114 research outputs found
Analysis of in-situ observations in the Strait of Gibraltar
During the EU-project CANIGO intensive ship-board observations were carried out in April 1996 and October 1997 in order to observe the spatial and temporal variability of the flow, of the internal bore and of the water mass
structure in the Strait of Gibraltar. An inverse model for the current and interfacefluctuations was developed to remove tidal currents from the measurement and to calculate the volume transport for the in and outflow seperately. In addition traveltime measurements across the strait have been analysed to test the suitability of acoustical instruments for a longterm monitoring of the exchange through the strait
Striking Photospheric Abundance Anomalies in Blue Horizontal-Branch Stars in Globular Cluster M13
High-resolution optical spectra of thirteen blue horizontal-branch (BHB)
stars in the globular cluster M13 show enormous deviations in element
abundances from the expected cluster metallicity. In the hotter stars (T_eff >
12000 K), helium is depleted by factors of 10 to 100 below solar, while iron is
enhanced to three times the solar abundance, two orders of magnitude above the
canonical metallicity [Fe/H] ~= -1.5 dex for this globular cluster. Nitrogen,
phosphorus, and chromium exhibit even more pronounced enhancements, and other
metals are also mildly overabundant, with the exception of magnesium, which
stays very near the expected cluster metallicity. These photospheric anomalies
are most likely due to diffusion --- gravitational settling of helium, and
radiative levitation of the other elements --- in the stable radiative
atmospheres of these hot stars. The effects of these mechanisms may have some
impact on the photometric morphology of the cluster's horizontal branch and on
estimates of its age and distance.Comment: 11 pages, 1 Postscript figure, uses aaspp4.sty, accepted for
publication in ApJ Letter
Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15
High-resolution optical spectra of eighteen blue horizontal-branch (BHB)
stars in the globular cluster M15 indicate that their stellar rotation rates
and photospheric compositions vary strongly as a function of effective
temperature. Among the cooler stars in the sample, at Teff ~ 8500 K, metal
abundances are in rough agreement with the canonical cluster metallicity, and
the v sin i rotations appear to have a bimodal distribution, with eight stars
at v sin i < 15 km/s and two stars at v sin i ~ 35 km/s. Most of the stars at
Teff > 10000 K, however, are slowly rotating, v sin i < 7 km/s, and their iron
and titanium are enhanced by a factor of 300 to solar abundance levels.
Magnesium maintains a nearly constant abundance over the entire range of Teff,
and helium is depleted by factors of 10 to 30 in three of the hotter stars.
Diffusion effects in the stellar atmospheres are the most likely explanation
for these large differences in composition. Our results are qualitatively very
similar to those previously reported for M13 and NGC 6752, but with even larger
enhancement amplitudes, presumably due to the increased efficiency of radiative
levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar
rotation explicitly preventing the onset of the diffusion mechanisms among a
subset of the hotter stars.Comment: 11 pages, 1 figure, 1 table, accepted to ApJ
The NextGen Model Atmosphere grid: II. Spherically symmetric model atmospheres for giant stars with effective temperatures between 3000 and 6800~K
We present the extension of our NextGen model atmosphere grid to the regime
of giant stars. The input physics of the models presented here is nearly
identical to the NextGen dwarf atmosphere models, however spherical geometry is
used self-consistently in the model calculations (including the radiative
transfer). We re-visit the discussion of the effects of spherical geometry on
the structure of the atmospheres and the emitted spectra and discuss the
results of NLTE calculations for a few selected models.Comment: ApJ, in press (November 1999), 13 pages, also available at
http://dilbert.physast.uga.edu/~yeti/PAPERS and at
ftp://calvin.physast.uga.edu/pub/preprints/NG-giants.ps.g
The analysis of spectra of novae taken near maximum
A project to analyze ultraviolet spectra of novae obtained at or near maximum optical light is presented. These spectra are characterized by a relatively cool continuum with superimposed permitted emission lines from ions such as Fe II, Mg II, and Si II. Spectra obtained late in the outburst show only emission lines from highly ionized species and in many cases these are forbidden lines. The ultraviolet data will be used with calculations of spherical, expanding, stellar atmospheres for novae to determine elemental abundances by spectral line synthesis. This method is extremely sensitive to the abundances and completely independent of the nebular analyses usually used to obtain novae abundances
The first million years of the Sun: A calculation of formation and early evolution of a solar-mass star
We present the first coherent dynamical study of the cloud
fragmentation-phase, collapse and early stellar evolution of a solar mass star.
