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The Stellar Population and Origin of the Mysterious High-Latitude Star-forming Cloud CG 12
The mysterious high Galactic latitude cometary globule CG 12 has beenobserved with the ACIS detector on board the Chandra X-Ray Observatory.We detect 128 X-ray sources, of which half are likely young stars formedwithin the globule's head. This new population of >~50 T Tauri starsand one new embedded protostar is far larger than the previouslyreported few intermediate-mass and two protostellar members of thecloud. Most of the newly discovered stars have masses 0.2-0.7Msolar, and 9%-15% have K-band excesses from innerprotoplanetary disks. X-ray properties provide an independent distanceestimate consistent with CG 12's unusual location >~200 pc above theGalactic plane. The star formation efficiency in CG 12 appears to be15%-35%, far above that seen in other triggered molecular globules. Themedian photometric age found for the T Tauri population assuming Siesset al. (2000) isochrones is ~4 Myr with a large spread of <1-20 Myrand ongoing star formation in the molecular cores. The stellar age andspatial distributions are inconsistent with a simple radiation-drivenimplosion (RDI) model and suggest either that CG 12 is an atypicallylarge shocked globule or that it has been subject to several distinctepisodes of triggering and ablation. We report a previously unnoticedgroup of B-type stars northwest of CG 12 that may be the remnants of anOB association that produced multiple supernova explosions that couldhave shocked and ablated the cloud over a 15-30 Myr period. HD 120958(B3e), the most luminous member of the group, may be currently drivingan RDI shock into the CG 12 cloud.
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