954 research outputs found
Radial velocities of pulsating subdwarf B stars: KPD 2109+4401 and PB 8783
High-speed spectroscopy of two pulsating subdwarf B stars, KPD 2109+4401 and
PB 8783, is presented. Radial motions are detected with the same frequencies as
reported from photometric observations and with amplitudes of ~2 km/sec in two
or more independent modes. These represent the first direct observations of
surface motion due to multimode non-radial oscillations in subdwarf B stars. In
the case of the sdB+F binary PB 8783, the velocities of both components are
resolved; high-frequency oscillations are found only in the sdB star and not
the F star. There also appears to be evidence for mutual motion of the binary
components. If confirmed, it implies that the F-type companion is >~1.2 times
more massive than the sdB star, while the amplitude of the F star acceleration
over 4 hours would constrain the orbital period to lie between 0.5 and 3.2d
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
Submillimeter Atmospheric Transparency at Maunakea, at the South Pole, and at Chajnantor
For a systematic assessment of submillimeter observing conditions at
different sites, we constructed tipping radiometers to measure the broad band
atmospheric transparency in the window around 350 m wavelength. The
tippers were deployed on Maunakea, Hawaii, at the South Pole, and in the
vicinity of Cerro Chajnantor in northern Chile. Identical instruments permit
direct comparison of these sites. Observing conditions at the South Pole and in
the Chajnantor area are better than on Maunakea. Simultaneous measurements with
two tippers demonstrate conditions at the summit of Cerro Chajnantor are
significantly better than on the Chajnantor plateau.Comment: Accepted by PAS
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