198 research outputs found
Modeling the Arctic Freshwater System and its integration in the global system: Lessons learned and future challenges
This is the final version of the article. Available from the publisher via the DOI in this record.Numerous components of the Arctic freshwater system (atmosphere, ocean, cryosphere, and terrestrial hydrology) have experienced large changes over the past few decades, and these changes are projected to amplify further in the future. Observations are particularly sparse, in both time and space, in the polar regions. Hence, modeling systems have been widely used and are a powerful tool to gain understanding on the functioning of the Arctic freshwater system and its integration within the global Earth system and climate. Here we present a review of modeling studies addressing some aspect of the Arctic freshwater system. Through illustrative examples, we point out the value of using a hierarchy of models with increasing complexity and component interactions, in order to dismantle the important processes at play for the variability and changes of the different components of the Arctic freshwater system and the interplay between them. We discuss past and projected changes for the Arctic freshwater system and explore the sources of uncertainty associated with these model results. We further elaborate on some missing processes that should be included in future generations of Earth system models and highlight the importance of better quantification and understanding of natural variability, among other factors, for improved predictions of Arctic freshwater system change.The first two authors have contributed
equally to the publication. The Arctic
Freshwater Synthesis has been
sponsored by the World Climate
Research Programmeâs Climate and the
Cryosphere project (WCRP-CliC), the
International Arctic Science Committee
(IASC), and the Arctic Monitoring and
Assessment Programme (AMAP). C.L.
acknowledges support from the UK
Natural Environment Research Council.
M.M.H. acknowledges support from NSF
PLR-1417642. D.M.L. is supported by
funding from the U.S. Department of
Energy BER, as part of its Climate Change
Prediction Program, Cooperative
Agreement DE-FC03-97ER62402/A010,
and NSF grants AGS-1048996,
PLS-1048987, and PLS-1304220. J.A.S. is
supported by Natural Environment
Research Council grant NE/J019585/1.
Y.D. is supported by Environment
Canadaâs Northern Hydrology program.
We acknowledge the World Climate
Research Programmeâs Working Group
on Coupled Modelling, which is responsible
for CMIP, and we thank the climate
modeling groups for producing and
making available their model output. For
CMIP, the U.S. Department of Energyâs
Program for Climate Model Diagnosis
and Intercomparison provides
coordinating support and led
development of software infrastructure
in partnership with the Global
Organization for Earth System Science
Portals. The CMIP data and CESM-LE data
are available through the relevant Web
data portal
Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN and HNC isotopologues
The 15N isotopologue abundance ratio measured today in different bodies of
the solar system is thought to be connected to 15N-fractionation effects that
would have occured in the protosolar nebula. The present study aims at putting
constraints on the degree of 15N-fractionation that occurs during the
prestellar phase, through observations of D, 13C and 15N-substituted
isotopologues towards B1b. Both molecules from the nitrogen hydride family,
i.e. N2H+ and NH3, and from the nitrile family, i.e. HCN, HNC and CN, are
considered in the analysis. As a first step, we model the continuum emission in
order to derive the physical structure of the cloud, i.e. gas temperature and
H2 density. These parameters are subsequently used as an input in a non-local
radiative transfer model to infer the radial abundances profiles of the various
molecules. Our modeling shows that all the molecules are affected by depletion
onto dust grains, in the region that encompasses the B1-bS and B1-bN cores.
While high levels of deuterium fractionation are derived, we conclude that no
fractionation occurs in the case of the nitrogen chemistry. Independently of
the chemical family, the molecular abundances are consistent with 14N/15N~300,
a value representative of the elemental atomic abundances of the parental gas.
The inefficiency of the 15N-fractionation effects in the B1b region can be
linked to the relatively high gas temperature ~17K which is representative of
the innermost part of the cloud. Since this region shows signs of depletion
onto dust grains, we can not exclude the possibility that the molecules were
previously enriched in 15N, earlier in the B1b history, and that such an
enrichment could have been incorporated into the ice mantles. It is thus
necessary to repeat this kind of study in colder sources to test such a
possibility.Comment: accepted in A&
Depletion of chlorine into HCl ice in a protostellar core
The freezeout of gas-phase species onto cold dust grains can drastically
alter the chemistry and the heating-cooling balance of protostellar material.
