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SN 2011dh
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Title: Spectropolarimetry of SN 2011dh in M51: geometric insights on a Type IIb supernova progenitor and explosion
Author: Jon C. Mauerhan, G. Grant Williams, Douglas C. Leonard, Paul S. Smith, Alexei V. Filippenko, Nathan Smith, Jennifer L. Hoffman, Leah Huk, Kelsey I. Clubb, Jeffrey M. Silverman, S. Bradley Cenko, Peter Milne, Avishay Gal-Yam, Sagi Ben-Ami

We present seven epochs of spectropolarimetry of the Type IIb supernova (SN) 2011dh in M51, spanning 86 days of its evolution. The first epoch was obtained 9 days after the explosion, when the photosphere was still in the depleted hydrogen layer of the stripped-envelope progenitor. Continuum polarisation is securely detected at the level of P~0.5% through day 14 and appears to diminish by day 30, which is different from the prevailing trends suggested by studies of other core-collapse SNe. Time-variable modulations in P and position angle are detected across P-Cygni line features. H-alpha and HeI polarisation peak after 30 days and exhibit position angles roughly aligned with the earlier continuum, while OI and CaII appear to be geometrically distinct. We discuss several possibilities to explain the evolution of the continuum and line polarisation, including the potential effects of a tidally deformed progenitor star, aspherical radioactive heating by fast-rising plumes of Ni-56 from the core, oblique shock breakout, or scattering by circumstellar material. While these possibilities are plausible and guided by theoretical expectations, they are not unique solutions to the data. The construction of more detailed hydrodynamic and radiative-transfer models that incorporate complex aspherical geometries will be required to further elucidate the nature of the polarised radiation from SN 2011dh and other Type IIb supernovae.

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RE: Supernova 2011dh
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Title: Imaging the Expanding Shell of SN 2011dh
Author: A. de Witt, M. F. Bietenholz, A. Kamble, A. M. Soderberg, A. Brunthaler, B. Zauderer, N. Bartel, M. P. Rupen

We report on third epoch VLBI observations of the radio-bright supernova SN 2011dh located in the nearby galaxy (7.8 Mpc) M51. The observations took place at t=453 d after the explosion and at a frequency of 8.4 GHz. We obtained a fairly well resolved image of the shell of SN 2011dh, making it one of only six recent supernovae for which resolved images of the ejecta are available. By fitting a spherical shell model directly to the visibility measurements we determine the angular radius of SN 2011dh's radio emission to be 636±29 µas . At a distance of 7.8 Mpc, this angular radius corresponds to a linear radius of (7.4±0.3) x 1016 cm and an average expansion velocity since the explosion of 18900^{+2800}_{-2400} kms-1. We also calculated more precise radius measurements for the earlier VLBI observations and we show that all the measured values of the radius of the emission region, up to t=453 d, are still almost perfectly consistent with those derived from fitting synchrotron self-absorbed models to the radio spectral energy distribution. We find that SN 2011dh's radius evolves in a power-law fashion, with R~t0.961±0.011, implying almost free expansion.

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ATel 6375: Detection of a Blue Point Source at the Location of Supernova 2011dh
 


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A Supergiant Star Goes Missing, and a Supernova Mystery Is Solved

Shortly after the light from an exploding star there reached Earth at the end of May 2011, amateur reports of the cataclysm began pouring in to the Central Bureau for Astronomical Telegrams, a clearinghouse for new telescope data. Soon the explosion was assigned the official designation supernova 2011dh.
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Title: One year of monitoring of the Type IIb supernova SN 2011dh
Authors: D. K. Sahu, G. C. Anupama, N. K. Chakradhari

Optical UBVRI photometry and low resolution spectroscopy of the type IIb supernova SN 2011dh in M51 are presented, covering the first year after the explosion. The light curve and spectral evolution are discussed. The early phase light curve evolution of SN 2011dh is very similar to SN 1993J and SN 2008ax. In the late phase, however, SN 2011dh declines faster than SN 1993J. The late phase decline in the B-band is steeper than in the R and I bands, indicating the possibility of dust formation. With a peak V-band absolute magnitude of M_V = -17.123±0.18 mag, SN 2011dh is a marginally faint type IIb event. The reddening corrected colour curves of SN 2011dh are found to be redder than other well studied type IIb supernovae. The bolometric light curve indicates ~ 0.09 solar masses of ^{56}Ni is synthesised during the explosion. The HeI lines were detected in the spectra during the rise to maximum. The nebular spectra of SN 2011dh show a box shaped emission in the red wing of the [OI] 6300-6363 \AA\ feature, that is attributed to H\alpha emission from a shock excited circumstellar material. The analysis of nebular spectra indicates that ~ 0.2 solar masses of oxygen was ejected during the explosion. Further, the [CaII]/[OI] line ratio in the nebular phase is ~ 0.7, indicating a progenitor with a main sequence mass of 10-15 solar masses.

