112 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
Spectral Analysis of the Chandra Comet Survey
We present results of the analysis of cometary X-ray spectra with an extended
version of our charge exchange emission model (Bodewits et al. 2006). We have
applied this model to the sample of 8 comets thus far observed with the Chandra
X-ray observatory and ACIS spectrometer in the 300-1000 eV range. The surveyed
comets are C/1999 S4 (LINEAR), C/1999 T1 (McNaught-Hartley), C/2000 WM1
(LINEAR), 153P/2002 (Ikeya-Zhang), 2P/2003 (Encke), C/2001 Q4 (NEAT), 9P/2005
(Tempel 1) and 73P/2006-B (Schwassmann-Wachmann 3) and the observations include
a broad variety of comets, solar wind environments and observational
conditions. The interaction model is based on state selective, velocity
dependent charge exchange cross sections and is used to explore how cometary
X-ray emission depend on cometary, observational and solar wind
characteristics. It is further demonstrated that cometary X-ray spectra mainly
reflect the state of the local solar wind. The current sample of Chandra
observations was fit using the constrains of the charge exchange model, and
relative solar wind abundances were derived from the X-ray spectra. Our
analysis showed that spectral differences can be ascribed to different solar
wind states, as such identifying comets interacting with (I) fast, cold wind,
(II), slow, warm wind and (III) disturbed, fast, hot winds associated with
interplanetary coronal mass ejections. We furthermore predict the existence of
a fourth spectral class, associated with the cool, fast high latitude wind.Comment: 16 pages, 16 figures, and 7 Tables; accepted A&A (Due to space
limits, this version has lower resolution jpeg images.
Rebirth of X-ray Emission from the Born-Again Planetary Nebula A 30
The planetary nebula (PN) A30 is believed to have undergone a very late
thermal pulse resulting in the ejection of knots of hydrogen-poor material.
Using HST images we have detected the angular expansion of these knots and
derived an age of 850+280-150 yr. To investigate the spectral and spatial
properties of the soft X-ray emission detected by ROSAT, we have obtained
Chandra and XMM-Newton observations of A30. The X-ray emission from A30 can be
separated into two components: a point-source at the central star and diffuse
emission associated with the hydrogen-poor knots and the cloverleaf structure
inside the nebular shell. To help us assess the role of the current stellar
wind in powering this X-ray emission, we have determined the stellar parameters
of the central star of A 30 using a non-LTE model fit to its optical and UV
spectrum. The spatial distribution and spectral properties of the diffuse X-ray
emission is suggestive that it is generated by the post-born-again and present
fast stellar winds interacting with the hydrogen-poor ejecta of the born-again
event. This emission can be attributed to shock-heated plasma, as the
hydrogen-poor knots are ablated by the stellar winds, under which circumstances
the efficient mass-loading of the present fast stellar wind raises its density
and damps its velocity to produce the observed diffuse soft X-rays. Charge
transfer reactions between the ions of the stellar winds and material of the
born-again ejecta has also been considered as a possible mechanism for the
production of diffuse X-ray emission, and upper limits on the expected X-ray
production by this mechanism have been derived. The origin of the X-ray
emission from the central star of A 30 is puzzling: shocks in the present fast
stellar wind and photospheric emission can be ruled out, while the development
of a new, compact hot bubble confining the fast stellar wind seems implausible.Comment: 29 pages, 11 figures, 4 tables; accepted for publication by Ap
Solar system X‐rays from charge exchange processes
While X‐ray astronomy began in 1962 and has made fast progress since then in expanding our knowledge about where in the Universe X‐rays are generated by which processes, it took one generation before the importance of a fundamentally different process was recognized. This happened in our immediate neighborhood, when in 1996 comets were discovered as a new class of X‐ray sources, directing our attention to charge exchange reactions. Charge exchange is fundamentally different from other processes which lead to the generation of X‐rays, because the X‐rays are not produced by hot electrons, but by ions picking up electrons from cold gas. Thus it opens up a new window, making it possible to detect cool gas in X‐rays (like in comets), while all the other processes require extremely high temperatures or otherwise extreme conditions. After having been overlooked for a long time, the astrophysical importance of charge exchange for the generation of X‐rays is now receiving increased general attention. In our solar system, charge exchange induced X‐rays have now been established to originate in comets, in all the planets from Venus to Jupiter, and even in the heliosphere itself. In addition to that, evidence for this X‐ray emission mechanism has been found at various locations across the Universe. Here we summarize the current knowledge about solar system X‐rays resulting from charge exchange processes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/91180/1/324_ftp.pd
Cometary charge exchange diagnostics in UV and X‐ray
Since the initial discovery of cometary charge exchange emission, more than 20 comets have been observed with a variety of X‐ray 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 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/91138/1/335_ftp.pd
Chandra Observations of Comets 8P/Tuttle and 17P/Holmes during Solar Minimum
We present results for Chandra observations of comets, 17P/Holmes (17P) and
8P/Tuttle (8P). 17P was observed for 30 ksec right after its major outburst, on
31 Oct 2007 (10:07 UT) and comet 8P/Tuttle was observed in 2008 January for 47
ksec. During the two Chandra observations, 17P was producing at least 100 times
more water than 8P but was 2.2 times further away from the Sun. Also, 17P is
the first comet observed at high latitude (+19.1 degrees) during solar minimum,
while 8P was observed at a lower solar latitude (3.4 degrees). The X-ray
spectrum of 17P is unusually soft with little significant emission at energies
above 500 eV. Depending on our choice of background, we derive a 300 to 1000 eV
