2023

Abstract: While nuclei lighter than iron are fused over the course of typical stellar evolution, almost half of the elements heavier than iron are created through the rapid neutron capture process (r-process). These nuclei are thought to be produced in magnetized outflows from neutron-rich explosive events including compact mergers and core-collapse supernovae. In this talk, I will discuss the potential of neutrino-driven winds from strongly magnetized and rapidly rotating protomagnetars as plausible sites for r-process nucleosynthesis. As heavy nuclei can eventually produce ultra-high energy cosmic rays, we examine the acceleration and survival conditions for these nuclei. We also explore the propagation of these jets within Wolf-Rayet stars and blue/red supergiants. In particular, we analyze the criteria for a successful jet breakout, maximum energy deposited into the cocoon and structural stability of these magnetized jets. We show that high-energy neutrinos can be produced for extended progenitors like blue/red supergiants and estimate the detectability of these neutrinos with IceCube-Gen2.

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Abstract: Massive O and B stars release a significant portion, approximately 50%, of their original mass into space through stellar winds throughout their lifespan. This continuous mass loss plays a crucial role in shaping their evolution and the turbulence within nearby interstellar gas clouds. However, simulating magnetic massive stars remains challenging due to limitations in the employed simulation tools. The recent development of the Riemann-Geomesh MHD code allows for the handling of high rotation rates and large magnetic tilt angles, facilitating a comprehensive analysis of stellar winds and the X-ray emission from magnetic massive stars.

The initial set of simulations utilizes an isothermal MHD approach to investigate the overall dynamics of magnetically channeled winds under varying conditions, including different magnetic strengths, magnetic tilt angles and high rotation rates. These simulations are run until a quasi-steady state is reached, exhibiting the dynamics of magnetic channeling and centrifugal breakout events in the mass outflow. The mass outflow is confined by the magnetic field loops that form the closed magnetosphere of the star. The cataloged results provide valuable perspective on the variations in the rates of angular momentum loss and mass loss across different configurations of rotation rate, magnetic field strength, and large magnetic tilt angles.

Additionally, a second set of simulations focuses specifically on studying the high-temperature X-ray emission from θ1 Orionis C, a magnetic massive star. These simulations incorporate the energy equation and cooling terms, enabling accurate modeling of the formation and subsequent cooling of dense shock regions. Remarkably, the simulated data consistently reproduces the observed features, such as the peaks and positive slope in the X-ray emission measure distribution of θ1 Orionis C, as obtained from Chandra data. Furthermore, the total X-ray luminosity obtained from the simulated data aligns well with the observed range of θ1 Orionis C.

Abstract: Universe has several poorly constrained periods over its evolution. Formation of first stars and galaxies, followed by reionization of the intergalactic medium is one such epoch. Often referred to as "cosmic dawn", observing this period is extremely challenging due to the faint nature of the signals originating from it. The redshifted 21-cm line from neutral hydrogen offers one such possibility to observe the cosmic dawn and extract information about the nature of first stars and galaxies. In this talk, I will discuss our attempts at detecting the 21-cm signal from cosmic dawn. I will elaborate the challenges involved in detecting faint cosmological signals, and how our in-house designed experiments address those challenges. I will finally discuss the recent results from our observations, and explore how 21-cm signal can also be employed to probe other cosmic mysteries at different redshifts. 

Abstract: This work focuses on the gravitational interactions of astrophysical systems. In particular,  we focus on the triple system dynamics, including mildly hierarchical three body secular dynamics, as well as precession induced resonances of binaries under the perturbation of a third companion. We apply our theoretical investigations of these physical processes to wide-orbit planetary systems and black hole binaries embedded in AGN disks. More specifically, we consider the secular dynamics of a test particle in a mildly-hierarchical configuration. We find the limit within which the secular approximation is reliable, present resonances and chaotic regions using surface of sections, and characterize regions of phase space that allow large eccentricity and inclination variations. Finally, we apply the secular results to the outer solar system. We focus on the distribution of extreme trans-neptunian objects (eTNOs) under the perturbation of a possible outer planet (Planet-9), and find that in addition to a low inclination Planet-9, a polar or a counter-orbiting one could also produce pericenter clustering of eTNOs, while the polar one leads to a wider spread of eTNO inclinations.

