268 research outputs found
A decreased probability of habitable planet formation around low-mass stars
Smaller terrestrial planets (< 0.3 Earth masses) are less likely to retain
the substantial atmospheres and ongoing tectonic activity probably required to
support life. A key element in determining if sufficiently massive "sustainably
habitable" planets can form is the availability of solid planet-forming
material. We use dynamical simulations of terrestrial planet formation from
planetary embryos and simple scaling arguments to explore the implications of
correlations between terrestrial planet mass, disk mass, and the mass of the
parent star. We assume that the protoplanetary disk mass scales with stellar
mass as Mdisk ~ f Mstar^h, where f measures the relative disk mass, and 1/2 < h
< 2, so that disk mass decreases with decreasing stellar mass. We consider
systems without Jovian planets, based on current models and observations for M
stars. We assume the mass of a planet formed in some annulus of a disk with
given parameters is proportional to the disk mass in that annulus, and show
with a suite of simulations of late-stage accretion that the adopted
prescription is surprisingly accurate. Our results suggest that the fraction of
systems with sufficient disk mass to form > 0.3 Earth mass habitable planets
decreases for low-mass stars for every realistic combination of parameters.
This "habitable fraction" is small for stellar masses below a mass in the
interval 0.5 to 0.8 Solar masses, depending on disk parameters, an interval
that excludes most M stars. Radial mixing and therefore water delivery are
inefficient in lower-mass disks commonly found around low-mass stars, such that
terrestrial planets in the habitable zones of most low-mass stars are likely to
be small and dry.Comment: Accepted to ApJ. 11 pages, 6 figure
Brown Bear Cognition and Welfare
The welfare of large-brained wide-roaming carnivores in captivity has been of longstanding public and professional concern. Bears are one such example. Ursids have unexpectedly large relative brain sizes, indeed showing brain size increases similar to those observed in canids. Bears also have considerable space requirements, with extensive home range sizes seen in the wild. Species with complex requirements often face difficulties when placed in artificial environments. Cognitive enrichment, however, may work to ameliorate this by providing stimulating mental challenges for individuals living in zoological settings. This case study explores the proposed benefits of cognitive trials. To do so, we exposed 17 captive European brown bears ( Ursus arctos arctos ) housed in UK zoos to two experimental tasks (1) a puzzle box and (2) an object-manipulation set-up. A total of 14 of the bears voluntarily engaged with the puzzle box, while 15 individuals interacted with the object-manipulation set-up. The bears that failed to interact with the tests were likely dissuaded due to the collective nature of testing, a lack of motivation or fear of novel objects. Cognitive stimulation, such as that provided in our study, offers one means of improving the welfare of captive bears. We advocate the use of cognition studies for zoo-housed bears to enrich their lived-experiences when housed in zoological settings. Information © The Authors 202
The formation and habitability of terrestrial planets in the presence of hot jupiters
`Hot jupiters,' giant planets with orbits very close to their parent stars,
are thought to form farther away and migrate inward via interactions with a
massive gas disk. If a giant planet forms and migrates quickly, the
planetesimal population has time to re-generate in the lifetime of the disk and
terrestrial planets may form (Armitage 2003). We present results of simulations
of terrestrial planet formation in the presence of hot jupiters, broadly
defined as having orbital radii <= 0.5 AU. We show that terrestrial planets
similar to those in the Solar System can form around stars with hot jupiters,
and can have water contents equal to or higher than the Earth's. For small
orbital radii of hot jupiters (e.g. 0.15, 0.25 AU) potentially habitable
planets can form, but for semi-major axes of 0.5 AU or greater their formation
is suppressed. We show that the presence of an outer giant planet such as
Jupiter does not enhance the water content of the terrestrial planets, but
rather decreases their formation and water delivery timescales. We speculate
that asteroid belts may exist interior to the terrestrial planets in systems
with hot jupiters.Comment: 5 pages, 2 color figures in emulate ApJ style submitted to Icaru
Monitoring Global Freshwater Resources with GRACE
Freshwater resources include surface waters, groundwater, and seasonal snowpack. Given adequate ground based measurements, all of these can be monitored effectively, however, outside of the developed world such measurements often are not systematic and the data not centralized, and as a result reports of freshwater availability may be largely anecdotal. Even in the developed world it can be difficult to quantify changes in groundwater storage over large scales. Owing to its global coverage, satellite remote sensing has become a valuable tool for freshwater resources assessment. In particular, the Gravity Recovery and Climate Experiment (GRACE) has demonstrated an unequaled ability to monitor total terrestrial water storage including groundwater at regional to continental scales. In this presentation we will identify apparent trends in terrestrial water storage observed by GRACE over the past nine years and attempt to explain their origins and predict whether they are likely to continue. Trends in certain regions where groundwater extraction has significantly depleted aquifers, including northern India and California, will be discussed in detail
Habitable Planet Formation in Binary-Planetary Systems
Recent radial velocity observations have indicated that Jovian-type planets
can exist in moderately close binary star systems. Numerical simulations of the
dynamical stability of terrestrial-class planets in such environments have
shown that, in addition to their giant planets, these systems can also harbor
Earth-like objects. In this paper, we study the late stage of terrestrial
planet formation in such binary-planetary systems, and present the results of
the simulations of the formation of Earth-like bodies in their habitable zones.
