Early O-type Stars Research Articles

A massive Keplerian protostellar disk with flyby-induced spirals in the Central Molecular Zone

Accretion disks are an essential component in the paradigm of the formation of low-mass stars. Recent observations further identify disks surrounding low-mass pre-main-sequence stars perturbed by flybys. Whether disks around more massive stars evolve in a similar manner has become an urgent question. We report the discovery of a Keplerian disk of a few solar masses surrounding a 32 M⊙ protostar in the Sagittarius C cloud around the Galactic Centre. The disk is gravitationally stable with two embedded spirals. A combined analysis of analytical solutions and numerical simulations demonstrates that the most likely scenario to form the spirals is through external perturbations induced by a close flyby, and one such perturber with the expected parameters is identified. The massive, early O-type star embedded in this disk forms in a similar manner as do low-mass stars, in the sense of not only disk-mediated accretion, but also flyby-impacted disk evolution. A massive and expansive Keplerian disk surrounding a massive O star in the Galaxy’s Central Molecular Zone is gravitationally stable. The disk features two embedded spirals, probably generated by external perturbations from a nearby gas condensation.

LBV phenomenon and binarity: The environment of HR Car

Luminous blue variable stars (LBVs) are of great interest in massive-star evolution as they experience very high mass-loss episodes within short periods of time. HR Car is a famous member of this class in the Galaxy. It has a large circumstellar nebula and has also been confirmed as being in a binary system. One means of gaining information about the evolutionary status and physical nature of LBVs is studying their environments. We investigated the stellar content within ∼100 pc of HR Car and also its circumstellar nebula. Very Large Telescope Multi Unit Spectroscopic Explorer (MUSE) observations of a 2′×2′ region around the star highlight the incompleteness of stellar classification for stars with magnitudes of V > 13 mag. Eight B0 to B9 stars have been identified which may lie in close spatial vicinity to HR Car. For a region with a radius of r = 1.2° (∼100 pc at a distance of 4.8 kpc) around HR Car, existing catalogs list several late O-type and early B-type stars, but only one early O-type star. Given the relatively low stellar and nebular masses in the HR Car system, no early O-type stars and only a few late O-type stars would be expected in association with HR Car. Instead, HR Car’s location in a point vector diagram suggests that HR Car is not isolated, but is part of a moving group with a population of B-type stars in a spiral arm, and it has not received a strong kick from a supernova explosion of a companion star or a merger event. Potential binary evolution pathways for the HR Car system cannot be fully explored because of the unknown nature of the companion star. Furthermore, the MUSE observations reveal the presence of a fast outflow and “bullets” that have been ejected at intervals of about 400 years. These features may have been caused by recurrent mass transfer in the system.

Low-metallicity massive single stars with rotation

Context. Metal-poor massive stars are assumed to be progenitors of certain supernovae, gamma-ray bursts, and compact object mergers that might contribute to the early epochs of the Universe with their strong ionizing radiation. However, this assumption remains mainly theoretical because individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the Small Magellanic Cloud. Aims. Here we explore the predictions of the state-of-the-art theories of stellar evolution combined with those of stellar atmospheres about a certain type of metal-poor (0.02 Z⊙) hot massive stars, the chemically homogeneously evolving stars that we call Transparent Wind Ultraviolet INtense (TWUIN) stars. Methods. We computed synthetic spectra corresponding to a broad range in masses (20−130 M⊙) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage. We investigated the influence of mass loss and wind clumping on spectral appearance and classified the spectra according to the Morgan-Keenan (MK) system. Results. We find that TWUIN stars show almost no emission lines during most of their core hydrogen-burning lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission, and even some metal lines (oxygen and carbon, but still almost no nitrogen) are detected. Mass loss and clumping play a significant role in line formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early-O type giant or supergiant, and we find Wolf–Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early-O type star observed in a low-metallicity galaxy could be the result of chemically homogeneous evolution and might therefore be the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe because they are hot without showing prominent emission lines during most of their lifetime.

