68 research outputs found
Predictors of clinically significant postprocedural hypotension after carotid endarterectomy and carotid angioplasty with stenting
ObjectivesSignificant hypotension after carotid endarterectomy (CEA) and carotid angioplasty with stenting (CAS) has been correlated with adverse outcomes. The objective of this study was to determine risk factors that predict hypotension after patients undergo CEA and CAS.MethodsThe review included 1474 CEA patients and 157 CAS patients who underwent procedures from 2002 to 2008. Specific patient characteristics, such as comorbid diseases, degree of carotid stenosis, presence of neurologic symptoms, and preprocedure medications, were assessed. Also reviewed were specific postprocedural clinical outcomes, including hypotension requiring pressors, myocardial infarction, stroke, death, and hospital length of stay.ResultsThe incidence of clinically significant hypotension was 12.6% in CEA patients and 35% in CAS patients (P < .001). Clinically significant hypotension was correlated with increased postprocedural myocardial infarction (2.1% vs 0.5%, P = .022), increased mortality (2.1% vs 0.1%, P < .001), and length of stay >2 days (46.3% vs 27.4%, P = .01). Hypotension was not associated with increased postprocedural strokes (0.8% vs 0.6%, P = .75) or recurrent neurologic symptoms (0.4% vs 0.3%, P = .55). Preoperative nitrate use predicted a greater incidence of postprocedural hypotension (P = .043). A history of tobacco use was correlated with postprocedure hypotension (P = .033). Preprocedural strokes, the use of calcium channel blockers, β-blockers, angiotensin-converting enzyme inhibitors, prior myocardial infarction, degree of preprocedural carotid stenosis, type of stent, previous ipsilateral and contralateral interventions, and female gender did not correlate with postprocedural hypotension (P >.05).ConclusionsPostprocedural hypotension occurs more commonly with CAS than CEA and is associated with increased postprocedural myocardial infarction and length of stay, and death. Nitrates and tobacco use predict a higher incidence of postprocedural hypotension. High-risk patients should be aggressively managed to prevent the increased morbidity and mortality due to postprocedural hypotension
The Spectra of T Dwarfs I: Near-Infrared Data and Spectral Classification
We present near-infrared spectra for a sample of T dwarfs, including eleven
new discoveries made using the Two Micron All Sky Survey. These objects are
distinguished from warmer (L-type) brown dwarfs by the presence of methane
absorption bands in the 1--2.5 \micron spectral region. A first attempt at a
near-infrared classification scheme for T dwarfs is made, based on the
strengths of CH and HO bands and the shapes of the 1.25, 1.6, and 2.1
\micron flux peaks. Subtypes T1 V through T8 V are defined, and spectral
indices useful for classification are presented. The subclasses appear to
follow a decreasing T scale, based on the evolution of CH and
HO bands and the properties of L and T dwarfs with known distances.
However, we speculate that this scale is not linear with spectral type for cool
dwarfs, due to the settling of dust layers below the photosphere and subsequent
rapid evolution of spectral morphology around T 1300--1500 K.
Similarities in near-infrared colors and continuity of spectral features
suggest that the gap between the latest L dwarfs and earliest T dwarfs has been
nearly bridged. This argument is strengthened by the possible role of CH as
a minor absorber shaping the K-band spectra of the latest L dwarfs. Finally, we
discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue
near-infrared colors (J-K = 0.24) due to suppression of the 2.1
\micron peak. The feature is likely caused by enhanced collision-induced
H absorption in a high pressure or low metallicity photosphere.Comment: 74 pages including 26 figures, accepted by ApJ v563 December 2001;
full paper including all of Table 3 may be downloaded from
http://www.gps.caltech.edu/~pa/adam/classification ;also see submission
010844
Masses, Radii, and Cloud Properties of the HR 8799 Planets
The near-infrared colors of the planets directly imaged around the A star HR
8799 are much redder than most field brown dwarfs of the same effective
temperature. Previous theoretical studies of these objects have concluded that
the atmospheres of planets b, c, and d are unusually cloudy or have unusual
cloud properties. Some studies have also found that the inferred radii of some
or all of the planets disagree with expectations of standard giant planet
evolution models. Here we compare the available data to the predictions of our
own set of atmospheric and evolution models that have been extensively tested
against observations of field L and T dwarfs, including the reddest L dwarfs.
