Nature of W51e2: Massive cores at different phases of star formation

Abstract

We present high-resolution continuum images of the W51e2 complex processed from archival data of the Submillimeter Array (SMA) at 0.85 and 1.3mm and the Very Large Array at 7 and 13mm. We also made line images and profiles of W51e2 for three hydrogen radio recombination lines (RRLs; H26α, H53α, and H66α) and absorption of two molecular lines of HCN(4-3) and CO(2-1). At least four distinct continuum components have been detected in the 3″ region of W51e2 from the SMA continuum images at 0.85 and 1.3mm with resolutions of 03 × 02 and 14 × 07, respectively. The west component, W51e2-W, coincides with the ultracompact H II region reported from previous radio observations. The H26α line observation reveals an unresolved hyper-compact ionized core (<006 or <310AU) with a high electron temperature of 1.2 × 104K, with the corresponding emission measure EM >7 × 1010 pc cm-6 and the electron density Ne >7 × 106 cm-3. The inferred Lyman continuum flux implies that the H II region W51e2-W requires a newly formed massive star, an O8 star or a cluster of B-type stars, to maintain the ionization. W51e2-E, the brightest component at 0.85mm, is located 09 east from the hyper-compact ionized core. It has a total mass of 140 M ⊙ according to our spectral energy distribution analysis and a large infall rate of >1.3 × 10-3 M ⊙ yr-1 inferred from the absorption of HCN. W51e2-E appears to be the accretion center in W51e2. Given the fact that no free-free emission and no RRLs have been detected, the massive core of W51e2-E appears to host one or more growing massive proto-stars. Located 2″ northwest from W51e2-E, W51e2-NW is detected in the continuum emission at 0.85 and 1.3mm. No continuum emission has been detected at λ ≥ 7mm. Along with the maser activities previously observed, our analysis suggests that W51e2-NW is at an earlier phase of star formation. W51e2-N is located 2″ north of W51e2-E and has only been detected at 1.3mm with a lower angular resolution (∼1″), suggesting that it is a primordial, massive gas clump in the W51e2 complex. © 2010. The American Astronomical Society. All rights reserved..