From dust to planets -- II. Effects of wide binary companions and external photoevaporation on planetesimal and embryo formation

Abstract

More than half of Solar-type stars are found in binary systems. The numbers of exoplanets within binary systems in s-type orbits now numbers over 700. However, whilst the numbers have increased, there still does not exist a global model of planet formation for wide binary systems, where there does for single stars and circumbinary systems. As a precursor to such a model, that includes the necessary physical planet formation processes, it is important to understand how an outer binary companion affects the evolution of circumstellar discs, and the formation of planetesimals and planetary embryos. The main mechanism for which these processes are affected, is through truncation of the protoplanetary disc outer edges. In this paper, we determine these effects, whilst also comparing them to the effects of external photoevaporation that competes to truncate protoplanetary discs. We find that disc truncation by both a binary companion and external photoevaporation significantly reduces the efficiency to which planetary embryos are able to accrete pebbles and grow into terrestrial mass planets. This is due to the pebble supply being cut off as the pebble production front reaches the disc outer edge before planets are able to significantly increase in mass. This hindrance to planet formation occurs when the truncation radius due to the binary companion is below $\sim 30$ au, corresponding to binary separations of $\sim90$ au for equal mass, circular binary stars. For separations greater than 300 au, planet formation operates similar to that around single stars. Our results highlight the detrimental effects of a binary companion for intermediate binaries, that can provide possible explanations for the dearth of multiple planets within binary systems of separations $<100$ au

Figures (12)

• Figure 1: Temporal evolution of gas surface densities for viscous discs truncated by an outer companion at distances of $20 \, {\rm au}$ (left-hand panel), $100 \, {\rm au}$ (middle panel), and $500 \, {\rm au}$ (right-hand panel). All discs evolved in a $100 \rm G_0$ environment with the viscous parameter $\alpha=10^{-3}$. Note that the line colours correspond to different simulation times in each panel (with the respective times shown in the panel legends), due to the differences in disc lifetimes.
• Figure 2: Temporal evolution of gas surface densities for viscous discs in different UV environments of $100 \rm G_0$ (left-hand panel), $10^3 \rm G_0$ (middle panel), and $10^4 \rm G_0$. The discs were all initially truncated by an outer companion to $200 \, {\rm au}$, whilst $\alpha = 10^{-3}$. Note that the line colours correspond to different simulation times in each panel (with the respective times shown in the panel legends), due to the differences in disc lifetimes.
• Figure 3: Temporal evolution of the disc outer radius for all of the discs shown in Figs. \ref{['fig:disc_trunc']} and \ref{['fig:disc_g0']}. The black dashed line shows the pebble production front over time.
• Figure 4: Pebble cutoff time as a function of the UV environment. The truncation radius due to the outer companion is highlighted by the different colours showing $r_{\rm t}=10\, {\rm au}$ (blue), $r_{\rm t}=20\, {\rm au}$ (red), $r_{\rm t}=50\, {\rm au}$ (yellow), $r_{\rm t}=100\, {\rm au}$ (purple) and $r_{\rm t}=250\, {\rm au}$ (green). The strength of the viscous turbulent parameter $\alpha$ is shown by the different line styles with $\alpha=10^{-3}$ shown by solid lines, $\alpha=10^{-3.5}$ by dashed lines, and $\alpha=10^{-4}$ by the dotted lines.
• Figure 5: Pebble cutoff time as a function of the truncation radius due to the outer companion. Diferent UV field strengths are highlighted by the different colours showing $10\rm G_0$ (blue), $10^2\rm G_0$ (red), $10^3\rm G_0$ (yellow), $10^4\rm G_0$ (purple) and $10^5\rm G_0$ (green). The strength of the viscous turbulent parameter $\alpha$ is shown by the different line styles with $\alpha=10^{-3}$ shown by solid lines, $\alpha=10^{-3.5}$ by dashed lines, and $\alpha=10^{-4}$ by the dotted lines..