221 research outputs found
New 3-D gas density maps of NaI and CaII interstellar absorption within 300pc
We present new high resolution (R>50,000) absorption measurements of the NaI
doublet (5889 - 5895A) along 482 nearby sight-lines, in addition to 807 new
measurements of the CaII K (3933A) absorption line. We have combined these new
data with previously reported measurements to produce a catalog of absorptions
towards a total of 1857 early-type stars located within 800pc of the Sun. Using
these data we have determined the approximate 3-dimensional spatial
distribution of neutral and partly ionized interstellar gasdensity within a
distance-cube of 300pc from the Sun. All newly recorded spectra were analyzed
by means of a multi-component line profile-fitting program, in most cases using
simultaneous fits to the line doublets. Normalized absorption profiles were
fitted by varying the velocity, doppler width and column density for all
intervening interstellar clouds. The resulting total column densities were then
used in conjunction with the Hipparcos distances of the target stars to
construct inversion maps of the 3-D spatial density distribution of the NaI and
CaII bearing gas. A plot of the equivalent width of NaI versus distance reveals
a wall of neutral gas at ~80pc that can be associated with the boundary wall to
the central rarefied Local Cavity region. In contrast, a similar plot for the
equivalent width of CaII shows no sharply increasing absorption at 80pc, but
instead we observe a slowly increasing value of CaII equivalent width with
increasing sight-line distance sampled.Comment: A&A accepte
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
Gas Absorption Detected from the Edge-on Debris Disk Surrounding HD32297
Near-infrared and optical imaging of HD32297 indicate that it has an edge-on
debris disk, similar to beta Pic. I present high resolution optical spectra of
the NaI doublet toward HD32297 and stars in close angular proximity. A
circumstellar absorption component is clearly observed toward HD32297 at the
stellar radial velocity, which is not observed toward any of its neighbors,
including the nearest only 0.9 arcmin away. An interstellar component is
detected in all stars >90 pc, including HD32297, likely due to the interstellar
material at the boundary of the Local Bubble. Radial velocity measurements of
the nearest neighbors, BD+07 777s and BD+07 778, indicate that they are
unlikely to be physically associated with HD32297. The measured circumstellar
column density around HD32997, log N(NaI) ~ 11.4, is the strongest NaI
absorption measured toward any nearby main sequence debris disk, even the
prototypical edge-on debris disk, beta Pic. Assuming that the morphology and
abundances of the gas component around HD32297 are similar to beta Pic, I
estimate an upper limit to the gas mass in the circumstellar disk surrounding
HD32297 of ~0.3 M_Earth.Comment: 13 pages, 2 figures; Accepted for publication in ApJ Letter
BULLDOZER, A FREE OPEN SOURCE SCALABLE SOFTWARE FOR DTM EXTRACTION
The paper introduces a software called Bulldozer, designed to extract a Digital Terrain Model (DTM) from a Digital Surface Model (DSM) obtained from various sensors. The software is based on a modified version of the multi-scale Drap Cloth principle to process noisy DSMs of any size, employing a tiling strategy and a stability margin to ensure consistent results. A parameter called max object size is introduced to differentiate objects from the ground during the drap cloth process. Gravity steps and ground distance sampling resolution are adjusted based on the input DSM. No-data and noisy values present in the DSM are detected and converted into no-data values to improve the quality of the Cloth simulation. The paper describes a memory-aware parallel execution strategy using both the multiprocessing and the shared memory Python modules. A benchmark dataset has been created to analyze the results and compare them with alternative approaches and reference datasets. Bulldozer offers an extensive Python API. It is open-source and available on PyPi and GitHub. Additionally, a QGIS plugin has been developed
Reachability problems for products of matrices in semirings
We consider the following matrix reachability problem: given square
matrices with entries in a semiring, is there a product of these matrices which
attains a prescribed matrix? We define similarly the vector (resp. scalar)
reachability problem, by requiring that the matrix product, acting by right
multiplication on a prescribed row vector, gives another prescribed row vector
(resp. when multiplied at left and right by prescribed row and column vectors,
gives a prescribed scalar). We show that over any semiring, scalar reachability
reduces to vector reachability which is equivalent to matrix reachability, and
that for any of these problems, the specialization to any is
equivalent to the specialization to . As an application of this result and
of a theorem of Krob, we show that when , the vector and matrix
reachability problems are undecidable over the max-plus semiring
. We also show that the matrix, vector, and scalar
reachability problems are decidable over semirings whose elements are
``positive'', like the tropical semiring .Comment: 21 page
High ions towards white dwarfs: circumstellar line shifts and stellar temperature
Based on a compilation of OVI, CIV, SiIV and NV data from IUE, FUSE, GHRS,
STIS, and COS, we derive an anti- correlation between the stellar temperature
and the high ion velocity shift w.r.t. to the photosphere, with positive (resp.
negative) velocity shifts for the cooler (resp. hotter) white dwarfs. This
trend probably reflects more than a single process, however such a dependence
on the WD's temperature again favors a CS origin for a very large fraction of
those ion absorptions, previously observed with IUE, HST-STIS, HST-GHRS, FUSE,
and now COS, selecting objects for which absorption line radial velocities,
stellar effective temperature and photospheric velocity can be found in the
literature. Interestingly, and gas in near-equilibrium in the star vicinity. It
is also probably significant that the temperature that corresponds to a null
radial velocity, i.e. \simeq 50,000K, also corresponds to the threshold below
which there is a dichotomy between pure or heavy elements atmospheres as well
as some temperature estimates for and a form of balance between radiation
pressure and gravitation. This is consistent with ubiquitous evaporation of
orbiting dusty material. Together with the fact that the fraction of stars with
(red-or blue-) shifted lines and the fraction of stars known to possess heavy
species in their atmosphere are of the same order, such a velocity-temperature
relationship is consistent with quasi-continuous evaporation of orbiting CS
dusty material, followed by accretion and settling down in the photosphere. In
view of these results, ion measurements close to the photospheric or the IS
velocity should be interpreted with caution, especially for stars at
intermediate temperatures. While tracing CS gas, they may be erroneously
attributed to photospheric material or to the ISM, explaining the difficulty of
finding a coherent pattern of the high ions in the local IS 3D distribution.Comment: Accepted by A&A. Body of paper identical to v1. This submission has a
more appropriate truncation of the original abstrac
Graph products of spheres, associative graded algebras and Hilbert series
Given a finite, simple, vertex-weighted graph, we construct a graded
associative (non-commutative) algebra, whose generators correspond to vertices
and whose ideal of relations has generators that are graded commutators
corresponding to edges. We show that the Hilbert series of this algebra is the
inverse of the clique polynomial of the graph. Using this result it easy to
recognize if the ideal is inert, from which strong results on the algebra
follow. Non-commutative Grobner bases play an important role in our proof.
There is an interesting application to toric topology. This algebra arises
naturally from a partial product of spheres, which is a special case of a
generalized moment-angle complex. We apply our result to the loop-space
homology of this space.Comment: 19 pages, v3: elaborated on connections to related work, added more
citations, to appear in Mathematische Zeitschrif
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
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