5,297 research outputs found
Apparatus to Determine Static and Dynamic Elastic Moduli
Conclusions 1. The apparatus makes it possible to concurrently measure ES and ED of cylindrical rock specimens under triaxial conditions in the NX Hoek cell. 2. Accuracy of the apparatus is comparable to usual dynamic modulus testing. 3. Preliminary test results for one particular rock type, show that under triaxial conditions the differences between E D and E S are smaller at high confining pressures than at low confining pressures. Circumstantial evidence suggests that microcracks and microfissures are responsible for this phenomenon. Further work needs to be undertaken in other rock types to substantiate these preliminary findings
A detailed X-ray investigation of zeta Puppis IV. Further characterization of the variability
Previously, the X-ray emission of zeta Puppis was found to be variable with
light curves harbouring "trends" with a typical timescale longer than the
exposure length. The origin of these changes was proposed to be linked to
large-scale structures in the wind, but further characterization of the
variability at high energies was needed. Since then, a number of new X-ray
observations have become available. Furthermore, a cyclic behaviour with a
1.78d period was identified in long optical photometric runs, which is thought
to be associated with the launching mechanism of large-scale wind structures.
We analysed these new X-ray data, revisited the old data, and compared X-ray
with optical data, including when simultaneous. We found that the behaviour in
X-rays cannot be explained in terms of a perfect clock because the amplitude
and shape of its variations change with time. For example, zeta Puppis was much
more strongly variable between 2007 and 2011 than before and after this
interval. Comparing the X-ray spectra of the star at maximum and minimum
brightness yields no compelling difference beyond the overall flux change: the
temperatures, absorptions, and line shapes seem to remain constant, well within
errors. The only common feature between X-ray datasets is that the variation
amplitudes appear maximum in the medium (0.6-1.2keV) energy band. Finally, no
clear and coherent correlation can be found between simultaneous X-ray and
optical data. Only a subgroup of observations may be combined coherently with
the optical period of 1.78d, although the simultaneous optical behaviour is
unknown. The currently available data do not reveal any obvious, permanent, and
direct correlation between X-ray and optical variations. The origin of the
X-ray variability therefore still needs to be ascertained, highlighting the
need for long-term monitoring in multiwavelengths, i.e. X-ray, UV, and optical.Comment: accepted for publication by A&
Amplitude variability in satellite photometry of the non-radially pulsating O9.5V star zeta Oph
We report a time-series analysis of satellite photometry of the non-radially
pulsating Oe star zeta Oph, principally using data from SMEI obtained
2003--2008, but augmented with MOST and WIRE results. Amplitudes of the
strongest photometric signals, at 5.18, 2.96, and 2.67/d, each vary
independently over the 6-year monitoring period (from ca. 30 to <2 mmag at
5.18/d), on timescales of hundreds of days. Signals at 7.19/d and 5.18/d have
persisted (or recurred) for around two decades. Supplementary spectroscopic
observations show an H-alpha emission episode in 2006; this coincided with
small increases in amplitudes of the three strongest photometric signals.Comment: MNRAS, in pres
Hercules X-1: Empirical Models of UV Emission Lines
The UV emission lines of Hercules X-1, resolved with the HST GHRS and STIS,
can be divided into broad (FWHM 750 km/s) and narrow (FWHM 150 km/s)
components. The broad lines can be unambiguously identified with emission from
an accretion disk which rotates prograde with the orbit. The narrow lines,
previously identified with the X-ray illuminated atmosphere of the companion
star, are blueshifted at both phi=0.2 and phi=0.8 and the line flux at phi=0.2
is 0.2 of the flux at phi=0.8. Line ratio diagnostics show that the density of
the narrow line region is log n=13.4+/-0.2 and the temperature is
T=1.0+/-0.2x10^5 K. The symmetry of the eclipse ingress suggests that the line
emission on the surface of the disk is left-right symmetric relative to the
orbit. Model fits to the O V, Si IV, and He II line profiles agree with this
