Dave van Buren

Bow shock models of ultracompact H II regions

This paper presents models of ultracompact H II regions as the bow shocks formed by massive stars, with strong stellar winds, moving supersonically through molecular clouds. The morphologies, sizes and brightnesses of observed objects match the models well. Plausible models are provided for the ultracompact H II regions G12.21 - 0.1, G29.96 - 0.02, G34.26 + 0.15, and G43.89 - 0.78. To do this, the equilibrium shape of the wind-blown shell is calculated, assuming momentum conservation. Then the shell is illuminated with ionizing radiation from the central star, radiative transfer for free-free emission through the shell is performed, and the resulting object is visualized at various angles for comparison with radio continuum maps. The model unifies most of the observed morphologies of ultracompact H II regions, excluding only those objects with spherical shells. Ram pressure confinement greatly lengthens the life of ultracompact H II regions, explaining the large number that exist in the Galaxy despite their low apparent kinematic ages. 32 refs.

Bow shock models for the velocity structure of ultracompact H II regions

The velocity structure of ultracompact H II regions is modeled assuming that O stars moving supersonically through molecular clouds sweep up bow shocks to produce the observed objects. The expected radio recombination line emission is calculated for the case of an optically thin continuum and the strong effect of changing the viewing angle is shown. The kinematic information removes the degeneracy with ram pressure of a previous model, allowing measurement of stellar velocity vectors and cloud densities. A detailed model for G29.96-0.02 shows good agreement with observations by Wood and Churchwell, supporting the bow shock hypothesis. It is found that the exciting star of G29 is moving at 20 km/s relative to the gas, suggesting that O stars acquire a large velocity dispersion early in their lives.

A more direct measure of supernova rates in starburst galaxies

We determine ages for young supernova remnants in the starburst galaxies M82 and NGC 253 by applying Chevaliers model for radio emission from supernova blast waves expanding into the ejecta of their precursor stars. Absolute ages are determined by calibrating the model with radio observations of Cas A. We derive supernova rates of 0.10 and 0.08/yr for M82 and NGC 253, respectively. Assuming L (sub FIR) to be proportional to the supernova rate, we find r(sub SN) approximately equal 2 x 10(exp -12) x L(sub FIR), solar yr(exp -1) for these archetypal starburst galaxies. This approach is unique in that the supernova rate is derived from direct observation of supernova remnants rather than from star formation rates and an assumed initial mass function (IMF). We suggest that the approach presented here can be used to derive star-formation rates that are more directly related to observable quantities than those derived by other methods. We find that the supernova rate, far infrared (FIR) luminosity, and dynamical mass of the M82 starburst place few constraints on the initial mass function (IMF) slope and mass limits.

AN UPPER LIMIT ON THE CONTRIBUTION OF GALACTIC FREE-FREE EMISSION TO THE COSMIC MICROWAVE BACKGROUND NEAR THE NORTH CELESTIAL POLE

We have observed the region of the sky north of 81 degrees declination with a wide-angle CCD camera and narrow-band (1 nm) H-alpha filter. After subtracting the stellar background using offband images and smoothing to 0.1 degree resolution, we set an upper limit on the anistropy in the H-alpha emission at this angular scale of 1.3 Rayleigh. At degree angular scales, the upper limit is 0.5 R, which corresponds to an anisotropy in the brightness temperature of the free-free emission at 32 GHz of 3 muK. Thus no more than 7% of the 44 muK anistropy observed by Netterfield et al. (1995) can be due to free-free emission by Galactic hydrogen.

Infrared supernovae in starbursts

The problem of uniquely confirming that the luminosity source of starburst galaxies is a young population of massive stars is considered. Unambiguous detection of the supernova explosion associated with a massive stellar population would provide proof of the starburst hypothesis. High spatial resolution narrow-band infrared imaging of starburst galaxies directly detects the cobalt synthesized in Type II supernova explosions. Coupled with observations of other infrared lines and continuum, progenitor masses can be at least roughly estimated. A statistically large sample of starburst supernovae will lead to an average starburst initial mass function. Standard candles can also be constructed, based on both individual and populations of supernovae. With current and planned instruments, K-band can be found out to cosmological distances. 27 references.

