http://hdl.handle.net/2248/93 Thu, 16 Jul 2026 06:48:11 GMT 2026-07-16T06:48:11Z http://hdl.handle.net/2248/9010 Title: Chemodynamic studies of the galaxy Authors: Deepak Abstract: Based on data from large spectroscopic surveys like the GALactic Archaeology with HERMES (GALAH) and the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST), along with astrometric and photometric data from the Gaia survey, we have addressed three major questions in the field of the chemical and dynamical evolution of the Galaxy. The first of these three questions is understanding the evolution of lithium (Li) in the Galaxy. For this, we used the spectroscopic data from the GALAH survey along with astrometric and photometric data from the Gaia survey. We found that Li has significantly increased in the Galactic disc compared to the observed value in the metal-poor halo stars and the theoretically predicted primordial value from the Standard Big Bang Nucleosynthesis models. Search for an explanation for such large-scale Li enrichment throughout the Galactic disc led us to study the evolution of Li in low-mass stars (the second problem we have addressed). According to the standard evolutionary models, Li gets depleted as stars ascend on the red-giant branch (RGB). However, about a per cent of giants have been found to have a thousandfold higher Li than the value predicted by standard models. These rare giants with high Li, well known as the Li-rich giants, have puzzled astronomers for over four decades since their first discovery by Wallerstein and Sneden (1982). To understand the evolution of Li in low-mass stars, we used data from the GALAH and Gaia surveys. From the second data release of the GALAH survey, we discovered 335 new Li-rich giants (with A(Li) ≥ 1.8 dex), of which 20 are super Li-rich with A(Li) ≥ 3.2 dex. We further discovered that almost all of these Li-rich giants are core He-burning (CHeB) giant stars, and Li enrichment in red giants is likely associated with the He-core flash. We also found that the Li-rich giants do not belong to any particular dynamic group in the Galaxy and are found throughout the Galaxy. However, they are more prevalent among giants of the Galactic thin disc than the thick disc and halo. We further found that chemically Li-rich and normal giants are similar except for Li abundance. We also found that the probability of becoming a Li-rich giant is approximately independent of a star’s mass, although the majority of the Li-rich giants are found to be low mass (M ≤ 2 M⊙). The frequency of occurrence of Lienriched giants among normal giants is about one per cent and slightly dependent on metallicity. Li-enriched and normal giants are also found to have a similar projected rotational velocity, suggesting that Li-enrichment in giants is not linked to scenarios such as mergers and tidal interaction between binary stars. To find more clues about the origin of Li enrichment in giants, we studied the correlation between giants’ Li abundance and asteroseismic parameters. Data for the CHeB giants suggest a decrease in A(Li) with an increase in the gravity mode period spacing ∆Π1, which is known to increase with time at the start of the CHeB phase suggesting the enriched Li in He-core burning giants decreases as stars evolve. Based on asteroseismic data also, we found no evidence of Li enrichment at the luminosity bump. In conclusion, these studies have helped to uncover the almost four-decade-old mystery of the origin of Li-rich giant stars, and now we know that all the low-mass giants experience Li enrichment during the CHeB phase to a varying degree. Lastly, to uncover the formation and evolution history of the Galaxy, we studied the chemical and kinematic properties of various pro-grade (like the Splash) and retrograde (like Gaia-Enceladus/Sausage (GE/S), Thamnos and Sequoia) substructures in the Galactic halo along with the Galactic thin and thick disc. We also studied the age distributions of all these Galactic components. We found that the star formation in the Splash, which is the major in situ component of the halo and has a median [Fe/H] of −0.75±0.24 dex, peaked about 13 Gyr ago. On the other hand, the star formation in the GE/S, which is the halo’s largest accreted component and has a median [Fe/H] of −1.31 ± 0.23 dex, peaked about 1.5 Gyr later than the Splash. The Galactic thin and thick discs are found to have peak ages of about 5.0 and 11.5 Gyr, respectively. The GE/S and Sequoia are also found to