354 research outputs found
High Contrast Imaging of the Close Environment of HD 142527 -
Context. It has long been suggested that circumstellar disks surrounding
young stars may be the signposts of planets, and still more since the recent
discoveries of embedded substellar companions. The planet-disk interaction may
create, according to models, large structures, gaps, rings or spirals, in the
disk. In that sense, the Herbig star HD 142527 is particularly compelling as,
its massive disk displays intriguing asymmetries that suggest the existence of
a dynamical peturber of unknown nature. Aims. Our goal was to obtain deep
thermal images of the close circumstellar environment of HD 142527 to re-image
the reported close-in structures (cavity, spiral arms) of the disk and to
search for stellar and substellar companions that could be connected to their
presence. Results. The circumstellar environment of HD 142527 is revealed at an
unprecedented spatial resolution down to the sub arcsecond level for the first
time at 3.8 microns. Our images reveal important radial and azimuthal
asymmetries which invalidate an elliptical shape for the disk as previously
proposed. It rather suggests a bright inhomogeneous spiral arm plus various
fainter spiral arms. We also confirm an inner cavity down to 30 AU and two
important dips at position angles of 0 and 135 deg. The detection performance
in angular differential imaging enables the exploration of the planetary mass
regime for projected physical separations as close as 40 AU. The use of our
detection map together with Monte Carlo simulations sets stringent constraints
on the presence of planetary mass, brown dwarf or stellar companions as a
function of the semi-major axis. They severely constrain the presence of
massive giant planets with semi-major axis beyond 50AU, i.e. probably within
the large disk's cavity that radially extends up to 145 AU or even further
outside.Comment: 8 pages, 7 figures, accepted in A&
Testing the universality of star formation - II. Comparing separation distributions of nearby star-forming regions and the field
We have measured the multiplicity fractions and separation distributions of
seven young star-forming regions using a uniform sample of young binaries. Both
the multiplicity fractions and separation distributions are similar in the
different regions. A tentative decline in the multiplicity fraction with
increasing stellar density is apparent, even for binary systems with
separations too close (19-100au) to have been dynamically processed. The
separation distributions in the different regions are statistically
indistinguishable over most separation ranges, and the regions with higher
densities do not exhibit a lower proportion of wide (300-620au) relative to
close (62-300au) binaries as might be expected from the preferential
destruction of wider pairs. Only the closest (19-100au) separation range, which
would be unaffected by dynamical processing, shows a possible difference in
separation distributions between different regions. The combined set of young
binaries, however, shows a distinct difference when compared to field binaries,
with a significant excess of close (19-100au) systems among the younger
binaries. Based on both the similarities and differences between individual
regions, and between all seven young regions and the field, especially over
separation ranges too close to be modified by dynamical processing, we conclude
that multiple star formation is not universal and, by extension, the star
formation process is not universal.Comment: accepted for publication in MNRA
Towards precise ages and masses of free floating planetary mass brown dwarfs
© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical SocietyMeasurement of the substellar initial mass function (IMF) in very young clusters is hampered by the possibility of the age spread of clustermembers. This is particularly serious for candidate planetary mass objects (PMOs), which have a very similar location to older and more massive brown dwarfs on the Hertzsprung-Russell Diagram (HRD). This degeneracy can be lifted by the measurement of gravity-sensitive spectral features. To this end we have obtained mediumresolution (R ~ 5000) Near-infrared Integral Field Spectrometer (NIFS) K-band spectra of a sample of late M-/early L-type dwarfs. The sample comprises old field dwarfs and very young brown dwarfs in the Taurus association and in the σ Orionis cluster. We demonstrate a positive correlation between the strengths of the 2.21 μm Na I doublet and the objects' ages. We demonstrate a further correlation between these objects' ages and the shape of their K-band spectra. We have quantified this correlation in the form of a new index, the H2(K) index. This index appears to be more gravity-sensitive than the Na I doublet and has the advantage that it can be computed for spectra where gravity-sensitive spectral lines are unresolved, while it is also more sensitive to surface gravity at very young ages (<10 Myr) than the triangular H-band peak. Both correlations differentiate young objects from field dwarfs, while the H2(K) index can distinguish, at least statistically, populations of ~1Myr objects from populations of ~10 Myr objects. We applied the H2(K) index to NIFS data for one Orion nebula cluster (ONC) PMO and to previously published low-resolution spectra for several other ONC PMOs where the 2.21 μm Na I doublet was unresolved and concluded that the average age of the PMOs is ~1Myr.Peer reviewe
