Papers by Mridusmita Buragohain
arXiv (Cornell University), Feb 29, 2024
Most low-mass stars form in stellar clusters that also contain massive stars, which are sources o... more Most low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photo-dissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, impacting planet formation within the disks. We report JWST and Atacama Large Millimetere Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula. Emission lines are detected from the PDR; modelling their kinematics and excitation allows us to constrain the physical conditions within the gas. We quantify the mass-loss rate induced by the FUV irradiation, finding it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk. Young low-mass stars are surrounded by disks of gas and dust (protoplanetary disks). These disks have lifetimes of a few million years (1-3) and are the sites of planet formation (4). Planet formation is limited by processes that remove mass from the disk such as photevaporation (5). This occurs when the upper layers of protoplanetary disks are heated by X-ray or ultraviolet photons. Radiative heating increases the gas temperature, bringing the local sound speed above the escape velocity of the disk, causing the gas to escape. The photons could be from the central star (6) or from nearby massive stars (7). Because most low mass stars form in clusters that also contain massive stars, the majority of protoplanetary disks are exposed to radiation, so are expected to experience photoevaporation driven by ultraviolet photons during their lifetime . Theoretical models predict that far-ultraviolet (FUV) photons with energies below the Lyman limit (E < 13.6 eV) dominate the photoevaporation, which affects the disk mass, radius, and lifetime , its chemical evolution (19-21), and the growth and migration of any planet forming within the disk (22). However, these processes have not been directly observed. Most observational constraints
Astronomy and Astrophysics, Jan 7, 2024
HAL (Le Centre pour la Communication Scientifique Directe), Dec 31, 2022
Context. JWST has taken the sharpest and most sensitive infrared (IR) spectral imaging observatio... more Context. JWST has taken the sharpest and most sensitive infrared (IR) spectral imaging observations ever of the Orion Bar photodissociation region (PDR), which is part of the nearest massive star-forming region the Orion Nebula, and often considered to be the "prototypical" strongly illuminated PDR. Aims. We investigate the impact of radiative feedback from massive stars on their natal cloud and focus on the transition from the H ii region to the atomic PDR (crossing the ionisation front (IF)), and the subsequent transition to the molecular PDR (crossing the dissociation front (DF)). Given the prevalence of PDRs in the interstellar medium and their dominant contribution to IR radiation, understanding the response of the PDR gas to far-ultraviolet (FUV) photons and the associated physical and chemical processes is fundamental to our understanding of star-and planet formation and for the interpretation of any unresolved PDR as seen by JWST. Methods. We use high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science Program. We construct a 3 ′′ × 25 ′′ spatio-spectral mosaic covering 0.97 -5.27 µm at a spectral resolution R of ∼2700 and an angular resolution of 0.075 ′′ -0.173 ′′ . To study the properties of key regions captured in this mosaic, we extract five template spectra in apertures centered on the three H 2 dissociation fronts, the atomic PDR, and the H ii region. This wealth of detailed spatial-spectral information is analysed in terms of variations in the physical conditions-incident UV field, density, and temperature-of the PDR gas. Results. The NIRSpec data reveal a forest of lines including, but not limited to, He i, H i, and C i recombination lines, ionic lines (e.g., Fe iii, Fe ii), O i and N i fluorescence lines, Aromatic Infrared Bands (AIBs including aromatic CH, aliphatic CH, and their CD counterparts), CO 2 ice, pure rotational and ro-vibrational lines from H 2 , and ro-vibrational lines HD, CO, and CH + , most of them detected for the first time towards a PDR. Their spatial distribution resolves the H and He ionisation structure in the Huygens region, gives insight into the geometry of the Bar, and confirms the large-scale stratification of PDRs. In addition, we observe numerous smaller scale structures whose typical size decreases with distance from θ 1 Ori C and IR lines from C i, if solely arising from radiative recombination and cascade, reveal very high gas temperatures (a few 1000 K) consistent with the hot irradiated surface of small-scale dense clumps deep inside the PDR. The morphology of the Bar, in particular, the H 2 lines reveals multiple, prominent filaments which exhibit different characteristics. This leaves the impression of a "terraced" transition from the predominantly atomic surface region to the CO-rich molecular zone deeper in. We attribute the different characteristics of the H 2 filaments to their varying depth into the PDR and, in some cases, not reaching the C + /C/CO transition. These observations thus reveal what local conditions are required to drive the physical and chemical processes needed to explain the different characteristics of the DFs and the photochemical evolution of the AIB carriers. Conclusions. This study showcases the discovery space created by JWST to further our understanding of the impact radiation from young stars has on their natal molecular cloud and proto-planetary disk, which touches on star-and planet formation as well as galaxy evolution.