Unlike some previous studies we require mutually consistent choices for
effective temperature, gravity, cloud properties, and planetary radius. This
procedure thus yields plausible values for the masses, effective temperatures,
and cloud properties of all three planets. We find that the cloud properties of
the HR 8799 planets are not unusual but rather follow previously recognized
trends, including a gravity dependence on the temperature of the L to T
spectral transition--some reasons for which we discuss. We find the inferred
mass of planet b is highly sensitive to whether or not we include the H and K
band spectrum in our analysis. Solutions for planets c and d are consistent
with the generally accepted constraints on the age of the primary star and
orbital dynamics. We also confirm that, like in L and T dwarfs and solar system
giant planets, non-equilibrium chemistry driven by atmospheric mixing is also
important for these objects. Given the preponderance of data suggesting that
the L to T spectral type transition is gravity dependent, we present an
exploratory evolution calculation that accounts for this effect. Finally we
recompute the the bolometric luminosity of all three planets.Comment: 52 pages, 12 figures, Astrophysical Journal, in press. v2 features
minor editorial updates and correction
An Initial Survey of White Dwarfs in the Sloan Digital Sky Survey
An initial assessment is made of white dwarf and hot subdwarf stars observed
in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees),
observed much like the full survey will be, 269 white dwarfs and 56 hot
subdwarfs are identified spectroscopically where only 44 white dwarfs and 5 hot
subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and
DB (helium) types. In addition, in the full survey to date, a number of WDs
have been found with uncommon spectral types. Among these are blue DQ stars
displaying lines of atomic carbon; red DQ stars showing molecular bands of C_2
with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na,
and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic
field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry
alone allows identification of stars hotter than 12000 K, and the density of
these stars for 15<g<20 is found to be ~2.2 deg^{-2} at Galactic latitudes
29-62 deg. Spectra are obtained for roughly half of these hot stars. The
spectra show that, for 15<g<17, 40% of hot stars are WDs and the fraction of
WDs rises to ~90% at g=20. The remainder are hot sdB and sdO stars.Comment: Accepted for AJ; 43 pages, including 12 figures and 5 table
Discoveries from a Near-infrared Proper Motion Survey using Multi-epoch 2MASS Data
We have conducted a 4030-square-deg near-infrared proper motion survey using
multi-epoch data from the Two Micron All-Sky Survey (2MASS). We find 2778
proper motion candidates, 647 of which are not listed in SIMBAD. After
comparison to DSS images, we find that 107 of our proper motion candidates lack
counterparts at B-, R-, and I-bands and are thus 2MASS-only detections. We
present results of spectroscopic follow-up of 188 targets that include the
infrared-only sources along with selected optical-counterpart sources with
faint reduced proper motions or interesting colors. We also establish a set of
near-infrared spectroscopic standards with which to anchor near-infrared
classifications for our objects. Among the discoveries are six young field
brown dwarfs, five "red L" dwarfs, three L-type subdwarfs, twelve M-type
subdwarfs, eight "blue L" dwarfs, and several T dwarfs. We further refine the
definitions of these exotic classes to aid future identification of similar
objects. We examine their kinematics and find that both the "blue L" and "red
L" dwarfs appear to be drawn from a relatively old population. This survey
provides a glimpse of the kinds of research that will be possible through
time-domain infrared projects such as the UKIDSS Large Area Survey, various
VISTA surveys, and WISE, and also through z- or y-band enabled, multi-epoch
