Correcting for the Effects of Interstellar Extinction

This paper addresses the issue of how best to correct astronomical data for the wavelength-dependent effects of Galactic interstellar extinction. The main general features of extinction from the IR through the UV are reviewed, along with the nature of observed spatial variations. The enormous range of extinction properties found in the Galaxy, particularly in the UV spectral region, is illustrated. Fortunately, there are some tight constraints on the wavelength dependence of extinction and some general correlations between extinction curve shape and interstellar environment. These relationships provide some guidance for correcting data for the effects of extinction. Several strategies for dereddening are discussed along with estimates of the uncertainties inherent in each method. In the Appendix, a new derivation of the wavelength dependence of an average Galactic extinction curve from the IR through the UV is presented, along with a new estimate of how this extinction law varies with the parameter R = A(V)/E(B-V). These curves represent the true monochromatic wavelength dependence of extinction and, as such, are suitable for dereddening IR--UV spectrophotometric data of any resolution, and can be used to derive extinction relations for any photometry system.Comment: To appear in PASP (January 1999) 14 pages including 4 pages of figures Uses emulateapj style. PASP, in press (January 1999

A New Measurement of the Average FUV Extinction Curve

We have measured the extinction curve in the far-ultraviolet wavelength region of (900 -- 1200 A) using spectra obtained with the Berkeley EUV/FUV spectrometer during the ORFEUS-I and the ORFEUS-II missions in 1993 and 1996. From the complete sample of early-type stars observed during these missions, we have selected pairs of stars with the same spectral type but different reddenings to measure the differential FUV extinction. We model the effects of molecular hydrogen absorption and exclude affected regions of the spectrum to determine the extinction from dust alone. We minimize errors from inaccuracies in the cataloged spectral types of the stars by making our own determinations of spectral types based on their IUE spectra. We find substantial scatter in the curves of individual star pairs and present a detailed examination of the uncertainties and their effects on each extinction curve. We find that, given the potentially large uncertainties inherent in using the pair method at FUV wavelengths, a careful analysis of measurement uncertainties is critical to assessing the true dust extinction. We present a new measurement of the average far-ultraviolet extinction curve to the Lyman limit; our new measurement is consistent with an extrapolation of the standard extinction curve of Savage & Mathis (1979).Comment: 13 pages text, 7 figures 4 tables. Sent as gzipped tar, with ms.tex and 7 figure

Ultraviolet Extinction Properties in the Milky Way

We have assembled a homogeneous database of 417 ultraviolet (UV) extinction curves for reddened sightlines having International Ultraviolet Explorer (IUE) spectra. We have combined these with optical and 2MASS photometry allowing estimates of the ratio of total-to-selective extinction, R(V), for the entire sample. Fitzpatrick-Massa (FM) parameters have also been found for the entire sample. This is the largest study of parameterized UV extinction curves yet published and it covers a wide range of environments, from dense molecular clouds to the diffuse interstellar medium (ISM), with extinctions A(V) ranging from 0.50 to 4.80. It is the first to extend far beyond the solar neighborhood and into the Galaxy at large, with 30 sightlines having distances > 5 kpc. Previously, the longest sightlines with FM parameters and R(V) extended ~ 1 kpc. We find that (1.) the CCM extinction law applies for 93% of the sightlines, implying that dust processing in the Galaxy is efficient and systematic; (2.) the central wavelength of the 2175 A bump is constant; (3.) the 2175 A bump width is dependent on environment. Only four sightlines show systematic deviations from CCM, HD 29647, 62542, 204827, and 210121. These sightlines all sample dense, molecule-rich clouds. The new extinction curves and values of R(V) allow us to revise the CCM law.Comment: 32 pages, 12 figure

The Spectroscopy of Plasma Evolution from Astrophysical Radiation Mission

The Spectroscopy of Plasma Evolution from Astrophysical Radiation (or the Far-ultraviolet Imaging Spectrograph) instruments, flown aboard the STSAT-1 satellite mission, have provided the first large-area spectral mapping of the cosmic far ultraviolet (FUV, lambda 900-1750 Ang) background. We observe diffuse radiation from hot (10^4 to 10^6 K) and ionized plasmas, molecular hydrogen, and dust scattered starlight. These data provide for the unprecedented detection and discovery of spectral emission from a variety of interstellar environments, including the general medium, molecular clouds, supernova remnants, and super-bubbles. We describe the mission and its data, present an overview of the diffuse FUV sky's appearance and spectrum, and introduce the scientific findings detailed later in this volume

Radiative Transfer Analysis of Far-UV Background Observations Obtained with the Far-Ultraviolet Space Telescope (FAUST)

