Spectroscopic and photometric evidence of two stellar populations in the Galactic globular cluster NGC 6121 (M 4)

By A. Marino, S. Villanova, G. Piotto, A. Milone, Y. Momany, L. Bedin, A. Medling. 2008

301 citations recorded by the source corpus.

Aims. We present abundance analysis based on high resolution spectra of 105 isolated red giant branch (RGB) stars in the Galactic Globular Cluster NGC 6121 (M 4). Our aim is to study its star population in the context of the multi-population phenomenon recently discovered to affect some Globular Clusters. Methods. The data have been collected with FLAMES+UVES, the multi-fiber high resolution facility at the ESO/VLT@UT2 telescope. Analysis was performed under LTE approximation for the following elements: O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Ba, and NLTE corrections were applied to those (Na, Mg) strongly affected by departure from LTE. Spectroscopic data were coupled with high-precision wide-field UBVIC photometry from WFI@2.2 m telescope and infrared JHK photometry from 2MASS. Results. We derived an average [Fe/H] = −1.07 ± 0.01 (internal error), and an α enhancement of [α/Fe] =+ 0.39 ± 0.05 dex (internal error). We confirm the presence of an extended Na-O anticorrelation, and find two distinct groups of stars with significantly different Na and O content. We find no evidence of a Mg-Al anticorrelation. By coupling our results with previous studies on the CN band strength, we find that the CN strong stars have higher Na and Al content and are more O depleted than the CN weak ones. The two groups of Na-rich, CN-strong and Na-poor, CN-weak stars populate two different regions along the RGB. The Na-rich group defines a narrow sequence on the red side of the RGB, while the Na-poor sample populate a bluer, more spread portion of the RGB. In the U vs. U − B color magnitude diagram the RGB spread is present from the base of the RGB to the RGB-tip. Apparently, both spectroscopic and photometric results imply the presence of two stellar populations in M 4. We briefly discuss the possible origin of these populations.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/1876cddf0e06c5f1fea00dba22aad09253a6bc2e

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In Astronomy and Astrophysics

