The M 4 Core Project with HST – II. Multiple stellar populations at the bottom of the main sequence
- A. P. Milone1,★,
- A. F. Marino1,
- L. R. Bedin2,
- G. Piotto2,3,
- S. Cassisi4,
- A. Dieball5,
- J. Anderson6,
- H. Jerjen1,
- M. Asplund1,
- A. Bellini6,
- K. Brogaard7,8,
- A. Dotter1,
- M. Giersz9,
- D. C. Heggie10,
- C. Knigge5,
- R. M. Rich11,
- M. van den Berg12,13 and
- R. Buonanno4
- 1Research School of Astronomy & Astrophysics, Australian National University, Mt Stromlo Observatory, via Cotter Rd, Weston, ACT 2611, Australia
- 2Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, Padova, I-35122 Padova, Italy
- 3Dipartimento di Fisica e Astronomia ‘Galileo Galilei’, Univ. di Padova, Vicolo dell'Osservatorio 3, Padova, I-35122 Padova, Italy
- 4INAF-Osservatorio Astronomico di Collurania, via Mentore Maggini, I-64100 Teramo, Italy
- 5Department of Physics and Astronomy, University Southampton, Southampton SO17 1BJ, UK
- 6Space Telescope Science Institute, 3800 San Martin Drive, Baltimore, MD 21218, USA
- 7Department of Physics and Astronomy, Aarhus University, Ny Munkegade, DK-8000 Aarhus C, Denmark
- 8Department of Physics and Astronomy, University of Victoria, PO Box 3055, Victoria, B.C., V8W 3P6, Canada
- 9Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, PL-00-716, Warsaw, Poland
- 10School of Mathematics and Maxwell Institute for Mathematical Sciences, University of Edinburgh, Kings Buildings, Edinburgh EH9-3JZ, UK
- 11Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
- 12Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, the Netherlands
- 13Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, 02138 MA, USA
- ↵★E-mail: milone{at}mso.anu.edu.au
- Accepted 2014 January 6.
- Received 2013 December 29.
- In original form 2013 November 21.
- First published online February 17, 2014.
Abstract
The M 4 Core Project with HST is designed to exploit the Hubble Space Telescope (HST) to investigate the central regions of M 4, the Globular Cluster closest to the Sun. In this paper we combine optical and near-infrared photometry to study multiple stellar populations in M 4. We detected two sequences of M-dwarfs containing ∼38 per cent (MSI) and ∼62 per cent (MSII) of MS stars below the main-sequence (MS) knee. We compare our observations with those of NGC 2808, which is the only other GCs where multiple MSs of very low-mass stars have been studied to date. We calculate synthetic spectra for M-dwarfs, assuming the chemical composition mixture inferred from spectroscopic studies of stellar populations along the red giant branch, and different helium abundances, and we compare predicted and observed colours. Observations are consistent with two populations, one with primordial abundance and another with enhanced nitrogen and depleted oxygen.
Key words
- © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
