arXiv subjects

Alan P. Boss

Publications and source records attributed to Alan P. Boss.

At least 19 recordsLinked to original sources

arXiv2026

Gas Giants Formed by Gravitational Instability May Accrete Atmospheres with Super-Stellar Carbon to Oxygen Ratios

Characterizing the atmospheric compositions of exoplanets, along with determining properties such as their mass, mean density, and orbital configuration, is 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 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 super-stellar 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 ($\sim$ 135 K) and CO$_2$ ($\sim$ 47 K) ices. Solids that avoid fragmentation and grow rapidly to cm-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 $\sim$ 7 au from a solar-mass protostar. Orbits inside $\sim$ 7 au, however, could result in the accretion of disk gas with nearly stellar C/O.

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arXiv2026

A universal brown dwarf desert formed between planets and stars

Giant planets and brown dwarfs play a crucial role in star and planet formation, as they are situated at the boundary between planets and stars with uncertain formation mechanisms. Previous observational searches for the formation boundary were hampered by the lack of large unified samples of wide-orbit giant planets and substellar companions. A combined analysis of radial velocity and astrometry mitigates this problem and has significantly enlarged the sample. Here we present a rigorous statistical analysis of the sample of 55 giant planets, brown dwarfs and low-mass stellar companions orbiting FGK stars. We quantitatively analyze the occurrence rates of brown dwarfs and identify a distinct brown dwarf desert at approximately $30\,M_\mathrm{J}$, with no evidence of disappearance up to 20 au. Unlike previous studies that predicted a declining planet occurrence rate beyond the water-ice line, we identify a new population of giant planets and low-mass brown dwarfs in this region. The metallicity and eccentricity trends in our sample suggest that these are the consequences of two different formation scenarios. Our combined population synthesis model successfully accounts for the observed brown dwarf desert, supporting the dual formation hypothesis.

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arXiv2025

Are We There Yet? Challenges in Quantifying the Frequency of Earth Analogs in the Habitable Zone

Searching for life elsewhere in the universe is one of the most highly prioritized pursuits in astronomy today. However, the ability to observe evidence of Earth-like life through biosignatures is limited by the number of planets in the solar neighborhood with conditions similar to Earth. The occurrence rate of Earth-like planets in the habitable zones of Sun-like stars, $η_{\oplus}$, is therefore crucial for addressing the apparent lack of consensus on its value in the literature. Here we present a review of the current understanding of $η_{\oplus}$. We first provide definitions for parameters that contribute to $η_{\oplus}$. Then, we discuss the previous and current estimated parameter values and the context of the limitations on the analyses that produced these estimates. We compile an extensive list of the factors that go into any calculation of $η_{\oplus}$, and how detection techniques and surveys differ in their sensitivity and ability to accurately constrain $η_{\oplus}$. Understanding and refining the value of $η_{\oplus}$ is crucial for upcoming missions and telescopes, such as the planned Habitable Worlds Observatory and the Large Interferometer for Exoplanets, which aim to search for biosignatures on exoplanets in the solar neighborhood.

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arXiv2023

Forming Gas Giants Around a Range of Protostellar M-dwarfs by Gas Disk Gravitational Instability

Recent discoveries of gas giant exoplanets around M-dwarfs (GEMS) from transiting and radial velocity (RV) 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 $M_\odot$ to 0.5 $M_\odot$. 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 $M_\odot$ to 0.05 $M_\odot$ while the disk to star mass ratios explored range from 0.04 to 0.3. The models have varied initial outer disk temperatures (10 K to 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 $M_\odot$ are needed for the GDGI mechanism to form gas giant protoplanets around M-dwarfs.

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arXiv2023

Enabling Exoplanet Demographics Studies with Standardized Exoplanet Survey Meta-Data

Goal 1 of the National Academies of Science, Engineering and Mathematics Exoplanet Science Strategy is "to understand the formation and evolution of planetary systems as products of the process of star formation, and characterize and explain the diversity of planetary system architectures, planetary compositions, and planetary environments produced by these processes", with the finding that "Current knowledge of the demographics and characteristics of planets and their systems is substantially incomplete." One significant roadblock to our ongoing efforts to improve our demographics analyses is the lack of comprehensive meta-data accompanying published exoplanet surveys. The Exoplanet Program Analysis Group (ExoPAG) Science Interest Group 2: Exoplanet Demographics has prepared this document to provide guidance to survey architects, authors, referees and funding agencies as to the most valuable such data products for five different exoplanet detection techniques - transit, radial velocity, direct imaging, microlensing and astrometry. We find that making these additional data easily available would greatly enhance the community's ability to perform robust, reproducible demographics analyses, and make progress on achieving the most important goals identified by the exoplanet and wider astronomical community.

