C. Cacciari

Measuring helium abundance difference in giants of NGC 2808

Context. Multiple populations have been detected in several globular clusters (GC) that do not display a spread in metallicity. Unusual features of their observed colour–magnitude diagrams (CMD) can be interpreted in terms of differences in the helium content of the stars belonging to the sub-populations. Aims. Even if evidence gathered so far is compelling, differences in He abundance have never been directly observed. We attempt to measure these differences in two giant stars of NGC 2808 with very similar astrophysical parameters but different Na and O abundances, hence that presumably belong to different sub-populations, by directly comparing their He I 10 830 A lines. Methods. The He 10 830 A line forms in the upper chromosphere. Our detailed models derive the chromospheric structure using the Ca II and Hα chromospheric lines, and simulate the corresponding He I 10 830 line profiles as a function of the helium abundance. We show that, at a given value of He abundance, the He I 10 830 equivalent width cannot significantly change without a corresponding much larger change in the Ca II chromospheric lines. We have used the VLT-CRIRES to obtain high-resolution spectra in the 10 830 A region, and the VLT-UVES to obtain spectra of the Ca II and Hα lines of our target stars. Results. The two target stars have very similar Ca II and Hα chromospheric lines, but different appearances in the He region. One line, blueshifted by 17 km s −1 with respect to the He 10 830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas the Na-poor star spectrum is consistent with a non-detection. From a detailed chromospheric modeling, we show that the difference in the spectra is consistent and most closely explained by an He abundance difference between the two stars of ΔY ≥ 0.17. Our optical observations bracket the infrared ones over a range of about 50 days and we do not observe any substantial variability in the Ca II and Hα lines. Conclusions. We provide direct evidence of a significant He line strength difference in giant stars of NGC 2808 belonging to different sub-populations, which had been previously detected by other photometric and spectroscopic means. The use of appropriate model chromospheres allows us for the first time to provide an approximate quantitative estimate of this difference, which is clearly consistent with the expected difference in abundance required by the stellar evolution theory to account for the observed peculiarities of this cluster’s cmD.

Blue Stragglers in the Galactic globular clusters M3: Evidence for two populations

A new VI-CCD survey of the central regions of the Galactic globular cluster M3 has been made in order to search for Blue Straggler Stars (BSS). About 9000 stars within r<250″ from the center have been measured down to the turnoff level, yielding about 70 new BSS candidates. The analysis of a complete sample of bright BSS (B<18.6), identified in this study and in previous surveys, clearly shows that the radial distribution of the BSS is nonmonotonic. There is a distinct dip in the distribution between 4r c (∼100″) and 8r c (∼200″). Taken at face value, this result is direct evidence that two distinct populations of BSS exist in M3 or, alternatively, that radially selective effects have been at work in the survival mechanisms or in the BSS dynamics within the cluster

The Young Globular Clusters of the Milky Way and the Local Group Galaxies: Playing with Great Circles

The small group of Galactic Globular Clusters (GGC) (Pal 12, Terzan 7, Ruprecht 106, Arp 2) recently discovered to be significantly younger (by ~3-4 Gyr) than the average cluster population of the Galaxy are shown to lie near planes passing in the vicinity of some satellite galaxies of the Milky Way and through the Galactic Centre itself. Assuming that these configurations represent a fossil record of interactions between the Galaxy and its companions from which these clusters originated, we identified, along one of them, another candidate ``young GGC, i.e. IC4499, whose Color-Magnitude Diagram is presented. Various hypotheses on the possible origin of ``young GGC are also briefly discussed within a framework where the location on preferential planes may be seen as a general characteristic for the Local Group members.

The Andromeda project I. Deep HST-WFPC2 V, I photometry of 16 fields toward the disk and the halo of the M 31 galaxy. Probing the stellar content and metallicity distribution

We have obtained HST-WFPC2xa0F555W andxa0F814W photometry for 16xa0fields in the vicinity of the luminous nearby spiral galaxy Mxa031, sampling the stellar content of the disk and the halo at different distances from the center, from ~20 to ~150xa0arcmin (i.e. ~4.5 to 35xa0kpc), down to limitingxa0 V and I xa0magnitudes ofxa0~27. u2029The Color–Magnitude Diagramsxa0(CMD) obtained for each field show the presence of complex stellar populations, including an intermediate age/young population and older populations with a wide range of metallicity. Those fields superposed on the disk of Mxa031 generally show a blue plume of stars which we identify with main sequence members. According to this interpretation, we find that the star formation rate over the last 0.5xa0Gyr has varied dramatically with location in the disk. u2029The most evident feature of all the CMDs is a prominent Red Giant Branchxa0(RGB) with a descending tip in the V xa0band, characteristic of metallicity higher thanxa01/10 Solar. A red clump is clearly detected in all of the fields, and a weak blue horizontal branch is frequently present. u2029The metallicity distributions, obtained by comparison of the RGBxa0stars with globular cluster templates, all show a long, albeit scantly populated, metal-poor tail and a main component peaking at

On the blue stragglers and horizontal branch morphology in Galactic globular clusters : some speculations and a new working scenario

rm [Fe/H] sim - ,0.6

The M(v)^HB Verses [FE/H] Calibration. I. HST Color-Magnitude Diagrams of Eight Globular Clusters in M31

. The most noteworthy characteristic of the abundance distributions is their overall similarity in all the sampled fields, covering a wide range of environments and galactocentric distances. Nevertheless, a few interesting differences and trends emerge from the general uniformity of the metallicity distributions. For example, the medianxa0[Fe/H] shows a slight decrease with distance along the minor axisxa0( Y ) up to

Hubble Space Telescope WFPC2 Color-Magnitude Diagrams for Globular Clusters in M31*

Ysimeq 20 arcmin

Modelling chromospheric line profiles in NGC 2808 : evidence of mass loss from RGB stars

, but the metallicity gradient completely disappears beyond this limit. Also, in some fields a very metal-rich (

Mass motions and chromospheres of RGB stars in the globular cluster NGC 2808

rm [Fe/H] ge -0.2

RR Lyrae Variables in the Globular Clusters of M31: A First Detection of Likely Candidates

) component is clearly present. u2029Whereas the fraction of metal-poor stars seems to be approximately constant (within few percent) in all fields, the fraction of metal-rich and, especially, very-metal-rich stars varies with position and seems to be more prominent in those fields superposed on the disk and/or with the presence of streams or substructures (e.g. Ibata et al. 2001). This might indicate and possibly trace interaction effects with some companion, e.g.xa0Mxa032.