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Important progress in the field of the Interstellar Medium (ISM) in general, and the Local Interstellar Medium (LISM) in particular, has been largely due to the recent launch of orbiting satellite missions and earth-bound observations with unprecedented sensitivity and high spectral or spatial resolution. However, the interpretation of a wealth of new data is still based to a great extent on models...
Constraints on the ambient (circumsolar) interstellar medium can be derived from observations of interstellar neutrals or their derivatives in the heliosphere. New results have been recently obtained which, when combined with optical and UV observations of the nearby stars (HST-GHRS, EUVE), remove long-standing contradictions and allow us to infer for our local cloud, pressure, ionization and limits...
We propose a new method to constrain the actual state of the interstellar cloud that surrounds the solar system. Using Voyager UVS Lyman-α sky maps and the powerful principle of invariance, we derive the H distribution all along the spacecraft path. Provided current models of the heliopause interface between the solar and the interstellar winds, we extrapolate this distribution to farther distances...
Ultraviolet absorption lines from carbon atoms within a neutral cloud inside or near the edge of the Local Bubble can reveal the thermal pressure of the gas, since the relative populations of the fine-structure levels are determined by a balance between collisional excitation and radiative decay. Features from a C I multiplet appearing in the uv spectrum of the star δ Cyg (l = 97°, b = +10°, d = 52...
We report on the status of a long-term program of mapping the velocity structure of the local ISM using ultra-high resolution observations of CaII K-line absorption towards nearby B and A-type stars. Absorption components common to the local interstellar cloud (LIC) have been detected in many (but not all) lines-of-sight. Our preliminary results indicate that the local ISM has a complex velocity pattern...
The EURD instrument has been designed to measure diffuse emission in the extreme ultraviolet (350–1100 Å). This new design provides an unprecedented 4–5 Å spectral resolution and 200 photons/sec/cm2/sr sensitivity after only 100 hours of observations. One of the goals of this project is to search for spectral lines of highly ionized species from the high temperature component (105 – 106 K) of the...
New results from EUVE are reviewed, with emphasis on the ionization state of the ISM and the question of thermal pressure balance between warm clouds and the hot intercloud medium. A description of what may be expected from the next generation of experiments, some to be launched shortly, will also be presented.
We present results from a search for FUV emission from the diffuse ISM conducted with an orbital FUV spectrometer, DUVE, which was launched in July, 1992. The DUVE spectrometer, which covers the band from 950 Å to 1080 Å with 3.2 Å resolution, observed a region of low neutral hydrogen column density near the south galactic pole for a total effective integration time of 1583 seconds. the only emission...
We have used the Deep Survey telescope of EUVE to investigate shadows in the diffuse EUV/Soft X-ray background cast by clouds in the interstellar medium. We confirm the existence of a shadow previously reported, and provide evidence for two new shadows. We used IRAS data to identify the clouds producing these shadows and to determine their optical depth to EUV radiation. The EUV-absorbing clouds are...
The first X-ray-opaque cloud in the direction of Auriga lies at 180 pc. A tenuous cloud, whose distance is derived from IUE absorption line studies as well as from an analysis of ROSAT data, is embedded at 55 pc. The X-ray emitting gas has a temperature of 105.9–6.0 K. The pressure is 1.6 104 K cm−3.
We have studied the EUV spectra of 13 DA white dwarfs, observed by the EUVE satellite, paying attention to the possible sources of absorbing material along the lines-of-sight in both the local interstellar medium and in the photospheres of the stars themselves. The range of interstellar column densities seen are consistent with our previous understanding of the local distribution of material. Absorption...
During the second flight of the ORFEUS-SPAS satellite (Nov./Dec. 96) high resolution (λ/Δλ = 10,000) Echelle-spectra of BD + 28° 4211 in the wavelength regime 912 – 1400 Å have been taken. Deuterium can be clearly identified in the ORFEUS II Echelle-spectra of this star. For the first time it was possible to take spectra of faint, not redshifted objects in the far ultraviolet with a sufficient spectral...
Observations of the diffuse EUV background towards 138 different directions using the spectrometers aboard the Extreme Ultraviolet Explorer satellite (EUVE) have been combined into a spectrum from 150Å to 730Å and represent an effective exposure of 18 million seconds. There is no significant evidence for any line flux from any source other than the geocorona. These results are inconsistent with the...
The Diffuse X-ray Spectrometer (DXS) obtained spectra of the low energy X-ray (44 – 83 Å) diffuse background near the galactic plane from galactic longitudes 150° ≲ l ≲ 300° with ≲ 3 Å spectral resolution and ∼ 15° angular resolution. Thus, DXS measured X-ray spectra that arise almost entirely from within the Local Bubble. The DXS spectra show emission lines and emission-line blends, indicating that...
Competing models for the origin of the local component of the diffuse X-ray background span more than an order of magnitude in temperature. We propose to test these models with a new instrument, the GRating Array Diffuse EUV Spectrometer (GRADES), which is sensitive to diffuse emission in the 90 – 260 Å bandpass, and offers a peak spectral resolution of about λ/127.
We plan to observe hot gas in the local interstellar medium (LISM) at high resolution using a rocket-borne spectrograph that can simultaneously observe OVI, NV and CIV at resolutions of 1 – 2 km s−1. Data from a single flight will have sufficient signal to noise ratio to detect a broad 50 km s−1 component down to a column density of N ≥ 0.5 × 1013 cm−2. High-velocity components as fast as 300 km s...
Nearby late-B and early-A stars show clear IS absorptions in Mg II, C II and other lines (IUE spectra). Some of them, in the Sco Cen direction, also show absorption features at C IV and Si IV line wavelengths. We discuss the possible formation of these lines in an intermediate temperature gas region between the dense and cold H I wall and the hot ISM, a region that could be the collision interaction...
The X-ray sky at 1/4 keV is completely dominated by diffuse emission. It has become clear that it originates as at least three separate components: local emission within the nearest ≈ 100 pc from the Sun, halo emission from beyond most of the neutral material of the Galactic disk, and the superposition of unresolved extragalactic sources. The only way to determine the temperatures and relative emission...
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