86 research outputs found
Beef cattle production in crop-livestock-forestry systems.
The use of integrated systems had been presented as an option to improve livestock production systems. This study aimed to evaluate beef cattle production systems. The experiment was carried out at Embrapa Agrossilvipastoril, Sinop/MT, from August/2015 to July/2016, follow a randomized complete block design, with four systems and four replications, totaling 16 experimental units (2 ha each). Four production systems were evaluated: livestock (L) ? palisade grass (Brachiaria brizantha cv. Marandu) pastures; livestock-forestry (LF) integration ? palisade grass pastures with triple rows of eucalyptus (Eucalyptus urograndis, H13 clone), spaced 3 m between trees, 3.5 m between rows, and 30 m between ranks and height of 18 m; crop-livestock (CL) integration ? soybean crop (Glycine max L.), succeeded by maize (Zea mays L.) as the second crop snow along with palisade grass, which was used in the first 2 years to create a straw coverge of the soil for the no-till system implemented, and then used as first year pasture; and crop-livestock-forestry (CLF) - first year palisade grass pastures, after two years of crop as describe in CL system, with single rows of eucalyptus, spaced 3 m between trees, 37 m between ranks and height of 18 m
Spectrally resolved interferometric observations of α Cephei and physical modeling of fast rotating stars
This is the final version of the article. Available from the publisher via the DOI in this record.Context. When a given observational quantity depends on several stellar physical parameters, it is generally very difficult to obtain observational constraints for each of them individually. Therefore, we studied under which conditions constraints for some individual parameters can be achieved for fast rotators, knowing that their geometry is modified by the rapid rotation which causes a non-uniform surface brightness distribution.
Aims. We aim to study the sensitivity of interferometric observables on the position angle of the rotation axis (PA) of a rapidly rotating star, and whether other physical parameters can influence the determination of PA, and also the influence of the surface differential rotation on the determination of the β exponent in the gravity darkening law that enters the interpretation of interferometric observations, using α Cep as a test star.
Methods. We used differential phases obtained from observations carried out in the Hα absorption line of α Cep with the VEGA/CHARA interferometer at high spectral resolution, R = 30 000 to study the kinematics in the atmosphere of the star.
Results. We studied the influence of the gravity darkening effect (GDE) on the determination of the PA of the rotation axis of α Cep and determined its value, PA = −157-10°+17°. We conclude that the GDE has a weak influence on the dispersed phases. We showed that the surface differential rotation can have a rather strong influence on the determination of the gravity darkening exponent. A new method of determining the inclination angle of the stellar rotational axis is suggested. We conclude that differential phases obtained with spectro-interferometry carried out on the Hα line can in principle lead to an estimate of the stellar inclination angle i. However, to determine both i and the differential rotation parameter α, lines free from the Stark effect and that have collision-dominated source functions are to be preferred.VEGA is a collaboration between CHARA and
OCA/LAOG/CRAL/LESIA that has been supported by the French programs
PNPS and ASHRA, by INSU and by the Région PACA. The project has
obviously benefitted from the strong support of the OCA and CHARA technical
teams. The CHARA Array is operated with support from the National Science
Foundation through grant AST-0908253, the W. M. Keck Foundation, the
NASA Exoplanet Science Institute, and from Georgia State University. This
work has made use of the BeSS database, operated at GEPI, Observatoire de
Meudon, France: http://basebe.obspm.fr, use of the Jean-Marie Mariotti
Center SearchCal service1 co-developed by FIZEAU and LAOG, and of
CDS Astronomical Databases SIMBAD and VIZIER2. We are grateful to an
anonymous referee for her/his valuable suggestions that helped to improve the
presentation of our results
Probing the atmosphere of a solar-like star by galactic microlensing at high magnification
We report a measurement of limb darkening of a solar-like star in the very
high magnification microlensing event MOA 2002-BLG-33. A 15 hour deviation from
the light curve profile expected for a single lens was monitored intensively in
V and I passbands by five telescopes spanning the globe. Our modelling of the
light curve showed the lens to be a close binary system whose centre-of-mass
passed almost directly in front of the source star. The source star was
identified as an F8-G2 main sequence turn-off star. The measured stellar
profiles agree with current stellar atmosphere theory to within ~4% in two
passbands. The effective angular resolution of the measurements is <1
micro-arcsec. These are the first limb darkening measurements obtained by
microlensing for a Solar-like star.Comment: Accepted for publication in A&A Letters. 5 pages, 2 embedded colour
ps figures plus 1 jpg figure. Version with all figures embedded available
from: http://www.roe.ac.uk/~iab/moa33paper
A near-infrared interferometric survey of debris disc stars. II. CHARA/FLUOR observations of six early-type dwarfs
