111 research outputs found
Carbon Nitrogen, and Oxygen Galactic Gradients: A Solution to the Carbon Enrichment Problem
Eleven models of Galactic chemical evolution, differing in the carbon,
nitrogen,and oxygen yields adopted, have been computed to reproduce the
Galactic O/H values obtained from H II regions. All the models fit the oxygen
gradient, but only two models fit also the carbon gradient, those based on
carbon yields that increase with metallicity due to stellar winds in massive
stars (MS) and decrease with metallicity due to stellar winds in low and
intermediate mass stars (LIMS). The successful models also fit the C/O versus
O/H evolution history of the solar vicinity obtained from stellar observations.
We also compare the present day N/H gradient and the N/O versus O/H and the
C/Fe, N/Fe, O/Fe versus Fe/H evolution histories of the solar vicinity
predicted by our two best models with those derived from H II regions and from
stellar observations. While our two best models fit the C/H and O/H gradients
as well as the C/O versus O/H history, only Model 1 fits well the N/H gradient
and the N/O values for metal poor stars but fails to fit the N/H values for
metal rich stars. Therefore we conclude that our two best models solve the C
enrichment problem, but that further work needs to be done on the N enrichment
problem. By adding the C and O production since the Sun was formed predicted by
Models 1 and 2 to the observed solar values we find an excellent agreement with
the O/H and C/H values of the solar vicinity derived from H II regions O and C
recombination lines. One of the most important results of this paper is that
the fraction of carbon due to MS and LIMS in the interstellar medium is
strongly dependent on time and on the galactocentric distance; at present about
half of the carbon in the interstellar medium of the solar vicinity has been
produced by MS and half by LIMS.Comment: 34 pages, 6 tables, 7 figures. Accepted for publication in Ap
Dark Matter and the Chemical Evolution of Irregular Galaxies
We present three types of chemical evolution models for irregular galaxies:
closed-box with continuous star formation rates (SFRs), closed-box with
bursting SFRs, and O-rich outflow with continuous SFRs. We discuss the chemical
evolution of the irregular galaxies NGC 1560 and II Zw 33, and a ``typical''
irregular galaxy. The fraction of low-mass stars needed by our models is larger
than that derived for the solar vicinity, but similar to that found in globular
clusters. For our typical irregular galaxy we need a mass fraction of about 40%
in the form of substellar objects plus non baryonic dark matter inside the
Holmberg radius, in good agreement with the results derived for NGC 1560 and II
Zw 33 where we do have an independent estimate of the mass fraction in non
baryonic dark matter. Closed-box models are better than O-rich outflow models
in explaining the C/O and Z/O observed values for our typical irregular galaxy.Comment: 14 pages, 2 figure, uses emulateapj.sty package. ApJ in press. New
models were added. The order of Tables has been correcte
Planetary nebulae abundances and stellar evolution
A summary is given of planetary nebulae abundances from ISO measurements. It
is shown that these nebulae show abundance gradients (with galactocentric
distance), which in the case of neon, argon, sulfur and oxygen (with four
exceptions) are the same as HII regions and early type star abundance
gradients. The abundance of these elements predicted from these gradients at
the distance of the Sun from the center are exactly the solar abundance. Sulfur
is the exception to this; the reason for this is discussed. The higher solar
neon abundance is confirmed; this is discussed in terms of the results of
helioseismology. Evidence is presented for oxygen destruction via ON cycling
having occurred in the progenitors of four planetary nebulae with bilobal
structure. These progenitor stars had a high mass, probably greater than 5
solar masses. This is deduced from the high values of He/H and N/H found in
these nebulae. Formation of nitrogen, helium and carbon are discussed. The high
mass progenitors which showed oxygen destruction are shown to have probably
destroyed carbon as well. This is probably the result of hot bottom burning.Comment: 9 pages, 2 Tables, 8 figures, A&A in pres
Carbon in Spiral Galaxies from Hubble Space Telescope Spectroscopy
We present measurements of the gas-phase C/O abundance ratio in six H II
regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet
spectroscopy using the Faint Object Spectrograph on the Hubble Space Telescope.
The C/O ratios increase systematically with O/H in both galaxies, from log C/O
approximately -0.8 at log O/H = -4.0 to log C/O approx. -0.1 at log O/H = -3.4.