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arXiv2022

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, FFP). Previous calculations with the EDTONS fixed grid three dimensional (3D) hydrodynamics code found that protoplanets with masses from 0.01 $M_\oplus$ to 3 $M_{Jup}$ could undergo chaotic orbital evolutions in MGU disks for $\sim$ 1000 yrs without undergoing monotonic inward or outward migration. Here the Enzo 2.5 adaptive mesh refinement (AMR) 3D hydrodynamics code is used to follow the formation and orbital evolution of protoplanets in MGU disks for up to 2000 yrs. 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 to 2000 yrs 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.

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arXiv2022

Possible Implications of Relatively High Levels of Initial $^{60}$Fe in Iron Meteorites for the Non-Carbonaceous -- Carbonaceous Meteorite Dichotomy and Solar Nebula Formation

Cook et al. (2021) found that iron meteorites have an initial abundance ratio of the short-lived isotope $^{60}$Fe to the stable isotope $^{56}$Fe of $^{60}$Fe/$^{56}$Fe $\sim$ $(6.4 \pm 2.0) \times 10^{-7}$. This appears to require the injection of live $^{60}$Fe from a Type II supernova (SN II) into the presolar molecular cloud core, as the observed ratio is over a factor of ten 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 $^{60}$Fe level, and in addition provides a physical mechanism for explaining the non-carbonaceous -- 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 $^{60}$Fe, 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 km/s, sufficient for the protostar/disk system to encounter within $\sim$ 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.

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arXiv2021

Flux-Limited Diffusion Approximation Models of Giant Planet Formation by Disk Instability. II. Quadrupled Spatial Resolution

While collisional accumulation is nearly universally accepted as the formation mechanism of rock and ice worlds, the situation regarding gas giant planet formation is more nuanced. Gas accretion by solid cores formed by collisional accumulation is the generally favored mechanism, but observations increasingly suggest that gas disk gravitational instability might explain the formation of at least the massive or wide-orbit gas giant exoplanets. This paper continues a series aimed at refining three-dimensional (3D) hydrodynamical models of disk instabilities, where the handling of the gas thermodynamics is a crucial factor. Boss (2017, 2019, 2021) used the $β$ cooling approximation (Gammie 2001) to calculate 3D models of disks with initial masses of 0.091 $M_\odot$ extending from 4 to 20 au around 1 $M_\odot$ protostars. Here we employ 3D flux-limited diffusion (FLD) approximation models of the same disks, in order to provide a superior treatment of disk gas thermodynamics. The new models have quadrupled spatial resolution compared to previous 3D FLD models (Boss 2008, 2012), in both the radial and azimuthal spherical coordinates, resulting in the highest spatial resolution 3D FLD models to date. The new models continue to support the hypothesis that such disks can form self-gravitating, dense clumps capable of contracting to form gas giant protoplanets, and suggest that the FLD models yield similar numbers of clumps as $β$ cooling models with $β\sim$ 1 to $\sim$ 10, including the critical value of $β$ = 3 for fragmentation proposed by Gammie (2001).

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arXiv2021

Optimized modeling of Gaia-Hipparcos astrometry for the detection of the smallest cold Jupiter and confirmation of seven low mass companions

To fully constrain the orbits of low mass circumstellar companions, we conduct combined analyses of the radial velocity data as well as the Gaia and Hipparcos astrometric data for eight nearby systems. Our study shows that companion-induced position and proper motion differences between Gaia and Hipparcos are significant enough to constrain orbits of low mass companions to a precision comparable with previous combined analyses of direct imaging and radial velocity data. We find that our method is robust to whether we use Gaia DR2 or Gaia EDR3, as well as whether we use all of the data, or just proper motion differences. In particular, we fully characterize the orbits of HD 190360 b and HD 16160 C for the first time. With a mass of 1.8$\pm$0.2$m_{\rm Jup}$ and an effective temperature of 123-176 K and orbiting around a Sun-like star, HD 190360 b is the smallest Jupiter-like planet with well-constrained mass and orbit, belonging to a small sample of fully characterized Jupiter analogs. It is separated from its primary star by 0.25$''$ and thus may be suitable for direct imaging by the CGI instrument of the Roman Space Telescope.

