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A census of the Carina Nebula - I. Cumulative energy input from massive stars
The Carina Nebula (NGC 3372) is our richest nearby laboratory in whichto study feedback through ultraviolet radiation and stellar winds fromvery massive stars during the formation of an OB association, at anearly phase in the evolution of the surrounding proto-superbubble beforesupernova explosions have influenced the environment. This feedback istriggering successive generations of new star formation around theperiphery of the nebula, while simultaneously evaporating the gas anddust reservoirs out of which young stars are trying to accrete material.This paper takes inventory of the combined effect from all the knownmassive stars that power the Carina Nebula through their total ionizingflux and integrated mechanical energy from their stellar winds. Carinais close enough and accessible enough that spectral types for individualstars are available, and many close binary and multiple systems haverecently been spatially resolved, so that one can simply add them.Adopting values from the literature for corresponding spectral types,the present-day total ionizing photon luminosity produced by the 65 Ostars and three WNL stars in Carina is QH~=1051s-1, the total bolometric luminosity of allstars earlier than B2 is 2.5 × 107Lsolar,and the total mechanical luminosity of stellar winds is LSW~=105Lsolar. The total QH was about 25per cent higher when η Carinae was on the main sequence, before itand its companion were surrounded by its obscuring dust shell; for thefirst 3Myr, the net ionizing flux of the 70 O stars in Carina was about150 times greater than in the Orion Nebula. About400-500Msolar has been contributed to the HII region bystellar wind mass-loss during the past 3Myr. Values for QHand LSW are also given for the individual clusters Tr14, 15and 16, and Bo10 and 11, which are more relevant on smaller spatialscales than the total values for the whole nebula.
| 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.
| Can single O stars produce non-thermal radio emission?
We present a model for the non-thermal radio emission from presumablysingle O stars, in terms of synchrotron emission from relativisticelectrons accelerated in wind-embedded shocks. These shocks areassociated with an unstable, chaotic wind. The main improvement withrespect to earlier models is the inclusion of the radial dependence ofthe shock velocity jump and compression ratio, based on one-dimensionaltime-dependent hydrodynamical simulations. The decrease of the velocityjump and the compression ratio as a function of radius produces arapidly decreasing synchrotron emissivity. This effectively prohibitsthe models from reproducing the spectral shape of the observednon-thermal radio emission. We investigate a number of “escaperoutes” by which the hydrodynamical predictions might bereconciled with the radio observations. We find that the observedspectral shape can be reproduced by a slower decline of the compressionratio and the velocity jump, by the re-acceleration of electrons in manyshocks or by adopting a lower mass-loss rate. However, none of theseescape routes are physically plausible. In particular, re-accelerationby feeding an electron distribution through a number of shocks, is incontradiction with current hydrodynamical simulations. Thesehydrodynamical simulations have their limitations, most notably the useof one-dimensionality. At present, it is not feasible to performtwo-dimensional simulations of the wind out to the distances requiredfor synchrotron-emission models. Based on the current hydrodynamicmodels, we suspect that the observed non-thermal radio emission from Ostars cannot be explained by wind-embedded shocks associated with theinstability of the line-driving mechanism. The most likely alternativemechanism is synchrotron emission from colliding winds. That would implythat all O stars with non-thermal radio emission should be members ofbinary or multiple systems.
| 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.
