Astronomy and Astrophysics Review manuscript No. (will be inserted by the editor) Joachim Puls Jorick S. Vink Francisco Najarro· · Mass loss from hot massive stars Received: date Abstract Mass loss is a key process in the evolution of massive stars, and must be understood quantitatively if it is to be successfully included in broader as- trophysical applications such as galactic and cosmic evolution and ionization. In this review, we discuss various aspects of radiation driven mass loss, both from the theoretical and the observational side. We focus on developments in the past decade, concentrating on the winds from OB-stars, with some excur- sions to the winds from Luminous Blue Variables (including super-Eddington, continuum-driven winds), winds from Wolf-Rayet stars, A-supergiants and Cen- tral Stars of Planetary Nebulae. After recapitulating the 1-D, stationary standard model of line-driven winds, extensions accounting for rotation and magnetic fields are discussed. Stationary wind models are presented that provide theoretical pre- dictions for the mass-loss rates as a function of spectral type, metallicity, and the proximity to the Eddington limit. 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 from corresponding campaigns (in- cluding the VLT-FLAMES survey of massive stars) with theoretical predictions, featuring the mass loss-metallicity dependence. Subsequently, we concentrate on two urgent problems, weak winds and wind-clumping, that have been identified from various diagnostics and that challenge our present understanding of radia- tion driven winds. We discuss the problems of “measuring” mass-loss rates from weak winds and the potential of the NIR Brα -line as a tool to enable a more pre- arXiv:0811.0487v1 [astro-ph] 4 Nov 2008 Joachim Puls Universit¨atssternwarte M¨unchen, Scheinerstr. 1, D-81679 M¨unchen, Germany E-mail: [email protected] Jorick S. Vink Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland E-mail: [email protected] Francisco Najarro Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain E-mail: [email protected] 2 J. Puls, J.S. Vink & F. Najarro cise quantification, and comment on physical explanations for mass-loss rates that are much lower than predicted by the standard model. Wind-clumping, conven- tionally interpreted as the consequence of a strong instability inherent to radiative line-driving, has severe implications for the interpretation of observational diag- nostics, since derived mass-loss rates are usually overestimated when clumping is present but ignored in the analyses. Depending on the specific diagnostics, such overestimates can amount to factors of 2 to 10, 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 inter- acting regions, and complete this review with a discussion of recent progress on the X-ray line emission from massive stars. Such emission is thought to originate both from magnetically confined winds and from non-magnetic winds, in the lat- ter case related to the line-driven instability and/or clump-clump collisions. We highlight as to how far the analysis of such X-ray line emission can give further clues regarding an adequate description of wind clumping. Keywords hydrodynamics stars: atmospheres stars: early-type stars: mass loss stars: winds, outflows · · · · 1 Introduction Within the last decade, our understanding of the physics of massive stars has sig- nificantly improved. It has been realized that massive stars are critical agents in galactic evolution, during both the present epoch as well as in the early Universe, and they are regarded as “cosmic engines” (Bresolin et al 2008). For instance, a population of very massive, First Stars (for a recent review, see Johnson et al 2008) is thought to play a dominant role in the reionization of the Universe and its first enrichment with metals. Already with the next generation of telescopes, the integrated light from these First Generations of Stars might become observable in the near-infrared (NIR), by means of Lyα (Barton et al 2004) and He II λ1640 (Kudritzki and The GSMT Science Working Group 2003) emission from the sur- rounding interstellar medium, if the corresponding initial mass function (IMF) is indeed top-heavy (Abel et al 2000, Bromm et al 2001). A proper knowledge of the mass-loss mechanisms during such early stages is crucial to understand the interplay of these First Stars with their environments. Another highlight concerns the long gamma-ray bursters (GRBs), which are likely the result of the terminal collapse of massive stars at low metallicity. GRBs may also become important tracers of the star formation history of the Universe at high redshift. Again, mass loss plays a dominant role in the evolution of angular-momentum loss, thus con- trolling whether the star will become such a burster. There are three factors