Corradi & Schwarz ( 1993, hereafter CS93) obtained images and long-slit spectra of M圜n 18 and concluded from optical and near-infrared diagnostics that it is a young PN. Schwarz, Corradi, & Melnick (1992) obtained ground-based images that show a striking resemblance to the triple-ring nebula around SN 1987A ( Burrows et al. M圜n 18 (PK 307-4☁) is a planetary nebula (PN) named after its discoverers, Mayall & Cannon (1940). Tablada 1006, 58090 Morelia, Michoacán, Mexico.Ĥ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218.ĥ Department of Astronomy, University of Wisconsin at Madison, 475 North Charter Street, Madison, WI 53706.Ħ Department of Physics and Astronomy, Arizona State University, Tyler Mall, Tempe, AZ 85287.ħ Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109.Ĩ Department of Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213-3890.ĩ Department of Astronomy, New Mexico State University, Box 30001, Department 4500, Las Cruces, NM 88003-8001.ġ0 Mount Stromlo and Siding Spring Observatories, Australian National University, Weston Creek Post Office, ACT 2611, Australia. Key words: circumstellar matter ISM: kinematics and dynamics planetary nebulae: individual (M圜n 18) stars: AGB and post-AGB stars: mass lossġ Jet Propulsion Laboratory, Mail Stop 183-900, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109.Ģ Infrared Processing and Analysis Center, Mail Stop 100-22, California Institute of Technology, Pasadena, CA 91125.ģ Instituto de Astronomía, Universidad Nacional Autónoma de México, J. Although none are found to be completely satisfactory, those involving a binary central star probably offer the best hope of successful explanation. We discuss several mechanisms for producing the offset of the central star. However, the complex inner nebular structure of M圜n 18 and the offset of the central star from the center of the nebula remain a mystery. The hourglass shape of the main nebula is consistent with the predictions of the generalized interacting-winds hypothesis for planetary nebula formation. The outer and inner hourglass, and ring 1 and ring 2, all have different centers, and none are coincident with the central star. Ring 1 is a bright elliptical ring, and ring 2 a smaller, higher excitation ring. In the complex central region of the nebula we find a small, inner hourglass structure and two rings. A series of arcs appear to be etched on the walls of the hourglass near its rims. Although from the ground M圜n 18 appeared to have a triple-ring structure similar to SN 1987A, the HST images show that M圜n 18 has an overall hourglass shape. We have obtained emission-line and continuum images of the young planetary nebula M圜n 18 with the Wide Field Planetary Camera 2 on the Hubble Space Telescope ( HST). Received 1999 February 25 accepted 1999 March 15 H OLTZMAN, 9 J OHN K RIST, 4 AND J EREMY R. R AGHVENDRA S AHAI, 1 A DITYA D AYAL, 1, 2 A LAN M. Sahai et al., The Etched Hourglass Nebula.
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