Surface abundances of CNO in Galactic O-stars: a pilot study with FASTWIND

Context. Rotational mixing is known to significantly affect the evolution of massive stars; however, we still lack a consensus regarding the various possible modeling approaches and mixing recipes describing this process. The empirical investigation of surface abundances of carbon, nitrogen, and oxygen (CNO) in large samples of O- and B-type stars will be essential for providing meaningful observational constraints on the different available stellar evolution models. Aims. Setting up and testing adequate tools to perform CNO surface abundance determinations for large samples of O-type stars, by means of the fast performance, NLTE, unified model atmosphere code FASTWIND. Methods. We have developed a set of semi-automatic tools for measuring and analyzing the observed equivalent widths of strategic optical C, N, and O lines from different ions. Our analysis strategy is based on a χ2 minimization of weighted differences between observed and synthetic equivalent widths, the latter computed from tailored model grids. We have paid special attention to the (significant) errors introduced by typical uncertainties in stellar parameters. In this pilot study, we describe these tools, and test their performance and reliability using a set of high quality spectra of a sample of 18 presumably single Galactic O-type stars with low projected rotational velocities (v sin i≲100 km s−1), and previously determined stellar parameters. In particular, we have compared the outcome of our analysis with results from existing studies and theoretical stellar evolution models. Results. Most of our results for carbon and nitrogen agree, within the errors, with both theoretical expectations and literature values. While many cooler dwarfs display C and N abundances close to solar, some of the early- and mid-O dwarfs – and most supergiants – show significant enrichment in N and depletion in C. Our results for oxygen in late-O dwarfs are, however, unexpectedly low, possibly indicating deficiencies in the adopted oxygen model atom. For all other objects, no systematic problems in their oxygen content have been identified. Specific stars in our sample show peculiarities in their abundances, and we suggest hypotheses regarding their origin. Conclusions. Our method is (almost) ready to be applied to large samples of late and mid O-type stars – although the oxygen model atom needs to be improved and carefully tested first. For early O-type stars (O4 and hotter), a simultaneous UV analysis seems to be inevitable, due to the scarcity and weakness of optical C and O lines. This will necessarily imply a more complex modeling, additionally accounting for the effects of X-rays from wind-embedded shocks and wind inhomogeneities.

Ongoing star formation at the outskirts of Sextans A: Spectroscopic detection of early-O type stars.

With both nebular- and stellar-derived abundances of $\lesssim$ 1/10 Zsun and low foreground extinction, Sextans A is a prime candidate to replace the Small Magellanic Cloud as reservoir of metal-poor massive stars and reference to study the metal-poor Universe. We report the discovery of two early-O type stars in Sextans A, the earliest O-stars with metallicity < 1/7 Zsun known to date, and two additional O9 stars. Colour-excess estimates towards individual targets, enabled by spectral typing, manifest that internal reddening is neither uniform nor negligible. The four targets define a new region of star formation far from the optically-brightest centre of the galaxy and from its conspicuous HII shells, but not devoid of neutral hydrogen. In fact, we detect a spatial correlation between OB-stars and HI in Sextans A and other dIrr's that leads us to propose that the neutral phase may be fundamental to star formation in low-density environments. According to the existing evidence at least two of the targets formed in isolation, thus suggestive of an stochastic sampling of the initial mass function that would enable low-mass galaxies like Sextans A to form very massive stars. The discovery of these four stars provide spatially-resolved, spectroscopic confirmation of recent findings suggesting that dwarf galaxies can sustain star formation despite the low density of the gas phase.

