Issue |
A&A
Volume 397, Number 1, January I 2003
|
|
---|---|---|
Page(s) | 237 - 247 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20021468 | |
Published online | 11 December 2002 |
The eclipsing millisecond pulsar PSR J1740-5340 and its red straggler companion*
Astronomical Institute, Utrecht University, PO Box 80 000, 3508 TA Utrecht, The Netherlands
Corresponding authors: J. Orosz, J.A.Orosz@astro.uu.nl orosz@sciences.sdsu.edu mhvk@astro.utoronto.ca
Received:
7
June
2002
Accepted:
7
October
2002
We present a high-resolution echelle spectrum taken with the
Very Large Telescope and analyse archival Hubble Space Telescope
photometry of the recently identified companion of the eclipsing
millisecond radio pulsar PSR J1740-5340 in the globular cluster NGC 6397.
From the spectrum, we show that the companion is metal poor,
as expected for a member of NGC 6397. Using synthetic
photometry, and assuming a true distance modulus of mag
and a colour excess of
, we derive a radius of
and an effective temperature of
, implying a luminosity of
(
errors). These properties make it similar to the so-called
“sub-subgiants” in the old open cluster M 67 and “red
stragglers” in 47 Tuc, which have luminosities comparable to
those of turn-off stars, but cooler temperatures and larger radii.
The light curve of the companion is well described by ellipsoidal
variations, and despite the incomplete (~60%) phase
coverage, we are able to derive good constraints on a number of the
system parameters. In particular, for the inclination we find a
2-σ lower limit of
(
at 3σ).
Assuming a pulsar mass of
, this
implies a companion mass in the range
. Combined with the photometric constraint, we
find a best fit for
and
. We infer a Roche lobe filling factor by
radius of ~97%.
Surprisingly, we find no evidence whatsoever for irradiation of the
companion, despite the high inferred rotational energy loss of the
pulsar (
). We
discuss possible reasons, but find most lacking. We hypothesise that
the system is a triple, and that the acceleration due to a third body
in a wide orbit around the binary led to an overestimate of the
intrinsic spin-down rate and hence the spin-down luminosity. This can
be tested by further timing observations. We also discuss two other
puzzles, viz., the system's location far outside the cluster core and
the companion's large radius and luminosity. We suggest that the
system was formed in a binary-binary encounter in the core, due to
which the system acquired a substantial velocity, and the companion –
which must have been a somewhat evolved turn-off star – lost much of
its envelope. We suggest other “red stragglers” and “sub-subgiants”
might have formed by similarly drastic encounters.
Key words: binaries: close / pulsars: individual: PSR J1740-5340 / globular clusters: individual: NGC 6397
© ESO, 2003
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