157 research outputs found
Phase behaviour of coarse-grained fluids
Soft condensed matter structures often challenge us with complex many-body
phenomena governed by collective modes spanning wide spatial and temporal
domains. In order to successfully tackle such problems mesoscopic
coarse-grained (CG) statistical models are being developed, providing a
dramatic reduction in computational complexity. CG models provide an
intermediate step in the complex statistical framework of linking the
thermodynamics of condensed phases with the properties of their constituent
atoms and molecules. These allow us to offload part of the problem to the CG
model itself and reformulate the remainder in terms of reduced CG phase space.
However, such exchange of pawns to chess pieces, or ``Hamiltonian
renormalization'', is a radical step and the thermodynamics of the primary
atomic and CG models could be markedly different. Here, we present a
comprehensive study of the phase diagram including binodal and interfacial
properties of a novel soft CG model, which includes finite-range attraction and
supports liquid phases. Although the model is rooted in similar arguments to
the Lennard-Jones (LJ) atomic pair potential, its phase behaviour is
qualitatively different from that of LJ and features several anomalies such as
an unusually broad liquid range, change in concavity of the liquid coexistence
branch with variation of the model parameters, volume contraction on fusion,
temperature of maximum density in the liquid phase and negative thermal
expansion in the solid phase. These results provide new insight into the
connection between simple potential models and complex emergent condensed
matter phenomena.Comment: 10 pages, full pape
Electronic effects in high-energy radiation damage in tungsten
Although the effects of the electronic excitations during high-energy
radiation damage processes are not currently understood, it is shown that their
role in the interaction of radiation with matter is important. We perform
molecular dynamics simulations of high-energy collision cascades in
bcc-tungsten using the coupled two-temperature molecular dynamics (2T-MD) model
that incorporates both the effects of electronic stopping and electron-phonon
interaction. We compare the combination of these effects on the induced damage
with only the effect of electronic stopping, and conclude in several novel
insights. In the 2T-MD model, the electron-phonon coupling results in less
damage production in the molten region and in faster relaxation of the damage
at short times. These two effects lead to significantly smaller amount of the
final damage at longer times.Comment: 7 pages, 7 figure
Exploratory Study of the X-Ray Properties of Quasars With Intrinsic Narrow Absorption Lines
We have used archival Chandra and XMM-Newton observations of quasars hosting
intrinsic narrow UV absorption lines (intrinsic NALs) to carry out an
exploratory survey of their X-ray properties. Our sample consists of three
intrinsic-NAL quasars and one "mini-BAL" quasar, plus four quasars without
intrinsic absorption lines for comparison. These were drawn in a systematic
manner from an optical/UV-selected sample. The X-ray properties of
intrinsic-NAL quasars are indistinguishable from those of "normal" quasars. We
do not find any excess absorption in quasars with intrinsic NALs, with upper
limits of a few times 10^22 cm^-2. We compare the X-ray and UV properties of
our sample quasars by plotting the equivalent width and blueshift velocity of
the intrinsic NALs and the X-ray spectral index against the "optical-to-X-ray"
slope, alpha-ox. When BAL quasars and other AGNs with intrinsic NALs are
included, the plots suggest that intrinsic-NAL quasars form an extension of the
BAL sequences and tend to bridge the gap between BAL and "normal" quasars.
