The Distance to SS 433/W50 and its Interaction with the Interstellar Medium

The distance to the relativistic jet source SS 433 and the related supernova remnant W50 is re-examined using new observations of H I in absorption from the VLA, H I in emission from the GBT, and CO emission from the FCRAO. The new measurements show H I in absorption against SS 433 to a velocity of 75 km s but not to the velocity of the tangent point, which bounds the kinematic distance at 5.5 6 dk < 6.5 kpc. This is entirely consistent with a 5.5 ± 0.2 kpc distance determined from light travel-time arguments (Blundell & Bowler 2004). The H I emission map shows evidence of interaction of the lobes of W50 with the interstellar medium near the adopted systemic velocity of VLSR = 75 km s . The western lobe sits in a cavity in the H I emission near the Galactic plane, while the eastern lobe terminates at an expanding H I shell. The expanding shell has a radius of 40 pc, contains 8 ± 3 × 10 M⊙ of H I and has a measured kinetic energy of 3 ± 1.5 × 10 ergs. There may also be a static H I ring or shell around the main part of W50 itself at an LSR velocity of 75 km s, with a radius of 70 pc and a mass in H I of 3.5 − 10 × 10 M⊙. We do not find convincing evidence for the interaction of the system with any molecular cloud or with H I at other velocities. The H I emission data suggest that SS 433 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 6 10 Myr at which time the more massive member became a supernova. New astrometric data on SS 433 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 6 1× 10 yr.