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The surprising magnetic topology of τ Sco: fossil remnant or dynamo output?
We report the discovery of a medium-strength (~0.5 kG) magnetic field onthe young, massive star τ Sco (B0.2V), which becomes thethird-hottest magnetic star known. Circularly polarized Zeemansignatures are clearly detected in observations collected mostly withthe ESPaDOnS spectropolarimeter, recently installed on the 3.6-mCanada-France-Hawaii Telescope; temporal variability is also clearlyestablished in the polarimetry, and can be unambiguously attributed torotational modulation with a period close to 41 d. Archival ultraviolet(UV) spectra confirm that this modulation repeats over time-scales ofdecades, and refine the rotation period to 41.033 +/- 0.002 d.Despite the slow rotation rate of τ Sco, we none the less succeed inreconstructing the large-scale structure of its magnetic topology. Wefind that the magnetic structure is unusually complex for a hot star,with significant power in spherical-harmonic modes of degree up to 5.The surface topology is dominated by a potential field, although amoderate toroidal component is probably present. We fail to detectintrinsic temporal variability of the magnetic structure over the 1.5-yrperiod of our spectropolarimetric observations (in agreement with thestable temporal variations of the UV spectra), and infer that anydifferential surface rotation must be very small.The topology of the extended magnetic field that we derive from thephotospheric magnetic maps is also more complex than a global dipole,and features in particular a significantly warped torus of closedmagnetic loops encircling the star (tilted at about 90° to therotation axis), with additional, smaller, networks of closed-fieldlines. This topology appears to be consistent with the exceptional X-rayproperties of τ Sco and also provides a natural explanation of thevariability observed in wind-formed UV lines. Although we cannotcompletely rule out the possibility that the field is produced throughdynamo processes of an exotic kind, we conclude that its magnetic fieldis most probably a fossil remnant from the star formation stage.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Science de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii.E-mail: donati@ast.obs-mip.fr (J-FD); idh@star.ucl.ac.uk (IDH);mmj@st-andrews.ac.uk (MMJ); petit@ast.obs-mip.fr (PP);claude.catala@obspm.fr (CC); jlandstr@uwo.ca (JDL);jean-claude.bouret@oamp.fr (J-CB); evelyne.alecian@obspm.fr (EA);jrb3@st-andrews.ac.uk (JRB); forveill@cfht.hawaii.edu (TF);fpaletou@ast.obs-mip.fr (FP); manset@cfht.hawaii.edu (NM)
| 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.
| C II Abundances in Early-Type Stars: Solution to a Notorious Non-LTE Problem
We address a long-standing discrepancy between non-LTE analyses of theprominent C II 4267 and 6578/82 Å multiplets in early-type stars.A comprehensive non-LTE model atom of C II is constructed based oncritically selected atomic data. This model atom is used for anabundance study of six apparently slow-rotating main-sequence and giantearly B-type stars. High-resolution and high S/N spectra allow us toderive highly consistent abundances not only from the classical featuresbut also from up to 18 additional C II lines in the visual-including twoso far unreported emission features equally well reproduced in non-LTE.These results require the stellar atmospheric parameters to bedetermined with care. A homogeneous (slightly) subsolar present-daycarbon abundance from young stars in the solar vicinity (in associationsand in the field) of log(C/H)+12=8.29+/-0.03 is indicated.
| The Dust-scattered X-Ray Halo around Swift GRB 050724
This paper discusses the X-ray halo around the Swift γ-ray burstGRB 050724 (z=0.258), detected by the Swift X-Ray Telescope. The halo,which forms a ring around the fading X-ray source, expands to a radiusof 200" within 8 ks of the burst, exactly as expected for small-angleX-ray scattering by Galactic dust along the line of sight to acosmologically distant GRB. The expansion curve and radial profile ofthe halo constrain the scattering dust to be concentrated at a distanceof D=139+/-9 pc (from Earth) in a cloud/sheet of thicknessΔD<22 pc. The halo was observed only out to scattering anglesof 200", for which the scattering is dominated by the largest grains,with a maximum size estimated to be amax~0.4-0.5 μm. Thescattering-to-extinction ratio was estimated to beτscat/AV>~0.022 this is a lower limit tothe true value because contribution from smaller grains, which scatterto larger angles, was not directly observed. The line of sight to theGRB passes close to the Ophiuchus molecular cloud complex, whichprovides a plausible site for the scattering dust.
