A purely geometric distance to the binary star Atlas, a member of the Pleiades

We present radial velocity and new interferometric measurements of the double star Atlas, which permit, with the addition of published interferometric data, to precisely derive the orbital parameters of the binary system and the masses of the components. The derived semi-major axis, compared with its measured angular size, allows to determine a distance to Atlas of 132+-4 pc in a purely geometrical way. Under the assumption that the location of Atlas is representative of the average distance of the cluster, we confirm the distance value generally obtained through main sequence fitting, in contradiction with the early Hipparcos result (118.3+-3.5 pc).Comment: 5 pages, 3 figures, accepted for publication in A&A Letter

Radio Sources in Low-Luminosity Active Galactic Nuclei. I. VLA Detections of Compact, Flat-Spectrum Cores

We report a 0.2" resolution, 15 GHz survey of a sample of 48 low-luminosity active galactic nuclei with the Very Large Array. Compact radio emission has been detected in 57% (17 of 30) of LINERs and low-luminosity Seyferts, at least 15 of which have a flat to inverted radio spectrum (alpha > -0.3). The compact radio cores are found in both type 1 (i.e. with broad Halpha) and type 2 (without broad Halpha) nuclei. The 2 cm radio power is significantly correlated with the emission-line ([OI] lambda6300) luminosity. While the present observations are consistent with the radio emission originating in star-forming regions, higher resolution radio observations of 10 of the detected sources, reported in a companion paper (Falcke et al. 2000), show that the cores are very compact (= 10^8K) and probably synchrotron self-absorbed, ruling out a starburst origin. Thus, our results suggest that at least 50% of low-luminosity Seyferts and LINERs in the sample are accretion powered, with the radio emission presumably coming from jets or advection-dominated accretion flows. We have detected only 1 of 18 `transition' (i.e. LINER + HII) nuclei observed, indicating their radio cores are significantly weaker than those of `pure' LINERs.Comment: To appear in the Astrophysical Journal, October 20, 200

Carnets de bord en sciences humaines : genèse, dynamique et fin d'une revue

Ce dernier numéro est l'occasion de faire un bilan et d'analyser les raisons qui nous ont conduits à décider de mettre un terme à l'existence d'une revue que nous avons animée pendant près de dix ans. Fidèles à la ligne éditoriale, nous dévoilons les coulisses de la production de Carnets de bord. Chemin faisant, nous nous livrons à un exercice d'auto-analyse susceptible d'éclairer la fragilité de ce genre d'initiatives intellectuelles dans un contexte universitaire où les critères managériaux d'évaluation ont largement gagné du terrain

Distribution of Faraday Rotation Measure in Jets from Active Galactic Nuclei II. Prediction from our Sweeping Magnetic Twist Model for the Wiggled Parts of AGN Jets and Tails

Distributions of Faraday rotation measure (FRM) and the projected magnetic field derived by a 3-dimensional simulation of MHD jets are investigated based on our "sweeping magnetic twist model". FRM and Stokes parameters were calculated to be compared with radio observations of large scale wiggled AGN jets on kpc scales. We propose that the FRM distribution can be used to discuss the 3-dimensional structure of magnetic field around jets and the validity of existing theoretical models, together with the projected magnetic field derived from Stokes parameters. In the previous paper, we investigated the basic straight part of AGN jets by using the result of a 2-dimensional axisymmetric simulation. The derived FRM distribution has a general tendency to have a gradient across the jet axis, which is due to the toroidal component of the magnetic field generated by the rotation of the accretion disk. In this paper, we consider the wiggled structure of the AGN jets by using the result of a 3-dimensional simulation. Our numerical results show that the distributions of FRM and the projected magnetic field have a clear correlation with the large scale structure of the jet itself, namely, 3-dimensional helix. Distributions, seeing the jet from a certain direction, show a good matching with those in a part of 3C449 jet. This suggests that the jet has a helical structure and that the magnetic field (especially the toroidal component) plays an important role in the dynamics of the wiggle formation because it is due to a current-driven helical kink instability in our model.Comment: Accepted for publication in Ap

Anatomy of helical relativistic jets: The case of S5 0836+710

Helical structures are common in extragalactic jets. They are usually attributed in the literature to periodical phenomena in the source (e.g., precession). In this work, we use VLBI data of the radio-jet in the quasar S5 0836+710 and hypothesize that the ridge-line of helical jets like this corresponds to a pressure maximum in the jet and assume that the helically twisted pressure maximum is the result of a helical wave pattern. For our study, we use observations of the jet in S5 0836+710 at different frequencies and epochs. The results show that the structures observed are physical and not generated artificially by the observing arrays. Our hypothesis that the observed intensity ridge-line can correspond to a helically twisted pressure maximum is confirmed by our observational tests. This interpretation allows us to explain jet misalignment between parsec and kiloparsec scales when the viewing angle is small, and also brings us to the conclusion that high-frequency observations may show only a small region of the jet flow concentrated around the maximum pressure ridge-line observed at low frequencies. Our work provides a potential explanation for the apparent transversal superluminal speeds observed in several extragalactic jets by means of transversal shift of an apparent core position with time.Comment: Accepted for publication in the Astrophysical Journa

High-Resolution Imaging of Molecular Gas and Dust in the Antennae (NGC 4038/39): Super Giant Molecular Complexes

