Chemical survey of Class I protostars with the IRAM-30 m

• Published in: Astronomy and astrophysics (Berlin) 2022-03, Vol.659, p.A67
• Main Authors: Mercimek, S., Codella, C., Podio, L., Bianchi, E., Chahine, L., Bouvier, M., López-Sepulcre, A., Neri, R., Ceccarelli, C.
• Format: Article
• Language: English
• Subjects: Accretion disks; Astrochemistry; Astrophysics; Binary stars; Chemical composition; Chemistry; Comets; Complexity; Deuteration; Envelopes; Extrasolar planets; Hydrogen sulfide; Interstellar chemistry; Local thermodynamic equilibrium; Molecular chains; Object recognition; Organic chemistry; Planet formation; Protoplanetary disks; Protostars; Sciences of the Universe; Star & galaxy formation; Star formation; Stellar cores; Velocity gradient
• ISSN: 0004-6361; 1432-0746; 1432-0756
• DOI: 10.1051/0004-6361/202141790

Context. Class I protostars are a bridge between Class 0 protostars (≤10 5 yr old), and Class II (≥10 6 yr) protoplanetary disks. Recent studies show gaps and rings in the dust distribution of disks younger than 1 Myr, suggesting that planet formation may start already at the Class I stage. To understand what chemistry planets will inherit, it is crucial to characterize the chemistry of Class I sources and to investigate how chemical complexity evolves from Class 0 protostars to protoplanetary disks. Aims. There are two goals: (i) to perform a census of the molecular complexity in a sample of four Class I protostars, and (ii) to compare the data with the chemical compositions of earlier and later phases of the Sun-like star formation process. Methods. We performed IRAM-30 m observations at 1.3 mm towards four Class I objects (L1489-IRS, B5-IRS1, L1455-IRS1, and L1551-IRS5). The column densities of the detected species were derived assuming local thermodynamic equilibrium (LTE) or large velocity gradients (LVGs). Results. We detected 27 species: C-chains, N-bearing species, S-bearing species, Si-bearing species, deuterated molecules, and interstellar complex organic molecules (iCOMs; CH 3 OH, CH 3 CN, CH 3 CHO, and HCOOCH 3 ). Among the members of the observed sample, L1551-IRS5 is the most chemically rich source. Different spectral profiles are observed: (i) narrow lines (~1 km s −1 ) towards all the sources, (ii) broader lines (~4 km s −1 ) towards L1551-IRS5, and (iii) line wings due to outflows (in B5-IRS1, L1455-IRS1, and L1551-IRS5). Narrow c-C 3 H 2 emission originates from the envelope with temperatures of 5–25 K and sizes of ~2′′−10′′. The iCOMs in L1551-IRS5 reveal the occurrence of hot corino chemistry, with CH 3 OH and CH 3 CN lines originating from a compact (~0.′′15) and warm ( T > 50 K) region. Finally, OCS and H 2 S seem to probe the circumbinary disks in the L1455-IRS1 and L1551-IRS5 binary systems. The deuteration in terms of elemental D/H in the molecular envelopes is: ~10−70% (D 2 CO/H 2 CO), ~5−15% (HDCS/H 2 CS), and ~1−23% (CH 2 DOH/CH 3 OH). For the L1551-IRS5 hot corino we derive D/H ~2% (CH 2 DOH/CH 3 OH). Conclusions. Carbon chain chemistry in extended envelopes is revealed towards all the sources. In addition, B5-IRS1, L1455-IRS1, and L1551-IRS5 show a low-excitation methanol line that is narrow and centered at systemic velocity, suggesting an origin from an extended structure, plausibly UV-illuminated. The abundance ratios of CH