510 research outputs found
Accretion Disc Evolution in Single and Binary T Tauri Stars
We present theoretical models for the evolution of T Tauri stars surrounded
by circumstellar discs. The models include the effects of pre-main-sequence
stellar and time dependent disc evolution, and incorporate the effects of
stellar magnetic fields acting on the inner disc. For single stars, consistency
with observations in Taurus-Auriga demands that disc dispersal occurs rapidly,
on much less than the viscous timescale of the disc, at roughly the epoch when
heating by stellar radiation first dominates over internal viscous dissipation.
Applying the models to close binaries, we find that because the initial
conditions for discs in binaries are uncertain, studies of extreme mass ratio
systems are required to provide a stringent test of theoretical disc evolution
models. We also note that no correlation of the infra-red colours of T Tauri
stars with their rotation rate is observed, in apparent contradiction to the
predictions of simple magnetospheric accretion models.Comment: 11 pages, MNRAS in pres
Carbon Rich Extremely Metal Poor Stars: Signatures of Population-III AGB stars in Binary Systems
We use the Cambridge stellar evolution code STARS to model the evolution and
nucleosynthesis of zero-metallicity intermediate-mass stars. We investigate the
effect of duplicity on the nucleosynthesis output of these systems and the
potential abundances of the secondaries. The surfaces of zero-metallicity stars
are enriched in CNO elements after second dredge up. During binary interaction,
such as Roche lobe overflow or wind accretion, metals can be released from
these stars and the secondaries enriched in CNO isotopes. We investigate the
formation of the two most metal poor stars known, HE 0107-5240 and HE
1327-2326. The observed carbon and nitrogen abundances of HE 0107-5240 can be
reproduced by accretion of material from the companion-enhanced wind of a seven
solar star after second dredge-up, though oxygen and sodium are underproduced.
We speculate that HE 1327-2326, which is richer in nitrogen and strontium, may
similarly be formed by wind accretion in a later AGB phase after third
dredge-up.Comment: 16 pages, 1 figure, 7 tables, accepted by MNRA
A Complete N-body Model of the Old Open Cluster M67
The old open cluster M67 is an ideal testbed for current cluster evolution
models because of its dynamically evolved structure and rich stellar
populations that show clear signs of interaction between stellar, binary and
cluster evolution. Here we present the first truly direct N-body model for M67,
evolved from zero age to 4 Gyr taking full account of cluster dynamics as well
as stellar and binary evolution. Our preferred model starts with 12000 single
stars and 12000 binaries placed in a Galactic tidal field at 8.0 kpc from the
Galactic Centre. Our choices for the initial conditions and for the primordial
binary population are explained in detail. At 4 Gyr, the age of M67, the total
mass has reduced by 90% as a result of mass loss and stellar escapes. The mass
and half-mass radius of luminous stars in the cluster are a good match to
observations although the model is more centrally concentrated than
observations indicate. The stellar mass and luminosity functions are
significantly flattened by preferential escape of low-mass stars. We find that
M67 is dynamically old enough that information about the initial mass function
is lost, both from the current luminosity function and from the current mass
fraction in white dwarfs. The model contains 20 blue stragglers at 4 Gyr which
is slightly less than the 28 observed in M67. Nine are in binaries. The blue
stragglers were formed by a variety of means and we find formation paths for
the whole variety observed in M67. Both the primordial binary population and
the dynamical cluster environment play an essential role in shaping the
population. A substantial population of short-period primordial binaries (with
periods less than a few days) is needed to explain the observed number of blue
stragglers in M67.Comment: 32 pages, 17 figures, submitted to MNRA
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