IIAP Ph.D. Theses

A multi wavelength study of novae in M31

Fri, 01 Aug 2025 00:00:00 GMT

A multi wavelength study of novae in M31 Basu, Judhajeet 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. Thesis Supervisor Dr. Sudhanshu Barway

Kinematics and thermodynamics of coronal mass ejections

Mon, 01 Sep 2025 00:00:00 GMT

Kinematics and thermodynamics of coronal mass ejections Khuntia, Soumyaranjan 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. Thesis Supervisor Dr. Wageesh Mishra

Deciphering the details of recent interactions in the magellanic clouds

Mon, 01 Sep 2025 00:00:00 GMT

Deciphering the details of recent interactions in the magellanic clouds Hota, Sipra 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. Thesis Supervisor Prof. Annapurni Subramaniam

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

Sun, 01 Jun 2025 00:00:00 GMT

Stellar population in the Magellanic Clouds: Star formation and evolution in diverse environments Dhanush, S. R 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. Thesis Supervisor Prof. Annapurni Subramaniam