313 research outputs found
Surprisingly Little O VI Emission Arises in the Local Bubble
This paper reports the first study of the O VI resonance line emission (1032,
1038 Angstroms) originating in the Local Bubble (or Local Hot Bubble)
surrounding the solar neighborhood. In spite of the fact that O VI absorption
within the Local Bubble has been observed, no resonance line emission was
detected during our 230 ksec Far Ultraviolet Spectroscopic Explorer observation
toward a ``shadowing'' filament in the southern Galactic hemisphere. As a
result, tight 2 sigma upper limits are set on the intensities in the 1032 and
1038 Angstrom emission lines: 500 and 530 photons cm^{-2} s^{-1} sr^{-1},
respectively. These values place strict constraints on models and simulations.
They suggest that the O VI-bearing plasma and the X-ray emissive plasma reside
in distinct regions of the Local Bubble and are not mixed in a single plasma,
whether in equilibrium with T ~ 10^6 K or highly overionized with T ~ 4 to 6 x
10^4 K. If the line of sight intersects multiple cool clouds within the Local
Bubble, then the results also suggest that hot/cool transition zones differ
from those in current simulations. With these intensity upper limits, we
establish limits on the electron density, thermal pressure, pathlength, and
cooling timescale of the O VI-bearing plasma in the Local Bubble. Furthermore,
the intensity of O VI resonance line doublet photons originating in the
Galactic thick disk and halo is determined (3500 to 4300 photons cm^{-2} s^{-1}
sr^{-1}), and the electron density, thermal pressure, pathlength, and cooling
timescale of its O VI-bearing plasma are calculated. The pressure in the
Galactic halo's O VI-bearing plasma (3100 to 3800 K cm^{-3}) agrees with model
predictions for the total pressure in the thick disk/lower halo. We also report
the results of searches for other emission lines.Comment: accepted by ApJ, scheduled for May 2003, replacement astro-ph
submission corrects typos and grammatical errors in original versio
Microstructure of the Local Interstellar Cloud and the Identification of the Hyades Cloud
We analyze high-resolution UV spectra of the Mg II h and k lines for 18
members of the Hyades Cluster to study inhomogeneity along these proximate
lines of sight. The observations were taken by the Space Telescope Imaging
Spectrograph (STIS) instrument on board the Hubble Space Telescope (HST). Three
distinct velocity components are observed. All 18 lines of sight show
absorption by the Local Interstellar Cloud (LIC), ten stars show absorption by
an additional cloud, which we name the Hyades Cloud, and one star exhibits a
third absorption component. The LIC absorption is observed at a lower radial
velocity than predicted by the LIC velocity vector derived by Lallement &
Bertin (1992) and Lallement et al. (1995), (v(predicted LIC) - v(observed LIC)
= 2.9 +/- 0.7 km/s), which may indicate a compression or deceleration at the
leading edge of the LIC. We propose an extention of the Hyades Cloud boundary
based on previous HST observations of other stars in the general vicinity of
the Hyades, as well as ground-based Ca II observations. We present our fits of
the interstellar parameters for each absorption component. The availability of
18 similar lines of sight provides an excellent opportunity to study the
inhomogeneity of the warm, partially ionized local interstellar medium (LISM).
