THE FORMATION OF PRIMORDIAL STARSStahler, Steven W.
doi: 10.1086/131877pmid: N/A
The problem of star formation from gas of primordial composition is discussed. The thermal evolution and fragmentation of primordial clouds, and the early evolution of the resulting stars, have been followed in calculations done with Palla (Arcetri) and Salpeter (Cornell). Although the clouds lacked the coolants provided by metals in present-day molecular clouds, they could have fragmented to low- and intermediate-mass stars if they contracted to sufficiently high density. Primordial protostars collapse faster and have larger hydrostatic covers than their Population I counterparts. The shock radiation emanating from these cores is optically visible, with an effective temperature of about 5000 K. The pre-main-sequence contraction phase resembles that in present-day stars, despite very different initial conditions.
T TAURI VARIABLES.Herbst, William
doi: 10.1086/131878pmid: N/A
A review is given of the variability characteristics of T Tauri stars monitored at Van Vleck Observatory. A list of T Tauri stars with known rotation periods is compiled and used to compare limits on the radii, derived from v sin i measures, with expectation based on the luminosities and effective temperatures of the stars. The large-amplitude quasi-perodic or aperiodic component of the light variation, which is more typical of the T Tauri phenomenon than periodic behavior, is discussed. Correlations between brightness and color and brightness and spectral change are reviewed. A typical time scale of 30 to 60 days is suggested for the phenomenon in the Van Vleck Observatory sample. A plausible interpretation of the variability is given, which involves the growth and decay of active regions on the photosphere of the star.
HII REGIONS AND STAR FORMATION IN GALAXIES.Hodge, Paul
doi: 10.1086/131879pmid: N/A
H II regions in galaxies, as loci of one type of star-forming event, show differences with Hubble type that can provide insight into environmental influences on the characteristics of star formation. This paper reviews the way that the number, location, size distribution, luminosity function, and structure of H II regions differ for galaxies of different type.
CLUSTERS OF THE SMALL MAGELLANIC CLOUD. I. A CATALOG OF 213 NEW CLUSTERS.Hodge, Paul
doi: 10.1086/131907pmid: N/A
A series of limiting plates of the Small Magellanic Cloud taken with the CTIO 4-m telescope has been examined to find all previously cataloged star clusters and all additional recognizable clusters, most of which turn out to be very small and faint. Because this survey reaches stars in the SMC as faint as absolute magnitudes of MB = +4, any significant star cluster not older than the universe should show up, and so these new data should be a nearly complete sample of all surviving clusters in most of the regions investigated. Data on the positions, diameters, and estimates of the brightest blue stars are given for the 213 new clusters. The total number of cataloged SMC cluster candidates is now 601. It is estimated that if similar deep surveys were to be made of the rest of the SMC the total number of detectable clusters would be approximately 900. Taking incompleteness into account, an estimate of the total SMC cluster population is approximately 2000.
SPECTROSCOPY OF HOT HORIZONTAL-BRANCH STARS IN GLOBULAR CLUSTERS. I. NGC 6397.Glaspey, J. W.; Demers, S.; Moffat, A. F. J.; Michaud, G.
doi: 10.1086/131908pmid: N/A
Spectra have been obtained, with moderate signal-to-noise ratio, for four, blue horizontal-branch (BHB) stars in the metal-poor globular cluster NGC 6397 using a photon-counting detector. These photometrically selected stars have effective temperatures near 12,000 K. The observed H-delta profiles suggest values of log g near 2.8, putting these stars above the main sequence of population I. At an apparent signal-to-noise ratio of 25:1 between 3900 A and 4200 A, the only identifiable absorption lines other than the Balmer lines are weak He I 4026 A and Ca II 3933 A (presumably interstellar). The scatter in the observed strengths of He I 4026 A for several field stars makes it uncertain whether or not He I is underabundant in these cluster BHB stars. An examination of the cores of the H-delta profiles suggests that the cluster HB stars are probably slow rotators.
SPECTROSCOPY OF SIX X-RAY-SELECTED BL LACERTAE CANDIDATES.Margon, Bruce; Boroson, Todd A.; Chanan, Gary A.; Thompson, Ian B.; Schneider, Donald P.
doi: 10.1086/131909pmid: N/A
Results of a continuing program aimed at extending the small list of X-ray-selected BL Lac objects are reported. High-quality spectra have been obtained of six faint blue objects that lie within the positional error boxes of X-ray sources discovered serendipitously by the Einstein Observatory. Three of the objects are found to be previously uncataloged low-red-shift quasi-stellar objects, including one formerly suggested as BL Lac candidate. Two are faint galactic stars, while the final object has a featureless spectrum, and thus remains a candidate. Although X-ray selection may ultimately be an effective means of discovering faint BL Lac objects, such sources are evidently rare at the X-ray flux levels attainable by the Einstein Observatory.
THE LYMAN-ALPHA FOREST IN THE QUASAR B2 1225+317 FROM 3140 A TO 3940 A.Boulade, O.; Vigroux, L.; Kunth, D.; Sargent, W. L. W.
doi: 10.1086/131911pmid: N/A
The authors have obtained a spectrum of the quasar B2 1225+317 from 3140 Å to 3940 Å using the new "UV prime" spectrograph at the 3.6-m CFH telescope. A signal-to-noise ratio larger than 25 was achieved with an exposure of only 90 minutes using an RCA CCD. They produce a new line list of the Lyman-alpha forest that is complete to about 0.15 Å rest equivalent width. They confirm that the number density of the Lα lines increases steeply for rest equivalent widths smaller than 0.20 Å as suggested by previous workers. The new equipment is very suited for quick quasar survey and in particular for a Mg II search at z < 0.40.