arXiv subjects

Kinsuk Acharyya

Publications and source records attributed to Kinsuk Acharyya.

At least 19 recordsLinked to original sources

arXiv2026

Delayed Methane Detection in 3I/ATLAS: GCR-Driven Subsurface Stratification and Interstellar Exposure Age Constraints

Interstellar comet 3I/ATLAS shows unusual volatile activity, including strong H$_2$O, CO, and CO$_2$ production and a delayed post-perihelion detection of CH$_4$. In this work, we investigate the delayed CH$_4$ detection in an H$_2$O, CO$_2$, and CO ice matrix using a shape model coupled with a thermophysical nucleus model that incorporates interstellar processing by Galactic Cosmic Rays. We track the sublimation, trapping, depletion, and release of these volatiles as a function of heliocentric distance and explore the relevant parameter space through 12000 model runs to identify viable combinations of primordial volatile inventory and effective GCR exposure age that reproduce the observed production rates of these volatiles. The viable models favour a volatile-rich primordial inventory consistent with formation in an ultra-cold ($T < 20$ K) region beyond the CO snowline, followed by an effective GCR processing exposure of 0.87--3.1 Gyr during interstellar passage. GCR processing produces a continuous compositional gradient and converts part of the CO reservoir into secondary CO$_2$ and depletes near-surface CH$_4$ through radiolytic conversion. The delayed CH$_4$ detection can be explained by the survival of CH$_4$ trapped within the deeper H$_2$O matrix and its subsequent release as subsurface heating progresses after perihelion. The timing of CH$_4$ sublimation can be used to estimate the effective GCR processing age of 3I/ATLAS. The posterior distributions also reveal a degeneracy between primordial dust content and GCR exposure, where dust-rich, weakly processed and ice-rich, strongly irradiated evolutionary pathways produce processed subsurface layers with similar thermophysical properties.

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arXiv2025

Research on the Interstellar Medium and Star Formation in the Galaxy: An Indian Perspective

Although the star formation process has been studied for decades, many important aspects of the physics involved remain unsolved. Recent advancement of instrumentation in the infrared, far-infrared and sub-millimetre wavelength regimes have contributed to a significantly improved understanding of processes in the interstellar medium (ISM) leading to star formation. The future of research on the ISM and star formation looks exciting with instruments like the JWST, ALMA, etc., already contributing to the topic by gathering high-resolution high-sensitivity data and with several larger ground- and space-bound facilities either being planned or constructed. India has a sizable number of astronomers engaged in research on topics related to the ISM and star formation. In this white paper invited by the Astronomical Society of India to prepare a vision document for Indian astronomy, we review the Indian contributions to the global understanding of the star formation process and suggest areas that require focused efforts both in creating observing facilities and in theoretical front in India, in order to improve the impact of our research in the coming decades.

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arXiv2024

The extent of formation of organic molecules in the comae of comets showing relatively high activity

Comets are a rich reservoir of complex organic molecules. Ground and space-based observatories have recently greatly enhanced the cometary molecular inventory. Although these molecules' origin is believed to be the cometary nucleus, they can be partially synthesised in the coma. We studied organic molecules' nucleus versus coma origins for various initial conditions, using a multifluid chemical-hydrodynamical model and an updated chemical network. For the study, we considered four comets [C/1996 B2 (Hyakutake), C/2012 F6 (Lemmon), C/2013 R1 (Lovejoy), and C/2014 Q2 (Lovejoy)] due to their relatively high activity and observation of large number organics species. We emphasised on the C-H-O and N-bearing species, including the simplest amino acid, glycine. We discuss the formation pathways of the organics and the conditions for their formation in the coma and find that the abundance varies from one comet to another due to differences in the initial conditions, relative abundances of the reactants and temperature. We compare the organic abundances when they are present as parent volatiles to their formation solely due to gas-phase chemistry. Their abundance purely due to the coma chemistry is moderately to significantly lower compared to that when they are parent volatiles. However, we find that the production rates of some of the coma-synthesised organic molecules can reach peak values of ~ 1e22 - 1e26 molecules/s, which is in the realm of detection by in situ/space-based observations, and can therefore be important considering future missions to comets. We also compare our modeled abundances with those observed in 67P/C-G by Rosetta, which detected several organics at a large heliocentric distance and low production rate.

