Wind Variability in the B Supergiant HD\ 64760

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Session 54 -- Interacting Binaries: Hot Stars
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[54.03] Wind Variability in the B Supergiant HD\ 64760

D. Massa (ARC), R. Prinja (UCL), A. Fullerton (Bartol)

We present a 6 day IUE\ time series of the rapidly rotating B0.5 Ib, HD\ 64760 ($v\sin i=238$ km/s). B0-1 supergiants, such as HD\ 64760, have several spectral diagnostics which are advantageous for studying wind variability. Some of these are: well-developed, but unsaturated wind lines spanning a wide range of ionization stages, making it possible to examine the ionization structure of wind variability; strong photospheric lines which are sensitive temperature and surface gravity diagnostics, with the strongest lines also showing wind effects, and; large radii and low terminal speeds, making the flow times of their winds (which scale as $R_{\odot }/v_\infty $) longer and wind variability easier to observe.

Our observations of HD\ 64760 revealed: continuous wind activity % throughout the entire 6 day run, including a particularly strong event during the second day, low velocity gas in the Al III lines indicating material moving outward from $v\simeq 0$ km/s at the beginning of the day 2 event, a\ photospheric connection between the ``photospheric'' Si III $% \lambda \lambda $1300 triplets (6 ev above ground) and the beginning of the large day 2 event, and verification of the wind-rotation connection, because HD\ 64760 and 2 other rapidly rotating B\ supergiants observed at the same time all showed continuous wind activity. Two aspects of the time series are especially interesting. First, ratios of the Si IV\ wind optical depths with those of Si III and N V\ show that the ionization of the wind did not change at the beginning of the day 2 event, so the increased absorption indicates an increase in the wind material. Second, as the absorption associated with the day 2 event moved out in the wind it narrowed in velocity and strengthened (as expected), but then suddenly became dramatically weaker during day 4. Again, ionic ratios show no dramatic ionization shift, indicating the wind material simply disappeared from the line of sight. Together, these observations suggest that the wind activity is physically tied to the base of the wind, implying that there are ``features'' on the stellar surface.

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