3,379 research outputs found
Stellar laboratories. V. The Xe VI ultraviolet spectrum and the xenon abundance in the hot DO-type white dwarf RE0503-289
For the spectral analysis of spectra of hot stars with a high resolution and
high signal-to-noise ratio (S/N), advanced non-local thermodynamic equilibrium
(NLTE) model atmospheres are mandatory. These are strongly dependent on the
reliability of the atomic data that are used for their calculation.
Reliable Xe VI oscillator strengths are used to identify Xe lines in the
ultraviolet spectrum of the DO-type white dwarf RE0503-289 and to determine its
photospheric Xe abundance.
We publish newly calculated oscillator strengths that are based on a recently
measured Xe VI laboratory line spectrum. These strengths were used to consider
their radiative and collisional bound-bound transitions in detail in our NLTE
stellar-atmosphere models to analyze Xe VI lines exhibited in high-resolution
and high S/N UV observations of RE0503-289.
We identify three hitherto unknown Xe VI lines in the ultraviolet spectrum of
RE0503-289 and confirm the previously measured photospheric Xe abundance of
this white dwarf (log Xe = -4.2 +/- 0.6).
Reliable measurements and calculations of atomic data are prerequisite for
stellar-atmosphere modeling. Observed Xe VI line profiles in the ultraviolet
spectrum of the white dwarf RE0503-289 were well reproduced with the newly
calculated Xe VI oscillator strengths.Comment: 3 pages, 4 figure
Interpretation of High Energy String Scattering in terms of String Configurations
High energy string scattering at fixed momentum transfer, known to be
dominated by Regge trajectory exchange, is interpreted by identifying families
of string states which induce each type of trajectory exchange. These include
the usual leading trajectory and its daughters as
well as the ``sister'' trajectories and their
daughters. The contribution of the sister to high energy scattering
is dominated by string excitations in the mode. Thus, at large ,
string scattering is dominated by wee partons, consistently with a picture of
string as an infinitely composite system of ``constituents'' which carry zero
energy and momentum.Comment: 14 pages, phyzzx, psfig required, Florida Preprint UFIFT-94-
Complete spectral energy distribution of the hot, helium-rich white dwarf RX J0503.9-2854
In the line-of-sight toward the DO-type white dwarf RX J0503.9-2854, the
density of the interstellar medium (ISM) is very low, and thus the
contamination of the stellar spectrum almost negligible. This allows us to
identify many metal lines in a wide wavelength range from the extreme
ultraviolet to the near infrared. In previous spectral analyses, many metal
lines in the ultraviolet spectrum of RX J0503.9-2854 have been identified. A
complete line list of observed and identified lines is presented here. We
compared synthetic spectra that had been calculated from model atmospheres in
non-local thermodynamical equilibrium, with observations. In total, we
identified 1272 lines (279 of them were newly assigned) in the wavelength range
from the extreme ultraviolet to the near infrared. 287 lines remain
unidentified. A close inspection of the EUV shows that still no good fit to the
observed shape of the stellar continuum flux can be achieved although He, C, N,
O, Al, Si, P, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Cr, Ni Zn, Ga, Ge, As, Kr, Zr, Mo,
Sn, Xe, and Ba are included in the stellar atmosphere models. There are two
possible reasons for the deviation between observed and synthetic flux in the
EUV. Opacities from hitherto unconsidered elements in the model-atmosphere
calculation may be missing and/or the effective temperature is slightly lower
than previously determined.Comment: 92 pages, 45 figure
Nutrient Limitation of Periphyton in a Spring-Fed, Coastal Stream in Florida, USA.
There is strong evidence to suggest that ground-water
nitrate concentrations have increased in recent years and
further increases are expected along portions of the central
Gulf coast of Florida. Much of the nitrate enriched groundwater
is discharged into surface waters through numerous
freshwater springs that are characteristic of the area and the
potential for eutrophication of their receiving waters is a
legitimate concern. To test the potential effects of elevated
nutrient concentrations on the periphyton community an
in
situ
nutrient addition experiment was conducted in the
spring-fed Chassahowitzka River, FL, USA, during the summer
of 1999. Plastic tubes housing arrays of glass microscope
slides were suspended in the stream. Periphyton colonizing
the microscope slides was subjected to artificial increases in
nitrogen, phosphorus or a combination of both. Slides from
each tube were collected at 3- to 4- day intervals and the
periphyton communities were measured for chlorophyll concentration.