have a distinct chemical evolution from the Splash, whose chemical composition is found to be similar to the metal-poor stars of the thick disc. In conclusion, our studies support the idea of galaxy formation by hierarchical clustering in a Lambda cold dark matter universe. Description: © Indian Institute of Astrophysics; Thesis Supervisor Prof. Bacham E. Reddy Fri, 01 Apr 2022 00:00:00 GMT http://hdl.handle.net/2248/9010 2022-04-01T00:00:00Z http://hdl.handle.net/2248/8925 Title: A multi wavelength study of novae in M31 Authors: Basu, Judhajeet Abstract: We present a comprehensive multi-wavelength study of extragalactic novae, with particular emphasis on rapidly recurring systems and their ultraviolet (UV) properties. Novae are thermonuclear runaway reactions that occur on the semi-degenerate surfaces of accreting white dwarf stars in close binary systems. These are important astrophysical laboratories for understanding binary evolution, accretion physics, shock formation, and galactic chemical enrichment. Through systematic observations using space-based UV and X-ray telescopes, along with ground-based optical observations, this work contributes to our understanding of nova physics across different evolutionary phases and recurrence timescales. We present the first systematic UV survey of novae in M31 using archival UVIT data. Through analysis of 19 different fields covering significant portions of M31, we developed background subtraction techniques to detect faint novae, particularly in the bright galactic bulge region. The resulting catalogue includes 42 novae detected in both outburst and quiescent phases. Quiescent novae showed signs of an accretion disc, whereas erupting novae showed signs of a shrinking photosphere. This work addressed the observational gap in systematic quiescent UV studies of extragalactic novae. The focus then shifts to M31N 2008-12a, a remarkable rapidly recurring nova with annual eruptions from 2008 to 2024. Through multi-wavelength observations covering eight consecutive eruptions (2017-2024), we investigated the unparalleled consistency in light curve morphology, spectroscopic properties, and supersoft Xray evolution. The analysis reveals that M31N 2008-12a hosts a white dwarf near the Chandrasekhar limit, making it a potential Type Ia supernova progenitor. We found strong evidence for jet-like bipolar structures that provide insights into nova explosion geometry. Anti-correlated UV and X-ray fluxes during the supersoft phase hinted towards disc activity resuming soon after each eruption. This leads to a study on rapidly recurring systems, focusing on LMCN 1968-12a, the first discovered extragalactic recurrent nova. Using simultaneous UV and X-ray observations of its 2024 eruption, we investigated the survival of the accretion disk following nova outbursts. The multi-wavelength light curves reveal distinct emission components: super soft X-rays from the white dwarf surface at temperatures of ∼ 106 K, and UV emission from an irradiated accretion disk at ∼ 2 × 104 K. The persistence of UV emission during the plateau phase provides strong evidence for disk survival and rapid reformation within days of the eruption. We end the study of individual events with a detailed study of the slow classical nova AT 2023tkw, discovered by the GROWTH-India Telescope in M31. The nova exhibited a complex light curve with multiple peaks and dips. Optical infrared observations reveal internal shock mechanisms responsible for the observed light curve variability, challenging traditional nova models and demonstrating the importance of shock-induced heating in nova explosions. The thesis ends with a summary of the key outputs from the thesis. We also briefly describe how synergies between upcoming and modern space-based UV and X-ray observatories and ground-based optical, infrared and radio observatories will lead to a comprehensive multi-wavelength understanding of novae populations, and will also be a key to unlocking the mysteries of outlier events. These results contribute to our understanding of nova physics, particularly the role of accretion discs in rapid recurrence, the nature of massive white dwarf systems, shock mechanisms in classical novae, and the potential connections between rapidly recurring novae and Type Ia supernova progenitors. Through this comprehensive approach, the work successfully covers a diverse class of novae ranging from slow classical systems to the fastest-recurring extragalactic