I am leading an integrated astronomy and planetary science program (Figure 1) of (a) pan-spectral astronomical observations (mainly at millimeter/sub-millimeter; near, mid and far-infrared wavelengths) from pre-stellar cores, to protostars and their associated shocks, protoplanetary disks, and ultimately to the bodies of the Solar System, (b) models of chemical and dynamical transport in the early Solar System, and (d) realistic numerical simulations of inaccessible cosmic environments. Synergistic integration of these areas is essential for testing whether delivery of the building blocks of life – exogenous water and prebiotic organics – enabled the emergence and development of life on Earth. Here, I give a brief summary of some of my past research projects in the context of Astrochemistry, Star and Planetary formation:Figure 1: Schematic summary of the formation of a star and a planetary system, like the Solar System, passes through five fundamental phases (Caselli and Ceccarelli 2012)
Star formation and Astrochemistry/Cosmochemistry: There is an intimate connection between the early solar nebula and the interstellar medium from which it formed. Thus, the study of current star-forming environments can tell us much about how we came to be. The combination of rapidly improving observational tools and increasingly sophisticated theory can help us to understand the physical processes associated with the assembly of Sun-like stars and their attendant planetary systems.
Interstellar clouds could plausibly be the initial formation sites for the prebiotic molecules that may have been delivered to the early Earth by comets (Mumma and Charnley 2011). Because of its relative closeness and rich molecular complexity, Taurus Molecular Cloud-1 (TMC-1) has been extensively observed to study the chemical processes taking place in dark clouds. I was a second lead member of the team who developed a local thermodynamical equilibrium radiative transfer model coupled with a Bayesian statistical method, which takes into account outliers to analyze the data from the Nobeyama Radio Observatory (NRO 45m) spectral survey of TMC-1 between 8 and 50 GHz. We computed the abundances relative to molecular hydrogen of 57 molecules in TMC-1 along with their associated uncertainty. This work is serving as a new reference (InterStellar Abundances Database) set of abundances for TMC-1 which can be used to benchmark the physical and chemical conditions of dark molecular clouds (Gratier, Majumdar et al. 2016). Molecular isotopic ratios are also keys to understanding the origin and early evolution of the solar system since isotopic fractionation chemistry is highly environment specific and can leave an imprint on solar system bodies. For the same source, I derived isotopic (D/H) ratios for many molecules that can be compared with the various family of comets and can be linked together to build up a coherent picture of the history of the Solar-System formation (Majumdar, Gratier et al. 2017a). I developed an isotope-fractionation reaction network (deuspin.kida.uva.2016) for this purpose, which is available in the KIDA database.Figure 2: (Top) 500 micron dust continuum emission seen by the SPIRE instrument on the Herschel Space Observatory. (Middle) Map of integrated intensity of NH3 (1,1). (Bottom) Map of integrated intensity of CCS J = 2-1 (Seo, Majumdar et al. 2019)
I did spectral survey of CCS and HC7N in the L1495-B218 filaments in the Taurus molecular cloud (Figure 2). I observed strong CCS emission in both evolved and young regions and weak emission in two evolved regions. HC7N emission is observed only in L1495A-N and L1521D. I find that CCS and HC7N intensity peaks do not coincide with NH3 or dust continuum intensity peaks. I also find that the fractional abundance of CCS does not show a clear correlation with the dynamical evolutionary stage of dense cores. My findings and chemical modeling indicate that the fractional abundances of CCS and HC7N are sensitive to the initial gas-phase C/O ratio, and they are good tracers of young condensed gas only when the initial C/O is close to solar value. Kinematic analysis using multiple lines including NH3, HC7N, CCS, CO, HCN, and HCO+ suggests that there may be three different star formation modes (Figure 3) in the L1495-B218 filaments. At the hub of the filaments, L1495A/B7N has formed a stellar cluster with large-scale inward flows (fast mode), while L1521D, a core embedded in a filament, is slowly contracting due to its self-gravity (slow mode). There is also one isolated core that appears to be marginally stable and may undergo quasi-static evolution (isolated mode) (Seo, Majumdar et al. 2019). But one of the key questions is how a prestellar core collapses/fragments and forms a protostar or protostars. Particularly, the formation of a first hydrostatic core and fragmentation within a prestellar core have never been observed. From my observations, I also found a fragmenting dense core candidate, L1495A-N, which has two condensations/fragments (Majumdar et al. 2020).Figure 3: The three modes of star formation. The first (fast) mode describes star formation at the hub, where the column density is highest and the global gravitational potential well is deepest. Large-scale flows drive continuous star formation in relatively short time and form a stellar cluster/association. The second (slow) mode elaborates the formation of the dense core chains within filaments due to gravitational fragmentation of filaments and localized star formation within each core. The third (isolated) mode shows formation of an isolated dense core and localized star formation removed from filamentary structures, which is often discussed as conventional star formation (e.g., L1544, Bok globules) (Seo, Majumdar et al. 2019)
Spectral Astronomical Observations (Low Mass and High Mass Star Forming Regions) using Single Dish Telescopes and Interferometers: I did a IRAM 30m spectral survey of a solar-type class 0 protostar IRAS 16293-2422 to understand: (a) Where are the elements of life during star formation? (b) Can a part of the molecular content be preserved during the star formation process and incorporated into comets? The major results of this spectral survey are: For the first time, I have detected organosulfur CH3SH in IRAS 16293-2422 (Majumdar et al. 2016) (Figure 4). This detection of CH3SH indicates that there may be several new families of sulphur bearing molecules (which could form starting from CH3SH which have not been detected or looked for yet and may give clues to the where is the element sulphur in star forming regions. This survey also led to the subsequent discovery of CH3SH in comet 67P/Churyumov-Gerasimenko by Rosetta (Figure 5c). Comparison of CH3SH/H2S ratios from our observation and comet 67P argued about the theory of protostellar inheritance of organic material. Recently, Majumdar et al. (2018) also assess whether such an inheritance is likely in the case of the precursor of amino acids CH3NCO by comparing the CH3NCO/HNCO ratio with ALMA observation in IRAS 16293-2422 and Rosetta Philae lander measurement on comet 67P.
Despite a wealth of fragmentary studies, the characterization and understanding of the chemical evolution of the youngest protostars, Class 0 objects, still remains poorly understood. I am currently leading spectral survey of Class 0 objects NGC 1333 IRAS 4A, L1448 IRS3B, L1448 IRS2, NGC 1333 IRAS 4B, IC348MMS and NGC 1333 IRAS 2A in the Perseus star-forming region to gain new insights into the formation and chemical composition of these youngest stellar systems. Since amino acids are the building blocks of life, the search for these molecules and their complex organic precursors at different stages of star and planet formation is one of the exciting topics in modern astronomy and planetary science. I am also involved in the large survey of complex organic molecules in high mass star forming regions as a co-investigator (Suzuki et al. 2019, Majumdar et al. 2020). This survey resulted in the detection of methanimine (CH2NH) in eight high mass star forming regions. In the past, CH2NH was referred as a possible precursor for the simplest amino acid glycine. ALMA provides greater sensitivity and spatial resolution and I am leading an ALMA cycle 5 observational project to locate glycine and its precursors in high mass star forming regions (Majumdar et al. 2020 in prep.).Figure 4: (Top) Continuum images of IRAS 16293-2422 at 3.0, 1.3 and 0.87 mm (ALMA Bands 3, 6 and 7). (Bottom) First detection of CH3SH in IRAS 16293-2422 (Majumdar et al. 2016)
