100 research outputs found
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 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
adde
Nova nucleosynthesis and Galactic evolution of the CNO isotopes
We study the role played both by novae and single stars in enriching the ISM
of the Galaxy with CNO group nuclei, in the framework of a detailed successful
model for the chemical evolution of both the Galactic halo and disc. Once all
the nucleosynthesis sources of CNO elements are taken into account, we conclude
that 13C, 15N and 17O are likely to have both a primary and a secondary origin,
in contrast to previous beliefs. Given the uncertainties still present in the
computation of theoretical stellar yields, our results can be used to put
constraints on stellar evolution and nucleosynthesis models.Comment: 17 pages, 9 figures, accepted for publication in MNRA
Manganese evolution in Omega Centauri: a clue to the cluster formation mechanisms?
We model the evolution of manganese relative to iron in the progenitor system of the globular cluster Omega Centauri by means of a self-consistent chemical evolution model. We use stellar yields that already reproduce the measurements of [Mn/Fe] versus [Fe/H] in Galactic field disc and halo stars, in Galactic bulge stars and in the Sagittarius dwarf spheroidal galaxy. We compare our model predictions to the Mn abundances measured in a sample of 10 red giant members and six subgiant members of ω Cen. The low values of [Mn/Fe] observed in a few, metal-rich stars of the sample cannot be explained in the framework of our standard, homogeneous chemical evolution model. Introducing cooling flows that selectively bring to the cluster core only the ejecta from specific categories of stars does not help to heal the disagreement with the observations. The capture of field stars does not offer a viable explanation either. The observed spread in the data and the lowest [Mn/Fe] values could, in principle, be understood if the system experienced inhomogeneous chemical evolution. Such an eventuality is qualitatively discussed in this paper. However, more measurements of Mn in ω Cen stars are needed to settle the issue of Mn evolution in this cluste
On the origin of the helium-rich population in the peculiar globular cluster Omega Centauri
In this contribution we discuss the origin of the extreme helium-rich stars
which inhabit the blue main sequence (bMS) of the Galactic globular cluster
Omega Centauri. In a scenario where the cluster is the surviving remnant of a
dwarf galaxy ingested by the Milky Way many Gyr ago, the peculiar chemical
composition of the bMS stars can be naturally explained by considering the
effects of strong differential galactic winds, which develop owing to multiple
supernova explosions in a shallow potential well.Comment: 2 pages, 1 figure, to appear in the Proceedings of IAU Symposium No.
268, Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds., Cambridge Univ. Press
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