Measurement of [HDCO]/[H2CO] Ratios in the Envelopes of Extremely Cold Protostars in Orion

Abstract: We present observations of HDCO and H2CO emission toward a sample of 15 Class 0 protostars in the Orion A and B clouds. Of these, eleven protostars are Herschel-identified PACS Bright Red Sources (PBRS) and four are previously identified protostars. Our observations revealed the chemical properties of the PBRS envelope for the first time. The column densities of HDCO and H2CO are derived from single dish observations at an angular resolution of ~20 arcsec (~8400 AU). The degree of deuteration in H2CO ([HDCO]/[H2CO]) was estimated to range from 0.03 to 0.31. The deuterium fractionation of most PBRS (70%) is similar to that of the non-PBRS sources. Three PBRS (30%) exhibit high deuterium fractionation, larger than 0.15. The large variation of the deuterium fractionation of H2CO in the whole PBRS sample may reflect the diversity in the initial conditions of star forming cores. There is no clear correlation between the [HDCO]/[H2CO] ratio and the evolutionary sequence of protostars.