D. Van Buren

Bow shocks and bubbles are seen around hot stars by IRAS

Examination of the IRAS all-sky imagery reveals extended, arcuate, and ringlike features associated with hot luminous stars. They fall into a number of classes: stellar wind bow shocks, stellar wind bubbles, dust shells, dust heated by isolated B stars, bright rims, and dust in H II regions. Here, some objects are discussed in which the star exercises structural control over the spatial distribution of dust: bow shocks, bubbles, and radiation pressure-driven shells. A list of the 15 most prominent objects is presented, a few prototypes are shown, and their characteristics are explained in terms of thermal emission processes and gasdynamics.

Cometary compact H II regions are stellar-wind bow shocks

Comet-shaped H II regions, like G34.3 + 0.2, are easily explained as bow shocks created by wind-blowing massive stars moving supersonically through molecular clouds. The required velocities of the stars through dense clumps are less than about 10 km/s, comparable to the velocity dispersion of stars in OB associations. An analytic model of bow shocks matches the gross characteristics seen in the radio continuum and the velocity structure inferred from hydrogen recombination and molecular line observations. The champagne flow model cannot account for these structures. VLBI observations of masers associated with the shells of cometary compact H II regions should reveal tailward proper motions predominantly parallel to the shell, rather than perpendicular. It is predicted that over a decade baseline, high signal-to-noise VLA observations of this class of objects will show headward pattern motion in the direction of the symmetry axis, but not expansion. Finally, shock-generated and coronal infrared lines are also predicted. 57 refs.

Astrometric Observation of MACHO Gravitational Microlensing

Following previous suggestions of other researchers, this paper discusses the prospects for astrometric observation of MACHO gravitational microlensing events. We derive the expected astrometric observables for a simple microlensing event with either a dark or self-luminous lens and demonstrate that accurate astrometry can determine the lens mass, distance, and proper motion in a very general fashion. In particular, we argue that in limited circumstances ground-based, narrow-angle differential astrometric techniques are sufficient to measure the lens mass directly and other lens properties (distance, transverse motion) by applying an independent model for the source distance and motion. We investigate the sensitivity of differential astrometry in determining lens parameters by Monte Carlo methods and derive a quasi-empirical relationship between astrometric accuracy and mass uncertainty.

Shocked molecular gas around the extremely young source IRAS 03282+3035

We present observations of the shock tracers H2 and SiO around the young stellar object IRAS 03282+3035. This unusual low-luminosity (L approximately = 2 solar luminosity) source drives a strong highly collimated CO outflow, and it is one of the youngest stellar objects known so far. The near-infrared H2 emission, tracing 2000 K gas, comes from extremely high velocity CO bullets along the axis of the blueshifted lobe of the outflow. The millimeter SiO emission, tracing roughly 100 K gas, arises from lower velocity material at the end of the outflow lobe. The lack of high-temperature and high-velocity gas at the end of the outflow lobe indicates there is no not bow shock at the outflow termination. In the context of current jet models this appears to rule out a bow shock driven by a steady state jet. Possible explanations for the structure include a time-dependent jet or a jet dominated by turbulent entrainment.

Gravitational scattering of asteroids onto neutron stars as a cause of. gamma. -ray bursts

We propose a mechanism by which asteroids may be scattered onto neutron stars causing impulsive and perhaps all ..gamma..-ray bursts. Asteroids which remain bound to the neutron star after it is formed can, through close encounters with planets, enter a region of magnetic drag near the star. This region is larger than the stars collisional cross section; it is approx.10/sup 10/ cm in radius and hence produces an interesting rate if even only a small fraction of all neutron stars are responsible for these bursts. We have interpreted the 1979 March 5 bursts in this context and can satisfy all of the observational constraints except for its suggested association with N49 in the LMC. Our model predicts that ..gamma..-ray bursts have a low luminosity (10/sup 35/ ergs s/sup -1/) X-ray precursor of fairly long duration (10/sup 4/ s).

