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Mode of access: World Wide Web. / Thesis (Ph. D.)--University of Hawaii at Manoa, 2005. / Includes bibliographical references. / Electronic reproduction. / Also available by subscription via World Wide Web / xvii, 261 leaves, bound ill. (some col.) 29 cm

Thesis (Ph. D.)--University of Canterbury, 2008. / Typescript (photocopy). Includes bibliographical references (p. 206-218). Also available via the World Wide Web.

Thesis (Ph. D.)--Georgia State University, 2009. / Title from file title page. Harold A. McAlister, committee chair; Russel J. White, Brian D. Mason, Douglas R. Gies, David W. Latham, A.G. Unil Perera, Todd J. Henry, committee members. Description based on contents viewed Aug. 24, 2009. Includes bibliographical references (p. 391-410).

Thesis (Ph.D.) - University of St Andrews, May 2008.

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

Herbig-Haro (HH) objects are small nebulous regions of shock excited gas associated with bipolar outflows from newly formed stars. This dissertation presents an extensive set of observational data on Herbig-Haro objects, including deep CCD images, medium resolution long-slit spectra, and high resolution echelle spectra. The CCD survey indicates that HH objects cannot be identified reliably on the basis of morphology alone. The spectral line data show that HH objects exhibit enormous linewidths even though the objects are only about 1500 AU in size. The spectra sometimes have two velocity peaks, and show spatial separation of the high and low radial velocity gas. A radiative bowshock model constructed from a series of planar shock models accounts for the large linewidths, unusual line profiles, line ratios, and spatial structure seen in HH objects. A simple formula is derived that relates the shock velocity and orientation angle of a radiating bowshock to the observed maximum and minimum radial velocities seen in a line profile. The bowshock forms around a 'bullet' of material ejected from the forming star. The most likely acceleration mechanism for the bullet seems to be a breakup of a collimated stellar jet.

A survey of the Monoceros OB1 dark cloud has been made for molecular outflows and young stellar objects. In all, nine molecular outflows and thirty far-infrared sources were identified in a portion of the cloud composed of about 3 x 10⁴ M(⊙)of material. Statistical arguments suggest that 90% of the far-infrared sources actually are young stellar objects embedded in the cloud. If the star formation rate in the Mon OB1 cloud is roughly constant with time then molecular outflows in the cloud should be able to support it against collapse due to gravity. This suggests that the birthrate of outflows in the solar neighborhood is very high. In fact, regardless of considerations of cloud support, the large number of outflows identified in the Mon OB1 cloud and the propensity of the youngest stellar objects in the cloud to be associated with outflows suggest that outflows have a high birthrate in the solar neighborhood and are part of a common stage in early stellar evolution. The young stellar objects identified in the cloud can be fit into a spectral classification system. In fact, in terms of spectral slopes, far-infrared luminosity, and source size the properties of the objects are consistent with expectations if the system represents an evolutionary sequence. It is also found that the outflow phase in early stellar evolution tends to occur at about the time that young stellar objects lose a large fraction of their circumstellar envelopes. As a result it seems likely that outflows play an important role in sweeping out the circumstellar gas around many young stellar objects and may, in fact, play an important part in the evolutionary transition between the protostellar and stellar stages of evolution.

We present a near-infrared study of the stars forming in the ρ Ophiuchi dark cloud and a mid- to far-infrared study of their environment. We determine that the total cloud luminosity matches that of the known embedded sources, ruling out the existence of a numerous but faint low mass embedded population. IRAS and optically thin C¹⁸O column density data are used to evaluate dust grain sizes and compositions via competing grain models. Radiative modleing shows that a standard power law distribution of graphite and silicate grains is responsible for IRAS 60 and 100 μm band emissions. These grains are heated to about one tenth of the cloud's depth in the core region. Their optical depths closely follow molecular column density structure, but these grains are considerably colder than the molecular gas. We detect 481 sources in the J,H, or K bands in a 0.184 deg.² survey region in the cloud. Approximately 79% of the embedded 3 band (JHK) detected sources have near-infrared color indices greater than the local background population, suggesting that many of these objects are in pre-main-sequence evolutionary phases. The reddest of these sources are grouped in a high column and spatial density area within the survey region. Sources in this area have a normal power-law K luminosity function which is consistent with a theoretical model of a standard mass function and an age of 10⁵ yr. Sources exterior to this area have a luminosity function with an excess of intermediate luminosity sources that significantly differs from the luminosity function of the interior region. We interpret this non-standard luminosity function and the wide range of source reddenings in this peripheral region to be indicators of a considerable age (10⁶ yr) or age spread among sources there. We estimate that the cloud's star formation efficiency is currently greater than or equal to 25%. These newly discovered young stellar sources provide a statistically significant sample for studies of the cloud's embedded population and support established ideas of bound cluster formation and star formation bursts within the cloud.

High signal-to-noise optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green ultraviolet excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The stellar effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously from a detailed analysis of hydrogen and helium absorption line profiles. The derived temperatures and gravities place the subluminous B stars in the theoretical H-R diagram along and bounded below by theoretical sequences of the zero-age extended horizontal branch, lending strong support to the hypothesis that these stars are composed of helium-burning cores of ∼0.5 $M(⊙) overlain by very thin layers of hydrogen (≲0.02 M(⊙)). Various scenarios for their past evolutionary history are examined in the context of their probable future evolution into white dwarfs of lower than average mass. The derived distances above the Galactic plane support a scale height for the population of z₀ = 285 pc, consistent with the identification of their progenitor stars as members of the old disk population. Radial velocities of sdB and sdO stars are analyzed to infer their kinematic characteristics. The results for the sdB stars are inconclusive, but for the sdO stars the results also are consistent with the population belonging to the older part of the thin disk.