973 research outputs found
Accretion-Induced Lithium Line Enhancements in Classical T Tauri Stars: RW Aur
It is widely accepted that much of the stochastic variability of T Tauri
stars is due to accretion by a circumstellar disk. The emission line spectrum
as well as the excess continuum emission are common probes of this process. In
this communication, we present additional probes of the circumstellar
environment in the form of resonance lines of low ionization potential
elements. Using a set of 14 high resolution echelle observations of the
classical T Tauri star (CTTS), RW Aur, taken between 1986 and 1996, we
carefully measure the continuum veiling at each epoch by comparing more than
500 absorption lines with those of an appropriate template. This allows us to
accurately subtract out the continuum emission and to recover the underlying
photospheric spectrum. In doing so, we find that selected photospheric lines
are enhanced by the accretion process, namely the resonance lines of LiI and
KI. A resonance line of TiI and a low excitation potential line of CaI also
show weak enhancements. Simple slab models and computed line bisectors lead us
to propose that these line enhancements are markers of cool gas at the
beginning of the accretion flow which provides an additional source of line
opacity. These results suggest that published values of surface lithium
abundances of classical T Tauri stars are likely to be overestimated. This
would account for the various reports of surface lithium abundances in excess
of meteoritic values among the extreme CTTS. Computing LTE lithium abundances
of RW Aur in a low and then high accretion state yields abundances which vary
by one order of magnitude. The low accretion state lithium abundance is
consistent with theoretical predictions for a star of this age and mass while
the high accretion state spectrum yields a super-meteoritic lithium abundance.Comment: 28 pages, 8 figures, accepted by Ap
Uso de ninhos de cupin como fonte de matéria orgânica em sistemas de produção agrosilviculturais na Amazônia
The growth of two annual crops, okra (Abelmoschus escutentus) and egg-plant (Solatium melongena) and one perennial crop, andiroba (Carapa guianensis, a native forest tree of Amazonia) under different treatments with organic manure derived from termite nest material of wood-feeding Nasutitermes species was tested (randomized block design). The use of 25-100 g of nest material gave no significant increase in okra productivity, and 25-200 g gave no significant response in andiroba. The combined use of NPK with 200 g of nest material gave a significant higher production in egg-plant (total number and total fresh weight of fruits) when compared to the control (without fertilizer) and to the treatment with NPK only.The results suggest the possibility to use termite nest material to enhance crop production in Amazonia, particularly in combination with low amounts of mineral fertilizer. Research lines for further investigations are outlined.Foi avaliado crescimento de duas espécies agriculturais anuais, quiabo (Abelmoschus esculentus) e berinjela (Solatium melongena), e de uma espécie perene, andiroba (Carapa guianensis, uma árvore nativa da Amazônia) sob diferentes tratamentos com matéria orgânica derivada de material de cupinzeiro de espécies xilófagas de Nasutitermes (desenho de bloco randomizado). O uso de 25-100 g de material de termiteiro não levou a um incremento significativo da produtividade em quiabo, e 25-200 g não resultou numa resposta significativa em andiroba. O uso combinado de NPK com 200 g de ninho de cupim resultou numa produção significantemente maior em S. melongena (número total e peso fresco total de frutos) se comparado com o controle (sem fertilizante nenhum) e com o tratamento de NPK apenas. Os resultados sugerem a possibilidade de usar material de cupinzeiro para melhorara produção agrossilvicultural na Amazônia, especialmente em combinação com pequenas quantidades de fertilizante mineral Linhas de pesquisa para futuras investigações são apresentadas
False positive probabilties for all Kepler Objects of Interest: 1284 newly validated planets and 428 likely false positives
We present astrophysical false positive probability calculations for every
Kepler Object of Interest (KOI)---the first large-scale demonstration of a
fully automated transiting planet validation procedure. Out of 7056 KOIs, we
determine that 1935 have probabilities <1% to be astrophysical false positives,
and thus may be considered validated planets. 1284 of these have not yet been
validated or confirmed by other methods. In addition, we identify 428 KOIs
likely to be false positives that have not yet been identified as such, though
some of these may be a result of unidentified transit timing variations. A side
product of these calculations is full stellar property posterior samplings for
every host star, modeled as single, binary, and triple systems. These
calculations use 'vespa', a publicly available Python package able to be easily
applied to any transiting exoplanet candidate.Comment: 20 pages, 8 figures. Published in ApJ. Instructions to reproduce
results can be found at https://github.com/timothydmorton/koi-fp
Possible Disintegrating Short-Period Super-Mercury Orbiting KIC 12557548
We report here on the discovery of stellar occultations, observed with
Kepler, that recur periodically at 15.685 hour intervals, but which vary in
depth from a maximum of 1.3% to a minimum that can be less than 0.2%. The star
that is apparently being occulted is KIC 12557548, a K dwarf with T_eff = 4400
K and V = 16. Because the eclipse depths are highly variable, they cannot be
due solely to transits of a single planet with a fixed size. We discuss but
dismiss a scenario involving a binary giant planet whose mutual orbit plane
precesses, bringing one of the planets into and out of a grazing transit. We
also briefly consider an eclipsing binary, that either orbits KIC 12557548 in a
hierarchical triple configuration or is nearby on the sky, but we find such a
scenario inadequate to reproduce the observations. We come down in favor of an
explanation that involves macroscopic particles escaping the atmosphere of a
