Magnetohydrodynamic Simulations of the Formation of Molecular Clouds toward the Stellar Cluster Westerlund 2: Interaction of a Jet with a Clumpy Interstellar Medium

Abstract

The formation mechanism of CO clouds observed with NANTEN2 and Mopra telescope toward the stellar cluster Westerlund 2 is studied by three-dimensional magnetohydrodynamic (MHD) simulations taking into account the interstellar cooling. These molecular clouds show a peculiar shape composing of an arcshaped cloud in one side of a TeV γ-ray source HESS J1023-575 and a linear distribution of clouds (jet clouds) in another side. We propose that these clouds are formed by the interaction of a jet with interstellar neutral hydrogen (HI) clumps. By studying the dependence of the shape of dense cold clouds formed by shock compression and cooling on the filling factor of HI clumps, we found that the density distribution of HI clumps determines the shape of molecular clouds formed by the jet-cloud interaction; arc-clouds are formed when the filling factor is large. On the other hand, when the filling factor is small, molecular clouds align with the jet. The jet propagates faster in models with small filling factors.

A Small-scale Investigation of Molecular Emission toward the Tip of the Western Lobe of W50/SS 433

“…Three clumps in regions N1, N2, and N3 are all located near the tip of the lobe. Such an arc-like spatial distribution of molecular gas seems somewhat similar to the simulational result with a large filling factor of HI clumps (Asahina et al 2017).…”

“…The clumps in regions N2 and N3, as well as in N1, are all located near the tip of the western X-ray or radio lobe ( § 3.1). An MHD simulation suggests that MCs can form by the jet-HI clump interaction: when the filling factor of the HI clumps the jet interacts with is large, arc-like distributed MCs with thicknesses of ∼ 1 pc form (Asahina et al 2017). Compared with the observed arc-like spatial distribution of the clumps in this work, the simulation could imply that the filling factor of the HI clumps is large to the west of SS 433, under the scenario that the clumps are potentially produced by jet-HI interaction.…”

“…The two clumps were selected based on the following two considerations. (1) They are located at low Galactic latitude, where the HI density should be higher than at the other side, making jet-induced CO formation more plausible (e.g., Asahina et al 2017). (2) Since they are located at the tip of the radio emission, any possible morphological correspondence could more easily be observed.…”

“…The interaction between jets and the ISM has also been studied by hydrodynamic and magnetohydrodynamic (MHD) simulations, and it is shown that MCs can be formed (e.g., Asahina et al 2014Asahina et al , 2017 with either an arc-like shape or a line-like shape that aligns with the jet, depending on the filling factor of the HI clumps that the jet interacts with (Asahina et al 2017). Although the assumed velocities of the jets in the simulations (about hundreds of km s −1 ) are much smaller than that for SS 433 (∼ 0.26 c; Margon & Anderson 1989), these results have demonstrated that the jets of SS 433 do have the potential to induce MC formation, especially when penetrating into a dense HI environment (e.g., the western lobe of W50, which is closer to the disk of the Milky Way).…”

“…There is some observational evidence for judging the interaction with MCs, such as molecular line broadening, high high-to-low excitation line ratio, and morphological agreement (Jiang et al 2010;Chen et al 2014). Motivated by the observational study of the 10 molecular clumps in Yamamoto et al (2008) and simulational studies of distribution of the molecular gas interacting with a jet (e.g., Asahina et al 2014Asahina et al , 2017, we have performed an IRAM 30m observation toward 2 of the 10 clumps (SS 433-N3 and part of SS 433-N2 in Yamamoto et al 2008) in molecular lines. The two clumps are located at the western tip of the X-ray and radio emission, which are suggested to result from the interaction of the jet of SS 433 and the surrounding medium (e.g., Safi-Harb & Ögelman 1997;Dubner et al 1998).…”