Binary companions triggering fragmentation in self-gravitating discs

Abstract

Observations of systems hosting close in (<1 AU) giant planets and brown dwarfs (M ≳ 7 MJup) find an excess of binary star companions, indicating that stellar multiplicity may play an important role in their formation. There is now increasing evidence that some of these objects may have formed via fragmentation in gravitationally unstable discs. We present a suite of 3D smoothed particle hydrodynamics (SPH) simulations of binary star systems with circumprimary self-gravitating discs, which include a realistic approximation to radiation transport, and extensively explore the companion’s orbital parameter space for configurations which may trigger fragmentation. We identify a ‘sweet spot’ where intermediate separation binary companions (100 AU ≲ a ≲ 400 AU) can cause a marginally stable disc to fragment. The exact range of ideal binary separations is a function of the companion’s eccentricity, inclination and mass. Heating is balanced by efficient cooling, and fragmentation occurs inside a spiral mode driven by the companion. Short separation, disc penetrating binary encounters (a ≲ 100 AU) are prohibitive to fragmentation, as mass stripping and disc heating quench any instability. This is also true of binary companions with high orbital eccentricities (e ≳ 0.75). Wide separation companions (a ≳ 500 AU) have little effect on the disc properties for the setup parameters considered here. The sweet spot found is consistent with the range of binary separations which display an excess of close in giant planets and brown dwarfs. Hence we suggest that fragmentation triggered by a binary companion may contribute to the formation of these substellar objects.

Companion mass limits for 17 binary systems obtained with binary differential imaging and MagAO/Clio

“…Fontanive et al (2019) found an inflated binary fraction of 80% with separations from 20-10,000 AU for stars hosting close in higher-mass planetary and brown dwarf companions (7-60 M Jup ). Cadman et al (2022) showed that the binary companions can trigger instability and fragmentation in gravitationally unstable disks, leading to formation of these giant planet and brown dwarf companions in outer regions of the disk, which somehow move to the close-in orbits currently observed. However other studies have concluded that the frequency of planets in binaries is not statistically different from that of single stars (e.g.…”

DMPP-3: confirmation of short-period S-type planet(s) in a compact eccentric binary star system, and warnings about long-period RV planet detections

“…Despite the often disastrous influence close-in eccentric binary companions are understood to have on planet formation (exemplified by the dearth of planets found in binaries with separation 10 au seen in Fig. 1), the occurrence rate for hot Jupiter planet hosts having a stellar companion is twice as high as the binarity ratio of field stars with projected separations 20 -10000 au (Cadman et al 2022). A secondary star can modify the projected spin-orbit angle between primary star and planet (observed through measuring the Rossiter-McLaughlin effect), exciting secular evolution and interactions that cause a departure from coplanarity for all orbiting bodies (Martin & Lubow 2018;Franchini et al 2020;Moe & Kratter 2021;Best & Petrovich 2022).…”

Visual Orbits and Alignments of Planet-hosting Binary Systems

“…Theoretical studies show that a close stellar companion can impact planets through the truncation or misalignment of the protoplanetary disk (Artymowicz & Lubow 1994;Kraus et al 2012;Martin et al 2014), the formation and migration of gas giant planets (Dawson & Johnson 2018;Fontanive et al 2019), and the scattering of planets in unstable triple-star systems (Thebault & Haghighipour 2015). For example, recent simulations of protoplanetary disks around the primary star in wide binary systems (with separations a = 100-400 au) often result in the disk fragmentation needed to form giant planets (Cadman et al 2022). Modeling also predicts that the shape and size of the companion's orbit can play a significant role in planet formation, such that close, eccentric, or highly inclined companions could hinder planet formation (Holman & Wiegert 1999;Quintana et al 2002;Jang-Condell 2015;Cadman et al 2022).…”

“…For example, recent simulations of protoplanetary disks around the primary star in wide binary systems (with separations a = 100-400 au) often result in the disk fragmentation needed to form giant planets (Cadman et al 2022). Modeling also predicts that the shape and size of the companion's orbit can play a significant role in planet formation, such that close, eccentric, or highly inclined companions could hinder planet formation (Holman & Wiegert 1999;Quintana et al 2002;Jang-Condell 2015;Cadman et al 2022).…”