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Dark Collapse of Gas Giant Planets

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Dark Collapse of Gas Giant Planets DARK COLLAPSE OF GAS GIANT PLANETS Sergei Nayakshin, University of Leicester Seung-Hoon Cha, Mark Fletcher “GI works for far away planets only” CA (Core Accretion) <— GI (Gravitational Instability)—> ✤ Gammie 01, Rice + 05, Rafikov 05: fragmentation at a > 50 AU or so ✤ Meru (talk here) a ~ 20 AU fragmentation GI PLANET MIGRATION 1. GI gives birth to fragments in the outer disc 2. Fragments migrate inward in ~ 10 orbits Boley+ 2010 Cha & Nayakshin 11 –34– Vorobyov & Basu 06 Fig. 2.— ConfigurationMachida of protostellar outflow + at t c =11 843 yr is shown by yellow volume, in which color indicates outflow speed. The density distributions (colors) are projected on each wall surface. The velocity vectors (arrows) on the equatorial plane are plotted on bottom surface. The magnetic field lines integrated from each planet are plotted by yellow lines. The box size is 100 AU. Migration of GI planets may explain all giant planets, including hot Jupiters Giant planets need ~ 1 Myr to cool ✤ GI fragments are basically First Cores (Larson 69, Masunaga & Inutsuka 00) 4 S. Nayakshin PTEP 2012,01A307 S.Inutsuka ✤ Fragments must heat up to ~ 2000tions K mayto becollapse the origin of or ”hot” else sub-giant they planets are such disrupted as hot Super-Earths or hot Neptunes. Another point worth noting from figure 1 is that the as- a = 10 AU trophysical metal components of the protoplanet (that is, all ] 1000 elements heavier than H and He) can settleEffective into the centre Ratio of Specific Heats of it provided that condensation temperature of the compo- Jupiter γ =1.1 < γ =4/3 a = 1 AU nents is higher than the central temperature of theeff planet. eff 100 In this paper we limit our attention to three dominant grain species: water ice, organics called CHON, and a mix of Fe Second and silicates (cf. 4.3 below). The condensation tempera- Core § o tures for these species are marked approximately by the po- 10 a = 0.05 AU = 7/5 sition of the respective text in the bottom panelγeff of figure 1. Second Collapse Planet Radius [R Note that the planet spends little time in the cold config- uration, since it cools relatively rapidly initially. From this 1 one can expect water to be the least able to condense down in TD planets, whereas Fe and silicates be the most able to Dissociation of H2 Fe and silicates γeff = 5/3 do so (this agrees with results of Forgan & Rice 2013b, that Ebind = 4.48 eV 1000 TD cores are mainly composedFirst of rocks Collapse ). Grain sedimentation may lead to core formationFirst in- Core side of the protoplanetdense (e.g., McCrea & Williams 1965; CHON (organics) Boss 1997; Helledcore & Schubert 2008). If the gaseous com- Downloaded from ponent of the protoplanet is then disrupted by the tides, the nearly “naked” core survives, since its density is much Temperature [K] Water Ice higher. Such disruptions may be an alternative to CA origin for terrestrial-likeFig. 1. Temperature planets (Kuiper evolution 1951; at the Boley center et of al. the 2010; gravitationally collapsing cloud obtained by Masunaga and 100 Nayakshin 2010a). http://ptep.oxfordjournals.org/ Inutsuka in their radiation hydrodynamical calculation of protostellar collapse in spherical symmetry [19]. The 0.0 0.5 1.0 1.5 2.0 first collapse phase corresponds to the formation of the first core,whichconsistsmainlyofhydrogenmolecules. time [Myr] Inutsuka 2012 The dissociation of hydrogen molecules triggers the second collapse,whicheventuallyproducesthesecond 2.3 TDcore modelor a protostar. possible Each outcomes phase of the temperature evolution is characterized by the effective ratio of specific Figure 1. Radiative cooling of an isolated Jupiter mass gas giant heats, γeff. planet. The planet is coreless and of Solar composition. The upper It is clear that the fate of a gas fragment formed in the outer panel shows planet’s radius (black curve). The three horizontal disc by GI is sealed (Boley et al. 2010; Forgan & Rice 2013b) lines depict the planet’s Hills radius if the planet were in orbit by the ratios of the various time scales: tcoll,theplanet’scon- around a 1 M mass star at distances indicated just above the tractionρ and1g/cc, collapse and time the second scale; tmigr adiabatic, the planet’s core or migra- a protostar forms. Further periods of evolution include a by guest on June 27, 2015 ∼ lines. The lower panel shows evolution of the central temperature tion timeT-Tauri scale; phase and tsed on, the the grain Hayashi growth track, and the sedimentation timescale of which is about two orders of magnitude longer of the planet. During the first Million years, the planet is dom- time scale. Qualitatively, there are several possibilities:5 inated by molecular hydrogen. The three di↵erent grain species than the dynamical timescale ( 10 year) of protostellar collapse, which is only accessible by steady ∼ considered here condense out below temperatures indicated ap- (i) Ifstate the calculations migration time (see, is the e.g., shortest the review of the by three, Chabrier the and Baraffe [21]). The time evolution of the SED proximately by the location of the species name in the panel. fragment is disrupted with essentially nothing remaining of it; obtained from the self-consistent RHD calculation is also shown in Ref. [19]. Molecular emission (ii) lineif tsed profiles<tmigr of<t variouscoll, then important grains sediment species have down also be- been calculated in self-consistent dynamical mod- 2.2 Tidal disruption of protoplanets fore theels planet [22]. is Recent disrupted. 