1,721,723 research outputs found
Observational diagnostics of accretion on young stars and brown dwarfs
No full textI present a summary of recent observational constraints on the accretion properties of young stars and brown dwarfs with focus on the high-energy emission. In their T Tauri phase young stars assemble a few percent of their mass by accretion from a disk. Various observational signatures of disks around pre-main sequence stars and the ensuing accretion process are found in the IR and optical regime: e.g. excess emission above the stellar photosphere, strong and broad emission lines, optical veiling. At high energies evidence for accretion is less obvious, and the X-ray emission from stars has historically been ascribed to magnetically confined coronal plasmas. While being true for the bulk of the emission, new insight obtained from XMM-Newton and Chandra observations has unveiled contributions from accretion and outflow processes to the X-ray emission from young stars. Their smaller siblings, the brown dwarfs, have been shown to undergo a T Tauri phase on the basis of optical/IR observations of disks and measurements of accretion rates. Most re-cently, first evidence was found for X-rays produced by accretion in a young brown dwarf, complementing the suspected analogy between stars and substellar objects
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
Time-resolved Spectroscopy of DROXO X-ray Sources: Flares and Fe Kα emission
No full textWe present a systematic search for Fe Kα emission from young stellar objects of the ρ Ophiuchi star forming region observed in the Deep Rho Ophiuchi XMM-Newton Observation
The Great Flare of 2021 November 19 on AD Leonis: Simultaneous XMM-Newton and TESS observations
Full text linkWe present a detailed analysis of a superflare on the active M dwarf star AD Leonis. The event presents a rare case of a stellar flare that was simultaneously observed in X-rays (with XMM-Newton) and in the optical (with the Transiting Exoplanet Survey Satellite, TESS). The radiated energy in the 0.2 - 12 keV X-ray band (1.26 +/- 0.01 x 10(33) erg) and the bolometric value (E-F,E-bol=5.57 +/- 0.03 x 10(33) erg) place this event at the lower end of the superflare class. The exceptional photon statistics deriving from the proximity of AD Leo has enabled measurements in the 1 - 8 angstrom GOES band for the peak flux (X1445 class) and integrated energy (E-F,E-GOES=4.30 +/- 0.05 x 10(32) erg), which enables a direct comparison with data on flares from our Sun. From extrapolations of empirical relations for solar flares, we estimate that a proton flux of at least 10(5)cm(-2)s(-1)sr(-1) accompanied the radiative output. With a time lag of 300 s between the peak of the TESS white-light flare and the GOES band flare peak as well as a clear Neupert effect, this event follows the standard (solar) flare scenario very closely. Time-resolved spectroscopy during the X-ray flare reveals, in addition to the time evolution of plasma temperature and emission measure, a temporary increase in electron density and elemental abundances, and a loop that extends into the corona by 13% of the stellar radius (4 x 10(9) cm). Independent estimates of the footprint area of the flare from TESS and XMM-Newton data suggest a high temperature of the optical flare (25000 K), but we consider it more likely that the optical and X-ray flare areas represent physically distinct regions in the atmosphere of AD Leo
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
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
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
The enigmatic young brown dwarf binary FU Tau: accretion and activity
No full textFUTau belongs to a rare class of young, wide brown dwarf binaries. We have resolved the system in a Chandra X-ray observation and detected only the primary, FUTauA. Hard X-ray emission, presumably from a corona, is present but, unexpectedly, we also detect a strong and unusually soft component from FUTauA. Its X-ray properties, so far unique among brown dwarfs, are very similar to those of the TTauri star TWHya. The analogy with TWHya suggests that the dominating soft X-ray component can be explained by emission from accretion shocks. However, the typical free-fall velocities of a brown dwarf are too low for an interpretation of the observed X-ray temperature as a post-shock region. On the other hand, velocities in excess of the free-fall speed are derived from archival optical spectroscopy, and independent pieces of evidence for strong accretion in FUTauA are found in optical photometry. The high X-ray luminosity of FUTauA coincides with a high bolometric luminosity confirming an unexplained trend among young brown dwarfs. In fact, FUTauA is overluminous with respect to evolutionary models while FUTauB is on the 1-Myr isochrone suggesting non-contemporaneous formation of the two components in the binary. The extreme youth of FUTauA could be responsible for its peculiar X-ray properties, in terms of atypical magnetic activity or accretion. Alternatively, rotation and magnetic field effects may reduce the efficiency of convection which in turn affects the effective temperature and radius of FUTauA, shifting its position in the Hertzsprung-Russell diagram. Although there is no direct proof of this latter scenario so far, we present arguments for its plausibility
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