1700-37 withrespect to Sco OB1 is 75 km s-1. This runaway velocityindicates that the progenitor of the compact X-ray source lost about 7Msun during the (assumed symmetric) supernova explosion. Thesystem's kinematical age is about 2 +/- 0.5 million years which marksthe date of the supernova explosion forming the compact object. Thepresent age of Sco OB1 is la 8 Myr; its suggested core, NGC 6231, seemsto be somewhat younger ( ~ 5 Myr). If HD 153919/4U1700-37 was born as amember of Sco OB1, this implies that the initially most massive star inthe system terminated its evolution within la 6 million years,corresponding to an initial mass ga 30 Msun. With theseparameters the evolution of the binary system can be constrained. Basedon data obtained with ESA's astrometric satellite Hipparcos.

Extended optical spectroscopic monitoring of wind structure in HD 152408
New perspectives are provided on significant spatial structure andtemporal variability in the near-star wind regions (i.e. < 3 R_star )of the massive luminous star HD 152408 (classified as O8:Iafpe orWN9ha). This study is primarily based on the analysis of high-qualityéchelle spectra secured during 21 nights between 1999 July toAugust, using the Landessternwarte-developed (fibre-fed) FEROSinstrument on the ESO 1.52-m telescope. These extended time-series data,with a total simultaneous wavelength coverage of lambda lambda3600 -9200Å, were exploited to monitor absorption and emission fluctuations(of ~ 5-10% of the line flux) in several He i and Balmer lines, togetherwith more deep-seated (near-photosphere) disturbances in weaker metallicemission and absorption lines. Organised large-scale wind structure inHD 152408 is principally betrayed by sequential episodes of discreteabsorption and emission features, which migrate from near zero velocityto almost the wind terminal velocity. This evolution is extremely slow,however, typically spanning ~ 4 days for an individual episode. Wedemonstrate that the blue-shifted sorption episodes in He i are veryclosely mirrored (in velocity and time) by absorption features (i.e.reduced not enhanced flux) in the blue wings of the mainly recombinationformed broad Hα emission line. The implication is that there isdetailed balancing between ground state photoionization andrecombination in the substantially optically thick Balmer lines.Surprisingly, the velocity behaviour of the red-ward and blue-wardmigrating features is highly asymmetric, such that the mean accelerationof the former is less than 50% of the latter. Fourier analysis reveals amodulation time-scale for the wind activity of ~ 7.7 days, plus itsharmonic at 3.9 days. The longer period is ~ 28 times greater than thecharacteristic radial wind flow time of HD 152408. We also detect a ~1.5 day periodic variation in the radial velocity of the weak C ivlambda lambda 5801, 5812 absorption lines, which are the closestapproximation to ``pure'' photospheric lines in the optical spectrum ofHD 152408. The wind-formed optical lines of HD 152408 are also affectedby fluctuations in the central peak emission, particularly evident inHα where the equivalent width may vary by up to 20%. Data securedbetween 1995 and 1999 reveal, however, that the epoch-to-epoch meanprofiles are remarkably similar. Non-LTE steady-state stellar atmospheremodels are used to synthesis profiles to match representative Hαand He i lambda 5876 line profiles. Only a slow (tailored) velocity law(compared to beta =1) provides a good match to the Hα emissionpeak and wings, but the models predict excess He i absorption. Theobserved extreme Hα emission variations can be reproduced by thesynthetic profiles with an implied +/-10% variation in mass-loss rate.The results on optical line profile variability in HD 152408 arediscussed in the context of models for co-rotating interaction regions(CIRs) in the wind. Several constraints are provided that argue againstsimple velocity fields in such streams, including (i) the slowacceleration of features to high velocities, within ~ 3 R_star , (ii)the strong asymmetry in projected acceleration of the approaching andreceding stream material, (iii) Balmer line absorption effects in theapproaching material, (iv) the rise of localised features from very lowvelocities, and (v) the stability of the large-scale CIRs againstturbulent small-scale wind structure. We suggest that it may be worthexploring hydrodynamic simulations of CIRs that incorporate differentvelocity fields on the leading (faster accelerating; blue-wardabsorption) and trailing (slower accelerating; red-ward emission) edgesof the spiral structures. Based on observations collected at theEuropean Southern Observatory, Chile (ESO 63.H--0080(A)).

