Investigating the kinematics of molecular gas in cometary globule L1616

Abstract

LDN 1616 is a cometary globule located approximately 8◦ west of the Orion OB1 associations. The massive OB stars in the Orion belt region act as catalysts, triggering the star formation activity observed in the L1616 region which is a photodissociation region (PDR). This paper provides an in-depth analysis of gas kinematics within the L1616 PDR, leveraging the Heterodyne Array Receiver Programme on the James Clerk Maxwell Telescope to observe 13CO and C18O J = 3 → 2 emissions. Employing the Clumpfind algorithm on the C18O emission data, we identify three distinct clumps within this PDR. For each of these clumps, we derive key physical parameters, including the mean kinetic temperature, optical depth, and velocity dispersion. In addition, we compute the non-thermal velocity dispersion and Mach number, providing critical insights into the turbulent dynamics of the gas. Comprehensive evaluation of mass, including virial and energy budget evaluations, are conducted to assess the gravitational stability and star-forming potential of the identified clumps. While previousstudies have proposed that radiation-driven implosion (RDI) is the dominant mechanism initiating star formation in LDN 1616, our results suggest that the clumps may represent pre-existing substructures within the PDR. This interpretation is supported by our estimation of a relatively low interstellar radiation field (G0), which, although insufficient to form clumps independently, may enhance gravitational instability through additional compression. Thus, our findings offer a more nuanced perspective on the role of RDI, highlighting its capacity to trigger star formation by amplifying the instability of pre-existing clumpy structures in PDRs like LDN 1616.