Hot molecular cores in the W49A molecular cloud complex

Abstract

We present a comprehensive dataset of hot molecular cores (HMCs) in the W49A molecular cloud complex based on high-resolution ALMA observations, including the 1.3~mm continuum, 12 molecular lines, and the H30alpha recombination line. In total, 18 HMCs are identified in the CH$_{3}$CN ($J_K=12_3-11_3$) map, together with 20 continuum sources in the 1.3~mm map. Ten HMCs have peaks coincident within 0\farcs1 of the 1.3~mm continuum peaks, indicating that thermal dust emission dominates the 1.3~mm emission for these sources. Correlation analyses of the line luminosities suggest a common structural picture for HMCs, in which five distinct regions with different physical and chemical properties coexist: hot and dense gas (CH$_3$CN, HC$_3$N, HNCO, OCS, H$_2$CO), outflow gas (SiO, SO), envelope gas (SO$_2$, CH$_3$OH), extended gas ($^{13}$CS, DCN, C$^{18}$O), and ionized gas (H30alpha). We find an empirical relation $X(\mathrm{CH_3CN})=2.5\times10^{-7}\exp[-490/T_\mathrm{rot}]$ between the fractional abundance and rotation temperature of CH$_{3}$CN, suggesting that $T\gtrsim200$--300~K is required to achieve high abundances of $\sim10^{-7}$. We find that HMCs without embedded H/UCHII regions are more numerous than, or at least comparable in number to, HMCs with such regions, suggesting that the former may have longer lifetimes ($\sim$10$^{5}$~yr) than the latter ($\sim$10$^{4}$~yr). We discuss the implications of these results for the core accretion and competitive accretion models.