| Early Infrared Spectral Development of V1187 Scorpii (Nova Scorpii 2004 No. 2)
We report on an unprecedented infrared time series of spectra of V1187Sco, a very fast ONeMg nova. The observations covered a 56 day period(2004 August 6-September 30) starting 2 days after the nova's peakbrightness. Time evolution of the spectra revealed changing linestrengths and profiles on timescales of less than a day to weeks as thenova evolved from early postmaximum to early coronal phases. When ourground-based optical and Spitzer Space Telescope data were combined, thewavelength coverage of 0.38-36 μm allowed an accurate spectral energydistribution to be derived when it was about 6 weeks after outburst.Developing double structure in the He I lines showed them changing fromnarrow to broad in only a few days. Using the O I lines in combinationwith the optical spectra, we derived a reddening of E(B-V)=1.56+/-0.08and a distance of 4.9+/-0.5 kpc. Modeling of the ejected materialstrongly suggested that it was geometrically thick withΔR/R=0.8-0.9 (more of a wind than a shell) and a low fillingfactor of order a few percent. The line shapes were consistent with acylindrical jet, bipolar, or spherical Hubble flow expansion with amaximum speed of about -3000 km s-1. The central peakappeared to be more associated with the spherical component, while thetwo peaks (especially in Hβ) suggested a ring with either a lowervelocity component or with its axis inclined to the line of sight.
| 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.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| Radial velocity measurements of B stars in the Scorpius-Centaurus association
We derive single-epoch radial velocities for a sample of 56 B-type starsmembers of the subgroups Upper Scorpius, Upper Centaurus Lupus and LowerCentaurus Crux of the nearby Sco-Cen OB association. The radial velocitymeasurements were obtained by means of high-resolution echelle spectravia analysis of individual lines. The internal accuracy obtained in themeasurements is estimated to be typically 2-3 km s-1, butdepends on the projected rotational velocity of the target. Radialvelocity measurements taken for 2-3 epochs for the targets HD 120307, HD142990 and HD 139365 are variable and confirm that they arespectroscopic binaries, as previously identified in the literature.Spectral lines from two stellar components are resolved in the observedspectra of target stars HD 133242, HD 133955 and HD 143018, identifyingthem as spectroscopic binaries.
| Detailed spectroscopic analysis of the Trapezium cluster stars inside the Orion nebula. Rotational velocities, stellar parameters, and oxygen abundances
We present the results of a spectroscopic analysis of the Trapeziumcluster stars inside the Orion nebula. The rotational velocities wereobtained using the Fourier analysis method, and we found agreement withvalues derived by the usual method based on linewidth measurements. Therotational velocity derived for θ1 Ori C by thismethod is consistent with the variability of some of its spectralfeatures that have a period of 15.42 days. By means of the fit of H, HeI, and He II observed profiles with Fastwind synthetic profiles, stellarparameters and wind characteristics were derived. This methodology letus estimate the errors associated with these parameters, and we foundthat macroturbulence effects have to be included for a good fit to theHe I-II lines in the spectrum of θ1 Ori C. By means ofa very accurate study, oxygen abundances were derived for the threeB0.5V stars θ1 Ori A, D, and θ2 OriB. Final abundances are consistent with the nebular gas-phase resultspresented in Esteban et al. (2004) and are lower than those given byCunha & Lambert (1994). Our results suggest a lower dust depletionfactor of oxygen than previous estimations for the Orion nebula.
