It has been suspected already more than thirty year ago that NGC 1851 is not chemically homogeneous. By now, it is clear that NGC 1851 has two distinct subgiant branches, however explanations of their origin so far lack consensus. Some authors explain them by two generations of stars, the first being primordial, while the second one being born from the ejecta of a fraction of the stars of the earlier one. There are suggestions that NGC 1851 might be the result of a merger of two globular clusters, or originated as the nucleus of a dwarf galaxy that was captured by the Milky Way. In this presentation, we provide a crucial missing information on CNO abundances determined in a homogeneous way for a large sample of NGC 1851 giants. High-resolution spectra were obtained with the VLT UVES spectrograph in a framework of the Gaia-ESO Survey. From the investigated stars we clearly see that the two NGC 1851 populations have by about 0.2 dex difference in metallicity, the sums of C+O+N differ by about 0.1 dex, the mean C/N ratio values differ by about 0.5 dex, chemical elements that are insensitive to internal stellar mixing show normal Galactic abundances of the corresponding metallicities. This leads us to the conclusion that NGC 1851 is a binary cluster. We provide interpretations of these and other results of the study.
The Gaia-ESO Spectroscopic Survey: CNO abundances in giants of the multiple-population globular cluster NGC 1851
Lardo, CarmelaMembro del Collaboration Group
;
2015
Abstract
It has been suspected already more than thirty year ago that NGC 1851 is not chemically homogeneous. By now, it is clear that NGC 1851 has two distinct subgiant branches, however explanations of their origin so far lack consensus. Some authors explain them by two generations of stars, the first being primordial, while the second one being born from the ejecta of a fraction of the stars of the earlier one. There are suggestions that NGC 1851 might be the result of a merger of two globular clusters, or originated as the nucleus of a dwarf galaxy that was captured by the Milky Way. In this presentation, we provide a crucial missing information on CNO abundances determined in a homogeneous way for a large sample of NGC 1851 giants. High-resolution spectra were obtained with the VLT UVES spectrograph in a framework of the Gaia-ESO Survey. From the investigated stars we clearly see that the two NGC 1851 populations have by about 0.2 dex difference in metallicity, the sums of C+O+N differ by about 0.1 dex, the mean C/N ratio values differ by about 0.5 dex, chemical elements that are insensitive to internal stellar mixing show normal Galactic abundances of the corresponding metallicities. This leads us to the conclusion that NGC 1851 is a binary cluster. We provide interpretations of these and other results of the study.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
