Mass loss is a key process in the evolution of massive stars, and must be understood quantitatively to be successfully included in broader astrophysical applications. In this review, we discuss various aspects of radiation driven mass loss, both from the theoretical and the observational side. We focus on winds from OB-stars, with some excursions to the Luminous Blue Variables, Wolf- Rayet stars, A-supergiants and Central Stars of Planetary Nebulae. After reca- pitulating the 1-D, stationary standard model of line-driven wind, extensions accounting for rotation and magnetic fields are discussed. The relevance of the so-called bi-stability jump is outlined. We summarize diagnostical methods to infer wind properties from observations, and compare the results with theore- tical predictions, featuring the massloss-metallicity dependence. Subsequently, we concentrate on two urgent problems which challenge our present understanding of radiation driven winds: weak winds and wind- clumping. We discuss problems of measuring mass-loss rates from weak winds and the potential of NIR- spectroscopy. Wind-clumping has severe implications for the interpretation of observational diagnostics, as derived mass-loss rates can be overestimated by factors of 2 to 10 if clumping is ignored, and we describe ongoing attempts to allow for more uniform results. We point out that independent arguments from stellar evolution favor a moderate reduction of present- day mass-loss rates. We also consider larger scale wind structure, interpreted in terms of co-rotating interacting regions, and complete this review with a discussion of recent progress on the X-ray line emission from massive stars, highlighting as to how far the analysis of such X-ray line emission can give further clues regarding an adequate description of wind clumping. (Abridged abstract)Comment: Astronomy and Astrophysics Review (accepted

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,Ṁ ∝ 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.

Context. The nuclear star cluster of the Galaxy is an important template for understanding its extragalactic counterparts, which can currently not be resolved into individual stars. Important drawbacks of observations of the Galactic center are, however, the presence of strong and spatially highly variable interstellar extinction and extreme crowding of the sources, which makes the use of adaptive optics techniques necessary. Both points pose serious obstacles to precise photometry that is needed for analyzing the stellar population. Aims. The aims of this work are to provide accurate photometry in multiple near-infrared broadband filters, to determine the powerlaw index of the extinction-law toward the central parsec of the Galaxy, to provide measurements of the absolute extinction toward the Galactic center, and finally to measure the spatial variability of extinction on arcsecond scales. Methods. We use observations of the central parsec of the Milky Way that were obtained with the near-infrared camera and adaptive optics system NAOS/CONICA at the ESO VLT unit telescope 4. The photometric method takes into account anisoplanatic effects and limits the corresponding systematic uncertainties to 2%. Absolute values for the extinction in the H, Ks, and L -bands as well as of the power-law indices of the H to Ks and Ks to L extinction-laws are measured based on the well-known properties of red clump stars. Extinction maps are derived based on H − Ks and Ks − L colors. Results. We present Ks-band photometry for ∼7700 stars, and additionally photometry for stars detected in the H and/or L -bands. From a number of recently published values we compute a mean distance of the Galactic center of R 0 = 8.03 ± 0.15 kpc, which has an uncertainty of just 2%. Based on this R 0 and on the RC method, we derive absolute mean extinction values toward the central parsec of the Galaxy of A H = 4.48 ± 0.13 mag, A Ks = 2.54 ± 0.12 mag, and A L = 1.27 ± 0.18 mag. We estimate values of the power-law indices of the extinction-law of α H−Ks = 2.21 ± 0.24 and α Ks−L = 1.34 ± 0.29. A Ks-band extinction map for the Galactic center is computed based on this extinction law and on stellar H − Ks colors. Both its statistical and systematic uncertainties are estimated to be <10%. Extinction in this map derived from stellar color excesses is found to vary on arcsecond scales, with a mean value of A Ks = 2.74 ± 0.30 mag. Mean extinction values in a circular region with 0.5 radius centered on Sagittarius A* are A H,SgrA * = 4.35 ± 0.12, A Ks,SgrA * = 2.46 ± 0.03, and A L ,SgrA * = 1.23 ± 0.08.

We present and use new spectra and narrow-band images, along with previously published broad-band images, of stars in the Arches cluster to extract photometry, astrometry, equivalent width, and velocity information. The data are interpreted with a wind/atmosphere code to determine stellar temperatures, luminosities, mass-loss rates, and abundances. We have doubled the number of known emission-line stars, and we have also made the first spectroscopic identification of the main sequence for any population in the Galactic Center. We conclude that the most massive stars are bona-fide Wolf-Rayet (WR) stars and are some of the most massive stars known, having M_{initial} > 100 Msun, and prodigious winds, Mdot > 10^{-5} Msun yr^{-1}, that are enriched with helium and nitrogen; with these identifications, the Arches cluster contains about 5% of all known WR stars in the Galaxy. We find an upper limit to the velocity dispersion of 22 kms^{-1}, implying an upper limit to the cluster mass of 7(10^4) Msun within a radius of 0.23 pc; we also estimate the bulk heliocentric velocity of the cluster to be v_{cluster,odot} approximately +95 kms^{-1}

