J. Deca, T. R. Marsh, R. H. Østensen, L. Morales-Rueda, C. M. Copperwheat, R. A. Wade, M. A. Stark, P. F. L. Maxted, G. Nelemans, U. Heber
About 50% of all known hot subdwarf B stars (sdBs) reside in close (short
period) binaries, for which common envelope ejection is the most likely
formation mechanism. However, Han et al. 2003 predict that the majority of sdBs
should form through stable mass transfer leading to long period binaries.
Determining orbital periods for these systems is challenging and while the
orbital periods of ~100 short period systems have been measured, there are no
periods measured above 30 days. As part of a large program to characterise the
orbital periods of subdwarf B binaries and their formation history, we have
found that PG1018-047 has an orbital period of 760\pm6 days, easily making it
the longest period ever detected for a subdwarf B binary. Exploiting the Balmer
lines of the subdwarf primary and the narrow absorption lines of the companion
present in the spectra, we derive the radial velocity amplitudes of both stars,
and estimate the mass ratio M_{MS}/M_{sdB} = 1.6\pm0.2. From the combination of
visual and infrared photometry, the spectral type of the companion star is
determined to be mid K.
View original:
http://arxiv.org/abs/1111.3576
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