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\begin{document}

\title{Pre-SN Mass-loss
%\thanks{Grants or other notes
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\author{Matteo Cantiello \and Raffaella Margutti \and Jeremiah Murphy}

\institute{Matteo Cantiello \at Affiliation not available \and Raffaella Margutti \at Affiliation not available \and Jeremiah Murphy \at Affiliation not available}

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\begin{abstract}
Our goal is to reconstruct pre-SN mass-loss history using radio variability observations and constrain the dominant mass-loss mechanism during the last few hundred years of massive star evolution. We will draw from existing observations to re-design the strategy for future observing campaigns.%
\end{abstract}%




\section{Introduction}

Observations are painting a complex picture of massive stars at the end of their lives. Contrary to expectations, massive stars have been found to experience major eruptions in the years preceding their explosion as supernovae (SNe). This sequence of eruptions was \emph{not predicted} on theoretical grounds and is \emph{not explained} by our current understanding of the physical mechanisms that drive the mass loss in evolved massive stars \cite{2014ARA&A..52..487S}. 

In this respect, two key observational findings are relevant: (i) The direct detection of luminous precursors in the month before the 
major explosion of the H-rich SN2009ip associated with the ejection of $\sim0.1\,\rm{M_{\odot}}$ of material \citep{2014ApJ...780...21M}. (ii) Evidence for significant modulations in the radio light-curves of hydrogen-stripped SNe  \citep[e.g. SNe Ib/c;][]{2012ApJ...752...17W}. 
%(i.e. SNe Ib/c; Wellons et al., 2012, Milisavljevic et al., 2013). 
This latter finding indicates that a complex environment, sculpted and enriched by a significantly \emph{non-steady} mass loss of the progenitor system in the years before the explosion, surrounds some Type Ib/c SNe.

The two findings above suggest that massive stars might lose their hydrogen envelopes through eruptive mass ejections (instead of steady winds, as it has been assumed so far) on time scales of months to decades before the core-collapse (i.e. much shorter than previously thought). To test this idea here we propose a focused collaborative effort to probe the life of massive stars in the last  centuries before death. Our goal is to reconstruct the density profile and mass-loss history of core-collapse SNe (i) using the entire sample of existing radio observations, (ii) treating the dynamics of the SN shock interaction with the complex medium in a self-consistent way and (iii) by finally connecting our findings to the physics that regulates the time-variable mass loss in massive stars.\selectlanguage{english}
\begin{figure}[h!]
\begin{center}
\includegraphics[width=1.00\columnwidth]{figures/mdotplot/mdotplot}
\caption{{\label{fig} The density profile around a nearby Ib SN as probed by recent radio observations (black solid line) [i] violates the expectations from the standard theory of line driven winds (orange line), [ii] requires a different physical explanation and [iii] motivates a deep and rigorous search for density enhancements in the environment of core-collapse SNe.  The overarching goal of the project is to provide a state-of-the art view of our current understanding of mass-loss from massive stars. Using existing data we will design the strategy of future follow up in the non-thermal part of the spectrum based on the parts of the parameter space that has not been sampled so far.%
}}
\end{center}
\end{figure}

\section{Steady vs Eruptive Massloss}
To help constrain mass-loss theory, we first need to understand the mass-loss history of massive stars.  In particular, we need to know the mass-loss rate ($\dot{M}$) and velocity as a function of time before explosion.
 
The mass loss rates during the last few hundred years of evolution of some core collapse supernova progenitors seem to violate the maximum values allowed by line-driven winds ($\dot{M} \sim 10^{-4}$M$_\odot$yr$^{-1}$, \cite{Smith_2006}, see also Fig.~\ref{fig}). 

Intense stellar mass loss during the final years before core collapse could be caused by 
internal gravity waves excited by core convection during Neon and Oxygen fusion \cite{Quataert_2012}. This mechanism is able to explain  mass loss rates of $\sim 0.01 - 1 $M$_\odot$yr$^{-1}$ and velocities of $\sim 200 - 500$ km s$^{-1}$  (Quataert et al. 2015). Most importantly, the model predicts a correlation between the energy associated with pre-SN mass ejection and the time to core collapse, with the most intense mass loss preferentially occurring closer to core collapse \cite{Shiode_2013}. 

