Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Wind signatures in the X-ray emission-line profiles of the late-O supergiant ζ Orionis
X-ray line-profile analysis has proved to be the most direct diagnosticof the kinematics and spatial distribution of the very hot plasma aroundO stars. The Doppler-broadened line profiles provide information aboutthe velocity distribution of the hot plasma, while thewavelength-dependent attenuation across a line profile providesinformation about the absorption to the hot plasma, thus providing astrong constraint on its physical location. In this paper, we applyseveral analysis techniques to the emission lines in the Chandra HighEnergy Transmission Grating Spectrometer (HETGS) spectrum of the late-Osupergiant ζ Ori (O9.7 Ib), including the fitting of a simpleline-profile model. We show that there is distinct evidence forblueshifts and profile asymmetry, as well as broadening in the X-rayemission lines of ζ Ori. These are the observational hallmarks of awind-shock X-ray source, and the results for ζ Ori are very similarto those for the earlier O star, ζ Pup, which we have previouslyshown to be well fit by the same wind-shock line-profile model. The moresubtle effects on the line-profile morphologies in ζ Ori, ascompared to ζ Pup, are consistent with the somewhat lower densitywind in this later O supergiant. In both stars, the wind optical depthsrequired to explain the mildly asymmetric X-ray line profiles implyreductions in the effective opacity of nearly an order of magnitude,which may be explained by some combination of mass-loss rate reductionand large-scale clumping, with its associated porosity-based effects onradiation transfer. In the context of the recent reanalysis of thehelium-like line intensity ratios in both ζ Ori and ζ Pup, andalso in light of recent work questioning the published mass-loss ratesin OB stars, these new results indicate that the X-ray emission fromζ Ori can be understood within the framework of the standardwind-shock scenario for hot stars.
| Discovery of a strong magnetic field on the O star HD 191612: new clues to the future of θ1 Orionis C*
From observations made with the ESPaDOnS spectropolarimeter, recentlyinstalled on the 3.6-m Canada-France-Hawaii Telescope, we report thediscovery of a strong magnetic field in the Of?p spectrum variable HD191612 - only the second known magnetic O star (followingθ1 Ori C). The stability of the observed Zeemansignature over four nights of observation, together with thenon-rotational shape of line profiles, argues that the rotation periodof HD 191612 is significantly longer than the 9-d value previouslyproposed. We suggest that the recently identified 538-d spectralvariability period is the rotation period, in which case the observedline-of-sight magnetic field of -220 +/- 38 G implies a large-scalefield (assumed dipolar) with a polar strength of about -1.5 kG. Ifconfirmed, this scenario suggests that HD 191612 is, essentially, anevolved version of the near-zero-age main-sequence magnetic O starθ1 Ori C, but with an even stronger field (about 15 kGat an age similar to that of θ1 Ori C). We suggestthat the rotation rate of HD 191612, which is exceptionally slow byaccepted O-star standards, could be due to angular momentum dissipationthrough a magnetically confined wind.
| 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
| 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.
| Winds from OB Stars: A Two-Component Scenario?
X-ray spectroscopy of several OB stars with massive winds has revealedthat many X-ray line profiles exhibit unexpectedly small blueshifts andare almost symmetric. Moreover, the hottest X-ray lines appear tooriginate closest to the star. These properties appear to beinconsistent with the standard model of X-rays originating in shockedmaterial in line-driven spherically symmetric winds. Here we raise thequestion, can the X-ray line data be understood in terms of atwo-component wind? We consider a scenario in which one component of thewind is a standard line-driven wind that emerges from a broad range oflatitudes centered on the equator. The second component of the windemerges from magnetically active regions in extensive polar caps. Theexistence of such polar caps is suggested by a recent model of dynamoaction in massive stars. We describe how the two-component model isconsistent with a variety of observational properties of OB star winds.
