Showing papers by "Alan P. Boss published in 2006"
TL;DR: In this paper, the authors re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star.
Abstract: Stable, hydrogen-burning, M dwarf stars comprise about 75% of all stars in the Galaxy. They are extremely long-lived and because they are much smaller in mass than the Sun (between 0.5 and 0.08 MSun), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially-habitable, wet planets residing within their habitable zones, which are only ~ 1/5 to 1/50 of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 MEarth orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone doesn't necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity, or thermal and non-thermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life.
285 citations
TL;DR: In this paper, a suite of three-dimensional, radiative, gravitational hydrodynamical models is presented that calculates the evolution of initially marginally gravitationally unstable disks with masses of 0.021-0.065 M orbiting around stars with masses between 0.1 and 0.5 M, respectively.
Abstract: Extrasolar planet surveys have begun to detect gas giant planets in orbit around M dwarf stars. While the frequency of gas giant planets around M dwarfs so far appears to be lower than that around G dwarfs, it is clearly not zero. Previous work has shown that the core accretion mechanism does not seem to be able to form gas giant planets around M dwarfs, because the time required for core formation scales with the orbital period, which lengthens for lower mass stars, resulting in failed (gas-poor) cores unless the gaseous protoplanetary disk survives for >10 Myr. Disk instability, on the other hand, is rapid enough (~103 yr) that it should be able to form gas giant protoplanets around even low-mass stars well before the gaseous disk disappears. A new suite of three-dimensional, radiative, gravitational hydrodynamical models is presented that calculates the evolution of initially marginally gravitationally unstable disks with masses of 0.021-0.065 M☉ orbiting around stars with masses of 0.1 and 0.5 M☉, respectively. The models show that gas giant planets are indeed likely to form by the disk instability mechanism in orbit around M dwarf stars, the opposite of the prediction for formation by the core accretion mechanism. This difference offers another observational test for discriminating between these two theoretical end members for giant planet formation. Ongoing and future extrasolar planet searches around M dwarfs by spectroscopy, microlensing, photometry, and astrometry offer the opportunity to help decide between the dominance of the two mechanisms.
146 citations
TL;DR: In this article, a suite of three-dimensional radiative gravitational hydrodynamics models suggesting that binary stars may be quite capable of forming planetary systems similar to our own is presented.
Abstract: Gas giant planets have been discovered in binary or triple star systems with a range of semimajor axes. We present a new suite of three-dimensional radiative gravitational hydrodynamics models suggesting that binary stars may be quite capable of forming planetary systems similar to our own. One difference between the new and previous calculations is the inclusion of artificial viscosity in the previous work, leading to significant conversion of disk kinetic energy into thermal energy in shock fronts and elsewhere. New models are presented showing how vigorous artificial viscosity can help to suppress clump formation. The new models with binary companions do not employ any explicit artificial viscosity and also include the third (vertical) dimension in the hydrodynamic calculations, allowing for transient phases of convective cooling. The new calculations of the evolution of initially marginally gravitationally stable disks show that the presence of a binary star companion may actually help to trigger the formation of dense clumps that could become giant planets. Earth-like planets would form much later in the inner disk regions by the traditional collisional accumulation of progressively larger, solid bodies. We also show that in models without binary companions, which begin their evolution as gravitationally stable disks, the disks evolve to form dense rings, which then break up into self-gravitating clumps. The latter models suggest that the evolution of any self-gravitating disk with sufficient mass to form gas giant planets is likely to lead to a period of disk instability, even in the absence of a trigger such as a binary star companion.
112 citations
TL;DR: In this paper, the authors show that super-Earths could also have been formed by the competing mechanism of disk instability, coupled with photoevaporative loss of their gaseous envelopes by a strong external source of UV radiation, i.e., an O star.
Abstract: While the recent microlensing discoveries of super-Earths orbiting two M dwarf stars have been taken as support for the core accretion mechanism of giant planet formation, we show here that these planets could also have been formed by the competing mechanism of disk instability, coupled with photoevaporative loss of their gaseous envelopes by a strong external source of UV radiation, i.e., an O star. M dwarfs that form in regions of future high-mass star formation would then be expected to have super-Earths orbiting at distances of several AU and beyond, while those that form in regions of low-mass star formation would be expected to have gas giants at those distances. Given that most stars are born in the former rather than in the latter regions, M dwarfs should have significantly more super-Earths than gas giants, as seems to be indicated by the microlensing surveys.
76 citations
TL;DR: In this article, a suite of three-dimensional radiative, gravitational hydrodynamical models is used to show that gas giant planets are unlikely to form by the disk-instability mechanism at distances of ~100-200 AU from young stars.
Abstract: A new suite of three-dimensional radiative, gravitational hydrodynamical models is used to show that gas giant planets are unlikely to form by the disk-instability mechanism at distances of ~100-200 AU from young stars. A similar result seems to hold for the core accretion mechanism. These results appear to be consistent with the paucity of detections of gas giant planets on wide orbits by infrared imaging surveys, and they also imply that if the object orbiting GQ Lupus is a gas giant planet, it most likely did not form at a separation of ~100 AU. Instead, a wide planet around GQ Lup must have undergone a close encounter with a third body that tossed the planet outward to its present distance from its protostar. If it exists, the third body may be detectable by NASA's Space Interferometry Mission.