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arXiv2021

The Effect of the Approach to Gas Disk Gravitational Instability on the Rapid Formation of Gas Giant Planets. II. Quadrupled Spatial Resolution

Observations support the hypothesis that gas disk gravitational instability might explain the formation of massive or wide-orbit gas giant exoplanets. The situation with regard to Jupiter-mass exoplanets orbiting within $\sim$ 20 au is more uncertain. Theoretical models yield divergent assessments often attributed to the numerical handling of the gas thermodynamics. Boss (2019) used the $β$ cooling approximation to calculate three dimensional hydrodynamical models of the evolution of disks with initial masses of 0.091 $M_\odot$ extending from 4 to 20 au around 1 $M_\odot$ protostars. The models considered a wide range (1 to 100) of $β$ cooling parameters and started from an initial minimum Toomre stability parameter of $Q_i = 2.7$ (gravitationally stable). The disks cooled down from initial outer disk temperatures of 180 K to as low as 40 K as a result of the $β$ cooling, leading to fragmentation into dense clumps, which were then replaced by virtual protoplanets (VPs) and evolved for up to $\sim$ 500 yr. The present models test the viability of replacing dense clumps with VPs by quadrupling the spatial resolution of the grid once dense clumps form, sidestepping in most cases VP insertion. After at least $\sim$ 200 yr of evolution, the new results compare favorably with those of Boss (2019): similar numbers of VPs and dense clumps form by the same time for the two approaches. The results imply that VP insertion can greatly speed disk instability calculations without sacrificing accuracy.

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arXiv2020

Formation and evolution of the local interstellar environment: combined constraints from nucleosynthetic and X-ray data

Several observations suggest that the Solar system has been located in a region affected by massive stellar feedback for at least a few Myr; these include detection of live $^{60}\text{Fe}$ in deep-sea archives and Antarctic snow, the broad angular distribution of $^{26}\text{Al}$ around the Galactic plane seen in all-sky $γ$-ray maps, and the all-sky soft X-ray background. However, our position inside the Galactic disc makes it difficult to fully characterise this environment, and our limited time baseline provides no information about its formation history or relation to large-scale Galactic dynamics. We explore these questions by using an $N$-body+hydrodynamics simulation of a Milky-Way-like galaxy to identify stars on Sun-like orbits whose environments would produce conditions consistent with those we observe. We find that such stars are uncommon but not exceptionally rare. These stars are found predominantly near the edges of spiral arms, and lie inside kpc-scale bubbles that are created by multiple generations of star formation in the arm. We investigate the stars' trajectories and find that the duration of the stay in the bubble ranges from 20 Myr to 90 Myr. The duration is governed by the crossing time of stars across the spiral arm. This is generally shorter than the bubble lifetime, which is $\sim 100$ Myr as a result of the continuous gas supply provided by the arm environment.

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arXiv2019

PEXO: a global modeling framework for nanosecond timing, microsecond astrometry, and $μ$m/s radial velocities

The ability to make independent detections of the signatures of exoplanets with complementary telescopes and instruments brings a new potential for robust identification of exoplanets and precision characterization. We introduce PEXO, a package for Precise EXOplanetology to facilitate the efficient modeling of timing, astrometry, and radial velocity data, which will benefit not only exoplanet science but also various astrophysical studies in general. PEXO is general enough to account for binary motion and stellar reflex motions induced by planetary companions and is precise enough to treat various relativistic effects both in the solar system and in the target system. We also model the post-Newtonian barycentric motion for future tests of general relativity in extrasolar systems. We benchmark PEXO with the pulsar timing package TEMPO2 and find that PEXO produces numerically similar results with timing precision of about 1 ns, space-based astrometry to a precision of 1 $μ$as, and radial velocity of 1 $μ$m/s and improves on TEMPO2 for decade-long timing data of nearby targets, due to its consideration of third-order terms of Roemer delay. PEXO is able to avoid the bias introduced by decoupling the target system and the solar system and to account for the atmospheric effects which set a practical limit for ground-based radial velocities close to 1 cm/s. Considering the various caveats in barycentric correction and ancillary data required to realize cm/s modeling, we recommend the preservation of original observational data. The PEXO modeling package is available at GitHub (https://github.com/phillippro/pexo).

