Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
Variations in D/H and D/O from New Far Ultraviolet Spectroscopic Explorer Observations
We use data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to determine the interstellar abundances of D I, N I, O I, Fe II,and H2 along the sight lines to WD 1034+001, BD +39 3226, andTD1 32709. Our main focus is on determining the D/H, N/H, O/H, and D/Oratios along these sight lines, with logN(H)>20.0, that probe gaswell outside of the Local Bubble. Hubble Space Telescope (HST) andInternational Ultraviolet Explorer (IUE) archival data are used todetermine the H I column densities along the WD 1034+001 and TD1 32709sight lines, respectively. For BD +39 3226, a previously published N(HI) is used. We find(D/H)×105=2.14+0.53-0.45,1.17+0.31-0.25, and1.86+0.53-0.43 and(D/O)×102=6.31+1.79-1.38,5.62+1.61-1.31, and7.59+2.17-1.76 for the WD 1034+001, BD +39 3226,and TD1 32709 sight lines, respectively (all 1 σ). The scatter inthese three D/H ratios exemplifies the scatter that has been found byother authors for sight lines with column densities in the range19.2
| A Rich Population of X-Ray-emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1
Recent optical and infrared studies have revealed that the heavilyreddened starburst cluster Westerlund 1 (Wd 1) contains at least 22Wolf-Rayet (W-R) stars, constituting the richest W-R population of anyGalactic cluster. We present results of a sensitive Chandra X-rayobservation of Wd 1 that detected 12 of the 22 known W-R stars and themysterious emission-line star W9. The fraction of detected WN stars isnearly identical to that of WC stars. The WN stars WR-A and WR-B, aswell as W9, are exceptionally luminous in X-rays and have similar hard,heavily absorbed X-ray spectra with strong Si XIII and S XV emissionlines. The luminous high-temperature X-ray emission of these three starsis characteristic of colliding-wind binary systems, but their binarystatus remains to be determined. Spectral fits of the X-ray-brightsources WR-A and W9 with isothermal plane-parallel shock models requirehigh absorption column densities, log NH=22.56(cm-2), and yield characteristic shock temperatureskTs~3 keV (Ts~35 MK).
| An Ultraviolet to Mid-Infrared Study of the Physical and Wind Properties of HD 164270 (WC9) and Comparison to BD +30 3639 ([WC9])
We present new Spitzer IRS observations of HD 164270 (WC9, WR103). Aquantitative analysis of the UV, optical, near-, and mid-IR spectra ofHD 164270 is presented, allowing for line blanketing and wind clumping,revealing T*~48 kK, logL/Lsolar~4.9, andM˙~10-5 Msolar yr-1 for a volumefilling factor of f~0.1. Our models predict that He is partiallyrecombined in the outer stellar wind, such that recent radio-derivedmass-loss rates of WC9 stars have been underestimated. We obtainC/He~0.2 and O/He~0.01 by number from optical diagnostics. Mid-IRfine-structure lines of [Ne II] 12.8 μm and [S III] 18.7 μm areobserved, with [Ne III] 15.5 μm and [S IV] 10.5 μm absent. Fromthese we obtain Ne/He~Ne+/He=2.2×10-3 bynumber, 7 times higher than the solar value (as recently derived byAsplund et al.), and S/He~S2+/He=5.1×10-5 bynumber. From a comparison with similar results for other WC subtypes weconclude that WC9 stars are as chemically advanced as earlier subtypes.We consider why late WC stars are exclusively observed inhigh-metallicity environments. In addition, we compare theUV/optical/mid-IR spectroscopic morphology of HD 164270 with theplanetary nebula central star BD +30 3639 ([WC9]). Their UV and opticalsignatures are remarkably similar, such that our quantitativecomparisons confirm similarities in stellar temperature, wind densities,and chemistry first proposed by Smith & Aller, in spite ofcompletely different evolutionary histories, with HD 164270 presently afactor of 10 more massive than BD +30 3639. At mid-IR wavelengths, thedust from the dense young nebula of BD +30 3639 completely dominates itsappearance, in contrast with HD 164270.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| Probing the wind-wind collision in γ2 Velorum with high-resolution Chandra X-ray spectroscopy: evidence for sudden radiative braking and non-equilibrium ionization
We present a new analysis of an archived Chandra HETGS X-ray spectrum ofthe WR+O colliding wind binary γ2 Velorum. The spectrumis dominated by emission lines from astrophysically abundant elements:Ne, Mg, Si, S and Fe. From a combination of broad-band spectral analysisand an analysis of line flux ratios we infer a wide range oftemperatures in the X-ray-emitting plasma (~4-40 MK). As in thepreviously published analysis, we find the X-ray emission lines areessentially unshifted, with a mean FWHM of 1240 +/- 30 kms-1. Calculations of line profiles based on hydrodynamicalsimulations of the wind-wind collision predict lines that areblueshifted by a few hundred km s-1. The lack of any observedshift in the lines may be evidence of a large shock-cone openinghalf-angle (>85°), and we suggest this may be evidence of suddenradiative braking. From the R and G ratios measured from He-likeforbidden-intercombination-resonance triplets we find evidence that theMgXI emission originates from hotter gas closer to the O star than theSiXIII emission, which suggests that non-equilibrium ionization may bepresent.
