207 research outputs found
The surprisingly low carbon mass in the debris disk around HD 32297
Gas has been detected in a number of debris disks. It is likely secondary,
i.e. produced by colliding solids. Here, we report ALMA Band 8 observations of
neutral carbon in the CO-rich debris disk around the 15--30 Myr old A-type star
HD 32297. We find that C is located in a ring at 110 au with a FWHM
of 80 au, and has a mass of M.
Naively, such a surprisingly small mass can be accumulated from CO
photo-dissociation in a time as short as 10 yr. We develop a simple
model for gas production and destruction in this system, properly accounting
for CO self-shielding and shielding by neutral carbon, and introducing a
removal mechanism for carbon gas. We find that the most likely scenario to
explain both C and CO observations, is one where the carbon gas is rapidly
removed on a timescale of order a thousand years and the system maintains a
very high CO production rate of 15 M Myr, much higher
than the rate of dust grind-down. We propose a possible scenario to meet these
peculiar conditions: the capture of carbon onto dust grains, followed by rapid
CO re-formation and re-release. In steady state, CO would continuously be
recycled, producing a CO-rich gas ring that shows no appreciable spreading over
time. This picture might be extended to explain other gas-rich debris disks.Comment: accepted for publication in the Ap
Evidence for an Edge-On Disk around the Young Star MWC 778 from Infrared Imaging and Polarimetry
MWC 778 is an unusual and little-studied young stellar object located in the
IC 2144 nebula. Recent spectroscopy by Herbig and Vacca (2008) suggested the
presence of an edge-on circumstellar disk around it. We present near-infrared
adaptive optics imaging polarimetry and mid-infrared imaging which directly
confirm the suspected nearly-edge-on disk around MWC 778 (i ~ 70-80 degrees)
plus reveal a more extensive envelope pierced by bipolar outflow cavities. In
addition, our mid-infrared images and near-infrared polarization maps detect a
spiral-shaped structure surrounding MWC 778, with arms that extend beyond 6" on
either side of the star.
Although MWC 778 has previously been classified as an Herbig Ae/Be star, the
properties of its central source (including its spectral type) remain fairly
uncertain. Herbig & Vacca (2008) suggested an F or G spectral type based on the
presence of metallic absorption lines in the optical spectrum, which implies
that MWC 778 may belong to the fairly rare class of Intermediate-Mass T Tauri
Stars (IMTTSs) which are the evolutionary precursors to Herbig Ae/Be objects.
Yet its integrated bolometric luminosity, > 750 L_sun (for an assumed distance
of 1 kpc) is surprisingly high for an F or G spectral type, even for an IMTTS.
We speculate on several possible explanations for this discrepancy, including
its true distance being much closer than 1 kpc, the presence of a binary
companion, and/or a non-stellar origin for the observed absorption lines.Comment: Accepted to AJ. 11 pages, 3 figure
Study of Gluon versus Quark Fragmentation in and Events at \sqrt{s}=10 GeV
Using data collected with the CLEO II detector at the Cornell Electron
Storage Ring, we determine the ratio R(chrg) for the mean charged multiplicity
observed in Upsilon(1S)->gggamma events, to the mean charged multiplicity
observed in e+e- -> qqbar gamma events. We find R(chrg)=1.04+/-0.02+/-0.05 for
jet-jet masses less than 7 GeV.Comment: 15 pages, postscript file also available through
http://w4.lns.cornell.edu/public/CLN
Variability of Disk Emission in Pre-Main Sequence and Related Stars. I. HD 31648 and HD 163296 - Isolated Herbig Ae Stars Driving Herbig-Haro Flows
Infrared photometry and spectroscopy covering a time span of a quarter
century are presented for HD 31648 (MWC 480) and HD 163296 (MWC 275). Both are
isolated Herbig Ae stars that exhibit signs of active accretion, including
driving bipolar flows with embedded Herbig-Haro (HH) objects. HD 163296 was
found to be relatively quiescent photometrically in its inner disk region, with
the exception of a major increase in emitted flux in a broad wavelength region
centered near 3 microns in 2002. In contrast, HD 31648 has exhibited sporadic
