arXiv subjects

Milan Sil

Publications and source records attributed to Milan Sil.

17 recordsLinked to original sources

arXiv2026

Chemistry and ro-vibrational excitation of CH$^+$ in the planetary nebula NGC 7027

Small carbon hydride cations, such as the methylidyne ion (CH$^+$), play an important role in the chemistry of the interstellar medium (ISM). They participate in gas-phase reaction networks leading to the formation of hydrocarbon species that act as precursors to more complex organic molecules. CH$^+$ is a highly reactive ion that is rapidly destroyed by H, H$_2$, and free electrons, making its excitation challenging to model. Its level populations depend not only on radiative and inelastic processes but also on chemical formation and destruction rates, a mechanism known as chemical pumping. We investigate this effect using a new set of ab initio state-resolved ro-vibrational (reactive and inelastic) collision data to model the observed CH$^+$ emission. Multiple rotational and ro-vibrational transitions of CH$^+$ detected toward the planetary nebula NGC 7027 are analyzed. The chemical structure of CH$^+$ is modeled with the CLOUDY code using updated reaction rates, providing the temperature and density structure across the nebula. A non-local thermodynamic equilibrium (NLTE) analysis is performed using CLOUDY and the single-zone RADEX code with a comprehensive set of spectroscopic and collisional data. In addition, chemical formation and destruction processes are implemented in RADEX and explored via Markov Chain Monte Carlo sampling. The CLOUDY model reproduces the observed CH$^+$ line fluxes within a factor of 1.3 on average. It indicates that rotational and ro-vibrational lines arise from physically distinct regions, primarily differing in temperature. RADEX models show that chemical pumping significantly enhances populations above ($\upsilon = 0, J = 1$), strongly increasing ro-vibrational emission, especially in the $\upsilon =2 \to 1$ band. Single-zone models remain limited, highlighting the need for full 1D modeling including all excitation processes.

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arXiv2025

Binding energy distributions of alcohols, thiols, and their precursors on interstellar water ice surfaces

Binding energy (BE) is a critical parameter in astrochemical modeling, governing the retention of species on interstellar dust grains and their subsequent chemical evolution. However, conventional models often rely on single-valued BEs, overlooking the intrinsic distribution arising from diverse adsorption sites. In this study, we present BEs for monohydric alcohols, thiols, and their plausible precursors, including aldehydes and thioaldehydes. We incorporate a distribution of BEs to capture the realistic variation in adsorption strengths. The quantum chemical calculations provide a range of BE values rather than a single estimate, ensuring a more precise description of molecular diffusion and surface chemistry. The BE trend of analogous species provides qualitative insight into the dominant reaction pathways and key precursors that drive the formation of larger molecules under interstellar conditions. Oxygen-bearing species generally exhibit higher BEs than their sulfur analogues, primarily due to stronger interactions, further influencing molecular adsorption and reactivity. We implemented BE distributions in astrochemical models, revealing significant effects on predicted abundances and establishing a more accurate framework for future astrochemical modeling.

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arXiv2025

Metallic species in interstellar medium: Astrochemical modeling

Metal-bearing species in diffuse or molecular clouds are often overlooked in astrochemical modeling except for the charge exchange process. However, catalytic cycles involving these metals can affect the abundance of other compounds. We prepared a comprehensive chemical network for Na, Mg, Al, Fe, K, and Si-containing species. Assuming water as the major constituent of interstellar ice in dark clouds, quantum chemical calculations were carried out to estimate the binding energy of important metallic species, considering amorphous solid water as substrate. Significantly lower binding energies (approximately 5 to 6 times) were observed for Na and Mg, while the value for Fe was roughly 4 times higher than what was used previously. Here, we calculated binding energy values for Al and K, for which no prior guesses were available. The total dipole moments and enthalpies of formation for several newly included species are unknown. Furthermore, the assessment of reaction enthalpies is necessary to evaluate the feasibility of the new reactions under interstellar conditions. These parameters were estimated and subsequently integrated into models. Some additional species that were not included in the UMIST/KIDA database have been introduced. The addition of these new species, along with their corresponding reactions, appears to significantly affect the abundances of related species. Some key reactions that significantly influence general metal-related chemistry include: $\rm{M^+ + H_2 \rightarrow M{H_2}^+ + h\nu}$, $\rm{MH + O \rightarrow MO + H}$ ($\rm{M \ = \ Fe, \ Na, \ Mg, \ Al, \ K}$), and $\rm{M_1^+ + M_2H \rightarrow M_1H + M_2^+}$ (where $\rm{M_1 \neq M_2}$, $M_1, \ M_2 \ = \ Na, \ Mg, \ Al, \ K, \ Fe$). Significant changes were observed in magnesium and sodium-bearing cyanides, isocyanides, and aluminum fluoride when additional reaction pathways were considered.