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arXiv2024

Signature of Vertical Mixing in Hydrogen-dominated Exoplanet Atmospheres

Vertical mixing is a crucial disequilibrium process in exoplanet atmospheres, significantly impacting chemical abundance and observed spectra. While current state-of-the-art observations have detected its signatures, the effect of vertical mixing on atmospheric spectra varies widely based on planetary parameters. In this study, we explore the influence of disequilibrium chemistry across a parameter space that includes eddy diffusion, surface gravity, internal and equilibrium temperature, and metallicity. We also assess the effectiveness of retrieval models in constraining the eddy diffusion coefficient. By running numerous 1D chemical kinetics models, we investigate the impact of vertical mixing on the transmission spectrum. We also built a custom fast-forward disequilibrium model, which includes vertical mixing using the quenching approximation and calculates the model abundance orders of magnitude faster than the chemical kinetics model. We coupled this forward model with an open source atmospheric retrieval code and used it on the JWST simulated output data of our chemical kinetics model and retrieved eddy diffusion coefficient, internal temperature and atmospheric metallicity. We find that there is a narrow region in the parameters space in which vertical mixing has a large effect on the atmospheric transmission spectrum. In this region of the parameter space, the retrieval model can put high constraints on the transport strength and provide optimal exoplanets to study vertical mixing. Also, the NH3 abundance can be used to constrain the internal temperature for equilibrium temperature T_equi > 1400 K.

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arXiv2023

Using a quench level approximation to estimate the effect of metallicity on N-bearing species abundances in H2-dominated atmospheres

Variations in atmospheric elemental nitrogen can considerably affect the abundance of major nitrogen-bearing species such as NH$_3$ and HCN. Also, due to vertical mixing and photochemistry, their abundance deviates from the thermochemical equilibrium. The goal of this study is to understand the effect of atmospheric metallicity on the composition of NH$_3$, N$_2$, and HCN over a large parameter space in the presence of vertical mixing which, when combined with the work on CHO-bearing species in Soni and Acharyya (2023) can provide a comprehensive understanding of the effect of atmospheric metallicity. We used quenching approximations and a full chemical kinetics model for the calculations, and a comparison between these two methods was made. For generating thermal profiles, petitRADTRANS code is used. Chemical timescales of NH$_3$ and N$_2$ are found to be complex functions of metallicity, while HCN is inversely proportional. Using NH$_3$ and CO quenched abundances, the HCN quenched abundance can be constrained since it remains in equilibrium with NH$_3$, CO, and H$_2$O. Quenched NH$_3$ increases with increasing K$_{zz}$ untill a particular point, after which it becomes independent of vertical mixing. There is a sweet spot in the K$_{zz}$ parameter space to maximize the quenched HCN for a given T$_{int}$ and T$_{equi}$; the parameter space moves towards the lower equilibrium temperature, and HCN abundance increases with metallicity. Finally, we used the dataset of quenched abundances to provide a list of potential candidates in which HCN observation can be possible.

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arXiv2023

The Effect of Metallicity on the Non-Equilibrium Abundance of Hydrogen Dominated Exoplanet Atmosphere

The atmospheric metallicity greatly influences the composition of exoplanet atmospheres. The effect of metallicity on the thermochemical equilibrium is well studied, though its effect on the disequilibrium abundance is loosely constrained. In this study, we have used the quenching approximation to study the effect of metallicity on the quenched abundance for a range of parameters (temperature: 500-2500 K, pressure: 10$^{-4}$-10$^3$ bar, metallicity: 0.1-1000 $\times$ solar metallicity). We determine the chemical timescale by finding rate limiting steps in a reduced chemical network with a network analysis tool and the thermochemical equilibrium abundance. The equilibrium abundance results are similar to the literature. The CO, H$_2$O, and CO$_2$ abundances increase with metallicity in the parameter range considered. The CH$_4$ abundance increases with metallicity for CO/CH$_4$ $<$ 1 and is unaffected for CO/CH$_4$ $>$ 1. The chemical timescale of CO shows minimal change with the metallicity, while the CH$_4$ chemical timescale is inversely proportional to atmospheric metallicity. The quench level of CO shifts into the high-pressure region, and the quench level of CH$_4$ shows complex behavior with metallicity. We benchmarked the quenching approximation with the 1D photochemistry-transport model for two test exoplanets (GJ 1214 b and HD 189733 b) and found it to be in good agreement. We also found that the quenching approximation is a powerful tool to constrain atmospheric parameters. We demonstrated this by constraining the metallicity and transport strength for the test exoplanets HR 8799 b, HD 189733 b, GJ 436 b, and WASP-39 b.