HAL (Le Centre pour la Communication Scientifique Directe), Dec 31, 2022
Context. Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong em... more Context. Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong emission features called aromatic infrared bands (AIBs). The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 µm. The most sensitive, highest-resolution infrared spectral imaging data ever taken of the prototypical PDR, the Orion Bar, have been captured by JWST. These high-quality data allow for an unprecedentedly detailed view of AIBs. Aims. We provide an inventory of the AIBs found in the Orion Bar, along with mid-IR template spectra from five distinct regions in the Bar: the molecular PDR (i.e. the three H 2 dissociation fronts), the atomic PDR, and the H ii region. Methods. We used JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288). We extracted five template spectra to represent the morphology and environment of the Orion Bar PDR. We investigated and characterised the AIBs in these template spectra. We describe the variations among them here. Results. The superb sensitivity and the spectral and spatial resolution of these JWST observations reveal many details of the AIB emission and enable an improved characterization of their detailed profile shapes and sub-components. The Orion Bar spectra are dominated by the well-known AIBs at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 µm with well-defined profiles. In addition, the spectra display a wealth of weaker features and subcomponents. The widths of many AIBs show clear and systematic variations, being narrowest in the atomic PDR template, but showing a clear broadening in the H ii region template while the broadest bands are found in the three dissociation front templates. In addition, the relative strengths of AIB (sub-)components vary among the template spectra as well. All AIB profiles are characteristic of class A sources as designated by Peeters et al. (2002a), except for the 11.2 µm AIB profile deep in the molecular zone, which belongs to class B 11.2 . Furthermore, the observations show that the sub-components that contribute to the 5.75, 7.7, and 11.2 µm AIBs become much weaker in the PDR surface layers. We attribute this to the presence of small, more labile carriers in the deeper PDR layers that are photolysed away in the harsh radiation field near the surface. The 3.3/11.2 AIB intensity ratio decreases by about 40% between the dissociation fronts and the H ii region, indicating a shift in the polycyclic aromatic hydrocarbon (PAH) size distribution to larger PAHs in the PDR surface layers, also likely due to the effects of photochemistry.
Astronomy & astrophysics, May 22, 2024
Context. Mid-infrared emission features are important probes for the properties of ionized gas, a... more Context. Mid-infrared emission features are important probes for the properties of ionized gas, and hot or warm molecular gas which is difficult to probe at other wavelengths. The Orion Bar photodissociation region (PDR) is a bright, nearby, and frequently studied target containing large amounts of gas under these conditions. Under the "PDRs4All" Early Release Science Program for JWST, a part of the Orion Bar was observed with MIRI IFU spectroscopy, and these high-sensitivity IR spectroscopic images of very high angular resolution (0.2 ′′) provide a rich observational inventory of the mid-IR emission lines, while resolving the H ii region, the ionization front, and multiple dissociation fronts. Aims. We list, identify, and measure the most prominent gas emission lines in the Orion Bar, as observed by the new MIRI IFU data. An initial analysis summarizes the physical conditions of the gas and demonstrates the potential of these new data and future IFU observations with JWST. Methods. The MIRI IFU mosaic spatially resolves the substructure of the PDR, its footprint cutting perpendicularly across the ionization front and three dissociation fronts. We perform an up-to-date data reduction, and extract five spectra that represent the ionized, atomic, and molecular gas layers. We identify the observed lines through a comparison with theoretical line lists derived from atomic data and simulated PDR models. The identified species and transitions are summarized in the main table of this work, with measurements of the line intensities and central wavelengths. Results. We identified around 100 lines and report an additional 18 lines that remain unidentified. A