We present new aperture synthesis CO maps of the Antennae (NGC 4038/39) obtained with the Caltech Millimeter Array. These sensitive images show molecular emission associated with the two nuclei and a partial ring of star formation to the west of NGC 4038, as well as revealing the large extent of the extra-nuclear region of star formation (the ``overlap region''), which dominates the CO emission from this system. The largest molecular complexes have masses of 3-6x10^8 M_sun, typically an order of magnitude larger than the largest structures seen to date in more quiescent galaxy disks. The extremely red luminous star clusters identified previously with HST are well-correlated with the CO emission, which supports the conclusion that they are highly embedded young objects rather than old globular clusters. There is an excellent correlation between the CO emission and the 15 micron emission seen with ISO, particularly for the brightest regions. The most massive complexes in the overlap region have similar [NeIII]/[NeII] ratios, which implies that all these regions are forming many massive stars. However, only the brightest mid-infrared peak shows strong, rising continuum emission longward of 10 microns, indicative of very small dust grains heated to high temperatures by their proximity to nearby luminous stars. Since these grains are expected to be removed rapidly from the immediate environment of the massive stars, it is possible that this region contains very young (< 1 Myr) sites of star formation. Alternatively, fresh dust grains could be driven into the sphere of influence of the massive stars, perhaps by the bulk motions of two giant molecular complexes. The kinematics and morphology of the CO emission in this region provide some support for this second scenario.Comment: Accepted for publication in The Astrophysical Journal, 13 pages, 5 figures, higher quality color images available at http://www.astro.cornell.edu/staff/vassilis/papers/ngc4038_co.ps.g

Graphene-based modulation-doped superlattice structures

The electronic transport properties of graphene-based superlattice structures are investigated. A graphene-based modulation-doped superlattice structure geometry is proposed and consist of periodically arranged alternate layers: InAs/graphene/GaAs/graphene/GaSb. Undoped graphene/GaAs/graphene structure displays relatively high conductance and enhanced mobilities at elevated temperatures unlike modulation-doped superlattice structure more steady and less sensitive to temperature and robust electrical tunable control on the screening length scale. Thermionic current density exhibits enhanced behaviour due to presence of metallic (graphene) mono-layers in superlattice structure. The proposed superlattice structure might become of great use for new types of wide-band energy gap quantum devices.Comment: 5 figure

Detection of an iron K Emission Line from the LINER NGC 4579

We present the results of an ASCA observation of the LINER NGC 4579. A point-like X-ray source is detected at the nucleus with a 2-10 keV luminosity of 1.5x10^41 ergs/s assuming a distance of 16.8 Mpc. The X-ray spectrum is represented by a combination of a power-law with a photon index of ~1.7 and soft thermal component with kT~0.9 keV. An iron K emission line is detected at 6.73+/-0.13 keV (rest frame) with an equivalent width of 490 +180/-190 eV and is statistically significant at more than 99.9 % confidence. The line center energy is consistent with Helium-like iron and is significantly higher than 6.4 keV which is expected from fluorescence by "cold" (or a lower ionization state of) iron. The iron line profile shows no significant red tail in contrast to Seyfert 1 galaxies although the statistics are limited. The line center energy, equivalent width, and profile are consistent with an origin in an ionized accretion disk. However the large mass accretion rate necessary to ionize the accretion disk is not consistent with the observed luminosity and normal accretion models.Comment: 15 pages, 5 figures, to appear in The Astrophysical Journa

Evidence For A Precessing Accretion Disk in the Nucleus of NGC 1097

We present new spectroscopic observations of the LINER (and now Seyfert 1) nucleus of NGC 1097, and discuss the evolution of its broad, double-peaked Balmer lines. When originally discovered in 1991, the red peak of the double-peaked H-alpha line was stronger than the blue, while by 1994 the H-alpha profile had become almost symmetric and the integrated line flux had decreased to half its original value. Our new spectrum, taken in 1996, shows that the broad, double-peaked lines have returned to almost their original strengths, the profiles of H-beta and H-alpha are identical to within errors, and the broad-line emitting region is unreddened. However, the profile of the Balmer lines is now such that the blue peak is stronger than the red, opposite to the asymmetry observed in 1991. Various models are considered for the observed behavior, all assuming that the emission lines originate in an accretion disk. We present a refined version of the precessing, planar, elliptical accretion ring model proposed by Storchi-Bergmann et al. and Eracleous et al. This model provides an acceptable fit to the line profiles. We also consider the possibility that the line profile evolution results from a precessing warp in the disk, induced by irradiation from the center, and show that the range of radii and precession time scales expected in this model are consistent with the observations. The sudden appearance of the "disk-like" broad line profiles in NGC 1097 could have resulted from the formation of a new accretion disk due to, for example, the tidal disruption of a star, or the illumination of a pre-existing disk by a transient ionizing source at the center of the disk.Comment: Accepted for publication in the Astrophysical Journal. TeX file with 5 postscript figures embeded using psfig.tex, 13 page

Properties of the H-alpha-emitting Circumstellar Regions of Be Stars

Long-baseline interferometric observations obtained with the Navy Prototype Optical Interferometer of the H-alpha-emitting envelopes of the Be stars eta Tauri and beta Canis Minoris are presented. For compatibility with the previously published interferometric results in the literature of other Be stars, circularly symmetric and elliptical Gaussian models were fitted to the calibrated H-alpha observations. The models are sufficient in characterizing the angular distribution of the H-alpha-emitting circumstellar material associated with these Be stars. To study the correlations between the various model parameters and the stellar properties, the model parameters for eta Tau and beta CMi were combined with data for other Be stars from the literature. After accounting for the different distances to the sources and stellar continuum flux levels, it was possible to study the relationship between the net H-alpha emission and the physical extent of the H-alpha-emitting circumstellar region. A clear dependence of the net H-alpha emission on the linear size of the emitting region is demonstrated and these results are consistent with an optically thick line emission that is directly proportional to the effective area of the emitting disk. Within the small sample of stars considered in this analysis, no clear dependence on the spectral type or stellar rotation is found, although the results do suggest that hotter stars might have more extended H-alpha-emitting regions.Comment: 24 pages, 16 figures, accepted for publication in Ap