2,997 research outputs found
Photoionisation and Heating of a Supernova Driven, Turbulent, Interstellar Medium
The Diffuse Ionised Gas (DIG) in galaxies traces photoionisation feedback
from massive stars. Through three dimensional photoionisation simulations, we
study the propagation of ionising photons, photoionisation heating and the
resulting distribution of ionised and neutral gas within snapshots of
magnetohydrodynamic simulations of a supernova driven turbulent interstellar
medium. We also investigate the impact of non-photoionisation heating on
observed optical emission line ratios. Inclusion of a heating term which scales
less steeply with electron density than photoionisation is required to produce
diagnostic emission line ratios similar to those observed with the Wisconsin
H{\alpha} Mapper. Once such heating terms have been included, we are also able
to produce temperatures similar to those inferred from observations of the DIG,
with temperatures increasing to above 15000 K at heights |z| > 1 kpc. We find
that ionising photons travel through low density regions close to the midplane
of the simulations, while travelling through diffuse low density regions at
large heights. The majority of photons travel small distances (< 100pc);
however some travel kiloparsecs and ionise the DIG.Comment: 10 pages, 13 figures, accepted to MNRA
2-D and 3-D Radiation Transfer Models of High-Mass Star Formation
2-D and 3-D radiation transfer models of forming stars generally produce
bluer 1-10 micron colors than 1-D models of the same evolutionary state and
envelope mass. Therefore, 1-D models of the shortwave radiation will generally
estimate a lower envelope mass and later evolutionary state than
multidimensional models. 1-D models are probably reasonable for very young
sources, or longwave analysis (wavelengths > 100 microns). In our 3-D models of
high-mass stars in clumpy molecular clouds, we find no correlation between the
depth of the 10 micron silicate feature and the longwave (> 100 micron) SED
(which sets the envelope mass), even when the average optical extinction of the
envelope is >100 magnitudes. This is in agreement with the observations of
Faison et al. (1998) of several UltraCompact HII (UCHII) regions, suggesting
that many of these sources are more evolved than embedded protostars.
We have calculated a large grid of 2-D models and find substantial overlap
between different evolutionary states in the mid-IR color-color diagrams. We
have developed a model fitter to work in conjunction with the grid to analyze
large datasets. This grid and fitter will be expanded and tested in 2005 and
released to the public in 2006.Comment: 10 pages, 8 figures, to appear in the proceedings of IAU Symp 227,
Massive Star Birth: A Crossroads of Astrophysics, (Cesaroni R., Churchwell
E., Felli M., Walmsley C. editors
2-D Radiative Transfer in Protostellar Envelopes: II. An Evolutionary Sequence
We present model spectral energy distributions, colors, polarization, and
images for an evolutionary sequence of a low-mass protostar from the early
collapse stage (Class 0) to the remnant disk stage (Class III). We find a
substantial overlap in colors and SEDs between protostars embedded in envelopes
(Class 0-I) and T Tauri disks (Class II), especially at mid-IR wavelengths.
Edge-on Class I-II sources show double-peaked spectral energy distributions,
with a short-wavelength hump due to scattered light and the long-wavelength
hump due to thermal emission. These are the bluest sources in mid-IR
color-color diagrams.
Since Class 0 and I sources are diffuse, the size of the aperture over which
fluxes are integrated has a substantial effect on the computed colors, with
larger aperture results showing significantly bluer colors. This causes overlap
in color-color diagrams between all evolutionary states, especially in the
mid-IR. However the near-IR polarization of the Class 0 sources is much higher
than the Class I-II sources, providing a means to separate these evolutionary
states.
We varied the grain properties in the circumstellar envelope, allowing for
larger grains in the disk midplane and smaller in the envelope. We find that
grain growth in disks of Class I sources can be detected at wavelengths greater
than 100 m.
