We use a combination of data acquired with the Advanced Camera for Survey on board the Hubble Space Telescope and the Large Binocular Camera (LBC-blue) mounted on the Large Binocular Telescope to ...sample the main sequence (MS) stars of the globular cluster (GC) NGC 5466 in the mass range 0.3 < M/M sub(middot in circle) < 0.8. We derive the cluster's Luminosity Function (LF) in several radial regions, from the center of the cluster out to the tidal radius. After corrections for incompleteness and field contamination, this was compared to theoretical LFs, obtained by multiplying a simple power-law mass function in the form dN/dm proportional, variant m super(alpha) by the derivative of the mass- luminosity relationship of the best-fit isochrone. We find that alpha varies from -0.6 in the core region to -1.9 in the outer region. This fact allows us to prove by observation that the stars in NGC 5466 have experienced the effects of mass segregation. We compare the radial variation of alpha from the center out to 5 core radii (r sub(c)) in NGC 5466 and the GC M10, finding that the gradient of alpha in the first 5r sub(c) is more than a factor of 2 shallower in NGC 5466 than in M10, in line with the differences in the clusters' relaxation timescales. NGC 5466 is dynamically younger than M10, with two-body relaxation processes only recently starting to shape the distribution of MS stars. This result fully agrees with the conclusion obtained in our previous works on the radial distribution of blue straggler stars, further confirming that this can be used as an efficient clock to measure the dynamical age of stellar systems.
Two independent studies recently uncovered two distinct populations among giants in the distant, massive globular cluster (GC) NGC 2419. One of these populations has normal magnesium (Mg) and ...potassium (K) abundances for halo stars: enhanced Mg and roughly solar K. The other population has extremely depleted Mg and very enhanced K. To better anchor the peculiar NGC 2419 chemical composition, we have investigated the behavior of K in a few red giant branch stars in NGC 6752, NGC 6121, NGC 1904, and omega Cen. To verify that the high K abundances are intrinsic and not due to some atmospheric features in giants, we also derived K abundances in less evolved turn-off and subgiant stars of clusters 47 Tuc, NGC 6752, NGC 6397, and NGC 7099. We normalized the K abundance as a function of the cluster metallicity using 21 field stars analyzed in a homogeneous manner. For all GCs of our sample, the stars lie in the K-Mg abundance plane on the same locus occupied by the Mg-normal population in NGC 2419 and by field stars. This holds for both giants and less-evolved stars. At present, NGC 2419 seems unique among GCs.
Abstract
We have derived from VIMOS spectroscopy the radial velocities for a sample of 71 stars selected from CFHT/Megacam photometry around the Galactic globular cluster NGC 7492. In the resulting ...velocity distribution, it is possible to distinguish two relevant non-Galactic kinematic components along the same line of sight: a group of stars at 〈vr〉 ∼ 125 km s−1 which is compatible with the velocity of the old leading arm of the Sagittarius tidal stream, and a larger number of objects at 〈vr〉 ∼ −110 km s−1 that might be identified as members of the trailing wrap of the same stream. The systemic velocity of NGC 7492 set at vr ∼ −177 km s−1 differs significantly from that of both components, thus our results confirm that this cluster is not one of the globular clusters deposited by the Sagittarius dwarf spheroidal in the Galactic halo, even if it is immersed in the stream. A group of stars with 〈vr〉 ∼ − 180 km s−1 might be comprised of cluster members along one of the tidal tails of NGC 7492.
We obtained FLAMES GIRAFFE+UVES spectra for both first- and second-generation red giant branch (RGB) stars in the globular cluster (GC) NGC 362 and used them to derive abundances of 21 atomic species ...for a sample of 92 stars. The surveyed elements include proton-capture (O, Na, Mg, Al, Si), α-capture (Ca, Ti), Fe-peak (Sc, V, Mn, Co, Ni, Cu), and neutron-capture elements (Y, Zr, Ba, La, Ce, Nd, Eu, Dy). The analysis is fully consistent with that presented for twenty GCs in previous papers of this series. Stars in NGC 362 seem to be clustered into two discrete groups along the Na-O anti-correlation with a gap at O/Na ~ 0 dex. Na-rich, second generation stars show a trend to be more centrally concentrated, although the level of confidence is not very high. When compared to the classical second-parameter twin NGC 288 with similar metallicity but different horizontal branch type and a much lower total mass, the proton-capture processing in stars of NGC 362 seems to be more extreme, confirming previous analysis. We discovered the presence of a secondary RGB sequence, which is redder than the bulk of the RGB. A preliminary estimate shows that this sequence comprises about 6% of RGB stars. Our spectroscopic data and literature photometry indicate that this sequence is populated almost exclusively by giants rich in Ba and is probably rich in all s-process elements, as found in other clusters. In this regard, NGC 362 joins previously studied GCs like NGC 1851, NGC 6656 (M 22), and NGC 7089 (M 2).