result, but fits to the N V lines suggest that the receding side of the disk is
brighter. We note that there are narrow absorption components in the N V lines
with blueshifts of 500 km/s.Comment: To be published in the Astrophysical Journa
FUSE Observations of a Full Orbit of Hercules X-1: Signatures of Disk, Star, and Wind
We observed an entire 1.7 day orbit of the X-ray binary Hercules X-1 with the
Far Ultraviolet Spectroscopic Explorer (FUSE). Changes in the O VI 1032,1037
line profiles through eclipse ingress and egress indicate a Keplerian accretion
disk spinning prograde with the orbit. These observations may show the first
double-peaked accretion disk line profile to be seen in the Hercules X-1
system. Doppler tomograms of the emission lines show a bright spot offset from
the Roche lobe of the companion star HZ Her, but no obvious signs of the
accretion disk. Simulations show that the bright spot is too far offset from
the Roche lobe to result from uneven X-ray heating of its surface. The absence
of disk signatures in the tomogram can be reproduced in simulations which
include absorption from a stellar wind. We attempt to diagnose the state of the
emitting gas from the C III 977, C III 1175, and N III 991 emission lines. The
latter may be enhanced through Bowen fluorescence.Comment: Accepted for publication in The Astrophysical Journa
Fundamental stellar parameters of zeta Pup and gamma^2 Vel from HIPPARCOS data
We report parallax measurements by the HIPPARCOS satellite of zeta Puppis and
gamma^2 Velorum. The distance of zeta Pup is d=429 (+120/ -77) pc, in agreement
with the commonly adopted value to Vela OB2. However, a significantly smaller
distance is found for the gamma^2 Vel system: d=258 (+41/-31) pc. The total
mass of gamma^2 Vel derived from its parallax, the angular size of the
semi-major axis as measured with intensity interferometry, and the period is
M(WR+O)=29.5 (+/-15.9) Msun. This result favors the orbital solution of Pike et
al. (1983) over that of Moffat et al. (1986). The stellar parameters for the O
star companion derived from line blanketed non-LTE atmosphere models are:
Teff=34000 (+/-1500) K, log L/Lsun=5.3 (+/-0.15) from which an evolutionary
mass of M=29 (+/-4) Msun and an age of 4.0 (+0.8/-0.5) Myr is obtained from
single star evolutionary models. With non-LTE model calculations including He
and C we derive a luminosity log L/Lsun~4.7 (+/-0.2) for the WR star. The
mass-luminosity relation of hydrogen-free WR stars implies a mass of M(WR)~5
(+/-1.5) Msun. From our data we favor an age of ~10 Myr for the bulk of the
Vela OB2 stars. Evolutionary scenarios for zeta Pup and gamma^2 Vel are
discussed in the light of our results.Comment: Submitted to ApJ Letters (misprints corrected
On the Nature of Andromeda IV
Lying at a projected distance of 40' or 9 kpc from the centre of M31,
Andromeda IV is an enigmatic object first discovered during van den Bergh's
search for dwarf spheroidal companions to M31. Being bluer, more compact and
higher surface brightness than other known dwarf spheroidals, it has been
suggested that And IV is either a relatively old `star cloud' in the outer disk
of M31 or a background dwarf galaxy. We present deep HST WFPC2 observations of
And IV and the surrounding field which, along with ground-based long-slit
spectroscopy and Halpha imagery, are used to decipher the true nature of this
puzzling object. We find compelling evidence that And IV is a background galaxy
seen through the disk of M31. The moderate surface brightness (SB(V)~24), very
blue colour (V-I<~0.6), low current star formation rate (~0.001 solar mass/yr)
and low metallicity (~10% solar) reported here are consistent with And IV being
a small dwarf irregular galaxy, perhaps similar to Local Group dwarfs such as
IC 1613 and Sextans A. Although the distance to And IV is not tightly
constrained with the current dataset, various arguments suggest it lies in the
range 5<~D<~8 Mpc, placing it well outside the confines of the Local Group. It
may be associated with a loose group of galaxies, containing major members UGC
64, IC 1727 and NGC 784. We report an updated position and radial velocity for
And IV.Comment: 26 pages, LaTex with 9 figures (including 6 jpg plates). Accepted for
publication in A
B stars as a diagnostic of star-formation at low and high redshift
We have extended the evolutionary synthesis models by Leitherer et al.