Observations of larger samples of intrinsic-NAL quasars are needed to verify
these conclusions. We also test two competing scenarios for the location of the
NAL gas in an accretion-disk wind. Our results strongly support a location of
the NAL gas at high latitudes above the disk, closer to the disk axis than the
dense BAL wind. We detect excess X-ray absorption only in Q0014+8118, which
does not host intrinsic NALs. The absorbing medium very likely corresponds to
an intervening system at z=1.1, which also produces strong absorption lines in
the rest-frame UV spectrum of this quasar. In the appendix we discuss the
connection between UV and X-ray attenuation and its effect on alpha-ox.Comment: Accepted by the Astrophysical Journa
Far Ultraviolet Absolute Flux of alpha Virginis
We present the far ultraviolet spectrum of alpha Virginis taken with EURD
spectrograph on-board MINISAT-01. The spectral range covered is from ~900 to
1080 A with 5 A spectral resolution. We have fitted Kurucz models to IUE
spectra of alpha Vir and compared the extension of the model to our wavelengths
with EURD data. This comparison shows that EURD fluxes are consistent with the
prediction of the model within 20-30%, depending on the reddening assumed. EURD
fluxes are consistent with Voyager observations but are ~60% higher than most
previous rocket observations of alpha Vir.Comment: 13 pages, 4 figures. Submitted to The Astrophysical Journa
Resolved Spectroscopy of the Narrow-Line Region in NGC 1068. II. Physical Conditions Near the NGC 1068 ``Hot-Spot''
The physical conditions near the optical continuum peak (``hot spot'') in the
inner narrow line region (NLR) of the Seyfert 2 galaxy, NGC 1068. Spectra were
taken with HST/STIS through the 0.1X52 arcsec slit, covering the full STIS 1200
to 10000 Angstrom waveband, and are from a region that includes the hot spot,
extending 0.2, or ~ 14 pc (for H= 75 km/sec/Mpc). Perhaps the most striking
feature of these spectra is the presence of strong coronal emission lines,
including [S XII] 7611 which has hitherto only been identified in spectra of
the solar corona. There is an apparent correlation between ionization energy
and velocity of the emission lines with respect to the systemic velocity of the
host galaxy, with the coronal lines blueshifted, most other high excitation
lines near systemic, and some of the low ionization lines redshifted. From the
results of our modeling, we find that the emission-line gas consists of three
principal components: 1) one in which most of the strong emission-lines, such
as [O III] 5007, [Ne V] 3426, C IV 1550, arise, 2) a more tenuous, highly
ionized component, which is the source of the coronal-line emission, and 3) a
component, which is not co-planar with the other two, in which the low
ionization and neutral lines, such as [N II] 6548 and [O I] 6300, are formed.
The first two components are directly ionized by the EUV-Xray continuum emitted
by the central source, while the low ionization gas is ionized by a combination
of highly absorbed continuum radiation and a small fraction of unabsorbed
continuum scattered by free electrons associated with the hot spot. The
combination of covering factor and Thomson optical depth of the high ionization
components is insufficient to scatter the observed fraction of continuum
radiation into our line-of-sight.Comment: 42 pages, Latex, includes 5 figures (postscript), to appear in the
Astrophysical Journa
Cosmological Effects of the First Stars: Evolving Spectra of Population III
The first stars hold intrinsic interest for their uniqueness and for their
potential importance to galaxy formation, chemical enrichment, and feedback on
the intergalactic medium (IGM). The metal-free composition of the first stars
restricts the stellar energy source to proton-proton burning rather than the
more efficient CNO cycle. Consequently they are hotter, smaller, and have
harder spectra than their present-day counterparts of finite metallicity. We
present new results from a continuing study of metal-free stars from a
cosmological point of view. We have calculated evolving spectra of Pop III
clusters, derived from a grid of zero-metallicity stellar evolutionary tracks.
