Stanley E. Kurtz

The Early Evolution of Massive Stars: Radio Recombination Line Spectra

Velocity shifts and differential broadening of radio recombination lines are used to estimate the densities and velocitiesof theionizedgasinseveralhypercompactandultracompactHiiregions.ThesesmallHiiregionsarethought tobeattheirearliestevolutionaryphaseandassociatedwiththeyoungestmassivestars.Theobservationssuggestthat these H ii regions are characterized by high densities, supersonic flows, and steep density gradients, consistent with accretion and outflows that would be associated with the formation of massive stars. Subject headingg: H ii regions — stars: early-type — stars: formation

The Effects of Dust on Compact and Ultracompact H II Regions

We calculate numerical models of dusty H II regions using the Cloudy photoionization code with a grain size distribution. Dust sublimation causes a depletion of grain sizes and types within the ionized region, with large graphite grains being able to exist closer to the star than smaller graphite grains or silicate grains. We investigate the time-dependent hydrodynamic expansion of dusty H II regions and find that the fraction of ionizing photons absorbed by dust decreases with time. Furthermore, dusty H II regions stall earlier and at smaller radii than their dust-free counterparts. Comparison is made between our models and observable parameters, such as the electron density. We find that the electron density in dusty ionized regions estimated from radio continuum observations is likely to be an overestimate, and we quantify the discrepancy. Finally, we calculate the infrared emission from dusty H II regions and their surrounding circumnebular dust shells using the DUSTY code. We find that the far-infrared emission depends strongly on the parameters assumed for the circumnebular dust shell.

Spiral Density Wave Shock-induced Star Formation at High Galactic Latitudes

We have modeled the gas response to a spiral density wave (SDW) in a thick, magnetized galactic disk. The inclusion in the model of the vertically extended galactic warm ionized gas layer alters the conventional view of the SDW scenario for star formation: whereas marked density enhancements still occur in the midplane, the shock and a prominent high column density structure extend to high z (the height above the galactic midplane) above the arm. We argue that if the SDW mechanism indeed triggers molecular cloud and star formation, it should do so not only at the midplane but also at distances well above the star-forming thin disk of the conventional picture. The resulting structure resembles a hydraulic jump, or bore, in which gas entering the spiral arm rises suddenly on the upstream side of the arm, then accelerates and angles downward, finally landing on a large downfall region downstream of the arm.

From Ultracompact to Extended H II Regions. II. Cloud Gravity and Stellar Motion

The dynamical evolution of HII regions with and without stellar motion in dense, structured molecular clouds is studied. Clouds are modeled in hydrostatic equilibrium, with gaussian central cores and external halos that obey r**-2 and r**-3 density power laws. The cloud gravity is included as a time-independent, external force. Stellar velocities of 0, 2, 8, and 12 km/s are considered. When stellar motion is included, stars move from the central core to the edge of the cloud, producing transitions from ultracompact to extended HII regions as the stars move into lower density regions. The opposite behavior occurs when stars move toward the cloud cores. The main conclusion of our study is that ultracompact HII regions are pressure-confined entities while they remain embedded within dense cores. The confinement comes from ram and/or ambient pressures. The survival of ultracompact regions depends on the position of the star with respect to the core, the stellar life-time, and the core crossing time. Stars with velocities less than the cloud dispersion velocity can produce cometary shapes smaller than 0.1 pc at times of 20,000 yr or more. The sequence Ultracompact to Compact to Extended HII region shows a variety of unpredictable structures due to ionization-shock front instability. Some ultracompact HII regions with a core-halo morphology might be explained by self-blocking effects, when stars overtake and ionize leading, piled-up clumps of neutral gas. We use thermal energy to support the cloud against gravity; the results remain the same if other types of isotropic cloud support are used.

WEAK AND COMPACT RADIO EMISSION IN EARLY HIGH-MASS STAR-FORMING REGIONS. I. VLA OBSERVATIONS

We present a high sensitivity radio continuum survey at 6 and 1.3

A Catalog of 44 GHz Methanol Masers in Massive Star-forming Regions. IV. The High-mass Protostellar Object Sample

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High-resolution Observations of the Massive Protostar in IRAS 18566+0408

cm using the Karl G. Jansky Very Large Array towards a sample of 58 high-mass star forming regions. Our sample was chosen from dust clumps within infrared dark clouds with and without IR sources (CMC-IRs, CMCs, respectively), and hot molecular cores (HMCs), with no previous, or relatively weak radio continuum detection at the

Long-term Variability of H2CO Masers in Star-forming Regions

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The Extended Emission of Ultracompact HII Regions: An Overview and New Observations

mJy level. Due to the improvement in the continuum sensitivity of the VLA, this survey achieved map rms levels of

The Hot Molecular Core of G12.21–0.10: NH 3 (4, 4) Observations

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