Early-Time Polarization of the Type II-Plateau Supernova SN 2012aw

ATel #4033; D. C. Leonard (San Diego State University), G. Pignata (Universidad Andres Bello), L. Dessart (Laboratoire d'Astrophysique de Marseille/California Institute of Technology), D. Hillier (University of Pittsburgh), C. Horst (San Diego State University), J. M. Fedrow (San Diego State University), L. Brewer (San Diego State University)
on 11 Apr 2012; 22:06 UT
Credential Certification: D. C. Leonard (leonard@astro.caltech.edu)

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We obtained spectropolarimetry of the nearby Type II-Plateau supernova SN 2012aw (CBET #3054) on Apr. 1 UT (roughly 17 days after explosion; ATEL #3996) with the ESO Very Large Telescope (+ FORS2 in polarimetric mode; range 430-920 nm, resolution 1.2 nm). A preliminary reduction of the data reveals a spectrum that continues (CBET #3054) to resemble those of Type II-Plateau supernovae (SN II-P) at similarly early epochs, presenting a smooth, blue continuum with broad P-Cyg features corresponding to H_alpha, Fe II (516.9 nm), H_beta, and the Ba II (455.4 nm) blend all evident. The ejecta velocity, as deduced from the minimum of H_beta, is about 9000 km/s.

The observed spectropolarimetry (i.e., uncorrected for interstellar polarization -- ISP) are characterized by significant polarization that decreases smoothly from ~0.7% at 430 nm to ~0.1% at 920 nm, with marked modulations of up to ~0.3% occurring across the P-Cyg line features. The strong wavelength dependence of the continuum polarization is unlike that observed in previous SNe II-P (or that expected for simple electron-scattering atmospheres; see, however, Dessart & Hillier 2011, MNRAS, 415, 3497, for how wavelength-dependent albedo at the photosphere could introduce a wavelength-dependent continuum polarization), and suggests that some portion may not be intrinsic to the SN. As a first attempt at disentangling the intrinsic SN polarization from the observed signal, we successfully fit a "Serkowski ISP Law" to the data that is consistent with the assumption that the SN light is intrinsically unpolarized at locations near the emission peaks of the H_alpha and H_beta P-Cyg profiles (and thus that any polarization observed in these regions is due to ISP; e.g., Chornock et al. 2010, ApJ, 713, 1363; Leonard et al. 2006, Nature, 440, 505). Removal of this ISP results in inferred intrinsic polarization for SN 2012aw in a wavelength-independent continuum polarization (p ~ 0.3%) with pronounced depolarizations across the P-Cyg profiles, similar to that seen in other SNe II-P found to be polarized at such early times (e.g., SN 2008bk, Leonard et al. 2012, ArXiv e-prints 1109.5406; SN 2004et, Leonard et al. 2009, BAAS, 213, 490.07). This intrinsic polarization is quite large for an SN II-P at such an early phase, and implies substantial asymmetries in the outer ejecta (and poses a challenge for existing models to reproduce at such early times; see Dessart & Hillier 2011).

We point out, however, that the derived ISP is somewhat surprising, since it is characterized by a rather large peak value (~0.8%; for an estimated total reddening towards SN 2012aw of approximately E(B-V) = 0.1 mag [Fraser et al. 2012, ApJL, submitted; arXiv:1204.1523], this inferred ISP implies a fairly large polarization efficiency for the interstellar dust) and somewhat atypical wavelength of maximum (~370 nm). While such peculiarities are not unheard of when considering supernova ISP (see, e.g., Patat et al. 2009, A & A, 508, 229, and references therein), this ISP estimate must certainly be verified by further observations and theoretical modeling, both to check the validity of the assumptions being made in its derivation as well as to investigate sources other than ISP as the cause of the observed polarization.

We note that optical photometry obtained at Mount Laguna Observatory's 1-m reflector confirms that SN 2012aw remains on a photometric plateau at visual wavelengths, with an estimated apparent brightness of R ~ 12.9 mag (calibrated against local field stars taken from the NOMAD catalog; Zacharias et al. 2005) at the epoch the spectropolarimetry data were taken.

Further spectropolarimetry observations from VLT are planned, and we gratefully acknowledge the effort of the entire observing staff at ESO-Paranal for the prompt execution of our spectropolarimetry ToO request. D.C.L. thanks the NSF for support through AST-1009571.