184 research outputs found
On the Decades-Long Stability of the Interstellar Wind through the Solar System
We have revisited the series of observations recently used to infer a
temporal variation of the interstellar helium flow over the last forty years.
Concerning the recent IBEX-Lo direct detection of Helium neutrals, there are
two types of precise and unambiguous measurements which do not rely on the
exact response of the instrument: the count rate maxima as a function of the
spin angle, which determines the ecliptic latitude of the flow, and the count
rate maxima as a function of IBEX longitude, which determines a tight
relationship between the ecliptic longitude of the flow and its velocity far
from the Sun. These measurements provide parameters (and couples of parameters
in the second case) remarkably similar to the canonical, old values. In
contrast, the preferential choice of a lower velocity and higher longitude
reported before from IBEX data is based only on the count rate variation (at
each spin phase maximum) as a function of the satellite longitude, when
drifting across the region of high fluxes. We have examined the consequences of
dead time counting effects, and conclude that their inclusion at a realistic
level is sufficient to reconcile the data with the old parameters, calling for
further investigations. We discuss the analyses of the STEREO pickup ion (PUI)
data and argue that the statistical method that has been preferred to infer the
neutral flow longitude (instead of the more direct method based on the PUI
maximum flux directions), is not appropriate. Moreover, transport effects may
have been significant at the very weak solar activity level of 2007-2009, in
which case the longitudes of the PUI maxima are only upper limits on the flow
longitude. Finally, we found that the use of some flow longitude determinations
based on UV glow data are not adequate. At variance with recent conclusions we
find no evidence for a temporal variability of the interstellar helium flow.Comment: 8 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Automated measurements of diffuse interstellar bands in early-type star spectra Correlations with the Color Excess
Stellar spectroscopic surveys may bring useful statistical information on the
links between Diffuse Interstellar Bands (DIBs) and interstellar environment.
DIB databases can also be used as a complementary tool for locating
interstellar (IS) clouds. Our goal is to develop fully automated methods of DIB
measurements to be applied to extensive data from stellar surveys. We present a
method appropriate for early-type nearby stars, its application to
high-resolution spectra of 130 targets recorded with ESO FEROS spectrograph,
and comparisons with other determinations. Using a DIB average profile deduced
from the most reddened stars, we performed an automated fitting of a
combination of a smooth stellar continuum, the DIB profile, and, when
necessary, a synthetic telluric transmission. Measurements are presented for 16
DIBs in the optical domain that could be extracted automatically: 4726.8,
4762.6, 4963.9, 5780.4, 5797.1, 5849.8, 6089.8, 6196.0, 6203.0-6204.5, 6269.8,
6283.8, 6379.3, 6445.3, 6613.6, 6660.7, and 6699.3 {\AA}.Comment: (A&A accepted
A catalog of 1.5273 micron diffuse interstellar bands based on APOGEE hot telluric calibrators
High resolution stellar spectroscopic surveys provide massive amounts of
diffuse interstellar bands (DIBs) measurements. Data can be used to study the
distribution of the DIB carriers and those environmental conditions that favor
their formation. In parallel, recent studies have also proved that DIBs
extracted from stellar spectra constitute new tools for building the 3D
structure of the Galactic Interstellar Medium (ISM). The amount of details on
the structure depends directly on the quantity of available lines of sight
(LOS). Therefore there is a need to construct databases of high-quality DIB
measurements as large as possible. We aim at providing the community with a
catalog of high-quality measurements of the 1.5273 micron DIB towards a large
fraction of the Apache Point Observatory Galactic Evolution Experiment (APOGEE)
hot stars observed to correct for the telluric absorption and not used for ISM
studies so far. This catalog would complement the extensive database recently
extracted from the APOGEE observations and used for 3D ISM mapping. We devised
a method to fit the stellar continuum of the hot calibration stars and
extracted the DIB from the normalized spectrum. Severe selection criteria based
on the absorption characteristics are applied to the results. In particular
limiting constraints on the DIB widths and Doppler shifts are deduced from the
HI 21 cm measurements, following a new technique of decomposition of the
emission spectra. From ~16 000 available hot telluric spectra we have extracted
~ 6700 DIB measurements and their associated uncertainties. The statistical
properties of the extracted absorptions are examined and our selection criteria
are shown to provide a robust dataset. The resulting catalog contains the DIB
total equivalent widths, central wavelengths and widths. We briefly illustrate
its potential use for the stellar and interstellar communities.Comment: The Astrophysical Journal Supplement, in press, 35 pages, 14 figure
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
Solar wind charge exchange X-ray emission from Mars Model and data comparison
Aims. We study the soft X-ray emission induced by charge exchange (CX)
collisions between solar-wind, highly charged ions and neutral atoms of the
Martian exosphere. Methods. A 3D multi species hybrid simulation model with
improved spatial resolution (130 km) is used to describe the interaction
between the solar wind and the Martian neutrals. We calculated velocity and
density distributions of the solar wind plasma in the Martian environment with
realistic planetary ions description, using spherically symmetric exospheric H
and O profiles. Following that, a 3D test-particle model was developed to
compute the X-ray emission produced by CX collisions between neutrals and solar
wind minor ions. The model results are compared to XMM-Newton observations of
Mars. Results. We calculate projected X-ray emission maps for the XMM-Newton
observing conditions and demonstrate how the X-ray emission reflects the
Martian electromagnetic structure in accordance with the observed X-ray images.
