11 research outputs found
An improved model of the Edgeworth-Kuiper debris disk
(Abridged) We access the expected EKB dust disk properties by modeling. We
treat the debiased population of the known transneptunian objects (TNOs) as
parent bodies and generate the dust with our collisional code. The resulting
dust distributions are modified to take into account the influence of
gravitational scattering and resonance trapping by planets on migrating dust
grains as well as the effect of sublimation. A difficulty is that the amount
and distribution of dust are largely determined by sub-kilometer-sized bodies.
These are directly unobservable, and their properties cannot be accessed by
collisional modeling, because objects larger than 10...60m in the present-day
EKB are not in a collisional equilibrium. To place additional constraints, we
use in-situ measurements of the New Horizons spacecraft within 20AU. We show
that the TNO population has to have a break in the size distribution at s<70km.
However, even this still leaves us with several models that all correctly
reproduce a nearly constant dust impact rates in the region of giant planet
orbits and do not violate the constraints from the non-detection of the EKB
dust thermal emission by the COBE spacecraft. The modeled EKB dust disks, which
conform to the observational constraints, can either be transport-dominated or
intermediate between the transport-dominated and collision-dominated regime.
The in-plane optical depth of such disks is tau(r>10AU)~10^-6 and their
fractional luminosity is f_d~10^-7. Planets and sublimation are found to have
little effect on dust impact fluxes and dust thermal emission. The spectral
energy distribution of an EKB analog, as would be seen from 10pc distance,
peaks at wavelengths of 40...50\mum at F~0.5mJy, which is less than 1% of the
photospheric flux at those wavelengths. Therefore, exact EKB analogs cannot be
detected with present-day instruments such as Herschel/PACS.Comment: 10 pages, 8 figures, accepted for publication in Astronomy and
Astophysic
The Edgeworth-Kuiper debris disk
(Abridged) The Edgeworth-Kuiper belt with its presumed dusty debris is a
natural reference for extrsolar debris disks. We employ a new algorithm to
eliminate the inclination and the distance selection effects in the known TNO
populations to derive expected parameters of the "true" EKB. Its estimated mass
is M_EKB=0.12 M_earth, which is by a factor of \sim 15 larger than the mass of
the EKB objects detected so far. About a half of the total EKB mass is in
classical and resonant objects and another half is in scattered ones. Treating
the debiased populations of EKB objects as dust parent bodies, we then
"generate" their dust disk with our collisional code. Apart from accurate
handling of collisions and direct radiation pressure, we include the
Poynting-Robertson (P-R) drag, which cannot be ignored for the EKB dust disk.
Outside the classical EKB, the radial profile of the optical depth
approximately follows tau \sim r^-2 which is roughly intermediate between the
slope predicted analytically for collision-dominated (r^-1.5) and
transport-dominated (r^-2.5) disks. The cross section-dominating grain size
still lies just above the blowout size (\sim 1...2 \microm), as it would
without the P-R transport. However, if the EKB were by one order of magnitude
less massive, the optical depth profile would fall off as tau \sim r^-3, and
the cross section-dominating grain size would shift from \sim 1...2\microm to
~100 \microm. These properties are seen if dust is assumed to be generated only
by known TNOs. If the solar system were observed from outside, the thermal
emission flux from the EKB dust would be about two orders of magnitude lower
than for solar-type stars with the brightest known infrared excesses observed
from the same distance. Herschel and other new-generation facilities should
reveal extrasolar debris disks nearly as tenuous as the EKB disk. The
Herschel/PACS instrument should be able to detect disks at a \sim 1...2M_EKB
level.Comment: 18 pages, 14 figures, accepted for publication in A&
A spectroscopic study of the plasma generated in a thallium arc. Transition probabilities for several lines of Tl I
