2022A&A...659A..67M

Context. Class I protostars are a bridge between Class 0 protostars (≤10⁵ yr old), and Class II (≥10⁶ 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; CH₃OH, CH₃CN, CH₃CHO, and HCOOCH₃). 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-C₃H₂ 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 CH₃OH and CH₃CN lines originating from a compact (∼0.''15) and warm (T > 50 K) region. Finally, OCS and H₂S 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% (D₂CO/H₂CO), ∼5-15% (HDCS/H₂CS), and ∼1-23% (CH₂DOH/CH₃OH). For the L1551-IRS5 hot corino we derive D/H ∼2% (CH₂DOH/CH₃OH). 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 CH₃CN, CH₃CHO, and HCOOCH₃ with respect to CH₃OH measured towards the L1551-IRS5 hot corino are comparable to that estimated at earlier stages (prestellar cores, Class 0 protostars), and to that found in comets. The deuteration in our sample is also consistent with the values estimated for sources at earlier stages. These findings support the inheritance scenario from prestellar cores to the Class I phase when planets start forming.