1,230 research outputs found
HST Spectrophotometry and Models for Solar Analogs
Absolute flux distributions for seven solar analog stars are measured from
0.3 to 2.5 \mu m by HST spectrophotometry.In order to predict the longer
wavelength mid-IR fluxes that are required for JWST calibration, the HST SEDs
are fit with Castelli & Kurucz model atmospheres; and the results are compared
with fits from the MARCS model grid. The rms residuals in 10 broad band bins
are all <0.5% for the best fits from both model grids. However, the fits differ
systematically: The MARCS fits are 40-100 K hotter in T_{eff}, 0.25-0.80 higher
in log g, 0.01-0.10 higher in log z, and 0.008-0.021 higher in the reddening
E(B-V), probably because their specifications include different metal
abundances. Despite these differences in the parameters of the fits, the
predicted mid-IR fluxes differ by only ~1%; and the modeled flux distributions
of these G stars have an estimated ensemble accuracy of 2% out to 30 \mu m.Comment: 19 pages, 2 tables, 7 figures; to appear in AJ 2010 Apri
NICMOS Spectrophotometry and Models for A-Stars
Absolute flux distributions for eight stars are well measured from 0.8-2.5mu
m with NICMOS grism spectrophotometry at a resolution of R~100 and an accuracy
of 1-2%. These SEDs are fit with Castelli & Kurucz model atmospheres; and the
results are compared with the Cohen-Walker-Witteborn (CWW) template models for
the same stars. In some cases, the T_{eff}, log g, and log z parameters of the
best fitting model differ by up to 1000 K from the earlier CWW model. However,
differences in the continua of the modeled IR flux distributions from 0.4-40mu
m are always less than the quoted CWW uncertainty of 5% because of compensating
changes in the measured extinction. At wavelengths longward of the 2.5mu m
NICMOS limit, uncertainties still approach 5%, because A-star models are not
yet perfect. All of these A stars lie in the JWST continuous viewing zone and
will be important absolute flux standards for the 0.8-30mu m JWST wavelength
range.Comment: 20 pages, 6 figures, 2 tables; to be published in AJ, 2008 Septembe
Copernicus observations of interstellar absorption at Lyman alpha
Column densities NH of atomic hydrogen have been derived for 40 OB stars from spectral scans at Lyman alpha obtained by the Copernicus (OAO-3) satellite. The stars are all between 60 and 1100 pc away with a range of mean densities n sub H of 0.01 to 2.5 atoms cm-3. The gas to color-excess ratio in clouds varies from 1 to 3 times the mean outside of clouds. The presence of molecular hydrogen correlates with E(B-V), but the best tracer for H2 is atomic hydrogen. The mean density of the gas for all 40 stars is much smaller than the mean of 0.7 atoms cm-3 obtained from 21-cm observations, because the brightest stars with less than average amounts of matter in the line of sight were selected for observation
Absolute calibration in the 1750 - 3350 A region
The absolute flux measurements in the rocket ultraviolet made by Bohlin, Frimout, and Lillie (BFL) are revised using a more correct treatment of the air extinction that enters the air calibration of their instrument. The absorption by molecular oxygen and ozone, Rayleigh scattering, and extinction by aerosols is tabulated for general use in ultraviolet calibrations performed in air. The revised absolute flux of eta UMa and final fluxes for alpha Lyr and zeta Oph are presented in the 1750-3350 A region. The absolute flux of the star eta UMa is compared to four other independent determinations in the 1200-3400 A region and a maximum difference of 35% is found near 1500 A between the OAO-2 and Apollo 17 fluxes. The rocket measurements of BFL, the ANS and TD-1 satellite data, and the Apollo 17 data are compared to the ultraviolet fluxes from the OAO-2, demonstrating a photometric reproducibility of about + or - 3 percent. Therefore, all four sets of spectrophotometry can be reduced to a common absolute scale
Results of basic improvements to the extraction of spectra from IUE images
Results of two methods of extracting spectra from IUE images are compared. The first method, which is presently implemented, performs a geometric correction of the image followed by a photometric correction. The spectral data are then extracted using a slit with an effective width and sampling interval of 2.4A for the SWP camera and 3.7A for the LWR camera in low dispersion. The second method performs the photometric correction without doing a geometric correction. The spectral data are then extracted from the photometrically corrected image by an extraction slit, which follows the spectral orders in the nongeometrically corrected space, with an effective width and sampling interval 1/2 that of the present method
Absolute Flux Calibration of the IRAC Instrument on the Spitzer Space Telescope using Hubble Space Telescope Flux Standards
The absolute flux calibration of the James Webb Space Telescope will be based
on a set of stars observed by the Hubble and Spitzer Space Telescopes. In order
