99 research outputs found
Chemical Evolution of the Magellanic Clouds: analytical models
We have extended our analytical chemical evolution modelling ideas for the
Galaxy to the Magellanic Clouds. Unlike previous authors, we assume neither a
steepened IMF nor selective galactic winds, since among the alpha-particle
elements only oxygen shows a large deficit relative to iron and a similar
deficit is also found in Galactic supergiants. Thus we assume yields and time
delays identical to those we previously assumed for the solar neighbourhood. We
include inflow and non-selective galactic winds and consider both smooth and
bursting star formation rates, the latter giving a better fit to the
age-metallicity relations. We predict essentially solar abundance ratios for
primary elements and these seem to fit most of the data within their
substantial scatter. Our LMC model also gives a remarkably good fit to the
anomalous Galactic halo stars discovered by Nissen & Schuster (1997).
Our models predict current ratios of SNIa to core-collapse supernova rates
enhanced by 50 per cent and 25 per cent respectively relative to the solar
neighbourhood, in fair agreement with ratios found by Cappellaro et al. (1993)
for Sdm-Im relative to Sbc galaxies, but these ratios are sensitive to detailed
assumptions about the bursts. We consider that the high SNIa/SNII ratios found
in some late-type galaxies are related to their star formation history rather
than to any differences in the IMF.Comment: 9 pages, 11 encapsulated figures, submitted to MNRA
High resolution spectroscopic study of red clump stars in the Galaxy: iron group elements
The main atmospheric parameters and abundances of the iron group elements
(vanadium, chromium, iron, cobalt and nickel) are determined for 62 red giant
"clump" stars revealed in the Galactic field by the Hipparcos orbiting
observatory. The stars form a homogeneous sample with the mean value of
temperature T=4750 +- 160K, of surface gravity log g = 2.41 +- 0.26 and the
mean value of metallicity [Fe/H] = -0.04 +- 0.15 dex. A Gaussian fit to the
[Fe/H] distribution produces the mean [Fe/H] = -0.01 dex and dispersion of
[Fe/H] = 0.08 dex. The near-solar metallicity and small dispersion of [Fe/H] of
clump stars of the Galaxy obtained in this work confirm the theoretical model
of the Hipparcos clump by Girardi & Salaris (2001). This suggests that nearby
clump stars are (in the mean) relatively young objects, reflecting mainly the
near-solar metallicities developed in the local disk during the last few Gyrs
of its history. We find iron group element to iron abundance ratios in clump
giants to be close to solar.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Searching for Milky Way twins: Radial abundance distribution as a strict criterion
We search for Milky Way-like galaxies among a sample of approximately 500
galaxies. The characteristics we considered of the candidate galaxies are the
following: stellar mass M_star, optical radius R_25, rotation velocity V_rot,
central oxygen abundance (O/H)_0, and abundance at the optical radius
(O/H)_R25. If the values of R_25 and M_star of the galaxy were close to that of
the Milky Way, then the galaxy was referred to as a structural Milky Way
analogue (sMWA). The oxygen abundance at a given radius of a galaxy is defined
by the evolution of that region, and we then assumed that the similarity of
(O/H)_0 and (O/H)_R25 in two galaxies suggests a similarity in their evolution.
