Probing the CO and methanol snow lines in young protostars. Results from
  the CALYPSO IRAM-PdBI survey
  Context. "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. Aims. We aim at using the information on
the sublimation regions of different kinds of ices to understand the chemistry
of the envelope, its temperature and density structure, and the history of the
accretion process. Methods. As part of the CALYPSO IRAM Large Program, we have
obtained observations of C$^{18}$O, N$_2$H$^+$ and CH$_3$OH 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.
Results. We observe an anti-correlation of C$^{18}$O and N$_2$H$^+$ in NGC
1333-IRAS4A, NGC 1333-IRAS4B, L1157, and L1448C, with N$_2$H$^+$ forming a ring
around the centrally peaked C$^{18}$O emission due to N$_2$H$^+$ being
chemically destroyed by CO. The emission regions of models and observations
match for a CO binding energy of 1200 K, which is higher than the binding
energy of pure CO ices ($\sim$855 K). Furthermore, we find very low CO
abundances inside the snow lines in our sources, about an order of magnitude
lower than the total CO abundance observed in the gas on large scales in
molecular clouds before depletion sets in. Conclusions. The high CO binding
energy may hint at CO being frozen out in a polar ice environment like
amorphous water ice or in non-polar CO$_2$-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. (Abbr. Version)
