1,407 research outputs found
Pseudo Goldstone Bosons Phenomenology in Minimal Walking Technicolor
We construct the non-linear realized Lagrangian for the Goldstone Bosons
associated to the breaking pattern of SU(4) to SO(4). This pattern is expected
to occur in any Technicolor extension of the standard model featuring two Dirac
fermions transforming according to real representations of the underlying gauge
group. We concentrate on the Minimal Walking Technicolor quantum number
assignments with respect to the standard model symmetries. We demonstrate that
for, any choice of the quantum numbers, consistent with gauge and Witten
anomalies the spectrum of the pseudo Goldstone Bosons contains electrically
doubly charged states which can be discovered at the Large Hadron Collider.Comment: 25 pages, 5 figure
Photochemistry of the PAH pyrene in water ice: the case for ion-mediated solid-state astrochemistry
Context. Icy dust grains play an important role in the formation of complex
inter- and circumstellar molecules. Observational studies show that polycyclic
aromatic hydrocarbons (PAHs) are abundantly present in the ISM in the gas
phase. It is likely that these non-volatile species freeze out onto dust grains
as well and participate in the astrochemical solid-state network, but
experimental PAH ice studies are largely lacking. Methods. Near UV/VIS
spectroscopy is used to track the in situ VUV driven photochemistry of pyrene
containing ices at temperatures ranging from 10 to 125 K. Results. The main
photoproducts of VUV photolyzed pyrene ices are spectroscopically identified
and their band positions are listed for two host ices, \water and CO. Pyrene
ionisation is found to be most efficient in \water ices at low temperatures.
The reaction products, triplet pyrene and the 1-hydro-1-pyrenyl radical are
most efficiently formed in higher temperature water ices and in low temperature
CO ice. Formation routes and band strength information of the identified
species are discussed. Additionally, the oscillator strengths of Py, Py^+ and
PyH are derived and a quantitative kinetic analysis is performed by fitting a
chemical reaction network to the experimental data. Conclusions. Pyrene is
efficiently ionised in water ice at temperatures below 50 K. Hydrogenation
reactions dominate the chemistry in low temperature CO ice with trace amounts
of water. The results are put in an astrophysical context by determining the
importance of PAH ionisation in a molecular cloud. The photoprocessing of a
sample PAH in ice described in this manuscript indicates that PAH
photoprocessing in the solid state should also be taken into account in
astrochemical models.Comment: 11 pages, 8 figures, accepted for publication in A&
Increased HCO production in the outer disk around HD 163296
Three formaldehyde lines were observed (HCO 3--2, HCO
3--2, and HCO 3--2) in the protoplanetary disk
around the Herbig Ae star HD 163296 with ALMA at 0.5 arcsecond (60 AU) spatial
resolution. HCO 3--2 was readily detected via imaging, while
the weaker HCO 3--2 and HCO 3--2 lines
required matched filter analysis to detect. HCO is present throughout most
of the gaseous disk, extending out to 550 AU. An apparent 50 AU inner radius of
the HCO emission is likely caused by an optically thick dust continuum. The
HCO radial intensity profile shows a peak at 100 AU and a secondary bump at
around 300 AU, suggesting increased production in the outer disk. Different
parameterizations of the HCO abundance were compared to the observed
visibilities with minimization, using either a characteristic
temperature, a characteristic radius or a radial power law index to describe
the HCO chemistry. Similar models were applied to ALMA Science Verification
data of CO. In all modeling scenarios, fits to the HCO data show an
increased abundance in the outer disk. The overall best-fit HCO model shows
a factor of two enhancement beyond a radius of 27020 AU, with an inner
abundance of . The HCO emitting region has a lower
limit on the kinetic temperature of K. The CO modeling suggests
an order of magnitude depletion in the outer disk and an abundance of in the inner disk. The increase in HCO outer disk emission
could be a result of hydrogenation of CO ices on dust grains that are then
sublimated via thermal desorption or UV photodesorption, or more efficient
gas-phase production beyond about 300 AU if CO is photodisocciated in this
region
Laboratory H2O:CO2 ice desorption data: entrapment dependencies and its parameterization with an extended three-phase model
Ice desorption affects the evolution of the gas-phase chemistry during the
protostellar stage, and also determines the chemical composition of comets
forming in circumstellar disks. From observations, most volatile species are
found in H2O-dominated ices. The aim of this study is first to experimentally
determine how entrapment of volatiles in H2O ice depends on ice thickness,
mixture ratio and heating rate, and second, to introduce an extended
three-phase model (gas, ice surface and ice mantle) to describe ice mixture
desorption with a minimum number of free parameters. Thermal H2O:CO2 ice
desorption is investigated in temperature programmed desorption experiments of
thin (10 - 40 ML) ice mixtures under ultra-high vacuum conditions. Desorption
is simultaneously monitored by mass spectrometry and reflection-absorption
infrared spectroscopy. The H2O:CO2 experiments are complemented with selected
H2O:CO, and H2O:CO2:CO experiments. The results are modeled with rate equations
that connect the gas, ice surface and ice mantle phases through surface
desorption and mantle-surface diffusion. The fraction of trapped CO2 increases
with ice thickness (10 - 32 ML) and H2O:CO2 mixing ratio (5:1 - 10:1), but not
with one order of magnitude different heating rates. The fraction of trapped
CO2 is 44 - 84 % with respect to the initial CO2 content for the investigated
experimental conditions. This is reproduced quantitatively by the extended
three-phase model that is introduced here. The H2O:CO and H2O:CO2:CO
experiments are consistent with the H2O:CO2 desorption trends, suggesting that
the model can be used for other ice species found in the interstellar medium to
significantly improve the parameterization of ice desorption.Comment: 12 pages, 9 figures, published in A&
Quantification of segregation dynamics in ice mixtures
(Abridged) The observed presence of pure CO2 ice in protostellar envelopes is
attributed to thermally induced ice segregation, but a lack of quantitative
experimental data has prevented its use as a temperature probe. Quantitative
segregation studies are also needed to characterize diffusion in ices, which
underpins all ice dynamics and ice chemistry. This study aims to quantify the
segregation mechanism and barriers in different H2O:CO2 and H2O:CO ice mixtures
covering a range of astrophysically relevant ice thicknesses and mixture
ratios. The ices are deposited at 16-50 K under (ultra-)high vacuum conditions.
