315 research outputs found
X-ray Line Emission from the Hot Stellar Wind of theta 1 Ori C
We present a first emission line analysis of a high resolution X-ray spectrum
of the stellar wind of theta 1 Ori C obtained with the High Energy Transmission
grating Spectrometer onboard the Chandra X-ray Observatory. The spectra are
resolved into a large number of emission lines from H- and He-like O, Ne, Mg,
Si, S, Ar and Fe ions. The He-like Fe XXV and Li-like Fe XXIV appear quite
strong indicating very hot emitting regions. From H/He flux ratios, as well as
from Fe He/Li emission measure ratios we deduce temperatures ranging from 0.5
to 6.1 x 10^7 K. The He-triplets are very sensitive to density as well. At
these temperatures the relative strengths of the intercombination and forbidden
lines indicate electron densities well above 10^12 cm^-3. The lines appear
significantly broadened from which we deduce a mean velocity of 770 km/s with a
spread between 400 and 2000 km/s. Along with results of the deduced emission
measure we conclude that the X-ray emission could originate in dense and hot
regions with a characteristic size of less then 4 x 10^10 cm.Comment: 4 pages, 3 figure
Observation of lattice waves through observation of the photoluminescence Blinking in InGaN Quantum Well devices
The photoluminescence of III-V wide band-gap semiconductors as InGaN is
characterized by local intensity fluctuations, known as 'blinking points', that
despite decades of research are not yet completely understood. In this letter
we report experimental data and a theoretical interpretation that suggests they
are caused by the interference of thermal vibrations of the Quantum Well
lattice. With far-field optical tests we could observe the lower frequency tail
of these interference waves and study their dynamics as they propagate up to
distances of several tens of microns.Comment: 4 pages, 4 figure
High-Resolution Chandra X-Ray Imaging And Spectroscopy Of The Sigma Orionis Cluster
We present results of a 90 ks Chandra X-ray observation of the young sigma Orionis cluster ( age similar to 3 Myr) obtained with the HETGS. We use the high-resolution grating spectrum and moderate-resolution CCD spectrum of the massive central star sigma Ori AB (O9.5 V + B0.5 V) to test wind shock theories of X-ray emission and also analyze the high spatial resolution zero-order ACIS-S image of the central cluster region. Chandra detected 42 X-ray sources on the primary CCD (ACIS-S3). All but five have near-IR or optical counterparts and about one-fourth are variable. Notable high-mass stellar detections are sigma Ori AB, the magnetic B star sigma Ori E, and the B5 V binary HD 37525. Most of the other detections have properties consistent with lower mass K- or M-type stars. We present the first X-ray spectrum of the unusual infrared source IRS 1, located approximate to 3 \u27\u27 north of sigma Ori AB. Its X-ray properties and elongated mid-IR morphology suggest that it is an embedded low-mass T Tauri star whose disk/envelope is being photoevaporated by sigma Ori AB. We focus on the radiative wind shock interpretation of the soft luminous X-ray emission from sigma Ori AB, but also consider possible alternatives including magnetically confined wind shocks and colliding wind shocks. Its emission lines show no significant asymmetries or centroid shifts and are moderately broadened to HWHM approximate to 264 km s(-1), or one-fourth the terminal wind speed. Forbidden lines in He-like ions are formally undetected, implying strong UV suppression. The Mg XI triplet forms in the wind acceleration zone within one stellar radius above the surface. These X-ray properties are consistent in several respects with the predictions of radiative wind shock theory for an optically thin wind, but explaining the narrow line widths presents a challenge to the theory
Discovery of 9 Ly alpha emitters at redshift z~3.1 using narrow-band imaging and VLT spectroscopy
Narrow-band imaging surveys aimed at detecting the faint emission from the
5007 [O III] line of intracluster planetary nebulae in Virgo also probe high
redshift z=3.1 Ly alpha emitters. Here we report on the spectroscopic
identification of 9 Ly alpha emitters at z=3.13, obtained with the FORS
spectrograph at Unit 1 of the ESO Very Large Telescope (VLT UT1). The spectra
of these high redshift objects show a narrow, isolated Ly alpha emission with
very faint (frequently undetected) continuum, indicating a large equivalent
width. No other features are visible in our spectra. Our Ly alpha emitters are
quite similar to those found by Hu (1998), Cowie & Hu (1998) and Hu et al.
