159 research outputs found
The Impact of Stellar Surface Magnetoconvection and Oscillations on the Detection of Temperate, Earth-Mass Planets Around Sun-Like Stars
Detecting and confirming terrestrial planets is incredibly difficult due to
their tiny size and mass relative to Sun-like host stars. However, recent
instrumental advancements are making the detection of Earth-like exoplanets
technologically feasible. For example, Kepler and TESS photometric precision
means we can identify Earth-sized candidates (and PLATO in the future will add
many long-period candidates to the list), while spectrographs such as ESPRESSO
and EXPRES (with an aimed radial velocity precision [RV] near 10 cm/s) mean we
will soon reach the instrumental precision required to confirm Earth-mass
planets in the habitable zones of Sun-like stars. However, many astrophysical
phenomena on the surfaces of these host stars can imprint signatures on the
stellar absorption lines used to detect the Doppler wobble induced by planetary
companions. The result is stellar-induced spurious RV shifts that can mask or
mimic planet signals. This review provides a brief overview of how stellar
surface magnetoconvection and oscillations can impact low-mass planet
confirmation and the best-tested strategies to overcome this astrophysical
noise. These noise reduction strategies originate from a combination of
empirical motivation and a theoretical understanding of the underlying physics.
The most recent predications indicate that stellar oscillations for Sun-like
stars may be averaged out with tailored exposure times, while granulation may
need to be disentangled by inspecting its imprint on the stellar line profile
shapes. Overall, the literature suggests that Earth-analog detection should be
possible, with the correct observing strategy and sufficient data collection.Comment: 14 pages, 10 figures, invited review article for a Special Issue on
the "Detection and Characterization of Extrasolar Planets" in Geosciences,
Guest Editors: M. Oshagh, M. and J. Martinez-Frias, accepted on 17 January
201
Refined architecture of the WASP-8 system: a cautionary tale for traditional Rossiter-McLaughlin analysis
Probing the trajectory of a transiting planet across the disk of its star
through the analysis of its Rossiter-McLaughlin effect can be used to measure
the differential rotation of the host star and the true obliquity of the
system. Highly misaligned systems could be particularly conducive to these
mesurements, which is why we reanalysed the HARPS transit spectra of WASP-8b
using the 'Rossiter-McLaughlin effect reloaded' (reloaded RM) technique. This
approach allows us to isolate the local stellar CCF emitted by the
planet-occulted regions. As a result we identified a 35% variation in the
local CCF contrast along the transit chord, which might trace a deepening of
the stellar lines from the equator to the poles. Whatever its origin, such an
effect cannot be detected when analyzing the RV centroids of the
disk-integrated CCFs through a traditional velocimetric analysis of the RM
effect. Consequently it injected a significant bias into the results obtained
by Queloz et al. (2010) for the projected rotational velocity (1.59 km/s) and the sky-projected
obliquity (-123.0). Using our
technique, we measured these values to be =
1.900.05 km/s and = -143.0. We
found no compelling evidence for differential rotation of the star, although
there are hints that WASP-8 is pointing away from us with the stellar poles
rotating about 25% slower than the equator. Measurements at higher accuracy
during ingress/egress will be required to confirm this result. In contrast to
the traditional analysis of the RM effect, the reloaded RM technique directly
extracts the local stellar CCFs, allowing us to analyze their shape and to
measure their RV centroids, unbiased by variations in their contrast or FWHM.Comment: Accepted for publication in A&A. 12 page
Understanding Astrophysical Noise from Stellar Surface Magneto-Convection
To obtain cm/s precision, stellar surface magneto-convection must be
disentangled from observed radial velocities (RVs). In order to understand and
remove the convective signature, we create Sun-as-a-star model observations
based on a 3D magnetohydrodynamic solar simulation. From these Sun-as-a-star
model observations, we find several line characteristics are correlated with
the induced RV shifts. The aim of this campaign is to feed directly into future
high precision RV studies, such as the search for habitable, rocky worlds, with
forthcoming spectrographs such as ESPRESSO.Comment: 6 pages, 3 figures; presented at the 18th Cambridge Workshop on Cool
Stars, Stellar Systems, and the Sun (CoolStars18); to appear in the
proceedings of Lowell Observatory (9-13 June 2014), edited by G. van Belle &
H. Harris. Updated with correct y-axis units on righthand plot in figure
A cautionary tale: limitations of a brightness-based spectroscopic approach to chromatic exoplanet radii
Determining wavelength-dependent exoplanet radii measurements is an excellent
way to probe the composition of exoplanet atmospheres. In light of this, Borsa
et al. (2016) sought to develop a technique to obtain such measurements by
comparing ground-based transmission spectra to the expected brightness
variations during an exoplanet transit. However, we demonstrate herein that
this is not possible due to the transit light curve normalisation necessary to
remove the effects of the Earth's atmosphere on the ground-based observations.
