2,911 research outputs found
Precision multi-epoch astrometry with VLT cameras FORS1/2
We investigate the astrometric performance of the FORS1 and FORS2 cameras of
the VLT at long time scales with emphasis on systematic errors which normally
prevent attainning a precision better than 1mas. The study is based on multi-
epoch time series of observations of a single sky region imaged with a time
spacing of 2-6 years at FORS1 and 1-5 months at FORS2. We performed a detailed
analysis of a random error of positions that was shown to be dominated by the
uncertainty of the star photocenter determination. The component of the random
error corresponding to image motion was found to be caused primarily by optical
aberrations and variations of atmospheric PSF size but not by the effect of
atmospheric image motion. Comparison of observed and model annual/monthly epoch
average positions yielded estimates of systematic errors for which temporal
properties and distribution in the CCD plane are given. At frame center, the
systematic component is about 25 mu-as. Systematic errors are shown to be
caused mainly by a combined effect of the image asymmetry and seeing variations
which therefore should be strongly limited to avoid generating random and
systematic errors. For a series of 30 images, we demonstrated presicion of
about 50 mu-as stable on daily, monthly, and annual time scales. Relative
proper motion and trigonometric parallaxes of stars in the center of the test
field were derived with a precision of 20 mu-as/yr and 40 mu-as for 17-19 mag
stars.Comment: 16 pages, 16 figures, 4 tables, accepted in A&A; typos and language
corrections; version sent to the printe
Astrometric planet search around southern ultracool dwarfs III. Discovery of a brown dwarf in a 3-year orbit around DE0630-18
Using astrometric measurements obtained with the FORS2/VLT camera, we are
searching for low-mass companions around 20 nearby ultracool dwarfs. With a
single-measurement precision of 0.1 milli-arcseconds, our survey is sensitive
to a wide range of companion masses from planetary companions to binary
systems. Here, we report the discovery and orbit characterisation of a new
ultracool binary at a distance of 19.5 pc from Earth that is composed of the
M8.5-dwarf primary DE0630-18 and a substellar companion. The nearly edge-on
orbit is moderately eccentric (e=0.23) with an orbital period of 1120 d, which
corresponds to a relative separation in semimajor axis of approximately 1.1 AU.
We obtained a high-resolution optical spectrum with UVES/VLT and measured the
system's heliocentric radial velocity. The spectrum does not exhibit lithium
absorption at 670.8 nm, indicating that the system is not extremely young. A
preliminary estimate of the binary's physical parameters tells us that it is
composed of a primary at the stellar-substellar limit and a massive brown-dwarf
companion. DE0630-18 is a new very low-mass binary system with a
well-characterised orbit.Comment: 4 pages, 7 figures. Accepted for publication in A&
Astrometric orbit of a low-mass companion to an ultracool dwarf
Little is known about the existence of extrasolar planets around ultracool
dwarfs. Furthermore, binary stars with Sun-like primaries and very low-mass
binaries composed of ultracool dwarfs show differences in the distributions of
mass ratio and orbital separation that can be indicative of distinct formation
mechanisms. Using FORS2/VLT optical imaging for high precision astrometry we
are searching for planets and substellar objects around ultracool dwarfs to
investigate their multiplicity properties for very low companion masses. Here
we report astrometric measurements with an accuracy of two tenths of a
milli-arcsecond over two years that reveal orbital motion of the nearby L1.5
dwarf DENIS-P J082303.1-491201 located at 20.77 +/- 0.08 pc caused by an unseen
companion that revolves about its host on an eccentric orbit in 246.4 +/- 1.4
days. We estimate the L1.5 dwarf to have 7.5 +/- 0.7 % of the Sun's mass that
implies a companion mass of 28 +/- 2 Jupiter masses. This new system has the
smallest mass ratio (0.36 +/- 0.02) of known very low-mass binaries with
characterised orbits. With this discovery we demonstrate 200 micro-arcsecond
astrometry over an arc-minute field and over several years that is sufficient
to discover sub-Jupiter mass planets around ultracool dwarfs. We also show that
the achieved parallax accuracy of < 0.4 % makes it possible to remove distance
as a dominant source of uncertainty in the modelling of ultracool dwarfs.Comment: 9 pages, 8 figures, accepted for publication in Astronomy and
