LOW-VELOCITY SHOCKS TRACED BY EXTENDED SiO EMISSION ALONG THE W43 RIDGES: WITNESSING THE FORMATION OF YOUNG MASSIVE CLUSTERS

Abstract

The formation of high-mass stars is tightly linked to that of their parental clouds. Here, we focus on the high-density parts of W43, a molecular cloud undergoing an efficient event of star formation. Using a column density image derived from Herschel continuum maps, we identify two high-density filamentary clouds, called the W43-MM1 and W43-MM2 ridges. Both have gas masses of 2.1 × 10 4 M and 3.5 × 10 4 M above >10 23 cm −2 and within areas of ∼6 and ∼14 pc 2 , respectively. The W43-MM1 and W43-MM2 ridges are structures that are coherent in velocity and gravitationally bound, despite their large velocity dispersion measured by the N 2 H + (1-0) lines of the W43-HERO IRAM large program. Another intriguing result is that these ridges harbor widespread (∼10 pc 2) bright SiO (2-1) emission, which we interpret to be the result of low-velocity shocks (10 km s −1). We measure a significant relationship between the SiO (2-1) luminosity and velocity extent and show that it distinguishes our observations from the high-velocity shocks associated with outflows. We use state-of-the-art shock models to demonstrate that a small percentage (10%) of Si atoms in low-velocity shocks, observed initially in gas phase or in grain mantles, can explain the observed SiO column density in the W43 ridges. The spatial and velocity overlaps between the ridges of high-density gas and the shocked SiO gas suggest that ridges could be forming via colliding flows driven by gravity and accompanied by low-velocity shocks. This mechanism may be the initial conditions for the formation of young massive clusters.

The earliest phases of high-mass star formation, as seen in NGC 6334 by Herschel-HOBYS

“…described in Appendix B of Nguyen-Luong et al (2013) to produce full images without saturation (see Figs. C.3-C.5).…”

“…(1)−(2) of Nguyen-Luong et al (2013) to derive a mass of ∼15 000 M and a density of 2.5 × 10 5 cm −3 for the NGC 6334 ridge. With the same assumptions, the NGC 6334 hubs have masses of ∼2200 and ∼2800 M and densities around 10 5 cm −3 .…”

“…Recent studies showed that the formation of ridges and massive stellar clusters are dynamical events, which are tightly linked (e.g., Nguyen-Luong et al 2013;Louvet et al 2016). Indeed, both ridges and hubs are observed to form through a global free-fall collapse (e.g., Schneider et al 2010b;Hill et al 2011;Hennemann et al 2012), followed by intense formation of high-mass stars quoted as mini-starbursts (e.g., Motte et al 2003;Nguyen Luong et al 2011;Louvet et al 2014).…”

“…The details of this process are under investigation (see recent reviews by, e.g., Tan et al 2014;Krumholz 2015;Motte et al 2017) and an increasing number of studies have suggested that they form through dynamical processes initiated by cloud formation (e.g., Schneider et al 2010a;Csengeri et al 2011;Peretto et al 2013;Nguyen-Luong et al 2013).…”

Impacts of pure shocks in the BHR71 bipolar outflow

“…They can be associated with the formation of "ridges" at the convergence region of molecular clouds (e.g. W43, Nguyen-Luong et al 2013), to the jets and outflow systems related to the formation of low-to high-mass stars (from low, e.g. Gomez-Ruiz et al 2013; to high, Leurini et al 2013;through intermediate, Gómez-Ruiz et al 2012; for examples, also see Arce et al 2007;Frank et al 2014;or Tan et al 2014 for reviews), or to supernova remnants (SNRs) interacting with molecular clouds (e.g.…”