Astrochemistry as a probe of initial conditions of high-mass pre-stellar clumps
Our understanding of the initial conditions of high-mass star formation are far from complete. Molecular line observations (such as MALT90 survey) are useful tools to probe the physical and chemical conditions of these clumps (such as densities, temperatures, ages, cosmic-ray ionization rates etc.). Some of the recent studies performed detailed chemical modeling of these clumps to interpret the molecular line emission but the effect of radiative transfer was not considered. However, molecular abundances estimated without radiative transfer analysis have significant uncertainties. We plan to bridge this gap by combining both chemical and radiative transfer models along with the available data from various Galactic plane surveys (such as ATLASGAL, MALT90, Hi-GAL) to estimate the abundances by comparing the simulated and observed spectra. We used Spitzer and Herschel 70 micron images to identify the pre-stellar clumps among the dust condensations detected by the ATLASGAL survey. Herschel observation were also used to generate the column density and temperature maps which then helped to construct the 3D physical models of these clumps. We used the chemical evolutionary code Saptarsy to calculate the chemical evolution of these clumps over the physical frameworks. To explore the chemical variation in these clumps, we constructed a grid of models by varying the density, temperature, initial abundance, CR-ionization rate etc. and calculated the molecular abundances. Finally, we performed radiative transfer modeling to generate the simulated spectra for some of the selected lines that are covered by MALT90 survey (such as J=1--0 transitions of HCN, HNC, N2H+, HCO+ etc.). We then compared the observed and simulated spectra and iterated over the input physio-chemical parameters until a satisfactory match were obtained. We find that the chemical complexity, relative molecular abundances and line widths of selected transitions also increase with dust temperature. There are some pre-stellar clumps where emission line of a particular molecule among the 'fav-four' molecules is not detected. These sources are important in order to understand the impact of large scale environment on the evolution of pre-stellar clumps. We will present the results of our ongoing study and discuss about some of the useful tracers that probe the physio-chemical conditions of high-mass pre-stellar clumps at different evolutionary stages.