Optical Discovery of an Apparent Galactic Supernova Remnant G159.6+7.3

Deep Halpha images of portions of a faint 3 x 4 degree Halpha shell centered at l = 159.6 deg, b = 7.3 deg seen on the Virginia Tech Spectral Line Survey images revealed the presence of several thin emission filaments along its eastern limb. Low-dispersion optical spectra of two of these filaments covering the wavelength range of 4500 - 7500 Angstroms show narrow Halpha line emissions with velocities around -170 +/- 30 km/s. Both the morphology and spectra of these filaments are consistent with a Balmer dominated shock interpretation and we propose these optical filaments indicate that the large Halpha emission shell is a previously unrecognized supernova remnant. ROSAT All Sky Survey images indicate the possible presence of extremely faint, diffuse emission from the shell's central region. The shell's location more than seven degrees off the Galactic plane in a region of relatively low interstellar density may account for the lack of any reported associated nonthermal radio emissions. The rare discovery of a Galactic SNR at optical wavelengths suggests that additional high latitude SNRs may have escaped radio and X-ray detection.Comment: 6 pages, 6 figures, accepted to the Astronomical Journa

The Nature of the Ultraluminous Oxygen-Rich Supernova Remnant in NGC 4449

Optical images and spectra, both ground based and taken by Hubble Space Telescope (HST), of the young, luminous O-rich supernova remnant in the irregular galaxy NGC 4449 are presented. HST images of the remnant and its local region were obtained with the ACS/WFC using filters F435W, F555W, F814W (B, V, and I, respectively), F502N ([O III]), F658N (Halpha + [N II]), F660N ([N II]), and F550M (line-free continuum). These images show an unresolved remnant (FWHM < 0.05 arcsec) located within a rich cluster of OB stars which itself is enclosed by a nearly complete interstellar shell seen best in Halpha + [N II] emission approximately 8'' x 6'' (150 pc x 110 pc) in size. The remnant and its associated OB cluster are isolated from two large nearby H II regions. The ACS [O III] image shows the remnant may be partially surrounded by a clumpy ring of emission approximately 1'' (~20 pc) in diameter. Recent ground-based spectra of the remnant reveal (1) the emergence of broad, blueshifted emission lines of [S II] 6716, 6731, [Ar III] 7136, and [Ca II] 7291, 7324 which were not observed in spectra taken in 1978 -- 1980; (2) faint emission at 6540 -- 6605 A centered about Halpha and [N II] 6548, 6583 with an expansion velocity of 500 +/- 100 km/s; and (3) excess emission around 4600 -- 4700 A suggestive of a Wolf-Rayet population in the remnant's star cluster. We use these new data to re-interpret the origin of the remnant's prolonged and bright luminosity and propose that the remnant is strongly interacting with dense, circumstellar wind loss material from a ~20 Msolar progenitor star.Comment: 20 pages, 10 figures. Matches version published in Ap

A Detailed Kinematic Map of Cassiopeia A's Optical Main Shell and Outer High-Velocity Ejecta

We present three-dimensional kinematic reconstructions of optically emitting material in the young Galactic supernova remnant Cassiopeia A (Cas A). These Doppler maps have the highest spectral and spatial resolutions of any previous survey of Cas A and represent the most complete catalog of its optically emitting material to date. We confirm that the bulk of Cas A's optically bright ejecta populate a torus-like geometry tilted approximately 30 degrees with respect to the plane of the sky with a -4000 to +6000 km/s radial velocity asymmetry. Near-tangent viewing angle effects and an inhomogeneous surrounding CSM/ISM environment suggest that this geometry and velocity asymmetry may not be faithfully representative of the remnant's true 3D structure or the kinematic properties of the original explosion. The majority of the optical ejecta are arranged in several well-defined and nearly circular ring-like structures with diameters between approximately 30 arcsec (0.5 pc) and 2 arcmin (2 pc). These ejecta rings appear to be a common phenomenon of young core-collapse remnants and may be associated with post-explosion input of energy from plumes of radioactive 56Ni-rich ejecta that rise, expand, and compress non-radioactive material. Our optical survey also encompassed Cas A's faint outlying ejecta knots and exceptionally high-velocity NE and SW streams of S-rich debris often referred to as `jets'. These outer knots, which exhibit a chemical make-up suggestive of an origin deep within the progenitor star, appear to be arranged in opposing and wide-angle outflows with opening half-angles of approximately 40 degrees.Comment: 15 pages, 11 figures, accepted by ApJ, associated movie files can be found at https://www.cfa.harvard.edu/~dmilisav

