An explanation for the curious mass loss history of massive stars: from OB stars, through Luminous Blue Variables to Wolf-Rayet stars

The stellar winds of massive stars show large changes in mass-loss rates and terminal velocities during their evolution from O-star through the Luminous Blue Variable phase to the Wolf-Rayet phase. The luminosity remains approximately unchanged during these phases. These large changes in wind properties are explained in the context of the radiation driven wind theory, of which we consider four different models. They are due to the evolutionary changes in radius, gravity and surface composition and to the change from optically thin (in continuum) line driven winds to optically thick radiation driven winds.Comment: Accepted for publication in Astronomy and Astrophysics (Letter to the Editor

Supernova Hosts for Gamma-Ray Burst Jets: Dynamical Constraints

I constrain a possible supernova origin for gamma-ray bursts by modeling the dynamical interaction between a relativistic jet and a stellar envelope surrounding it. The delay in observer's time introduced by the jet traversing the envelope should not be long compared to the duration of gamma-ray emission; also, the jet should not be swallowed by a spherical explosion it powers. The only stellar progenitors that comfortably satisfy these constraints, if one assumes that jets move ballistically within their host stars, are compact carbon-oxygen or helium post-Wolf-Rayet stars (type Ic or Ib supernovae); type II supernovae are ruled out. Notably, very massive stars do not appear capable of producing the observed bursts at any redshift unless the stellar envelope is stripped prior to collapse. The presence of a dense stellar wind places an upper limit on the Lorentz factor of the jet in the internal shock model; however, this constraint may be evaded if the wind is swept forward by a photon precursor. Shock breakout and cocoon blowout are considered individually; neither presents a likely source of precursors for cosmological GRBs. These envelope constraints could conceivably be circumvented if jets are laterally pressure-confined while traversing the outer stellar envelope. If so, jets responsible for observed GRBs must either have been launched from a region several hundred kilometers wide, or have mixed with envelope material as they travel. A phase of pressure confinement and mixing would imprint correlations among jets that may explain observed GRB variability-luminosity and lag-luminosity correlations.Comment: 17 pages, MNRAS, accepted. Contains new analysis of pressure-confined jets, of jets that experience oblique shocks or mix with their cocoons, and of cocoons after breakou

Neon Abundances from a Spitzer/IRS Survey of Wolf-Rayet Stars

We report on neon abundances derived from {\it Spitzer} high resolution spectral data of eight Wolf-Rayet (WR) stars using the forbidden line of [\ion{Ne}{3}] 15.56 microns. Our targets include four WN stars of subtypes 4--7, and four WC stars of subtypes 4--7. We derive ion fraction abundances $\gamma$ of Ne^{2+} for the winds of each star. The ion fraction abundance is a product of the ionization fraction $Q_{\rm i}$ in stage i and the abundance by number ${\cal A}_E$ of element E relative to all nuclei. Values generally consistent with solar are obtained for the WN stars, and values in excess of solar are obtained for the WC stars.Comment: to appear in Astrophysical Journa

Nonthermal gamma-ray and X-ray flashes from shock breakout in gamma-ray bursts/supernovae

Thermal X-ray emission which is simultaneous with the prompt gamma-rays has been detected for the first time from a supernova connected with a gamma-ray burst (GRB), namely GRB060218/SN2006aj. It has been interpreted as arising from the breakout of a mildly relativistic, radiation-dominated shock from a dense stellar wind surrounding the progenitor star. There is also evidence for the presence of a mildly relativistic ejecta in GRB980425/SN1998bw, based on its X-ray and radio afterglow. Here we study the process of repeated bulk Compton scatterings of shock breakout thermal photons by the mildly relativistic ejecta. During the shock breakout process, a fraction of the thermal photons would be repeatedly scattered between the pre-shock material and the shocked material as well as the mildly relativistic ejecta and, as a result, the thermal photons get boosted to increasingly higher energies. This bulk motion Comptonization mechanism will produce nonthermal gamma-ray and X-ray flashes, which could account for the prompt gamma-ray burst emission in low-luminosity supernova-connected GRBs, such as GRB060218. A Monte Carlo code has been developed to simulate this repeated scattering process, which confirms that a significant fraction of the thermal photons get "accelerated" to form a nonthermal component, with a dominant luminosity. This interpretation for the prompt nonthermal emission of GRB060218 may imply that either the usual internal shock emission from highly relativistic jets in these low-luminosity GRBs is weak, or alternatively, that there are no highly relativistic jets in this peculiar class of bursts.Comment: Accepted for publication in ApJ; Introduction expanded, references added, conclusions unchanged; total 7 pages including 2 color figures and 1 tabl