This is because the recoverable exoplanet radius is set by the planet-to-star
radius ratio within the transit light curve; we demonstrate this both
analytically and with simulated planet transits, as well as through a
reanalysis of the HD 189733b data.Comment: 5 pages, 2 figures, 1 table, accepted to A&
A geometrical origin for the covariant entropy bound
Causal diamond-shaped subsets of space-time are naturally associated with
operator algebras in quantum field theory, and they are also related to the
Bousso covariant entropy bound. In this work we argue that the net of these
causal sets to which are assigned the local operator algebras of quantum
theories should be taken to be non orthomodular if there is some lowest scale
for the description of space-time as a manifold. This geometry can be related
to a reduction in the degrees of freedom of the holographic type under certain
natural conditions for the local algebras. A non orthomodular net of causal
sets that implements the cutoff in a covariant manner is constructed. It gives
an explanation, in a simple example, of the non positive expansion condition
for light-sheet selection in the covariant entropy bound. It also suggests a
different covariant formulation of entropy bound.Comment: 20 pages, 8 figures, final versio
Orbital misalignment of the super-Earth Men c with the spin of its star
Planet-planet scattering events can leave an observable trace of a planet's
migration history in the form of orbital misalignment with respect to the the
stellar spin axis, which is measurable from spectroscopic timeseries taken
during transit. We present high-resolution spectroscopic transits observed with
ESPRESSO of the close-in super-Earth Men c. The system also contains an
outer giant planet on a wide, eccentric orbit, recently found to be inclined
with respect to the inner planetary orbit. These characteristics are
reminiscent of past dynamical interactions. We successfully retrieve the
planet-occulted light during transit and find evidence that the orbit of
Men c is moderately misaligned with the stellar spin axis with ().
This is consistent with the super-Earth Men c having followed a
high-eccentricity migration followed by tidal circularisation, and hints that
super-Earths can form at large distances from their star. We also detect clear
signatures of solar-like oscillations within our ESPRESSO radial velocity
timeseries, where we reach a radial velocity precision of cm/s. We
model the oscillations using Gaussian processes and retrieve a frequency of
maximum oscillation, Hz. These
oscillations makes it challenging to detect the Rossiter-McLaughlin effect
using traditional methods. We are, however, successful using the reloaded
Rossiter-McLaughlin approach. Finally, in an appendix we also present updated
physical parameters and ephemerides for Men c from a Gaussian process
transit analysis of the full TESS Cycle 1 data.Comment: 20 pages, 11 figures. Published in MNRA
Geometric entropy, area, and strong subadditivity
The trace over the degrees of freedom located in a subset of the space
transforms the vacuum state into a density matrix with non zero entropy. This
geometric entropy is believed to be deeply related to the entropy of black
holes. Indeed, previous calculations in the context of quantum field theory,
where the result is actually ultraviolet divergent, have shown that the
geometric entropy is proportional to the area for a very special type of
subsets. In this work we show that the area law follows in general from simple
considerations based on quantum mechanics and relativity. An essential
ingredient of our approach is the strong subadditive property of the quantum
mechanical entropy.Comment: Published versio
Stellar surface magneto-convection as a source of astrophysical noise II. Center-to-limb parameterisation of absorption line profiles and comparison to observations
Manifestations of stellar activity (such as star-spots, plage/faculae, and
convective flows) are well known to induce spectroscopic signals often referred
to as astrophysical noise by exoplanet hunters. For example, setting an
ultimate goal of detecting true Earth-analogs demands reaching radial velocity
(RV) precisions of ~9 cm/s. While this is becoming technically feasible with
the latest generation of highly stabilised spectrographs, it is astrophysical
noise that sets the true fundamental barrier on attainable RV precisions. In
this paper we parameterise the impact of solar surface magneto-convection on
absorption line profiles, and extend the analysis from the solar disc centre
(Paper I) to the solar limb. Off disc-centre, the plasma flows orthogonal to
the granule tops begin to lie along the line-of-sight and those parallel to the
granule tops are no longer completely aligned with the observer. Moreover, the
granulation is corrugated and the granules can block other granules, as well as
the intergranular lane components. Overall, the visible plasma flows and
geometry of the corrugated surface significantly impact the resultant line
profiles and induce centre-to-limb variations in shape and net position. We
detail these herein, and compare to various solar observations. We find our
granulation parameterisation can recreate realistic line profiles and induced
radial velocity shifts, across the stellar disc, indicative of both those found
in computationally heavy radiative 3D magnetohydrodynamical simulations and
empirical solar observations.Comment: 17 pages, 14 figures, accepted to Ap
Stellar Surface Magnetoconvection as a Source of Astrophysical Noise. III. Sun-as-a-Star Simulations and Optimal Noise Diagnostics
Stellar surface magnetoconvection (granulation) creates asymmetries in the
observed stellar absorption lines that can subsequently manifest themselves as
spurious radial velocities shifts. In turn, this can then mask the
Doppler-reflex motion induced by orbiting planets on their host stars, and
represents a particular challenge for determining the masses of low-mass,
long-period planets. Herein, we study this impact by creating Sun-as-a-star
observations that encapsulate the granulation variability expected from 3D
magnetohydrodynamic simulations. These Sun-as-a-star model observations are in
good agreement with empirical observations of the Sun, but may underestimate
the total variability relative to the quiet Sun due to the increased magnetic
field strength in our models. We find numerous line profile characteristics
linearly correlate with the disc-integrated convection-induced velocities.
Removing the various correlations with the line bisector, equivalent width, and
the V_asy indicator may reduce ~50-60% of the granulation noise in the measured
velocities. We also find that simultaneous photometry may be a key diagnostic,
as our proxy for photometric brightness also allowed us to remove ~50% of the
granulation-induced radial velocity noise. These correlations and
granulation-noise mitigations breakdown in the presence of low instrumental
resolution and/or increased stellar rotation, as both act to smooth the
observed line profile asymmetries.Comment: 18 pages + 15 pages references/appendix, 24 figures, 6 tables,
accepted to Ap
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