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Posts: 128137
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Supernova 2011dh Progenitor
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Title: The Progenitor of Supernova 2011dh Has Vanished
Authors: Schuyler D. Van Dyk (1), WeiKang Zheng (2), Kelsey I. Clubb (2), Alexei V. Filippenko (2), S. Bradley Cenko (2), Nathan Smith (3), Ori D. Fox (2), Patrick L. Kelly (2), Isaac Shivvers (2), Mohan Ganeshalingam (4) ((1) Spitzer Science Center/Caltech, (2) UC Berkeley, (3) University of Arizona, (4) Lawrence Berkeley National Lab)

We conducted Hubble Space Telescope (HST) Snapshot observations of the Type IIb Supernova (SN) 2011dh in M51 at an age of ~641 days with the Wide Field Camera 3. We find that the yellow supergiant star, clearly detected in pre-SN HST images, has disappeared, implying that this star was almost certainly the progenitor of the SN. Interpretation of the early-time SN data which led to the inference of a compact nature for the progenitor, and to the expected survival of this yellow supergiant, is now clearly incorrect. We also present ground-based UBVRI light curves obtained with the Katzman Automatic Imaging Telescope (KAIT) up to SN age ~70 days. From the light-curve shape including the very late-time HST data, and from recent interacting binary models for SN 2011dh, we estimate that a putative surviving companion star to the now deceased yellow supergiant could be detectable by late 2013, especially in the ultraviolet. No obvious light echoes are detectable yet in the SN environment.

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Posts: 128137
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Supernova 2011dh
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Title: Optical and near-infrared observations of SN 2011dh - The first 100 days
Authors: M. Ergon, J. Sollerman, M. Fraser, A. Pastorello, S. Taubenberger, N. Elias-Rosa, M. Bersten, A. Jerkstrand, S. Benetti, M.T. Botticella, C. Fransson, A. Harutyunyan, R. Kotak, S. Smartt, S. Valenti, F. Bufano, E. Cappellaro, M. Fiaschi, A. Howell, E. Kankare, L. Magill, S. Mattila, R. Naves, P. Ochner, J. Ruiz, K. Smith, L. Tomasella, M. Turatto

We present optical and near-infrared (NIR) photometry and spectroscopy of the Type IIb supernova (SN) 2011dh for the first 100 days. We complement our extensive dataset with SWIFT ultra-violet (UV) and Spitzer mid-infrared (MIR) data to build a UV to MIR bolometric lightcurve using both photometric and spectroscopic data. Hydrodynamical modelling of the SN based on this bolometric lightcurve have been presented in Bersten (2012). We find that the absorption minimum for the hydrogen lines is never seen below ~11000 km/s but approaches this value as the lines get weaker. This suggests that the interface between the helium core and hydrogen rich envelope is located near this velocity in agreement with the Bersten et al. (2012) He4R270 ejecta model. Spectral modelling of the hydrogen lines using this ejecta model supports the conclusion and we find a hydrogen mass of 0.01-0.04 solar masses to be consistent with the observed spectral evolution. We estimate that the photosphere reaches the helium core at 5-7 days whereas the helium lines appear between ~10 and ~15 days, close to the photosphere and then move outward in velocity until ~40 days. This suggests that increasing non-thermal excitation due to decreasing optical depth for the gamma-rays is driving the early evolution of these lines. We also provide and discuss pre- and post-explosion observations of the SN site which shows a reduction by 60-75 percent in flux at the position of the yellow supergiant coincident with SN 2011dh. The V band decline between Jan 20 and Apr 14 2013 is 0.76 mag which is consistent with the remaining flux being emitted by the SN. Hence we find that the star was indeed the progenitor of SN 2011dh as previously suggested by Maund et al. (2011) and which is also consistent with the results from the hydrodynamical modelling.