| Bright OB stars in the Galaxy. III. Constraints on the radial stratification of the clumping factor in hot star winds from a combined Hα, IR and radio analysis
Context: .Recent results strongly challenge the canonical picture ofmassive star winds: various evidence indicates that currently acceptedmass-loss rates, {dot M}, may need to be revised downwards, by factorsextending to one magnitude or even more. This is because the mostcommonly used mass-loss diagnostics are affected by "clumping"(small-scale density inhomogeneities), influencing our interpretation ofobserved spectra and fluxes. Aims: .Such downward revisions wouldhave dramatic consequences for the evolution of, and feedback from,massive stars, and thus robust determinations of the clumping propertiesand mass-loss rates are urgently needed. We present a first attemptconcerning this objective, by means of constraining the radialstratification of the so-called clumping factor. Methods: .To thisend, we have analyzed a sample of 19 Galactic O-type supergiants/giants,by combining our own and archival data for Hα, IR, mm and radiofluxes, and using approximate methods, calibrated to more sophisticatedmodels. Clumping has been included into our analysis in the"conventional" way, by assuming the inter-clump matter to be void.Because (almost) all our diagnostics depends on the square of density,we cannot derive absolute clumping factors, but only factors normalizedto a certain minimum. Results: .This minimum was usually found tobe located in the outermost, radio-emitting region, i.e., the radiomass-loss rates are the lowest ones, compared to {dot M} derived fromHα and the IR. The radio rates agree well with those predicted bytheory, but are only upper limits, due to unknown clumping in the outerwind. Hα turned out to be a useful tool to derive the clumpingproperties inside r < 3{ldots}5 Rstar. Our most importantresult concerns a (physical) difference between denser and thinnerwinds: for denser winds, the innermost region is more strongly clumpedthan the outermost one (with a normalized clumping factor of 4.1± 1.4), whereas thinner winds have similar clumping properties inthe inner and outer regions. Conclusions: .Our findings arecompared with theoretical predictions, and the implications arediscussed in detail, by assuming different scenarios regarding the stillunknown clumping properties of the outer wind.
| The magnetic field and confined wind of the O star θ1 Orionis C
Aims.In this paper we confirm the presence of a globally-ordered,kG-strength magnetic field in the photosphere of the young O starθ1 Orionis C, and examine the properties of itsoptical line profile variations. Methods: .A new series ofhigh-resolution MuSiCoS Stokes V and I spectra has been acquired whichsamples approximately uniformly the rotational cycle ofθ1 Orionis C. Using the Least-Squares Deconvolution(LSD) multiline technique, we have succeeded in detecting variableStokes V Zeeman signatures associated with the LSD mean line profile.These signatures have been modeled to determine the magnetic fieldgeometry. We have furthermore examined the profile variations of linesformed in both the wind and photosphere using dynamic spectra.Results: .Based on spectrum synthesis fitting of the LSD profiles, wedetermine that the polar strength of the magnetic dipole component is1150 B_d 1800 G and that the magnetic obliquity is 27° β 68°, assuming i=45± 20°. The best-fitvalues for i=45° are Bd = 1300 ± 150~G~ (1σ)and β = 50° ± 6° (1σ). Our data confirm theprevious detection of a magnetic field in this star, and furthermoredemonstrate the sinusoidal variability of the longitudinal field andaccurately determine the phases and intensities of the magnetic extrema.The analysis of "photospheric" and "wind" line profile variationssupports previous reports of the optical spectroscopic characteristics,and provides evidence for infall of material within the magneticequatorial plane.
| Multi-periodic photospheric pulsations and connected wind structures in HD 64760
We report on the results of an extended optical spectroscopic monitoringcampaign on the early-type B supergiant HD 64760(B0.5 Ib) designed to probe the deep-seated origin of spatial windstructure in massive stars. This new study is based on high-resolutionechelle spectra obtained with the Feros instrument at ESO La Silla. 279spectra were collected over 10 nights between February 14 and 24, 2003.From the period analysis of the line-profile variability of thephotospheric lines we identify three closely spaced periods around 4.810h and a splitting of ±3%. The velocity - phase diagrams of theline-profile variations for the distinct periods reveal characteristicprograde non-radial pulsation patterns of high order corresponding topulsation modes with l and m in the range 6{-}10. A detailed modeling ofthe multi-periodic non-radial pulsations with the Bruce and Kyliepulsation-model codes (Townsend 1997b, MNRAS, 284, 839) favors eitherthree modes with l=-m and l=8,6,8 or m=-6 and l=8,6,10 with the secondcase maintaining the closely spaced periods in the co-rotating frame.The pulsation models predict photometric variations of 0.012{-}0.020 magconsistent with the non-detection of any of the spectroscopic periods byphotometry. The three pulsation modes have periods clearly shorter thanthe characteristic pulsation time scale and show small horizontalvelocity fields and hence are identified as p-modes. The beating of thethree pulsation modes leads to a retrograde beat pattern with tworegions of constructive interference diametrically opposite on thestellar surface and a beat period of 162.8 h (6.8 days). This beatpattern is directly observed in the spectroscopic time series of thephotospheric lines. The wind-sensitive lines display features ofenhanced emission, which appear to follow the maxima of the photosphericbeat pattern. The pulsation models predict for the two regionsnormalized flux amplitudes of A=+0.33,-0.28, sufficiently large to raisespiral co-rotating interaction regions (Cranmer & Owocki 1996, ApJ,462, 469). We further investigate the observed Hα wind-profilevariations with a simple rotating wind model with wind-densitymodulations to simulate the effect of possible streak lines originatingfrom the localized surface spots created by the NRP beat pattern. It isfound that such a simple scenario can explain the time scales and somebut not all characteristics of the observed Hα line-profilevariations.
