1,116 research outputs found
Complex organic molecules in comets C/2012 F6 (Lemmon) and C/2013 R1 (Lovejoy): detection of ethylene glycol and formamide
A spectral survey in the 1 mm wavelength range was undertaken in the
long-period comets C/2012 F6 (Lemmon) and C/2013 R1 (Lovejoy) using the 30 m
telescope of the Institut de radioastronomie millim\'etrique (IRAM) in April
and November-December 2013. We report the detection of ethylene glycol
(CHOH) (aGg' conformer) and formamide (NHCHO) in the two comets.
The abundances relative to water of ethylene glycol and formamide are 0.2-0.3%
and 0.02% in the two comets, similar to the values measured in comet C/1995 O1
(Hale-Bopp). We also report the detection of HCOOH and CHCHO in comet
C/2013 R1 (Lovejoy), and a search for other complex species (methyl formate,
glycolaldehyde).Comment: Accepted for publication as a Letter in Astronomy and Astrophysic
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&
Isotopic ratios of H, C, N, O, and S in comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy)
The apparition of bright comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy) in
March-April 2013 and January 2015, combined with the improved observational
capabilities of submillimeter facilities, offered an opportunity to carry out
sensitive compositional and isotopic studies of the volatiles in their coma. We
observed comet Lovejoy with the IRAM 30m telescope between 13 and 26 January
2015, and with the Odin submillimeter space observatory on 29 January - 3
February 2015. We detected 22 molecules and several isotopologues. The
HO and HO production rates measured with Odin follow a
periodic pattern with a period of 0.94 days and an amplitude of ~25%. The
inferred isotope ratios in comet Lovejoy are O/O = 499 24
and D/H = 1.4 0.4 in water, S/S = 24.7
3.5 in CS, all compatible with terrestrial values. The ratio
C/C = 109 14 in HCN is marginally higher than terrestrial
and N/N = 145 12 in HCN is half the Earth ratio. Several
upper limits for D/H or 12C/13C in other molecules are reported. From our
observation of HDO in comet C/2014 Q2 (Lovejoy), we report the first D/H ratio
in an Oort Cloud comet that is not larger than the terrestrial value. On the
other hand, the observation of the same HDO line in the other Oort-cloud comet,
C/2012 F6 (Lemmon), suggests a D/H value four times higher. Given the previous
measurements of D/H in cometary water, this illustrates that a diversity in the
D/H ratio and in the chemical composition, is present even within the same
dynamical group of comets, suggesting that current dynamical groups contain
comets formed at very different places or times in the early solar system.Comment: Accepted for publication in Astronomy and Astrophysic
Velocity-resolved [CII] emission and [CII]/FIR Mapping along Orion with Herschel
We present the first 7.5'x11.5' velocity-resolved map of the [CII]158um line
toward the Orion molecular cloud-1 (OMC-1) taken with the Herschel/HIFI
instrument. In combination with far-infrared (FIR) photometric images and
velocity-resolved maps of the H41alpha hydrogen recombination and CO J=2-1
lines, this data set provides an unprecedented view of the intricate
small-scale kinematics of the ionized/PDR/molecular gas interfaces and of the
radiative feedback from massive stars. The main contribution to the [CII]
luminosity (~85%) is from the extended, FUV-illuminated face of the cloud
G_0>500, n_H>5x10^3 cm^-3) and from dense PDRs (G_0~10^4, n_H~10^5 cm^-3) at
the interface between OMC-1 and the HII region surrounding the Trapezium
cluster. Around 15% of the [CII] emission arises from a different gas component
without CO counterpart. The [CII] excitation, PDR gas turbulence, line opacity
(from [13CII]) and role of the geometry of the illuminating stars with respect
to the cloud are investigated. We construct maps of the [CII]/FIR and FIR/M_Gas
ratios and show that [CII]/FIR decreases from the extended cloud component
(10^-2-10^-3) to the more opaque star-forming cores (10^-3-10^-4). The lowest
values are reminiscent of the "[CII] deficit" seen in local ultra-luminous IR
galaxies hosting vigorous star formation. Spatial correlation analysis shows
that the decreasing [CII]/FIR ratio correlates better with the column density
of dust through the molecular cloud than with FIR/M_Gas. We conclude that the
[CII] emitting column relative to the total dust column along each line of
sight is responsible for the observed [CII]/FIR variations through the cloud.Comment: 21 pages, 17 figures. Accepted for publication in the Astrophysical
Journal (2015 August 12). Figures 2, 6 and 7 are bitmapped to lower
resolution. This is version 2 after minor editorial changes. Notes added
after proofs include
Resonant Magnetization Tunneling in Mn12 Acetate: The Absence of Inhomogeneous Hyperfine Broadening
We present the results of a detailed study of the
thermally-assisted-resonant-tunneling relaxation rate of Mn12 acetate as a
