* Astronomy

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RE: Supernova 2011fe

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Title: BVRI Photometry of SN 2011fe in M101
Authors: Michael W. Richmond, Horace A. Smith

We present BVRI photometry of supernova 2011fe in M101 from 2.9 to 182 days after the explosion. The light curves and colour evolution show that SN 2011fe belongs to the "normal" subset of type Ia supernovae, with \Delta m_{15}(B) = 1.21 ± 0.03 mag. After correcting for extinction and adopting a distance modulus of (m - M) = 29.10 mag to M101, we derive absolute magnitudes M_B = -19.21, M_V = -19.19, M_R = -19.18 and M_I = -18.94. We compare visual measurements of this event to our CCD photometry and find evidence for a systematic difference based on colour.

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OU Telescope spies supernova

A robotic astronomical telescope operated by The Open University on the Spanish island of Mallorca was responsible for a one-off image of a supernova, leading to crucial information on the nature of these exploding stars.
The image was taken by a PhD student who happened to be "in the right place at the right time" and was able to gain sophisticated detail of these captivating stars.
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'White dwarf' triggers giant supernovae explosion

Some of the brightest known explosions in the universe, type 1a supernovae, are triggered by ancient, dense stars called white dwarfs, a new study has revealed.
These supernovae are such regular features of the Universe that astrophysicists use them to measure cosmic distances. But still the causes behind these giant explosions are still unknown.
Now, scientists at the Weizmann Institute of Science, as part of an international effort to study supernovae, are beginning to clear up the mystery of why certain stars explode in a brilliant display at the ends of their lives.
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Title: The supernova Ia 2011fe in M101, its tip of the red-giant branch (TRGB) distance, and the value of H_0
Authors: G.A. Tammann, B. Reindl (Department of Physics and Astronomy, Univ. of Basel)

The light curve parameters of the normal type Ia SN2011fe are derived from the rich archive of the AAVSO. This leads, together with the TRGB distance modulus of (m-M) = 29.39 ± 0.05 of the parent galaxy M101, to maximum magnitudes of the unreddened SN of M_B = -19.45 ± 0.08, M_V = -19.46 ± 0.08, and M_I = -19.25 +/- 0.06 (for the standard decline rate of Delta m_15 = 1.1). When these values are inserted into the Hubble line defined by 62 SNe Ia with 3000 < v < 20,000 km/s - and considering also four other SNe Ia with TRGB distances - one obtains a large-scale value of the Hubble constant of H_0 = 64.3 ± 1.9 ± 3.2. This value can be much improved in the future by using only TRGB distances of SNe Ia.

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Title: Multi-epoch Spectropolarimetry of SN 2011fe
Authors: Paul S. Smith, G. Grant Williams, Nathan Smith, Peter A. Milne, Buell T. Jannuzi, E. M. Green

We present multiple spectropolarimetric observations of the nearby Type Ia supernova SN 2011fe in M101, obtained before, during, and after the time of maximum apparent visual brightness. SN 2011fe exhibits time-dependent polarization in both the continuum and strong absorption lines. At all epochs, red wavelengths exhibit a degree of continuum polarisation of 0.2-0.4 per cent, likely indicative of persistent asymmetry in the electron-scattering photosphere. However, the degree of polarisation across the Si II 6355 absorption line varies dramatically from epoch to epoch. Before maximum, Si II 6355 shows enhanced polarisation at the same position angle (PA) as the polarised continuum. During two epochs near maximum, however, Si II 6355 absorption has a lower degree of polarisation, with a PA that is 90deg from the continuum. After maximum, the absorption feature has the same degree of polarisation and PA as the adjacent continuum. Another absorption feature in the blue (either Si II 5051 or a blend with Fe II lines) shows qualitatively similar changes, although the changes are shifted in time to an earlier epoch. This behaviour is similar to that seen in broad absorption-line quasars, where the polarisation in absorption features has been interpreted as the line absorbing some of the unpolarised continuum flux. This behaviour, along with the 90deg shifts of the polarisation PA with time, imply a time-dependent large-scale asymmetry in the explosion.