| γ-rays from cascades in close massive binaries containing energetic pulsars
Some massive binaries should contain energetic pulsars which injectrelativistic leptons from their inner magnetospheres and/or pulsar windregions. If the binary system is compact enough, then these leptons caninitiate inverse Compton (IC) e+/- pair cascades in theanisotropic radiation field of a massive star. γ-rays can beproduced in the IC cascade during its development in a pulsar windregion and above a shock in a massive star wind region where thepropagation of leptons is determined by the structure of a magneticfield around the massive star. For a binary system with specificparameters, we calculate phase-dependent spectra and fluxes ofγ-rays escaping as a function of the inclination angle of thesystem and for different assumptions on injection conditions of theprimary leptons (their initial spectra and location of the shock insidethe binary). We conclude that the features of γ-ray emission fromsuch massive binaries containing energetic pulsars should allow us toobtain important information on the acceleration of particles by thepulsars, and on interactions of a compact object with the massive starwind. Predicted γ-ray light curves and spectra in the GeV and TeVenergy ranges from such binary systems within our Galaxy and MagellanicClouds should be observed by future AGILE and GLAST satellites andlow-threshold Cherenkov telescopes, such as MAGIC, HESS, VERITAS orCANGAROO III.
| FUSE Determination of a Low Deuterium Abundance along an Extended Sight Line in the Galactic Disk
We present a study of the deuterium abundance along the extended sightline toward HD 90087 with the Far Ultraviolet Spectroscopic Explorer(FUSE). HD 90087 is a O9.5 III star located in the Galactic disk at adistance of ~2.7 kpc away from the Sun. Both in terms of distance andcolumn densities, HD 90087 has the longest and densest sight lineobserved in the Galactic disk for which a deuterium abundance has beenmeasured from ultraviolet absorption lines so far. Because manyinterstellar clouds are probed along this sight line, possiblevariations in the properties of individual clouds should be averagedout. This would yield a deuterium abundance that is characteristic ofthe interstellar medium on scales larger than the Local Bubble. The FUSEspectra of HD 90087 show numerous blended interstellar and stellarfeatures. We have measured interstellar column densities of neutralatoms, ions, and molecules by simultaneously fitting the interstellarabsorption lines detected in the different FUSE channels. As far aspossible, saturated lines were excluded from the fits in order tominimize possible systematic errors. IUE (International UltravioletExplorer) archival data are also used to measure neutral hydrogen. Wereport D/O=(1.7+/-0.7)×10-2 andD/H=(9.8+/-3.8)×10-6 (2 σ). Our new resultsconfirm that the gas-phase deuterium abundance in the distantinterstellar medium is significantly lower than the one measured withinthe Local Bubble. We supplement our study with a revision of the oxygenabundance toward Feige 110, a moderately distant (~200 pc) sdOB star,located ~150 pc below the Galactic plane. Excluding saturated lines fromthe fits of the FUSE spectra is critical; this led us to derive an O Icolumn density about 2 times larger than the one previously reported forFeige 110. The corresponding updated D/O ratio on this sight line isD/O=(2.6+/-1.0)×10-2 (2 σ), which is lower thanthe one measured within the Local Bubble. The data set available nowoutside the Local Bubble, which is based primarily on FUSE measurements,shows a contrast between the constancy of D/O and the variability ofD/H. As oxygen is considered to be a good proxy for hydrogen within theinterstellar medium, this discrepancy is puzzling.
