1,412 research outputs found
The CO-H2 conversion factor of diffuse ISM: Bright 12CO emission also traces diffuse gas
We show that the XCO factor, which converts the CO luminosity into the column
density of molecular hydrogen has similar values for dense, fully molecular gas
and for diffuse, partially molecular gas. We discuss the reasons of this
coincidence and the consequences for the understanding of the interstellar
medium.Comment: 5 pages, 1 PostScript figure. To be published in the proceedings of
the Zermatt 2010 conference: "Conditions and impact of star formation: New
results with Herschel and beyond". Uses EAS LaTeX macro
The CARD/RCA Water Sector Model
Water has become a major input into agriculture production in the United States. Irrigation in the Western United States has been important in crop production and will continue to be important. Irrigation is also becoming more important in areas of the southeast. The use and conservation of water, as well as the importance of irrigation in the conservation of soil, are areas of concern as outlined in the 1977 Soil and Water Resources Conservation Act. Therefore, it is necessary for the CARD/RCA programming models to incorporate a water sector
The Water Sector for the CARD/RCA 85 Model
Availability of water for agricultural use is one of the major factors determining agricultural production in the Western United States and is becoming an important factor in areas of the Southwest. In addition, water use and conservation is an important concern of the 1977 Soil and Water Resources Conservation Act. Thus, it is necessary to build an agricultural water sector for use in the CARD/RAC85 programming models
Testimony of Burton C. English before the Joint Economic Committee and the Agriculture Subcommittee in Washington, D.C.
Good morning, ladies and gentlemen. My name is Burton C. English. I am a staff economist with the Center for Agricultural Development located in Ames, Iowa. My area of expertise is in agricultural economics and policy with a special emphasis in soil conservation. I wish to thank you for inviting me here to testify. You will note that the testimony that I am presenting here was written by Earl O. Heady and myself. I send his apologies for not being able to make this hearing, but his schedule would not permit it
On the Relationship Between Molecular Hydrogen and Carbon Monoxide Abundances in Molecular Clouds
The most usual tracer of molecular gas is line emission from CO. However, the
reliability of that tracer has long been questioned in environments different
from the Milky Way. We study the relationship between H2 and CO abundances
using a fully dynamical model of magnetized turbulence coupled to a chemical
network simplified to follow only the dominant pathways for H2 and CO formation
and destruction, and including photodissociation using a six-ray approximation.
We find that the abundance of H2 is primarily determined by the amount of time
available for its formation, which is proportional to the product of the
density and the metallicity, but insensitive to photodissociation.
Photodissociation only becomes important at extinctions under a few tenths of a
visual magnitude, in agreement with both observational and prior theoretical
work. On the other hand, CO forms quickly, within a dynamical time, but its
abundance depends primarily on photodissociation, with only a weak secondary
dependence on H2 abundance. As a result, there is a sharp cutoff in CO
abundance at mean visual extinctions A_V < 3. At lower values of A_V we find
that the ratio of H2 column density to CO emissivity X_CO is proportional to
A_V^(-3.5). This explains the discrepancy observed in low metallicity systems
between cloud masses derived from CO observations and other techniques such as
infrared emission. Our work predicts that CO-bright clouds in low metallicity
systems should be systematically larger or denser than Milky Way clouds, or
both. Our results further explain the narrow range of observed molecular cloud
column densities as a threshold effect, without requiring the assumption of
virial equilibrium.Comment: 16 pages, 11 figures. Updated to match version accepted by MNRA
Focus, Vol. 1 No. 1
A literary magazine of student writing published by the Department of English of Stephen F. Austin State College.https://scholarworks.sfasu.edu/focus/1000/thumbnail.jp
Is molecular gas necessary for star formation?
On galactic scales, the surface density of star formation appears to be well
correlated with the surface density of molecular gas. This has lead many
authors to suggest that there exists a causal relationship between the chemical
state of the gas and its ability to form stars -- in other words, the
assumption that the gas must be molecular before star formation can occur. We
test this hypothesis by modelling star formation within a dense cloud of gas
with properties similar to a small molecular cloud using a series of different
models of the chemistry, ranging from one in which the formation of molecules
is not followed and the gas is assumed to remain atomic throughout, to one that
tracks the formation of both H2 and CO. We find that presence of molecules in
the gas has little effect on the ability of the gas to form stars: star
formation can occur just as easily in atomic gas as in molecular gas. At low
densities (< 10^4 cm^-3), the gas is able to cool via C+ fine-structure
emission almost as efficiently as via CO rotational line emission, while at
higher densities, the main cooling process involves the transfer of energy from
gas to dust, meaning that the presence of molecules is again unimportant.