The isolation of luminous blue variables: on subdividing the sample

A debate has arisen concerning the fundamental nature of LBVs) and their role in stellar evolution. While Smith and Tombleson proposed that their isolated environments indicate that LBVs must be largely the product of binary evolution, Humphreys et al. have recently expressed the view that the traditional single-star view still holds if one appropriately selects a subsample of LBVs. This paper finds the claim of Humphreys et al. to be quantitatively unjustified. A statistical test of "candidate" as opposed to "confirmed" LBVs shows no significant difference ($<$1$\sigma$) between their environments. Even if the sample is further subdivided as proposed, the three most luminous LBVs are spatially dispersed similar to late O-type dwarfs, which have much longer lifetimes than expected for classical LBVs. Lower-luminosity LBVs have a distribution associated with red supergiants (RSGs), but these RSGs are dominated by stars of 10-15 M$_{\odot}$ initial mass, with much longer lifetimes than expected for those lower-luminosity LBVs. If one's view is restricted to the highest-luminosity LBVs, then the appropriate comparison is with early O-type stars that are their presumed progenitors, when this is done, it is clear that even the high-luminosity LBVs are more dispersed than expected. Humphreys et al. also suggest that velocities of LBVs support the single-star view, being inconsistent with runaways. A quantitative analysis of the radial velocity distribution of LBVS in M31 and M33 contradicts this, modest runway speeds expected from mass gainers in binary evolution are consistent with the observed velocities, although the data lack the precision to discriminate.

Red supergiants and the past of Cygnus OB2

Context. Red supergiants are the evolved descendants of massive stars with initial masses between 7 and 40 M-circle dot. Their brightness makes them easily detectable in the near infrared, making them useful probes of star formation that occurred several tens of Myr ago. Aims. We investigate the past star formation history of Cygnus OB2, the nearest very massive OB association, using red supergiants as a probe. Our aim is to confirm the evidence, found by previous studies, that star formation in the Cygnus OB2 region started long before the latest burst that gave rise to the dense aggregate of early O-type stars that dominate the appearance of the association at present. Methods. Near-infrared star counts in the Cygnus region reveal moderate evidence for a peak in the areal density of bright, reddened stars approximately coincident with Cygnus OB2. A total of 11 sources are found within a circle of 1 degrees radius centered on the association, of which 4 are non-supergiants based on existing observations. Near-infrared spectroscopy is presented of the remaining seven candidates, including four that have been already classified as M supergiants in the literature. Results. We confirm the presence of seven red supergiants in the region and argue that they are probably physically associated with Cygnus OB2. Their location is roughly coincident with that of the older population identified by previous studies, supporting the scenario in which the main star formation activity in the association has been shifting toward higher Galactic longitudes with time. Their luminosities are compared with the predictions of evolutionary tracks with and without rotation to estimate the mass of their progenitors and ages. In this way, we confirm that massive star formation was already taking place in the area of Cygnus OB2 over 20 Myr ago, and we estimate that the star formation rate in the latest 6 Myr represents a six-fold increase over the massive star formation rate at the time when the progenitors of the current red supergiants were formed. Conclusions. The Cygnus OB2 association has a history of star formation extending into the past for at least about 20 Myr, probably dovetailing with the general history of star formation in the region that gave rise to other associations like the neighboring Cygnus OB9. The sustained massive star formation history also argues for a long lifetime of the giant molecular complex from which Cygnus OB2 formed, whose remnants constitute the present-day Cygnus X complex.

Global collapse of molecular clouds as a formation mechanism for the most massive stars

