1,721,084 research outputs found
A Stellar Census Of The Tucana-Horologium Moving Group
Get PDFWe report the selection and spectroscopic confirmation of 129 new late-type (SpT = K3-M6) members of the Tucana-Horologium moving group, a nearby (d similar to 40 pc), young (tau similar to 40 Myr) population of comoving stars. We also report observations for 13 of the 17 known Tuc-Hor members in this spectral type range, and that 62 additional candidates are likely to be unassociated field stars; the confirmation frequency for new candidates is therefore 129/191 = 67%. We have used radial velocities, Ha emission, and Li-6708 absorption to distinguish between contaminants and bona fide members. Our expanded census of Tuc-Hor increases the known population by a factor of similar to 3 in total and by a factor of similar to 8 for members with SpT >= K3, but even so, the K-M dwarf population of Tuc-Hor is still markedly incomplete. Our expanded census allows for a much more detailed study of Tuc-Hor than was previously feasible. The spatial distribution of members appears to trace a two-dimensional sheet, with a broad distribution in X and Y, but a very narrow distribution (+/- 5 pc) in Z. The corresponding velocity distribution is very small, with a scatter of +/- 1.1 km s(-1) about the mean UVW velocity for stars spanning the entire 50 pc extent of Tuc-Hor. We also show that the isochronal age (tau similar to 20-30 Myr) and the lithium depletion boundary age (tau similar to 40 Myr) disagree, following the trend in other pre-main-sequence populations for isochrones to yield systematically younger ages. The H alpha emission line strength follows a trend of increasing equivalent width with later spectral type, as is seen for young clusters. We find that moving group members have been depleted of measurable lithium for spectral types of K7.0-M4.5. None of our targets have significant infrared excesses in the WISE W3 band, yielding an upper limit on warm debris disks of F < 0.7%. Finally, our purely kinematic and color-magnitude selection procedure allows us to test the efficiency and completeness for activity-based selection of young stars. We find that 60% of K-M dwarfs in Tuc-Hor do not have ROSAT counterparts and would have been omitted in X-ray-selected samples. In contrast, GALEX UV-selected samples using a previously suggested criterion for youth achieve completeness of 77% and purity of 78%, and we suggest new SpT-dependent selection criteria that will yield > 95% completeness for tau similar to 40 Myr populations with GALEX data available.Clay FellowshipAstronom
Three Wide Planetary-Mass Companions To fw Tau, ROXs 12, And ROXs 42B
Get PDFWe report the discovery of three planetary-mass companions (M = 6-20 M-Jup) in wide orbits (rho similar to 150-300 AU) around the young stars FW Tau (Taurus-Auriga), ROXs 12 (Ophiuchus), and ROXs 42B (Ophiuchus). All three wide planetary-mass companions (PMCs) were reported as candidate companions in previous binary survey programs, but then were neglected for > 10 yr. We therefore obtained followup observations that demonstrate that each candidate is comoving with its host star. Based on the absolute M-K' magnitudes, we infer masses (from hot-start evolutionary models) and projected separations of 10 +/- 4 M-Jup and 330 +/- 30 AU for FW Tau b, 16 +/- 4 M-Jup and 210 +/- 20 AU for ROXs 12, and 10 +/- 4 M-Jup and 140 +/- 10 AU for ROXs 42B b. We also present similar observations for 10 other candidates that show that they are unassociated field stars, as well as multicolor JHK'L' near-infrared photometry for our new PMCs and for five previously identified substellar or planetary-mass companions. The near-infrared photometry for our sample of eight known and new companions generally parallels the properties of free-floating, low-mass brown dwarfs in these star-forming regions. However, five of the seven objects with M < 30 M-Jup are redder in K' - L' than the distribution of young free-floating counterparts of similar J - K' color. We speculate that this distinction could indicate a structural difference in circumplanetary disks, perhaps tied to higher disk mass since at least two of the objects in our sample are known to be accreting more vigorously than typical free-floating counterparts.Clay FellowshipNASA through Hubble Fellowship 51257.01STScINASA NAS 5-26555Australian Research Council's Discovery program DP1094977Macquarie University Research Development GrantNASA through Hubble FellowshipNASA through the Sagan Fellowship programALMA-CONICYT fund 31120009NSF AST09-09222NASA Keck PI Data AwardAstronom
Dynamical Masses of Young M Dwarfs: Masses and Orbital Parameters of GJ 3305 AB, the Wide Binary Companion To the Imaged Exoplanet Host 51 Eri
Get PDFWe combine new high resolution imaging and spectroscopy from Keck/NIRC2, Discovery Channel Telescope/DSSI, and Keck/HIRES with published astrometry and radial velocities to measure individual masses and orbital elements of the GJ 3305 AB system, a young (similar to 20 Myr) M+M binary (unresolved spectral type M0) member of the beta Pictoris moving group comoving with the imaged exoplanet host 51 Eri. We measure a total system mass of 1.11 +/- 0.04 M-circle dot, a period of 29.03 +/- 0.50 year, a semimajor axis of 9.78 +/- 0.14 AU, and an eccentricity of 0.19 +/- 0.02. The primary component has a dynamical mass of 0.67 +/- 0.05 M-circle dot and the secondary has a mass of 0.44 +/- 0.05 M-circle dot. The recently updated BHAC15 models are consistent with the masses of both stars to within 1.5 sigma. Given the observed masses the models predict an age of the GJ 3305 AB system of 37 +/- 9 Myr. Based on the observed system architecture and our dynamical mass measurement, it is unlikely that the orbit of 51 Eri b has been significantly altered by the Kozai-Lidov mechanism.National Science Foundation DGE-1144469National Aeronautics and Space AdministrationDiscovery CommunicationsJohn Templeton FoundationAstronom
