A Pre‐Protostellar Core in L1551

Large-field surveys of NH 3 , C 2 S, 13 CO, and C 18 O in the L1551 dark cloud have revealed a prolate, pre-protostellar molecular core (L1551 MC) in a relatively quiescent region to the northwest of the well-known IRS 5 source. The kinetic temperature is measured to be 9 K, the total mass is ~2 M ☉ , and the average particle density is 10 4 -10 5 cm -3 . L1551 MC is 2 25 × 1 11 in projection, oriented at a position angle of 133°. The turbulent motions are on the order of the sound speed in the medium and contain 4% of the gravitational energy, E grav , of the core. The angular momentum vector is projected along the major axis of L1551 MC, corresponding to a rotational energy of 2.5 × 10 -3 i | E grav |. The thermal energy constitutes about a third of | E grav |, and the virial mass is approximately equal to the total mass. L1551 MC is gravitationally bound and in the absence of strong, ~160 μG magnetic fields will likely contract on a ~0.3 Myr timescale. The line profiles of many molecular species suggest that the cold quiescent interior is surrounded by a dynamic, perhaps infalling envelope that is embedded within the ambient molecular gas of L1551.