1,721,119 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
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Signs of similar stellar obliquity distributions for hot and warm Jupiters orbiting cool stars
Get PDFTransiting giant planets provide a natural opportunity to examine stellar obliquities, which offer clues about the origin and dynamical histories of close-in planets. Hot Jupiters orbiting Sun-like stars show a tendency for obliquity alignment, which suggests that obliquities are rarely excited or that tidal realignment is common. However, the stellar obliquity distribution is less clear for giant planets at wider separations where realignment mechanisms are not expected to operate. In this work, we uniformly derive line-of-sight inclinations for 47 cool stars (T [subscript eff] < 6200 K) harboring transiting hot and warm giant planets by combining rotation periods, stellar radii, and v sin i measurements. Among the systems that show signs of spin-orbit misalignment in our sample, three are identified as being misaligned here for the first time. Of particular interest are Kepler-1654, one of the longest-period (1047 d; 2.0 AU) giant planets in a misaligned system, and Kepler-30, a multi-planet misaligned system. By comparing the reconstructed underlying inclination distributions, we find that the inferred minimum misalignment distributions of hot Jupiters spanning a/R [subscript ∗] = 3–20 (≈ 0.01–0.1 AU) and warm Jupiters spanning a/R [subscript ∗] = 20–400 (≈ 0.1–1.9 AU) are in good agreement. With 90% confidence, at least 24 [superscript +7 subscript −10]% of warm Jupiters and 14 [superscript +5 subscript −8]% of hot Jupiters around cool stars are misaligned by at least 10◦. Most stars harboring warm Jupiters are therefore consistent with spin-orbit alignment. The similarity of hot and warm Jupiter misalignment rates suggests that either the occasional misalignments are primordial and originate in misaligned disks, or the same underlying processes that create misaligned hot Jupiters also lead to misaligned warm Jupiters.Astronom
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2MASS J04435750+3723031 B : a young companion at the substellar boundary with potential membership in the [beta] Pictoris moving group
Get PDFWe present a detailed characterization of 2MASS J04435750+3723031 B, a low-mass companion orbiting an M2 star at 7.6" (550 AU) with potential membership in the 23 Myr β Pictoris moving group. Using near-infrared spectroscopy of the companion from IRTF/SpeX we have found a spectral type of M6 ± 1 and indications of youth through age-sensitive absorption lines and a low surface gravity index (VL-G). A young age is supported by Hα emission and lithium absorption in the host star. A comparison to atmospheric models reveals an effective temperature of 2800 ± 100 K with a low surface gravity of 4.0 ± 0.5 dex for the companion. This system has been previously proposed as a member of the β Pictoris moving group. Here we re-evaluate the membership of this system and find that it is a marginally consistent kinematic match to the β Pictoris moving group using Gaia parallaxes and new radial velocities for the host and companion. If this object does belong to the β Pictoris moving group, it would be a kinematic outlier and the companion would be over--luminous compared to other similar ultracool objects like PZ Tel B; this would suggest 2M0443+3723 B could be a close brown dwarf binary, and would make it the sixth substellar companion in this group. If 2M0443+3723 AB does not belong to any moving group then its age is more uncertain. In this case it is still young (< 30 Myr), and the implied mass would be between ~ 30--110 M [subscript Jup]. To test this hypothesis, we acquired NIR AO images with Keck II/NIRC2, but they do not reveal a binary to the companion that is resolvable down to the diffraction limit of ~ 3 AUAstronom
Variations on the Author
Get PDF“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
Get PDFWe 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
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The Epoch of Giant Planet Migration planet search program : near-infrared radial velocity jitter of young Sun-like stars
