3,577 research outputs found
Do Two Temperature Debris Disks Have Multiple Belts?
We present a study of debris disks whose spectra are well modelled by dust
emission at two different temperatures. These disks are typically assumed to be
a sign of multiple belts, which in only a few cases have been confirmed via
high resolution observations. We first compile a sample of two-temperature
disks to derive their properties, summarised by the ratios of the warm and cool
component temperatures and fractional luminosities. The ratio of warm to cool
temperatures is constant in the range 2-4, and the temperatures of both warm
and cool components increases with stellar mass. We then explore whether this
emission can arise from dust in a single narrow belt, with the range of
temperatures arising from the size variation of grain temperatures. This model
can produce two-temperature spectra for Sun-like stars, but is not supported
where it can be tested by observed disk sizes and far-IR/mm spectral slopes.
Therefore, while some two-temperature disks arise from single belts, it is
probable that most have multiple spatial components. These disks are plausibly
similar to the outer Solar System's configuration of Asteroid and
Edgeworth-Kuiper belts separated by giant planets. Alternatively, the inner
component could arise from inward scattering of material from the outer belt,
again due to intervening planets. In either case, we suggest that the ratio of
warm/cool component temperatures is indicative of the scale of outer planetary
systems, which typically span a factor of about ten in radius.Comment: accepted to MNRA
Constraining the orbits of sub-stellar companions imaged over short orbital arcs
Imaging a star's companion at multiple epochs over a short orbital arc
provides only four of the six coordinates required for a unique orbital
solution. Probability distributions of possible solutions are commonly
generated by Monte Carlo (MCMC) analysis, but these are biased by priors and
may not probe the full parameter space. We suggest alternative methods to
characterise possible orbits, which compliment the MCMC technique. Firstly the
allowed ranges of orbital elements are prior-independent, and we provide means
to calculate these ranges without numerical analyses. Hence several interesting
constraints (including whether a companion even can be bound, its minimum
possible semi-major axis and its minimum eccentricity) may be quickly computed
using our relations as soon as orbital motion is detected. We also suggest an
alternative to posterior probability distributions as a means to present
possible orbital elements, namely contour plots of elements as functions of
line of sight coordinates. These plots are prior-independent, readily show
degeneracies between elements and allow readers to extract orbital solutions
themselves. This approach is particularly useful when there are other
constraints on the geometry, for example if a companion's orbit is assumed to
be aligned with a disc. As examples we apply our methods to several imaged
sub-stellar companions including Fomalhaut b, and for the latter object we show
how different origin hypotheses affect its possible orbital solutions. We also
examine visual companions of A- and G-type main sequence stars in the
Washington Double Star Catalogue, and show that per cent must be
unbound.Comment: Accepted for publication in MNRA
Herschel Observations of Debris Discs Orbiting Planet-hosting Subgiants
Debris discs are commonly detected orbiting main-sequence stars, yet little
is known regarding their fate as the star evolves to become a giant. Recent
observations of radial velocity detected planets orbiting giant stars highlight
this population and its importance for probing, for example, the population of
planetary systems orbiting intermediate mass stars. Our Herschel survey
observed a subset of the Johnson et al program subgiants, finding that 4/36
exhibit excess emission thought to indicate debris, of which 3/19 are
planet-hosting stars and 1/17 are stars with no current planet detections.
