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Detection of a hot core in the intermediate-mass Class 0 protostar NGC 7129-FIRS 2

Abstract
We report high angular resolution ( at 1.3 mm) observations of the Class 0 intermediate-mass (IM) protostar NGC 7129-FIRS 2 using the Plateau de Bure Interferometer. Our observations show the existence of an intense unresolved source in the continuum at 1.3 mm and 3 mm at the position of the Class 0 object. In addition, compact CH₃CN emission is detected at this position. The high rotational temperature derived from the CH₃CN lines (  K), as well as the enhanced CH₃CN fractional abundance (X(CH₃CN) ~ ), shows the existence of a hot core in this IM young stellar object. This is to our knowledge the first IM hot core detected so far. Interferometric maps of the region in the CH₃OH 5_kk' 4_kk' and D₂CO 4₀₄ 3₀₃ lines are also presented in this paper. The methanol emission presents two condensations, one associated with the hot core, which was very intense in the high upper state energy lines (E_u>100 K), and the other associated with the bipolar outflow which dominates the emission in the low excitation lines. Enhanced CH₃OH abundances (X(CH₃OH) ~ - a few 10^-7) were measured in both components. While intense D₂CO emission was detected towards the hot core, the N₂D⁺ line was not detected in our interferometric observations. The different behaviors of D₂CO and N₂D⁺ emissions suggest different formation mechanisms for the two species and different deuteration processes for H₂CO and N₂H⁺ (surface and gas-phase chemistry, respectively). Finally, the spectrum of the large bandwidth correlator shows a forest of lines at the hot core position, revealing that this object is extraordinarily rich in complex molecules. For deeper insight into the chemistry of complex molecules, we compared the fractional abundances of the complex O- and N- bearing species in FIRS 2 with those in hot corinos and massive hot cores. Within the large uncertainty involved in fractional abundance estimates towards hot cores, we did not detect any variation in the relative abundances of O- and N-bearing molecules ([CH₃CN] /[CH₃OH] ) with the hot core luminosity. However, the O-bearing species H₂CO and HCOOH seemed to be more abundant in low and intermediate mass stars than in massive star-forming regions. We propose that this could be the consequence of a different grain mantle composition in low and massive star-forming regions.