75 citations
TL;DR: In this article, the authors show that super-Earths could also have been formed by the competing mechanism of disk instability, coupled with photoevaporative loss of their gaseous envelopes by a strong external source of UV radiation, i.e., an O star.
Abstract: While the recent microlensing discoveries of super-Earths orbiting two M dwarf stars have been taken as support for the core accretion mechanism of giant planet formation, we show here that these planets could also have been formed by the competing mechanism of disk instability, coupled with photoevaporative loss of their gaseous envelopes by a strong external source of UV radiation, i.e., an O star. M dwarfs that form in regions of future high-mass star formation would then be expected to have super-Earths orbiting at distances of several AU and beyond, while those that form in regions of low-mass star formation would be expected to have gas giants at those distances. Given that most stars are born in the former rather than in the latter regions, M dwarfs should have significantly more super-Earths than gas giants, as seems to be indicated by the microlensing surveys.
56 citations
TL;DR: In this article, a 3D, nonlocal thermodynamic equilibrium (LTE) radiative transfer code was combined with 3D finite differences hydrodynamical code to model molecular emission lines from the vicinity of a 1.4MJ self-gravitating gas giant planet.
Abstract: In the era of high-resolution submillimeter interferometers, it will soon be possible to observe the neutral circumstellar medium directly involved in gas giant planet (GGP) formation at physical scales previously unattainable. In order to explore possible signatures of GGP formation via disk instabilities, we have combined a three-dimensional (3D), nonlocal thermodynamic equilibrium (LTE) radiative transfer code with a 3D, finite differences hydrodynamical code to model molecular emission lines from the vicinity of a 1.4MJ self-gravitating proto-GGP. Here we explore the properties of rotational transitions of the commonly observed dense gas tracer, HCO+. Our main results are as follows: (1) Very high lying HCO+ transitions (e.g., HCO+ J = 7-6) can trace dense clumps around circumstellar disks. Depending on the molecular abundance, the proto-GGP may be directly imageable by the Atacama Large Millimeter Array (ALMA). (2) HCO+ emission lines are heavily self-absorbed through the proto-GGP's dense molecular core. This signature is nearly ubiquitous and only weakly dependent on assumed HCO+ abundances. The self-absorption features are most pronounced at higher angular resolutions. Dense clumps that are not self-gravitating only show minor self-absorption features. (3) Line temperatures are highest through the proto-GGP at all assumed abundances and inclination angles. Conversely, due to self-absorption in the line, the velocity-integrated intensity may not be. High angular resolution interferometers such as the Submillimeter Array (SMA) and ALMA may be able to differentiate between competing theories of GGP formation.
36 citations
TL;DR: In this article, a high-precision Doppler search for giant planets orbiting a well-defined sample of metal-poor dwarfs in the field is described, which constitutes a fundamental test of theoretical predictions, which will help discriminate between proposed giant planetformationand migrationmodels.
Abstract: We describe a high-precision Doppler search for giant planets orbiting a well-defined sample of metal-poor dwarfs in the field. This experiment constitutes a fundamental test of theoretical predictions, which will help discriminate betweenproposedgiant planetformationandmigrationmodels.Wepresentdetailsofthesurvey,aswellasanoverall assessment of the quality of our measurements, making use of the results for stars that show no significant velocity variation. Subject headingg planetary systems: formation — stars: abundances — stars: statistics — techniques: radial velocities
29 citations
TL;DR: In this article, the authors show that the efficiency of large-scale mixing and transport is sufficient to spatially homogenize an initially highly spatially heterogeneous nebula to dispersions of ~10% about the mean value of 26Al/27Al on time scales of thousands of years.
Abstract: The degree of isotopic spatial heterogeneity in the solar nebula has long been a puzzle, with different isotopic systems implying either large-scale initial spatial homogeneity (e.g., 26Al chronometry) or a significant amount of preserved heterogeneity (e.g., ratios of the three stable oxygen isotopes, 16O, 17O, and 18O). We show here that in a marginally gravitationally unstable (MGU) solar nebula, the efficiency of large-scale mixing and transport is sufficient to spatially homogenize an initially highly spatially heterogeneous nebula to dispersions of ~10% about the mean value of 26Al/27Al on time scales of thousands of years. A similar dispersion would be expected for 17O/16O and 18O/16O ratios produced by ultraviolet photolysis of self-shielded molecular CO gas at the surface of the outer solar nebula. In addition to preserving a chronological interpretation of initial 26Al/27Al ratios and the self-shielding explanation for the oxygen isotope ratios, these solar nebula models offer a self-consistent environment for achieving large-scale mixing and transport of thermally annealed dust grains, shock-wave processing of chondrules and refractory inclusions, and giant planet formation.