novae known, providing insights into the opposite ends of the spectrum of nova behaviour and evolution. Description: Thesis Supervisor Dr. Sudhanshu Barway Fri, 01 Aug 2025 00:00:00 GMT http://hdl.handle.net/2248/8925 2025-08-01T00:00:00Z http://hdl.handle.net/2248/8924 Title: Kinematics and thermodynamics of coronal mass ejections Authors: Khuntia, Soumyaranjan Abstract: Coronal Mass Ejections (CMEs) are powerful solar eruptions that expel magnetized plasma into the heliosphere, often triggering severe space weather disturbances at Earth. While their large-scale kinematics and magnetic structures are well studied, the internal thermodynamic evolution of CMEs, from the low corona to 1 au, remains inadequately understood. This thesis aims to bridge this gap through a comprehensive investigation that integrates analytical modeling, remote observations, in situ measurements, and statistical analysis. One of the core focuses of the thesis is the development and refinement of the Flux Rope Internal State (FRIS) model, an analytical framework that derives internal thermodynamic parameters, such as polytropic index, heating rate, and internal forces, using the remotely measurable kinematic quantities from coronagraphs (SOHO/LASCO and STEREO/COR). The original formulation of FRIS is critically re-evaluated, and key mathematical inconsistencies are corrected to ensure physical self-consistency in the derived quantities. This improved model is then applied to selected CMEs with contrasting kinematics, revealing detailed insights into their thermal state evolution and internal force balance during the early propagation phase. These results are further compared with in situ analyses of two selected ICMEs, where sustained heating is indicated by near-isothermal effective polytropic indices (Γ ≈ 0.88 & 0.76) and supported by turbulence diagnostics, such as inertial and dissipation-scale spectral slopes, low magnetic compressibility, and enhanced intermittency in the sheath and post- ICME regions, reinforcing the continuity of heating from the low corona to 1au. Beyond case-specific implementation, a focused investigation of nine Earth-directed, fast CMEs using the improved FRIS model and 3D kinematics derived from the Graduated Cylindrical Shell (GCS) reconstruction reveals a two-phase thermal evolution: an initial heat-release phase (Γ > 5/3) followed by a transition into a nearly isothermal heating state (Γ ≈ 0.8–1.2) at heliocentric distances between 3–7 𝑅⊙. This evolution is closely coupled with the CMEs’ expansion behavior, where CMEs with slower expansion acceleration show less pronounced temperature drops before reaching isothermal conditions. A detailed internal force balance analysis indicates that Lorentz forces act to suppress expansion during the early propagation phase, whereas thermal pressure and centrifugal forces drive the expansion, with thermal pressure becoming the dominant contributor at larger heliocentric distances. Differential emission measure (DEM) analysis of the CME source regions using SDO/AIA EUV data corroborates the presence of intrinsically hot plasma (2.8–7.2 MK), in agreement with the FRIS-derived These findings challenge the often assumed constancy of Γ in CME modeling and highlight the necessity of incorporating its dynamic evolution into global MHD simulations and predictive tools. The statistical component of the thesis analyzes CMEs across Solar Cycles 23, 24, and the ascending phase of Cycle 25 using in situ observations at 1 au from missions such as Wind and ACE. Within a polytropic framework, the thermal state of magnetic ejecta (MEs) is characterized using event-wise median proton polytropicindices (Γ𝑝), allowing classification into heating and cooling categories. MEs predominantly exhibit non-equilibrium thermal behavior, with a substantial fraction (∼45%) undergoing net heating during interplanetary propagation, especially near solar maxima. In contrast, cooling MEs maintain nearly constant and elevated Γ𝑝 values (∼2), indicating enhanced energy loss beyond adiabatic expectations or possible thermal retention from their eruption. A distinct solar cycle modulation is observed in Γ𝑝, proton temperature, and expansion speed, with the median Γ𝑝 increasing from 1.46 in Solar Cycle 23 to 1.87 in Cycle 24, suggesting a shift toward more cooling-dominated states over time. Despite thermodynamic similarities between magnetic clouds (MCs) and non-cloud ejecta, MCs consistently emerge as the most