Kinematics of Shocked Molecular Gas Adjacent to the Supernova Remnant W44

Abstract

We mapped molecular gas toward the supernova remnant W44 in the HCO + J = 1-0 line with the Nobeyama Radio Observatory 45 m telescope and in the CO J = 3-2 line with the Atacama Submillimeter Telescope Experiment 10 m telescope. High-velocity emission wings were detected in both lines over the area where the radio shell of W44 overlaps the molecular cloud in the plane of the sky. We found that the average velocity distributions of the wing emission can be fitted by a uniform expansion model. The best-fit expansion velocities are 12.2±0.3 km s −1 and 13.2±0.2 km s −1 in HCO + and CO, respectively. The non-wing CO J = 3-2 component is also fitted by the same model with an expansion velocity of 4.7 ± 0.1 km s −1 . This component might be dominated by a post shock higher-density region where the shock velocity had slowed down. The kinetic energy of shocked molecular gas is estimated to be (3.5±1.3) × 10 49 erg. Adding this and the energy of the previously identified HI shell, we concluded that (1.2±0.2) × 10 50 erg has been converted into gas kinetic energy from the initial baryonic energy of the W44 supernova. We also found ultra-high-velocity CO J = 3-2 wing emission with a velocity width of ∼ 100 km s −1 at (l, b) = (+34. • 73, −0. • 47). The origin of this extremely high-velocity wing is a mystery.

Kinematics of Ultra-High-Velocity Gas in the Expanding Molecular Shell Adjacent to the W44 Supernova Remnant

“…The kinematics of the Bullet differ distinctly from those of the expanding molecular shell. Interestingly, a radio continuum blob (Jones et al 1993) and an H 2 ro-vibrational line nebulosity (Reach et al 2005) overlap with the Bullet in the plane of the sky (Sashida et al 2013). Despite the high significance of the Bullet in the CO J=3-2 data, neither its nature nor origin have been totally explained.…”

“…Shocked molecular gas is well traced by 25 OH 1720 MHz maser spots (Claussen et al 1997), several high-velocity wings of CO emission (Seta et al 2004), and by a faint spatially-extended broad CO and HCO + emissions (Sashida et al 2013). Such OH maser and broad emissions arise from a thin expanding shell of shocked molecular gas with the expansion velocity of 13.2 km s −1 (Sashida et al 2013). The total kinetic energy, E kin = (1-3)×10 50 erg, corresponds to the yield from a supernova explosion of 0.1-0.3.…”

“…The W44 SNR interacts with the adjacent giant molecular cloud (Seta et al 1998(Seta et al , 2004. Shocked molecular gas is well traced by 25 OH 1720 MHz maser spots (Claussen et al 1997), several high-velocity wings of CO emission (Seta et al 2004), and by a faint spatially-extended broad CO and HCO + emissions (Sashida et al 2013). Such OH maser and broad emissions arise from a thin expanding shell of shocked molecular gas with the expansion velocity of 13.2 km s −1 (Sashida et al 2013).…”

“…Throughout this study, the distance to the Bullet (W44 SNR) is assumed to be 3 kpc. (Sashida et al 2013). The velocity range for the integration is from V LSR = +20 km s −1 to +60 km s −1 .…”

“…White contours outline VLA radio continuum emission at 20 cm (Jones et al 1993). The orange ellipse shows the best-fit model of uniformly expanding shell calculated from the spatially-extended wings (Sashida et al 2013). …”