665 research outputs found
Searching for a dusty cometary belt around TRAPPIST-1 with ALMA
Low-mass stars might offer today the best opportunities to detect and characterize planetary systems, especially those harbouring close-in low-mass temperate planets. Among those stars, TRAPPIST-1 is exceptional since it has seven Earth-sized planets, of which three could sustain liquid water on their surfaces. Here we present new and deep ALMA observations of TRAPPIST-1 to look for an exo-Kuiper belt which can provide clues about the formation and architecture of this system. Our observations at 0.88 mm did not detect dust emission, but can place an upper limit of 23 µJy if the belt is smaller than 4 au, and 0.15 mJy if resolved and 100 au in radius. These limits correspond to low dust masses of ̃10-5 to 10-2 M⊕, which are expected after 8 Gyr of collisional evolution unless the system was born with a >20 M⊕ belt of 100 km-sized planetesimals beyond 40 au or suffered a dynamical instability. This 20 M⊕ mass upper limit is comparable to the combined mass in TRAPPIST-1 planets, thus it is possible that most of the available solid mass in this system was used to form the known planets. A similar analysis of the ALMA data on Proxima Cen leads us to conclude that a belt born with a mass ≳1 M⊕ in 100 km-sized planetesimals could explain its putative outer belt at 30 au. We recommend that future characterizations of debris discs around low-mass stars should focus on nearby and young systems if possible
Line-profile tomography of exoplanet transits I: The Doppler shadow of HD 189733b
We present a direct method for isolating the component of the starlight
blocked by a planet as it transits its host star, and apply it to spectra of
the bright transiting planet HD 189733b. We model the global shape of the
stellar cross-correlation function as the convolution of a limb-darkened
rotation profile and a gaussian representing the Doppler core of the average
photospheric line profile. The light blocked by the planet during the transit
is a gaussian of the same intrinsic width, whose trajectory across the line
profile yields a precise measure of the misalignment angle and an independent
measure of v sin I. We show that even when v sin I is less than the width of
the intrinsic line profile, the travelling Doppler "shadow" cast by the planet
creates an identifiable distortion in the line profiles which is amenable to
direct modelling. Direct measurement of the trajectory of the missing starlight
yields self-consistent measures of the projected stellar rotation rate, the
intrinsic width of the mean local photospheric line profile, the projected
spin-orbit misalignment angle, and the system's centre-of-mass velocity.
Combined with the photometric rotation period, the results give a geometrical
measure of the stellar radius which agrees closely with values obtained from
high-precision transit photometry if a small amount of differential rotation is
present in the stellar photosphere.Comment: 8 pages, 5 figures, 2 tables; accepted by MNRA
Gaia's potential for the discovery of circumbinary planets
The abundance and properties of planets orbiting binary stars - circumbinary planets - are largely unknown because they are difficult to detect with currently available techniques. Results from the Kepler satellite and other studies indicate a minimum occurrence rate of circumbinary giant planets of ∼10 per cent, yet only a handful are presently known. Here, we study the potential of ESA's Gaia mission to discover and characterize extrasolar planets orbiting nearby binary stars by detecting the binary's periodic astrometric motion caused by the orbiting planet. We expect that Gaia will discover hundreds of giant planets around binaries with FGK-dwarf primaries within 200pc of the Sun, if we assume that the giant planet mass distribution and abundance are similar around binaries and single stars. If on the other hand all circumbinary gas giants have masses lower than two Jupiter masses, we expect only four detections. Gaia is critically sensitive to the properties of giant circumbinary planets and will therefore make the detailed study of their population possible. Gaia's precision is such that the distribution in mutual inclination between the binary and planetary orbital planes will be obtained. It also possesses the capacity to establish the frequency of planets across the Hertzsprung-Russell diagram, both as a function of mass and of stellar evolutionary state from pre-main sequence to stellar remnants. Gaia's discoveries can reveal whether a second epoch of planetary formation occurs after the red giant phas
Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman- line
The TRAPPIST-1 system offers the opportunity to characterize terrestrial,
potentially habitable planets orbiting a nearby ultracool dwarf star. We
performed a four-orbit reconnaissance with the Space Telescope Imaging
Spectrograph onboard the Hubble Space Telescope to study the stellar emission
at Lyman-, to assess the presence of hydrogen exospheres around the two
inner planets, and to determine their UV irradiation. We detect the
Lyman- line of TRAPPIST-1, making it the coldest exoplanet host star
for which this line has been measured. We reconstruct the intrinsic line
profile, showing that it lacks broad wings and is much fainter than expected
from the stellar X-ray emission. TRAPPIST-1 has a similar X-ray emission as
Proxima Cen but a much lower Ly- emission. This suggests that
TRAPPIST-1 chromosphere is only moderately active compared to its transition
region and corona. We estimated the atmospheric mass loss rates for all
planets, and found that despite a moderate extreme UV emission the total XUV
irradiation could be strong enough to strip the atmospheres of the inner
planets in a few billions years. We detect marginal flux decreases at the times
of TRAPPIST-1b and c transits, which might originate from stellar activity, but
could also hint at the presence of extended hydrogen exospheres. Understanding
the origin of these Lyman- variations will be crucial in assessing the
atmospheric stability and potential habitability of the TRAPPIST-1 planets.Comment: Published in A&A as a Letter to the Edito
New evidence about HW Vir's circumbinary planets from Hipparcos-Gaia astrometry and a reanalysis of the eclipse timing variations using nested sampling
The post common-envelope eclipsing binary HW Virginis has had many
circumbinary companions proposed based on eclipse timing variations. Each
proposed solution has lacked in predictability and orbital stability, leaving
the origin of the eclipse timing variations an active area of research.