geoeffective structures, characterized by enhanced magnetic fields, low plasma beta, and suppressed Γ𝑝 values. Further, superposed epoch analyses (SEA) are performed on categorized ICME events, which reveal systematic variations in thermal state, plasma temperature, and magnetic field strength across distinct ICME substructures, including the pre-ICME, sheath, magnetic ejecta, and post-ICME regions. Notably, High-impact ICMEs are predominantly found to be thermally heated, with lower Γ𝑝 values,stronger sheath compression, elevated bulk speeds, and trailing high-speed solar wind, all contributing to enhanced geomagnetic storm potential. Finally, a detailed study of CME–CME interactions associated with the 10 May 2024 great storm shows that such interactions significantly modify both kinematics and thermodynamics of the involved structures. The resulting complex ejecta at 1 au exhibit enhanced magnetic fields, heat-release-dominated electrons,and bimodal ion thermal states. FRIS model analysis reveals diverse thermal behaviors among the individual CMEs, with most exhibiting a transition to an isothermal state by 6–9 𝑅⊙, except in cases of suppressed expansion. The findings underscore that CME–CME interactions can obscure one-to-one comparisons between solar-origin properties and in situ measurements, while also enhancing internal heating, structural complexity, and geo effectiveness. This thesis concludes by synthesizing results from analytical modeling, selected case studies, and long-term statistical analyses to construct a unified understanding of CME thermodynamics in the low corona and at 1 au. The findings reveal that CME thermal evolution is highly variable and depends on factors such as expansion dynamics, Solar Cycle phase, and interplanetary interactions. By integrating remote-sensing and in situ observations within a physically consistent thermodynamic framework, this work successfully bridges the gap between observational regimes, revealing the dynamic interplay between CME kinematics and internal thermal evolution. These insights not only advance the fundamental understanding of CME physics but also offer new pathways for improving space weather forecasting through diagnostics based on CME thermal state. Description: Thesis Supervisor Dr. Wageesh Mishra Mon, 01 Sep 2025 00:00:00 GMT http://hdl.handle.net/2248/8924 2025-09-01T00:00:00Z http://hdl.handle.net/2248/8923 Title: Deciphering the details of recent interactions in the magellanic clouds Authors: Hota, Sipra Abstract: The Small Magellanic Cloud (SMC), a dwarf galaxy in the local neighborhood, offers an accessible laboratory for exploring star formation processes in a metal-poor environment.The SMC’s structure, stellar populations, and kinematics bear clear evidence of its recent dynamical encounters with the Large Magellanic Cloud (LMC) and the Milky Way. In particular, the young massive stars formed in these disturbed conditions serve as crucial tracers of galaxy interactions, stellar feedback, and hierarchical star formation. However, until recently, the far ultraviolet (FUV) view of the SMC has been limited by spatial resolution and incomplete coverage. In this thesis, we employ data from the Ultra Violet Imaging Telescope (UVIT) onboard AstroSat together with complementary optical and near-infrared surveys (Gaia, SMASH, VMC) to present the most comprehensive FUV-based study of young stellar populations in the SMC to date. As the first step, we constructed a catalog of ∼76,800 SMC FUV stars, of which ∼62,900 are identified as probable SMC main body members. This catalog includes UV, optical, and IR fluxes, enabling the study of stellar populations across multiple evolutionary stages. Based on the Gaia optical color–magnitude diagram (CMD), FUV stars were classified into four young populations: Young 1, Young 2, Young 3, and Blue Loop.Their spatial distribution reveals a highly irregular and clumpy morphology, withstructures such as a broken bar, a shell-like feature, and the inner Wing. The Young1, Young 2, and Young 3 populations are concentrated in the eastern and northeasternSMC, while the Blue Loop, Young 2 and Young 3 dominate the southwest. Proper motion analysis demonstrates that stars younger than ∼150 Myr show evidence of east–west kinematic stretching, consistent with signatures of the recent LMC–SMC encounter, while no strong perturbation is seen along declination. To probe star formation closely, we analyzed clustering of stars with ages