NGC 146: a young open cluster with a Herbig Be star and intermediate mass pre-main sequence stars

¹ Indian Institute of Astrophysics, II Block Koramangala, Bangalore 560034, India e-mail: purni@iiap.res.in
² CREST Campus, Indian Institute of Astrophysics, Hosakote, Bangalore, India e-mail: dks@crest.ernet.in
³ Aryabhatta Research Institute of Observational Studies (ARIES), Manora peak, Nainital 263129, India e-mail: sagar@upso.ernet.in
⁴ VSRTP Student, Mother Theresa University, Kodaikanal, India

Received: 12 January 2005
Accepted: 11 May 2005

Abstract

We present UBV CCD photometry and low-resolution spectra of stars in the field of the young open cluster NGC 146. UBV photometry of 434 stars were used to estimate the reddening of 0.55 ± 0.04 mag and BV photometry of 976 stars were used to estimate a distance modulus of  = 12.7 ± 0.2 mag, corresponding to a distance of 3470 pc. We estimated 10–16 Myr as the turn-off age for the upper main sequence of the cluster using isochrones and synthetic colour magnitude diagrams. We identified two B type stars with H_α in emission and located on the MS using slit-less spectra. A higher resolution spectrum of the brighter Be star indicated the presence of a number of emission lines, with some lines showing the signature of gas infall. This star was found to be located in the region of Herbig Ae/Be stars in the  vs.  colour–colour diagram. Thus, we identify this star as a Herbig Be star. On the other hand, 54 stars were found to show near infrared excess, of which 17 were found to be located in the region of Herbig Ae/Be stars and 18 stars were found to be located in the region of Be stars in the NIR colour–colour diagram. Thus NGC 146 is a young cluster with a large number of intermediate mass pre-main sequence stars. The turn-on age of the cluster is found to be ~3 Myr. Though NGC 146 shows an older turn off, the bulk of stars in this cluster seems to belong to the younger population of 3 Myr.