577 research outputs found
Chandra's Close Encounter with the Disintegrating Comets 73P/2006 (Schwassmann--Wachmann--3) Fragment B and C/1999 S4 (LINEAR)
On May 23, 2006 we used the ACIS-S instrument on the Chandra X-ray
Observatory (CXO) to study the X-ray emission from the B fragment of comet
73P/2006 (Schwassmann-Wachmann 3) (73P/B). We obtained a total of 20 ks of CXO
observation time of Fragment B, and also investigated contemporaneous ACE and
SOHO solar wind physical data. The CXO data allow us to spatially resolve the
detailed structure of the interaction zone between the solar wind and the
fragment's coma at a resolution of ~ 1,000 km, and to observe the X-ray
emission due to multiple comet--like bodies. We detect a change in the spectral
signature with the ratio of the CV/OVII line increasing with increasing
collisional opacity as predicted by Bodewits \e (2007). The line fluxes arise
from a combination of solar wind speed, the species that populate the wind and
the gas density of the comet. We are able to understand some of the observed
X-ray morphology in terms of non-gravitational forces that act upon an actively
outgassing comet's debris field. We have used the results of the Chandra
observations on the highly fragmented 73P/B debris field to re-analyze and
interpret the mysterious emission seen from comet C/1999 S4 (LINEAR) on August
1st, 2000, after the comet had completely disrupted. We find the physical
situations to be similar in both cases, with extended X-ray emission due to
multiple, small outgassing bodies in the field of view. Nevertheless, the two
comets interacted with completely different solar winds, resulting in
distinctly different spectra.Comment: accepted by ApJ, 44 Pages, including 4 tables and 14 figure
The ion-induced charge-exchange X-ray emission of the Jovian Auroras: Magnetospheric or solar wind origin?
A new and more comprehensive model of charge-exchange induced X-ray emission,
due to ions precipitating into the Jovian atmosphere near the poles, has been
used to analyze spectral observations made by the Chandra X-ray Observatory.
The model includes for the first time carbon ions, in addition to the oxygen
and sulfur ions previously considered, in order to account for possible ion
origins from both the solar wind and the Jovian magnetosphere. By comparing the
model spectra with newly reprocessed Chandra observations, we conclude that
carbon ion emission provides a negligible contribution, suggesting that solar
wind ions are not responsible for the observed polar X-rays. In addition,
results of the model fits to observations support the previously estimated
seeding kinetic energies of the precipitating ions (~0.7-2 MeV/u), but infer a
different relative sulfur to oxygen abundance ratio for these Chandra
observations.Comment: 11 pages, 2 figures, 2 tables, submitted to ApJ Lette
Transience of hot dust around sun-like stars
There is currently debate over whether the dust content of planetary systems
is stochastically regenerated or originates in planetesimal belts evolving in
steady state. In this paper a simple model for the steady state evolution of
debris disks due to collisions is developed and confronted with the properties
of the emerging population of 7 sun-like stars that have hot dust <10AU. The
model shows there is a maximum possible disk mass at a given age, since more
massive primordial disks process their mass faster. The corresponding maximum
dust luminosity is f_max=0.00016r^(7/3)/t_age. The majority (4/7) of the hot
disks exceed this limit by >1000 and so cannot be the products of massive
asteroid belts, rather the following systems must be undergoing transient
events characterized by an unusually high dust content near the star: eta
Corvi, HD69830, HD72905 and BD+20307. It is also shown that the hot dust cannot
originate in a recent collision in an asteroid belt, since there is also a
maximum rate at which collisions of sufficient magnitude to reproduce a given
dust luminosity can occur. Further it is shown that the planetesimal belt
feeding the dust in these systems must be located further from the star than
the dust, typically at >2AU. Other notable properties of the 4 hot dust systems
are: two also have a planetesimal belt at >10AU (eta Corvi and HD72905); one
has 3 Neptune mass planets at <1AU (HD69830); all exhibit strong silicate
features in the mid-IR. We consider the most likely origin for the dust in
these systems to be a dynamical instability which scattered planetesimals
inwards from a more distant planetesimal belt in an event akin to the Late
Heavy Bombardment in our own system, the dust being released from such
planetesimals in collisions and possibly also sublimation.Comment: 16 pages, accepted by ApJ, removed HD128400 as hot dust candidat
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
Resolving the terrestrial planet forming regions of HD113766 and HD172555 with MIDI
We present new MIDI interferometric and VISIR spectroscopic observations of
HD113766 and HD172555. Additionally we present VISIR 11um and 18um imaging
observations of HD113766. These sources represent the youngest (16Myr and 12Myr
old respectively) debris disc hosts with emission on <<10AU scales. We find
that the disc of HD113766 is partially resolved on baselines of 42-102m, with
variations in resolution with baseline length consistent with a Gaussian model
for the disc with FWHM of 1.2-1.6AU (9-12mas). This is consistent with the
VISIR observations which place an upper limit of 0."14 (17AU) on the emission,
with no evidence for extended emission at larger distances. For HD172555 the
MIDI observations are consistent with complete resolution of the disc emission
on all baselines of lengths 56-93m, putting the dust at a distance of >1AU
(>35mas). When combined with limits from TReCS imaging the dust at ~10um is
constrained to lie somewhere in the region 1-8AU. Observations at ~18um reveal
extended disc emission which could originate from the outer edge of a broad
disc, the inner parts of which are also detected but not resolved at 10um, or
from a spatially distinct component. These observations provide the most
accurate direct measurements of the location of dust at 1-8AU that might
originate from the collisions expected during terrestrial planet formation.
These observations provide valuable constraints for models of the composition
of discs at this epoch and provide a foundation for future studies to examine
in more detail the morphology of debris discs.Comment: 22 pages, 19 figures, accepted for publication in MNRA
- …