830 research outputs found
The Dynamical Implications of Multiple Stellar Formation Events in Galactic Globular Clusters
Various galactic globular clusters display abundance anomalies that affect
the morphology of their colour-magnitude diagrams. In this paper we consider
the possibility of helium enhancement in the anomalous horizontal branch of NGC
2808. We examine the dynamics of a self-enrichment scenario in which an initial
generation of stars with a top-heavy initial mass function enriches the
interstellar medium with helium via the low-velocity ejecta of its asymptotic
giant branch stars. This enriched medium then produces a second generation of
stars which are themselves helium-enriched. We use a direct N-body approach to
perform five simulations and conclude that such two-generation clusters are
both possible and would not differ significantly from their single-generation
counterparts on the basis of dynamics. We find, however, that the stellar
populations of such clusters would differ from single-generation clusters with
a standard initial mass function and in particular would be enhanced in white
dwarf stars. We conclude, at least from the standpoint of dynamics, that
two-generation globular clusters are feasible.Comment: 24 pages, 7 figures, 3 tables. Accepted for publication in Ap
The Core Composition of a White Dwarf in a Close Double Degenerate System
We report the identification of the double degenerate system NLTT 16249 that
comprises a normal, hydrogen-rich (DA) white dwarf and a peculiar,
carbon-polluted white dwarf (DQ) showing photospheric traces of nitrogen. We
disentangled the observed spectra and constrained the properties of both
stellar components. In the evolutionary scenario commonly applied to the
sequence of DQ white dwarfs, both carbon and nitrogen would be dredged up from
the core. The C/N abundance ratio (~ 50) in the atmosphere of this unique DQ
white dwarf suggests the presence of unprocessed material (14N) in the core or
in the envelope. Helium burning in the DQ progenitor may have terminated early
on the red-giant branch after a mass-ejection event leaving unprocessed
material in the core although current mass estimates do not favor the presence
of a low-mass helium core. Alternatively, some nitrogen in the envelope may
have survived an abridged helium-core burning phase prior to climbing the
asymptotic giant-branch. Based on available data, we estimate a relatively
short orbital period (P <~ 13 hrs) and on-going spectroscopic observations will
help determine precise orbital parameters.Comment: Accepted for publication in ApJ Letter
An ALMA view of CS and SiS around oxygen-rich AGB stars
We aim to determine the distributions of molecular SiS and CS in the
circumstellar envelopes of oxygen-rich asymptotic giant branch stars and how
these distributions differ between stars that lose mass at different rates. In
this study we analyse ALMA observations of SiS and CS emission lines for three
oxygen-rich galactic AGB stars: IK Tau, with a moderately high mass-loss rate
of M yr, and W Hya and R Dor with low mass loss
rates of M yr. These molecules are usually
more abundant in carbon stars but the high sensitivity of ALMA allows us to
detect their faint emission in the low mass-loss rate AGB stars. The high
spatial resolution of ALMA also allows us to precisely determine the spatial
distribution of these molecules in the circumstellar envelopes. We run
radiative transfer models to calculate the molecular abundances and abundance
distributions for each star. We find a spread of peak SiS abundances with
for R Dor, for W Hya, and for
IK Tau relative to H. We find lower peak CS abundances of
for R Dor, for W Hya and
for IK Tau, with some stratifications in the abundance
distributions. For IK Tau we also calculate abundances for the detected
isotopologues: CS, SiS, SiS, SiS, SiS,
SiS, and SiS. Overall the isotopic ratios we derive
for IK Tau suggest a lower metallicity than solar.Comment: 16 page
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
Nucleosynthetic history of elements in the Galactic disk [X/Fe]-age relations from high-precision spectroscopy
Context. The chemical composition of stars is intimately linked to the formation and evolution of the Galaxy.
Aims. We aim to trace the chemical evolution of the Galactic disk through the inspection of the [X/Fe]–age relations of 24 species from C to Eu.
Methods. Using high-resolution and high signal-to-noise UVES spectra of nine solar twins, we obtained precise estimates of stellar ages and chemical abundances. These determinations have been integrated with additional accurate age and abundance determinations from recent spectroscopic studies of solar twins existing in the literature, comprising superb abundances with 0.01 dex precision. Based on this data set, we outlined the [X/Fe]–age relations over a time interval of 10 Gyr.
Results. We present the [X/Fe] – age relations for 24 elements (C, O, Na, Mg, Al, Si, S, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Ba, La, Ce, Nd, and Eu). Each different class of elements showed a distinct evolution with time that relies on the different characteristics, rates, and timescales of the nucleosynthesis sites from which they are produced. The α-elements are characterized by a [X/Fe] decrease with time. Strikingly, the opposite behavior is observed for Ca. The iron-peak elements show an early [X/Fe] increase followed by a decrease towards the youngest stars. The [X/Fe] for the n-capture elements decrease with age. We also found that both [Mg/Y] and [Al/Y] are precise stellar clocks, with [Al/Y] showing the steepest dependence on age.
