Evidence for a Massive Protocluster in S255N

Abstract

S255N is a luminous far-infrared source that contains many indications of active star formation but lacks a prominent near-infrared stellar cluster. We present mid-infrared through radio observations aimed at exploring the evolutionary state of this region. Our observations include 1.3 mm continuum and spectral line data from the Submillimeter Array, Very Large Array 3.6 cm continuum and 1.3 cm water maser data, and multicolor IRAC images from the Spitzer Space Telescope. The cometary morphology of the previously known UCH II region G192.584-0.041 is clearly revealed in our sensitive, multiconfiguration 3.6 cm images. The 1.3 mm continuum emission has been resolved into three compact cores, all of which are dominated by dust emission and have radii <7000 AU. The mass estimates for these cores range from 6 to 35 Msolar. The centroid of the brightest dust core (SMA1) is offset by 1.1'' (2800 AU) from the peak of the cometary UCH II region and exhibits the strongest HC3N, CN, and DCN line emission in the region. SMA1 also exhibits compact CH3OH, SiO, and H 2CO emission and likely contains a young hot core. We find spatial and kinematic evidence that SMA1 may contain further multiplicity, with one of the components coincident with a newly detected H2O maser. There are no mid-infrared point-source counterparts to any of the dust cores, further suggesting an early evolutionary phase for these objects. The dominant mid-infrared emission is a diffuse, broadband component that traces the surface of the cometary UCH II region but is obscured by foreground material on its southern edge. An additional 4.5 μm linear feature emanating to the northeast of SMA1 is aligned with a cluster of methanol masers and likely traces a outflow from a protostar within SMA1. Our observations provide direct evidence that S255N is forming a cluster of intermediate- to high-mass stars.