Our maps confirm that X-ray images are a powerful tool for the study of solar
wind - planetary interfaces. However, the simulation results reveal several
quantitative discrepancies compared to the observations. Typical solar wind and
neutral coronae conditions corresponding to the 2003 observation period of Mars
cannot reproduce the high luminosity or the corresponding very extended halo
observed with XMM-Newton. Potential explanations of these discrepancies are
discussed.Comment: 10 pages, 5 figures, accepted for publication in Astron. Astrophysic
Extracting interstellar diffuse absorption bands from cool star spectra: Application to bulge clump giants in Baade's window
Interstellar diffuse bands are usually extracted from hot star spectra
because they are characterized by smooth continua. It introduces a strong
limitation on the number of available targets, and reduces potential studies of
the IS matter and the use of absorptions for cloud mapping. We have developed a
new automatic fitting method appropriate to interstellar absorptions in spectra
of cool stars that possess stellar atmospheric parameters. We applied this
method to the extraction of three DIBs in high resolution VLT FLAMES/GIRAFFE
spectra of red clump stars from the bulge. By combining all stellar synthetic
spectra, HITRAN-LBLRTM atmospheric transmission spectra and diffuse band
empirical absorption profiles, we determine the 6196, 6204, and 6284 A DIB
strength toward the 219 target stars and discuss the sources of uncertainties.
In order to test the sensitivity of the DIB extraction, we intercompare the
three results and compare the DIB equivalent widths with the reddening derived
from an independent extinction map based on OGLE photometric data. Most stellar
spectra could be well reproduced by the composite stellar, atmospheric and
interstellar models. Measurement uncertainties on the EWs are smaller for the
broad and strong 6284 A DIB, and are of the order of 10-15%. Uncertainties on
the two narrow and weaker DIBs are larger, as expected, and found to be highly
variable from one target to the other. They strongly depend on the radial
velocity of the star . DIB-DIB correlations among the three bands demonstrate
that a meaningful signal is extracted. For the 6284 and 6204 A DIBs, the
star-to-star variability of the equivalent width (EW) also reflects features of
the OGLE extinction map...Comment: Accepted for publication in A&A. 14 pages+appendix, 15 figures and 3
table
The solar wind as seen by SOHO/SWAN since 1996: comparison with SOHO/LASCO C2 coronal densities
We update the SOHO/SWAN H Lyman-alpha brightness analysis to cover the
1996-2008 time interval. A forward model applied to the intensity maps provides
the latitude and time dependence of the interstellar Hydrogen ionisation rate
over more than a full solar cycle. The hydrogen ionisation, being almost
entirely due to charge-exchange with solar wind ions, reflects closely the
solar wind flux. Our results show that the solar wind latitudinal structure
during the present solar minimum is strikingly different from the previous
minimum, with a much wider slow solar wind equatorial belt which persists until
at least the end of 2008. We compute absolute values of the in-ecliptic H
ionisation rates using OMNI solar wind data and use them to calibrate our
ionisation rates at all heliographic latitudes. We then compare the resulting
fluxes with the synoptic LASCO/C2 electron densities at 6 solar radii. The two
time-latitude patterns are strikingly similar over all the cycle. This
comparison shows that densities at 6 solar radii can be used to infer the solar
wind type close to its source, with high (resp. low) densities tracing the slow
(resp. fast) solar wind, simply because the density reflects at which altitude
occurs the acceleration. The comparison between the two minima suggests that
the fast polar wind acceleration occurs at larger distance during the current
minimum compared to the previous one. This difference, potentially linked to
the magnetic field decrease or(and) the coronal temperature decrease should be
reproduced by solar wind expansion models.Comment: Proceedings of the SOHO-23 conference, Sept 21-25, 200
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