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Description
The origin of the globular cluster (GC) NGC3201 is under debate. Its retrograde orbit points to an extragalactic origin, but no further chemical evidence supports this idea. Light-element chemical abundances are useful to tag GCs and can be used to shed light in this discussion. Recently it was shown that the CN and CH indices are useful to identify anomalous GCs out of typical Milky Way GCs. A possible origin of anomalous clusters is the merger of two GCs and/or nucleus of a dwarf galaxy. We aim at deriving CN and CH band strengths for red giant stars in NGC3201 and compare with photometric indices and high-resolution spectroscopy and discuss in the context of GC chemical tagging. We measure molecular band indices of S(3839) and G4300 for CN and CH, respectively from low-resolution spectra of red giant stars. Gravity and temperature effects are removed. Photometric indices are used to indicate further chemical information on C+N+O or s-process element abundances, not derived from low-resolution spectra. We found three groups on the CN-CH distribution. A main sequence (S1), a secondary less-populated sequence (S2), and a group of peculiar (pec) CN-weak and CH-weak stars, one of which was previously known. The three groups seem to have different C+N+O and/or s-process element abundances, to be confirmed by high-resolution spectroscopy. These are typical characteristics of anomalous GCs. The CN distribution of NGC3201 is quadrimodal, which is more common in anomalous clusters. However, NGC3201 does not belong to the trend of anomalous GCs in the mass-size relation. NGC3201 shows signs that it can be chemically tagged as anomalous: unusual CN-CH relation, indications that pec-S1-S2 is an increasing sequence of C+N+O or s-process element abundances, and a multimodal CN distribution that seems to correlate with s-process element abundances. The differences are: it has a debatable Fe-spread and it does not follow the trend of mass-size of all anomalous clusters. Three scenarios are postulated here: (i) if the sequence pec-S1-S2 has increasing C+N+O and s-process element abundances, NGC3201 would be the first anomalous GC out of the mass-size relation; (ii) if the abundances are almost constant, NGC3201 would be the first non-anomalous GC with multiple CN-CH anti-correlation groups, or (iii) it would be the first anomalous GC without variations in C+N+O and s-process element abundances. In all cases, the definition of anomalous clusters and the scenario where they have an extragalactic origin must be revised.
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