Astrophysics. The reduced astrometry data will be made publically available
through the CD
Astrometric planet search around southern ultracool dwarfs II: Astrometric reduction methods and a deep astrometric catalogue
We describe the astrometric reduction of images obtained with the FORS2/VLT
camera in the framework of an astrometric planet search around 20
M/L-transition dwarfs. We present the correction of systematic errors, the
achieved astrometric performance, and a new astrometric catalogue containing
the faint reference stars in 20 fields located close to the Galactic plane. We
detected three types of systematic errors in the FORS2 astrometry: the relative
motion of the camera's two CCD chips, errors that are correlated in space, and
an error contribution of yet unexplained origin. The relative CCD motion has
probably a thermal origin and usually is 0.001-0.010 px (~0.1-1 mas), but
sometimes amounts to 0.02-0.05 px (3-6 mas). This instability and
space-correlated errors are detected and mitigated using reference stars. The
third component of unknown origin has an amplitude of 0.03-0.14 mas and is
independent of the observing conditions. We find that a consecutive sequence of
32 images of a well-exposed star over 40 min at 0.6" seeing results in a median
r.m.s. of the epoch residuals of 0.126 mas. Overall, the epoch residuals are
distributed according to a normal law with a chi2~1. We compiled a catalogue of
12000 stars with I-band magnitudes of 16-22 located in 20 fields, each covering
~2x2'. It contains I-band magnitudes, ICRF positions with 40-70 mas precision,
and relative proper motions and absolute trigonometric parallaxes with a
precision of 0.1 mas/yr and 0.1 mas at the bright end, respectively.Comment: 17 pages, 19 figures, 4 tables, accepted for publication in A&A on
March 14, 201
Astrometric planet search around southern ultracool dwarfs I: First results, including parallaxes of 20 M8-L2 dwarfs
Extrasolar planet searches targeting very low-mass stars and brown dwarfs are
hampered by intrinsic or instrumental limitations. Time series of astrometric
measurements with precisions better than one milli-arcsecond can yield new
evidence on the planet occurrence around these objects. We present first
results of an astrometric search for planets around 20 nearby dwarf stars with
spectral types M8-L2. Over a timespan of two years, we obtained I-band images
of the target fields with the FORS2 camera at the Very Large Telescope. Using
background stars as references, we monitored the targets' astrometric
trajectories, which allowed us to measure parallax and proper motions, set
limits on the presence of planets, and to discover the orbital motions of two
binary systems. We determined trigonometric parallaxes with an average accuracy
of 0.09 mas (~0.2 %) resulting in a reference sample for the study of ultracool
dwarfs at the M/L transition, whose members are located at distances of 9.5-40
pc. This sample contains two newly discovered tight binaries (DE0630-18 and
DE0823-49) and one previously known wide binary (DE1520-44). Only one target
shows I-band variability >5 mmag r.m.s. We derived planet exclusion limits that
set an upper limit of 9 % to the occurrence of giant planets with masses >5
MJup in intermediate-separation (0.01-0.8 AU) orbits around M8-L2 dwarfs. We
demonstrated that astrometric observations with an accuracy of 120 micro-arcsec
over two years are feasible from the ground and can be used for a planet search
survey. The detection of two tight very low-mass binaries showed that our
search strategy is efficient and may lead to the detection of planetary-mass
companions through follow-up observations.Comment: 19 pages. Accepted to A&A on March 10, 2014. This is the accepted
version of the paper that includes minor changes and language editin
The Spitzer search for the transits of HARPS low-mass planets - I. No transit for the super-Earth HD 40307b
We have used Spitzer and its IRAC camera to search for the transit of the
super-Earth HD 40307b. The transiting nature of the planet could not be firmly
discarded from our first photometric monitoring of a transit window because of
the uncertainty coming from the modeling of the photometric baseline. To obtain
a firm result, two more transit windows were observed and a global Bayesian
analysis of the three IRAC time series and the HARPS radial velocities was
performed. Unfortunately, any transit of the planet during the observed phase
window is firmly discarded, while the probability that the planet transits but
that the eclipse was missed by our observations is nearly negligible (0.26%).Comment: Submitted to A&
