1,238 research outputs found
Thermohaline instability and rotation-induced mixing II- Yields of 3He for low- and intermediate-mass stars
Context. The 3He content of Galactic HII regions is very close to that of the
Sun and the solar system, and only slightly higher than the primordial 3He
abundance as predicted by the standard Big Bang nucleosynthesis. However, the
classical theory of stellar evolution predicts a high production of 3He by
low-mass stars, implying a strong increase of 3He with time in the Galaxy. This
is the well-known "3He problem". Aims. We study the effects of thermohaline and
rotation-induced mixings on the production and destruction of 3He over the
lifetime of low- and intermediate-mass stars at various metallicities. Methods.
We compute stellar evolutionary models in the mass range 1 to 6M\odot for four
metallicities, taking into account thermohaline instability and
rotation-induced mixing. For the thermohaline diffusivity we use the
prescription based on the linear stability analysis, which reproduces Red Giant
Branch (RGB) abundance patterns at all metallicities. Rotation-induced mixing
is treated taking into account meridional circulation and shear turbulence. We
discuss the effects of these processes on internal and surface abundances of
3He and on the net yields. Results. Over the whole mass and metallicity range
investigated, rotation-induced mixing lowers the 3He production, as well as the
upper mass limit at which stars destroy 3He. For low-mass stars, thermohaline
mixing occuring beyond the RGB bump is the dominant process in strongly
reducing the net 3He yield compared to standard computations. Yet these stars
remain net 3He producers. Conclusions. Overall, the net 3He yields are strongly
reduced compared to the standard framework predictions
Signs of strong Na and K absorption in the transmission spectrum of WASP-103b
Context: Transmission spectroscopy has become a prominent tool for
characterizing the atmospheric properties on close-in transiting planets.
Recent observations have revealed a remarkable diversity in exoplanet spectra,
which show absorption signatures of Na, K and , in some cases
partially or fully attenuated by atmospheric aerosols. Aerosols (clouds and
hazes) themselves have been detected in the transmission spectra of several
planets thanks to wavelength-dependent slopes caused by the particles'
scattering properties. Aims: We present an optical 550 - 960 nm transmission
spectrum of the extremely irradiated hot Jupiter WASP-103b, one of the hottest
(2500 K) and most massive (1.5 ) planets yet to be studied with this
technique. WASP-103b orbits its star at a separation of less than 1.2 times the
Roche limit and is predicted to be strongly tidally distorted. Methods: We have
used Gemini/GMOS to obtain multi-object spectroscopy hroughout three transits
of WASP-103b. We used relative spectrophotometry and bin sizes between 20 and 2
nm to infer the planet's transmission spectrum. Results: We find that WASP-103b
shows increased absorption in the cores of the alkali (Na, K) line features. We
do not confirm the presence of any strong scattering slope as previously
suggested, pointing towards a clear atmosphere for the highly irradiated,
massive exoplanet WASP-103b. We constrain the upper boundary of any potential
cloud deck to reside at pressure levels above 0.01 bar. This finding is in line
with previous studies on cloud occurrence on exoplanets which find that clouds
dominate the transmission spectra of cool, low surface gravity planets while
hot, high surface gravity planets are either cloud-free, or possess clouds
located below the altitudes probed by transmission spectra.Comment: Accepted for publication in A&
1995, Spatial and temporal variability of late Neogene equatorial Pacific carbonate
High-resolution, continuous records of GRAPE wet bulk density (a carbonate proxy) from Ocean Drilling Program Leg 138 provide one the opportunity for a detailed study of eastern equatorial Pacific Ocean carbonate sedimentation during the last 6 m.y. The transect of sites drilled spans both latitude and longitude in the eastern equatorial Pacific from 90° to 110°W and from 5°S to 10°N. Two modes of variability are resolved through the use of Empirical Orthogonal Function (EOF) analysis. In the presence of large tectonic and climatic boundary condition changes over the last 6 m.y., the dominant mode of spatial variability in carbonate sedimentation is remarkably constant. The first mode accounts for over 50% of the variance in the data, and is consistent with forcing by equatorial divergence. This mode characterizes both carbonate concentration and carbonate mass accumulation rate time