| Bulk Velocities, Chemical Composition, and Ionization Structure of the X-Ray Shocks in WR 140 near Periastron as Revealed by the Chandra Gratings
The Wolf-Rayet WC7+O4-5 binary WR 140 went through the periastronpassage of its 8 yr eccentric binary orbit in early 2001 as the twostars made their closest approach. Both stars have powerful supersonicstellar winds that crash into each other between the stars to produceX-rays. Chandra grating observations were made when the X-rays were attheir peak, making WR 140 the brightest hot-star X-ray source in the skyand giving the opportunity to study the velocity profiles of lines, allof which were resolved and blueshifted before periastron. In the generalcontext of shock physics, the measurements constrain the flow of hot gasand where different ions were made. The brightness of lines relative tothe strong continuum in conjunction with plasma models gives interimabundance estimates for eight different elements in WC-type materialincluding an Ne/S ratio in good agreement with earlier long-wavelengthmeasurements. The lower velocity widths of cool ions imply a plasma thatwas not in equilibrium, probably due to the collisionless nature of theshock transitions and the slow character of both the postshock energyexchange between ions and electrons and subsequent ionization. Electronheat conduction into fast-moving preshock gas was absent, probablysuppressed by the magnetic field involved in WR 140's synchrotronemission. After periastron, the spectrum was weaker due mainly toabsorption by cool Wolf-Rayet star material.
| The D/H Ratio toward PG 0038+199
We determine the D/H ratio in the interstellar medium toward the DOwhite dwarf PG 0038+199 using spectra from the Far UltravioletSpectroscopic Explorer (FUSE), with ground-based support from KeckHIRES. We employ curve-of-growth, apparent optical depth, andprofile-fitting techniques to measure the column densities and limits ofmany other species (H2, Na I, C I, C II, C III, N I, N II, OI, Si II, P II, S III, Ar I, and Fe II), which allows us to determinerelated ratios such as D/O, D/N, and the H2 fraction. Ourefforts are concentrated on measuring gas-phase D/H, which is key tounderstanding Galactic chemical evolution, and comparing it topredictions from big bang nucleosynthesis. We find column densitieslogN(HI)=20.41+/-0.08, logN(DI)=15.75+/-0.08, andlogN(H2)=19.33+/-0.04, yielding a molecular hydrogen fractionof 0.14+/-0.02 (2 σ errors), with an excitation temperature of143+/-5 K. The high H I column density implies that PG 0038+199 liesoutside of the Local Bubble; we estimate its distance to be297+164-104 pc (1 σ).[DI+HD]/[HI+2H2] toward PG 0038+199 is1.91+0.52-0.42×10-5 (2 σ).There is no evidence of component structure on the scale ofΔv>8 km s-1, based on Na I, but there is marginalevidence for structure on smaller scales. The D/H value is high comparedto the majority of recent D/H measurements but consistent with thevalues for two other measurements at similar distances. D/O is inagreement with other distant measurements. The scatter in D/H valuesbeyond ~100 pc remains a challenge for Galactic chemical evolution.This paper is dedicated in memory of Ervin J. Williger, father of thefirst author, who passed away on 2003 September 13. His enthusiasticsupport and encouragement were essential to its successful completion.Based on data from the Far Ultraviolet Spectroscopic Explorer and the W.M. Keck Observatory.
| The Neon Abundance of Galactic Wolf-Rayet Stars
The fast, dense winds that characterize Wolf-Rayet (W-R) stars obscuretheir underlying cores and complicate the verification of evolving coreand nucleosynthesis models. Core evolution can be probed by measuringabundances of wind-borne nuclear-processed elements, partiallyovercoming this limitation. Using ground-based mid-infrared spectroscopyand the 12.81 μm [Ne II] emission line measured in four Galactic W-Rstars, we estimate neon abundances and compare them to long-standingpredictions from evolved-core models. For the WC star WR 121, thisabundance is found to be >~11 times the cosmic value, in goodagreement with predictions. For the three less-evolved WN stars, littleneon enhancement above cosmic values is measured, as expected. Wediscuss the impact of clumping in W-R winds on this measurement and thepromise of using metal abundance ratios to eliminate sensitivity to winddensity and ionization structure.
| Evidence of Correlated Titanium and Deuterium Depletion in the Galactic Interstellar Medium
Current measurements indicate that the deuterium abundance in diffuseinterstellar gas varies spatially by a factor of ~4 among sight linesextending beyond the Local Bubble. One plausible explanation for thescatter is the variable depletion of D onto dust grains. To test thisscenario, we have obtained high signal-to-noise, high- resolutionprofiles of the refractory ion Ti II along seven Galactic sight lineswith D/H ranging from 0.65 to 2.1×10-5. Thesemeasurements, acquired with the recently upgraded Keck/HIRESspectrometer, indicate a correlation between Ti/H and D/H at the betterthan 95% confidence level Therefore, our observations support theinterpretation that D/H scatter is associated with differentialdepletion. We note, however, that Ti/H values taken from the literaturedo not uniformly show the correlation. Finally, we identify significantcomponent-to-component variations in the depletion levels amongindividual sight lines and discuss complications arising from thisbehavior.
