Person:

Oberg, Karin

Complex organic molecules (COMs) have been observed toward several low-mass young stellar objects (LYSOs). Small and heterogeneous samples have so far precluded conclusions on typical COM abundances, as well as the origin(s) of abundance variations between sources. We present observations toward 16 deeply embedded (Class 0/I) low-mass protostars using the IRAM 30 m telescope. We detect CH 2 CO, CH 3 CHO, CH 3 OCH 3 , CH 3 OCHO, CH 3 CN, HNCO, and HC 3 N toward 67%, 37%, 13%, 13%, 44%, 81%, and 75% of sources, respectively. Median column densities derived using survival analysis range between 6.0 × 10 10 cm −2 (CH 3 CN) and 2.4 × 10 12 cm −2 (CH 3 OCH 3 ), and median abundances range between 0.48% (CH 3 CN) and 16% (HNCO) with respect to CH 3 OH. Column densities for each molecule vary by about one order of magnitude across the sample. Abundances with respect to CH 3 OH are more narrowly distributed, especially for oxygen-bearing species. We compare observed median abundances with a chemical model for low-mass protostars and find fair agreement, although some modeling work remains to bring abundances higher with respect to CH 3 OH. Median abundances with respect to CH 3 OH in LYSOs are also found to be generally comparable to observed abundances in hot cores, hot corinos, and massive YSOs. Compared with comets, our sample is comparable for all molecules except HC 3 N and CH 2 CO, which likely become depleted at later evolutionary stages.

Context: The gas and dust in circumstellar disks provide the raw materials to form planets. The study of organic molecules and their building blocks in such disks offers insight into the origin of the prebiotic environment of terrestrial planets. Aims: We aim to determine the distribution of formaldehyde, H2CO, in the disk around HD 163296 to assess the contribution of gas- and solid-phase formation routes of this simple organic. Methods: Three formaldehyde lines were observed (H2CO 303–202, H2CO 322–221, and H2CO 321–220) in the protoplanetary disk around the Herbig Ae star HD 163296 with ALMA at ~0.5″ (60 AU) spatial resolution. Different parameterizations of the H2CO abundance were compared to the observed visibilities, using either a characteristic temperature, a characteristic radius or a radial power law index to describe the H2CO chemistry. Similar models were applied to ALMA Science Verification data of C18O. In each scenario, χ2 minimization on the visibilities was used to determine the best-fit model in each scenario. Results: H2CO 303–202 was readily detected via imaging, while the weaker H2CO 322–221 and H2CO 321–220 lines required matched filter analysis to detect. H2CO is present throughout most of the gaseous disk, extending out to ~550 AU. An apparent 50 AU inner radius of the H2CO emission is likely caused by an optically thick dust continuum. The H2CO radial intensity profile shows a peak at ~100 AU and a secondary bump at ~300 AU, suggesting increased production in the outer disk. In all modeling scenarios, fits to the H2CO data show an increased abundance in the outer disk. The overall best-fit H2CO model shows a factor of two enhancement beyond a radius of 270 ± 20 AU, with an inner abundance (relative to H2) of 2 − 5 × 10-12. The H2CO emitting region has a lower limit on the kinetic temperature of T> 20 K. The C18O modeling suggests an order of magnitude depletion of C18O in the outer disk and an abundance of 4 − 12 × 10-8 in the inner disk. Conclusions: There is a desorption front seen in the H2CO emission that roughly coincides with the outer edge of the 1.3 millimeter continuum. The increase in H2CO outer disk emission could be a result of hydrogenation of CO ices on dust grains that are then sublimated via thermal desorption or UV photodesorption. Alternatively, there could be more efficient gas-phase production of H2CO beyond ~300 AU if CO is photodisocciated in this region.