arXiv subjects

L. Sbordone

Publications and source records attributed to L. Sbordone.

117 records · Page 7Linked to original sources

arXiv2005

A flash in the dark: UVES/VLT high resolution spectroscopy of GRB afterglows

We present the first high resolution (R=20000--45000, corresponding to 14 km/s at 4200A to 6.6 km/s at 9000A) observations of the optical afterglow of Gamma Ray Bursts. GRB020813 and GRB021004 were observed by UVES@VLT 22.19 hours and 13.52 hours after the trigger, respectively. These spectra show that the inter--stellar matter of the GRB host galaxies is complex, with many components contributing to each main absorption system, and spanning a total velocity range of up to about 3000 km/s. Several narrow components are resolved down to a width of a few tens of km/s. In the case of GRB021004 we detected both low and high ionization lines. Combined with photoionization results obtained with CLOUDY, the ionization parameters of the various systems are consistent with a remarkably narrow range with no clear trend with system velocity. This can be interpreted as due to density fluctuations on top of a regular R^-2 wind density profile.

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arXiv2005

A search for Interstellar absorptions towards Sgr dSph

We searched for Na I D1,D2 interstellar absorption lines towards 12 giant stars of the Sgr dwarf spheroidal and 3 stars of the associated globular cluster Terzan 7, observed at high resolution with the UVES spectrograph at the 8.2 m Kueyen VLT telescope. These stars are not an ideal background source for the analysis of interstellar (IS) absorption lines since they have been observed in the framework of a stellar abundances study. However they are sufficiently luminous to allow a decent S/N ratio at a resolution of about 43000, i.e. ~7 km/s, which allows an exploratory study. We detected two distinct groups of IS absorptions, a ``local'' group, with radial velocities ranging from -43 km/s to 50 km/s, and a high velocity group with radial velocities ranging from 150 km/s to 165 km/s. Likely the high velocity components are due to gas falling on the Sgr dSph since have been observed only in 2 stars which are separated by only 1.5' in the sky. We argue that the observations suggest a stripping of gas due to the passage of Sgr through the Galactic disc.

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arXiv2004

UVES observations of the Canis Major overdensity

We present the first detailed chemical abundances for three giant stars which are candidate members of the Canis Major overdensity, obtained by using FLAMES-UVES at VLT. The stars, in the background of the open cluster NGC 2477, have radial velocities compatible with a membership to this structure. However, due to Galactic disc contamination, radial velocity by itself is unable to firmly establish membership. The metallicities span the range -0.5 < [Fe/H] < +0.1. Assuming that at least one of the three stars is indeed a member of CMa implies that this structure has undergone a high level of chemical processing, comparable to that of the Galactic disc. The most metal-rich star of the sample, EIS 6631, displays several abundance ratios which are remarkably different from those of Galactic stars: [alpha/Fe] ~-0.2, [Cu/Fe] ~+0.25, [La/Fe]~+0.6, [Ce/Fe]~+0.8 and [Nd/Fe]~+0.6. These ratios make it likely that this star was formed in an external galaxy.

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arXiv2004

UVES detailed chemical abundances in the Sgr dSph and the CMa overdensity

We present detailed Chemical abundances for 12 stars in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) and 3 in the Canis Mayor Overdensity (CMa), based on UVES spectra. Sgr dSph stars present high metallicity ([Fe/H] between -1 and 0) low [alpha/Fe] ratios, overabundant La, Ce and Nd, underabundant Na, Al,Ni and Zn. Zn underabundance in particular is interesting, since it may undermine the parallel drawn between dSph and DLAs, where Zn is used as a proxy for Fe. Of the three CMa stars, one is likely a Milky Way (MW) interloper, the other two present solar metallicity, underabundant alpha elements and overabundant La, Ce and Nd. These features are strongly resembling what found in Sgr dSph. Moreover, a significant Cu overabundance is detected ([Cu/Fe] ~ +0.25), unusual for MW disk stars. In our opinion these results strongly point toward the presence, in the CMa overdensity, of an accreted stellar population of extragalactic origin.