| Proper Motions of the HH 47 Jet Observed with the Hubble Space Telescope
We present a proper-motion study of the shock waves within the classicstellar jet HH 47 based on Hubble Space Telescope (HST) Hα and [SII] images of the region taken over two epochs. Individual knots withinthe jet and in the bow shock/Mach disk working surface of HH 47A movesignificantly in the 5 yr that separate the images, and the excellentspatial resolution of HST makes it possible to measure the propermotions with enough precision to easily observe differential motionsthroughout the flow. The bright portion of the jet emerges at37.5d+/-2.5d from the plane of the sky with an average velocity of 300km s-1. Dynamical ages of the shock waves in the jet rangefrom a few decades for knots recently ejected by the source to ~1300 yrfor the faint extended bow shock HH 47D. The jet curves, but motions ofknots in the jet are directed radially away from the exciting source,and velocity variability in the flow drives the shock waves that heatthe jet internally. The jet orientation angle varies with time by about15° and currently points to the northwestern portion of a cavityoutlined by a reflection nebula, where a quasi-stationary shock deflectsthe jet. The major working surface HH 47A is more complex than a simplebow shock/Mach disk and contains numerous clumps that move relative toone another with velocities of ~+/-40 km s-1. Small clumps orinstabilities affect the Mach disk, and dense clumps may move all theway through the working surface to cause the bumpy morphology seen atthe bow shock. A localized area between the bow shock and Mach diskvaries significantly between the two sets of images.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
| Low-Mass Star Formation in the Gum Nebula: The CG 30/31/38 Complex
We present photometric and spectroscopic results for the low-masspre-main-sequence (PMS) stars with spectral types K-M in the cometaryglobule (CG) 30/31/38 complex. We obtained multiobject high-resolutionspectra for the targets selected as possible PMS stars frommultiwavelength photometry. We identified 11 PMS stars brighter thanV=16.5 with ages <~5 Myr at a distance of approximately 200 pc. Thespatial distribution of the PMS stars, CG clouds, and ionizing sources(O stars and supernova remnants) suggests a possible triggered origin ofthe star formation in this region. We confirm the youth of thephotometrically selected PMS stars using the lithium abundances. Theradial velocities of the low-mass PMS stars are consistent with those ofthe cometary globules. Most of the PMS stars show weak Hα emissionwith Wλ(Hα)<10 Å. Only one out of the11 PMS stars shows a moderate near-IR excess, which suggests a shortsurvival time (t<5 Myr) of circumstellar disks in this star-formingenvironment. In addition, we find five young late-type stars and one Aestar that have no obvious relation to the CG 30/31/38 complex. We alsodiscuss a possible scenario of the star formation history in the CG30/31/38 region.
| 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.
| Asphericity and clumpiness in the winds of Luminous Blue Variables
We present the first systematic spectropolarimetric study of LuminousBlue Variables (LBVs) in the Galaxy and the Magellanic Clouds, in orderto investigate the geometries of their winds. We find that at least halfof our sample show changes in polarization across the strong Hαemission line, indicating that the light from the stars is intrinsicallypolarized and therefore that asphericity already exists at the base ofthe wind. Multi-epoch spectropolarimetry on four targets revealsvariability in their intrinsic polarization. Three of these, AG Car, HRCar and P Cyg, show a position angle (PA) of polarization which appearsrandom with time. Such behaviour can be explained by the presence ofstrong wind-inhomogeneities, or “clumps” within the wind.Only one star, R 127, shows variability at a constant PA, and henceevidence for axi-symmetry as well as clumpiness. However, if viewed atlow inclination, and at limited temporal sampling, such a wind wouldproduce a seemingly random polarization of the type observed in theother three stars. Time-resolved spectropolarimetric monitoring of LBVsis therefore required to determine if LBV winds are axi-symmetric ingeneral. The high fraction of LBVs (>50%) showing intrinsicpolarization is to be compared with the lower ~20-25% for similarstudies of their evolutionary neighbours, O supergiants and Wolf-Rayetstars. We anticipate that this higher incidence is due to the lowereffective gravities of the LBVs, coupled with their variabletemperatures within the bi-stability jump regime. This is alsoconsistent with the higher incidence of wind asphericity that we find inLBVs with strong Hα emission and recent (last ~10 years) strongvariability.