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arXiv2019

The Effect of the Approach to Gas Disk Gravitational Instability on the Rapid Formation of Gas Giant Planets

Observational evidence suggests that gas disk instability may be responsible for the formation of at least some gas giant exoplanets, particularly massive or distant gas giants. With regard to close-in gas giants, Boss (2017) used the $β$ cooling approximation to calculate hydrodynamical models of inner gas disk instability, finding that provided disks with low values of the initial minimum Toomre stability parameter (i.e., $Q_i < 2$ inside 20 au) form, fragmentation into self-gravitating clumps could occur even for $β$ as high as 100 (i.e., extremely slow cooling). Those results implied that the evolution of disks toward low $Q_i$ must be taken into account. This paper presents such models: initial disk masses of 0.091 $M_\odot$ extending from 4 to 20 au around a 1 $M_\odot$ protostar, with a range (1 to 100) of $β$ cooling parameters, the same as in Boss (2017), but with all the disks starting with $Q_i = 2.7$, i.e., gravitationally stable, and allowed to cool from their initial outer disk temperature of 180 K to as low as 40 K. All the disks eventually fragment into at least one dense clump. The clumps were again replaced by virtual protoplanets (VPs) and the masses and orbits of the resulting ensemble of VPs compare favorably with those of Boss (2017), supporting the claim that disk instability can form gas giants rapidly inside 20 au, provided that sufficiently massive protoplanetary disks exist.

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arXiv2019

Imaging Giant Protoplanets with the ELTs

We have now accumulated a wealth of observations of the planet-formation environment and of mature planetary systems. These data allow us to test and refine theories of gas-giant planet formation by placing constraints on the conditions and timescale of this process. Yet a number of fundamental questions remain unanswered about how protoplanets accumulate material, their photospheric properties and compositions, and how they interact with protoplanetary disks. While we have begun to detect protoplanet candidates during the last several years, we are presently only sensitive to the widest separation, highest mass / accretion rate cases. Current observing facilities lack the angular resolution and inner working angle to probe the few-AU orbital separations where giant planet formation is thought to be most efficient. They also lack the contrast to detect accretion rates that would form lower mass gas giants and ice giants. Instruments and telescopes coming online over the next decade will provide high contrast in the inner giant-planet-forming regions around young stars, allowing us to build a protoplanet census and to characterize planet formation in detail for the first time.

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arXiv2018

Triggering Collapse of the Presolar Dense Cloud Core and Injecting Short-Lived Radioisotopes with a Shock Wave. VI. Protostar and Protoplanetary Disk Formation

Cosmochemical evaluations of the initial meteoritical abundance of the short-lived radioisotope (SLRI) $^{26}$Al have remained fairly constant since 1976, while estimates for the initial abundance of the SLRI $^{60}$Fe have varied widely recently. At the high end of this range, $^{60}$Fe initial abundances have seemed to require $^{60}$Fe nucleosynthesis in a core collapse supernova, followed by incorporation into primitive meteoritical components within $\sim$ 1 Myr. This paper continues the detailed exploration of this classical scenario, using models of the self-gravitational collapse of molecular cloud cores that have been struck by suitable shock fronts, leading to the injection of shock front gas into the collapsing cloud through Rayleigh-Taylor fingers formed at the shock-cloud interface. As before, these models are calculated using the FLASH three dimensional, adaptive mesh refinement (AMR), gravitational hydrodynamical code. While the previous models used FLASH 2.5, the new models employ FLASH 4.3, which allows sink particles to be introduced to represent the newly formed protostellar object. Sink particles permit the models to be pushed forward farther in time to the phase where a $\sim 1 M_\odot$ protostar has formed, orbited by a rotating protoplanetary disk. These models are thus able to define what type of target cloud core is necessary for the supernova triggering scenario to produce a plausible scheme for the injection of SLRIs into the presolar cloud core: a $\sim 3 M_\odot$ cloud core rotating at a rate of $\sim 3 \times 10^{-14}$ rad s$^{-1}$ or higher.

