2018A&A...618A.145O

Context. Intermediate-mass (IM) protostars provide a bridge between the low- and high-mass protostars. Despite their relevance, little is known about their chemical diversity.
Aims. We want to investigate the molecular richness towards the envelope of I-M protostars and to compare their properties with those of low- and high-mass sources.
Methods. We have selected the isolated IM Class 0 protostar Cep E-mm to carry out an unbiased molecular survey with the IRAM 30 m telescope between 72 and 350GHz with an angular resolution lying in the range 7-34". Our goal is to obtain a census of the chemical content of the protostellar envelope. These data were complemented with NOEMA observations of the spectral bands 85.9-89.6GHz and 216.8-220.4GHz at angular resolutions of 2.3" and 1.4", respectively.
Results. The 30 m spectra show bright emission of O- and N-bearing complex organic molecules (COMs): CH₃OH and its rare isotopologues CH₂DOH and ¹³CH₃OH, CH₃CHO, CH₃OCH₃, CH₃COCH₃, HCOOH, HCOOCH₃, H₂CCO, NH₂CHO, CH₃CN, C₂H₃CN, C₂H₅CN, HNCO and H₂CO. We identify up to three components in the spectral signature of COMs: an extremely broad line (eBL) component associated with the outflowing gas (FWHM>7km/s), a narrow line (NL) component (FWHM<3km/s) associated with the cold envelope, and a broad line (BL) component (FWHM~=5.5km/s) which traces the signature of a hot corino. The eBL and NL components are detected only in molecular transitions of low excitation and dominate the emission of CH₃OH. The BL component is detected in highly excited gas (E_up > 100 K). The NOEMA observations reveal Cep E-mm as a binary protostellar system, whose components, Cep E-A and Cep E-B, are separated by ~=1.7". Cep E-A dominates the core continuum emission and powers the long-studied, well-known, high-velocity jet associated with HH377. The lower flux source Cep E-B powers another high-velocity molecular jet, reaching velocities of ~=80km/s, which propagates in a direction close to perpendicular with respect to the Cep E-A jet. Our interferometric maps show that the emission of COMs arises from a region of ~=0.7"size around Cep E-A, and corresponds to the BL component detected with the IRAM 30 m telescope. On the contrary, no COM emission is detected towards Cep E-B. We have determined the rotational temperature (T_rot) and the molecular gas column densities from a simple population diagram analysis or assuming a given excitation temperature. Rotational temperatures of COMs emission were found to lie in the range 20-40K with column densities ranging from a few times 10¹⁵cm^–2 for O-bearing species, down to a few times 10¹⁴cm^–2 for N-bearing species. Molecular abundances are similar to those measured towards other low- and intermediate-mass protostars. Ketene (H₂CCO) appears as an exception, as it is found significantly more abundant towards Cep E-A. High-mass hot cores are significantly less abundant in methanol and N-bearing species are more abundant by two to three orders of magnitude.
Conclusions. Cep E-mm reveals itself as a binary protostellar system with a strong chemical differentiation between both cores. Only the brightest component of the binary is associated with a hot corino. Its properties are similar to those of low-mass hot corinos.