1,076 research outputs found
Debris discs around M stars: non-existence versus non-detection
Motivated by the reported dearth of debris discs around M stars, we use
survival models to study the occurrence of planetesimal discs around them.
These survival models describe a planetesimal disc with a small number of
parameters, determine if it may survive a series of dynamical processes and
compute the associated infrared excess. For the WISE satellite, we demonstrate
that the dearth of debris discs around M stars may be attributed to the small
semi-major axes generally probed if either: 1. the dust grains behave like
blackbodies emitting at a peak wavelength coincident with the observed one; 2.
or the grains are hotter than predicted by their blackbody temperatures and
emit at peak wavelengths that are shorter than the observed one. At these small
distances from the M star, planetesimals are unlikely to survive or persist for
time scales of 300 Myr or longer if the disc is too massive. Conversely, our
survival models allow for the existence of a large population of low-mass
debris discs that are too faint to be detected with current instruments.
However, our interpretation becomes less clear and large infrared excesses are
allowed if only one of these scenarios holds: 3. the dust grains are hotter
than blackbody and predominantly emit at the observed wavelength; 4. or are
blackbody in nature and emit at peak wavelengths longer than the observed one.
Both scenarios imply that the parent planetesimals reside at larger distances
from the star than inferred if the dust grains behaved like blackbodies. In all
scenarios, we show that the infrared excesses detected at 22 and 70 microns
from AU Mic are easily reconciled with its young age. We elucidate the
conditions under which stellar wind drag may be neglected when considering dust
populations around M stars. The WISE satellite should be capable of detecting
debris discs around young M stars with ages on the order of 10 Myr.Comment: Accepted by MNRAS. 11 pages, 9 figure
Layered Quantum Key Distribution
We introduce a family of QKD protocols for distributing shared random keys
within a network of users. The advantage of these protocols is that any
possible key structure needed within the network, including broadcast keys
shared among subsets of users, can be implemented by using a particular
multi-partite high-dimensional quantum state. This approach is more efficient
in the number of quantum channel uses than conventional quantum key
distribution using bipartite links. Additionally, multi-partite
high-dimensional quantum states are becoming readily available in quantum
photonic labs, making the proposed protocols implementable using current
technology.Comment: 11 pages, 5 figures. In version 2 we extended section 4 about the
dimension-rate trade-off and corrected minor error
On the gap-opening criterion of migrating planets in protoplanetary disks
We perform two-dimensional hydrodynamical simulations to quantitatively
explore the torque balance criterion for gap-opening (as formulated by Crida et
al. 2006) in a variety of disks when considering a migrating planet. We find
that even when the criterion is satisfied, there are instances when planets
still do not open gaps. We stress that gap-opening is not only dependent on
whether a planet has the ability to open a gap, but whether it can do so
quickly enough. This can be expressed as an additional condition on the
gap-opening timescale versus the crossing time, i.e. the time it takes the
planet to cross the region which it is carving out. While this point has been
briefly made in the previous literature, our results quantify it for a range of
protoplanetary disk properties and planetary masses, demonstrating how crucial
it is for gap-opening. This additional condition has important implications for
the survival of planets formed by core accretion in low mass disks as well as
giant planets or brown dwarfs formed by gravitational instability in massive
disks. It is particularly important for planets with intermediate masses
susceptible to Type III-like migration. For some observed transition disks or
disks with gaps, we expect that estimates on the potential planet masses based
on the torque balance gap-opening criterion alone may not be sufficient. With
consideration of this additional timescale criterion theoretical studies may
find a reduced planet survivability or that planets may migrate further inwards
before opening a gap.Comment: Accepted by ApJ, 22 pages, 13 figures, 6 table
Evolutionary models of cold and low-mass planets: Cooling curves, magnitudes, and detectability
Future instruments like NIRCam and MIRI on JWST or METIS at the ELT will be
able to image exoplanets that are too faint for current direct imaging
instruments. Evolutionary models predicting the planetary intrinsic luminosity
as a function of time have traditionally concentrated on gas-dominated giant
planets. We extend these cooling curves to Saturnian and Neptunian planets. We
simulate the cooling of isolated core-dominated and gas giant planets with