changes in the entire 3-13 micron region throughout this span of time. In both
stars the changes in the 1-5 micron flux indicate structural changes in the
region of the disk near the dust sublimation zone, possibly causing its
distance from the star to vary with time. Repeated thermal cycling through this
region will result in the preferential survival of large grains, and an
increase in the degree of crystallinity. The variability observed in these
objects has important consequences for the interpretation of other types of
observations. For example, source variability will compromise models based on
interferometry measurements unless the interferometry observations are
accompanied by nearly-simultaneous photometric data.Comment: 55 pages, 18 figures, 2 tables, Accepted by Ap
Safety and efficacy of Y-90 microsphere treatment in patients with primary and metastatic liver cancer: The tumor selectivity of the treatment as a function of tumor to liver flow ratio
BACKGROUND: Treatment records and follow-up data on 40 patients with primary and metastatic liver malignancies who underwent a single whole-liver treatment with Y-90 resin microspheres (SIR-Spheres(® )Sirtex Medical, Lake Forest, IL) were retrospectively reviewed. The objective of the study was to evaluate the anatomic and physiologic determinants of radiation dose distribution, and the dose response of tumor and liver toxicity in patients with liver malignancies who underwent hepatic arterial Y-90 resin microsphere treatment. METHODS: Liver and tumor volume calculations were performed on pre-treatment CT scans. Fractional tumor and liver flow characteristics and lung shunt fractions were determined using hepatic arterial Tc-99m MAA imaging. Absorbed dose calculations were performed using the MIRD equations. Liver toxicity was assessed clinically and by liver function tests. Tumor response to therapy was assessed by CT and/or tumor markers. RESULTS: Of the 40 patients, 5 had hepatocellular cancer (HCC), and 35 had metastatic liver tumors (15 colorectal cancer, 10 neuroendocrine tumors, 4 breast cancer, 2 lung cancer, 1 ovarian cancer, 1 endometrial cancer, and 2 unknown primary adenocarcinoma). All patients were treated in a salvage setting with a 3 to 80 week follow-up (mean: 19 weeks). Tumor volumes ranged from 15.0 to 984.2 cc (mean: 294.9 cc) and tumor to normal liver uptake ratios ranged from 2.8 to 15.4 (mean: 5.4). Average administered activity was 1.2 GBq (0.4 to 2.4 GBq). Liver absorbed doses ranged from 0.7 to 99.5 Gy (mean: 17.2 Gy). Tumor absorbed doses ranged from 40.1 to 494.8 Gy (mean: 121.5 Gy). None of the patients had clinical venoocclusive disease or therapy-induced liver failure. Seven patients (17.5 %) had transient and 7 patients (17.5 %) had persistent LFT abnormalities. There were 27 (67.5%) responders (complete response, partial response, and stable disease). Tumor response correlated with higher tumor flow ratio as measured by Tc-99m MAA imaging. CONCLUSION: Doses up to 99.5 Gy to uninvolved liver are tolerated with no clinical venoocclusive disease or liver failure. The lowest tumor dose producing a detectable response is 40.1 Gy. The utilization of MAA-based imaging techniques to determine tumor and liver blood flow for clinical treatment planning and the calculation of administered activity may improve clinical outcomes
Does Time Since Immigration Modify Neighborhood Deprivation Gradients in Preterm Birth? A Multilevel Analysis
Immigrants’ health is jointly influenced by their pre- and post-migration exposures, but how these two influences operate with increasing duration of residence has not been well-researched. We aimed to examine how the influence of maternal country of birth and neighborhood deprivation effects, if any, change over time since migration and how neighborhood effects among immigrants compare with those observed in the Canadian-born population. Birth data from Ontario hospital records (2002–2007) were linked with an official Canadian immigration database (1985–2000). The outcome measure was preterm birth. Neighborhoods were ranked according to a neighborhood deprivation index developed for Canadian