BHASWATI MOOKERJEA

Search for Small hydrides and carbon chains in the ism : Absorption spectroscopy

Sub-millimeter observing facilities in India

RESEARCH INTERESTS

The interstellar medium (ISM) or the gas and dust between the stars is the most important component of galaxies since it is responsible for forming the stars that are the dominant sources of energy. While most mass of the galaxies are currently understood to be in the form of dark matter particles that are (nearly) collisionless, it is the baryons (regular matter) and electrons, accounting for 10% of the mass, that determine the visible appearance of galaxies and are responsible for nearly all of the energy emitted by galaxies. At early times, the baryonic mass in the galaxies was primarily in the gas in the ISM. As the galaxies evolved the ISM is gradually converted to stars, some of the interstellar gas may get ejected from the galaxy in the form of galactic wind, or stripped from the galaxy by the intergalactic medium (IGM). Similarly, infalling gas from the IGM may add to the mass of the ISM. At the present epoch, in our Galaxy, the Milky Way most of the baryons are incorporated into stars or stellar remnants. But even today nearly 10% of the baryons in the Milky Way are in the ISM.

My research broadly aims at

Understanding the workings of the ISM, its distribution and conditions (temperature, density, ionization etc) which finally lead to the formation of stars.
Understanding the formation of high mass stars

Since most of the ISM consists of dust and gas which are accessible only at wavelengths between 60 micron to 3mm, I use observatories with photometers (cameras) and spectrometers operational at these wavelengths. The work involves analysis and visualization of spectroscopic data in combination with theoretical tools and models to (i) identify large-scale structures (filaments) in which stars form and study their properties, (ii) study the physical properties (temperature, density) and chemical stratification in the clouds arising from the impact of radiation fields (PDRs) and kinematics (velocity distribution) (iii) detect molecules in diffuse interstellar gas.

For details of my work see below.

Study of photon dominated regions

The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet ( 6 eV<E<13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the ISM in which the heating and chemistry are regulated by far ultraviolet photons are termed Photo-Dissociation Regions or Photon Dominated (PDRs). I study the properties of PDRs to quantify the radiative and mechanical feedback of high mass stars on the ambient ISM to understand the role of high mass stars in inducing or inhibiting the formation of future generation of stars.

Selected publications on this topic:

Mookerjea, B., Sandell, G., Güsten, R. et al. "Constraining the geometry of the reflection nebula NGC 2023 with [O I]: emission & absorption", Monthly Notices of the Royal Astronomical Society, 525, 5468, 2023, doi:10.1093/mnras/stad2644
Mookerjea, B., “Star Formation Triggered by the Expanding Bubble S111”, The Astrophysical Journal, vol. 926, no. 1, 2022. doi:10.3847/1538-4357/ac4258.
Mookerjea, B., Sandell, G., Veena, V. S. et al. “Distribution of ionized, atomic, and PDR gas around S 1 in ρ Ophiuchus”, Astronomy and Astrophysics, vol. 648, 2021. doi:10.1051/0004-6361/202040217.
Mookerjea, B., Sandell, G., Güsten, R., Riquelme, D., Wiesemeyer, H., and Chambers, E., “Opening the Treasure Chest in Carina”, Astronomy and Astrophysics, vol. 626, 2019. doi:10.1051/0004-6361/201935482.
Mookerjea, B., Israel, F., Kramer, C. et al. “Velocity resolved [C II] spectroscopy of the center and the BCLMP 302 region of M 33 (HerM 33es)”, Astronomy and Astrophysics, vol. 586, 2016. doi:10.1051/0004-6361/201527366.
Sandell, G., Mookerjea, B., Güsten, R., Requena-Torres, M. A., Riquelme, D., and Okada, Y., “High spectral and spatial resolution observations of the PDR emission in the NGC 2023 reflection nebula with SOFIA and APEX”, Astronomy and Astrophysics, vol. 578, 2015. doi:10.1051/0004-6361/201525881.
Mookerjea, B., Ossenkopf, V., Ricken, O. et al. “The structure of hot gas in Cepheus B”, Astronomy and Astrophysics, vol. 542, 2012. doi:10.1051/0004-6361/201218923.
Mookerjea, B., Kramer, C., Roellig, M. et al. “The Herschel M 33 extended survey (HerM33es): PACS spectroscopy of the star-forming region BCLMP 302”, Astronomy and Astrophysics, vol. 532, 2011. doi:10.1051/0004-6361/201116447.
Mookerjea, B., Kantharia, N. G., Roshi, D. A., and Masur, M., “CI 492GHz mapping towards Cas A”, Monthly Notices of the Royal Astronomical Society, vol. 371, no. 2, pp. 761–768, 2006. doi:10.1111/j.1365-2966.2006.10700.x.
Mookerjea, B., Kramer, C., Röllig, M., and Masur, M., “Study of photon dominated regions in Cepheus B”, Astronomy and Astrophysics, vol. 456, no. 1, pp. 235–244, 2006. doi:10.1051/0004-6361:20064991.
Kramer, C., Mookerjea B., Bayet, E. et al “Photon dominated regions in the spiral arms of M 83 and M 51”, Astronomy and Astrophysics, vol. 441, no. 3, pp. 961–973, 2005. doi:10.1051/0004-6361:20053358.
Mookerjea, B., Ghosh, S. K., Kaneda, H. et al. “Mapping of large scale 158 mu m [CII] line emission: Orion A”, Astronomy and Astrophysics, vol. 404, pp. 569–578, 2003. doi:10.1051/0004-6361:20030537.

