389 research outputs found
Connecting the primordial and Galactic deuterium abundances
The deuterium abundances inferred from observations of the interstellar
medium within 1-2 kpc of the Sun range over a factor of three and the
corresponding oxygen abundances show an even larger dispersion. While the lower
D (and O) abundances likely result from depletion onto dust, the higher D
abundances are consistent with the BBN-predicted primordial D abundance and
chemical evolution models of the Galaxy with infall of primordial or nearly
primordial material. The large ranges in deuterium and oxygen abundances
suggest that the effects of depletion and/or infall have not been homogenized
in the local interstellar medium.Comment: 5 pages, 4 figures; accepted for publication in MNRA
Formation and evolution of late-type dwarf galaxies. I. NGC 1705 and NGC 1569
(Abridged.) We present one-zone chemical evolution models for two dwarf
starburst galaxies, NGC 1705 and NGC 1569. Using information about the past
star formation history and initial mass function of the systems previously
obtained from Hubble Space Telescope colour-magnitude diagrams, we identify
possible scenarios of chemical enrichment and development of galactic winds. In
order not to overestimate the current metallicity of the interstellar gas
inferred from H II region spectroscopy, we suggest that the winds efficiently
remove from the galaxies the metal-rich ejecta of dying stars. Conversely,
requiring the final mass of neutral gas to match the value inferred from 21-cm
observations implies a relatively low efficiency of interstellar medium
entrainment in the outflow, thus confirming previous findings that the winds
driving the evolution of typical starbursts are differential. These conclusions
could be different only if the galaxies accrete huge fractions of unprocessed
gas at late times. By assuming standard stellar yields we obtain a good fit to
the observed nitrogen to oxygen ratio of NGC 1569, while the mean N/O ratio in
NGC 1705 is overestimated by the models. Reducing the extent of hot bottom
burning in low-metallicity intermediate-mass stars does not suffice to solve
the problem. Localized self-pollution from stars more massive than 60 MSun in
NGC 1705 and/or funneling of larger fractions of nitrogen through its winds are
then left to explain the discrepancy between model predictions and
observations. Inspection of the log(N/O) vs. log(O/H)+12 diagram for a sample
of dwarf irregular and blue compact dwarf galaxies in the literature favours
the latter hypothesis.Comment: 20 pages, 10 figures, accepted for publication on MNRA
Chemical enrichment in very low-metallicity environments: Bootes I
We present different chemical evolution models for the ultrafaint dwarf
galaxy Bootes I. We either assume that the galaxy accretes its mass through
smooth infall of gas of primordial chemical composition (classical models) or
adopt mass accretion histories derived from the combination of merger trees
with semi-analytical modelling (cosmologically-motivated models). Furthermore,
we consider models with and without taking into account inhomogeneous mixing in
the ISM within the galaxy. The theoretical predictions are then compared to
each other and to the body of the available data. From this analysis, we
confirm previous findings that Bootes I has formed stars with very low
efficiency but, at variance with previous studies, we do not find a clear-cut
indication that supernova explosions have sustained long-lasting galactic-scale
outflows in this galaxy. Therefore, we suggest that external mechanisms such as
ram pressure stripping and tidal stripping are needed to explain the absence of
neutral gas in Bootes I today.Comment: 13 pages, 11 figures, accepted for publication in MNRA
Chemical evolution in a model for the joint formation of quasars and spheroids
Direct and indirect pieces of observational evidence point to a strong
connection between high-redshift quasars and their host galaxies. In the
framework of a model where the shining of the quasar is the episode that stops
the formation of the galactic spheroid inside a virialized halo, it has been
proven possible to explain the submillimetre source counts together with their
related statistics and the local luminosity function of spheroidal galaxies.
The time delay between the virialization and the quasar manifestation required
to fit the counts is short and incresing with decresing the host galaxy mass.
In this paper we compute the detailed chemical evolution of gas and stars
inside virialized haloes in the framework of the same model, taking into
account the combined effects of cooling and stellar feedback. Under the
assumption of negligible angular momentum, we are able to reproduce the main
observed chemical properties of local ellipticals. In particular, by using the
same duration of the bursts which are required in order to fit the
submillimetre source counts, we recover the observed increase of the Mg/Fe
ratio with galactic mass. Since for the most massive objects the assumed
duration of the burst is Tburst < 0.6 Gyr, we end up with a picture for
elliptical galaxy formation in which massive spheroids complete their assembly
at early times, thus resembling a monolithic collapse, whereas smaller galaxies
are allowed for a more prolonged star formation, thus allowing for a more
complicated evolutionary history.Comment: 17 pages, 11 figures, accepted for publication in MNRA
Light element evolution resulting from WMAP data
The recent determination of the baryon-to-photon ratio from WMAP data by
Spergel et al. (2003) allows one to fix with unprecedented precision the
primordial abundances of the light elements D, 3He, 4He and 7Li in the
framework of the standard model of big bang nucleosynthesis. We adopt these
primordial abundances and discuss the implications for Galactic chemical
evolution, stellar evolution and nucleosynthesis of the light elements. The
model predictions on D, 3He and 4He are in excellent agreement with the
available data, while a significant depletion of 7Li in low-metallicity stars
is required to reproduce the Spite plateau.Comment: 10 pages, 6 figures, accepted for publication in MNRAS. Minor changes
(Figure 2: the observationally inferred range of D/H variation in the LISM
has been modified - only the most reliable data are shown); some references
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