8,095 research outputs found
Detection of circumstellar CH2CHCN, CH2CN, CH3CCH and H2CS
We report on the detection of vinyl cyanide (CH2CHCN), cyanomethyl radical
(CH2CN), methylacetylene (CH3CCH) and thioformaldehyde (H2CS) in the C-rich
star IRC +10216. These species, which are all known to exist in dark clouds,
are detected for the first time in the circumstellar envelope around an AGB
star. The four molecules have been detected trough pure rotational transitions
in the course of a 3 mm line survey carried out with the IRAM 30-m telescope.
The molecular column densities are derived by constructing rotational
temperature diagrams. A detailed chemical model of the circumstellar envelope
is used to analyze the formation of these molecular species. We have found
column densities in the range 5 x 10^(12)- 2 x 10^(13) cm^(-2), which
translates to abundances relative to H2 of several 10^(-9). The chemical model
is reasonably successful in explaining the derived abundances through gas phase
synthesis in the cold outer envelope. We also find that some of these
molecules, CH2CHCN and CH2CN, are most probably excited trough infrared pumping
to excited vibrational states. The detection of these species stresses the
similarity between the molecular content of cold dark clouds and C-rich
circumstellar envelopes. However, some differences in the chemistry are
indicated by the fact that in IRC +10216 partially saturated carbon chains are
present at a lower level than those which are highly unsaturated, while in
TMC-1 both types of species have comparable abundances.Comment: 9 pages, 5 figures; accepted for publication in A&
Theoretical and experimental study of AC loss in HTS single pancake coils
The electromagnetic properties of a pancake coil in AC regime as a function
of the number of turns is studied theoretically and experimentally.
Specifically, the AC loss, the coil critical current and the voltage signal are
discussed. The coils are made of Bi2Sr2Ca2Cu3O10/Ag (BiSCCO) tape, although the
main qualitative results are also applicable to other kinds of superconducting
tapes, such as coated conductors. The AC loss and the voltage signal are
electrically measured using different pick up coils with the help of a
transformer. One of them avoids dealing with the huge coil inductance. Besides,
the critical current of the coils is experimentally determined by conventional
DC measurements. Furthermore, the critical current, the AC loss and the voltage
signal are simulated, showing a good agreement with the experiments. For all
simulations, the field dependent critical current density inferred from DC
measurements on a short tape sample is taken into account.Comment: 22 pages, 15 figures; contents extended (sections 3.2 and 4); one new
figure (figure 5) and two figures replaced (figures 3 and 8); typos
corrected; title change
High-J v=0 SiS Maser Emission in IRC+10216: A New Case of Infrared Overlaps
We report on the first detection of maser emission in the J=11-10, J=14-13
and J=15-14 transitions of the v=0 vibrational state of SiS toward the C-rich
star IRC+10216. These masers seem to be produced in the very inhomogeneous
region between the star and the inner dust formation zone, placed at 5-7 R*,
with expansion velocities below 10 km/s. We interpret the pumping mechanism as
due to overlaps between v=1-0 ro-vibrational lines of SiS and mid-IR lines of
C2H2, HCN and their 13C isotopologues. The large number of overlaps found
suggests the existence of strong masers for high-J v=0 and v=1 SiS transitions,
located in the submillimeter range. In addition, it could be possible to find
several rotational lines of the SiS isotopologues displaying maser emission.Comment: 4 pages, 1 figure, published in the ApJ Letter
The abundances of polyacetylenes towards CRL618
We present a mid-infrared high spectral resolution spectrum of CRL618 in the
frequency ranges 778-784 and 1227-1249 cm^-1 (8.01-8.15 and 12.75-12.85 um)
taken with the Texas Echelon-cross-Echelle Spectrograph (TEXES) and the
Infrared Telescope Facility (IRTF). We have identified more than 170
ro-vibrational lines arising from C2H2, HCN, C4H2, and C6H2. We have found no
unmistakable trace of C8H2. The line profiles display a complex structure
suggesting the presence of polyacetylenes in several components of the
circumstellar envelope (CSE). We derive total column densities of 2.5 10^17,
3.1 10^17, 2.1 10^17, 9.3 10^16 cm^-2, and < 5 10^16 cm^-2 for HCN, C2H2, C4H2,
C6H2, and C8H2, respectively. The observations indicate that both the
rotational and vibrational temperatures in the innermost CSE depend on the
molecule, varying from 100 to 350 K for the rotational temperatures and 100 to
500 K for the vibrational temperatures. Our results support a chemistry in the
innermost CSE based on radical-neutral reactions triggered by the intense UV
radiation field.Comment: 9 pages, 4 figures, 1 table; accepted for publication in The
Astrophysical Journa
Multiple Charge State Beam Acceleration at Atlas
A test of the acceleration of multiple charge-state uranium beams was
performed at the ATLAS accelerator. A 238U+26 beam was accelerated in the ATLAS
PII linac to 286 MeV (~1.2 MeV/u) and stripped in a carbon foil located 0.5 m
from the entrance of the ATLAS Booster section. A 58Ni9+ 'guide' beam from the
tandem injector was used to tune the Booster for 238U+38. All charge states
from the stripping were injected into the booster and accelerated. Up to 94% of
the beam was accelerated through the Booster linac, with losses mostly in the
lower charge states. The measured beam properties of each charge state and a
comparison to numerical simulations are reported in this paper.Comment: LINAC2000, MOD0
Magnetic structure and orbital ordering in BaCoO3 from first-principles calculations
Ab initio calculations using the APW+lo method as implemented in the WIEN2k
code have been used to describe the electronic structure of the
quasi-one-dimensional system BaCoO3. Both, GGA and LDA+U approximations were
employed to study different orbital and magnetic orderings. GGA predicts a
metallic ground state whereas LDA+U calculations yield an insulating and
ferromagnetic ground state (in a low-spin state) with an alternating orbital
ordering along the Co-Co chains, consistent with the available experimental
data.Comment: 8 pages, 9 figure
Clues to NaCN formation
ALMA is providing us essential information on where certain molecules form.
Observing where these molecules emission arises from, the physical conditions
of the gas, and how this relates with the presence of other species allows us
to understand the formation of many species, and to significantly improve our
knowledge of the chemistry that occurs in the space. We studied the molecular
distribution of NaCN around IRC +10216, a molecule detected previously, but
whose origin is not clear. High angular resolution maps allow us to model the
abundance distribution of this molecule and check suggested formation paths. We
modeled the emission of NaCN assuming local thermal equilibrium (LTE)
conditions. These profiles were fitted to azimuthal averaged intensity profiles
to obtain an abundance distribution of NaCN. We found that the presence of NaCN
seems compatible with the presence of CN, probably as a result of the
photodissociation of HCN, in the inner layers of the ejecta of IRC +10216.
However, similar as for CH 3 CN, current photochemical models fail to reproduce
this CN reservoir. We also found that the abundance peak of NaCN appears at a
radius of 3 x 10 15 cm, approximately where the abundance of NaCl, suggested to
be the parent species, starts to decay. However, the abundance ratio shows that
the NaCl abundance is lower than that obtained for NaCN. We expect that the LTE
assumption might result in NaCN abundances higher than the real ones. Updated
photochemical models, collisional rates, and reaction rates are essential to
determine the possible paths of the NaCN formation.Comment: 7 pages, 10 figures. Accepted for publication in A&A letter
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