17,326 research outputs found
Revisiting the radial abundance gradients of nitrogen and oxygen of the Milky Way
We present spectra obtained with the 10.4 m Gran Telescopio Canarias
telescope of 13 Galactic HII regions, most of them of very low ionisation
degree. The objects are located along the Galactic disc, with R_G from 5.7 to
16.1 kpc. We determine T_e([NII]) for all of them. We obtain - for the first
time - a radial abundance gradient of N that is independent on the ionisation
correction factor. The radial distribution of the N/O ratio is almost flat,
indicating that the bulk of N is not formed by standard secondary processes. We
have made a reassessment of the radial O abundance gradient combining our
results with previous similar ones by Esteban et al. (2017); producing a
homogeneous dataset of 35 HII regions with direct determinations of the
electron temperature. We report the possible presence of a flattening or drop
of the O abundance in the inner part of the Galactic disc. This result confirms
previous findings from metallicity distributions based on Cepheids and red
giants. Finally, we find that the scatter of the N and O abundances of HII
regions with respect to the gradient fittings is not substantially larger than
the observational uncertainties, indicating that both chemical elements seem to
be well mixed in the interstellar gas at a given distance along the Galactic
discComment: 24 pages, 13 figures, 6 tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Societ
Casimir Energy of an irregular membrane
We compute the Casimir energy which arises in a bi-dimensional surface due to
the quantum fluctuations of a scalar field. We assume that the boundaries are
irregular and the field obeys Dirichlet condition. We re-parametrize the
problem to one which has flat boundary conditions and the irregularity is
treated as a perturbation in the Laplace-Beltrami operator which appears.
Later, to compute the Casimir energy, we use zeta function regularization. It
is compared the results coming from perturbation theory with the WKB method.Comment: 9 page
Topological Field Configurations in the Presence of Isospin Chemical Potential
We analyze the stability of different topological solutions in Quantum Field
Theory when an isospin chemical potential is included. We work in the
limit when temperature vanishes. We find that static vortex solutions in
do exist. However, the 't Hooft-Polyakov monopole in is no longer
stable, as soon as the chemical potential acquires a finite value. In the case
of the Skyrmion, this topological solution still exists for finite , up to
a certain critical value.Comment: 4 pages, no figure
The production of strong broad He II emission after the tidal disruption of a main-sequence star by a supermassive black hole
The tidal disruption event (TDE) PS1-10jh lacked strong Balmer lines but
showed strong, broad, He II emission both before maximum light and for at least
8 months thereafter. Gezari et al. interpreted this as evidence for the
disruption of a rare hydrogen-deficient star. However, Guillochon et al. have
argued instead that the disrupted star was a normal main-sequence star and that
the strength of the He II emission compared with the Balmer lines is a result
the emission being similar to the broad-line region (BLR) of an AGN, but
lacking the outer, lower-ionization BLR gas. We show that the profile of He II
4686 in PS1-10jh is similar to the blueshifted profiles of high-ionization
lines in AGNs. We find an He II 4686/Halpha ratio for PS1-10jh of ~ 3.7. We
show that both the high-velocity gas of the inner BLR of normal AGNs and the
spectra of type II-P supernovae right after shock break out also produce very
high He II 4686/Halpha ratios. A high He II 4686/Halpha ratio can thus be
produced with a solar H/He abundance ratio. We demonstrate from photoionization
modelling that the estimated He II 4686/Halpha ratio can be produced with a BLR
truncated before the He++ Stroemgren length if the density is ~ 10^{11}
cm^{-3}. The similarity of the He II 4686 emission in PS1-10jh to the emission
from the inner BLRs of AGNs supports the idea that the emission after a TDE
event is similar to that of normal AGNs.Comment: Submitted to MNRAS. 5 pages. 4 figure
Noncommutativity and the Aharonov-Bohm Effect
The possibility of detecting noncommutive space relics is analyzed by using
the Aharonov-Bohm effect. If space is non-commutative, it turns out that the
holonomy receives kinematical corrections that tend to diffuse the fringe
pattern. This fringe pattern has a non-trivial energy dependence and,
therefore, one could observe noncommutative effects by modifying the energy of
the incident electrons beam in the Tonomura experimental arrangementComment: Original title changed, several parts rewritten and a section about a
possible experimental detection of spatial noncommutativity is adde
Charmonium in the vector channel at finite temperature from QCD sum rules
Thermal Hilbert moment QCD sum rules are used to obtain the temperature
dependence of the hadronic parameters of charmonium in the vector channel, i.e.