High-precision interferometric observations of six early-type main sequence
stars known to harbour cold debris discs have been obtained in the
near-infrared K band with the FLUOR instrument at the CHARA Array. The measured
squared visibilities are compared to the expected visibility of the stellar
photospheres based on theoretical photospheric models taking into account
rotational distortion, searching for potential visibility reduction at short
baselines due to circumstellar emission. Our observations bring to light the
presence of resolved circumstellar emission around one of the six target stars
(zeta Aql) at the 5 sigma level. The morphology of the emission source cannot
be directly constrained because of the sparse spatial frequency sampling of our
interferometric data. Using complementary adaptive optics observations and
radial velocity measurements, we find that the presence of a low-mass companion
is a likely origin for the excess emission. The potential companion has a
K-band contrast of four magnitudes, a most probable mass of about 0.6 Msun, and
is expected to orbit between about 5.5 AU and 8 AU from its host star assuming
a purely circular orbit. Nevertheless, by adjusting a physical debris disc
model to the observed Spectral Energy Distribution of the zeta Aql system, we
also show that the presence of hot dust within 10 AU from zeta Aql, producing a
total thermal emission equal to 1.69 +- 0.31% of the photospheric flux in the K
band, is another viable explanation for the observed near-infrared excess. Our
re-interpretation of archival near- to far-infrared photometric measurements
shows however that cold dust is not present around zeta Aql at the sensitivity
limit of the IRS and MIPS instruments onboard Spitzer, and urges us to remove
zeta Aql from the category of bona fide debris disc stars.Comment: 14 pages, accepted for publication in A&
Direct constraint on the distance of y2 Velorum from AMBER/VLTI observations
In this work, we present the first AMBER observations, of the Wolf-Rayet and
O (WR+O) star binary system y2 Velorum. The AMBER instrument was used with the
telescopes UT2, UT3, and UT4 on baselines ranging from 46m to 85m. It delivered
spectrally dispersed visibilities, as well as differential and closure phases,
with a resolution R = 1500 in the spectral band 1.95-2.17 micron. We interpret
these data in the context of a binary system with unresolved components,
neglecting in a first approximation the wind-wind collision zone flux
contribution. We show that the AMBER observables result primarily from the
contribution of the individual components of the WR+O binary system. We discuss
several interpretations of the residuals, and speculate on the detection of an
additional continuum component, originating from the free-free emission
associated with the wind-wind collision zone (WWCZ), and contributing at most
to the observed K-band flux at the 5% level. The expected absolute separation
and position angle at the time of observations were 5.1±0.9mas and
66±15° respectively. However, we infer a separation of
3.62+0.11-0.30 mas and a position angle of 73+9-11°. Our analysis thus
implies that the binary system lies at a distance of 368+38-13 pc, in agreement
with recent spectrophotometric estimates, but significantly larger than the
Hipparcos value of 258+41-31 pc
The VLT-FLAMES Tarantula Survey: XXV. Surface nitrogen abundances of O-type giants and supergiants
Context. Theoretically, rotation-induced chemical mixing in massive stars has far reaching evolutionary consequences, affecting the
sequence of morphological phases, lifetimes, nucleosynthesis, and supernova characteristics.
Aims. Using a sample of 72 presumably single O-type giants to supergiants observed in the context of the VLT-FLAMES Tarantula
Survey (VFTS), we aim to investigate rotational mixing in evolved core-hydrogen burning stars initially more massive than 15 M� by
analysing their surface nitrogen abundances.
Methods. Using stellar and wind properties derived in a previous VFTS study we computed synthetic spectra for a set of up to
21 N ii-v lines in the optical spectral range, using the non-LTE atmosphere code FASTWIND. We constrained the nitrogen abundance
by fitting the equivalent widths of relatively strong lines that are sensitive to changes in the abundance of this element. Given the quality
of the data, we constrained the nitrogen abundance in 38 cases; for 34 stars only upper limits could be derived, which includes almost
all stars rotating at 3e sin i > 200 km s−1
.
Results. We analysed the nitrogen abundance as a function of projected rotation rate 3e sin i and confronted it with predictions of rotational
mixing. We found a group of N-enhanced slowly-spinning stars that is not in accordance with predictions of rotational mixing
in single stars. Among O-type stars with (rotation-corrected) gravities less than log gc = 3.75 this group constitutes 30−40 percent
of the population. We found a correlation between nitrogen and helium abundance which is consistent with expectations, suggesting
that, whatever the mechanism that brings N to the surface, it displays CNO-processed material. For the rapidly-spinning O-type stars
we can only provide upper limits on the nitrogen abundance, which are not in violation with theoretical expectations. Hence, the data
cannot be used to test the physics of rotation induced mixing in the regime of high spin rates.
