Enlarging habitable zones around binary stars in hostile environments

Abstract

Habitable zones are regions around stars where large bodies of liquid water can be sustained on a planet or satellite. As many stars form in binary systems with nonzero eccentricity, the habitable zones around the component stars of the binary can overlap and be enlarged when the two stars are at periastron (and less often when the stars are at apastron). We perform N-body simulations of the evolution of dense star-forming regions and show that binary systems where the component stars originally have distinct habitable zones can undergo interactions that push the stars closer together, causing the habitable zones to merge and become enlarged. Occasionally, overlapping habitable zones can occur if the component stars move further apart, but the binary becomes more eccentric. Enlargement of habitable zones happens to 1-2 binaries from an average initial total of 352 in each simulated star-forming region, and demonstrates that dense star-forming regions are not always hostile environments for planet formation and evolution.

Long-term stability of planets in and around binary stars

“…In previous work (Wootton & Parker 2019) we demonstrated that several binaries in each star-forming region could experience a hardening interaction that causes the two components to move closer together (especially at periastron). These simulations were of highly substructured (D = 1.6) regions with a population of field-like binaries.…”

“…Given that dynamical encounters can alter the orbits of binary systems, and that planets form early on in the star formation process, a significant fraction of binary systems that host stable planets could be made unstable by encounters in dense star-forming regions. In previous work (Wootton & Parker 2019) we have demonstrated that dynamical encounters can push close (< 10 au) binaries together, so that the size of the habitable zone around the binary system increases. In this paper, we examine the effects of dynamical encounters on the long-term stability of planets in and around binary systems to determine the change (if significant) in the fraction of binary systems that could host stable planets.…”

“…We also calculate any change in the habitable zone around binary systems, following the method outlined in Wootton & Parker (2019). This involves defining an isophote, i.e.…”

“…to the orbit of the binary as a result of dynamical interactions can cause the two stars to move closer together, increasing the size of the isophote-based habitable zone. In Wootton & Parker (2019) we performed this calculation for the binaries in the first suite of our N-body simulations (field binaries in a highly substructured [D = 1.6] region) and for comparison we will extend this analysis to the smooth (D = 3.0) star-forming regions and the simulations with the Kroupa (1995a) 'universal' initial binary population. The boundaries of the habitable zone around a binary are calculated using the following equation:…”

“…6. For comparison, we show via the faded symbols the results from the substructured regions first presented in Wootton & Parker (2019). In the substructured regions, a total of 354 binary systems across a suite of 20 statistically identical simulations had their isophote habitable zones enlarged, whereas for the smooth regions it is only 75 systems (again, across 20 simulations).…”