Leveraging the catalogue of \textit{Hipparcos} and \textit{Gaia} proper motion
anomalies, we show there is slight evidence for a circumbinary companion
orbiting HW Vir. We place an upper limit in mass for such a companion which
excludes some previously claimed companions. We also apply this method to V471
Tauri and confirm the non-detection of a previously claimed brown dwarf. We
adapt the {\tt kima} nested sampling code to analyse eclipse timing variations
and re-analyse archival data on HW Vir, varying the order of the ephemeris that
we fit for and the amount of the data that we use. Although signals are clearly
present, we find two signals around 2500 and 4000 day periods that are not
coherent between different \textit{chunks} of the data, so are likely to not be
of planetary origin. We analyse the whole dataset and find the best solution to
contain four signals. Of these four we argue the outermost is the most
compatible with astrometry and thus the most likely to be of planetary nature.
We posit the other three pseudo-periodic signals are caused by physical
processes on the white dwarf. The eventual release of the full \textit{Gaia}
epoch astrometry is a promising way to confirm whether circumbinary planets
exist around HW Vir (and other similar systems), and explore white dwarf
physics.Comment: 10 pages, 5 figures, 5 tables, published in Monthly Notices of the
Royal Astronomical Societ
Dusty circumbinary discs: inner cavity structures and stopping locations of migrating planets
We present the results of two-fluid hydrodynamical simulations of
circumbinary discs consisting of gas and dust, with and without embedded
planets, to examine the influence of the dust on the structure of the tidally
truncated inner cavity and on the parking locations of migrating planets. In
this proof-of-concept study, we consider Kepler-16 and -34 analogues, and
examine dust fluids with Stokes numbers in the range and dust-to-gas ratios of 0.01 and 1. For the canonical dust-to-gas
ratio of 0.01, we find the inclusion of the dust has only a minor effect on the
cavity and stopping locations of embedded planets compared to dust-free
simulations. However, for the enhanced dust-to-gas ratio of unity, assumed to
arise because of significant dust drift and accumulation, we find that the dust
can have a dramatic effect by shrinking and circularising the inner cavity,
which brings the parking locations of planets closer to the central binary.
This work demonstrates the importance of considering both gas and dust in
studies of circumbinary discs and planets, and provides a potential means of
explaining the orbital properties of circumbinary planets such as Kepler-34b,
which have hitherto been difficult to explain using gas-only hydrodynamical
simulations.Comment: Accepted for publication in MNRAS, 20 pages, 17 figure
Dust accretion in binary systems: implications for planets and transition discs
The presence of planets in binary systems poses interesting problems for planet formation theories, both in cases where planets must have formed in very compact discs around the individual stars and where they are located near the edge of the stable circumbinary region, where in situ formation is challenging. Dust dynamics is expected to play an important role in such systems, since dust trapping at the inner edge of circumbinary discs could aid in situ formation, but would simultaneously starve the circumstellar discs of the solid material needed to form planets. Here we investigate the dynamics of dust in binary systems using smoothed particle hydrodynamics. We find that all our simulations tend towards dust trapping in the circumbinary disc, but the time-scale on which trapping begins depends on binary mass ratio (q) and eccentricity as well as the angular momentum of the infalling material. For q ≳ 0.1, we find that dust can initially accrete on to the circumstellar discs, but as the circumbinary cavity grows in radius, dust eventually becomes trapped in the circumbinary disc. For q = 0.01, we find that increasing the binary eccentricity increases the time required for dust trapping to begin. However, even this longer time-scale is likely to be shorter than the planet formation time-scale in the inner disc and is insufficient to explain the observed pre-transitional discs. This indicates that increase in companion eccentricity alone is not enough to allow significant transfer of solids from the outer to the inner disc
- …