(1998). Using simple population synthesis models, on the assumption that these
sources are regions of star formation, we conclude that the nebulae are nearly
optically thick and must have a very low dust content, in order to explain the
high observed Ly alpha equivalent widths. For the cosmological and star
formation parameters we adopted, the total stellar mass produced would seem to
correspond to the formation of rather small galaxies, some of which are perhaps
destined to merge. The implied star formation density in our sampled comoving
volume is probably somewhat smaller than, but of the same order of magnitude as
the star formation density at z=3 derived by other authors from Lyman-break
galaxy surveys. This result agrees with the expectation that the Ly alpha
emitters are a low-metallicity (or low-dust) tail in a distribution of star
forming regions at high redshifts. Finally, the Ly alpha emitters may
contribute as many H-ionizing photons as QSOs at z=3.Comment: 26 pages, 17 Postscript figures, ApJ in pres
On the Importance of the Interclump Medium for Superionization: O VI Formation in the Wind of Zeta Pup
We have studied superionization and X-ray line formation in the spectra of
Zeta Pup using our new stellar atmosphere code (XCMFGEN) that can be used to
simultaneously analyze optical, UV, and X-ray observations. Here, we present
results on the formation of the O VI ll1032, 1038 doublet. Our simulations,
supported by simple theoretical calculations, show that clumped wind models
that assume void in the interclump space cannot reproduce the observed O VI
profiles. However, enough O VI can be produced if the voids are filled by a low
density gas. The recombination of O VI is very efficient in the dense material
but in the tenuous interclump region an observable amount of O VI can be
maintained. We also find that different UV resonance lines are sensitive to
different density regimes in Zeta Pup : C IV is almost exclusively formed
within the densest regions, while the majority of O VI resides between clumps.
N V is an intermediate case, with contributions from both the tenuous gas and
clumps.Comment: Accepted for publication in ApJL, 4 pages with 3 figure
SwitchWare: Accelerating Network Evolution (White Paper)
We propose the development of a set of software technologies ( SwitchWare ) which will enable rapid development and deployment of new network services. The key insight is that by making the basic network service selectable on a per user (or even per packet) basis, the need for formal standardization is eliminated. Additionally, by making the basic network service programmable, the deployment times, today constrained by capital funding limitations, are tremendously reduced (to the order of software distribution times). Finally, by constructing an advanced, robust programming environment, even the service development time can be reduced.
A SwitchWare switch consists of input and output ports controlled by a software-programmable element; programs are contained in sequences of messages sent to the SwitchWare switch\u27s input ports, which interpret the messages as programs. We call these Switchlets . This accelerates the pace of network evolution, as evolving user needs can be immediately reflected in the network infrastructure. Immediate reconfigurability enhances the adaptability of the network infrastructure in the face of unexpected situations. We call a network built from SwitchWare switches an active network
Molecular Dynamics for Fermions
The time-dependent variational principle for many-body trial states is used
to discuss the relation between the approaches of different molecular dynamics
models to describe indistinguishable fermions. Early attempts to include
effects of the Pauli principle by means of nonlocal potentials as well as more
recent models which work with antisymmetrized many-body states are reviewed
under these premises.
Keywords: Many-body theory; Fermion system; Molecular dynamics; Wave-packet
dynamics; Time-dependent variational principle; Statistical properties;
Canonical ensemble; Ergodicity; Time averagingComment: 97 pages, 13 postscript figures. To be published in July 2000 issue
of Reviews of Modern Physics. More information at http://www-aix.gsi.de/~fmd
Deep CCD Surface Photometry of Galaxy Clusters I: Methods and Initial Studies of Intracluster Starlight
We report the initial results of a deep imaging survey of galaxy clusters.
