DSpace Community: Ph.D. Theses with IIAP affiliation are submitted here

Stellar population in the Magellanic Clouds: Star formation and evolution in diverse environments

Sun, 01 Jun 2025 00:00:00 GMT

Title: Stellar population in the Magellanic Clouds: Star formation and evolution in diverse environments Authors: Dhanush, S. R Abstract: The Magellanic Clouds (MCs) are the closest interacting satellite dwarf galaxies of the Milky Way (MW). The Magellanic system comprises the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), which are connected by the Magellanic Bridge, a structure composed of gas and stars. The recent proper motion (PM) studies of the MCs suggest that they are interacting not only with each other but also with the MW. The interactions experienced by the MCs have also produced a prominent tail of neutral hydrogen gas, known as the Magellanic Stream, which stretches across ∼ 200◦ of the southern sky. In addition, filamentary leading arms extend ahead of the Clouds’ motion, arching over the MW disk. Interactions between the MCs trigger star formation and impact the internal kinematics of both galaxies. Therefore, studying the stellar populations residing within these galaxies is essential to unraveling their evolution and interaction history. Star clusters serve as valuable tracers for unraveling the star formation history of galaxies. In the MCs, over 4000 clusters have been cataloged. Determining their ages provides insights into the cluster formation history (CFH) of the MCs. However, a comprehensive age estimation from the center to the outskirts of the LMC and SMC is still lacking. Such global age dating is essential for identifying the episodes of enhanced star/ cluster formation (CF) triggered by the LMC-SMC interactions. Also, modeling the internal kinematics of the LMC and SMC is crucial for revealing the disk response of the MCs to their mutual interaction. The goal of this thesis is to trace the CFH and model the kinematics of the MCs to gain insights into their interaction history. The analysis in our studies is based entirely on Gaia (Global Astrometric Interferometer for Astrophysics) Data Release 3 (DR3) data sets. In this study, we present a detailed view of CF to trace the evolution and interaction history of the MCs in the last 3.5 Gyr. We parameterized 1710 and 280 star clusters in the LMC and the SMC, where 847 and 113 clusters are newly characterized in the outer LMC and SMC, respectively. We estimated the age-extinction metallicitydistance parameters using an automated fitting of the color-magnitude diagram (CMD) after field star removal, followed by a Markov Chain Monte Carlo (MCMC) technique. We report a first-time detection of two synchronized CF peaks in the MCs at 1.5 }0.12 Gyr and 800 }60 Myr. We recommend that the choice of the metallicity (Z) values of isochrones for clusters with age ≤ 1 - 2 Gyr are ZLMC = 0.004 - 0.008 and ZSMC = 0.0016 - 0.004 for the LMC and SMC, respectively. We found evidence for spiral arms in the LMC, as traced by the cluster count profiles over the last 3.5 Gyr. The density maps provide evidence of ram-pressure stripping in the North-East of the LMC, a severe truncation of CF in the South of the LMC, and a radial shrinkage of CF in the SMC in the last 450 Myr. The last SMC-LMC interaction (∼ 150 Myr) resulted in a substantial CF in the North and Eastern SMC, with a marginal impact on the LMC. This study brings out the CF episodes in the MCs and their connection to the LMC-SMC-MW interactions. The internal kinematics of the LMC disk have been modeled by several studies using different tracers with varying coverage, resulting in a range of parameters. In this study, we modeled the LMC disk using 1705 star clusters and field stars, based on a robust MCMC method. The dependency of model parameters on the age, coverage, and strength of the clusters are also presented. This is the first comprehensive 2D kinematic study using star clusters. Red clump (RC) stars and young main-sequence stars are also modeled for comparison. The clusters and field stars are found to have distinctly different kinematic centers, disk inclination, position angle of the line of nodes, and scale radius. We also note a significant radial variation of the disk parameters. Clusters and young stars are found to have a large residual PM and a relatively large velocity dispersion when compared to the RC field population, which could be due to perturbation from the bar and spiral arms. We traced the presence of large residual PM and non-circular motion among clusters likely to be due to the bar and detected a decrease in the scale radius as a result of the possible evolution of the bar. The kinematically deviant clusters point to a spatio-temporal disturbance in the LMC disk, matching the expected impact parameter and time of the recent collision between the LMC and the SMC. Similar to the LMC, we modeled the kinematics of the SMC by analyzing the PM from nine different stellar populations, which include young main sequence stars (< 2 Gyr), RGB stars, RC stars, red giants with line-of-sight (LOS) velocities, and three groups of star clusters. We trace the evolution from a non-rotating flattened elliptical system as mapped by the old population to a rotating highly stretched disk structure as denoted by the young main sequence stars and clusters (< 400 Myr). We estimated that the inclination, i (∼ 58◦ to 82◦) decreases and the position angle, Θ (∼ 180◦ to 240◦) increases with age. We estimated an asymptotic velocity of ∼ 49 - 89 km s−1 with scale-radius of ∼ 6 - 9 kpc for the young main sequence populations with velocity dispersion of ∼ 11 km s−1, suggesting a rotation-supported disk structure. Our models estimate a LOS extension of ∼ 30 kpc, in agreement with observations. We identified four regions of the SMC showing anomalies in the residual PM, the East Anomaly (EA), South East Anomaly (SEA), South Anomaly (SA), and West Anomaly (WA). The SEA appears like an infalling feature and is identified for the first time. The tidal imprints observed in the residual PM of the SMC suggest that the recent interaction with the LMC considerably shapes its evolution. Description: Thesis Supervisor Prof. Annapurni Subramaniam