Efecto de la defoliación post-quema sobre el crecimiento de dos gramÃneas nativas en el Caldenal, Argentina
El crecimiento en los pastizales naturales semiáridos puede ser afectado por la longitud de tiempo que el pastoreo es excluido durante el perÃodo de regeneración post-fuego. El objetivo de este trabajo a campo fue estudiar el efecto de la defoliación post-fuego en diferentes estados fenológicos: vegetativo (defoliación temprana) y elongación de entrenudos (defoliación tardÃa) sobre el crecimiento de Piptochaetium napostaense y Poa ligularis, dos gramÃneas nativas perennes muy importantes en la región semiárida templada central de Argentina (Caldenal). En general, los tratamientos de defoliación post-quema no afectaron el número de hojas verdes en las macollas de P. napostaense. La defoliación temprana después del tratamiento de fuego generalmente redujo el número de hojas verdes en las macollas de P. ligularis con respecto a las plantas control. Contrariamente, la defoliación tardÃa usualmente incrementó el número de hojas verdes en las macollas de P. ligularis. Sin embargo, en ambos casos, muy pocas de las diferencias detectadas fueron significativas (p0,05) a aquellas de las plantas no defoliadas. Inmediatamente después de los tratamientos de defoliación temprana y tardÃa, las tasas de crecimiento relativas de la altura y la longitud verde total de las macollas de las plantas de ambas especies fueron reducidas (p0.05) than values on non-defoliated plants, respectively. Immediately after early- and late-season defoliation treatments, relative growth rates for tiller height and total green length were reduced (p<0.05) on plants of both species with respect to control plants. Our results suggest that a one-year-period without severe defoliations after fire would be at least necessary for not risking the persistence of these perennial grasses in the Caldenal community.Fil: Pelaez, Daniel Valerio. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones CientÃficas; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; ArgentinaFil: Boo, Roberto Miguel. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones CientÃficas; ArgentinaFil: Mayor, M. D.. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones CientÃficas; ArgentinaFil: Elia, Omar Raul. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; ArgentinaFil: Martinez, S. A.. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - BahÃa Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones CientÃficas; Argentin
The Spitzer search for the transits of HARPS low-mass planets - II. Null results for 19 planets
Short-period super-Earths and Neptunes are now known to be very frequent
around solar-type stars. Improving our understanding of these mysterious
planets requires the detection of a significant sample of objects suitable for
detailed characterization. Searching for the transits of the low-mass planets
detected by Doppler surveys is a straightforward way to achieve this goal.
Indeed, Doppler surveys target the most nearby main-sequence stars, they
regularly detect close-in low-mass planets with significant transit
probability, and their radial velocity data constrain strongly the ephemeris of
possible transits. In this context, we initiated in 2010 an ambitious Spitzer
multi-Cycle transit search project that targeted 25 low-mass planets detected
by radial velocity, focusing mainly on the shortest-period planets detected by
the HARPS spectrograph. We report here null results for 19 targets of the
project. For 16 planets out of 19, a transiting configuration is strongly
disfavored or firmly rejected by our data for most planetary compositions. We
derive a posterior probability of 83% that none of the probed 19 planets
transits (for a prior probability of 22%), which still leaves a significant
probability of 17% that at least one of them does transit. Globally, our
Spitzer project revealed or confirmed transits for three of its 25 targeted
planets, and discarded or disfavored the transiting nature of 20 of them. Our
light curves demonstrate for Warm Spitzer excellent photometric precisions: for
14 targets out of 19, we were able to reach standard deviations that were
better than 50ppm per 30 min intervals. Combined with its Earth-trailing orbit,
which makes it capable of pointing any star in the sky and to monitor it
continuously for days, this work confirms Spitzer as an optimal instrument to
detect sub-mmag-deep transits on the bright nearby stars targeted by Doppler
surveys.Comment: Accepted for publication in Astronomy and Astrophysics. 23 pages, 21
figure
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