488 research outputs found
Spitzer Infrared Spectrograph Observations of M, L, and T Dwarfs
We present the first mid-infrared spectra of brown dwarfs, together with
observations of a low-mass star. Our targets are the M3.5 dwarf GJ 1001A, the
L8 dwarf DENIS-P J0255-4700, and the T1/T6 binary system epsilon Indi Ba/Bb. As
expected, the mid-infrared spectral morphology of these objects changes rapidly
with spectral class due to the changes in atmospheric chemistry resulting from
their differing effective temperatures and atmospheric structures. By taking
advantage of the unprecedented sensitivity of the Infrared Spectrograph on the
Spitzer Space Telescope we have detected the 7.8 micron methane and 10 micron
ammonia bands for the first time in brown dwarf spectra.Comment: 4 pages, 2 figure
Information-theoretic interpretation of quantum error-correcting codes
Quantum error-correcting codes are analyzed from an information-theoretic
perspective centered on quantum conditional and mutual entropies. This approach
parallels the description of classical error correction in Shannon theory,
while clarifying the differences between classical and quantum codes. More
specifically, it is shown how quantum information theory accounts for the fact
that "redundant" information can be distributed over quantum bits even though
this does not violate the quantum "no-cloning" theorem. Such a remarkable
feature, which has no counterpart for classical codes, is related to the
property that the ternary mutual entropy vanishes for a tripartite system in a
pure state. This information-theoretic description of quantum coding is used to
derive the quantum analogue of the Singleton bound on the number of logical
bits that can be preserved by a code of fixed length which can recover a given
number of errors.Comment: 14 pages RevTeX, 8 Postscript figures. Added appendix. To appear in
Phys. Rev.
Diagnostic utility of B-type natriuretic peptide in critically ill patients with pulmonary edema: a prospective cohort study
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Nutrient turnover in forest floors of interior Alaska
In this study, we examined the loss of selected chemical elements, mass, and energy from decomposing quaking aspen (Populus tremuloides Michx.), paper birch (Betula papyrifera Marsh.), and alder (Alnus
crispa subsp. sinuata (Regel) Hult.) in upland aspen and birch stands near Fairbanks, Alaska. The following summary of the work,
much of which was conducted before funding of the present project, will appear this year as a short paper in Ecology. The paper is
included as part of this report because it describes experimental site locations and field and laboratory methods used in conducting this study. After the paper was prepared, additional sets of samples
were collected, which extended the duration of the study to 1,325 days. Regression analysis has now been conducted on all parameters originally discussed in the project proposal. These results, with the exception of those for Fe, Mn, Zn, and ash are summarized in Table 1. Further
examination of data for the later constituents is necessary before submitting these results. The equations relating energy loss to weight loss have also been extended to include data from the 1,325-day samples. Graphical presentation of this data will be available shortly
A Spitzer Infrared Spectrograph Spectral Sequence of M, L, and T Dwarfs
We present a low-resolution (R ≡ λ/Δλ ≈ 90), 5.5-38 μm spectral sequence of a sample of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The spectra exhibit prominent absorption bands of H_2O at 6.27 μm, CH_4 at 7.65 μm, and NH_3 at 10.5 μm and are relatively featureless at λ ≳ 15 μm. Three spectral indices that measure the strengths of these bands are presented; H_2O absorption features are present throughout the MLT sequence, while the CH_4 and NH_3 bands first appear at roughly the L/T transition. Although the spectra are, in general, qualitatively well matched by synthetic spectra that include the formation of spatially homogeneous silicate and iron condensate clouds, the spectra of the mid-type L dwarfs show an unexpected flattening from roughly 9 to 11 μm. We hypothesize that this may be a result of a population of small silicate grains that are not predicted in the cloud models. The spectrum of the peculiar T6 dwarf 2MASS J0937+2931 is suppressed from 5.5 to 7.5 μm relative to typical T6 dwarfs and may be a consequence of its mildly metal-poor/high surface gravity atmosphere. Finally, we compute bolometric luminosities of a subsample of the M, L, and T dwarfs by combining the IRS spectra with previously published 0.6-4.1 μm spectra and find good agreement with the values of Golimowski et al., who use L'- and M'-band photometry to account for the flux emitted at λ > 2.5 μm