urban areas and collapsed into tertiles of approximately equal size. Time since immigration was measured from the date of arrival to Canada to the date of delivery, ranging from 1 to 22 years. We used cross-classified random effect models to simultaneously account for the membership of births (N = 83,233) to urban neighborhoods (N = 1,801) and maternal countries of birth (N = 168). There were no differences in preterm birth between neighborhood deprivation tertiles among immigrants with less than 15 years of residence. Among immigrants with 15 years of stay or more, the adjusted absolute risk difference (ARD%, 95% confidence interval) between high-deprived (tertile 3) and low-deprived (tertile 1) neighborhoods was 1.86 (0.68, 2.98), while the ARD% observed among the Canadian-born (N = 314,237) was 1.34 (1.11, 1.57). Time since migration modifies the neighborhood deprivation gradient in preterm birth among immigrants living in Ontario cities. Immigrants reached the level of inequalities in preterm birth observed at the neighborhood level among the Canadian-born after 14 years of stay, but neighborhoods did not influence preterm birth among more recent immigrants, for whom the maternal country of birth was more predictive of preterm birth
Properties and distribution of paired candidate stony meteorites at Meridiani Planum, Mars
The Mars Exploration Rover Opportunity investigated four rocks, informally dubbed Barberton, Santa Catarina, Santorini, and Kasos, that are possible stony meteorites. Their chemical and mineralogical composition is similar to the howardite, eucrite, and diogenite group but with additional metal, similar to mesosiderite silicate clasts. Because of their virtually identical composition and because they appear to represent a relatively rare group of meteorites, they are probably paired. The four rocks were investigated serendipitously several kilometers apart, suggesting that Opportunity is driving across a larger population of similar rock fragments, maybe a meteorite strewn field. Small amounts of ferric Fe are a result of weathering. We did not observe evidence for fusion crusts. Four iron meteorites were found across the same area. Although mesosiderites are stony irons, a genetic link to these irons is unlikely. The stony meteorites probably fell later than the irons. The current atmosphere is sufficiently dense to land such meteorites at shallow entry angles, and it would disperse fragments over several kilometers upon atmospheric breakup. Alternatively, dispersion by spallation from an impacting meteoroid may have occurred. Santa Catarina and a large accumulation of similar rocks were found at the rim of Victoria crater. It is possible that they are associated with the impactor that created Victoria crater, but our limited knowledge about their distribution cannot exclude mere coincidence
The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 Micron Spectrum of the Planetary-Mass Companion VHS 1256-1257 b
We present the highest fidelity spectrum to date of a planetary-mass object.
VHS 1256 b is a 20 M widely separated (8\arcsec, a =
150 au), young, planetary-mass companion that shares photometric colors and
spectroscopic features with the directly imaged exoplanets HR 8799 c, d, and e.
As an L-to-T transition object, VHS 1256 b exists along the region of the
color-magnitude diagram where substellar atmospheres transition from cloudy to
clear. We observed VHS 1256~b with \textit{JWST}'s NIRSpec IFU and MIRI MRS
modes for coverage from 1 m to 20 m at resolutions of 1,000 -
3,700. Water, methane, carbon monoxide, carbon dioxide, sodium, and potassium
are observed in several portions of the \textit{JWST} spectrum based on
comparisons from template brown dwarf spectra, molecular opacities, and
atmospheric models. The spectral shape of VHS 1256 b is influenced by
disequilibrium chemistry and clouds. We directly detect silicate clouds, the
first such detection reported for a planetary-mass companion.Comment: Accepted ApJL Iterations of spectra reduced by the ERS team are
hosted at this link:
https://github.com/bemiles/JWST_VHS1256b_Reduction/tree/main/reduced_spectr
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