series. Variability in the first mode is highly coherent with insolation, indicating a strong linear relationship between equatorial Pacific car bonate sedimentation and Milankovitch variability. Frequency domain analysis indicates that the coupling to equatorial divergence in carbonate sedimentation is strongest in the precession band (19-23 k.y.) and weakest though present at lower frequencies. The second mode of variability has a consistent spatial pattern of east-west asymmetry over the past 4 m.y. only; prior to 4 Ma, a different mode of spatial variability may have been present, possibly suggesting influence by closure of the Isthmus of Panama or other tectonic changes. The second mode of variability may indicate influence by CaCO3 dissolution. The second mode of variability is not highly coherent with insolation. Comparison of the modes of carbonate variability to a 4 m.y. record of benthic δ 1 8 indicates that although overall correlation between carbonate and δ 1 8 is low, both modes of variability in carbonate sedimentation are coherent with δ 1 8 changes at some frequencies. The first mode of carbonate variability is coherent with Sites 846/849 δ 1 8 at the dominant insolation periods, and the second mode is coherent at 100 k.y. during the last 2 m.y. The coherence between carbonate sedimentation and δ 1 8 in both EOF modes suggests that multiple uncorrelated modes of variability operated within the climate system during the late Neogene
Artificial Incoherent Speckles Enable Precision Astrometry and Photometry in High-Contrast Imaging
State-of-the-art coronagraphs employed on extreme adaptive optics enabled instruments are constantly improving the contrast detection limit for companions at ever-closer separations from the host star. In order to constrain their properties and, ultimately, compositions, it is important to precisely determine orbital parameters and contrasts with respect to the stars they orbit. This can be difficult in the post-coronagraphic image plane, as by definition the central star has been occulted by the coronagraph. We demonstrate the flexibility of utilizing the deformable mirror in the adaptive optics system of the Subaru Coronagraphic Extreme Adaptive Optics system to generate a field of speckles for the purposes of calibration. Speckles can be placed up to 22.5 λ/D from the star, with any position angle, brightness, and abundance required. Most importantly, we show that a fast modulation of the added speckle phase, between 0 and π, during a long science integration renders these speckles effectively incoherent with the underlying halo. We quantitatively show for the first time that this incoherence, in turn, increases the robustness and stability of the adaptive speckles, which will improve the precision of astrometric and photometric calibration procedures. This technique will be valuable for high-contrast imaging observations with imagers and integral field spectrographs alike
Sodium atoms and clusters on graphite: a density functional study
Sodium atoms and clusters (N<5) on graphite (0001) are studied using density
functional theory, pseudopotentials and periodic boundary conditions. A single
Na atom is observed to bind at a hollow site 2.45 A above the surface with an
adsorption energy of 0.51 eV. The small diffusion barrier of 0.06 eV indicates
a flat potential energy surface. Increased Na coverage results in a weak
adsorbate-substrate interaction, which is evident in the larger separation from
the surface in the cases of Na_3, Na_4, Na_5, and the (2x2) Na overlayer. The
binding is weak for Na_2, which has a full valence electron shell. The presence
of substrate modifies the structures of Na_3, Na_4, and Na_5 significantly, and
both Na_4 and Na_5 are distorted from planarity. The calculated formation
energies suggest that clustering of atoms is energetically favorable, and that
the open shell clusters (e.g. Na_3 and Na_5) can be more abundant on graphite
than in the gas phase. Analysis of the lateral charge density distributions of
Na and Na_3 shows a charge transfer of about 0.5 electrons in both cases.Comment: 20 pages, 6 figure
Exploring dust around HD142527 down to 0.025" / 4au using SPHERE/ZIMPOL
We have observed the protoplanetary disk of the well-known young Herbig star
HD 142527 using ZIMPOL Polarimetric Differential Imaging with the VBB (Very
Broad Band, ~600-900nm) filter. We obtained two datasets in May 2015 and March
2016. Our data allow us to explore dust scattering around the star down to a
radius of ~0.025" (~4au). The well-known outer disk is clearly detected, at
higher resolution than before, and shows previously unknown sub-structures,
including spirals going inwards into the cavity. Close to the star, dust
