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

Taxing Intermediate Goods to Compensate for Distorting Taxes on Household Consumption

In contrast to the classic result in Diamond and Mirrlees (1971) that fiscal taxes should not be levied on intermediate use of goods, Newbury (1985) showed that, in a closed economy with Leontief technology, input taxes should be used to indirectly tax commodities that for some reason are untaxed in final consumption. This paper extends the Newbury result to more general cases; i.e., to open economies with substitution possibilities in the production functions. Moreover, it shows that the welfare maximizing proportion between the tax rate for intermediate use by firms and final demand by households declines with higher elasticities of substitution in production functions and with higher price elasticities in import demand functions and export supply functions. It also shows that the welfare maximizing proportion of tax rates between households and firms for one commodity will depend upon the corresponding proportion of tax rates for important substitutes for that commodity. These results are shown both in stylized Computable General Equilibrium (CGE) models and in an applied CGE model of the Swedish economy where the tax on electricity is used as an example.Optimal taxation; CGE-analysis

Stock portfolio structure of individual investors infers future trading behavior

Although the understanding of and motivation behind individual trading behavior is an important puzzle in finance, little is known about the connection between an investor's portfolio structure and her trading behavior in practice. In this paper, we investigate the relation between what stocks investors hold, and what stocks they buy, and show that investors with similar portfolio structures to a great extent trade in a similar way. With data from the central register of shareholdings in Sweden, we model the market in a similarity network, by considering investors as nodes, connected with links representing portfolio similarity. From the network, we find groups of investors that not only identify different investment strategies, but also represent groups of individual investors trading in a similar way. These findings suggest that the stock portfolios of investors hold meaningful information, which could be used to earn a better understanding of stock market dynamics.Comment: 9 pages, 4 figures, 1 tabl

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

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

Photometry of the outer solar corona from lunar based observations

Two-dimensional isophotes of outer solar corona derived from integrated vidicon pictures taken from lunar surface by Surveyors 6 and