EPoS Contribution
EPoS Contribution
CALYPSO studies of the CO snow line in young low-mass protostars

Sibylle Anderl
IPAG, Grenoble, FR
"Snow lines", marking regions where abundant volatiles freeze out onto the surface of dust grains, play an important role for planet growth and bulk composition in protoplanetary disks. They can already be observed in the envelopes of the much younger, low-mass Class 0 protostars that are still in their early phase of heavy accretion. The information on the sublimation regions of different kinds of ices can be used to understand the chemistry of the envelope, its temperature and density structure, and may even hint at the history of the accretion process. This information is crucial in order to get the full picture of the early protostellar collapse and the subsequent evolution of young protostars. As part of the CALYPSO IRAM Large Program, we have obtained observations of C18O, N2H+ and CH3OH towards nearby Class 0 protostars with the IRAM Plateau de Bure interferometer at sub-arcsecond resolution. For four of these sources we have modeled the emission using a chemical code coupled with a radiative transfer module. In these four sources, which are NGC 1333-IRAS4A, NGC 1333-IRAS4B, L1157, and L1448C, we observe an anti-correlation of C18O and N2H+ with N2H+ forming a ring (perturbed by the outflow) around the centrally peaked C18O emission. This emission morphology, which is due to N2H+ being chemically destroyed by CO, reveals the CO and N2 ice sublimation regions in these protostellar envelopes with unprecedented resolution. We also observe compact methanol emission towards three of the sources. Based on our chemical model and assuming temperature and density profiles from the literature, we find that for all four sources the CO snow line appears further inwards than expected from the binding energy of pure CO ices (~855 K). The emission regions of models and observations match for a higher value of the CO binding energy of 1200 K. With this value, the radius of the CO snow line corresponds to a dust temperature of ~24 K in our models. The binding energy for N2 ices is modeled at 1000 K, also higher than for pure N2 ices. Furthermore, we find very low CO abundances inside the snow lines in our sources, about an order of magnitude lower than the total CO abundance observed in the gas on large scales in molecular clouds before depletion sets in. The high CO binding energy may hint at CO being frozen out in a polar ice environment like amorphous water ice or in non-polar CO2-rich ice. The low CO abundances are comparable to values found in protoplanetary disks, which may indicate an evolutionary scenario where these low values are already established in the protostellar phase. While the emission morphology of these four sources can be well reproduced with temperature profiles based on their recent luminosities, this is not the case for the source IRAM 04191, which is surrounded by an extended N2H+ ring. In this talk, I will also discuss whether this emission morphology may trace a past accretion burst. Furthermore, an outlook will be given on the planned extension of these studies towards exploring the dynamics of the inner envelope.
Caption: Emission of N2H+ and C18O towards IRAS4B, IRAS4A, L1448C, L1157, and IRAM04191. Colour background: N2H+ (1-0) emission integrated over all seven hyperfine components. The wedges show the N2H+ intensity scale in Jy/beam*km/s. The contours show integrated emission of C18O (2-1) in steps of 6 sigma (IRAS4A and 4B) or 3 sigma (L1448C and L1157), starting at 3 sigma, with sigma=(0.032, 0.027, 0.028, 0.030, 0.016) Jy/beam*km/s for IRAS4B, IRAS4A, L1448C, L1157, and IRAM04191 respectively. The filled ellipses in the lower left corner of the panels indicate the synthesized beam sizes of the N2H+ observations at 3 mm. The dashed white lines illustrate the small-scale outflow directions, and the white crosses show positions of continuum emission peaks at 1 and 3 mm. Note the different map scale for IRAM04191 that was chosen in order to display the full extent of the ring-like structure in N2H+.
S. Maret, IPAG, Fr
and the CALYPSO team
Suggested Session: Cores and Collapse