On the Chandra X-ray Sources in the Galactic Center

Recent deep Chandra surveys of the Galactic center region have revealed the existence of a faint, hard X-ray source population. While the nature of this population is unknown, it is likely that several types of stellar objects contribute. For sources involving binary systems, accreting white dwarfs and accreting neutron stars with main sequence companions have been proposed. Among the accreting neutron star systems, previous studies have focused on stellar wind-fed sources. In this paper, we point out that binary systems in which mass transfer occurs via Roche lobe overflow (RLOF) can also contribute to this X-ray source population. A binary population synthesis study of the Galactic center region has been carried out, and it is found that evolutionary channels for neutron star formation involving the accretion induced collapse of a massive ONeMg white dwarf, in addition to the core collapse of massive stars, can contribute to this population. The RLOF systems would appear as transients with quiescent luminosities, above 2 keV, in the range from 10^31-10^32 ergs/s. The results reveal that RLOF systems primarily contribute to the faint X-ray source population in the Muno et al. (2003) survey and wind-fed systems can contribute to the less sensitive Wang et al. (2002) survey. However, our results suggest that accreting neutron star systems are not likely to be the major contributor to the faint X-ray source population in the Galactic center.Comment: 12 pages, 3 figures, 1 table ApJ in press (Dec 2004). Substantial change

A New Version of Reimers' law of Mass Loss Based on a Physical Approach

We present a new semi-empirical relation for the mass loss of cool stellar winds, which so far has frequently been described by "Reimers' law". Originally, this relation was based solely on dimensional scaling arguments without any physical interpretation. In our approach, the wind is assumed to result from the spill-over of the extended chromosphere, possibly associated with the action of waves, especially Alfven waves, which are used as guidance in the derivation of the new formula. We obtain a relation akin to the original Reimers law, but which includes two new factors. They reflect how the chromospheric height depends on gravity and how the mechanical energy flux depends, mainly, on effective temperature. The new relation is tested and sensitively calibrated by modelling the blue end of the Horizontal Branch of globular clusters. The most significant difference from mass loss rates predicted by the Reimers relation is an increase by up to a factor of 3 for luminous late-type (super-)giants, in good agreement with observations.Comment: 12 pages, 4 figures, accepted by ApJ Letter

On the Dynamic Stability of Cool Supergiant Atmospheres

We have developed a new formalism to compute the thermodynamic coefficient Gamma1 in the theory of stellar and atmospheric stability. We generalize the classical derivation of the first adiabatic index, which is based on the assumption of thermal ionization and equilibrium between gas and radiation temperature, towards an expression which incorporates photo-ionization due to radiation with a temperature T_rad different from the local kinetic gas temperature.Our formalism considers the important non-LTE conditions in the extended atmospheres of supergiant stars. An application to the Kurucz grid of cool supergiant atmospheres demonstrates that models with T_rad =~ T_eff between 6500 K and 7500 K become most unstable against dynamic perturbations, according to Ledoux' stability integral . This results from Gamma1 and acquiring very low values, below 4/3, throughout the entire stellar atmosphere, which causes very high gas compression ratios around these effective temperatures. Based on detailed NLTE-calculations, we discuss atmospheric instability of pulsating massive yellow supergiants, like the hypergiant rho Cas (Ia+), which exist in the extension of the Cepheid instability strip, near the Eddington luminosity limit.Comment: 54 pages including figures and the Appendix, 7 figures, Accepted for The Astrophysical Journal, Main Journal, 558, Sept. 200