| A spectroscopic search for the non-nuclear Wolf-Rayet population of the metal-rich spiral galaxy M 83
We present a catalogue of non-nuclear regions containing Wolf-Rayetstars in the metal-rich spiral galaxy M 83 (NGC 5236). From a total of283 candidate regions identified using He ii λ4686 imaging withVLT-FORS2, Multi Object Spectroscopy of 198 regions was carried out,confirming 132 WR sources. From this sub-sample, an exceptional contentof ~1035 ± 300 WR stars is inferred, with N(WC)/N(WN) ~ 1.2,continuing the trend to larger values at higher metallicity amongstLocal Group galaxies, and greatly exceeding current evolutionarypredictions at high metallicity. Late-type stars dominate the WCpopulation of M 83, with N(WC8-9)/N(WC4-7) = 9 and WO subtypes absent,consistent with metallicity dependent WC winds. Equal numbers of late toearly WN stars are observed, again in contrast to current evolutionarypredictions. Several sources contain large numbers of WR stars. Inparticular, #74 (alias region 35 from de Vaucouleurs et al.) contains~230 WR stars, and is identified as a Super Star Cluster from inspectionof archival HST/ACS images. Omitting this starburst cluster would resultin revised statistics of N(WC)/N(WN) ~ 1 and N(WC8-9)/N(WC4-7) ~ 6 forthe "quiescent" disk population. Including recent results for thenucleus and accounting for incompleteness in our spectroscopic sample,we suspect the total WR population of M 83 may exceed 3000 stars.
| Lower mass loss rates in O-type stars: Spectral signatures of dense clumps in the wind of two Galactic O4 stars
We have analyzed the far-ultraviolet spectrum of two Galactic O4 stars,the O4If+ supergiant HD 190429A and the O4V((f)) dwarf HD 96715, usingarchival FUSE and IUE data. We have conducted a quantitative analysisusing the two NLTE model atmosphere and wind codes, tlusty and cmfgen,which incorporate a detailed treatment of NLTE metal line blanketing.From the far-UV spectrum, we have derived the stellar and windparameters and the surface composition of the two stars. The surface ofHD 190429A has a composition typical of an evolved O supergiant(nitrogen-rich, carbon and oxygen-poor), while HD 96715 exhibits surfacenitrogen enhancement similar to the enrichment found in SMC O dwarfswhich has been attributed to rotationally-induced mixing. Followingstudies of Magellanic Cloud O stars, we find that homogeneous windmodels could not match the observed profile of O vλ1371 andrequire very low phosphorus abundance to fit the Pvλλ1118-1128 resonance lines. We show, on the other hand,that we are able to match the O v and P v lines using clumped windmodels. In addition to these lines, we find that N ivλ1718 isalso sensitive to wind clumping. For both stars, we have calculatedclumped wind models that match well all these lines from differentspecies and that remain consistent with Hα data. In particular, wehave achieved an excellent match of the P v resonance doublet,indicating that our physical description of clumping is adequate. Thesefits therefore provide a coherent and thus much stronger evidence ofwind clumping in O stars than earlier claims. We show that the successof the clumped wind models in matching these lines results fromincreased recombination in the clumps, hence from a better descriptionof the wind ionization structure. We find that the wind of these twostars is highly clumped, as expressed by very small volume fillingfactors, namely f_&infy; = 0.04 for HD 190429A and f_&infy; = 0.02 forHD 96715. In agreement with our analysis of SMC stars, clumping startsdeep in the wind, just above the sonic point. The most crucialconsequence of our analysis is that the mass loss rates of O stars needto be revised downward significantly, by a factor of 3 and more. Theselower mass loss rates will affect substantially the evolution of massivestars. Accounting for wind clumping is essential when determining thewind properties of O stars. Our study therefore calls for a fundamentalrevision in our understanding of mass loss and of O-type star stellarwinds.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the 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/431/773
| The influence of binaries on galactic chemical evolution
Understanding the galaxy in which we live is one of the greatintellectual challenges facing modern science. With the advent of highquality observational data, the chemical evolution modeling of ourgalaxy has been the subject of numerous studies in the last years.However, all these studies have one missing element which is theevolution of close binaries. Reason: their evolution is very complex andsingle stars only perhaps can do the job. (Un)Fortunately at present weknow that the majority of the observed stars are members of a binary ormultiple system and that certain objects can only be formed throughbinary evolution. Therefore galactic studies that do not account forclose binary evolution may be far from realistic.Because of the large expertise developed through the years in stellarevolution in general and binary evolution in particular at the BrusselsAstrophysical Institute, we found ourselves in a privileged position tobe the first to do chemical evolutionary simulations with the inclusionof detailed binary evolution. The complexity of close binary evolutionhas kept many astronomers from including binary stars into theirstudies. However, it is not always the easiest way of living that givesyou the most excitement and satisfaction.