masses of 5 Earthmasses to 2 Jupitermasses. The luminosity includes the
contribution from the cooling and contraction of the core and of the H/He
envelope, as well as radiogenic decay. For the atmosphere we use grey,
AMES-Cond, petitCODE, and HELIOS models. We consider solar and non-solar
metallicities as well as cloud-free and cloudy atmospheres. The most important
initial conditions, namely the core-to-envelope ratio and the initial
luminosity are taken from planet formation simulations based on the core
accretion paradigm. We first compare our cooling curves for Uranus, Neptune,
Jupiter, Saturn, GJ 436b, and a 5 Earthmass-planet with a 1% H/He envelope with
other evolutionary models. We then present the temporal evolution of planets
with masses between 5 Earthmasses and 2 Jupitermasses in terms of their
luminosity, effective temperature, radius, and entropy. We discuss the impact
of different post formation entropies. For the different atmosphere types and
initial conditions magnitudes in various filter bands between 0.9 and 30
micrometer wavelength are provided. Using black body fluxes and non-grey
spectra, we estimate the detectability of such planets with JWST. It is found
that a 20 (100) Earthmass-planet can be detected with JWST in the background
limit up to an age of about 10 (100) Myr with NIRCam and MIRI, respectively.Comment: Language corrected version and improved arrangements of figures,
online data at:
http://www.space.unibe.ch/research/research_groups/planets_in_time/numerical_data/index_eng.htm
Revisiting the Phase Curves of WASP-43b: Confronting Reanalyzed Spitzer Data with Cloudy Atmospheres
Recently acquired Hubble and Spitzer phase curves of the short-period hot
Jupiter WASP-43b make it an ideal target for confronting theory with data. On
the observational front, we re-analyze the 3.6 and 4.5 m Spitzer phase
curves and demonstrate that our improved analysis better removes residual red
noise due to intra-pixel sensitivity, which leads to greater fluxes emanating
from the nightside of WASP-43b, thus reducing the tension between theory and
data. On the theoretical front, we construct cloudfree and cloudy atmospheres
of WASP-43b using our Global Circulation Model (GCM), THOR, which solves the
non-hydrostatic Euler equations (compared to GCMs that typically solve the
hydrostatic primitive equations). The cloudfree atmosphere produces a
reasonable fit to the dayside emission spectrum. The multi-phase emission
spectra constrain the cloud deck to be confined to the nightside and have a
finite cloud-top pressure. The multi-wavelength phase curves are naturally
consistent with our cloudy atmospheres, except for the 4.5 m phase curve,
which requires the presence of enhanced carbon dioxide in the atmosphere of
WASP-43b. Multi-phase emission spectra at higher spectral resolution, as may be
obtained using the James Webb Space Telescope, and a reflected-light phase
curve at visible wavelengths would further constrain the properties of clouds
in WASP-43b.Comment: Accepted for publication in Ap
The Peculiar Atmospheric Chemistry of KELT-9b
The atmospheric temperatures of the ultra-hot Jupiter KELT-9b straddle the
transition between gas giants and stars, and therefore between two
traditionally distinct regimes of atmospheric chemistry. Previous theoretical
studies assume the atmosphere of KELT-9b to be in chemical equilibrium. Despite
the high ultraviolet flux from KELT-9, we show using photochemical kinetics
calculations that the observable atmosphere of KELT-9b is predicted to be close
to chemical equilibrium, which greatly simplifies any theoretical
interpretation of its spectra. It also makes the atmosphere of KELT-9b, which
is expected to be cloudfree, a tightly constrained chemical system that lends
itself to a clean set of theoretical predictions. Due to the lower pressures
probed in transmission (compared to emission) spectroscopy, we predict the
abundance of water to vary by several orders of magnitude across the
atmospheric limb depending on temperature, which makes water a sensitive
thermometer. Carbon monoxide is predicted to be the dominant molecule under a
wide range of scenarios, rendering it a robust diagnostic of the metallicity
when analyzed in tandem with water. All of the other usual suspects (acetylene,
ammonia, carbon dioxide, hydrogen cyanide, methane) are predicted to be
subdominant at solar metallicity, while atomic oxygen, iron and magnesium are
predicted to have relative abundances as high as 1 part in 10,000. Neutral
atomic iron is predicted to be seen through a forest of optical and
near-infrared lines, which makes KELT-9b suitable for high-resolution
ground-based spectroscopy with HARPS-N or CARMENES. We summarize future
observational prospects of characterizing the atmosphere of KELT-9b.Comment: Accepted by ApJ. 9 pages, 6 figures. Corrected minor errors in
Figures 1a and 1b (some line styles were switched by accident), text and
conclusions unchanged, these minor changes will be updated in final ApJ proo
HELIOS-Retrieval: An Open-source, Nested Sampling Atmospheric Retrieval Code, Application to the HR 8799 Exoplanets and Inferred Constraints for Planet Formation
