1,837 research outputs found
Post-Impact Thermal Evolution of Porous Planetesimals
Impacts between planetesimals have largely been ruled out as a heat source in
the early Solar System, by calculations that show them to be an inefficient
heat source and unlikely to cause global heating. However, the long-term,
localized thermal effects of impacts on planetesimals have never been fully
quantified. Here, we simulate a range of impact scenarios between planetesimals
to determine the post-impact thermal histories of the parent bodies, and hence
the importance of impact heating in the thermal evolution of planetesimals. We
find on a local scale that heating material to petrologic type 6 is achievable
for a range of impact velocities and initial porosities, and impact melting is
possible in porous material at a velocity of > 4 km/s. Burial of heated
impactor material beneath the impact crater is common, insulating that material
and allowing the parent body to retain the heat for extended periods (~
millions of years). Cooling rates at 773 K are typically 1 - 1000 K/Ma,
matching a wide range of measurements of metallographic cooling rates from
chondritic materials. While the heating presented here is localized to the
impact site, multiple impacts over the lifetime of a parent body are likely to
have occurred. Moreover, as most meteorite samples are on the centimeter to
meter scale, the localized effects of impact heating cannot be ignored.Comment: 38 pages, 9 figures, Revised for Geochimica et Cosmochimica Acta
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The spectral energy distribution of galaxies at z > 2.5: Implications from the Herschel/SPIRE color-color diagram
We use the Herschel SPIRE color-color diagram to study the spectral energy
distribution (SED) and the redshift estimation of high-z galaxies. We compiled
a sample of 57 galaxies with spectroscopically confirmed redshifts and SPIRE
detections in all three bands at , and compared their average SPIRE
colors with SED templates from local and high-z libraries. We find that local
SEDs are inconsistent with high-z observations. The local calibrations of the
parameters need to be adjusted to describe the average colors of high-z
galaxies. For high-z libraries, the templates with an evolution from z=0 to 3
can well describe the average colors of the observations at high redshift.
Using these templates, we defined color cuts to divide the SPIRE color-color
diagram into different regions with different mean redshifts. We tested this
method and two other color cut methods using a large sample of 783
Herschel-selected galaxies, and find that although these methods can separate
the sample into populations with different mean redshifts, the dispersion of
redshifts in each population is considerably large. Additional information is
needed for better sampling.Comment: 17 pages, 14 figures, accepted for publication in A&
Residence times of particles in diffusive protoplanetary disk environments
ABSTRACT The chemical and physical evolution of primitive materials in protoplanetary disks are determined by the types of environments they are exposed to and their residence times within each environment. Here, a method for calculating representative paths of materials in diffusive protoplanetary disks is developed and applied to understanding how the vertical trajectories that particles take impact their overall evolution. The methods are general enough to be applied to disks with uniform diffusivity, the so-called constant-α cases, and disks with a spatially varying diffusivity, such as expected in "layered-disks." The average long-term dynamical evolution of small particles and gaseous molecules is independent of the specific form of the diffusivity in that they spend comparable fractions of their lifetimes at different heights in the disk. However, the paths that individual particles and molecules take depend strongly on the form of the diffusivity leading to a different range of behavior of particles in terms of deviations from the mean. As temperatures, gas densities, chemical abundances, and photon fluxes will vary with height in protoplanetary disks, the different paths taken by primitive materials will lead to differences in their chemical and physical evolution. Examples of differences in gas phase chemistry and photochemistry are explored here. The methods outlined here provide a powerful tool that can be integrated with chemical models to understand the formation and evolution of primitive materials in protoplanetary disks on timescales of 10 5 -10 6 years
Residence Times of Particles in Diffusive Protoplanetary Disk Environments I. Vertical Motions
The chemical and physical evolution of primitive materials in protoplanetary
disks are determined by the types of environments they are exposed to and their
residence times within each environment. Here a method for calculating
representative paths of materials in diffusive protoplanetary disks is
developed and applied to understanding how the vertical trajectories that
particles take impact their overall evolution. The methods are general enough
to be applied to disks with uniform diffusivity, the so-called
"constant-" cases, and disks with a spatially varying diffusivity, such
as expected in "layered-disks." The average long-term dynamical evolution of
small particles and gaseous molecules is independent of the specific form of
the diffusivity in that they spend comparable fractions of their lifetimes at
different heights in the disk. However, the paths that individual particles and
molecules take depend strongly on the form of the diffusivity leading to a
different range of behavior of particles in terms of deviations from the mean.
As temperatures, gas densities, chemical abundances, and photon fluxes will
vary with height in protoplanetary disks, the different paths taken by
primitive materials will lead to differences in their chemical and physical
evolution. Examples of differences in gas phase chemistry and photochemistry
are explored here. The methods outlined here provide a means of integrating
particle dynamics with chemical models to understand the formation and
evolution of primitive materials in protoplanetary disks over timescales of
- years.Comment: Accepted to Ap
Lava channel formation during the 2001 eruption on Mount Etna: evidence for mechanical erosion
We report the direct observation of a peculiar lava channel that was formed
near the base of a parasitic cone during the 2001 eruption on Mount Etna.
Erosive processes by flowing lava are commonly attributed to thermal erosion.
