Astronomy and Astrophysics – Astronomy
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...481..479s&link_type=abstract
Astrophysical Journal v.481, p.479
Astronomy and Astrophysics
Astronomy
15
Stars: Emission-Line, Be, Stars: Individual Constellation Name: Lambda Eridani, Stars: Mass Loss, Stars: Oscillations, Ultraviolet: Stars, X-Rays: Stars
Scientific paper
We document the results of simultaneous wavelength monitoring of the B2e star lambda Eri. This campaign was carried out from ground stations and with the ROSAT, ASCA, IUE, and Voyager 2 space platforms during a week in 1995 February--March; a smaller follow-up was conducted in 1995 September. During the first of these intervals lambda Eri exhibited extraordinary wind and disk-ejection activity. The ROSAT/HRI X-ray light curves showed no large flares such as the one the ROSAT/PSPC observed in 1991. However, possible low-level fluctuations in the February--March ROSAT data occurred at the same time as unusual activity in H alpha , He I lambda 6678, He II lambda 1640, and the C IV doublet. For example, the hydrogen and helium lines exhibited an emission in the blue half of their profiles, probably lasting several hours. The C IV lines showed a strong high-velocity discrete absorption component (DAC) accompanied by unusually strong absorption at lower velocities. The helium line activity suggests that a mass ejection occurred at the base of the wind, while the strong C III (Voyager) and C IV (IUE) lines imply that shock interactions occurred in the wind flow. It is not clear that the X-ray elevations are directly related to the strong C IV absorptions because the former changed on a much more rapid timescale than absorptions in the C IV lines. Within hours of the mild X-ray flux variations found by ROSAT on February 28, the Voyager UV spectrometer (UVS) observed a "ringing" that decayed over three 3 hr cycles. The amplitude of these fluctuations was strong (50%) at 950--1100 A, decreased rapidly with wavelength, and faded to nondetection longward of 1300 Angstroms. Various considerations indicate that these continuum variations were not due to an instrumental pathology in the UVS. Rather, they appear to be due to a time-dependent flux deficit in the 950--1250 Angstroms region. We outline a scenario in which a dense plasma structure over the star's surface is heated and cooled quasi-periodically to produce such flux changes. Observations of new examples of this phenomenon are badly needed. Amateur astronomers can make a significant contribution to its understanding by searching for ringing in light curves of Be stars during their outburst phases. Finally, we draw attention to an increase in the emission of the H alpha line that occurred at about the time the far-ultraviolet ringing started. This increased emission hints that ~50,000 K plasma near the star's surface can influence the circumstellar disk at ~12 R* by its increased Lyman continuum flux.
Anandarao Boddapati G.
Chakraborty Amit
Corcoran Michael F.
Ezuka Hiroyuki
Hirata Ryuko
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