An Unbiased Spectral Line Survey toward R CrA IRS7B in the 345 GHz
  Window with ASTE
  We have conducted a spectral line survey in the 332 - 364 GHz region with the
ASTE 10 m telescope toward R CrA IRS7B, a low-mass protostar in the Class 0 or
Class 0/I transitional stage. We have also performed some supplementary
observations in the 450 GHz band. In total, 16 molecular species are identified
in the 332 - 364 GHz region. Strong emission lines of CN and CCH are observed,
whereas complex organic molecules and long carbon-chain molecules which are
characteristics of hot corino and warm carbon-chain chemistry (WCCC) source,
respectively, are not detected. The rotation temperature of CH3OH is evaluated
to be 31 K, which is significantly lower than that reported for the
prototypical hot corino IRAS 16293-2422 (~85 K). The deuterium fractionation
ratios for CCH and H2CO are obtained to be 0.038 and 0.050, respectively, which
are much lower than those in the hot corino. These results suggest a weak hot
corino activity in R CrA IRS7B. On the other hand, the carbon-chain related
molecules, CCH and c-C3H2, are found to be abundant. However, this source
cannot be classified as a WCCC source, since long carbon-chain molecules are
not detected. If WCCC and hot corino chemistry represent the two extremes in
chemical compositions of low-mass Class 0 sources, R CrA IRS7B would be a
source with a mixture of these two chemical characteristics. The UV radiation
from the nearby Herbig Ae star R CrA may also affect the chemical composition.
The present line survey demonstrates further chemical diversity in low-mass
star-forming regions.