The relative importance of primordial molecular cloud fragmentation versus large-scale accretion still remains to be assessed in the context of massive core/star formation. Studying the kinematics of the dense gas surrounding massive-star progenitors can tell us the extent to which large-scale flow of material impacts the growth in mass of star-forming cores. Here we present a comprehensive dataset of the 5500(+/-800) Msun infrared dark cloud SDC335.579-0.272 (hereafter SDC335) which exhibits a network of cold, dense, parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0 observations reveal two massive star-forming cores, MM1 and MM2, sitting at the centre of SDC335 where the filaments intersect. With a gas mass of 545(+770,-385) Msun contained within a source diameter of 0.05pc, MM1 is one of the most massive, compact protostellar cores ever observed in the Galaxy. As a whole, SDC335 could potentially form an OB cluster similar to the Trapezium cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense gas furthermore reveal that the kinematics of this hub-filament system are consistent with a global collapse of the cloud. These molecular-line data point towards an infall velocity V_{inf} =0.7(+/-0.2) km/s, and a total mass infall rate \dot{M}_{inf} = 2.5(+/-1.0) x 10^{-3} Msun/yr towards the central pc-size region of SDC335. This infall rate brings 750(+/-300) Msun of gas to the centre of the cloud per free-fall time (t_{ff}=3x10^5 yr). This is enough to double the mass already present in the central pc-size region in 3.5(+2.2,-1.0) x t_{ff}. These values suggest that the global collapse of SDC335 over the past million year resulted in the formation of an early O-type star progenitor at the centre of the cloud's gravitational potential well.

The interstellar medium and the massive stellar content towards the SNR G18.1−0.1 and neighbouring H ii regions

We perform a multiwavelength study toward the SNR G18.1-0.1 and nearby several HII regions (infrared dust bubbles N21 and N22, and the HII regions G018.149-00.283 and G18.197-00.181). Our goal is to provide observational evidence supporting that massive stars usually born in clusters from the same molecular cloud, which then produce, along their evolution, different neighboring objects such as HII regions, interstellar bubbles and supernova remnants. We suggest that the objects analysed in this work belong to a same complex located at the distance of about 4 kpc. Using molecular data we inspected the interstellar medium toward this complex and from optical and X-ray observations we looked for OB-type stars in the region. Analysing public 13CO J=1--0 data we found several molecular structures very likely related to the HII region/SNR complex. We suggest that the molecular gas is very likely being swept and shaped by the expansion of the HII regions. From spectroscopic optical observations obtained with the 2.15 m telescope at CASLEO, Argentina, we discovered three O-type stars very likely exciting the bubbles N21 and N22, and an uncatalogued HII region northward bubble N22, respectively. Also we found four B0-5 stars, one toward the bubble N22 and the others within the HII region G18.149-0.283. By inspecting the Chandra Source Catalog we found two point X-ray sources and we suggest that one of them is an early O-type star. Finally we inspected the large scale interstellar medium around this region. We discovered a big molecular shell of about 70 pc x 28 pc in which the analysed complex appears to be located in its southern border.

Young, massive star candidates detected throughout the nuclear star cluster of the Milky Way

Aims. Young, massive stars have been found at projected distances R < 0.5 pc from supermassive black hole, Sgr A* at the center of our Galay. In recent years, increasing evidence has been found for the presence of young, massive stars also at R > 0.5 pc. Our goal in this work is a systematic search for young, massive star candidates throughout the entire region within R ~ 2.5 pc of the black hole. Methods. The main criterion for the photometric identification of young, massive early-type stars is the lack of CO-absorption in the spectra. We used narrow-band imaging with VLT/ISAAC to search for young, massive stars within ~2.5 pc of Sgr A*. Results. We have found 63 early-type star candidates at R < 2.5 pc, with an estimated erroneous identification rate of only about 20%. Considering their K-band magnitudes and interstellar extinction, they are candidates for Wolf-Rayet stars, supergiants, or early O-type stars. Of these, 31 stars are so far unknown young, massive star candidates, all of which lie at R>0.5pc. The surface number density profile of the young, massive star candidates can be well fit by a single power-law, with Gamma = 1.6 +- 0.17 at R < 2.5 pc, which is significantly steeper than that of the late-type giants that make up the bulk of the observable stars in the NSC. Intriguingly, this power-law is consistent with the power-law that describes the surface density of young, massive stars in the same brightness range at R < 0.5 pc. Conclusions. The finding of a significant number of newly identified early-type star candidates at the Galactic center suggests that young, massive stars can be found throughout the entire cluster which may require us to modify existing theories for star formation at the Galactic center. Follow-up studies are needed to improve the existing data and lay the foundations for a unified theory of star formation in the Milky Way's NSC.

