Browsing by Author "Owen, James E."

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    Close-in Super-Earths: The first and the last stages of planet formation in an MRI-accreting disc
    Janković, Marija; Owen, James E.; Mohanty, Subhanjoy
    We explore in situ formation and subsequent evolution of close-in super-Earths and mini-Neptunes. We adopt a steady-state inner protoplanetary gas disc structure that arises from viscous accretion due to the magneto-rotational instability (MRI). We consider the evolution of dust in the inner disc, including growth, radial drift, and fragmentation, and find that dust particles that radially drift into the inner disc fragment severely due to the MRI-induced turbulence. This result has two consequences: (1) radial drift of grains within the inner disc is quenched, leading to an enhancement of dust in the inner regions that scales as dust-togas-mass-flux-ratio at ∼1 au; (2) however, despite this enhancement, planetesimal formation is impeded by the small grain size. Nevertheless, assuming that planetary cores are present in the inner disc, we then investigate the accretion of atmospheres on to cores and their subsequent photoevaporation. We then compare our results to the observed exoplanet mass-radius relationship. We find that (1) the low gas surface densities and high temperatures in the inner disc reduce gas accretion on to cores compared to the minimum mass solar nebula, preventing the cores from growing into hot Jupiters, in agreement with the data; (2) however, our predicted envelope masses are still typically larger than observed ones. Finally, we sketch a qualitative picture of how grains may grow and planetesimals form in the inner disc if grain effects on the ionization levels and the MRI and the back reaction of the dust on the gas (both neglected in our calculations) are accounted for.
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    Inside-out Planet Formation. V. Structure of the Inner Disk as Implied by the MRI
    Mohanty, Subhanjoy; Jankovic, Marija R.; Tan, Jonathan C.; Owen, James E.
    The ubiquity of Earth- to super-Earth-sized planets found very close to their host stars has motivated in situ formation models. In particular, inside-out planet formation is a scenario in which planets coalesce sequentially in the disk, at the local gas pressure maximum near the inner boundary of the dead zone. The pressure maximum arises from a decline in viscosity, going from the active innermost disk (where thermal ionization yields high viscosities via the magnetorotational instability [MRI]) to the adjacent dead zone (where the MRI is quenched). Previous studies of the pressure maximum, based on α-disk models, have assumed ad hoc values for the viscosity parameter α in the active zone, ignoring the detailed MRI physics. Here we explicitly couple the MRI criteria to the α-disk equations, to find steady-state solutions for the disk structure. We consider both Ohmic and ambipolar resistivities, a range of disk accretion rates (10−10–10−8 M⊙ yr−1), stellar masses (0.1–1 M⊙), and fiducial values of the non-MRI α-viscosity in the dead zone (αDZ = 10−5 to 10−3). We find that (1) a midplane pressure maximum forms radially outside the dead zone inner boundary; (2) Hall resistivity dominates near the inner disk midplane, perhaps explaining why close-in planets do not form in ∼50% of systems; (3) X-ray ionization can compete with thermal ionization in the inner disk, because of the low steady-state surface density there; and (4) our inner disks are viscously unstable to surface density perturbations.
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    MRI-active inner regions of protoplanetary discs – II. Dependence on dust, disc, and stellar parameters
    Jankovic, Marija R.; Mohanty, Subhanjoy; Owen, James E.; Tan, Jonathan C.
    Close-in super-Earths are the most abundant exoplanets known. It has been hypothesized that they form in the inner regions of protoplanetary discs, out of the dust that may accumulate at the boundary between the inner region susceptible to the magneto-rotational instability (MRI) and an MRI-dead zone further out. In Paper I, we presented a model for the viscous inner disc which includes heating due to both irradiation and MRI-driven accretion; thermal and non-thermal ionization; dust opacities; and dust effects on ionization. Here, we examine how the inner disc structure varies with stellar, disc, and dust parameters. For high accretion rates and small dust grains, we find that: (1) the main sources of ionization are thermal ionization and thermionic and ion emission; (2) the disc features a hot, high-viscosity inner region, and a local gas pressure maximum at the outer edge of this region (in line with previous studies); and (3) an increase in the dust-to-gas ratio pushes the pressure maximum outwards. Consequently, dust can accumulate in such inner discs without suppressing the MRI, with the amount of accumulation depending on the viscosity in the MRI-dead regions. Conversely, for low accretion rates and large dust grains, there appears to be an additional steady-state solution in which: (1) stellar X-rays become the main source of ionization; (2) MRI-viscosity is high throughout the disc; and (3) the pressure maximum ceases to exist. Hence, if planets form in the inner disc, larger accretion rates (and thus younger discs) are favoured.