ASSOCIATION OF INFRARED DARK CLOUD CORES WITH YSOS: STARLESS OR STARRED IRDC CORES

Abstract

In this paper we examined theassociation of InfraRed Dark Cloud (IRDC) with YSOs and the geometric properties of the IRDC cores. For this studya total of 13,650 IRDC were collected mainly from the catalogs of the IRDC published from other studies andpartially from our catalog of IRDC containing new 789 IRDC core candidates. The YSO candidates were searched for usingthe GLIMPSE, MSX, and IRAS point sources by the shape of their SED or using activity of water or methanol maser. The associationof the IRDC with these YSOs was checked by their line- of-sight coincidence within the dimension of the IRDC core.This work found that a total of 4,110 IRDC have YSO candidates while 9,540 IRDC have no indication of the existenceof YSOs. Considering the 12,200 IRDC within the GLIMPSE survey region for which the YSO candidates were determinedwith better sensitivity, we found that 4,098 IRDC (34%) have at least one YSO candidate and 1,072 among themseem to have embedded YSOs, while the rest 8,102 (66%) have no YSO candidate. Therefore, the ratio of [N(IRDC core with protostars)]/[N(IRDCcore without YSO)] for 12,200 IRDC is about 0.13. Taking into account this ratio and typical lifetime of high-massembedded YSOs, we suggest that the IRDC would spend about 104 ~ 105 years to form high-mass stars.However, we should note that the GLIMPSE point sources have a minimum detectable luminosity of about 1.2 Lq at a typical IRDC cores distanceof ~4 kpc. Therefore, the ratio given here should be a lower limit and the estimated lifetime of starless IRDC canbe an upper limit. The physical parameters of the IRDC somewhat vary depending on how many YSO candidates the IRDCcores contain. The IRDC with more YSOs tend to be larger, more elongated, and have better darkness contrast than theIRDC with fewer or no YSOs.