THE EVOLUTION OF PROTOPLANETARY DISKS IN THE ARCHES CLUSTER

Most stars form in a cluster environment. These stars are initially surrounded by discs from which potentially planetary systems form. Of all cluster environments starburst clusters are probably the most hostile for planetary systems in our Galaxy. The intense stellar radiation and extreme density favour rapid destruction of circumstellar discs via photoevaporation and stellar encounters. Evolving a virialized model of the Arches cluster in the Galactic tidal field we investigate the effect of stellar encounters on circumstellar discs in a prototypical starburst cluster. Despite its proximity to the deep gravitational potential of the Galactic centre only a moderate fraction of members escapes to form an extended pair of tidal tails. Our simulations show that encounters destroy one third of the circumstellar discs in the cluster core within the first 2.5 Myr of evolution, preferentially affecting the least and most massive stars. A small fraction of these events causes rapid ejection and the formation of a weaker second pair of tidal tails that is overpopulated by disc-poor stars. Two predictions arise from our study: (i) If not destroyed by photoevaporation protoplanetary discs of massive late B- and early O-type stars represent the most likely hosts of planet formation in starburst clusters. (ii) Multi-epoch K- and L-band photometry of the Arches cluster would provide the kinematically selected membership sample required to detect the additional pair of disc-poor tidal tails.

A spectroscopic investigation of early-type stars in the young open cluster Westerlund 2

The distance of the very young open cluster Westerlund 2, which contains the very massive binary system WR 20a and is likely associated with a TeV source, has been the subject of much debate. We attempt a joint analysis of spectroscopic and photometric data of eclipsing binaries in the cluster to constrain its distance. A sample of 15 stars, including three eclipsing binaries (MSP 44, MSP 96, and MSP 223) was monitored with the FLAMES multi-object spectrograph. The spectroscopic data are analysed together with existing BV photometry. The analysis of the three eclipsing binaries clearly supports the larger values of the distance, around 8 kpc, and rules out values of about 2.4 - 2.8 kpc that have been suggested in the literature. Furthermore, our spectroscopic monitoring reveals no clear signature of binarity with periods shorter than 50 days in either the WN6ha star WR 20b, the early O-type stars MSP 18, MSP 171, MSP 182, MSP 183, MSP 199, and MSP 203, or three previously unknown mid O-type stars. The only newly identified candidate binary system is MSP 167. The absence of a binary signature is especially surprising for WR 20b and MSP 18, which were previously found to be bright X-ray sources. The distance of Westerlund 2 is confirmed to be around 8 kpc as previously suggested based on the spectrophotometry of its population of O-type stars and the analysis of the light curve of WR 20a. Our results suggest that short-period binary systems are not likely to be common, at least not among the population of O-type stars in the cluster.

The Stellar Population of M17

The stellar content of M17 has been investigated by multicolor photometry and spectroscopy. Various independent estimates yield a distance of 2.1 ± 0.2 kpc. The ratio of total-to-selective extinction is R = 3.9. Within a projected area of 3.6 × 3.7 pc, there are several thousand stars. About 74% of them show infrared excess suggesting the presence of dense circumstellar material; the excess frequency is higher for fainter stars. The number of spectroscopically classified exciting stars could be enlarged from 13 to 46. The two central O4 stars are both spectroscopic binaries; multiplicity of other early O-type stars could also be established, increasing the number of high-mass stars even further. Our data suggest at least two episodes of star formation: There are about 500 ZAMS sources (2 < AV < 7)—among them many spectroscopically classified OB stars and a significant fraction of lower mass sources with infrared excess (~25%) and X-ray emission (~6%). About 3350 heavily reddened sources with 10 < AV < 40) are most likely deeply embedded pre-main-sequence objects with an age of less than 5 × 105 yr. This group contains about 47% sources with infrared excess and 12% X-ray emitters. Cluster members later than about A0 have not yet reached the main sequence. In addition, a group of 647 protostellar candidates (1.5 < K − L < 6.9) has been detected in the cluster center as well as in the northern and southwestern bar. This population of accreting protostars argues in favor of ongoing star formation triggered by the central O stars in M17.