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arXiv2025

Chemistry and ro-vibrational excitation of HeH$^+$ in the Planetary Nebula NGC 7027

HeH$^+$ belongs to the class of "reactive" ions that can be destroyed so quickly that chemical formation and destruction rates may compete with inelastic rates and should be considered when solving the statistical equilibrium equations. This so-called chemical "pumping" or "excitation" effect is investigated here for the first time in HeH$^+$. The chemical evolution of HeH$^+$ in NGC 7027 is modeled with the CLOUDY photoionization code using updated reaction rate coefficients. The non-LTE analysis of the three observed HeH$^+$ emission lines is then performed with the CLOUDY and RADEX codes using an extensive set of spectroscopic and inelastic collisional data suitable for the specific high-temperature environment of NGC 7027. In a second approach, chemical formation and destruction rates of HeH$^+$ are implemented in RADEX. This code is combined with MCMC sampling (performed on the RADEX-parameters space) to extract the best-fit HeH$^+$ column density and physical conditions from the observed line fluxes. The CLOUDY and RADEX non-LTE results are found to be in good agreement, and the $\upsilon=1-0 \ P(2)/P(1)$ line ratio is better than 20%. Agreement to better than a factor of 2.3 is obtained when including the reaction between He($2^3S$) and H as an additional source of HeH$^+$. The RADEX/MCMC model with chemical pumping is found to reproduce both the observed line fluxes and the line ratio to 20%. Our results suggest that additional HeH$^+$ lines must be detected in NGC 7027 to better constrain the physical conditions via non-LTE models. Uncertainties in collisional (reactive and inelastic) data of HeH$^+$ have been largely reduced in this work. The three observed lines are not sensitive to chemical pumping while excited "short-lived" levels are significantly overpopulated with respect to a non-LTE model neglecting chemical excitation.

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arXiv2024

Fate and detectability of rare gas hydride ions in nova ejecta: A case study with nova templates

HeH$^+$ was the first heteronuclear molecule to form in the metal-free Universe after the Big Bang. The molecule gained significant attention following its first circumstellar detection in the young and dense planetary nebula NGC 7027. We target some hydride ions associated with the noble gases (HeH$^+$, ArH$^+$, and NeH$^+$) to investigate their formation in harsh environments like the nova outburst region. We use a photoionization modeling (based on previously published best-fit physical parameters) of the moderately fast ONe type nova, QU Vulpeculae 1984, and the CO type novae, RS Ophiuchi and V1716 Scorpii. Our steady-state modeling reveals a convincing amount of HeH$^+$, especially in the dense clump of RS Ophiuchi and V1716 Scorpii. The calculated upper limit on the surface brightness of HeH$^+$ transitions suggests that the James Webb Space Telescope (JWST) could detect some of them, particularly in sources like RS Ophiuchi and V1716 Scorpii, which have similar physical and chemical conditions and evolution. It must be clearly noted that the sources studied are used as templates, and not as targets for observations. The detection of these lines could be useful for determining the physical conditions in similar types of systems and for validating our predictions based on new electron-impact ro-vibrational collisional data at temperatures of up to 20,000 K.

