384 research outputs found
Rapid Reservoir Sedimentation of Four Historic Thin Arch Dams in Australia
Waters flowing in streams and rivers have the ability to scour channel beds, to carry particles (heavier than water) and to deposit materials. This phenomenon of sediment transport can affect substantially the design of reservoirs. The paper describes four case studies of siltation which rendered useless water storage structures in less than 25 years. Although each dam had advanced structural features, the hydrology of the catchment and sediment transport processes were not properly taken into account. The study highlights practical situations in which a reservoir must be analysed as a complete system, taking into account structural features, hydraulics, hydrology, sediment transport, catchment erosion and catchment management. The case studies may used as teaching examples to increase student interest on the significance of sediment transport problems and to emphasise the design procedure to professionals. They may serve also to alert the community at large to basic errors caused by improper soil conservation policy and the inability to predict sediment-load process
The polar ring galaxy AM1934-563 revisited
We report long-slit spectroscopic observations of the dust-lane polar-ring
galaxy AM1934-563 obtained with the Southern African Large Telescope (SALT)
during its performance-verification phase. The observations target the spectral
region of the Ha, [NII] and [SII] emission-lines, but show also deep NaI
stellar absorption lines that we interpret as produced by stars in the galaxy.
We derive rotation curves along the major axis of the galaxy that extend out to
about 8 kpc from the center for both the gaseous and the stellar components,
using the emission and absorption lines. We derive similar rotation curves
along the major axis of the polar ring and point out differences between these
and the ones of the main galaxy. We identify a small diffuse object visible
only in Ha emission and with a low velocity dispersion as a dwarf HII galaxy
and argue that it is probably metal-poor. Its velocity indicates that it is a
fourth member of the galaxy group in which AM1934-563 belongs. We discuss the
observations in the context of the proposal that the object is the result of a
major merger and point out some observational discrepancies from this
explanation. We argue that an alternative scenario that could better fit the
observations may be the slow accretion of cold intergalactic gas, focused by a
dense filament of galaxies in which this object is embedded (abridged).Comment: 19 pages, 13 figures, submitted to MNRAS. Some figures were bitmapped
to reduce the size. Full resolution version is available from
http://www.saao.ac.za/~akniazev/pub/AM1934_563.pd
On the Correlation Between CO Absorption and Far-Ultraviolet Non-Linear Extinction Toward Galactic OB Stars
A sample of 59 sight lines to reddened Galactic OB stars was examined for
correlations of the strength of the CO Fourth Positive (A - X) absorption band
system with the ultraviolet interstellar extinction curve parameters. We used
archival high-dispersion NEWSIPS IUE spectra to measure the CO absorption for
comparison to parametric fits of the extinction curves from the literature. A
strong correlation with the non-linear far-UV curvature term was found with
greater absorption, normalized to E(B-V), being associated with more curvature.
A weaker trend with the linear extinction term was also found. Mechanisms for
enhancing CO in dust environments exhibiting high non-linear curvature are
discussed.Comment: 10 pages, including 6 figures. LaTeX2e (emulateapj5.sty). To appear
in ApJ, Sep 20, 200
Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
We have used archival far-ultraviolet spectra from observations made by
HST/STIS and FUSE to determine the column densities and rotational excitation
temperatures for CO and H2, respectively, along the lines of sight to 23
Galactic O and B stars. The sightlines have reddening values in the range
E(B-V)= 0.07-0.62, sampling the diffuse to translucent interstellar medium. We
find that the H2 column densities range from 5x10^18-8x10^20 cm^-2 and the CO
from upper limits around 2x10^12 cm^-2 to detections as high as 1.4x10^16
cm^-2. CO increases with increasing H2, roughly following a power law of factor
\~2. The CO/H2 column density ratio is thus not constant, and ranges from 10^-7
- 10^-5, with a mean value of 3x10^-6. The sample segregates into "diffuse" and
"translucent" regimes, the former having a molecular fraction less than ~0.25
and A_V/d<1 mag kpc^-1. The mean CO/H2 for these two regimes are 3.6x10^-7 and
9.3x10^-6, respectively, significantly lower than the canonical dark cloud
value of 10^-4. In six of the sightlines, 13CO is observed, and the isotopic
ratio we observe (~50-70) is consistent with, if perhaps a little below, the
average 12C/13C for the ISM at large. The average H2 rotational excitation
temperature is 74+/-24 K, in good agreement with previous studies, and the
average CO temperature is 4.1 K, with some sightlines as high as 6.4 K. The
higher excitation CO is observed with higher column densities, consistent with
the effects of photon trapping in clouds with densities in the 20-100 cm^-3
range. We discuss the implications for the structure of the diffuse/translucent
regimes of the interstellar medium and the estimation of molecular mass in
galaxies.Comment: emualateapj style, 6 figures, 3 tables, accepted on 21 Nov 2006 for
publication in The Astrophysical Journa
The metallicity extremes of the Sagittarius dSph using SALT spectroscopy of PNe
In this work we present the first spectroscopic results obtained with the
Southern African Large Telescope (SALT) telescope during its
perfomance-verification phase. We find that the Sagittarius dwarf spheroidal
galaxy (Sgr) Sgr contains a youngest stellar population with [O/H] -0.2 and age
t>1 Gyr, and an oldest population with [O/H]=-2.0. The values are based on
spectra of two planetary nebulae (PNe), using empirical abundance
determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He.