| Nonlocal radiative coupling in non monotonic stellar winds
There is strong observational evidence of shocks and clumping inradiation-driven stellar winds from hot, luminous stars. The resultingnon monotonic velocity law allows for radiative coupling between distantlocations, which is so far not accounted for in hydrodynamic windsimulations. In the present paper, we determine the Sobolev sourcefunction and radiative line force in the presence of radiative couplingin spherically symmetric flows, extending the geometry-free formalism ofRybicki & Hummer (1978, ApJ, 219, 654) to the case of three-pointcoupling, which can result from, e.g., corotating interaction regions,wind shocks, or mass overloading. For a simple model of an overloadedwind, we find that, surprisingly, the flow decelerates at all radiiabove a certain height when nonlocal radiative coupling is accountedfor. We discuss whether radiation-driven winds might in general not beable to re-accelerate after a non monotonicity has occurred in thevelocity law.
| 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.
| 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.
| Orbital parameters of the microquasar LS I +61 303
New optical spectroscopy of the high-mass X-ray binary microquasar LS I+61 303 is presented. Eccentric orbital fits to our radial velocitymeasurements yield updated orbital parameters in good agreement withprevious work. Our orbital solution indicates that the periastronpassage occurs at radio phase 0.23 and the X-ray/radio outbursts aretriggered 2.5-4 d after the compact star passage. The spectrum of theoptical star is consistent with a B0 V spectral type and contributes ~65per cent of the total light, the remainder being the result of emissionby a circumstellar disc. We also measure the projected rotationalvelocity to be v sini~= 113 km s-1.
| The epoch of the constellations on the Farnese Atlas and their origin in Hipparchus's lost catalogue
Not Available
| 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.
| Far-Ultraviolet and Optical Observations of BW Vulpeculae
We have compared the behavior of 39 Far-Ultraviolet SpectroscopicExplorer spectra through the pulsation cycle of the very large amplitudeβ Cephei star BW Vul with optical and UV (IUE) spectra, as well asoptical photometry. The FUSE light curve suggests a temperatureamplitude of about 4000 K, while the radial velocity curve is similar tocorresponding optical curves, except that the famous ``stillstand''feature, due to a shock wave caused by returning infall to theatmosphere from a previous cycle, is slightly weakened. We haveperformed line syntheses on a number of features in various spectralregions. We find that profiles of the red He I lines (λ6678 andλ5876) at ``faint star'' and ``bright star'' phases can besynthesized with standard Kurucz model atmospheres by assuming moderateand high microturbulences, respectively. For the faint-star profile, theline profiles are too deep to be explained other than by lowering theboundary temperature of a standard Kurucz model by 1000-2000 K. Thisresult is confirmed by line syntheses of the far-UV spectra at bothphases, which show that fits can be best achieved with models havingeffective temperatures 4000 K hotter than models fitting optical lines.This result is robust against non-LTE effects. The combined optical andfar-UV results suggest that the temperature distributions of both thelower and upper atmosphere are steeper than those computed in (static)Kurucz models. With ad hoc modifications to the atmospheric temperaturedistribution, we simulated the weakening of the temperature-sensitive CII λλ6578-6583 doublet during shock passages byintroducing a uniform shallow temperature gradient through the lineformation region. This result agrees with published hydrodynamic resultsthat these shocks produce a plateau-like elevation in gas variablesthrough the atmosphere.
| Dust Properties in the Far-Ultraviolet in Ophiuchus
We have derived the albedo (a) and phase function asymmetry factor (g)of interstellar dust grains at 1100 Å using archival Voyagerobservations of diffuse radiation in Ophiuchus. We have found that thegrains are highly forward-scattering, with g=0.55+/-0.25 anda=0.40+/-0.10. Even though most of the gas in this direction is in theOphiuchus molecular cloud, the diffuse FUV radiation is almost entirelydue to scattering in a relatively thin foreground cloud. This suggeststhat one cannot assume that the UV background is directly correlatedwith the total amount of gas in any direction.
