134 research outputs found
CNO abundances and carbon isotope ratios in evolved stars of the open clusters NGC 2324, NGC 2477, and NGC 3960
Our main aim is to determine carbon-to-nitrogen and carbon isotope ratios for
evolved giants in the open clusters NGC 2324, NGC 2477, and NGC 3960, which
have turn-off masses of about 2 Msun, and to compare them with predictions of
theoretical models. High-resolution spectra were analysed using a differential
synthetic spectrum method. Abundances of carbon were derived using the C2 Swan
(0,1) band heads at 5135 and 5635.5 A. The wavelength interval 7940-8130 A with
strong CN features was analysed to determine nitrogen abundances and carbon
isotope ratios. The oxygen abundances were determined from the [Oi] line at
6300 A. The mean values of the CNO abundances are [C/Fe]=-0.35+-0.06 (s.d.),
[N/Fe]=0.28+-0.05, and [O/Fe]=-0.02+-0.10 in seven stars of NGC 2324;
[C/Fe]=-0.26+-0.02, [N/Fe]=0.39+-0.04, and [O/Fe]=-0.11+-0.06 in six stars of
NGC 2477; and [C/Fe]=-0.39+-0.04, [N/Fe]=0.32+-0.05, and [O/Fe]=-0.19+-0.06 in
six stars of NGC 3960. The mean C/N ratio is equal to 0.92+-0.12, 0.91+-0.09,
and 0.80+-0.13, respectively. The mean 12C/13C ratio is equal to 21+-1, 20+-1,
and 16+-4, respectively. The 12C/13C and C/N ratios of stars in the
investigated open clusters were compared with the ratios predicted by stellar
evolution models. The mean values of the 12C/13C and C/N ratios in NGC 2324 and
NGC 2477 agree well with the first dredge-up and thermohaline-induced
extra-mixing models, which are similar for intermediate turn-off mass stars.
The 12C/13C ratios in the investigated clump stars of NGC 3960 span from 10 to
20. The mean carbon isotope and C/N ratios in NGC 3960 are close to predictions
of the model in which the thermohaline- and rotation-induced (if rotation
velocity at the zero-age main sequence was 30% of the critical velocity)
extra-mixing act together.Comment: 9 pages, 11 figures. arXiv admin note: text overlap with
arXiv:1603.0952
Two sequences of spiral galaxies with different shapes of the metallicity gradients
We considered two sequences of spiral galaxies with different shapes of the
radial gas-phase oxygen abundance distributions from the galaxies in the MaNGA
survey: (1) Galaxies in which the gradient is well approximated by a single
linear relation across the whole disc, that is, galaxies with an S (slope)
gradients, (2) galaxies in which the metallicity in the inner region of the
disc is at a nearly constant level and the gradient is negative at larger
radii, that is, galaxies with level-slope (LS) gradients. We also selected
galaxies with a nearly uniform oxygen abundance across the whole galaxy, that
is, galaxies with level (L) gradients that can be the final evolutionary stage
of the two galaxy sequences described above. The radial nitrogen abundance
distributions in galaxies with LS oxygen abundance distributions also show
breaks at radii smaller than the O/H distribution breaks. The observed
behaviour of the oxygen and nitrogen abundances with radius in these galaxies
can be explained by the time delay between the nitrogen and oxygen enrichment
together with the variation in the star formation history along the radius.
These galaxies clearly show the effect of the inside-out disc evolution model.
We find that the shape of the radial abundance distribution in a galaxy is not
related to its macroscopic characteristics (rotation velocity, stellar mass,
isophotal radius, and star formation rate). The correlations between the
gradient slopes and macroscopic characteristics of galaxies are weak in the
sense that the scatter of the points in each diagram is large. We also examined
the properties of the Milky Way in the context of the considered galaxy
samples.Comment: 20 pages, 19 figures, accepted to Astronomy and Astrophysic
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
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
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
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