surveys such as Pan-STARRS and LSST.Comment: To appear in the September 2010 issue of The Astrophysical Journal,
Supplement Serie
An Infrared High Proper Motion Survey Using 2MASS and SDSS: Discovery of M, L and T Dwarfs
A search of the Two Micron All Sky Survey and Sloan Digital Sky
Survey reveals 36 previously unknown high proper motion objects with
J<17. Their red-optical colors indicate that 27 are M dwarfs, 8 are
early-type L dwarfs, and 1 is a late-type T dwarf. The L dwarfs have J-Ks
colors near the extrema of known L dwarfs indicating that previous surveys for
L dwarfs using color as a selection criterion may be biased. Followup
near-infrared spectroscopy of 6 dwarfs confirm they are all late-type with
spectral types ranging from M8 to T4. Spectroscopy also shows that some of the
L dwarf spectra exhibit peculiar features similar to other peculiar "blue" L
dwarfs which may indicate that these dwarfs have a relatively condensate free
atmosphere or may be metal poor. Photometric distance estimates indicate that
22 of the new M, L and T dwarfs lie within 100 pc of the Sun with the newly
discovered T dwarf, 2MASS J10595185+3042059, located at about 25 pc. Based on
the colors and proper motions of the newly identified objects, several appear
to be good subdwarf candidates. The proper motions of known ultracool dwarfs
detected in our survey were also measured, including, for the first time, SDSS
J085834.42+325627.6 (T1), SDSS J125011.65+392553.9 (T4) and 2MASS
J15261405+2043414 (L7).Comment: 14 pages, 7 tables, 9 figures: Accepted for the January 2009 AJ issu
A Catalog of Spectroscopically Identified White Dwarf Stars in the First Data Release of the Sloan Digital Sky Survey
We present the full spectroscopic white dwarf and hot subdwarf sample from
the SDSS first data release, DR1. We find 2551 white dwarf stars of various
types, 240 hot subdwarf stars, and an additional 144 objects we have identified
as uncertain white dwarf stars. Of the white dwarf stars, 1888 are non-magnetic
DA types and 171, non-magnetic DBs. The remaining (492) objects consist of all
different types of white dwarf stars: DO, DQ, DC, DH, DZ, hybrid stars like
DAB, etc., and those with non-degenerate companions. We fit the DA and DB
spectra with a grid of models to determine the Teff and log(g) for each object.
For all objects, we provide coordinates, proper motions, SDSS photometric
magnitudes, and enough information to retrieve the spectrum/image from the SDSS
public database. This catalog nearly doubles the known sample of
spectroscopically-identified white dwarf stars. In the DR1 imaged area of the
sky, we increase the known sample of white dwarf stars by a factor of 8.5. We
also comment on several particularly interesting objects in this sample.Comment: To be published ApJ, 20May04. 52 pages, 13 figures, 12 tables. Full
tables are available at the (now-corrected) SDSS DR1 Value-added catalog at
http://www.sdss.org/dr1/products/value_added/index.htm
The 0.8-14.5 micron Spectra of Mid-L to Mid-T Dwarfs: Diagnostics of Effective Temperature, Grain Sedimentation, Gas Transport, and Surface Gravity
We present new 5.2-14.5 micron low-resolution spectra of 14 mid-L to mid-T
dwarfs, as well as 3.0-4.1 micron spectra for five of these dwarfs. These data
are supplemented by existing red and near-infrared spectra (0.6-2.5 micron), as
well as red through mid-infrared spectroscopy of seven other L and T dwarfs
presented by Cushing et al. (2008). We compare these spectra to those generated
from the model atmospheres of Saumon & Marley (2008). The models reproduce the
observed spectra well, except in the case of one very red L3.5 dwarf, 2MASS
J22244381-0158521. The broad wavelength coverage allows us to constrain almost
independently the four parameters used to describe these photospheres in our
models: effective temperature (Teff), surface gravity, grain sedimentation
efficiency (fsed) and vertical gas transport efficiency (Kzz).
