320 research outputs found
Shocks and a Giant Planet in the Disk Orbiting BP Piscium?
Spitzer IRS spectroscopy supports the interpretation that BP Piscium, a gas
and dust enshrouded star residing at high Galactic latitude, is a first-ascent
giant rather than a classical T Tauri star. Our analysis suggests that BP
Piscium's spectral energy distribution can be modeled as a disk with a gap that
is opened by a giant planet. Modeling the rich mid-infrared emission line
spectrum indicates that the solid-state emitting grains orbiting BP Piscium are
primarily composed of ~75 K crystalline, magnesium-rich olivine; ~75 K
crystalline, magnesium-rich pyroxene; ~200 K amorphous, magnesium-rich
pyroxene; and ~200 K annealed silica ('cristobalite'). These dust grains are
all sub-micron sized. The giant planet and gap model also naturally explains
the location and mineralogy of the small dust grains in the disk. Disk shocks
that result from disk-planet interaction generate the highly crystalline dust
which is subsequently blown out of the disk mid-plane and into the disk
atmosphere.Comment: 25 pages, 4 figures, 1 table. Accepted to Ap
Characterization of Dusty Debris Disks: the IRAS and Hipparcos Catalogs
Dusty debris disks around main-sequence stars are signposts for the existence
of planetesimals and exoplanets. From cross-correlating \hip stars with the
\iras catalogs, we identify 146 stars within 120 pc of Earth that show excess
emission at 60 \micron. This search took special precautions to avoid false
positives. Our sample is reasonably well distributed from late B to early
K-type stars, but it contains very few later type stars. Even though \iras flew
more than 20 years ago and many astronomers have cross-correlated its catalogs
with stellar catalogs, we were still able to newly identify debris disks at as
many as 33 main-sequence stars; of these, 32 are within 100 pc of Earth. The
power of an all-sky survey satellite like \iras is evident when comparing our
33 new debris disks with the total of only 22 dusty debris disk stars detected
first with the more sensitive, but pointed, satellite \iso. Our investigation
focuses on the mass, dimensions, and evolution of dusty debris disks.Comment: Changes made to table, figures and conclusion (v4); correction made
to Equation 9 (v3); added a paragraph and a reference to Section 3 (v2); ApJ
in pres
Warm dust in the terrestrial planet zone of a sun-like Pleiad: collisions between planetary embryos?
Only a few solar-type main sequence stars are known to be orbited by warm
dust particles; the most extreme is the G0 field star BD+20 307 that emits ~4%
of its energy at mid-infrared wavelengths. We report the identification of a
similarly dusty star HD 23514, an F6-type member of the Pleiades cluster. A
strong mid-IR silicate emission feature indicates the presence of small warm
dust particles, but with the primary flux density peak at the non-standard
wavelength of ~9 micron. The existence of so much dust within an AU or so of
these stars is not easily accounted for given the very brief lifetime in orbit
of small particles. The apparent absence of very hot (>~1000 K) dust at both
stars suggests the possible presence of a planet closer to the stars than the
dust. The observed frequency of the BD+20 307/HD 23514 phenomenon indicates
that the mass equivalent of Earth's Moon must be converted, via collisions of
massive bodies, to tiny dust particles that find their way to the terrestrial
planet zone during the first few hundred million years of the life of many
(most?) sun-like stars. Identification of these two dusty systems among
youthful nearby solar-type stars suggests that terrestrial planet formation is
common.Comment: ApJ in press, 19 pages including 3 figures and 2 tables, minor
changes to the tables and figure
Copious amounts of hot and cold dust orbiting the main sequence A-type stars HD 131488 and HD 121191
We report two new dramatically dusty main sequence stars: HD 131488 (A1 V) and HD 121191 (A8 V). HD 131488 is found to have substantial amounts of dust in its terrestrial planet zone (L IR/L bol ≈ 4 × 10-3), cooler dust farther out in its planetary sy
The Tucana/Horologium, Columba, AB Doradus, and Argus Associations: New Members and Dusty Debris Disks
We propose 35 star systems within ~70 pc of Earth as newly identified members
of nearby young stellar kinematic groups; these identifications include the
first A- and late-B type members of the AB Doradus moving group and field Argus
Association. All but one of the 35 systems contain a bright solar- or
earlier-type star that should make an excellent target for the next generation
of adaptive optics (AO) imaging systems on large telescopes. AO imaging has
revealed four massive planets in orbit around the {\lambda} Boo star HR 8799.
