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

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

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 $\mu$ is included. We work in the limit when temperature vanishes. We find that static vortex solutions in $2+1D$ do exist. However, the 't Hooft-Polyakov monopole in $3+1D$ 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 $\mu$, up to a certain critical value.Comment: 4 pages, no figure

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 $J$ / $\psi$ resonance mass, coupling (leptonic decay constant), total width, and continuum threshold. The continuum threshold $s_0$, 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 $s_0 = 4 m_Q^2$, with $m_Q$ the charm quark mass, at $T\simeq 1.22 T_c$. The rest of the hadronic parameters behave very differently from those of light-light and heavy-light quark systems. The $J$ / $\psi$ mass is essentially constant in a wide range of temperatures, while the total width grows with temperature up to $T \simeq 1.04 T_c$ beyond which it decreases sharply with increasing T. The resonance coupling is also initially constant and then begins to increase monotonically around $T \simeq T_c$. This behaviour of the total width and of the leptonic decay constant provides a strong indication that the $J$ / $\psi$ 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 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

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

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 $M$ 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

Thermodynamics of hot quantum scalar field in a (D+1) dimensional curved spacetime

We use the brick wall model to calculate the free energy of quantum scalar field in a curved spacetime (D +1) dimensions. We find the thermodynamics properties of quantum scalar field in several scenaries: Minkowski spacetime, Schwarzschild spacetime and BTZ spacetime. For the cases analysed, the thermodynamical properties of quantum scalar field is exactly with the reported. It was found that the entropy of the gas is proportional to the horizon area in a gravity field strong, which is consistent with the holographic principle