Near-infrared imaging of compact HII regions in Cygnus X

Abstract

Abstract. We present a near-infrared imaging survey of compact HII regions in the direction of the Cygnus X complex, for which no previous observations at those wavelengths have been published so far. The targets have been selected from a catalog of sources in that region having a thermal spectral energy distribution between 408 and 4800 MHz (Wendker et al. 1991), and an inferred angular size smaller than 5 arcmin across. We present images in the JHK filters, color-magnitude, and color-color diagrams for each region. We also suggest and apply a method for estimating the distance by comparing the dereddened H-band flux from all the stars in the area of the HII region and the radio-continuum flux. Many of the regions imaged are clearly associated with stellar aggregates with different degrees of concentration, whose components show varying amounts of extinction. Some objects are often found in the region of the (J − H), (H − K) diagram indicating excess emission of circumstellar nature. A detailed discussion on each object is provided in the context of existing published observations at different wavelengths, in particular regarding the existence of ultracompact components. A number of ultracompact HII regions are found to have clearly visible unresolved or nearly unresolved K-band counterparts characterized by very red H − K colors, suggesting that the extinction may be low enough in their direction so as to allow the direct observation of the star ionizing the ultracompact component.

Does Cyg OB2 harbour any open cluster?

“…The Cyg OB2 area (2 • diameter) is delimited by the circle and the objects projected inside it might be related to itself, to the general field of Cygnus X, or to the Perseus or Outer Arms in the background. The embedded cluster in G 80.35+0.72 (ECX 6-27 or 18P61), projected close to the nucleus of Cyg OB2, appears to be a background cluster in the Outer Arm (Comerón & Torra 2001). DR 7 (ECX6-18) might be in the Perseus Arm (Comerón & Torra 2001) or in Cygnus X (Dutra & Bica 2001).…”

“…The embedded cluster in G 80.35+0.72 (ECX 6-27 or 18P61), projected close to the nucleus of Cyg OB2, appears to be a background cluster in the Outer Arm (Comerón & Torra 2001). DR 7 (ECX6-18) might be in the Perseus Arm (Comerón & Torra 2001) or in Cygnus X (Dutra & Bica 2001). DR 15 (IRAS 20306+4005, ECX6-25) is an embedded cluster probably in Cygnus X (Comerón & Torra 2001), while DR 18 is a loose star-forming region in Cygnus X (Comerón & Torra 1999).…”

“…DR 7 (ECX6-18) might be in the Perseus Arm (Comerón & Torra 2001) or in Cygnus X (Dutra & Bica 2001). DR 15 (IRAS 20306+4005, ECX6-25) is an embedded cluster probably in Cygnus X (Comerón & Torra 2001), while DR 18 is a loose star-forming region in Cygnus X (Comerón & Torra 1999). DR 11 (ECX6-21) encloses a stellar group in Cygnus X (Comerón & Torra 2001).…”

“…DR 15 (IRAS 20306+4005, ECX6-25) is an embedded cluster probably in Cygnus X (Comerón & Torra 2001), while DR 18 is a loose star-forming region in Cygnus X (Comerón & Torra 1999). DR 11 (ECX6-21) encloses a stellar group in Cygnus X (Comerón & Torra 2001). The optical object Do 44 is a loose concentration of stars near the edge of Cyg OB2; it is probably part of Cyg OB2 but does not look like an open cluster.…”

“…It encompasses one of the richest concentrations of star forming regions in the Galaxy as a result of the Local Arm plunge into Cygnus and/or the existence of a coherent structure with enhanced star formation. Comerón & Torra (2001) carried out a near-infrared imaging survey of compact H II regions in Cygnus X using data from Observatorio del Teide and found new embedded clusters. Dutra & Bica (2001) found new embedded clusters and stellar groups in the direction of known optical and radio nebulae in the Galaxy, including the Cygnus X area, using the Two Micron All Sky Survey (hereafter 2MASS, Skrutskie et al 1997).…”