Beyond mildly hierarchical triple dynamics, we also propose a novel pathway through which compact binaries could merge due to eccentricity excitation, including in a near coplanar configuration. Mechanisms have been proposed to enhance the merger rate of stellar mass black hole binaries, such as the Von Zeipel-Lidov-Kozai mechanism (vZLK). However, high inclinations are required in order to greatly excite the eccentricity and to reduce the merger time through vZLK. Specifically, a compact binary migrating in an AGN disk could be captured in a precession-induced resonance, when the apsidial and nodal precession rates of the binary are commensurable to the orbital period around the supermassive black hole. We find 8 such resonances upto quardupole order of the Hamiltonian.  We show that if a binary is captured in these resonances and is migrating towards the companion, it can experience large eccentricity and inclination variations. Eccentricity is excited when the binary sweeps through the resonance which happens only when it migrates on a timescale 10-100 times the libration timescale of the resonance. Libration timescale decreases as the mass of the disk increases. The eccentricity excitation of the binary can reduce the merger timescale by a factor up to $10^{3−5}$.

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Abstract: PRATUSH -- Probing ReionizATion of the Universe using Signal from Hydrogen -- is a proposed cosmology experiment to detect the global red-shifted 21-cm signal from the Cosmic Dawn and Epoch of Reionization (CD/EoR). PRATUSH orbiting the Moon will seek to precisely measure the low-frequency radio sky-spectrum over 40 to 200 MHz. The scientific observations would be made in the radio-quiet region when in the farside of the Moon, and the data would be transmitted back to Earth when in the near-side. PRATUSH was proposed to the Indian Space Research Organization (ISRO) during a call for proposals in the announcement of opportunity for science payloads in 2018. PRATUSH is in the pre-project studies phase. In this talk, I will discuss the motivation for PRATUSH, its current status and future plans.

Abstract: Role of magnetic fields (B-fields) and turbulence in star formation is still under debate but considering the magnetised nature of molecular clouds, we expect B-fields to have a significant impact on this process. Observations of molecular clouds in different environments using different astronomical techniques helps in understanding the various important aspects of star formation. I mostly worked towards mapping B-fields in nearby low-mass star forming regions in different environments viz. molecular clouds in isolation and HII regions at their different evolutionary stages using multi band polarization observations. This was done using optical, near-IR, and sub-mm polarization observations to map B-fields from pc to sub-pc scales. In this talk, I will briefly present my work on B-fields in various Galactic regions i.e. cores, filaments, and HII regions.  We know that grain alignment is the key to understand interstellar dust polarization. Therefore, I will mostly emphasise on my recently accomplished work on investigation of grain alignment efficiency in PDRs using dust polarization measurements.

Abstract: The “quasar mode” (or radiative mode) of AGN feedback, operated through outflows, plays an essential role in the evolution of galaxies. Quasar outflows are detected as blue-shifted broad absorption lines in the UV/optical spectra of quasars. Thanks to the Sloan digital sky survey, ~100,000 broad absorption line quasars are available now for ensemble statistical studies. This rich dataset has also enabled us to identify some peculiar cases of these sources. In this talk, I will review the current understanding of the quasar outflows and present our recent efforts to understand (i) the nature of these sources and (ii) the primary driver for the variability in the absorption lines.

Abstract: Recent coordinated observations and interpretations of disparate "messenger" signals from GW170817-GRB170817A-EM170817 have inaugurated the era of multi-messenger transient gravitational wave (GW) astronomy. I will argue that the bright blazar OJ 287 should allow us to pursue persistent multi-messenger GW astronomy during the era of Square Kilometer Array. This is mainly due to the several successful multi-wavelength observational campaigns that allowed us to argue for the presence of a spinning supermassive black hole binary that spirals in due to the emission of nano-Hertz GWs in the central engine of a unique blazar OJ287. Our on-going efforts, relevant to both the Event Horizon Telescope consortium and the International Pulsar Timing Array consortium which aims to detect GWs from such massive BH systems in the coming years, will be also listed.