flux of 0.5 to 4.5 x 10^-13 ergs/cm2/sec, with over 90% of the emission in the
300 to 400 eV range. This corresponds to an X-ray luminosity between 0.4 to 3.3
x 10^15 ergs/sec. 17P's lack of X-rays in the 400 to 1000 eV range, in a simple
picture, may be attributed to the polar solar wind, which is depleted in highly
charged ions. 8P/Tuttle was much brighter, with an average count rate of 0.20
counts/s in the 300 to 1000 eV range. We derive an average X-ray flux in this
range of 9.4 x 10^-13 ergs/cm2/sec and an X-ray luminosity for the comet of 1.7
x 10^14 ergs/sec. The light curve showed a dramatic decrease in flux of over
60% between observations on January 1st and 4th. When comparing outer regions
of the coma to inner regions, its spectra showed a decrease in ratios of
CVI/CV, OVIII/OVII, as predicted by recent solar wind charge exchange emission
models. There are remarkable differences between the X-ray emission from these
two comets, further demonstrating the qualities of cometary X-ray observations,
and solar wind charge exchange emission in more general as a means of remote
diagnostics of the interaction of astrophysical plasmas.Comment: 37 Pages, 8 Tables, 11 Figures; Accepted in Astrophysical Journal
Supplement
WISE/NEOWISE observations of Active Bodies in the Main Belt
We report results based on mid-infrared photometry of 5 active main belt
objects (AMBOs) detected by the Wide-field Infrared Survey Explorer (WISE)
spacecraft. Four of these bodies, P/2010 R2 (La Sagra), 133P/Elst-Pizarro,
(596) Scheila, and 176P/LINEAR, showed no signs of activity at the time of the
observations, allowing the WISE detections to place firm constraints on their
diameters and albedos. Geometric albedos were in the range of a few percent,
and on the order of other measured comet nuclei. P/2010 A2 was observed on
April 2-3, 2010, three months after its peak activity. Photometry of the coma
at 12 and 22 {\mu}m combined with ground-based visible-wavelength measurements
provides constraints on the dust particle mass distribution (PMD), dlogn/dlogm,
yielding power-law slope values of {\alpha} = -0.5 +/- 0.1. This PMD is
considerably more shallow than that found for other comets, in particular
inbound particle fluence during the Stardust encounter of comet 81P/Wild 2. It
is similar to the PMD seen for 9P/Tempel 1 in the immediate aftermath of the
Deep Impact experiment. Upper limits for CO2 & CO production are also provided
for each AMBO and compared with revised production numbers for WISE
observations of 103P/Hartley 2.Comment: 32 Pages, including 5 Figure
P/2010A2 LINEAR - I: An impact in the Asteroid Main Belt
Comet P/2010A2 LINEAR is a good candidate for membership with the Main Belt
Comet family. It was observed with several telescopes (ESO NTT, La Silla;
Gemini North, Mauna Kea; UH 2.2m, Mauna Kea) from 14 Jan. until 19 Feb. 2010 in
order to characterize and monitor it and its very unusual dust tail, which
appears almost fully detached from the nucleus; the head of the tail includes
two narrow arcs forming a cross. The immediate surroundings of the nucleus were
found dust-free, which allowed an estimate of the nucleus radius of 80-90m. A
model of the thermal evolution indicates that such a small nucleus could not
maintain any ice content for more than a few million years on its current
orbit, ruling out ice sublimation dust ejection mechanism. Rotational spin-up
and electrostatic dust levitations were also rejected, leaving an impact with a
smaller body as the favoured hypothesis, and ruling out the cometary nature of
the object.
The impact is further supported by the analysis of the tail structure.
Finston-Probstein dynamical dust modelling indicates the tail was produced by a
single burst of dust emission. More advanced models, independently indicate
that this burst populated a hollow cone with a half-opening angle alpha~40degr
and with an ejection velocity v_max ~ 0.2m/s, where the small dust grains fill
the observed tail, while the arcs are foreshortened sections of the burst cone.
The dust grains in the tail are measured to have radii between a=1-20mm, with a
differential size distribution proportional to a^(-3.44 +/- 0.08). The dust
contained in the tail is estimated to at least 8x10^8kg, which would form a
sphere of 40m radius. Analysing these results in the framework of crater
physics, we conclude that a gravity-controlled crater would have grown up to
~100m radius, i.e. comparable to the size of the body. The non-disruption of
the body suggest this was an oblique impact.Comment: 15 pages, 11 figures, in pres
Airfall on Comet 67P/Churyumov-Gerasimenko
We here study the transfer process of material from one hemisphere to the
other (deposition of airfall material) on an active comet nucleus, specifically
67P/Churyumov-Gerasimenko. Our goals are to: 1) quantify the thickness of the
airfall debris layers and how it depends on the location of the target area, 2)
determine the amount of and ice that are lost
from icy dust assemblages of different sizes during transfer through the coma,
and 3) estimate the relative amount of vapor loss in airfall material after
deposition in order to understand what locations are expected to be more active
than others on the following perihelion approach.
We use various numerical simulations, that include orbit dynamics,
thermophysics of the nucleus and of individual coma aggregates, coma gas
kinetics and hydrodynamics, as well as dust dynamics due to gas drag, to
address these questions. We find that the thickness of accumulated airfall
material varies substantially with location, and typically is of the order
-. The airfall material preserves substantial amounts of
water ice even in relatively small (cm-sized) coma aggregates after a rather
long () residence in the coma. However, is lost
within a couple of hours even in relatively large (dm-sized) aggregates, and is
not expected to be an important component in airfall deposits. We introduce
reachability and survivability indices to measure the relative capacity of
different regions to simultaneously collect airfall and to preserve its water
ice until the next perihelion passage, thereby grading their potential of
contributing to comet activity during the next perihelion passage.Comment: 65 pages, 11 figures. Published manuscrip
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