Spectroscopic Confirmation Of Young Planetary-Mass Companions On Wide Orbits
Get PDFWe present moderate-resolution ( R similar to 4000-5000) near-infrared integral field spectroscopy of the young (1-5 Myr) 6-14 M-Jup companions ROXs 42B b and FW Tau b obtained with Keck/OSIRIS and Gemini-North/NIFS. The spectrum of ROXs 42B b exhibits clear signs of low surface gravity common to young L dwarfs, confirming its extreme youth, cool temperature, and low mass. Overall, it closely resembles the free-floating 4-7 M-Jup L-type Taurus member 2MASS J04373705+2331080. The companion to FW Tau AB is more enigmatic. Our optical and near-infrared spectra show strong evidence of outflow activity and disk accretion in the form of line emission from [S II], [O I], H alpha, Ca II, [Fe II], Pa beta, and H-2. The molecular hydrogen emission is spatially resolved as a single lobe that stretches approximate to 0 ''.1 (15 AU). Although the extended emission is not kinematically resolved in our data, its morphology resembles shock-excited H-2 jets primarily seen in young Class 0 and Class I sources. The near-infrared continuum of FW Tau b is mostly flat and lacks the deep absorption features expected for a cool, late-type object. This may be a result of accretion-induced veiling, especially in light of its strong and sustained Ha emission (EW(H alpha)greater than or similar to 290 angstrom). Alternatively, FW Tau b may be a slightly warmer (M5-M8) accreting low-mass star or brown dwarf (0.03-0.15 M circle dot) with an edge-on disk. Regardless, its young evolutionary stage is in stark contrast to its Class III host FW Tau AB, indicating a more rapid disk clearing timescale for the host binary system than for its wide companion. Finally, we present near-infrared spectra of the young (similar to 2-10 Myr) low-mass (12-15 M-Jup) companions GSC 6214-210 B and SR 12 C and find they best resemble low- gravity M9.5 and M9 substellar templates.NASA NNX11AC31GNSF AST09-09222Astronom
The Mass-Radius Relation Of Young Stars. I. Usco 5, An M4.5 Eclipsing Binary In Upper Scorpius Observed By K2
Get PDFWe present the discovery that UScoCTIO 5, a known spectroscopic binary in the Upper Scorpius star-forming region (P = 34 days, M-tot sin(i) = 0.64M(circle dot)), is an eclipsing system with both primary and secondary eclipses apparent in K2 light curves obtained during Campaign 2. We have simultaneously fit the eclipse profiles from the K2 light curves and the existing RV data to demonstrate that UScoCTIO 5 consists of a pair of nearly identical M4.5 stars with M-A = 0.329 +/- 0.002 M-circle dot, R-A = 0.834 +/- 0.006 R-circle dot, M-B = 0.317 +/- 0.002 M-circle dot, and R-B = 0.810 +/- 0.006 R-circle dot. The radii are broadly consistent with pre-main-sequence ages predicted by stellar evolutionary models, but none agree to within the uncertainties. All models predict systematically incorrect masses at the 25%-50% level for the HR diagram position of these mid-M dwarfs, suggesting significant modifications to mass-dependent outcomes of star and planet formation. The form of the discrepancy for most model sets is not that they predict luminosities that are too low, but rather that they predict temperatures that are too high, suggesting that the models do not fully encompass the physics of energy transport (via convection and/or missing opacities) and/or a miscalibration of the SpT-T-eff scale. The simplest modification to the models (changing T-eff to match observations) would yield an older age for this system, in line with the recently proposed older age of Upper Scorpius (tau similar to 11 Myr).NASA Science Mission directorateW. M. Keck FoundationAstronom
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
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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Mapping nearby young stellar structures and their star formation histories
Get PDFYoung stellar associations hold a star formation record that can persist for millions of years, revealing the progression of star formation long after the dispersal of the natal cloud. To identify nearby young stellar populations that trace this progression, we have designed a comprehensive framework for the identification of young stars, and use it to identify ~3x10⁴ candidate young stars within a distance of 333 pc using Gaia DR2. Applying the HDBSCAN clustering algorithm to this sample, we identify 27 top-level groups, nearly half of which have little to no presence in previous literature. Ten of these groups have visible substructure, including notable young associations such as Orion, Perseus, Taurus, and Sco-Cen. We provide a complete subclustering analysis on all groups with substructure, using age estimates to reveal each region's star formation history. The patterns we reveal include an apparent star formation origin for Sco-Cen along a semicircular arc, as well as clear evidence for sequential star formation moving away from that arc with a propagation speed of ~4 km s⁻¹ (~4 pc Myr⁻¹). We also identify earlier bursts of star formation in Perseus and Taurus that predate current, kinematically identical active star-forming events, suggesting that the mechanisms that collect gas can spark multiple generations of star formation, punctuated by gas dispersal and cloud regrowth. The large spatial scales and long temporal scales on which we observe star formation offer a bridge between the processes within individual molecular clouds and the broad forces guiding star formation at galactic scales.Astronom
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