Low-resolution spectroscopy of main sequence stars belonging to 12 Galactic globular clusters

Abstract

Context. Globular clusters show star-to-star abundance variations for light elements that are not yet well understood. The preferred explanation involves a self-enrichment scenario, within which two subsequent generations of stars co-exist in globular clusters. Observations of chemical abundances in the main sequence and sub-giant branch stars allow us to investigate the signature of this chemically processed material without the complicating effects caused by stellar evolution and internal mixing. Aims. Our main goal is to investigate the carbon-nitrogen anti-correlation with low-resolution spectroscopy of 20−50 stars fainter than the first dredge-up in seven Galactic globular clusters (NGC 288, NGC 1851, NGC 5927, NGC 6352, NGC 6388, and Pal 12) with different properties. We complemented our observations with 47 Tuc archival data, with four additional clusters from the literature (M 15, M 22, M 55, NGC 362), and with additional literature data on NGC 288. Methods. In this first paper, we measured the strengh of the CN and CH band indices, which correlate with the N and C abundances, and we investigated the anti-correlation and bimodality of these indices. We compared r CN , the ratio of stars belonging to the CN-strong and weak groups, with 15 different cluster parameters. Results. We clearly see bimodal anti-correlation of the CH and CN band stregths in the metal-rich clusters (Pal 12, 47 Tuc, NGC 6352, NGC 5927). Only M 15 among the metal-poor clusters shows a clearly bimodal anti-correlation. We found weak correlations (sligthly above 1σ) of r CN with the cluster orbital parameters, present-day total mass, cluster concentration, and age. Conclusions. Our findings support the self-enrichment scenario, and suggest that the occurrence of more than two major generations of stars in a GGC should be rare. Small additional generations (<10−20% of the total) would be difficult to detect with our samples. The first generation, which corresponds to the CN-weak stars, usually contains more stars than the second one ( r CN = 0.82 ± 0.29), as opposed to results based on the Na-O anti-correlations.

C and N abundances of main sequence and subgiant branch stars in NGC 1851

“…To these 46 spectra observed with Magellan, we added 47 other MS and SGB spectra from Pancino et al (2010) observed with the FORS2 multi-object spectrograph on the ESO VLT at the Paranal Observatory in Chile. We refer the reader to that paper for details of the FORS2 observations.…”

“…We refer the reader to that paper for details of the FORS2 observations. We reduced our data following the procedure described in Pancino et al (2010). For the data pre-reduction, we used the standard procedure for overscan correction and bias-subtraction with the routines available in the noao.imred.ccdred package in IRAF 2 .…”

“…While the detection of significant scatter in iron and/or n-capture elements associated to s-process is limited to a few clusters (e.g., Hesser et al 1982;Yong & Grundahl 2008;Marino et al 2009), star-to-star variations in abundances of the light elements (C, N, O, Na, Mg, Al), have been observed in stars of all evolutionary phases in the majority of GCs (e.g., Martell et al 2011;Pancino et al 2010;Kraft 1979;Ramírez & Cohen 2003;Kayser et al 2008;Carretta et al 2009). …”

“…While the above listed spectroscopic studies targeted evolved stars in NGC 1851, before this work, only Pancino et al (2010) analyzed a sample of unevolved stars in this clusters to provide index measurements, and we present for the first time an abundance analysis of MS, TO and SGB stars. In particular, we observed stars located on the faint-and bright-SGB and derived for them C and N abundances; aiming for the first time to provide insights on the chemical signature differences between the two discrete sequences on the SGB.…”

NGC 6362: The Least Massive Globular Cluster With Chemically Distinct Multiple Populations*

“…Star-to-star differences in the C and N abundances have been known for decades in GCs (see, e.g., Osborn 1971;Norris 1981;Martell & Smith 2009;Pancino et al 2010), with the detection of CN-weak and CN-strong stars. In recent years the use of high-resolution spectroscopy coupled with large samples of GC stars has established that old, massive GCs, both in our Galaxy (Carretta et al 2009; and in the Local Group satellites (Mucciarelli et al 2009), show star-to-star variations in some light elements: intrinsic scatters of Na and O abundances have been observed in all the GCs, variations of Al abundances in most of them, while star-to-star scatters in Mg abundances have been found only in some peculiar clusters.…”