scattering is detected at high signal-to-noise ratio, but it is unclear whether
the signal represents the inner disk, which has been linked to the two
prominent local minima in the scattering of the outer disk, interpreted as
shadows. An interpretation of an inclined inner disk combined with a dust halo
is compatible with both our and previous observations, but other arrangements
of the dust cannot be ruled out. Dust scattering is also present within the
large gap between ~30 and ~140au. The comparison of the two datasets suggests
rapid evolution of the inner regions of the disk, potentially driven by the
interaction with the close-in M-dwarf companion, around which no polarimetric
signal is detected.Comment: 11 pages, 7 figures, accepted for publication in A
VIBES: VIsual Binary Exoplanet survey with SPHERE Upper limits on wide S-planet and S-BD frequencies, triple system discovery, and astrometric confirmation of 20 stellar binaries and three triple systems
Recent surveys indicate that planets in binary systems are more abundant than
previously thought, which is in agreement with theoretical work on disc
dynamics and planet formation in binaries. In order to measure the abundance
and physical characteristics of wide-orbit giant exoplanets in binary systems,
we have designed the 'VIsual Binary Exoplanet survey with Sphere' (VIBES) to
search for planets in visual binaries. It uses the SPHERE instrument at VLT to
search for planets in 23 visual binary and four visual triple systems with ages
of <145 Myr and distances of <150 pc. We used the IRDIS dual-band imager on
SPHERE to acquire high-contrast images of the sample targets. For each binary,
the two components were observed at the same time with a coronagraph masking
only the primary star. For the triple star, the tight components were treated
as a single star for data reduction. This enabled us to effectively search for
companions around 50 individual stars in binaries and four binaries in triples.
We derived upper limits of 13.7\% for the frequency of sub-stellar
companions around primaries in visual binaries, 26.5\% for the fraction of
sub-stellar companions around secondaries in visual binaries, and an occurrence
rate of 9.0\% for giant planets and brown dwarfs around either component of
visual binaries. We have combined our observations with literature measurements
to astrometrically confirm, for the first time, that 20 binaries and two triple
systems, which were previously known, are indeed physically bound. Finally, we
discovered a third component of the binary HD~121336. The upper limits we
derived are compatible with planet formation through the core accretion and the
gravitational instability processes in binaries. These limits are also in line
with limits found for single star and circumbinary planet search surveys.Comment: Accepted for publication in Astronomy & Astrophysics on 18.09.2020 21
pages, 11 figure
The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits: II- Survey description, results and performances
In anticipation of the VLT/SPHERE planet imager guaranteed time programs, we
have conducted a preparatory survey of 86 stars between 2009 and 2013 in order
to identify new faint comoving companions to ultimately carry out a
comprehensive analysis of the occurence of giant planets and brown dwarf
companions at wide (10-2000 AU) orbits around young, solar-type stars. We used
NaCo at VLT to explore the occurrence rate of giant planets and brown dwarfs
between typically 0.1 and 8''. Diffraction-limited observations in H-band
combined with angular differential imaging enabled us to reach primary
star-companion brightness ratios as small as 10-6 at 1.5''. 12 systems were
resolved as new binaries, including the discovery of a new white dwarf
companion to the star HD8049. Around 34 stars, at least one companion candidate
was detected in the observed field of view. More than 400 faint sources were
detected, 90% of them in 4 crowded fields. With the exception of HD8049B, we
did not identify any new comoving companions. The survey also led to spatially
resolved images of the thin debris disk around HD\,61005 that have been
published earlier. Finally, considering the survey detection limits, we derive
a preliminary upper limit on the frequency of giant planets for semi-major axes
of [10,2000] AU: typically less than 15% between 100 and 500 AU, and less than
10% between 50 and 500 AU for exoplanets more massive than 5 MJup and 10 MJup
respectively, considering a uniform input distribution and with a confidence
level of 95%.Comment: 19 pages, 8 figures, 12 Tables, accepted to A&
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