However, field evidence strongly suggests that models of thermal erosion cannot
explain the formation of this channel. Here, we put forward the idea that the
essential erosion mechanism was abrasive wear. By applying a simple model from
tribology we demonstrate that the available data agree favorably with our
hypothesis. Consequently, we propose that erosional processes resembling the
wear phenomena in glacial erosion are possible in a volcanic environment.Comment: accepted for publication in Physical Review Letter
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Earth’s carbon deficit caused by early loss through irreversible sublimation
Carbon is an essential element for life, but its behavior during Earth’s accretion is not well understood. Carbonaceous grains in meteoritic and cometary materials suggest that irreversible sublimation, and not condensation, governs carbon acquisition by terrestrial worlds. Through astronomical observations and modeling, we show that the sublimation front of carbon carriers in the solar nebula, or the soot line, moved inward quickly so that carbon-rich ingredients would be available for accretion at 1 astronomical unit after the first million years. On the other hand, geological constraints firmly establish a severe carbon deficit in Earth, requiring the destruction of inherited carbonaceous organics in the majority of its building blocks. The carbon-poor nature of Earth thus implies carbon loss in its precursor material through sublimation within the first million years
Probing the Timescale of the 1.4 GHz Radio emissions as a Star formation tracer
Radio used as a star formation rate (SFR) tracer presents enormous advantages
by being unaffected by dust and radio sources being pinpointed at the
sub-arc-second level. The interpretation of the low frequency 1.4 GHz
luminosity is hampered by the difficulty in modeling the cosmic ray paths in
the interstellar medium, and their interactions with the magnetic field. In
this work, we compare the SFR derived from radio observations, and the ones
derived from spectral energy distribution (SED) modeling. We aim at better
understand the behavior of the SFR radio tracer, with a specific emphasis on
the link with star-formation histories. We used the SED modeling code Code
Investigating GALaxy Emission, CIGALE, with a non-parametric star formation
history model (SFH) and fit the data over the wavelength range from the
ultraviolet (UV) up to the mid-infrared (mid-IR). We interpret the difference
between radio and SED-based SFR tracers in the light of recent gradients in the
derived SFH. To validate the robustness of the results, we checked for any
remaining active galaxy nuclei (AGN) contribution and tested the impact of our
SFH modeling approach. Approximately 27% our galaxies present a radio SFR
(SFR) at least ten times larger than the instantaneous SFR from
SED-fitting (SFR). This trend affects primarily the galaxies that
show a declining SFH activity over the last 300 Myr. Both SFR indicators
converge toward a consistent value, when the SFHs are averaged over a period
larger than 150 Myr to derive SFR. Although the radio at low
frequency 1.4 GHz is a good tracer of the star formation activity of galaxies
with constant or increasing SFH, our results indicate that this is not the case
for galaxies that are quenching. Our analysis suggests that the star formation
time sensitivity of the radio low frequency could be longer than 150 Myr.Comment: 10 pages, 10 figure
Cooling of Dense Gas by H2O Line Emission and an Assessment of its Effects in Chondrule-Forming Shocks
We consider gas at densities appropriate to protoplanetary disks and
calculate its ability to cool due to line radiation emitted by H2O molecules
within the gas. Our work follows that of Neufeld & Kaufman (1993; ApJ, 418,
263), expanding on their work in several key aspects, including use of a much
expanded line database, an improved escape probability formulism, and the
inclusion of dust grains, which can absorb line photons. Although the escape
probabilities formally depend on a complicated combination of optical depth in
the lines and in the dust grains, we show that the cooling rate including dust
is well approximated by the dust-free cooling rate multiplied by a simple
function of the dust optical depth. We apply the resultant cooling rate of a
dust-gas mixture to the case of a solar nebula shock pertinent to the formation
of chondrules, millimeter-sized melt droplets found in meteorites. Our aim is
to assess whether line cooling can be neglected in chondrule-forming shocks or
if it must be included. We find that for typical parameters, H2O line cooling
shuts off a few minutes past the shock front; line photons that might otherwise
escape the shocked region and cool the gas will be absorbed by dust grains.
During the first minute or so past the shock, however, line photons will cool
the gas at rates ~ 10,000 K/hr, dropping the temperature of the gas (and most
likely the chondrules within the gas) by several hundred K. Inclusion of H2O
line cooling therefore must be included in models of chondrule formation by
nebular shocks.Comment: Accepted for publication in The Astrophysical Journa
Linking dust emission to fundamental properties in galaxies: The low-metallicity picture
In this work, we aim at providing a consistent analysis of the dust
properties from metal-poor to metal-rich environments by linking them to
fundamental galactic parameters. We consider two samples of galaxies: the Dwarf
Galaxy Survey (DGS) and KINGFISH, totalling 109 galaxies, spanning almost 2 dex
in metallicity. We collect infrared (IR) to submillimetre (submm) data for both
samples and present the complete data set for the DGS sample. We model the
observed spectral energy distributions (SED) with a physically-motivated dust
model to access the dust properties. Using a different SED model (modified
blackbody), dust composition (amorphous carbon), or wavelength coverage at
submm wavelengths results in differences in the dust mass estimate of a factor
two to three, showing that this parameter is subject to non-negligible
systematic modelling uncertainties. For eight galaxies in our sample, we find a
rather small excess at 500 microns (< 1.5 sigma). We find that the dust SED of
low-metallicity galaxies is broader and peaks at shorter wavelengths compared
to more metal-rich systems, a sign of a clumpier medium in dwarf galaxies. The
PAH mass fraction and the dust temperature distribution are found to be driven
mostly by the specific star-formation rate, SSFR, with secondary effects from
metallicity. The correlations between metallicity and dust mass or total-IR
luminosity are direct consequences of the stellar mass-metallicity relation.
The dust-to-stellar mass ratios of metal-rich sources follow the well-studied
trend of decreasing ratio for decreasing SSFR. The relation is more complex for
highly star-forming low-metallicity galaxies and depends on the chemical
evolutionary stage of the source (i.e., gas-to-dust mass ratio). Dust growth
processes in the ISM play a key role in the dust mass build-up with respect to
the stellar content at high SSFR and low metallicity. (abridged)Comment: 44 pages (20 pages main body plus 5 Appendices), 11 figures, 9
tables, accepted for publication in A&
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