Paul A. Crowther

Physical Properties of Wolf-Rayet Stars

AbstractThe striking broad emission line spectroscopic appearance of Wolf-Rayet (WR) stars has long defied analysis, owing to the extreme physical conditions within their line- and continuum-formin...

The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 M⊙ stellar mass limit

Spectroscopic analyses of hydrogen-rich WN5‐6 stars within the young star clusters NGC 3603 and R136 are presented, using archival Hubble Space Telescope and Very Large Telescope spectroscopy, and high spatial resolution near-IR photometry, including MultiConjugate Adaptive Optics Demonstrator (MAD) imaging of R136. We derive high stellar temperatures for the WN stars in NGC 3603 (T∗ ∼ 42±2 kK) and R136 (T∗ ∼ 53± 3 kK) plus clumping-corrected mass-loss rates of 2 ‐ 5 ×10 −5 M⊙ yr −1 which closely agree with theoretical predictions from Vink et al. These stars make a disproportionate contribution to the global ionizing and mechanical wind power budget of their host clusters. Indeed, R136a1 alone supplies ∼7% of the ionizing flux of the entire 30 Doradus region. Compar isons with stellar models calculated for the main-sequence evolution of 85 ‐ 500 M⊙ accounting for rotation suggest ages of ∼1.5 Myr and initial masses in the range 105 ‐ 170 M⊙ for three systems in NGC 3603, plus 165 ‐ 320 M⊙ for four stars in R136. Our high stellar masses are supported by consistent spectroscopic and dynamical mass determinations for the components of NGC 3603 A1. We consider the predicted X-ray luminosity of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars within such a high density environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa initial mass function, both NGC 3603 and R136 are consistent with an tentative upper mass limit of ∼300 M⊙. The Arches cluster is either too old to be used to diagnose the upper mass limit, exhibits a deficiency of very massive stars, or mo re likely stellar masses have been underestimated ‐ initial masses for the most luminous stars in the Arches cluster approach 200 M⊙ according to contemporary stellar and photometric results. The potential for stars greatly exceeding 150 M⊙ within metal-poor galaxies suggests that such pair-instab ility supernovae could occur within the local universe, as has been claimed for SN 2007bi.

Realistic ionizing fluxes for young stellar populations from 0.05 to 2 Z

We present a new grid of ionizing fluxes for O and Wolf‐Rayet (W‐R) stars for use with evolutionary synthesis codes and single-star H II region analyses. A total of 230 expanding, non-LTE, line-blanketed model atmospheres have been calculated for five metallicities (0.05, 0.2, 0.4, 1 and 2 Z� ) using the WM-BASIC code of Pauldrach, Hoffmann & Lennon for O stars and the CMFGEN code of Hillier & Miller for W‐R stars. The stellar wind parameters are scaled with metallicity for both O and W‐R stars. We compare the ionizing fluxes of the new models with the CoStar models of Schaerer & de Koter and the pure helium W‐R models of Schmutz, Leitherer & Gruenwald. We find significant differences, particularly above 54 eV, where the emergent flux is determined by the wind density as a function of metallicity. The new models have lower ionizing fluxes in the He I continuum with important implications for nebular line ratios. We incorporate the new models into the evolutionary synthesis code STARBURST99 and compare the ionizing outputs for an instantaneous burst and continuous star formation with the work of Schaerer & Vacca (SV98), and Leitherer et al. The changes in the output ionizing fluxes as a function of age are dramatic. We find that, in contrast to previous studies, nebular He II λ4686 will be at, or just below, the detection limit in low metallicity starbursts during

