Study of spatial structure and extinction of galactic star clusters

Abstract

Present thesis focuses on the optical photometric, astrometric and dynamical analysis of three southern namely NGC 6067, NGC 2506, IC 4651 and two northern namely King 12 and NGC 2355 galactic star clusters. These clusters have a wide range in age and location in the Galaxy, hence useful to understand different properties of the objects at different parts of the Galaxy. The present study is carried out using a combination of variety of data set, which include kinematical as well as photometric CCD data. In the present thesis, we have successfully separated the cluster members from the field stars and then conducted analysis using these members only. That is why our analysis is more accurate than the information available in the literature about these objects specially for IC 4651 which is highly obscured by the background stars in the disc of Galaxy. For this cluster, we have decontaminated the main sequence below V ∼ 16 mag for the first time. We also studied interstellar extinction towards twenty young open clusters using data available in literature. For all these clusters we conducted interstellar extinction analysis for the first time. Chapter 1 of the thesis introduces various problems we are interested in. We also discussed motivation of the present thesis and various techniques which can be used to analyze our object of interest. We have also given a review of available literature regarding our research work. The main problem is lack of observational data to study the structure of the Milky Way. We also discussed how the problem can be addressed by studying its constituents e.g. stars, dust and gas. We presented a short introduction of star clusters in this chapter which are our target objects. Chapter 2 is dedicated to the description of data sets, telescopes and detectors used for our study. We used raw fits images from 2.2 m MPG/ESO telescope mounted at la Silla Chile and 104-cm Sampurnanand telescope mounted at Manora peak, ARIES, Nainital. Wide Field Imager (WFI) CCD and PyLON CCD are used as detectors for 2.2 m MPG/ESO telescope and 104-cm Sampurnanand telescope respectively. We also discussed the methods of reduction of these CCD acquired raw images using different software packages such as IRAF, DAOPHOT II and astrometric software package. For astrometric data, we used five parameter Gaia mission data from its new release. 2MASS catalogue is used for data in near-IR filters JHK. For interstellar extinction study, we used data from WEBDA database in UBV RI filters. Analysis of optical and astrometric data for five open clusters NGC 6067, NGC 2506, IC 4651, King 12 and NGC 2355 is presented in Chapter 3. Stars in a cluster have similar motion and distance. Using this property, we separated cluster members from the field stars and then determined the fundamental and structural properties such as centre, radius, age, distance, metallicity and reddening for these clusters. We complemented our optical data with near-IR data from 2MASS for this analysis. We identified three blue straggler stars in the cluster NGC 2506, one straggler star in NGC 2355 and a binary sequence in IC 4651. In this chapter, we also studied the dynamical status of our objects of interest. We also determined completeness of our photometry. The luminosity function shows that the young clusters NGC 6067 and King 12 are still bound to their fainter members whereas old and intermediate clusters NGC 2506 and NGC 2355 have lost some of their fainter members due to dynamical evolution while the old cluster IC 4651 (younger than NGC 2506) has lost most of its low mass stars due to dynamical evolution and influence of Galactic tidal forces. We found that mass function of all the clusters becomes flatter on going towards the inner part of the cluster which gives a hint of mass segregation. We confirmed the presence of mass segregation in all the clusters by using cumulative radial distribution of the masses. We also found that all the clusters under study are dynamically relaxed except NGC 6067. We discussed the dynamics of five clusters under study in the Galaxy using Galactic potential models in Chapter 4. We found that all the clusters are moving in boxy pattern and circular orbits around the Galactic centre. Two clusters NGC 6067 and IC 4651 are orbiting inside the solar circle and near the Galactic plane hence highly affected by the Galactic tidal forces which can lead to a shorter life for these two clusters. Other three clusters NGC 2506, King 12 and NGC 2355 are orbiting outside the solar circle and far from Galactic plane except King 12 which is orbiting near the Galactic plane. So we can predict a longer life for the clusters NGC 2506 and NGC 2355. In Chapter 5, we studied interstellar extinction towards 20 young open clusters. We found presence of non-uniform extinction in 17 clusters. The dust grains in 12 clusters have similar properties as of the general interstellar matter while 6 clusters show anomalous behaviour. We found that the amount of dust and gas reduces with the age of the clusters. We also identified thirteen stars in three clusters which are having circumstellar dust around them.