PG 0900+400 (= BI Lyn) has been identified as a composite-spectrum binary by Ferguson, Green & Liebert (1984), who found that it consists of a hot subdwarf star (Teff ~ 31000 K) and an early K star contributing roughly equally at V. Observations of PG 0900+400 were carried out at the KPNO 2.1m telescope on 1995 March 8--11 UT in search of radial velocity variations that would show the system to have a short orbital period and thus to be a candidate product of common envelope evolution. Preliminary analysis showed no large velocity variations attributable to orbital motion in the Na I and He I D lines. An improved analysis of these and other (metal) lines will be presented. On the first, third, and fourth nights of the run, the Hα profile was in its ``normal'' state, showing weak emission wings displaced ~ 300 km s(-1) (at peak) to either side of an absorption core of variable depth and slightly variable central wavelength. On the second night, seven observations obtained over a 7-hour interval showed a dramatic strengthening of the Hα absorption profile. As the night progressed, the 100--200% ``excess'' equivalent width was displaced to the blue by successively larger velocities, ending with a terminal velocity (measured at continuum level) of ~ 700 km s(-1) . The absorption line depth measured as a fraction of continuum varied from 40% to 25% below continuum during this time. The redward emission wing was not much altered during this time. No equivalent event was seen in the D lines of either Na I or He I. Observation of such a strong and asymmetric absorption episode in Hα is to our knowledge unprecedented in similar binary systems. Interpretations of the event as a mass loss episode from the hot star, or alternatively as a giant flare event on the K star, will be discussed. Frequent monitoring of PG 0900+400 and similar objects is needed to characterize how often such events occur. Supported in part by an NSF REU Site grant to Penn State.
American Astronomical Society Meeting Abstracts #186
- Pub Date:
- May 1995