14,911 research outputs found
Ethyl 2-methyl-4-phenylÂpyrido[1,2-a]benzimidazole-3-carboxylÂate
The title compound, C21H18N2O2, was synthesized using a novel tandem annulation reaction between (1H-benzimidazol-2-yl)(phenÂyl)methanone and (E)-ethyl 4-bromoÂbut-2-enoate under mild conditions. The dihedral angles between the mean planes of the five-membered imidazole ring and the pyridine, benzene and phenyl rings are 0.45 (6), 1.69 (1) and 70.96 (8)°, respectively. In the crystal, molÂecules are linked through interÂmolecular C—H⋯N hydrogen bonds
On a refinement of Wilf-equivalence for permutations
Recently, Dokos et al. conjectured that for all , the patterns and
are -Wilf-equivalent. In this paper, we confirm this conjecture for all
and . In fact, we construct a descent set preserving bijection
between -avoiding permutations and -avoiding
permutations for all . As a corollary, our bijection enables us to
settle a conjecture of Gowravaram and Jagadeesan concerning the
Wilf-equivalence for permutations with given descent sets
Effects of turbulent dust grain motion to interstellar chemistry
Theoretical studies have revealed that dust grains are usually moving fast
through the turbulent interstellar gas, which could have significant effects
upon interstellar chemistry by modifying grain accretion. This effect is
investigated in this work on the basis of numerical gas-grain chemical
modeling. Major features of the grain motion effect in the typical environment
of dark clouds (DC) can be summarised as follows: 1) decrease of gas-phase
(both neutral and ionic) abundances and increase of surface abundances by up to
2-3 orders of magnitude; 2) shifts of the existing chemical jumps to earlier
evolution ages for gas-phase species and to later ages for surface species by
factors of about ten; 3) a few exceptional cases in which some species turn out
to be insensitive to this effect and some other species can show opposite
behaviors too. These effects usually begin to emerge from a typical DC model
age of about 10^5 yr. The grain motion in a typical cold neutral medium (CNM)
can help overcome the Coulomb repulsive barrier to enable effective accretion
of cations onto positively charged grains. As a result, the grain motion
greatly enhances the abundances of some gas-phase and surface species by
factors up to 2-6 or more orders of magnitude in the CNM model. The grain
motion effect in a typical molecular cloud (MC) is intermediate between that of
the DC and CNM models, but with weaker strength. The grain motion is found to
be important to consider in chemical simulations of typical interstellar
medium.Comment: 20 pages, 10 figures and 2 table
Recovering stellar population parameters via different population models and stellar libraries
Three basic ingredients are required to generate a simple stellar population
(SSP) library, i.e., an initial mass function (IMF), a stellar evolution
model/isochrones, and an empirical/theoretical stellar spectral library.
However, there are still some uncertainties to the determination and
understanding of these ingredients. We perform the spectral fitting to test the
relative parameter offsets between these uncertainties using two different
stellar population models, two different empirical stellar libraries, two
different isochrones, and the Salpeter and Chabrier IMFs. Based on these
setups, we select five SSP libraries generated with the Galaxev/STELIB and
Vazdekis/MILES models, and apply them to the pPXF full-spectrum fitting of both
MaNGA and mock spectra. We find that: 1) Compared to the Galaxev/STELIB model,
spectral fitting qualities with the Vazdekis/MILES model have significant
improvements for those metal-rich (especially over-solar) spectra, which cause
better reduced distributions and more precisely fitted absorption
lines. This might due to the lack of metal rich stars in the empirical STELIB
library, or code improvement of the Vazdekis model. 2) When applying the
Vazdekis/MILES model for spectral fitting, the IMF variation will lead to not
only a systematic offset in , but also offsets in age and metallicity,
and these offsets increase with increasing stellar population ages. However,
the IMF-variation caused metallicity offsets disappear in the case of
Galaxev/STELIB based libraries. 3) The Padova2000 model provides a better match
to the MaNGA galaxy spectra at [M/H], while the BaSTI model match the
local galaxy spectra better at [M/H]. Current tests suggest that
spectral fitting with the Vazdekis/MILES+BaSTI combination would be a better
choice for local galaxies.Comment: 19 pages, 17 figures, accepted for publication in MNRA
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