Binary interaction during Roche-lobe overflow (RLOF) or common envelope (CE) phase (e.g. Podsiadlowski et al. 1992) could also cause enhanced, irregular mass loss.
The binary interaction and the stellar death of one of the two stars are however not necessarily related events, so that we would not generally expect the two phenomena to be synchronized.
The expected rate of stars exploding during or shortly after the onset of binary interaction is small ($< 5$\%, De Mink, Priv. Comm.) and no correlation is expected between mass-loss rate and time to core collapse. 



 
%This energy can generate circumstellar environments with 10-3--1 solar masses reaching 100 AU before %explosion. We predict a correlation between the energy associated with pre-SN mass ejection and the %time to core collapse, with the most intense mass loss preferentially occurring closer to core collapse.
%\begin{itemize}
%\item \textbf{Binary Interactions}
%The ejection of the H-rich envelope can be the result of binary interaction during Roche-lobe overflow (RLOF) or common envelope (CE) phase (e.g. Podsiadlowski et al. 1992). Since the observed fraction of $\sim37\%$ of H-poor SNe among core-collapse explosions is too large to be reconciled with the evolution of single massive stars (e.g. Li et al. 2011, Smith et al. 2011), it is reasonable to expect that at least some SNe Ibc originate in binaries. The binary interaction and the stellar death of one of the two stars are however not necessarily related events, so that we would not generally expect the two phenomena to be synchronized. A stellar explosion can be driven by the CE evolution only under peculiar circumstances, like the inspiral of a compact object to the central core of the companion star (Chevalier 2012). As a result, simulations show that only a small fraction $\sim2-3$\% of SNe Ib/c progenitors to experience the ejection of the H envelope due to binary interaction in the last centuries before explosion (Chevalier 2012). 

%\item \textbf{Eruptive Massloss}
%\item \textbf{Pulsationally Enhanced Massloss}
%\end{itemize}

\section{Radio Observations as Probes of Pre-SN Mass-loss}
The radio emission in SNe is powered by synchrotron emission due to shock-accelerated relativistic electrons gyrating in the post-shock magnetic field. The synchrotron self-absorption (SSA) or free-free absorption (FFA) at low frequencies results in a spectral turnover. The evolution of the spectral peak frequency and luminosity directly depend on the density of the environment and radius of the blast-wave. Thus, by monitoring the evolution of the radio emission resulting from the SN shock interaction with the medium, it is possible to map the density of the environment sampled by the shock. Since the SN shock is typically $\geq 30$ times faster than the progenitor stellar wind, the radio emission from a SN shock effectively acts as a time machine and allows us to sample the pre-SN mass-loss history of the stellar progenitor in the centuries before the explosion. X-rays and radio emission from the SN shock interaction with the medium are currently the \emph{only} probes of this phase of evolution of evolved stars. We focus on the radio wavelength range as it offers the most homogeneous data set at the time of writing.

Motivated by the current discovery of strong interaction in an otherwise ordinary Ib SN at late times ($t>100$ days), here we propose the analysis of the entire data set of radio observations of core-collapse SNe and the self-consistent hydrodynamical modeling of the emission. The primary goal is to map the density and mass-loss history of evolved massive stars in the centuries before explosion and to connect these findings to the nuclear burning history of the progenitor star.

\section{Modeling the Supernova Shock-CSM Interaction}

The variable radio emission from core-collapse SNe encodes but does not directly yield the mass-loss history.  To infer the mass-loss history, we must first model the radio emission from a supernova shock overtaking the CSM.  For relatively simple shock-CSM interactions, \cite{Chevalier_1982} and \cite{Chevalier_1989} developed analytic solutions and set the language for shock-CSM interactions.  However, it is very likely that the mass-loss history is more complicated with multiple ejection events including some that are aspherical.  Therefore, we will need to model the shock-CSM interations with numerical simulations.


\section{Conclusions}
\begin{itemize}
Understanding the pre-explosion eruption is a major topic of the Scialog 2015 meeting, and one of the emerging themes is to measure the mass-loss history before explosion.  We propose to measure the mass-loss history for a large sample of core-collapse SNe.  Raffaella Margutti will orchestrate the observations, Jeremiah Murphy will simulate the shock-CSM interaction, and Matteo Cantiello will use these results to constrain mass-loss in stellar evolution theory. 
\end{itemize}

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