Constraining the Age and Distance of the Galactic Supernova Remnant G156.2+5.7 by H-alpha Expansion Measurements

We present deep H-alpha images of portions of the X-ray bright but optically faint Galactic supernova remnant G156.2+5.7, revealing numerous and delicately thin nonradiative filaments which mark the location of the remnant's forward shock. These new images show that these filaments have a complex structure not visible on previous lower resolution optical images. By comparing H-alpha images taken in 2004 at the McDonald Observatory and in 2015-2016 at the Kiso Observatory, we set a stringent 1-sigma upper limit of expansion to be 0.06 arcsec/yr. This proper motion, combined with a shock speed of 500 km/s inferred from X-ray spectral analyses, gives a distance of > 1.7 kpc. In addition, a simple comparison of expansion indices of several SNRs allows us to infer the age of the remnant to be a few 10,000 yr old. These estimates are more straightforward and reliable than any other previous studies, and clearly rule out a possibility that G156.2+5.7 is physically associated with part of the Taurus-Auriga cloud and dust complex at a distance of 200-300 pc.Comment: 16 pages, 5 figures, accepted for publication in The Astrophysical Journa

The Nature of the Strong 24 Μmspitzersource J222557+601148: Not a Young Galactic Supernova Remnant

The nebula J222557+601148, tentatively identified by Morris et al. as a young Galactic supernova remnant (SNR) from Spitzer Galactic First Look Survey images and a follow-up mid-infrared spectrum, is unlikely to be an SNR remnant based on Hα, [O III], [S II] images, and low-dispersion optical spectra. The object is seen in Hα and [O III] λ5007 images as a faint, roughly circular ring nebula with dimensions matching that seen in 24 μm Spitzer images. Low-dispersion optical spectra show it to have narrow Hα and [N II] λλ6548,6583 line emissions with no evidence of broad or high-velocity (v ≥ 300 km s–1) line emissions. The absence of any high-velocity optical features, the presence of relatively strong [N II] emissions, the lack of detected [S II] emission which would indicate the presence of shock-heated gas, plus no coincident X-ray or nonthermal radio emissions indicate that the nebula is unlikely to be an SNR, young or old. Instead, it is likely a faint, high-excitation planetary nebula (PN) as its elliptical morphology would suggest, lying at a distance ~2-3 kpc with unusual but not extraordinary mid-IR colors and spectra. We have identified an m r\u27 = 22.4 ± 0.2 star as a PN central star candidate

An HST Survey of The Highest-Velocity Ejecta in Cassiopeia A

We present Hubble Space Telescope WFC3/IR images of the Cassiopeia A supernova remnant that survey its high-velocity, S-rich debris in the NE jet and SW counterjet regions through [S iii] λλ9069, 9531 and [S ii] λλ10,287–10,370 line emissions. We identify nearly 3400 sulfur emitting knots concentrated in ~120° wide opposing streams, almost triple the number previously known. The vast majority of these ejecta knots lie at projected distances well out ahead of the remnant\u27s forward blast wave and main shell ejecta, extending to angular distance of to the NE and to the SW from the center of expansion. Such angular distances imply undecelerated ejecta knot transverse velocities of 15,600 and 12,700 km s−1, respectively, assuming an explosion date AD and a distance of 3.4 kpc. Optical spectra of knots near the outermost tip of the NE ejecta stream show strong emission lines of S, Ca, and Ar. We estimate a total mass ~0.1 and a kinetic energy of at least erg for S-rich ejecta in the NE jet and SW counterjet. Although their broadness and kinetic energy argue against the Cas A SN being a jet-induced explosion, the jets are kinematically and chemically distinct from the rest of the remnant. This may reflect an origin in a jet-like mechanism that accelerated interior material from a Si-, S-, Ar-, and Ca-rich region near the progenitor\u27s core up through the mantle and H-, He-, N-, and O-rich outer layers with velocities that greatly exceeded that of the rapidly expanding photosphere