Disk Composition and Planet Formation: Snowlines (transition from ice to gas) are key ingredients for planet formation. Providing observational constraints on the locations of the major snowlines (N2, CO, CO2 and H2O) is therefore crucial for fully connecting planet compositions to their formation mechanism. Unfortunately, the most important snowline, that of water, is very difficult to observe directly in protoplanetary disks due to its close proximity to the central star. On the other hand, CO2 cannot be observed in the gas phase through rotational transitions in the far-infrared or sub-millimeter range due to its lack of permanent dipole moment. It has to be observed through its vibrational transitions at near- and mid-infrared wavelengths. Recently, I observed protonated carbon dioxide (HOCO+) in the protostellar system IRAS 16293-2422 as a chemical probe of gas phase CO2. I also found that it chemical traces the water snowline because gaseous H2O is its one of the most abundant destroyer in warm dense gas. Observations of CO2 in exoplanet atmospheres is one of the major science themes for JWST. Currently, I am using ALMA data together with thermo-chemical model in protoplanetary disks to constrain the temperature structure, ionization fraction, H2O and CO2 snow lines, overall C/O ratio using this new chemical probe and linking them with future observations of CO2 atmospheres in exoplanets (Majumdar and Willacy 2020 in prep.). Studying different molecular species in protoplanetary disks is very useful to characterize the properties of these objects, which are the sites of planet formation. I am also involved as a co-investigator in a large spectral survey of H2S, CS, SO and SO2 in the dense disk around GG Tau A with the NOEMA interferometer. We detected H2S emission from the dense and cold ring orbiting around GG Tau A (Figure 5a). This is the first detection of H2S in a protoplanetary disk (Figure 5b). The H2S detection in GG Tau A is most likely linked to the much larger mass of this disk compared to that in other T Tauri systems (Phuong, Chapillon, Majumdar et al. 2018).Figure 5: (a) ALMA and IRAM images of GG Tau A (Dutrey et al. 2016) IRAM data: CO J=2-1 (Guilloteau and Dutrey 2001) and CS J=3-2 integrated line maps and 1.3 mm continuum map. ALMA data: CO and 13CO J=3-2 and CO J=6-5 (Dutrey et al., 2014) and CS J=7-6 (Phuong, Chapillon, Majumdar et al. 2018) integrated line emission, 0.8 and 0.45 mm continuum maps; (b) Integrated intensity maps of H2S in GG Tau A; (c) Sulfur budget in the comet 67P
Formation of a circumbinary planetary system: Half of the stars in our Galaxy are members of multiple systems and the Kepler telescope revealed that such systems can harbor exoplanets. Planet formation in such systems can be quite different from that around single stars, like the Sun, because of the gravitational disturbance due to the multiplicity. In the case of GG Tau, the planet is farther away from the central stars but this result is a first very important step towards the understanding of how such planetary systems can form and survive. To probe the planet formation process in multiple systems, we observed the GG Tau A system, a young binary (actually a hierarchical triple) system that is known to harbor a massive circumbinary ring surrounded by a shallower disk. An unusual 'hot spot' (twice warmer than the surrounding disk) was found in CO gas using ALMA, and later confirmed with other CO transitions. This "hot spot" suggested an internal heating source, perhaps an accreting protoplanet located at almost 10 times the Sun-Neptune distance. Our new observations reveal a striking spiral pattern in the outer disk starting from the hot spot location. This pattern is strikingly similar to that expected from the spiral density wave excited by a proto-planet. Two other spiral-like patterns are also found at larger distances, and could be explained by two other (yet unseen) proto-planets in a very special configuration. This configuration -known as an orbital resonance by the planetary astronomers- is expected to make the planetary system more stable. Figure 6: Discovery of a Circumbinary Planetary System: Spirals induced by the three putative planets, GG Tau Ac (black), GG Tau Ad (cyan), and GG Tau Ae (blue). The white ellipses indicate the approximate inner and outer edge of the dust ring, Phoung et al. (2020)
In summary, I am trying to advance our understanding of star and planet formation observationally and theoretically, which has been established not only by a well-organized research plan (for future planetary, astrochemistry, astrobiology, and exo-planetary research) but also by a good vision towards the future of these fields in Indian and international context.
Summary the future research projects (summary in Figure 6) in the planetary and exo-planetary field will be available after the approval from funding agencies such as DAE, ISRO, DST, CSIR and MOES (stay tuned!).