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arXiv2024

Experimental and computational study of ethanolamine ices at astrochemical conditions

Ethanolamine (NH2CH2CH2OH) has recently been identified in the molecular cloud G+0.693-0.027, situated in the SgrB2 complex in the Galactic center. However, its presence in other regions, and in particular in star-forming sites, is still elusive. Given its likely role as a precursor to simple amino acids, understanding its presence in the star-forming region is required. Here, we present the experimentally obtained temperature-dependent spectral features and morphological behavior of pure ethanolamine ices under astrochemical conditions in the 2 - 12 micro meter (MIR) and 120 - 230 nm (VUV) regions for the first time. These features would help in understanding its photochemical behavior. In addition, we present the first chemical models specifically dedicated to ethanolamine. These models include all the discussed chemical routes from the literature, along with the estimated binding energies and activation energies from quantum chemical calculations reported in this work. We have found that surface reactions: CH2OH + NH2CH2 --> NH2CH2CH2OH and NH2 + C2H4OH --> NH2CH2CH2OH in warmer regions (60-90 K) could play a significant role in the formation of ethanolamine. Our modeled abundance of ethanolamine complements the upper limit of ethanolamine column density estimated in earlier observations in hot core/corino regions. Furthermore, we provide a theoretical estimation of the rotational and distortional constants for various species (such as HNCCO, NH2CHCO, and NH2CH2CO) related to ethanolamine that have not been studied in existing literature. This study could be valuable for identifying these species in the future.

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arXiv2024

Assessing realistic binding energies of some essential interstellar radicals with amorphous solid water. A fully quantum chemical approach

In the absence of laboratory data, state-of-the-art quantum chemical approaches can provide estimates of the binding energy (BE) of interstellar species with grains. Without BE values, contemporary astrochemical models are compelled to utilize wild guesses, often delivering misleading information. Here, we employed a fully quantum chemical approach to estimate the BE of seven diatomic radicals - CH, NH, OH, SH, CN, NS, and NO - that play a crucial role in shaping the interstellar chemical composition, using a suitable amorphous solid water model as a substrate since water is the principal constituent of interstellar ice in dense and shielded regions. While the BEs are compatible with physisorption, the binding of CH in some sites shows chemisorption, in which a chemical bond to an oxygen atom of a water molecule is formed. While no structural change has been observed for the CN radical, it is believed that the formation of a hemibonded system between the outer layer of the water cluster and the radical is the reason for the unusually large BE in one of the binding sites considered in our study. A significantly lower BE for NO, consistent with recent calculations, is obtained, which helps explain the recently observed HONO/NH$_2$OH and HONO/HNO ratios in the low-mass hot corino IRAS 16293-2422 B with chemical models.

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arXiv2022

Physics And Chemistry Of Star Forming Region And Protoplanetary Disk

My thesis work aims to study the inter-relation between various physical and chemical conditions in a wide range of astrophysical environments. Our studied regions range from the super-hot regions (i.e., nebular, photon-dominated, or photodissociation regions, diffuse area, through which the lights of the background stars can reach us) to the super-cold regions (i.e., dense molecular clouds, proto-planetary disks, etc. where interstellar dust particles absorb all background visible and ultra-violet lights). The chemical complexity of the interstellar cloud gradually evolves due to the evolution in physical conditions. The dense molecular clouds are the birth sites of star-formation, where a wide variety of complex organic molecules are observed. Dust particles play an essential role in the formation of these complex organic molecules. During the warm-up stage of a star-forming region, the molecules formed during the cold phase start to return to the gas phase by various thermal and non-thermal evaporation processes. These complex molecules again freeze out to the outer part of the proto-planetary disk during the further evolved stage according to their condensation temperature and form the so-called snow-lines. The binding energies of these molecules with the prevailing dust particles play a crucial role in determining the structural information of this disk. Thus the binding energy of the molecules is vital to understand several critical aspects of the star and planet formation processes. In this thesis, I will discuss the chemical complexity obtained in a wide range of the star-forming region and whether this chemical complexity lead to biomolecules in space.