Given the small numbers involved, there is no evidence that the disc detection
rate around stars with planets is different to that around stars without
planets. Our detections provide a clear indication that large quantities of
dusty material can survive the stars' main-sequence lifetime and be detected on
the subgiant branch, with important implications for the evolution of planetary
systems and observations of polluted or dusty white dwarfs. Our detection rates
also provide an important constraint that can be included in models of debris
disc evolution.Comment: 12 pages, MNRAS, accepte
Resolved Imaging of the HR 8799 Debris Disk with Herschel
We present Herschel far-infrared and submillimeter maps of the debris disk
associated with the HR 8799 planetary system. We resolve the outer disk
emission at 70, 100, 160 and 250 um and detect the disk at 350 and 500 um. A
smooth model explains the observed disk emission well. We observe no obvious
clumps or asymmetries associated with the trapping of planetesimals that is a
potential consequence of planetary migration in the system. We estimate that
the disk eccentricity must be <0.1. As in previous work by Su et al. (2009), we
find a disk with three components: a warm inner component and two outer
components, a planetesimal belt extending from 100 - 310 AU, with some
flexibility (+/- 10 AU) on the inner edge, and the external halo which extends
to ~2000 AU. We measure the disk inclination to be 26 +/- 3 deg from face-on at
a position angle of 64 deg E of N, establishing that the disk is coplanar with
the star and planets. The SED of the disk is well fit by blackbody grains whose
semi-major axes lie within the planetesimal belt, suggesting an absence of
small grains. The wavelength at which the spectrum steepens from blackbody, 47
+/- 30 um, however, is short compared to other A star debris disks, suggesting
that there are atypically small grains likely populating the halo. The PACS
longer wavelength data yield a lower disk color temperature than do MIPS data
(24 and 70 um), implying two distinct halo dust grain populations.Comment: 13 pages, 8 figures (6 color), accepted for publication in the
Astrophysical Journa
An experimental study of the P-V-T-S relations of seawater
The compressibility of seawater has been determined as a function of salinity, pressure, and temperature. Specific volumes have been computed. The salinities, as measured on a University of Washington-type conductivity bridge, were 0.00, 30.52, 34.99, and 41.03‰. The pressure range was from r to r 300 bars and the temperature range from 0°C to 25°C. The experiments were carried out by measuring decrements of volume at 100-bar intervals spaced in 5° isotherms...
MOCCA College: An assessment of inferential narrative and expository comprehension
[ES] MOCCA-C is an assessment of adult reading ability designed for early diagnosis of reading problems, for formative assessment in reading intervention planning, for assessment of reading improvement over time, and for assessment of reading intervention outcomes. It uses both narrative and expository reading passages and it currently has four forms. Two goals of this research were to compare narrative and expository passages on (a) their difficulty and (b) their ability to discriminate between good and poor readers. An additional goal was to assess whether narrative and expository passages measure the same or different comprehension dimensions. A final goal was to assess the reliability of forms. We randomly assigned students to forms with between 274 – 279 college students per form. Across the several forms, results suggest that narrative passages are easier and better discriminate between good and poor readers. However, both narrative and expository passages measure a single dimension of ability. MOCCA-C scores are reliable. Implications for research and practice are discussed.Davison, ML.; Seipel, B.; Clinton, V.; Carlson, S.; Kennedy, P.; Kennedy, PC. (2020). MOCCA College: An assessment of inferential narrative and expository comprehension. En 6th International Conference on Higher Education Advances (HEAd'20). Editorial Universitat Politècnica de València. (30-05-2020):417-425. https://doi.org/10.4995/HEAd20.2020.11081OCS41742530-05-202
Spatially Resolved Images of Dust Belt(s) Around the Planet-hosting Subgiant Kappa CrB
We present Herschel spatially resolved images of the debris disc orbiting the
subgiant Kappa CrB. Not only are these the first resolved images of a debris
disc orbiting a subgiant, but Kappa CrB is a rare example of an intermediate
mass star where a detailed study of the structure of the planetary system can
be made, including both planets and planetesimal belt(s). The only way to
discover planets around such stars using the radial velocity technique is to
observe 'retired' A stars, which are cooler and slower rotators compared to
their main-sequence counterparts. A planetary companion has already been
detected orbiting the subgiant Kappa CrB, with revised parameters of m sin i =
2.1MJ and apl = 2.8AU (Johnson et al. 2008a). We present additional Keck I
HIRES radial velocity measurements that provide evidence for a second planetary
companion, alongside Keck II AO imaging that places an upper limit on the mass
of this companion. Modelling of our Herschel images shows that the dust is
broadly distributed, but cannot distinguish between a single wide belt (from 20
to 220AU) or two narrow dust belts (at around 40 and 165AU). Given the
existence of a second planetary companion beyond approximately 3AU it is
possible that the absence of dust within approximately 20AU is caused by
dynamical depletion, although the observations are not inconsistent with
depletion of these regions by collisional erosion, which occurs at higher rates
closer to the star.Comment: Updated abstrac
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