| The Physical Properties and Effective Temperature Scale of O-Type Stars as a Function of Metallicity. II. Analysis of 20 More Magellanic Cloud Stars and Results from the Complete Sample
In order to determine the physical properties of the hottest and mostluminous stars and understand how these properties change as a functionof metallicity, we have analyzed HST/UV and high-S/N optical spectra ofan additional 20 Magellanic Cloud stars, doubling the sample presentedin the first paper in this series. Our analysis uses non-LTEline-blanketed models that include spherical extension and thehydrodynamics of the stellar wind. In addition, our data set includesFUSE observations of O VI and HST near-UV He I and He II lines to testfor consistency of our derived stellar properties for a few stars. Theresults from the complete sample are as follows: (1) We present aneffective temperature scale for O stars as a function of metallicity. Wefind that the SMC O3-7 dwarfs are 4000 K hotter than Galactic stars ofthe same spectral type. The difference is in the sense expected due tothe decreased significance of line blanketing and wind blanketing at thelower metallicities that characterize the SMC. The temperaturedifference between the SMC and Milky Way O dwarfs decreases withdecreasing temperature, becoming negligible by spectral type B0, inaccord with the decreased effects of stellar winds at lower temperaturesand luminosities. The temperatures of the LMC stars appear to beintermediate between that of the Milky Way and SMC, as expected based ontheir metallicities. Supergiants show a similar effect but are roughly3000-4000 K cooler than dwarfs for early O stars, also with a negligibledifference by B0. The giants appear to have the same effectivetemperature scale as dwarfs, consistent with there being littledifference in the surface gravities. When we compare our scale to otherrecent modeling efforts, we find good agreement with some CMFGENresults, while other CMFGEN studies are discordant, although there arefew individual stars in common. WM-BASIC modeling by others has resultedin significantly cooler effective temperatures than what we find, asdoes the recent TLUSTY/CMFGEN study of stars in the NGC 346 cluster, butour results lead to a far more coeval placement of stars in the H-Rdiagram for this cluster. (2) We find that the wind momentum of thesestars scales with luminosity and metallicity in the ways predicted byradiatively driven wind theory, supporting the use of photosphericanalyses of hot luminous stars as a distance indicator for galaxies withresolved massive star populations. (3) A comparison of the spectroscopicmasses with those derived from stellar evolutionary theory showsrelatively good agreement for stars with effective temperatures below45,000 K; however, stars with higher temperatures all show a significantmass discrepancy, with the spectroscopic masses a factor of 2 or moresmaller than the evolutionary masses. This problem may in part be due tounrecognized binaries in our sample, but the result suggests a possiblesystematic problem with the surface gravities or stellar radii derivedfrom our models. (4) Our sample contains a large number of stars of theearliest O types, including those of the newly proposed O2 subtype. Weprovide the first quantitative descriptions of their defining spectralcharacteristics and investigate whether the new types are a legitimateextension of the effective temperature sequence. We find that the NIII/N IV emission line ratio used to define the new classes does not, byitself, serve as an effective temperature indicator within a givenluminosity class: there are O3.5 V stars that are as hot or hotter thanO2 V stars. However, the He I/He II ratio does not fair much better forstars this hot, as we find that He I λ4471/He II λ4542,usually taken primarily as a temperature indicator, becomes sensitive toboth the mass-loss rate and surface gravities for the hottest stars.This emphasizes the need to rely on all of the spectroscopic diagnosticlines, and not simply N III/N IV or even He I/He II, for these extremeobjects. (5) The two stars with the most discordant radial velocities inour sample happen to be O3 ``field stars,'' i.e., found far from thenearest OB associations. This provides the first compellingobservational evidence as to the origin of the field O stars in theMagellanic Clouds, i.e., that these are classic runaway OB stars,ejected from their birthplaces.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 (AURA), Inc.,under NASA contract NAS 5-26555. These observations are associated withprograms 6417, 7739, and 9412.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, operated for NASA by John Hopkins Universityunder NASA contract NAS5-32985. These observations are associated withprogram C002.
| 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.
| 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}.
| VLT K-band spectroscopy of massive stars deeply embedded in IRAS sources with UCHII colours
We have obtained high resolution (R = 10 000) K-band spectra ofcandidate young massive stars deeply embedded in (ultra-) compact H IIregions (UCHIIs). These objects were selected from a near-infraredsurvey of 44 fields centered on IRAS sources with UCHII colours. Often,the near-infrared counterpart of the IRAS source is a young embeddedcluster hosting massive stars. In these clusters, three types of objectsare identified. The first type (38 objects) consists of "naked" OB starswhose K-band spectra are dominated by photospheric emission. We classifythe K-band spectra of the OB-type cluster members using near-infraredclassification criteria. A few of them have a very early (O3-O4 V)spectral type, consistent with a young age of the embedded clusters. Thespectral classification provides an important constraint on the distanceto the embedded cluster. The ionising power of the population thusderived is compared to the information obtained from the infrared andradio flux of these sources. In most cases these two differentdeterminations of the ionising flux are consistent, from which weconclude that we have identified the ionising star(s) in about 50% ofthe embedded clusters. The second type (7 objects) are point sourcesassociated with UCHII radio emission, that exhibit nebular emissionlines in the near-infrared. Six of the objects in this group produce HeI emission indicative of an embedded O-type star. These objects are moreembedded than the OB stars and probably do not dominate the infraredflux as measured by IRAS. They may emit the bulk of their reprocessed UVradiation at mm wavelengths. The third type (20 objects) ischaracterised by broad (100-200 km s-1) Brγ emissionand no photospheric absorption profiles. Bik et al. (2005, A&A,submitted) show that these objects are massive YSO candidates surroundedby dense circumstellar disks.
