G337.342-0.119 (the "Pebble"): A Cold, Dense, High-Mass Molecular Cloud with Unusually Large Linewidths and a Candidate High-Mass Star Cluster Progenitor

Exactly how high-mass star clusters form, especially the young massive clusters (YMCs: age $<100$ Myr; mass $>10^4$ solar masses), remains an open problem, largely because they are so rare that examples of their cold, dense, molecuar progenitors remain elusive. The molecular cloud G337.342$-$0.119, the `Pebble,' is a candidate for such a cold progenitor. Although G337.342$-$0.119 was originally identified as four separate ATLASGAL clumps, the similarity in their molecular line velocities and linewidths in the MALT90 dataset demonstrate that these four clumps are in fact one single, coherent cloud. This cloud is unique in the MALT90 survey for its combination of both cold temperatures ($T_{dust} \sim 14$ K) and large linewidths $(\Delta V \sim 10$ km s$^{-1}$). The near/far kinematic distance ambiguity is difficult to resolve for G337.342$-$0.119. At the near kinematic distance (4.7 kpc), the mass is 5,000 solar masses and the size is $7\times2$ pc. At the far kinematic distance (11 kpc), the mass is 27,000 solar masses and the size is $15 \times 4$ pc. The unusually large linewidths of G337.342$-$0.119 are difficult to reconcile with a gravitationally bound system in equilibrium. If our current understanding of the Galaxy's Long Bar is approximately correct, G337.342$-$0.119 cannot be located at its end. Rather, it is associated with a large star-forming complex that contains multiple clumps with large linewidths. If G337.342$-$0.119 is a prototypical cold progenitor for a high-mass cluster, its properties may indicate that the onset of high-mass star cluster formation is dominated by extreme turbulence.