Abstract: Coronal Mass Ejections (CMEs) are the most energetic and episodic expulsions of magnetized plasma from the Sun. In the talk, I will begin with the current status of the problem and specific challenges holding back progress in better understanding the evolution of CMEs in the heliosphere. I will highlight our efforts in implementing the three-dimensional reconstruction techniques, primarily using wide field-of-view imager's observations combined with in situ observations and modeling. Our study shows that the kinematics of CMEs can change significantly in the interplanetary medium due to the interaction of multiple CMEs and the substantial aerodynamic drag they experience. The talk will also briefly cover my recent attempts to use the estimated kinematics of the CMEs as inputs to an analytical model to probe the thermodynamic evolution of CMEs at distances much closer to the Sun that have yet to be accessed by in situ spacecraft. The talk's end segment will show a glimpse of solar variability due to episodic CMEs and the ubiquitous quasi-study solar wind over the last two decades. Finally, I will conclude with the lesson learned from our studies and further improvements required in CME research, which have much wider application across astrophysics and the search for habitable exoplanets.

Abstract: Stars are born deeply ensconced in dense clouds of interstellar medium. Protostellar jets launched in the early evolutionary phase of these young stellar objects are targets of intense scrutiny, to understand the properties of the protostellar systems. We have investigated a few jets from massive embedded protostars through their interaction with the ambient medium, in radio and infrared wavebands and we trace emission from shocked regions where the jet impacts the medium. Our results from GMRT, India, at low radio frequencies have confirmed the presence of non-thermal synchrotron emission from these jets, unlike measurements at higher radio frequencies that usually trace thermal free-free emission. We assimilate the observed emission measurements within a numerical model that we have developed, which incorporate the effects of both thermal and non-thermal emission, to understand the properties of the jets and ambient medium in the vicinity.

Abstract: A central aspect of Astrobiology research concerns the delineation of life’s origin and its early evolution on our planet. This intriguing and complex story that is thought to have started several billions of years ago, continues to be a very fascinating scientific mystery. It involves characterising how the transition from chemistry to biology would have occurred on the early Earth. In this context, I will give an overview of the current understanding prevalent in the field while also sharing some of the contributions that the COoL lab@IISER Pune has made towards these attempts. Pertinently, discerning the transition from non-life to life also has important implications for understanding life’s presence/distribution in the Universe.

Abstract: 'Slow-roll' of the inflaton field during inflationary phase is an essential feature of the inflationary dynamics, as the inflationary phase can make a 'graceful exit' when the inflaton field ceases to slow-roll. The slow-rolling of the inflaton field demands the inflaton potential to be nearly flat. However, when the potential becomes extremely flat, such as near any inflection point, the dynamics of the inflaton deviates from slow-roll and enters a phase of so-called Ultra slow-roll. Having a phase of Ultra slow-roll during standard inflationary phase often turns out to be useful, because such an Ultra slow-roll phase can enhance the cosmological perturbations enough which can later collapse and give rise to Primordial Black Holes. Presently, Primordial Black Holes are considered as favoured candidates for the Dark Matter. 

  In this talk, we will ask the question: "What happens in Warm Inflation if the dynamics enters an extremely flat part of the potential?" Warm inflation is a variant inflationary scenario where a concurrent radiation bath is maintained throughout inflation due to the dissipation of the inflaton energy densities to the radiation bath. The system is studied under the assumption of near thermal equilibrium which is one of the essential features of Warm Inflation. We will discuss in this talk the challenges one faces in maintaining the thermal equilibrium of the system when it undergoes an evolution through an extremely flat region of the potential. Thus, realising an Ultra slow-roll phase while keeping the thermal equilibrium in Warm Inflation is a daunting task. We will then show that only a subclass of Warm Inflationary models can undergo Ultra slow-roll while maintaining the thermal equilibrium of the Warm Inflationary system. 

Abstract: Merging stars and compact objects offer extraordinary laboratories to study a wide array of physical phenomena like binary stellar evolution, dust formation, gravitational waves and the origins of the heaviest elements in the universe. Yet, many gaps persist in our understanding of the wide spectrum of outcomes resulting from these mergers, that range from peculiar, low-luminosity explosions to large-amplitude variable stars. In this talk, I will demonstrate how the landscape of mergers can be explored systematically using time-domain surveys. I will describe results from searches for merging massive stars, white dwarfs and neutron stars using the Zwicky Transient Facility (ZTF), Palomar Gattini IR (PGIR) and the newly commissioned Wide-Field Infrared Transient Explorer (WINTER) surveyors at the Palomar Observatory in California. For each class of mergers, I will discuss insights that these systematic samples provide about the formation, evolution and final fates of their progenitor binary systems. In closing, I will describe the exciting potential of the upcoming time-domain surveys in significantly improving our view of the merger landscape.