| 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.
| Chandra Observations of Associates of η Carinae. II. Spectra
The low-resolution X-ray spectra around η Car covering Trumpler 16and part of Trumpler 14 have been extracted from a Chandra CCD ACISimage. Various analysis techniques have been applied to the spectrabased on their count rates. The spectra with the greatest number ofcounts (HD 93162 = WR 25, HD 93129 AB, and HD 93250) have been fittedwith a wind model, which uses several components with differenttemperatures and depths in the wind. Weaker spectra have been fittedwith Raymond-Smith models. The weakest spectra are simply intercomparedwith strong spectra. In general, fits produce reasonable parametersbased on knowledge of the extinction from optical studies and on therange of temperatures for high- and low-mass stars. Direct comparisonsof spectra confirm the consistency of the fitting results and alsohardness ratios for cases of unusually large extinction in the clusters.The spectra of the low-mass stars are harder than the more massivestars. Stars in the sequence evolving from the main sequence (HD 93250)through the system containing the O supergiant (HD 93129 AB) and thenthrough the Wolf-Rayet stage (HD 93162), presumably ending in theextreme example of η Car, share the property of being unusuallyluminous and hard in X-rays. For these X-ray-luminous stars, their highmass and evolutionary status (from the very last stages of the mainsequence and beyond) is the common feature. Their binary status ismixed, and their magnetic status is still uncertain.Based on observations made with the Chandra X-Ray Observatory.
| Non-LTE Line Formation for Hydrogen Revisited
We discuss aspects of non-LTE line formation for hydrogen in early-typestars. We evaluate the effect of variations in the electron-impactexcitation cross sections in model atoms of differing complexity bycomparison with observation. While the Balmer lines are basicallyunaffected by the choice of atomic data, the Paschen, Brackett, andPfund series members allow us to discriminate between the differentmodels. Non-LTE calculations based on the widely used approximationformulae of Mihalas, Heasley, & Auer and of Johnson fail tosimultaneously reproduce the optical and IR spectra over the entireparameter range. The use of data from ab initio calculations up toprincipal quantum number n<=7 largely solves the problem. Werecommend a reference model using the available data. This model is ofgeneral interest because of the ubiquity of the hydrogen spectrum.
| The Effective Temperatures of Hot Stars. II. The Early-O Types
We derived the stellar parameters of a sample of Galactic early-O typestars by analyzing their UV and far-UV spectra from the Far UltravioletSpectroscopic Explorer (905-1187 Å), the International UltravioletExplorer, the Hubble Space Telescope STIS, and the Orbiting andRetrievable Far and Extreme Ultraviolet Spectrometer (1200-2000Å). The data have been modeled with spherical, hydrodynamic,line-blanketed, non-LTE synthetic spectra computed with the WM-BASICcode. We obtain effective temperatures ranging fromTeff=41,000 to 39,000 K for the O3-O4 dwarf stars andTeff=37,500 K for the only supergiant of the sample (O4If+). Our values are lower than those from previous empiricalcalibrations for early-O types by up to 20%. The derived luminosities ofthe dwarf stars are also lower by 6%-12% however, the luminosity of thesupergiant is in agreement with previous calibrations within the errorbars. Our results extend the trend found for later O types in a previouswork by Bianchi & Garcia.Based on observations with the NASA-CNES-CSA FUSE, which is operated byThe Johns Hopkins University under NASA contract NAS5-32985, on IUEobservations from the MAST and INES archives and on MAST archival datafrom the HST and the ORFEUS mission.
