W₄₀ region in the Gould belt: an embedded cluster and H_II region at the junction of filaments

K. K. Mallick, M. S. N. Kumar, D. K. Ojha, R. Bachiller, M. R. Samal, L. Pirogov

Abstract
We present a multiwavelength study of the W40 star-forming region using infrared (IR) observations in UKIRT JHK bands, Spitzer IRAC bands, and Herschel PACS bands; 2.12 µm H₂ narrow-band imaging; and radio continuum observations from GMRT (610 and 1280 MHz), in a field of view (FoV) of ∼ 34' × 40' . Archival Spitzer observations in conjunction with near-IR (NIR) observations are used to identify 1162 Class II/III and 40 Class I sources in the FoV. The nearest-neighbour stellar surface density analysis shows that majority of these young stellar objects (YSOs) constitute the embedded cluster centered on the high-mass source IRS 1A South. Some YSOs, predominantly younger population, are distributed along and trace the filamentary structures at lower stellar surface density. The cluster radius is obtained as 0.44 pc - matching well with the extent of radio emission - with a peak density of 650 pc^-2. The JHK data is used to map the extinction in the region which is subsequently used to compute the cloud mass. It has resulted in 126 M_☉ and 71 M_☉ for the central cluster and the northern IRS 5 region, respectively. H₂ narrow-band imaging displays significant emission, which prominently resembles fluorescent emission arising at the borders of dense regions. Radio continuum analysis shows this region as having blister morphology, with the radio peak coinciding with a protostellar source. Free-free emission spectral energy distribution (SED) analysis is used to obtain physical parameters of the overall photoionized region and the IRS 5 sub-region. This multiwavelength scenario is suggestive of star formation having resulted from merging of multiple filaments to form a hub. Star formation seems to have taken place in two successive epochs, with the first epoch traced by the central cluster and the high-mass star(s) - followed by a second epoch which is spreading into the filaments as uncovered by the Class I sources and even younger protostellar sources along the filaments. The IRS 5 H_II region displays indications of swept-up material which has possibly led to the formation of protostars.

Keywords
H II regions - infrared: ISM - ISM: bubbles - ISM: individual objects: W40 - radio continuum: ISM - stars: formation

The Astrophysical Journal
Volume 779, Page 113_1
December 2013

DOI: 10.1088/0004-637X/779/2/113
ADS Bibliographic code: 2013ApJ...779..113M