On the massive stellar population of the super star cluster Westerlund 1

We present new spectroscopic and photometric observations of the young Galactic open cluster Westerlund 1 (Wd 1) that reveal a unique population of massive evolved stars. We identify ∼200 cluster members and present spectroscopic clas- sifications for ∼25% of these. We find that all stars so classified are unambiguously post-Main Sequence objects, consistent with an apparent lack of an identifiable Main Sequence in our photometric data to V ∼ 20. We are able to identify rich popu- lations of Wolf Rayet stars, OB supergiants and short lived transitional objects. Of these, the latter group consists of both hot (Luminous Blue Variable and extreme B supergiant) and cool (Yellow Hypergiant and Red Supergiant) objects - we find that half the known Galactic population of YHGs resides within Wd 1. We obtain a mean V − MV ∼ 25 mag from the cluster Yellow Hypergiants, implying a Main Sequence turnoff at or below MV = −5 (O7 V or later). Based solely on the masses inferred for the 53 spectroscopically classified stars, we determine an absolute minimum mass of ∼1.5 × 10 3 Mfor Wd 1. However, considering the complete photometrically and spectroscopically selected cluster population and adopting a Kroupa IMF we infer a likely mass for Wd 1 of ∼10 5 M� , noting that inevitable source confusion and incompleteness are likely to render this an underestimate. As such, Wd 1 is the most massive compact young cluster yet identified in the Local Group, with a mass exceeding that of Galactic Centre clusters such as the Arches and Quintuplet. Indeed, the luminosity, inferred mass and com- pact nature of Wd 1 are comparable with those of Super Star Clusters - previously identified only in external galaxies - and is consistent with expectations for a Globular Cluster progenitor.We present new spectroscopic and photometric observations of the young Galactic open cluster Westerlund 1 (Wd 1) that reveal a unique population of massive evolved stars. We identify ~200 cluster members and present spectroscopic classifications for ~25% of these. We find that all stars so classified are unambiguously post-Main Sequence objects, consistent with an apparent lack of an identifiable Main Sequence in our photometric data to V~20. We are able to identify rich populations of Wolf Rayet stars, OB supergiants and short lived transitional objects. Of these, the latter group consists of both hot (Luminous Blue Variable and extreme B supergiants) and cool (Yellow Hypergiant and Red Supergiant) objects - we find that half the known Galactic population of YHGs resides within Wd1. We obtain a mean V-M_V ~25 mag from the cluster Yellow Hypergiants, implying a Main Sequence turnoff at or below M_V =-5 (O7 V or later). Based solely on the masses inferred for the 53 spectroscopically classified stars, we determine an absolute minimum mass of \~1.5 x 10^3 Msun for Wd 1. However, considering the complete photometrically and spectroscopically selected cluster population and adopting a Kroupa IMF we infer a likely mass for Wd 1 of ~10^5 Msun, noting that inevitable source confusion and incompleteness are likely to render this an underestimate. As such, Wd 1 is the most massive compact young cluster yet identified in the Local Group, with a mass exceeding that of Galactic Centre clusters such as the Arches and Quintuplet. Indeed, the luminosity, inferred mass and compact nature of Wd 1 are comparable with those of Super Star Clusters - previously identified only in external galaxies - and is consistent with expectations for a Globular Cluster progenitor.

The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars

We present a comprehensive study of the observational dependence of the mass-loss rate in stationary stellar winds of hot massive stars on the metal content of their atmospheres. The metal content of stars in the Magellanic Clouds is discussed, and a critical assessment is given of state-of-the-art mass-loss determinations of OB stars in these two satellite systems and the Milky-Way. Assuming a powerlaw dependence of mass loss on metal content, u M ∝ Z m , and adopting a theoretical relation between the terminal flow velocity and metal content, v∞ ∝ Z 0.13 (Leitherer et al. 1992, ApJ, 401, 596), we find m = 0.83 ± 0.16 for non-clumped outflows from an analysis of the wind momentum luminosity relation (WLR) for stars more luminous than 10 5.2 L� . Within the errors, this result is in agreement with the prediction m = 0.69 ± 0.10 by Vink et al. (2001, A&A, 369, 574). Absolute empirical values for the mass loss, based on Hα and ultraviolet (UV) wind lines, are found to be a factor of two higher than predictions in this high luminosity regime. If this difference is attributed to inhomogeneities in the wind, and this clumping does not impact the predictions, this would imply that luminous O and early-B stars have clumping factors in their Hα and UV line forming regions of about a factor of four. For lower luminosity stars, the winds are so weak that their strengths can generally no longer be derived from optical spectral lines (essentially Hα) and one must currently rely on the analysis of UV lines. We confirm that in this low-luminosity domain the observed Galactic WLR is found to be much steeper than expected from theory (although the specific sample is rather small), leading to a discrepancy between UV mass-loss rates and the predictions by a factor 100 at luminosities of L ∼ 10 4.75 L� , the origin of which is unknown. We emphasize that even if the current mass-loss rates of hot luminous stars would turn out to be overestimated as a result of wind clumping, but the degree of clumping would be rather independent of metallicity, the scalings derived in this study are expected to remain correct.

