208 research outputs found
XMM-Newton observations of HD189733 during planetary transits
We report on two XMM-Newton observations of the planetary host star HD189733.
The system has a close in planet and it can potentially affect the coronal
structure via interactions with the magnetosphere. We have obtained X-ray
spectra and light curves from EPIC and RGS on board XMM-Newton which we have
analyzed and interpreted. We reduced X-ray data from primary transit and
secondary eclipse occurred in April 17th 2007 and May 18th 2009, respectively.
In the April 2007 observation only variability due to weak flares is
recognized. In 2009 HD189733 exhibited a X-ray flux always larger than in the
2007 observation. The average flux in 2009 was higher than in 2007 observation
by a factor of 45%. During the 2009 secondary eclipse we observed a softening
of the X-ray spectrum significant at level of ~3 sigma. Further, we observed
the most intense flare recorded at either epochs. This flare occurred 3 ks
after the end of the eclipse.The flare decay shows several minor ignitions
perhaps linked to the main event and hinting for secondary loops that emit
triggered by the main loop. Magneto-Hydro-Dynamical (MHD) simulations show that
the magnetic interaction between planet and star enhances the density and the
magnetic field in a region comprised between the planet and the star because of
their relative orbital/rotation motion. X-ray observations and model
predictions are globally found in agreement, despite the quite simple MHD model
and the lack of precise estimate of parameters including the alignment and the
intensity of stellar and planetary magnetic fields. Future observations should
confirm or disprove this hypothesis, by determining whether flares are
systematically recurring in the light curve at the same planetary phase.Comment: Accepted for publication on The Astrophysical Journa
A Search for EUV Emission from Comets with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS)
We have obtained EUV spectra between 90 and 255 \AA of the cometsC/2002 T7
(LINEAR), C/2001 Q4 (NEAT), and C/2004 Q2 (Machholz) near their perihelion
passages in 2004 with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS).
We obtained contemporaneous data on Comet NEAT Q4 with the X-ray
Observatory ACIS instrument, marking the first simultaneous EUV and X-ray
spectral observations of a comet. The total CHIPS/EUV observing times were 337
ks for Q4, 234 ks for T7, and 483 ks for Machholz and for both CHIPS and
we calculate we have captured all the comet flux in the instrument
field of view. We set upper limits on solar wind charge exchange emission lines
of O, C, N, Ne and Fe occurring in the spectral bandpass of CHIPS. The spectrum
of Q4 obtained with can be reproduced by modeling emission lines of
C, N O, Mg, Fe, Si, S, and Ne solar wind ions. The measured X-ray emission line
intensities are consistent with our predictions from a solar wind charge
exchange model. The model predictions for the EUV emission line intensities are
determined from the intensity ratios of the cascading X-ray and EUV photons
arising in the charge exchange processes. They are compatible with the measured
limits on the intensities of the EUV lines. For comet Q4, we measured a total
X-ray flux of 3.7 ergs cm s, and derive from
model predictions a total EUV flux of 1.5 erg cm
s. The CHIPS observations occurred predominantly while the satellite was
on the dayside of Earth. For much of the observing time, CHIPS performed
observations at smaller solar angles than it was designed for and EUV emission
from the Sun scattered into the instrument limited the sensitivity of the EUV
measurements.Comment: 28 pages total, 4 tables, 7 figures. Accepted by The Astrophysical
Journa
Study of the April 20, 2007 CME-Comet Interaction Event with an MHD Model
This study examines the tail disconnection event on April 20, 2007 on comet
2P/Encke, caused by a coronal mass ejection (CME) at a heliocentric distance of
0.34 AU. During their interaction, both the CME and the comet are visible with
high temporal and spatial resolution by the STEREO-A spacecraft. Previously,
only current sheets or shocks have been accepted as possible reasons for comet
tail disconnections, so it is puzzling that the CME caused this event. The MHD
simulation presented in this work reproduces the interaction process and
demonstrates how the CME triggered a tail disconnection in the April 20 event.
