350 research outputs found
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
Orbital Parameters and Chemical Composition of Four White Dwarfs in Post-Common Envelope Binaries
We present FUSE observations of the hot white dwarfs in the post-common
envelope binaries Feige 24, EUVE J0720-317, BPM 6502, and EUVE J2013+400. The
spectra show numerous photospheric absorption lines which trace the white dwarf
orbital motion. We report the detection of C III, O VI, P V, and Si IV in the
spectra of Feige 24, EUVE J0720-317 and EUVE J2013+400, and the detection of C
III, N II, Si III, Si IV, and Fe III in the spectra of BPM 6502. Abundance
measurements support the possibility that white dwarfs in post-common envelope
binaries accrete material from the secondary star wind. The FUSE observations
of BPM 6502 and EUVE J2013+400 cover a complete binary orbit. We used the FUSE
spectra to measure the radial velocities traced by the white dwarf in the four
binaries, where the zero-point velocity were fixed using the ISM velocities in
the line of sight of the stellar systems. For BPM 6502 we determined a white
dwarf velocity semi-amplitude of K_WD = 18.6+/-0.5km/s, and with the velocity
semi-amplitude of the red dwarf companion (K_RD = 75.2+/-3.1 km/s), we estimate
the mass ratio to be q = 0.25+/-0.01. Adopting a spectroscopic mass
determination for the white dwarf, we infer a low secondary mass of M_RD =
0.14+/-0.01 M_solar. For EUVE J2013+400 we determine a white dwarf velocity
semi-amplitude of K_WD = 36.7+/-0.7 km/s. The FUSE observations of EUVE
J0720-317 cover approximately 30% of the binary period and combined with the
HST GHRS measurements (Vennes et al. 1999, ApJ 523, 386), we update the binary
properties. FUSE observations of Feige 24 cover approximately 60% of the orbit
and we combine this data set with HST STIS (Vennes et al. 2000, ApJ, 544, 423)
data to update the binary properties.Comment: Accepted for publication in Ap
Regular obstructed categories and TQFT
A proposal of the concept of -regular obstructed categories is given. The
corresponding regularity conditions for mappings, morphisms and related
structures in categories are considered. An n-regular TQFT is introduced. It is
shown the connection of time reversibility with the regularity.Comment: 22 pages in Latex. To be published in J. Math. Phy
FUSE Survey of the Local Interstellar Medium within 200 Parsec
We present FUSE observations of the interstellar gas toward 30 white dwarf
and 1 subdwarf (SdO) stars. These sightlines probe the Local Bubble (LB) and
the local interstellar medium (LISM) near the LB. Our survey detected only
diffuse H_2 molecular clouds along six sightlines. There is no evidence from
this study that H_2 exists well inside the perimeter of the LB. The kinematical
temperature for H_2 is less than the usual temperature observed in the local
interstellar clouds, implying different gas phases in the LISM. The relative
abundance ratios of Si II, P II, and Fe II give insight about the dust content.
These ratios vary, but are similar to the depletion patterns observed in warm
and halo diffuse clouds in more distant sightlines in the Galaxy. The N I/O I
and Ar I/O I ratios are significantly subsolar within the LB. Outside the LB a
larger scatter is observed from subsolar to solar. Because Ar and N are only
weakly depleted into dust grains if at all, the deficiencies of their neutral
forms are likely due to photoionization. The evidence for significant
ionization of N (and hence Ar) is strengthened by the detection and measurement
of N II, which is a dominant ion for this element toward many sightlines. These
observations imply that photoionization is the main ionization mechanism in the
LISM and do not support the existence of a highly ionized condition in the
past. In view of the variations observed in the different atomic and ionic
ratios, the photoionization conditions vary significantly in the LB and the
LISM. [Abridged.]Comment: Accepted for publication in the Ap
Aspects of the Mass Distribution of Interstellar Dust Grains in the Solar System from In-Situ Measurements
The in-situ detection of interstellar dust grains in the Solar System by the
dust instruments on-board the Ulysses and Galileo spacecraft as well as the
recent measurements of hyperbolic radar meteors give information on the
properties of the interstellar solid particle population in the solar vicinity.
Especially the distribution of grain masses is indicative of growth and
destruction mechanisms that govern the grain evolution in the interstellar
medium. The mass of an impacting dust grain is derived from its impact velocity
and the amount of plasma generated by the impact. Because the initial velocity
and the dynamics of interstellar particles in the Solar System are well known,
we use an approximated theoretical instead of the measured impact velocity to
derive the mass of interstellar grains from the Ulysses and Galileo in-situ
data. The revised mass distributions are steeper and thus contain less large
grains than the ones that use measured impact velocities, but large grains
still contribute significantly to the overall mass of the detected grains. The
flux of interstellar grains with masses is determined to
be . The comparison of radar data
with the extrapolation of the Ulysses and Galileo mass distribution indicates
that the very large () hyperbolic meteoroids detected by
the radar are not kinematically related to the interstellar dust population
detected by the spacecraft.Comment: 14 pages, 11 figures, to appear in JG
Comparing various multi-component global heliosphere models
Modeling of the global heliosphere seeks to investigate the interaction of
the solar wind with the partially ionized local interstellar medium. Models
that treat neutral hydrogen self-consistently and in great detail, together
with the plasma, but that neglect magnetic fields, constitute a sub-category
within global heliospheric models. There are several different modeling
strategies used for this sub-category in the literature. Differences and
commonalities in the modeling results from different strategies are pointed
out. Plasma-only models and fully self-consistent models from four research
groups, for which the neutral species is modeled with either one, three, or
four fluids, or else kinetically, are run with the same boundary parameters and
equations. They are compared to each other with respect to the locations of key
heliospheric boundary locations and with respect to the neutral hydrogen
content throughout the heliosphere. In many respects, the models' predictions
are similar. In particular, the locations of the termination shock agree to
within 7% in the nose direction and to within 14% in the downwind direction.
