Volume 573, January 2015
|Number of page(s)||16|
|Section||Galactic structure, stellar clusters and populations|
|Published online||19 December 2014|
Observing multiple stellar populations with VLT/FORS2
Dipartimento di Fisica e Astronomia “Galileo Galilei”Università di
Vicolo dell’Osservatorio 3,
e-mail: email@example.com, firstname.lastname@example.org
2 Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, 2611 Weston, ACT, Australia
e-mail: email@example.com; firstname.lastname@example.org
3 INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
4 Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
5 INAF – Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy
Received: 2 May 2014
Accepted: 23 October 2014
Aims. We present the photometric analysis of the external regions of three Galactic globular clusters: NGC 6121, NGC 6397, and NGC 6752. The main goal is the characterization of multiple stellar populations along the main sequence (MS) and the study of the radial trend of different populations hosted by the target clusters.
Methods. The data have been collected using the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) mounted at the Very Large Telescope (VLT) UT1 of the European Southern Observatory (ESO) in UBVI filters. From these data sets we extracted high-accuracy photometry and constructed color–magnitude diagrams (CMDs). We exploit appropriate combinations of colors and magnitudes, which are powerful tools for identifying multiple stellar populations, such as B versus U − B and V versus cU,B,I = (U − B) − (B − I) CMDs.
Results. We confirm previous findings of a split MS in NGC 6752 and NGC 6121. Apart from the extreme case of ω Centauri, this is the first detection of multiple MS from ground-based photometry. For NGC 6752 and NGC 6121, we compare the number ratio of the blue MS to the red MS in the cluster outskirts with the fraction of first and second generation stars measured in the central regions. There is no evidence for significant radial trend. The MS of NGC 6397 is consistent with a simple stellar population. We propose that the lack of multiple sequences is due both to observational errors and to the limited sensitivity of U,B,V,I photometry to multiple stellar populations in metal-poor GCs. Finally, we compute the helium abundance for the stellar populations hosted by NGC 6121 and NGC 6752, finding a mild (ΔY ~ 0.02) difference between stars in the two sequences.
Key words: globular clusters: individual: NGC 6752 / globular clusters: general / globular clusters: individual: NGC 6397 / globular clusters: individual: M 4
Based on observations at the European Southern Observatory using the Very Large Telescope on Cerro Paranal through ESO programme 089.D-0978 (P. I. G. Piotto).
Catalogs of photometry for all globular clusters presented are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/573/A70
© ESO, 2014
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