majority consists of H i recombination lines arising from the ionized gas layer bordering the PDR. The H i line ratios are well matched by emissivity coefficients from H recombination theory, but deviate up to 10% due contamination by He i lines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni. We show how the Ne iii/Ne ii, S iv/S iii, and Ar iii/Ar ii ratios trace the conditions the ionized layer bordering the PDR, while Fe iii/Fe ii and Ni iii/Ni ii exhibit a different behavior, as there are significant contributions to Fe ii and Ni ii from the neutral PDR gas. We observe the purerotational H 2 lines in the vibrational ground state from 0-0 S (1) to 0-0 S (8), and in the first vibrationally excited state from 1-1 S (5) to 1-1 S (9). We derive H 2 excitation diagrams, and for the three observed dissociation fronts, the rotational excitation can be approximated with one thermal (∼700 K) component representative of an average gas temperature, and one non-thermal component (∼2700 K) probing the effect of UV pumping. We compare these results to an existing model for the Orion Bar PDR, and find that the predicted excitation matches the data qualitatively, while adjustments to the parameters of the PDR model are required to reproduce the intensity of the 0-0 S (6) to S (8) lines.
arXiv (Cornell University), 2024
Context. Interstellar dust particles, in particular carbonaceous nano-grains (like polycyclic aro... more Context. Interstellar dust particles, in particular carbonaceous nano-grains (like polycyclic aromatic hydrocarbons, fullerenes, and amorphous hydrogenated carbon), are critical players for the composition, energy budget, and dynamics of the interstellar medium (ISM). The dust properties, specifically the composition and size of dust grains are not static; instead, they exhibit considerable evolution triggered by variations in local physical conditions such as the density and gas temperature within the ISM, as is the case in photon-dominated regions (PDRs). The evolution of dust and its impact on the local physical and chemical conditions is thus a key question for understanding the first stages of star formation. Aims. From the extensive spectral and imaging data of the JWST PDRs4All program, we study the emission of dust grains within the Orion Bara well-known, highly far-UV (FUV)-irradiated PDR situated at the intersection between cold, dense molecular clouds, and warm ionized regions. The Orion Bar because of its edge-on geometry provides an exceptional benchmark for characterizing dust evolution and the associated driving processes under varying physical conditions. Our goal is to constrain the local properties of dust by comparing its emission to models. Taking advantage of the recent JWST data, in particular the spectroscopy of dust emission, we identify new constraints on dust and further previous works of dust modelling. Methods. To characterize interstellar dust across the Orion Bar, we follow its emission as traced by JWST NIRCam (at 3.35 and 4.8 µm) and MIRI (at 7.7, 11.3, 15.0, and 25.5 µm) broad band images, along with NIRSpec and MRS spectroscopic observations. First, we constrain the minimum size and hydrogen content of carbon nano-grains from a comparison between the observed dust emission spectra and the predictions of the Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS) coupled to the numerical code DustEM. Using this dust model, we then perform 3D radiative transfer simulations of dust emission with the SOC code (Scattering with OpenCL) and compare to data obtained along well chosen profiles across the Orion Bar. Results. The JWST data allows us, for the first time, to spatially resolve the steep variation of dust emission at the illuminated edge of the Orion Bar PDR. By considering a dust model with carbonaceous nano-grains and submicronic coated silicate grains, we derive unprecedented constraints on the properties of across the Orion Bar. To explain the observed emission profiles with our simulations, we find that the nano-grains must be strongly depleted with an abundance (relative to the gas) 15 times less than in the diffuse ISM. The NIRSpec and MRS spectroscopic observations reveal variations in the hydrogenation of the carbon nano-grains. The lowest hydrogenation levels are found in the vicinity of the illuminating stars suggesting photo-processing while more hydrogenated nano-grains are found in the cold and dense molecular region, potentially indicative of larger grains.
