1,102 research outputs found
Probing exoplanetary materials using sublimating dust
Planetary systems consist of more than just planets orbiting a central star. They also include a wide range of smaller bodies, such as asteroids, comets, and interplanetary dust grains. All these materials can be investigated to increase our understanding of planetary systems. In the study of extrasolar systems, the dust component can be particularly useful, because it constitutes a relatively large cross-section, making it observationally accessible to modern telescopes. To correctly make the step from dust observations to inferring properties of an exoplanetary system, one must understand in detail how dust grains are produced, how they behave after being released, and how they are destroyed or removed. Understanding the physics of circumstellar dust is the subject of my thesis work. The thesis focusses specifically on dust grains orbiting extremely close to their host star, at only a few stellar radii, where temperatures can be reached that are sufficient to sublimate rocks. I studied two astrophysical problems in which dust sublimation is relevant. (1) Hot exozodiacal dust. This is dust located in the close vicinity of a star that can be detected through near-infrared interferometry. Some 10% to 30% of all stars seem to have such a population of dust, but its origin is still unclear. I tested possible mechanisms that could explain the phenomenon, and made an in-depth study of the inner parts of the Fomalhaut system. (2) Dusty tails of evaporating exoplanets. I demonstrated how the composition of dust released by an evaporating rocky exoplanet, which trails the planet in a comet-like tail, can be inferred from detailed shape of the transit light curve produced by these objects
Near-infrared emission from sublimating dust in collisionally active debris disks
Hot exozodiacal dust is thought to be responsible for excess near-infrared
(NIR) emission emanating from the innermost parts of some debris disks. The
origin of this dust, however, is still a matter of debate. We test whether hot
exozodiacal dust can be supplied from an exterior parent belt by
Poynting-Robertson (P-R) drag, paying special attention to the pile-up of dust
that occurs due to the interplay of P-R drag and dust sublimation.
Specifically, we investigate whether pile-ups still occur when collisions are
taken into account, and if they can explain the observed NIR excess. We compute
the steady-state distribution of dust in the inner disk by solving the
continuity equation. First, we derive an analytical solution under a number of
simplifying assumptions. Second, we develop a numerical debris disk model that
for the first time treats the complex interaction of collisions, P-R drag, and
sublimation in a self-consistent way. From the resulting dust distributions we
generate thermal emission spectra and compare these to observed excess NIR
fluxes. We confirm that P-R drag always supplies a small amount of dust to the
sublimation zone, but find that a fully consistent treatment yields a maximum
amount of dust that is about 7 times lower than that given by analytical
estimates. The NIR excess due this material is much smaller (<10^-3 for A-type
stars with parent belts at >1 AU) than the values derived from interferometric
observations (~10^-2). Pile-up of dust still occurs when collisions are
considered, but its effect on the NIR flux is insignificant. Finally, the
cross-section in the innermost regions is clearly dominated by barely bound
grains.Comment: 18 pages, 10 figures, A&A accepte
Chromatic transit light curves of disintegrating rocky planets
Context. Kepler observations have revealed a class of short period
exoplanets, of which Kepler-1520 b is the prototype, which have comet-like dust
tails thought to be the result of small, rocky planets losing mass. The shape
and chromaticity of the transits constrain the properties of the dust particles
originating from the planet's surface, offering a unique opportunity to probe
the composition and geophysics of rocky exoplanets.
Aims. We aim to approximate the average Kepler long-cadence light curve of
Kepler-1520 b and investigate how the optical thickness and transit
cross-section of a general dust tail can affect the observed wavelength
dependence and depth of transit light curves.
Methods. We developed a new 3D model that ejects sublimating particles from
the planet surface to build up a dust tail, assuming it to be optically thin,
and used 3D radiative transfer computations that fully treat scattering using
the distribution of hollow spheres (DHS) method, to generate transit light
curves between 0.45 and 2.5 m.