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arXiv2018

Dynamical Masses of Eps Ind B and C: Two Massive Brown Dwarfs at the Edge of the Stellar-Substellar Boundary

We report individual dynamical masses for the brown dwarfs Epsilon Indi B and C, which have spectral types of T1.5 and T6, respectively, measured from astrometric orbit mapping. Our measurements are based on a joint analysis of astrometric data from the Carnegie Astrometric Planet Search and the Cerro Tololo Inter-American Observatory Parallax Investigation as well as archival high resolution imaging, and use a Markov Chain Monte Carlo method. We find dynamical masses of 75.0+-0.82 Mjup for the T1.5 B component and 70.1+-0.68 Mjup for the T6 C component. These masses are surprisingly high for substellar objects and challenge our understanding of substellar structure and evolution. We discuss several evolutionary scenarios proposed in the literature and find that while none of them can provide conclusive explanations for the high substellar masses, evolutionary models incorporating lower atmospheric opacities come closer to approximating our results. We discuss the details of our astrometric model, its algorithm implementation, and how we determine parameter values via Markov Chain Monte Carlo Bayesian inference.

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arXiv2017

Astrometric Constraints on the Masses of Long-Period Gas Giant Planets in the TRAPPIST-1 Planetary System

Transit photometry of the M8V dwarf star TRAPPIST-1 (2MASS J23062928-0502285) has revealed the presence of at least seven planets with masses and radii similar to that of Earth orbiting at distances that might allow liquid water to be present on their surfaces. We have been following TRAPPIST-1 since 2011 with the CAPSCam astrometric camera on the 2.5-m du Pont telescope at the Las Campanas Observatory in Chile. In 2016 we noted that TRAPPIST-1 lies slightly farther away than previously thought, at 12.49 pc, rather than 12.1 pc. Here we examine fifteen epochs of CAPSCam observations of TRAPPIST-1, spanning the five years from 2011 to 2016, and obtain a revised trigonometric distance of $12.56 \pm 0.12$ pc. The astrometric data analysis pipeline shows no evidence for a long-period astrometric wobble of TRAPPIST-1. After proper motion and parallax are removed, residuals at the level of $\pm 1.3$ millarcsec (mas) remain. The amplitude of these residuals constrains the masses of any long-period gas giant planets in the TRAPPIST-1 system: no planet more massive than $\sim 4.6 M_{Jup}$ orbits with a 1 yr period, and no planet more massive than $\sim 1.6 M_{Jup}$ orbits with a 5 yr period. Further refinement of the CAPSCam data analysis pipeline, combined with continued CAPSCam observations, should either detect any long-period planets, or put an even tighter constraint on these mass upper limits.

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arXiv2017

Triggering Collapse of the Presolar Dense Cloud Core and Injecting Short-Lived Radioisotopes with a Shock Wave. V. Nonisothermal Collapse Regime

Recent meteoritical analyses support an initial abundance of the short-lived radioisotope $^{60}$Fe that may be high enough to require nucleosynthesis in a core collapse supernova, followed by rapid incorporation into primitive meteoritical components, rather than a scenario where such isotopes were inherited from a well-mixed region of a giant molecular cloud polluted by a variety of supernovae remnants and massive star winds. This paper continues to explore the former scenario, by calculating three dimensional, adaptive mesh refinement, hydrodynamical code (FLASH 2.5) models of the self-gravitational, dynamical collapse of a molecular cloud core that has been struck by a thin shock front with a speed of 40 km/sec, leading to the injection of shock front matter into the collapsing cloud through the formation of Rayleigh-Taylor fingers at the shock-cloud intersection. These models extend the previous work into the nonisothermal collapse regime using a polytropic approximation to represent compressional heating in the optically thick protostar. The models show that the injection efficiencies of shock front material are enhanced compared to previous models, which were not carried into the nonisothermal regime and so did not reach such high densities. The new models, combined with the recent estimates of initial $^{60}$Fe abundances, imply that the supernova triggering and injection scenario remains as a plausible explanation for the origin of the short-lived radioisotopes involved in the formation of our solar system.

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