3,701 research outputs found
On the lithium abundance dispersion in late-type Pleiades stars
I present the results of a programme to monitor the strengths of the Li I
6708A, K I 7699A and chromospheric Halpha lines in a group of cool Pleiades
stars. Consistent instrumentation and analysis techniques are used to show that
there is no Li I variability on timescales of 1 year that could possibly
account for the apparent spread in Li abundances seen in Pleiades stars between
effective temperatures of 4800-5200K. Comparison with published data reveals
tentative evidence for variability on 10 year timescales, but at a very low
level. The lack of chromospheric activity variability above levels of 20 to 30
percent makes it difficult however, to rule out evenly distributed magnetic
activity regions causing a scatter in the Li I line strengths at a given
abundance. The similar star to star scatter of K I line strengths in these and
published data reinforces the conclusion that it is still unsafe to attribute
the Li I line strength dispersion to a large variation in Li depletion at a
given mass.Comment: Latex 7 pages, 3 postscript figures to appear in MNRA
Using rotation, magnetic activity and lithium to estimate the ages of low mass stars
The rotation rate, level of magnetic activity and surface lithium abundance
are age-dependent quantities in stars of about a solar mass and below. The
physical reasons for the evolution of these phenomena are qualitatively
understood, but accurate quantitative models remain dependent on empirical
calibration using the Sun and stars of known age, chiefly in clusters. In this
work I review the status of these "empirical age indicators", outlining the
astrophysics of their time dependence, describing the measurements, assessing
the precision (and accuracy) of age estimates when applied to individual stars,
and identifying their principle limitations in terms of the mass and age ranges
over which they are useful. Finally, I discuss the "lithium depletion boundary"
technique which, in contrast to the empirical methods, appears to provide
robust, almost model-independent ages that are both precise and accurate, but
which is only applicable to coeval groups of stars.Comment: 38 pages; contribution to the proceedings of the 23rd Evry Schatzman
School on Stellar Astrophysics, "The Ages of Stars", Roscoff 2013, EAS
Publications Series, eds. C. Charbonnel et al. This version has minor
corrections/additions to the bibliograph
Ages and Age Spreads in Young Stellar Clusters
I review progress towards understanding the time-scales of star and cluster
formation and of the absolute ages of young stars. I focus in particular on the
areas in which Francesco Palla made highly significant contributions -
interpretation of the Hertzsprung-Russell diagrams of young clusters and the
role of photospheric lithium as an age diagnostic.Comment: To appear in "Francesco's Legacy: Star Formation in Space and Time",
Memorie della SAIt, in press. Eds. R. Cesaroni, E. Corbelli and D. Galli. 5p
The effect of starspots on the radii of low-mass pre-main sequence stars
A polytropic model is used to investigate the effects of dark photospheric
spots on the evolution and radii of magnetically active, low-mass (M<0.5Msun),
pre-main sequence (PMS) stars. Spots slow the contraction along Hayashi tracks
and inflate the radii of PMS stars by a factor of (1-beta)^{-N} compared to
unspotted stars of the same luminosity, where beta is the equivalent covering
fraction of dark starspots and N \simeq 0.45+/-0.05. This is a much stronger
inflation than predicted by the models of Spruit & Weiss (1986) for main
sequence stars with the same beta, where N \sim 0.2 to 0.3. These models have
been compared to radii determined for very magnetically active K- and M-dwarfs
in the young Pleiades and NGC 2516 clusters, and the radii of tidally-locked,
low-mass eclipsing binary components. The binary components and ZAMS K-dwarfs
have radii inflated by \sim 10 per cent compared to an empirical
radius-luminosity relation that is defined by magnetically inactive field
dwarfs with interferometrically measured radii; low-mass M-type PMS stars, that
are still on their Hayashi tracks, are inflated by up to \sim 40 per cent. If
this were attributable to starspots alone, we estimate that an effective spot
coverage of 0.35 < beta < 0.51 is required. Alternatively, global inhibition of
convective flux transport by dynamo-generated fields may play a role. However,
we find greater consistency with the starspot models when comparing the loci of
active young stars and inactive field stars in colour-magnitude diagrams,
particularly for the highly inflated PMS stars, where the large, uniform
temperature reduction required in globally inhibited convection models would
cause the stars to be much redder than observed.Comment: MNRAS in press, 13 page
A lithium depletion boundary age of 21 Myr for the Beta Pictoris moving group
Optical spectroscopy is used to confirm membership for 8 low-mass candidates
in the young Beta Pic moving group (BPMG) via their radial velocities,
chromospheric activity and kinematic parallaxes. We searched for the presence
of the Li I 6708A resonance feature and combined the results with literature
measurements of other BPMG members to find the age-dependent lithium depletion
boundary (LDB) -- the luminosity at which Li remains unburned in a coeval
group. The LDB age of the BPMG is 21 +/- 4 Myr and insensitive to the choice of
low-mass evolutionary models. This age is more precise, likely to be more
accurate, and much older than that commonly assumed for the BPMG. As a result,
substellar and planetary companions of BPMG members will be more massive than
previously thought.Comment: Accepted for MNRAS Letter
On the frequency of close binary systems among very low-mass stars and brown dwarfs
We have used Monte Carlo simulation techniques and published radial velocity
surveys to constrain the frequency of very low-mass star (VLMS) and brown dwarf
(BD) binary systems and their separation (a) distribution. Gaussian models for
the separation distribution with a peak at a = 4 au and 0.6 =< sigma(log(a/au))
=< 1.0 correctly predict the number of observed binaries, yielding a close
(a<2.6 au) binary frequency of 17-30 per cent and an overall VLMS/BD binary
frequency of 32-45 per cent. We find that the available N-body models of
VLMS/BD formation from dynamically decaying protostellar multiple systems are
excluded at >99 per cent confidence because they predict too few close binary
VLMS/BDs. The large number of close binaries and high overall binary frequency
are also very inconsistent with recent smoothed particle hydrodynamical
modelling and argue against a dynamical origin for VLMS/BDs.Comment: Accepted for publication in MNRAS letters. 5 pages, 2 figure
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