The nose locations of the heliopause agree to within 5%. The filtration of
neutral hydrogen from the interstellar medium into the inner heliosphere,
however, is model dependent, as are other neutral results including the
hydrogen wall. These differences are closely linked to the strength of the
interstellar bow shock. The comparison also underlines that it is critical to
include neutral hydrogen into global heliospheric models.Comment: 10 pages, 4 figures, submitted to a special section at A&A of an ISSI
team "Determination of the physical Hydrogen parameters of the LIC from
within the Heliosphere
Stellar Lyman-alpha Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres
We search the Hubble Space Telescope (HST) archive for previously unanalyzed
observations of stellar H I Lyman-alpha emission lines, our primary purpose
being to look for new detections of Lyman-alpha absorption from the outer
heliosphere, and to also search for analogous absorption from the astrospheres
surrounding the observed stars. The astrospheric absorption is of particular
interest because it can be used to study solar-like stellar winds that are
otherwise undetectable. We find and analyze 33 HST Lyman-alpha spectra in the
archive. All the spectra were taken with the E140M grating of the Space
Telescope Imaging Spectrograph (STIS) instrument on board HST. The HST/STIS
spectra yield 4 new detections of heliospheric absorption (70 Oph, Xi Boo, 61
Vir, and HD 165185) and 7 new detections of astrospheric absorption (EV Lac, 70
Oph, Xi Boo, 61 Vir, Delta Eri, HD 128987, and DK UMa), doubling the previous
number of heliospheric and astrospheric detections. When combined with previous
results, 10 of 17 lines of sight within 10 pc yield detections of astrospheric
absorption. This high detection fraction implies that most of the ISM within 10
pc must be at least partially neutral, since the presence of H I within the ISM
surrounding the observed star is necessary for an astrospheric detection. In
contrast, the detection percentage is only 9.7% (3 out of 31) for stars beyond
10 pc. Our Lyman-alpha analyses provide measurements of ISM H I and D I column
densities for all 33 lines of sight, and we discuss some implications of these
results. Finally, we measure chromospheric Lyman-alpha fluxes from the observed
stars. We use these fluxes to determine how Lyman-alpha flux correlates with
coronal X-ray and chromospheric Mg II emission, and we also study how
Lyman-alpha emission depends on stellar rotation.Comment: 56 pages, 15 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by ApJ
Consequences of a Change in the Galactic Environment of the Sun
The interaction of the heliosphere with interstellar clouds has attracted
interest since the late 1920's, both with a view to explaining apparent
quasi-periodic climate "catastrophes" as well as periodic mass extinctions.
Until recently, however, models describing the solar wind - local interstellar
medium (LISM) interaction self-consistently had not been developed. Here, we
describe the results of a two-dimensional (2D) simulation of the interaction
between the heliosphere and an interstellar cloud with the same properties as
currently, except that the neutral H density is increased from the present
value of n(H) ~ 0.2 cm^-3 to 10 cm^-3. The mutual interaction of interstellar
neutral hydrogen and plasma is included. The heliospheric cavity is reduced
considerably in size (approximately 10 - 14 AU to the termination shock in the
upstream direction) and is highly dynamical. The interplanetary environment at
the orbit of the Earth changes markedly, with the density of interstellar H
increasing to ~2 cm^-3. The termination shock itself experiences periods where
it disappears, reforms and disappears again. Considerable mixing of the shocked
solar wind and LISM occurs due to Rayleigh-Taylor-like instabilities at the
nose, driven by ion-neutral friction. Implications for two anomalously high
concentrations of 10Be found in Antarctic ice cores 33 kya and 60 kya, and the
absence of prior similar events, are discussed in terms of density enhancements
in the surrounding interstellar cloud. The calculation presented here supports
past speculation that the galactic environment of the Sun moderates the
interplanetary environment at the orbit of the Earth, and possibly also the
terrestrial climate.Comment: 23 pages, 2 color plates (jpg), 3 figures (eps
Perspectives on Interstellar Dust Inside and Outside of the Heliosphere
Measurements by dust detectors on interplanetary spacecraft appear to
indicate a substantial flux of interstellar particles with masses exceeding
10^{-12}gram. The reported abundance of these massive grains cannot be typical
of interstellar gas: it is incompatible with both interstellar elemental
abundances and the observed extinction properties of the interstellar dust
population. We discuss the likelihood that the Solar System is by chance
located near an unusual concentration of massive grains and conclude that this
is unlikely, unless dynamical processes in the ISM are responsible for such
concentrations. Radiation pressure might conceivably drive large grains into
"magnetic valleys". If the influx direction of interstellar gas and dust is
varying on a ~10 yr timescale, as suggested by some observations, this would
have dramatic implications for the small-scale structure of the interstellar
medium.Comment: 13 pages. To appear in Space Science Review
- âŠ