function of an external, longitudinal magnetic field and find that the data can
be fit extremely well to a Lorentzian function. No hint of inhomogeneous
broadening is found, even though some is expected from the Mn nuclear hyperfine
interaction. This inconsistency implies that the tunneling mechanism cannot be
described simply in terms of a random hyperfine field.Comment: Some minor revisions, title changed, updated figures, two added
notes, one added reference. RevTeX, 4 pages, 3 postscript figures. Submitted
to Rapid Communication
Effect of local Coulomb interactions on the electronic structure and exchange interactions in Mn12 magnetic molecules
We have studied the effect of local Coulomb interactions on the electronic
structure of the molecular magnet Mn12-acetate within the LDA+U approach. The
account of the on-site repulsion results in a finite energy gap and an integer
value of the molecule's magnetic moment, both quantities being in a good
agreement with the experimental results. The resulting magnetic moments and
charge states of non-equivalent manganese ions agree very well with
experiments. The calculated values of the intramolecular exchange parameters
depend on the molecule's spin configuration, differing by 25-30% between the
ferrimagnetic ground state and the completely ferromagnetic configurations. The
values of the ground-state exchange coupling parameters are in reasonable
agreement with the recent data on the magnetization jumps in megagauss magnetic
fields. Simple estimates show that the obtained exchange parameters can be
applied, at least qualitatively, to the description of the spin excitations in
Mn12-acetate.Comment: RevTeX, LaTeX2e, 4 EPS figure
Low energy magnetic excitations of the Mn_{12}-acetate spin cluster observed by neutron scattering
We performed high resolution diffraction and inelastic neutron scattering
measurements of Mn_{12}-acetate. Using a very high energy resolution, we could
separate the energy levels corresponding to the splitting of the lowest S
multiplet. Data were analyzed within a single spin model (S=10 ground state),
using a spin Hamiltonian with parameters up to 4^{th} order.
The non regular spacing of the transition energies unambiguously shows the
presence of high order terms in the anisotropy (D= -0.457(2) cm^{-1}, B_4^0 =
-2.33(4) 10^{-5}cm^{-1}).
The relative intensity of the lowest energy peaks is very sensitive to the
small transverse term, supposed to be mainly responsible for quantum tunneling.
This allows an accurate determination of this term in zero magnetic field
(B_4^4 = \pm 3.0(5) 10^{-5} cm^{-1}). The neutron results are discussed in view
of recent experiments and theories.Comment: 4 pages ? 3 figures, submitted to Physical Review Lette
Crossover between Thermally Assisted and Pure Quantum Tunneling in Molecular Magnet Mn12-Acetate
The crossover between thermally assisted and pure quantum tunneling has been
studied in single crystals of high spin (S=10) uniaxial molecular magnet Mn12
using micro-Hall-effect magnetometry. Magnetic hysteresis and relaxation
experiments have been used to investigate the energy levels that determine the
magnetization reversal as a function of magnetic field and temperature. These
experiments demonstrate that the crossover occurs in a narrow (0.1 K) or broad
(1 K) temperature interval depending on the magnitude of the field transverse
to the anisotropy axis.Comment: 5 pages, 4 figure
Spin Tunneling and Phonon-assisted Relaxation in Mn12-acetate
We present a comprehensive theory of the magnetization relaxation in a
Mn12-acetate crystal in the high-temperature regime (T>1 K), which is based on
phonon-assisted spin tunneling induced by quartic magnetic anisotropy and weak
transverse magnetic fields. The overall relaxation rate as function of the
longitudinal magnetic field is calculated and shown to agree well with
experimental data including all resonance peaks measured so far. The Lorentzian
shape of the resonances, which we obtain via a generalized master equation that
includes spin tunneling, is also in good agreement with recent data. We derive
a general formula for the tunnel splitting energy of these resonances. We show
that fourth-order diagonal terms in the Hamiltonian lead to satellite peaks. A
derivation of the effective linewidth of a resonance peak is given and shown to
agree well with experimental data. In addition, previously unknown spin-phonon
coupling constants are calculated explicitly. The values obtained for these
constants and for the sound velocity are also in good agreement with recent
data. We show that the spin relaxation in Mn12-acetate takes place via several
transition paths of comparable weight. These transition paths are expressed in
terms of intermediate relaxation times, which are calculated and which can be
tested experimentally.Comment: 18 pages, 22 EPS figures, REVTe
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