The VLT-FLAMES Tarantula Survey VIII. Multiplicity properties of the O-type star population

Context. The Tarantula Nebula in the Large Magellanic Cloud is our closest view of a starburst region and is the ideal environment to investigate important questions regarding the formation, evolution and final fate of the most massive stars. Aims. We analyze the multiplicity properties of the massive O-type star population observed through multi-epoch spectroscopy in the framework of the VLT-FLAMES Tarantula Survey. With 360 O-type stars, this is the largest homogeneous sample of massive stars analyzed to date. Methods. We use multi-epoch spectroscopy and variability analysis to identify spectroscopic binaries. We also use a Monte-Carlo method to correct for observational biases. By modeling simultaneously the observed binary fraction, the distributions of the amplitudes of the radial velocity variations and the distribution of the time scales of these variations, we constrain the intrinsic current binary fraction and period and mass-ratio distributions. Results. We observe a spectroscopic binary fraction of 0.35 ± 0.03, which corresponds to the fraction of objects displaying statistically significant radial velocity variations with an amplitude of at least 20 km s-1. We compute the intrinsic binary fraction to be 0.51 ± 0.04. We adopt power-laws to describe the intrinsic period and mass-ratio distributions: f(log 10P/d) ~ (log 10P/d)π (with log 10P/d in the range 0.15−3.5) and f(q) ~ qκ with 0.1 ≤ q = M2/M1 ≤ 1.0. The power-law indexes that best reproduce the observed quantities are π = −0.45 ± 0.30 and κ = −1.0 ± 0.4. The period distribution that we obtain thus favours shorter period systems compared to an Opik law (π = 0). The mass ratio distribution is slightly skewed towards low mass ratio systems but remains incompatible with a random sampling of a classical mass function (κ = −2.35). The binary fraction seems mostly uniform across the field of view and independent of the spectral types and luminosity classes. The binary fraction in the outer region of the field of view (r > 7.8′, i.e. ≈117 pc) and among the O9.7 I/II objects are however significantly lower than expected from statistical fluctuations. The observed and intrinsic binary fractions are also lower for the faintest objects in our sample (Ks > 15.5 mag), which results from observational effects and the fact that our O star sample is not magnitude-limited but is defined by a spectral-type cutoff. We also conclude that magnitude-limited investigations are biased towards larger binary fractions. Conclusions. Using the multiplicity properties of the O stars in the Tarantula region and simple evolutionary considerations, we estimate that over 50% of the current O star population will exchange mass with its companion within a binary system. This shows that binary interaction is greatly affecting the evolution and fate of massive stars, and must be taken into account to correctly interpret unresolved populations of massive stars.

A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds

We describe a moderate-resolution Far Ultraviolet Spectroscopic Explorer (FUSE) survey of H2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. Our survey is sensitive to N(H2) ≥ 1014 cm-2; the highest column densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions = 0.010 for the SMC and = 0.012 for the LMC, compared with = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Lyα in pencil beam sight lines as measures of N(H I). These results imply that the diffuse H2 masses of the LMC and SMC are 8 × 106 and 2 × 106 M☉, respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 × 10-18 cm3 s-1, and incident radiation fields ranging from 10-100 times the Galactic mean value. We find that these high-radiation, low formation rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H2 column densities in low rotational states (J = 0 and 1) to derive kinetic and/or rotational temperatures of diffuse interstellar gas, and we find that the distribution of rotational temperatures is similar to Galactic gas, with T01 = 82 ± 21 K for clouds with N(H2) ≥ 1016.5 cm-2. There is only a weak correlation between detected H2 and far-infrared fluxes as determined by IRAS, perhaps as a result of differences in the survey techniques. We find that the surface density of H2 probed by our pencil beam sight lines is far lower than that predicted from the surface brightness of dust in IRAS maps. We discuss the implications of this work for theories of star formation in low-metallicity environments.

