The silicate absorption profile in the ISM towards the heavily obscured nucleus of NGC 4418

The 9.7-micron silicate absorption profile in the interstellar medium provides important information on the physical and chemical composition of interstellar dust grains. Measurements in the Milky Way have shown that the profile in the diffuse interstellar medium is very similar to the amorphous silicate profiles found in circumstellar dust shells around late M stars, and narrower than the silicate profile in denser star-forming regions. Here, we investigate the silicate absorption profile towards the very heavily obscured nucleus of NGC 4418, the galaxy with the deepest known silicate absorption feature, and compare it to the profiles seen in the Milky Way. Comparison between the 8-13 micron spectrum obtained with TReCS on Gemini and the larger aperture spectrum obtained from the Spitzer archive indicates that the former isolates the nuclear emission, while Spitzer detects low surface brightness circumnuclear diffuse emission in addition. The silicate absorption profile towards the nucleus is very similar to that in the diffuse ISM in the Milky Way with no evidence of spectral structure from crystalline silicates or silicon carbide grains.Comment: 7 Pages, 3 figures. MNRAS in pres

The Radio Jet Associated with the Multiple V380 Ori System

The giant Herbig-Haro object 222 extends over $\sim$6$'$ in the plane of the sky, with a bow shock morphology. The identification of its exciting source has remained uncertain over the years. A non-thermal radio source located at the core of the shock structure was proposed to be the exciting source. However, Very Large Array studies showed that the radio source has a clear morphology of radio galaxy and a lack of flux variations or proper motions, favoring an extragalactic origin. Recently, an optical-IR study proposed that this giant HH object is driven by the multiple stellar system V380 Ori, located about 23$'$ to the SE of HH 222. The exciting sources of HH systems are usually detected as weak free-free emitters at centimeter wavelengths. Here we report the detection of an elongated radio source associated with the Herbig Be star or with its close infrared companion in the multiple V380 Ori system. This radio source has the characteristics of a thermal radio jet and is aligned with the direction of the giant outflow defined by HH~222 and its suggested counterpart to the SE, HH~1041. We propose that this radio jet traces the origin of the large scale HH outflow. Assuming that the jet arises from the Herbig Be star, the radio luminosity is a few times smaller than the value expected from the radio-bolometric correlation for radio jets, confirming that this is a more evolved object than those used to establish the correlation.Comment: 13 pages, 3 figure

DNA: From rigid base-pairs to semiflexible polymers

The sequence-dependent elasticity of double-helical DNA on a nm length scale can be captured by the rigid base-pair model, whose strains are the relative position and orientation of adjacent base-pairs. Corresponding elastic potentials have been obtained from all-atom MD simulation and from high-resolution structural data. On the scale of a hundred nm, DNA is successfully described by a continuous worm-like chain model with homogeneous elastic properties characterized by a set of four elastic constants, which have been directly measured in single-molecule experiments. We present here a theory that links these experiments on different scales, by systematically coarse-graining the rigid base-pair model for random sequence DNA to an effective worm-like chain description. The average helical geometry of the molecule is exactly taken into account in our approach. We find that the available microscopic parameters sets predict qualitatively similar mesoscopic parameters. The thermal bending and twisting persistence lengths computed from MD data are 42 and 48 nm, respectively. The static persistence lengths are generally much higher, in agreement with cyclization experiments. All microscopic parameter sets predict negative twist-stretch coupling. The variability and anisotropy of bending stiffness in short random chains lead to non-Gaussian bend angle distributions, but become unimportant after two helical turns.Comment: 13 pages, 6 figures, 6 table

Models of Metal Poor Stars with Gravitational Settling and Radiative Accelerations: I. Evolution and Abundance Anomalies

