Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.........6s&link_type=abstract
Thesis (PH.D.)--THE CATHOLIC UNIVERSITY OF AMERICA, 1995.Source: Dissertation Abstracts International, Volume: 56-02, Section: B
Other
2
Stellar Evolution
Scientific paper
Based on the study of 35 Herbig Ae/Be stars in the UV using IUE data, this study proposes an evolutionary sequence for the circumstellar matter surrounding these objects. This sequence has two distinct phases each with unique underlying physics: (1) the UV Broad Line stars have extremely broad bands of absorption arising mainly from the metastable states of Fe II and (2) the UV Narrow Line stars have more narrow Fe II absorption features and also features indicating the presence of shocks. The differences between the groups is illustrated by plotting the equivalent width of an Fe II feature against the equivalent width of C IV (a species that indicates the presence of shocks). In the Fe II/C IV plane, the stars form a sharp 'L' shape. Thus the plot is referred to as the "L-Diagram". Evidence from the UV indicates that the UV Broad Line stars evolve into the UV Narrow Line stars. This picture is compatible with models from the IR and other bandpasses. This implies that the circumstellar material around Herbig Ae/Be stars begins its evolution as a dense toroids with pronounced collimated outflow. Next it undergoes a transition and essentially no collimated outflow remains. At this point, strong shocks emerge. During the early phase, the outflow is probably collimated by magnetic fields. In the second phase, the magnetic field has collapsed and a boundary layer has been established. The remaining circumstellar material is accreted without significant contribution from magnetic fields.
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