The Magellanic Bridge cluster NGC 796: Deep optical AO imaging reveals the stellar content and initial mass function of a massive open cluster

NGC 796 is a massive young cluster located 59 kpc from us in the diffuse intergalactic medium of the 1/5-1/10 $Z_{\odot}$ Magellanic Bridge, allowing to probe variations in star formation and stellar evolution processes as a function of metallicity in a resolved fashion, providing a link between resolved studies of nearby solar-metallicity and unresolved distant metal-poor clusters located in high-redshift galaxies. In this paper, we present adaptive optics $gri$H$\alpha$ imaging of NGC 796 (at 0.5", which is ~0.14 pc at the cluster distance) along with optical spectroscopy of two bright members to quantify the cluster properties. Our aim is to explore if star formation and stellar evolution varies as a function of metallicity by comparing the properties of NGC 796 to higher metallicity clusters. We find from isochronal fitting of the cluster main sequence in the colour-magnitude diagram an age of 20$^{+12}_{-5}$ Myr. Based on the cluster luminosity function, we derive a top-heavy stellar initial mass function (IMF) with a slope $\alpha$ = 1.99$\pm$0.2, hinting at an metallicity and/or environmental dependence of the IMF which may lead to a top-heavy IMF in the early Universe. Study of the H$\alpha$ emission line stars reveals that Classical Be stars constitute a higher fraction of the total B-type stars when compared with similar clusters at greater metallicity, providing some support to the chemically homogeneous theory of stellar evolution. Overall, NGC 796 has a total estimated mass of 990$\pm200$ $M_{\odot}$, and a core radius of 1.4$\pm$0.3 pc which classifies it as a massive young open cluster, unique in the diffuse interstellar medium of the Magellanic Bridge.Comment: Accepted for publication in the Astrophysical Journal. Contains 14 pages, 11 figures, and 3 table

A pilot search for mm-wavelength recombination lines from emerging ionized winds in pre-planetary nebulae candidates

We report the results from a pilot search for radio recombination line (RRL) emission at millimeter wavelengths in a small sample of pre-planetary nebulae (pPNe) and young PNe (yPNe) with emerging central ionized regions. Observations of the H30\alpha, H31a, H39a, H41a, H48b, H49b, H51b, and H55g lines at 1 and 3mm have been performed with the IRAM 30 m radio telescope. These lines are excellent probes of the dense inner (<~150 au) and heavily obscured regions of these objects, where the yet unknown agents for PN-shaping originate. We detected mm-RRLs in three objects: CRL 618, MWC 922, and M 2-9. For CRL 618, the only pPN with previous published detections of H41a, H35a, and H30a emission, we find significant changes in the line profiles indicating that current observations are probing regions of the ionized wind with larger expansion velocities and mass-loss rate than ~29 years ago. In the case of MWC 922, we observe a drastic transition from single-peaked profiles at 3mm to double-peaked profiles at 1mm, which is consistent with maser amplification of the highest frequency lines; the observed line profiles are compatible with rotation and expansion of the ionized gas, probably arranged in a disk+wind system around a ~5-10 Msun central mass. In M 2-9, the mm-RRL emission appears to be tracing a recent mass outburst by one of the stars of the central binary system. We present the results from non-LTE line and continuum radiative transfer models, which enables us to constrain the structure, kinematics, and physical conditions (electron temperature and density) of the ionized cores of our sample. (abridged). We deduce mass-loss rates of ~1e-6-1e-7 Msun/yr, which are significantly higher than the values adopted by stellar evolution models currently in use and would result in a transition from the asymptotic giant branch to the PN phase faster than hitherto assumed.Comment: Accepted by Astronomy and Astrophysics. 28 pages, including figure

Quantum Interference and Decoherence in Single-Molecule Junctions: How Vibrations Induce Electrical Current

Quantum interference effects and decoherence mechanisms in single-molecule junctions are analyzed employing a nonequilibrium Green's function approach. Electrons tunneling through quasi-degenerate states of a nanoscale molecular junction exhibit interference effects. We show that electronic-vibrational coupling, inherent to any molecular junction, strongly quenches such interference effects. As a result, the electrical current can be significantly larger than without electronic-vibrational coupling. The analysis reveals that the quenching of quantum interference is particularly pronounced if the junction is vibrationally highly excited, e.g. due to current-induced nonequilibrium effects in the resonant transport regime.Comment: 11 pages, 4 figure

Comment on "Density Functional Simulation of a Breaking Nanowire"

In a recent Letter, Nakamura et al. [Phys. Rev. Lett. 82, 1538 (1999)] described first principles calculations for a breaking Na nanocontact. Their system consists of a periodic one-dimensional array of supercells, each of which contains 39 Na atoms, originally forming a straight, crystalline wire with a length of 6 atoms. The system is elongated by increasing the length of the unit cell. At each step, the atomic configuration is relaxed to a new local equilibrium, and the tensile force is evaluated from the change of the total energy with elongation. Aside from a discontinuity of the force occuring at the transition from a crytalline to an amorphous configuration during the early stages of elongation, they were unable to identify any simple correlations between the force and the number of electronic modes transmitted through the contact. An important question is whether their model is realistic, i.e., whether it can be compared to experimental results obtained for a single nanocontact between two macroscopic pieces of metal. In this Comment, we demonstrate that with such a small unit cell, the interference effects between neighboring contacts are of the same size as the force oscillations in a single nanocontact.Comment: 1 pag

Sobre el stock adamellítico de La Granja y el sector metamórfico encajante (Guadarrama Central)

[Resumen] El stock adamellítico de La Granja (Guadarrama Central) intruye en materiales metamórficos de alto grado (con cordierita-silnmanita-ortosa), provocando neoformación de andalucita. Se describen asímismo, algunas facies litológicas de «contacto» como son los gneises porfidoclásticos o los cinturones aplograníticos de borde del granitoide. Se describen los materiales metamórficos haciendo especial hincapié en los rasgos texturales de los mismos y en el fuerte control geoquímico que ejerce el medio petrológico a las diferentes blastesis minerales. El estudio geoquímico aunado al estructural nos define un carácter ortoderivado a las formaciones glandulares sálicas y leucogneísicas con las que forman Unidad. Además se apuntan pautas de variación geoquímica de los gneises porfidoclásticos de «contacto». El stock se define como adamellita de grano grueso porfídica de tendencias calco-alcalinas y como apéndice de los materiales de grano grueso leucocráticos de la vertiente meridional de la Sierra. Se describen las redes filonianas características del stock encontrando ciertas diferencias composicionales entre ambos materiales. Finalmente encontramos una variación geoquímica, clara en elementos menores, entre las facies de borde e interior del dique[Abstract] La Granja granitoid stock in Centrat Guadarrama (Spain) intrudes in high grade metamorphic rocks (cordierite-sillimanite-ortose) inducing neoformation of andalusite. There are lithological «contact» facies, such as porphyroclastic gneiss or aplogranitic belts of chilled margin.We describe here the metamorphic materials, in special the textural features and the geochemical control of mineral blastesis by the lithology. The geochemical and structural studies determine an orthogenesis for augen and leucocratic gneiss which belong to the same formation. We describe too, some geochemical features of the porphyroclastic gneiss. The porphyritic granitoid stock is adamellitic in composition (O-monzonite), coarse grained and has a calc-alcalic character. La Granja stock is the natural appendix of leucocratic coarse grained granites from the Sierra's southern side. In this paper we deal with the characteristic stock dike-swarm, finding compositional differences between both materials. Finally we find a clear geochemical variation, specially in minor elements, between margin and core in the dike