M. Tosi

Quantifying the uncertainties of chemical evolution studies II. Stellar yields

Context. Galactic chemical evolution models are useful tools for interpreting the large body of high-quality observational data on the chemical composition of stars and gas in galaxies that have become available in recent years. Aims. This is the second paper of a series that aims at quantifying the uncertainties in chemical evolution model predictions related to the underlying model assumptions. Specifically, it deals with the uncertainties due to the choice of the stellar yields. Methods. We adopted a widely used model for the chemical evolution of the Galaxy to test the effects of changing the stellar nucleosynthesis prescriptions on the predicted evolution of several chemical species. Up-to-date results from stellar evolutionary models were carefully taken into account. Results. We find that, except for a handful of elements whose nucleosynthesis in stars is well understood by now, large uncertainties still affect model predictions. This is especially true for the majority of the iron-peak elements, but also for much more abundant species such as carbon and nitrogen. The main causes of the mismatch we find among the outputs of different models assuming different stellar yields and among model predictions and observations are (i) the adopted location of the mass cut in models of type II supernova explosions; (ii) the adopted strength and extent of hot bottom burning in models of asymptotic giant branch stars; (iii) the neglection of the effects of rotation on the chemical composition of the stellar surfaces; (iv) the adopted rates of mass loss and of (v) nuclear reactions; and (vi) the different treatments of convection. Conclusions. Our results suggest that it is mandatory to include processes such as hot bottom burning in intermediate-mass stars and rotation in stars of all masses in accurate studies of stellar evolution and nucleosynthesis. In spite of their importance, both these processes still have to be better understood and characterized. As for massive stars, presupernova models computed with mass loss and rotation are available in the literature, but they still wait for a self-consistent coupling with the results of explosive nucleosynthesis computations.

Metal Abundances of Red Clump Stars in Open Clusters. I. NGC 6819

We present an analysis of high-dispersion spectra (R ~ 40,000) of three red clump stars in the old open cluster NGC 6819. The spectra were obtained with SARG, the high-dispersion spectrograph of the Telescopio Nazionale Galileo. The spectra were analyzed using both equivalent widths measured with an automatic procedure and comparisons with synthetic spectra. NGC 6819 is found to be slightly metal-rich ([Fe/H] = +0.09 ± 0.03, internal error); there are no previous high-resolution studies to compare. Most element-to-element abundance ratios are close to solar; we find a slight excess of Si and a significant Na overabundance. Our spectra can also be used to derive the interstellar reddening toward the cluster by comparing the observed colors with those expected from line excitation: we derive E(B-V) = 0.14 ± 0.04, in agreement with the most recent estimate for this cluster.

Age Determination of Six Intermediate-Age Small Magellanic Cloud Star Clusters with Hst/acs

We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color-magnitude diagrams (CMDs) extend ~3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC 419 where the Padova isochrones provided the best fit. Using Dartmouth isochrones we derive ages of 7.5 ± 0.5 Gyr (Lindsay 1), 6.5 ± 0.5 Gyr (Kron 3), 6 ± 0.5 Gyr (NGC 339), 6 ± 0.5 Gyr (NGC 416), and 6.5 ± 0.5 Gyr (Lindsay 38). The CMD of NGC 419 shows several main-sequence turnoffs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC 419. We confirm that the SMC contains several intermediate-age populous star clusters with ages unlike those of the Large Magellanic Cloud and the Milky Way. Interestingly, our intermediate-age star clusters have a metallicity spread of ~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally-concentrated blue straggler star candidates in NGC 416, while these are not present for the other clusters. Using the red clump magnitudes, we find that the closest cluster, NGC 419 (~50 kpc), and the farthest cluster, Lindsay 38 (~67 kpc), have a relative distance of ~17 kpc, which confirms the large depth of the SMC. The three oldest SMC clusters (NGC 121, Lindsay 1, and Kron 3) lie in the northwestern part of the SMC, while the youngest (NGC 419) is located near the SMC main body.