If the values of (O/H)_0 and (O/H)_R25 in the galaxy were close to that of the
Milky Way, then the galaxy was referred to as an evolutionary Milky Way
analogue (eMWA). If the galaxy was simultaneously an eMWA and sMWA, then the
galaxy was considered a Milky Way twin. We find that the position of the Milky
Way on the (O/H)_0 - (O/H)_R25 diagram shows a large deviation from the general
trend in the sense that the (O/H)_R25 in the Milky Way is appreciably lower
than in other galaxies of similar (O/H)_0. This feature of the Milky Way
evidences that its (chemical) evolution is not typical. We identify four
galaxies (NGC~3521, NGC~4651, NGC~2903, and MaNGA galaxy M-8341-09101) that are
simultaneously sMWA and eMWA and can therefore be considered as Milky Way
twins. In previous studies, Milky Way-like galaxies were selected using
structural and morphological characteristics, that is, sMWAs were selected. We
find that the abundances at the centre and at the optical radius (evolutionary
characteristics) provide a stricter criterion for selecting real Milky Way
twinsComment: Accepted to Astronomy and Astrophysics, 28 pages, 13 figure
Measuring Ages and Elemental Abundances from Unresolved Stellar Populations: Fe, Mg, C, N, and Ca
We present a method for determining mean light-weighted ages and abundances
of Fe, Mg, C, N, and Ca, from medium resolution spectroscopy of unresolved
stellar populations. The method, pioneered by Schiavon (2007), is implemented
in a publicly available code called EZ_Ages. The method and error estimation
are described, and the results tested for accuracy and consistency, by
application to integrated spectra of well-known Galactic globular and open
clusters. Ages and abundances from integrated light analysis agree with studies
of resolved stars to within +/-0.1 dex for most clusters, and to within +/-0.2
dex for nearly all cases. The results are robust to the choice of Lick indices
used in the fitting to within +/-0.1 dex, except for a few systematic
deviations which are clearly categorized. The realism of our error estimates is
checked through comparison with detailed Monte Carlo simulations. Finally, we
apply EZ_Ages to the sample of galaxies presented in Thomas et al. (2005) and
compare our derived values of age, [Fe/H], and [alpha/Fe] to their analysis. We
find that [alpha/Fe] is very consistent between the two analyses, that ages are
consistent for old (Age > 10 Gyr) populations, but show modest systematic
differences at younger ages, and that [Fe/H] is fairly consistent, with small
systematic differences related to the age systematics. Overall, EZ_Ages
provides accurate estimates of fundamental parameters from medium resolution
spectra of unresolved stellar populations in the old and intermediate-age
regime, for the first time allowing quantitative estimates of the abundances of
C, N, and Ca in these unresolved systems. The EZ_Ages code can be downloaded at
http://www.ucolick.org/~graves/EZ_Ages.htmlComment: Accepted to ApJ
Revisiting the Globular Cluster System of the Merger Remnant Elliptical NGC 3610
We have obtained Keck spectra of six candidate globular clusters (GCs) in the
dynamically young elliptical galaxy NGC 3610, supplementing our previous Keck
spectra of eight confirmed GCs (Strader et al. 2003). Five of our new
candidates are confirmed to be GCs. Of the thirteen GCs, eleven are located
within the K band effective radius of 7 kpc. Two of these thirteen clusters are
found to be young (~ 2 Gyr) and very metal-rich ([Z/H] ~ +0.5), three are old
and metal-poor, and the remaining eight clusters are old and metal-rich. The
ages of the young clusters are consistent with a recent spectroscopic age
estimate of 1.6+/-0.5 Gyr for the galaxy itself (Denicolo et al. 2003) and
suggest that these clusters formed in the disk-disk merger which likely created
NGC 3610. Intriguingly, both young GCs have [alpha/Fe] ~ +0.3, while the
majority of the old clusters are not alpha-enhanced, in contrast to Galactic
and M31 GCs, and contrary to predictions of nucleosynthetic calculations. The
two old subpopulations of GCs can be attributed to the merger progenitors. The
relative numbers of old and new metal-rich GCs are poorly constrained because
of the expected differences in radial distributions of the two subpopulations.
However, based on our spectroscopic results and a comparison of the Hubble
Space Telescope color magnitude diagram (Whitmore et al. 2002) with stellar
population models, we argue that more than half of the metal-rich GCs are
likely to be old.Comment: 21 pages, 6 figures, accepted for Jan 2004 A
Identifying Very Metal-Rich Stars with Low-Resolution Spectra: Finding Planet-Search Targets
We present empirical calibrations that estimate stellar metallicity,
effective temperature and surface gravity as a function of Lick/IDS indices.
These calibrations have been derived from a training set of 261 stars for which
(1) high-precision measurements of [Fe/H], T_eff and log g have been made using
spectral-synthesis analysis of HIRES spectra, and (2) Lick indices have also
been measured. Our [Fe/H] calibration, which has precision 0.07 dex, has
identified a number of bright (V < 9) metal-rich stars which are now being
screened for hot Jupiter-type planets. Using the Yonsei-Yale stellar models, we
show that the calibrations provide distance estimates accurate to 20% for
nearby stars.