| 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 Study of Herbig Ae/Be Stars
We present ASCA results of intermediate-mass pre-main-sequence (PMS)stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources,we detect 11 plausible X-ray counterparts. X-ray luminosities of thedetected sources in the 0.5-10 keV band are in the range oflogLX~30-32 ergs s-1, which is systematicallyhigher than those of low-mass PMS stars. This fact suggests that thecontribution of a possible low-mass companion is not large. Most of thebright sources show significant time variation; in particular, two HAeBestars-MWC 297 and TY CrA-exhibit flarelike events with long decaytimescales (e-folding time ~10-60 ks). These flare shapes are similar tothose of low-mass PMS stars. The X-ray spectra are successfullyreproduced by an absorbed one- or two-temperature thin-thermal plasmamodel. The temperatures are in the range of kT~1-5 keV, significantlyhigher than those of main-sequence OB stars (kT<1 keV). These X-rayproperties are not explained by wind-driven shocks, but are more likelydue to magnetic activity. On the other hand, the plasma temperaturerises as absorption column density increases or as HAeBe stars ascend toearlier phases. The X-ray luminosity reduces after stellar age of a fewtimes 106 yr. X-ray activity may be related to stellarevolution. The age of the activity decay is apparently near thetermination of jet or outflow activity. We thus hypothesize thatmagnetic activity originates from the interaction of the large-scalemagnetic fields coupled to the circumstellar disk. We also discussdifferences in X-ray properties between HAeBe stars and main-sequence OBstars.
| Optical Spectroscopy of the Surface Population of the ρ Ophiuchi Molecular Cloud: The First Wave of Star Formation
We present the results of optical spectroscopy of 139 stars obtainedwith the Hydra multiobject spectrograph. The objects extend over a 1.3deg2 area surrounding the main cloud of the ρ Ophcomplex. The objects were selected from narrowband images to haveHα in emission. Using the presence of strong Hα emission,lithium absorption, location in the Hertzsprung-Russell diagram, orpreviously reported X-ray emission, we were able to identify 88 objectsas young stars associated with the cloud. Strong Hα emission wasconfirmed in 39 objects with line widths consistent with their origin inmagnetospheric accretion columns. Two of the strongest emission-lineobjects are young, X-ray-emitting brown dwarf candidates with M8spectral types. Comparisons of the bolometric luminosities and effectivetemperatures with theoretical models suggest a median age for thispopulation of 2.1 Myr, which is significantly older than the agesderived for objects in the cloud core. It appears that these starsformed contemporaneously with low-mass stars in the Upper Scorpiussubgroup, likely triggered by massive stars in the Upper Centaurussubgroup.
| Non-LTE modelling of the He I 10830 Å line in early-type main sequence stars
The near-IR He I 10,830 Å transition is a highly sensitivediagnostic for non-LTE effects in astrophysical plasmas. So far, non-LTEline-formation computations have failed to quantitatively reproduceobservations of this line in the entire range of early-A to late-O mainsequence stars. It is shown that the non-LTE modelling was insufficient,for the most part, either because of inaccurate photoionizationcross-sections for the 2s ^3S state or because of neglecting lineblocking. New calculations based on state-of-the-art atomic data giveexcellent agreement with observation for the He I 10,830 Åfeature, while profiles of the He I lines in the visual are retained.
| Chandra observation of the fast X-ray transient IGR J17544-2619: evidence for a neutron star?
IGR J17544-2619 belongs to a distinct group of at least seven fast X-raytransients that cannot readily be associated with nearby flare stars orpre-main sequence stars and most probably are X-ray binaries with windaccretion. Sofar, the nature of the accretor has been determined in onlyone case (SAX J1819.3-2525/V4641 Sgr). We carried out a 20 ks ChandraACIS-S observation of IGR J17544-2619 which shows the source inquiescence going into outburst. The Chandra position confirms theprevious tentative identification of the optical counterpart, a blueO9Ib supergiant at 3 to 4 kpc (Pellizza, Chaty & Negueruela, inprep.). This is the first detection of a fast X-ray transient inquiescence. The quiescent spectrum is very soft. The photon index of5.9±1.2 (90% confidence error margin) is much softer than 6quiescent black hole candidates that were observed with Chandra ACIS-S(Kong et al. 2002, ApJ, 570, 277; Tomsick et al. 2003, ApJ, 599, L133).Assuming that a significant fraction of the quiescent photons comes fromthe accretor and not the donor star, we infer that the accretor probablyis a neutron star. A fit to the quiescent spectrum of the neutron staratmosphere model developed by Pavlov et al. (1994, A&A, 289, 837)and Zavlin et al. (1996, A&A, 315, 141) implies an unabsorbedquiescent 0.5-10 keV luminosity of(5.2±1.3)×1032 erg s-1. We speculateon the nature of the brief outbursts.