| The high energy X-ray tail of η Car revealed by BeppoSAX
We report on the June 2000 long (100 ks) BeppoSAX exposure that unveileda new very high energy component of the X-ray spectrum of η Carabove 10 keV extending to at least 50 keV. We find that the 2-150 keVspectrum is best reproduced by a thermal + non-thermal model. Thethermal component dominates the 2-10 keV spectral range withkTh = 5.5 ± 0.3 keV and log NHh = 22.68± 0.01. The spectrum displays a prominent iron emission linecentred at 6.70 keV. Its equivalent width of 0.94 keV, if produced bythe thermal source, gives a slightly sub-solar iron abundance ([Fe/H] =-0.15 ± 0.02). The high energy tail above 10 keV is best fittedby a power law with a photon index of 2.42 ± 0.04. The integrated13-150 keV luminosity of 12 Lȯ is comparable tothat of the 2-10 keV thermal component (19 Lȯ). Thepresent result can be explained, in the η Car binary star scenario,by Comptonisation of low frequency radiation by high energy electrons,probably generated in the colliding wind shock front, or ininstabilities in the wind of the S Dor primary star. It is possible thatthe high energy tail had largely weakened near the minimum of the 5.53yr cycle. Probably, the thermal component has a longer recovery timelike that of the highest excitation optical emission lines. Bothfeatures can be associated with the large absorption measured byBeppoSAX at phase 0.05.Based on space observations collected with the BeppoSAX X-Ray AstronomySatellite which is a program of the Agenzia Spaziale Italiana withparticipation of the Netherlands Agency for Aerospace Programs.
| Radio observations of HD 93129A: The earliest O star with the highest mass loss?
We present the results of radio continuum observations towards the opencluster Tr 14, where our main targets are the early-type O stars HD93129A/B and HD 93128. The observations were carried out at 3 cm (8.64GHz) and 6 cm (4.80 GHz) with the Australia Telescope Compact Array.Only HD 93129A (type O2 If*) was detected; we measure flux densities ofS3 cm = 2.0 ± 0.2 mJy and S6 cm = 4.1± 0.4 mJy. The resulting spectral index of α = -1.2± 0.3 (Sν ∝ να) indicatespredominantly non-thermal emission, suggesting HD 93129A may be a binarysystem. We propose that the observed 3 cm radio emission is mostlycoming from the non-thermal wind collision region of a binary, and, to alesser extent, from the thermal winds of the primary and secondary starsin HD 93129A. At a stellar distance of 2.8 kpc, we derive a mass-lossrate Mȯ = 5.1 × 10-5 Mȯyr-1, assuming the thermal fraction of the 3 cm emission is0.5.
| Stellar and wind parameters of Galactic O-stars. The influence of line-blocking/blanketing
We have re-analyzed the Galactic O-star sample from \citet{puls96} bymeans of line-blanketed NLTE model atmospheres in order to investigatethe influence of line-blocking/blanketing on the derived parameters. Theanalysis has been carried out by fitting the photospheric and wind linesfrom H and He. In most cases we obtained a good fit, but we have alsofound certain inconsistencies which are probably related to a stillinadequate treatment of the wind structure. These inconsistenciescomprise the line cores of Hγ and Hβ insupergiants (the synthetic profiles are too weak when the mass-loss rateis determined by matching Hα) and the ``generalizeddilution effect'' (cf. \citealt{vo89}) which is still present in He I4471 of cooler supergiants and giants.Compared to pure H/He plane-parallel models we found a decrease ineffective temperatures which is largest at earliest spectral types andfor supergiants (with a maximum shift of roughly 8000 K). This findingis explained by the fact that line-blanketed models of hot stars havephotospheric He ionization fractions similar to those from unblanketedmodels at higher Teff and higher log g. Consequently, anyline-blanketed analysis based on the He ionization equilibrium resultsin lower Teff-values along with a reduction of either log gor helium abundance (if the reduction of log g is prohibited by theBalmer line wings). Stellar radii and mass-loss rates, on the otherhand, remain more or less unaffected by line-blanketing.We have calculated ``new'' spectroscopic masses and compared them withprevious results. Although the former mass discrepancy \citep{h92}becomes significantly reduced, a systematic trend for masses below 50Msun seems to remain: The spectroscopically derived valuesare smaller than the ``evolutionary masses'' by roughly 10Msun. Additionally, a significant fraction of our samplestars stays over-abundant in He, although the actual values were foundto be lower than previously determined.Also the wind-momentum luminosity relation (WLR) changes because oflower luminosities and almost unmodified wind-momentum rates. Comparedto previous results, the separation of the WLR as a function ofluminosity class is still present but now the WLR for giants/dwarfs isconsistent with theoretical predictions.We argue that the derived mass-loss rates of stars withHα in emission are affected by clumping in the lowerwind region. If the predictions from different and independenttheoretical simulations (\citealt {Vink00, Paul03, puls03a}) that theWLR should be independent of luminosity class were correct, a typicalclumping factor <ρ2>/<ρ>2 ≈5 should be derived by ``unifying'' the different WLRs.Based upon observations obtained at the INT and the European SouthernObservatory, La Silla, Chile. The INT is operated on the island of LaPalma by the ING in the Spanish Observatorio de El Roque de losMuchachos of the Instituto de Astrofísica de Canarias.Appendix A in only available in electronic form athttp://www.edpsciences.org