We present an open-source retrieval code named HELIOS-Retrieval (hereafter
HELIOS-R), designed to obtain chemical abundances and temperature-pressure
profiles from inverting the measured spectra of exoplanetary atmospheres. In
the current implementation, we use an exact solution of the radiative transfer
equation, in the pure absorption limit, in our forward model, which allows us
to analytically integrate over all of the outgoing rays (instead of performing
Gaussian quadrature). Two chemistry models are considered: unconstrained
chemistry (where the mixing ratios are treated as free parameters) and
equilibrium chemistry (enforced via analytical formulae, where only the
elemental abundances are free parameters). The nested sampling algorithm allows
us to formally implement Occam's Razor based on a comparison of the Bayesian
evidence between models. We perform a retrieval analysis on the measured
spectra of the HR 8799b, c, d and e directly imaged exoplanets. Chemical
equilibrium is disfavored by the Bayesian evidence for HR 8799b, c and d. We
find supersolar C/O, C/H and O/H values for the outer HR 8799b and c
exoplanets, while the inner HR 8799d and e exoplanets have substellar C/O,
substellar C/H and superstellar O/H values. If these retrieved properties are
representative of the bulk compositions of the exoplanets, then they are
inconsistent with formation via gravitational instability (without late-time
accretion) and consistent with a core accretion scenario in which late-time
accretion of ices occurred differently for the inner and outer exoplanets. For
HR 8799e, we find that spectroscopy in the K band is crucial for constraining
C/O and C/H. HELIOS-R is publicly available as part of the Exoclimes Simulation
Platform (ESP; www.exoclime.org).Comment: 27 pages, 21 figures, 3 tables, published in A
High-Dimensional Pixel Entanglement: Efficient Generation and Certification
Photons offer the potential to carry large amounts of information in their
spectral, spatial, and polarisation degrees of freedom. While state-of-the-art
classical communication systems routinely aim to maximize this
information-carrying capacity via wavelength and spatial-mode division
multiplexing, quantum systems based on multi-mode entanglement usually suffer
from low state quality, long measurement times, and limited encoding capacity.
At the same time, entanglement certification methods often rely on assumptions
that compromise security. Here we show the certification of photonic
high-dimensional entanglement in the transverse position-momentum
degree-of-freedom with a record quality, measurement speed, and entanglement
dimensionality, without making any assumptions about the state or channels.
Using a tailored macro-pixel basis, precise spatial-mode measurements, and a
modified entanglement witness, we demonstrate state fidelities of up to 94.4%
in a 19-dimensional state-space, entanglement in up to 55 local dimensions, and
an entanglement-of-formation of up to 4 ebits. Furthermore, our measurement
times show an improvement of more than two orders of magnitude over previous
state-of-the-art demonstrations. Our results pave the way for noise-robust
quantum networks that saturate the information-carrying capacity of single
photons
A Comparative Study of Atmospheric Chemistry with VULCAN
We present an update of the open-source photochemical kinetics code VULCAN (Tsai et al. 2017; this https URL) to include C-H-N-O-S networks and photochemistry. Additional new features are advection transport, condensation, various boundary conditions, and temperature-dependent UV cross-sections. First, we validate our photochemical model for hot Jupiter atmospheres by performing an intercomparison of HD 189733b models between Moses et al. (2011), Venot et al. (2012), and VULCAN, to diagnose possible sources of discrepancy. Second, we set up a model of Jupiter extending from the deep troposphere to upper stratosphere to verify the kinetics for low temperature. Our model reproduces hydrocarbons consistent with observations, and the condensation scheme successfully predicts the locations of water and ammonia ice clouds. We show that vertical advection can regulate the local ammonia distribution in the deep atmosphere. Third, we validate the model for oxidizing atmospheres by simulating Earth and find agreement with observations. Last, VULCAN is applied to four representative cases of extrasolar giant planets: WASP-33b, HD 189733b, GJ 436b, and 51 Eridani b. We look into the effects of the C/O ratio and chemistry of titanium/vanadium species for WASP-33b; we revisit HD 189733b for the effects of sulfur and carbon condensation; the effects of internal heating and vertical mixing (Kzz) are explored for GJ 436b; we test updated planetary properties for 51 Eridani b with S8 condensates. We find sulfur can couple to carbon or nitrogen and impact other species such as hydrogen, methane, and ammonia. The observable features of the synthetic spectra and trends in the photochemical haze precursors are discussed for each case
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