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Register a new userMassive Star Cluster Populations in Irregulars as Probable Younger Counterparts of Old Metal-Rich Globular Cluster Populations in Spheroids
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Peak metallicities of metal-rich(MR) populations of GCs belonging to spheroids of different mass fall within the somewhat conservative -0.7<=[Fe/H]<=-0.3 range. Indeed, if possible age effects are taken into account,this metallicity range might become smaller. Irregulars, like the LMC, with longer timescales of their formation and lower star formation (SF) efficiency do not contain the old MRGCs with [Fe/H]>-1.0,but they are observed to form populations of young/ intermediate-age massive star clusters (MSCs) with masses exceeding 10^4 Msol. Their formation is widely believed to be accidental process fully depending on external factors. From analysis of data available on the populations and their hosts, including populous star clusters in the LMC, we find that their most probable mean metallicities fall within -0.7<=[Fe/H]<=-0.3, as the peak metallicities of MRGCs do, irrespective of sings of interaction. Moreover, both the disk giant metallicity distribution function (MDF) in the LMC and the MDFs for old giants in the halos of massive spheroids exhibit significant increasing toward [Fe/H]~-0.5. That is in agreement with a correlation found between SF activity in galaxies and their metallicity. The formation of both the old MRGCs in spheroids and MSC populations in irregulars probably occurs approximately at the same stage of the host galaxies chemical evolution and is related to the essentially increased SF activity in the hosts around the same metallicity that is achieved very soon in massive spheroids, later in lower-mass spheroids, and much more later in irregulars. (Abridged)
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We address the problem of the factors contributing to a peak color trend of old metal-rich globular cluster (MRGC) populations with mass of their hosts, early-type galaxies and spheroidal subsystems of spiral ones (spheroids). The color-mass trend is often converted to a metallicity-mass trend under the assumption that age effects are small or negligible. While direct estimates of the ages of MRGC populations neither can rule out nor reliably support the populations age trend, key data on timing of the formation of spheroids and other indirect evidence imply it in the sense: the more massive spheroid the older on average its MRGC population. We show that the contribution of an allowable age trend of the MRGC populations to their peak color trend can achieve up to ~50 % or so. In this event the comparable value of the color trend, ~30 %, is due to alpha-element ratio systematic variations of the order of Delta[alpha/Fe] ~ 0.1 to 0.2 dex because of a correlation between the [alpha/Fe] ratios and age. Hence a systematic variation of exactly [Fe/H] ratios may turn out to be less significant among the contributors, and its range many times lower, i.e. of the order of Delta[Fe/H] ~ 0.1 or even none, than the corresponding range deduced by assuming no age trend.
We present new spectroscopic observations of Mkn 309, a starburst galaxy with one of the largest WR populations known. A highly super solar metallicity is derived. Using additional objects we analyse a sample of five metal-rich WR galaxies with the main goal of constraining the basic properties of the massive star populations (IMF slope, M_up) and the star formation history (age, burst duration) of these objects by quantitative comparisons with evolutionary synthesis models. The following main results are obtained: 1) The observations are well explained by extended bursts of star formation or a superposition of several bursts. Ages and burst durations are estimated. This naturally explains both the observed WR populations (including WN and WC stars) and the presence of red supergiants. 2) The fitted SEDs indicate that the stellar light suffers from a smaller extinction than that of the gas, confirming independent earlier findings. 3) All the considered observational constraints are compatible with a Salpeter IMF extending to masses >~ 40 Msun. Adopting a conservative approach we derive a LOWER LIMIT of Mup >~ 30 Msun for the Salpeter IMF. From more realistic assumptions on the metallicity and SF history we favour a lower limit Mup >~ 30-40 Msun, which is also in agreement with Hbeta equivalent width measurements of metal-rich HII regions in spiral galaxies indicating an upper mass cut-off of at least ~ 35 - 50 Msun. Steep IMF slopes (alpha >~ 3.3) are very unlikely. (abridged/modified abstract)
The formation of populous secondary star cluster systems is a widespread phenomenon in mergers of gas-rich galaxies. Many, if not most, of those clusters are massive and compact enough to be young globular clusters (GCs). GC systems in most E/S0 galaxies feature bimodal color distributions with a fairly universal blue peak similar to the blue peak of halo GCs in the Milky Way (MW) and M31, and a variable red peak. Due to the well-known age -- metallicity degeneracy of optical broad-band colors, the metallicities and ages, and, hence, the origin of the red peak GCs are not yet known. We use evolutionary synthesis models for GC {bf systems} of various metallicities to study the time evolution of their luminosity functions (LFs) in various bands U,..., K and of their color distributions. By comparison with the universal blue peak GC population we investigate for which combinations of age and metallicity a second GC population can or cannot be identified in typical observations of GC color distributions and we discuss implications for the GC LF as a distance indicator.
The most massive star clusters include several generations of stars with a different chemical composition (mainly revealed by an Na-O anti-correlation) while low-mass star clusters appear to be chemically homogeneous. We are investigating the chemical composition of several clusters with masses of a few 10^4 Msun to establish the lower mass limit for the multiple stellar population phenomenon. Using FLAMES@VLT spectra we determine abundances of Fe, O, Na, and several other elements (alpha, Fe-peak, and neutron-capture elements) in the old open cluster Berkeley 39. This is a massive open cluster: M~10^4 Msun, approximately at the border between small globular clusters and large open clusters. Our sample size of about 30 stars is one of the largest studied for abundances in any open cluster to date, and will be useful to determine improved cluster parameters, such as age, distance, and reddening when coupled with precise, well-calibrated photometry. We find that Berkeley 39 is slightly metal-poor, <[Fe/H]>=-0.20, in agreement with previous studies of this cluster. More importantly, we do not detect any star-to-star variation in the abundances of Fe, O, and Na within quite stringent upper limits. The r.m.s. scatter is 0.04, 0.10, and 0.05 dex for Fe, O, and Na, respectively. This small spread can be entirely explained by the noise in the spectra and by uncertainties in the atmospheric parameters. We conclude that Berkeley 39 is a single-population cluster.
NGC 6791 is a unique stellar cluster, key to our understanding of both the multiple stellar population phenomenon and the evolution and assembly of the Galaxy. However, despite many investigations, its nature is still very controversial. Geisler et al. (2012) found evidence suggesting it was the first open cluster to possess multiple populations but several subsequent studies did not corroborate this. It has also been considered a member of the thin or thick disk or even the bulge, and both as an open or globular cluster or even the remnant of a dwarf galaxy. Here, we present and discuss detailed abundances derived from high resolution spectra obtained with UVES at VLT and HIRES at Keck of 17 evolved stars of this cluster. We obtained a mean [Fe/H]=+0.313+-0.005, in good agreement with recent estimates, and with no indication of star-to-star metallicity variation, as expected. We also did not find any variation in Na, in spite of having selected the very same stars as in Geisler et al. (2012), where a Na variation was claimed. This points to the presence of probable systematics in the lower resolution spectra of this very high metallicity cluster analysed in that work. In fact, we find no evidence for an intrinsic spread in any element, corroborating recent independent APOGEE data. The derived abundances indicate that NGC 6791 very likely formed in the Galactic Bulge and that the proposed association with the Thick Disk is unlikely, despite its present Galactic location. We confirm the most recent hypothesis suggesting that the cluster could have formed in the Bulge and radially migrated to its current location, which appears the best explanation for this intriguing object.
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