| XMM-Newton X-ray observations of the Carina nebula
We use new XMM-Newton observations to perform a detailed X-ray analysisof the Carina nebula region in the 0.3-12 keV energy range. Our sourcedetection yields 80 discrete X-ray sources, from which about 20 per centseem not to have optical counterparts. To get an idea of the energyspectrum of these sources, we construct an X-ray colour-colour diagramusing the energy bands 0.3-2, 2-4.5 and 4.5-12 keV. We analyse thespectra of the most intense X-ray sources associated with early-typestars, including the luminous blue variable η Carina and WR25. Weshow that the X-ray emission from these sources is well fitted bymultitemperature model spectra. We detect surprisingly intense X-rayemission at energies above 4 keV for some of the observed early-typestars, especially from CPD-59 2629 (Tr16-22) which presents particularlyhard X-ray emission. We detect intense soft X-ray emission, below <2keV, in HDE 303311, which presents an X-ray excess of about 100 timeshigher than has been observed in other O5V stars. We use these data toconstruct the Lx/Lbol relation for the 0.3-12 and3.0-12 keV energy ranges, for all the observed O-type stars, plus ηCarina and WR25. Most of the bright stars seem to agree with lowmetallicity spectral models. The Lx/Lbol ratio forO-type stars in the 0.3-12 keV range is well fitted by a constant~6.07.54.8× 10-7, in fairagreement with the canonical expression Lx/Lbol~ 2× 10-7 formerly estimated for the 0.3-2.4 keV energyband. In contrast, the Lx/Lbol relation for the3.0-12 keV range presents a strong deviation from the canonicalrelation, with a high dispersion of about four orders of magnitude. Wealso detect intrinsic X-ray time variability in seven sources, over thetime-scale of about 50 h covered by the observations. This includes anX-ray flare of about 2-h duration detected in DETWC Tr16J104429.2-594143, a source probably not physically associated with theCarina nebula. We discuss the different underlying physical mechanismsthat can be responsible for the X-ray emission from early-type stars.
| Imaging study of NGC 3372, the Carina nebula - I. UBVRIJHK photometry of Tr 14, Tr 15, Tr 16 and Car I
We present the results of a large-scale imaging photometric study of thestellar population in the northern part of NGC 3372 in the UBVRIJHKbands with a wavelength coverage from 0.33 to 2.5 μm. Theobservations were made at Las Campanas Observatory. The optical CCDmosaics cover an area approximately of 32 × 22 arcmin2centred between the Tr 14 and Tr 16 clusters. The survey was extended tocover 12 × 12 arcmin2 at the location of Tr 15.Near-infrared NICMOS3 mosaics covering the areas occupied by theseclusters were obtained in the JHK photometric bands. By means of starcounts in V, the centres and sizes of each cluster were redeterminedyielding: Tr 14 (r= 264 arcsec), Tr 15 (r= 320 arcsec) and Tr 16 (r= 320arcsec). It was confirmed that Cr 232 is not a true cluster. Multicolouroptical photometry was obtained for 4152 stars. Two colour andcolour-magnitude diagrams are presented and analysed for each individualcluster and compared to those of the field. We confirm the widespreadvariations in the dust density and also in the dust size distributionleading to widely different values of AV and reddening lawstowards Tr 14 and Tr 16. No spatial patterns were found for thesevariations. Spectroscopic parallaxes were computed and the results areconsistent with all three clusters being at a similar distance from theSun (= 2.7 kpc) but the data have shown very large scatter inboth AV and d. Analyses of the extinction-correctedcolour-magnitude diagrams suggest ages between 3 and 60 million yearsfor the stars in Tr 15 and between less than 1 and 6 million years forTr 14 and Tr 16. A small number of infrared-excess stars were found inTr 16 and Tr 14 but not in Tr 15. The distribution of stars in Tr 14seen in the near-infrared suggests that this cluster is partiallyembedded in a molecular cloud. This molecular cloud extends towards thewest reaching its highest density, marked by a CO peak emission, somethree arcmin to the south-west of the nucleus of Tr 14. The richultraviolet field created by the Tr 14 stars ionizes most of the visibleHII region in its vicinity and most of the radio HII region Car I.Evidence is found of ionization fronts leading into the molecular cloud,which appears to be `wrapping' the Tr 14 cluster. Deep JHK images of theCar I region reveal the presence of an embedded stellar populationilluminating a large infrared reflection nebula. It includes at leastone O9-B0 star associated with an ultracompact HII region. Nebulous 2.2μm emission from three of the mid-infrared sources in the Tr 14region is also found.