Challenges of tracing the molecular ISM

The terms atomic, ionized, molecular ISM primarily refers to the state of hydrogen (the main element of the ISM) in the gas. However the hydrogen molecule being homonuclear does not have spectral transitions that are excited , at the typical temperatures of the molecular ISM (10-30 K) and hence it can not be observed directly. As a result, alternative molecules which have transitions that can be excited are used to trace the molecular H2 in the ISM. Alternative molecules are explored as "surrogates" by comparing their abundances relative to the molecular H2 in regions with physical conditions conducive to the excitation of spectral lines from both H2 and this molecule and establishing a nearly "constant" value of the relative abundance.

The primary "surrogate" molecule has so far been CO, although it is known for some time that CO depletes onto dust grains in the cold ISM and hence is not detected in the spectra of such molecular regions. More recent studies of singly ionized carbon (C+) has revealed existence of molecular gas not detected in CO in diffuse regions where the FUV flux photodissociates CO to form C+ although hydrogen remains molecular. Additionally, the abundances of molecules such as CH, OH and HF are shown to have strong correlation with molecular hydrogen and the values of relative abundances also remain constant over a significant range of conditions in the ISM.

Relevant publications addressing this topic are:

Rawlins, K. and Mookerjea, B. "Investigating the OH-H2 relation in diffuse Galactic clouds", Monthly Notices of the Royal Astronomical Society, vol. 523, no.3, pp 4593-4602, 2023. doi:10.1093/mnras/stad1707
Mookerjea, B., “Revisiting the OH-CH correlation in diffuse clouds”, Monthly Notices of the Royal Astronomical Society, vol. 459, no. 3, pp. 2822–2826, 2016. doi:10.1093/mnras/stw872.
Mookerjea, B., Israel, F., Kramer, C. et al. “Velocity resolved [C II] spectroscopy of the center and the BCLMP 302 region of M 33 (HerM 33es)”, Astronomy and Astrophysics, vol. 586, 2016. doi:10.1051/0004-6361/201527366.

Search for Small hydrides and carbon chains in the ism : Absorption spectroscopy

Among the many molecules identified in the interstellar medium and circumstellar envelopes so far (over 140, not including isotopologues bearing rare isotopes) a significant subsample has been detected in the diffuse interstellar medium, ranging from H2 and HD (e.g. Rachford et al. 2002) to chemically rather complex species such as C3 (Oka et al. 2003), and c-C3H2 (Lucas and Liszt 2000). Probing the diffuse ISM molecules is an efficient method for studying the first steps of interstellar chemistry, and its interrelation with the interstellar medium dynamics and evolution. Many of the small (light) molecules such as the hydrides have pure rotational transitions in the far-infrared and submillimeter wavelengths and for the pure carbonchains molecules such as C3 and longer the ro-vibrational transitions are also in the far-infrared. Herschel Space Observatory had provided a unique opportunity to observe such molecules in the far-infrared with high velocity resolution, important to identify the components (clouds) along the lines of sight to bright continuum sources. I was a co-I of the Herschel Guaranteed Time Key Program "Probing the InterStellar Medium using Absorption Studies (PRISMAS)" (PI: M. Gerin). In particular, I led the analysis of the observations of the carbon chain molecules C3 together in collaboration with laboratory spectroscopists from Cologne and later Kassel.

