Douglas Owen Starkey Wood

Solar system-sized condensations in the Orion Nebula

New observations of 22 ultracompact radio sources toward the Trapezium cluster and toward the core of the Kleinmann-Low nebula are reported. The diameters of the sources range from less than 3.7 x 10 to the 14th cm to about 3.4 x 10 to the 15th cm. Their emission measures are typically greater than 10 to the 8th pc/cm exp 6, their electron densities are greater than or equal to 10 to the 6th/cu cm, and most are optically thin at 2 cm. Fifteen are associated with stars. Two interpretations for the optically visible nebular condensations are discussed. They may be dense, molecular globules embedded in the diffuse H II region with envelopes ionized by the Trapezium stars, or they may be low-mass, premain-sequence stars whose accretion disks are slowly being evaporated by the Trapezium stars. Properties and mass-loss rates are derived for the objects not associated with visible stars. 39 references.

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.

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.

The infrared emission from dust surrounding newly formed O stars

The dust cocoon around a newly formed 06 ZAMS star embedded in its natal molecular cloud is modeled for a wide range of radial density distribution and grain properties. The most important result of this investigation is that the only models that satisfy all the observational constraints require thin dust shells with relatively large inner radii composed of grains with about one-half the graphite/silicate abundance ratio of the MRN-DL mixture for the diffuse interstellar medium. It is also shown that constant density models produce the best overall fit to the observations; the dust cocoons are optically thick at wavelengths shortward of approximately 7 microns, and the warm dust cocoons are large; the average dust temperatures drop very steeply just outside the inner boundary of the dust shell to less than 100 K in less than 0.1 percent of the outer shell radius; the addition of water ice mantles to refractory grain cores produces a strong 3.07 micron absorption feature and weaker 12 and 45 micron absorption features; and all models imply rather massive dust shells. 37 refs.

The excitation, abundance, and distribution of HNCO in Sagittarius B2

Snyder and Buhl (1972) have discovered isocyanic acid (HNCO) in Sgr B2. It is pointed out that HNCO is a particularly interesting interstellar molecule because under certain conditions it can be employed in a study of the far-infrared radiation field of the cloud in which it resides. The present study was mainly conducted with the aim to establish empirically the extent to which HNCO is populated by radiative processes in a source with a measured far-infrared radiation field. It is attempted to deduce the radiation field from the HNCO excitation conditions. Attention is given to an energy-level diagram of HNCO in the ground electronic and vibrational states, the antenna and receiver properties, a table with the observed parameters toward Sagittarius B2, the velocity structure, the HNCO intensity distribution, a summary of the Sgr B2 properties, the population distribution and column density, and the excitation of HNCO. 27 references.

Flux densities of ultracompact H II regions at 7 millimeters

Observations of the 7-mm continuum are reported for 29 suspected ultracompact (UC) H II regions. Twenty-seven sources were detected, most of which had not previously been observed at 7 mm. These sources were found to be small (only eight were fully resolved by the 43-arcsec beam) and bright at mm wavelengths (greater than 0.5 Jy), making them easy to distinguish from associated diffuse free-free emission which can confuse single-dish observations made at cm wavelengths. The ionizing stars are mostly O5-O6, consistent with the detection limit of 0.5 Jy. It is found that dust in UC H II regions typically absorbs more than 50 percent of the stellar ionizing photons. Some of the sources reported here might also serve as flux and pointing calibrators at mm wavelengths. 19 references.

A single rotating disk of H I in Arp 299 (NGC 3690/IC 694)

Ten-arcsec resolution 21-cm spectral line observations of the merging galaxies NGC 3690/IC 694 are presented. Strong H I absorption was seen at three sites of activity identified previously by Gehrz, Sramek, and Weedman (1983), and regions of weak emission were found extending from the optical edges of the system. The absorption data set lower limits on the neutral hydrogen mass contained in the nuclei and in the overlap region. The emission shows evidence of a single rotating disk of H I surrounding the system out to a radius twice that of the optical radius. This single H I disk morphology indicates that IC 694 and NGC 3690 are coverage of Arp 299 which now spans from the UV to radio regimes, thus making it possible to compare the distribution and kinematics of atomic hydrogen with previous observations, and offering new insight into the effect of a collision on a progenitors interstellar medium. 12 refs.