Study of lithium and other related elements among evolved stars

Wed, 01 Jan 2025 00:00:00 GMT

Title: Study of lithium and other related elements among evolved stars Authors: Mallick, Anohita Abstract: The formation and evolution of elements is a fundamental problem in modern astrophysics. Lithium (Li), one of the four stable nuclides produced during Big Bang Nucleosynthesis (BBN), exhibits an observed abundance in many celestial bodies that deviates from theoretical predictions. In stars, the behavior of Li is particularly complex, as it is easily destroyed under high temperatures, where it undergoes proton capture to form stable helium nuclei. Consequently, Li cannot be preserved in the hot stellar interior, but is instead confined to the surface layers. If surface Li is transported to the interior during stellar evolution it is destroyed due to high temperatures, leading to a reduction in Li abundance in the star’s atmosphere. Standard stellar evolution models predict that stars with initial Li abundances comparable to the ISM value ( 3.3 dex) will show a Li abundance no greater than 1.5 dex once they reach the giant stage. However, observations have revealed a small number of giant stars, known as Li-rich giants, with Li abundances exceeding this threshold. Some of these stars even display Li abundances higher than the ISM value, and are referred to as super Li-rich giants. The presence of such stars presents a significant challenge to the standard stellar evolution models, suggesting the existence of additional mechanisms that can enrich Li in stars. Understanding these mechanisms is essential for both the formation of Li and the broader theories of stellar evolution, though it remains a challenging area of research. Significant progress has been made over the past four decades in the study of Li-rich giants. We have used large scale survey programs such as LAMOST and GALAH spectroscopic surveys, photometric survey of Kepler and TESS space telescopes, Gaia astrometry and high-resolution spectra using 2-m Himalayan Chandra Telescope (HCT) and the 10 m Hobby Eberly Telescope (HET). Our work discovered a number of new red clump super-Li rich giants with proper evolutionary phases assigned based on asteroseismic analysis. The extensive systematic survey based on an unbiased sample of giants common among Kepler and LAMOST surveys’ fields provided a key evidence that Li enhancement is only associated with the He-core burning phase post He-flash in low-mass stars. Another key result that emerged from this study is the location of the Li enrichment site in most Li-rich stars is the He-flashing phase, the transition between the evolution of stars from the end of RGB tip to the He-core burning period in the horizontal branch. Based on the analysis of spectroscopic and photometric data, we provided first-of-its-kind evidence in the form of a correlation between lithium abundances in giants and their masses derived using asteroseismology. Another significant part of this thesis is the study of photospheric Li abundances and its relation with the strength of chromospheric He I 10830Å. The subtle correlation is the another evidence that the He-flash is the source for Li enhancement and the enhanced activity in the chromosphere resulting stronger He I line among very high Li-rich giants. Description: Thesis Supervisor Prof. B. Eswar Reddy