below 150 Myr, identifying 236 stellar structures whose sizes span from two parsecs to three hundred parsecs. Their irregular morphologies,perimeter-area dimension (Dp = 1.46± 0.04), fractal dimensions (D2 ∼ 1.3–1.6), and log-normal surface density distributionclosely resemble the properties of a turbulent interstellar medium. These results provide strong evidence that star formation in the SMC is hierarchical in nature and regulated by supersonic turbulence, with galaxy interactions supplying the driving mechanism for this turbulent regime. Expanding the analysis to the northeastern SMC Shell region, we combined UVIT data with Gaia EDR3 to create an FUV–optical catalog of ∼14,400 stars. Overlaying isochrones on FUV–optical CMD reveals multiple star formation episodes, most notablyat ∼260 Myr, linked to the last close LMC–SMC encounter, and at ∼60 Myr,associated with the SMC’s close approach to the Milky Way. The FUV stellar surface density, together with the dispersion in proper motion, indicates that the inner SMC extends northeastward to about 2.2°. In the FUV stellar density map, we identify arm-like and arc-like features whose kinematics are comparable to those of its main body. These outer extensions represent spatial overdensities of stars formed during multiple episodes of star formation, but they do not exhibit any clear kinematic distinction.The median proper motion and velocity dispersion are comparable to those of the SMC main body, suggesting that this region has not undergone significant tidal influence. Expanding the analysis to the northeastern SMC Shell region, we combined UVIT data with Gaia EDR3 to create an FUV–optical catalog of ∼14,400 stars. Overlaying isochrones on FUV–optical CMD reveals multiple star formation episodes, most notably at ∼260 Myr, linked to the last close LMC–SMC encounter, and at ∼60 Myr,associated with the SMC’s close approach to the Milky Way. The FUV stellar surface density, together with the dispersion in proper motion, indicates that the inner SMC extends northeastward to about 2.2°. In the FUV stellar density map, we identify arm-like and arc-like features whose kinematics are comparable to those of its main body. These outer extensions represent spatial over densities of stars formed during multiple episodes of star formation, but they do not exhibit any clear kinematic distinction.The median proper motion and velocity dispersion are comparable to thoseof the SMC main body, suggesting that this region has not undergone significant tidal influence. Finally, to characterize the properties of the SMC’s young stars, we choose a sample region within the Shell where multifilter UVIT data are available for robustness. We constructed spectral energy distributions (SEDs) for 1348 stars in the Shell region,spanning 18 photometric bands from UV to IR. Using single- and double-component fits, we derived effective temperatures, luminosities, and radii, identifying 1242 single systems which are mainly main sequence B- and A-type stars (10,000–30,000 K, 3–8 M⊙) and 85 systems which are double systems. These double systems include 18 stripped star binary systems, 9 subgiant-giant binary systems, and 20 candidate binaries. We also found 38 double systems, which could be non-contact binary or a star with a circumstellar disk or line-of-sight projections within the SMC. The characterized single and double systems include known eclipsing binaries, emission line stars, photometric variables, and pulsating variables. Taken together, the results presented in this thesis deliver the most extensive FUV catalog of the SMC to date and provide a detailed view of the distribution, clustering, kinematics, and properties of its young stars. Our findings establish that the morphology and dynamics of massive stars are strongly shaped by LMC–SMC interactions,while their spatial substructure reflects turbulence-driven hierarchical star formation. By identifying binary systems and stripped stars, this work also lays critical groundworkfor spectroscopic follow-up studies, which are essential to fully constrain the role of binarity and feedback in metal-poor environments. Looking forward, future UV missions such as UVEX and INSIST will further expand upon these results, offering deeper insight into how massive stars regulate the galaxies’ evolution in the early Universe. Description: Thesis Supervisor Prof. Annapurni Subramaniam Mon, 01 Sep 2025 00:00:00 GMT http://hdl.handle.net/2248/8923 2025-09-01T00:00:00Z