We find that H-ionizing photon production from metal-free stellar clusters
takes twice as long as that of Pop II to decline to 1/10 its peak value. In
addition, metal-free stars produce substantially more photons than Pop II in
the HeII ( Ryd) continuum. We suggest that large Ly equivalent
widths ( Angstroms) may provide a means of detecting
metal-free stellar populations at high redshift, and that HeII recombination
lines (1640, 4686) may confirm identifications of Population
III. While Pop III clusters are intrinsically bluer than their Pop II
counterparts, nebular continuum emission makes up this difference and may
confuse attempts to discern Pop III stars with broadband colors. In a companion
paper, we explore the consequences of evolving spectra of Pop III for the
reionization of the IGM in both H and He. (ABRIDGED)Comment: Accepted by ApJ; 32 pages, 10 figures, with an updated discussion of
the effects of mass loss on Pop III and Pop II comparisons. For
full-resolution color figures, see http://casa.colorado.edu/~tumlinso/fs.htm
OSMO: Ontology for simulation, modelling, and optimization
This work describes the ontology OSMO, i.e., an ontologization and extension of MODA, a workflow metadata standard that constitutes a mandatory requirement within a number of European calls and projects in the context of materials modelling. OSMO was developed within the Horizon 2020 project VIMMP (Virtual Materials Marketplace) and is part of a larger effort in ontology engineering driven by the European Materials Modelling Council, with the Elementary Multiperspective Material Ontology (EMMO) as its core. As such, OSMO provides connections and alignments with other related domain ontologies in computational engineering, including the EMMO itself. This work summarizes the domain, purpose, and design choices underlying OSMO, commenting on the implementation of OSMO and its applications
Optical Identification of the Hardest X-ray Source in the ASCA Large Sky Survey
We report the optical identification of the hardest X-ray source (AX
J131501+3141) detected in an unbiased wide-area survey in the 0.5--10 keV band,
the ASCA Large Sky Survey. The X-ray spectrum of the source is very hard and is
well reproduced by a power-law component (Gamma = 1.5^+0.7_-0.6) with N_H =
6^+4_-2 *10^22 cm^-2 (Sakano et al. 1998). We have found a galaxy with R=15.62
mag near the center of the error circle for the X-ray source. The optical
spectrum of the galaxy shows only narrow emission lines whose ratios correspond
to those of a type 2 Seyfert galaxy at z = 0.072, implying an
absorption-corrected X-ray luminosity of 2*10^43 erg sec^-1 (2--10 keV) and M_B
= -20.93 mag. A radio point source is also associated with the center of the
galaxy. We thus identify the X-ray source with this galaxy as an obscured AGN.
The hidden nature of the nucleus of the galaxy in the optical band is
consistent with the X-ray spectrum. These results support the idea that the
obscured AGNs/QSOs contribute significantly to the cosmic X-ray background in
the hard band at the faint flux level.Comment: 20 pages with 5 postscript figures, uses aaspp4.sty, Ap.J. Accepte
Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. II. The IUE Campaign
PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was
monitored with the IUE satellite at ~1 hour time-resolution for ten nearly
uninterrupted days in May 1994. The campaign, which was coordinated with EUVE,
ROSAT, and ASCA monitoring, along with optical and radio observations from the
ground, yielded the largest set of spectra and the richest short time scale
variability information ever gathered for a blazar at UV wavelengths. The
source flared dramatically during the first day, with an increase by a factor
~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly
constant level for ~5 days, then flared with ~35% amplitude for two days. The
same variability was seen in both short- and long-wavelength IUE light curves,
with zero formal lag (~<2 hr), except during the rapid initial flare, when the
variations were not resolved. Spectral index variations were small and not
clearly correlated with flux. The flux variability observed in the present
monitoring is so rapid that for the first time, based on the UV emission alone,
the traditional Delta L/Delta t limit indicating relativistic beaming is
exceeded. The most rapid variations, under the likely assumption of synchrotron
radiation, lead to a lower limit of 1 G on the magnetic field strength in the
UV emitting region. These results are compared with earlier intensive
monitoring of PKS 2155-304 with IUE in November 1991, when the UV flux
variations had completely different characteristics.Comment: 45 pages, Latex, 11 PostScript figures, to appear in The
Astrophysical Journa
Spectral Energy Distributions of Weak Active Galactic Nuclei Associated With Low-Ionization Nuclear Emission Regions
We present a compilation of spectral energy distributions of 35 weak AGNs in
LINERs using recent data from the published literature. We make use of
previously published compilations of data, after complementing and extending
them with more recent data. The main improvement in the recent data is afforded
by high-spatial resolution observations with the Chandra X-Ray Observatory and
high-spatial resolution radio observations utilizing a number of facilities. In
addition, a considerable number of objects have been observed with the HST in
the near-IR through near-UV bands since the earlier compilations were
published. The data include upper limits resulting from either non-detections
or observations at low spatial resolution that do not isolate the AGN. For the
sake of completeness, we also compute and present a number of quantities from
the data, such as alpha-ox, bolometric corrections, bolometric luminosities,
Eddington ratios, and the average SED. We anticipate that these data will be
useful for a number of applications. In a companion paper, we use a subset of
these data ourselves to assess the energy budgets of LINERs.Comment: emulateapj format, 12 pages in total, to appear in ApJS, one large
table and one large figure abridged (will be available in electronic journal
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