Millimeter continuum observations of McNeil's Nebula Object

摘要

We performed 98 and 110 GHz continuum monitoring observations of the FU Orionis candidate, McNeil's Nebula Object (MNO), from 2004 February to May, with the Nobeyama Millimeter Array. In this study, we succeeded in detecting compact (similar to 3000AU) dust emission towards the position of LMZ 12, the millimeter counterpart of MNO. From monitoring observations, however, we could not find any distinct time variation of the total flux densities for the period from 2004 February to May. The Spectral Energy Distribution (SED) of MNO in lambda = 0.45-3 mm. which shows no increase in brightness after the outburst, is found to have a power-law form with all index of 2.5; the index corresponds to a beta index of 0.5 for the dust mass opacity coefficient. If we consider the continuum emission from MNO as an optically thin thermal one from the dust in the envelope or the outer cold region of the accretion disk around MNO, the total flux densities of the emission give a mass of the circumstellar material of 0.04 +/- 0.01 M-circle dot, which is slightly larger than the disk mass of T Tauri stars (similar to 0.01 M-circle dot). It is very likely, based on these results, that MNO experiences an FU Orionis outburst in the transient stage from a protostar to a T Tauri star. Furthermore, considering the non-detection of an millimeter outburst together with the outburst at optical and IR wavelengths, we suggest that the eruptive phenomenon occurs in the innermost hot region of the disk.