Complex Organic Molecules toward Embedded Low-mass Protostars^∗

Abstract

Complex organic molecules (COMs) have been observed towards 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 towards 16 deeply embedded (Class 0/I) low-mass protostars using the IRAM 30m telescope. We detect CH 2 CO, CH 3 CHO, CH 3 OCH 3 , CH 3 OCHO, CH 3 CN, HNCO, and HC 3 N towards 67%, 37%, 13%, 13%, 44%, 81%, and 75% of sources respectively. Median column densities derived using survival analysis range between 6.0x10 10 cm −2 (CH 3 CN) and 2.4x10 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 young stellar objects. 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.

Questioning the spatial origin of complex organic molecules in young protostars with the CALYPSO survey

“…The two sources of Bergner et al (2017) with detections of CH 3 OCH 3 and CH 3 OCHO have abundances relative to methanol of ∼0.09 and ∼0.06 for these two molecules, respectively, which is similar to what we obtain for group 3 for both molecules and group 2 for CH 3 OCH 3 , but a factor 3-5 higher than group 1 (Table 7). In addition, Bergner et al (2017) report a median value of CH 3 CHO abundance relative to methanol similar to the values of groups 2 and 3 but a factor ∼5 higher than group 1. Group 1 may thus represent sources that have undergone a different type of chemical processing of O-bearing COMs on small scale compared to the dominant processing in the cold envelope.…”

“…Group 1 may thus represent sources that have undergone a different type of chemical processing of O-bearing COMs on small scale compared to the dominant processing in the cold envelope. The situation is less clear for the N-bearing COMs: the CH 3 CN abundances relative to methanol derived by Bergner et al (2017) range from 0.005 to 0.05, which encompasses the values we derive for the three groups of the CA-LYPSO sample. Bergner et al (2017) found that methanol is well correlated with CH 3 CHO and CH 3 CN in their source sample.…”

“…The situation is less clear for the N-bearing COMs: the CH 3 CN abundances relative to methanol derived by Bergner et al (2017) range from 0.005 to 0.05, which encompasses the values we derive for the three groups of the CA-LYPSO sample. Bergner et al (2017) found that methanol is well correlated with CH 3 CHO and CH 3 CN in their source sample. We find a good correlation between methanol and CH 3 CN for the CA-LYPSO sources as well, but the correlation is much weaker between methanol and CH 3 CHO.…”

“…The CALYPSO interferometric data that we have used here probe only the inner parts of the envelopes because of the spatial filtering by the interferometer. Therefore, in order to compare the chemical composition derived from the CALYPSO survey to the one predicted by the models, we considered only the inner regions of the models of Bergner et al (2017). In practice, we integrated the modeled column densities of the molecules over the inner region where the abundance of methanol is the highest and forms a plateau (see Fig.…”

“…In practice, we integrated the modeled column densities of the molecules over the inner region where the abundance of methanol is the highest and forms a plateau (see Fig. 12 of Bergner et al 2017). The radius of this region is about 6 au and 20 au for the 1 L ⊙ and 10 L ⊙ simulations, respectively.…”