We have studied the optical spectra of a sample of 31 O-and early B-type stars in the Small Magellanic Cloud, 21 of which are associated with the young massive cluster NGC 346. Stellar parameters are determined using an automated fitting method (Mokiem et al. 2005, A&A, 441, 711), which combines the stellar atmosphere code fastwind (Puls et al. 2005, A&A, 435, 669) with the genetic algorithm based optimisation routine pikaia (Charbonneau 1995, ApJS, 101, 309). Comparison with predictions of stellar evolution that account for stellar rotation does not result in a unique age, though most stars are best represented by an age of 1-3 Myr. The automated method allows for a detailed determination of the projected rotational velocities. The present day v r sin i distribution of the 21 dwarf stars in our sample is consistent with an underlying rotational velocity (v r ) distribution that can be characterised by a mean velocity of about 160−190 km s −1 and an effective half width of 100−150 km s −1 . The v r distribution must include a small percentage of slowly rotating stars. If predictions of the time evolution of the equatorial velocity for massive stars within the environment of the SMC are correct (Maeder & Meynet 2001, A&A, 373, 555), the young age of the cluster implies that this underlying distribution is representative for the initial rotational velocity distribution. The location in the Hertzsprung-Russell diagram of the stars showing helium enrichment is in qualitative agreement with evolutionary tracks accounting for rotation, but not for those ignoring v r . The mass loss rates of the SMC objects having luminosities of log L /L 5.4 are in excellent agreement with predictions by Vink et al. (2001, A&A, 369, 574). However, for lower luminosity stars the winds are too weak to determineṀ accurately from the optical spectrum. Three targets were classifiedas Vz stars, two of which are located close to the theoretical zero-age main sequence. Three lower luminosity targets that were not classified as Vz stars are also found to lie near the ZAMS. We argue that this is related to a temperature effect inhibiting cooler from displaying the spectral features required for the Vz luminosity class.

We report on the unprecedented Red Supergiant (RSG) population of a massive young cluster, located at the base of the Scutum-Crux Galactic arm. We identify candidate cluster RSGs based on 2MASS photometry and medium resolution spectroscopy. With follow-up high-resolution spectroscopy, we use CObandhead equivalent width and high-precision radial velocity measurements to identify a core grouping of 26 physically-associated RSGs -the largest such cluster known to-date. Using the stars' velocity dispersion, and their inferred luminosities in conjuction with evolutionary models, we argue that the cluster has an initial mass of ∼40,000M , and is therefore among the most massive in the galaxy. Further, the cluster is only a few hundred parsecs away from the cluster of 14 RSGs recently reported by Figer et al (2006). These two RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the unique opportunity to study the pre-supernova evolution of massive stars, and the Blue-to Red-Supergiant ratio at uniform metallicity. We use GLIMPSE, MIPSGAL and MAGPIS survey data to identify several objects in the field of the larger cluster which seem to be indicative of recent region-wide starburst activity at the point where the Scutum-Crux arm intercepts the Galactic bulge. Future abundance studies of these clusters will therefore permit the study of the chemical evolution and metallicity gradient of the Galaxy in the region where the disk meets the bulge. arXiv:0708.0821v3 [astro-ph]

Abstract.We have analyzed six OB supergiants and one giant covering spectral types from O3 to B1 in the Galactic OB association Cyg OB2 by means of an updated version of FASTWIND (Santolaya-Rey et al. 1997) that includes an approximate treatment of metal line blocking and blanketing. This large coverage in spectral type allows us to derive a new temperature scale for Galactic O supergiants that is lower than the one obtained by using pure H-He models, either plane-parallel and hydrostatic or spherical with mass-loss. The lower temperatures are thus a combined effect of line blanketing and the large mass-loss rates. In some cases, the newly derived effective temperature is reduced by up to 8000 K. Changes are larger for earlier stars with large mass-loss rates. As a consequence, luminosities are modified as well, which results in a lower number of emerging ionizing photons and reduces the mass discrepancy. Although there are still significant differences between spectroscopic and evolutionary masses, we do not find any obvious systematic pattern of those differences. We derive mass-loss rates and the corresponding wind momentum-luminosity relation for the analyzed stars. Although consistent with previous results by Puls et al. (1996) for Galactic stars, our relation is better defined due to a reduction of errors related to stellar distances and points to a possible separation between extreme Of stars (Of + , Of * ) and stars with more moderate morphologies. However this finding is only tentative, as the statistics are still scarce.

Context. The 30 Doradus (30 Dor) region of the Large Magellanic Cloud, also known as the Tarantula nebula, is the nearest starburst region. It contains the richest population of massive stars in the Local Group, and it is thus the best possible laboratory to investigate open questions on the formation and evolution of massive stars. Aims. Using ground-based multi-object optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to establish the (projected) rotational velocity distribution for a sample of 216 presumably single O-type stars in 30 Dor. The sample is large enough to obtain statistically significant information and to search for variations among subpopulations -in terms of spectral type, luminosity class, and spatial location -in the field of view. Methods. We measured projected rotational velocities, e sin i, by means of a Fourier transform method and a profile fitting method applied to a set of isolated spectral lines. We also used an iterative deconvolution procedure to infer the probability density, P( e ), of the equatorial rotational velocity, e . Results. The distribution of e sin i shows a two-component structure: a peak around 80 km s −1 and a high-velocity tail extending up to ∼600 km s −1 . This structure is also present in the inferred distribution P( e ) with around 80% of the sample having 0 < e ≤ 300 km s −1 and the other 20% distributed in the high-velocity region. The presence of the low-velocity peak is consistent with what has been found in other studies for late O-and early B-type stars. Conclusions. Most of the stars in our sample rotate with a rate less than 20% of their break-up velocity. For the bulk of the sample, mass loss in a stellar wind and/or envelope expansion is not efficient enough to significantly spin down these stars within the first few Myr of evolution. If massive-star formation results in stars rotating at birth with a large portion of their break-up velocities, an alternative braking mechanism, possibly magnetic fields, is thus required to explain the present-day rotational properties of the O-type stars in 30 Dor. The presence of a sizeable population of fast rotators is compatible with recent population synthesis computations that investigate the influence of binary evolution on the rotation rate of massive stars. Even though we have excluded stars that show significant radial velocity variations, our sample may have remained contaminated by post-interaction binary products. That the highvelocity tail may be populated primarily (and perhaps exclusively) by post-binary interaction products has important implications for the evolutionary origin of systems that produce gamma-ray bursts.