22 research outputs found
Quantum mechanical analysis of the elastic propagation of electrons in the Au/Si system: application to Ballistic Electron Emission Microscopy
We present a Green's function approach based on a LCAO scheme to compute the
elastic propagation of electrons injected from a STM tip into a metallic film.
The obtained 2D current distribution in real and reciprocal space furnish a
good representation of the elastic component of Ballistic Electron Emission
Microscopy (BEEM) currents. Since this component accurately approximates the
total current in the near threshold region, this procedure allows --in contrast
to prior analyses-- to take into account effects of the metal band structure in
the modeling of these experiments. The Au band structure, and in particular its
gaps appearing in the [111] and [100] directions provides a good explanation
for the previously irreconcilable results of nanometric resolution and
similarity of BEEM spectra on both Au/Si(111) and Au/Si(100).Comment: 12 pages, 9 postscript figures, revte
A theoretical analysis of Ballistic Electron Emission Microscopy: k-space distributions and spectroscopy
We analyze BEEM experiments. At low temperatures and low voltages, near the
threshold value of the Schottky barrier, the BEEM current is dominated by the
elastic component. Elastic scattering by the lattice results in the formation
of focused beams and narrow lines in real space. To obtain the current injected
in the semiconductor, we compute the current distribution in reciprocal space
and, assuming energy and conservation. Our results show an
important focalization of the injected electron beam and explain the similarity
between BEEM currents for Au/Si(111) and Au/Si(100).Comment: 17 pages, 5 figures (postscript), Latex, APS,
http://www.icmm.csic.es/Pandres/pedro.htm. Appl. Surf. Sci. (in press
Refining the transit-timing and photometric analysis of TRAPPIST-1: Masses, Radii, densities, dynamics, and ephemerides
We have collected transit times for the TRAPPIST-1 system with the Spitzer
Space Telescope over four years. We add to these ground-based, HST and K2
transit time measurements, and revisit an N-body dynamical analysis of the
seven-planet system using our complete set of times from which we refine the
mass ratios of the planets to the star. We next carry out a photodynamical
analysis of the Spitzer light curves to derive the density of the host star and
the planet densities. We find that all seven planets' densities may be
described with a single rocky mass-radius relation which is depleted in iron
relative to Earth, with Fe 21 wt% versus 32 wt% for Earth, and otherwise
Earth-like in composition. Alternatively, the planets may have an Earth-like
composition, but enhanced in light elements, such as a surface water layer or a
core-free structure with oxidized iron in the mantle. We measure planet masses
to a precision of 3-5%, equivalent to a radial-velocity (RV) precision of 2.5
cm/sec, or two orders of magnitude more precise than current RV capabilities.
We find the eccentricities of the planets are very small; the orbits are
extremely coplanar; and the system is stable on 10 Myr timescales. We find
evidence of infrequent timing outliers which we cannot explain with an eighth
planet; we instead account for the outliers using a robust likelihood function.
We forecast JWST timing observations, and speculate on possible implications of
the planet densities for the formation, migration and evolution of the planet
system
A super-Earth and a sub-Neptune orbiting the bright, quiet M3 dwarf TOI-1266
We report the discovery and characterisation of a super-Earth and a sub-Neptune transiting the bright (K = 8.8), quiet, and nearby (37 pc) M3V dwarf TOI-1266. We validate the planetary nature of TOI-1266 b and c using four sectors of TESS photometry and data from the newly-commissioned 1-m SAINT-EX telescope located in San Pedro Mártir (México). We also include additional ground-based follow-up photometry as well as high-resolution spectroscopy and high-angular imaging observations. The inner, larger planet has a radius of R = 2.37_(−0.12)^(+0.16) R_⊕ and an orbital period of 10.9 days. The outer, smaller planet has a radius of R = 1.56_(−0.13)^(+0.15) R_⊕ on an 18.8-day orbit. The data are found to be consistent with circular, co-planar and stable orbits that are weakly influenced by the 2:1 mean motion resonance. Our TTV analysis of the combined dataset enables model-independent constraints on the masses and eccentricities of the planets. We find planetary masses of M_p = 13.5_(−9.0)^(+11.0) M_⊕ (<36.8 M_⊕ at 2-σ) for TOI-1266 b and 2.2_(−1.5)^(+2.0) M_⊕ (<5.7 M_⊕ at 2-σ) for TOI-1266 c. We find small but non-zero orbital eccentricities of 0.09_(−0.05)^(+0.06) (<0.21 at 2-σ) for TOI-1266 b and 0.04 ± 0.03 (< 0.10 at 2-σ) for TOI-1266 c. The equilibrium temperatures of both planets are of 413 ± 20 and 344 ± 16 K, respectively, assuming a null Bond albedo and uniform heat redistribution from the day-side to the night-side hemisphere. The host brightness and negligible activity combined with the planetary system architecture and favourable planet-to-star radii ratios makes TOI-1266 an exquisite system for a detailed characterisation