An Infrared Space Observatory Upper Limit to the Low-Mass Star Halo in theEdge-on Galaxy NGC 4565

We present deep ISOCAM observations taken at 4.5 μm (LW1) in search of a faint halo surrounding the edge-on spiral galaxy NGC 4565. Such a halo might exist if the massive halo that is needed to explain the flat rotation curve of this galaxy were attributable to a population of faint, red objects. The upper limit, 5.4 kJy sr-1 (3 σ) at 20 kpc, reported here excludes a halo consisting of late-M dwarfs distributed in a spherical halo but not of lower luminosity brown dwarf stars.

The 5° Diameter Ionized Halo of the Planetary Nebula Abell 36

We have observed an ionized halo surrounding the planetary nebula Abell 36. It is barrel-shaped, with dimensions 4° × 5°, which is 17 pc × 21 pc at a distance of 240 pc. With an average Hα surface brightness of 0.8 ± 0.2 rayleighs, the halos total Hα flux, 8.3 × 10-10 ergs cm-2 s-1, exceeds that of the previously known inner part of the nebula by a factor of 30. The ionized mass of the halo is 48 M⊙, where is the filling factor of the ionized gas. Velocity-resolved Hα spectra indicate that the halo is ambient interstellar matter and not matter ejected from the planetary nebula. The edges of the halo are evident in both 100 μm emission and red optical continuum.

A Wide-Angle Hα Image of the LMC

We are conducting a wide-angle Hα survey of the southern sky at CTIO using a robotic CCD camera. The survey consists of 283 fields covering the sky from δ = −90° to δ = +10°, with the same centers as those in the IRAS Sky Survey Atlas. As of July 1, 1998, it was about 45% complete. When all the images are obtained and fully processed, the survey will be made available to the scientific community on the web and on CD-ROM.

The volume filling factor of the infrared cirrus IS 0.2

The filling factor of the infrared cirrus in which luminous stars are embedded is estimated. The stars are assumed to be randomly situated with respect to the cirrus. A direct ratio of the number of stars with associated cirrus emission to those which in principle could be detected indicates that the cirrus filling factor is roughly 0.2.

First Results from the ISO-IRAS Faint Galaxy Survey*

We present the first results from the ISO-IRAS Faint Galaxy Survey (IIFGS), a program designed to obtain ISO observations of the most distant and luminous galaxies in the IRAS Faint Source Survey by filling short gaps in the ISO observing schedule with pairs of 12 μm ISOCAM and 90 μm ISOPHOT observations. As of 1997 October, over 500 sources have been observed, with an ISOCAM detection rate over 80%, covering over 1.25 deg^2 of sky to an 11.5 μm point-source completeness limit of approximately 1.0 mJy (corresponding to a ~10 σ detection sensitivity). Observations are presented for nine sources detected by ISOPHOT and ISOCAM early in the survey for which we have ground-based G- and I-band images and optical spectroscopy. The ground-based data confirm that the IIFGS strategy efficiently detects moderate-redshift (z = 0.11-0.38 for this small sample) strong emission line galaxies with L_(60 μm) ≳ 10^(11) L_☉; one of our sample has L_(60 μm) > 10^(12) L_☉ (H_0 = 75 km s^(-1) Mpc^(-1), Ω = 1). The infrared-optical spectral energy distributions are comparable to those of nearby luminous infrared galaxies, which span the range from pure starburst (e.g., Arp 220) to infrared QSO (Mrk 231). Two of the systems show signs of strong interaction, and four show active galactic nucleus (AGN)-like excitation; one of the AGNs, F15390+6038, which shows a high excitation Seyfert 2 spectrum, has an unusually warm far- to mid-infrared color and may be an obscured QSO. The IIFGS sample is one of the largest and deepest samples of infrared-luminous galaxies available, promising to be a rich sample for studying infrared-luminous galaxies up to z ~ 1 and for understanding the evolution of infrared galaxies and the star formation rate in the universe.