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arXiv2022

Astrochemical model to study the abundances of branched carbon-chain molecules in a hot molecular core with realistic binding energies

Straight-chain (normal-propyl cyanide, n - C3H7CN) and branched-chain (iso-propyl cyanide, i - C3H7CN) alkyl cyanides are recently identified in the massive star-forming regions (Sgr B2(N) and Orion). These branched-chain molecules indicate that the key amino acids (side-chain structures) may also be present in a similar region. The process by which this branching could propagate towards the higher-order (butyl cyanide, C4H9CN) is an active field of research. Since the grain catalysis process could have formed a major portion of these species, considering a realistic set of binding energies are indeed essential. We employ quantum chemical calculations to estimate the binding energy of these species considering water as a substrate because water is the principal constituent of this interstellar ice. We find significantly lower binding energy values for these species than were previously used. It is noticed that the use of realistic binding energy values can significantly change the abundance of these species. The branching is more favorable for the higher-order alkyl cyanides with the new binding energies. With the inclusion of our new binding energy values and one essential destruction reaction (i - C3H7CN + H -> CH3C(CH3)CN + H2, having an activation barrier of 947 K), abundances of t - C4H9CN dramatically increased.

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arXiv2021

Is there any linkage between interstellar aldehyde and alcohol?

It is speculated that there might be some linkage between interstellar aldehydes and their corresponding alcohols. Here, an observational study and astrochemical modeling are coupled together to illustrate the connection between them. The ALMA Cycle 4 data of a hot molecular core, G10.47+0.03 is utilized for this study. Various aldehydes (acetaldehyde, propanal, and glycolaldehyde), alcohols (methanol and ethylene glycol), and a ketone (acetone) are identified in this source. The excitation temperatures and the column densities of these species were derived via the rotation diagram method assuming LTE conditions. An extensive investigation is carried out to understand the formation of these species. Six pairs of aldehyde-alcohol: i) methanal and methanol; ii) ethanal and ethanol; iii) propanal and 1-propanol; iv) propenal and allyl alcohol; v) propynal and propargyl alcohol; vi) glycolaldehyde and ethylene glycol; vii) along with one pair of ketone-alcohol (acetone and isopropanol) and viii) ketene-alcohol (ethenone and vinyl alcohol) are considered for this study. Two successive hydrogenation reactions in the ice phase are examined to form these alcohols from aldehydes, ketone, and ketene, respectively. Quantum chemical methods are extensively executed to review the ice phase formation route and the kinetics of these species. Based on the obtained kinetic data, astrochemical modeling is employed to derive the abundances of these aldehydes, alcohols, ketone, and ketene in this source. It is seen that our model could successfully explain the observed abundances of various species in this hot molecular core.

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arXiv2021

Effect of binding energies on the encounter desorption

The abundance of interstellar ice constituents is usually expressed with respect to the water ice because, in denser regions, a significant portion of the interstellar grain surface would be covered by water ice. The binding energy (BE), or adsorption energy of the interstellar species regulates the chemical complexity of the interstellar grain mantle. Due to the high abundance of water ice, the BE of surface species with the water is usually provided and widely used in astrochemical modeling. However, the hydrogen molecules would cover some part of the grain mantle in the denser and colder part of the interstellar medium. Even at around ~ 10K, few atoms and simple molecules with lower adsorption energies can migrate through the surface. The BE of the surface species with H2 substrate would be very different from that of a water substrate. However, adequate information regarding these differences is lacking. Here, we employ the quantum chemical calculation to provide the BE of 95 interstellar species with H2 substrate. These are representative of the BEs of species to a H2 overlayer on a grain surface. On average, we notice that the BE with the H2 monomer substrate is almost ten times lower than the BE of these species reported earlier with the H2 O c-tetramer configuration. The encounter desorption of H and H2 was introduced (with ED (H, H2 ) =45 K and ED (H2 , H2 ) =23 K) to have a realistic estimation of the abundances of the surface species in the colder and denser region. Our quantum chemical calculations yield higher adsorption energy of H2 than that of H (ED (H, H2 ) = 23 - 25 K and ED (H2, H2 ) =67 - 79 K). We further implement an astrochemical model to study the effect of encounter desorption with the resent realistic estimation. The encounter desorption of the N atom (calculations yield ED (N, H2 ) =83 K) is introduced to study the differences with its inclusion.

