8,875 research outputs found
Fe-bump instability: the excitation of pulsations in subdwarf B and other low-mass stars
We consider the excitation of radial and non-radial oscillations in low-mass
B stars by the iron-bump opacity mechanism. The results are significant for the
interpretation of pulsations in subdwarf B stars, helium-rich subdwarfs and
extreme helium stars, including the EC14026 and PG1716 variables. We
demonstrate that, for radial oscillations, the driving mechanism becomes
effective by increasing the contrast between the iron-bump opacity and the
opacity from other sources. The location of the iron-bump instability boundary
depends on the mean molecular weight in the envelope and also on the radial
order of the oscillation. A bluer instability boundary is provided by
increasing the iron abundance alone, explaining the observed EC14026 variables,
and by higher radial order oscillations. We show that the coolest EC14026
variables may vary in the fundamental radial mode, but the hottest variables
must be of higher radial order. In considering non-radial oscillations, we
demonstrate that g-modes of high radial order and low spherical degree (l<4)
may be excited in some blue horizontal branch stars with near-normal
composition (Z=0.02). Additional iron enhancement extends the g-mode
instability zone to higher effective temperatures and also creates a p-mode
instability zone. With sufficient iron, the p-mode and g-mode instability zones
overlap, allowing a small region where the EC14026 and PG1716-type variability
can be excited simultaneously. However its location is roughly 5000 K too low
compared with the observed boundary between EC14026 and PG1716 variables.Comment: MNRAS, in press, 16 pages, 13 figure
K2 observations of the pulsating subdwarf B star EQ Piscium: an sdB+dM binary
K2, the two-wheel mission of the Kepler space telescope, observed the
pulsating subdwarf B star EQ Psc during engineering tests in 2014 February. In
addition to a rich spectrum of g-mode pulsation frequencies, the observations
demonstrate a light variation with a period of 19.2 h and a full amplitude of
2%. We suggest that this is due to reflection from a cool companion, making
EQ\,Psc the longest-period member of some 30 binaries comprising a hot subdwarf
and a cool dwarf companion (sdB+dM), and hence useful for exploring the
common-envelope ejection mechanism in low-mass binaries.Comment: Accepted for publication in MNRA
A three channel telemetry system
A three channel telemetry system intended for biomedical applications is described. The transmitter is implemented in a single chip using a 2 micron BiCMOS processes. The operation of the system and the test results from the latest chip are discussed. One channel is always dedicated to temperature measurement while the other two channels are generic. The generic channels carry information from transducers that are interfaced to the system through on-chip general purpose operational amplifiers. The generic channels have different bandwidths: one from dc to 250 Hz and the other from dc to 1300 Hz. Each generic channel modulates a current controlled oscillator to produce a frequency modulated signal. The two frequency modulated signals are summed and used to amplitude modulate the temperature signal which acts as a carrier. A near-field inductive link telemeters the combined signals over a short distance. The chip operates on a supply voltage anywhere from 2.5 to 3.6 Volts and draws less than 1 mA when transmitting a signal. The chip can be incorporated into ingestible, implantable and other configurations. The device can free the patient from tethered data collection systems and reduces the possibility of infection from subcutaneous leads. Data telemetry can increase patient comfort leading to a greater acceptance of monitoring
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