Methyl Formate in the NGC 2264 IRS 1 Region

Abstract

Millimeter-wave spectral lines of HCOOCH3 are observed toward a massive star-forming region, NGC 2264 IRS 1, with the Nobeyama 45 m radio telescope and Nobeyama Millimeter Array. The HCOOCH3 emission is not detected toward the dense core around IRS 1, which is the brightest IR source. However, it is definitively detected toward MMS 3, which is thought to contain a high-mass equivalent of a Class 0 protostar. The column density and the fractional abundance of HCOOCH3 in MMS 3 are found to be (4-30) × 1015 cm-2 and (0.7-5.3) × 10-8, respectively, assuming that the range of the excitation temperature is from 50 to 250 K. The fractional abundance is lower by an order of magnitude than that in the compact ridge of Orion KL. On the other hand, the upper limit to the fractional abundance toward IRS 1 is significantly lower than the abundance toward MMS 3. Since MMS 3 is less evolved than IRS 1, this result would indicate that HCOOCH3 preferentially exists in the younger stage of protostellar evolution, as in the case of low-mass star forming regions. The distribution of HCOOCH3 is found to be slightly offset from the dust continuum peak of MMS 3 by 13''. This situation is similar to that found in the compact ridge of Orion KL, which would provide us with an important clue in exploring its peculiar chemistry.