A spectroscopic and statistical study of Collinder 140⋆

Spectra of 140 stars in a 1.36 deg² field centred on Collinder 140 (Cr 140), 17 outside this field and 125 in two neighbouring fields of the same total area have been classified on the MK system. All stars were classified to a completeness limit of V = 14.35 – 0.65 S (where S is a numerical representation of spectral class). A statistical comparison of the cluster field with those outside the cluster shows that the cluster consists of 33 stars – 14 ± 5 F8 to K4 giants with $$V\,\leqslant\,7.2\,+\,0.65\,S\,\text{and}\,19\,\pm\,5\,\text{B2}$$ to A4 main sequence stars with V < 10.3. Few, if any, members are expected outside the above spectral and magnitude limits. This is in good agreement with the observational data which give 29 members found within the 1.36 deg² field and a further seven immediately outside it. The following cluster properties have been determined: distance $$410\,\pm \,30\,\text{pc};{E}_{B-V}=0.04;\text{age}\,20\,\pm \,6\,\text{Myr}$$. The F8 to K4 giant members appear anomalous in that they are too faint to be post-main sequence members and the turn-off age of the cluster is too old for them to be pre-main sequence, however the Poisson statistics argue strongly in favour of their membership. To resolve the nature of the latter it is essential that radial velocity and proper motion studies of stars in the region of Cr 140 be made. Until this has been done speculation on the astrophysical implications of the yellow giant members might well be a wasted effort.