(1999b) by including a new library of B stars generated from the IUE
high-dispersion spectra archive. We present the library and show how the
stellar spectral properties vary according to luminosity classes and spectral
types. We have generated synthetic UV spectra for prototypical young stellar
populations varying the IMF and the star formation law. Clear signs of age
effects are seen in all models. The contribution of B stars in the UV line
spectrum is clearly detected, in particular for greater ages when O stars have
evolved. With the addition of the new library we are able to investigate the
fraction of stellar and interstellar contributions and the variation in the
spectral shapes of intense lines. We have used our models to date the spectrum
of the local super star cluster NGC1705-1. Photospheric lines of CIII1247,
SiIII1417, and SV1502 were used as diagnostics to date the burst of NGC 1705-1
at 10 Myr. We have selected the star-forming galaxy 1512-cB58 as a first
application of the new models to high-z galaxies. This galaxy is at z=2.723, it
is gravitationally lensed, and its high signal-to-noise Keck spectrum show
features typical of local starburst galaxies, such as NGC 1705-1. Models with
continuous star formation were found to be more adequate for 1512-cB58 since
there are spectral features typical of a composite stellar population of O and
B stars. A model with Z =0.4Z_solar and an IMF with alpha=2.8 reproduces the
stellar features of the 1512-cB58 spectrum.Comment: 23 pages with figures, see
http://sol.stsci.edu/~demello/welcomeb.htm
Triggered Star Formation in a Massive Galaxy at z=3.8: 4C41.17
Spectropolarimetric observations obtained with the W. M. Keck Telescope of
the z=3.8 radio galaxy 4C41.17 show that the UV continuum emission from this
galaxy, which is aligned with the radio axis, is unpolarized (P[2sigma] <
2.4%). This implies that scattered AGN light, which is generally the dominant
contributor to the rest-frame UV emission in z~1 radio galaxies, is unlikely to
be a major component of the UV flux from 4C41.17. The spectrum shows absorption
lines that are similar to those detected in the spectra of the recently
discovered population of star forming galaxies at z~2-3. A galaxian outflow may
contribute partially to the low ionization absorption lines; however, the high
velocity wings of the high ionization lines are unlikely to be dominated by a
galaxian wind since the implied outflow mass is very large. The detection of
stellar absorption lines, the shape of the SiIV profile, the unpolarized
continuum, the inability of any AGN-related processes to account for the UV
flux, and the similarity of the UV continuum spectra of 4C41.17 and the nearby
starburst region NGC 1741B1 suggest that the UV light in 4C41.17 is dominated
by young stars. If so, the implied star-formation rate is roughly
140-1100Msun/yr. We discuss the possibility that star formation in 4C41.17 was
triggered by the radio source. Our data are consistent with the hypothesis that
4C41.17 is undergoing its major epoch of star formation at z~4, and that by z~1
it will have evolved to have spectral and morphological properties similar to
those observed in known z~1 powerful radio galaxies.Comment: 28 pages (Latex text + figures); Accepted for publication in The
Astrophysical Journal (Dec 1, 1997 issue
A Period and a Prediction for the Of?p Spectrum Alternator HD 191612
The observational picture of the enigmatic O-type spectrum variable HD191612
has been sharpened substantially. A symmetrical, low-amplitude light curve with
a period near 540 d has recently been reported from Hipparcos photometry. This
period satisfies all of the spectroscopy since at least 1982, including
extensive new observations during 2003 and 2004, and it has predicted the next
transition during September--October 2004. Measurements of the H alpha
equivalent width reveal a sharp emission peak in the phase diagram, in contrast
to the apparently sinusoidal light curve. The He II absorption-line strength is
essentially constant, while He I varies strongly, possibly filled in by
emission in the O6 state, thus producing the apparent spectral-type variations.
The O8 state appears to be the "normal" one. Two intermediate O7 observations
have been obtained, which fall at the expected phases, but these are the only
modern observations of the transitions so far. The period is too long for
rotation or pulsation; although there is no direct evidence as yet for a
companion, a model in which tidally induced oscillations drive an enhanced wind
near periastron of an eccentric orbit appears promising. Further observations
during the now predictable transitions may provide a critical test. Ultraviolet
and X-ray observations during both states will likely also prove illuminating.Comment: 7 pages, 3 figures, 1 table; scheduled for the 2004 December 10 issue
of ApJL, Vol. 617, No. 1. ApJ
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