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arXiv2004

ATLAS and SYNTHE under Linux

We have successfully ported under GNU Linux ATLAS 9, the widely used stellar atmosphere modeling code, as well as both the SYNTHE suite of programs, its ``companion'' for spectral synthesis, and WIDTH, used to derive chemical abundances from equivalent widths of spectral lines. The porting has been realized by using the Intel Fortran Compiler. Our aim was to port the codes with the minimum possible amount of modifications: full compatibility with the VMS version has been maintained, along with all the codes functionalities. Dramatic improvement in calculation speed with respect to the VMS version has been achieved. The full suite of codes is intended to be freely available to anyone.

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arXiv2003

The Sgr dSph hosts a metal-rich population

We report on abundances of O, Mg, Si, Ca and Fe for 10 giants in the Sgr dwarf spheroidal derived from high resolution spectra obtained with UVES at the 8.2m Kueyen-VLT telescope. The iron abundance spans the range -0.8 <[Fe/H] < 0.0 and the dominant population is relatively metal-rich with [Fe/H] -0.25. The alpha/Fe ratios are slightly subsolar, even at the lowest observed metallicities suggesting a slow or bursting star formation rate. From our sample of 12 giants (including the two observed by Bonifacio et al 2000) we conclude that a substantial metal rich population exists in Sgr, which dominates the sample. The spectroscopic metallicities allow one to break the age-metallicity degeneracy in the interpretation of the colour-magnitude diagram (CMD). Comparison of isochrones of appropriate metallicity with the observed CMD suggests an age of 1 Gyr or younger, for the dominant Sgr population sampled by us. We argue that the observations support a star formation that is triggered by the passage of Sgr through the Galactic disc, both in Sgr and in the disc. This scenario has also the virtue of explaining the mysterious ``bulge C stars'' as disc stars formed in this event. The interaction of Sgr with the Milky Way is likely to have played a major role in its evolution.

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arXiv2003

A common trend in the chemical evolution of Local Group dwarf spheroidals and Damped Ly-alpha systems

We compare chemical abundances of Local Group dwarf spheroidals, obtained from recent UVES/VLT observations, and of high redshift Damped Lyman alpha systems (DLAs), corrected for dust effects. We focus, in particular, on the abundance ratio between alpha-capture elements and iron, alpha/Fe, a well known indicator of chemical evolution. Comparison of the data in the plane alpha/Fe versus Fe/H shows a remarkable similarity between the dwarf spheroidals and the DLAs, suggestive of a common trend in their chemical evolution. At any given metallicity these two distinct types of astronomical targets show alpha/Fe ratios systematically lower than those of Milky Way stars. In terms of chemical evolution models, this suggests that, on average, dSph galaxies and DLA systems are characterized by lower, or more episodic, star formation rates than the Milky Way.

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arXiv2003

Chemical abundances in Terzan 7 from UVES spectra

We present chemical abundances for Mg, Si, Ca and Fe for three red giants in the sparse globular cluster Terzan 7, physically associated to the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), which is presently being tidally disrupted by the Milky Way. The data has been obtained with VLT-UVES and show a mean [Fe/H]=-0.57, with solar alpha content, mean [alpha/Fe]=-0.03. This enforces Ter 7's membership to the Sgr dSph system, which shows a similar pattern of abundance.

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arXiv2003

Sgr dSph: a bridge between Dwarf Galaxies and DLAs?

We present abundances for 12 giants in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) obtained from VLT - UVES spectra. Moving on a short period, polar orbit around the Milky Way, the Sgr dSph is undergoing tidal disruption and will eventually dissolve in the Halo.Our sample is dominated by a metal rich, alpha-element-poor population, indicative of long chemical processing of the dSph gas during a slow, probably bursting star formation history. This population is the most metal-rich ever observed in a Dwarf Galaxy of the Local Group, and has the lowest alpha-element content. Placing the known abundances of the LG dwarfs on the [alpha/Fe] vs. [Fe/H] plane allows now to recognize a well defined evolutive sequence, different from the one followed by the MW disk stars, but apparently superimposed to the one followed by many of the Damped Lyman Alpha systems.

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