Our image calculations predict that the diffuse emission from edge-on Class I
and II sources should be detectable in the mid-IR with the Space Infrared
Telescope Facility (SIRTF) in nearby star forming regions (out to several
hundred parsecs).Comment: A version with high-resolution images is available at
http://www.astro.wisc.edu/glimpse/glimpsepubs.htm
Infrared Spectroscopy of the Diffuse Ionized Halo of NGC 891
We present infrared spectroscopy from the Spitzer Space Telescope at one disk
position and two positions at a height of 1 kpc from the disk in the edge-on
spiral NGC 891, with the primary goal of studying halo ionization. Our main
result is that the [Ne III]/[Ne II] ratio, which provides a measure of the
hardness of the ionizing spectrum free from the major problems plaguing optical
line ratios, is enhanced in the extraplanar pointings relative to the disk
pointing. Using a 2D Monte Carlo-based photo-ionization code which accounts for
the effects of radiation field hardening, we find that this trend cannot be
reproduced by any plausible photo-ionization model, and that a secondary source
of ionization must therefore operate in gaseous halos. We also present the
first spectroscopic detections of extraplanar PAH features in an external
normal galaxy. If they are in an exponential layer, very rough emission
scale-heights of 330-530 pc are implied for the various features. Extinction
may be non-negligible in the midplane and reduce these scale-heights
significantly. There is little significant variation in the relative emission
from the various features between disk and extraplanar environment. Only the
17.4 micron feature is significantly enhanced in the extraplanar gas compared
to the other features, possibly indicating a preference for larger PAHs in the
halo.Comment: 35 pages in ApJ preprint format, 8 figures, accepted for publication
in ApJ. Minor change to Introduction to give appropriate credit to earlier,
related wor
High-Latitude HI in the Low Surface Brightness Galaxy UGC7321
From the analysis of sensitive HI 21-cm line observations, we find evidence
for vertically extended HI emission (|z|<~2.4 kpc) in the edge-on, low surface
brightness spiral galaxy UGC7321. Three-dimensional modelling suggests that the
HI disk of UGC7321 is both warped and flared, but that neither effect can fully
reproduce the spatial distribution and kinematics of the highest z-height gas.
We are able to model the high-latitude emission as an additional HI component
in the form of a ``thick disk'' or ``halo'' with a FWHM~3.3 kpc. We find
tentative evidence that the vertically extended gas declines in rotational
velocity as a function of z, although we are unable to completely rule out
models with constant V(z). In spite of the low star formation rate of UGC7321,
energy from supernovae may be sufficient to sustain this high-latitude gas.
However, alternative origins for this material, such as slow, sustained infall,
cannot yet be excluded.Comment: to appear in the August 20 Astrophysical Journal; 17 pages; version
with full resolution figures available at
http://cfa-www.harvard.edu/~lmatthew
The Infrared Properties of Super Star Clusters: Predictions from Three-Dimensional Radiative Transfer Models
With high-resolution infrared data becoming available that can probe the
formation of high-mass stellar clusters for the first time, models that make
testable predictions of these objects are necessary. We utilize a
three-dimensional radiative transfer code, including a hierarchically clumped
medium, to study the earliest stages of super star cluster evolution. We
explore a range of parameter space in geometric sequences that mimic the
evolution of an embedded super star cluster. The inclusion of a hierarchically
clumped medium can make the envelope porous, in accordance with previous models
and supporting observational evidence. The infrared luminosity inferred from
observations can differ by a factor of two from the true value in the clumpiest
envelopes depending on the viewing angle. The infrared spectral energy
distribution also varies with viewing angle for clumpy envelopes, creating a
range in possible observable infrared colors and magnitudes, silicate feature
depths and dust continua. General observable features of cluster evolution
differ between envelopes that are relatively opaque or transparent to
mid-infrared photons. The [70]-[160] color can be used to determine star
formation efficiency; the Spitzer IRAC/MIPS [8.0]-[24] color is able to
constrain Rin and Rout values; and the IRAC [3.6]-[5.8] color is sensitive to
the fraction of the dust distributed in clumps. Finally, in a comparison of
these models to data of ultracompact HII regions, we find good agreement,
suggesting that these models are physically relevant, and will provide useful
diagnostic ability for datasets of resolved, embedded SSCs with the advent of
high-resolution infrared telescopes like JWST.Comment: ApJ, accepted, to be published in the 729 -1 issue. 17 pages with 18
figure
Evolution of brown dwarf disks: A Spitzer survey in Upper Scorpius
We have carried out a Spitzer survey for brown dwarf (BD) disks in the ~5 Myr
old Upper Scorpius (UpSco) star forming region, using IRS spectroscopy from 8
