569 research outputs found
Variations of the HeII 1640A Line in B0e--B2.5e Stars
Using the IUE data archive, we have examined the SWP-camera echellograms of
74 B0--B2.5e stars for statistically significant fluctuations in the He II
("Halpha") 1640A line profile. In this sample we found that the HeII line is
occasionally variable in 10 stars over short to long timescales. The
HeII-variable stars discovered are lambda Eri, omega Ori, mu Cen, 6 Cep, HD
67536, psi-1 Ori, eta Cen, pi Aqr, 2 Vul, and 19 Mon. The most frequent two
types of variability are an extended blue wing absorption and a weakening of
the line along the profile. Other types of variability are a weak emission in
the red wing and occasionally a narrow emission feature. In the overwhelming
number of cases, the CIV resonance doublet exhibits a similar response; rarely,
it can exhibit a variation in the opposite sense. Similar responses are also
often seen in the SiIV doublet, and occasionally even the SiIII 1206A line. We
interpret the weakenings of HeII and of high-velocity absorptions of CIV to
localized decreases in the photospheric temperature, although this may not be a
unique interpretation. We discuss the variable blue wing absorptions and red
wing emissions in terms of changes in the velocity law and mass flux carried by
the wind. In the latter case, recent experimental models by Venero, Cidale, &
Ringuelet require that during such events the wind must be heated to 35kK at
some distance from the star.Comment: 15 pages (figures imbedded) in A&A format. Accepted by the Astronomy
& Astrophysic
Spectral Response of the Pulsationally-Induced Shocks in the Atmosphere of BW Vulpeculae
The star BW Vul excites an extremely strong radial pulsation that grows in
its envelope and is responsible for visible shock features in the continuum
flux and spectral line profiles emerging in the atmosphere At two phases
separated by 0.8 cycles. Material propelled upwards in the atmosphere from the
shock returns to the lower photosphere where it creates a second shock just
before the start of the next cycle. We have obtained three nights of echelle
data for this star over about 5 pulsation cycles (P = 0.201 days) in order to
evaluate the effects of on a number of important lines in the spectrum,
including the HeI 5875A and 6678A lines. These data were supplemented by
archival high-dispersion IUE (UV) data from 1994. A comparison of profiles of
the two HeI lines during the peak of the infall activity suggests that
differences in the development of the blue wing at this time are due to heating
and short-lived formations of an optically thin layer above the atmospheric
region compressed by the infall. This discovery and the well-known decreases in
equivalent widths of the CII 6578-83A doublet at the two shock phases, suggests
that shock flattens the temperature gradient and produces heating in heating
the upper atmosphere. Except for absorptions in the blue wings of the UV
resonance lines, we find no evidence for sequential shock delays arriving at
various regions of line formation of the photosphere (a "Van Hoof effect").
Phase lags cited by some former observers may be false indicators arising from
varying degrees of desaturation of multiple lines, such as for the red HeI
lines. In addition, an apparent lag in the equivalent width curve of lines
arising from less excited atomic levels could instead be caused by post-shock
cooling, followed by a rebound shock.Comment: 12 pages in Latex/MNRAS format, 9 eps-format figure
Is there a propeller neutron star in Cas?
Cas is the prototype of a small population of B0-B1.5 III-V
classical Be (cBe) stars that emit anomalous and hard X-rays with a unique
array of properties. Cas is known to host, like other cBe stars, a
decretion disk and also a low mass companion. Recently Postnov et al. have
posited that this companion is a magnetized rapidly spinning neutron star that
deflects direct gravitational accretion from a stellar/disk wind via the
"propeller mechanism." These authors state that the key X-ray observations are
"remarkably well produced" in this scenario. We reexamine this mechanism in
detail and conclude that there are a number of fatal objections in its
application to the Cas case. Among other considerations these issues
include the prediction under the propeller scenario of a much smaller
population of Cas stars than is observed and the lack of allowance for
observed correlations of X-ray and UV and/or optical properties over a variety
of timescales.Comment: 9 page
X-Ray Flare Characteristics in the B2e Star Lambda Eridani (ROSAT)
We document the results of a simultaneous wavelength monitoring on 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 February-March 1995; a smaller follow-up was conducted in September 1995. 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/PSCA 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 implies 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 UVS observed a "ringing" that decayed over three 3-hr. cycles. The amplitude of these fluctuations was strong (50%) at (lambda)(lambda)950-1100, decreased rapidly with wavelength, and faded to nondetection longward of (lambda)1300. 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 (lambda)(lambda)950-1250 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 FUV ringing started. This increased emission hints that approximately 50,000K plasma near the star's surface can influence the circumstellar disc at approximately 12R. by its increased Lyman continuum flux
X-Ray Flare Characteristics in lambda Eridani
This proposal was for a joint X-ray/ultraviolet/ground-based study of the abnormal Be star lambda Eri, which has previously shown evidence of X-ray flaring from ROSAT observations in 1991. The X-ray component consisted of observations from both the ASCA and ROSAT satellites
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