1,165 research outputs found
Diblock copolymers at a homopolymer-homopolymer-interface: a Monte Carlo simulation
The properties of diluted symmetric A-B diblock copolymers at the interface
between A and B homopolymer phases are studied by means of Monte Carlo (MC)
simulations of the bond fluctuation model. We calculate segment density
profiles as well as orientational properties of segments, of A and B blocks,
and of the whole chain. Our data support the picture of oriented ``dumbbells'',
which consist of mildly perturbed A and B Gaussian coils. The results are
compared to a self consistent field theory (SCFT) for single copolymer chains
at a homopolymer interface. We also discuss the number of interaction contacts
between monomers, which provide a measure for the ``active surface'' of
copolymers or homopolymers close to the interface
The Emergence of the Modern Universe: Tracing the Cosmic Web
This is the report of the Ultraviolet-Optical Working Group (UVOWG)
commissioned by NASA to study the scientific rationale for new missions in
ultraviolet/optical space astronomy approximately ten years from now, when the
Hubble Space Telescope (HST) is de-orbited. The UVOWG focused on a scientific
theme, The Emergence of the Modern Universe, the period from redshifts z = 3 to
0, occupying over 80% of cosmic time and beginning after the first galaxies,
quasars, and stars emerged into their present form. We considered
high-throughput UV spectroscopy (10-50x throughput of HST/COS) and wide-field
optical imaging (at least 10 arcmin square). The exciting science to be
addressed in the post-HST era includes studies of dark matter and baryons, the
origin and evolution of the elements, and the major construction phase of
galaxies and quasars. Key unanswered questions include: Where is the rest of
the unseen universe? What is the interplay of the dark and luminous universe?
How did the IGM collapse to form the galaxies and clusters? When were galaxies,
clusters, and stellar populations assembled into their current form? What is
the history of star formation and chemical evolution? Are massive black holes a
natural part of most galaxies? A large-aperture UV/O telescope in space
(ST-2010) will provide a major facility in the 21st century for solving these
scientific problems. The UVOWG recommends that the first mission be a 4m
aperture, SIRTF-class mission that focuses on UV spectroscopy and wide-field
imaging. In the coming decade, NASA should investigate the feasibility of an 8m
telescope, by 2010, with deployable optics similar to NGST. No high-throughput
UV/Optical mission will be possible without significant NASA investments in
technology, including UV detectors, gratings, mirrors, and imagers.Comment: Report of UV/O Working Group to NASA, 72 pages, 13 figures, Full
document with postscript figures available at
http://casa.colorado.edu/~uvconf/UVOWG.htm
The bright-end galaxy candidates at z ~ 9 from 79 independent HST fields
We present a full data analysis of the pure-parallel Hubble Space Telescope
(HST) imaging observations in the Brightest of Reionizing Galaxies Survey
(BoRG[z9]) in Cycle 22. The medium-deep exposures with five HST/WFC3IR+UVIS
filter bands from 79 independent sightlines (~370 arcmin^2) provide the least
biased determination of number density for z>9 bright galaxies against cosmic
variance. After a strict two-step selection for candidate galaxies, including
dropout color and photometric redshift analyses, and revision of previous BoRG
candidates, we identify one source at z~10 and two sources at z~9. The z~10
candidate shows evidence of line-of-sight lens magnification (mu~1.5), yet it
appears surprisingly luminous (MUV ~ -22.6\pm0.3 mag), making it one of the
brightest candidates at z > 8 known (~ 0.3 mag brighter than the z = 8.68
galaxy EGSY8p7, spectroscopically confirmed by Zitrin and collaborators). For z
~ 9 candidates, we include previous data points at fainter magnitudes and find
that the data are well fitted by a Schechter luminosity function with alpha ~
-2.1, MUV ~ -21.5 mag, and log phi ~ -4.5 Mpc^-3mag^-1, for the first time
without fixing any parameters. The inferred cosmic star formation rate density
is consistent with unaccelerated evolution from lower redshift.Comment: 18pages, 7figures, 6tables. accepted to the Astrophysical Journa
The Escape of Ionizing Photons from OB Associations in Disk Galaxies: Radiation Transfer Through Superbubbles
By solving the time-dependent radiation transfer problem of stellar radiation
through evolving superbubbles within a smoothly varying HI distribution, we
estimate the fraction of ionizing photons emitted by OB associations that
escapes the HI disk of our Galaxy into the halo and intergalactic medium (IGM).
We consider both coeval star-formation and a Gaussian star-formation history
with a time spread sigma_t = 2 Myr. We consider both a uniform H I distribution
and a two-phase (cloud/intercloud) model, with a negligible filling factor of
hot gas. We find that the shells of the expanding superbubbles quickly trap or
attenuate the ionizing flux, so that most of the escaping radiation escapes
shortly after the formation of the superbubble. For the coeval star-formation
history, the total fraction of Lyman Continuum photons that escape both sides
of the disk in the solar vicinity is f_esc approx 0.15 +/- 0.05. For the
Gaussian star formation history, f_esc approx 0.06 +/- 0.03, a value roughly a
factor of two lower than the results of Dove & Shull (1994), where superbubbles
were not considered. For a local production rate of ionizing photons Psi_LyC =
4.95 X 10^7 cm^{-2} s^{-1}, the flux escaping the disk is Phi_LyC approx
(1.5-3.0) X 10^6 cm^{-2} s^{-1} for coeval and Gaussian star formation,
comparable to the flux required to sustain the Reynolds layer.Comment: Revised version (expanded), accepted for publication by ApJ, 38
pages, 8 figures, aasms4.sty and aabib.sty files include
Metal Abundances in the Magellanic Stream
We report on the first metallicity determination for gas in the Magellanic
Stream, using archival HST GHRS data for the background targets Fairall 9, III
Zw 2, and NGC 7469. For Fairall 9, using two subsequent HST revisits and new
Parkes Multibeam Narrowband observations, we have unequivocally detected the
MSI HI component of the Stream (near its head) in SII1250,1253 yielding a
metallicity of [SII/H]=-0.55+/-0.06(r)+/-0.2(s), consistent with either an SMC
or LMC origin and with the earlier upper limit set by Lu et al. (1994). We also
detect the saturated SiII1260 line, but set only a lower limit of
[SiII/H]>-1.5. We present serendipitous detections of the Stream, seen in
MgII2796,2803 absorption with column densities of (0.5-1)x10^13 cm^-2 toward
the Seyfert galaxies III Zw 2 and NGC 7469. These latter sightlines probe gas
near the tip of the Stream (80 deg down-Stream of Fairall 9). For III Zw 2, the
lack of an accurate HI column density and the uncertain MgIII ionization
correction limits the degree to which we can constrain [Mg/H]; a lower limit of
[MgII/HI]>-1.3 was found. For NGC 7469, an accurate HI column density
determination exists, but the extant FOS spectrum limits the quality of the
MgII column density determination, and we conclude that [MgII/HI]>-1.5.
Ionization corrections associated with MgIII and HII suggest that the
corresponding [Mg/H] may range lower by 0.3-1.0 dex. However, an upward
revision of 0.5-1.0 dex would be expected under the assumption that the Stream
exhibits a dust depletion pattern similar to that seen in the Magellanic
Clouds. Remaining uncertainties do not allow us to differentiate between an LMC
versus SMC origin to the Stream gas.Comment: 30 pages, 8 figures, LaTeX (aaspp4), also available at
http://casa.colorado.edu/~bgibson/publications.html, accepted for publication
in The Astronomical Journa
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