In contrast to well-known species such as carbon monoxide (CO), the freezeout
of various carriers of elements with abundances has not yet been
well studied. Our aim here is to study the depletion of chlorine in the
protostellar core, OMC-2 FIR 4. We observed transitions of HCl and H2Cl+
towards OMC-2 FIR 4 using the Herschel Space Observatory and Caltech
Submillimeter Observatory facilities. Our analysis makes use of state of the
art chlorine gas-grain chemical models and newly calculated HCl-H
hyperfine collisional excitation rate coefficients. A narrow emission component
in the HCl lines traces the extended envelope, and a broad one traces a more
compact central region. The gas-phase HCl abundance in FIR 4 is 9e-11, a factor
of only 0.001 that of volatile elemental chlorine. The H2Cl+ lines are detected
in absorption and trace a tenuous foreground cloud, where we find no depletion
of volatile chlorine. Gas-phase HCl is the tip of the chlorine iceberg in
protostellar cores. Using a gas-grain chemical model, we show that the
hydrogenation of atomic chlorine on grain surfaces in the dark cloud stage
sequesters at least 90% of the volatile chlorine into HCl ice, where it remains
in the protostellar stage. About 10% of chlorine is in gaseous atomic form.
Gas-phase HCl is a minor, but diagnostically key reservoir, with an abundance
of <1e-10 in most of the protostellar core. We find the 35Cl/37Cl ratio in
OMC-2 FIR 4 to be 3.2\pm0.1, consistent with the solar system value.Comment: 13 pages, 12 figures, accepted for publication in A&
Astronomical identification of CN-, the smallest observed molecular anion
We present the first astronomical detection of a diatomic negative ion, the
cyanide anion CN-, as well as quantum mechanical calculations of the excitation
of this anion through collisions with para-H2. CN- is identified through the
observation of the J = 2-1 and J = 3-2 rotational transitions in the C-star
envelope IRC +10216 with the IRAM 30-m telescope. The U-shaped line profiles
indicate that CN-, like the large anion C6H-, is formed in the outer regions of
the envelope. Chemical and excitation model calculations suggest that this
species forms from the reaction of large carbon anions with N atoms, rather
than from the radiative attachment of an electron to CN, as is the case for
large molecular anions. The unexpectedly large abundance derived for CN-, 0.25
% relative to CN, makes likely its detection in other astronomical sources. A
parallel search for the small anion C2H- remains so far unconclusive, despite
the previous tentative identification of the J = 1-0 rotational transition. The
abundance of C2H- in IRC +10216 is found to be vanishingly small, < 0.0014 %
relative to C2H.Comment: 5 pages, 4 figures; accepted for publication in A&A Letter
A new ab initio potential energy surface for the collisional excitation of N2H(+) by H2
10 pags.; 14 figs.© 2015 AIP Publishing LLC. We compute a new potential energy surface (PES) for the study of the inelastic collisions between N2H+ and H2 molecules. A preliminary study of the reactivity of N2H+ with H2 shows that neglecting reactive channels in collisional excitation studies is certainly valid at low temperatures. The four dimensional (4D) N2H+âH2 PES is obtained from electronic structure calculations using the coupled cluster with single, double, and perturbative triple excitation level of theory. The atoms are described by the augmented correlation consistent triple zeta basis set. Both molecules were treated as rigid rotors. The potential energy surface exhibits a well depth of â2530 cmâ1. Considering this very deep well, it appears that converged scattering calculations that take into account the rotational structure of both N2H+ and H2 should be very difficult to carry out. To overcome this difficulty, the âadiabatic-hindered-rotorâ treatment, which allows para-H2(j = 0) to be treated as if it were spherical, was used in order to reduce the scattering calculations to a 2D problem. The validity of this approach is checked and we find that cross sections and rate coefficients computed from the adiabatic reduced surface are in very good agreement with the full 4D calculationsThis research was supported by the CNRS national program
âPhysique et Chimie du Milieu Interstellaire.â F.L. and
Y.K. also thank the Agence Nationale de la Recherche
(ANR-HYDRIDES), contract No. ANR-12-BS05-0011-01.
