arXiv subjects

C. Chiappini

Publications and source records attributed to C. Chiappini.

113 records · Page 7Linked to original sources

arXiv2004

A search for late-type supergiants in the inner regions of the Milky Way

We present the results of a narrow-band infrared imaging survey of a narrow strip (12' wide) around the galactic equator between 6 deg and 21 deg of galactic longitude aimed at detecting field stars with strong CO absorption, mainly late-type giants and supergiants. Our observations include follow-up low resolution spectroscopy (R = 980) of 191 selected candidates in the H and K bands. Most of these objects have photometric and spectroscopic characteristics consistent with them being red giants, and some display broad, strong absorption wings due to water vapor absorption between the H and K bands. We also identify in our sample 18 good supergiant candidates characterized by their lack of noticeable water absorption, strong CO bands in the H and K windows, and HK_S photometry suggestive of high intrinsic luminosity and extinctions reaching up to A_V ~40 mag. Another 9 additional candidates share the same features except for weak H2O absorption, which is also observed among some M supergiants in the solar neighbourhood. Interesting differences are noticed when comparing our stars to a local sample of late-type giants and supergiants, as well as to a sample of red giants in globular clusters of moderately subsolar metallicity and to a sample of bulge stars. (...) We propose that the systematic spectroscopic differences of our inner Galaxy stars are due to their higher metallicities that cause deeper mixing in their mantles, resulting in lower surface abundances of C and O and higher abundances of CN, which contribute to the strength of the CaI and NaI features at low resolution. Our results stress the limitations of using local stars as templates for the study of composite cool stellar populations such as central starbursts in galaxies (Abridged).

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arXiv2002

K dwarfs and the chemical evolution of the Solar cylinder

K-dwarfs have life-times older than the present age of the Galactic disc, and are thus ideal stars to investigate the disc's chemical evolution. We have developed several photometric metallicity indicators for K dwarfs, based an a sample of accurate spectroscopic metallicities for 34 disc and halo G and K dwarfs. The photometric metallicities lead us to develop a metallicity index for K dwarfs based only on their position in the colour absolute-magnitude diagram. Metallicities have been determined for 431 single K dwarfs drawn from the Hipparcos catalog, selecting the stars by absolute magnitude and removing multiple systems. The sample is essentially a complete reckoning of the metal content in nearby K dwarfs. We use stellar isochrones to mark the stars by mass, and select a subset of 220 of the stars which is complete in a narrow mass interval. We fit the data with a model of the chemical evolution of the Solar cylinder. We find that only a modest cosmic scatter is required to fit our age metallicity relation. The model assumes two main infall episodes for the formation of the halo-thick disc and thin disc respectively. The new data confirms that the solar neighbourhood formed on a long timescale of order 7 Gyr.

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arXiv2001

Mg_2-sigma in Early-Type Galaxies and Spiral Bulges

We analyze new measurements of the Mg_2 central line strength index and velocity dispersion (sigma) for the galaxies of the ENEAR survey. The observations are now complete (da Costa et al. 2000) and the sample contains 1223 early-type galaxies. We also analyze the line strength indices for a sample of 95 spiral bulges (from Sa to Sbc). For the early-type galaxies we find: i) that the Mg_2-sigma relation for Es and S0s are nearly the same, with both populations showing comparable scatter, and ii) a marginal difference in the slope of the Mg_2-sigma relation for cluster and field early-type galaxies. However, we suggest that before interpreting such a difference in the framework of a mass-metallicity relation, it is important to take into account the effects of rotation in the Mg_2-sigma relation. Our preliminary results indicate that once the rotation effects are minimized by choosing a sample containing only slow rotators, the Mg_2-sigma relation is similar both for isolated and clustered galaxies. More data on rotational velocities of early-type galaxies are needed to confirm this result. For spiral bulges, we find that their locus in the Mg_2-sigma plane lies always below the one occupied by early-type galaxies.

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arXiv2000

The mass surface density in the local disk and the chemical evolution of the Galaxy

We have studied the effect of adopting different values of the total baryonic mass surface density in the local disk at the present time in a model for the chemical evolution of the Galaxy. We have compared our model results with the G-dwarf metallicity distribution, the amounts of gas, stars, stellar remnants, infall rate and SN rate in the solar vicinity, and with the radial abundance gradients and gas distribution in the disk. This comparison strongly suggests that the value of the total baryonic mass surface density in the local disk which best fits the observational properties should lie in the range 50-75 Msun pc-2, and that values outside this range should be ruled out.

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arXiv1996

The Chemical Evolution of the Galaxy: the two-infall model

In this paper we present a new chemical evolution model for the Galaxy which assumes two main infall episodes for the formation of halo-thick disk and thin disk, respectively. We do not try to take into account explicitly the evolution of the halo but we implicitly assume that the timescale for the formation of the halo was of the same order as the timescale for the formation of the thick disk. The formation of the thin-disk is much longer than that of the thick disk, implying that the infalling gas forming the thin-disk comes not only from the thick disk but mainly from the intergalactic medium. The timescale for the formation of the thin-disk is assumed to be a function of the galactocentric distance, leading to an inside-out picture for the Galaxy building. The model takes into account the most up to date nucleosynthesis prescriptions and adopts a threshold in the star formation process which naturally produces a hiatus in the star formation rate at the end of the thick disk phase, as suggested by recent observations. The model results are compared with an extended set of observational constraints. Among these constraints, the tightest one is the metallicity distribution of the G-dwarf stars for which new data are now available. Our model fits very well these new data. We show that in order to reproduce most of these constraints a timescale $\le 1$ Gyr for the (halo)-thick-disk and of 8 Gyr for the thin-disk formation in the solar vicinity are required. We predict that the radial abundance gradients in the inner regions of the disk ($R< R_{\odot}$) are steeper than in the outer regions, a result confirmed by recent abundance determinations, and that the inner ones steepen in time during the Galactic lifetime.

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