the / resonance mass, coupling (leptonic decay constant), total
width, and continuum threshold. The continuum threshold , which signals
the end of the resonance region and the onset of perturbative QCD (PQCD),
behaves as in all other hadronic channels, i.e. it decreases with increasing
temperature until it reaches the PQCD threshold , with the
charm quark mass, at . The rest of the hadronic parameters
behave very differently from those of light-light and heavy-light quark
systems. The / mass is essentially constant in a wide range of
temperatures, while the total width grows with temperature up to beyond which it decreases sharply with increasing T. The resonance
coupling is also initially constant and then begins to increase monotonically
around . This behaviour of the total width and of the leptonic
decay constant provides a strong indication that the / resonance
might survive beyond the critical temperature for deconfinement.Comment: An error in the PQCD scattering term has been corrected. No changes
result, other than a slight reduction of the critical temperature. A few
clarifying paragraphs have been adde
The chemical composition of Galactic ring nebulae around massive stars
We present deep spectra of ring nebulae associated with Wolf-Rayet (WR) and
O-type stars: NGC 6888, G2.4+1.4, RCW 58, S 308, NGC 7635 and RCW 52. The data
have been taken with the 10m Gran Telescopio Canarias and the 6.5m Clay
Telescope. We extract spectra of several apertures in some of the objects. We
derive C and O abundances from faint recombination lines in NGC
6888 and NGC 7635, permitting to derive their C/H and C/O ratios and estimate
the abundance discrepancy factor (ADF) of O. The ADFs are larger than
the typical ones of normal HII regions but similar to those found in the
ionised gas of star-forming dwarf galaxies. We find that chemical abundances
are rather homogeneous in the nebulae where we have spectra of several
apertures: NGC 6888, NGC 7635 and G2.4+1.4. We obtain very high values of
electron temperature in a peripheral zone of NGC 6888, finding that shock
excitation can reproduce its spectral properties. We find that all the objects
associated with WR stars show N enrichment. Some of them also show He
enrichment and O deficiency as well as a lower Ne/O than expected, this may
indicate the strong action of the ON and NeNa cycles. We have compared the
chemical composition of NGC 6888, G2.4+1.4, RCW 58 and S 308 with the
nucleosynthesis predicted by stellar evolution models of massive stars. We find
that non-rotational models of stars of initial masses between 25 and 40 solar
masses seem to reproduce the observed abundance ratios of most of the nebulae.Comment: 26 pages, 12 pages, 13 tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Societ
The radial abundance gradient of chlorine in the Milky Way
We determine the radial abundance gradient of Cl in the Milky Way from HII
regions spectra. For the first time, the Cl/H ratios are computed by simply
adding ionic abundances and not using an ionization correction factor (ICF). We
use a collection of published very deep spectra of Galactic HII regions. We
have re-calculated the physical conditions, ionic and total abundances of Cl
and O using the same methodology and updated atomic data for all the objects.
We find that the slopes of the radial gradients of Cl and O are identical
within the uncertainties: -0.043 dex/kpc. This is consistent with a lockstep
evolution of both elements. We obtain that the mean value of the Cl/O ratio
across the Galactic disc is log(Cl/O) = -3.42 +/- 0.06. We compare our Cl/H
ratios with those determined from Cl++ abundances and using some available ICF
schemes of the literature. We find that our total Cl abundances are always
lower than the values determined using ICFs, indicating that those correction
schemes systematically overestimate the contribution of Cl+ and Cl+++ species
to the total Cl abundance. Finally, we propose an empirical ICF(Cl++) to
estimate the Cl/H ratio in HII regions.Comment: 9 pages, 2 figures, 10 tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Societ
Optimization and NP_R-Completeness of Certain Fewnomials
We give a high precision polynomial-time approximation scheme for the
supremum of any honest n-variate (n+2)-nomial with a constant term, allowing
real exponents as well as real coefficients. Our complexity bounds count field
operations and inequality checks, and are polynomial in n and the logarithm of
a certain condition number. For the special case of polynomials (i.e., integer
exponents), the log of our condition number is quadratic in the sparse
encoding. The best previous complexity bounds were exponential in the sparse
encoding, even for n fixed. Along the way, we extend the theory of
A-discriminants to real exponents and certain exponential sums, and find new
and natural NP_R-complete problems.Comment: 9 pages, 7 figures (3 of them tiny). This is close to the final
conference proceedings versio
Black Shells, Dirac's Field and the species problem
We describe a thermal atmosphere around a black hole as vacuum excitations
near to gravitational radius of a contracting thin black shell, i.e., in terms
of properties of the physical vacuum of fields around a thin shell of mass
collapsing from infinity to the Schwarzschild radius according to an external
stationary observer. A natural explanation is introduced for the necessary
cutoff using the equations of motion of the shells. We make a thermodynamic
description of a fermionic field near the gravitational radius. Then a solution
to the species problem for two fields, scalar one and spinor one, is proposed.
Finally we get the Bekenstein-Hawking entropy as entanglement entropy of a
thermal atmosphere, independent from number of fields.Comment: 31 pages, 11 figure
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