It is found that the CME disturbs the comet with a combination of a
sudden rotation of the interplanetary magnetic field (IMF), followed by a
gradual rotation. Such an interpretation applies our understanding
of solar wind-comet interactions to determine the \textit{in situ} IMF
orientation of the CME encountering Encke.Comment: 13 pages, 3 figures, accepted by the ApJ Letter
Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ~12 Myr HD172555 System
The fine dust detected by IR emission around the nearby Beta Pic analogue
star HD172555 is very peculiar. The dust mineralogy is composed primarily of
highly refractory, non-equilibrium materials, with approximately three-quarters
of the Si atoms in silica (SiO2) species. Tektite and obsidian lab thermal
emission spectra (non-equilibrium glassy silicas found in impact and magmatic
systems) are required to fit the data. The best-fit model size distribution for
the observed fine dust is dn/da = a-3.95 +/- 0.10. This steep a size
distribution, with abundant micron-sized particles, argues for a fresh source
of material within the last 0.1 Myr. The location of the dust with respect to
the star is at 5.8 +/- 0.6 AU (equivalent to 1.9 +/- 0.2 AU from the Sun),
within the terrestrial planet formation region but at the outer edge of any
possible terrestrial habitability zone. The mass of fine dust is 4 x 10^19 - 2
x 10^20 kg, equivalent to a 150 - 200 km radius asteroid. Significant emission
features centered at 4 and 8 um due to fluorescing SiO gas are also found.
Roughly 10^22 kg of SiO gas, formed by vaporizing silicate rock, is also
present in the system, and a separate population of very large, cool grains,
massing 10^21 - 10^22 kg and equivalent to the largest sized asteroid currently
found in the Solar System's main asteroid belt, dominates the solid
circumstellar material by mass. The makeup of the observed dust and gas, and
the noted lack of a dense circumstellar gas disk, strong primary x-ray
activity, or an extended disk of Beta-meteroids argues that the source of the
observed circumstellar materials is a giant hypervelocity (> 10 km sec^-1)
impact between large rocky planetesimals, similar to the ones which formed the
Moon and which stripped the surface crustal material off of Mercury's surface.Comment: 48 Pages, 8 Figures, 4 Tables Accepted for Publication in the
Astrophysical Journal 13-Jun-2009 References, Figures Updated 16-Jun-200
Cometary charge exchange diagnostics in UV and Xâray
Since the initial discovery of cometary charge exchange emission, more than 20 comets have been observed with a variety of Xâray and UV observatories. This observational sample offers a broad variety of comets, solar wind environments and observational conditions. It clearly demonstrates that solar wind charge exchange emission provides a wealth of diagnostics, which are visible as spatial, temporal, and spectral emission features. We review the possibilities and limitations of each of those in this contribution (© 2012 WILEYâVCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/91138/1/335_ftp.pd
Mid-infrared spectra of the shocked Murchison CM chondrite: comparison with astronomical observations of dust in debris disks
We present laboratory mid-infrared transmission/absorption spectra obtained from matrix of the hydrated Murchison CM meteorite experimentally shocked at peak pressures of 10-49 GPa, and compare them to astronomical observations of circumstellar dust in different stages of the formation of planetary systems. The laboratory spectra of the Murchison samples exhibit characteristic changes in the infrared features. A weakly shocked sample (shocked at 10 GPa) shows almost no changes from the unshocked sample dominated by hydrous silicate (serpentine). Moderately shocked samples (21-34 GPa) have typical serpentine features gradually replaced by bands of amorphous material and olivine with increasing shock pressure. A strongly shocked sample (36 GPa) shows major changes due to decomposition of the serpentine and due to devolatilization. A shock melted sample (49 GPa) shows features of olivine recrystallized from melted material. The infrared spectra of the shocked Murchison samples show similarities to astronomical spectra of dust in various young stellar objects and debris disks. The spectra of highly shocked Murchison samples (36 and 49 GPa) are similar to those of dust in the debris disks of HD113766 and HD69830, and the transitional disk of HD100546. The moderately shocked samples (21-34 GPa) exhibit spectra similar to those of dust in the debris disks of Beta Pictoris and BD+20307, and the transitional disk of GM Aur. An average of the spectra of all Murchison samples (0-49 GPa) has a similarity to the spectrum of the older proto-planetary disk of SU Auriga. In the gas-rich transitional and protoplanetary disks, the abundances of amorphous silicates and gases have widely been considered to be a primary property. However, our study suggests that impact processing may play a significant role in generating secondary amorphous silicates and gases in those disks. Infrared spectra of the shocked Murchison samples also show similarities to the dust fromcomets (C/2002 V1, C/2001 RX14, 9P/Tempel 1, and Hale Bopp), suggesting that the comets also contain shocked Murchison-like material
Spitzer observations of the asteroid-comet transition object and potential spacecraft target 107P (4015) Wilson-Harrington
Context. Near-Earth asteroid-comet transition object 107P/ (4015)
Wilson-Harrington is a possible target of the joint European Space Agency (ESA)
and Japanese Aerospace Exploration Agency (JAXA) Marco Polo sample return
mission. Physical studies of this object are relevant to this mission, and also
to understanding its asteroidal or cometary nature. Aims. Our aim is to obtain
significant new constraints on the surface thermal properties of this object.