The very massive X-ray bright binary system Wack 2134 (= WR 21a)★

Fil: Niemela, Virpi Sinikka. Universidad Nacional de la Plata. Facultad de Ciencias Astronomicas y Geofisicas; Argentina

The young Galactic star cluster [DBS2003] 179

Recent near- and mid-infrared surveys have brought evidence that the Milky Way continues to form massive clusters. We carry out a program to determine the basic physical properties of the new massive cluster candidate [DBS2003]179. Medium-resolution K-band spectra and deep near-infrared images of [DBS2003]179 were used to derive the spectral types of eight member stars, and to estimate the distance and reddening to the cluster. Seven of ten stars with spectra show emission lines. Comparison with template spectra indicated that they are early O-type stars. The mean radial velocity of the cluster is Vrad=-77+-6 km/s. Knowing the spectral types of the members and the color excesses, we determined extinction Av~16.6 and distance modulus (m-M)0~14.5 mag (D~7.9 kpc). The presence of early O-stars and a lack of red supergiants suggests a cluster age of 2-5Myr. The total cluster mass is approximated to 0.7x10^4 Msun and it is not yet dynamically relaxed. The candidate [DBS2003]179 further increases the family of the massive young clusters in the Galaxy, although it appears less massive than the prototypical starburst clusters.

Chandra monitoring of the very massive binary WR20a and the young massive cluster Westerlund 2

Context. Westerlund 2 is a young, massive, obscured stellar cluster of our Galaxy. It harbors the most massive star with well determined parameters, WR20a (82+83M⊙), a dozen very early O-type stars (O3‐7), and a wealth of PMS stars. Although of importance, this cluster is still not well known. Aims. The high-energy properties of this cluster, especially tho se of its early-type stars are examined in details. The varia bility of the X-ray sources is investigated. Methods. A monitoring of the field was performed using three Chandra ob servations. This dataset probes daily as well as monthly to yearly timescales and provides the deepest X-ray view of the cluster to date. Results. The two Wolf-Rayet stars WR20a (WN6ha+WN6ha) and WR20b (WN6ha) were analyzed in detail. They are both very luminous and display very hard spectra, but WR20b does not seem to vary. On the contrary, WR20a, a known eclipsing, collidingwind binary, brightens in the X-ray domain during the eclipses, i.e. when the collision is seen face-on. This can be expla ined by the properties of the wind-wind collision zone, whose high density leads to a large absorbing column (2× 10 24 cm −2 ). All twelve O-type stars previously classified spectroscopi cally, two eclipsing binaries previously identified and nin e newly identified O-type star candidates are detected in the high energy domain; ten of them could be analyzed spectroscopically. Four are overluminous, but the others present typical LX/LBOL ratios, suggesting that several O-type objects are actuall y binaries. Variability at the∼ 2σ level was detected for a majority of the sources, of unknown origin for the putatively single objects. Faint, soft, diffuse emission pervades the entire field-of-view but no clear s tructure can be identified, even at the position of a blister proposed to be at the origin of the TeV source HESS J1023−575. Finally, the X-ray properties of PMS objects were also investigated, in particular the brightest flaring ones. They prov ided an additional argument in favor of a large distance (∼8kpc) for the cluster.

Forming an early O-type star through gas accretion?