We detected the CCC molecule along lines of sight to multiple Galactic far-infrared continuum sources. The velocity resolution allowed us to identify that the absorption arose from the gas associated with the background source, which is a high-mass star forming region. Modelling of the column densities derived suggested a Warm Carbon Chain Chemistry being in action that leads to the re-generation of CCC at later times by evaporation of methane ice from the dust grains in the star forming regions. In a follow up work with the Stratospheric Observatory For Infrared Astronomy we also detected the 13C isotopes of CCC, viz., C13CC and 13CCC and found that the column density ratio N(13CCC)/N(C13CC) was different from the statistically predicted value and proposed that the discrepant abundance ratio arises due to the lower zero-point energy of C13CC, which makes position-exchange reaction converting 13CCC to C13CC energetically favorable.

Selected publications relevant to this area:

Giesen, T. F., Mookerjea, B., Fuchs, G. “First detection of the carbon chain molecules 13CCC and C13CC towards SgrB2(M)”, Astronomy and Astrophysics, vol. 633, 2020. doi:10.1051/0004-6361/201936538.
Vastel, C., Mookerjea, B., Pety, J., and Gerin, M., “Deuterium fractionation of a distant cold dark cloud along the line of sight of W51”, Astronomy and Astrophysics, vol. 597, 2017. doi:10.1051/0004-6361/201629289.
Persson, C. M., Gerin, M., Mookerjea, B. et al. “First detection of [N II] 205 μm absorption in interstellar gas. Herschel-HIFI observations towards W 31C, W 49N, W 51, and G34.3+0.1”, Astronomy and Astrophysics, vol. 568, 2014. doi:10.1051/0004-6361/201423997.
Mookerjea, B., Hassel, G., Gerin, M. et al. “Chemistry of C3 and carbon chain molecules in DR21(OH)”, Astronomy and Astrophysics, vol. 546, 2012. doi:10.1051/0004-6361/201219287.
Mookerjea, B., Giesen, T. F., Stutzki, J. et al. “Excitation and abundance of C3 in star forming cores. Herschel/HIFI observations of the sight-lines to W31C and W49N”, Astronomy and Astrophysics, vol. 521, 2010. doi:10.1051/0004-6361/201015095.

Formation of high mass stars

Star formation is a multi-scale process that occurs in large (L ∼10 pc), dense (n~100 cm^-3 ), and cold (T ∼ 10 K) giant molecular clouds (GMCs) that have a predominantly hierarchical structure with increasing densities toward smaller scales that are the birth sites of stars. Stars span a large range of masses from ∼0.08 Msun to 200 Msun. Typically, high-mass and low-mass stars are separated at the mass at which stellar death results in supernovae (SNe) explosions. Low-mass stars are the main stellar constituent of galaxies since they dominate the initial mass function (IMF) and therefore dominate the star formation process, and are also the sites of planet formation. In contrast, high-mass stars represent only ∼1% of the stellar population in star-forming galaxies by number but they have a much more dramatic effect on their natal environments with their intense radiation fields, fast stellar winds, and subsequent SNe explosions. This feedback has direct implications for both star and galaxy formation since stellar feedback may be responsible for the dissolution of star clusters and the destruction of the GMCs out of which they form. This makes the study of the birth and life of high-mass stars important.

However, the study of the formation of high-mass stars using observations is hindered by (i) the fact such high-mass stars being fewer and consequently distant, (ii) their formation in clustered environments that are characterized by higher surface densities and larger velocity dispersions than nearby low-mass star forming regions. It is still highly debated if high-mass star formation is simply a scaled up version of low-mass star formation in which massive stars form via the monolithic collapse of massive pre-stellar cores that are supported by turbulence and/or magnetic fields rather than thermal motions or if they form via larger scale accretion flows due to gravity or converging, inertial flows that naturally occur in supersonic turbulence from the surrounding molecular cloud.

I am currently pursuing a program based on sub- and millimeter spectroscopic observations of high-mass star forming regions located on large-scale filaments merging to or branching out from hubs in regions forming stellar groups.

Mookerjea, B.,Veena, V. , Güsten, R. et al. "Spiral structure and massive star formation in the hub-filament-system G326.27-0.49", Monthly Notices of the Royal Astronomical Society, vol. 520, issue 2, pp.2517-2533, 2023, 10.1093/mnras/stad215

Sub-millimeter observing facilities in India

I am currently leading the Indian Submillimeter Astronomy Alliance formed to support activities related to the upcoming sub-millimeter observing facility in the Himalayas in India.
My group is collaborating with SAC-ISRO, Ahmedabad on the science utilization of the sub-mm facility as well as development of second-generation instruments for the telescope
I have initiated a multi-institute effort to characterize high altitude (>5000m) sites in India to establish their potential for hosting sub-mm (and optical/near-infrared) observing facilities.