Results. We show that the transit depth is wavelength independent for
optically thick tails, potentially explaining why only some observations
indicate a wavelength dependence. From the 3D nature of our simulated tails, we
show that their transit cross-sections are related to the component of particle
ejection velocity perpendicular to the planet's orbital plane and use this to
derive a minimum ejection velocity of 1.2 kms. To fit the average
transit depth of Kepler-1520 b of 0.87%, we require a high dust mas-loss rate
of 7 80 M Gyr which implies planet lifetimes that may be
inconsistent with the observed sample. Therefore, these mass-loss rates should
be considered to be upper limits.Comment: 22 pages, 22 figures, accepted for publication in A&
Dusty tails of evaporating exoplanets. I. Constraints on the dust composition
Recently, two exoplanet candidates have been discovered, KIC 12557548b and
KOI-2700b, whose transit profiles show evidence for a comet-like tail of dust
trailing the planet, thought to be fed by the evaporation of the planet's
surface. We aim to put constraints on the composition of the dust ejected by
these objects from the shape of their transit light curves. We derive a
semi-analytical expression for the attenuation of dust cross-section in the
tail, incorporating the sublimation of dust grains as well as their drift away
from the planet. This expression shows that the length of the tail is highly
sensitive to the sublimation properties of the dust material. We compute tail
lengths for several possible dust compositions, and compare these to
observational estimates of the tail lengths of KIC 12557548b and KOI-2700b,
inferred from their light curves. The observed tail lengths are consistent with
dust grains composed of corundum (Al2O3) or iron-rich silicate minerals (e.g.,
fayalite, Fe2SiO4). Pure iron and carbonaceous compositions are disfavoured. In
addition, we estimate dust mass loss rates of 1.7 +/- 0.5 M_earth/Gyr for KIC
12557548b, and > 0.007 M_earth/Gyr (1-sigma lower limit) for KOI-2700b.Comment: 10 pages, 7 figures, A&A accepte
A non-homogeneous Semi-Markov model for Interval Censoring
Previous approaches to modelling interval-censored data have often relied on
assumptions of homogeneity in the sense that the censoring mechanism, the
underlying distribution of occurrence times, or both, are assumed to be
time-invariant. In this work, we introduce a model which allows for
non-homogeneous behaviour in both cases. In particular, we outline a censoring
mechanism based on semi-Markov processes in which interval generation is
assumed to be time-dependent and we propose a Markov point process model for
the underlying occurrence time distribution. We prove the existence of this
process and derive the conditional distribution of the occurrence times given
the intervals. We provide a framework within which the process can be
accurately modelled, and subsequently compare our model to homogeneous
approaches by way of a parametric example.Comment: 21 pages, 4 figure
An Inner Gaseous Disk around the Herbig Be Star MWC 147
We present high-spectral-resolution, optical spectra of the Herbig Be star
MWC 147, in which we spectrally resolve several emission lines, including the
[O I] lines at 6300 and 6363\deg. Their highly symmetric, double-peaked line
profiles indicate that the emission originates in a rotating circumstellar
disk. We deconvolve the Doppler-broadened [O I] emission lines to obtain a
measure of emission as a function of distance from the central star. The
resulting radial surface brightness profiles are in agreement with a disk
structure consisting of a flat, inner, gaseous disk and a flared, outer, dust
disk. The transition between these components at 2 to 3 AU corresponds to the
estimated dust sublimation radius. The width of the double-peaked Mg II line at
4481\deg suggests that the inner disk extends to at least 0.10 AU, close to the
corotation radius.Comment: accepted for ApJ Letters (Oct. 2010
Modelling the KIC8462852 light curves : compatibility of the dips and secular dimming with an exocomet interpretation
