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Bulk Properties and Evolution of Jupiter-Family Comet Nuclei
This thesis presents the results from photometric time-series observations of Jupiter-family comets (JFCs). A method was developed that allows mutual absolute photometric calibration of data taken at different epochs with different instruments and results in absolute-calibration uncertainty of âŒ0.02 mag. The method was applied to various datasets with the goal of studying the rotation rates and surface properties of ten comets.Previously published properties of JFCs were collected and complemented by new results. The resulting comprehensive sample was used to study the ensemble properties of JFC nuclei. It confirmed the cut-off in bulk density at âŒ0.6 g cmâ3 and provided evidence for a lower limit on the bulk tensile strength of 10-25 Pa.New lightcurves of three JFCs were used to look for spin changes over their last orbits. None of the observed comets had detectable period changes, and strict conservative upper limits were set. Comparing these results with all eight other JFCs with measured rotational changes suggests that large JFCs are less likely to undergo rotationally-driven mass-loss, and are therefore more likely to survive more perihelion passages than smaller nuclei. This conclusion is supported by evidence from the cumulative size distributions of JFCs and dormant comets, as well as from recent dynamical simulations.This work almost doubled the sample of JFCs with both albedos and phase-function slopes known. The extended sample shows a possible correlation of increasing phase-function slopes for larger geometric albedos, which can be interpreted as an evolutionary trend for JFCs. According to this hypothesis, newly activated JFCs have higher albedos and steeper phase functions, which gradually decrease due to sublimation-driven erosion. If confirmed, this correlation could be used to analyse surface erosion from the ground and to distinguish between dormant comets and asteroids
Viscous Kelvin-Helmholtz instabilities in highly ionised plasmas
Transport coefficients in highly ionised plasmas like the intra-cluster
medium (ICM) are still ill-constrained. They influence various processes, among
them the mixing at shear flow interfaces due to the Kelvin-Helmholtz
instability (KHI). The observed structure of potential mixing layers can be
used to infer the transport coefficients, but the data interpretation requires
a detailed knowledge of the long-term evolution of the KHI under different
conditions. Here we present the first systematic numerical study of the effect
of constant and temperature-dependent isotropic viscosity over the full range
of possible values. We show that moderate viscosities slow down the growth of
the KHI and reduce the height of the KHI rolls and their rolling-up.
Viscosities above a critical value suppress the KHI. The effect can be
quantified in terms of the Reynolds number Re = U{\lambda}/{\nu}, where U is
the shear velocity, {\lambda} the perturbation length, and {\nu} the kinematic
viscosity. We derive the critical Re for constant and temperature dependent,
Spitzer-like viscosities, an empirical relation for the viscous KHI growth time
as a function of Re and density contrast, and describe special behaviours for
Spitzer-like viscosities and high density contrasts. Finally, we briefly
discuss several astrophysical situations where the viscous KHI could play a
role, i.e., sloshing cold fronts, gas stripping from galaxies, buoyant
cavities, ICM turbulence, and high velocity clouds.Comment: Accepted by MNRAS. 22 pages, 21 figure
Stripped elliptical galaxies as probes of ICM physics: II. Stirred, but mixed? Viscous and inviscid gas stripping of the Virgo elliptical M89
Elliptical galaxies moving through the intra-cluster medium (ICM) are
progressively stripped of their gaseous atmospheres. X-ray observations reveal
the structure of galactic tails, wakes, and the interface between the galactic
gas and the ICM. This fine-structure depends on dynamic conditions (galaxy
potential, initial gas contents, orbit in the host cluster), orbital stage
(early infall, pre-/post-pericenter passage), as well as on the still
ill-constrained ICM plasma properties (thermal conductivity, viscosity,
magnetic field structure). Paper I describes flow patterns and stages of
inviscid gas stripping. Here we study the effect of a Spitzer-like temperature
dependent viscosity corresponding to Reynolds numbers, Re, of 50 to 5000 with
respect to the ICM flow around the remnant atmosphere. Global flow patterns are
independent of viscosity in this Reynolds number range. Viscosity influences
two aspects: In inviscid stripping, Kelvin-Helmholtz instabilities (KHIs) at
the sides of the remnant atmosphere lead to observable horns or wings.