We confirm the high abundances of PN StWr2-21 with 12+log(O/H) = 8.57+/-0.02
dex. The other PN studied, BoBn1, is an extraordinary object in that the neon
abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn1 yield
the original stellar metallicity, corresponding to 12+log(O/H) = 6.72+/-0.16
dex which is 1/110 of the solar value. The actual [O/H] is much higher: third
dredge-up enriched the material by a factor of ~12 in oxygen, ~240 in nitrogen
and ~70 in neon. Neon as well as nitrogen and oxygen content may have been
produced in the intershell of low-mass AGB stars. Well defined broad WR lines
are present in the spectrum of StWr2-21 and absent in the spectrum of BoBn1.
This puts the fraction of [WR]-type central PNe stars to 67% for dSph galaxies.Comment: 14 pages, 4 figures, accepted to MNRA
Far-Ultraviolet Dust Albedo Measurements in the Upper Scorpius Cloud Using the SPINR Sounding Rocket Experiment
The Spectrograph for Photometric Imaging with Numeric Reconstruction (SPINR)
sounding rocket experiment was launched on 2000 August 4 to record
far-ultraviolet (912-1450 A) spectral and spatial information for the giant
reflection nebula in the Upper Scorpius region. The data were divided into
three arbitrary bandpasses (912-1029 A, 1030-1200 A, and 1235-1450 A) for which
stellar and nebular flux levels were derived. These flux measurements were used
to constrain a radiative transfer model and to determine the dust albedo for
the Upper Scorpius region. The resulting albedos were 0.28+/-0.07 for the
912-1029 A bandpass, 0.33+/-0.07 for the 1030-1200 A bandpass, and 0.77+/-0.13
for the 1235-1450 A bandpass
Rocket and FUSE Observations of IC 405: Differential Extinction and Fluorescent Molecular Hydrogen
We present far-ultraviolet spectroscopy of the emission/reflection nebula IC
405 obtained by a rocket-borne long-slit spectrograph and the Far Ultraviolet
Spectroscopic Explorer. Both data sets show a rise in the ratio of the nebular
surface brightness to stellar flux (S/F_*) of approximately two orders of
magnitude towards the blue end of the far-UV bandpass. Scattering models using
simple dust geometries fail to reproduce the observed S/F_* for realistic grain
properties. The high spectral resolution of the FUSE data reveals a rich
fluorescent molecular hydrogen spectrum ~1000" north of the star that is
clearly distinguished from the steady blue continuum. The S/F_* remains roughly
constant at all nebular pointings, showing that fluorescent molecular hydrogen
is not the dominant cause for the blue rise. We discuss three possible
mechanisms for the ``Blue Dust'': differential extinction of the dominant star
(HD 34078), unusual dust grain properties, and emission from nebular dust. We
conclude that uncertainties in the nebular geometry and the degree of dust
clumping are most likely responsible for the blue rise. As an interesting
consequence of this result, we consider how IC 405 would appear in a spatially
unresolved observation. If IC 405 was observed with a spatial resolution of
less than 0.4 pc, for example, an observer would infer a far-UV flux that was
2.5 times the true value, giving the appearance of a stellar continuum that was
less extinguished than radiation from the surrounding nebula, an effect that is
reminiscent of the observed ultraviolet properties of starburst galaxies.Comment: 8 pages, 9 figures, using emulateapj, ApJ - accepte
Masculinity as Governance: police, public service and the embodiment of authority, c. 1700-1850
About the book: Public Men offers an introduction to an exciting new field: the history of masculinities in the political domain and will be essential reading for students and specialists alike with interests in gender or political culture. By building upon new work on gender and political culture, these new case studies explore the gendering of the political domain and the masculinities of the men who have historically dominated it. As such, Public Men is a major contribution to our understanding of the history of Britain between the Eighteenth and the Twentieth centuries
