631 research outputs found
Modelling Grassland Ecosystems
In this contribution a view of the promise and difficulties of modelling grassland is given. This is largely centred around work with a grassland ecosystem simulator known as the Hurley Pasture Model.
A brief introduction sets forth possible reasons for building a large ecosystem model, and stresses the importance of modelling objectives. It is suggested that a model is de rigeur for any research programme which aims to take a firm grasp of the complex responses of grassland. Mechanistic models are required to provide the understanding needed for intelligent and flexible management of grassland, whatever the prevailing environmental or economic objectives. The models are necessarily large, reflecting the complexity of the ‘real’ system, and, in a sense, are ‘big’ science. The challenge is to develop models of ‘engineering strength’. This requires an appropriate research environment, which should be reasonably stable, multidisciplinary, well-connected to experimental programmes, and permit adequate support for the three essential legs of an ecosystem model: development, documentation, and application. Some modelling researchers are dismayed by the wasteful fragmentation of many plant ecosystem modelling research programmes.
Next an outline account of the Hurley Pasture Model (HPM) is given. Most plant ecosystem models are now quite similar at the qualitative level, and few would dispute the statement that a reasonable level of consensus is emerging. The HPM is a standard model of the genre. It comprises plant, animal, soil and water submodels. To-date there is no phenology submodel. There are environmental and management drivers, the former accepting monthly, daily or diurnal data, the latter permitting the simulation of fertilizer, grazing, and cutting scenarios, more or less ad libitum.
Recent developments of the HPM include a submodel to take account of acclimation of photosynthesis to light, nitrogen, carbon dioxide (‘down-regulation’) and temperature; and a simple method of using the HPM to simulate legume dynamics in a grass-legume pasture.
Finally, some applications of the model are presented, relating to fertilizer application, grazing, harvesting, and climate change. These are to illustrate the scope of the model, for both application and understanding. The last application shows that in grassland ecosystems climate change responses can be greatly affected by (i) a variable legume content; (ii) management, e.g. how the crop is grazed or cut; and (iii) water stress, as occurs in southern Britain. The impact of climate change on grassland ecosystems is of particular interest. It is known that, in a constant climate, a grassland ecosystem can take hundreds of years to come to equilibrium. Experiments cannot address the problem directly. Short-term experiments can give very variable responses, depending on conditions, which are often misleading, even opposite in sign from long-term responses. Mechanistic models provide a clear framework for unifying these variable results, understanding why they arise, and making predictions about the future time course of plant ecosystems. There seems to be no other way of doing this work
How to Measure Subdiffusion Parameters
We propose a method to measure the subdiffusion parameter and
subdiffusion coefficient which are defined by means of the
relation where
denotes a mean square displacement of a random walker starting from at
the initial time . The method exploits a membrane system where a substance
of interest is transported in a solvent from one vessel to another across a
thin membrane which plays here only an auxiliary role. We experimentally study
a diffusion of glucose and sucrose in a gel solvent, and we precisely determine
the parameters and , using a fully analytic solution of
the fractional subdiffusion equation.Comment: short version of cond-mat/0309072, to appear in Phys. Rev. Let
Measuring subdiffusion parameters
We propose a method to extract from experimental data the subdiffusion
parameter and subdiffusion coefficient which are defined by
means of the relation where
denotes a mean square displacement of a random walker starting from
at the initial time . The method exploits a membrane system where a
substance of interest is transported in a solvent from one vessel to another
across a thin membrane which plays here only an auxiliary role. Using such a
system, we experimentally study a diffusion of glucose and sucrose in a gel
solvent. We find a fully analytic solution of the fractional subdiffusion
equation with the initial and boundary conditions representing the system under
study. Confronting the experimental data with the derived formulas, we show a
subdiffusive character of the sugar transport in gel solvent. We precisely
determine the parameter , which is smaller than 1, and the subdiffusion
coefficient .Comment: 17 pages, 9 figures, revised, to appear in Phys. Rev.