| O stars with weak winds: the Galactic case
We study the stellar and wind properties of a sample of Galactic Odwarfs to track the conditions under which weak winds (i.e. mass lossrates lower than 10-8 Mȯ yr-1)appear. The sample is composed of low and high luminosity dwarfsincluding Vz stars and stars known to display qualitatively weak winds.Atmosphere models including non-LTE treatment, spherical expansion andline blanketing are computed with the code CMFGEN (Hillier & Miller1998, ApJ, 496, 407). Both UV and Hα lines are used to derive windproperties while optical H and He lines give the stellar parameters. Wefind that the stars of our sample are usually 1 to 4 Myr old. Mass lossrates of all stars are found to be lower than expected from thehydrodynamical predictions of Vink et al. (2001, A&A, 369, 574). Forstars with log {L}/{Lȯ} 5.2, the reduction is byless than a factor 5 and is mainly due to the inclusion of clumping inthe models. For stars with log {L}/{Lȯ} 5.2 thereduction can be as high as a factor 100. The inclusion of X-rayemission (possibly due to magnetic mechanisms) in models with lowdensity is crucial to derive accurate mass loss rates from UV lines,while it is found to be unimportant for high density winds. The modifiedwind momentum - luminosity relation shows a significant change of slopearound this transition luminosity. Terminal velocities of low luminositystars are also found to be low. Both mass loss rates and terminalvelocities of low L stars are consistent with a reduced line forceparameter α. However, the physical reason for such a reduction isstill not clear although the finding of weak winds in Galactic starsexcludes the role of a reduced metallicity. There may be a link betweenan early evolutionary state and a weak wind, but this has to beconfirmed by further studies of Vz stars. X-rays, through the change inthe ionisation structure they imply, may be at the origin of a reductionof the radiative acceleration, leading to lower mass loss rates. Abetter understanding of the origin of X-rays is of crucial importancefor the study of the physics of weak winds.
| Spectral analysis of early-type stars using a genetic algorithm based fitting method
We present the first automated fitting method for the quantitativespectroscopy of O- and early B-type stars with stellar winds. The methodcombines the non-LTE stellar atmosphere code fastwind from Puls et al.(2005, A&A, 435, 669) with the genetic algorithm based optimizationroutine pikaia from Charbonneau (1995, ApJS, 101, 309), allowing for ahomogeneous analysis of upcoming large samples of early-type stars (e.g.Evans et al. 2005, A&A, 437, 467). In this first implementation weuse continuum normalized optical hydrogen and helium lines to determinephotospheric and wind parameters. We have assigned weights to theselines accounting for line blends with species not taken into account,lacking physics, and/or possible or potential problems in the modelatmosphere code. We find the method to be robust, fast, and accurate.Using our method we analysed seven O-type stars in the young cluster CygOB2 and five other Galactic stars with high rotational velocities and/orlow mass loss rates (including 10 Lac, ζ Oph, and τ Sco) thathave been studied in detail with a previous version of fastwind. Thefits are found to have a quality that is comparable or even better thanproduced by the classical “by eye” method. We defineerrorbars on the model parameters based on the maximum variations ofthese parameters in the models that cluster around the global optimum.Using this concept, for the investigated dataset we are able to recovermass-loss rates down to ~6 × 10-8~Mȯyr-1 to within an error of a factor of two, ignoringpossible systematic errors due to uncertainties in the continuumnormalization. Comparison of our derived spectroscopic masses with thosederived from stellar evolutionary models are in very good agreement,i.e. based on the limited sample that we have studied we do not findindications for a mass discrepancy. For three stars we findsignificantly higher surface gravities than previously reported. Weidentify this to be due to differences in the weighting of Balmer linewings between our automated method and “by eye” fittingand/or an improved multidimensional optimization of the parameters. Theempirical modified wind momentum relation constructed on the basis ofthe stars analysed here agrees to within the error bars with thetheoretical relation predicted by Vink et al. (2000, A&A, 362, 295),including those cases for which the winds are weak (i.e. less than a fewtimes 10-7 Mȯ yr-1).