Increasing viscosity suppresses KHIs of increasing length scale, and thus
observable horns and wings. Furthermore, in inviscid stripping, stripped
galactic gas can mix with the ambient ICM in the galaxy's wake. This mixing is
suppressed increasingly with increasing viscosity, such that viscously stripped
galaxies have long X-ray bright, cool wakes. We provide mock X-ray images for
different stripping stages and conditions. While these qualitative results are
generic, we tailor our simulations to the Virgo galaxy M89 (NGC 4552), where
Re~ 50 corresponds to a viscosity of 10% of the Spitzer level. Paper III
compares new deep Chandra and archival XMM-Newton data to our simulations.Comment: ApJ in press. 16 pages, 16 figures. Text clarified, conclusions
unchange
Stripped elliptical galaxies as probes of ICM physics: I. Tails, wakes, and flow patterns in and around stripped ellipticals
Elliptical cluster galaxies are progressively stripped of their atmospheres
due to their motion through the intra-cluster medium (ICM). Deep X-ray
observations reveal the fine-structure of the galaxy's remnant atmosphere and
its gas tail and wake. This fine-structure depends on dynamic conditions
(galaxy potential, initial gas contents, orbit through the host cluster),
orbital stage (early infall, pre-/post-pericenter passage), and ICM plasma
properties (thermal conductivity, viscosity, magnetic field structure). We aim
to disentangle dynamic and plasma effects in order to use stripped ellipticals
as probes of ICM plasma properties. This first paper of a series investigates
the hydrodynamics of progressive gas stripping by means of inviscid
hydrodynamical simulations. We distinguish a long-lasting initial relaxation
phase and a quasi-steady stripping phase. During quasi-steady stripping, the
ICM flow around the remnant atmosphere resembles the flow around solid bodies,
including a `deadwater' region in the near wake. Gas is stripped from the
remnant atmosphere predominantly at its sides via Kelvin-Helmholtz
instabilities. The downstream atmosphere is largely shielded from the ICM wind
and thus shaped into a tail. Observationally, both, this `remnant tail' and the
stripped gas in the wake can appear as a `tail', but only in the wake can
galactic gas mix with the ambient ICM. While the qualitative results are
generic, the simulations presented here are tailored to the Virgo elliptical
galaxy M89 (NGC 4552) for the most direct comparison to observations. Papers II
and III of this series describe the effect of viscosity and compare to Chandra
and XMM-Newton observations, respectively.Comment: ApJ, in press. 19 pages, 13 figures. Clarifications added, text
restructured. Conclusions unchange
Properties of the Bare Nucleus of Comet 96P/Machholz 1
We observed comet 96P/Machholz 1 on a total of nine nights before and after perihelion during its 2017/2018 apparition. Both its unusually small perihelion distance and the observed fragmentation during multiple apparitions make 96P an object of great interest. Our observations show no evidence of a detectable dust coma, implying that we are observing a bare nucleus at distances ranging from 2.3 to 3.8 au. Based on this assumption, we calculated its color and found average values of g'âr' = 0.50 ± 0.04, r'âi' = 0.17 ± 0.03, and i'âz' = 0.06 ± 0.04. These are notably more blue than those of the nuclei of other Jupiter-family and long-period comets. Furthermore, assuming a bare nucleus, we found an equivalent nuclear radius of 3.4 ± 0.2 km with an axial ratio of at least 1.6 ± 0.1. The lightcurve clearly displays one large peak, one broad flat peak, and two distinct troughs, with a clear asymmetry that suggests that the shape of the nucleus deviates from that of a simple triaxial ellipsoid. This asymmetry in the lightcurve allowed us to constrain the nuclear rotation period to 4.10 ± 0.03 hr and 4.096 ± 0.002 hr before and after perihelion, respectively. Within the uncertainties, 96P's rotation period does not appear to have changed throughout the apparition, and we conclude a maximum possible change in rotation period of 130 s. The observed properties were compared to those of comet 322P and interstellar object 1I/'Oumuamua in an attempt to study the effects of close perihelion passages on cometary surfaces and their internal structure and the potential interstellar origin of 96P
Rotation of Cometary Nuclei New Lightcurves and an Update of the Ensemble Properties of Jupiter-Family Comets
We report new lightcurves and phase functions for nine Jupiter-family comets
(JFCs). They were observed in the period 2004-2015 with various ground
telescopes as part of the Survey of Ensemble Physical Properties of Cometary
Nuclei (SEPPCoN) as well as during devoted observing campaigns. We add to this
a review of the properties of 35 JFCs with previously published rotation
properties.
The photometric time-series were obtained in Bessel R, Harris R and SDSS r'
filters and were absolutely calibrated using stars from the Pan-STARRS survey.
This specially-developed method allowed us to combine data sets taken at
different epochs and instruments with absolute-calibration uncertainty down to
0.02 mag. We used the resulting time series to improve the rotation periods for
comets 14P/Wolf, 47P/Ashbrook-Jackson, 94P/Russell, and 110P/Hartley 3 and to
determine the rotation rates of comets 93P/Lovas and 162P/Siding-Spring for the
first time. In addition to this, we determined the phase functions for seven of
the examined comets and derived geometric albedos for eight of them.
We confirm the known cut-off in bulk densities at 0.6 g
if JFCs are strengthless. Using the model of Davidsson
(2001) for prolate ellipsoids with typical density and elongations, we conclude
that none of the known JFCs require tensile strength larger than 10-25 Pa to
remain stable against rotational instabilities. We find evidence for an
increasing linear phase function coefficient with increasing geometric albedo.
