Massive expanding torus and fast outflow in planetary nebula NGC 6302

Abstract

We present interferometric observations of ${}^{12}$CO and ${}^{13}$CO $J$=2$-$1 emission from the butterfly-shaped, young planetary nebula NGC 6302. The high angular resolution and high sensitivity achieved in our observations allow us to resolve the nebula into two distinct kinematic components: (1) a massive expanding torus seen almost edge-on and oriented in the North-South direction, roughly perpendicular to the optical nebula axis. The torus exhibits very complex and fragmentated structure; (2) high velocity molecular knots moving at high velocity, higher than 20 \kms, and located in the optical bipolar lobes. These knots show a linear position-velocity gradient (Hubble-like flow), which is characteristic of fast molecular outflow in young planetary nebulae. From the low but variable ${}^{12}$CO/${}^{13}$CO $J$=2$-$1 line intensity ratio we conclude that the ${}^{12}$CO $J$=2$-$1 emission is optically thick over much of the nebula. Using the optically thinner line ${}^{13}$CO $J$=2$-$1 we estimate a total molecular gas mass of $\sim$ 0.1 M${}_{\odot }$, comparable to the ionized gas mass; the total gas mass of the NGC 6302 nebula, including the massive ionized gas from photon dominated region, is found to be $\sim$ 0.5 M${}_{\odot }$. From radiative transfer modelling we infer that the torus is seen at inclination angle of 75${}^{\circ }$ with respect to the plane of the sky and expanding at velocity of 15 \kms. Comparison with recent observations of molecular gas in NGC 6302 is also discussed.

24 pages, 7 figures, accepted for publication in Astrophysical Journal

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