1. Jay Anderson
2. R. Gratton
3. E. Carretta
4. A. Bragaglia
5. G. Piotto
6. A. Milone
7. L. Bedin
8. F. D’Antona

In Instrumentation

1. R. Gratton
2. E. Carretta
3. Jay Anderson
4. A. Bragaglia
5. F. D’Antona
6. G. Piotto
7. A. Milone
8. L. Bedin

Citations in the corpus

• Na-O Anticorrelation and HB. VII. The chemical composition of first and second-generation stars in 15 globular clusters from GIRAFFE spectra (2009, methodology): “The recent paper by Marino et al. (2008) found that the dichotomy in chemistry (mainly in O, Na and N content) between two generic populations in NGC 6121 was also visible as a different photometric location along the RGB of the two groups.”
• A WFC3/HST VIEW OF THE THREE STELLAR POPULATIONS IN THE GLOBULAR CLUSTER NGC 6752 (2013, background): “3Note however that, in some cases, multimodal distribution in Na and O have been detected also from high-resolution spectroscopic (see e.g. Yong et al. 2008, Marino et al. 2008, Lind et al. 2011 for the cases of NGC6752, M4, and NGC6397).”
• Photometric signatures of multiple stellar populations in galactic globular clusters (2011, background): “…Piotto et al. 2007), NGC 1851 (double SGB and RGB Milone et al. 2008; Han et al. 2009), M4, NGC 3201 and NGC 1261 (two red giant branch populations; Marino et al. 2008; Kravtsov et al. 2010a,b), as well as in NGC 6752 and 47 Tuc (clear spread of the MS, also RGB spread in NGC 6752; Anderson et…”
• THE ACS SURVEY OF GALACTIC GLOBULAR CLUSTERS. IX. HORIZONTAL BRANCH MORPHOLOGY AND THE SECOND PARAMETER PHENOMENON (2009, background): “Marino et al. (2008) presented spectroscopic evidence for a bimodal abundance distribution along the RGB of M 4 as well as a broadening of the RGB in U − B.”
• Properties of stellar generations in globular clusters and relations with global parameters (2010, methodology): “We employed good quality CMDs (Bellazzini et al. 2001 for NGC 288; Marino et al. 2008 for NGC 6121; Rosenberg et al. 1999 for NGC 6254; Momany et al. 2003 for NGC 6752; Vargas Alvarez & Sandquist 2007 for NGC 6809) with the same HB models (Cassisi et al. 1999) as used by those authors.”
• Multiple populations in globular clusters (2012, methodology): “A few sparse studies, e.g. Marino et al. (2008) for M 4, seem to indicate a separation between the two stellar generations, but the road is still open to many interpretations.”
• The second and third parameters of the horizontal branch in globular clusters (2010, methodology): “…this data set with a few GCs from the literature (Shetrone & Keane 2000 for NGC 362; Sneden et al. 2004 and Cohen & Melendez 2005 for NGC 5272 and NGC 6205; Sneden et al. 1991, 2000 for NGC 6341; Marino et al. 2008 for NGC 6121; Marino et al. 2009 for NGC 6656; and Yong et al. 2005 for NGC 6752).”
• The SUMO project: a SUrvey of Multiple pOpulations in globular clusters (2013, methodology): “In case of NGC 6121, stars with available spectroscopic measurements of Na and O from Marino et al. (2008) and Carretta et al. (2009) are plotted as circles and triangles respectively, while the Na-O nticorrelation determined by these authors is shown in the two insets.”
• The Hubble Space TelescopeUV Legacy Survey of Galactic Globular Clusters – V. Constraints on formation scenarios (2015, background): “(For a previous example of spectroscopic discreteness see Marino et al. 2008) Discreteness is quite powerful at discriminating among competing scenarios and therefore we expand on it further in Appendix A. • Supernova avoidance.”
• MULTIPLE STELLAR POPULATIONS IN 47 Tucanae (2011, methodology): “Other examples are given by Marino et al. (2008), who showed that the presence of two groups of stars with different C, N, Na, and O content is at the root of the difference in the (U − B) color of the RGB stars in NGC 6121 (M4).”
• CHEMICAL ABUNDANCES FOR 855 GIANTS IN THE GLOBULAR CLUSTER OMEGA CENTAURI (NGC 5139) (2010, background): “…have revealed that several of the most massive known globular clusters contain multiple main sequence, subgiant, and/or red giant branch (RGB) populations (Piotto et al. 2007; Marino et al. 2008; Milone et al. 2008; Anderson et al. 2009; Moretti et al. 2009; Piotto 2009; Milone et al. 2010).”
• The two metallicity groups of the globular cluster M 22: a chemical perspective (2011, methodology): “15 23 giant branches (e.g. Marino et al. 2008, Lee et al. 2009, Lind et al. 2011a).”
• A double stellar generation in the globular cluster NGC 6656 (M 22): Two stellar groups with different iron and s-process element abundances (2009, methodology): “A rough estimate of errors in the atmospheric parameters can be made as in Marino et al. (2008), assuming that stars of the same V magnitude (corrected for differential reddening) have the same parameters.”
• Abundance patterns of multiple populations in globular clusters: a chemical evolution model based on yields from AGB ejecta (2010, background): “Their results, however, are in contrast with the abundance patterns in M4 (NGC 6121) in which the maximum [Al/Fe] is ∼0.8, and the minimum is [Al/Fe]∼ 0.6 (Ivans et al. 1999; Marino et al. 2008a), so that this cluster does not show great Al variations, if any.”
• THE ARAUCARIA PROJECT. THE DISTANCE TO THE SMALL MAGELLANIC CLOUD FROM LATE-TYPE ECLIPSING BINARIES (2013, unknown): “See Marino et al. (2008) for a more detailed explanation of the method we used to measure the EWs and Villanova et al. (2010) for a description of the line list that was used.”
• The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – IX. The Atlas of multiple stellar populations (2016, background): “From Marino et al. (2008),chemical analysis is available for eleven stars in common with our WFC3/UVIS sample of RGB stars.”
• Multiple stellar populations in Magellanic Cloud clusters - I. An ordinary feature for intermediate age globulars in the LMC? (2008, background): “In a spectro-photometric study of M4, Marino et al. (2008) detect a bimodal RGBs, and demostrate that it is due to a bimodal distribution of CN, Na, O, also indicating that this relatively-small cluster contains multiple populations.”
• THE HUBBLE SPACE TELESCOPE UV LEGACY SURVEY OF GALACTIC GLOBULAR CLUSTERS. I. OVERVIEW OF THE PROJECT AND DETECTION OF MULTIPLE STELLAR POPULATIONS (2014, background): “…that can be ascribed to the proto-galactic interstellar matter out of which theyformed, the second-generation (2G) stars can be depleted in C and O, enhanced in N and Na (e.g., Carretta et al. 2009, Marino et al. 2008, 2011a 2011b, 2012 2014), and enhanced in He (Milone et al. 2012a, 2012b, 2014a).”