to 12\mu m and MIPS photometry at 24\mu m. Our sample consists of 35 confirmed
very low mass members of UpSco. Thirteen objects in this sample show clear
excess flux at 24\mu m, explained by dust emission from a circum-sub-stellar
disk. Objects without excess emission either have no disks at all or disks with
inner opacity holes of at least ~5 AU radii. Our disk frequency of 37\pm 9% is
higher than what has been derived previously for K0-M5 stars in the same region
(on a 1.8 sigma confidence level), suggesting a mass-dependent disk lifetime in
UpSco. The clear distinction between objects with and without disks as well as
the lack of transition objects shows that disk dissipation inside 5 AU occurs
rapidly, probably on timescales of <~10^5 years. For the objects with disks,
most SEDs are uniformly flat with flux levels of a few mJy, well modeled as
emission from dusty disks affected by dust settling to the midplane, which also
provides indirect evidence for grain growth. The silicate feature around 10\mu
m is either absent or weak in our SEDs, arguing for a lack of hot, small dust
grains. Compared with younger objects in Taurus, BD disks in UpSco show less
flaring. Taken together, these results clearly demonstrate that we see disks in
an advanced evolutionary state: Dust settling and grain growth are ubiquituous
in circum-sub-stellar disks at ages of 5 Myr, arguing for planet forming
processes in BD disks. For almost all our targets, results from high-resolution
spectroscopy and high-spatial resolution imaging have been published before,
thus providing a large sample of BDs for which information about disks,
accretion, and binarity is available. (abridged)Comment: 39 pages, 7 figures, accepted for publication in Ap
North American Land Data Assimilation System: A Framework for Merging Model and Satellite Data for Improved Drought Monitoring
Drought is a pervasive natural climate hazard that has widespread impacts on human activity and the environment. In the United States, droughts are billion-dollar disasters, comparable to hurricanes and tropical storms and with greater economic impacts than extratropical storms, wildfires, blizzards, and ice storms combined (NCDC, 2009). Reduction of the impacts and increased preparedness for drought requires the use and improvement of monitoring and prediction tools. These tools are reliant on the availability of spatially extensive and accurate data for representing the occurrence and characteristics (such as duration and severity) of drought and their related forcing mechanisms. It is increasingly recognized that the utility of drought data is highly dependent on the application (e.g., agricultural monitoring versus water resource management) and time (e.g., short- versus long-term dryness) and space (e.g., local versus national) scales involved. A comprehensive set of drought indices that considers all components of the hydrological–ecological–human system is necessary. Because of the dearth of near-real-time in situ hydrologic data collected over large regions, modeled data are often useful surrogates, especially when combined with observations from remote sensing and in situ sources.
This chapter provides an overview of drought-related activities associated with the North American Land Data Assimilation System (NLDAS), which purports to provide an incremental step toward improved drought monitoring and forecasting. The NLDAS was originally conceived to improve short-term weather forecasting by providing better land surface initial conditions for operational weather forecast models. This reflects increased recognition of the role of land surface water and energy states, such as surface temperature, soil moisture, and snowpack, to atmospheric processes via feedbacks through the coupling of the water and energy cycles. Phase I of the NLDAS (NLDAS-1; Mitchell et al., 2004) made tremendous progress toward developing an operational system that gave high-resolution land hydrologic products in near real time. The system consists of multiple land surface models (LSMs) that are driven by an observation-based meteorological data set both in real time and retrospectively. This work resulted in a series of scientific papers that evaluated the retrospective data (meteorology and model output) in terms of their ability to reflect observations of the water and energy cycles and the uncertainties in the simulations as measured by the spread among individual models (Pan et al., 2003; Robock et al., 2003; Sheffield et al., 2003; Lohmann et al., 2004; Mitchell et al., 2004; Schaake et al., 2004). These evaluations led to the implementation of significant improvements to the LSMs in the form of new model physics and adjustments to parameter values and to the methods and input meteorological data (Xia et al., 2012). The system has since expanded in scope to include model intercomparison studies, real-time monitoring, and hydrologic prediction and has inspired other activities such as high-resolution land surface modeling and global land data assimilation systems (e.g., the Global Land Data Assimilation System [GLDAS], Rodell et al., 2004; the Land Information System [LIS], Kumar et al., 2006)
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