428 research outputs found
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
TESS uncloaks the secondaries in hydrogen-deficient binaries
Sgr is the prototype of four known hydrogen-deficient binary (HdB)
systems. These are characterised by a hydrogen-deficient A-type primary,
variable hydrogen emission lines, and a normally unseen secondary presumed to
be an upper main-sequence star. Orbital periods range from tens of days to 360
d. TESS observations of all four HdBs show a flux variation with well-defined
period in the range 0.5 -- 0.9 d, too short to be associated with the
supergiant primary, and more likely to be the rotation period of the secondary
and associated with a chemical surface asymmetry or a low-order non-radial
oscillation. The observed rotation period supports a recent analysis of the
Sgr secondary. The observations give a direct glimpse of the
secondary in all four systems, and should help to explain how the primary has
been stripped to become a low-mass hydrogen remnant.Comment: MNRAS: Accepted 2022 November 05. Submitted 2022 October 22, 5 page
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