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arXiv2022

The mid-infrared molecular inventory towards Orion IRc2

We present the first high spectral resolution mid-infrared survey in the Orion BN/KL region, covering 7.2 to 28.3 micron. With SOFIA/EXES we target the enigmatic source Orion IRc2. While this is in the most prolifically studied massive star-forming region, longer wavelengths and molecular emission lines dominated previous spectral surveys. The mid-infrared observations in this work access different components and molecular species in unprecedented detail. We unambiguously identify two new kinematic components, both chemically rich with multiple molecular absorption lines. The "blue clump" has vLSR = -7.1 \pm 0.7 km/s and the "red clump" 1.4 \pm 0.5 km/s. While the blue and red clumps have similar temperatures and line widths, molecular species in the blue clump have higher column densities. They are both likely linked to pure rotational H2 emission also covered by this survey. This work provides evidence for the scenario that the blue and red clumps are distinct components unrelated to the classic components in the Orion BN/KL region. Comparison to spectroscopic surveys towards other infrared targets in the region show that the blue clump is clearly extended. We analyze, compare, and present in depth findings on the physical conditions of C2H2, 13CCH2, CH4, CS, H2O, HCN, H13CN, HNC, NH3, and SO2 absorption lines and an H2 emission line associated with the blue and red clumps. We also provide limited analysis of H2O and SiO molecular emission lines towards Orion IRc2 and the atomic forbidden transitions [FeII], [SI], [SIII], and [NeII].

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arXiv2022

A possible solution for the faint young Sun paradox: Clues from the exoplanetary data

Faint young Sun paradox (FYSP) is one of the unsolved problem in solar physics. The present study aims to get a possible solution for the FYSP through sun-like G stars and their exoplanetary systems. Using physical properties of exoplanetary data, an empirical relationship between the rate of mass loss ($\frac{dM}{dt}$) with stellar mass (M$_{\star}$) and age ({\em t}) is obtained. We found mass loss rate varies with stellar mass as $\propto$ $(M_{\star}/M_\odot)^{-3.788}$ and proportional to the age as $\propto$ t$^{-1.25}$, which indicates rate of mass loss is higher during early evolutionary stages. Then we applied mass loss corrections to stellar masses of G-type stars with planets and obtained their initial masses at the early evolutionary stages. Subsequently, we applied these relationships to calculate the mass loss rate and mass of Sun at the early evolutionary stage, which is found to be $\sim$ 10$^{-11}$ solar mass per year and $\sim$ (1.061$\pm$0.006) solar mass respectively. The higher solar mass can probably alleviate the problem of the faint young Sun paradox. Then the estimated initial stellar masses of the host stars are used to obtain a best power law relationship with the planetary masses that supports the hypothesis that the {\em massive stars harbour massive planets.} Finally, by using the same empirical power law, planetary mass in the vicinity of Sun is estimated to be $\sim$ (0.84$\pm$0.19) Jupiter mass, which is much higher compared to the present solar terrestrial planetary mass. Hence, this study also suggests that there is a missing planetary mass in the vicinity of the Sun, which can solve the FYSP problem.