An observational perspective into the nature of short-plateau Type II supernovae

Tue, 01 Oct 2024 00:00:00 GMT

Title: An observational perspective into the nature of short-plateau Type II supernovae Authors: Teja, Rishabh Singh Abstract: Supernovae (SNe, singular SN), the endpoints of stellar evolution, have fascinated humans since time immemorial, with recorded evidence dating back to two millennia. Continuing this quest, modern astronomy and astrophysics have come far in understanding these events and not mere recording for bookkeeping purposes. Astronomers are widely using these violent cosmic deaths in many domains of astronomy. Some common themes are to study galactic chemical evolution, measure cosmological distances independent of other measurements, study natal kicks to compact objects, e.g., white dwarfs, neutron stars, and black holes, sources of various heavier elements, and a few others. These explosions also help to understand the stellar evolution, formation of neutron stars and black holes, host environment, and mass loss mechanisms. Further, SNe are considered one of the sources of gravitational wave radiation and high-energy gamma rays. Even in interdisciplinary areas, their use is becoming eminent in studying metal enrichment in the universe and the possible effects of a nearby SN on Earth and its biosphere. Evidently, SNe play a crucial role in astrophysics, and hence their study becomes equally essential. Their studies as single events are equally important as studying them as a population. Numerous events do not fit the orthodox classification schemes and require a different explanation with thorough studies. Studying such peculiar events in great detail for better classification and judiciously weeding out contaminant sources from larger samples used in other astrophysical domains is crucial. The recent advent of extensive surveys such as All-Sky Automated Survey for Supernovae (ASASSN), Asteroid Terrestrial-impact Last Alert System (ATLAS), Zwicky Transient Facility (ZTF), etc., have led to numerous discoveries, and upcoming surveys like Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) would only add more. These require detailed spectroscopic and multi-band photometric follow-up for a clear understanding. The James Webb Space Telescope (JWST) has recently opened up the SNe domain to much higher redshifts. Apart from their observational studies, it is critical to model these events with the existing tools to get a detailed picture of them. Only with enough observations, analysis, and modeling of these events could we get much information about them. Studying these single events in great detail is necessary to gather better statistics and say more about different trends in these populations. Hence, we take up this work to observe, analyze, and model such explosions in detail to improve our understanding using observation facilities available in national and international domains. We attempt to analyze and model these events for in-depth understanding using open-source tools as widely as possible. The thesis is focused on the rarely occurring Type IIP supernovae with shorter plateau duration in their light curve evolution. Various modeling works have theorized that these events could originate from higher-mass red supergiants (RSGs). In this work, we study and understand several short-plateau Type II SNe in detail using multiwavelength observation and 1-D hydrodynamical modeling. It has enabled us to show alternate origins of these events based on observational evidence backed by detailed modeling of these rare events. Further, a great deal of diversity has also been revealed among these events. The thesis makes use of various national and international observational facilities, both ground and space-based, such as in optical 2-m Himalayan Chandra Telescope, 0.7-m GROWTH India Telescope, in ultraviolet AstroSat, Swift/UVOT and 1.5-m near-infrared KANATA telescope from Japan. Other publicly available data are also used whenever available. We have used various analytical and modeling tools to estimate several explosion parameters and attempted to know about the progenitor and its properties. The thesis presents detailed studies of SN 2018gj, SN 2020jfo, and SN 2021wvw short plateau SNe, including SN 2023ixf, the nearby decadal supernova. Description: Thesis Supervisor Prof. D.K. Sahu