We determine young star properties as the consequence of the parent cloud
evolution. Mass, luminosity and effective temperature in the first million
years of the proto-Sun result from gravitational fragmentation of a molecular
cloud region that produces a cluster of prestellar clumps. We calculate the
global dynamical behavior of the cloud using isothermal 3D hydrodynamics and
follow the evolution of individual protostars in detail using a 1D
radiation-fluid-dynamic system of equations that comprises a correct standard
solar model solution, as a limiting case. We calculate the pre-main sequence
(PMS) evolutionary tracks of a solar mass star in a dense stellar cluster
environment and compare it to one that forms in isolation. Up to an age of
950.000 years differences in the accretion history lead to significantly
different temperature and luminosity evolution. As accretion fades and the
stars approach their final masses the two dynamic PMS tracks converge. After
that the contraction of the quasi-hydrostatic stellar interiors dominate the
overall stellar properties and proceed in very similar ways. Hence the position
of a star in the Hertzsprung-Russell diagram becomes a function of age and mass
only. However, our quantitative description of cloud fragmentation, star
formation and early stellar evolution predicts substantial corrections to the
classical, i.e. hydrostatic and initially fully convective models: At an age of
1 million years the proto-Sun is twice as bright and 500 Kelvin hotter than
according to calculations that neglect the star formation process.Comment: Four pages, accepted for publication in ApJ Letter
Application of Airborne Infrared Remote Sensing to the Study of Ocean Submesoscale Eddies
This paper explores the use of infrared remote sensing methods to examine submesoscale eddies that recur downstream of a deep-water island (Santa Catalina, CA). Data were collected using a mid-wave infrared camera deployed on an aircraft flown at an altitude of 3.7 km, and research boats made nearly simultaneous measurements of temperature and current profiles. Structure within the thermal field is generally adequate as a tracer of surface fluid motions, though the imagery needs to be processed in a novel way to preserve the smallest-scale tracer patterns. In the case we focus on, the eddy is found to have a thermal signature of about 1 km in diameter and a cyclonic swirling flow. Vorticity is concentrated over a smaller area of about 0.5 km in diameter. The Rossby number is 27, indicating the importance of the centrifugal force in the dynamical balance of the eddy. By approximating the eddy as a Rankine vortex, an estimate of upward doming of the thermocline (about 14 m at the center) is obtained that agrees qualitatively with the in-water measurements. Analysis also shows an outward radial flow that creates areas of convergence (sinking flow) along the perimeter of the eddy. The imagery also reveals areas of localized vertical mixing within the eddy thermal perimeter, and an area of external azimuthal banding that likely arises from flow instability
A Self-Consistent NLTE-Spectra Synthesis Model of FeLoBAL QSOs
We present detailed radiative transfer spectral synthesis models for the Iron
Low Ionization Broad Absorption Line (FeLoBAL) active galactic nuclei (AGN)
FIRST J121442.3+280329 and ISO J005645.1-273816. Detailed NLTE spectral
synthesis with a spherically symmetric outflow reproduces the observed spectra
very well across a large wavelength range. While exact spherical symmetry is
probably not required, our model fits are of high quality and thus very large
covering fractions are strongly implied by our results. We constrain the
kinetic energy and mass in the ejecta and discuss their implications on the
accretion rate. Our results support the idea that FeLoBALs may be an
evolutionary stage in the development of more ``ordinary'' QSOs.Comment: Accepted for publication in ApJ/removed misleading remarks about
CLOUDY in section
Horizon effects with surface waves on moving water
Surface waves on a stationary flow of water are considered, in a linear model
that includes the surface tension of the fluid. The resulting gravity-capillary
waves experience a rich array of horizon effects when propagating against the
flow. In some cases three horizons (points where the group velocity of the wave
reverses) exist for waves with a single laboratory frequency. Some of these
effects are familiar in fluid mechanics under the name of wave blocking, but
other aspects, in particular waves with negative co-moving frequency and the
Hawking effect, were overlooked until surface waves were investigated as
examples of analogue gravity [Sch\"utzhold R and Unruh W G 2002 Phys. Rev. D 66
044019]. A comprehensive presentation of the various horizon effects for
gravity-capillary waves is given, with emphasis on the deep water/short
wavelength case kh>>1 where many analytical results can be derived. A
similarity of the state space of the waves to that of a thermodynamic system is
pointed out.Comment: 30 pages, 15 figures. Minor change
The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni
The dwarf nova SS Cygni is a close binary star consisting of a K star transferring mass to a white dwarf by way of an accretion disk. We have obtained new spectroscopic observations of SS Cyg with the Hobby-Eberly Telescope (HET). Fits of synthetic spectra for Roche-lobe-filling stars to the absorption-line spectrum of the K star yield the amplitude of the K star's radial velocity curve and the mass ratio: K_{K} = 162.5 +/- 1.0 km/s and q= M_{K} /M_{wd} = 0.685 +/- 0.015. The fits also show that the accretion disk and white dwarf contribute a fraction f = 0.535 +/- 0.075 of the total flux at 5500 angstroms. Taking the weighted average of our results with previously published results obtained using similar techniques, we find = 163.7 +/- 0.7 km/s and = 0.683 +/- 0.012. The orbital light curve of SS Cyg shows an ellipsoidal variation diluted by light from the disk and white dwarf. From an analysis of the ellipsoidal variations we limit the orbital inclination to the range 45 deg. <= i <= 56 deg. The derived masses of the K star and white dwarf are M_{K} = 0.55 +/- 0.13 M_sun and M_{wd} = 0.81 +/- 0.19 M_sun, where the uncertainties are dominated by systematic errors in the orbital inclination. The K star in SS Cyg is 10% to 50% larger than an unevolved star with the same mass and thus does not follow the mass-radius relation for Zero-Age Main-Sequence stars; nor does it follow the ZAMS mass/spectral-type relation. Its mass and spectral type are, however, consistent with models in which the core hydrogen has been significantly depleted
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