This paper outlines the second tier of the screening of planet-search targets
by the N2K Consortium, a project designed to identify the stars most likely to
harbor extrasolar planets. Discoveries by the N2K Consortium include the
transiting hot Saturn HD 149026 b (Sato et al. 2005, astro-ph/0507009) and HD
88133 b (Fischer et al. 2005). See Ammons et al. (2005, In Press) for a
description of the first tier of N2K metallicity screening, calibrations using
broadband photometry.Comment: Accepted for publication in the Astrophysical Journa
UBVI CCD Photometry of the Old Open Cluster Berkeley 17
Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley 17.
Color-magnitude diagrams (CMDs) are constructed and reach well past the
main-sequence turn-off for both clusters. Cluster ages are determined by means
of isochrone fitting to the cluster CMDs. These fits are constrained to agree
with spectroscopic metallicity and reddening estimates. Cluster ages are
determined to be 7.0+/-0.5 Gyr for NGC 188, and 10.0+/- 1.0 Gyr for Berkeley
17, where the errors refer to uncertainties in the relative age determinations.
These ages are compared to the ages of relatively metal-rich inner halo/thick
disk globular clusters and other old open clusters. Berkeley 17 and NGC 6791
are the oldest open clusters with an age of 10 Gyr. They are 2 Gyr younger than
the thick disk globular clusters. These results confirm the status of Berkeley
17 as one of the oldest known open cluster in the Milky Way, and its age
provides a lower limit to the age of the Galactic disk.Comment: to appear in AJ; 28 pages, 9 figure
Chemical composition of red horizontal branch stars in the thick disk of the Galaxy
High-resolution spectra of 13 core helium-burning stars in the thick disk of
the Galaxy have been obtained with the SOFIN spectrograph on the Nordic Optical
Telescope to investigate abundances of up to 22 chemical elements. Abundances
of carbon were studied using the C_2 Swan (0,1) band head at 5635.5 A. The
wavelength interval 7980-8130 A with strong CN features was analysed in order
to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen
abundances were determined from the O I line at 6300 A. Abundances in the
investigated stars suggest that carbon is depleted by about 0.3 dex, nitrogen
is enhanced by more than 0.4 dex and oxygen is unaltered. The 12C/13C ratios
are lowered and lie between values 3 and 7 which is in agreement with "cool
bottom processing" predictions (Boothroyd & Sackmann 1999). The C/N ratios in
the investigated stars are lowered to values between 0.7 and 1.2 which is less
than present day theoretical predictions and call for further studies of
stellar mixing processes. Abundance ratios of O, Mg, Eu and other heavy
chemical elements to iron in the investigated stars show a pattern
characteristic of thick disk stars. The results provide evidence that the thick
disk population has a distinct chemical history from the thin disk. The onset
of the bulk of SN Ia is suggested to appear around [Fe/H] = -0.6 dex.Comment: A&A LaTeX, 11 pages, 9 enclosed figures, Astronomy & Astrophysics, in
pres
The Indo-U.S. Library of Coude Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9m Coud\'e Feed telescope
at Kitt Peak National Observatory. This telescope feeds the coud\'e
spectrograph of the 2.1m telescope. The spectra have been obtained with the #5
camera of the coud\'e spectrograph and a Loral 3K X 1K CCD. Two gratings have
been used to provide spectral coverage from 3460 \AA to 9464 \AA, at a
resolution of 1\AA FWHM and at an original dispersion of 0.44 \AA/pixel.
For 885 stars we have complete spectra over the entire 3460 \AA to 9464 \AA
wavelength region (neglecting small gaps of 50 \AA), and partial spectral
coverage for the remaining stars. The 1273 stars have been selected to provide
broad coverage of the atmospheric parameters T, log g, and [Fe/H], as
well as spectral type. The goal of the project is to provide a comprehensive
library of stellar spectra for use in the automated classification of stellar
and galaxy spectra and in galaxy population synthesis. In this paper we discuss
the characteristics of the spectral library, viz., details of the observations,
data reduction procedures, and selection of stars. We also present a few
illustrations of the quality and information available in the spectra. The
first version of the complete spectral library is now publicly available from
the National Optical Astronomy Observatory (NOAO) via FTP and HTTP.Comment: 18 pages, 6 figures, 4 table
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