Generic Evolution Of Deuterium And Helium-3

The primordial abundances of deuterium and of helium-3, produced during big bang nucleosynthesis, depend sensitively on the baryon density. Thus, the observed abundances of D and \he may provide useful ``baryometers'' provided the evolution from primordial to present (or, presolar nebula) abundances is understood. Inevitably, the derivation of primordial from observed abundances requires the intervention of a model for galactic evolution and, so, the inferred primordial abundances are, necessarily, model dependent. Here, an analytic framework for the evolution of D and \he is presented which is ``generic'' in the sense that it should describe the results of any specific galactic evolution model. The ``effective \he survival fraction'', $\Gamma _3$, is the one free parameter which is model specific. Solar system and interstellar data are used to infer upper and lower bounds to the primordial deuterium mass fraction $(X_{2P})$ as a function of $\Gamma _3$ and, these bounds are used to constrain the present baryon-to-photon ratio $(\eta)$ and baryon density $(\Omega_B)$. For $\Gamma _3 \geq$ 1/4 it is found that (from D and \he alone): $3.1 \leq \eta_{10} \leq 9.0$; $0.045 \leq \Omega_B h^2_{50} \leq 0.133$ (where $H_0 = 50h_{50}$ km$\,$s$^{-1}$ Mpc$^{-1}$)

Star formation histories of dwarf galaxies from the Colour-Magnitude diagrams of their resolved stellar populations

In this tutorial paper we summarize how the star formation (SF) history of a galactic region can be derived from the colour-magnitude diagram (CMD) of its resolved stars. The procedures to build synthetic CMDs and to exploit them to derive the SF histories (SFHs) are described, as well as the corresponding uncertainties. The SFHs of resolved dwarf galaxies of all morphological types, obtained from the application of the synthetic CMD method, are reviewed and discussed. In short: 1) Only early-type galaxies show evidence of long interruptions in the SF activity; late-type dwarfs present rather continuous, or gasping, SF regimes; 2) A few early-type dwarfs have experienced only one episode of SF activity concentrated at the earliest epochs, whilst many others show extended or recurrent SF activity; 3) No galaxy experiencing now its first SF episode has been found yet; 4) No frequent evidence of strong SF bursts is found; 5) There is no significant difference in the SFH of dwarf irregulars and blue compact dwarfs, except for the current SF rates. Implications of these results on the galaxy formation scenarios are briefly discussed.Comment: 29 pages. Tutorial Review to appear in the special issue "Dwarf-Galaxy Cosmology" in Advances in Astronom

Intermediate age open clusters: Collinder 110

We present CCD BV photometry of the intermediate age open cluster Collinder 110, a nearby, scarcely populated, and poorly studied system. There is no literature information on the metallicity, so we tested several possibilities, and found a slight evidence of sub-solar abundances. Using the synthetic Colour - Magnitude Diagrams technique we estimate the following parameters: age between 1.1 and 1.5 Gyr, reddening 0.38 <~ E(B-V) <~ 0.45, distance modulus (m-M)o between 11.45 and 11.7 if the cluster metallicity is sub-solar.Comment: 8 pages, 6 figures, MNRAS, in pres

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