Shocking Clouds in the Cygnus Loop

With Hubble Space Telescope Wide-Field Planetary Camera 2 observations of the Cygnus Loop supernova remnant, we examine the interaction of an interstellar cloud with the blast wave on physical scales of 10 cm. The shock front is distorted, revealing both edge-on and face-on views of filaments and diffuse emission, similar to those observed on larger scales at lower resolution. We identify individual shocks in the cloud of density n ≈ 15 cm−3 having velocity vs ≈ 170 km s−1. We also find the morphologically unusual diffuse Balmer-dominated emission of faster shocks in a lower-density region. The obstacle diffracts these shocks, so they propagate at oblique angles with respect to the primary blast wave. The intricate network of diffuse and filamentary Hα emission arises during the early stage of interaction between the cloud and blast wave, demonstrating that complex shock propagation and emission morphology occur before the onset of instabilities that destroy clouds completely. Subject headings: ISM: individual(Cygnus Loop) — shock waves — supernova remnants 1. the southeast knot of the cygnus loop Supernova remnants and the interstellar medium act upon each other reciprocally. The energy of supernova remnants (SNRs) heats and ionizes the interstellar medium (ISM), and their blast waves govern mass exchange between the hot, warm, and cool phases of the ISM. In turn, the extant ISM determines the evolution of SNR blast waves as they propagate through the environment it provides. Thus, in order to understand the large-scale structure of the ISM, we must also discern the nature of shock evolution in inhomogeneous media. The Cygnus Loop supernova remnant provides an ideal laboratory in which to examine these issues. It is relatively unobscured, with E(B − V ) = 0.08 (Parker 1967), and nearby (440 −100 pc; Blair et al. 1999), so 1 ′′ corresponds to 7 × 10 cm. It is a middle-aged supernova remnant, not only in terms of absolute lifetime, τ = 8000 yr (Levenson et al. 1998, scaling for the revised distance), but more importantly because the interaction of the blast wave with the ISM dominates its evolution and its appearance at all wavelengths. At optical wavelengths, the outstanding features are due to decelerated shocks that propagate through dense interstellar clouds. Shocks that are reflected off the cloud surfaces propagate back through previously shocked material, further heating and compressing it, enhancing X-ray emission (Hester & Cox 1986). One example of this characteristic interaction is the southeast knot. Fesen, Kwitter, & Downes (1992) drew attention to this apparently insignificant feature in the optical and X-ray emission, which has a small angular extent compared to the diameter of the Cygnus Loop. In contrast to the SNR as a whole, the optical appearance of the southeast knot suggests that it represents only a very minor enhancement in the local ISM, or that the interaction is very young. Fesen et al. (1992) support the former interpretation, proposing that this represents the late stage of interaction with a small cloud that has been completely engulfed by the blast wave and is in the process of being shredded by fluid instabilities (Klein, McKee, & Colella 1994). X-ray imaging contradicts this interpretation, however, because the knot is located at the apex of a large-scale (0.5 degree) indentation in the eastern limb as traced by very low surface brightness X-ray emission (Graham et al. 1995; Levenson, Graham, & Snowden 1999). Thus, the obstacle is certainly large, extending at least 5 pc along the line of sight, and the interaction is at an early stage. The optical emission is confined to a 2′ × 4′ region, but the apparent insignificance of the southeast knot may belie its importance. The highest surface brightest optical and X-ray emitting regions of the Cygnus Loop are associated with the well-developed reflected and transmitted shocks that form in mature cloud–blast-wave interactions (?, e.g.,)]Hes94. The morphology of these bright regions is notoriously complex because multiple shocks are present along the same line of sight. Only in a few cases is the geometry unambiguous, such as the western limb (Levenson et al. 1996). In addition, the development of fluid dynamic instabilities into the non-linear regime during the late phase of interaction adds to the challenge of interpreting the more prominent regions. These difficulties suggest that focussing on very recent interactions, although intrinsically fainter, may provide useful insights into the sizes, 1 Based on observations with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-2655.