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arXiv2022

Understanding the impact of diffusion of CO in the astrochemical models

The mobility of lighter species on the surface of interstellar dust grains plays a crucial role in forming simple through complex molecules. Carbon monoxide is one of the most abundant molecules, its surface diffusion on the grain surface is essential to forming many molecules. Recent laboratory experiments found a diverse range of diffusion barriers for CO on the grain surface, their use can significantly impact the abundance of several molecules. The impact of different diffusion barriers of CO, in the astrochemical models, is studied to understand its effect on the abundance of solid CO and the species for which it is a reactant partner. A gas-grain network is used for three different physical conditions; cold-core and warm-up models with slow and fast heating rates. Two different ratios (0.3 and 0.5) between diffusion and desorption barrier are utilized for all the species. For each physical condition and ratio, six different models are run by varying diffusion barriers of CO. Solid CO abundance for the models with the lowest diffusion barrier yields less than 0.1% of water ice for cold clouds and a maximum of 0.4% for slow and fast warm-up models. Also, solid CO$_2$ in dense clouds is significantly overproduced (140 % of water). The abundance of H$_2$CO and CH$_3$OH showed an opposite trend, and HCOOH, CH$_3$CHO, NH$_2$CO, and CH$_3$COCH$_3$ are produced in lower quantities for models with low diffusion barriers for CO. Considerable variation in abundance is observed between models with the high and low diffusion barrier. Models with higher diffusion barriers provide a relatively better agreement with the observed abundances when compared with the models having lower diffusion barriers.

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arXiv2020

The First Mid-Infrared Detection of HNC in the Interstellar Medium: Probing the Extreme Environment Towards the Orion Hot Core

We present the first mid-infrared (MIR) detections of HNC and H13CN in the interstellar medium, and numerous, resolved HCN rovibrational transitions. Our observations span 12.8 to 22.9 micron towards the hot core Orion IRc2, obtained with the Echelon-Cross-Echelle Spectrograph aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Exceptional, ~5 km/s, resolution distinguishes individual rovibrational transitions of the HNC and HCN P, Q, and R branches; and the H13CN R branch. This allows direct measurement of the species' excitation temperatures, column densities, and relative abundances. HNC and H13CN exhibit a local standard rest velocity of -7 km/s that may be associated with an outflow from nearby radio source I and an excitation temperature of about 100 K. We resolve two velocity components for HCN, the primary component also being at -7 km/s with temperature 165 K. The hottest component, which had never before been observed, is at 1 km/s with temperature 309 K. This is the closest component to the hot core's centre measured to date. The derived 12C/13C=13 is below expectation for Orion's Galactocentric distance, but the derived HCN/HNC=72 is expected for this extreme environment. Compared to previous sub-mm and mm observations, our SOFIA line survey of this region shows that the resolved MIR molecular transitions are probing a distinct physical component and isolating the chemistry closest to the hot core.

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arXiv2018

High Spectral Resolution SOFIA/EXES Observations of C2H2 towards Orion-IRc2

We present high-spectral resolution observations from 12.96 - 13.33 microns towards Orion IRc2 using the mid-infrared spectrograph, EXES, on SOFIA. These observations probe the physical and chemical conditions of the Orion hot core, which is sampled by a bright, compact, mid-infrared background continuum source in the region, IRc2. All ten of the rovibrational C2H2 transitions expected in our spectral coverage, are detected with high S/N, yielding continuous coverage of the R-branch lines from J=9-8 to J=18-17, including both ortho and para species. Eight of these rovibrational transitions are newly reported detections. The isotopologue, 13CCH2, is clearly detected with high signal-to-noise. This enabled a direct measurement of the 12C/13C isotopic ratio for the Orion hot core of 14 +/- 1 and an estimated maximum value of 21. We also detected several HCN rovibrational lines. The ortho and para C2H2 ladders are clearly separate and tracing two different temperatures, 226 K and 164 K, respectively, with a non-equilibrium ortho to para ratio (OPR) of 1.7 +\- 0.1. Additionally, the ortho and para V_LSR values differ by about 1.8 +/- 0.2 km/s, while, the mean line widths differ by 0.7 +/- 0.2 km/s, suggesting that these species are not uniformly mixed along the line of sight to IRc2. We propose that the abnormally low C2H2 OPR could be a remnant from an earlier, colder phase, before the density enhancement (now the hot core) was impacted by shocks generated from an explosive event 500 yrs ago.