Abstract: Geodesy is the science of measuring Earth’s shape, size, orientation, and gravity in a 4-D (space-time) coordinate system. This field is at least 2500 years old and is an ally of astronomy. However, the major advances in Geodesy have been made in the last 4 decades after the advent of the satellite era. The most influential development occurred with the launch of GRACE satellite mission in 2002. The concept of this satellite mission is unique and distinctly different from any satellite mission. It provides changes in the gravitational field of the Earth at monthly scale, which has been used to study the mass transport in the Earth system. In this talk, I will shed light on GRACE satellite mission, its working principle, major breakthroughs, and its limitations. Towards the end, I will introduce a new emerging field, Relativistic Geodesy, and talk about opportunities and challenges.

Abstract: The Galactic Warm Ionized Medium (WIM) is one of the four atomic phases of the Galaxy, containing most of the ionized gas (and not HII regions).  The consensus view is that the WIM is powered by ionizing radiation leaking from HII regions. However, we lack clear understanding of how the ionizing radiation diffuses to great distances.  The traditional probes are H-alpha (degree scale; Fabry-Perot) and pulsar dispersion measure (point measurement). Fabry-Perot observations have been conducted in a few optical nebular and auroral lines of metals.  Here, I discuss the diagnostic power of  IFUs on large space- (JWST) & ground-based (Keck) optical telescopes that allow us to study the WIM on scales of arseconds. I will fortify this conclusion by presenting detections of the Galactic diffuse medium via mid-IR fine structure lines with the MIRI-MRS IFU on JWST. I will end the talk reviewing an old but important topic: the ionization fraction of H and He in the WIM.

Abstract: To understand black hole growth, we need to understand the physical processes that drive the accretion of the gas on to the black hole and how they evolve over time. Stellar-mass black holes in X-ray Binaries (XRBs) exhibit extreme spectral state transitions that occur on observable timescales and as a function of accretion rate. A comparison of spectral state changes between stellar and supermassive black holes can inform our understanding of AGN accretion. However, observable timescales for state transitions of AGN are typically not attainable, but can be explored with a large sample of AGN. Here, I will present our analysis of a sample of 3500+ AGN with simultaneous UV and X-ray observations from the XMM-Newton and Neil Gehrels Swift satellites, complemented with radio, optical and infrared data. Our results establish that AGN and XRBs display analogous spectral states, most notably linking the radio emission from the relativistic jet with the energetic emission from the X-ray corona and thermal emission from an accretion disc. I will highlight our unique result demonstrating how the AGN radio morphology correlates with the accretion state change, analogous to the presence of radio jets during a typical XRB outburst. Further, I will present the salient results of our analysis of how different AGN properties, viz. nuclear obscuration, Seyfert type, optical classification are correlated with their observed accretion states, along with their relation to the star formation and stellar masses of their host-galaxies, hinting at the co-evolution of AGN and host galaxy with feeding and feedback.

Abstract: The search for planets beyond our solar system is one of the biggest scientific quests at present. The first exoplanet detection by the radial velocity (RV) method nearly three decades back was awarded the Nobel prize in Physics. Since then, the number of exoplanets found has increased exponentially with nearly 5000 exoplanet detections till date. We have learnt that diverse planetary systems form with orbits, densities, and planetary atmospheres that are very different from the planets in our solar system. In my talk, I will discuss planet demographics across the stellar spectral range focusing on large planets around the solar-type stars and smaller planets around the late spectral type stars. Despite the advent of large telescopes and precise spectrographs, the detection of smaller planets is largely limited by the intrinsic stellar jitter. I will talk about the ways that we devise to disentangle this stellar jitter from the planet signature. I will finally highlight the role of small to medium-size telescopes for exoplanet characterization in the era of current and future space missions like TESS and PLATO.