In 1992 the Far-Ultraviolet Space Telescope (FAUST) provided measurements of the ultraviolet (140-180nm) diffuse sky background at high, medium, and low Galactic latitudes. A significant fraction of the detected radiation was found to be of Galactic origin, resulting from scattering by dust in the diffuse interstellar medium. To simulate the radiative transfer in the Galaxy, we employed a Monte Carlo model which utilized a realistic, non-isotropic radiation field based on the measured fluxes (at 156nm) and positions of 58,000 TD-1 stars, and a cloud structure for the interstellar medium. The comparison of the model predictions with the observations led to a separation of the Galactic scattered radiation from an approximately constant background, attributed to airglow and extragalactic radiation, and to a well constrained determination of the dust scattering properties. The derived dust albedo a = 0.45 +/- 0.05 is substantially lower than albedos derived for dust in dense reflection nebulae and star-forming regions, while the phase function asymmetry g = 0.68 +/- 0.10 is indicative of a strongly forward directed phase function. We show the highly non-isotropic phase function to be responsible, in conjunction with the non-isotropic UV radiation field, for the wide range of observed correlations between the diffusely scattered Galactic radiation and the column densities of neutral atomic hydrogen. The low dust albedo is attributed to a size distribution of grains in the diffuse medium with average sizes smaller than those in dense reflection nebulae.Comment: 35 pages, 10 figures included, to be published in the Ap

Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships

(Abridged) We carried out a comprehensive far-ultraviolet (UV) survey of ^12CO and H_2 column densities along diffuse molecular Galactic sight lines in order to explore in detail the relationship between CO and H_2. We measured new CO abundances from HST spectra, new H_2 abundances from FUSE data, and new CH, CH^+, and CN abundances from the McDonald and European Southern Observatories. A plot of log N(CO) versus log N(H_2) shows that two power-law relationships are needed for a good fit of the entire sample, with a break located at log N(CO, cm^-2) = 14.1 and log N(H_2) = 20.4, corresponding to a change in production route for CO in higher-density gas. Similar logarithmic plots among all five diatomic molecules allow us to probe their relationships, revealing additional examples of dual slopes in the cases of CO versus CH (break at log N = 14.1, 13.0), CH^+ versus H_2 (13.1, 20.3), and CH^+ versus CO (13.2, 14.1). These breaks are all in excellent agreement with each other, confirming the break in the CO versus H_2 relationship, as well as the one-to-one correspondence between CH and H_2 abundances. Our new sight lines were selected according to detectable amounts of CO in their spectra and they provide information on both lower-density (< 100 cm^-3) and higher-density diffuse clouds. The CO versus H_2 correlation and its intrinsic width are shown to be empirically related to the changing total gas density among the sight lines of the sample. We employ both analytical and numerical chemical schemes in order to derive details of the molecular environments. In the low-density gas, where equilibrium-chemistry studies have failed to reproduce the abundance of CH^+, our numerical analysis shows that nonequilibrium chemistry must be employed for correctly predicting the abundances of both CH^+ and CO.Comment: 40 pages in emulateapj style, to appear in the Astrophysical Journa

Spectroscopic survey of Kepler stars. I. HERMES/Mercator observations of A- and F-type stars

The Kepler space mission provided near-continuous and high-precision photometry of about 207 000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Fe i, and Fe ii lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λ Boo types. The distribution of the projected rotational velocity, vsin i, is typical for A and F stars and ranges from 8 to about 280 km s−1, with a mean of 134 km s−1

Effect of manipulation of primary tumour vascularity on metastasis in an adenocarcinoma model

One explanation for the clinical association between tumour vascularity and probability of metastasis is that increased primary tumour vascularity enhances haematogenous dissemination by offering greater opportunity for tumour cell invasion into the circulation (intravasation). We devised an experimental tumour metastasis model that allowed manipulation of primary tumour vascularity with differential exposure of the primary and metastatic tumour site to angiogenic agents. We used this model to assess the effects of local and systemic increases in the level of the angiogenic agent basic fibroblast growth factor on metastasis. BDIX rats with implanted hind limb K12/TR adenocarcinoma tumours received either intratumoural or systemic, basic fibroblast growth factor or saline infusion. Both intratumoural and systemic basic fibroblast growth factor infusion resulted in significant increases in tumour vascularity, blood flow and growth, but not lung metastasis, compared with saline-infused controls. Raised basic fibroblast growth factor levels and increase in primary tumour vascularity did not increase metastasis. The clinical association between tumour vascularity and metastasis is most likely to arise from a metastatic tumour genotype that links increased tumour vascularity with greater metastatic potential