On the observability of bow shocks of Galactic runaway OB stars

Abstract

Massive stars that have been ejected from their parent cluster and\nsupersonically sailing away through the interstellar medium (ISM) are\nclassified as exiled. They generate circumstellar bow shock nebulae that can be\nobserved. We present two-dimensional, axisymmetric hydrodynamical simulations\nof a representative sample of stellar wind bow shocks from Galactic OB stars in\nan ambient medium of densities ranging from n_ISM=0.01 up to 10.0/cm3.\nIndependently of their location in the Galaxy, we confirm that the infrared is\nthe most appropriated waveband to search for bow shocks from massive stars.\nTheir spectral energy distribution is the convenient tool to analyze them since\ntheir emission does not depend on the temporary effects which could affect\nunstable, thin-shelled bow shocks. Our numerical models of Galactic bow shocks\ngenerated by high-mass (~40 Mo) runaway stars yield H$\\alpha$ fluxes which\ncould be observed by facilities such as the SuperCOSMOS H-Alpha Survey. The\nbrightest bow shock nebulae are produced in the denser regions of the ISM. We\npredict that bow shocks in the field observed at Ha by means of\nRayleigh-sensitive facilities are formed around stars of initial mass larger\nthan about 20 Mo. Our models of bow shocks from OB stars have the emission\nmaximum in the wavelength range 3 <= lambda <= 50 micrometer which can be up to\nseveral orders of magnitude brighter than the runaway stars themselves,\nparticularly for stars of initial mass larger than 20 Mo.\n