| An Atlas of Far-Ultraviolet Spectra of Wolf-Rayet Stars from the FUSE Satellite
We present an atlas of far-ultraviolet spectra of 21 Wolf-Rayet (WR)stars in the Galaxy and Large and Small Magellanic Clouds, secured withthe Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The datacover the wavelength range of 912-1190 Å at a spectral resolutionof 0.1 Å and span examples of most subtypes in the WN and WCsequences. We discuss the FUV spectral morphology of the different WRsequences, emphasizing the wide range of ions and chemical speciesexhibiting well-developed P Cygni profiles and emission lines in thiswavelength range. For WN stars the relative strengths of C IV/C III, NIII/N II, P V/P IV, and Si IV/Si III show a decrease in strength of thehigh ions from WN3 to WN11 complemented by an increase in the lower ionsat later types. The ``super ions'' of O VI and S VI are consideredphotoionized wind features for WN3-WN6 stars, probably the result ofAuger ionization in WN7-WN9 stars, and probably absent at WN10-WN11. TheWN5h star Sk 41 in the SMC shows relatively weaker features, which canbe ascribed to the effects of a global galaxy metal deficiency. For theWC stars, a similar pattern of wind ionization-linked strengths in theemissions and P Cygni profiles is present, particularly evident in therelative strengths of lines in P V, S IV, Si IV, and Si III. O VI, and SVI features are only seen in the earliest WC subtypes. The high carbonabundance in WC stars is reflected by the presence of strong C IV and CIII lines throughout the sequence. We present new estimates of the windterminal velocities from measurements of saturated absorption componentsobserved in a wide range of I.P. species. Considerable revisions tov&infy; for the WN3 and WN5 (SMC) stars in our sample and,in particular for the WN10 and WN11 stars are found. The latter make useof the unique availability of the N II resonance line in the FUSEwaveband.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| 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.
| The Deuterium-to-Hydrogen Ratio in a Low-Metallicity Cloud Falling onto the Milky Way
Using Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble SpaceTelescope observations of the QSO PG 1259+593, we detect D I Lymanseries absorption in high-velocity cloud Complex C, a low-metallicitygas cloud falling onto the Milky Way. This is the first detection ofatomic deuterium in the local universe in a location other than thenearby regions of the Galactic disk. We construct a velocity model forthe sight line based on the numerous O I absorption lines detected inthe ultraviolet spectra. We identify eight absorption-line components,two of which are associated with the high-velocity gas in Complex C at~-128 and ~-112 km s-1. A new Westerbork Synthesis RadioTelescope (WSRT) interferometer map of the H I 21 cm emission toward PG1259+593 indicates that the sight line passes through a compactconcentration of neutral gas in Complex C. We use the WSRT data togetherwith single-dish data from the Effelsberg 100 m radio telescope toestimate the H I column density of the high-velocity gas and toconstrain the velocity extents of the H I Lyman series absorptioncomponents observed by FUSE. We find N(HI)=(9.0+/-1.0)×1019 cm-2, N(DI)=(2.0+/-0.6)×1015 cm-2, and N(OI)=(7.2+/-2.1)×1015 cm-2 for the Complex Cgas (68% confidence intervals). The corresponding light-elementabundance ratios are D/H=(2.2+/-0.7)×10-5,O/H=(8.0+/-2.5)×10-5, and D/O=0.28+/-0.12. Themetallicity of Complex C gas toward PG 1259+593 is approximately 1/6solar, as inferred from the oxygen abundance[O/H]=-0.79+/-0.120.16. While we cannot rule out avalue of D/H similar to that found for the local ISM (i.e.,D/H~1.5×10-5), we can confidently exclude values as lowas those determined recently for extended sight lines in the Galacticdisk (D/H<1×10-5). Combined with the sub-solarmetallicity estimate and the low nitrogen abundance, this conclusionlends support to the hypothesis that Complex C is located outside theMilky Way, rather than inside in material recirculated between theGalactic disk and halo. The value of D/H for Complex C is consistentwith the primordial abundance of deuterium inferred from recentWilkinson Microwave Anisotropy Probe observations of the cosmicmicrowave background and simple chemical evolution models that predictthe amount of deuterium astration as a function of metallicity.
