5,803 research outputs found
Evidence of ongoing radial migration in NGC 6754: Azimuthal variations of the gas properties
Understanding the nature of spiral structure in disk galaxies is one of the
main, and still unsolved questions in galactic astronomy. However, theoretical
works are proposing new testable predictions whose detection is becoming
feasible with recent development in instrumentation. In particular, streaming
motions along spiral arms are expected to induce azimuthal variations in the
chemical composition of a galaxy at a given galactic radius. In this letter we
analyse the gas content in NGC 6754 with VLT/MUSE data to characterise its 2D
chemical composition and H line-of-sight velocity distribution. We find
that the trailing (leading) edge of the NGC 6754 spiral arms show signatures of
tangentially-slower, radially-outward (tangentially-faster, radially-inward)
streaming motions of metal-rich (poor) gas over a large range of radii. These
results show direct evidence of gas radial migration for the first time. We
compare our results with the gas behaviour in a -body disk simulation
showing spiral morphological features rotating with a similar speed as the gas
at every radius, in good agreement with the observed trend. This indicates that
the spiral arm features in NGC 6754 may be transient and rotate similarly as
the gas does at a large range of radii.Comment: 8 pages, 4 figures, accepted for publication in ApJL 2016 September
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Physical interpretation of NUT solution
We show that the well-known NUT solution can be correctly interpreted as
describing the exterior field of two counter-rotating semi-infinite sources
possessing negative masses and infinite angular momenta which are attached to
the poles of a static finite rod of positive mass.Comment: 7 pages, 1 figure, submitted to Classical and Quantum Gravit
Micromammal biostratigraphy of the Alcoi Basin (eastern Spain) : remarks on the Pliocene record of the Iberian Peninsula
The study of 13 micromammal localities in the southern section of the Gormaget ravine (Alcoi Basin, SE Spain) and another 4 localities in the northern section has allowed us to define four local biozones in the dawn of the Pliocene, possibly recording the Mio-Pliocene boundary. The great density of localities close to the Mio-Pliocene boundary has enabled us to achieve a great resolution in the biozonation of the earliest Pliocene, only comparable in the Iberian Peninsula to the record of the Teruel Basin (NE Spain). We interpret these biozones in the light of the Neogene Mammal units and the European Land Mammal Ages, and correlate them with other local biozones defined for the same time span in the Iberian Peninsula
Logarithmic potential of Hermite polynomials and information entropies of the harmonic oscillator eigenstates
13 pages, 1 figure.-- PACS nrs.: 03.65.Ge, 02.10.Nj, 02.10.Sp.MR#: MR1471913 (99c:81031)Zbl#: Zbl 0891.33007The problem of calculating the information entropy in both position and momentum spaces for the nth stationary state of the one-dimensional quantum harmonic oscillator reduces to the evaluation of the logarithmic potential V_n(t)=-\int e\sp {-x\sp 2}H_n\sp 2(x)\log -t x at the zeros of the Hermite polynomial Hn(x). Here, a closed analytical expression for Vn(t) is obtained, which in turn yields an exact analytical expression for the entropies when the exact location of the zeros of Hn(x) is known. An inequality for the values of Vn(t) at the zeros of Hn(x) is conjectured, which leads to a new, nonvariational, upper bound for the entropies. Finally, the exact formula for Vn(t) is written in an alternative way, which allows the entropies to be expressed in terms of the even-order spectral moments of the Hermite polynomials. The asymptotic (n>>1) limit of this alternative expression for the entropies is discussed, and the conjectured upper bound for the entropies is proved to be asymptotically validThe author gratefully acknowledges the financial support from the Fundació Aula (Barcelona, Spain).Publicad
On interrelations between Sibgatullin's and Alekseev's approaches to the construction of exact solutions of the Einstein-Maxwell equations
The integral equations involved in Alekseev's "monodromy transform" technique
are shown to be simple combinations of Sibgatullin's integral equations and
normalizing conditions. An additional complex conjugation introduced by
Alekseev in the integrands makes his scheme mathematically inconsistent;
besides, in the electrovac case all Alekseev's principal value integrals
contain an intrinsic error which has never been identified before. We also
explain how operates a non-trivial double-step algorithm devised by Alekseev
for rewriting, by purely algebraic manipulations and in a different (more
complicated) parameter set, any particular specialization of the known
analytically extended N-soliton electrovac solution obtained in 1995 with the
aid of Sibgatullin's method.Comment: 7 pages, no figures, section II extende
Singular sources in the Demianski-Newman spacetimes
The analysis of singular regions in the NUT solutions carried out in the
recent paper (Manko and Ruiz, 2005 Class. Quantum Grav. 22, p.3555) is now
extended to the Demianski-Newman vacuum and electrovacuum spacetimes. We show
that the effect which produces the NUT parameter in a more general situation
remains essentially the same as in the purely NUT solutions: it introduces the
semi-infinite singularities of infinite angular momenta and positive or
negative masses depending on the interrelations between the parameters; the
presence of the electromagnetic field additionally endows the singularities
with electric and magnetic charges. The exact formulae describing the mass,
charges and angular momentum distributions in the Demianski-Newman solutions
are obtained and concise general expressions P_n=(m+i\nu)(ia)^n,
Q_n=(q+ib)(ia)^n for the entire set of the respective Beig-Simon multipole
moments are derived. These moments correspond to a unique choice of the
integration constant in the expression of the metric function \omega which is
different from the original choice made by Demianski and Newman.Comment: 22 pages, 5 figures; submitted to Classical and Quantum Gravit
Considerations on Field Methodology for Macrofungi Studies in Fragmented Forests of Mediterranean Agricultural Landscapes
The methodology used for the determination of macrofungal diversity in Mediterranean areas differs in the time of sampling and the number of years displayed, making it difficult to compare results. Furthermore, the results could be refuted because the studies are being conducted over an insufficient number of years or without considering the variation of the meteorological conditions from one year to the next and its effects on fruiting time, which might not fit the sampling. In order to optimize field work on fungal fruiting in Mediterranean environments dominated by holm oak (Quercus ilex L.), a weekly field analysis of macrofungal diversity from February 2009 to June 2013 was carried out in a Mediterranean holm oak forest in the middle-west of the Iberian Peninsula. The results revealed that fruiting bodies appeared throughout the year and that there was a delay in autumn fruiting, overlapping with spring. All this seems to indicate that weekly collection throughout the year and for a period of two years could be sufficient to estimate the macrofungal biodiversity of this ecosystem. © 2022 by the authors. Licensee MDPI, Basel, Switzerland
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