The Resolved Stellar Population of the Poststarburst Galaxy NGC 1569

We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) photometry of the resolved stellar population in the poststarburst galaxy NGC 1569. The color-magnitude diagram (CMD) derived in the F439W and F555W photometric bands contains ~2800 stars with a photometric error of ≤0.2 mag down to mF439, mF555 26 and is complete for mF555 23. Adopting the literature-distance modulus and reddening, our CMD samples stars more massive than ~4 M☉, allowing us to study the star formation (SF) history over the last ~0.15 Gyr. The data are interpreted using theoretical simulations based on stellar evolutionary models. The synthetic diagrams include photometric errors and incompleteness factors. Testing various sets of tracks, we find that the ability of the models to reproduce the observed features in the CMD is strictly related to the shape of the blue loops of the sequences with masses around 5 M☉. The field of NGC 1569 experienced a global SF burst of 0.1 Gyr duration, ending ~5-10 Myr ago. During the burst, the SF rate was approximately constant, and, if quiescent periods occurred, they lasted less than ~10 Myr. The level of the SF rate was very high; for a single-slope initial mass function (IMF) ranging from 0.1 to 120 M☉, we find values of 3, 1, and 0.5 M☉ yr-1 for α = 3, 2.6, and 2.35 (Salpeter), respectively. When scaled for the surveyed area, these rates are approximately 100 times larger than found in the most active dwarf irregulars in the Local Group. The data are consistent with a Salpeter IMF, though our best models indicate slightly steeper exponents. We discuss the implications of our results in the general context of the evolution of dwarf galaxies.

THE STAR FORMATION HISTORY OF I Zw 18

The star formation history in I Zw 18 has been inferred from Hubble Space Telescope Wide Field Planetary Camera 2 archival data. This is done by comparing the derived V, B-V and V, V-I color-magnitude diagrams and luminosity functions with synthetic ones, based on various sets of stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the field of I Zw 18 allow for a look-back time up to 1 Gyr. We find that the main body is not experiencing its first episode of star formation. Instead, it has been forming stars over the last 0.5?1 Gyr, at a rate of ~(1?2) ? 10-2 M? yr-1 kpc-2. A more intense activity of (6?16) ? 10-2 M? yr-1 kpc-2 has taken place between 15 and 20 Myr ago. For the secondary body, the look-back time is 0.2 Gyr at most and the uncertainty is much higher because of the shallower diagrams and the small number of resolved stars. The derived range of star formation rate is (3?10) ? 10-3 M? yr-1 kpc-2. The IMF providing the best fit to the observed stellar populations in the main body has a slope 1.5, much flatter than in any similar galaxy analyzed with the same method. In the secondary body, it is peaked at ? 2.2, closer to Salpeters slope (? = 2.35).

The Star Formation History of IZw18

The star formation history in I Zw 18 has been inferred from Hubble Space Telescope Wide Field Planetary Camera 2 archival data. This is done by comparing the derived V, B-V and V, V-I color-magnitude diagrams and luminosity functions with synthetic ones, based on various sets of stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the field of I Zw 18 allow for a look-back time up to 1 Gyr. We find that the main body is not experiencing its first episode of star formation. Instead, it has been forming stars over the last 0.5?1 Gyr, at a rate of ~(1?2) ? 10-2 M? yr-1 kpc-2. A more intense activity of (6?16) ? 10-2 M? yr-1 kpc-2 has taken place between 15 and 20 Myr ago. For the secondary body, the look-back time is 0.2 Gyr at most and the uncertainty is much higher because of the shallower diagrams and the small number of resolved stars. The derived range of star formation rate is (3?10) ? 10-3 M? yr-1 kpc-2. The IMF providing the best fit to the observed stellar populations in the main body has a slope 1.5, much flatter than in any similar galaxy analyzed with the same method. In the secondary body, it is peaked at ? 2.2, closer to Salpeters slope (? = 2.35).

