Envelope structure of deeply embedded young stellar objects in the SerpensMolecular Cloud

Aperture-synthesis and single-dish (sub-) millimeter molecular-line and con tinuum observations reveal in great detail the envelope structure of deeply embedded young stellar objects (SMM 1 = FIRS 1, SMM 2, SMM 3, SMM 4) in th e densely star-forming Serpens Molecular Cloud. SMM 1, 3, and 4 show partia lly resolved (>2 " = 800 AU) continuum emission in the beam of the Owens Va lley Millimeter Array at lambda= 3.4-1.4 mm. The continuum visibilities acc urately constrain the density structure in the envelopes, which can be desc ribed by a radial power law with slope -2.0 +/- 0.5 on scales of 300 to 800 0 AU. Inferred envelope masses within a radius of 8000 AU are 8.7, 3.0, and 5.3 M-. for SMM 1, 3, and 4, respectively. A point source with 20%-30% of the total flux at 1.1 mm is required to lit the observations on long baseli nes, corresponding to warm envelope material within similar to 100 AU or a circumstellar disk. No continuum emission is detected interferometrically t oward SMM 2, corresponding to an upper limit of 0.2 M-. assuming T-d= 24 K. The lack of any compact dust emission suggests that the SMM 2 core does no t contain a central protostar. Aperture-synthesis observations of the (CO)- C-13, (CO)-O-18, HCO+, (HCO+)-C-13, HCN, (HCN)-C-13, N2H+ 1-0, SiO 2-1, and SO 2(2)-1(1) transitions reveal compact emission toward SMM 1, 3, and 4. S MM 2 shows only a number of clumps scattered throughout the primary field o f view, supporting the conclusion that this core does not contain a central star. The compact molecular emission around SMM 1, 3, and 4 traces 5 "-10 " (2000-4000 AU) diameter cores that correspond to the densest regions of t he envelopes, as well as material directly associated with the molecular ou tflow. Especially prominent are the optically thick HCN and HCO+ lines that show up brightly along the walls of the outflow cavities. SO and SiO trace shocked material, where their abundances may be enhanced by 1-2 orders of magnitude over dark-cloud values. A total of 31 molecular transitions have been observed with the James Clerk Maxwell and Caltech Submillimeter telesc opes in the 230, 345, 490, and 690 GHz atmospheric windows toward all four sources, containing, among others, lines of CO, HCO+, HCN, H2CO, SiO, SO, a nd their isotopomers. These lines show 20-30 km s(-1) wide line wings, deep and narrow (1-2 km s(-1)) self-absorption, and 2-3 km s(-1) FWHM line core s. The presence of highly excited lines like (CO)-C-12 4-3 and 6-5, (CO)-C- 13 6-5 and several H2CO transitions indicates the presence of material with temperatures greater than or similar to 100 K. Monte Carlo calculations of the molecular excitation and line transfer show that the envelope model de rived from the dust emission can successfully reproduce the observed line i ntensities. The depletion of CO in the cold gas is modest compared to value s inferred in objects like NGC 1333 IRAS 4, suggesting that the phase of la rge depletions through the entire envelope is short lived and may be influe nced by the local star formation density. Emission in high-excitation lines of CO and H2CO requires the presence of a small amount of similar to 100 K material comprising less than 1% of the total envelope mass and probably a ssociated with the outflow or the innermost region of the envelope. The der ived molecular abundances in the warm (T-kin > 20 K) envelope are similar t o those found toward other class 0 YSOs like IRAS 16293-2422, though some s pecies appear enhanced toward SMM 1. Taken together, the presented observations and analysis provide the first c omprehensive view of the physical and chemical structure of the envelopes o f deeply embedded young stellar objects in a clustered environment on scale s between 1000 and 10,000 AU.