ACHANDRAOBSERVATION OF THE OBSCURED STAR-FORMING COMPLEX W40

Abstract

The young stellar cluster illuminating the W40 H ii region, one of the nearest massive star-forming regions, has been observed with the ACIS detector on board the Chandra X-ray Observatory. Due to its high obscuration, this is a poorly studied stellar cluster with only a handful of bright stars visible in the optical band, including three OB stars identified as primary excitation sources. We detect 225 X-ray sources, of which 85% are confidently identified as young stellar members of the region. Two potential distances of the cluster, 260 pc and 600 pc, are used in the paper. Supposing the X-ray luminosity function to be universal, it supports a 600 pc distance as a lower limit for W40 and a total population of at least 600 stars down to 0.1 M under the assumption of a coeval population with a uniform obscuration. In fact, there is strong spatial variation in K s -band-excess disk fraction and non-uniform obscuration due to a dust lane that is identified in absorption in optical, infrared, and X-ray. The dust lane is likely part of a ring of material which includes the molecular core within W40. In contrast to the likely ongoing star formation in the dust lane, the molecular core is inactive. The star cluster has a spherical morphology, an isothermal sphere density profile, and mass segregation down to 1.5 M . However, other cluster properties, including a 1 Myr age estimate and ongoing star formation, indicate that the cluster is not dynamically relaxed. X-ray diffuse emission and a powerful flare from a young stellar object are also reported.

The formation of active protoclusters in the Aquila rift: a millimeter continuum view

“…4.1.1) or more massive star forming regions (see for example Nguyen Luong et al 2011). However, Kuhn et al (2010) argue that the high disk fraction they find thanks to K-band excess measurements would support an age for the W40 cluster of only ∼1 Myr, which would lead to a upper-limit SFR of ∼57 M Myr −1 pc −2 . For comparison, the SFR in the Serpens Main cluster, i.e.…”

“…However, assuming a standard IMF, the total stellar mass is roughly proportional to the total bolometric luminosity of the YSO population, which also scales as d 2 . Therefore, we argue 4 Note that if we take into account the additional Class III objects identified in X-rays by Kuhn et al (2010) (∼109 Chandra YSOs without Kband excess), then the SFE goes up to ∼26% and the upper-limit of the SFR to ∼120 M Myr −1 pc −2 (assuming a lower-limit age of ∼1 Myr, see Kuhn et al (2010)). …”

“…Indeed, the Lyman-continuum luminosity towards the HII region of W40 (∼1.5 × 10 48 photons s −1 ) inferred from the radio continuum emission by Altenhoff et al (1970) could be produced by a single O9 V star (Smith et al 1985). In a recent study, Kuhn et al (2010) found 93 YSOs with NIR K-band excess belonging to the W40 region, based on their Chandra observations toward the young cluster. However, their census did not distinguish between Class I, Class II, and later evolutionary stages.…”

“…Also based on extinction measurements (Smith et al 1985) and radio spectral line data (Vallee 1987;Vallee et al 1992), the nearby HII region W40 has usually been considered to be at a distance ranging from 300 to 700 pc. A recent Chandra study by Kuhn et al (2010) emphasizes, on the basis of X-ray luminosity function arguments, a most-likely distance of ∼600 pc for the W40 cluster, although not excluding the possibility of a smaller (∼300 pc) distance.…”

“…3. It mainly features a ring-like structure following the outer border of the W40 HII region (Smith et al 1985;Vallee 1987;Kuhn et al 2010), and spatially coincident with the CO emission mapped by Zhu et al (2006). In addition to this western dense shell, the map shows elongated structures (also containing compact sources) in the eastern part of the photo-dissociated region, while the central part of the infrared cluster containing most of the pre-main sequence objects (Kuhn et al 2010) does not show any evidence for cold dust emission at 1.2 mm.…”