| Determination of the Mass Loss Rate and the Terminal Velocity of Stellar Winds. I. Genetic Algorithm for Automatic Line Profile Fitting
A new method for automatic fitting of Pline profiles in UV spectra ofstellar winds is presented. The line source function is calculated usingSobolev's approximation and the emergent flux is obtained by exactintegration of the equation of the radiation transport (similar to theSEI method described by Lamers et al. (1987)). The quality of the fit isevaluated using the likelihood estimator. The maximization of thelikelihood is done by a genetic algorithm. The advantages of our methodwith respect to other similar approaches are its robustness and itsinsensibility to the initial guess. In addition, the algorithmguarantees the localization of the global maximum of the likelihoodhypersurface, which is not the case for classical minimizationalgorithms. Here we present an implementation of the genetic algorithmfor line profile fitting, its tests on both synthetic and real data andan estimation of the confidence limits of the results.
| Looking for Discrete UV Absorption Features in the Early-Type Eclipsing Binaries μ1 Scorpii and AO Cassiopeiae
A search for discrete absorption components in the ultraviolet spectraof the early-type binaries μ1 Scorpii and AO Cassiopeiaehas been undertaken by analyzing material secured with the InternationalUltraviolet Explorer satellite during an exclusively assigned intervalof nearly 50 hr. While the spectra of μ1 Sco definitely donot show the presence of such lines, the spectra of AO Cas do confirmthem and permit us to draw some conclusions about where they may beformed.
| A Survey of N IV and O IV Features near 3400 Å in O2-O5 Spectra
We have conducted a survey of little-known N IV and O IV multiplets near3400 Å in an extensive sample of well-classified, very earlyO-type spectra. The initial motivation was to search for additionaluseful classification criteria for these types, but an unexpected resultis the high sensitivity of these features to evolutionary CNOprocessing. We have found a useful discriminant between O2 and latertypes in the relative strengths of the O IV multiplets, one of which issubject to selective emission in the hottest spectra; the overallstrengths of these lines also decrease between spectral types O4 and O5.More remarkable, however, are the variations in the N/O ratios amongboth individual stars and clusters. For instance, several O4 If+ spectrahave very large ratios, while main-sequence stars in the Carina Nebulagenerally have smaller values than others of the same spectral types inother regions. These effects correspond to different degrees of mixingof processed material as a function of evolutionary age and initialrotational velocities; the second effect provides significant furtherevidence that very massive stars mix while still on the main sequence.Thus, further analysis of these features will likely provide valuablediagnostics of important evolutionary parameters.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| 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.
| On the feasibility of detection of neutron star companions to OB runaways using Gaia astrometry
For an illustrative sample of classical OB runaway stars, we examine thecapability of the upcoming Gaia satellite to detect compact companionsby the use of astrometric techniques. For the OB runaway stars in oursample, we estimate initial system parameters and consider the modifyingevolutionary effects of mass transfer and supernova explosion of theprimary. The possible system configurations that follow from this, andthe expected Gaia accuracy, determine the likelihood of detecting amovement of the photocentre due to an unseen companion. As the size ofthe natal kick imparted to the core of the exploding star is increasedthe overall probability of detecting a neutron star companion decreasesas more systems become disrupted. The overall detection probabilitiesfor our illustrative sample range from 2% to 27%, which imply thatwithin a distance of approximately 5 kpc from the Sun around 48detections of compact companions to runaway stars can be expected. Forcomparison, around 15% of High Mass X-ray Binaries would exhibit wobblesdetectable with Gaia.
| 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.
