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"Boss, Alan"
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Forming Gas Giants around a Range of Protostellar M-dwarfs by Gas Disk Gravitational Instability
Recent discoveries of gas giant exoplanets around M-dwarfs from transiting and radial velocity surveys are difficult to explain with core-accretion models. We present here a homogeneous suite of 162 models of gravitationally unstable gaseous disks. These models represent an existence proof for gas giants more massive than 0.1 Jupiter masses to form by the gas disk gravitational instability (GDGI) mechanism around M-dwarfs for comparison with observed exoplanet demographics and protoplanetary disk mass estimates for M-dwarf stars. We use the Enzo 2.6 adaptive mesh refinement (AMR) 3D hydrodynamics code to follow the formation and initial orbital evolution of gas giant protoplanets in gravitationally unstable gaseous disks in orbit around M-dwarfs with stellar masses ranging from 0.1 M ⊙ to 0.5 M ⊙. The gas disk masses are varied over a range from disks that are too low in mass to form gas giants rapidly to those where numerous gas giants are formed, therefore revealing the critical disk mass necessary for gas giants to form by the GDGI mechanism around M-dwarfs. The disk masses vary from 0.01 M ⊙ to 0.05 M ⊙ while the disk to star mass ratios explored the range from 0.04 to 0.3. The models have varied initial outer disk temperatures (10–60 K) and varied levels of AMR grid spatial resolution, producing a sample of expected gas giant protoplanets for each star mass. Broadly speaking, disk masses of at least 0.02 M ⊙ are needed for the GDGI mechanism to form gas giant protoplanets around M-dwarfs.
Journal Article
Gas Giants Formed by Gravitational Instability May Accrete Atmospheres with Superstellar Carbon-to-oxygen Ratios
Characterizing the atmospheric compositions of exoplanets, along with determining properties such as their mass, mean density, and orbital configuration, are thought to be an effective means for differentiating between various formation and evolution scenarios. Exoplanet atmospheric C/O ratios, when compared to host star C/O ratios, have been advanced as discriminators of gas giant formation and evolution scenarios in the context of the core accretion mechanism. Gas giants formed by the gas disk gravitational instability (GDGI), on the other hand, are thought to have atmospheres with C/O ratios identical to their host stars. We examine this assumption through analysis of fully three-dimensional radiative hydrodynamics models of the GDGI in the flux-limited diffusion approximation. We show here that GDGI protoplanets may be able to form and accrete disk gas with superstellar C/O ratios, as a result of their formation and orbital evolution in a disk with midplane temperatures in the range of the evaporation temperatures of water (∼135 K) and CO2 (∼47 K) ices. Solids that avoid fragmentation and grow rapidly to centimeter size could be transported inward to the central protostar or outward to the edge of the disk considerably faster than the disk gas is dissipated, leading to the preferential accretion of C-rich disk gas compared to the O-rich ices, provided that the protoplanet’s orbit remains outside ∼7 au from a solar-mass protostar. Orbits inside ∼7 au, however, could result in the accretion of disk gas with a nearly stellar C/O.
Journal Article
Orbital Migration of Protoplanets in a Marginally Gravitationally Unstable Disk. II. Migration, Merging, and Ejection
Protoplanets formed in a marginally gravitationally unstable (MGU) disk by either core accretion or disk instability will be subject to dynamical interactions with massive spiral arms, possibly resulting in inward or outward orbital migration, mergers with each other, or even outright ejection from the protoplanetary system. The latter process has been hypothesized as a possible formation scenario for the unexpectedly high frequency of unbound gas giant exoplanets (free floating planets, FFPs). Previous calculations with the EDTONS fixed grid three-dimensional (3D) hydrodynamics code found that protoplanets with masses from 0.01 M ⊕ to 3 M Jup could undergo chaotic orbital evolutions in MGU disks for ∼1000 yr without undergoing monotonic inward or outward migration. Here the Enzo 2.5 adaptive mesh refinement 3D hydrodynamics code is used to follow the formation and orbital evolution of protoplanets in MGU disks for up to 2000 yr. The Enzo results confirm the basic disk fragmentation results of the EDTONS code, as well as the absence of monotonic inward or outward orbital migration. In addition, Enzo allows protoplanet mergers to occur, unlike EDTONS, resulting in a significant decrease in the number of protoplanets that survive for 1000–2000 yr in the Enzo models. These models also imply that gas giants should be ejected frequently in MGU disks that fragment into large numbers of protoplanets, supporting ejection as a possible source mechanism for the observed FFPs.
Journal Article
The Roman Microlensing Survey: Confirmation or Refutation of Gas Giant Exoplanet Formation Theories
Exoplanet research has moved from the discovery of new classes of planets toward creating a census of exoplanet population demographics across the spectrum of exoplanet and host star masses. This census will constrain possible exoplanet formation and evolution theories. Direct imaging and ground-based microlensing surveys excel at detecting long-period exoplanets. The Roman Space Telescope microlensing survey (RMS) will help complete the census by possibly discovering hundreds of long-period and free-floating exoplanets. Long-period gas giants are difficult to form by the classic core-accretion mechanism, whereas the gas disk gravitational instability (GDGI) mechanism can form gas giants at distances of 10 au and beyond for AFGKM host stars. The Enzo 2.6 adaptive mesh refinement three-dimensional hydrodynamics code is used to model the formation of gas giant protoplanets by GDGI around solar-mass protostars, for future comparison with the RMS, which will provide convincing evidence about whether GDGI is needed to explain exoplanet demographics. Previous Enzo models have shown that a robust GDGI is able to form enough gas giants in a single system to result in the ejection of a significant fraction within ∼2000 yr, a likely source of free-floating exoplanets. The present models investigate the GDGI outcomes for a larger range of initial protoplanetary disk masses and outer disk temperatures than in the previous work of A. P. Boss, resulting in the formation of abundant substellar companions with masses in the range of ∼0.1 to ∼100 MJup on orbits ranging from ∼3 to ∼30 au.