| Chandra Observations of Associates of η Carinae. I. Luminosities
The region around the η Carinae Nebula has three OB associations,which contain a Wolf-Rayet star and several massive O3 stars. An earlyChandra ACIS-I image was centered on η Car and includes Trumpler 16and part of Trumpler 14. The Chandra image confirms the well-knownresult that O and very early B stars are X-ray sources withLX~=10-7Lbol over an X-ray luminosityrange of about 100. Two new, anomalously strong X-ray sources have beenfound among the hot star population: Tr 16-244, a heavily reddened O3 Istar, and Tr 16-22, a heavily reddened O8.5 V star. Two stars have anunusually large LX/Lbol: HD 93162, a Wolf-Rayetstar (and possible binary), and Tr 16-22, a possible colliding-windbinary. In addition, a population of sources associated with cool starsis detected. In the color-magnitude diagram, these X-ray sources sitabove the sequence of field stars in the Carina arm. The OB stars are onaverage more X-ray-luminous than the cool star X-ray sources. X-raysources among A stars have X-ray luminosities similar to those of coolerstars and may be due to cooler companions. Upper limits are presentedfor B stars that are not detected in X-rays. These upper limits are alsothe upper limits for any cool companions that the hot stars may have.Hardness ratios are presented for the most luminous sources in bands0.5-0.9, 0.9-1.5, and 1.5-2.04 keV. The available information on thebinary nature of the hot stars is discussed, but binarity does notcorrelate with X-ray strength in a simple way.Based on observations made with the Chandra X-Ray Observatory.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| The Carina Spiral Feature: Strömgren-Hβ photometry approach. I. The photometric data-base
A data-base collating all uvbybeta photometry available at present forO-B9 stars brighter than 10th visual magnitude in the field of theCarina Spiral Feature is presented. The completeness and homogeneity ofthe data-base are discussed.Based on CDS data.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/523
| XMM-Newton high-resolution X-ray spectroscopy of the Wolf-Rayet object WR 25 in the Carina OB1 association
We report the analysis of the first high-resolution X-ray spectra of theWolf-Rayet (WR) object WR 25 (HD 93162, WN6ha+O4f) obtained with theReflection Grating Spectrometers (RGS) and the European Photon ImagingCameras (EPIC-MOS and PN) CCD spectrometers on board the XMM-Newtonsatellite. The spectrum exhibits bright emission lines of the H- andHe-like ions of Ne, Mg, Si and S, as well as Fe XVII i to Fe XX and FeXXV lines. Line fluxes have been measured. The RGS and EPIC spectrahave been simultaneously fitted to obtain self-consistent temperatures,emission measures, and elemental abundances. Strong absorption by thedense WR stellar wind and the interstellar medium (ISM) is observedequivalent to NH = 7 x 1021 cm-2.Multi-temperature (DEM) fitting yields two dominant components aroundtemperatures of 7.0 and 32 MK, respectively. The XMM intrinsic (i.e.unabsorbed, corrected for the stellar wind absorption and the absorptionof ISM) X-ray luminosity of WR 25 is Lx(0.5-10 keV) = 1.3 x1034 erg s-1, and Lx(0.5-10 keV) = 0.85x 1034 erg s-1, (when correcting for the ISM only)assuming d=3.24 kpc. The obtained chemical abundances are subsolar,except for S. This may be real, but could equally well be due to a weakcoupling to the continuum, which is strongly influenced by theabsorption column density and the subtracted background. The expectedhigh N-abundance, as observed in the optical wavelength region, couldnot be confirmed due to the strong wind absorption, blocking out itsspectral signature. The presence of the Fe XXV emission-line complex at~ 6.7 keV is argued as being indicative for colliding winds inside aWR+O 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).
