Karen M. Strom

Circumstellar material associated with solar-type pre-main-sequence stars - A possible constraint on the timescale for planet building

The observed frequency distribution of near-IR excesses for a sample of 83 stars in the Taurus-Auriga star-forming complex is used to provide an estimate of the number of solar-type premain-sequence (PMS) stars surrounded by circumstellar disks. The results suggest that if all such PMS stars are initially surrounded by disks, then disk lifetimes must range from much less than 3 x 10 to the 6th yr to about 10 to the 7th yr, providing a constraint on the time available for planet building. PMS stars with small near-IR excesses, but significant mid-IR and far-IR excesses, provide some evidence for changes in disk structure with time. 55 refs.

An IRAS survey of the Taurus-Auriga molecular cloud

IRAS data are used to search for young premain-sequence stars not previously associated with molecular cloud cores in the Taurus-Auriga region. NIR photometry and optical spectroscopy suggest that many of the objects are young stars. The sample includes six new embedded sources with luminosities comparable to that of the average T Tauri star, suggesting that surveys for premain-sequence stars in the cloud are essentially complete for luminosities greater than 0.5 solar luminosities. A disagreement is found between accretion rates derived from the duration of the embedded phase and those derived from the bolometric luminosity. It is found that this disagreement may be reconciled if a star accretes most of its mass in a time that is short compared to the duration of the embedded phase or if the ages of T Tauri stars have been underestimated. 62 refs.

A sensitive 10-micron search for emission arising from circumstellar dust associated with solar-type pre-main-sequence stars

The presence and evolutionary timescales of circumstellar disks surrounding solar-type premain-sequence stars are studied using excess IR radiation above photospheric levels. The analysis is based on optical photometry, published near-IR fluxes, IRAS fluxes, and 10-micron flux measurements of 20 stars in Taurus-Auriga obtained with the NASA IR telescope facility. About half of the stars with ages less than 3 Myr show excess 2.2 and 10 micron emission, consistent with emission from optically thick disks extending inward to the stellar surface. At ages of about 10 Myr, less than 10 percent of the sample stars show evidence of dust emission from optically thick disks. It is concluded that the timescale over which disks survive as IR-luminous, optically thick structures is less than 10 Myr. Also, evidence for inner holes in premain-sequence stars surrounded by optically thick disks is discussed. 18 refs.

Are wide pre-main-sequence binaries coeval?

We have observed a sample of 39 wide (projected separations 400-6000 AU) pre-main-sequence binary pairs spectroscopically and with optical and near-infrared images. The observations enable us to place 26 of the pairs in an H-R diagram and to determine masses and ages of the primary and secondary according to three sets of pre-main-sequence evolutionary tracks. In two-thirds of the cases the primary and secondary lie along the same isochrone to within the observational errors. However, real age differences appear for about one-third of our sample pairs-there is no set of nonintersecting theoretical isochrones that can make the primary and secondary have the same age for all pairs in our sample. In the cases where there are significant age differences between the component stars, the less massive star is usually younger than the more massive star. There is no correlation of the age differences with the presence or absence of accretion disks around the young stars. Hence, while disk accretion may affect the evolutionary tracks of the pre-main-sequence stars H-R diagram, we see no clear evidence of this effect among the pairs in our sample. The age differences also do not depend systematically on the apparent separation, the mass ratio, or the ages of stars.

Forbidden line and H-alpha profiles in T Tauri star spectra - A probe of anisotropic mass outflows and circumstellar disks

The results of a high-resolution spectroscopic study of 10 T Tauri stars (TTS) and two Herbig emission stars are presented based on red echelle spectra including the lines of forbidden O I 6300 A, forbidden N II 6584 A, forbidden S II 6716, 6731 A, as well as H-alpha. The forbidden lines display a continuous progression of profile types. The velocity structure in the forbidden lines is critically examined and compared to computed line profiles for a number of different wind models. Constant velocity spherical or conical winds fail to reproduce the observed line profiles, which are better explained by a wind with a latitude-dependent velocity field. A prediction of the wind model correlating the velocity of the reversal at H-alpha and the forbidden line velocity structure as a function of the view angle to the star is explored. Estimates of the average densities and sizes for the TTS forbidden emission regions are presented, and mass-loss rates are computed. IRAS far-IR fluxes are used to estimate the disk sizes.