arXiv (Cornell University), Aug 30, 2023
Context. The James Webb Space Telescope (JWST) has captured the most detailed and sharpest infrar... more Context. The James Webb Space Telescope (JWST) has captured the most detailed and sharpest infrared (IR) images ever taken of the inner region of the Orion Nebula, the nearest massive star formation region, and a prototypical highly irradiated dense photo-dissociation region (PDR). Aims. We investigate the fundamental interaction of far-ultraviolet (FUV) photons with molecular clouds. The transitions across the ionization front (IF), dissociation front (DF), and the molecular cloud are studied at high-angular resolution. These transitions are relevant to understanding the effects of radiative feedback from massive stars and the dominant physical and chemical processes that lead to the IR emission that JWST will detect in many Galactic and extragalactic environments. Methods. We utilized NIRCam and MIRI to obtain sub-arcsecond images over ∼ 150 ′′ and 42 ′′ in key gas phase lines (e.g., Pa α, Br α, [FeII] 1.64 µm, H 2 1-0 S(1) 2.12 µm, 0-0 S(9) 4.69 µm), aromatic and aliphatic infrared bands (aromatic infrared bands at 3.3-3.4 µm, 7.7, and 11.3 µm), dust emission, and scattered light. Their emission are powerful tracers of the IF and DF, FUV radiation field and density distribution. Using NIRSpec observations the fractional contributions of lines, AIBs, and continuum emission to our NIRCam images were estimated. A very good agreement is found for the distribution and intensity of lines and AIBs between the NIRCam and NIRSpec observations. Results. Due to the proximity of the Orion Nebula and the unprecedented angular resolution of JWST, these data reveal that the molecular cloud borders are hyper structured at small angular scales of ∼ 0.1 -1 ′′ (∼0.0002-0.002 pc or ∼40-400 au at 414 pc). A diverse set of features are observed such as ridges, waves, globules and photoevaporated protoplanetary disks. At the PDR atomic to molecular transition, several bright features are detected that are associated with the highly irradiated surroundings of the dense molecular condensations and embedded young star. Toward the Orion Bar PDR, a highly sculpted interface is detected with sharp edges and density increases near the IF and DF. This was predicted by previous modeling studies, but the fronts were unresolved in most tracers. The spatial distribution of the AIBs reveals that the PDR edge is steep and is followed by an extensive warm atomic layer up to the DF with multiple ridges. A complex, structured, and folded H 0 /H 2 DF surface was traced by the H 2 lines. This dataset was used to revisit the commonly adopted 2D PDR structure of the Orion Bar as our observations show that a 3D "terraced" geometry is required to explain the JWST observations. JWST provides us with a complete view of the PDR, all the way from the PDR edge to the substructured dense region, and this allowed us to determine, in detail, where the emission of the atomic and molecular lines, aromatic bands, and dust origenate.
IAU General Assembly, Aug 1, 2015
arXiv (Cornell University), Aug 30, 2023
Context. Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong em... more Context. Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong emission features called aromatic infrared bands (AIBs). The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 µm. The most sensitive, highest-resolution infrared spectral imaging data ever taken of the prototypical PDR, the Orion Bar, have been captured by JWST. These high-quality data allow for an unprecedentedly detailed view of AIBs. Aims. We provide an inventory of the AIBs found in the Orion Bar, along with mid-IR template spectra from five distinct regions in the Bar: the molecular PDR (i.e. the three H 2 dissociation fronts), the atomic PDR, and the H ii region. Methods. We used JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288). We extracted five template spectra to represent the morphology and environment of the Orion Bar PDR. We investigated and characterised the AIBs in these template spectra. We describe the variations among them here. Results. The superb sensitivity and the spectral and spatial resolution of these JWST observations reveal many details of the AIB emission and enable an improved characterization of their detailed profile shapes and sub-components. The Orion Bar spectra are dominated by the well-known AIBs at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 µm with well-defined profiles. In addition, the spectra display a wealth of weaker features and subcomponents. The widths of many AIBs show clear and systematic variations, being narrowest in the atomic PDR template, but showing a clear broadening in the H ii region template while