which have contributed to the aforementioned progress: (i) new observational facilities such as ground-based 10-m class telescopes equip- ped with multi-object spectrographs, and space-born observatories with spectro- scopic capabilities in the ultraviolet (HST, FUSE, GALEX), infrared (SPITZER), X-ray (XMM-NEWTON, CHANDRA), and in the γ-ray (HETE-2, SWIFT) domains; (ii) the development of simulations of complex physical processes in the interior Mass loss from hot massive stars 3 and outer envelopes of massive stars, such as rotation and magnetic fields; and (iii) the advancement of model atmospheres and radiative transfer techniques. Modeling the atmospheres of hot stars is a tremendous challenge due to severe departures from Local Thermodynamic Equilibrium (LTE) because of the intense radiation, low densities, and the presence of supersonic velocity fields initiated by the transfer of momentum from the stellar radiation field to the atmospheric plasma. Radiation-driven winds are fundamentally important, in providing energy (kinetic and ionizing radiation) and momentum input into the ISM, in creating wind-blown bubbles and circumstellar shells, and in triggering star formation. They can affect stellar evolution by modifying evolutionary timescales, surface abundances, and stellar luminosities. The mass loss does not only influence the stellar evolution but also the atmospheric structure, and winds need to be properly modeled to derive the correct stellar parameters by means of quantitative spec- troscopy. The last review on radiation-driven winds from hot stars was given by Kudritzki and Puls (2000) who concentrated on the derivation and calibration of the so-called wind- momentum luminosity relation (WLR) and its potential application as an extra- galactic distance indicator. Thereafter, efficient methods have been developed that allow for theoretical predictions of mass-loss rates as a function of stellar param- eters in the complete upper HRD (Vink et al 2000, 2001), and these can be used to check our standard picture of radiation-driven winds through comparison with empirical results. Owing to the enormous improvements in observational capabil- ities and diagnostic methods, a number of puzzling phenomena have meanwhile emerged, and this has resulted in exciting speculations and theoretical develop- ments which are by no means settled. To name two of the most prominent, there is the so-called weak-wind problem which indicates that the mass-loss rates from late O-/early B-type dwarfs might be a factor of 10 to 100 lower than theoreti- cally expected, challenging our understanding of radiation-driven winds. Second, there is the issue of wind-clumping which refers to small-scale density inhomo- geneities distributed across the wind. Because clumping “contaminates” almost all our mass-loss sensitive diagnostics, it might result in severe down-sizing of previously derived mass-loss rates, and consequently impact significantly our un- derstanding of stellar and galactic evolution. It is the goal of this review to provide an overview of these developments, to outline their implications, and to indicate possible directions for future work. Because of the breadth of the topic and its complexity, we cannot cover all as- pects. Instead, we primarily focus our review on the winds from single, “normal” hot massive stars in well-established evolutionary stages such as OB-dwarfs, gi- ants, and supergiants, and on some of the most relevant aspects of the winds from Luminous Blue Variables (LBVs) and Wolf-Rayet (WR) stars (detailed in a re- cent review by Crowther 2007). We also consider the winds from Central Stars of Planetary Nebulae (CSPN) which are thought to obey similar physical rules as their massive O-star counterparts. For a review on physical processes related to wind-wind collisions in binary systems, we refer to De Becker (2007). This review is organized in such a way that most chapters can be read inde- pendently from each other. Sect. 2.1 provides the basic line-driven wind theory in terms of a 1-D, stationary model with statistically distributed lines. In Sect. 2.2 this “standard model” is extended by accounting for the interaction with rotation 4 J. Puls, J.S. Vink & F. Najarro and magnetic fields, which are not only important when the field strengths are large, as it is the case for few “normal”1 OB stars, but also when the wind density is quite low and relatively modest fields are present. In Sect. 3, we compare and contrast the various stationary wind models pro- viding theoretical predictions for the mass-loss rates as a function of spectral type, metallicity, and the proximity to the Eddington limit. We focus on predictions ob- tained by Monte Carlo methods for stars in various evolutionary phases (OBA, LBV, WR) and for various chemical mixtures.