| On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking intoaccount the effects induced by the fast rotation on their fundamentalparameters. All program stars were observed in the BCDspectrophotometric system in order to minimize the perturbationsproduced by the circumstellar environment on the spectral photosphericsignatures. This is one of the first attempts at determining stellarmasses and ages by simultaneously using model atmospheres andevolutionary tracks, both calculated for rotating objects. The stellarages (τ) normalized to the respective inferred time that eachrotating star can spend in the main sequence phase (τ_MS) reveal amass-dependent trend. This trend shows that: a) there are Be starsspread over the whole interval 0 τ/τ_MS 1 of themain sequence evolutionary phase; b) the distribution of points in the(τ/τMS,M/Mȯ) diagram indicates thatin massive stars (M 12~Mȯ) the Be phenomenon ispresent at smaller τ/τ_MS age ratios than for less massive stars(M 12~Mȯ). This distribution can be due to: i)higher mass-loss rates in massive objets, which can act to reduce thesurface fast rotation; ii) circulation time scales to transport angularmomentum from the core to the surface, which are longer the lower thestellar mass.
| Quantitative H and K band spectroscopy of Galactic OB-stars at medium resolution
In this paper we have analyzed 25 Galactic O and early B-stars by meansof H and K band spectroscopy, with the primary goal to investigate towhat extent a lone near-IR spectroscopy is able to recover stellar andwind parameters derived in the optical. Most of the spectra have beentaken with subaru-ircs, at an intermediate resolution of 12 000, andwith a very high S/N, mostly on the order of 200 or better. In order tosynthesize the strategic H/He lines, we have used our recent,line-blanketed version of fastwind (Puls et al. 2005, A&A, 435,669). In total, seven lines have been investigated, where for two starswe could make additional use of the Hei2.05 singlet which has beenobserved with irtf-cshell. Apart from Brγ and Heii2.18, the otherlines are predominately formed in the stellar photosphere, and thusremain fairly uncontaminated from more complex physical processes,particularly clumping. First we investigated the predicted behaviour ofthe strategic lines. In contradiction to what one expects from theoptical in the O-star regime, almost all photospheric H/Hei/Heii H/Kband lines become stronger if the gravity decreases. Concerning H andHeii, this finding is related to the behaviour of Stark broadening as afunction of electron density, which in the line cores is different formembers of lower (optical) and higher (IR) series. Regarding Hei, thepredicted behaviour is due to some subtle NLTE effects resulting in astronger overpopulation of the lower level when the gravity decreases.We have compared our calculations with results from the alternative NLTEmodel atmosphere code cmfgen (Hillier & Miller 1998, ApJ, 496, 407).In most cases, we found reasonable or nearly perfect agreement. Only theHei2.05 singlet for mid O-types suffers from some discrepancy, analogouswith findings for the optical Hei singlets. For most of our objects, weobtained good fits, except for the line cores of Brγ in earlyO-stars with significant mass-loss. Whereas the observations showBrγ mostly as rather symmetric emission lines, the models predicta P Cygni type profile with strong absorption. This discrepancy (whichalso appears in lines synthesized by cmfgen) might be an indirect effectof clumping. After having derived the stellar and wind parameters fromthe IR, we have compared them to results from previous optical analyses.Overall, the IR results coincide in most cases with the optical oneswithin the typical errors usually quoted for the correspondingparameters, i.e., an uncertainty in T_eff of 5%, in log g of 0.1 dex andin {dot M} of 0.2 dex, with lower errors at higher wind densities.Outliers above the 1-σ level where found in four cases withrespect to log g and in two cases for {dot M}.
