Study of milky way halo stars and connection to globular clusters

Abstract

The thesis aims to study the chemical abundances of very metal poor Milky Way halo stars and Globular cluster stars, to understand their possible common origin. Halo stars and globular clusters belong to the oldest stellar population of the Galaxy and detailed Chemical tagging of these populations can addresses several intriguing problems in the area of galaxy formation and globular cluster formation and evolution. In this study we have used low and high resolution spectroscopic abundance analysis to address possible connection between halo stars and globular clusters. In order to achieve this, We carried out high resolution spectroscopic survey using the Hanle echelle spectrograph at 2m Himalayan Chandra telescope. We also use low resolution spectra of Halo stars and globular clusters from Sloan Digital Sky Survey (SDSS). The thesis describes detailed abundances analysis of about 50 stars, in the metallicity range of halo and Globular clusters. These are selected from bright SDSS-MARVELS pre-survey data. All these objects are newly discovered VMP or EMP stars and their detailed chemical abundances are studied for the first time in the this work. These results are presented in detail in individual chapters.

The results include two stars (EMP & CEMP-no) in the metallicity range < -3.0 has already been published in ApJ. The other interesting results of the study are three globular cluster escapees that show the typical light-element anomalies (CH-CN, Na- O, Mg-Al anti-correlations) associated with second generation GC stars. Two of them

are RGBs and one is a blue straggler star, which is a rare class of object among halo stars. It is also the most metal poor Gc escapee discovered till date. The blue straggler shows strong overabundances of Na([Na/Fe]=+1.50) along with a very depleted Mg and Ca([Mg/Fe]=-0.30; [Ca/Fe]=-0.50). Lithium is also be detected in two of these GC escapees which is very important to constrain the nature of the original polluters of the GC. These objects are bright for detailed isotopic abundances studies with 8-10 class telescopes, which will provide insights to the origin of globular cluster abundance anomaly and efforts are underway. Additional interesting objects include discovery of an r−process rich star with [Eu- /Fe]=+0.9 and [Th/Fe]=1.28 and other R-I and R-II stars with +0.30<[Eu/Fe]<+1.2. We also identified two CEMP-no stars and two CEMP-s stars. One of the CEMP-s star ([Fe/H]=-2.3,[C/Fe]=0.87) shows a rather unusually high abundance of n-capture like [Ba/Fe]=1.67 and [Eu/Fe]=0.78 and uniform enhancement in all heavy elements. This could be a signature of NS-NS merger events that produce a blue Kilo-Novae and synthesis light r-process elements or i-process. We have also conducted a comparative study of CEMP-no and EMP stars using their heavy element enrichment and Lithium.

Lithium was detected in CEMP giants and dwarfs and they exhibit the expected deple- tion from Spite plateau as they ascend the giant branch. We found Lithium to have a similar distribution among CEMP-no and EMP stars. Lithium was also detected (A(Li)=1.60) in one of the r-I stars which a rare occurrence. We also present heavy element abundance among GC stars using low resolution SDSS data and compare them with Halo stars to understand the common origin. We have used the archival data of ESO and SDSS to study the key neutron capture elements like Sr and Ba for globular cluster stars. A spectral grid was developed over wide ranges of temperature,log(g),]Fe/H],[Sr/Fe] and [Ba/Fe]. We tried to explore the common origin for the Halo stars and individual clusters. Neutron star mergers were found to the chief contributor for globular clusters whereas Halo stars showed both NS-NS mergers and supernovae to play a key role in different paradigms.