Nucleosynthesis in Massive Stars With Improved Nuclear and Stellar Physics

We present the first calculations to follow the evolution of all stable nuclei and their radioactive progenitors in stellar models computed from the onset of central hydrogen burning through explosion as Type II supernovae. Calculations are performed for Pop I stars of 15, 19, 20, 21, and 25 M_sun using the most recently available experimental and theoretical nuclear data, revised opacity tables, neutrino losses, and weak interaction rates, and taking into account mass loss due to stellar winds. A novel ``adaptive'' reaction network is employed with a variable number of nuclei (adjusted each time step) ranging from about 700 on the main sequence to more than 2200 during the explosion. The network includes, at any given time, all relevant isotopes from hydrogen through polonium (Z=84). Even the limited grid of stellar masses studied suggests that overall good agreement can be achieved with the solar abundances of nuclei between 16O and 90Zr. Interesting discrepancies are seen in the 20 M_sun model and, so far, only in that model, that are a consequence of the merging of the oxygen, neon, and carbon shells about a day prior to core collapse. We find that, in some stars, most of the ``p-process'' nuclei can be produced in the convective oxygen burning shell moments prior to collapse; in others, they are made only in the explosion. Serious deficiencies still exist in all cases for the p-process isotopes of Ru and Mo.Comment: 53 pages, 17 color figures (3 as separate GIF images), slightly extended discussion and references, accepted by Ap

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

The hypergiant HR 8752 evolving through the yellow evolutionary void

Context. We study the time history of the yellow hypergiant HR 8752 based on high-resolution spectra (1973-2005), the observed MK spectral classification data, B - V- and V-observations (1918-1996) and yet earlier V-observations (1840-1918).<br>Aims. Our local thermal equilibrium analysis of the spectra yields accurate values of the effective temperature (T-eff), the acceleration of gravity (g), and the turbulent velocity (v(t)) for 26 spectra. The standard deviations average are 82 K for T-eff, 0.23 for log g, and 1.1 km s(-1) for v(t).<br>Methods. A comparison of B-V observations, MK spectral types, and T-eff-data yields E(B-V), "intrinsic" B-V, T-eff, absorption A(V), and the bolometric correction BC. With the additional information from simultaneous values of B-V, V, and an estimated value of R, the ratio of specific absorption to the interstellar absorption parameter E(B - V), the "unreddened" bolometric magnitude m(bol),(0) can be determined. With Hipparcos distance measurements of HR 8752, the absolute bolometric magnitude M-bol,M-0 can be determined.<br>Results. Over the period of our study, the value of T-eff gradually increased during a number of downward excursions that were observable over the period of sufficient time coverage. These observations, together with those of the effective acceleration g and the turbulent velocity v(t), suggest that the star underwent a number of successive gas ejections. During each ejection, a pseudo photosphere was produced of increasingly smaller g and higher v(t) values. After the dispersion into space of the ejected shells and after the restructuring of the star's atmosphere, a hotter and more compact photosphere became visible. From the B - V and V observations, the basic stellar parameters, T-eff, log M/M-circle dot, log L/L-circle dot, and log R/R-circle dot are determined for each of the observational points. The results show the variation in these basic stellar parameters over the past near-century.<br>Conclusions. We show that the atmospheric instability region in the HR-diagram that we baptize the yellow evolutionary void actually consists of two parts. We claim that the present observations show that HR 8752 is presently climbing out of the "first" instability region and that it is on its way to stability, but in the course of its future evolution it still has to go through the second potential unstable region

On the role of continuum-driven eruptions in the evolution of very massive stars and Population III stars

We suggest that the mass lost during the evolution of very massive stars may be dominated by optically thick, continuum-driven outbursts or explosions, instead of by steady line-driven winds. In order for a massive star to become a WR star, it must shed its H envelope, but new estimates of the effects of clumping in winds indicate that line driving is vastly insufficient. We discuss massive stars above roughly 40-50 Msun, for which the best alternative is mass loss during brief eruptions of luminous blue variables (LBVs). Our clearest example of this phenomenon is the 19th century outburst of eta Car, when the star shed 12-20 Msun or more in less than a decade. Other examples are circumstellar nebulae of LBVs, extragalactic eta Car analogs (``supernova impostors''), and massive shells around SNe and GRBs. We do not yet fully understand what triggers LBV outbursts, but they occur nonetheless, and present a fundamental mystery in stellar astrophysics. Since line opacity from metals becomes too saturated, the extreme mass loss probably arises from a continuum-driven wind or a hydrodynamic explosion, both of which are insensitive to metallicity. As such, eruptive mass loss could have played a pivotal role in the evolution and fate of massive metal-poor stars in the early universe. If they occur in these Population III stars, such eruptions would profoundly affect the chemical yield and types of remnants from early SNe and hypernovae.Comment: 4 pages, 1 figure, accepted by ApJ Letter