Evolutionary models have been calculated for Pop II stars of 0.5 to 1.0$M_\odot$ from the pre-main-sequence to the lower part of the giant branch. Rosseland opacities and radiative accelerations were calculated taking into account the concentration variations of 28 chemical species, including all species contributing to Rosseland opacities in the OPAL tables. The effects of radiative accelerations, thermal diffusion and gravitational settling are included. While models were calculated both for Z=0.00017 and 0.0017, we concentrate on models with Z=0.00017 in this paper. These are the first Pop II models calculated taking radiative acceleration into account. It is shown that, at least in a 0.8$M_\odot$ star, it is a better approximation not to let Fe diffuse than to calculate its gravitational settling without including the effects of $g_{rad}(Fe)$. In the absence of any turbulence outside of convection zones, the effects of atomic diffusion are large mainly for stars more massive than 0.7$M_\odot$. Overabundances are expected in some stars with \teff \ge 6000K. Most chemical species heavier than CNO are affected. At 12 Gyr, overabundance factors may reach 10 in some cases (e.g. for Al or Ni) while others are limited to 3 (e.g. for Fe). The calculated surface abundances are compared to recent observations of abundances in globular clusters as well as to observations of Li in halo stars. It is shown that, as in the case of Pop I stars, additional turbulence appears to be present.Comment: 40 pages, 17 color figures, to appear in The Astrophysical Journal, April 2002 (paper with original high resolution figures can be found at http://www.cerca.umontreal.ca/~richer/Fichiersps/popII_1.ps

Collins functions for pions from SIDIS and new e+e- data: a first glance at their transverse momentum dependence

New data from Belle and BaBar Collaborations on azimuthal asymmetries, measured in e+e- annihilations into pion pairs at Q^2=112 GeV^2, allow to take the first, direct glance at the transverse momentum dependence of the Collins functions, in addition to their z dependence. These data, together with available Semi-Inclusive Deep Inelastic Scattering (SIDIS) data on the Collins asymmetry, are simultaneously analysed in the framework of the generalised parton model assuming two alternative Q^2 evolution schemes and exploiting two different parameterisations for the Collins functions. The corresponding results for the transversity distributions are presented. Analogous data, newly released by the BESIII Collaboration, on e+e- annihilations into pion pairs at the lower Q^2 of 13 GeV^2, offer the possibility to explore the sensitivity of these azimuthal correlations on transverse momentum dependent evolution effects.Comment: 20 pages, 15 figures, 4 table

Chandra Observation of PSR B1823-13 and its Pulsar Wind Nebula

We report on an observation of the Vela-like pulsar B1823-13 and its synchrotron nebula with Chandra.The pulsar's spectrum fits a power-law model with a photon index Gamma_PSR=2.4 for the plausible hydrogen column density n_H=10^{22} cm^{-2}, corresponding to the luminosity L_PSR=8*10^{31} ergs s^{-1} in the 0.5-8 keV band, at a distance of 4 kpc. The pulsar radiation likely includes magnetospheric and thermal components, but they cannot be reliably separated because of the small number of counts detected and strong interstellar absorption. The pulsar is surrounded by a compact, 25''x 10'', pulsar wind nebula (PWN) elongated in the east-west direction, which includes a brighter inner component, 7''x 3'', elongated in the northeast-southwest direction. The slope of the compact PWN spectrum is Gamma_comp=1.3, and the 0.5-8 keV luminosity is L_comp~3*10^{32} ergs s^{-1}. The compact PWN is surrounded by asymmetric diffuse emission (extended PWN) seen up to at least 2.4' south of the pulsar, with a softer spectrum (Gamma_ext=1.9), and the 0.5-8 keV luminosity L_ext~10^{33}-10^{34} ergs s^{-1}. We also measured the pulsar's proper motion using archival VLA data: \mu_\alpha=23.0+/-2.5 mas yr^{-1}, \mu_\delta=-3.9+/-3.3 mas yr^{-1}, which corresponds to the transverse velocity v_perp=440 km s^{-1}. The direction of the proper motion is approximately parallel to the elongation of the compact PWN, but it is nearly perpendicular to that of the extended PWN and to the direction towards the center of the bright VHE gamma-ray source HESS J1825-137, which is likely powered by PSR B1823-13.Comment: 13 pages, 8 figures and 3 tables; submitted to Ap

The exceptional Herbig Ae star HD101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star

We obtained high-resolution, high signal-to-noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period P_rot=13.86d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non-statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current-driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate-mass stars could be an alternative to a frozen-in fossil field.Comment: 7 pages, 6 figures, 1 table, to appear in Astronomische Nachrichte