This paper shows how the dips and secular dimming in the KIC8462852 light curve can originate in circumstellar material distributed around a single elliptical orbit (e.g. exocomets). The expected thermal emission and wavelength dependent dimming is derived for different orbital parameters and geometries, including dust that is optically thick to stellar radiation, and for a size distribution of dust with realistic optical properties. We first consider dust distributed evenly around the orbit, then show how to derive its uneven distribution from the optical light curve and to predict light curves at different wavelengths. The fractional luminosity of an even distribution is approximately the level of dimming times stellar radius divided by distance from the star at transit. Non-detection of dust thermal emission for KIC8462852 thus provides a lower limit on the transit distance to complement the 0.6 au upper limit imposed by 0.4 d dips. Unless the dust distribution is optically thick, the putative 16 per cent century-long secular dimming must have disappeared before the WISE 12 mum measurement in 2010, and subsequent 4.5 mum observations require transits at >0.05 au. However, self-absorption of thermal emission removes these constraints for opaque dust distributions. The passage of dust clumps through pericentre is predicted to cause infrared brightening lasting tens of days and dimming during transit, such that total flux received decreases at wavelengths <5 mum, but increases to potentially detectable levels at longer wavelengths. We suggest that lower dimming levels than seen for KIC8462852 are more common in the Galactic population and may be detected in future transit surveys
Modelling the KIC8462852 light curves : compatibility of the dips and secular dimming with an exocomet interpretation
This paper shows how the dips and secular dimming in the KIC8462852 light curve can originate in circumstellar material distributed around a single elliptical orbit (e.g. exocomets). The expected thermal emission and wavelength dependent dimming is derived for different orbital parameters and geometries, including dust that is optically thick to stellar radiation, and for a size distribution of dust with realistic optical properties. We first consider dust distributed evenly around the orbit, then show how to derive its uneven distribution from the optical light curve and to predict light curves at different wavelengths. The fractional luminosity of an even distribution is approximately the level of dimming times stellar radius divided by distance from the star at transit. Non-detection of dust thermal emission for KIC8462852 thus provides a lower limit on the transit distance to complement the 0.6 au upper limit imposed by 0.4 d dips. Unless the dust distribution is optically thick, the putative 16 per cent century-long secular dimming must have disappeared before the WISE 12 mum measurement in 2010, and subsequent 4.5 mum observations require transits at >0.05 au. However, self-absorption of thermal emission removes these constraints for opaque dust distributions. The passage of dust clumps through pericentre is predicted to cause infrared brightening lasting tens of days and dimming during transit, such that total flux received decreases at wavelengths <5 mum, but increases to potentially detectable levels at longer wavelengths. We suggest that lower dimming levels than seen for KIC8462852 are more common in the Galactic population and may be detected in future transit surveys
Luminosity- and morphology-dependent clustering of galaxies
How does the clustering of galaxies depend on their inner properties like
morphological type and luminosity? We address this question in the mathematical
framework of marked point processes and clarify the notion of luminosity and
morphological segregation. A number of test quantities such as conditional
mark-weighted two-point correlation functions are introduced. These descriptors
allow for a scale-dependent analysis of luminosity and morphology segregation.
Moreover, they break the degeneracy between an inhomogeneous fractal point set
and actual present luminosity segregation. Using the Southern Sky Redshift
Survey~2 (da Costa et al. 1998, SSRS2) we find both luminosity and
morphological segregation at a high level of significance, confirming claims by
previous works using these data (Benoist et al. 1996, Willmer et al. 1998).
Specifically, the average luminosity and the fluctuations in the luminosity of
pairs of galaxies are enhanced out to separations of 15Mpc/h. On scales smaller
than 3Mpc/h the luminosities on galaxy pairs show a tight correlation. A
comparison with the random-field model indicates that galaxy luminosities
depend on the spatial distribution and galaxy-galaxy interactions. Early-type
galaxies are also more strongly correlated, indicating morphological
segregation. The galaxies in the PSCz catalog (Saunders et al. 2000) do not
show significant luminosity segregation. This again illustrates that mainly
early-type galaxies contribute to luminosity segregation. However, based on
several independent investigations we show that the observed luminosity
segregation can not be explained by the morphology-density relation alone.Comment: aastex, emulateapj5, 20 pages, 13 figures, several clarifying
comments added, ApJ accepte
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