Abstract: Energy transport across a wide range of dynamical scales in the intracluster medium (and generally in gaseous halos) is one of the most interesting topics in current research and future interest. Hot baryons, visible in the X-rays, need to be stably sustained against radiative cooling over a large inner fraction of the cluster virial radius. A historical motivation has been the lack of sufficient observed cold gas in the cluster cores that is expected in the absence of efficient heating. Quantitatively, there is enough energy from active galactic nuclei to solve the problem at the simplest level, but the complexity of how that energy flows around is not well understood. Multiple transport mechanisms are being actively discussed including long wavelength, nearly isotropic sound waves, anisotropic heat conduction only along local magnetic fields (depending on the local temperature gradient), generation and dissipation of volume-filling turbulence, etc. While sound waves and turbulence have been strong contenders, thermal conduction has been claimed to be further suppressed by gyro-scale whistlers that scatter thermal electrons efficiently in the weakly magnetised ICM. In the latter scenario, thermal instability domain may be enhanced leading to excess and/or smaller scale cold gas. This further implies that observations may need to account for excess cold/mixed phase gas. In my talk, I will discuss these topics of energy transport in the ICM and the consequences. 

Abstract: It is estimated that the solar surface is covered by millions of (almost ballistic) plasma jets at any given time that are believed to be conduits for transfer of mass and momentum especially to the fast solar wind in the coronal holes. About 2 decades ago it was conjectured that these jets are powered by the energy of the bubbling solar convection beneath. However, in later years the narrative has taken a hyperbole to instead include complex microphysics of the solar chromosphere in order explain a certain energetic variety of these spicules (a.k.a type-II) which are believed to be more important than their lethargic counterparts (type I). In our recent work, we combine solar observations, 2D numerical simulations and laboratory experiments to show that observed features of spicule jets can be clearly explained by the inclusion of realistic solar convection in presence of magnetic fields. We further find that our results contradict the existing paradigm of bi-modality (types I vs II) of solar spicules. Our 3D magneto-convection simulations also bring forth a surprising topological possibility- that spicules may not be conical 1D plasma needles dotting the solar surface but, rather like folds in the 2D plasma drapery embedded in the 3D solar atmosphere.

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Abstract: To understand the structuring and dynamics of the upper photosphere and the chromosphere of the Sun, we need to improve and extend the existing numerical radiation-magnetohydrodynamical (MHD) simulations. In the solar chromosphere, radiative energy transport is dominated by only the strongest spectral lines. For these lines, the approximation of local thermo-dynamic equilibrium (LTE) is known to be very inaccurate, and a state of equilibrium cannot be assumed in general. To calculate the radiative energy transport under these conditions, the population evolution equation must be evaluated including all time dependent terms. To this end, we have developed a non-LTE non-equilibrium radiative transfer (RT) module to the well-known MHD code MURaM. In this module we have developed a numerical method to solve the evolution equation for the atomic level populations in a time-implicit way, keeping all time dependent terms to first order. Our main non-equilibrium treatment is of the Hydrogen bound and free states. For the equation of state, to determine kinetic temperature, we treat the Hydrogen molecular evolution also in non-equilibrium. The other elements comprising the gas are treated in LTE. Finally, the pressure and the radiative flux divergence from the RT module are provided to the MHD equations, to evolve the MHD and the radiative quantities self-consistently and iteratively. The module is developed for one, two and three dimensions (1d,2d, 3d) but currently tested in 1d. In this talk, I will describe the method, discuss equilibrium solutions and show some results of the dynamic evolution.

Abstract: The energy produced by supernovae or supermassive black holes plays a major role in dictating the evolution of galaxies. The produced energy is directly responsible for heating the star-forming gas in the interstellar medium or stopping the gas to reach the galaxy thus suppressing overall star formation. The interaction between the produced energy and the interstellar medium is often mediated by bubbles and winds, such as the gamma-ray bubbles (known as the Fermi Bubbles) in our Galaxy. Understanding such processes require multi-wavelength observations and a detailed theoretical understanding of these bubbles and winds. In the talk, I will present some of my recent works to understand how bubbles and winds produced by supernovae and supermassive black holes interact with the interstellar medium and their observational signatures. I will also present my works on the state-of-the-art numerical simulations that have been successful to discover so far unknown properties in these systems and are bringing us much closer to bridging the gap between theory and observations in the interstellar medium.