The optical emission spectra (2000–15 000) A of a plasma produced an a Tl arc lamp have been recorded and analysed; using the series nd 2D3/2`5/2 → 6p 2P Q3/2 and ns 2SL/2 → 6p 2P Q3/2 we have obtained that the electron density is of the order of 10L4 cm−3 and the excitation temperature is (2880 ± 50) K. Relative transition probabilities for 26 lines from excited levels near the ionization limit of Tl I have been determined from line intensities
Debris disk size distributions: steady state collisional evolution with P-R drag and other loss processes
We present a new scheme for determining the shape of the size distribution,
and its evolution, for collisional cascades of planetesimals undergoing
destructive collisions and loss processes like Poynting-Robertson drag. The
scheme treats the steady state portion of the cascade by equating mass loss and
gain in each size bin; the smallest particles are expected to reach steady
state on their collision timescale, while larger particles retain their
primordial distribution. For collision-dominated disks, steady state means that
mass loss rates in logarithmic size bins are independent of size. This
prescription reproduces the expected two phase size distribution, with ripples
above the blow-out size, and above the transition to gravity-dominated
planetesimal strength. The scheme also reproduces the expected evolution of
disk mass, and of dust mass, but is computationally much faster than evolving
distributions forward in time. For low-mass disks, P-R drag causes a turnover
at small sizes to a size distribution that is set by the redistribution
function (the mass distribution of fragments produced in collisions). Thus
information about the redistribution function may be recovered by measuring the
size distribution of particles undergoing loss by P-R drag, such as that traced
by particles accreted onto Earth. Although cross-sectional area drops with
1/age^2 in the PR-dominated regime, dust mass falls as 1/age^2.8, underlining
the importance of understanding which particle sizes contribute to an
observation when considering how disk detectability evolves. Other loss
processes are readily incorporated; we also discuss generalised power law loss
rates, dynamical depletion, realistic radiation forces and stellar wind drag.Comment: Accepted for publication by Celestial Mechanics and Dynamical
Astronomy (special issue on EXOPLANETS
Dusty Planetary Systems
Extensive photometric stellar surveys show that many main sequence stars show
emission at infrared and longer wavelengths that is in excess of the stellar
photosphere; this emission is thought to arise from circumstellar dust. The
presence of dust disks is confirmed by spatially resolved imaging at infrared
to millimeter wavelengths (tracing the dust thermal emission), and at optical
to near infrared wavelengths (tracing the dust scattered light). Because the
expected lifetime of these dust particles is much shorter than the age of the
stars (>10 Myr), it is inferred that this solid material not primordial, i.e.
the remaining from the placental cloud of gas and dust where the star was born,
but instead is replenished by dust-producing planetesimals. These planetesimals
are analogous to the asteroids, comets and Kuiper Belt objects (KBOs) in our
Solar system that produce the interplanetary dust that gives rise to the
zodiacal light (tracing the inner component of the Solar system debris disk).
The presence of these "debris disks" around stars with a wide range of masses,
luminosities, and metallicities, with and without binary companions, is
evidence that planetesimal formation is a robust process that can take place
under a wide range of conditions. This chapter is divided in two parts. Part I
discusses how the study of the Solar system debris disk and the study of debris
disks around other stars can help us learn about the formation, evolution and
diversity of planetary systems by shedding light on the frequency and timing of
planetesimal formation, the location and physical properties of the
planetesimals, the presence of long-period planets, and the dynamical and
collisional evolution of the system. Part II reviews the physical processes
that affect dust particles in the gas-free environment of a debris disk and
their effect on the dust particle size and spatial distribution.Comment: 68 pages, 25 figures. To be published in "Solar and Planetary
Systems" (P. Kalas and L. French, Eds.), Volume 3 of the series "Planets,
Stars and Stellar Systems" (T.D. Oswalt, Editor-in-chief), Springer 201
Herschel's "Cold Debris Disks": Background Galaxies or Quiescent Rims of Planetary Systems?
