X-Ray Imaging of Planetary Nebulae with Wolf-Rayet-type Central Stars: Detection of the Hot Bubble in NGC 40

Rodolfo Montez Jr., University of Rochester
Joel H. Kastner, Rochester Institute of Technology
Orsola De Marco, American Museum of Natural History
Noam Soker, Technion-Israel Institute of Technology

© 2005 The American Astronomical Society.

This research was supported by NASA through Chandra award number GO4–5169X issued to Rochester Institute of Technology by the Chandra X-ray Observatory Center, which is operated by Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8–03060. O.D. is grateful to Janet Jeppson Asimov for financial support.

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We present the results of Chandra X-Ray Observatory observations of the planetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-type Wolf-Rayet central stars that are currently driving fast (!1000 km s^-1), massive winds into denser, slow-moving (~10 kms^-1)material ejected during recently terminated asymptotic giant branch (AGB) evolutionary phases. Hence, these observations provide key tests of models of wind-wind interactions in PNs. In NGC 40, we detect faint, diffuse X-ray emission distributed within a partial annulus that lies nested within a ~40" diameter ring of nebulosity observed in optical and near-infrared images. Hen 2-99 is not detected. The inferred X-ray temperature (Tx ~10^6 K) and luminosity (Lx ~2 x10^30 ergs s^-1) of NGC 40 are the lowest measured thus far for any PN displaying diffuse X-ray emission. These results, combined with the ringlike morphology of the X-ray emission from NGC 40, suggest that its X-ray emission arises from a ‘‘hot bubble’’ that is highly evolved and is generated by a shocked, quasi-spherical fast wind from the central star, as opposed to AGB or post-AGB jet activity. In contrast, the lack of detectable X-ray emission from Hen 2-99 suggests that this PN has yet to enter a phase of strong wind-wind shocks.