1,591 research outputs found
High spectral resolution observations of HNC3 and HCCNC in the L1544 prestellar core
HCCNC and HNC3 are less commonly found isomers of cyanoacetylene, HC3N, a
molecule that is widely found in diverse astronomical sources. We want to know
if HNC3 is present in sources other than the dark cloud TMC-1 and how its
abundance is relative to that of related molecules. We used the ASAI unbiased
spectral survey at IRAM 30m towards the prototypical prestellar core L1544 to
search for HNC3 and HCCNC which are by-product of the HC3NH+ recombination,
previously detected in this source. We performed a combined analysis of
published HNC3 microwave rest frequencies with thus far unpublished millimeter
data because of issues with available rest frequency predictions. We determined
new spectroscopic parameters for HNC3, produced new predictions and detected it
towards L1544. We used a gas-grain chemical modelling to predict the abundances
of N-species and compare with the observations. The modelled abundances are
consistent with the observations, considering a late stage of the evolution of
the prestellar core. However the calculated abundance of HNC3 was found 5-10
times higher than the observed one. The HC3N, HNC3 and HCCNC versus HC3NH+
ratios are compared in the TMC-1 dark cloud and the L1544 prestellar core.Comment: Accepted in MNRAS letters. 5 pages plus 2 additional pages for the
on-line materia
Field-Induced Quasiparticle Excitation in Ca(AlSi): Evidence for unconventional Superconductivity
The temperature () and magnetic field () dependence of the magnetic
penetration depth, , in Ca(AlSi) exhibits
significant deviation from that expected for conventional BCS superconductors.
In particular, it is inferred from a field dependence of () at 2.0 K that the quasiparticle excitation is strongly enhanced by the
Doppler shift. This suggests that the superconducting order parameter in
Ca(AlSi) is characterized by a small energy scale
K originating either from anisotropy or multi-gap
structure.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
The experimental challenge of detecting solar axion-like particles to test cosmological ALP-photon oscillation hypothesis
We consider possible experimental tests of recent hypotheses suggesting that
TeV photons survive the pair production interaction with extragalactic
background light over cosmological distances by converting to axion-like
particles (ALPs) in galactic magnetic fields. We show that proposed giant
ultra-low background scintillation detectors will even have a difficult time
reaching the present CAST sensitivity, which is one to two orders of magnitude
less sensitive than necessary for a meaningful test of the ALP-photon
oscillation hypothesis. Potential alternative tests are briefly discussed.Comment: 4 pages, no figure
Trends in Molecular Emission from Different Extragalactic Stellar Initial Mass Functions
Banerji et al. (2009) suggested that top-heavy stellar Initial Mass Functions
(IMFs) in galaxies may arise when the interstellar physical conditions inhibit
low-mass star formation, and they determined the physical conditions under
which this suppression may or may not occur. In this work, we explore the
sensitivity of the chemistry of interstellar gas under a wide range of
conditions. We use these results to predict the relative velocity-integrated
antenna temperatures of the CO rotational spectrum for several models of high
redshift active galaxies which may produce both top-heavy and unbiased IMFs. We
find that while active galaxies with solar metallicity (and top-heavy IMFs)
produce higher antenna temperatures than those with sub-solar metallicity (and
unbiased IMFs) the actual rotational distribution is similar. The high-J to
peak CO ratio however may be used to roughly infer the metallicity of a galaxy
provided we know whether it is active or quiescent. The metallicity strongly
influences the shape of the IMF. High order CO transitions are also found to
provide a good diagnostic for high far-UV intensity and low metallicity
counterparts of Milky Way type systems both of which show some evidence for
having top-heavy IMFs. We also compute the relative abundances of molecules
known to be effective tracers of high density gas in these galaxy models. We
find that the molecules CO and CS may be used to distinguish between solar and
sub-solar metallicity in active galaxies at high redshift whereas HCN, HNC and
CN are found to be relatively insensitive to the IMF shape at the large visual
magnitudes typically associated with extragalactic sources.Comment: 26 Pages, 8 Figures, Accepted for publication in Ap
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Observations of five molecular species in absorption towards Sagittarius B2
Seven diffuse molecular clouds have been detected in absorption, using the Sgr B2 star-formation region was used as a source of background continuum emission. Transitions were observed at frequencies around 49, 85 and 98 GHz, from CS, C34S, H13CN, H13CO+, SiO and C3H2. Clouds detected in absorption include the "nuclear disk", the 3 kpc expanding arm, spiral arms in the Galactic Plane, and two unidentified regions. The nuclear disk line profile was found to be inconsistent with homogeneous disk or bar models, instead suggesting irregular perturbations of the gas within a few hundred pc of the Galactic Centre.
Absorption in CS was detected in two different rotational transitions, leading to reliable estimates of the physical parameters of the clouds. In particular, exitation temperaturers could be estimated, instead of assumed values being used, as was the case in previous studies. Results from an LTE analysis and from LVG modelling show that the absorption lines are mostly optically thin, with molecular column densities ~1012-14cm-2 per cloud. Excitation temperatures as high as 5K were found, inconsistent with heating by the 2.7K cosmic background radiation alone. Cloud densities were estimated at nH2~104cm-3, or less if the gas is highly subthermalised
B12Hn and B12Fn: planar vs icosahedral structures
Using density functional theory and quantum Monte Carlo calculations, we show that B12Hn and B12Fn (n = 0 to 4) quasi-planar structures are energetically more favorable than the corresponding icosahedral clusters. Moreover, we show that the fully planar B12F6 cluster is more stable than the three-dimensional counterpart. These results open up the possibility of designing larger boron-based nanostructures starting from quasi-planar or fully planar building blocks
Atomic Carbon and CO Isotope Emission in the Vicinity of DR15
We present observations of the 3P1-3P0 fine structure transition of atomic
carbon [CI], the J=3-2 transition of CO, as well as of the J=1-0 transitions of
13CO and C18O toward DR15, an HII region associated with two mid-infrared dark
clouds (IRDCs). The 13CO and C18O J=1-0 emissions closely follow the dark
patches seen in optical wavelength, showing two self-gravitating molecular
cores with masses of 2000 Msun and 900 Msun, respectively, at the positions of
the catalogued IRDCs.
Our data show a rough spatial correlation between [CI] and 13CO J=1-0. Bright
[CI] emission occurs in relatively cold gas behind the molecular cores, neither
in highly excited gas traced by CO J=3-2 emission nor in HII region/molecular
cloud interface. These results are inconsistent with those predicted by
standard photodissociation region (PDR) models, suggesting an origin for
interstellar atomic carbon unrelated to photodissociation processes.Comment: 11 pages Latex, 6 figures, Accepted for publication in The
Astrophysical Journa
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