An Infrared Colour Discriminator for Giant and Supergiant Optical M Stars

An infrared colour - MK spectral type diagram of optical M giants, M Miras and M supergiants is presented. The area in the diagram with m 12 − m 25 > 0.70 and MK spectral type earlier than M4 is occupied by early type optical M supergiants. The region of spectral type later than M4 is occupied by optical late M giants (Mira variables) and late M supergiants. The area in the diagram with a colour range of 0.15 < m 12 − m 25 < 0.70 is occupied by optical giants (Mira Variables) and supergiants (M supergiants with blue companions and probably some M supergiants in the evolutionary stage pre or post M supergiants with large infrared excesses). Finally, the area in the diagram with m 12 − m 25 < 0.15 is clearly occupied by M giants without circumstellar dust shells. The colour distribution of the M giants, M supergiants and M Miras in the colour - spectral type diagram suggests that we can use the colour properties as search criteria for (i) early M supergiants with colours of m 12 − m 25 > 0.70, (ii) late M supergiants with colours of m 12 − m 25 > 0.70, (iii) M giants with m 12 − m 25 < 0.15, (iv) Mira candidates using IRAS point sources which are also identified as optical M stars.