Quantifying the uncertainties of chemical evolution studies I. Stellar lifetimes and initial mass function

Stellar lifetimes and initial mass function are basic ingredients of chemical evolution models, for which different recipes can be found in the literature. In this paper, we quantify the effects on chemical evolution studies of the uncertainties in these two parameters. We concentrate on chemical evolution models for the Milky Way, because of the large number of good observational constraints. Such chemical evolution models have already ruled out significant temporal variations for the stellar initial mass function in our own Galaxy, with the exception perhaps of the very early phases of its evolution. Therefore, here we assume a Galactic initial mass function constant in time. Through an accurate comparison of model predictions for the Milky Way with carefully selected data sets, it is shown that specific prescriptions for the initial mass function in particular mass ranges should be rejected. As far as the stellar lifetimes are concerned, the major differences among existing prescriptions are found in the range of very low-mass stars. Because of this, the model predictions differ widely for those elements which are produced mostly by very long-lived objects, as for instance 3 He and 7 Li. However, we conclude that model predictions of several important observed quantities, constraining the plausible Galactic formation scenarios, are fairly robust with respect to changes in both the stellar mass spectrum and lifetimes. For instance, the metallicity distribution of low-mass stars is nearly unaffected by these changes, since its shape is dictated mostly by the time scale for thin-disk formation.

The Bologna Open Cluster Chemical Evolution Project: Midterm Results from the Photometric Sample

We describe a long-term project aimed at deriving information on the chemical evolution of the Galactic disk from a large sample of open clusters. The main property of this project is that all clusters are analyzed in a homogeneous way to guarantee the robustness of the ranking in age, distance, and metallicity. Special emphasis is devoted to the evolution of the earliest phases of the Galactic disk evolution, for which clusters have superior reliability with respect to other types of evolution indicators. The project is twofold: on one hand we derive the age, distance, and reddening (and indicative metallicity) by interpreting deep and accurate photometric data with stellar evolution models, and on the other hand, we derive the chemical abundances from high-resolution spectroscopy. Here we describe our overall goals and approaches and report on the midterm project status of the photometric part, with 16 clusters already studied, covering an age interval from 0.1 to 6 Gyr and galactocentric distances from 6.6 to 21 kpc. The importance of quantifying the theoretical uncertainties by deriving the cluster parameters with various sets of stellar models is emphasized. Stellar evolution models assuming overshooting from convective regions appear to better reproduce the photometric properties of the cluster stars. The examined clusters show a clear metallicity dependence on the galactocentric distance and no dependence on age. The tight relation between cluster age and magnitude difference between the main-sequence turnoff and the red clump is confirmed.

I Zw 18 Revisited with HST ACS and Cepheids: New Distance and Age*

We present new V- and I-band HST ACS photometry of I Zw 18, the most metal-poor blue compact dwarf (BCD) galaxy in the nearby universe. It has been argued in the past that I Zw 18 is a very young system that started forming stars only 500 Myr ago, but other work has hinted that older (1 Gyr) red giant branch (RGB) stars may also exist. Our new data, once combined with archival HST ACS data, provide a deep and uncontaminated optical color-magnitude diagram (CMD) that now strongly indicates an RGB. The RGB tip (TRGB) magnitude yields a distance modulus (m - M)0 = 31.30 ± 0.17, i.e., D = 18.2 ± 1.5 Mpc. The time-series nature of our observations allows us to also detect and characterize for the first time three classical Cepheids in I Zw 18. The time-averaged Cepheid V and I magnitudes are compared to the VI reddening-free Wesenheit relation predicted from new nonlinear pulsation models specifically calculated at the metallicity of I Zw 18. For the one bona fide classical Cepheid with a period of 8.63 days this implies a distance modulus (m - M)0 = 31.42 ± 0.26. The other two Cepheids have unusually long periods (125.0 and 129.8 days) but are consistent with this distance. The coherent picture that emerges is that I Zw 18 is farther away than previously assumed and older than suggested by some previous works. The presence of an RGB population rules out the possibility that I Zw 18 is a truly primordial galaxy formed recently (z 0.1) in the local universe.

The metallicity of the old open cluster ngc 6791

We have observed four red clump stars in the very old, metal-rich open cluster NGC 6791 to derive its metallicity using the high-resolution spectrograph SARG mounted on the Galileo National Telescope (TNG). Using a spectrum synthesis technique, we obtain an average value of [Fe/H] = +0.47 (±0.04, rms = 0.08) dex. Our method was tested on μ Leo, a well-studied, metal-rich field giant. We also derive average oxygen and carbon abundances for NGC 6791 from synthesis of [O I] at 6300 A and C2 at 5086 A, finding [O/Fe] -0.3 and [C/Fe] -0.2.