509 research outputs found
Aditional Ultra-High-Resolution Observations of Ca+ Ions in the Local Insterstellar Medium
We present ultra-high-resolution (0.35 km sâ1 FWHM) observations of the interstellar Ca K line towards seven nearby stars. The spectral resolution was sufficient to resolve the line profiles fully, thereby enabling us to detect hitherto unresolved velocity components, and to obtain accurate measurements of the velocity dispersions (b values). Absorption components with velocities similar to those expected for the Local Interstellar Cloud (LIC) and the closely associated âG cloudâ were identified towards six of the seven stars. However, in most cases the b values deduced for these components were significantly larger than the b â 2.2 km sâ1 (i.e. Tk â 7000 K, vt â 1 km sâ1) expected for the LIC, and it is argued that this results from the presence of additional, spectrally unresolved, components having similar velocities and physical conditions. For two stars (ÎŽ Vel and α Pav) we detect interstellar components with much smaller b values (1.1 ± 0.3 and 0.8 ± 0.1 km sâ1, respectively) than are expected for low-density clouds within the Local Bubble. In the case of the narrow α Pav component, we also find an anomalously large Na i/Ca ii column density ratio, which is indicative of a relatively high density. Thus it is possible that, in addition to LIC-type clouds, the local interstellar medium contains a population of previously undetected cooler and denser interstellar clouds
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
OVII and OVIII line emission in the diffuse soft X-ray background: heliospheric and galactic contributions
We study the 0.57 keV (O VII triplet) and 0.65 keV (O VIII) diffuse emission
generated by charge transfer collisions between solar wind (SW) oxygen ions and
interstellar H and He neutral atoms in the inner Heliosphere. These lines which
dominate the 0.3-1.0 keV energy interval are also produced by hot gas in the
galactic halo (GH) and possibly the Local Interstellar Bubble (LB). We
developed a time-dependent model of the SW Charge-Exchange (SWCX) X-ray
emission, based on the localization of the SW Parker spiral at each instant. We
include input SW conditions affecting three selected fields, as well as
shadowing targets observed with XMM-Newton, Chandra and Suzaku and calculate
X-ray emission fot O VII and O VIII lines. We determine SWCX contamination and
residual emission to attribute to the galactic soft X-ray background. We obtain
ground level intensities and/or simulated lightcurves for each target and
compare to X-ray data. The local 3/4 keV emission (O VII and O VIII) detected
in front of shadowing clouds is found to be entirely explained by the CX
heliospheric emission. No emission from the LB is needed at these energies.
Using the model predictions we subtract the heliospheric contribution to the
measured emission and derive the halo contribution. We also correct for an
error in the preliminary analysis of the Hubble Deep Field North (HDFN).Comment: 21 pages (3 on-line), 10 figures (4 on-line), accepted for
publication in Astronomy and Astrophysic
The Velocity Distribution of the Nearest Interstellar Gas
The bulk flow velocity for the cluster of interstellar cloudlets within about
30 pc of the Sun is determined from optical and ultraviolet absorption line
data, after omitting from the sample stars with circumstellar disks or variable
emission lines and the active variable HR 1099. Ninety-six velocity components
towards the remaining 60 stars yield a streaming velocity through the local
standard of rest of -17.0+/-4.6 km/s, with an upstream direction of l=2.3 deg,
b=-5.2 deg (using Hipparcos values for the solar apex motion). The velocity
dispersion of the interstellar matter (ISM) within 30 pc is consistent with
that of nearby diffuse clouds, but present statistics are inadequate to
distinguish between a Gaussian or exponential distribution about the bulk flow
velocity. The upstream direction of the bulk flow vector suggests an origin
associated with the Loop I supernova remnant. Groupings of component velocities
by region are seen, indicating regional departures from the bulk flow velocity
or possibly separate clouds. The absorption components from the cloudlet
feeding ISM into the solar system form one of the regional features. The
nominal gradient between the velocities of upstream and downstream gas may be
an artifact of the Sun's location near the edge of the local cloud complex. The
Sun may emerge from the surrounding gas-patch within several thousand years.Comment: Typographical errors corrected; Five tables, seven figures;
Astrophysical Journal, in pres
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
Two-component model of the interaction of an interstellar cloud with surrounding hot plasma
We present a two-component gasdynamic model of an interstellar cloud embedded
in a hot plasma. It is assumed that the cloud consists of atomic hydrogen gas,
interstellar plasma is quasineutral. Hydrogen atoms and plasma protons interact
through a charge exchange process. Magnetic felds and radiative processes are
ignored in the model. The influence of heat conduction within plasma on the
interaction between a cloud and plasma is studied. We consider the extreme case
and assume that hot plasma electrons instantly heat the plasma in the
interaction region and that plasma flow can be described as isothermal. Using
the two-component model of the interaction of cold neutral cloud and hot
plasma, we estimate the lifetime of interstellar clouds. We focus on the clouds
typical for the cluster of local interstellar clouds embedded in the hot Local
Bubble and give an estimate of the lifetime of the Local interstellar cloud
where the Sun currently travels. The charge transfer between highly charged
plasma ions and neutral atoms generates X-ray emission. We assume typical
abundance of heavy ions for the Local Bubble plasma and estimate the X-ray
emissivity due to charge exchange from the interface between cold neutral cloud
and hot plasma. Our results show that charge exchange X-ray emission from the
neutral-plasma interfaces can be a non-negligible fraction of the observed
X-ray emission.Comment: 9 pages, 7 figure
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
Exploring Interstellar Titanium and Deuterium Abundances and Other Correlations
The origin of the observed variability of the gas-phase D/H ratio in the
local interstellar medium is still debated, and in particular the role of
deuterium depletion onto dust grains. Here we extend the study of the
relationship between deuterium and titanium, a refractory species and tracer of
elemental depletion, and explore other relationships. We have acquired high
resolution spectra for nine early-type stars using the VLT/UVES spectrograph,
and detected the absorption lines of interstellar TiII. Using a weighted
orthogonal distance regression (ODR) code and a special method to treat non
symmetric errors, we compare the TiII columns with the corresponding HI, DI and
also OI columns. We perform in parallel the same comparisons for available FeII
data. We find a significant correlation between TiII/HI and D/H in our data
set, and, when combined with published results, we confirm and better constrain
a previously published trend and extend it to low HI columns. We exclude
uncertainties in HI and OI columns as the main contributor to the derived
metals-deuterium correlations by showing that the TiII/HI ratio is positively
correlated with DI/OI. We find a similar correlation between FeII/HI and
DI/OI.The TiII gradients are similar or slightly smaller than for FeII, while
one would expect larger variations on the basis of the higher condensation
temperature of titanium. However we argue that ionisation effects introduce
biases that affect iron and not titanium and may explain the gradient
similarity. We find a less significant negative correlation between the TiII/DI
ratio and the hydrogen column, possibly a sign of different evaporation of D
and metals according to the cloud properties
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
- âŠ