We used FLAMES+GIRAFFE (Medusa mode) at the VLT to obtain moderately high resolution spectra for 30 red horizontal branch (RHB) stars, 4 RR Lyrae variables, and 17 blue horizontal branch (BHB) stars ...in the low-concentration, moderately metal-rich globular cluster NGC 6723. The spectra were optimized to derive O and Na abundances. In addition, we obtained abundances for other elements, including N, Fe, Mg, Ca, Ni, and Ba. We used these data to discuss the evidence of a connection between the distribution of stars along the horizontal branch (HB) and the multiple populations that are typically present in globular clusters. Since in many other clusters this fraction is about two thirds, we conclude that the fraction of first/second generation in globular clusters may be strongly variable. The reason for this spread is yet to be understood. Finally, we found a high Ba abundance, with a statistically significant radial abundance gradient.
Information on globular clusters (GC) formation mechanisms can be gathered by studying the chemical signature of the multiple populations that compose these stellar systems. In particular, we ...investigate the anti-correlations among O, Na, Al, and Mg to explore the influence of cluster mass and environment on GCs in the Milky Way and in extragalactic systems. We present here the results obtained on NGC 6139, which, on the basis of its horizontal branch morphology, has been proposed to be dominated by first-generation stars. In our extensive study based on high-resolution spectroscopy, the first for this cluster, we found a metallicity of Fe/H = −1.579 ± 0.015 ± 0.058 (rms = 0.040 dex, 45 bona fide member stars) on the UVES scale defined by our group. The stars in NGC 6139 show a chemical pattern normal for GCs, with a rather extended Na-O (and Mg-Al) anti correlation. NGC 6139 behaves as expected from its mass and contains a large portion (about two thirds) of second-generation stars.
We report the detection of a pair of degree-long tidal tails associated with the globular cluster Palomar 14, using images obtained at the Canada-France-Hawaii Telescope. We reveal a power-law ...departure from a King profile at large distances to the cluster center. The density map constructed with the optimal matched filter technique shows a nearly symmetrical and elongated distribution of stars on both sides of the cluster, forming an S-shape characteristic of mass loss. This evidence may be the telltale signature of tidal stripping in action. This, together with its large Galactocentric distance, imposes strong constraints on its orbit and/or origin: (1) it must follow an external orbit confined to the peripheral region of the Galactic halo and/or (2) it formed in a satellite galaxy later accreted by the Milky Way.
We used deep observations collected with Advanced Camera for Surveys (ACS) at Hubble Space Telescope (HST) to derive the fraction of binary systems in a sample of 13 low-density Galactic globular ...clusters. By analysing the colour distribution of main-sequence stars we derived the minimum fraction of binary systems required to reproduce the observed colour–magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6 per cent within the core radius. The estimated global fractions of binary systems range from 10 to 50 per cent depending on the cluster. A dependence of the relative fraction of binary systems on the cluster age has been detected, suggesting that the binary disruption process within the cluster core is active and can significantly reduce the binary content in time.
To check the Impact of the multiple population scenario for globular clusters on their horizontal branch (HB) we present an analysis of the composition of 110 red HB (RHB) stars in 47 Tucanae and of ...61 blue HB (BHB) and 30 RHB stars in M 5. In 47 Tue we found tight relations between the colours of the stars and their abundances of p-capture elements. This strongly supports the idea that the He content - which is expected to be closely correlated with the abundances of p-capture elements - is the third parameter (after overall metallicity and age) that determines the colour of HB stars. Finally, we found a C-star on the HB of 47 Tue and a Ba-rich, fast-rotating, likely binary star on the HB of MS. These stars are among the brightest and coolest HB stars.
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