Segregation is then monitored at 23-70 K as a function of time, through
infrared spectroscopy. Thin (8-37 ML) H2O:CO2/CO ice mixtures segregate
sequentially through surface processes, followed by an order of magnitude
slower bulk diffusion. Thicker ices (>100 ML) segregate through a fast bulk
process. The thick ices must therefore be either more porous or segregate
through a different mechanism, e.g. a phase transition. The segregation
dynamics of thin ices are reproduced qualitatively in Monte Carlo simulations
of surface hopping and pair swapping. The experimentally determined
surface-segregation rates for all mixture ratios follow the Ahrrenius law with
a barrier of 1080[190] K for H2O:CO2 and 300[100] K for H2O:CO mixtures. During
low-mass star formation H2O:CO2 segregation will be important already at 30[5]
K. Both surface and bulk segregation is proposed to be a general feature of ice
mixtures when the average bond strengths of the mixture constituents in pure
ice exceeds the average bond strength in the ice mixture.Comment: Accepted for publication in A&A. 25 pages, including 13 figure
DCO, DCN and ND reveal three different deuteration regimes in the disk around the Herbig Ae star HD163296
The formation pathways of deuterated species trace different regions of
protoplanetary disks and may shed light into their physical structure. We aim
to constrain the radial extent of main deuterated species; we are particularly
interested in spatially characterizing the high and low temperature pathways
for enhancing deuteration of these species. We observed the disk surrounding
the Herbig Ae star HD 163296 using ALMA in Band 6 and obtained resolved
spectral imaging data of DCO (=3-2), DCN (=3-2) and ND
(=3-2). We model the radial emission profiles of DCO, DCN and
ND, assuming their emission is optically thin, using a parametric model
of their abundances and radial excitation temperature estimates. DCO can be
described by a three-region model, with constant-abundance rings centered at 70
AU, 150 AU and 260 AU. The DCN radial profile peaks at about ~60 AU and
ND is seen in a ring at ~160 AU. Simple models of both molecules using
constant abundances reproduce the data. Assuming reasonable average excitation
temperatures for the whole disk, their disk-averaged column densities (and
deuterium fractionation ratios) are 1.6-2.6 cm
(0.04-0.07), 2.9-5.2 cm (0.02) and 1.6-2.5 cm (0.34-0.45) for DCO, DCN and ND, respectively.
Our simple best-fit models show a correlation between the radial location of
the first two rings in DCO and the DCN and ND abundance
distributions that can be interpreted as the high and low temperature
deuteration pathways regimes. The origin of the third DCO ring at 260 AU is
unknown but may be due to a local decrease of ultraviolet opacity allowing the
photodesorption of CO or due to thermal desorption of CO as a consequence of
radial drift and settlement of dust grains
The ancient heritage of water ice in the solar system
Identifying the source of Earth's water is central to understanding the
origins of life-fostering environments and to assessing the prevalence of such
environments in space. Water throughout the solar system exhibits
deuterium-to-hydrogen enrichments, a fossil relic of low-temperature,
ion-derived chemistry within either (i) the parent molecular cloud or (ii) the
solar nebula protoplanetary disk. Utilizing a comprehensive treatment of disk
ionization, we find that ion-driven deuterium pathways are inefficient,
curtailing the disk's deuterated water formation and its viability as the sole
source for the solar system's water. This finding implies that if the solar
system's formation was typical, abundant interstellar ices are available to all
nascent planetary systems.Comment: 33 pages, 7 figures including main text and supplementary materials.
Published in Scienc
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