50 research outputs found
Schwarzschild Models for the Galaxy
Schwarzschild's orbit-superposition technique is the most developed and
well-tested method available for constraining the detailed mass distributions
of equilibrium stellar systems. Here I provide a very short overview of the
method and its existing implementations, and briefly discuss their viability as
a tool for modeling the Galaxy using Gaia data.Comment: 2 pages; invited review at "Modelling the Galaxy in the era of Gaia",
proceedings of Joint Discussion 5 at the XXVIIth IAU General Assembly (Rio de
Janeiro, August 2009); James Binney, Ian F. Corbett, eds
On Collision Course: The Nature of the Binary Star Cluster NGC 2006 / SL 538
The LMC hosts a rich variety of star clusters seen in close projected
proximity. Ages have been derived for few of them showing differences up to few
million years, hinting at being binary star clusters. However, final
confirmation needs to be done through spectroscopic analysis. Here we focus on
the LMC cluster pair NGC2006-SL538 and aim to determine whether the star
cluster pair is a bound entity (binary star cluster) or a chance alignment.
Using the MIKE echelle spectrograph at LCO we have acquired integrated-light
spectra for each cluster. We have measured radial velocities by two methods: a)
direct line profile measurement yields v km/s for NGC2006 and
km/s for SL538. b) By comparing observed spectra with
synthetic bootstrapped spectra yielding km/s for NGC2006 and
km/s for SL538. Finally when spectra are directly compared,
we find a km/s. Full-spectrum SED fits reveal that the
stellar population ages lie in the range 13-21 Myr with a metallicity of
Z=0.008. We find indications for differences in the chemical abundance patterns
as revealed by the helium absorption lines between the two clusters. The
dynamical analysis shows that the two clusters are likely to merge within the
next 150 Myr. The NGC2006-SL538 cluster pair shows radial velocities,
stellar population and dynamical parameters consistent with a gravitational
bound entity. We conclude that this is a genuine binary cluster pair, and we
propose that their differences in ages and stellar population chemistry is most
likely due to variances in their chemical enrichment history within their
environment. We suggest that their formation may have taken place in a loosely
bound star-formation complex which saw initial fragmentation but then had its
clusters become a gravitationally bound pair by tidal capture.Comment: Accepted for publication in Astronomy & Astrophysics. 15 pages, 10
figures in low resolutio
Lithium abundance patterns of late-F stars: an in-depth analysis of the lithium desert
We address the existence and origin of the lithium (Li) desert, a region in
the Li - T_eff plane sparsely populated by stars. Here we analyze some of the
explanations that have been suggested for this region, including mixing in the
late main sequence, a Li dip origin for stars with low Li abundances in the
region, and a possible relation with the presence of planets. To study the Li
desert, we measured the atmospheric parameters and Li abundance of 227 late-F
dwarfs and subgiants, chosen to be in the T_eff range of the desert and without
previous Li abundance measurements. Subsequently, we complemented those with
literature data to obtain a homogeneous catalog of 2318 stars, for which we
compute masses and ages. We characterize stars surrounding the region of the Li
desert. We conclude that stars with low Li abundances below the desert are more
massive and more evolved than stars above the desert. Given the unexpected
presence of low Li abundance stars in this effective temperature range, we
concentrate on finding their origin. We conclude that these stars with low Li
abundance do not evolve from stars above the desert: at a given mass, stars
with low Li (i.e., below the desert) are more metal-poor. Instead, we suggest
that stars below the Li desert are consistent with having evolved from the Li
dip, discarding the need to invoke additional mixing to explain this feature.
Thus, stars below the Li desert are not peculiar and are only distinguished
from other subgiants evolved from the Li dip in that their combination of
atmospheric parameters locates them in a range of effective temperatures where
otherwise only high Li abundance stars would be found (i.e., stars above the
desert).Comment: Accepted for publication in A&