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arXiv2021

Chemical complexity of phosphorous bearing species in various regions of the Interstellar medium

Phosphorus related species are not known to be as omnipresent in space as hydrogen, carbon, nitrogen, oxygen, and sulfur-bearing species. Astronomers spotted very few P-bearing molecules in the interstellar medium and circumstellar envelopes. Limited discovery of the P-bearing species imposes severe constraints in modeling the P-chemistry. In this paper, we carry out extensive chemical models to follow the fate of P-bearing species in diffuse clouds, photon-dominated or photodissociation regions (PDRs), and hot cores/corinos. We notice a curious correlation between the abundances of PO and PN and atomic nitrogen. Since N atoms are comparatively abundant in diffuse clouds and PDRs than in the hot core/corino region, PO/PN reflects < 1 in diffuse clouds, << 1 in PDRs, and > 1 in the late warm-up evolutionary phase of the hot core/corino regions. During the end of the post-warm-up phase, we obtain PO/PN > 1 for hot core and < 1 for its low mass analog. We employ a radiative transfer model to investigate the transitions of some of the P-bearing species in diffuse cloud and hot core regions and estimate the line profiles. Our study estimates the required integration time to observe these transitions with ground-based and space-based telescopes. We also carry out quantum chemical computation of the infrared features of PH3 along with various impurities. We notice that SO2 overlaps with the PH3 bending-scissoring modes around ~ (1000 - 1100) cm-1. We also find that the presence of CO2 can strongly influence the intensity of the stretching modes around ~ 2400 cm-1 of PH3 .

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arXiv2020

Exploring the Possibility of Identifying Hydride and Hydroxyl Cations of Noble Gas Species in the Crab Nebula Filament

The first identification of the argonium ion (ArH+) towards the Crab Nebula supernova remnant was proclaimed by the Herschel in the sub-millimeter and far-infrared domain. Very recently the discovery of the hydro-helium cation (HeH+) in the planetary nebula (NGC 7027) has been reported by using the SOFIA. The elemental abundance of neon is much higher than that of the argon. However, the presence of neonium ions (NeH+) is yet to be confirmed in space. Though the hydroxyl radicals (OH) are very abundant either in neutral or in the cationic form, hydroxyl cations of such noble gases (i.e., ArOH+, NeOH+, and HeOH+) are yet to be identified in space. Here, we employ a spectral synthesis code to examine the chemical evolution of the hydride and hydroxyl cations of the various isotopes of Ar, Ne, and He in the Crab Nebula filament and calculate their line emissivity and intrinsic line surface brightness. We successfully explain the observed surface brightness of two transitions of ArH+ (617 and 1234 GHz), one transition of OH+ (971 GHz), and one transition of H2 (2.12 micrometer). We also explain the observed surface brightness ratios between various molecular and atomic transitions. We find that our model reproduces the overall observed features when a hydrogen number density of ~10^4-10^6 cm^-3 and a cosmic-ray ionization rate per H2 of ~10^-11-10^-10 s^-1 are chosen. We discuss the possibility of detecting some hydride and hydroxyl cations in the Crab and diffuse cloud environment. Some transitions of these molecules are highlighted for future astronomical detection.

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arXiv2020

A Systematic Study on the Absorption Features of Interstellar Ices in Presence of Impurities

Spectroscopic studies play a key role in the identification and analysis of interstellar ices and their structure. Some molecules have been identified within the interstellar ices either as pure, mixed, or even as layered structures. Absorption band features of water ice can significantly change with the presence of different types of impurities (CO, CO2, CH3OH, H2CO, etc.). In this work, we carried out a theoretical investigation to understand the behavior of water band frequency, and strength in the presence of impurities. The computational study has been supported and complemented by some infrared spectroscopy experiments aimed at verifying the effect of HCOOH, NH3 , and CH3 OH on the band profiles of pure H2O ice. Specifically, we explored the effect on the band strength of libration, bending, bulk stretching, and free-OH stretching modes. Computed band strength profiles have been compared with our new and existing experimental results, thus pointing out that vibrational modes of H2O and their intensities can change considerably in the presence of impurities at different concentrations. In most cases, the bulk stretching mode is the most affected vibration, while the bending is the least affected mode. HCOOH was found to have a strong influence on the libration, bending, and bulk stretching band profiles. In the case of NH3, the free-OH stretching band disappears when the impurity concentration becomes 50%. This work will ultimately aid a correct interpretation of future detailed spaceborne observations of interstellar ices by means of the upcoming JWST mission.

