The evoluation of young stellar object disks and their environment

By carrying out direct imaging and spectroscopic observations of young, pre-main sequence stars in nearby molecular clouds researchers have begun: to define the frequency with which disks of approximately solar system size and mass form around young stars, and to understand the timescale for disk evolution; to characterize the early radiation (ultraviolet and keV particle) environment of circumstellar disks through study of evolution of stellar winds, wind/disk interactions and the UV and optical emission characteristics of young stars; and to understand the evolution of the solid and gaseous constituents of disks through observations of absorption features in circumstellar gas, broad emission features produced by organic compounds on grain surfaces, and absorption features (e.g., ice) produced in grain mantles. These programs offer the possibilty of relating results from astrophysical studies of the environment to newly-formed stars to the record of planet formation preserved in the solar system. Researchers completed a spectroscopic survey of 30 T Tauri stars with ages ranging from approximately 2 x 10 to the 5th power to 3 x 10 to the 7th power years. From analysis of (OI) and (SII) emission lines, it was concluded that all but two of the stars in our sample are surrounded by optically opaque disks of dimension approximately 50 AU. The two remaining objects show evidence consistent with partial disk clearing (at an age approximately 3 x 10 to the 6th power year). R and I band CFHT observations yielded detections of disks of dimension approximately 100 AU surrounding 5 additional T Tauri stars

The evolution of young stellar object disks and their environment

The main efforts were directed towards determining the frequency of disk occurrence and the timescales for disk evolution for solar-type and intermediate mass stars. The results of the investigation showed that optically thick disks are accretion disks. The projected accomplishments are also discussed

The properties and environment of primitive solar nebulae as deduced from observations of solar-type pre-main sequence stars

The following topics were discussed: (1) current observation evidence for the presence of circumstellar disks associated with solar type pre-main sequence (PMS) stars; (2) the properties of such disks; and (3) the disk environment

Demographics of Transition Objects

The unusual properties of transition objects (young stars with an optically thin inner disc surrounded by an optically thick outer disc) suggest that significant disc evolution has occured in these systems. We explore the nature of these systems by examining their demographics, specifically their stellar accretion rates (Mdot) and disc masses (Mdisc) compared to those of accreting T Tauri stars of comparable age. We find that transition objects in Taurus occupy a restricted region of the Mdot vs. Mdisc plane. Compared to non-transition single stars in Taurus, they have stellar accretion rates that are typically ~10 times lower at the same disc mass and median disc masses ~4 times larger. These properties are anticipated by several proposed planet formation theories and suggest that the formation of Jovian mass planets may play a significant role in explaining the origin of at least some transition objects. Considering transition objects as a distinct demographic group among accreting T Tauri stars leads to a tighter relationship between disc masses and stellar accretion rates, with a slope between the two quantities that is close to the value of unity expected in simple theories of disc accretion.Comment: 11 pages, 4 figures, to appear in MNRA

Analysis of F and G Subdwarfs. II. A Model-Atmosphere Abundance Analysis of the Subdwarfs HD 140283 and HD 19445

A grid of model atmospheres has been used to perform an abundance analysis of these two extreme Population II stars. The abundances determined from the models confirm the general results of previous investigations, namely that metals are deficient by a factor of about 100 relative to average Population I abundances. A marginal deficiency of s-process material relative to the average deficiency is found; the carbon deficiencies are comparable to those of the other elements and for HD 19445 the α-process elements are enhanced. From the analysis it also appears that convection plays a significant role in determining the temperature structure of subdwarf atmospheres

Near–Infrared Classification Spectroscopy: \u3ci\u3eH\u3c/i\u3e–band Spectra of Fundamental MK Standards

We present a catalog of H-band spectra for 85 stars of approximately solar abundance observed at a resolving power of 3000 with the KPNO Mayall 4 m Fourier Transform Spectrometer. The atlas covers spectral types O7M5 and luminosity classes IV as defined in the MK system. We identify both atomic and molecular indices and line ratios that are temperature and luminosity sensitive, allowing spectral classification to be carried out in the H-band. The line ratios permit spectral classification in the presence of continuum excess emission, which is commonly found in premain-sequence or evolved stars. We demonstrate that with spectra of R = 1000 obtained at signal-to-noise ratio \u3e50, it is possible to derive spectral types within ±2 subclasses for late-type stars. These data are available electronically through the Astronomical Data Center in addition to being served on the World Wide Web

Analysis of F and G Subdwarfs. II. A Model-Atmosphere Abundance Analysis of the Subdwarfs HD 140283 and HD 19445

A grid of model atmospheres has been used to perform an abundance analysis of these two extreme Population II stars. The abundances determined from the models confirm the general results of previous investigations, namely that metals are deficient by a factor of about 100 relative to average Population I abundances. A marginal deficiency of s-process material relative to the average deficiency is found; the carbon deficiencies are comparable to those of the other elements and for HD 19445 the α-process elements are enhanced. From the analysis it also appears that convection plays a significant role in determining the temperature structure of subdwarf atmospheres