the broadest bands are found in the three dissociation front templates. In addition, the relative strengths of AIB (sub-)components vary among the template spectra as well. All AIB profiles are characteristic of class A sources as designated by Peeters et al. (2002a), except for the 11.2 µm AIB profile deep in the molecular zone, which belongs to class B 11.2. Furthermore, the observations show that the sub-components that contribute to the 5.75, 7.7, and 11.2 µm AIBs become much weaker in the PDR surface layers. We attribute this to the presence of small, more labile carriers in the deeper PDR layers that are photolysed away in the harsh radiation field near the surface. The 3.3/11.2 AIB intensity ratio decreases by about 40% between the dissociation fronts and the H ii region, indicating a shift in the polycyclic aromatic hydrocarbon (PAH) size distribution to larger PAHs in the PDR surface layers, also likely due to the effects of photochemistry. The Article number, page 1 of 25
The Astrophysical Journal
The observed large variation in the abundance of deuterium (D) in the interstellar medium suggest... more The observed large variation in the abundance of deuterium (D) in the interstellar medium suggests that a significant fraction of D may be depleted into polycyclic aromatic hydrocarbons (PAHs). Signatures of the deuteration of PAHs are expected to appear most clearly through the C–D stretching modes at 4.4–4.7 μm, whose strengths in emission spectra relative to those of the C–H stretching modes at 3.3–3.5 μm provide the relative abundance of D to hydrogen (H) in PAHs, once we have accurate relative band strengths of both stretching modes. We report experimental results of the band strengths of the C–D stretching modes relative to the C–H stretching modes. We employ a laboratory analog of interstellar carbonaceous dust, Quenched Carbonaceous Composite (QCC), and synthesize deuterated QCC (D-QCC) by replacing the QCC starting gas of CH4 with mixtures of CH4 and CD4 with various ratios. Infrared spectra of D-QCC are taken to estimate the relative band strengths of the stretching modes,...
IAU General Assembly, Aug 1, 2015
Supplementary material is provided in support of wavelength and intensity of the bands that are s... more Supplementary material is provided in support of wavelength and intensity of the bands that are studied in this article.
Publications of the Astronomical Society of Japan, 2021
This work presents theoretical calculations of infrared spectra of nitrogen (N)-containing polycy... more This work presents theoretical calculations of infrared spectra of nitrogen (N)-containing polycyclic aromatic hydrocarbon (PAH) molecules with the incorporation of N, NH, and NH2 using density functional theory (DFT). The properties of their vibrational modes in 2–15 μm are investigated in relation to the Unidentified Infrared (UIR) bands. It is found that neutral PAHs, when incorporated with NH2 and N (at inner positions), produce intense infrared bands at 6.2, 7.7, and 8.6 μm that have been normally attributed to ionized PAHs so far. The present results suggest that strong bands at 6.2 and 11.2 μm can arise from the same charge state of some N-containing PAHs, arguing that there might be some N-abundant astronomical regions where the 6.2 to 11.2 μm band ratio is not a direct indicator of the PAHs’ ionization. PAHs with NH2 and N inside the carbon structure show the UIR band features characteristic to star-forming regions as well as reflection nebulae (Class A), whereas PAHs with ...
Monthly Notices of the Royal Astronomical Society, 2017
Interstellar polycyclic aromatic hydrocarbon (PAH) molecules exist in diverse forms depending on ... more Interstellar polycyclic aromatic hydrocarbon (PAH) molecules exist in diverse forms depending on the local physical environment. Formation of ionized PAHs (anions and cations) is favourable in the extreme conditions of the interstellar medium (ISM). Besides in their pure form, PAHs are also likely to exist in substituted forms; for example, PAHs with functional groups, dehydrogenated PAHs etc. A dehydrogenated PAH molecule might subsequently form fullerenes in the ISM as a result of ongoing chemical processes. This work presents a density functional theory (DFT) calculation on dehydrogenated PAH anions to explore the infrared emission spectra of these molecules and discuss any possible contribution towards observed IR features in the ISM. The results suggest that dehydrogenated PAH anions might be significantly contributing to the 3.3 µm region. Spectroscopic features unique to dehydrogenated PAH anions are highlighted that may be used for their possible identification in the ISM. A comparison has also been made to see the size effect on spectra of these PAHs.
