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Triggered Star Formation by Massive Stars
We present our diagnosis of the role that massive stars play in theformation of low- and intermediate-mass stars in OB associations (theλ Ori region, Ori OB1, and Lac OB1 associations). We find thatthe classical T Tauri stars and Herbig Ae/Be stars tend to line upbetween luminous O stars and bright-rimmed or comet-shaped clouds; thecloser to a cloud the progressively younger they are. Our positional andchronological study lends support to the validity of theradiation-driven implosion mechanism, where the Lyman continuum photonsfrom a luminous O star create expanding ionization fronts to evaporateand compress nearby clouds into bright-rimmed or comet-shaped clouds.Implosive pressure then causes dense clumps to collapse, prompting theformation of low-mass stars on the cloud surface (i.e., the bright rim)and intermediate-mass stars somewhat deeper in the cloud. These starsare a signpost of current star formation; no young stars are seenleading the ionization fronts further into the cloud. Young stars inbright-rimmed or comet-shaped clouds are likely to have been formed bytriggering, which would result in an age spread of several megayearsbetween the member stars or star groups formed in the sequence.
| Dust around young stars. Photopolarimetric observations of the T Tauri star BM Andromedae.
The results of photoelectric UBV observations of the T Tauri typevariable BM And made during 1983-1991, as well as of simultaneousphotopolarimetric UBVRI observations of this star for 1990-93 are given.In the course of these observations both the brightest (V=11.6)mag andthe weakest (V=14.1)mag states of this star were registered. It is shownthat the initial reddening of the star during a minimum is stopped atthe same brightness level and the star can be bluer in the color U-Bduring the deepest part of minimum. The decrease of brightness of BM Andis accompanied by an increase of linear polarization (up to 3-7%)simultaneously in all UBVRI bands. These changes agree with thoseobserved by Kardopolov & Rspaev (1990) and are, in their nature,similar to those observed in Herbig Ae stars with non-periodicAlgol-type minima. The similar behavior both of the color index and ofthe polarization of BM And and of these stars indicates that the stellarradiation scattered by dust particles in the circumstellar disk is thesource of the intrinsic polarization and reduced blue radiation observedat the deepest minima. This intrinsic component of the linearpolarization was separated from the observed one. Study of theinterstellar (IS) polarization of the nearby stars shows that the ISmagnetic field in this region has a regular structure, and the vector ofintrinsic linear polarization of BM And is parallel to the lines ofmagnetic force. The latter means that the circumstellar disk of BM Andlies in the plane normal to the IS magnetic field if the disk isoptically thin. Such an orientation points to the important role of themagnetic field during the initial stage of gravitational collapse of theprotostellar cloud from which BM And was formed.
| The unusual cometary star-forming region G110-13
We present far-IR, radio continuum, and spectral line observations of anunusual, highly elongated, comet-shaped molecular cloud, located about100 pc from the Galactic plane. The presence of three late B-type starsembedded within, or adjacent to, this low-mass cloud implies astar-forming efficiency that may be as high as 30 percent. Severalmechanisms that may have been responsible for its unusual morphology andhigh star-forming efficiency will be described and evaluated. Althoughram-pressure resulting from the rapid motion of this cloud through theinterstellar medium could explain its streamlined appearance, there isevidence that G110-13 is the compression front formed by a recent cloudcollision.
| Observational studies relating to star formation. II.
Abstract image available at:http://adsabs.harvard.edu/abs/1969AJ.....74.1021A
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