50,581 research outputs found
Accounting for Multiplicity in Calculating Eta Earth
Using the updated exoplanet population parameters of our previous study,
which includes the planetary radius updates from Gaia DR2 and an inferred
multiplicity distribution, we provide a revised calculation.
This is achieved by sampling planets from our derived population model and
determining which planets meet our criterion for habitability. To ensure robust
results, we provide probabilities calculated over a range of upper radius
limits. Our most optimistic criterion for habitability provides an
value of . We also
consider the effects of multiplicity and the number of habitable planets each
system may contain. Our calculation indicates that of GK dwarfs
have more than one planet within their habitable zone. This optimistic
habitability criterion also suggests that of solar-like stars
will harbor 5 or more habitable planets. These tightly packed highly habitable
system should be extremely rare, but still possible. Even with our most
pessimistic criterion we still expect that of solar-like stars
harbor more than one habitable planet.Comment: 7 pages, 1 figure; Accepted for publication in MNRA
Co- and counter-helicity interaction between two adjacent laboratory prominences
The interaction between two side-by-side solar prominence-like plasmas has been studied using a four-electrode magnetized plasma source that can impose a wide variety of surface boundary conditions. When the source is arranged to create two prominences with the same helicity (co-helicity), it is observed that helicity transfer from one prominence to the other causes the receiving prominence to erupt sooner and faster than the transmitting prominence. When the source is arranged to create two prominences with opposite helicity (counter-helicity), it is observed that upon merging, prominences wrap around each other to form closely spaced, writhing turns of plasma. This is followed by appearance of a distinct bright region in the middle and order of magnitude higher emission of soft x rays. The four-electrode device has also been used to change the angle of the neutral line and so form more pronounced S-shapes
Dissipation Layers in Rayleigh-B\'{e}nard Convection: A Unifying View
Boundary layers play an important role in controlling convective heat
transfer. Their nature varies considerably between different application areas
characterized by different boundary conditions, which hampers a uniform
treatment. Here, we argue that, independent from boundary conditions,
systematic dissipation measurements in Rayleigh-B\'enard convection capture the
relevant near-wall structures. By means of direct numerical simulations with
varying Prandtl numbers, we demonstrate that such dissipation layers share
central characteristics with classical boundary layers, but, in contrast to the
latter, can be extended naturally to arbitrary boundary conditions. We validate
our approach by explaining differences in scaling behavior observed for no-slip
and stress-free boundaries, thus paving the way to an extension of scaling
theories developed for laboratory convection to a broad class of natural
systems
The scalar perturbation spectral index n_s: WMAP sensitivity to unresolved point sources
Precision measurement of the scalar perturbation spectral index, n_s, from
the Wilkinson Microwave Anisotropy Probe temperature angular power spectrum
requires the subtraction of unresolved point source power. Here we reconsider
this issue. First, we note a peculiarity in the WMAP temperature likelihood's
response to the source correction: Cosmological parameters do not respond to
increased source errors. An alternative and more direct method for treating
this error term acts more sensibly, and also shifts n_s by ~0.3 sigma closer to
unity. Second, we re-examine the source fit used to correct the power spectrum.
This fit depends strongly on the galactic cut and the weighting of the map,
indicating that either the source population or masking procedure is not
isotropic. Jackknife tests appear inconsistent, causing us to assign large
uncertainties to account for possible systematics. Third, we note that the WMAP
team's spectrum was computed with two different weighting schemes: uniform
weights transition to inverse noise variance weights at l = 500. The fit
depends on such weighting schemes, so different corrections apply to each
multipole range. For the Kp2 mask used in cosmological analysis, we prefer
source corrections A = 0.012 +/- 0.005 muK^2 for uniform weighting and A =
0.015 +/- 0.005 muK^2 for N_obs weighting. Correcting WMAP's spectrum
correspondingly, we compute cosmological parameters with our alternative
likelihood, finding n_s = 0.970 +/- 0.017 and sigma_8 = 0.778 +/- 0.045 . This
n_s is only 1.8 sigma from unity, compared to the ~2.6 sigma WMAP 3-year
result. Finally, an anomalous feature in the source spectrum at l<200 remains,
most strongly associated with W-band.Comment: 9 pages, 10 figures, 3 tables. Submitted to Ap
Changing Face of the Extrasolar Giant Planet, HD 209458b
High-resolution atmospheric flow simulations of the tidally-locked extrasolar
giant planet, HD 209458b, show large-scale spatio-temporal variability. This is
in contrast to the simple, permanent day/night (i.e., hot/cold) picture. The
planet's global circulation is characterized by a polar vortex in motion around
each pole and a banded structure corresponding to ~3 broad zonal (east-west)
jets. For very strong jets, the circulation-induced temperature difference
between moving hot and cold regions can reach up to ~1000 K, suggesting that
atmospheric variability could be observed in the planet's spectral and
photometric signatures.Comment: 6 pages, 1 ps figure, 2 low-res color figures (JPEG). Figure 3
updated. Contact authors for hi-res versions of color figures. Accepted for
publication in ApJ
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