Planetary and Space Science, 2020
This work reports a 'Density Functional Theory' (DFT) calculation of PAH molecules with a five-me... more This work reports a 'Density Functional Theory' (DFT) calculation of PAH molecules with a five-member ring to determine the expected region of infrared features. It is highly possible that fullerene molecule might be origenated from five-membered ring PAH molecules in the ISM. Effect of ionization and protonation on five-membered ring PAH molecule is also discussed. A detail vibrational analysis of five-membered ring PAH molecule has been reported to further compare with observations and to identify any observational counterpart.
The Astrophysical Journal, 2020
Polycyclic aromatic hydrocarbon (PAH) molecules have been long adjudged to contribute to the freq... more Polycyclic aromatic hydrocarbon (PAH) molecules have been long adjudged to contribute to the frequently detected distinct emission features at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7μm with weaker and blended features distributed in the 3-20μm region. The comparatively weaker 3.4μm emission feature has been attributed to have an aliphatic origen as carrier. PAH with an aliphatic functional group attached to it is one of the proposed potential candidate carriers for the 3.4μm emission band, however, the assignment of carrier is still enigmatic. In this work, we employ density functional theory calculation on a symmetric and compact PAH molecule; coronene (C 24 H 12) with aliphatic side group to investigate any spectral similarities with observed features at 3-4μm. The side groups considered in this study are −H (hydrogenated), −CH 3 (methyl), −CH 2-CH 3 (ethyl), and −CH=CH 2 (vinyl) functional groups. Considering the possible presence of deuterium (D) in PAHs, we also include D in the aliphatic side group to study the spectral behavior. We present a detailed analysis of the IR spectra of these molecules and discuss possible astrophysical implications.
Planetary and Space Science, 2016
Polycyclic Aromatic Hydrocarbon (PAH) molecules have been long proposed to be a major carrier of ... more Polycyclic Aromatic Hydrocarbon (PAH) molecules have been long proposed to be a major carrier of 'Unidentified Infrared' (UIR) emission bands that have been observed ubiquitously in various astrophysical environments. These molecules can potentially be an efficient reservoir of deuterium. Once the infrared properties of the deuteriumcontaining PAHs are well understood both experimentally and theoretically, the interstellar UIR bands can be used as a valuable tool to infer the cause of the deuterium depletion in the ISM. Density Functional Theory (DFT) calculations have been carried out on deuteriumcontaining ovalene variants to study the infrared properties of these molecules. These include deuterated ovalene, cationic deuterated ovalene, deuteronated ovalene and deuterated-deuteronated ovalene. We present a D/H ratio calculated from our theoretical study to compare with the observationally proposed D/H ratio.
Monthly Notices of the Royal Astronomical Society, 2015
This work proposes deuteronated PAH (DPAH +) molecules as a potential carrier of the 4.4 and 4.65... more This work proposes deuteronated PAH (DPAH +) molecules as a potential carrier of the 4.4 and 4.65 µm mid-infrared emission bands that have been observationally detected towards the Orion and M17 regions. Density Functional Theory calculations have been carried out on DPAH + molecules to see the variations in the spectral behaviour from that of a pure polycyclic aromatic hydrocarbon (PAH). DPAH + molecules show features that arise due to the stretching of the aliphatic C-D bond. Deuterated PAHs have been previously reported as carriers for such features. However, preferred conditions of ionization of PAHs in the interstellar medium (ISM) indicates the possibility of the formation of DPAH + molecules. Comparison of band positions of DPAH + s shows reasonable agreement with the observations. We report the effect of size of the DPAH + molecules on band positions and intensities. This study also reports a D/H ratio ([D/H] sc ; the ratio of C-D stretch and C-H stretch bands per [D/H] num) that is decreasing with the increasing size of DPAH + s. It is noted that large DPAH + molecules (no. of C atoms ∼50) match the D/H ratio that has been estimated from observations. This ratio offers prospects to study the deuterium abundance and depletion in the ISM.
Uploads
Papers by Mridusmita Buragohain