A multiwavelength census of stellar contents in the young cluster NGC 1624

Abstract

We present a comprehensive multiwavelength analysis of the young cluster NGC 1624 associated with the HII region Sh2-212 using optical UBVRI photometry, optical spectroscopy and GMRT radio continuum mapping along with the near-infrared (NIR) JHK archival data. From optical observations of the massive stars, reddening E(B − V) and distance to the cluster are estimated to be 0.76–1.00 mag and 6.0 ± 0.8 kpc, respectively. The present analysis yields a spectral class of O6.5V for the main ionizing source of the region, and the maximum postmain- sequence age of the cluster is estimated as ∼4Myr. Detailed physical properties of the young stellar objects (YSOs) in the region are analysed using a combination of optical/NIR colour–colour and colour–magnitude diagrams. The distribution of YSOs in the (J − H)/ (H − K) NIR colour–colour diagram shows that a majority of them have Av ≤ 4mag. However, a few YSOs show Av values higher than 4 mag. Based on the NIR excess characteristics, we identified 120 probable candidate YSOs in this region, which yield a disc frequency of ∼20 per cent. However, this should be considered as a lower limit. These YSOs are found to have an age spread of ∼5Myr with a median age of ∼2–3 Myr and a mass range of ∼0.1– 3.0M_. A significant number of YSOs are located close to the cluster centre and we detect an enhanced density of reddened YSOs located/projected close to the molecular clumps detected by Deharveng et al. at the periphery of NGC 1624. This indicates that the YSOs located within the cluster core are relatively older in comparison to those located/projected near the clumps. From the radio continuum flux, the spectral class of the ionizing source of the ultracompact HII (UCH II) region at the periphery of Sh2-212 is estimated to be ∼B0.5V. From the optical data, the slope of the mass function (MF) _, in the mass range 1.2 _ M/M_ < 27, can be represented by a single power law with a slope −1.18 ± 0.10, whereas the NIR data in the mass range 0.65 _ M/M_ < 27 yield _ =−1.31 ± 0.15. Thus the MF agrees fairly with the Salpeter value. The slope of the K-band luminosity function (KLF) for the cluster is found to be 0.30 ± 0.06, which is in agreement with the values obtained for other young clusters.