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Why do Molecular Clouds Give Birth to Objects like Stars

We present preliminary results of an ultraviolet survey of stellar winds in 62 Be and 43 normal B stars covering spectral types BO to B5 and luminosity classes III-V. The data were obtained with the International Ultraviolet Explorer Satellite. We find that the wind absorption seen in the resonance lines of C IV in Be stars is often the result of multiple superposed blueshifted absorption components. We confirm the results of Henrichs (1984), who used B and V photometry of the globular cluster NGC 362, obtained with the CTIO 4-m PFCCD, shows a well-defined main sequence from the turnoff, near V = 18.5 and (B - V) = 0.45, to V ~ 22. The main-sequence intrinsic width is less than ± 0.010 mag in (B — V), limiting the spread in metal abundance among unevolved main-sequence stars to A[Fe/H] < ±0.09 dex. For our three 3' X 5' fields, centered approximately 7' from the cluster center, there are no blue-straggler candidates, and the distribution of stellar colors about the main sequence indicates there to be at most a small fraction of binary systems on the main sequence. The best match to the isochrones of Vandenberg and Bell (1985, Ap. J. Suppl., 58, 561) is for the case [Fe/H] = —1.0, a helium mass fraction, Y, between 0.26 and 0.30, and an age of ~ 14 Gyr. The data can be matched to slightly more metal poor or rich isochrones by making small adjustments to the accepted values for distance modulus and reddening, or considering smaller values for the mixing length parameter. For these cases, the indicated helium abundance remains relatively high, and the age ranges from 14-17 Gyr. tion of velocity variations. These results make it very unlikely that the accretion disk is fed by a companion star, but are consistent with the idea that the accreted material arises from a remnant disk of protostellar material. The broad-line region of the Galactic center has been mapped in the Ba line of atomic hydrogen with a 2.5 arc sec beam and at a velocity resolution of400 km sec 1 . The high-velocity line emission is found to be spatially re-solved, with a characteristic FWHM of 3 arc sec. The peak intensity of this emission is coincident with the position of the source 1RS 16 Center. The broad line appears to be symmetric about V LSR = 0 at all positions, suggesting that the high-velocity gas motions have no rotational component. If the broad-line emission is due to outflow then the mass-loss rate from 1RS 16 Center is— 10 3 2J£ 0 yr -1 . If it is caused by infall onto a central object, the mass of that object is — 6 X 10 6 The By line at 1RS 16 Center has also been found to have broad wings. Measurements of both Brackett line intensities at a number of positions in the Galactic center show that the reddening toward the central 0.5 arc minute is approximately uniform, corresponding to an extinction of A v — 30 mag. The intensity of the He I line at 2.06 |xm peaks at 1RS 16. The ionization state of the gas also appears to be maximum near 1RS 16. The nature of the source (or sources) giving rise to the broad-line emission in the Galactic center presently remains unknown. It has been suggested that a supermassive black hole (— 4 X 10 6 3K 0 ) resides in the Galactic center region conclusion is on the of gas and the assump-tion that the motions of the gas clouds are in response the gravitational potential. We have conducted a detailed spectroscopic and imaging study of the optical emission associated with the young stellar object IRS5 in the Lynds 1551 star-forma-tion region. This source has already been found to possess a bipolar molecular outflow, several blueshifted Herbig-Haro (H-H) objects, an optical emission-line and radio-continuum-emitting “jet” and a circumstellar disk. The major results of our study of this prototype bipolar outflow source are as follows. Infrared spectroscopy of low-luminosity YSOs in the Taurus and Ophiuchus clouds indicates that 7 of the 13 objects observed show line excess. In two cases the line strengths are greater than predicted from Balmer ionization. A model invoking thermal emission is energetically possible but the energy source is not readily identifiable. Rotational energy in a circumstellar disk is a possible energy source. The CMa R1 association is embedded in a curved region of dark nebulosity which is part of a larger-scale ring structure thought to be a relatively old supernova remnant. We present linear polarization measurements at optical wavelengths of stars which sample the CMa R1 dark cloud. The observed polarizations generally follow the curvature of the dark cloud and are found to be consistent with models of shock-wave compression of an initial interstellar magnetic field by a nearby supernova. These results provide further evidence that star formation in CMa R1 was triggered by cloud collapse due to an external source of compression rather than quiescent collapse along magnetic field lines. the and/or declining in accord with their respec-tive ionization potentials. Here we illustrate this sequence and extend it to type B8, establishing that the same systematic behavior continues through the B supergiant