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Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| 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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