Mapping Observations of Peptide-like Molecules around Sagittarius B2

Abstract

Peptide-like molecules, which have a close connection with the origin of life, have been detected in the Universe. Mapping observations of HCONH _2 and CH _3 CONH _2 , two of the simplest peptide-like molecules, are performed toward the Sagittarius B2 (Sgr B2) complex with the IRAM 30 m telescope. Seven transitions of HCONH _2 and five transitions of CH _3 CONH _2 are used in the analysis. The spatial distributions of the excitation temperature and column density of HCONH _2 in the molecular envelope of Sgr B2 are obtained by rotation diagrams. Assuming the same excitation temperature of HCONH _2 , the column densities of CH _3 CONH _2 are also calculated. The results show that the excitation temperature ranges from 6 to 46 K in the molecular envelope of Sgr B2. The abundance ratios between HCONH _2 and CH _3 CONH _2 are calculated to explore the relationship between them, as are those between HCONH _2 and HNCO. The abundance ratio of CH _3 CONH _2 /HCONH _2 varies from 10% to 20%, while that of HCONH _2 /HNCO ranges from 1.5% to 10%. CH _3 CONH _2 is enhanced with respect to HCONH _2 in the northwest region of Sgr B2. One transition of H ^13 CONH _2 is detected toward 12 positions of Sgr B2, from which a ^12 C/ ^13 C ratio of 28.7 is obtained. A time-dependent chemical model with a short-duration X-ray burst is used to explain the observed abundances of HCONH _2 and CH _3 CONH _2 , with the best-fitting result at T _dust = 53–56 K. More chemical reactions are required to be included in the model since the modeled abundance is lower than the observed one at the observed T _dust .

Keywords

• Interstellar molecules
• Abundance ratios
• Isotopic abundances
• Molecular clouds