We present high angular resolution (∼3) and sensitive 1.3 mm continuum, cyanogen (CN) and vinyl cyanide (C 2 H 3 CN) line observations made with the Submillimeter Array (SMA) toward one of most highly obscured objects of the W51 IRS2 region, W51 North. We find that the CN line exhibits a pronounced inverse P-Cygni profile indicating that the molecular gas is falling into this object with a mass accretion rate between 4 and 7 x 10 -2 M ⊙ yr -1 . The C 2 H 3 CN traces an east-west rotating molecular envelope that surrounds either a single obscured (proto)star with a kinematic mass of 40 M ⊙ or a small central cluster of B-type stars and that is associated with a compact high velocity bipolar outflow traced by H 2 O masers and SiO molecular emission. We thus confirm that the W51 North region is part of the growing list of young massive star forming regions that have been associated with infalling motion and with high mass accretion rates (∼10 -2 -10 -4 M ⊙ yr -1 ), strengthening the evidence that massive stars can form with very high accretion rates sufficient to quench the formation of a UCHII region.

Early-type stars in the core of the young open cluster Westerlund 2

Aims. The properties of the early-type stars in the core of the Westerlund2 cluster are examined in order to establish a link between the cluster and the very massive Wolf-Rayet binary WR20a as well as the HII complex RCW49. Methods. Photometric monitoring as well as spectroscopic observations of Westerlund2 are used to search for light variability and to establish the spectral types of the early-type stars in the cluster core. Results. The first light curves of the eclipsing binary WR20a in B and V filters are analysed and a distance of 8kpc is inferred. Three additional eclipsing binaries, which are probable late O or early B-type cluster members, are discovered, but none of the known early O-type stars in the cluster displays significant photometric variability above 1% at the 1-sigma level. The twelve brightest O-type stars are found to have spectral types between O3 and O6.5, significantly earlier than previously thought. Conclusions. The distance of the early-type stars in Westerlund2 is established to be in excellent agreement with the distance of WR20a, indicating that WR20a actually belongs to the cluster. Our best estimate of the cluster distance thus amounts to 8.0pm1.4kpc. Despite the earlier spectral types, the currently known population of early-type stars in Westerlund2 does not provide enough ionizing photons to account for the radio emission of the RCW49 complex. This suggests that there might still exist a number of embedded early O-stars in RCW49.

The Effective Temperatures of Hot Stars. II. The Early‐O Types

We derive the stellar parameters of a sample of Galactic early-O type stars by analysing their UV and Far-UV spectra from FUSE (905-1187A), IUE, HST-STIS and ORFEUS (1200-2000A). The data have been modeled with spherical, hydrodynamic, line-blanketed, non-LTE synthetic spectra computed with the WM-basic code. We obtain effective temperatures ranging from Teff = 41,000 K to 39,000 K for the O3-O4 dwarf stars, and Teff = 37,500 K for the only supergiant of the sample (O4 If+). Our values are lower than those from previous empirical calibrations for early-O types by up to 20%. The derived luminosities of the dwarf stars are also lower by 6 to 12%; however, the luminosity of the supergiant is in agreement with previous calibrations within the error bars. Our results extend the trend found for later-O types in a previous work by Bianchi & Garcia.

The Stellar Content of Obscured Galactic Giant H [CSC]ii[/CSC] Regions. III. W31

We present near-infrared (J, H, and K) photometry and moderate-resolution (λ/Δλ = 3000) K-band spectroscopy of the embedded stellar cluster in the giant H II region W31. Four of the brightest five cluster members are early O-type stars based on their spectra. We derive a spectrophotometric distance for W31 of 3.4 ± 0.3 kpc using these new spectral types and infrared photometry. The brightest cluster source at K is a red object that lies in the region of the J-H versus H-K color-color plot inhabited by stars with excess emission in the K band. This point source has an H- plus K-band spectrum that shows no photospheric features, which we interpret as being the result of veiling by local dust emission. Strong Brackett series emission and permitted Fe II emission are detected in this source; the latter feature is suggestive of a dense inflow or outflow. The near-infrared position of this red source is consistent with the position of a 5 GHz thermal radio source seen in previous high angular resolution VLA images. We also identify several other K-band sources containing excess emission with compact radio sources. These objects may represent stars in the W31 cluster still embedded in their birth cocoons.