TY - JOUR

T1 - Molecular freeze-out as a tracer of the thermal and dynamical evolution of pre- And protostellar cores

AU - Jørgensen, J. K.

AU - Schöier, F. L.

AU - Van Dishoeck, E. F.

PY - 2005/5/1

Y1 - 2005/5/1

N2 - Radiative transfer models of multi-transition observations are used to determine molecular abundances as functions of position in pre- and protostellar cores. The data require a "drop" abundance profile with radius, with high abundances in the outermost regions probed by low excitation 3 mm lines, and much lower abundances at intermediate zones probed by higher frequency lines. The results are illustrated by detailed analysis of CO and HCO + lines for a subset of objects. We propose a scenario in which the molecules are frozen out in a region of the envelope where the temperature is low enough (≲40 K) to prevent immediate desorption, but where the density is high enough (>104-105 cm-3) that the freeze-out timescales are shorter than the lifetime of the core. The size of the freeze-out zone is thereby a record of the thermal and dynamical evolution of the cores. Fits to CO data for a sample of 16 objects indicate that the size of the freeze-out zone decreases significantly between class 0 and I objects, explaining the variations in, for example, CO abundances with envelope masses. However, the corresponding timescales are 105±0.5 years, with no significant difference between class 0 and I objects. These timescales suggest that the dense pre-stellar phase with heavy depletions lasts only a short time, of the order of 105 yr, in agreement with recent chemical-dynamical models.

AB - Radiative transfer models of multi-transition observations are used to determine molecular abundances as functions of position in pre- and protostellar cores. The data require a "drop" abundance profile with radius, with high abundances in the outermost regions probed by low excitation 3 mm lines, and much lower abundances at intermediate zones probed by higher frequency lines. The results are illustrated by detailed analysis of CO and HCO + lines for a subset of objects. We propose a scenario in which the molecules are frozen out in a region of the envelope where the temperature is low enough (≲40 K) to prevent immediate desorption, but where the density is high enough (>104-105 cm-3) that the freeze-out timescales are shorter than the lifetime of the core. The size of the freeze-out zone is thereby a record of the thermal and dynamical evolution of the cores. Fits to CO data for a sample of 16 objects indicate that the size of the freeze-out zone decreases significantly between class 0 and I objects, explaining the variations in, for example, CO abundances with envelope masses. However, the corresponding timescales are 105±0.5 years, with no significant difference between class 0 and I objects. These timescales suggest that the dense pre-stellar phase with heavy depletions lasts only a short time, of the order of 105 yr, in agreement with recent chemical-dynamical models.

KW - ISM: abundances

KW - ISM: molecules

KW - Stars: formation

UR - http://www.scopus.com/inward/record.url?scp=18744389168&partnerID=8YFLogxK

U2 - 10.1051/0004-6361:20042092

DO - 10.1051/0004-6361:20042092

M3 - Journal article

AN - SCOPUS:18744389168

VL - 435

SP - 177

EP - 182

JO - Astronomy & Astrophysics

JF - Astronomy & Astrophysics

SN - 0004-6361

IS - 1

ER -