A rocky planet transiting a nearby low-mass star
M-dwarf stars -- hydrogen-burning stars that are smaller than 60 per cent of
the size of the Sun -- are the most common class of star in our Galaxy and
outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M
dwarfs host Earth-sized planets in great numbers: the average number of M-dwarf
planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per
star. The nearest such planets known to transit their star are 39 parsecs away,
too distant for detailed follow-up observations to measure the planetary masses
or to study their atmospheres. Here we report observations of GJ 1132b, a
planet with a size of 1.2 Earth radii that is transiting a small star 12
parsecs away. Our Doppler mass measurement of GJ 1132b yields a density
consistent with an Earth-like bulk composition, similar to the compositions of
the six known exoplanets with masses less than six times that of the Earth and
precisely measured densities. Receiving 19 times more stellar radiation than
the Earth, the planet is too hot to be habitable but is cool enough to support
a substantial atmosphere, one that has probably been considerably depleted of
hydrogen. Because the host star is nearby and only 21 per cent the radius of
the Sun, existing and upcoming telescopes will be able to observe the
composition and dynamics of the planetary atmosphere.Comment: Published in Nature on 12 November 2015, available at
http://dx.doi.org/10.1038/nature15762. This is the authors' version of the
manuscrip
Dynamical Masses of Young Stars I:Discordant Model Ages of Upper Scorpius
We present the results of a long term orbit monitoring program, using sparse
aperture masking observations taken with NIRC2 on the Keck-II telescope, of
seven G to M-type members of the Upper Scorpius subgroup of the Sco-Cen OB
association. We present astrometry and derived orbital elements of the binary
systems we have monitored, and also determine the age, component masses,
distance and reddening for each system using the orbital solutions and
multi-band photometry, including Hubble Space Telescope photometry, and a
Bayesian fitting procedure. We find that the models can be forced into
agreement with any individual system by assuming an age, but that age is not
consistent across the mass range of our sample. The G-type binary systems in
our sample have model ages of ~11.5 Myr, which is consistent with the latest
age estimates for Upper Scorpius, while the M-type binary systems have
significantly younger model ages of ~7 Myr. Based on our fits, this age
discrepancy in the models corresponds to a luminosity under-prediction of
0.8-0.15 dex, or equivalently an effective temperature over-prediction of
100-300 K for M-type stars at a given premain-sequence age. We also find that
the M-type binary system RXJ 1550.0-2312 has an age (~16 Myr) and distance (~90
pc) indicating that it is either a nearby young binary system or a member of
the Upper-Centaurus-Lupus subgroup with a 57% probability of membership.Comment: 16 pages, 8 figures, 9 tables, accepted for publication in Ap
On the binary frequency of the lowest mass members of the pleiades with hubble space telescope wide field camera 3
E. V. Garcia, et al., “On the Binary Frequency of the Lowest mass Members of the Pleiades with Hubble Space Telescope Wide Field Camera 3”, The Astrophysical Journal, Vol. 804(1), May 2015. © 2015. The American Astronomical Society.We present the results of a Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging survey of 11 of the lowest mass brown dwarfs in the Pleiades known (25-40 MJup). These objects represent the predecessors to T dwarfs in the field. Using a semi-empirical binary point-spread function (PSF)-fitting technique, we are able to probe to 0.″ 03 (0.75 pixel), better than 2x the WFC3/UVIS diffraction limit. We did not find any companions to our targets. From extensive testing of our PSF-fitting method on simulated binaries, we compute detection limits which rule out companions to our targets with mass ratios of 0.7 and separations 4 AU. Thus, our survey is the first to attain the high angular resolution needed to resolve brown dwarf binaries in the Pleiades at separations that are most common in the field population. We constrain the binary frequency over this range of separation and mass ratio of 25-40 MJup Pleiades brown dwarfs to bePeer reviewe
Neurocytotoxic effects of iron-ions on the developing brain measured in vivo using medaka (Oryzias latipes), a vertebrate model
Purpose: Exposure to heavy-ion radiation is considered a critical health risk on long-term space missions. The developing central nervous system (CNS) is a highly radiosensitive tissue; however, the biological effects of heavy-ion radiation, which are greater than those of low-linear energy transfer (LET) radiation, are not well studied, especially in vivo in intact organisms. Here, we examined the effects of iron-ions on the developing CNS using vertebrate organism, fish embryos of medaka (Oryzias latipes)
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