2007MNRAS.381..881L

The distance to the relativistic jet source SS433 and the related supernova remnant W50 is re-examined using new observations of HI in absorption from the Very Large Array, HI in emission from the Green Bank Telescope, and ¹²CO emission from the Five College Radio Astronomy Observatory. The new measurements show HI in absorption against SS433 to a velocity of 75 km/s but not to the velocity of the tangent point, which bounds the kinematic distance at 5.5 ≤ d_k< 6.5 kpc. This is entirely consistent with a 5.5 ±0.2 kpc distance determined from light traveltime arguments by Blundell & Bowler. The HI emission map shows evidence of interaction of the lobes of W50 with the interstellar medium near the adopted systemic velocity of V_LSR= 75 km/s. The western lobe sits in a cavity in the HI emission near the Galactic plane, while the eastern lobe terminates at an expanding HI shell. The expanding shell has a radius of 40 pc, contains 8±3x10³M_☉ of HI and has a measured kinetic energy of 3±1.5x10⁴⁹ erg. There may also be a static HI ring or shell around the main part of W50 itself at a local standard of rest (LSR) velocity of 75 km/s, with a radius of 70 pc and a mass in HI of 3.5-10x10⁴M_☉. We do not find convincing evidence for the interaction of the system with any molecular cloud or with HI at other velocities. The HI emission data suggest that SS433 lies in an interstellar environment substantially denser than average for its distance from the Galactic plane.

This Population I system, now about 200 pc below the Galactic plane, most likely originated as a runaway O-star binary ejected from a young cluster in the plane. Given a modest ejection velocity of ≥30 km/s, the binary could have reached its present location in ≤10 Myr at which time the more massive member became a supernova (SN). New astrometric data on SS433 show that the system now has a peculiar velocity of a few tens of km/s in the direction of the Galactic plane. From this peculiar velocity and the symmetry of the W50 remnant, we derive a time since the SN of ≤1x10⁵ yr.