We consider a circumprimary disk of Moon- to Mars-sized objects and numerically
integrate the orbits of these bodies at the presence of the Jovian-type planet
of the system and for different values of the mass, semimajor axis, and orbital
eccentricity of the secondary star. Results indicate that, Earth-like objects,
with substantial amounts of water, can form in the habitable zone of the
primary star. Simulations also indicate that, by transferring angular momentum
from the secondary star to protoplanetary objects, the giant planet of the
system plays a key role in the radial mixing of these bodies and the water
contents of the final terrestrial planets. We will discuss the results of our
simulation and show that the formation of habitable planets in binary-planetary
systems is more probable in binaries with moderate to large perihelia.Comment: 27 pages, 11 figures, submitted for publicatio
The Search for other Earths: limits on the giant planet orbits that allow habitable terrestrial planets to form
Gas giant planets are far easier than terrestrial planets to detect around
other stars, and are thought to form much more quickly than terrestrial
planets. Thus, in systems with giant planets, the late stages of terrestrial
planet formation are strongly affected by the giant planets' dynamical
presence. Observations of giant planet orbits may therefore constrain the
systems that can harbor potentially habitable, Earth-like planets. We present
results of 460 N-body simulations of terrestrial accretion from a disk of Moon-
to Mars-sized planetary embryos. We systematically vary the orbital semimajor
axis of a Jupiter-mass giant planet between 1.6 and 6 AU, and eccentricity
between 0 and 0.4. We find that for Sun-like stars, giant planets inside
roughly 2.5 AU inhibit the growth of 0.3 Earth-mass planets in the habitable
zone. If planets accrete water from volatile-rich embryos past 2-2.5 AU, then
water-rich habitable planets can only form in systems with giant planets beyond
3.5 AU. Giant planets with significant orbital eccentricities inhibit both
accretion and water delivery. The majority of the current sample of extra-solar
giant planets appears unlikely to form habitable planets.Comment: Accepted by ApJ Letter
Mechanisms of Groucho-mediated repression revealed by genome-wide analysis of Groucho binding and activity
Antibody validation (A) Chromatin isolated and sheared exactly as for the ChIP-seq analysis was subjected to immunoprecipitation with the indicated amounts (in μl) of affinity purified antibody against the Gro GP domain used for the ChIP-seq analysis, and then probed in a western blot with both an anti-Gro monoclonal antibody (mAb) or the anti-GP antibody. The band indicated by the asterisk is a cross-reacting protein that is recognized in the western blot but that is not efficiently immunoprecipitated by the anti-GP antibody. Ab HC – antibody heavy chain. (B) Heat map showing overlap (Jacard similarity coefficient [96]) between the peaks called in the duplicate ChIP-seq experiments at each time point. (C) Representative genome browser tracts comparing duplicate ChIP-seq experiments. (D and E) Comparison of Gro binding patterns obtained by ChIP-seq using our anti-GP antibody with that obtained by ChIP-chip (0–12 hr embryos; modENCODE #597) and ChIP-seq (white pre-pupae; modENCODE #4981) using independently derived antibodies [40]. (PDF 588 kb
Born extra-eccentric: A broad spectrum of primordial configurations of the gas giants that match their present-day orbits
In a recent paper we proposed that the giant planets' primordial orbits may
have been eccentric (~0.05), and used a suite of dynamical simulations to show
outcomes of the giant planet instability that are consistent with their
present-day orbits. In this follow-up investigation, we present more
comprehensive simulations incorporating superior particle resolution, longer
integration times, and eliminating our prior means of artificially forcing
