628 research outputs found
Energy-limited escape revised
Gas planets in close proximity to their host stars experience
photoevaporative mass loss. The energy-limited escape concept is generally used
to derive estimates for the planetary mass-loss rates. Our photoionization
hydrodynamics simulations of the thermospheres of hot gas planets show that the
energy-limited escape concept is valid only for planets with a gravitational
potential lower than ergg because in these planets the radiative energy input is
efficiently used to drive the planetary wind. Massive and compact planets with
ergg
exhibit more tightly bound atmospheres in which the complete radiative energy
input is re-emitted through hydrogen Ly and free-free emission. These
planets therefore host hydrodynamically stable thermospheres. Between these two
extremes the strength of the planetary winds rapidly declines as a result of a
decreasing heating efficiency. Small planets undergo enhanced evaporation
because they host expanded atmospheres that expose a larger surface to the
stellar irradiation. We present scaling laws for the heating efficiency and the
expansion radius that depend on the gravitational potential and irradiation
level of the planet. The resulting revised energy-limited escape concept can be
used to derive estimates for the mass-loss rates of super-Earth-sized planets
as well as massive hot Jupiters with hydrogen-dominated atmospheres.Comment: 5 pages, 5 figures, accepted for publication in A&
High-energy irradiation and mass loss rates of hot Jupiters in the solar neighborhood
Giant gas planets in close proximity to their host stars experience strong
irradiation. In extreme cases photoevaporation causes a transonic, planetary
wind and the persistent mass loss can possibly affect the planetary evolution.
We have identified nine hot Jupiter systems in the vicinity of the Sun, in
which expanded planetary atmospheres should be detectable through Lyman alpha
transit spectroscopy according to predictions. We use X-ray observations with
Chandra and XMM-Newton of seven of these targets to derive the high-energy
irradiation level of the planetary atmospheres and the resulting mass loss
rates. We further derive improved Lyman alpha luminosity estimates for the host
stars including interstellar absorption. According to our estimates WASP-80 b,
WASP-77 b, and WASP-43 b experience the strongest mass loss rates, exceeding
the mass loss rate of HD 209458 b, where an expanded atmosphere has been
confirmed. Furthermore, seven out of nine targets might be amenable to Lyman
alpha transit spectroscopy. Finally, we check the possibility of angular
momentum transfer from the hot Jupiters to the host stars in the three binary
systems among our sample, but find only weak indications for increased stellar
rotation periods of WASP-77 and HAT-P-20.Comment: 11 pages, 5 figures, accepted for publication in A&
Temporal coding in the hippocampus
There is a large body of evidence that the hippocampus is involved in temporal
aspects of memory. It remains unclear what neural processes within the hippocampus
contribute to this ability. The following experiments aim to quantify and qualify these
neural processes while rats perform temporal memory tasks. First we examined the firing
of neurons in the hippocampus while rats compared a current series of odors to a learned
sequence of odors. We found evidence of neural correlates which might represent
whether a stimulus odor was in the correct ordinal sequence or not. Next we examined
the delay intervals in between learned sequences of events with the goal of identifying the
origin of “time cells” in the hippocampus. We used a delayed alternating T-maze task
that our lab has used before to record time cells in area CA1 of the hippocampus. We
found time cells in CA3, one of the major inputs to CA1 and demonstrated that they
behave in many ways like place cells previously observed in these two regions. Time
cells had previously been reported to occur only when an animal is engaged in a task with
memory load. We demonstrated that memory load isn't necessary to observe time cells.
