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PHYSICAL PARAMETERS FOR SUBDWARF B STARS WITH
COMPOSITE SPECTRA
R. AZNAR CUADRADO, C.S. JEFFERY
Armagh Observatory, College Hill, Armagh BT61 9DG, N.
Ireland
For many years, the evolutionary origins of subdwarf B stars remained a
mystery. Observations of sdB stars with composite spectra and theoretical
considerations suggested that binary star evolution should play a major
role, but proof that a large fraction are binaries has taken a decade to
establish. Whilst many sdB stars have recently been recognised to be in
short-period binaries with unseen companions [8], we have focused on sdB
stars in which the spectrum of the secondary can be seen [5]. We previously
studied these systems by means of their ux distributions [1] and shown
that the secondaries are probably main-sequence G stars. This result has
been conrmed from analysis of their optical and near-infrared spectra [2].
The infrared triplet of ionized calcium was recognised as a good in-
dicator of a late-type companion in composite sdB spectra [5] and is an
excellent measure of cool star surface gravity. Our aim was to measure the
eective temperatures, surface gravities, and surface helium abundances of
the sdB stars in our sample, and the eective temperatures, surface gravi-
ties and radius ratios of their companions. This would enable us to identify
the companions and estimate their masses.
Observations were obtained with the Isaac Newton and William Her-
schel Telescopes at the La Palma Observatory in 1997 and 1998. Spectra
were obtained in the blue (4000 4700  A) and near-infrared (8000 8800  A)
and mostly at a resolution R  5 000. The measurement of atmospheric
parameters was achieved by nding the best-t models within a model grid
using  2 minimization. Model atmospheres and synthetic spectra for the
sdB stars were computed using STERNE and SPECTRUM [6]. Cool star
spectra were computed using Kurucz model atmospheres and SYNTHE [7].
 2 minimization was carried out using SFIT [6].
Full results are reported in [2]. We nd that sdB stars in composite
systems coincide in T e and log g with those of sdB stars with non-composite
spectra both in our own sample and in independent samples. The cool

2
companions are seen to be G stars on the main sequence. According to [3],
such stars are produced by stable Roche Lobe over ow whilst the more
massive star was close to the tip of the red-giant branch and are expected
to have very long orbital periods.
It is well known that sdB stars typically have lower than normal surface
helium abundances y = n He =nH due to the downward diusion of helium
in the stellar atmosphere [9]. For all composite systems in our sample, we
found y  0:01. The majority of the remainder have 0:01  y  1:94. It
has been proposed [10] that sdB stars may be divided into three groups of
helium stars, one of which we further subdivide:
{ i) sdB stars with single spectra showing no radial velocity changes,
{ ii) sdB stars with single spectra showing large velocity variations and pe-
riods of hours to days, a) having a low-mass main-sequence companion and
b) having a white dwarf companion,
{ iii) sdB stars with composite spectra showing small or no velocity varia-
tions and periods  years.
Combining our measurements of y with previous measurements [8], we nd
that group (i) all have y  0:01, group (ii) have 0:01  y  0:03 and group
(iii) all have y  0:01. In order to explain this phenomenon we suggest that:
{ i) single sdBs are formed from HeWD+HeWD mergers [4],[11] and have
a smaller hydrogen reservoir than other sdBs, so y reaches some minimum
even with diusion.
{ ii) for sdBs in short-period orbits, tidal perturbations occur at intervals
shorter than the diusion timescale (10 5 y) and so diusion is disrupted.
{ iii) sdBs in synchronous long-period orbits experience a much lower tidal
disruption and diusion is most eective at reducing surface helium.
The statistical basis for this hypothesis requires improvement. Its conr-
mation will be important for understanding sdB evolution.
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