THE UNIQUE Na:O ABUNDANCE DISTRIBUTION IN NGC 6791: THE FIRST OPEN(?) CLUSTER WITH MULTIPLE POPULATIONS
- Departamento de Astronomia, Universidad de Concepcion, Casilla 160-C (Chile)
- European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago 19 (Chile)
- Department of Astronomy, Indiana University, 727 East Third Street, Bloomington, IN 47405-7105 (United States)
- Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Los Angeles, CA 90095-1547 (United States)
- Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States)
Almost all globular clusters investigated exhibit a spread in their light element abundances, the most studied being an Na:O anticorrelation. In contrast, open clusters show a homogeneous composition and are still regarded as Simple Stellar Populations. The most probable reason for this difference is that globulars had an initial mass high enough to retain primordial gas and ejecta from the first stellar generation and thus formed a second generation with a distinct composition, an initial mass exceeding that of open clusters. NGC 6791 is a massive open cluster and warrants a detailed search for chemical inhomogeneities. We collected high-resolution, high signal-to-noise spectra of 21 members covering a wide range of evolutionary status and measured their Na, O, and Fe content. We found [Fe/H] = +0.42 {+-} 0.01, in good agreement with previous values, and no evidence for a spread. However, the Na:O distribution is completely unprecedented. It becomes the first open cluster to show intrinsic abundance variations that cannot be explained by mixing, and thus the first discovered to host multiple populations. It is also the first star cluster to exhibit two subpopulations in the Na:O diagram with one being chemically homogeneous while the second has an intrinsic spread that follows the anticorrelation so far displayed only by globular clusters. NGC 6791 is unique in many aspects, displaying certain characteristics typical of open clusters, others more reminiscent of globulars, and yet others, in particular its Na:O behavior investigated here, that are totally unprecedented. It clearly had a complex and fascinating history.
- OSTI ID:
- 22078547
- Journal Information:
- Astrophysical Journal Letters, Vol. 756, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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