We find that these structures are roughly homogeneous. The measured Mg II
column densities do not vary by more than a factor of 2 for angular separations
of < 8 degrees, which at the outer edge of the LIC correspond to physical
separations of < 0.6 pc.Comment: 35 pages, 11 figures, AASTEX v.5.0 plus EPSF extensions in mkfig.sty;
accepted by Ap
Distance to the northern high-latitude HI shells
A detailed 3D distribution of interstellar matter in the solar neighborhood
is increasingly necessary. As part of a 3D mapping program, we aim at assigning
a precise distance to the high-latitude HI gas in particular the northern part
(b \geq 55^{circ}) of the shell associated with the conspicuous radio continuum
Loop I. This shell is thought to be the expanding boundary of an interstellar
bubble inflated and recently reheated by the strong stellar winds of the nearby
Scorpius-Centaurus OB. We recorded high-resolution spectra of 30 A-type target
stars located at various distances in the direction of the northern part of
Loop I. Interstellar NaI 5889-5895 and CaII K-H 3934-3968 {\AA} are modeled and
compared with the HI emission spectra from the LAB Survey. About two-thirds of
our stellar spectra possess narrow interstellar lines. Narrow lines are located
at the velocity of the main, low-velocity Loop 1 HI shell ([-6,+1] km/s in the
LSR). Using Hipparcos distances to the target stars, we show that the closest
boundary of the b geq+70^{\circ} part of this low-velocity Loop I arch is
located at of 98 \pm 6 pc. The corresponding interval for the lower-latitude
part (55^{\circ} \leq b \leq 70^{\circ}) is 95-157 pc. However, since the two
structures are apparently connected, the lower limit is more likely. At
variance with this shell, the second HI structure, which is characterized by
LSR Doppler velocities centered at -30 km/s, is NOT detected in any of the
optical spectra. It is located beyond 200 parsecs or totally depleted in NaI
and CaII. We discuss these results in the light of spherical expanding shells
and show that they are difficult to reconcile with simple geometries and a
nearby shell center close to the Plane. Instead, this high-latitude gas seems
to extend the inclined local chimney wall to high distances from the Plane.Comment: Astronomy & Astrophysics (A&A in press
The Ionization of the Local Interstellar Medium, as Revealed by FUSE Observations of N, O and Ar toward White Dwarf Stars
FUSE spectra of the white dwarf stars G191-B2B, GD 394, WD 2211-495 and WD
2331-475 cover the absorption features out of the ground electronic states of N
I, N II, N III, O I and Ar I in the far ultraviolet, providing new insights on
the origin of the partial ionization of the Local Interstellar Medium (LISM),
and for the case of G191-B2B, the interstellar cloud that immediately surrounds
the solar system. Toward these targets the interstellar abundances of Ar I, and
sometimes N I, are significantly below their cosmic abundances relative to H I.
In the diffuse interstellar medium, these elements are not likely to be
depleted onto dust grains. Generally, we expect that Ar should be more strongly
ionized than H (and also O and N whose ionizations are coupled to that of H via
charge exchange reactions) because the cross section for the photoionization of
Ar I is very high. Our finding that Ar I/H I is low may help to explain the
surprisingly high ionization of He in the LISM found by other investigators.
Our result favors the interpretation that the ionization of the local medium is
maintained by a strong EUV flux from nearby stars and hot gases, rather than an
incomplete recovery from a past, more highly ionized condition.Comment: 13 pages, 2 figures. To appear in a special issue of the
Astrophysical Journal Letters devoted to the first scientific results from
the FUSE missio
Deuterium Toward WD1634-573: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission
We use Far Ultraviolet Spectrocopic Explorer (FUSE) observations to study
interstellar absorption along the line of sight to the white dwarf WD1634-573
(d=37.1+/-2.6 pc). Combining our measurement of D I with a measurement of H I
from Extreme Ultraviolet Explorer data, we find a D/H ratio toward WD1634-573
of D/H=(1.6+/-0.5)e-5. In contrast, multiplying our measurements of D I/O
I=0.035+/-0.006 and D I/N I=0.27+/-0.05 with published mean Galactic ISM gas
phase O/H and N/H ratios yields D/H(O)=(1.2+/-0.2)e-5 and
D/H(N)=(2.0+/-0.4)e-5, respectively. Note that all uncertainties quoted above
are 2 sigma. The inconsistency between D/H(O) and D/H(N) suggests that either
the O I/H I and/or the N I/H I ratio toward WD1634-573 must be different from
the previously measured average ISM O/H and N/H values. The computation of
D/H(N) from D I/N I is more suspect, since the relative N and H ionization
states could conceivably vary within the LISM, while the O and H ionization
states will be more tightly coupled by charge exchange.Comment: 23 pages, 5 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by ApJ Supplemen
The Origin of Radio Scintillation In the Local Interstellar Medium
We study three quasar radio sources (B1257-326, B1519-273, and J1819+385)
that show large amplitude intraday and annual scintillation variability
produced by the Earth's motion relative to turbulent-scattering screens located