Journal Article
Formation of Giant Planets by Gas Disk Gravitational Instability on Wide Orbits around Protostars with Varied Masses. II. Quadrupled Spatial Resolution and Beta Cooling
Exoplanet demographics are sufficiently advanced to provide important constraints on theories of planet formation. While core and pebble accretion are preferred for rocky and icy planets, there appears to be a need for gas disk gravitational instability (GDGI) to play a role in the formation of M-dwarf gas giants and those orbiting at large distances. Here we present GDGI models that go beyond those presented by Boss (2011) dealing with the formation of wide-orbit gas giants. The new models use quadrupled spatial resolution, in both the radial and azimuthal directions, to reduce the effects of finite spatial resolution. The new models also employ the β cooling approximation, instead of the diffusion approximation used by Boss (2011), in order to push the models further in time. As in Boss (2011), the central protostars have masses of 0.1, 0.5, 1.0, 1.5, or 2.0 M ⊙, surrounded by disks with masses ranging from 0.019 M ⊙ to 0.21 M ⊙. For each case, two models are computed, one with an initial minimum Toomre Q stability value ranging from 1.1 to 1.7, and one with a higher initial disk temperature, resulting in the initial minimum Q ranging from 2.2 to 3.4. These new models continue to show that GDGI can explain the formation of gas giants at distances of ∼30 to ∼50 au on eccentric orbits (e less than ∼0.2), though the number formed drops to 0 as the protostar mass decreases to 0.1 M ⊙.
Journal Article
Corporate Social Responsibility and the Benefits of Employee Trust: A Cross-Disciplinary Perspective
Research on corporate social responsibility (CSR) has tended to focus on external stakeholders and outcomes, revealing little about internal effects that might also help explain CSR-firm performance linkages and the impact that corporate marketing strategies can have on internal stakeholders such as employees. The two studies (N = 1,116 and N = 2,422) presented in this article draw on theory from both corporate marketing and organizational behavior (OB) disciplines to test the general proposition that employee trust partially mediates the relationship between CSR and employee attitudinal and behavioral outcomes. Both studies provide evidence in support of these general relationships. Theoretical and practical implications of these findings are discussed in the context of CSR and corporate marketing research.
Journal Article
Possible Implications of Relatively High Levels of Initial 60Fe in Iron Meteorites for the Noncarbonaceous–Carbonaceous Meteorite Dichotomy and Solar Nebula Formation
Cook et al. found that iron meteorites have an initial abundance ratio of the short-lived isotope 60Fe to the stable isotope 56Fe of 60Fe/56Fe ∼ (6.4 ± 2.0) × 10−7. This appears to require the injection of live 60Fe from a Type II supernova (SN II) into the presolar molecular cloud core, as the observed ratio is over a factor of 10 times higher than would be expected to be found in the ambient interstellar medium (ISM) as a result of galactic chemical evolution. The supernova triggering and injection scenario offers a ready explanation for an elevated initial 60Fe level, and in addition provides a physical mechanism for explaining the noncarbonaceous–carbonaceous (NC–CC) dichotomy of meteorites. The NC–CC scenario hypothesizes the solar nebula first accreted material that was enriched in supernova-derived nuclides, and then later accreted material depleted in supernova-derived nuclides. While the NC–CC dichotomy refers to stable nuclides, not short-lived isotopes like 60Fe, the SN II triggering hypothesis provides an explanation for the otherwise unexplained change in nuclides being accreted by the solar nebula. Three-dimensional hydrodynamical models of SN II shock-triggered collapse show that after triggering collapse of the presolar cloud core, the shock front sweeps away the local ISM while accelerating the resulting protostar/disk to a speed of several kilometers per second, sufficient for the protostar/disk system to encounter within ∼1 Myr the more distant regions of a giant molecular cloud complex that might be expected to have a depleted inventory of supernova-derived nuclides.
Journal Article
Searching for Giant Exoplanets around M-dwarf Stars (GEMS) I: Survey Motivation
Recent discoveries of transiting giant exoplanets around M-dwarf stars (GEMS), aided by the all-sky coverage of TESS, are starting to stretch theories of planet formation through the core-accretion scenario. Recent upper limits on their occurrence suggest that they decrease with lower stellar masses, with fewer GEMS around lower-mass stars compared to solar-type. In this paper, we discuss existing GEMS both through confirmed planets, as well as protoplanetary disk observations, and a combination of tests to reconcile these with theoretical predictions. We then introduce the Searching for GEMS survey, where we utilize multidimensional nonparameteric statistics to simulate hypothetical survey scenarios to predict the required sample size of transiting GEMS with mass measurements to robustly compare their bulk-density with canonical hot Jupiters orbiting FGK stars. Our Monte Carlo simulations predict that a robust comparison requires about 40 transiting GEMS (compared to the existing sample of ∼15) with 5σ mass measurements. Furthermore, we discuss the limitations of existing occurrence estimates for GEMS and provide a brief description of our planned systematic search to improve the occurrence rate estimates for GEMS.
Journal Article