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 OB Zoo: A Digital Atlas of Peculiar Spectra
A digital atlas of 20 high-luminosity, peculiar OB spectra in the3800-4900 Å range is presented. The atlas is organized anddiscussed in terms of the following four categories: WN-A or WNL stars,OB Iape or very late WN (WNVL) stars, iron stars, and B-supergiantluminous blue variables (LBVs). Several objects in the earliercategories are also active or quiescent LBVs. Some (but not all) ofthese objects have been well studied, and extensive references areprovided, as are comprehensive spectral-line identifications. Severalnew morphological relationships among the objects have been recognizedthrough this presentation. In particular, attention is drawn to theoccurrence of spatial pairing between nearly identical, unusual spectra,which may have implications for a particular mode of massive-starformation. This small sample includes one or both members of at leastfive such pairs. Physical explanations of these peculiar, likelytransitional spectra and the relationships among them are essential fora complete understanding of massive stellar evolution.

WR 146 - observing the OB-type companion
We present new radio and optical observations of the colliding-windsystem WR146 aimed at understanding the nature of the companion to theWolf-Rayet (WR) star and the collision of their winds. The radioobservations reveal emission from three components: the WR stellar wind,the non-thermal wind-wind interaction region and, for the first time,the stellar wind of the OB companion. This provides the uniquepossibility of determining the mass-loss rate and terminal wind velocityratios of the two winds, independent of distance. Respectively, theseratios are 0.20+/-0.06 and 0.56+/-0.17 for the OB-companion starrelative to the WR star. A new optical spectrum indicates that thesystem is more luminous than had been believed previously. We deducethat the `companion' cannot be a single, low-luminosity O8 star assuggested previously, but is either a high-luminosity O8 star, orpossibly an O8+WC binary system.

Quantitative analysis of WC stars: constraints on neon abundances from ISO-SWS spectroscopy
Neon abundances are derived in four Galactic WC stars -γ2Vel (WR11, WC8+O7.5III), HD156385 (WR90, WC7),HD192103 (WR135, WC8) and WR146 (WC5+O8) - using mid-infraredfine-structure lines obtained with ISO-SWS. Stellar parameters for eachstar are derived using the non-local thermodynamic equilibrium modelatmospheric code of Hillier & Miller, together with ultraviolet(IUE), optical (INT, AAT) and infrared (UKIRT, ISO) spectroscopy. In thecase of γ2Vel, we adopt very recent results from DeMarco et al., who followed an identical approach. ISO-SWS data setsreveal the [Neiii] 15.5-μm line in each of our targets, while [Neii]12.8μm, [Siv] 10.5μm and [Siii] 18.7μm are observed solely inγ2Vel. Using a method updated from Barlow et al. toaccount for clumped winds, we derive Ne/He=(3-4)×10-3by number, plus S/He=6×10-5 for γ2Vel.Neon is highly enriched, such that Ne/S in γ2Vel iseight times higher than cosmic values. However, observed Ne/He ratiosare a factor of 2 lower than predictions of current evolutionary modelsof massive stars. An imprecise mass loss and distance were responsiblefor the much greater discrepancy in neon content identified by Barlow etal. Our sample of WC5-8 stars span a narrow range in T* (=55-71kK), withno trend towards higher temperature at earlier spectral type, supportingearlier results for a larger sample by Koesterke & Hamann. Stellarluminosities range from 100000 to 500000Lsolar, while10-5.1 <= M/(Msolar yr-1) <=10-4.5, adopting clumped winds, in which volume fillingfactors are 10per cent. In all cases, wind performance numbers are lessthan 10, significantly lower than recent estimates. Carbon abundancesspan 0.08<=C/He<=0.25 by number, while oxygen abundances remainpoorly constrained.

Submit a new article


Related links

  • - No Links Found -
Submit a new link


Member of following groups:


Observation and Astrometry data

Constellation:Scorpius
Right ascension:16h51m33.70s
Declination:-41°13'50.0"
Apparent magnitude:5.22
Distance:2083.333 parsecs
Proper motion RA:-0.3
Proper motion Dec:0.3
B-T magnitude:5.29
V-T magnitude:5.249

Catalogs and designations:
Proper Names
HD 1989HD 151804
TYCHO-2 2000TYC 7872-2284-1
USNO-A2.0USNO-A2 0450-25798697
BSC 1991HR 6245
HIPHIP 82493

→ Request more catalogs and designations from VizieR