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Physical parameters for subdwarf B stars with composite spectra
R. Aznar Cuadrado, C.S. Je ery
Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
rea@star.arm.ac.uk, csj@star.arm.ac.uk
Abstract
Intermediate-resolution spectra have been obtained for a
number of subdwarf B stars having both single and com-
posite spectra. Physical parameters have been determined
for the sdB stars and, in composite-spectrum systems,
their cool companions. For these binaries, we have
developed a method which uses the blue-optical spectrum
to determine the e ective temperatures of both stars, the
surface gravity of the hot stars and the radius ratio of the
system. The surface gravity of the cool star is measured
using the infrared calcium triplet. The surface gravities
of these cool companions identify them as main-sequence
stars with masses in the range 0:8 1:2M . There
is evidence that the composite-spectrum sdBs are more
helium-poor than single-spectrum sdBs.
Introduction
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 be a ma-
jor contributor, but proof that a large fraction are bi-
naries has taken a decade to establish. Whilst others
have recently identi ed many sdB stars in short-period
binaries with unseen companions, we have focused on
those sdB stars in which the spectrum of the secondary
can be seen. We previously studied these systems by
means of their ux distributions (Aznar Cuadrado &
Je ery 2001) and shown that the secondaries are prob-
ably main-sequence G stars. In this study, published
recently in full by Aznar Cuadrado & Je ery (2002),
we report an analysis of their optical and near-infrared
spectra.
Observations
Observations were obtained at the Isaac Newton and
William Herschel Telescopes at the La Palma Obser-
vatory 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.
All spectra were reduced using standard procedures.
Spectral Analysis
Figure 1. Normalized red spectrum of the compos­
ite PG 2110+127 around the infrared calcium triplet
(histogram) together with the best fit model spectrum
(polyline).
The infrared triplet of ionized calcium was recognised
as a good indicator of a late-type companion in com-
posite sdB spectra by Je ery & Pollacco (1998) and is
an excellent measure of cool star surface gravity. Our
aim was to the measure the e ective temperatures,
surface gravities, surface helium abundances and ra-
dius ratios of the sdB stars and their companions in
our sample. This was achieved by nding the best- t
model spectra within a model grid using  2 minimiza-
tion. Model atmospheres and synthetic spectra for the
sdB stars were computed using STERNE and SPEC-
TRUM (Je ery et al. 2001). Cool star spectra were
computed using Kurucz model atmospheres and SYN-
THE (Kurucz 1993). The  2 minimization was carried
out using SFIT. One result is shown in Figs. 1 and 2.
Table 1. Results for composite sdB stars
Star T eff:1 log g 1 y 1 T eff:2 log g 2 R 2 =R 1
PG 0110+262 21 000 5.17 <0.01 5 250 4.53 3.2
PG 0749+658 25 400 5.70 <0.01 5 000 4.58 3.5
PG 1104+243 32 850 5.40 0.01 6 400 4.30 5.9
PG 1701+359 32 500 5.75 <0.01 6 000 4.60 2.7
PG 1718+519 29 000 6.00 <0.01 5 200 4.55 4.8
PG 2110+127 26 500 5.20 <0.01 5 400 4.40 4.7
PG 2135+045 28 400 4.80 <0.01 5 000 4.40 3.1
PG 2148+095 30 000 4.90 <0.01 5 700 4.40 3.0
Composite sdB stars
Figure 3. Position of single (filled circles) and com­
posite sdB stars (filled triangles) in the (log g 1 --T eff:1 )
diagram. Open circles represent the position of an
homogeneous (Maxted et al. 2001).
Figure 4. Position of cool companions to composite
sdB stars in the (log g 2 --T eff:2 ) diagram. Labels refer
to sequential number in Table 1.
We nd that sdB stars in composite systems coincide
in T eff and log g with those of sdB stars with non-
composite spectra both in our own sample and in in-
dependent samples (Fig. 3). The cool companions are
seen to be G stars on the main sequence (Fig. 4).
Helium Abundances
We also measured the surface helium abundance y =
n He =n H for both composite and single sdB stars in
our sample. For all composite systems, y  0:01.
For other sdBs, the majority have 0:01  y  1:94.
Following Sa er et al. (2001) we consider three
groups of helium stars, one of which we subdivide;
Figure 2. Normalized blue spectrum of the composite
PG 2110+127 (a) together with the best fit composite
model spectrum (b) formed by adding models with (c)
and (d) with a radius ratio R 2 /R 1 =4.7. Model spectra
have been velocity shifted and degraded to match the
observations.  marks a CCD defect.
i) sdB stars with single spectra showing no radial ve-
locity changes,
ii) sdB stars with single spectra showing large velocity
variations and periods of hours to days,
iia) as above having a low-mass mains-sequence com-
panion,
iib) as above having a white dwarf companion,
iii) sdB stars with composite spectra showing small or
no velocity variations and periods  years.
Together with the results of Maxted et al. (2001), our
measurements show that group (i) all have y  0:01,
group (ii) have 0:01  y  0:03 and group (iii) all have
y  0:01.
We suggest that the y measurements may be ex-
plained for each group as follows:
i) single sdBs are formed from HeWD+HeWD mergers
(Iben 1990, Saio & Je ery 2000) and have a smaller
hydrogen reservoir than other sdBs, so y reaches some
minimum even with di usion.
ii) for sdBs in short-period orbits, tidal perturbations
occur at intervals shorter than the di usion timescale
(10 5 y) and so di usion is disrupted.
iii) sdBs in synchronous long-period orbits experience
low tidal disruption and di usion is most e ective at
reducing surface helium.
sdBs with y !1 (negligible hydrogen) are considered
in a separate poster (Ahmad et al. 2002).
References
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