Infrared Spectral Energy Distributions of Nearby Galaxies
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out a
comprehensive multi-wavelength survey on a sample of 75 nearby galaxies. The
1-850um spectral energy distributions are presented using broadband imaging
data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. The infrared colors derived
from the globally-integrated Spitzer data are generally consistent with the
previous generation of models that were developed based on global data for
normal star-forming galaxies, though significant deviations are observed.
Spitzer's excellent sensitivity and resolution also allow a detailed
investigation of the infrared spectral energy distributions for various
locations within the three large, nearby galaxies NGC3031 (M81), NGC5194 (M51),
and NGC7331. Strong correlations exist between the local star formation rate
and the infrared colors f_nu(70um)/f_nu(160um) and f_nu(24um)/f_nu(160um),
suggesting that the 24 and 70um emission are useful tracers of the local star
formation activity level. Preliminary evidence indicates that variations in the
24um emission, and not variations in the emission from polycyclic aromatic
hydrocarbons at 8um, drive the variations in the f_nu(8.0um)/f_nu(24um) colors
within NGC3031, NGC5194, and NGC7331. If the galaxy-to-galaxy variations in
spectral energy distributions seen in our sample are representative of the
range present at high redshift then extrapolations of total infrared
luminosities and star formation rates from the observed 24um flux will be
uncertain at the factor-of-five level (total range). The corresponding
uncertainties using the redshifted 8.0um flux (e.g. observed 24um flux for a
z=2 source) are factors of 10-20. Considerable caution should be used when
interpreting such extrapolated infrared luminosities.Comment: 32 pages including 16 figures; accepted for publication in the
Astrophysical Journa
The Opacity of Spiral Galaxy Disks VIII: Structure of the Cold ISM
The quantity of dust in a spiral disk can be estimated using the dust's
typical emission or the extinction of a known source. In this paper, we compare
two techniques, one based on emission and one on absorption, applied on
sections of fourteen disk galaxies. The two measurements reflect, respectively
the average and apparent optical depth of a disk section. Hence, they depend
differently on the average number and optical depth of ISM structures in the
disk. The small scale geometry of the cold ISM is critical for accurate models
of the overall energy budget of spiral disks. ISM geometry, relative
contributions of different stellar populations and dust emissivity are all free
parameters in galaxy Spectral Energy Distribution (SED) models; they are also
sometimes degenerate, depending on wavelength coverage. Our aim is to constrain
typical ISM geometry. The apparent optical depth measurement comes from the
number of distant galaxies seen in HST images through the foreground disk. We
measure the IR flux in images from the {\it Spitzer} Infrared Nearby Galaxy
Survey in the same section of the disk that was covered by HST. A physical
model of the dust is fit to the SED to estimate the dust surface density, mean
temperature, and brightness in these disk sections. The surface density is
subsequently converted into the average optical depth estimate. The two
measurements generally agree. The ratios between the measured average and
apparent optical depths of the disk sections imply optically thin clouds in
these disks. Optically thick disks, are likely to have more than a single cloud
along the line-of-sight.Comment: 31 pages, 5 figures, 4 tables, accepted for publication in A
Mid-Infrared IRS Spectroscopy of NGC 7331: A First Look at the SINGS Legacy
The nearby spiral galaxy NGC 7331 was spectrally mapped from 5-38um using all
modules of Spitzer's IRS spectrograph. A strong new dust emission feature,
presumed due to PAHs, was discovered at 17.1um. The feature's intensity is
nearly half that of the ubiquitous 11.3um band. The 7-14um spectral maps
revealed significant variation in the 7.7 and 11.3um PAH features between the
stellar ring and nucleus. Weak [OIV] 25.9um line emission was found to be
centrally concentrated in the nucleus, with an observed strength over 10% of
the combined neon line flux, indicating an AGN or unusually active massive star
photo-ionization. Two [SIII] lines fix the characteristic electron density in
the HII regions at n_e < ~200 cm^-3. Three detected H_2 rotational lines,
tracing warm molecular gas, together with the observed IR continuum, are
difficult to match with standard PDR models. Either additional PDR heating or
shocks are required to simultaneously match lines and continuum.Comment: 6 pages, 5 figures, accepted for publication in ApJS Spitzer Special