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Title: A Compact Degenerate Primary-Star Progenitor of SN 2011fe
Authors: Joshua S. Bloom, Daniel Kasen, Ken J. Shen, Peter E. Nugent, Nathaniel R. Butler, Melissa L. Graham, D. Andrew Howell, Ulrich Kolb, Stefan Holmes, Carole Haswell, Vadim Burwitz, Juan Rodriguez, Mark Sullivan

While a white dwarf is, from a theoretical perspective, the most plausible primary star in Type Ia supernova (SN Ia), many other candidates have not been formally ruled out. Shock energy deposited in the envelope of any exploding primary contributes to the early SN brightness and, since this radiation energy is degraded by expansion after the explosion, the diffusive luminosity depends on the initial primary radius. We present a new non-detection limit of the nearby SN Ia 2011fe, obtained what appears to be just 4 hours after explosion, allowing us to directly constrain the initial primary radius, R_p. Coupled with the non-detection of a quiescent X-ray counterpart and the inferred synthesised Ni mass, we show that R_p 10,000 gm cm^{-3}, and that the effective temperature must be less than a few x 10^5 K. This rules out hydrogen burning main sequence stars and giants. Constructing the helium-burning main sequence and carbon-burning main sequence, we find such objects are also excluded. By process of elimination, we find that only degeneracy-supported compact objects---WDs and neutron stars---are viable as the primary star of SN 2011fe. With few caveats, we also restrict the companion (secondary) star radius to R_ c

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Title: Supernova 2011fe from an Exploding Carbon-Oxygen White Dwarf Star
Authors: Peter E. Nugent, Mark Sullivan, S. Bradley Cenko, Rollin C. Thomas, Daniel Kasen, D. Andrew Howell, David Bersier, Joshua S. Bloom, S. R. Kulkarni, Michael T. Kandrashoff, Alexei V. Filippenko, Jeffrey M. Silverman, Geoffrey W. Marcy, Andrew W. Howard, Howard T. Isaacson, Kate Maguire, Nao Suzuki, James E. Tarlton, Yen-Chen Pan, Lars Bildsten, Benjamin J. Fulton, Jerod T. Parrent, David Sand, Philipp Podsiadlowski, Federica B. Bianco, Benjamin Dilday, Melissa L. Graham, Joe Lyman, Phil James, Mansi M. Kasliwal, Nicholas M. Law, Robert M. Quimby, Isobel M. Hook, Emma S. Walker, Paolo Mazzali, Elena Pian, Eran O. Ofek, Avishay Gal-Yam, Dovi Poznanski

Type Ia supernovae (SNe Ia) have been used empirically as standardised candles to reveal the accelerating universe even though fundamental details, such as the nature of the progenitor system and how the star explodes, remained a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary could be anything from a main sequence star to a red giant, or even another white dwarf. The uncertainty stems from the fact that no recent SN Ia has been discovered close enough to detect the stars before explosion. Here we report early observations of SN 2011fe (PTF11kly) in M101 at a distance of 6.4 Mpc, the closest SN Ia in the past 25 years. We find that the exploding star was likely a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was most likely a main sequence star. Early spectroscopy shows high-velocity oxygen that varies on a time scale of hours and extensive mixing of newly synthesized intermediate mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions.

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Title: On the Nature of the Progenitor of the Type Ia SN2011fe in M101
Authors: Jifeng Liu, Rosanne Di Stefano, Tao Wang, Maxwell Moe

The explosion of a Type Ia supernova, SN 2011ef, in the nearby Pinwheel galaxy (M101 at 6.4 Mpc) provides an opportunity to study pre-explosion images and search for the progenitor, which should consist of a white dwarf (WD), possibly surrounded by an accretion disk, in orbit with another star. We report on our use of deep Chandra observations to limit the luminosity and temperature of the pre-explosion white dwarf (WD). It is found that if the spectrum was a blackbody, then WDs of highest possible temperatures and luminosities are excluded but, even if the WD was emitting at the Eddington luminosity, values of kT less than roughly 60 eV are permitted. This allows the progenitor to be an accreting nuclear-burning WD with an expanded photosphere. Pre-SN HST observations were used to derive a lower limit of about 10 eV for the expanded photosphere. Li et al.\, (2011) have already ruled out the possibility of a giant donor. We consider the combined emission from the WD, disk, and donor, and find that even the combined emission from a bright subgiant, WD and disk would not likely have been observed prior to explosion, and neither would some local candidates for the nuclear-burning WD model.