(abridged) Infrared excesses associated with debris disk host stars detected so far peak at wavelengths around ~100{\mu}m or shorter. However, six out of 31 excess sources in the Herschel OTKP DUNES have been seen to show significant - and in some cases extended - excess emission at 160{\mu}m, which is larger than the 100{\mu}m excess. This excess emission has been suggested to stem from debris disks colder than those known previously. Using several methods, we re-consider whether some or even all of the candidates may be associated with unrelated galactic or extragalactic emission and conclude that it is highly unlikely that none of the candidates represents a true circumstellar disk. For true disks, both the dust temperatures inferred from the SEDs and the disk radii estimated from the images suggest that the dust is nearly as cold as a blackbody. This requires the grains to be larger than ~100{\mu}m, regardless of their material composition. To explain the dearth of small grains, we explore several conceivable scenarios: transport-dominated disks, disks of low dynamical excitation, and disks of unstirred primordial macroscopic grains. Our qualitative analysis and collisional simulations rule out the first two of these scenarios, but show the feasibility of the third one. We show that such disks can survive for gigayears, largely preserving the primordial size distribution. They should be composed of macroscopic solids larger than millimeters, but smaller than kilometers in size. Thus planetesimal formation, at least in the outer regions of the systems, has stopped before "cometary" or "asteroidal" sizes were reached
Herschel discovery of a new class of cold, faint debris discs
We present Herschel PACS 100 and 160 micron observations of the solar-type
stars alpha Men, HD 88230 and HD 210277, which form part of the FGK stars
sample of the Herschel Open Time Key Programme (OTKP) DUNES (DUst around NEarby
S tars). Our observations show small infrared excesses at 160 micron for all
three stars. HD 210277 also shows a small excess at 100 micron, while the 100
micron fluxes of alpha Men and HD 88230 agree with the stellar photospheric
predictions. We attribute these infrared excesses to a new class of cold, faint
debris discs. alpha Men and HD 88230 are spatially resolved in the PACS 160
micron images, while HD 210277 is point-like at that wavelength. The projected
linear sizes of the extended emission lie in the range from ~ 115 to ~ 250 AU.
The estimated black body temperatures from the 100 and 160 micron fluxes are
22 K, while the fractional luminosity of the cold dust is
Ldust/Lstar ~ 10E-6, close to the luminosity of the Solar-System's Kuiper belt.
These debris discs are the coldest and faintest discs discovered so far around
mature stars and cannot easily be explained by invoking "classical" debris disc
models.Comment: Accepted by Astronomy and Astrophysics Letter
DUst around NEarby Stars. The survey observational results
<p>Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system counterparts are the asteroid and Edgeworth-Kuiper belts.</p><p>Aims. The DUNES survey aims at detecting extra-solar analogues to the Edgeworth-Kuiper belt around solar-type stars, putting in this way the solar system into context. The survey allows us to address some questions related to the prevalence and properties of planetesimal systems.</p><p>Methods. We used Herschel/PACS to observe a sample of nearby FGK stars. Data at 100 and 160 mu m were obtained, complemented in some cases with observations at 70 mu m, and at 250, 350 and 500 mu m using SPIRE. The observing strategy was to integrate as deep as possible at 100 mu m to detect the stellar photosphere.</p><p>Results. Debris discs have been detected at a fractional luminosity level down to several times that of the Edgeworth-Kuiper belt. The incidence rate of discs around the DUNES stars is increased from a rate of similar to 12.1% +/- 5% before Herschel to similar to 20.2% +/- 2%. A significant fraction (similar to 52%) of the discs are resolved, which represents an enormous step ahead from the previously known resolved discs. Some stars are associated with faint far-IR excesses attributed to a new class of cold discs. Although it cannot be excluded that these excesses are produced by coincidental alignment of background galaxies, statistical arguments suggest that at least some of them are true debris discs. Some discs display peculiar SEDs with spectral indexes in the 70-160 mu m range steeper than the Rayleigh-Jeans one. An analysis of the debris disc parameters suggests that a decrease might exist of the mean black body radius from the F-type to the K-type stars. In addition, a weak trend is suggested for a correlation of disc sizes and an anticorrelation of disc temperatures with the stellar age.</p>