A Spitzer Infrared Spectrograph (IRS) Spectral Sequence of M, L, and T Dwarfs
We present a low-resolution (R = 90), 5.5-38 micron spectral sequence of a
sample of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS)
onboard the Spitzer Space Telescope. The spectra exhibit prominent absorption
bands of H_2O at 6.27 microns, CH_4 at 7.65 microns, and NH_3 at 10.5 microns
and are relatively featureless at lambda > 15 microns. Three spectral indices
that measure the strengths of these bands are presented; H_2O absorption
features are present throughout the MLT sequence while the CH_4 and NH_3 bands
first appear at roughly the L/T transition. Although the spectra are, in
general, qualitatively well matched by synthetic spectra that include the
formation of spatially homogeneous silicate and iron condensate clouds, the
spectra of the mid-type L dwarfs show an unexpected flattening from roughly 9
to 11 microns. We hypothesize that this may be a result of a population of
small silicate grains that are not predicted in the cloud models. The spectrum
of the peculiar T6 dwarf 2MASS J0937+2931 is suppressed from 5.5-7.5 microns
relative to typical T6 dwarfs and may be a consequence of its mildly
metal-poor/high surface gravity atmosphere. Finally, we compute bolometric
luminosities of a subsample of the M, L, and T dwarfs by combining the IRS
spectra with previously published 0.6-4.1 micron spectra and find good
agreement with the values of Golimowski et al. who use L'- and M'-band
photometry and to account for the flux emitted at lambda > 2.5 microns.Comment: Accepted for publication in the Astrophysical Journa
Characterizing two solar-type Kepler subgiants with asteroseismology: KIC10920273 and KIC11395018
Determining fundamental properties of stars through stellar modeling has
improved substantially due to recent advances in asteroseismology. Thanks to
the unprecedented data quality obtained by space missions, particularly CoRoT
and Kepler, invaluable information is extracted from the high-precision stellar
oscillation frequencies, which provide very strong constraints on possible
stellar models for a given set of classical observations. In this work, we have
characterized two relatively faint stars, KIC10920273 and KIC11395018, using
oscillation data from Kepler photometry and atmospheric constraints from
ground-based spectroscopy. Both stars have very similar atmospheric properties;
however, using the individual frequencies extracted from the Kepler data, we
have determined quite distinct global properties, with increased precision
compared to that of earlier results. We found that both stars have left the
main sequence and characterized them as follows: KIC10920273 is a
one-solar-mass star (M=1.00 +/- 0.04 M_sun), but much older than our Sun
(t=7.12 +/- 0.47 Gyr), while KIC11395018 is significantly more massive than the
Sun (M=1.27 +/- 0.04 M_sun) with an age close to that of the Sun (t=4.57 +/-
0.23 Gyr). We confirm that the high lithium abundance reported for these stars
should not be considered to represent young ages, as we precisely determined
them to be evolved subgiants. We discuss the use of surface lithium abundance,
rotation and activity relations as potential age diagnostics.Comment: 12 pages, 3 figures, 5 tables. Accepted by Ap
Detection of Potential Transit Signals in the First Three Quarters of Kepler Mission Data
We present the results of a search for potential transit signals in the first
three quarters of photometry data acquired by the Kepler Mission. The targets
of the search include 151,722 stars which were observed over the full interval
and an additional 19,132 stars which were observed for only 1 or 2 quarters.
From this set of targets we find a total of 5,392 detections which meet the
Kepler detection criteria: those criteria are periodicity of the signal, an
acceptable signal-to-noise ratio, and a composition test which rejects spurious
detections which contain non-physical combinations of events. The detected
signals are dominated by events with relatively low signal-to-noise ratio and
by events with relatively short periods. The distribution of estimated transit
depths appears to peak in the range between 40 and 100 parts per million, with
a few detections down to fewer than 10 parts per million. The detected signals
are compared to a set of known transit events in the Kepler field of view which
were derived by a different method using a longer data interval; the comparison
shows that the current search correctly identified 88.1% of the known events. A
tabulation of the detected transit signals, examples which illustrate the
analysis and detection process, a discussion of future plans and open,
potentially fruitful, areas of further research are included
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