THE TRANSITIONAL STRIPPED-ENVELOPE SN 2008ax: SPECTRAL EVOLUTION AND EVIDENCE FOR LARGE ASPHERICITY

Journal Article · Tue Sep 20 00:00:00 EDT 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/739/1/41; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)· OSTI ID:21587502

Chornock, R; Filippenko, A V; Li, W; Modjaz, M; Silverman, J M; Steele, T N ^[1]; Marion, G H ^[2]; Foley, R J ^[3]; Rafelski, M ^[4]; Becker, G D ^[5]; De Vries, W H ^[6]; Garnavich, P ^[7]; Jorgenson, R A ^[8]; Lynch, D K; Rudy, R J; Russell, R W ^[9]; Malec, A L; Murphy, M T ^[10]; Moran, E C ^[11]; Stockton, A ^[12]

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1. Department of Astronomy, University of California, Berkeley, CA 94720-3411 (United States)
2. The George P. and Cynthia W. Mitchell Institute for Fundamental Physics and Astronomy, Texas A and M University, Department of Physics and Astronomy, 4242 TAMU, College Station, TX 77843-4242 (United States)
3. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 (United States)
4. Department of Physics, and Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093-0424 (United States)
5. Kavli Institute for Cosmology and Institute of Astronomy, Cambridge CB3 0HA (United Kingdom)
6. Department of Physics, University of California, Davis, CA 95616 (United States)
7. Physics Department, University of Notre Dame, Notre Dame, IN 46556 (United States)
8. Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA (United Kingdom)
9. Aerospace Corporation, Mail Stop M2-266, Los Angeles, CA 90009-2957 (United States)
10. Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Victoria 3122 (Australia)
11. Astronomy Department, Wesleyan University, Middletown, CT 06459 (United States)
12. Institute for Astronomy, University of Hawaii, Honolulu, HI 96822 (United States)

Supernova (SN) 2008ax in NGC 4490 was discovered within hours after shock breakout, presenting the rare opportunity to study a core-collapse SN beginning with the initial envelope-cooling phase immediately following shock breakout. We present an extensive sequence of optical and near-infrared spectra, as well as three epochs of optical spectropolarimetry. Our initial spectra, taken two days after shock breakout, are dominated by hydrogen Balmer lines at high velocity. However, by maximum light, He I lines dominated the optical and near-infrared spectra, which closely resembled those of normal Type Ib supernovae (SNe Ib) such as SN 1999ex. This spectroscopic transition defines Type IIb SNe, but the strong similarity of SN 2008ax to normal SNe Ib beginning near maximum light, including an absorption feature near 6270 A due to H{alpha} at high velocities, suggests that many objects classified as SNe Ib in the literature may have ejected similar amounts of hydrogen as SN 2008ax, roughly a few x 0.01 M{sub sun}. Only the unusually early discovery of SN 2008ax allowed us to observe the spectroscopic signatures of the hydrogen-rich outer ejecta. Early-time spectropolarimetry (six and nine days after shock breakout) revealed strong line polarization modulations of 3.4% across H{alpha}, indicating the presence of large asphericities in the outer ejecta and possibly that the spectrum of SN 2008ax could be dependent on the viewing angle. After removal of interstellar polarization, the continuum shares a common polarization angle with the hydrogen, helium, and oxygen lines, while the calcium and iron absorptions are oriented at different angles. This is clear evidence of deviations from axisymmetry even in the outer ejecta. Intrinsic continuum polarization of 0.64% only nine days after shock breakout shows that the outer layers of the ejecta were quite aspherical. A single epoch of late-time spectropolarimetry as well as the shapes of the nebular line profiles demonstrate that asphericities extended from the outermost layers all the way down to the center of this core-collapse SN. SN 2008ax may in some ways be an extragalactic analog of the explosion giving rise to Cassiopeia A, which has recently been determined to be a remnant of an SN IIb.

• OSTI ID: 21587502
• Journal Information: Astrophysical Journal, Vol. 739, Issue 1; Other Information: DOI: 10.1088/0004-637X/739/1/41; Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ABSORPTION
BALMER LINES
HELIUM
HYDROGEN
INFRARED SPECTRA
NEAR INFRARED RADIATION
OXYGEN
POLARIZATION
SUPERNOVAE
BINARY STARS
ELECTROMAGNETIC RADIATION
ELEMENTS
ERUPTIVE VARIABLE STARS
FLUIDS
GASES
INFRARED RADIATION
NONMETALS
RADIATIONS
RARE GASES
SORPTION
SPECTRA
STARS
VARIABLE STARS