1,221 research outputs found
Biochemical processes in sagebrush ecosystems: Interactions with terrain
The objectives of a biogeochemical study of sagebrush ecosystems in Wyoming and their interactions with terrain are as follows: to describe the vegetational pattern on the landscape and elucidate controlling variables, to measure the soil properties and chemical cycling properties associated with the vegetation units, to associate soil properties with vegetation properties as measured on the ground, to develop remote sensing capabilities for vegetation and surface characteristics of the sagebrush landscape, to develop a system of sensing snow cover and indexing seasonal soil to moisture; and to develop relationships between temporal Thematic Mapper (TM) data and vegetation phenological state
Can stellar activity make a planet seem misaligned?
Several studies have shown that the occultation of stellar active regions by
the transiting planet can generate anomalies in the high-precision transit
light curves, and these anomalies may lead to an inaccurate estimate of the
planetary parameters (e.g., the planet radius). Since the physics and geometry
behind the transit light curve and the Rossiter- McLaughlin effect
(spectroscopic transit) are the same, the Rossiter-McLaughlin observations are
expected to be affected by the occultation of stellar active regions in a
similar way. In this paper we perform a fundamental test on the spin-orbit
angles as derived by Rossiter-McLaughlin measurements, and we examine the
impact of the occultation of stellar active regions by the transiting planet on
the spin-orbit angle estimations. Our results show that the inaccurate
estimation on the spin-orbit angle due to stellar activity can be quite
significant (up to 30 degrees), particularly for the edge-on, aligned, and
small transiting planets. Therefore, our results suggest that the aligned
transiting planets are the ones that can be easily misinterpreted as misaligned
owing to the stellar activity. In other words, the biases introduced by
ignoring stellar activity are unlikely to be the culprit for the highly
misaligned systems.Comment: 8 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Solving mazes with memristors: a massively-parallel approach
Solving mazes is not just a fun pastime. Mazes are prototype models in graph theory, topology, robotics, traffic optimization, psychology, and in many other areas of science and technology. However, when maze complexity increases their solution becomes cumbersome and very time consuming. Here, we show that a network of memristors - resistors with memory - can solve such a non-trivial problem quite easily. In particular, maze solving by the network of memristors occurs in a massively parallel fashion since all memristors in the network participate simultaneously in the calculation. The result of the calculation is then recorded into the memristors’ states, and can be used and/or recovered at a later time. Furthermore, the network of memristors finds all possible solutions in multiple-solution mazes, and sorts out the solution paths according to their length. Our results demonstrate not only the first application of memristive networks to the field of massively-parallel computing, but also a novel algorithm to solve mazes which could find applications in different research fields
A Survey for Spectroscopic Binaries Among Very Low-Mass Stars
We report on the results of a survey for radial velocity variability in a
heterogeneous sample of very low-mass stars and brown dwarfs. One
distinguishing characteristic of the survey is its timespan, which allows an
overlap between spectroscopic binaries and those which can be found by high
angular-resolution imaging. We are able to place a new constraint on the total
binary fraction in these objects, which suggests that they are more likely the
result of extending the same processes at work at higher masses into this mass
range, rather than a distinct mode of formation. Our basic result is that there
are out of 53, or % spectroscopic binaries in the
separation range 0-6 AU, nearly as many as resolved binaries. This leads to an
estimate of an upper limit of % for the binary fraction of VLM
objects (it is an upper limit because of the possible overlap between the
spectroscopic and resolved populations). A reasonable estimate for the very
low-mass binary fraction is %. We consider several possible separation
and frequency distributions, including the same one as found for GK stars, a
compressed version of that, a version of the compressed distribution truncated
at 15 AU, and a theoretical distribution which considers the evaporation of
small-N clusters. We conclude that the latter two bracket the observations,
which may mean that these systems form with intrinsically smaller separations
due to their smaller mass, and then are truncated due to their smaller binding
energy. We do not find support for the ``ejection hypothesis'' as their
dominant mode of formation, particularly in view of the similarity in the total
binary fraction compared with slightly more massive stars, and the difficulty
this mechanism has in producing numerous binary systems.Comment: 36 pages, accepted for publication in AJ, abstract shortened for
arXiv.or
A high resolution spectral atlas of brown dwarfs
We present a UVES/VLT high resolution atlas of three L dwarfs and one T dwarf
system, spectral classes at which most of the objects are brown dwarfs. Our
atlas covers the optical region from H up to the near infrared at 1
m. We present spectral details of ultra-cool atmospheres at very high
resolution () and compare the spectra to model calculations. Our
comparison shows that molecular features from VO and CaH, and atomic features
from Cs and Rb are reasonably well fit by current models. On the other hand,
features due to TiO, CrH, and water, and atomic Na and K reveal large
discrepancies between model calculations and our observations.Comment: 17 pages, 11 figures, accepted by A&A, reduced figure quality for
arXi
Extremely low long‐term erosion rates around the Gamburtsev Mountains in interior East Antarctica
The high elevation and rugged relief (>3 km) of the Gamburtsev Subglacial Mountains (GSM) have long been considered enigmatic. Orogenesis normally occurs near plate boundaries, not cratonic interiors, and large‐scale tectonic activity last occurred in East Antarctica during the Pan‐African (480–600 Ma). We sampled detrital apatite from Eocene sands in Prydz Bay at the terminus of the Lambert Graben, which drained a large pre‐glacial basin including the northern Gamburtsev Mountains. Apatite fission‐track and (U‐Th)/He cooling ages constrain bedrock erosion rates throughout the catchment. We double‐dated apatites to resolve individual cooling histories. Erosion was very slow, averaging 0.01–0.02 km/Myr for >250 Myr, supporting the preservation of high elevation in interior East Antarctica since at least the cessation of Permian rifting. Long‐term topographic preservation lends credence to postulated high‐elevation mountain ice caps in East Antarctica since at least the Cretaceous and to the idea that cold‐based glaciation can preserve tectonically inactive topography
Chromospheric Variability in SDSS M Dwarfs. II. Short-Timescale H-alpha Variability
[Abridged] We present the first comprehensive study of short-timescale
chromospheric H-alpha variability in M dwarfs using the individual 15 min
spectroscopic exposures for 52,392 objects from the Sloan Digital Sky Survey.