Issu
The Calibration of Mid-Infrared Star Formation Rate Indicators
With the goal of investigating the degree to which the mid-infrared emission
traces the star formation rate (SFR), we analyze Spitzer 8 um and 24 um data of
star-forming regions in a sample of 33 nearby galaxies with available
HST/NICMOS images in the Paschen-alpha (1.8756 um) emission line. The galaxies
are drawn from the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample, and
cover a range of morphologies and a factor ~10 in oxygen abundance. Published
data on local low-metallicity starburst galaxies and Luminous Infrared Galaxies
are also included in the analysis. Both the stellar-continuum-subtracted 8 um
emission and the 24 um emission correlate with the extinction-corrected
Pa-alpha line emission, although neither relationship is linear. Simple models
of stellar populations and dust extinction and emission are able to reproduce
the observed non-linear trend of the 24 um emission versus number of ionizing
photons, including the modest deficiency of 24 um emission in the low
metallicity regions, which results from a combination of decreasing dust
opacity and dust temperature at low luminosities. Conversely, the trend of the
8 um emission as a function of the number of ionizing photons is not well
reproduced by the same models. The 8 um emission is contributed, in larger
measure than the 24 um emission, by dust heated by non-ionizing stellar
populations, in agreement with previous findings. Two SFR calibrations, one
using the 24 um emission and the other using a combination of the 24 um and
H-alpha luminosities (Kennicutt et al. 2007), are presented. No calibration is
presented for the 8 um emission, because of its significant dependence on both
metallicity and environment. The calibrations presented here should be directly
applicable to systems dominated by on-going star formation.Comment: 67 pages, 15 figures, accepted for publication on the Astrophysical
Journal; replacement contains: correction to equation 8; important tweaks to
equation 9; various typos correcte
Star Formation Thresholds in Galactic Disks
We report the first results of a detailed study of the star formation law in
a sample of 32 nearby spiral galaxies with well-measured rotation curves, HI
and H (as traced by CO) surface density profiles, and new \Ha CCD
photometry. Our results strongly support the view that the formation of
gravitationally bound interstellar clouds regulates the onset of widespread
star formation -- at least in the outer regions of galactic disks.Comment: Will appear in July 1 ApJ. Abbreviated abstract. Postscript version
available at http://www.astro.caltech.edu/~clm
Warm Dust and Spatially Variable PAH Emission in the Dwarf Starburst Galaxy NGC 1705
We present Spitzer observations of the dwarf starburst galaxy NGC 1705
obtained as part of SINGS. The galaxy morphology is very different shortward
and longward of ~5 microns: short-wavelength imaging shows an underlying red
stellar population, with the central super star cluster (SSC) dominating the
luminosity; longer-wavelength data reveals warm dust emission arising from two
off-nuclear regions offset by ~250 pc from the SSC. These regions show little
extinction at optical wavelengths. The galaxy has a relatively low global dust
mass (~2E5 solar masses, implying a global dust-to-gas mass ratio ~2--4 times
lower than the Milky Way average). The off-nuclear dust emission appears to be
powered by photons from the same stellar population responsible for the
excitation of the observed H Alpha emission; these photons are unassociated
with the SSC (though a contribution from embedded sources to the IR luminosity
of the off-nuclear regions cannot be ruled out). Low-resolution IRS
spectroscopy shows moderate-strength PAH emission in the 11.3 micron band in
the eastern peak; no PAH emission is detected in the SSC or the western dust
emission complex. There is significant diffuse 8 micron emission after scaling
and subtracting shorter wavelength data; the spatially variable PAH emission
strengths revealed by the IRS data suggest caution in the interpretation of
diffuse 8 micron emission as arising from PAH carriers alone. The metallicity
of NGC 1705 falls at the transition level of 35% solar found by Engelbracht and
collaborators; the fact that a system at this metallicity shows spatially
variable PAH emission demonstrates the complexity of interpreting diffuse 8
micron emission. A radio continuum non-detection, NGC 1705 deviates
significantly from the canonical far-IR vs. radio correlation. (Abridged)Comment: ApJ, in press; please retrieve full-resolution version from
http://www.astro.wesleyan.edu/~cannon/pubs.htm
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