We acknowledge Laurent Pagani for stimulating this work.Peer Reviewe
Stratified NH and ND emission in the prestellar core 16293E in L1689N
Context. High degrees of deuterium fractionation are commonly found in cold prestellar cores and in the envelopes around young protostars. As it brings strong constraints to chemical models, deuterium chemistry is often used to infer core history or molecule formation pathways. Whereas a large number of observations are available regarding interstellar deuterated stable molecules, relatively little is known about the deuteration of hydride radicals, as their fundamental rotational transitions are at high frequencies where the atmosphere is mostly opaque.
Aims. Nitrogen hydride radicals are important species in nitrogen chemistry, as they are thought to be related to ammonia formation. Observations have shown that ammonia is strongly deuterated, with [NH_2D]/[NH_3] ~ 10%. Models predict similarly high [ND]/[NH] ratios, but so far only one observational determination of this ratio is available, towards the envelope of the protostar IRAS16293-2422. To test model predictions, we aim here to determine [ND]/[NH] in a dense, starless core.
Methods. We observed NH and ND in 16293E with the HIFI spectrometer on board the Herschel Space Observatory as part of the CHESS guaranteed time key programme, and derived the abundances of these two species using a non local thermodynamic equilibrium radiative transfer model.
Results. Both NH and ND are detected in the source, with ND in emission and NH in absorption against the continuum that arises from the cold dust emission. Our model shows, however, that the ND emission and the NH absorption originate from different layers in the cloud, as further evidenced by their different velocities. In the central region of the core, we can set a lower limit to the [ND]/[NH] ratio of âł2%. This estimate is consistent with recent pure gas-phase models of nitrogen chemistry
Sex differences and effects of prenatal exposure to excess testosterone on ventral tegmental area dopamine neurons in adult sheep
Prenatal testosterone (T) excess in sheep results in a wide array of reproductive neuroendocrine deficits and alterations in motivated behavior. The ventral tegmental area (VTA) plays a critical role in reward and motivated behaviors and is hypothesised to be targeted by prenatal T. Here we report a sex difference in the number VTA dopamine cells in the adult sheep, with higher numbers of tyrosine hydroxylase (TH)âimmunoreactive (âir) cells in males than females. Moreover, prenatal exposure to excess T during either gestational days 30â90 or 60â90 resulted in increased numbers of VTA THâir cells in adult ewes compared to control females. Stereological analysis confirmed significantly greater numbers of neurons in the VTA of males and prenatal Tâtreated ewes, which was primarily accounted for by greater numbers of THâir cells. In addition, immunoreactivity for TH in the cells was denser in males and prenatal Tâtreated females, suggesting that sex differences and prenatal exposure to excess T affects both numbers of cells expressing TH and the protein levels within dopamine cells. Sex differences were also noted in numbers of THâir cells in the substantia nigra, with more cells in males than females. However, prenatal exposure to excess T did not affect numbers of THâir cells in the substantia nigra, suggesting that this sex difference is organised independently of prenatal actions of T. Together, these results demonstrate sex differences in the sheep VTA dopamine system which are mimicked by prenatal treatment with excess T.We report a sex difference in ventral tegmental area (VTA) dopamine cells in the adult sheep with higher numbers of tyrosine hydroxylase (TH)âimmunoreactive cells in males than females. Moreover, prenatal exposure to excess T during gestational days 30â90 or 60â90 caused increased numbers of VTA THâimmunoreactive cells in adult ewes compared to control females. Sex differences were also demonstrated in the substantia nigra, but prenatal T had no effect on TH in this area. Results indicate that sex differences and prenatal exposure to excess T affects both numbers of cells expressing TH and the protein levels in the VTA.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111123/1/ejn12871.pd
Herschel spectral-mapping of the Helix Nebula (NGC 7293): Extended CO photodissociation and OH+ emission
The Helix Nebula (NGC 7293) is the closest planetary nebulae. Therefore, it
is an ideal template for photochemical studies at small spatial scales in
planetary nebulae. We aim to study the spatial distribution of the atomic and
the molecular gas, and the structure of the photodissociation region along the
western rims of the Helix Nebula as seen in the submillimeter range with
Herschel. We use 5 SPIRE FTS pointing observations to make atomic and molecular
spectral maps. We analyze the molecular gas by modeling the CO rotational lines
using a non-local thermodynamic equilibrium (non-LTE) radiative transfer model.
For the first time, we have detected extended OH+ emission in a planetary
nebula. The spectra towards the Helix Nebula also show CO emission lines (from
J= 4 to 8), [NII] at 1461 GHz from ionized gas, and [CI] (2-1), which together
with the OH+ lines, trace extended CO photodissociation regions along the rims.
