Stars with Plumbing Issues: The Formation of Collimated Outflows on Common-Envelope Simulations and Comparison to Water Fountains Observations

Abstract

Common-envelope evolution (CEE) is one of the biggest open questions in binary stellar evolution, despite being the main channel for the formation of close binaries. One of the main reasons CEE is difficult to model is the lack of direct observations that could constrain numerical simulations. One exception is luminous red novae, which are thought to represent CEEs that end in mergers. Unfortunately, there are no confirmed direct detections of ongoing events that result in the survival of a close binary, and we must rely on observations of post-CEE systems. Among these, planetary nebulae (PNe) are particularly important because their morphologies can probe how the envelope is ejected. However, post-CEE PNe do not reflect the ejected envelope in its pristine form, as winds from the central core also affect their morphology. In this context, Water Fountains (WFs), a class of objects proposed to form during CEE, provide an ideal comparison. They are identified by their collimated water masers, and most are still in the post-AGB phase. As such, WFs provide some of the best observational constraints for simulations, since they likely capture a snapshot of the envelope ejection while it is still happening. In this paper, we show that the formation of a circumbinary disk with collimated outflows surrounding the central binary arises naturally from hydrodynamical simulations of CEE, and that their morphology and kinematics are consistent with observations of WFs. We also present insights into how the properties of WFs may provide clues to understanding how CEE proceeds and help guide future simulations.

Figures (13)

• Figure 1: Density distribution for a $2~\text{M}_\odot$ RGB star in a binary system with mass ratio $q=0.5$. Top: Q050 run, HLLC Riemann solver; bottom: Q050-HLL run, HLL Riemann solver. Each panel shows both $y$-$x$ and $y$-$z$ views at times $t = 40.3$, $100.7$, $171.1$, $211.4$, $302$, and $503.4$ days. Each pannel have the $\epsilon$ at that given time. The cyan '$\times$' marks the center of mass of the core+companion duo.
• Figure 2: Envelope mass distribution per $\cos(\theta)$ for a radius of 700 $R_{\odot}$ around the center (which is the size of the box of Fig. \ref{['fig:projection_plots']}) of mass of core+companion due for the 2 M$_\odot$ RGB star, q = 0.5, at 40 (blue), 300 (red) and 800 (orange) days. Left panel: Q050 run, HLLC solver. Right panel: Q050-HLL run, HLL solver.
• Figure 3: Evolution of the unbound fraction (red) and orbital separation (blue) for a $2~M_{\odot}$ RGB star with $q=0.5$. Solid and dashed lines represent simulations using the HLLC (Q050 run) and HLL (Q050-HLL run) Riemann solvers, respectively. The markers 'x' and '*' indicate the times at which $\epsilon=0.01$, $0.001$, and $0.0001$ are reached for the HLLC and HLL runs, respectively.
• Figure 4: Left: Density distribution for a $2\,\text{M}_\odot$ RGB star in a binary system with mass ratio $q=0.5$ in the $y$--$z$ plane at $t=402.7$ days. The cyan circle marks the spherical slice with radius $100\,R_\odot$ around the center in which the radial-velocity Gaussian is calculated over $\theta$ (see Sec. \ref{['sec:outflows']}). The red lines mark the slice of the cylinder with radius $300\,R_\odot$ in which the density Gaussian is calculated over $z$ (see Sec. \ref{['sec:density']}). Right: Gaussian fits for the velocity distribution over $\theta$ (top) and the density distribution over $z$ (bottom).
• Figure 5: Left: Disk height $H$ as a function of radius for the runs Q025, Q050, Q075, Q100, and Q050-NCR at $t=500$ days (faint lines), together with Q050-T100 at $t=500$ and $1500$ days (red and blue bright lines, respectively). The black dashed line shows the best linear fit to the Q050-T100 profile at $t=500$ days. Right: Disk thickness $H/R$ as a function of radius for the same runs.