instabilities to occur at specified times by shifting a planets' position in
its orbit. While we find that the residual phase of planetary migration only
minimally alters the the planets' ultimate eccentricities, our work uncovers
several intriguing outcomes in realizations where Jupiter and Saturn are born
with extremely large eccentricities (~0.10 and ~0.25, respectively). In
successful simulations, the planets' orbits damp through interactions with the
planetesimal disk prior to the instability, thus loosely replicating the
initial conditions considered in our previous work. Our results therefore
suggest an even wider range of plausible evolutionary pathways are capable of
replicating Jupiter and Saturn's modern orbital architecture.Comment: 12 pages, 3 figures, 2 tables, accepted for publication in Icaru
Terrestrial Planet Formation in Disks with Varying Surface Density Profiles
The ``minimum-mass solar nebula'' (MMSN) model estimates the surface density
distribution of the protoplanetary disk by assuming the planets to have formed
in situ. However, significant radial migration of the giant planets likely
occurred in the Solar system, implying a distortion in the values derived by
the MMSN method. The true density profiles of protoplanetary disks is therefore
uncertain. Here we present results of simulations of late-stage terrestrial
accretion, each starting from a disk of planetary embryos. We assume a
power-law surface density profile that varies with heliocentric distance r as
r^-alpha, and vary alpha between 1/2 and 5/2 (alpha = 3/2 for the MMSN model).
We find that for steeper profiles (higher values of alpha), the terrestrial
planets (i) are more numerous, (ii) form more quickly, (iii) form closer to the
star, (iv) are more massive, (v) have higher iron contents, and (vi) have lower
water contents. However, the possibility of forming potentially habitable
planets does not appear to vary strongly with alpha.Comment: 10 pages, 5 figures in emulateapj style. tp appear in Oct 20, 2005,
issue of Ap
Stringent Limits on the Polarized Submillimeter Emission from Protoplanetary Disks
We present arcsecond-resolution Submillimeter Array (SMA) polarimetric
observations of the 880 um continuum emission from the protoplanetary disks
around two nearby stars, HD 163296 and TW Hydrae. Although previous
observations and theoretical work have suggested that a 2-3% polarization
fraction should be common for the millimeter continuum emission from such
disks, we detect no polarized continuum emission above a 3-sigma upper limit of
7 mJy in each arcsecond-scale beam, or <1% in integrated continuum emission. We
compare the SMA upper limits with the predictions from the exploratory Cho &
Lazarian (2007) model of polarized emission from T Tauri disks threaded by
toroidal magnetic fields, and rule out their fiducial model at the ~10-sigma
level. We explore some potential causes for this discrepancy, focusing on model
parameters that describe the shape, magnetic field alignment, and size
distribution of grains in the disk. We also investigate related effects like
the magnetic field strength and geometry, scattering off of large grains, and
the efficiency of grain alignment, including recent advances in grain alignment
theory, which are not considered in the fiducial model. We discuss the impact
each parameter would have on the data and determine that the suppression of
polarized emission plausibly arises from rounding of large grains, reduced
efficiency of grain alignment with the magnetic field, and/or some degree of
magnetic field tangling (perhaps due to turbulence). A poloidal magnetic field
geometry could also reduce the polarization signal, particularly for a face-on
viewing geometry like the TW Hya disk. The data provided here offer the most
stringent limits to date on the polarized millimeter-wavelength emission from
disks around young stars.Comment: 15 pages, 6 figures, accepted for publication in Ap
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