to cross-calibrate the two facilities, several A, G, and white dwarf (WD) stars
are observed with both Spitzer and Hubble and are the prototypes for a set of
JWST calibration standards. The flux calibration constants for the four Spitzer
IRAC bands 1-4 are derived from these stars and are 2.3, 1.9, 2.0, and 0.5%
lower than the official cold-mission IRAC calibration of Reach et al. (2005),
i.e. in agreement within their estimated errors of ~2%. The causes of these
differences lie primarily in the IRAC data reduction and secondarily in the
SEDs of our standard stars. The independent IRAC 8 micron band-4 fluxes of
Rieke et al. (2008) are about 1.5 +/- 2% higher than those of Reach et al. and
are also in agreement with our 8 micron result.Comment: 16 pages, 6 figure
International Ultraviolet Explorer Atlas of O-typespectra from 1200 to 1900 Angstrom
The IUE archives provide an unprecedented sample of uniform, high-quality ultraviolet stellar spectra. In particular, they contain high-resolution SWP data for nearly 200 different O stars. We have undertaken a survey of the 1200-1900 A region in about 120 of them having homogeneous optical spectral classifications to investigate systematically the behavior of the ultraviolet features, incuding the prominent stellar wind profiles and the degree to which they correlate with the optical types. The standard extracted spectrograms have been rebinned to a constant wavelength resolution of 0.25A and uniformly normalized (not dereddened) at the GSFC RDAF. They are then plotted at 10A/cm, with reseau, photometric quality and echelle order junction flags available. This atlas contains such plots for about 100 stars, arranged in spectral-type, luminosity and peculiar object sequences. The results show a high degree of correlation between the ultraviolet features, both photospheric and stellar-wind, and the optical classifications for the majority of the O-type stars
The virtual observatory service TheoSSA: Establishing a database of synthetic stellar flux standards. I. NLTE spectral analysis of the DA-type white dwarf G 191-B2B
H-rich, DA-type white dwarfs are particularly suited as primary standard
stars for flux calibration. State-of-the-art NLTE models consider opacities of
species up to trans-iron elements and provide reliable synthetic
stellar-atmosphere spectra to compare with observation.
We establish a database of theoretical spectra of stellar flux standards that
are easily accessible via a web interface.
In the framework of the Virtual Observatory, the German Astrophysical Virtual
Observatory developed the registered service TheoSSA. It provides easy access
to stellar spectral energy distributions (SEDs) and is intended to ingest SEDs
calculated by any model-atmosphere code. In case of the DA white dwarf G
191-B2B, we demonstrate that the model reproduces not only its overall
continuum shape but also the numerous metal lines exhibited in its ultraviolet
spectrum.
TheoSSA is in operation and contains presently a variety of SEDs for DA white
dwarfs. It will be extended in the near future and can host SEDs of all primary
and secondary flux standards. The spectral analysis of G 191-B2B has shown that
our hydrostatic models reproduce the observations best at an effective
temperature of 60000 +/- 2000K and a surface gravity of log g = 7.60 +/- 0.05.
We newly identified Fe VI, Ni VI, and Zn IV lines. For the first time, we
determined the photospheric zinc abundance with a logarithmic mass fraction of
-4.89 (7.5 times solar). The abundances of He (upper limit), C, N, O, Al, Si,
O, P, S, Fe, Ni, Ge, and Sn were precisely determined. Upper abundance limits
of 10% solar were derived for Ti, Cr, Mn, and Co.
The TheoSSA database of theoretical SEDs of stellar flux standards guarantees
that the flux calibration of all astronomical data and cross-calibration
between different instruments can be based on the same models and SEDs
calculated with different model-atmosphere codes and are easy to compare.Comment: 42 pages, 27 figure
Effects of temperature fluctuations of IUE data quality
Analysis of IUE calibration lamp images shows that variation in the temperature of the scientific instrument causes shifts in the location of the spectral format with respect to the reseau grid on the detector and in the location of the reseaux themselves. In high dispersion, a camera head amplifier temperature difference of 6C corresponds to a shift of 4 pixels in the spectral format for LWR and 2 pixels for SWP along the dispersion direction. Shifts perpendicular to the disperson (for the same temperature difference) are less than one pixel for both cameras. In low dispersion spectra, the shifts are similar but orthogonal to those described above with the larger motion lying in the direction perpendicular to the dispersion. In both dispersion modes, the observed shifts are apparently independent of wavelength. In high dispersion, the constant pixel shift mimics a constant velocity error
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