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Abstract: Stellar feedback is one of the most important ingredients in the evolution of the interstellar medium (ISM). Massive stars inject immense amounts of energy into their surroundings through stellar winds (mechanical feedback) and through emitting energetic photons (radiative feedback). Mechanical energy input pushes the gas into different physical structures and the radiative energy input heats up the surrounding medium by ionizing and dissociating various species. Quantifying stellar feedback is crucial to understand its impact on various physical and chemical processes in the ISM. FEEDBACK is a recent observational survey performed using the SOFIA telescope to observe C+ in 11 sources of our Galaxy. I will present the first results of this survey emphasizing the need of multi-wavelength observations and machine learning to understand our Galaxy better.

Abstract: Metal-poor stars of $\lesssim 0.8 ~\mathrm{M}_\odot$ that have [Fe/H]~$\lesssim -2.5$ are thought to have formed within $\sim 1$ Gyr after the Big Bang. Because of their low mass, they have very long lifetimes and are still around today. The surface composition of these stars is a fossil record of interstellar gas in the early Galaxy from which they were formed and are crucial for studying the early Galactic and chemical evolution. Abundance patterns of elements observed in these stars provide a unique probe for studying the nucleosynthesis and constraining the nature of the first and early massive stars. Interestingly, elements heavier than Fe group such as Sr, Ba, and Pb, which are primarily produced by neutron capture processes, are ubiquitous in these stars. Furthermore, their abundance patterns show a large variation that seems to indicate that all types of neutron capture processes, rapid, intermediate, and slow, operated in the early Galaxy. The sites for neutron capture processes that can operate at such early times however are still a major puzzle. I will discuss the current status of the sites heavy element synthesis in the early Galaxy including recent results from my works.

Abstract: There are several classes of modified gravity theories whose extra degrees of freedom are not completely screened in the interiors of stellar and substellar objects. Such theories predict an altered hydrostatic equilibrium condition inside these objects. Moreover, the interior structures of these objects might have a small pressure anisotropy induced by several physical processes, including rotation and magnetic fields. All these effects, both individually and in conjunction with each other, induce changes in the predicted stellar observables. Such changes also have an impact on different phases of the stellar life cycle, starting from its birth to its death. The aim of this talk will be to understand these and predict new constraints using stellar physics within the framework of degenerate higher-order scalar-tensor theories beyond Horndeski.

Abstract: Magnetic braking, a mechanism with which angular momentum is lost from the system, governs the evolution of binary stars such as cataclysmic variables (CVs) and AM Canum Venaticorum (AM CVn) stars, as well as regulates the spin-down of single stars, like our Sun. However, a physical phenomenon governing such a braking mechanism is still not well understood. In this talk, I will present our physically motivated Double Dynamo formalism of magnetic braking, which explains the prominent features of CVs and AM CVns. In addition, I will touch upon how this model can be used to address the problem of stellar spin-down and its possible implications on habitability.

Abstract: Primordial non-Gaussianity arising from inflationary models is a unique probe of non-trivial dynamics of the inflaton field and its possible interactions with other fields. Often when examining and constraining the scalar non-Gaussianity arising from inflation, certain templates are adopted for the scalar non-Gaussianity parameter f_NL. The current constraints from cosmic microwave background (CMB) on such templates of f_NL provide weak bounds on their amplitudes. However, f_NL shall lead to indirect imprints on the scalar power spectrum through the higher-order loop-level contributions and we may expect effective constraints from them. In this talk, I shall discuss our recent examinations of such non-Gaussian imprints on observables namely, the angular spectrum of CMB and the spectral density of scalar-induced secondary gravitational waves. I shall present the interesting insights provided by this method, particularly for realistic models that do not conform to conventional templates of f_NL. I shall illustrate that the non-Gaussian contributions to observable spectra are of strengths comparable to the Gaussian contributions and sensitive to model parameters that are degenerate at the level of the Gaussian power spectrum. Finally, I shall conclude with an outlook of exciting possibilities of different implementations of this method.