A Neutron Star with a Massive Progenitor in Westerlund 1

We report the discovery of an X-ray pulsar in the young, massive Galactic star cluster Westerlund 1. We detected a coherent signal from the brightest X-ray source in the cluster, CXO J164710.2–455216, during two Chandra observations on 2005 May 22 and June 18. The period of the pulsar is 10.6107(1) s. We place an upper limit to the period derivative of u P 1M⊙. Taken together, the properties of the pulsar indicate that it is a magnetar. The rarity of slow X-ray pulsars and the position of CXO J164710.2–455216 only 1.6 ′ from the core of Westerlund 1 indicates that it is a member of the cluster with >99.97% confidence. Westerlund 1 contains 07V stars with initial masses Mi�35M⊙ and >50 post-main-sequence stars that indicate the cluster is 4±1 Myr old. Therefore, the progenitor to this pulsar had an initial mass Mi>40M⊙. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars. Subject headings: X-rays: stars — neutron stars — open clusters and associations: individual (Westerlund 1)

A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy ⋆

New NTT/SOFI imaging and spectroscopy of the Wolf-Rayet population in Westerlund 1 are presented. Narrow-band near-IR imaging together with follow up spectroscopy reveals four new Wolf-Rayet stars, of which three were independently identified recently by Groh et al., bringing the confirmed Wolf-Rayet content to 24 (23 excludin g source S) ‐ representing 8% of the known Galactic Wolf-Rayet population ‐ comprising 8 WC stars and 16 (15) WN stars. Revised coordinates and near-IR photometry are presented, whilst a quantitative nearIR spectral classification scheme for Wolf-Rayet stars is pr esented and applied to members of Westerlund 1. Late subtypes are dominant, with no subtypes earlier than WN5 or WC8 for the nitrogen and carbon sequences, respectively. A qualitative inspection of the WN stars suggests that most (�75%) are highly H-deficient. The Wolf-Rayet binary fraction is high (>62%), on the basis of dust emission from WC stars, in addition to a significant WN binary fraction from hard X-ray detections according to Clark et al. We exploit the large WN population of Westerlund 1 to reassess its distance (�5.0 kpc) and extinction (AKS � 0.96 mag), such that it is located at the edge of the Galactic bar, with an oxygen metallicity �60% higher than Orion. The observed ratio of WR stars to red and yellow hypergiants, N(WR)/N(RSG+YHG)� 3, favours an age of�4.5‐5.0 Myr, with individual Wolf-Rayet stars descended from progenitors of initial mass � 40 55M⊙. Qualitative estimates of current masses for non-dusty, H-free WR stars are presented, revealing 10 18M⊙, such that �75% of the initial stellar mass has been removed via stellar winds or close binary evolution. We present a revision to the cluster turn-off mass for other Milky Way clusters in which Wolf-Rayet stars are known, based upon the latest temperature calibration for OB stars. Finally, c omparisons between the observed WR population and subtype distribution in Westerlund 1 and instantaneous burst evolutionary synthesis models are presented.

Stellar and wind properties of LMC WC4 stars A metallicity dependence for Wolf-Rayet mass-loss rates ?

We use ultraviolet space-based (FUSE, HST) and optical/IR ground-based (2.3 m MSSSO, NTT) spectroscopy to determine the physical parameters of six WC4-type Wolf-Rayet stars in the Large Magellanic Cloud. Stellar parameters are revised significantly relative to Grafener et al. (1998) based on improved observations and more sophisticated model atmosphere codes, which account for line blanketing and clumping. We find that stellar luminosities are revised upwards by up to 0.4 dex, with surface abundances spanning a lower range of 0.1 C/He 0.35 (20-45% carbon by mass) and O/He 0.06 (10% oxygen by mass). Relative to Galactic WC5-8 stars at known distance, and analysed in a similar manner, LMC WC4 stars possess systematically higher stellar luminosities,0.2 dex lower wind densities, yet a similar range of surface chemistries. We illustrate how the classification Ciii5696 line is extremely sensitive to wind density, such that this is the principal dierence between the subtype distribution of LMC and Galactic early-type WC stars. Temperature dierences do play a role, but carbon abundance does not aect WC spectral types. We illustrate the eect of varying temperature and mass-loss rate on the WC spectral type for HD 32257 (WC4, LMC) and HD 156385 (WC7, Galaxy) which possess similar abundances and luminosities. Using the latest evolutionary models, pre-supernova stellar masses in the range 11-19 M are anticipated for LMC WC4 stars, with 7-14 M for Galactic WC stars with known distances. These values are consistent with pre-cursors of bright type- Ic supernovae such as SN 1998bw (alias GRB 980425) for which a minimum total mass of C and O of 14 M has been independently derived.