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arXiv2020

Identification of pre-biotic molecules containing Peptide-like bond in a hot molecular core, G10.47+0.03

After hydrogen, oxygen, and carbon, nitrogen is one of the most chemically active species in the interstellar medium (ISM). Nitrogen bearing molecules have great importance as they are actively involved in the formation of biomolecules. Therefore, it is essential to look for nitrogen-bearing species in various astrophysical sources, specifically around high-mass star-forming regions where the evolutionary history is comparatively poorly understood. In this paper, we report the observation of three potential pre-biotic molecules, namely, isocyanic acid (HNCO), formamide (NH2CHO), and methyl isocyanate (CH3NCO), which contain peptide-like bonds (-NH-C(=O)-) in a hot molecular core, G10.47+0.03 (hereafter, G10). Along with the identification of these three complex nitrogen-bearing species, we speculate their spatial distribution in the source and discuss their possible formation pathways under such conditions. The rotational diagram method under the LTE condition has been employed to estimate the excitation temperature and the column density of the observed species. Markov Chain Monte Carlo method was used to obtain the best suited physical parameters of G10 as well as line properties of some species. We also determined the hydrogen column density and the optical depth for different continuum observed in various frequency ranges. Finally, based on these observational results, we have constructed a chemical model to explain the observational findings. We found that HNCO, NH2CHO, and CH3NCO are chemically linked with each other.

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arXiv2018

An Approach to Estimate the Binding energy of Interstellar Species

One of the major obstacles to accurately model the interstellar chemistry is an inadequate knowledge about the binding energy (BE) of interstellar species with dust grains. In denser region of molecular cloud, where very complex chemistry is active, interstellar dust is predominantly covered by H2O molecules and thus it is essential to know the interaction of gas phase species with water ice to trace realistic physical and chemical processes. To this effect, we consider water (cluster) ice to calculate the BE of several atoms, molecules, and radicals of astrochemical interest. Systematic studies have been carried out to come up with a relatively more accurate BE of astrophysically relevant species on water ice. We increase the size of the water cluster methodically to capture the realistic situation. Sequentially one, three, four, five and six water molecules are considered to represent water ice analogue in increasing order of complexity. We notice that for most of the species considered here, as we increase the cluster size, our calculated BE value starts to converge towards the experimentally obtained value. More specifically, our computed results with water c-pentamer (average deviation from experiment ~ 15.8%) and c-hexamer (chair) (average deviation from experiment ~ 16.7%) configuration are found to be more nearer as the experimentally obtained value than other water clusters considered.

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arXiv2017

Chemical modeling for predicting the abundances of certain aldimines and amines in hot cores

We consider six isomeric groups (CH3N, CH5N, C2H5N, C2H7N, C3H7N and C3H9N) to review the presence of amines and aldimines within the interstellar medium (ISM). Each of these groups contains at least one aldimine or amine. Methanimine (CH2NH) from CH3N and methylamine (CH3NH2) from CH5N isomeric group were detected a few decades ago. Recently, the presence of ethanimine (CH3CHNH) from C2H5N isomeric group has been discovered in the ISM. This prompted us to investigate the possibility of detecting any aldimine or amine from the very next three isomeric groups in this sequence: C2H7N, C3H7N and C3H9N. We employ high-level quantum chemical calculations to estimate accurate energies of all the species. According to enthalpies of formation, optimized energies, and expected intensity ratio, we found that ethylamine (precursor of glycine) from C2H7N isomeric group, (1Z)-1-propanimine from C3H7N isomeric group, and trimethylamine from C3H9N isomeric group are the most viable candidates for the future astronomical detection. Based on our quantum chemical calculations and from other approximations (from prevailing similar types of reactions), a complete set of reaction pathways to the synthesis of ethylamine and (1Z)-1-propanimine is prepared. Moreover, a large gas-grain chemical model is employed to study the presence of these species in the ISM. Our modeling results suggest that ethylamine and (1Z)-1-propanimine could efficiently be formed in hot-core regions and could be observed with present astronomical facilities. Radiative transfer modeling is also implemented to additionally aid their discovery in interstellar space.

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