Dust Properties Towards Galactic Anti-Center Direction

Abstract

The starlight passing through the interstellar medium becomes linearly polarized due to the differential extinction from the alignment of asymmetric dust grains in the medium. Both macroscopic and microscopic information about the medium can be achieved with the polarimetric study. By macroscopic properties, we mean the projected direction of the magnetic field at the region whether it is a macroscopic field in the Galaxy or the local magnetic fields of the cloud, whereas the microscopic properties are the size distribution, shape, and efficiency of dust grains. Thus the polarimetric technique is assumed as a more reliable technique to get information about dust grains. Furthermore, polarimetry is useful in explaining the membership of stars in the cluster and in detecting the location of energetic phenomena that happened in the history of a cluster. The thesis aims to see the distribution of dust grain’s properties towards the Galactic anti-center direction. Open star clusters are selected as samples for the study. We have observed four open star clusters (NGC 2345, NGC 1817, Alessi 1, and NGC 7380) in multi-bands using ARIES imaging polarimeter from 104-cm telescope of ARIES. We have also included eleven clusters observed in the past to complete the study. Firstly, we have analyzed 197 stars in the region of the cluster NGC 2345, 125 stars in NGC 1817, 73 stars in Alessi 1, and 72 stars in the cluster region NGC 7380 to study the polarization properties towards the direction of these clusters. The degree of polarization towards these lines of sight is due to the foreground dust concentration, while few stars are found to have an intrinsic component in the polar ization e.g. 30 stars in NGC 2345, 11 stars in NGC 1817, 8 stars in Alessi 1 and only one star in NGC 7380 are found intrinsically polarized. Both NGC 1817 and Alessi 1 clusters are located below the Galactic plane. The indication of the dust layer in front of NGC 1817 and Alessi 1 is not found. However, one layer before NGC 2345 at a distance of 1.2 kpc and two dust layers in front of NGC 7380 at distances of 0.7 and 1.3 kpc are noticed. Stokes plane analysis is used for membership information for these clusters. We have calculated the membership probability of members of the cluster NGC 2345 using the polarimetric technique and we found a good corre lation between our results and previously estimated probabilities of members. The maximum polarization for cluster NGC 2345 and NGC 7380 are found as 1.55 % and 1.71 %, while a lower value is found for clusters NGC 1817 (0.93%) and Alessi 1 (0.83%) clusters. The size distribution of dust grains towards these lines of sight is similar to that of general interstellar medium i.e. the wavelength for maximum polarization varies from 0.54 µm to 0.59 µm, which corresponds to the average size of aligned grains as 0.14 - 0.16 µm. Polarizing efficiency towards these four clusters is found close to the average efficiency found in the Galaxy. In the case of the cluster NGC 2345, less polarizing efficiency of intra-cluster medium is noticed. A power law relation of polarizing efficiency with extinction is seen with a slope of -0.67. Vari ations of polarizing efficiency and wavelength for maximum polarization are found with extinction and radial distance from cluster center which led to the conclusion for dust grains alignment through radiative torque alignment theory. The orienta tion of dust grains is found nearly parallel to the Galactic magnetic field towards the direction of NGC 2345, Alessi 1, and NGC 7380 clusters, while for cluster NGC 1817, orientation is found to deviate from Galactic parallel indicating perturbation of dust in the region. During the Stokes plane analysis of the region for cluster NGC 1817, two groups of stars with different polarimetric characteristics are noticed. The nonrelaxed dust cloud and the combination of this cloud with relaxed interstellar dust layers seem to be the reason behind the polarization of these groups of stars. The overall distribution of polarization towards the Galactic anti-center direc tion shows a higher value in the Galactic plane and decreases while going above and below the plane. Also, the dust layer is found to be present in front of those clusters which lie near the Galactic plane. To conclude the thesis, the information regarding the distribution of dust grains, their size distribution, alignment, polar izing efficiency, foreground dust concentration, and membership information of the cluster are extracted using the polarimetric study of open star clusters.