An Unusual Blueshifted H$\alpha$ Absorption Episode in the Binary Star PG~0900+400
Session 47 -- Binary Stars
Display presentation, Thursday, June 15, 1995, 9:20am - 4:00pm

## [47.01] An Unusual Blueshifted H$\alpha$ Absorption Episode in the Binary Star PG~0900+400

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 ($T_{eff} \approx 31000$~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 H$\alpha$ profile was in its normal'' state, showing weak emission wings displaced $\sim$300 km~s$^{-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 H$\alpha$ 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 $\sim$700 km~s$^{-1}$. The absorption line depth measured as a fraction of continuum varied from 40\% to 25\% below continuum during this time. The redward emission wing was not much altered during this time. No equivalent event was seen in the D lines of either Na I or He I.
Observation of such a strong and asymmetric absorption\/ episode in H$\alpha$ 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.