A Molecular Line Observation toward Massive Clumps Associated with Infrared Dark Clouds

Abstract

We have surveyed the N2H+ J=1-0, HC3N J=5-4, CCS JN=43-32, NH3 (J,K) = (1, 1), (2, 2), (3, 3), and CH3OH J=7-6 lines toward the55 massive clumps associated with infrared dark clouds by using theNobeyama Radio Observatory 45 m telescope and the Atacama SubmillimeterTelescope Experiment 10 m telescope. The N2H+,HC3N, and NH3 lines are detected toward most ofthe objects. On the other hand, the CCS emission is detected toward noneof the objects. The [CCS]/[N2H+] ratios are foundto be mostly lower than unity even in the Spitzer 24 mum dark objects.This suggests that most of the massive clumps are chemically moreevolved than the low-mass starless cores. The CH3OH emissionis detected toward 18 out of 55 objects. All theCH3OH-detected objects are associated with the Spitzer 24mum sources, suggesting that star formation has already started in allthe CH3OH-detected objects. The velocity widths of theCH3OH JK=70-60 A+and 7-1-6-1 E lines are broader than those ofN2H+ J=1-0. The CH3OHJK=70-60 A+ and7-1-6-1 E lines tend to have broader line width inthe MSX dark objects than in the others, the former being younger orless luminous than the latter. The origin of the broad emission isdiscussed in terms of the interaction between an outflow and an ambientcloud.