THE RUNTS OF THE LITTER: WHY PLANETS FORMED THROUGH GRAVITATIONAL INSTABILITY CAN ONLY BE FAILED BINARY STARS

By K. Kratter, R. Murray-Clay, A. Youdin. 2009

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Recent direct imaging discoveries suggest a new class of massive, distant planets around A stars. These widely separated giants have been interpreted as signs of planet formation driven by gravitational instability, but the viability of this mechanism is not clear-cut. In this paper, we first discuss the local requirements for fragmentation and the initial fragment mass scales. We then consider whether the fragment's subsequent growth can be terminated within the planetary mass regime. Finally, we place disks in the larger context of star formation and disk evolution models. We find that in order for gravitational instability to produce planets, disks must be atypically cold in order to reduce the initial fragment mass. In addition, fragmentation must occur during a narrow window of disk evolution, after infall has mostly ceased, but while the disk is still sufficiently massive to undergo gravitational instability. Under more typical conditions, disk-born objects will likely grow well above the deuterium burning planetary mass limit. We conclude that if planets are formed by gravitational instability, they must be the low-mass tail of the distribution of disk-born companions. To validate this theory, ongoing direct imaging surveys must find a greater abundance of brown dwarf and M-star companions to A stars. Their absence would suggest planet formation by a different mechanism such as core accretion, which is consistent with the debris disks detected in these systems.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/a49bd745396535db154cc403bc5defe14af67d28

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Citations in the corpus

Discovery and spectroscopy of the young jovian planet 51 Eri b with the Gemini Planet Imager (2015, unknown): “…imaged planets are almost all too bright for “cold-start” except for very specific accretion shock properties, though their formation is also difficult to explain through global instability which should operate preferentially at higher masses and large semi-major axis separation ( 18 , 19 ).”
Forming the cores of giant planets from the radial pebble flux in protoplanetary discs (2014, background): “However, planetary-mass objects are difficult too form at these orbital distances (Kratter et al. 2010) and numerical simulations show that it is difficult not to catastrophically disrupt multiple systems formed by self-gravity (Vorobyov 2013).”
Dynamics of Protoplanetary Disks (2010, background): “Kratter et al. (2010) simulated the evolution of protostellar disks formed from rapid infall, assuming that the disk behaves isothermally prior to fragmentation.”
The Close Binary Fraction of Solar-type Stars Is Strongly Anticorrelated with Metallicity (2018, methodology): “We begin with an expression for the disk midplane equilibrium temperature (see Kratter et al. 2008, 2010b), s t t = + +⎜ ⎟ ⎛ ⎝ ⎞ ⎠ ( )T F F 3 8 1 2 , 64 visc irrad where p = W˙ ( )F M3 4 , 7visc 2 * *s m p = ⎜ ⎟ ⎡ ⎣ ⎢⎢ ⎛ ⎝⎜ ⎞ ⎠⎟ ⎛ ⎝ ⎞ ⎠ ⎤ ⎦ ⎥⎥ ( )F k G M L r4 1 , 8birrad 3 1 7 2 7 3 7 4 * * = ˙ (…”
A COMBINED SUBARU/VLT/MMT 1–5 μm STUDY OF PLANETS ORBITING HR 8799: IMPLICATIONS FOR ATMOSPHERIC PROPERTIES, MASSES, AND FORMATION (2011, background): “The mass ratio and semimajor axis distribution of wide planets and low-mass brown dwarfs may help constrain the formation mechanism for HR 8799’s planets (Kratter et al. 2010).”
THE EXOPLANET CENSUS: A GENERAL METHOD APPLIED TO KEPLER (2011, unknown): “If this interpretation holds, it would add support to the core accretion hypothesis (Pollack et al. 1996), though this theory is not seriously challenged at masses below MJup and possibly much higher (Kratter et al. 2010).”
THE FORMATION OF LOW-MASS BINARY STAR SYSTEMS VIA TURBULENT FRAGMENTATION (2010, background): “…suggested that Electronic address: soffner@cfa.harvard.edu this process was limited to large mass ratio systems, recent work such as Stamatellos & Whitworth (2009) and Kratter et al. (2010a) has shown that when disks continue to be fed at their outer edges, the companions can grow substantially.”
Characterization of exoplanets from their formation - II. The planetary mass-radius relationship (2012, background): “The effective optical depth can be estimated as τeff = τ + 1 τ (Rafikov 2005), so that the cooling timescale τcool of a gas parcel is approximatively (Kratter et al. 2010) τcool = 3γadΣc2sτeff 32(γad − 1)σT 4mid .”
Clumps in the outer disk by disk instability: Why they are initially gas giants and the legacy of disruption (2009, result): “This has led to recent claims (Kratter et al., 2009) that, even initially, gas giant-mass clumps should be atypical, which is inconsistent with our analysis and our simulation data.”
TWO WIDE PLANETARY-MASS COMPANIONS TO SOLAR-TYPE STARS IN UPPER SCORPIUS (2010, background): “Another plausible formation scenario for wide ULMCs appears to be formation like stars from the fragmen- tation of protostellar clouds and massive circumstellar disks (e.g. Kratter et al. 2010).”
Gravitational Instabilities in Circumstellar Disks (2016, methodology): “In this case, simulations suggest that disk undergo strong m = 1 dominated transport, and in many cases fragmentation into stellar companions (Adams et al. 1989; Kratter et al. 2010a).”
Characterization of exoplanets from their formation - I. Models of combined planet formation and evolution (2012, background): “But these discoveries, like the planetary system around HR 8799 (Marois et al. 2008, 2010), have triggered numerous theoretical studies regarding the formation of these objects (e.g., Dodson-Robinson et al. 2009; Kratter et al. 2010).”
PLANETS AROUND LOW-MASS STARS (PALMS). IV. THE OUTER ARCHITECTURE OF M DWARF PLANETARY SYSTEMS (2014, background): “…collapse of a massive protoplanetary disk has consumed much of the discussion about planet formation for the past decade, especially after the discovery of the HR 8799 planets (e.g., Boss 2007; Durisen et al. 2007; Boley 2009; Dodson-Robinson et al. 2009; Nero & Bjorkman 2009; Kratter et al. 2010).”
THE TW Hya DISK AT 870 μm: COMPARISON OF CO AND DUST RADIAL STRUCTURES (2011, background): “If true, gas densities at large disk radii may be higher than typically assumed, with profound implications for facilitating giant planet formation by gravitational instability (e.g., Boley 2009; Kratter et al. 2010; Boss 2011).”
ORBITAL MOTION OF HR 8799 b, c, d USING HUBBLE SPACE TELESCOPE DATA FROM 1998: CONSTRAINTS ON INCLINATION, ECCENTRICITY, AND STABILITY (2011, background): “The formation mechanisms for these planets either using gravitational instability or core accretion is an active topic of research (Rafikov 2005; Kratter et al. 2010; Boss 2011; Currie et al. 2011a).”