OB STARS AND STELLAR BOW SHOCKS IN CYGNUS-X: A NOVEL LABORATORY ESTIMATING STELLAR MASS LOSS RATES

Abstract

We use mid-infrared images from the Spitzer Space Telescope Cygnus X Legacy Survey to search for stellar bow shocks (BSs), a signature of early-type "runaway" stars with high space velocities. We identify ten arc-shaped nebulae containing centrally located stars as candidate BSs. New spectroscopic observations of five stars show that all are late-O to early-B dwarfs, while one is a previously classified B0.2 giant. These stars have moderate radial velocities, differing by ΔV< 10 km s−1 from members of the Cygnus OB2 Association. The spectral energy distributions (SEDs) of the other four stars are consistent with late-O to early-B dwarfs at the nominal ∼1.6 kpc distance of Cyg OB2. Our morphologically selected sample of BS candidates encompasses diverse physical phenomena. Three of the stars appear to be pre-main-sequence objects on the basis of rising SEDs in the mid-IR, and their nebulae may be photon-dominated regions illuminated by the central star but shaped by external sources such as winds from Cyg OB2. Four objects have ambiguous classification. These may be partial dust shells or bubbles. We conclude that three of the objects are probable BSs, based on their morphological similarity to analytic prescriptions. Their nebular morphologies reveal no systematic pattern of orientations that might indicate either a population of stars ejected from or large-scale hydrodynamic outflows from Cyg OB2. The fraction of runaways among OB stars near Cyg OB2 identified either by radial velocity or BS techniques is ∼0.5%, much smaller than the ∼8% estimated among field OB stars. We discuss possible reasons for this difference. We also obtained a heliocentric radial velocity for the previously known BS star, BD+43°3654, of −66.2 ± 9.4 km s−1, solidifying its runaway status and implying a space velocity of 77 ± 10 km s−1. We use the principles of momentum-driven BSs in conjunction with the observed sizes, BS luminosities and SEDs, and dust/polycyclic aromatic hydrocarbon emission models to arrive at a novel method for estimating stellar mass loss rates. Derived mass loss rates range between 10−7 and few ×10−6 M☉ yr−1 for the three O5V–∼B2V stars identified as generating BSs. These values are at the upper range of, but broadly consistent with, estimates from other methods. We calculate a relatively large mass loss rate of 160 × 10−6 M☉ yr−1 for O4If star BD+43°3654 using the same method.