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| The relation between far-UV and visible extinctions
For directions of sufficient reddening (/E(B-V)>~0.25), there is asimple relation between the slope of the extinction curve in the far-UVand /E(B-V). Regardless of direction, the far-UV extinction curve isproportional to 1/λn e-2E(B-V)/λ(/λ in μm, /n=4), in accordance with the idea that reddenedstars spectra are contaminated by scattered light (Zagury, 2001b). Thisrelation is not compatible with the standard theory of extinction whichstates that far-UV and visible extinctions are due to different classesof particle. In that model the two (far-UV and visible) extinctions varythus independently according to the proportion of each type of particle.In preceding papers I have shown that the standard theory cannot explainUV observations of nebulae, and is contradicted by the UV spectra ofstars with very low reddening: for how long shall the standard theory beconsidered as the interpretation of the extinction curve?
| An Atlas of Galactic OB Spectra Observed with the Far Ultraviolet Spectroscopic Explorer
An atlas of far-ultraviolet spectra of 45 Galactic OB stars observedwith the Far Ultraviolet Spectroscopic Explorer is presented. The atlascovers the wavelength region between 912 and 1185 Å with aneffective spectral resolution of 0.12 Å. Systematic trends in themorphology and strength of stellar features are discussed. Particularattention is drawn to the variations of the C III λ1176, S IVλλ1063, 1073, and P V λλ1118, 1128 lineprofiles as a function of temperature and luminosity class; and the lackof a luminosity dependence associated with O VI λλ1032,1038. Numerous interstellar lines are also identified. Based onobservations made with the NASA-CNES-CSA Far Ultraviolet SpectroscopicExplorer. FUSE is operated for NASA by the Johns Hopkins Universityunder NASA contract NAS5-32985.
| A New Spectral Classification System for the Earliest O Stars: Definition of Type O2
High-quality, blue-violet spectroscopic data are collected for 24 starsthat have been classified as type O3 and that display the hallmark N IVand N V lines. A new member of the class is presented; it is the secondknown in the Cyg OB2 association, and only the second in the northernhemisphere. New digital data are also presented for several of the otherstars. Although the data are inhomogeneous, the uniform plots bysubcategory reveal some interesting new relationships. Several issuesconcerning the classification of the hottest O-type spectra arediscussed, and new digital data are presented for the five original O3dwarfs in the Carina Nebula, in which the N IV, N V features are veryweak or absent. New spectral types O2 and O3.5 are introduced here assteps toward resolving these issues. The relationship between thederived absolute visual magnitudes and the spectroscopic luminosityclasses of the O2-O3 stars shows more scatter than at later O types, atleast partly because some overluminous dwarfs are unresolved multiplesystems, and some close binary systems of relatively low luminosity andmass emulate O3 supergiant spectra. However, it also appears that thebehavior of He II λ4686, the primary luminosity criterion atlater O types, responds to other phenomena in addition to luminosity atspectral types O2-O3. There is evidence that these spectral types maycorrespond to an immediate pre-WN phase, with a correspondingly largerange of luminosities and masses. A complete census of spectraclassified into the original O3 subcategories considered here (notincluding intermediate O3/WN types or O3 dwarfs without N IV, N Vfeatures) totals 45 stars; 34 of them belong to the Large MagellanicCloud and 20 of the latter to 30 Doradus.
| Near IR spectroscopy of active galactic nuclei
Using the VLT together with the near infrared instrument ISAAC, we haveobtained medium spectral and high spatial resolution observations of asample of nearby Seyfert galaxies in the H-band. This band isparticularly suited for stellar population studies since the stellarcomponent dominates over the AGN nucleus. The H-band also includes thepeak contribution from cool stars. The AGN spectra are very rich instrong metallic lines which are sensitive to stellar luminosity class.For 4 out of 5 galaxies the central velocity dispersions are found to besignificantly lower than reported in previous studies. Gradients in thestellar population within the central regions were searched for,together with evidence for dilution of the stellar spectral featureswithin the nucleus. Based on observations collected at the Very LargeTelescope (UT1) of the European Southern Observatory, Paranal, Chile,ESO NO 63.A-0366.