The initial mass function and global rates of mass, momentum, and energy input to the interstellar medium via stellar winds

Using the Michigan HD catalog volumes I--III, the all-sky sample of O stars of Garmany, Conti, and Chiosi, Luckes map of the distribution of obscuring material within 2 kpc, and an amalgam of recent stellar evolution calculations, the number of stars formed kpc/sup -2/ yr/sup -1/ (log (M/M/sub sun/))/sup -1/ (IMF) is psi = 5.4 x 10/sup -4/(M/M/sub sun/)/sup -1.03/. A calibration of mass-loss rates with stellar parameters based on published data yields m = 2.0 x 10/sup -13/(L/L/sub sun/)/sup 1.25/M/sub sun/ yr/sup -1/. Energy injection into the ISM by winds and supernovae balances mechanical energy dissipation via cloud-cloud collisions. For stars M>5 M/sub sun/ there is near balance between the rate at which mass is turned into stars and the rate at which it is lost from them, implying small remnant masses.

A Near- and Mid-Infrared Study of the Interacting Galaxy Pair UGC 12914/12915: “Taffy”

The system consists of two counter-rotating spirals having suffered a nearly face-on collsion only ~10(sup 7) years ago.

Detection of flare like events and their relationship to presumed spot regions on V471 Tau - A solar-stellar connection

Since its discovery as an eclipsing binary with a white dwarf companion by Nelson and Young (1970), V471 Tau (BD + 16 deg 516) has been an object of considerable importance. The K dwarf companion has been the subject of intensive studies because it demonstrates many of the properties normally attributed to classical RS CVn stars. In the present investigation, it is demonstrated that the K dwarf companion exhibits also flaring properties which characterize the BY Dra stars. In a sense, V471 Tau may be the critical link by providing information that the distinctions between RS CVn phenomena and BY Dra phenomena are artificial and misleading, arising entirely from historical processes. Observations in the optical and the X-ray wavelength regions are reported. A study of the occurrence times of flare events in terms of the ephemeris of the photometric wave which is known to migrate through the light curve of V471 Tau reveals a strong correlation, suggesting that the flares occur near regions of the K dwarf where spot groups are inferred.

Optical Observations of the Binary Pulsar System PSR B1718–19: Implications for Tidal Circularization

We report on Keck and Hubble Space Telescope optical observations of the eclipsing binary pulsar system PSR B1718-19, in the direction of the globular cluster NGC 6342. These reveal a faint star (mF702W = 25.21 ± 0.07; Vega system) within the pulsars 05 radius positional error circle. This may be the companion. If it is a main-sequence star in the cluster, it has radius RC 0.3 R☉, temperature Teff 3600 K, and mass MC 0.3 M☉. In many formation models, however, the pulsar (spun-up by accretion or newly formed) and its companion are initially in an eccentric orbit. If so, for tidal circularization to have produced the present-day highly circular orbit, a large stellar radius is required, i.e., the star must be bloated. Using constraints on the radius and temperature from the Roche and Hayashi limits, we infer from our observations that RC 0.44 R☉ and Teff 3300 K. Even for the largest radii, the required efficiency of tidal dissipation is larger than expected for some prescriptions.

The [N II] Kinematics of R Aquarii

We report a kinematic study of the symbiotic star system R Aqr derived from [N II] λ6584 emission observations with a Fabry-Perot imaging spectrometer. The [N II] spatial structure of the R Aqr jet, first observed circa 1977, and surrounding hourglass-shaped nebulosity, due to an explosion ~660 yr ago, are derived from 41 velocity planes spaced at ~12 km s-1 intervals. Fabry-Perot imagery shows that the elliptical nebulosity making up the waist of the hourglass shell is consistent with a circular ring expanding radially at 55 km s-1 as seen at an inclination angle i ~ 70°. With respect to the position of R Aqr, at optical and radio wavelengths the jet is made up of a northeast component and a less intense, more amorphous southwest component. The [N II] Fabry-Perot data cube demonstrates that the two jet components possess quite different intensity-velocity structures which represent collimated flow in opposite directions. We offer an idealized schematic model for the R Aqr jet motion which results in a small-scale helical structure forming around a larger scale helical path. The implications of such a jet model are discussed. We present a movie showing a side-by-side comparison of the spatial structure of the model and the data as a function of the 41 velocity planes.