Presentation #113.08 in the session “Galaxy Morphology and Star Formation”.
Massive stars contribute significantly to the evolution of galaxies by injecting radiative and mechanical energy into the ISM. This energy input stirs the environment around massive stars through stellar winds, ionization and heating of the gas, and through supernovae explosions, all processes that considerably change the chemical composition and structure of the ISM in their neighborhood (see overviews in Tielens 2010 and Draine 2010). The interaction of massive stars with their surroundings can affect the star formation process either by quenching star formation or by triggering it: formation of cloud and inter cloud phases in the ISM, leading to disruption of molecular clouds, can result in halting star formation, while compression of the surrounding gas or the propagation of photoionisation induced shocks can set of star formation (Elmegreen & Lada 1977). Massive stars give rise to bright HII regions and photodissociation regions (PDRs). HII regions comprise hot ionized gas irradiated by strong ultraviolet (UV, hν > 13.6 eV) radiation from one or more nearby hot luminous stars. PDRs are at the interface of these HII regions and the cool molecular cloud shielded from UV radiation from the illuminating star (Hollenbach et al. 1999). In PDRs, the thermal and chemical processes are regulated by far-UV (FUV, 6 eV < hν < 13.6 eV) photons. The lagoon Nebula, Messier 8 (M8), is one of the brightest massive star-forming regions known in our galaxy and it hosts high-UV flux PDRs and HII regions. We performed a detailed (imaging and line) survey toward M8, using the world’s leading FIR and submm wavelength observing facilities like the SOFIA, APEX and IRAM 30 m telescopes. We described the morphology of the volume around the bright stellar system Herschel 36, which is main ionizing source of M8 and determined the physical conditions of the gas surrounding it (Tiwari et al.2018). We reported a small hydrocarbon (C2H, c-C3H2) study toward this region and shed some light on the formation process of hydrocarbons in M8-Main, which is a high UV flux PDR with G0 ~ 105 in Habing units (Tiwari et al. 2019). I will summarize the above findings in M8 and will present our recent work (Tiwari et al. submitted in A&A) on studying the environment of M8 East, which is at an earlier stage of massive star formation. We analysed the J = 1-0 transitions of CO, HCN, HCO+, HNC, and N2H+ to characterize the energetics, density and chemistry of the region. Examining the ancillary data in IR wavelengths, allowed us to identify the ionization front and by comparing the spatial distribution of the YSO population in M8 East, we constrained the speed of the ionization front.