Initially the planets were of uncertain mass due in large part to the uncertain
age of the star. We find that HR 8799 is a likely member of the ~30 Myr old
Columba Association implying planet masses ~6 times that of Jupiter. We
consider Spitzer Space Telescope MIPS photometry of stars in the ~30 Myr old
Tucana/Horologium and Columba Associations, the ~40 Myr old field Argus
Association, and the ~70 Myr old AB Doradus moving group. The percentage of
stars in these young stellar groups that display excess emission above the
stellar photosphere at 24 and 70 \mu m wavelengths - indicative of the presence
of a dusty debris disk - is compared with corresponding percentages for members
of 11 open clusters and stellar associations with ages between 8 and 750 Myr,
thus elucidating the decay of debris disks with time.Comment: Accepted for publication in Ap
The First Spatially Resolved Mid-IR Spectra of NGC 1068 Obtained at Diffraction-limited Resolution with LWS at Keck I Telescope
We present spatially resolved mid-IR spectra of NGC 1068 with a
diffraction-limited resolution of 0.25\arcsec using the Long Wavelength
Spectrometer (LWS) at the Keck I telescope. The mid-infrared image of NGC 1068
is extended along the N-S direction. Previous imaging studies have shown the
extended regions are located inside the ionization cones indicating that the
mid-infrared emission arises perhaps from the inner regions of the narrow-line
clouds instead of the proposed dusty torus itself. The spatially resolved
mid-IR spectra were obtained at two different slit position angles, +8.0 and
-13.0 degrees across the elongated regions in the mid-IR. From these spectra,
we found only weak silicate absorption toward the northern extended regions but
strong in the nucleus and the southern extended regions. This is consistent
with a model of a slightly inclined cold obscuring torus which covers much of
the southern regions but is behind the northern extension. While a detailed
analysis of the spectra requires a radiative transfer model, the lack of
silicate emission from the northern extended regions prompts us to consider a
dual dust population model as one of the possible explanations in which a
different dust population exists in the ionization cones compared to that in
the dusty torus. Dust inside the ionization cones may lack small silicate
grains giving rise to only a featureless continuum in the northern extended
regions while dust in the dusty torus has plenty of small silicate grains to
produce the strong silicate absorption lines towards the nucleus and the
southern extended regions.Comment: Accepted to Ap
Comparison of early-, late-, and non-participants in a school-based asthma management program for urban high school students
<p>Abstract</p> <p>Background</p> <p>To assess bias and generalizability of results in randomized controlled trials (RCT), investigators compare participants to non-participants or early- to late-participants. Comparisons can also inform the recruitment approach, especially when working with challenging populations, such as urban adolescents. In this paper, we describe characteristics by participant status of urban teens eligible to participate in a RCT of a school-based, web-based asthma management program.</p> <p>Methods</p> <p>The denominator for this analysis was all students found to be eligible to participate in the RCT. Data were analyzed for participants and non-participants of the RCT, as well as for students that enrolled during the initially scheduled recruitment period (early-participants) and persons that delayed enrollment until the following fall when recruitment was re-opened to increase sample size (late-participants). Full Time Equivalents (FTEs) of staff associated with recruitment were estimated.</p> <p>Results</p> <p>Of 1668 teens eligible for the RCT, 386 enrolled early, and 36 enrolled late, leaving 1246 non-participants. Participants were younger (p < 0.01), more likely to be diagnosed, use asthma medication, and have moderate-to-severe disease than non-participants, odds ratios (95% Confidence Intervals) = 2.1(1.7-2.8), 1.7(1.3-2.1), 1.4(1.0-1.8), respectively. ORs were elevated for the association of late-participation with Medicaid enrollment, 1.9(0.7-5.1) and extrinsic motivation to enroll, 1.7(0.6-5.0). Late-participation was inversely related to study compliance for teens and caregivers, ORs ranging from 0.1 to 0.3 (all p-values < 0.01). Early- and late-participants required 0.45 FTEs/100 and 3.3 FTEs/100, respectively.</p> <p>Conclusions</p> <p>Recruitment messages attracted youth with moderate-to-severe asthma, but extending enrollment was costly, resulting in potentially less motivated, and certainly less compliant, participants. Investigators must balance internal versus external validity in the decision to extend recruitment. Gains in sample size and external validity may be offset by the cost of additional staff time and the threat to internal validity caused by lower participant follow-up.</p> <p>Trial Registration</p> <p>ClinicalTrials.gov: <a href="http://www.clinicaltrials.gov/ct2/show/NCT00201058">NCT00201058</a></p
Gas and Dust Associated with the Strange, Isolated, Star BP Piscium
We have carried out a multiwavelength observational campaign demonstrating
some of the remarkable properties of the infrared-bright variable star BP Psc.
Surrounded by a compact dusty, gaseous disk, this little-studied late-G (or
early-K) type star emits about 75% of its detected energy flux at infrared
wavelengths. Evidence for accretion of gas in conjunction with narrow bi-polar
jets and Herbig-Haro objects is apparently consistent with classification of BP
Psc as a pre-main sequence star, as postulated in most previous studies. If
young, then BP Psc would be one of the nearest and oldest known classical T
Tauri stars. However, such an evolutionary classification encounters various
problems that are absent or much less severe if BP Psc is instead a luminosity
class III post-main sequence star. In this case, it would be the first known
example of a first ascent giant surrounded by a massive molecular disk with
accompanying rapid gas accretion and prominent jets and HH objects. In this
model, the genesis of the massive dusty gaseous disk could be a consequence of
the envelopment of a low mass companion star. Properties in the disk may be
conducive to the current formation of planets, a gigayear or more after the
formation of BP Psc itself.Comment: Accepted for Astrophysical Journal New version with minor changes:
includes fixing a typo on the 3rd line of the paragraph that follows Equa 4
and adding a new reference (Nordhaus and Blackman 2006
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