| Shocked Clouds in the Vela Supernova Remnant
Unusually strong high-excitation C I has been detected in 11 lines ofsight through the Vela supernova remnant (SNR) by means of UV absorptionline studies of IUE data. Most of these lines of sight lie near thewestern edge of the bright X-ray region of the SNR in a spatiallydistinct band approximately 1° by 4° oriented approximatelynorth-south. The high-excitation C I (denoted C I* and C I**) isinterpreted as evidence of a complex of shocked dense clouds interactingwith the SNR, because of the high pressures indicated in this region. Tofurther analyze the properties of this region of enhanced C I* and CI**, we present new HIRES-processed IRAS data of the entire Vela SNR. Atemperature map calculated from the HIRES IRAS data, based on atwo-component dust model, reveals the signature of hot dust at severallocations in the SNR. The hot dust is anticorrelated spatially withX-ray emission, as would be expected for a dusty medium interacting witha shock wave. The regions of hot dust are strongly correlated withoptical filaments, supporting a scenario of dense clouds interior to theSNR that have been shocked and are now cooling behind the supernovablast wave. With few exceptions, the lines of sight to the stronghigh-excitation C I pass through regions of hot dust and opticalfilaments. Possible mechanisms for the production of the unexpectedlylarge columns of high-excitation C I are discussed. Dense clouds on theback western hemisphere of the remnant may explain the relatively lowX-ray emission in the western portion of the Vela SNR due to the slowerforward shock velocity in regions where the shock has encountered thedense clouds. An alternate explanation for the presence of ground-stateand excited-state neutrals, as well as ionized species, along the sameline of sight is a magnetic precursor that heats and compresses the gasahead of the shock.
| Two New Low Galactic D/H Measurements from the Far Ultraviolet Spectroscopic Explorer
We analyze interstellar absorption observed toward two subdwarf O stars,JL 9 and LS 1274, using spectra taken by the Far UltravioletSpectroscopic Explorer (FUSE). Column densities are measured for manyatomic and molecular species (H I, D I, C I, N I, O I, P II, Ar I, FeII, and H2), but our main focus is on measuring the D/Hratios for these extended lines of sight, as D/H is an importantdiagnostic for both cosmology and Galactic chemical evolution. We findD/H=(1.00+/-0.37)×10-5 toward JL 9 andD/H=(0.76+/-0.36)×10-5 toward LS 1274 (2 σuncertainties). With distances of 590+/-160 and 580+/-100 pc,respectively, these two lines of sight are currently among the longestGalactic lines of sight with measured D/H. With the addition of thesemeasurements, we see a significant tendency for longer Galactic lines ofsight to yield low D/H values, consistent with previous inferences aboutthe deuterium abundance from D/O and D/N measurements. Short lines ofsight with H I column densities of logN(HI)<19.2 suggest that thegas-phase D/H value within the Local Bubble is(D/H)LBg=(1.56+/-0.04)×10-5. However, thefour longest Galactic lines of sight with measured D/H, which haved>500 pc and logN(HI)>20.5, suggest a significantly lower valuefor the true local disk gas-phase D/H value,(D/H)LDg=(0.85+/-0.09)×10-5. Oneinterpretation of these results is that D is preferentially depletedonto dust grains relative to H and that longer lines of sight thatextend beyond the Local Bubble sample more depleted material. In thisscenario, the higher Local Bubble D/H ratio is actually a betterestimate than (D/H)LDg for the true local disk D/H,(D/H)LD. However, if (D/H)LDg is different from(D/H)LBg simply because of variable astration and incompleteinterstellar medium mixing, then (D/H)LD=(D/H)LDg.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE), which is operated for NASA by JohnsHopkins University under NASA contract NAS5-32985.
| Detection possibility of the pair-annihilation neutrinos from the neutrino-cooled pre-supernova star
The signal produced in neutrino observatories by the pair-annihilationneutrinos emitted from a 20 Msolar pre-supernova star at thesilicon burning phase is estimated. The spectrum of the neutrinos withan average energy ~2 MeV is calculated with the use of the Monte Carlomethod. A few relevant reactions for neutrinos and anti-neutrinos inmodern detectors are considered. The most promising results are fromν¯e+p-->n+e+ reaction. During theSi-burning phase we expect 1.27 neutrons/day/kton of water to beproduced by neutrinos from a star located at a distance of 1 kpc. Smalladmixture of effective neutron-absorbers as e.g. NaCl orGdCl3 makes these neutrons easily visible because ofCherenkov light produced by electrons which were hit by ~8 MeV photoncascade emitted by Cl or Gd nuclei. The estimated rate of neutronproduction for SNO and Super-Kamiokande is, respectively, 2.2 and 41events per day for a star at 1 kpc. For future detectors UNO andHyper-Kamiokande we expect 5.6 and 6.9 events per day even for a star 10kpc away. This would make it possible to foresee a massive star death afew days before its core collapse. Importance of such a detection fortheoretical astrophysics is discussed.