The sample of L3.5 to T5.5 dwarfs spans the range 1800 < Teff < 1000 K, with
an L-T transition (spectral types L7-T4) that lies between 1400 K and 1100K for
dwarfs with typical near-infrared colors; bluer and redder dwarfs can be 100 K
warmer or cooler, respectively, when using infrared spectral types. In this
model analysis, fsed increases rapidly between types T0 and T4, indicating that
increased sedimentation can explain the rapid disappearance of clouds at this
stage of brown dwarf evolution. There is a suggestion that the transition to
dust-free atmospheres happens at lower temperatures for lower gravity dwarfs.Comment: 53 pages, 16 figures, to be published in the Astrophysical Journa
The Mass-Luminosity Relation in the L/T Transition:Individual Dynamical Masses for the New J-Band Flux Reversal Binary SDSSJ105213.51+442255.7AB
We have discovered that SDSSJ105213.51+442255.7 (T0.51.0) is a binary in
Keck laser guide star adaptive optics imaging, displaying a large J-to-K-band
flux reversal (J = -0.450.09 mag, K = 0.520.05 mag).
We determine a total dynamical mass from Keck orbital monitoring (885
) and a mass ratio by measuring the photocenter orbit from
CFHT/WIRCam absolute astrometry ( = 0.780.07). Combining these
provides the first individual dynamical masses for any field L or T dwarfs,
493 for the L6.51.5 primary and 393
for the T1.51.0 secondary. Such a low mass ratio for a nearly equal
luminosity binary implies a shallow massluminosity relation over the L/T
transition (log/log). This
provides the first observational support that cloud dispersal plays a
significant role in the luminosity evolution of substellar objects. Fully
cloudy models fail our coevality test for this binary, giving ages for the two
components that disagree by 0.2 dex (2.0). In contrast, our observed
masses and luminosities can be reproduced at a single age by "hybrid"
evolutionary tracks where a smooth change from a cloudy to cloudless
photosphere around 1300 K causes slowing of luminosity evolution. Remarkably,
such models also match our observed JHK flux ratios and colors well. Overall,
it seems that the distinguishing features SDSSJ1052+4422AB, like a J-band flux
reversal and high-amplitude variability, are normal for a field L/T binary
caught during the process of cloud dispersal, given that the age
(1.11 Gyr) and surface gravity (log = 5.05.2) of
SDSSJ1052+4422AB are typical for field ultracool dwarfs.Comment: Accepted to ApJ; 33 pages, 8 figures, 5 table
Further Defining Spectral Type "Y" and Exploring the Low-mass End of the Field Brown Dwarf Mass Function
We present the discovery of another seven Y dwarfs from the Wide-field
Infrared Survey Explorer (WISE). Using these objects, as well as the first six
WISE Y dwarf discoveries from Cushing et al., we further explore the transition
between spectral types T and Y. We find that the T/Y boundary roughly coincides
with the spot where the J-H colors of brown dwarfs, as predicted by models,
turn back to the red. Moreover, we use preliminary trigonometric parallax
measurements to show that the T/Y boundary may also correspond to the point at
which the absolute H (1.6 um) and W2 (4.6 um) magnitudes plummet. We use these
discoveries and their preliminary distances to place them in the larger context
of the Solar Neighborhood. We present a table that updates the entire stellar
and substellar constituency within 8 parsecs of the Sun, and we show that the
current census has hydrogen-burning stars outnumbering brown dwarfs by roughly
a factor of six. This factor will decrease with time as more brown dwarfs are
identified within this volume, but unless there is a vast reservoir of cold
brown dwarfs invisible to WISE, the final space density of brown dwarfs is
still expected to fall well below that of stars. We also use these new Y dwarf
discoveries, along with newly discovered T dwarfs from WISE, to investigate the
field substellar mass function. We find that the overall space density of
late-T and early-Y dwarfs matches that from simulations describing the mass
function as a power law with slope -0.5 < alpha < 0.0; however, a power-law may
provide a poor fit to the observed object counts as a function of spectral type
because there are tantalizing hints that the number of brown dwarfs continues
to rise from late-T to early-Y. More detailed monitoring and characterization
of these Y dwarfs, along with dedicated searches aimed at identifying more
examples, are certainly required.Comment: 91 pages, 15 figures, accepted for publication in The Astrophysical
Journa
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