The M4 Core Project with HST is designed to exploit the Hubble Space Telescope to investigate the central regions of M4, the Globular Cluster closest to the Sun. In this paper we combine optical and near-infrared photometry to study multiple stellar populations in M4. We detected two sequences of M-dwarfs containing ~38% (MS_I) and ~62% (MS_II) of MS stars below the main-sequence (MS) knee. We compare our observations with those of NGC2808, 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 colors. Observations are consistent with two populations, one with primordial abundance and another with enhanced nitrogen and depleted oxygen.
@article{arxiv.1401.1091,
title = {The M4 Core Project with HST -- II. Multiple Stellar Populations at the Bottom of the Main Sequence},
author = {A. P. Milone and A. F. Marino and L. R. Bedin and G. Piotto and S. Cassisi and A. Dieball and J. Anderson and H. Jerjen and M. Asplund and A. Bellini and K. Brogaard and A. Dotter and M. Giersz and D. C. Heggie and C. Knigge and R. M. Rich and M. van den Berg and R. Buonanno},
journal= {arXiv preprint arXiv:1401.1091},
year = {2015}
}
Comments
8 pages, 6 figure, accepted for publication in MNRAS