Our observations of the similarities between place and time cells led us to conjecture that
the hippocampus might process space and time similarly. In a final study I examined time
vii
cell firing properties with an aim at constraining models of time cells. We defined time
cells in several ways including a new methodology that is promising as a future unbiased
selection criteria. All of our findings help further elucidate several different ways that
neural coding in the hippocampus contributes to temporal processing
Coronal X-ray emission and planetary irradiation in HD 209458
HD 209458 is one of the benchmark objects in the study of hot Jupiter
atmospheres and their evaporation through planetary winds. The expansion of the
planetary atmosphere is thought to be driven by high-energy EUV and X-ray
irradiation. We obtained new Chandra HRC-I data, which unequivocally show that
HD 209458 is an X-ray source. Combining these data with archival XMM-Newton
observations, we find that the corona of HD 209458 is characterized by a
temperature of about 1 MK and an emission measure of 7e49 cm^-3, yielding an
X-ray luminosity of 1.6e27 erg/s in the 0.124-2.48 keV band. HD 209458 is an
inactive star with a coronal temperature comparable to that of the inactive Sun
but a larger emission measure. At this level of activity, the planetary
high-energy emission is sufficient to support mass-loss at a rate of a few
times 1e10 g/s.Comment: Accepted for publication in A&
TPCI: The PLUTO-CLOUDY Interface
We present an interface between the (magneto-) hydrodynamics code PLUTO and
the plasma simulation and spectral synthesis code CLOUDY. By combining these
codes, we constructed a new photoionization hydrodynamics solver: The
PLUTO-CLOUDY Interface (TPCI), which is well suited to simulate
photoevaporative flows under strong irradiation. The code includes the
electromagnetic spectrum from X-rays to the radio range and solves the
photoionization and chemical network of the 30 lightest elements. TPCI follows
an iterative numerical scheme: First, the equilibrium state of the medium is
solved for a given radiation field by CLOUDY, resulting in a net radiative
heating or cooling. In the second step, the latter influences the (magneto-)
hydrodynamic evolution calculated by PLUTO. Here, we validated the
one-dimensional version of the code on the basis of four test problems:
Photoevaporation of a cool hydrogen cloud, cooling of coronal plasma, formation
of a Stroemgren sphere, and the evaporating atmosphere of a hot Jupiter. This
combination of an equilibrium photoionization solver with a general MHD code
provides an advanced simulation tool applicable to a variety of astrophysical
problems.Comment: 13 pages, 10 figures, accepted for publication in A&
Science-based restoration monitoring of coastal habitats, Volume Two: Tools for monitoring coastal habitats
Healthy coastal habitats are not only important ecologically; they also support healthy coastal communities and improve the quality of people’s lives. Despite their many benefits and values, coastal habitats have been systematically modified, degraded, and destroyed throughout the United States and its protectorates beginning with European colonization in the 1600’s (Dahl 1990). As a result, many coastal habitats around the United States are in desperate need of restoration. The monitoring of restoration projects, the focus of this document, is necessary to ensure that restoration efforts are successful, to further the science, and to increase the efficiency of future restoration efforts
Определение условий перехода вязкой суспензии в вязкопластинчатый материал на сите виброгрохота
На підставі рішення рівнянь Нав’є-Стокса для установленої течії в’язкої сировини, що не стискується, по похилій ситовій поверхні аналітично визначені умови переходу вугільної шламової суспензії у в’язкопластичний матеріал при зневоднені на ситі віброгрохота.На основе решения уравнений Навье-Стокса для установившегося течения вязкой несжигаемой жидкости по наклонной ситовой поверхности аналитически определены условия перехода угольной шламовой суспензии в вязкопластинчатый материал при обезвоживании на сите виброгрохота
Nonspatial sequence coding in CA1 neurons
The hippocampus is critical to the memory for sequences of events, a defining feature of episodic memory. However, the fundamental neuronal mechanisms underlying this capacity remain elusive. While considerable research indicates hippocampal neurons can represent sequences of locations, direct evidence of coding for the memory of sequential relationships among nonspatial events remains lacking. To address this important issue, we recorded neural activity in CA1 as rats performed a hippocampus-dependent sequencememory task. Briefly, the task involves the presentation of repeated sequences of odors at a single port and requires rats to identify each item as “in sequence” or “out of sequence”. We report that, while the animals’ location and behavior remained constant, hippocampal activity differed depending on the temporal context of items—in this case, whether they were presented in or out of sequence. Some neurons showed this effect across items or sequence positions (general sequence cells), while others exhibited selectivity for specific conjunctions of item and sequence position information (conjunctive sequence cells) or for specific probe types (probe-specific sequence cells). We also found that the temporal context of individual trials could be accurately decoded from the activity of neuronal ensembles, that sequence coding at the single-cell and ensemble level was linked to sequence memory performance, and that slow-gamma oscillations (20–40 Hz) were more strongly modulated by temporal context and performance than theta oscillations (4–12 Hz). These findings provide compelling evidence that sequence coding extends beyond the domain of spatial trajectories and is thus a fundamental function of the hippocampus
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