Cooling by H2 is particularly inefficient, accounting for as little as 1
percent of the overall cooling in the cloud. Rather than the chemical makeup,
we find that the most important factor controlling the rate of star formation
is the ability of the gas to shield itself from the interstellar radiation
field. As this is also a prerequisite for the survival of molecules within the
gas, our results support a picture in which molecule formation and the
formation of cold gas are both correlated with the column density of the cloud
-- and thus its ability to shield itself -- rather than being directly
correlated with each other.Comment: 13 pages, 9 figures. Added new section on density PDF, other minor
revisions to match version accepted by MNRA
Cost-effectiveness of non-invasive methods for assessment and monitoring of liver fibrosis and cirrhosis in patients with chronic liver disease: systematic review and economic evaluation
BACKGROUND: Liver biopsy is the reference standard for diagnosing the extent of fibrosis in chronic liver disease; however, it is invasive, with the potential for serious complications. Alternatives to biopsy include non-invasive liver tests (NILTs); however, the cost-effectiveness of these needs to be established. OBJECTIVE: To assess the diagnostic accuracy and cost-effectiveness of NILTs in patients with chronic liver disease. DATA SOURCES: We searched various databases from 1998 to April 2012, recent conference proceedings and reference lists. METHODS: We included studies that assessed the diagnostic accuracy of NILTs using liver biopsy as the reference standard. Diagnostic studies were assessed using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS-2) tool. Meta-analysis was conducted using the bivariate random-effects model with correlation between sensitivity and specificity (whenever possible). Decision models were used to evaluate the cost-effectiveness of the NILTs. Expected costs were estimated using a NHS perspective and health outcomes were measured as quality-adjusted life-years (QALYs). Markov models were developed to estimate long-term costs and QALYs following testing, and antiviral treatment where indicated, for chronic hepatitis B (HBV) and chronic hepatitis C (HCV). NILTs were compared with each other, sequential testing strategies, biopsy and strategies including no testing. For alcoholic liver disease (ALD), we assessed the cost-effectiveness of NILTs in the context of potentially increasing abstinence from alcohol. Owing to a lack of data and treatments specifically for fibrosis in patients with non-alcoholic fatty liver disease (NAFLD), the analysis was limited to an incremental cost per correct diagnosis. An analysis of NILTs to identify patients with cirrhosis for increased monitoring was also conducted. RESULTS: Given a cost-effectiveness threshold of £20,000 per QALY, treating everyone with HCV without prior testing was cost-effective with an incremental cost-effectiveness ratio (ICER) of £9204. This was robust in most sensitivity analyses but sensitive to the extent of treatment benefit for patients with mild fibrosis. For HBV [hepatitis B e antigen (HBeAg)-negative)] this strategy had an ICER of £28,137, which was cost-effective only if the upper bound of the standard UK cost-effectiveness threshold range (£30,000) is acceptable. For HBeAg-positive disease, two NILTs applied sequentially (hyaluronic acid and magnetic resonance elastography) were cost-effective at a £20,000 threshold (ICER: £19,612); however, the results were highly uncertain, with several test strategies having similar expected outcomes and costs. For patients with ALD, liver biopsy was the cost-effective strategy, with an ICER of £822. LIMITATIONS: A substantial number of tests had only one study from which diagnostic accuracy was derived; therefore, there is a high risk of bias. Most NILTs did not have validated cut-offs for diagnosis of specific fibrosis stages. The findings of the ALD model were dependent on assuptions about abstinence rates assumptions and the modelling approach for NAFLD was hindered by the lack of evidence on clinically effective treatments. CONCLUSIONS: Treating everyone without NILTs is cost-effective for patients with HCV, but only for HBeAg-negative if the higher cost-effectiveness threshold is appropriate. For HBeAg-positive, two NILTs applied sequentially were cost-effective but highly uncertain. Further evidence for treatment effectiveness is required for ALD and NAFLD. STUDY REGISTRATION: This study is registered as PROSPERO CRD42011001561. FUNDING: The National Institute for Health Research Health Technology Assessment programme
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