SCYON Abstract

Received on March 4 2001

Multicolour CCD Photometric and Mass function study of the Distant Southern Open Star Clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253

Authors	Ram Sagar^1,3, U. Munari ² and K. S. de Boer ³
Affiliation	¹ State Observatory, Manora Peak, Naini Tal, 263129, India, and Indian Institute of Astrophysics, Bangalore, 560034, India, ² Osservatorio Astronomico di Padova, Sede di Asiago, I-36012 Asiago (VI), Italy ³ Sternwarte der Universität Bonn, Auf dem Hügel 71, D-53121, Bonn, Germany
Accepted by	Monthly Notices Royal Astronomical Society
Contact	sagar@upso.ernet.in
URL
Links	NGC 3105 / NGC 3603 / NGC 4815 / NGC 6253 / Hogg 15 / Melotte 105 / Pismis 20

Abstract

We derive cluster parameters and mass functions from new UBVRI CCD photometric observations of ~ 3,500 stars reaching down to V ~ 20 mag for the distant southern open star clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253. For NGC 3105 and Hogg 15, CCD data has been presented for the first time. The reddening is non-uniform across the face of young (age < 300 Myr) clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15 and Pismis 20 with average values of E(B-V) = 1.06, 1.44, 0.52, 1.15 and 1.20 mag respectively, while it is uniform with average values of E(B-V) = 0.72 and 0.20 mag for the older clusters NGC 4815 and NGC 6253 respectively. The values of colour excess ratios indicate presence of normal interstellar reddening across the cluster regions studied here.

Well defined main sequences can be seen in all the clusters. However, main sequence turn-off points and subgiant branches are well defined only in the older clusters NGC 4815 and NGC 6253. The distances to the clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253 are 9.5 ± 1.5, 7.2 ± 1.2, 2.3 ± 0.2, 3.0 ± 0.3, 2.75 ± 0.2, 3.55 ± 0.35 and 1.8 ± 0.12 kpc respectively, while the corresponding ages derived using theoretical convective core overshooting stellar evolutionary isochrones are 25 ± 10, 3 ± 2, 250 ± 30, 6 ± 2, 400 ± 50, 6 ± 2 and 2500 ± 600 Myr respectively.

In the mass range ~ 1 - 75 M_O, the mass functions of all clusters except for NGC 6253 have been studied. The slopes of their mass spectrum agree within errors with the Salpeter value (1.35). The slope of the MF for stars more massive than 10 M_O is almost the same as for the lower mass stars.

The MF slopes of clusters younger than 500 Myr seems to have no dependence on Galactic longitude, Galacto-centric distance and cluster age. As the inherent uncertainties in the mass function determinations of young galactic star clusters can produce internal scatter which is larger than the external ones, we conclude that above 1 M_O, initial mass function is universal with a slope of Salpeter value.