| Chandra X-ray observations of the young stellar cluster NGC 6193 in the Ara OB1 association
A 90-ks Chandra High Energy Transmission Grating observation of theyoung stellar cluster NGC 6193 in the southern Ara OB1 associationdetected 43 X-ray sources in a 2 × 2 arcmin2 coreregion centred on the massive O stars HD 150135 (O6.5V) and HD 150136(O3 + O6V). The cluster is dominated by exceptionally bright X-rayemission from the two O stars, which are separated by only 10 arcsec.The X-ray luminosity of HD 150136 is logLX= 33.39 (ergs-1), making it one of the most luminous O-star X-ray sourcesknown. All of the fainter X-ray sources in the core region havenear-infrared (near-IR) counterparts, but existing JHK photometryprovides little evidence for near-IR excesses. These core sources havetypical mean photon energies ~ 2 keV and about one-third arevariable. It is likely that some are young low-mass stars in thecluster, but cluster membership remains to be determined. Gratingspectra show that the X-ray properties of HD 150135 and HD 150136 aresimilar, but not identical. Both have moderately broadened unshiftedemission lines and their emission is dominated by cool plasma at kT~ 0.3keV, pointing to a wind-shock origin. However, the emission of HD 150136is slightly hotter and four times more luminous than its optical twin HD150135. We discuss the possibility that a radiative colliding wind shockcontributes to the prodigious X-ray output of the short-period (2.66 d)spectroscopic binary HD 150136. A surprising result is that the X-rayemission of HD 150136 is slowly variable on a time-scale of <1 d. Theorigin of the variability is not yet known but the observed behavioursuggests that it is an occultation effect.
| 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.
| Dynamo-generated magnetic fields at the surface of a massive star
Spruit has shown that an astrophysical dynamo can operate in thenon-convective material of a differentially rotating star as a result ofa particular instability in the magnetic field (the Tayler instability).By assuming that the dynamo operates in a state of marginal instability,Spruit has obtained formulae which predict the equilibrium strengths ofazimuthal and radial field components in terms of local physicalquantities. Here, we apply Spruit's formulae to our previously publishedmodels of rotating massive stars in order to estimate Tayler dynamofield strengths. There are no free parameters in Spruit's formulae. Inour models of 10- and 50-Msolar stars on the zero-age mainsequence, we find internal azimuthal fields of up to 1 MG, and internalradial components of a few kG. Evolved models contain weaker fields. Inorder to obtain estimates of the field strength at the stellar surface,we examine the conditions under which the Tayler dynamo fields aresubject to magnetic buoyancy. We find that conditions for Taylerinstability overlap with those for buoyancy at intermediate to highmagnetic latitudes. This suggests that fields emerge at the surface of amassive star between magnetic latitudes of about 45° and the poles.We attempt to estimate the strength of the field which emerges at thesurface of a massive star. Although these estimates are very rough, wefind that the surface field strengths overlap with values which havebeen reported recently for line-of-sight fields in several O and Bstars.
| Visual Star Colours from Instrumental Photometry
In order to display graphically the visual colours of stars and otherastronomical objects, photometric broadband R, V, B colours are used toproxy for the r, g, b colours of the three visual sensors of the eye.From photometric Johnson B-V and V-R colour indices, R, V, and Bmagnitudes (V = 0) are calculated, and from these the respectivebrightnesses (r, v = 1 = g, and b) are calculated. After suitablenormalization these are then placed in a ternary diagram having r, g,and b as the vertices. All B-V and V-R are adjusted so that the Sunfalls in the same place as a blackbody at 5800 K. The resulting ternaryplot shows all of its objects (stars, planets) in their visual coloursat their relative positions in the ternary diagram. The star coloursdisplayed on a computer monitor screen or as a print with a colourprinter are more vivid than the usual visual impressions of isolatedstars, undoubtedly because of properties of the dark-adapted eye, butdouble-star pairs with contrasting colours correspond nicely totelescopic visual impressions.
| Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
| Polarization Effects in the Radiation of Magnetized Envelopes and Extended Accretion Structures
We have calculated the degree and position angle of the polarization ofradiation scattered in a magnetized, optically thin or optically thickenvelope around a central source, taking into account Faraday rotationof the plane of polarization during the propagation of the scatteredradiation and the finite size of the radiation source. The wavelengthdependence of the degree of polarization can be used to estimate themagnetic field of the source (a star, the region around a neutron star,or a black hole), and we have used our calculations to estimate themagnetic fields in a number of individual objects: several hot O andWolf-Rayet stars, compact objects in X-ray close binaries with blackholes (SS 433, Cyg X-1), and supernovae. The spectrum of the linearpolarization can be used to determine the magnetic field in the vicinityof a central supermassive black hole, where the polarized opticalradiation is generated. In a real physical model, this value can beextrapolated to the region of the last stable orbit. In the future, theproposed technique will make it possible to directly estimate themagnetic field in the region of the last stable orbit of a supermassiveblack hole using X-ray polarimetry.
