Towards Understanding The B[e] Phenomenon: IV. Modeling of IRAS 00470+6429

FS CMa type stars are a recently described group of objects with the B[e] phenomenon that exhibit strong emission-line spectra and strong IR excesses. In this paper we report the first attempt for a detailed modeling of IRAS 00470+6429, for which we have the best set of observations. Our modeling is based on two key assumptions: the star has a main-sequence luminosity for its spectral type (B2) and the circumstellar envelope is bimodal, composed of a slowly outflowing disk-like wind and a fast polar wind. Both outflows are assumed to be purely radial. We adopt a novel approach to describe the dust formation site in the wind that employs timescale arguments for grain condensation and a self-consistent solution for the dust destruction surface. With the above assumptions we were able to reproduce satisfactorily many observational properties of IRAS 00470+6429, including the H line profiles and the overall shape of the spectral energy distribution. Our adopted recipe for dust formation proved successful in reproducing the correct amount of dust formed in the circumstellar envelope. Possible shortcomings of our model, as well as suggestions for future improvements, are discussed.Comment: 11 pages, 7 figures, accepted for publication in The Astrophysical Journa

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

Polarization and kinematics in Cygnus A

From optical spectropolarimetry of Cygnus A we conclude that the scattering medium in the ionization cones in Cygnus A is moving outward at a speed of 170+-34 km/s, and that the required momentum can be supplied by the radiation pressure of an average quasar. Such a process could produce a structure resembling the observed ionization cones, which are thought to result from shadowing by a circumnuclear dust torus. We detect a polarized red wing in the [O III] emission lines arising from the central kiloparsec of Cygnus A. This wing is consistent with line emission created close to the boundary of the broad-line region.Comment: 5 pages, accepted for publication in MNRAS letter

Numerical Modeling of Eta Carinae Bipolar Outflows

In this paper, we present two-dimensional gas dynamic simulations of the formation and evolution of the eta-Car bipolar outflows. Adopting the interacting nonspherical winds model, we have carried out high-resolution numerical simulations, which include explicitly computed time-dependent radiative cooling, for different possible scenarios of the colliding winds. In our simulations, we consider different degrees of non-spherical symmetry for the pre-outburst wind and the great eruption of the 1840s presented by the eta-Car wind. From these models, we obtain important differences in the shape and kinematical properties of the Homunculus structure. In particular, we find an appropriate combination of the wind parameters (that control the degree of non-spherical symmetry) and obtain numerical experiments that best match both the observed morphology and the expansion velocity of the eta-Car bipolar shell. In addition, our numerical simulations show the formation of a bipolar nebula embedded within the Homunculus (the little Homunculus) developed from a secondary eruptive event suffered by the star in the 1890s, and also the development of tenuous, high velocity ejections in the equatorial region that result from the impact of the eruptive wind of the 1840s with the pre-outburst wind and that could explain some of the high speed features observed in the equatorial ejecta. The models were, however, unable to produce equatorial ejections associated to the second eruptive event.Comment: 33 pages, 9 figures, accepted by the Astrophysical Journa

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

Evidence for Environmentally Dependent Cluster Disruption in M83

Using multi-wavelength imaging from the Wide Field Camera 3 on the Hubble Space Telescope we study the stellar cluster populations of two adjacent fields in the nearby face-on spiral galaxy, M83. The observations cover the galactic centre and reach out to ~6 kpc, thereby spanning a large range of environmental conditions, ideal for testing empirical laws of cluster disruption. The clusters are selected by visual inspection to be centrally concentrated, symmetric, and resolved on the images. We find that a large fraction of objects detected by automated algorithms (e.g. SExtractor or Daofind) are not clusters, but rather are associations. These are likely to disperse into the field on timescales of tens of Myr due to their lower stellar densities and not due to gas expulsion (i.e. they were never gravitationally bound). We split the sample into two discrete fields (inner and outer regions of the galaxy) and search for evidence of environmentally dependent cluster disruption. Colour-colour diagrams of the clusters, when compared to simple stellar population models, already indicate that a much larger fraction of the clusters in the outer field are older by tens of Myr than in the inner field. This impression is quantified by estimating each cluster's properties (age, mass, and extinction) and comparing the age/mass distributions between the two fields. Our results are inconsistent with "universal" age and mass distributions of clusters, and instead show that the ambient environment strongly affects the observed populations.Comment: 6 pages, 3 figures, MNRAS in pres

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

Runaway Massive Binaries and Cluster Ejection Scenarios

The production of runaway massive binaries offers key insights into the evolution of close binary stars and open clusters. The stars HD 14633 and HD 15137 are rare examples of such runaway systems, and in this work we investigate the mechanism by which they were ejected from their parent open cluster, NGC 654. We discuss observational characteristics that can be used to distinguish supernova ejected systems from those ejected by dynamical interactions, and we present the results of a new radio pulsar search of these systems as well as estimates of their predicted X-ray flux assuming that each binary contains a compact object. Since neither pulsars nor X-ray emission are observed in these systems, we cannot conclude that these binaries contain compact companions. We also consider whether they may have been ejected by dynamical interactions in the dense environment where they formed, and our simulations of four-body interactions suggest that a dynamical origin is possible but unlikely. We recommend further X-ray observations that will conclusively identify whether HD 14633 or HD 15137 contain neutron stars.Comment: Accepted to ApJ, 11 page