1,721,036 research outputs found
The kinematic relationship between disk and jet in the DG Tauri system
No full textWe present high angular resolution millimeter wavelength continuum and \cii observations of the circumstellar disk surrounding the T Tauri star DG Tauri. We show that the velocity pattern in the inner regions of the disk is consistent with Keplerian rotation about a central 0.67 M_sun star. The disk rotation is also consistent with the toroidal velocity pattern in the initial channel of the optical jet, as inferred from HST spectra of the first de-projected 100 AU from the source. Our observations support the tight relationship between disk and jet kinematics postulated by the popular magneto-centrifugal models for jet formation and collimation
Exploring the feedback of asymmetric jets on the orbital motions in protoplanetary disks
Full text linkProtoplanetary disks are often associated with powerful bipolar jets. In most cases the two jet lobes carry a different amount of linear momentum. We investigate the dynamical feedback of such an asymmetric jet on its launch region in the disk. We adopt a Hamiltonian formulation and solve for the departures from the initial Keplerian orbits with a symplectic integrator. The back-reaction effect produces a shift in the position of the orbits toward the weaker jet lobe, deforming the shape of the inner disk. The loci of the orbits oscillate quasiperiodically, alternating radial and vertical displacements. The amplitude is a small fraction of the disk thickness, and is proportional to the momentum imbalance. Such motions can contribute to the onset of turbulence, and to the mixing of molecular material
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A resolved outflow of matter from a brown dwarf
No full textThe birth of stars involves not only accretion but also, counter-intuitively, the expulsion of matter in the form of highly supersonic outflows. Although this phenomenon has been seen in young stars, a fundamental question is whether it also occurs among newborn brown dwarfs: these are the so-called `failed stars', with masses between stars and planets, that never manage to reach temperatures high enough for normal hydrogen fusion to occur. Recently, evidence for accretion in young brown dwarfs has mounted, and their spectra show lines that are suggestive of outflows. Here we report spectro-astrometric data that spatially resolve an outflow from a brown dwarf. The outflow's characteristics appear similar to, but on a smaller scale than, outflows from normal young stars. This result suggests that the outflow mechanism is universal, and perhaps relevant even to the formation of planets
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Unveiling the Launching Region of YSO Jets with AMBER
No full textThe interplay between accretion and ejection in the environment of young stellar objects (YSOs) is believed to be a crucial element in the star formation process. Since most of the properties of the models are set up in the first few AUs from the source (below the so-called Alfvèn surface), to validate and constrain the models observationally we need very high angular resolution. With HST (resolution ~ 0.''1, i.e. about 14 AU in Taurus) we have been able to access the external border of the acceleration region, for jets in the Taurus-Auriga cloud. Here we see an onion-like kinematic structure in the first 200 AU of the flow, and indications for rotation around the symmetry axis for the resolved low/moderate velocity component. We have now planned observations with AMBER on the VLTI to investigate at 1 mas resolution (in J) the core of the central engine, down to 0.1 AU from the source. Here we describe a joint project by several Institutes in the AMBER consortium dedicated to the study of the morphology and detailed kinematics of a few selected targets. On one hand we will use the large UV coverage of the ATs to explore at medium spectral resolution the structure of the flow. On the other hand the large collection area of the UTs combined with the high resolution mode (R=10000) of AMBER will allow us to search for interesting kinematic features, among which signatures of rotation around the axis, that would constitute an important validation of the proposed models for the jet launching
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Discovery of a Bipolar Outflow from 2MASSW J1207334-393254, a 24 M Brown Dwarf
No full textThe 24 MJup brown dwarf 2MASS 1207-3932 has for some time been known to show clear signs of classical T Tauri-like accretion. Through analysis of its oxygen forbidden emission, we have discovered that it is driving a bipolar outflow. Blue- and redshifted components to the [O I] λ6300 forbidden emission line are seen at velocities of -8 and +4 km s-1 (on either side of the systemic velocity). Spectroastrometry recovers the position of both components relative to the brown dwarf at ~0.08" (in opposing directions). A position-velocity diagram of the line region supports the spectroastrometric results. The Hα and He I λ6678 lines were also analyzed. These line regions are not offset with respect to the continuum, ruling out the presence of spectroastrometric artifacts and underlining the validity of the [O I] λ6300 results. The low radial velocity of the outflow and the relatively large offsets are consistent with 2MASS 1207-3932 having a near edge-on disk as proposed by Scholz et al. 2MASS 1207-3932 is now the smallest mass galactic object known to drive an outflow. The age of the TW Hydrae association (~8 Myr) also makes this one of the oldest objects with a resolved jet. This discovery not only highlights the robustness of the outflow mechanism over an enormous range of masses but also suggests that it may even be feasible for young giant planets with accretion disks to drive outflows
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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