| A Medium Resolution Near-Infrared Spectral Atlas of O and Early-B Stars
We present intermediate-resolution (R~8000-12,000) high signal-to-noise(S/N) H- and K-band spectroscopy of a sample of 37 optically visiblestars, ranging in spectral type from O3 to B3 and representing mostluminosity classes. Spectra of this quality can be used to constrain thetemperature, luminosity, and general wind properties of OB stars, whenused in conjunction with sophisticated atmospheric model codes. Mostimportant is the need for moderately high resolutions (R>=5000) andvery high signal-to-noise (S/N>=150) spectra for a meaningful profileanalysis. When using near-infrared spectra for a classification system,moderately high signal-to-noise (S/N~100) is still required, though theresolution can be relaxed to just a thousand or two. In the Appendix weprovide a set of very high-quality near-infrared spectra of Brackettlines in six early-A dwarfs. These can be used to aid in the modelingand removal of such lines when early-A dwarfs are used for telluricspectroscopic standards.
| 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.
| Chandra HETGS Multiphase Spectroscopy of the Young Magnetic O Star θ1 Orionis C
We report on four Chandra grating observations of the oblique magneticrotator θ1 Ori C (O5.5 V), covering a wide range ofviewing angles with respect to the star's 1060 G dipole magnetic field.We employ line-width and centroid analyses to study the dynamics of theX-ray-emitting plasma in the circumstellar environment, as well asline-ratio diagnostics to constrain the spatial location, and globalspectral modeling to constrain the temperature distribution andabundances of the very hot plasma. We investigate these diagnostics as afunction of viewing angle and analyze them in conjunction with new MHDsimulations of the magnetically channeled wind shock mechanism onθ1 Ori C. This model fits all the data surprisinglywell, predicting the temperature, luminosity, and occultation of theX-ray-emitting plasma with rotation phase.
| Ion-by-Ion Differential Emission Measure Determination of Collisionally Ionized Plasma. II. Application to Hot Stars
In a previous paper we have described a technique to derive constraintson the differential emission measure (DEM) distribution, a measure ofthe temperature distribution, of collisionally ionized hot plasmas fromtheir X-ray emission line spectra. We apply this technique to theChandra HETGS spectra of all of the nine hot stars available to us atthe time that this project was initiated. We find that DEM distributionsof six of the seven O stars in our sample are very similar, but thatθ1 Ori C has an X-ray spectrum characterized by highertemperatures. The DEM distributions of both of the B stars in our samplehave lower magnitudes than those of the O stars, and one, τ Sco, ischaracterized by higher temperatures than the other, β Cru. Theseresults confirm previous work in which high temperatures have been foundfor θ1 Ori C and τ Sco and taken as evidence forchanneling of the wind in magnetic fields, the existence of which isrelated to the stars' youth. Our results demonstrate the utility of ourmethod for deriving temperature information for large samples of X-rayemission-line spectra.
| X-Ray Counterparts of Runaway Stars
An X-ray search for possible compact companions of runaway OB stars hasbeen conducted using pointed ROSAT observations. Of a list of 71 runawaystars, ROSAT exposures were available for 24, of which 13 are detected.These numbers are nearly 3 times larger than for a previously studiedEinstein sample, and spectral information is exploited as well.Luminosities, hardness ratios, and long-term variability are as fornormal OB stars and do not suggest the presence of collapsed companions.A result like this is often interpreted as support for dynamicalejection from a dense group rather than a supernova event in a binary asa production process for runaway stars. There are, however, severalcircumstances that may adversely affect the observability of a compactcompanion, or after a supernova explosion systems may be disruptedbecause of the large natal kick velocity imparted to the neutron star asa result of asymmetries in the explosions. It is noted that there isactually evidence for both of these production routes and that they maybe expected to occur sequentially in the evolution of OB associations.
| 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.
| Phosphorus in the diffuse interstellar medium
We present FUSE and HST/STIS measurements of the P ii column densitytoward Galactic stars. We analyzed P ii through the profile fitting ofthe unsaturated λ1125 and λ1533 lines and derived columndensities integrated along the sightlines as well as in individualresolved components. We find that phosphorus is not depleted along thosesightlines sampling the diffuse neutral gas. We also investigate thecorrelation existing between P ii and O i column densities and find thatthere is no differential depletion between these two specie.Furthermore, the ratio N(P ii)/N(O i) is consistent with the solar P/Ovalue, implying that P ii and O i coexist in the same gaseous phase andare likely to evolve in parallel. We argue that phosphorus, as traced byP ii, is an excellent neutral oxygen tracer in various physicalenvironments, except when ionization corrections are a significantissue. Thus, P ii lines (observable with FUSE, HST/STIS, or withVLT/UVES for the QSO sightlines) are particularly useful as a proxy forO i lines when these are saturated or blended.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
INHABITED SKY