Methods. We present mid-infrared photometry in two filters (16 and 22 microns)
obtained with NASA's Spitzer Space Telescope on February 12, 2007, and results
from the application of the Near Earth Asteroid Thermal Model (NEATM).We
obtained high S/N in two mid-IR bands allowing accurate measurements of its
thermal emission. Results. We obtain a well constrained beaming parameter (eta
= 1.39 +/- 0.26) and obtain a diameter and geometric albedo of D = 3.46 +/-
0.32 km, and pV = 0.059 +/- 0.011. We also obtain similar results when we apply
this best-fitting thermal model to single-band mid-IR photometry reported by
Campins et al. (1995), Kraemer et al. (2005) and Reach et al. (2007).
Conclusions. The albedo of 4015 Wilson-Harrington is low, consistent with those
of comet nuclei and primitive C-, P-, D-type asteorids. We establish a rough
lower limit for the thermal inertia of W-H of 60 Jm^-2s^(-0.5)K^-1 when it is
at r=1AU, which is slightly over the limit of 30 Jm^-2s^(-0.5)K-1 derived by
Groussin et al. (2009) for the thermal inertia of the nucleus of comet
22P/Kopff.Comment: 4 pages, 1 figure and 3 tables. Paper accepted for publicatio
Sub-mm observations and modelling of Vega type stars
We present new sub-mm observations and modelling of Vega excess stars, using
realistic dust grain models. For resolved disks, we find that different objects
require very different dust grain properties in order to simultaneously fit the
image data and SED. Fomalhaut and Vega require solid dust grains, whilst HR4796
and HD141569 can only be fitted using porous grains. The older stars tend to
have less porous grains than younger stars, which may indicate that collisions
have compacted the dust grains. Eps Eri appears to be deficient in small dust
grains compared to our best fitting model. This may be due to factors which
affect the size distribution of grains close to the radiation pressure blowout
limit. Alternatively, this discrepancy may be due to some external influence on
the disk (e.g. a planet). When the model is applied to unresolved targets, an
estimate of the disk size can be made. However, the large diversity in dust
composition for the resolved disks means that we cannot make a reliable
assumption as to the composition of the grains in an unresolved disk, and there
is corresponding uncertainty in the disk size. In addition, the poor fit for
Eps Eri shows that the model cannot always account for the SED even if the disk
size is known. These two factors mean that it may not be possible to determine
a disk's size without actually resolving it.Comment: 15 pages, 15 figures, accepted by MNRAS. Revised Eps Eri modelling to
show larger range of minimum size cutoffs with porous grains, Figure
Constraints on the circumstellar dust around KIC 8462852
© The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.We present millimetre (SMA) and sub-millimetre (SCUBA-2) continuum observations of the peculiar star KIC 8462852 which displayed several deep and aperiodic dips in brightness during the Kepler mission. Our observations are approximately confusion-limited at 850 m and are the deepest millimetre and sub-millimetre photometry of the star that has yet been carried out. No significant emission is detected towards KIC 8462852. We determine upper limits for dust between a few 10 M and 10 M for regions identified as the most likely to host occluding dust clumps and a total overall dust budget of 7.7 M within a radius of 200 AU. Such low limits for the inner system make the catastrophic planetary disruption hypothesis unlikely. Integrating over the Kepler lightcurve we determine that at least 10 M of dust is required to cause the observed Q16 dip. This is consistent with the currently most favoured cometary breakup hypothesis, but nevertheless implies the complete breakup of 30 Comet 1/P Halley type objects. Finally, in the wide SCUBA-2 field-of-view we identify another candidate debris disc system that is potentially the largest yet discovered.Peer reviewedFinal Published versio
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