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arXiv2016

Sticking of molecules on non-porous amorphous water ice

Accurate modeling of physical and chemical processes in the interstellar medium requires detailed knowledge of how atoms and molecule adsorb on dust grains. However, the sticking coefficient, a number between 0 and 1 that measures the first step in the interaction of a particle with a surface, is usually assumed in simulations of ISM environments to be either 0.5 or 1. Here we report on the determination of the sticking coefficient of H$_2$, D$_2$, N$_2$, O$_2$, CO, CH$_4$, and CO$_2$ on non-porous amorphous solid water (np-ASW). The sticking coefficient was measured over a wide range of surface temperatures using a highly collimated molecular beam. We showed that the standard way of measuring the sticking coefficient --- the King-Wells method --- leads to the underestimation of trapping events in which there is incomplete energy accommodation of the molecule on the surface. Surface scattering experiments with the use of a pulsed molecular beam are used instead to measure the sticking coefficient. Based on the values of the measured sticking coefficient we suggest a useful general formula of the sticking coefficient as a function of grain temperature and molecule-surface binding energy. We use this formula in a simulation of ISM gas-grain chemistry to find the effect of sticking on the abundance of key molecules both on grains and in the gas-phase.

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arXiv2016

Binding Energy of Molecules on Water Ice: Laboratory Measurements and Modeling

We measured the binding energy of N$_2$, CO, O$_2$, CH$_4$, and CO$_2$ on non-porous (compact) amorphous solid water (np-ASW), of N$_2$ and CO on porous amorphous solid water (p-ASW), and of NH$_3$ on crystalline water ice. We were able to measure binding energies down to a fraction of 1\% of a layer, thus making these measurements more appropriate for astrochemistry than the existing values. We found that CO$_2$ forms clusters on np-ASW surface even at very low coverages. The binding energies of N$_2$, CO, O$_2$, and CH$_4$ decrease with coverage in the submonolayer regime. Their values at the low coverage limit are much higher than what is commonly used in gas-grain models. An empirical formula was used to describe the coverage dependence of the binding energies. We used the newly determined binding energy distributions in a simulation of gas-grain chemistry for cold cloud and hot core models. We found that owing to the higher value of desorption energy in the sub-monlayer regime a fraction of all these ices stays much longer and up to higher temperature on the grain surface compared to the single value energies currently used in the astrochemical models.

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arXiv2013

Deep observations of O2 toward a low-mass protostar with Herschel-HIFI

According to traditional gas-phase chemical models, O2 should be abundant in molecular clouds, but until recently, attempts to detect interstellar O2 line emission with ground- and space-based observatories have failed. Following the multi-line detections of O2 with low abundances in the Orion and rho Oph A molecular clouds with Herschel, it is important to investigate other environments, and we here quantify the O2 abundance near a solar-mass protostar. Observations of O2, at 487 GHz toward a deeply embedded low-mass Class 0 protostar, NGC 1333-IRAS 4A, are presented, using the HIFI instrument on the Herschel Space Observatory. Complementary data of the chemically related NO and CO molecules are obtained as well. The high spectral resolution data are analysed using radiative transfer models to infer column densities and abundances, and are tested directly against full gas-grain chemical models. The deep HIFI spectrum fails to show O2 at the velocity of the dense protostellar envelope, implying one of the lowest abundance upper limits of O2/H2 at <6x10^-9 (3 sigma). However, a tentative (4.5 sigma) detection of O2 is seen at the velocity of the surrounding NGC 1333 molecular cloud, shifted by 1 km/s relative to the protostar. For the protostellar envelope, pure gas-phase models and gas-grain chemical models require a long pre-collapse phase (~0.7-1x10^6 years), during which atomic and molecular oxygen are frozen out onto dust grains and fully converted to H2O, to avoid overproduction of O2 in the dense envelope. The same model also reproduces the limits on the chemically related NO molecule. The tentative detection of O2 in the surrounding cloud is consistent with a low-density PDR model with small changes in reaction rates. The low O2 abundance in the collapsing envelope around a low-mass protostar suggests that the gas and ice entering protoplanetary disks is very poor in O2.

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arXiv2010

Effects of Initial Condition and Cloud Density on the Composition of the Grain Mantle

Evolution of grain mantles in various interstellar environment is studied. We concentrate mainly on water, methanol, carbon di-oxide, which constitute nearly 90% of the grain mantle. We investigate how the production rates of these molecules depend on the relative gas phase abundances of oxygen and carbon monoxide and constrain the relevant parameter space which reproduces these molecules closed to the observed abundances. Allowing to accrete only H, O and CO on the grains and using the Monte-Carlo method we follow the chemical processes for a few million years. We allow formation of multi-layers on the grains and incorporate the freeze-out effects of accreting O and CO. We find that the formation of these molecules depends on the initial conditions as well as the average cloud density. Specifically, when the number density of accreting O is less than 3 times more than that of CO, methanol is always over-produced. Using available reaction pathways it appears to be difficult to match the exact observed abundances of all the three molecules simultaneously. Only in a narrow region of parameter space all these three molecules are produced within the observed limit. In addition to this, we found that the incorporation of the freeze-outs of O and CO leads to almost steady state on the grain surface. The mantle thickness grows anywhere between 60 to 500 layers in a period of two million years. In addition, we consider a case where the gas number density changes with time due to gradual collapse of the molecular cloud and present the evolution of composition of different species as a function of radius of the collapsing cloud.