within a few parsecs of the Sun. We find that the lines of sight to these
sources pass through the edges of partially ionized warm interstellar clouds
where two or more clouds may interact. From the gas flow vectors of these
clouds, we find that the relative radial and transverse velocities of these
clouds are large and could generate the turbulence that is responsible for the
observed scintillation. For all three sight lines the flow velocities of nearby
warm local interstellar clouds are consistent with the fits to the transverse
flows of the radio scintillation signals.Comment: 16 pages, 5 figures; Accepted for publication in Ap
Local ISM 3D Distribution and Soft X-ray Background Inferences for Nearby Hot Gas
Three-dimensional (3D) interstellar medium (ISM) maps can be used to locate not only interstellar (IS) clouds, but also IS bubbles between the clouds that are blown by stellar winds and supernovae, and are filled by hot gas. To demonstrate this, and to derive a clearer picture of the local ISM, we compare our recent 3D IS dust distribution maps to the ROSAT diffuse Xray background maps after removal of heliospheric emission. In the Galactic plane, there is a good correspondence between the locations and extents of the mapped nearby cavities and the soft (0.25 keV) background emission distribution, showing that most of these nearby cavities contribute to this soft X-ray emission. Assuming a constant dust to gas ratio and homogeneous 106 K hot gas filling the cavities, we modeled in a simple way the 0.25 keV surface brightness along the Galactic plane as seen from the Sun, taking into account the absorption by the mapped clouds. The data-model comparison favors the existence of hot gas in the solar neighborhood, the so-called Local Bubble (LB). The inferred mean pressure in the local cavities is found to be approx.9,400/cu cm K, in agreement with previous studies, providing a validation test for the method. On the other hand, the model overestimates the emission from the huge cavities located in the third quadrant. Using CaII absorption data, we show that the dust to CaII ratio is very small in those regions, implying the presence of a large quantity of lower temperature (non-X-ray emitting) ionized gas and as a consequence a reduction of the volume filled by hot gas, explaining at least part of the discrepancy. In the meridian plane, the two main brightness enhancements coincide well with the LB's most elongated parts and chimneys connecting the LB to the halo, but no particular nearby cavity is found towards the enhancement in the direction of the bright North Polar Spur (NPS) at high latitude. We searched in the 3D maps for the source regions of the higher energy (0.75 keV) enhancements in the fourth and first quadrants. Tunnels and cavities are found to coincide with the main bright areas, however no tunnel nor cavity is found to match the low-latitude b > or approx. 8deg, brightest part of the NPS. In addition, the comparison between the 3D maps and published spectral data favors a NPS central source region location beyond 230 pc, i.e. at larger distance than usually considered. Those examples illustrate the potential use of more detailed 3D distributions of the nearby ISM for the interpretation of the diffuse soft X-ray background
The Structure of the Local Interstellar Medium V: Electron Densities
We present a comprehensive survey of CII* absorption detections toward stars
within 100 pc in order to measure the distribution of electron densities
present in the local interstellar medium (LISM). Using high spectral resolution
observations of nearby stars obtained by GHRS and STIS onboard the Hubble Space
Telescope, we identify 13 sight lines with 23 individual CII* absorption
components, which provide electron density measurements, the vast majority of
which are new. We employ several strategies to determine more accurate CII
column densities from the saturated CII resonance line, including, constraints
of the line width from the optically thin CII* line, constraints from
independent temperature measurements of the LISM gas based on line widths of
other ions, and third, using measured SII column densities as a proxy for CII
column densities. The sample of electron densities appears consistent with a
log-normal distribution and an unweighted mean value of n_e(CII_SII) =
0.11^+0.10_-0.05 cm^-3. Seven individual sight lines probe the Local
Interstellar Cloud (LIC), and all present a similar value for the electron
density, with a weighted mean of n_e(LIC) = 0.12 +/- 0.04 cm^-3. The Hyades
Cloud, a decelerated cloud at the leading edge of the platoon of LISM clouds,
has a significantly higher electron density than the LIC. Observed toward
G191-B2B, the high electron density may be caused by the lack of shielding from
such a strong radiation source. Given some simple assumptions, the range of
observed electron densities translates into a range of thermal pressures, P/k =
3300^+5500_-1900 K cm^-3. This work greatly expands the number of electron
density measurements and provides important constraints on the ionization,
abundance, and evolutionary models of the local interstellar medium. (abridged)Comment: 41 pages, 9 figures; Accepted for publication in Ap
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