Our sample contains about 10^3-10^4 objects per spectral type bin in the range
M0-M9, with a total of about 206,000 spectra and a typical number of 3
exposures per object (ranging up to a maximum of 30 exposures). Using this
extensive data set we find that about 16% of the sources exhibit H-alpha
emission in at least one exposure, and of those about 45% exhibit H-alpha
emission in all of the available exposures. Within the sample of objects with
H-alpha emission, only 26% are consistent with non-variable emission,
independent of spectral type. The H-alpha variability, quantified in terms of
the ratio of maximum to minimum H-alpha equivalent width (R_EW), and the ratio
of the standard deviation to the mean (sigma_EW/), exhibits a rapid rise
from M0 to M5, followed by a plateau and a possible decline in M9 objects. In
particular, R_EW increases from a median value of about 1.8 for M0-M3 to about
2.5 for M7-M9, and variability with R_EW>10 is only observed in objects later
than M5. For the combined sample we find that the R_EW values follow an
exponential distribution with N(R_EW) exp[-(R_EW-1)/2]; for M5-M9 objects the
characteristic scale is R_EW-1\approx 2.7, indicative of stronger variability.
In addition, we find that objects with persistent H-alpha emission exhibit
smaller values of R_EW than those with intermittent H-alpha emission. Based on
these results we conclude that H-alpha variability in M dwarfs on timescales of
15 min to 1 hr increases with later spectral type, and that the variability is
larger for intermittent sources.Comment: Submitted to ApJ; 20 pages, 15 figure
Large-scale magnetic topologies of late M dwarfs
We present here the final results of the first spectropolarimetric survey of
a small sample of active M dwarfs, aimed at providing observational constraints
on dynamo action on both sides of the full-convection threshold (spectral type
M4). Our two previous studies (Donati et al. 2008b; Morin et al. 2008b) were
focused on early and mid M dwarfs. The present paper examines 11 fully
convective late M dwarfs (spectral types M5-M8). Tomographic imaging techniques
were applied to time-series of circularly polarised profiles of 6 stars, in
order to infer their large-scale magnetic topologies. For 3 other stars we
could not produce such magnetic maps, because of low variability of the Stokes
V signatures, but were able to derive some properties of the magnetic fields.
We find 2 distinct categories of magnetic topologies: on the one hand strong
axisymmetric dipolar fields (similar to mid M dwarfs), and on the other hand
weak fields generally featuring a significant non-axisymmetric component, and
sometimes a significant toroidal one. Comparison with unsigned magnetic fluxes
demonstrates that the second category of magnetic fields shows less
organization (less energy in the large scales), similarly to partly convective
early M dwarfs. Stars in both categories have similar stellar parameters, our
data do not evidence a separation between these 2 categories in the
mass-rotation plane. We also report marginal detection of a large-scale
magnetic field on the M8 star VB 10 featuring a significant toroidal
axisymmetric component, whereas no field is detectable on VB 8 (M7).Comment: 26 pages, 16 figures, 9 tables, 11 tables in appendix. Accepted for
publication in MNRA
The Effects of Close Companions (and Rotation) on the Magnetic Activity of M Dwarfs
We present a study of close white dwarf and M dwarf (WD+dM) binary systems
and examine the effect that a close companion has on the magnetic field
generation in M dwarfs. We use a base sample of 1602 white dwarf -- main
sequence binaries from Rebassa et al. to develop a set of color cuts in GALEX,
SDSS, UKIDSS, and 2MASS color space to construct a sample of 1756 WD+dM
high-quality pairs from the SDSS DR8 spectroscopic database. We separate the
individual WD and dM from each spectrum using an iterative technique that
compares the WD and dM components to best-fit templates. Using the absolute
height above the Galactic plane as a proxy for age, and the H{\alpha} emission
line as an indicator for magnetic activity, we investigate the age-activity
relation for our sample for spectral types \leqM7. Our results show that
early-type M dwarfs (\leqM4) in close binary systems are more likely to be
active and have longer activity lifetimes compared to their field counterparts.
However, at a spectral type of M5 (just past the onset of full convection in M
dwarfs), the activity fraction and lifetimes of WD+dM binary systems becomes
more comparable to that of the field M dwarfs. One of the implications of
having a close binary companion is presumed to be increased stellar rotation
through disk-disruption, tidal effects, or angular momentum exchange. Thus, we
interpret the similarity in activity behavior between late-type dMs in WD+dM
pairs and late-type field dMs to be due to a decrease in sensitivity in close
binary companions (or stellar rotation), which has implications for the nature
of magnetic activity in fully-convective stars. (Abridged)Comment: 21 pages, 19 figures, emulateapj style, accepted to Astronomical
Journal June 28, 201
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