Tracing the origin of heavy elements through metal-poor stars

Fri, 01 Nov 2024 00:00:00 GMT

Title: Tracing the origin of heavy elements through metal-poor stars Authors: Saraf, Pallavi Abstract: The hierarchical mass assembly of galaxies and the formation timescales of their substructures are important topics in astrophysics and cosmology. These aspects can be explored in the local universe by studying resolved stars. By analyzing individual stars and stellar populations, one can estimate their ages. Although the color-magnitude diagram of co-evolved stellar populations aids in age determination, dating field stars is more complex. Typically, the iron content (or metallicity) of stars serves as an age indicator. However, metallicity is influenced by the history and rate of star formation. Analyzing multiple chemical elements can provide additional insights. Yet, using elements heavier than iron, particularly those formed by rapid neutron capture, hinges on understanding their astrophysical origins. Identifying these origins remains a significant challenge and a key question in nuclear astrophysics. This thesis investigates chemical compositions of metal-poor stars, aiming to understand their origins and chemical evolution. Halo stars, which are some of the Galaxy’s oldest, offer insights into the astrophysical sites of element production due to their minimal pollution by progenitor stars. Utilizing high-resolution spectroscopic analysis through telescopes such as the 10-m GTC, 8.4-m VLT, 10-m KECK, and 2-m HCT, and supplementing with low-resolution spectra from LAMOST, this study analyzes the detailed abundances of approximately 50 stars. These stars, with metallicities ranging from −3.2 to −1.8, include many classified as very metal-poor and extremely metalpoor, with their chemical compositions examined for the first time. The findings are systematically presented across the thesis. The initial findings of the thesis include a comprehensive abundance analysis of four r-process enhanced (RPE) stars, utilizing the HORuS spectrograph at 10-m GTC. These stars are in the metallicity range of −2.3 to −1.9. The high SNR of the spectrograph enable determining the abundances of 16 light elements and 20 neutroncapture elements, including thir peak element, Os. We identified Th in two objects, with [Th/Fe] values of 0.65 and 0.6, respectively, which helped us estimate their ages. The study discusses the metallicity trends of elements such as Mg, Sr, Ba, Eu, Os, and Th in both r-II and r-I objects, using a compilation of RPE objects from existing literature. We carried out a detailed line-by-line differential analysis comparing a moderately RPE object (r-I: HD107752) with an extremely RPE object (r-II: CS31082-0001) to explore the potential shared origins of their heavy element nucleosynthesis. This part of the study utilized high-resolution and high SNR spectra from the ESO-VLT’s UVES instrument, sourced from the ESO data archive. We identified three distinct patterns in the differential abundance analysis. The similar abundances of light elements up to zinc in both stars suggest a shared origin for these elements, with no odd-even variation observed. In the case of neutron-capture elements, r-I stars exhibit slightly depleted light r-process elements and more significantly depleted heavier r-process elements, challenging the theory of a single production site. We also provide plausible scenarios for their r-process enrichment. Additionally, we performed a kinematic analysis of nearly 466 r-process-enhanced stars compiled from literature, examining their origins and locations within the Milky Way. We compare the significance of our orbit-based classification of stars into different Galactic components with the Toomre diagram-based classification. Our findings indicate that RPE stars are equally distributed between the disk and halo of the Galaxy. We also utilized archival data from the ESO and KECK telescopes to explore similarities between CEMP-r/s and r-process objects and to investigate the origin of thorium in CEMP-r/s stars using machine learning algorithms. We observed that r-I and r-II stars do not distinctly separate into two groups; rather, there is an intermediate group that may consist of diluted or mixed stars. Our analysis indicate that CEMP-r/s and RPE stars are separate classes of objects. Additionally, we used the HESP, installed on the Himalayan Chandra Telescope to observe some metal-poor stars (the HESP-GOMPA Survey). This led to the discovery of numerous very metal-poor, extremely metal-poor, CEMP-r, and RPE stars. We also study the kinematics of these stars. These findings are detailed in Chapter 7 of the thesis. Our study indicates that within our range of metallicities, neutron stars mergers and supernovae are the primary sites contributing to the chemical composition of halo stars. Description: Thesis Supervisor Prof. T. Sivarani