Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997mnras.285..657o&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 285, Issue 3, pp. 657-672.
Astronomy and Astrophysics
Astronomy
55
Binaries: General, Stars: Individual: Ec 20117-4014, Stars: Individual: Ec 14026-2647, Stars: Individual: Ec 10228-0905, Stars: Individual: Pb 8783, Stars: Oscillations
Scientific paper
A new class of pulsating stars, the EC 14026 stars, has recently been announced. We report the discovery of rapid oscillations in EC 20117-4014. The oscillations are multiperiodic with three periods detected so far: 137.3, 142.1 and 158.7s. The peak-to-peak amplitude of these oscillations is ~1 per cent. Along with the spectrum and UBV colours, this implies that EC 20117-4014 is another member of the new class. Interestingly, there is evidence for period/phase changes in the main 137.3-s period, as no linear ephemeris can be found that will fit the collection of light curves we have obtained. We interpret the oscillations as pulsations of an sdB star in a binary with an F/G companion, as indicated by the optical spectra. In order to derive atmospheric parameters of the component stars, low- (~3.5A) and intermediate- (~1A) resolution optical spectroscopy is presented for the first four EC 14026 stars to be discovered: EC 14026-2647, EC 10228-0905, EC 20117-4014 and PB 8783. Their spectra are modelled as the combination of sdB and F/G stars using a grid of high-gravity model atmospheres to represent the former, and observed spectra to represent the latter. The spectral types of the late-type companions are estimated to be in the range early F to early G. Atmospheric parameters for the sdB components of the four stars are derived and found to be similar: log g~6.0 and T_eff~35 000K. If the late-type companions are on or near the main sequence, their distances agree with those estimated for the sdB components, assuming that the masses of the latter are ~0.5Msolar. The evolutionary status of the four stars strongly suggests that the sdB component has suffered substantial mass loss by Roche lobe overflow, along the lines originally proposed by Mengel et al. This hypothesis can be tested by using the orbital time delay in the pulsations to measure the binary period and separation. The discovery of pulsations in sdB stars is important: asteroseismological studies to probe their structure and evolutionary time-scale can now be contemplated.
Kilkenny Dave
Koen Chris
Lynas-Gray Anthony E.
O'Donoghue Darragh
Stobie Robert S.
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