On the Mass-loss Rate of Massive Stars in the Low-metallicity Galaxies IC 1613, WLM, and NGC 3109

Tramper, F.; Sana, H.; de Koter, A.; Kaper, L.

The Astrophysical Journal Letters, Volume 741, Issue 1, L8 (2011)

ADS – Journal – arXiv


We present a spectroscopic analysis of Very Large Telescope/X-Shooter observations of six O-type stars in the low-metallicity (Z ~ 1/7 Z sun) galaxies IC 1613, WLM, and NGC 3109. The stellar and wind parameters of these sources allow us, for the first time, to probe the mass loss versus metallicity dependence of stellar winds at metallicities below that of the Small Magellanic Cloud (at Z ~ 1/5 Z sun) by means of a modified wind momentum versus luminosity diagram. The wind strengths that we obtain for the objects in WLM and NGC 3109 are unexpectedly high and do not agree with theoretical predictions. The objects in IC 1613 tend toward a higher than expected mass-loss rate, but remain consistent with predictions within their error bars. We discuss potential systematic uncertainties in the mass-loss determinations to explain our results. However, if further scrutinization of these findings point towards an intrinsic cause for this unexpected sub-SMC mass-loss behavior, implications would include a higher than anticipated number of Wolf-Rayet stars and Ib/Ic supernovae in low-metallicity environments, but a reduced number of long-duration gamma-ray bursts produced through a single-star evolutionary channel.

Keywords: galaxies: individual: IC1613 WLM NGC3109; stars: massive; stars: mass-loss; stars: winds; outflows; techniques: spectroscopic; Astrophysics – Solar and Stellar Astrophysics

Corona, Jet, and Relativistic Line Models for Suzaku/RXTE/Chandra-HETG Observations of the Cygnus X-1 Hard State

Nowak, Michael A.; Hanke, Manfred; Trowbridge, Sarah N.; Markoff, Sera B.; Wilms, Jörn; Pottschmidt, Katja; Coppi, Paolo; Maitra, Dipankar; Davis, John E.; Tramper, Frank

The Astrophysical Journal, Volume 728, Issue 1, 13 (2011)

ADS – Journal – arXiv


Using Suzaku and the Rossi X-ray Timing Explorer (RXTE), we have conducted a series of four simultaneous observations of the galactic black hole candidate Cyg X-1 in what were historically faint and spectrally hard “low states.” Additionally, all of these observations occurred near superior conjunction with our line of sight to the X-ray source passing through the dense phases of the “focused wind” from the mass donating secondary. One of our observations was also simultaneous with observations by the Chandra-High Energy Transmission Grating (HETG). These latter spectra are crucial for revealing the ionized absorption due to the secondary’s focused wind. Such absorption is present and must be accounted for in all four spectra. These simultaneous data give an unprecedented view of the 0.8-300 keV spectrum of Cyg X-1, and hence bear upon both corona and X-ray emitting jet models of black hole hard states. Three models fit the spectra well: coronae with thermal or mixed thermal/non-thermal electron populations and jets. All three models require a soft component that we fit with a low temperature disk spectrum with an inner radius of only a few tens of GM/c 2. All three models also agree that the known spectral break at 10 keV is not solely due to the presence of reflection, but each gives a different underlying explanation for the augmentation of this break. Thus, whereas all three models require that there is a relativistically broadened Fe line, the strength and inner radius of such a line is dependent upon the specific model, thus making premature line-based estimates of the black hole spin in the Cyg X-1 system. We look at the relativistic line in detail, accounting for the narrow Fe emission and ionized absorption detected by HETG. Although the specific relativistic parameters of the line are continuum dependent, none of the broad line fits allow for an inner disk radius that is >40 GM/c 2.

Keywords: accretion; accretion disks; black hole physics; radiation mechanisms: non-thermal; X-rays: binaries; Astrophysics – High Energy Astrophysical Phenomena