Conclusions. Knowledge of the [X/Fe]-age relations is a gold mine from which we can achieve a great understanding of the processes that governed the formation and evolution of the Milky Way. Through the reverse engineering of these relations we will be able to put strong constraints on the nature of the stellar formation history, the SNe rates, the stellar yields, and the variety of the SNe progenitors
HI in four star-forming low-luminosity E/S0 and S0 galaxies
We present HI data cubes of four low-luminosity early-type galaxies which are
currently forming stars. These galaxies have absolute magnitudes in the range
M_B=-17.9 to -19.9 (H_o=50 km/s/Mpc). Their HI masses range between a few times
10^8 and a few times 10^9 M_sun and the corresponding values for M_HI/L_B are
between 0.07 and 0.42, so these systems are HI rich for their morphological
type. In all four galaxies, the HI is strongly centrally concentrated with high
central HI surface densities, in contrast to what is typically observed in more
luminous early-type galaxies. In two galaxies (NGC 802 and ESO 118-G34), the
kinematics of the HI suggests that the gas is in a strongly warped disk, which
we take as evidence for recent accretion of HI. In the other two galaxies (NGC
2328 and ESO 027-G21) the HI must have been part of the systems for a
considerable time. The HI properties of low-luminosity early-type galaxies
appear to be systematically different from those of many more luminous
early-type galaxies, and we suggest that these differences are due to a
different evolution of the two classes. The star formation history of these
galaxies remains unclear. Their UBV colours and Halpha emission-line strengths
are consistent with having formed stars at a slowly-declining rate for most of
the past 10^10 years. However, the current data do not rule out a small burst
of recent star formation overlaid on an older stellar population.Comment: To appear in AJ, LateX, figures in gif format, paper also available
at http://www.nfra.nl/~morganti/LowLu
18 Sco: a solar twin rich in refractory and neutron-capture elements. Implications for chemical tagging
We study with unprecedented detail the chemical composition and stellar
parameters of the solar twin 18 Sco in a strictly differential sense relative
to the Sun. Our study is mainly based on high resolution (R ~ 110 000) high S/N
(800-1000) VLT UVES spectra, which allow us to achieve a precision of about
0.005 dex in differential abundances. The effective temperature and surface
gravity of 18 Sco are Teff = 5823+/-6 K and log g = 4.45+/-0.02 dex, i.e., 18
Sco is 46+/-6 K hotter than the Sun and log g is 0.01+/-0.02 dex higher. Its
metallicity is [Fe/H] = 0.054+/-0.005 dex and its microturbulence velocity is
+0.02+/-0.01 km/s higher than solar. Our precise stellar parameters and
differential isochrone analysis show that 18 Sco has a mass of 1.04+/-0.02M_Sun
and that it is ~1.6 Gyr younger than the Sun. We use precise HARPS radial
velocities to search for planets, but none were detected. The chemical
abundance pattern of 18 Sco displays a clear trend with condensation
temperature, showing thus higher abundances of refractories in 18 Sco than in
the Sun. Intriguingly, there are enhancements in the neutron-capture elements
relative to the Sun. Despite the small element-to-element abundance differences
among nearby n-capture elements (~0.02 dex), we successfully reproduce the
r-process pattern in the solar system. This is independent evidence for the
universality of the r-process. Our results have important implications for
chemical tagging in our Galaxy and nucleosynthesis in general.Comment: ApJ, in pres
C3PO: Towards a complete census of co-moving pairs of stars. I. High precision stellar parameters for 250 stars
We conduct a line-by-line differential analysis of a sample of 125 co-moving
pairs of stars (dwarfs and subgiants near solar metallicity). We obtain high
precision stellar parameters with average uncertainties in effective
temperature, surface gravity and metallicity of 16.5 K, 0.033 dex and 0.014
dex, respectively. We classify the co-moving pairs of stars into two groups,
chemically homogeneous (conatal; |Delta[Fe/H]| 0.04 dex) and
inhomogeneous (non-conatal), and examine the fraction of chemically homogeneous
pairs as a function of separation and effective temperature. The four main
conclusions from this study are: (1) A spatial separation of \ds = 10 AU is
an approximate boundary between homogeneous and inhomogeneous pairs of stars,
and we restrict our conclusions to only consider the 91 pairs with \ds
10 AU; (2) There is no trend between velocity separation and the fraction
of chemically homogeneous pairs in the range \dv 4 \kms; (3) We confirm
that the fraction of chemically inhomogeneous pairs increases with increasing
\teff\ and the trend matches a toy model of that expected from planet
ingestion; (4) Atomic diffusion is not the main cause of the chemical
inhomogeneity. A major outcome from this study is a sample of 56 bright
co-moving pairs of stars with chemical abundance differences 0.02 dex
(5\%) which is a level of chemical homogeneity comparable to that of the Hyades
open cluster. These important objects can be used, in conjunction with star
clusters and the \gaia\ ``benchmark'' stars, to calibrate stellar abundances
from large-scale spectroscopic surveys.Comment: MNRAS in press (see source file for full versions of long tables
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