Pulsation-Initiated Mass Loss in Luminous Blue Variables: A Parameter Study

Luminous blue variables (LBVs) are characterized by semi-periodic episodes of enhanced mass-loss, or outburst. The cause of these outbursts has thus far been a mystery. One explanation is that they are initiated by kappa-effect pulsations in the atmosphere caused by an increase in luminosity at temperatures near the so-called ``iron bump'' (T ~ 200,000 K), where the Fe opacity suddenly increases. Due to a lag in the onset of convection, this luminosity can build until it exceeds the Eddington limit locally, seeding pulsations and possibly driving some mass from the star. We present some preliminary results from a parameter study focusing on the conditions necessary to trigger normal S-Dor type (as opposed to extreme eta-Car type) outbursts. We find that as Y increases or Z decreases, the pulsational amplitude decreases and outburst-like behavior, indicated by a large, sudden increase in photospheric velocity, becomes likes likely.Comment: 6 pages, 4 figures, to be published in the Proceedings of Massive Stars as Cosmic Engines, IAU Symp 250, ed. F. Bresolin, P. A. Crowther, & J. Puls (Cambridge Univ. Press

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

Characteristics of a Family with the MEN-2A Syndrome

Results of screening and follow-up in a family with the MEN-2A syndrome are described. Fourteen (83%) subjects at risk were affected with medullary thyroid carcinoma (MTC). Asymptomatic pheochromocytoma and parathyroid hyperplasia were found in two instances each. Intrathyroidal MTC without metastases was found in the four youngest subjects and was associated with normal basal but abnormal pentagastrin-stimulated calcitonin (CT) levels. In the ten subjects with abnormal basal CT levels, metastatic spread to at least regional lymph nodes was found. In eight of these ten subjects, total thyroidectomy with excision of affected lymph nodes and additional radioiodine did not return CT levels to normal postoperatively. In the other two subjects, the CT levels returned to normal after thyroidectomy and an ablative dose of radioiodine, and remained normal during follow-up. This study confirms the view that family screening at a young age promotes the detection of MTC at a curable stage. Radioiodine as an adjunct to surgery may be of value in the prevention of tumor recurrence in patients with normal postoperative CT levels but residual thyroid tissue

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