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arXiv2008

Formation of water and methanol in star forming molecular clouds

We study the formation of water and methanol in the dense cloud conditions to find the dependence of its production rate on the binding energies, reaction mechanisms, temperatures, and grain site number. We wish to find the effective grain surface area available for chemical reaction and the effective recombination timescales as functions of grain and gas parameters. We used a Monte Carlo simulation to follow the chemical processes occurring on the grain surface. We find that the formation rate of various molecules is strongly dependent on the binding energies. When the binding energies are high, it is very difficult to produce significant amounts of the molecular species. Instead, the grain is found to be full of atomic species. The production rates are found to depend on the number density in the gas phase. We show that the concept of the effective grain surface area, which we introduced in our earlier work, plays a significant role in grain chemistry. We compute the abundance of water and methanol and show that the results strongly depend on the density and composition in the gas phase, as well as various grain parameters. In the rate equation, it is generally assumed that the recombination efficiencies are independent of the grain parameters, and the surface coverage. Presently, our computed parameter $α$ for each product is found to depend on the accretion rate, the grain parameters and the surface coverage of the grain. We compare our results obtained from the rate equation and the one from the effective rate equation, which includes $α$. At the end we compare our results with the observed abundances.

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arXiv2008

Recombination Efficiency of Molecular Hydrogen on Interstellar Grains-II A Numerical Study

A knowledge of the recombination time on the grain surfaces has been a major obstacle in deciding the production rate of molecular hydrogen and other molecules in the interstellar medium. We present a numerical study to compute this time for molecular hydrogen for various cloud and grain parameters. We also find the time dependence, particularly when a grain is freshly injected into the system. Apart from the fact that the recombination times seem to be functions of the grain parameters such as the activation barrier energy, temperature etc, our result also shows the dependence on the number of sites in the grain $S$ and the effective accretion rate per site $a_s$ of atomic hydrogen. Simply put, the average time that a pair of atomic hydrogens will take to produce one molecular hydrogen depends on how heavily the grain is already populated by atomic and molecular hydrogens and how fast the hopping and desorption times are. We show that if we write the average recombination time as $T_r \sim S^α/A_H$, where, $A_H$ is the hopping rate, then $α$ could be much greater than 1 for all astrophysically relevant accretion rates. Thus the average formation rate of $H_2$ is also dependent on the grain parameters, temperature and the accretion rate. We believe that our result will affect the overall rate of the formation of complex molecules such as methanol which require successive hydrogenation on the grain surfaces in the interstellar medium.

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arXiv2008

Time evolution of simple molecules during proto-star collapse

We study the formation and evolution of several molecules in a collapsing interstellar cloud using a reasonably large reaction network containing more then four hundred atomic and molecular species. We employ a time dependent, spherically symmetric, hydrodynamics code to follow the hydrodynamic and chemical evolution of the collapsing cloud. The flow is assumed to be self-gravitating. We use two models to study the hydrodynamic evolution: in the first model, we inject matter into an initially low density region and in the second model, we start with a constant density cloud and let it collapse due to self-gravity. We study the evolution of the central core for both the cases. We include the grain chemistry to compute the formation of molecular hydrogen and carried out the effect of gas and grain chemistry at each time step. We follow the collapse for more than $10^{14}$s (about 3 million years) and present the time evolution of the globally averaged abundances of various simple but biologically important molecules, such as glycine, alanine etc. We compare our results with those obtained from observations found that for lighter molecules the agreement is generally very good. For complex molecules we tend to under predict the abundances. This indicates that other pathways could be present to form these molecules or more accurate reaction rates were needed. Keywords: hydrodynamics; star formation; ISM; chemical evolution

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