* Astronomy

Blobrana · L

Caldwell 73

Blobrana · L

RE: NGC 1851

Blobrana · L

Title: The Horizontal Branch of NGC 1851: Constraints from its RR Lyrae Variables
Authors: Andrea Kunder, Maurizio Salaris, Santi Cassisi, Roberto de Propris, Alistair Walker, Peter B. Stetson, Marcio Catelan, Pia Amigo

We use the pulsational properties of the RR Lyrae variables in the globular cluster NGC 1851 to obtain detailed constraints of the various sub-stellar populations present along its horizontal branch. On the basis of detailed synthetic horizontal branch modelling, we find that minor helium variations (Y~0.248-0.280) are able to reproduce the observed periods and amplitudes of the RR Lyrae variables, as well as the frequency of fundamental and first-overtone RR Lyrae stars. Comparison of number ratios amongst the blue and red horizontal branch components and the two observed subgiant branches also suggest that the RR Lyrae variables originated from the progeny of the bright subgiant branch. The RR Lyrae variables with a slightly enhanced helium (Y~0.270-0.280) have longer periods at a given amplitude, as is seen with Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with Y~0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI) variables. This correlation does suggest that the pulsational properties of RR Lyrae stars can be very useful for tracing the various sub-populations and can provide suitable constraints on the multiple population phenomenon. It appears of great interest to explore whether this conclusion can be generalised to other globular clusters hosting multiple populations.

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Title: Spectroscopic hint of a cold stream in the direction of the globular cluster NGC 1851
Authors: A. Sollima, R. G. Gratton, J. A. Carballo-Bello, D. Martinez-Delgado, E. Carretta, A. Bragaglia, S. Lucatello, J. Penarrubia

We present the results of a spectroscopic survey performed in the outskirts of the globular cluster NGC1851 with VIMOS@VLT. The radial velocities of 107 stars in a region between 12' and 33' around the cluster have been derived. We clearly identify the cluster stellar population over the entire field of view, indicating the presence of a significant fraction of stars outside the tidal radius predicted by King models. We also find tentative evidence of a cold (sigma_v< 20 km/s) peak in the distribution of velocities at v_r~180 km/s constituted mainly by Main Sequence stars whose location in the colour-magnitude diagram is compatible with a stream at a similar distance of this cluster. If confirmed, this evidence would strongly support the extra-Galactic origin of this feature.

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Title: Spectroscopic analysis of the two subgiant branches of the globular cluster NGC1851
Authors: R. G. Gratton, S. Villanova, S. Lucatello, A. Sollima, D. Geisler, E. Carretta, S. Cassisi, A. Bragaglia

NGC1851 possibly shows a spread in [Fe/H], but the relation between this spread and the division in the SGB is unknown. We obtained blue (3950-4600 A) intermediate resolution (R~8,000) spectra for 47 stars on the bright and 30 on the faint SGB of NGC 1851 (b-SGB and f-SGB, respectively). The determination of the atmospheric parameters to extremely high internal accuracy leads to small errors when comparing different stars in the cluster. We found that the b-SGB is slightly more metal-poor than the f-SGB, with [Fe/H]=-1.227±0.009 and [Fe/H]=-1.162± 0.012, respectively. This implies that the f-SGB is only slightly older by ~0.6 Gyr than the b-SGB if the total CNO abundance is constant. There are more C-normal stars in the b-SGB than in the f-SGB. This is consistent with what is found for HB stars, if b-SGB are the progenitors of red HB stars, and f-SGB those of blue HB ones. The abundances of the n-capture elements Sr and Ba have a bimodal distribution, reflecting the separation between f-SGB (Sr and Ba-rich) and b-SGB stars (Sr and Ba-poor). In both groups, there is a clear correlation between [Sr/Fe] and [Ba/Fe], suggesting that there is a real spread in the abundances of n-capture elements. There is some correlation between C and Ba abundances, while the same correlation for Sr is much more dubious. We identified six C-rich stars, which have a moderate overabundance of Sr and Ba and rather low N abundances. This group of stars might be the progenitors of these on the anomalous RGB in the (v, v-y) diagram. These results are discussed within different scenarios for the formation of NGC1851. It is possible that the two populations originated in different regions of an inhomogeneous parent object. However, the striking similarity with M22 calls for a similar evolution for these two clusters. Deriving reliable CNO abundances for the two sequences would be crucial.

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