The primary goals of this survey are to quantify the amount of intracluster
light as a function of cluster properties, and to quantify the frequency of
tidal debris. We outline the techniques needed to perform such a survey, and we
report findings for the first two galaxy clusters in the survey: Abell 1413,
and MKW 7 . These clusters vary greatly in richness and structure. We show that
our surface photometry reliably reaches to a surface brightness of \mu_v = 26.5
mags per arcsec. We find that both clusters show clear excesses over a
best-fitting r^{1/4} profile: this was expected for Abell 1413, but not for MKW
7. Both clusters also show evidence of tidal debris in the form of plumes and
arc-like structures, but no long tidal arcs were detected. We also find that
the central cD galaxy in Abell 1413 is flattened at large radii, with an
ellipticity of , the largest measured ellipticity of any cD galaxy
to date.Comment: 58 pages, 24 figures, accepted for publication in the Astrophysical
Journal. Version has extremely low resolution figures to comply with 650k
limit. High resolution version is available at
http://burro.astr.cwru.edu/johnf/icl1.ps.gz Obtaining high resolution version
is strongly reccomende
On the role of continuum-driven eruptions in the evolution of very massive stars and Population III stars
We suggest that the mass lost during the evolution of very massive stars may
be dominated by optically thick, continuum-driven outbursts or explosions,
instead of by steady line-driven winds. In order for a massive star to become a
WR star, it must shed its H envelope, but new estimates of the effects of
clumping in winds indicate that line driving is vastly insufficient. We discuss
massive stars above roughly 40-50 Msun, for which the best alternative is mass
loss during brief eruptions of luminous blue variables (LBVs). Our clearest
example of this phenomenon is the 19th century outburst of eta Car, when the
star shed 12-20 Msun or more in less than a decade. Other examples are
circumstellar nebulae of LBVs, extragalactic eta Car analogs (``supernova
impostors''), and massive shells around SNe and GRBs. We do not yet fully
understand what triggers LBV outbursts, but they occur nonetheless, and present
a fundamental mystery in stellar astrophysics. Since line opacity from metals
becomes too saturated, the extreme mass loss probably arises from a
continuum-driven wind or a hydrodynamic explosion, both of which are
insensitive to metallicity. As such, eruptive mass loss could have played a
pivotal role in the evolution and fate of massive metal-poor stars in the early
universe. If they occur in these Population III stars, such eruptions would
profoundly affect the chemical yield and types of remnants from early SNe and
hypernovae.Comment: 4 pages, 1 figure, accepted by ApJ Letter
Low mass loss rates in O-type stars: Spectral signatures of dense clumps in the wind of two Galactic O4 stars
We have analyzed the far-UV spectrum of two Galactic O4 stars, the O4If+
supergiant HD190429A and the O4V((f)) dwarf HD96715, using archival FUSE and
IUE data. We have conducted a quantitative analysis based on the two NLTE model
atmosphere and wind codes, TLUSTY and CMFGEN. We have derived the stellar and
wind parameters and the surface composition of the two stars. The surface of
HD190429A has a composition typical of an evolved O supergiant (N-rich, C and
O-poor), while HD96715 exhibits surface N enhancement similar to the enrichment
found in SMC O dwarfs and attributed to rotationally-induced mixing. We find
that homogeneous wind models could not match the observed profile of O V1371
and require very low phosphorus abundance to fit the P V1118-1128 resonance
lines. However, we are able to match the O V and P V lines using clumped wind
models. We find that N IV1718 is also sensitive to wind clumping. For both
stars, we have calculated clumped wind models that match well all these lines
from different species and that remain consistent with Halpha data. These fits
therefore provide a coherent and thus much stronger evidence of wind clumping
in O stars than earlier claims. We find that the wind of these two stars is
highly clumped, as expressed by very small volume filling factors, namely
f=0.04 for HD190429A and f=0.02 for HD96715. In agreement with our analysis of
SMC stars, clumping starts deep in the wind, just above the sonic point. The
most crucial consequence of our analysis is that the mass loss rates of O stars
need to be revised downward significantly, by a factor of 3 and more.
Accounting for wind clumping is essential when determining the wind properties
of O stars. Our study therefore calls for a fundamental revision in our
understanding of mass loss and of O-type star winds. (abridged)Comment: To appear in Astronomy & Astrophysics; 16 pages; accepted version
after minor revisio
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