The addition of approximately 10 μg/L of phosphate
above ambient concentrations significantly increased
the amount of periphyton on artificial substrates relative to
controls; the addition of approximately 100 μg/L of nitrate
above ambient concentrations did not. The findings from
this experiment implicated phosphorus, rather than
nitrogen, as the nutrient that potentially limits periphyton
growth in this system.(PDF contains 4 pages.
Microscopic modeling of photoluminescence of strongly disordered semiconductors
A microscopic theory for the luminescence of ordered semiconductors is
modified to describe photoluminescence of strongly disordered semiconductors.
The approach includes both diagonal disorder and the many-body Coulomb
interaction. As a case study, the light emission of a correlated plasma is
investigated numerically for a one-dimensional two-band tight-binding model.
The band structure of the underlying ordered system is assumed to correspond to
either a direct or an indirect semiconductor. In particular, luminescence and
absorption spectra are computed for various levels of disorder and sample
temperature to determine thermodynamic relations, the Stokes shift, and the
radiative lifetime distribution.Comment: 35 pages, 14 figure
Stellar laboratories. IX. New Se V, Sr IV - VII, Te VI, and I VI oscillator strengths and the Se, Sr, Te, and I abundances in the hot white dwarfs G191-B2B and RE 0503-289
To analyze spectra of hot stars, advanced non-local thermodynamic equilibrium
(NLTE) model-atmosphere techniques are mandatory. Reliable atomic data is for
the calculation of such model atmospheres.
We aim to calculate new Sr IV - VII oscillator strengths to identify for the
first time Sr spectral lines in hot white dwarf (WD) stars and to determine the
photospheric Sr abundances. o measure the abundances of Se, Te, and I in hot
WDs, we aim to compute new Se V, Te VI, and I VI oscillator strengths.
To consider radiative and collisional bound-bound transitions of Se V, Sr IV
- VII, Te VI, and I VI in our NLTE atmosphere models, we calculated oscillator
strengths for these ions.
We newly identified four Se V, 23 Sr V, 1 Te VI, and three I VI lines in the
ultraviolet (UV) spectrum of RE0503-289. We measured a photospheric Sr
abundance of 6.5 +3.8/-2.4 x 10**-4 (mass fraction, 9500 - 23800 times solar).
We determined the abundances of Se (1.6 +0.9/-0.6 x 10**-3, 8000 - 20000), Te
(2.5 +1.5/-0.9 x 10**-4, 11000 - 28000), and I (1.4 +0.8/-0.5 x 10**-5, 2700 -
6700). No Se, Sr, Te, and I line was found in the UV spectra of G191-B2B and we
could determine only upper abundance limits of approximately 100 times solar.
All identified Se V, Sr V, Te VI, and I VI lines in the UV spectrum of
RE0503-289 were simultaneously well reproduced with our newly calculated
oscillator strengths.Comment: 26 pages, 5 figure
Disk evolution in the Ori OB1 association
We analyze multi-band photometry of a subsample of low mass stars in the
associations Ori OB1a and 1b discovered during the CIDA Orion Variability
Survey, which have ages of 7 - 10 Myr and 3 - 5 Myr, respectively. We obtained
UBVRcIc photometry at Mt. Hopkins for 6 Classical T Tauri stars (CTTS) and 26
Weak T Tauri stars (WTTS) in Ori OB1a, and for 21 CTTS and 2 WTTS in Ori OB1b.
We also obtained L band photometry for 14 CTTS at Mt. Hopkins, and 10um and
18um photometry with OSCIR at Gemini for 6 CTTS; of these, all 6 were detected
at 10um while only one was detected at 18um. We estimate mass accretion rates
from the excess luminosity at U, and find that they are consistent with
determinations for a number of other associations, with or without high mass
star formation. The observed decrease of mass accretion rate with age is
qualitatively consistent with predictions of viscous evolution of accretion
disks. We find an overall decrease of disk emission from Taurus to Ori OB1b to
Ori OB1a. This decrease implies that significant grain growth and settling
towards the midplane has taken place in the inner disks of Ori OB1. We compare
the SED of the star detected at both 10um and 18um with disk models for similar
stellar and accretion parameters. We find that the low <= 18 um fluxes of this
Ori OB1b star cannot be due to the smaller disk radius expected from viscous
evolution in the presence of the FUV radiation fields from the OB stars in the
association. Instead, we find that the disk of this star is essentially a flat
disk, with little if any flaring, indicating a a significant degree of dust
settling towards the midplane, as expected from dust evolution in
protoplanetary disks.Comment: 35 pages, 11 figures, to appear in the Astronomical Journal. Full
resolution figures in http://www.cida.ve/~briceno/publications
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