A Dor - An Eclipsing Subdwarf - Brown Dwarf Binary

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by  T. Rauch

To appear in “Compact Binaries in the Galaxy and Beyond (2004)” RevMexAA(SC)
AA DOR — AN ECLIPSING SUBDWARF – BROWN DWARF BINARY
T. Rauch1,2
AADor is an eclipsing, close, post common-
envelope binary (PCEB) consisting of a sdOB pri-
marystarandanunseensecondarywithanextraor-
dinary small mass (M2 ≈ 0.066M⊙) – formally a
browndwarf(seeRauch2004fordetails). Inaspec-
tral analysis of AADor, Rauch (2000) determined
4
0 Teff = 42kK and logg = 5.2 (cgs). The determi-
0 nation of the components’ masses by comparison of
2 these results with evolutionary models of Driebe et

n al. (1998) shows a discrepancy to masses derived
a from radial-velocity and the eclipse curves (Hilditch
J
etal. 2003)–logg =5.5wouldbenecessaryinorder

2 to achieve an intersection at M1 ≈0.330M⊙.

Possible reasons for this discrepancy may be too
1
v optimistic error ranges in Rauch (2000) or in the
6 analysis of light curve and radial-velocity curve, or
0 that the evolutionarymodels of Driebe et al. (1998)
0 arenotappropriateinthecaseofAADorsincethese
1
are post-RGB models for non-CE stars.
0
4 Since the decrement of the hydrogen Balmer
0 series is a sensitive indicator for logg, 107 high-
/ resolution´echelle spectra with short exposure times
h
p (180 sec) have been taken in Jan 2001 with UVES
- (UV-visual ´echelle spectrograph) attached to the
o
ESO VLT. Additional medium-resolution longslit
r
t spectra have been taken at the 2.3m telescope at
s
a SSO in Sept 2003with the DBS (double beam spec-
: trograph). However,the analysisofboth, the UVES
v
i spectra (Rauch & Werner 2003) and the DBS spec-
X
tra, shows that a logg higher than 5.2 results in a
r worse fit to the observation.
a
Sincethesecondaryisheatedbyirradiationofthe
primary up to ≈ 20kK, one can expect a weak Hβ Fig. 1. Section of the UVES spectra around Hβ com-
emission in the UVES spectra. However, we do not pared with synthetic spectra. A weak Hβ emission is
find any signature of the secondary. The emission used torepresent thesecondary’s radiation.
in the line core of Hβ (Fig.1) comes clearly from
pears possible that this is one of the main reasons
the primary, its phase dependence is likely due to
for the disagreement in the mass-radius relation de-
an irradiation effect of the heated secondary on the
scribed above.
primary which increases its T by ≈ 7kK, i.e. the
eff
primary–takenasanisolatedstar–wouldhaveonly REFERENCES
Teff ≈ 35kK, resulting in a ≈ 10% smaller mass. A Driebe, T., Sch¨onberner, D., Bl¨ocker, T., & Herwig, F.
phase-dependent spectral analysis is presently per- 1998, A&A,339, 129
formed in order to investigate on this effect. It ap- Hilditch, R.W., Kilkenny, D., & Lynas-Gray, A.E., Hill,
G. 2003, MNRAS,344, 644
1Dr.-Remeis-Sternwarte,Sternwartstraße7,D-96049Bam- Rauch,T. 2000, A&A,356, 665
berg,Germany([email protected]). Rauch,T. 2004, Ap&SS,in press
2Institut fu¨r Astronomie und Astrophysik, Sand 1, D- Rauch,T., & Werner, K. 2003, A&A,400, 271
72076Tu¨bingen,Germany.
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