slowly disintegrating planet not much larger than Mercury. The particles could
take the form of micron-sized pyroxene or aluminum oxide dust grains. The
planetary surface is hot enough to sublimate and create a high-Z atmosphere;
this atmosphere may be loaded with dust via cloud condensation or explosive
volcanism. Atmospheric gas escapes the planet via a Parker-type thermal wind,
dragging dust grains with it. We infer a mass loss rate from the observations
of order 1 M_E/Gyr, with a dust-to-gas ratio possibly of order unity. For our
fiducial 0.1 M_E planet, the evaporation timescale may be ~0.2 Gyr. Smaller
mass planets are disfavored because they evaporate still more quickly, as are
larger mass planets because they have surface gravities too strong to sustain
outflows with the requisite mass-loss rates. The occultation profile evinces an
ingress-egress asymmetry that could reflect a comet-like dust tail trailing the
planet; we present simulations of such a tail.Comment: 14 pages, 7 figures; submitted to ApJ, January 10, 2012; accepted
March 21, 201
Photometric Variability in Kepler Target Stars: The Sun Among Stars -- A First Look
The Kepler mission provides an exciting opportunity to study the lightcurves
of stars with unprecedented precision and continuity of coverage. This is the
first look at a large sample of stars with photometric data of a quality that
has heretofore been only available for our Sun. It provides the first
opportunity to compare the irradiance variations of our Sun to a large cohort
of stars ranging from vary similar to rather different stellar properties, at a
wide variety of ages. Although Kepler data is in an early phase of maturity,
and we only analyze the first month of coverage, it is sufficient to garner the
first meaningful measurements of our Sun's variability in the context of a
large cohort of main sequence stars in the solar neighborhood. We find that
nearly half of the full sample is more active than the active Sun, although
most of them are not more than twice as active. The active fraction is closer
to a third for the stars most similar to the Sun, and rises to well more than
half for stars cooler than mid K spectral types.Comment: 13 pages, 4 figures, accepted to ApJ Letter
Riverscape properties contribute to the origin and structure of a hybrid zone in a Neotropical freshwater fish
Understanding the structure of hybrid zones provides valuable insights about species boundaries and speciation, such as the evolution of barriers to gene flow and the strength of selection. In river networks, studying evolutionary processes in hybrid zones can be especially challenging, given the influence of past and current river properties along with biological species- specific traits. Here, we suggest that a natural hybrid zone between two divergent lineages of the sexually dimorphic Neotropical fish Nematocharax venustus was probably established by secondary contact as a result of a river capture event between the Contas and Pardo river basins. This putative river capture is supported by hydrogeological evidence of elbows of capture, wind gaps and geological faults. The morphological (colour pattern) and genetic (mtDNA and RADseq) variation reveal a clinal transition between parental lineages along the main river, with predominance of F2 hybrids at the centre of the hybrid zone, absence of early generation backcrosses and different levels of hybridization in the tributaries. We highlight that different sources of information are crucial for understanding how the riverscape spatial history influences the connectivity between and within river systems and, consequently, the dynamics of gene flow between freshwater lineages/species.River networks are spatially and temporally dynamic environments that impose challenges to the study of hybrid zones. Here, we elucidate the morphological and genetic structure of a hybrid zone generated by secondary contact between lineages of a Neotropical fish genus and discuss the possible role of past and current river properties (including river captures) in its origin and structure.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163541/3/jeb13689_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163541/2/jeb13689-sup-0001-AppendixS1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163541/1/jeb13689.pd
Theoretical Spectra of Terrestrial Exoplanet Surfaces
We investigate spectra of airless rocky exoplanets with a theoretical
framework that self-consistently treats reflection and thermal emission. We
find that a silicate surface on an exoplanet is spectroscopically detectable
via prominent Si-O features in the thermal emission bands of 7 - 13 \mu m and
15 - 25 \mu m. The variation of brightness temperature due to the silicate
features can be up to 20 K for an airless Earth analog, and the silicate
features are wide enough to be distinguished from atmospheric features with
relatively high-resolution spectra. The surface characterization thus provides
a method to unambiguously identify a rocky exoplanet. Furthermore,
identification of specific rocky surface types is possible with the planet's
reflectance spectrum in near-infrared broad bands. A key parameter to observe
is the difference between K band and J band geometric albedos (A_g (K)-A_g
(J)): A_g (K)-A_g (J) > 0.2 indicates that more than half of the planet's
surface has abundant mafic minerals, such as olivine and pyroxene, in other
words primary crust from a magma ocean or high-temperature lavas; A_g (K)-A_g
(J) < -0.09 indicates that more than half of the planet's surface is covered or
partially covered by water ice or hydrated silicates, implying extant or past
water on its surface. Also, surface water ice can be specifically distinguished
by an H-band geometric albedo lower than the J-band geometric albedo. The
surface features can be distinguished from possible atmospheric features with
molecule identification of atmospheric species by transmission spectroscopy. We
therefore propose that mid-infrared spectroscopy of exoplanets may detect rocky
surfaces, and near-infrared spectrophotometry may identify ultramafic surfaces,
hydrated surfaces and water ice.Comment: Accepted for publication on the Ap
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