The estimated OH+ column density is (1-10)x1e12 cm-2. The CH+ (1-0) line was
not detected at the sensitivity of our observations. Non-LTE models of the CO
excitation were used to constrain the average gas density (n(H2)=(1-5)x1e5
cm-3) and the gas temperature (Tk= 20-40 K). The SPIRE spectral-maps suggest
that CO arises from dense and shielded clumps in the western rims of the Helix
Nebula whereas OH+ and [CI] lines trace the diffuse gas and the UV and X-ray
illuminated clumps surface where molecules reform after CO photodissociation.
[NII] traces a more diffuse ionized gas component in the interclump medium.Comment: Accepted for publication in Astronomy and Astrophysic
Nitrogen isotopic ratios in Barnard 1: a consistent study of the N_2H^+, NH_3, CN, HCN, and HNC isotopologues
Context. The ^(15)N isotopologue abundance ratio measured today in different bodies of the solar system is thought to be connected to ^(15)N-fractionation effects that would have occurred in the protosolar nebula.
Aims. The present study aims at putting constraints on the degree of 15N-fractionation that occurs during the prestellar phase, through observations of D, ^(13)C, and ^(15)N-substituted isotopologues towards B1b. Molecules both from the nitrogen hydride family, i.e. N2H+, and NH3, and from the nitrile family, i.e. HCN, HNC, and CN, are considered in the analysis.
Methods. As a first step, we modelled the continuum emission in order to derive the physical structure of the cloud, i.e. gas temperature and H_2 density. These parameters were subsequently used as input in a non-local radiative transfer model to infer the radial abundance profiles of the various molecules.
Results. Our modelling shows that all the molecules are affected by depletion onto dust grains in the region that encompasses the B1-bS and B1-bN cores. While high levels of deuterium fractionation are derived, we conclude that no fractionation occurs in the case of the nitrogen chemistry. Independently of the chemical family, the molecular abundances are consistent with ^(14)N/^(15)N ~ 300, a value representative of the elemental atomic abundances of the parental gas.
Conclusions. The inefficiency of the ^(15)N-fractionation effects in the B1b region can be linked to the relatively high gas temperature ~17 K, which is representative of the innermost part of the cloud. Since this region shows signs of depletion onto dust grains, we cannot exclude the possibility that the molecules were previously enriched in ^(15)N, earlier in the B1b history and that such an enrichment could have been incorporated into the ice mantles. It is thus necessary to repeat this kind of study in colder sources to test such a possibility
Similar levels of deuteration in the pre-stellar core L1544 and the protostellar core HH211
In the centre of pre-stellar cores, deuterium fractionation is enhanced due
to the low temperatures and high densities. Therefore, the chemistry of
deuterated molecules can be used to study the earliest stages of star
formation. We analyse the deuterium fractionation of simple molecules,
comparing the level of deuteration in the envelopes of the pre-stellar core
L1544 in Taurus and the protostellar core HH211 in Perseus. We used single-dish
observations of CCH, HCN, HNC, HCO, and their C-, O- and
D-bearing isotopologues, detected with the Onsala 20m telescope. We derived the
column densities and the deuterium fractions of the molecules. Additionally, we
used radiative transfer simulations and results from chemical modelling to
reproduce the observed molecular lines. We used new collisional rate
coefficients for HNC, HNC, DNC, and DCN that consider the hyperfine
structure of these molecules. We find high levels of deuteration for CCH (10%)
in both sources, consistent with other carbon chains, and moderate levels for
HCN (5-7%) and HNC (8%). The deuterium fraction of HCO is enhanced towards
HH211, most likely caused by isotope-selective photodissociation of CO.
Similar levels of deuteration show that the process is likely equally efficient
towards both cores, suggesting that the protostellar envelope still retains the
chemical composition of the original pre-stellar core. The fact that the two
cores are embedded in different molecular clouds also suggests that
environmental conditions do not have a significant effect on the deuteration
within dense cores. Radiative transfer modelling shows that it is necessary to
include the outer layers of the cores to consider the effects of extended
structures. Besides HCO observations, HCN observations towards L1544 also
require the presence of an outer diffuse layer where the molecules are
relatively abundant.Comment: 27 pages, 17 figures, accepted for publication in A&
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