3D modeling There is therefore of protostellar a remnant, radiation hydrodynamics can be found, e.g., in Refs. The key point to observe from the top panel of figure 1 a solid core (Boley et al. 2010), possibly surrounded by a [9–12,23–26]. is just how extended the planet is at birth (Rp 1AU, post-disruption atmosphere remaining bound to the core if ⇠ or about 2000RJ ), and that it takes over a Million years the latter is massive enough; for the planet to contract to radii comparable to that of (iii) If tcoll <tmigr, then the planet collapses before it is Jupiter. The red solid horizontal lines show the Hill’s ra- disrupted, and survives as as a gas giant planet, provided it dius, RH , of the planet at three distances from a M =1M is not3. disrupted Resistive in the magnetohydrodynamics ”hot” region or pushed all the of way protostellar collapse 2 ⇤ into the star. host star. When Rp is larger when RH , tidal forces from In this section and the following sections we mainly explain the results of resistive magnetohydro- the star exceed self-gravity of the planet, and it can be dis- (iv) If grain sedimentation time is the shortest of the rupted, as in the original Kuiper (1951) scenario. Figure 1 three,dynamical and the core simulations mass is significant, in the seriesMcore of papersa few by to Machida et al. [27–34,53] ⇠ thus shows that the planet can be tidally disrupted at the ten or more Earth masses, then the grain component can planet-star separations 1 < a < 10 AU in the pre-collapse a↵ect the whole fragment by either triggering its collapse stage. In addition, the planet⇠ can⇠ be tidally disrupted in the (Nayakshin3.1. et Basic al. 2014) equations or destruction (Nayakshin & Cha post-collapse stage in the innermost ”hot” region of the disc, 2012). a < 0.1 AU, provided the collapse happened very recently In the early phase of protostellar collapse, the gas remains mostly neutral, since ionization is mainly (no⇠ more than 105 years earlier). Nayakshin (2011c); Clearly,due to it cosmic is essential rays that to be provide able to a calculate small ionization not only degree in molecular density. The timescale of col- ⇠ Nayakshin & Lodato (2012) proposed that such hot disrup- these three time scales but also the detailed planetary struc- ture aslisional well as the coupling protoplanetary between disc charged evolution. particles The planet and neutral particles is relatively short compared with formation/destructionthe self-gravitational road map collapse outlined timescale. above Therefore, is only a the dynamics of weakly ionized gas in the proto- 2 this condition may be even stricter for young rotating planets, very rough guess as to what may actually happen in a real- > stellar collapse calculation can be described by a 1-fluid resistive MHD equation based on the strong e.g., Rp 0.5RH may be sufficient for disruption, see 4.5 below istic disc and planet setting. ⇠ § c 2008 RAS, MNRAS 000,1–21 5/25 1110 FISCHER & VALENTI Vol. 622 TABLE 3 Stars with Uniform Planet Detectability Star ID Planet/Star HD 142 ............................... P HD 2039 ............................. P HD 4203 ............................. P HD 8574 ............................. P HD 10697 ........................... P Note.—Table 3 is published in its entirety in the electronic edition of the Astrophysical Journal. A portion is shown here for guidance regarding its form and content. more than 100 metal-poor stars (Mayor et al. 2003). In addition, a Fig. 5.—Same results as Fig. 4, but divided into 0.1 dex metallicity bins. The Doppler survey of 150 low-metallicity stars has been underway increasing trend in the fraction of stars with planets as a function of metallicity is at Keck for the past two years (Sozzetti et al. 2004). No planets well fitted with a power law, yielding the probability that an FGK-type star has a 2:0 gas giant planet: (planet) 0:03 (NFe=NH)=(NFe=NH) . have been announced from either of these surveys, suggesting P ¼ ½ that the rate of occurrence of Jovian-mass planets with orbital periods less than 3 yr does not exceed (and is likely lower than) often as they orbit solar-metallicity stars, it seems very likely that afewpercentaroundmetal-poorstars.
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