The median linear phase function coefficient for JFCs is 0.046 mag/deg and the
median geometric albedo is 4.2 per cent.Comment: 38 pages, accepted for publication in MNRA
Implications of the Small Spin Changes Measured for Large Jupiter-Family Comet Nuclei
Rotational spin-up due to outgassing of comet nuclei has been identified as a possible mechanism for considerable mass-loss and splitting. We report a search for spin changes for three large Jupiter-family comets (JFCs): 14P/Wolf, 143P/Kowal-Mrkos, and 162P/Siding Spring. None of the three comets has detectable period changes, and we set conservative upper limits of 4.2 (14P), 6.6 (143P) and 25 (162P) minutes per orbit. Comparing these results with all eight other JFCs with measured rotational changes, we deduce that none of the observed large JFCs experiences significant spin changes. This suggests that large comet nuclei are less likely to undergo rotationally-driven splitting, and therefore more likely to survive more perihelion passages than smaller nuclei. We find supporting evidence for this hypothesis in the cumulative size distributions of JFCs and dormant comets, as well as in recent numerical studies of cometary orbital dynamics. We added 143P to the sample of 13 other JFCs with known albedos and phase-function slopes. This sample shows a possible correlation of increasing phase-function slopes for larger geometric albedos. Partly based on findings from recent space missions to JFCs, we hypothesise that this correlation corresponds to an evolutionary trend for JFCs. We propose that newly activated JFCs have larger albedos and steeper phase functions, which gradually decrease due to sublimation-driven erosion. If confirmed, this could be used to analyse surface erosion from ground and to distinguish between dormant comets and asteroids
A seven square degrees survey for galaxy-scale gravitational lenses with the HST imaging archive
We present the results of a visual search for galaxy-scale gravitational
lenses in nearly 7 square degrees of Hubble Space Telescope (HST) images. The
dataset comprises the whole imaging data ever taken with the Advanced Camera
for Surveys (ACS) in the filter F814W (I-band) up to August 31st, 2011, i.e.
6.03 square degrees excluding the field of the Cosmic Evolution Survey (COSMOS)
which has been the subject of a separate visual search. In addition, we have
searched for lenses in the whole Wide Field Camera 3 (WFC3) near-IR imaging
dataset in all filters (1.01 square degrees) up to the same date. Our primary
goal is to provide a sample of lenses with a broad range of different
morphologies and lens-source brightness contrast in order estimate a lower
limit to the number of galaxy-scale strong lenses in the future Euclid survey
in its VIS band. Our criteria to select lenses are purely morphological as we
do not use any colour or redshift information.The final candidate selection is
very conservative hence leading to a nearly pure but incomplete sample. We find
49 new lens candidates: 40 in the ACS images and 9 in the WFC3 images. Out of
these, 16 candidates are secure lenses owing to their striking morphology, 21
more are very good candidates, and 12 more have morphologies compatible with
gravitational lensing but also compatible with other astrophysical objects. It
is therefore insensitive to cosmic variance and allows to estimate the number
of galaxy-scale strong lenses on the sky for a putative survey depth, which is
the main result of the present work. Because of the incompleteness of the
sample, the estimated lensing rates should be taken as lower limits. Using
these, we anticipate that a 15 000 square degrees space survey such as Euclid
will find at least 60 000 galaxy-scale strong lenses down to a limiting AB
magnitude of I = 24.5 (10-sigma) or I = 25.8 (3-sigma).Comment: 13 pages, 12 figures, Accepted for publication in MNRA
The Thermal, Mechanical, Structural, and Dielectric Properties of Cometary Nuclei After Rosetta
The physical properties of cometary nuclei observed today relate to their complex history and help to constrain their formation and evolution. In this article, we review some of the main physical properties of cometary nuclei and focus in particular on the thermal, mechanical, structural and dielectric properties, emphasising the progress made during the Rosetta mission. Comets have a low density of 480±220 kgmâ3 and a low permittivity of 1.9â2.0, consistent with a high porosity of 70â80%, are weak with a very low global tensile strength â1mâ2sâ1/2 that allowed them to preserve highly volatiles species (e.g. CO, CO2, CH4, N2) into their interior since their formation. As revealed by 67P/Churyumov-Gerasimenko, the above physical properties vary across the nucleus, spatially at its surface but also with depth. The broad picture is that the bulk of the nucleus consists of a weakly bonded, rather homogeneous material that preserved primordial properties under a thin shell of processed material, and possibly covered by a granular material; this cover might in places reach a thickness of several meters. The properties of the top layer (the first meter) are not representative of that of the bulk nucleus. More globally, strong nucleus heterogeneities at a scale of a few meters are ruled out on 67Pâs small lobe
Correction to: The Thermal, Mechanical, Structural, and Dielectric Properties of Cometary Nuclei After Rosetta
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