The Type IIn supernova 1994W: evidence for the explosive ejection of a circumstellar envelope

Abstract

We present and analyse spectra of the Type IIn supernova (SN) 1994W obtained between 18 and 203 d after explosion. During the luminous phase (first 100 d) the line profiles are composed of three major components: (i) narrow P‐Cygni lines with the absorption minima at −700 km s−1; (ii) broad emission lines with blue velocity at zero intensity ∼4000 km s−1; and (iii) broad, smooth wings extending out to at least ∼5000 km s−1, most apparent in Hα. These components are identified with an expanding circumstellar (CS) envelope, shocked cool gas in the forward post‐shock region, and multiple Thomson scattering in the CS envelope, respectively. The absence of broad P‐Cygni lines from the SN is the result of the formation of an optically thick, cool, dense shell at the interface of the ejecta and the CS envelope. Models of the SN deceleration and Thomson scattering wings are used to recover the density (n≈ 109 cm−3), radial extent [∼(4–5) × 1015 cm] and Thomson optical depth (τT≳ 2.5) of the CS envelope during the first month. The plateau‐like SN light curve is reproduced by a hydrodynamical model and is found to be powered by a combination of internal energy leakage after the explosion of an extended pre‐SN (∼1015 cm) and subsequent luminosity from CS interaction. The pre‐explosion kinematics of the CS envelope is recovered, and is close to homologous expansion with outer velocity ∼1100 km s−1 and a kinematic age of ∼1.5 yr. The high mass (∼0.4 M⊙) and kinetic energy (∼2 × 1048 erg) of the CS envelope, combined with low age, strongly suggest that the CS envelope was explosively ejected ∼1.5 yr prior to the SN explosion.

Numerical simulations of superluminous supernovae of type IIn

“…We believe this hybrid line formation is at the origin of the very large Hα line width in SN 2010jl, as well as the blueshift of its peak emission. Interestingly, the modest peak luminosities and the absence of broad lines at all times in events like SN 1994W (Chugai et al 2004;Dessart et al 2009) or SN 2011ht (Humphreys et al 2012) are perplexing because the emission from the CDS should be seen once the CSM becomes transparent.…”

“…This has been discussed in the past for the bolometric luminosity or the colours (see, e.g., Chugai et al 2004;Woosley et al 2007;van Marle et al 2010;Moriya et al 2013b). Here, we repeat such explorations to investigate the impact on spectral signatures as well.…”

“…It has also revealed an intriguing diversity, spanning a range of luminosities, duration, line profile morphology, with events like SNe 1988Z (Stathakis & Sadler 1991;Turatto et al 1993), 1994W (Sollerman et al 1998;Chugai et al 2004), 2006gy , and the enigmatic SN 2009ip (Mauerhan et al 2013;Pastorello et al 2013;Margutti et al 2014). The origin of Type IIn SNe is associated with the interaction of fast material with previously-expelled slowly-expanding material (Grasberg & Nadezhin 1986).…”

“…It is either based on the results of radiation hydrodynamics simulations but limited to a single line, e.g., Hα (Chugai et al 2004), or based on an ad-hoc atmospheric structure and the assumption of steady-state radiation through a diffusive optically thick inner boundary (Dessart et al 2009).…”

“…Numerical simulations of interacting SNe are generally undertaken with radiation hydrodynamics (Chugai et al 2004;Woosley et al 2007;Moriya et al 2013b;Whalen et al 2013), hydrodynamics combined with a parameterised cooling function (van Marle et al 2010;Chen et al 2014), or pure hydrodynamics (by assuming adiabaticity; see, e.g., Blondin et al 1996). When studying super-luminous SNe IIn, radiation hydrodynamics with a non-grey (multi-group) treatment is a must because the radiation contribution is a sizeable fraction of the pressure/energy in the problem, the characteristic temperatures of the radiation and of the gas may differ strongly, and the emerging radiation provides essential signatures characterising the event.…”