Stagnation and Infall of Dense Clumps in the Stellar Wind of tau Scorpii

Observations of the B0.2V star tau Scorpii have revealed unusual stellar wind characteristics: red-shifted absorption in the far-ultraviolet O VI resonance doublet up to +250 km/s, and extremely hard X-ray emission implying gas at temperatures in excess of 10^7 K. We describe a phenomenological model to explain these properties. We assume the wind of tau Sco consists of two components: ambient gas in which denser clumps are embedded. The clumps are optically thick in the UV resonance lines primarily responsible for accelerating the ambient wind. The reduced acceleration causes the clumps to slow and even infall, all the while being confined by the ram pressure of the outflowing ambient wind. We calculate detailed trajectories of the clumps in the ambient stellar wind, accounting for a line radiation driving force and the momentum deposited by the ambient wind in the form of drag. We show these clumps will fall back towards the star with velocities of several hundred km/sec for a broad range of initial conditions. The infalling clumps produce X-ray emitting plasmas with temperatures in excess of (1-6)x10^7 K in bow shocks at their leading edge. The infalling material explains the peculiar red-shifted absorption wings seen in the O VI doublet. The required mass loss in clumps is 3% - 30% ofthe total mass loss rate. The model developed here can be generally applied to line-driven outflows with clumps or density irregularities. (Abstract Abridged)Comment: To appear in the ApJ (1 May 2000). 24 pages, including 6 embedded figure

Circumstellar grain extinction properties of recently discovered post AGB stars

The circumstellar grains of two hot evolved post asymptotic giant branch (post AGB) stars, HD 89353 and HD 213985 were examined. From ultraviolet spectra, energy balance of the flux, and Kurucz models, the extinction around 2175 A was derived. With visual spectra, an attempt was made to detect 6614 A diffuse band absorption arising from the circumstellar grains so that we could examine the relationship of these features to the infrared features. For both stars, we did not detect any diffuse band absorption at 6614 A, implying the carrier of this diffuse band is not the carrier of the unidentified infrared features not of the 2175 A bump. The linear ultraviolet extinction of the carbon-rich star HD 89353 was determined to continue across the 2175 A region with no sign of the bump; for HD 213985 it was found to be the reverse: a strong, wide bump in the mid-ultraviolet. The 213985 bump was found to be positioned at 2340 A, longward of its usual position in the interstellar medium. Since HD 213985 was determined to have excess carbon, the bump probably arises from a carbonaceous grain. Thus, in view of the ultraviolet and infrared properties of the two post AGB stars, ubiquitous interstellar infrared emission features do not seem to be associated with the 2175 A bump. Instead, the infrared features seem related to the linear ultraviolet extinction component: hydrocarbon grains of radius less than 300 A are present with the linear HD 89353 extinction; amorphous anhydrous carbonaceous grains of radius less than 50 A might cause the shifted ultraviolet extinction bump of HD 213985

Low-Mass Pre-Main Sequence Stars in the Large Magellanic Cloud - III: Accretion Rates from HST-WFPC2 Observations

We have measured the present accretion rate of roughly 800 low-mass (~1-1.4 Mo) pre-Main Sequence stars in the field of Supernova 1987A in the Large Magellanic Cloud (LMC, Z~0.3 Zo). It is the first time that this fundamental parameter for star formation is determined for low-mass stars outside our Galaxy. The Balmer continuum emission used to derive the accretion rate positively correlates with the Halpha excess. Both these phenomena are believed to originate from accretion from a circumstellar disk so that their simultaneous detection provides an important confirmation of the pre-Main Sequence nature of the Halpha and UV excess objects, which are likely to be the LMC equivalent of Galactic Classical TTauri stars. The stars with statistically significant excesses are measured to have accretion rates larger than 1.5x10^{-8}Mo/yr at an age of 12-16 Myrs. For comparison, the time scale for disk dissipation observed in the Galaxy is of the order of 6 Myrs. Moreover, the oldest Classical TTauri star known in the Milky Way (TW Hydrae, with 10 Myrs of age) has a measured accretion rate of only 5x10^{-10} Mo/yr, ie 30 times less than what we measure for stars at a comparable age in the LMC. Our findings indicate that metallicity plays a major role in regulating the formation of low-mass stars.Comment: Accepted for publication in the Astrophysical Journal (10 June 2004), 28 pages, 9 figures. Typo corrected in the abstract on 21 February 200

The Missing Luminous Blue Variables and the Bistability Jump

We discuss an interesting feature of the distribution of luminous blue variables on the H-R diagram, and we propose a connection with the bistability jump in the winds of early-type supergiants. There appears to be a deficiency of quiescent LBVs on the S Dor instability strip at luminosities between log L/Lsun = 5.6 and 5.8. The upper boundary, is also where the temperature-dependent S Dor instability strip intersects the bistability jump at about 21,000 K. Due to increased opacity, winds of early-type supergiants are slower and denser on the cool side of the bistability jump, and we postulate that this may trigger optically-thick winds that inhibit quiescent LBVs from residing there. We conduct numerical simulations of radiation-driven winds for a range of temperatures, masses, and velocity laws at log L/Lsun=5.7 to see what effect the bistability jump should have. We find that for relatively low stellar masses the increase in wind density at the bistability jump leads to the formation of a modest to strong pseudo photosphere -- enough to make an early B-type star appear as a yellow hypergiant. Thus, the proposed mechanism will be most relevant for LBVs that are post-red supergiants. Yellow hypergiants like IRC+10420 and rho Cas occupy the same luminosity range as the ``missing'' LBVs, and show apparent temperature variations at constant luminosity. If these yellow hypergiants do eventually become Wolf-Rayet stars, we speculate that they may skip the normal LBV phase, at least as far as their apparent positions on the HR diagram are concerned.Comment: 20 pages, 4 figs, accepted by Ap

The Baltimore and Utrecht models for cluster dissolution

The analysis of the age distributions of star cluster samples of different galaxies has resulted in two very different empirical models for the dissolution of star clusters: the Baltimore model and the Utrecht model. I describe these two models and their differences. The Baltimore model implies that the dissolution of star clusters is mass independent and that about 90% of the clusters are destroyed each age dex, up to an age of about a Gyr, after which point mass-dependent dissolution from two-body relaxation becomes the dominant mechanism. In the Utrecht model, cluster dissolution occurs in three stages: (i) mass-independent infant mortality due to the expulsion of gas up to about 10 Myr; (ii) a phase of slow dynamical evolution with strong evolutionary fading of the clusters lasting up to about a Gyr; and (iii) a phase dominated by mass dependent-dissolution, as predicted by dynamical models. I describe the cluster age distributions for mass-limited and magnitude-limited cluster samples for both models. I refrain from judging the correctness of these models.Comment: 3 pages, 1 figure, to appear in "Young Massive Star Clusters - Initial Conditions and Environment", 2008, Astrophysics and Space Science, Eds. E. Perez, R. de Grijs and R.M. Gonzalez Delgad

Blue Variable Stars from the MACHO database I: Photometry and Spectroscopy of the LMC sample

We present the photometric properties of 1279 blue variable stars within the LMC. Photometry is derived from the MACHO database. The lightcurves of the sample exhibit a variety of quasi-periodic and aperiodic outburst behavior. A characteristic feature of the photometric variation is that the objects are reddest when at maximum outburst. A subset of 102 objects were examined spectroscopically. Within this subset, 91% exhibited Balmer emission in at least one epoch, in some cases with spectacular spectral variability. The variability observed in the sample is consistent with the establishment and maintenance of the Be phenomenon.Comment: 19 pages, AJ accepte