| An XMM-Newton observation of the multiple system HD 167971 (O5-8V + O5-8V + (O8I)) and the young open cluster NGC 6604
We discuss the results of two XMM-Newton observations of the opencluster NGC 6604 obtained in April and September 2002. We concentratemainly on the multiple system HD 167971 (O5-8V + O5-8V + (O8I)). Thesoft part of the EPIC spectrum of this system is thermal with typicaltemperatures of about 2 × 106 to 9 ×106 K. The nature (thermal vs. non-thermal) of the hard partof the spectrum is not unambiguously revealed by our data. If theemission is thermal, the high temperature of the plasma (~2.3 ×107 to 4.6 × 107 K) would be typical of whatshould be expected from a wind-wind interaction zone within a longperiod binary system. This emission could arise from an interactionbetween the combined winds of the O5-8V + O5-8V close binary system andthat of the more distant O8I companion. Assuming instead that the hardpart of the spectrum is non-thermal, the photon index would be rathersteep (~3). Moreover, a marginal variability between our two XMM-Newtonpointings could be attributed to an eclipse of the O5-8V + O5-8V system.The overall X-ray luminosity points to a significant X-ray luminosityexcess of about a factor 4 possibly due to colliding winds. ConsideringHD 167971 along with several recent X-ray and radio observations, wepropose that the simultaneous observation of non-thermal radiation inthe X-ray (below 10.0 keV) and radio domains appears rather unlikely.Our investigation of our XMM-Newton data of NGC 6604 reveals a rathersparse distribution of X-ray emitters. Including the two brightnon-thermal radio emitters HD 168112 and HD 167971, we present a list of31 X-ray sources along with the results of the cross-correlation withoptical and infrared catalogues. A more complete spectral analysis ispresented for the brightest X-ray sources. Some of the members of NGC6604 present some characteristics suggesting they may be pre-mainsequence star candidates.
| XMM-Newton observations of β Centauri (B1 III): The temperature structure in the hot plasma and the photosphere-wind connection
We present XMM-Newton observations of the B giant β Centauri (B1III). The spectra are rich of spectral lines from a wide range ofionization stages which indicate temperatures in the range ~0.1-0.6 keV.Simultaneous fits to the rgs, epic-mos, and epic-pn spectra yield threeplasma temperatures (0.1, 0.2, and 0.6 keV), emission measures, andelemental abundances which are quite close to solar values. Thesetemperatures are confirmed by DEM modeling. According to the derivedmodels the intrinsic source X-ray luminosity in the energy range 0.3-10keV is Lx = 10 × 1030 erg s-1 ata distance of 161 pc. An analysis of the X-ray light curve suggests thatthe photospheric variability does not have much of an effect on theproperties of the X-ray luminosity. The sensitivity of the He-likeforbidden and intercombination lines to a strong ultraviolet stellarradiation field is used to constrain the radial distances at which thelines of Ne ix, O vii, and N vi originate.
| A non-LTE analysis of the spectra of two narrow lined main sequence stars in the SMC
An analysis of high-resolution VLT/UVES spectra of two B-type mainsequence stars, NGC 346-11 and AV 304, in the Small Magellanic Cloud(SMC), has been undertaken, using the non-LTE tlusty model atmospheresto derive the stellar parameters and chemical compositions of each star.The chemical compositions of the two stars are in reasonable agreement.Moreover, our stellar analysis agrees well with earlier analyses of H IIregions. The results derived here should be representative of thecurrent base-line chemical composition of the SMC interstellar medium asderived from B-type stars.
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| Takýmyýldýz: | Akrep |
| Sað Açýklýk: | 16h35m53.00s |
| Yükselim: | -28°12'58.0" |
| Görünürdeki Parlaklýk: | 2.82 |
| Uzaklýk: | 131.752 parsek |
| özdevim Sað Açýklýk: | -9.6 |
| özdevim Yükselim: | -21.5 |
| B-T magnitude: | 2.536 |
| V-T magnitude: | 2.759 |
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