Infrared luminosity functions for the young stellar population associated with the L1641 molecular cloud

Results are reported of a deep near-IR imaging survey which samples 0.77 sq deg of the L1641 star-forming complex and reaches 5 sigma limits at J (1.25 micron,), H (1.65 micron), and K (2.2 microns) of 16.8, 15.8, and 14.7 mag, respectively. A population of about 1500 stars spread throughout the cloud, seven small aggregates comprised of 10-50 stars whose typical projected surface densities exceed that of the distributed population by factors of 4-10, and a heretofore unknown, partially embedded dense cluster comprised of about 150 stars are identified. Analysis of the distribution of (J-H), (H-K), and (R-I) colors for these groups suggest that, in all cases, the stellar populations are dominated by solar-type PMS stars which appear to contain a mix of objects analogous to weak-line T Tauri stars. The present observed luminosity functions are based on reddening-corrected J-band magnitude.

A study of the stellar population in the Chamaeleon dark clouds

The properties of the stellar population in the Chamaeleon dark clouds are discussed. Spectral energy distributions, based on the extant photometric and spectroscopic data base and IRAS fluxes measured from coadded data taken at the position of each star, and spectral types allow placement of the stars in an H-R diagram. The age and mass distributions and the luminosity function for the Chamaeleon stars are compared to those in the Taurus-Auriga dark clouds and are found to be similar. A small subsample (eight of 36) of the Chamaeleon stars show unusual spectral energy distributions which seem best interpreted as arising from circumstellar disks whose inner regions (R(in)) is less than 30-50 stellar radii) area devoid of material. The X-ray properties of this sample of premain-sequence objects are compared to those of other premain-sequence samples, as well as to the Hyades and the Pleiades main-sequence stars.

THE MAGNETIC EVOLUTION OF THE TAURUS MOLECULAR CLOUDS .1. LARGE-SCALE PROPERTIES

Finely sampled maps of the (C-13)O J = 1-0 emission from five dark clouds within the Taurus molecular cloud complex were made, and polarization measurements of the optical emission were obtained from background and embedded stars to determine the direction of the interstellar magnetic field towards these regions. The clouds are found to have flattened morphologies with the direction of their minor axis parallel to the direction of the magnetic field, as expected if the lateral contraction of the gas is inhibited by magnetic pressures. In addition, each cloud appears to be rotating about an axis parallel to the magnetic field direction as predicted by detailed calculations of the magnetic braking process. It is believed that the magnetic field has played a significant part in the evolution of these regions, which are characterized by mean densities of about 1000-5000/cu cm. In addition, the clouds are determined to be in virial equilibrium based on the extent and mean line-width of (C-13)O J = 1-0 emission and the masses derived from molecular hydrogen column densities. 57 references.

Optical manifestations of mass outflows from young stars - At atlas of CCD images of Herbig-Haro objects

An attempt is made to provide deep CCD images suitable for tracing the shock-excited and scattered-light components characterizing extended Herbig-Haro complexes. Tabular data include astrometric positions for all stars, shock-excited emission knots, prominent scattered-light patches, as well as an identification for all H-alpha emission objects within the CCD images. It is concluded that: (1) optical outflows are not always accompanied by molecular flows, (2) optical outflows show a higher degree of collimation than their molecular counterparts, and (3) the Herbig-Haro phenomenon is not restricted to low-mass young stellar objects.

Moderate-resolution spectral standards from lambda 5600 to lambda 9000

We present a grid of stellar classification spectra of moderate resolution (R approximately 1500) in the range lambda lambda 5600-9000 A, compiled from high signal-to noise spectra of 275 stars, most in the open clusters Praesepe and M67. The grid covers dwarfs from types B8 through M5, giants from G8 through M7, and subgiants from F5 through K0. We catalog atomic and molecular absorption features useful for stellar classification, and demonstrate the use of luminosity-sensitive features to distinguish between late-type dwarf and giant stars. The entire database is made available in digital format on anonymous ftp and through the World Wide Web.