A Case Study of Low-mass Star Formation

This article synthesizes observational data from an extensive program aimed toward a comprehensive understanding of star formation in a low-mass star-forming molecular cloud. New observations and published data spanning from the centimeter wave band to the near infrared reveal the high and low density molecular gas, dust, and pre-main sequence stars in L1551. The total cloud mass of ∼ 160M contained within a 0.9 pc has a dynamical timescale, tdyn = 1.1 Myr. Thirty-five pre-main sequence stars with masses from ∼ 0.1 to 1.5M are selected to be members of the L1551 association constituting a total of 22±5M of stellar mass. The observed star formation efficiency, SFE = 12%, while the total efficiency, SFEtot, is estimated to fall between 9 and 15%. L1551 appears to have been forming stars for several tdyn with the rate of star formation increasing with time. Star formation has likely progressed from east to west, and there is clear evidence that another star or stellar system will form in the high column density region to the northwest of L1551 IRS5. High-resolution, wide-field maps of L1551 in CO isotopologue emission display the structure of the molecular cloud at 1600 AU physical resolution. The CO emission clearly reveals the disruption of the ambient cloud by outflows in the line core and traces the interface between regions of outflow and quiescent gas in the line wings. Kinetic energy from outflows is being deposited back into the cloud on a physical scale λpeak ≈ 0.05 pc at a rate, Ėinput ≈ 0.05L . The remaining energy afforded by the full mechanical luminosity of outflow in L1551 destroys the cloud or is otherwise lost to the greater interstellar medium. The CO emission is optically thin and traces well the turbulent velocity structure of the cloud. The total turbulent energy is close to what is expected from virial equilibrium. The turbulent velocities exist primarily on small scales in the cloud and the energy spectrum of turbulent fluctuations, E(k) ∝ k−β , is derived by various methods to have β ≈ 1–2. The turbulent dissipation rate estimated using the results of current numerical simulations is Ėdiss ≈ Ėinput. This study reveals that stellar feedback is a significant factor in the evolution of the L1551 cloud. Subject headings: stars: formation — ISM: clouds — stars: pre-main sequence — ISM: individual(L1551) — ISM: structure — radio lines: ISM — techniques: interferometric