Mid Infrared View of the High Mass Star Formation Region W51A

TL;DR

This study uses mid-infrared observations from Spitzer and Gemini to identify and analyze ultracompact HII regions and massive young stellar objects in the W51A star formation complex, revealing their properties and embedded nature.

Contribution

It provides the first detailed mid-infrared imaging and spectral energy distribution modeling of W51A's star-forming cores, identifying new stellar candidates and their characteristics.

Findings

Identified mid-infrared counterparts of 8 ultracompact HII regions.

Revealed the central core with massive young stellar candidates.

Estimated spectral types ranging from B3 to O5, mostly B1.

Abstract

In this paper we present the results of a mid infrared study of G49.5-0.4, or W51A, part of the massive starbirth complex W51. Combining public data from the $Spitzer$ IRAC camera, and Gemini mid infrared camera T-ReCS at 7.73, 9.69, 12.33 and 24.56 \micron, with spatial resolution of $\sim$0.5\arcsec, we have identified the mid infrared counterparts of 8 ultracompact HII regions, showing that two radio sources are deeply embedded in molecular clouds and another is a cloud of ionized gas. From the T-ReCS data we have unveiled the central core of W51 region, revealing massive young stellar candidates. We modeled the spectral energy distribution of the detected sources suggesting the embedded objects are sources with spectral types ranging from B3 to O5, but the majority of the fits indicate stellar objects with B1 spectral types. We also present an extinction map of IRS~2, showing that a region with lower extinction corresponds to the region where a proposed jet of gas has impacted the foreground cloud. From this map, we also derived the total extinction towards the enigmatic source IRS~2E, which amounts to $\sim$60 magnitudes in the $V$ band. We calculated the color temperature due to thermal emission of the circumstellar dust of the detected sources; the temperatures are in the interval of $\sim$100 -- 150 K, which corresponds to the emission of dust located at 0.1 pc from the central source. Finally, we show a possible mid infrared counterpart of a detected source at mm wavelengths that was found by \cite{zap08,zap09} to be a massive young stellar object undergoing a high accretion rate.