A super-Earth and a sub-Neptune orbiting the bright, quiet M3 dwarf TOI-1266
We report the discovery and characterisation of a super-Earth and a
sub-Neptune transiting the bright (), quiet, and nearby (37 pc) M3V
dwarf TOI-1266. We validate the planetary nature of TOI-1266 b and c using four
sectors of TESS photometry and data from the newly-commissioned 1-m SAINT-EX
telescope located in San Pedro M\'artir (Mexico). We also include additional
ground-based follow-up photometry as well as high-resolution spectroscopy and
high-angular imaging observations. The inner, larger planet has a radius of
R and an orbital period of 10.9 days. The
outer, smaller planet has a radius of R on
an 18.8-day orbit. The data are found to be consistent with circular, co-planar
and stable orbits that are weakly influenced by the 2:1 mean motion resonance.
Our TTV analysis of the combined dataset enables model-independent constraints
on the masses and eccentricities of the planets. We find planetary masses of
= (
at 2-) for TOI-1266 b and
( at 2-) for TOI-1266
c. We find small but non-zero orbital eccentricities of
( at 2-) for TOI-1266 b and ( at
2-) for TOI-1266 c. The equilibrium temperatures of both planets are of
K and K, respectively, assuming a null Bond albedo and
uniform heat redistribution from the day-side to the night-side hemisphere. The
host brightness and negligible activity combined with the planetary system
architecture and favourable planet-to-star radii ratios makes TOI-1266 an
exquisite system for a detailed characterisation
Green's function calculation of Ballistic Electron Emission Microscopy currents (BEEM v2.1)
We describe the FORTRAN-90 program BEEM v2.1 for the computation of real and reciprocal space currents in Ballistic Electron Emission Microscopy (BEEM), and Spectroscopy (BEES). Based on a LCAO Hamiltonian, the equilibrium one-particle Green's functions of a semi-infinite crystal are obtained using a decimation technique. By means of Keldysh's formalism, the STM tip is coupled to this crystal, and expressions for BEEM currents in real and reciprocal space are obtained. Finally, a Dyson-like equation and a self-consistent charge neutrality condition are applied to incorporate reconstructed or relaxed layers in the surface region. Running demos for Au and CoSi2 are provided
Co-orbital exoplanets from close-period candidates: the TOI-178 case
International audienc
The 0.8-4.5μm broadband transmission spectra of TRAPPIST-1 planets
International audienceThe TRAPPIST-1 planetary system represents an exceptional opportunity for the atmospheric characterization of temperate terrestrial exoplanets with the upcoming James Webb Space Telescope (JWST). Assessing the potential impact of stellar contamination on the planets' transit transmission spectra is an essential precursor step to this characterization. Planetary transits themselves can be used to scan the stellar photosphere and to constrain its heterogeneity through transit depth variations in time and wavelength. In this context, we present our analysis of 169 transits observed in the optical from space with K2 and from the ground with the SPECULOOS and Liverpool telescopes. Combining our measured transit depths with literature results gathered in the mid/near-IR with Spitzer/IRAC and HST/WFC3, we construct the broadband transmission spectra of the TRAPPIST-1 planets over the 0.6-4.5 m spectral range. While planets b, d, and f spectra show some structures at the 200-300ppm level, the four others are globally flat. Even if we cannot discard their instrumental origins, two scenarios seem to be favored by the data: a stellar photosphere dominated by a few high-latitude giant (cold) spots, or, alternatively, by a few small and hot (3500-4000K) faculae. In both cases, the stellar contamination of the transit transmission spectra is expected to be less dramatic than predicted in recent papers. Nevertheless, based on our results, stellar contamination can still be of comparable or greater order than planetary atmospheric signals at certain wavelengths. Understanding and correcting the effects of stellar heterogeneity therefore appears essential to prepare the exploration of TRAPPIST-1's with JWST