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As already indicated, V652Her is an important EHe star because its
relatively short pulsation period and large amplitude pulsations allow
its overall dimensions to be determined with high precision. The
pulsation properties have been determined from visual and ultraviolet
spectrophotometry and from spectroscopy by Landolt (1975), Hill et
al. (1981), Lynas-Gray et al (1984) and Jeffery & Hill (1986). The
simple saw-tooth shape of the radial-velocity curve implies that the
pulsation can be divided quite simply into a short impulse phase
lasting
cycles, followed by a near free-fall phase for the
remainder of the cycle. Combining these data, Lynas-Gray et al. (1984)
obtained a direct measurement of the radius
and
mass
. A refinement of the
measurement of
led Jeffery et al. (1999) to obtain
, with
K,
(cgs) and
. In contrast to most
EHes, V652Her is nitrogen rich and carbon and oxygen poor, implying
that its surface is predominantly CNO-processed. The period change
discovered and refined by Kilkenny & Lynas-Gray (1982,1984) and
Kilkenny et al. (1996) translates into a contraction rate
yr
, together with nonlinear terms
and
.
With a lower luminosity and a purely CNO-processed surface, the
evolutionary status of V652Her has long been regarded as possibly
quite different to most EHes. Jeffery (1984) constructed a set of
highly artificial ``helium horizontal branch models'' in which a
0.5
helium-burning core was surrounded by an envelope with a
very low hydrogen abundance. Because of the low hydrogen-abundance,
the luminosity of the H-burning shell at the core-envelope interface
was very high and the star evolved rapidly towards the helium
main-sequence. Whilst able to match
and surface
composition, these models could only suggest a possible structure for
V652Her, rather than explain its origin.
Other highly artificial models in the horizontal-branch family have been constructed, notably by Sweigart (1997), but fail to provide either a hydrogen-poor surface or a self-consistent explanation of their origin. Similarly, no ``final-flash'' models have been computed which match the observed properties of V652Her.
Saio & Nomoto (1998) made the first successful models for the merger
of two carbon-oxygen white dwarfs, and prompted Saio & Jeffery (2000)
to attempt models for the merger of two helium white dwarfs. Following
orbital decay, the less massive white dwarf in a double-degenerate
suffers total tidal disruption on a dynamical timescale; the debris
forms a thick disk around the surviving white dwarf. The latter then
accretes matter from the disk until the envelope is sufficiently
massive that nuclear reactions, in this case
burning, are
initiated at the core-envelope interface. At this point, the star
expands to become a cool helium giant. Heating of the core surface by
the nuclear-burning shell, or flame, lifts the local
electron-degeneracy so that the flame migrates inwards. Because the
flame migration proceeds stepwise, the surface evolution follows a
series of loops of increasing
and decreasing
, until the flame
reaches the core centre, whereupon the star assumes the structure of a
helium main-sequence star or hot subdwarf. A schematic of the
evolution is shown in Fig. 2.
The evolution sequence for a 0.476
helium white dwarf
accreting 0.233
helium-rich debris passes exactly through the
observed locus for V752Her. To within the numerical uncertainty of
the calculations, this model also has the correct pulsation
properties,
and
. As yet, the higher order terms
and
(Kilkenny et al. 1996) cannot be
reproduced.
![]() |
Star | ![]() |
![]() |
![]() |
HD168476=PVTel | 0.95 | 0.85 | 0.82 |
BD+1![]() |
1.09 | 0.93 | 0.03 |
LSIV-1![]() |
0.66 | 0.94 | 0.76 |