A translucent interstellar cloud at z=2.69: CO, H2 and HD in the line-of-sight to SDSS J123714.60+064759.5
We present the analysis of a sub-DLA system (log N(H^0)=20.0+/-0.15,
z_abs=2.69) toward SDSS J123714+064759 (z_em=2.78). Using the VLT/UVES and
X-shooter spectrographs, we detect H2, HD and CO molecules in absorption with
log N(H2,HD,CO)=(19.21,14.48,14.17). The overall metallicity of the system is
super-solar ([Zn/H]=+0.34) and iron is highly depleted ([Fe/Zn]=-1.39),
revealing metal-rich and dusty gas. The strongest H2 component does not
coincide with the centre of the HI absorption. This implies that the molecular
fraction in this component, f=2N(H2)/(2N(H2)+N(H^0)), is larger than the mean
molecular fraction =1/4 in the system. This is supported by the detection of
Cl^0 associated with this H2-component having N(Cl^0)/N(Cl^+)>0.4. Since Cl^0
is tied up to H2 by charge exchange reactions, this means that the molecular
fraction in this component is not far from unity. The size of the molecular
cloud is probably smaller than 1pc. Both the CO/H2=10^-5 and CO/C^0~1 ratios
for f>0.24 indicate that the cloud classifies as translucent, i.e., a regime
where carbon is found both in atomic and molecular form. The corresponding
extinction, Av=0.14, albeit lower than the definition of a translucent
sightline (based on extinction properties), is high for the observed H^0 column
density. This means that intervening clouds with similar local properties but
with larger column densities could be missed by current magnitude-limited QSO
surveys. The excitation of CO is dominated by radiative interaction with the
Cosmic Microwave Background Radiation (CMBR) and we derive Tex(CO)=10.5+0.8-0.6
K when TCMBR(z=2.69)=10.05 K is expected. The astration factor of deuterium
-with respect to the primordial D/H ratio- is only about 3. This can be the
consequence of accretion of unprocessed gas from the intergalactic medium onto
the associated galaxy. [abridged]Comment: 17 pages, 21 figures, 8 tables, accepted for publication in A&
Far-ultraviolet Spectroscopy of Venus and Mars at 4 A Resolution with the Hopkins Ultraviolet Telescope on Astro-2
Far-ultraviolet spectra of Venus and Mars in the range 820-1840 A at 4 A
resolution were obtained on 13 and 12 March 1995, respectively, by the Hopkins
Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the
Space Shuttle Endeavour. Longward of 1250 A, the spectra of both planets are
dominated by emission of the CO Fourth Positive band system and strong OI and
CI multiplets. In addition, CO Hopfield-Birge bands, B - X (0,0) at 1151 A and
C - X (0,0) at 1088 A, are detected for the first time, and there is a weak
indication of the E - X (0,0) band at 1076 A in the spectrum of Venus. The B -
X band is blended with emission from OI 1152. Modeling the relative intensities
of these bands suggests that resonance fluorescence of CO is the dominant
source of the emission, as it is for the Fourth Positive system. Shortward of
Lyman-alpha, other emission features detected include OII 834, OI lambda 989,
HI Lyman-beta, and NI 1134 and 1200. For Venus, the derived disk brightnesses
of the OI, OII, and HI features are about one-half of those reported by Hord et
al. (1991) from Galileo EUV measurements made in February 1990. This result is
consistent with the expected variation from solar maximum to solar minimum. The
ArI 1048, 1066 doublet is detected only in the spectrum of Mars and the derived
mixing ratio of Ar is of the order of 2%, consistent with previous
determinations.Comment: 8 pages, 5 figures, accepted for publication in ApJ, July 20, 200
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