The Neon Abundance of Galactic Wolf-Rayet Stars

The fast, dense winds which characterize Wolf-Rayet (WR) stars obscure their underlying cores, and complicate the verification of evolving core and nucleosynthesis models. Core evolution can be probed by measuring abundances of wind-borne nuclear processed elements, partially overcoming this limitation. Using ground-based mid-infrared spectroscopy and the 12.81um [NeII] emission line measured in four Galactic WR stars, we estimate neon abundances and compare to long-standing predictions from evolved-core models. For the WC star WR121, this abundance is found to be >~11x the cosmic value, in good agreement with predictions. For the three less-evolved WN stars, little neon enhancement above cosmic values is measured, as expected. We discuss the impact of clumping in WR winds on this measurement, and the promise of using metal abundance ratios to eliminate sensitivity to wind density and ionization structure.Comment: Accepted for publication in ApJ; 9 pages, 2 color figures, 4 table

The most massive progenitors of neutron stars: CXO J164710.2-455216

The evolution leading to the formation of a neutron star in the very young Westerlund 1 star cluster is investigated. The turnoff mass has been estimated to be 35 Msun, indicating a cluster age ~ 3-5 Myr. The brightest X-ray source in the cluster, CXO J164710.2-455216, is a slowly spinning (10 s) single neutron star and potentially a magnetar. Since this source was argued to be a member of the cluster, the neutron star progenitor must have been very massive (M_zams > 40 Msun) as noted by Muno et al. (2006). Since such massive stars are generally believed to form black holes (rather than neutron stars), the existence of this object poses a challenge for understanding massive star evolution. We point out while single star progenitors below M_zams < 20 Msun form neutron stars, binary evolution completely changes the progenitor mass range. In particular, we demonstrate that mass loss in Roche lobe overflow enables stars as massive as 50-80 Msun, under favorable conditions, to form neutron stars. If the very high observed binary fraction of massive stars in Westerlund 1 (> 70 percent) is considered, it is natural that CXO J164710.2-455216 was formed in a binary which was disrupted in a supernova explosion such that it is now found as a single neutron star. Hence, the existence of a neutron star in a given stellar population does not necessarily place stringent constraints on progenitor mass when binary interactions are considered. It is concluded that the existence of a neutron star in Westerlund 1 cluster is fully consistent with the generally accepted framework of stellar evolution.Comment: 5 pages of text and 4 figures (submitted to Astrophysical Journal

Chemical Abundances in the Secondary Star in the Black Hole Binary A0620-00

Using a high resolution spectrum of the secondary star in the black hole binary A0620-00, we have derived the stellar parameters and veiling caused by the accretion disk in a consistent way. We have used a chi^2 minimization procedure to explore a grid of 800 000 LTE synthetic spectra computed for a plausible range of both stellar and veiling parameters. Adopting the best model parameters found, we have determined atmospheric abundances of Fe, Ca, Ti, Ni and Al. The Fe abundance of the star is [Fe/H]=0.14 +- 0.20. Except for Ca, we found the other elements moderately over-abundant as compared with stars in the solar neighborhood of similar iron content. Taking into account the small orbital separation, the mass transfer rate and the mass of the convection zone of the secondary star, a comparison with element yields in supernova explosion models suggests a possible explosive event with a mass cut comparable to the current mass of the compact object. We have also analyzed the Li abundance, which is unusually high for a star of this spectral type and relatively low mass.Comment: 32 pages, 5 tables and 11 figures, uses rotate.st

GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor

The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly continuous monitoring of its early optical afterglow decay, as well as high-quality optical spectra that determined a redshift of z3=2.328 for its host galaxy, an active starburst galaxy with strong Lyman-alpha emission and several absorption lines. Spectral observations show multiple absorbers at z3A=2.323, z3B= 2.317, and z3C= 2.293 blueshifted by 450, 990, and 3,155 km/s respectively relative to the host galaxy Lyman-alpha emission. We argue that these correspond to a fragmented shell nebula that has been radiatively accelerated by the gamma-ray burst (GRB) afterglow at a distance greater than 0.3 pc from a Wolf-Rayet star progenitor. The chemical abundance ratios indicate that the nebula is overabundant in carbon and silicon. The high level of carbon and silicon is consistent with a swept-up shell nebula gradually enriched by a WCL progenitor wind over the lifetime of the nebula prior to the GRB onset. The detection of statistically significant fluctuations and color changes about the jet-like optical decay further supports this interpretation since fluctuations must be present at some level due to inhomogeneities in a clumpy stellar wind medium or if the progenitor has undergone massive ejection prior to the GRB onset. This evidence suggests that the mass-loss process in a Wolf-Rayet star might lead naturally to an iron-core collapse with sufficient angular momentum that could serve as a suitable GRB progenitor.Comment: Replaced with version accepted by ApJ; 40 pages, 9 figure

X-ray Emission from Wind Blown Bubbles. III. ASCA SIS Observations of NGC6888

We present ASCA SIS observations of the wind-blown bubble NGC6888. Owing to the higher sensitivity of the SIS for higher energy photons compared to the ROSAT PSPC, we are able to detect a T ~ 8x10^6 K plasma component in addition to the T ~ 1.3x10^6 K component previously detected in PSPC observations. No significant temperature variations are detected within NGC6888. Garcia-Segura & Mac Low's (1995) analytical models of WR bubbles constrained by the observed size, expansion velocity, and mass of the nebular shell under-predict the stellar wind luminosity, and cannot reproduce simultaneously the observed X-ray luminosity, spectrum, surface brightness profile, and SIS count rate of NGC6888's bubble interior. The agreement between observations and expectations from models can be improved if one or more of the following ad hoc assumptions are made: (1) the stellar wind luminosity was weaker in the past, (2) the bubble is at a special evolutionary stage and the nebular shell has recently been decelerated to 1/2 of its previous expansion velocity, and (3) the heat conduction between the hot interior and the cool nebular shell is suppressed. Chandra and XMM-Newton observations with high spatial resolution and high sensitivity are needed to determine accurately the physical conditions NGC6888's interior hot gas for critical comparisons with bubble models.Comment: 24 pages, 6 figures; accepted for Astrophysical Journal, Nov 1, 2005 issu

Effects of the stellar wind on X-ray spectra of Cygnus X-3

We study X-ray spectra of Cyg X-3 from BeppoSAX, taking into account absorption and emission in the strong stellar wind of its companion. We find the intrinsic X-ray spectra are well modelled by disc blackbody emission, its upscattering by hot electrons with a hybrid distribution, and by Compton reflection. These spectra are strongly modified by absorption and reprocessing in the stellar wind, which we model using the photoionization code cloudy. The form of the observed spectra implies the wind is composed of two phases. A hot tenuous plasma containing most of the wind mass is required to account for the observed features of very strongly ionized Fe. Small dense cool clumps filling <0.01 of the volume are required to absorb the soft X-ray excess, which is emitted by the hot phase but not present in the data. The total mass-loss rate is found to be (0.6--1.6) x 10^-5 solar masses per year. We also discuss the feasibility of the continuum model dominated by Compton reflection, which we find to best describe our data. The intrinsic luminosities of our models suggest that the compact object is a black hole.Comment: MNRAS, in pres