| NGC 604, the Scaled OB Association (SOBA) Prototype. I. Spatial Distribution of the Different Gas Phases and Attenuation by Dust
We have analyzed Hubble Space Telescope and ground-based data tocharacterize the different gas phases and their interaction with themassive young cluster in NGC 604, a giant H II region in M33. The warmionized gas is made out of two components: a high-excitation, highsurface brightness H II surface located at the faces of the molecularclouds directly exposed to the ionizing radiation of the central scaledOB association (SOBA); and a low-excitation, low surface brightness halothat extends to much larger distances from the ionizing stars. Thecavities created by the winds and supernova explosions are filled withX-ray-emitting coronal gas. The nebular lines emitted by the warm gasexperience a variable attenuation as a consequence of the dustdistribution, which is patchy in the plane of the sky and with cloudsinterspersed among emission-line sources in the same line of sight. Theoptical depth at Hα as measured from the ratio of the thermalradio continuum to Hα shows a very good correlation with the totalCO (1-->0) column, indicating that most of the dust resides in thecold molecular phase. The optical depth at Hα as measured from theratio of Hα to Hβ also correlates with the CO emission butnot as strongly as in the previous case. We analyze the differencebetween those two measurements, and we find that <~11% of the H IIgas is hidden behind large-optical-depth molecular clouds; we pinpointthe positions in NGC 604 where that hidden gas is located. We detect twocandidate compact H II regions embedded inside the molecular cloud; bothare within short distance of WR/Of stars, and one of them is locatedwithin 16 pc of a red supergiant. We estimate the age of the mainstellar generation in NGC 604 to be ~3 Myr from the ionization structureof the H II region, a value consistent with previous age measurements.The size of the main cavity is smaller than that predicted byextrapolating from single-star wind-blown bubbles; possible explanationsfor this effect are presented.
| 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.
| Line profile variations in WR+O binary systems. I. The code and basic predictions
We compute the P Cyg line profiles formed by the stellar winds of binarysystems containing a Wolf-Rayet and an O-type star, incorporating theeffects due to wind eclipses and wind-wind collisions. The contributionfrom both stellar winds to the P Cygni emission lines is modeled fordifferent orbital phases. The opacity and the source function arecalculated assuming a simplified atom and the Sobolev approximation, andthe emission-line profile is calculated by exact radiative transferthrough the 3D geometry wind. We analyze the cases of a P Cygni linethat is formed only in the WR wind, and the case of a line formed inboth the WR and the O-star winds. The line-profile variations that arepredicted by this model are presented. When compared with observations,the synthetic profiles and their phase-dependent variability provide anestimate for the opening angle of the WWC shock cone and the velocitylaw of both stellar winds. Ultraviolet (UV) observations of the binarysystem γ2 Vel are used to illustrate how the modelpredictions can be applied to the observational data.
| Wind clumping and the wind-wind collision zone in the Wolf-Rayet binary γ2 Velorum observations at high and low state. XMM-Newton observations at high and low state
We present XMM-Newton observations of γ2 Velorum (WR11, WC8+O7.5III, P = 78.53 d), a nearby Wolf-Rayet binary system, at itsX-ray high and low states. At high state, emission from a hotcollisional plasma dominates from about 1 to 8 keV. At low state,photons between 1 and 4 keV are absorbed. The hot plasma is identifiedwith the shock zone between the winds of the primary Wolf-Rayet star andthe secondary O giant. The absorption at low state is interpreted asphotoelectric absorption in the Wolf-Rayet wind. This absorption allowsus to measure the absorbing column density and to derive a mass lossrate .M = 8 × 10-6 Mȯ yr-1for the WC8 star. This mass loss rate, in conjunction with a previousWolf-Rayet wind model, provides evidence for a clumped WR wind. Aclumping factor of 16 is required. The X-ray spectra below 1 keV (12Å) show no absorption and are essentially similar in both states.There is a rather clear separation in that emission from a plasma hotterthan 5 MK is heavily absorbed in low state while the cooler plasma isnot. This cool plasma must come from a much more extended region thanthe hot material. The Neon abundance in the X-ray emitting material is2.5 times the solar value. The unexpected detection of C V (25.3Å) and C VI (31.6 Å) radiative recombination continua atboth phases indicates the presence of a cool (40 000 K)recombination region located far out in the binary system.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
Kataloglar ve belirtme:
|