22 citations
Posted Content•
TL;DR: In this article, the authors discuss the wide range of calculations undertaken by themselves and others using various numerical techniques, and report preliminary results from a large multi-code collaboration, including triggering mechanisms for GI's, disk heating and cooling, orbital survival of dense clumps, interactions of solids with GI-driven waves and shocks, and hybrid scenarios where GI's facilitate core accretion.
Abstract: Protoplanetary gas disks are likely to experience gravitational instabilites (GI's) during some phase of their evolution Density perturbations in an unstable disk grow on a dynamic time scale into spiral arms that produce efficient outward transfer of angular momentum and inward transfer of mass through gravitational torques In a cool disk with rapid enough cooling, the spiral arms in an unstable disk form self-gravitating clumps Whether gas giant protoplanets can form by such a disk instability process is the primary question addressed by this review We discuss the wide range of calculations undertaken by ourselves and others using various numerical techniques, and we report preliminary results from a large multi-code collaboration Additional topics include -- triggering mechanisms for GI's, disk heating and cooling, orbital survival of dense clumps, interactions of solids with GI-driven waves and shocks, and hybrid scenarios where GI's facilitate core accretion The review ends with a discussion of how well disk instability and core accretion fare in meeting observational constraints
17 citations
TL;DR: The Origins Billions Star Survey (OBSS) as mentioned in this paper is a mission concept addressing the astrophysics of extrasolar planets, Galactic structure, the Galactic halo and tidal streams, the Local Group and local supercluster of galaxies, dark matter, star formation, open clusters, the solar system, and the celestial reference frame by determining the position, parallax, and proper motion, as well as photometry, for billions of stars down to 23rd visual magnitude.
Abstract: The Origins Billions Star Survey is a mission concept addressing the astrophysics of extrasolar planets, Galactic structure, the Galactic halo and tidal streams, the Local Group and local supercluster of galaxies, dark matter, star formation, open clusters, the solar system, and the celestial reference frame by determining the position, parallax, and proper motion, as well as photometry, for billions of stars down to 23rd visual magnitude. It is capable of surveying the entire celestial sphere or dwelling on a star field by varying the cadence of observations. The mission's ability to measure objects fainter than 17th magnitude allows a large number of extragalactic compact objects to be observed, making the astrometric measurements absolute. The project mission accuracy is comparable to Gaia for a survey mission. Improved accuracy can be achieved by dwelling on a particular star field or by using the Gaia positions at 14th magnitude to improve the positions of objects at the 18th–23rd visual magnitudes.
TL;DR: In this article, a high-precision Doppler search for giant planets orbiting a well-defined sample of metal-poor dwarfs in the field is described, which constitutes a fundamental test of theoretical predictions which will help discriminate between proposed giant planet formation and migration models.
Abstract: We describe a high-precision Doppler search for giant planets orbiting a well-defined sample of metal-poor dwarfs in the field. This experiment constitutes a fundamental test of theoretical predictions which will help discriminate between proposed giant planet formation and migration models. We present here details on the survey as well as an overall assessment of the quality of our measurements, making use of the results for the stars that show no significant velocity variation.
Journal Article•
01 Dec 2006
TL;DR: The two competing theories for the formation of gas giant planets, core accretion and disk instability, appear to have testable predictions: (i) Core accretion seems to require exceptionally long-lived disks, implying that gas giants should be somewhat rare, while disk instability can occur in even the shortest-lived disk, indicating that gas giant should be abundant.
Abstract: The discovery of the first planetary companion to a solar-type star by Mayor and Queloz (1995) launched the extrasolar planetary systems era. Observational and theoretical progress in this area has been made at a breathtaking pace since 1995, as evidenced by this workshop. We now have a large and growing sample of extrasolar gas giant planets with which to test our theories of their formation and evolution. The two competing theories for the formation of gas giant planets, core accretion and disk instability, appear to have testable predictions: (i) Core accretion seems to require exceptionally long-lived disks, implying that gas giants should be somewhat rare, while disk instability can occur in even the shortest-lived disk, implying that gas giants should be abundant. The ongoing census of gas giants by the spectroscopic search programs will determine the frequency of gas giants on Jupiter-like orbits within the next decade. (ii) Core accretion takes millions of years to form gas giants, while disk instability forms gaseous protoplanets in thousands of years. Determining the epoch of gas giant planet formation by searching for astrometric wobbles indicative of gas giant companions around young stars with a range of ages (∼ 0. 1M yr to∼ 10 Myr) should be possible with the Space Interferometry Mission (SIM). (iii) Core accretion would seem to be bolstered by a higher ratio of dust to gas, whereas disk instability occurs equally well for a range of dust opacities. Determining whether a high primordial metallicity is necessary for gas giant planet formation can be accomplished by spectroscopic and astrometric searches for gas giants around metal-poor stars. Eventually, ice giant planets will be detectable as well. If ice giants are found to be much more frequent that gas giants, this may imply that core accretion occurs, but usually fails to form a gas giant. Terrestrial planets will be detected through photometry by Kepler and Eddington, astrometry by SIM, and imaging by Terrestrial Planet Finder and Darwin. Ultimately these detections will clarify the process of Earth formation by collisional accumulation, the only contending theory.