| BeppoSAX broad X-ray range observations of eta Carinae during high and low spectroscopic states
We present BeppoSAX spectra of the extremely luminous and massive objecteta Car observed in a very broad X-ray range (0.1-200 keV) during highstate (December 1996) and egress from the last low state (March 1998).Both spectra are composed of at least two components, a soft one withkTs < 0.7 keV, and a hard with kTh =4.7 keV. Wefound in March 1998 a large flux defect in the 1.5-4 keV range, whilethe flux remained constant below 1.5 keV and above 5 keV. We attributethis defect to a x3.5 increase of the absorbing matter in front of thehard component, while its temperature and unabsorbed luminosity werenearly the same in the two epochs. In December 1996 the PDS X-ray fluxin the 13-20 keV range is larger than the extrapolated hard spectrum,indicating the presence of an even harder additional component, whichpossibly declined during the March 1998 low state. Conversely, we findthat at that time, the flux of the 6.7 keV iron line was 40% stronger.Coordinated optical and NIR spectroscopic observations indicate that inMarch 1998 eta Car was still in a state of low excitation of theemission line spectrum, with extended P Cygni absorptions. These resultsindicate that after the X-ray flux minimum, the hard component recoveredits high state luminosity more rapidly than the high ionization spectrallines, but in the meantime it was partly occulted by a large amount ofabsorbing matter placed in front of the source. These results arediscussed in the framework of the proposed binary model of eta Car.Based on space observations collected with the BeppoSAX X-Ray AstronomySatellite which is a program of the Agenzia Spaziale Italiana withparticipation of the Netherlands Agency for Aerospace Programs, and onspectroscopic observations obtained at the Laboratório Nacionalde Astrofísica (LNA/MCT), Brazil, and at the Complejo AstronomicoEl Leoncito (CASLEO), Argentina.
| In hot pursuit of the hidden companion of eta Carinae: An X-ray determination of the wind parameters
We present X-ray spectral fits to a recently obtained Chandra gratingspectrum of eta Carinae, one of the most massive and powerful stars inthe Galaxy and which is strongly suspected to be a colliding wind binarysystem. Hydrodynamic models of colliding winds are used to generatesynthetic X-ray spectra for a range of mass-loss rates and windvelocities. They are then fitted against newly acquired Chandra gratingdata. We find that due to the low velocity of the primary wind (~500kmps), most of the observed X-ray emission appears to arise from theshocked wind of the companion star. We use the duration of thelightcurve minimum to fix the wind momentum ratio at eta = 0.2. We arethen able to obtain a good fit to the data by varying the mass-loss rateof the companion and the terminal velocity of its wind. We find that{dot M}2 ~ 10-5 Msunyr-1 andvinfty_2 ~ 3000 kmps. With observationally determined valuesof ~500-700 kmps for the velocity of the primary wind, our fit implies aprimary mass-loss rate of {dot M}1 ~ 2.5 x 10-4Msunyr-1. This value is smaller than commonlyinferred, although we note that a lower mass-loss rate can reduce someof the problems noted by Hillier et al. (\cite{HDIG2001}) when a valueas high as 10-3 Msunyr-1 is used. Thewind parameters of the companion are indicative of a massive star whichmay or may not be evolved. The line strengths appear to show slightlysub-solar abundances, although this needs further confirmation. Based onthe over-estimation of the X-ray line strengths in our model, andre-interpretation of the HST/FOS results, it appears that the Homunculusnebula was produced by the primary star.
| Optical spectroscopy of X-Mega targets - II. The massive double-lined O-type binary HD 93205
A new high-quality set of orbital parameters for the O-typespectroscopic binary HD 93205 has been obtained combining échelleand coudé CCD observations. The radial velocity orbits derivedfrom the Heii λ4686Å (primary component) and Heiλ4471Å (secondary component) absorption lines yieldsemi-amplitudes of 133+/-2 and 314+/-2kms-1 for each binarycomponent, resulting in minimum masses of 31 and 13Msolar(q=0.42). We also confirm for the binary components the spectralclassification of O3V+O8V previously assigned. Assuming for the O8Vcomponent a `normal' mass of 22-25Msolar we would derive forthe primary O3V a mass of `only' 52-60Msolar and aninclination of about 55° for the orbital plane. We have alsodetermined for the first time a period of apsidal motion for thissystem, namely 185+/-16yr using all available radial velocity data setsof HD 93205 (from 1975 to 1999). Phase-locked variations of the X-rayemission of HD 93205 consisting of a rise of the observed X-ray fluxnear periastron passage are also discussed.
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