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Posts: 128137
Date:
SN 2011dh
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Observationally Confirmed Supernova Explosion of a Yellow Supergiant Star

Observational results of the Hubble Space Telescope announced in March 2013 confirmed the theoretical prediction by the Bersten team at the Kavli Institute for the Physics and Mathematics of the Universe that the yellow supergiant star found at the location of supernova SN 2011dh in the famous nearby galaxy M51 was indeed the star that exploded.
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Posts: 128137
Date:
Supernova 2011dh
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Title: Type IIb Supernova SN 2011dh: Spectra and Photometry from the Ultraviolet to the Near-Infrared
Authors: G. H. "Howie'' Marion, Jozsef Vinko, Robert P. Kirshner, Ryan J. Foley, Perry Berlind, Allyson Bieryla, Joshua S. Bloom, Michael L. Calkins, Peter Challis, Roger A. Chevalier, Ryan Chornock, Chris Culliton, Jason L. Curtis, Gilbert A. Esquerdo, Mark E. Everett, Emilio E. Falco, Kevin France, Claes Fransson, Andrew S. Friedman, Peter Garnavich, Bruno Leibundgut, Samuel Meyer, Nathan Smith, Alicia M. Soderberg, Jesper Sollerman, Dan L. Starr, Tamas Szklenar, Katalin Takats, J. Craig Wheeler

We report spectroscopic and photometric observations of the Type IIb SN 2011dh obtained between 4 and 34 days after the estimated date of explosion (May 31.5 UT). These data cover a wide wavelength range from 2000 Angstroms in the ultraviolet (UV) to 2.4 microns in the near-infrared (NIR). NIR spectra reveal helium in the atmosphere 3 days before the B-band maximum and confirm the classification of SN 2011dh as a Type IIb. Optical spectra provide line profiles and velocity measurements of H I, He I, Ca II and Fe II. UV spectra obtained with the Space Telescope Imaging Spectrograph show that the UV flux for SN 2011dh is low compared to other SN IIb. H I features are strong early and weaken during the period of our observations. He I features are first detected eleven days after the explosion and become progressively stronger. For all phases at which He is detected, the hydrogen line-forming region is separated from the helium layer by about 4000 km/s. This velocity gap is consistent with a H-rich shell surrounding the progenitor and we estimate the shell mass for SN 2011dh relative to other SN IIb. Both H I and He I maintain a constant velocity after B-max while Ca II and Fe II velocities continue to decline. Light curves are presented for twelve passbands. SN 2011dh reached a maximum bolometric luminosity of 1.7 x 10^{42} erg/s, about 23 days after the explosion. NIR emission contributes more than 35% of the total bolometric flux. We compare our results to other studies of SN 2011dh and to other Type IIb SN. We find that SN IIb with different masses for the H shell have similar peak luminosities, decline rates and colours from the time of maximum. That suggests that the progenitors inside of the H shells are close to the same composition and mass. These data will provide firm constraints for models of SN 2011dh based on the explosion of a yellow supergiant.

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Posts: 128137
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RE: Possible supernova in Messier 51
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Title: Injection and Acceleration of Electrons at A Strong Shock: Radio and X-ray Study of Young Supernova 2011dh
Authors: Keiichi Maeda

In this paper, we develop a model for the radio and X-ray emissions from Type IIb Supernova (SN IIb) 2011dh in the first 100 days after the explosion, and investigate a spectrum of relativistic electrons accelerated at a strong shock wave. The widely-accepted theory of the particle acceleration, so-called diffusive shock acceleration (DSA) or Fermi mechanism, requires seed electrons with modest energy with gamma ~ 1 - 100, and little is known about this pre-acceleration mechanism: We derive the energy distribution of relativistic electrons in this pre-accelerated energy regime. We find that the efficiency of the electron acceleration must be low, i.e., epsilon_e <~ 0.01 as compared to the conventionally assumed value of epsilon_e ~ 0.1. Furthermore, independently from the low value of epsilon_e, we find that the X-ray luminosity cannot be attributed to any emission mechanisms suggested so far as long as these electrons follow the conventionally-assumed single power-law distribution. A consistent view between the radio and X-ray can only be obtained if the pre-acceleration injection spectrum peaks at gamma ~ 20-30 and then only a fraction of these electrons eventually experience the DSA-like acceleration toward the higher energy -- then the radio and X-ray properties are explained through the synchrotron and inverse Compton mechanisms, respectively. Our findings support the idea that the pre-acceleration of the electrons is coupled with the generation/amplification of the magnetic field.

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