1,695 research outputs found
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
HD 145263: Spectral Observations of Silica Debris Disk Formation via Extreme Space Weathering?
We report here time domain infrared spectroscopy and optical photometry of
the HD145263 silica-rich circumstellar disk system taken from 2003 through
2014. We find an F4V host star surrounded by a stable, massive 1e22 - 1e23 kg
(M_Moon to M_Mars) dust disk. No disk gas was detected, and the primary star
was seen rotating with a rapid ~1.75 day period. After resolving a problem with
previously reported observations, we find the silica, Mg-olivine, and
Fe-pyroxene mineralogy of the dust disk to be stable throughout, and very
unusual compared to the ferromagnesian silicates typically found in primordial
and debris disks. By comparison with mid-infrared spectral features of
primitive solar system dust, we explore the possibility that HD 145263's
circumstellar dust mineralogy occurred with preferential destruction of
Fe-bearing olivines, metal sulfides, and water ice in an initially comet-like
mineral mix and their replacement by Fe-bearing pyroxenes, amorphous pyroxene,
and silica. We reject models based on vaporizing optical stellar megaflares,
aqueous alteration, or giant hypervelocity impacts as unable to produce the
observed mineralogy. Scenarios involving unusually high Si abundances are at
odds with the normal stellar absorption near-infrared feature strengths for Mg,
Fe, and Si. Models involving intense space weathering of a thin surface patina
via moderate (T < 1300 K) heating and energetic ion sputtering due to a stellar
superflare from the F4V primary are consistent with the observations. The space
weathered patina should be reddened, contain copious amounts of nanophase Fe,
and should be transient on timescales of decades unless replenished.Comment: 41 Pages, 5 Figures, 5 Tables, Accepted for publication in the
Astrophysical Journa
And then there were four: a study of UK market concentration - causes, consequences and the scope for market adjustment
While concentration measures are a good indicator of market structure, the link with competitiveness is more complex than often assumed. In particular, the modern theory of industrial organisation makes no clear statement regarding the impact of concentration on competition - the focus of this paper is concentration and no inferences are made about competitive aspects of the market. The extent and nature of concentration within the UK listed company audit market as at April, 2002 and, pro forma, after the collapse of Andersen is documented and analysed in detail (by firm, market segment and industry sector). The largest four firms held 90 per cent of the market (based on audit fees) in 2002, rising to 96 per cent with the demise of Andersen. A single firm, Pricewaterhouse-Coopers, held 70 per cent or more of the share of six out of 38 industry sectors, with a share of 50 per cent up to 70 per cent in a further seven sectors. The provision of non-audit services (NAS) by incumbent auditors is also considered. As at April 2002, the average ratio of non-audit fees (paid to auditor) to audit fees was 208 per cent, and exceeded 300 per cent in seven sectors. It is likely, however, that disposals by firms of their management consultancy and outsource firms, combined with the impact of the Smith Report on audit committees will serve to reduce these ratios. Another finding is that audit firms with expertise in a particular sector appeared to earn significantly higher nonaudit fees from their audit clients in that sector. The paper thus provides a solid empirical basis for debate. The subsequent discussion considers the implications for companies and audit firms of the high level of concentration in the current regulatory climate, where no direct regulatory intervention is planned
Stable Coronal X-Ray Emission Over Twenty Years of XZ Tau
XZ Tau AB is a frequently observed binary YSO in the Taurus Molecular Cloud;
XZ Tau B has been classified as an EXOr object. We present new
Chandra/HETG-ACIS-S observations of XZ Tau AB, complemented with variability
monitoring of the system with XMM-Newton, to constrain the variability of this
system and identify high-resolution line diagnostics to better understand the
underlying mechanisms that produce the X-rays. We observe two flares with
XMM-Newton, but find that outside of these flares the coronal X-ray spectrum of
XZ Tau AB is consistent over twenty years of observations. We compare the
ensemble of XZ Tau X-ray observations over time with the scatter across stars
observed in point-in-time observations of the Orion Nebula Cluster and find
that both overlap in terms of plasma properties, i.e., some of the scatter
observed in the X-ray properties of stellar ensembles stems from intrinsic
source variability.Comment: Accepted for publication in the Astronomical Journal. 19 pages, 11
figure
The Properties of X-ray Luminous Young Stellar Objects in the NGC 1333 and Serpens Embedded Clusters
We present Chandra X-ray data of the NGC 1333 embedded cluster, combining
these data with existing Chandra data, Sptizer photometry and ground based
spectroscopy of both the NGC 1333 & Serpens North clusters to perform a
detailed study of the X-ray properties of two of the nearest embedded clusters
to the Sun. In NGC 1333, a total of 95 cluster members are detected in X-rays,
of which 54 were previously identified with Spitzer. Of the Spitzer sources, we
detect 23% of the Class I protostars, 53% of the Flat Spectrum sources, 52% of
the Class II, and 50% of the Transition Disk YSOs. Forty-one Class III members
of the cluster are identified, bringing the total identified YSO population to
178. The X-ray Luminosity Functions (XLFs) of the NGC 1333 and Serpens clusters
are compared to each other and the Orion Nebula Cluster. Based on this
comparison, we obtain a new distance for the Serpens cluster of 360+22/-13 pc.
The X-ray luminosity was found to depend on the bolometric luminosity as in
previous studies of other clusters, and that Lx depends primarily on the
stellar surface area. In the NGC 1333 cluster, the Class III sources have a
somewhat higher X-ray luminosity for a given surface area. We also find
evidence in NGC 1333 for a jump in the X-ray luminosity between spectral types
of M0 and K7, we speculate that this may result from the presence of radiative
zones in the K-stars. The gas column density vs. extinction in the NGC 1333 was
found to be N_H = 0.89 +/- 0.13 x 10^22 A_K, this is lower than expected of the
standard ISM but similar to that found previously in the Serpens Cloud Core.Comment: 58 pages, 14 figures, accepted by A
- …