1,721,082 research outputs found
Toward Direct Imaging of Low-mass Gas-Giant Planets with the James Webb Space Telescope
No full textISSN:0074-1809ISSN:1743-9213ISSN:1743-921
Modeling The Detectability Of Exoplanets For The Palomar Extreme Adaptive Optics Palm-3000 System.
No full textPoster I presented at the 218th Meeting of the American Astronomical Society.
Abstract:
In this study, we present the projected capabilities and detection limitations for the PALM-3000 Extreme Adaptive Optics system to directly image super-Jupiter planets around nearby stars. PALM-3000 is the new adaptive optics system for the Palomar 5 m telescope, which employs a 3366 - actuator deformable mirror and is expected to deliver contrast ratios near 107 at 1 arc second from bright stars. The PALM-3000 system will be an upgrade from the current Adaptive Optics system at Palomar and commissioning will begin in the summer of 2011. Planetary and orbital parameters (mass, eccentricity, semi-major axis) were randomly sampled from known distributions, which have been established or extrapolated from radial velocity observations. Parent stars were modeled in accordance with the stellar initial mass function (IMF) and given randomized ages and distances from Earth. Probability for detection was modeled using a Monte-Carlo simulation written in IDL. Projected contrast curves in the H band for PALM-3000 were used as the constraints for planetary detection
Planetary Monte-Carlo: Detection efficiency of PALM-3000
Get PDFMini presentation given to tri-state area astronomers at the American Museum of Natural History on preliminary results of a monte-carlo simulation to optimize targets for direct imaging of exoplanets using the new P3K and Vortex Coronagraph
Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph. II. Emergence and Sedimentation of Silicate Clouds in L Dwarfs, and Analysis of the Full M5--T9 Field Dwarf Spectroscopic Sample
Get PDFWe present a uniform analysis of all mid-infrared spectra of
field M5--T9 dwarfs obtained with the Spitzer Infrared Spectrograph (IRS). The
sample contains 113 spectra out of which 12 belong to late-M dwarfs, 69 to L
dwarfs, and 32 to T dwarfs. Sixty-eight of these spectra are presented for the
first time. We measure strengths of the main absorption bands in the IRS
spectra, namely HO at 6.25 m, CH at 7.65 m, NH at 10.5
m, and silicates over 8--11 m. Water absorption is present in all
spectra and strengthens with spectral type. The onset of methane and ammonia
occurs at the L8 and T2.5 types, respectively, although ammonia can be
detectable as early as T1.5. Silicate absorption sets in at spectral type L2,
is on average the strongest in L4--L6 dwarfs, and disappears past L8. However,
silicate absorption can also be absent from the spectra at any L subtype. We
find a positive correlation between the silicate absorption strength and the
excess (deviation from median) near-infrared colour at a given L subtype, which
supports the idea that variations of silicate cloud thickness produce the
observed colour scatter in L dwarfs. We also find that variable L3--L7 dwarfs
are twice more likely to have above-average silicate absorption than
non-variables. The ensemble of results solidifies the evidence for silicate
condensate clouds in the atmospheres of L dwarfs, and for the first time
observationally establishes their emergence and sedimentation between effective
temperatures of 2000 K and 1300 K, respectively.Comment: Accepted for publication in MNRA
Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph -- III. Dust Grains in Young L Dwarf Atmospheres Are Heavier
Get PDFAnalysis of all archival 5--14 micron spectra of field ultracool dwarfs from
the Infrared Spectrograph on the Spitzer Space Telescope has shown that
absorption by silicates in the 8--11 micron region is seen in most L-type (1300
K to 2200 K) dwarfs. The absorption is caused by silicate-rich clouds in the
atmospheres of L dwarfs and is strongest at L4--L6 spectral types. Herein we
compare averages of the mid-infrared silicate absorption signatures of L3--L7
dwarfs that have low (10 cm s) vs.\ high
(10 cm s) surface gravity. We find that the silicate
absorption feature is sensitive to surface gravity and indicates a difference
in grain size and composition between dust condensates in young and old mid-L
dwarfs. The mean silicate absorption profile of low-gravity mid-L dwarfs
matches expectations for 1 micron-sized amorphous iron- and
magnesium-bearing pyroxene (MgFeSiO) grains. High-gravity mid-L
dwarfs have silicate absorption better represented by smaller (0.1
m) and more volatile amorphous enstatite (MgSiO) or SiO grains. This
is the first direct spectroscopic evidence for gravity-dependent sedimentation
of dust condensates in ultracool atmospheres. It confirms theoretical
expectations for lower sedimentation efficiencies in low-gravity atmospheres
and independently confirms their increased dustiness.Comment: Accepted for publication in MNRA
Brown Dwarf Companions to Young Solar Analogs: An Adaptive Optics Survey Using Palomar and Keck
Get PDFWe present results from an adaptive optics survey conducted with the Palomar and Keck telescopes over 3 years, which measured the frequency of stellar and sub-stellar companions to Sun-like stars. The survey sample contains 266 stars in the 3-10000 million year age range at heliocentric distances between 8 and 200 parsecs and with spectral types between F5-K5. A sub-sample of 101 stars, between 3-500 million years old, were observed in deep exposures with a coronagraph to search for faint sub-stellar companions. A total of 288 candidate companions were discovered around the sample stars, which were re-imaged at subsequent epochs to determine physical association with the candidate host stars by checking for common proper motion. Benefiting from a highly accurate astrometric calibration of the observations, we were able to successfully apply the common proper motion test in the majority of the cases, including stars with proper motions as small as 20 milli-arcseconds/year.
The results from the survey include the discovery of three new brown dwarf companions (HD 49197B, HD 203030B, and ScoPMS 214B), 43 new stellar binaries, and a triple system. The physical association of an additional, a priori-suspected, candidate sub-stellar companion to the star HII 1348 is astrometrically confirmed. The newly-discovered and confirmed young brown dwarf companions span a range of spectral types between M5 and T0.5, and will be of prime significance for constraining evolutionary models of young brown dwarfs and extra-solar planets.
Based on the 3 new detections of sub-stellar companions in the 101 star sub-sample and following a careful estimate of the survey incompleteness, a Bayesian statistical analysis shows that the frequency of 0.012-0.072 solar-mass brown dwarfs in 30-1600 AU orbits around young solar analogs is 6.8% (-4.9%, +8.3\%; 2-sigma limits). While this is a factor of 3 lower than the frequency of stellar companions to G-dwarfs in the same orbital range, it is significantly higher than the frequency of brown dwarfs in 0-3 AU orbits discovered through precision radial velocity surveys. It is also fully consistent with the observed frequency of 0-3 AU extra-solar planets. Thus, the result demonstrates that the radial-velocity "brown dwarf desert" does not extend to wide separations, contrary to previous belief.</p
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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