The assessment of the near infrared identification of carbon stars^*,**

I. The Local Group galaxies WLM, IC 10 and NGC 6822

¹ INAF, Osservatorio Astronomico di Roma, Viale del Parco Mellini 84, 00136 Roma, Italy e-mail: battinel@oarhp1.rm.astro.it
² Département de Physique, Université de Montréal, C.P.6128, Succursale Centre-Ville, Montréal, Qc, H3C 3J7, Canada e-mail: demers@astro.umontreal.ca
³ INAF - Instituto di Radioastronomia, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: filippo@arcetri.astro.it

Received: 30 April 2007
Accepted: 18 July 2007

Abstract

Context.The selection of AGB C and M stars from NIR colours has been done in recent years using adjustable criteria that are in needs of standardization if one wants to compare, in a coherent manner, properties of various populations.

Aims.We intend to assess the NIR colour technique to identify C and M stars.

Methods.We compare the NIR colours of several C stars previously identified from spectroscopy or narrow band techniques in WLM, IC 10 and NGC 6822.

Results.We demonstrate that very few M stars have but a non negligible number of C stars are bluer than this limit. Thus, counts of M and C stars based on such limit do not produce pure samples.

Conclusions.C/M ratios determined from NIR colours must be regarded as underestimates mainly because the M numbers include many warm C stars and also K stars if no blue limit is considered.