Get PDFI present early results from the Epoch of Giant Planet Migration program, a precise RV survey of over one hundred intermediate-age (~20–200 Myr) G and K dwarfs with the Habitable–Zone Planet Finder spectrograph (HPF) at McDonald Observatory's Hobby-Eberly Telescope (HET). The goals of this program are to determine the timescale and dominant physical mechanism of giant planet migration interior to the water ice line of Sun-like stars. Here, I summarize results from the first 14 months of this program, with a focus on my custom RV pipeline for HPF, a measurement of the intrinsic near-infrared RV activity of young Solar analogs, and modeling the underlying population-level distribution of stellar jitter. I demonstrate on-sky stability at the sub-2 m s⁻¹ level for the K2 standard HD 3765 using a least-squares matching method to extract precise RVs. Based on a subsample of 29 stars with at least three RV measurements from my program, I find a median RMS level of 34 m s⁻¹ . This is nearly a factor of 2 lower than the median RMS level in the optical of 60 m s⁻¹ for a comparison sample with similar ages and spectral types as my targets. The observed near-infrared jitter measurements for this subsample are well reproduced with a log-normal parent distribution with μ = 4.15 and σ = 1.02. Finally, by compiling RMS values from previous planet search programs, I show that near-infrared jitter for G and K dwarfs generally decays with age in a similar fashion to optical wavelengths, albeit with a shallower slope and lower overall values for ages [less than or approximately equal to] 1 Gyr.Astronom
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Dynamical mass of the young substellar companion HD 984 B
Get PDFModel-independent masses of substellar companions are critical tools to validate models of planet and brown dwarf cooling, test their input physics, and determine the formation and evolution of these objects. In this work, we measure the dynamical mass and orbit of the young substellar companion HD 984 B. We obtained new high-contrast imaging of the HD 984 system with Keck/NIRC2 which expands the baseline of relative astrometry from 3 to 8 years. We also present new radial velocities of the host star with the Habitable-Zone Planet Finder spectrograph at the Hobby-Eberly Telescope. Furthermore, HD 984 exhibits a significant proper motion difference between Hipparcos and Gaia EDR3. Our joint orbit fit of the relative astrometry, proper motions, and radial velocities yields a dynamical mass of 61 ± 4 M [subscript Jup] for HD 984 B, placing the companion firmly in the brown dwarf regime. The new fit also reveals a higher eccentricity for the companion (e = 0.76 ± 0.05) compared to previous orbit fits. Given the broad age constraint for HD 984, this mass is consistent with predictions from evolutionary models. HD 984 B’s dynamical mass places it among a small but growing list of giant planet and brown dwarf companions with direct mass measurementsAstronom
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Deep H [alpha] imaging survey of IC 348 with the Hubble Space Telescope : demographics of accreting protoplanets on wide orbits
Get PDFTraditional direct imaging exoplanet surveys are limited by their reliance on thermal emission, resulting in a potentially biased understanding of planet demographics at wide separations if a substantial fraction of giant planets lose energy during the accretion phase and form with low temperatures and luminosities. Strong H [alpha] emission from young accreting planets can be used to overcome these limitations. This approach provides an alternative assessment of planet occurrence rates regardless of their formation history and, with sufficient sensitivity, can enable direct imaging of sub-Saturn-mass planets for the first time. We present results from a deep H [alpha] imaging survey of 227 members of the ~2 Myr old star-forming region IC 348 using HST/WFC3-UVIS. Employing a deep learning-based image classification model to differentiate real sources from false positives due to cosmic rays, we identify one new brown dwarf companion candidate and provide upper limits on H [alpha] line luminosities, accretion luminosities, and mass accretion rates for non-detections. By adopting a framework that accounts for formation conditions as well as the accretion mechanisms, we compute sensitivity to protoplanet masses and separations for each target and for the entire survey. The average sensitivities achieved in our survey demonstrate that using H [alpha] imaging with HST is an efficient method for exoplanet searches and establishing population constraints. Using stellar formation and accretion models, we measure the occurrence rate of planetary-mass companions between 5-13 M [subscript] Jup at 100-1000 au to be less than 1.76%—-similar to rates from direct imaging surveys at infrared wavelengths. This suggests that there is no hidden population of cold-start planets assuming accretion in this range is steady rather than stochastic.Astronom
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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