C/N shows no correlation with O/H. The rate of increase of C/O is somewhat
uncertain because of uncertainty as to the appropriate UV reddening law, and
uncertainty in the metallicity dependence on grain depletions. However, the
trend of increasing C/O with O/H is clear, confirming and extending the trend
in C/O indicated previously from observations of irregular galaxies. Our data
indicate that the radial gradients in C/H across spiral galaxies are steeper
than the gradients in O/H. Comparing the data to chemical evolution models for
spiral galaxies shows that models in which the massive star yields do not vary
with metallicity predict radial C/O gradients that are much flatter than the
observed gradients. The most likely hypothesis at present is that stellar winds
in massive stars have an important effect on the yields and thus on the
evolution of carbon and oxygen abundances. C/O and N/O abundance ratios in the
outer disks of spirals determined to date are very similar to those in dwarf
irregular galaxies. This implies that the outer disks of spirals have average
stellar population ages much younger than the inner disks.Comment: 38 pages, 9 postscript figures, uses aaspp4.sty. Accepted for
publication in The Astrophysical Journa
Stellar populations of Globular Clusters in the Elliptical galaxy NGC1407
We present high-quality, Keck spectroscopic data for a sample of 20 globular
clusters (GCs) in the massive E0 galaxy NGC1407. A subset of twenty
line-strength indices of the Lick/IDS system have been measured for both the GC
system and the central integrated star-light of the galaxy. Ages, metallicities
and [alpha/Fe] ratios have been derived using several different approaches. The
majority GCs in NGC1407 studied are old, follow a tight metallicity sequence
reaching values slightly above solar, and exhibit mean [alpha/Fe] ratios of ~
0.3 dex. In addition, three GCs are formally derived to be young (~ 4 Gyr), but
we argue that they are actually old GCs hosting blue horizontal branches. We
report, for the first time, evidence for the existence of two
chemically-distinct subpopulations of metal-rich (MR) GCs. We find some MR GCs
exhibit significantly larger [Mg/Fe] and [C/Fe] ratios. Different star
formation time-scales are proposed to explain the correlation between Mg and C
abundances. We also find striking CN overabundances over the entire GC
metallicity range. Interestingly, the behavior of C and N in metal-poor (MP)
GCs clearly deviates from the one in MR GCs. In particular, for MR GCs, N
increases dramatically while C essentially saturates. This may be interpreted
as a consequence of the increasing importance of the CNO cycle with increasing
metallicity.Comment: 53 pages, 11 figures, 5 tables. Accepted for publication in The
Astronomical Journa
The Chemical Evolution of Helium
We report on measurements of the 4He abundance toward the outer Galaxy HII
region S206 with the NRAO Green Bank telescope. Observations of hydrogen and
helium radio recombination lines between 8-10 GHz were made toward the peak
radio continuum position in S206. We derive 4He/H = 0.08459 +/- 0.00088
(random) +/- 0.0010 (known systematic), 20% lower than optical recombination
line results. It is difficult to reconcile the large discrepancy between the
optical and radio values even when accounting for temperature, density, and
ionization structure or for optical extinction by dust. Using only M17 and S206
we determine dY/dZ = 1.41 +/- 0.62 in the Galaxy, consistent with standard
chemical evolution models. High helium abundances in the old stellar population
of elliptical galaxies can help explain the increase in UV emission with
shorter wavelength between 2000 and 1200 Angstrom, called the UV-upturn or UVX.
Our lower values of dY/dZ are consistent with a normal helium abundance at
higher metallicity and suggest that other factors, such as a variable red giant
branch mass-loss with metallicity, may be important. When combined with 4He
abundances in metal poor galaxy HII regions, Magellanic cloud HII regions, and
M17 that have been determined from optical recombination lines including the
effects of temperature fluctuations, our radio 4He/H abundance ratio for S206
is consistent with a helium evolution of dY/dZ = 1.6. A linear extrapolation to
zero metallicity predicts a 4He/H primordial abundance ratio about 5% lower
than that given by the Wilkinson Microwave Anisotropy Probe and standard Big
Bang nucleosynthesis. The measured 4He abundances may be systematically
underestimated by a few percent if clumping exists in these HII regions.Comment: 19 pages, 3 figures (accepted for publication in the AJ
HII Region Metallicity Distribution in the Milky Way Disk
The distribution of metals in the Galaxy provides important information about
galaxy formation and evolution. HII regions are the most luminous objects in
the Milky Way at mid-infrared to radio wavelengths and can be seen across the
entire Galactic disk. We used the NRAO Green Bank Telescope (GBT) to measure
radio recombination line and continuum emission in 81 Galactic HII regions. We
calculated LTE electron temperatures using these data. In thermal equilibrium
metal abundances are expected to set the nebular electron temperature with high
abundances producing low temperatures. Our HII region distribution covers a
large range of Galactocentric radius (5 to 22 kpc) and samples the Galactic
azimuth range 330 degree to 60 degree. Using our highest quality data (72
objects) we derived an O/H Galactocentric radial gradient of -0.0383 +/- 0.0074
dex/kpc. Combining these data with a similar survey made with the NRAO 140 Foot
telescope we get a radial gradient of -0.0446 +/- 0.0049 dex/kpc for this
larger sample of 133 nebulae. The data are well fit by a linear model and no
discontinuities are detected. Dividing our sample into three Galactic azimuth
regions produced significantly different radial gradients that range from -0.03
to -0.07 dex/kpc. These inhomogeneities suggest that metals are not well mixed
at a given radius. We stress the importance of homogeneous samples to reduce
the confusion of comparing data sets with different systematics. Galactic
chemical evolution models typically derive chemical evolution along only the
radial dimension with time. Future models should consider azimuthal evolution
as well.Comment: Accepted for publication in Ap
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