Conclusions. While the surface abundances of 60−70 percent of presumed single O-type giants to supergiants behave in conformity
with expectations, at least 30−40 percent of our sample can not be understood in the current framework of rotational mixing for single
stars. Even though we have excluded stars showing radial velocity variations, of our sample may have remained contaminated by postinteraction
binary products. Hence, it is plausible that effects of binary interaction need to be considered to understand their surface
properties. Alternatively, or in conjunction, the effects of magnetic fields or alternative mass-loss recipes may need to be invoked
Asteroseismology and Interferometry
Asteroseismology provides us with a unique opportunity to improve our
understanding of stellar structure and evolution. Recent developments,
including the first systematic studies of solar-like pulsators, have boosted
the impact of this field of research within Astrophysics and have led to a
significant increase in the size of the research community. In the present
paper we start by reviewing the basic observational and theoretical properties
of classical and solar-like pulsators and present results from some of the most
recent and outstanding studies of these stars. We centre our review on those
classes of pulsators for which interferometric studies are expected to provide
a significant input. We discuss current limitations to asteroseismic studies,
including difficulties in mode identification and in the accurate determination
of global parameters of pulsating stars, and, after a brief review of those
aspects of interferometry that are most relevant in this context, anticipate
how interferometric observations may contribute to overcome these limitations.
Moreover, we present results of recent pilot studies of pulsating stars
involving both asteroseismic and interferometric constraints and look into the
future, summarizing ongoing efforts concerning the development of future
instruments and satellite missions which are expected to have an impact in this
field of research.Comment: Version as published in The Astronomy and Astrophysics Review, Volume
14, Issue 3-4, pp. 217-36
Near-Infrared interferometry of Eta Carinae with high spatial and spectral resolution using the VLTI and the AMBER instrument
We present the first NIR spectro-interferometry of the LBV Eta Carinae. The K
band observations were performed with the AMBER instrument of the ESO Very
Large Telescope Interferometer using three 8.2m Unit Telescopes with baselines
from 42 to 89m. The aim of this work is to study the wavelength dependence of
Eta Car's optically thick wind region with a high spatial resolution of 5 mas
(11 AU) and high spectral resolution. The medium spectral resolution
observations (R=1,500) were performed in the wavelength range around both the
HeI 2.059 micron and the Br gamma 2.166 micron emission lines, the high
spectral resolution observations (R=12,000) only in the Br gamma line region.
In the K-band continuum, a diameter of 4.0 +/-0.2 mas (Gaussian FWHM, fit range
28-89m) was measured for Eta Car's optically thick wind region. If we fit
Hillier et al. (2001) model visibilities to the observed AMBER visibilities, we
obtain 50 % encircled-energy diameters of 4.2, 6.5 and 9.6mas in the 2.17
micron continuum, the HeI, and the Br gamma emission lines, respectively. In
the continuum near the Br gamma line, an elongation along a position angle of
120+/-15 degrees was found, consistent with previous VLTI/VINCI measurements by
van Boekel et al. (2003). We compare the measured visibilities with predictions
of the radiative transfer model of Hillier et al. (2001), finding good
agreement. Furthermore, we discuss the detectability of the hypothetical hot
binary companion. For the interpretation of the non-zero differential and
closure phases measured within the Br gamma line, we present a simple geometric
model of an inclined, latitude-dependent wind zone. Our observations support
theoretical models of anisotropic winds from fast-rotating, luminous hot stars
with enhanced high-velocity mass loss near the polar regions.Comment: 22 pages, 14 figures, 2 tables; A&A in pres
Determination of stellar shape in microlensing event MOA 2002-BLG-33
We report a measurement of the shape of the source star in microlensing event
MOA 2002-BLG-33. The lens for this event was a close binary whose
centre-of-mass passed almost directly in front of the source star. At this
time, the source star was closely bounded on all sides by a caustic of the
lens. This allowed the oblateness of the source star to be constrained. We
found that a/b = 1.02^{+0.04}_{-0.02} where a and b are its semi-major and
semi-minor axes respectively. The angular resolution of this measurement is
approximately 0.04 microarcsec. We also report HST images of the event that
confirm a previous identification of the source star as an F8-G2 turn-off
main-sequence star.Comment: 8 pages, 5 figures, Accepted by A&
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