1,105 research outputs found
2MASS J05162881+2607387: A New Low-Mass Double-Lined Eclipsing Binary
We show that the star known as 2MASS J05162881+2607387 (hereafter J0516) is a
double-lined eclipsing binary with nearly identical low-mass components. The
spectroscopic elements derived from 18 spectra obtained with the High
Resolution Spectrograph on the Hobby-Eberly Telescope during the Fall of 2005
are K_1=88.45 +/- 0.48 km/s and K_2=90.43 +/- 0.60 km/s, resulting in a mass
ratio of$q=K_1/K_2 = 0.978 +/- 0.018 and minimum masses of M_1 sin^{3}i=0.775
+/- 0.016 solar masses and M_2 sin^{3}i=0.759 +/- 0.012 solar masses,
respectively. We have extensive differential photometry of J0516 obtained over
several nights between 2004 January-March (epoch 1) and 2004 October-2005
January plus 2006 January (epoch 2) using the 1m telescope at the Mount Laguna
Observatory. The source was roughly 0.1 mag brighter in all three bandpasses
during epoch 1 when compared to epoch 2. Also, phased light curves from epoch 1
show considerable out-of-eclipse variability, presumably due to bright spots on
one or both stars. In contrast, the phased light curves from epoch 2 show
little out-of-eclipse variability. The light curves from epoch 2 and the radial
velocity curves were analyzed using our ELC code with updated model atmospheres
for low-mass stars. We find the following: M_1=0.787 +/- 0.012 solar masses,
R_1=0.788 +/- 0.015 solar radii, M_2=0.770 +/- 0.009 solar masses, and
R_2=0.817 +/- 0.010 solar radii. The stars in J0516 have radii that are
significantly larger than model predictions for their masses, similar to what
is seen in a handful of other well-studied low-mass double-lined eclipsing
binaries. We compiled all recent mass and radius determinations from low-mass
binaries and determine an empirical mass-radius relation of the form R = 0.0324
+ 0.9343M + 0.0374M^2, where the quantities are in solar units.Comment: 16 pages, 10 figures (Figure 1 has degraded quality), to appear in
Ap
Evaluating irreversible social harms
In this paper we investigate how irreversible social harms should be evaluated from an ethical perspective. First, we define a general notion of irreversibility, drawing on discussions in ecology and economics. This notion is relational in the sense that 'irreversibility' is always 'irreversibility for a certain party'. We also note that a change may be more or less difficult to reverse, with full reversibility and irreversibility as two extremes. Second, we examine what can make an irreversible change a harm, and why these kinds of harms have particular ethical significance. Here we draw on discussions from ethics, particularly regarding the Capability Approach. We also show how our notion of irreversibility connects to, and can add to, discussions in the fields of development studies and disaster management, particularly on the concept of resilience. Third, we suggest how potentially irreversible harms can be recognised and dealt with in policy-making. Finally, we show how our framework can be applied by evaluating the land acquisition process of two biofuel producers in Tanzania
Direct N-body Modelling of Stellar Populations: Blue Stragglers in M67
We present a state-of-the-art N-body code which includes a detailed treatment
of stellar and binary evolution as well as the cluster dynamics. This code is
ideal for investigating all aspects relating to the evolution of star clusters
and their stellar populations. It is applicable to open and globular clusters
of any age. We use the N-body code to model the blue straggler population of
the old open cluster M67. Preliminary calculations with our binary population
synthesis code show that binary evolution alone cannot explain the observed
numbers or properties of the blue stragglers. On the other hand, our N-body
model of M67 generates the required number of blue stragglers and provides
formation paths for all the various types found in M67. This demonstrates the
effectiveness of the cluster environment in modifying the nature of the stars
it contains and highlights the importance of combining dynamics with stellar
evolution. We also perform a series of N = 10000 simulations in order to
quantify the rate of escape of stars from a cluster subject to the Galactic
tidal field.Comment: 26 pages, 18 figures, accepted for publication in 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
A more detailed look at the Opacities for Enriched Carbon and Oxygen Mixtures
We have included opacity tables in our stellar evolution code that enable us
to accurately model the structure of stars composed of mixtures with carbon and
oxygen independently enhanced relative to solar. We present tests to
demonstrate the effects of the new tables. Two of these are practical examples,
the effect on the evolution of a thermally pulsing asymptotic giant branch star
and a Wolf-Rayet Star. The changes are small but perceptible.Comment: 6 pages, 3 figures, 5 tables. Refereed version with corrections
resubmitted to MNRA
Modelling the observed properties of carbon-enhanced metal-poor stars using binary population synthesis
The stellar population in the Galactic halo is characterised by a large
fraction of CEMP stars. Most CEMP stars are enriched in -elements (CEMP-
stars), and some of these are also enriched in -elements (CEMP- stars).
One formation scenario proposed for CEMP stars invokes wind mass transfer in
the past from a TP-AGB primary star to a less massive companion star which is
presently observed. We generate low-metallicity populations of binary stars to
reproduce the observed CEMP-star fraction. In addition, we aim to constrain our
wind mass-transfer model and investigate under which conditions our synthetic
populations reproduce observed abundance distributions. We compare the CEMP
fractions and the abundance distributions determined from our synthetic
populations with observations. Several physical parameters of the binary
stellar population of the halo are uncertain, e.g. the initial mass function,
the mass-ratio and orbital-period distributions, and the binary fraction. We
vary the assumptions in our model about these parameters, as well as the wind
mass-transfer process, and study the consequent variations of our synthetic
CEMP population. The CEMP fractions calculated in our synthetic populations
vary between 7% and 17%, a range consistent with the CEMP fractions among very
metal-poor stars recently derived from the SDSS/SEGUE data sample. The results
of our comparison between the modelled and observed abundance distributions are
different for CEMP- stars and for CEMP- stars. For the latter, our
simulations qualitatively reproduce the observed distributions of C, Na, Sr,
Ba, Eu, and Pb. Contrarily, for CEMP- stars our model cannot reproduce the
large abundances of neutron-rich elements such as Ba, Eu, and Pb. This result
is consistent with previous studies, and suggests that CEMP- stars
experienced a different nucleosynthesis history to CEMP- stars.Comment: 17 pages, 11 figures, accepted for publication on Astronomy and
Astrophysic
A Photometric Technique to Search for Be Stars in Open Clusters
We describe a technique to identify Be stars in open clusters using Stromgren
b, y, and narrow-band Halpha photometry. We first identify the B-type stars of
the cluster using a theoretical isochrone fit to the (b-y, y) color-magnitude
diagram. The strongest Be stars are easily identified in a (b-y, y-Halpha)
color-color diagram, but those with weaker Halpha emission (classified as
possible Be star detections) may be confused with evolved or foreground stars.
Here we present such photometry plus Halpha spectroscopy of members of the
cluster NGC 3766 to demonstrate the accuracy of our technique. Statistical
results on the relative numbers of Be and B-type stars in additional clusters
will be presented in a future paper.Comment: 15 pages, 6 figures, 1 table. Accepted by Ap
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