122 research outputs found
The relation between column densities of interstellar OH and CH molecules
We present a new, close relation between column densities of OH and CH
molecules based on 16 translucent sightlines (six of them new) and confirm the
theoretical oscillator strengths of the OH A--X transitions at 3078 and 3082
\AA (0.00105, 0.000648) and CH B--X transitions at 3886 and 3890 \AA, (0.00320,
0.00210), respectively. We also report no difference between observed and
previously modelled abundances of the OH molecule.Comment: 4 pages, 0 figures, accepted for publication in MNRA
A Validated Finite Element Model for Room Acoustic Treatments with Edge Absorbers
Porous acoustic absorbers have excellent properties in the low-frequency
range when positioned in room edges, therefore they are a common method for
reducing low-frequency reverberation. However, standard room acoustic
simulation methods such as ray tracing and mirror sources are invalid for low
frequencies in general which is a consequence of using geometrical methods,
yielding a lack of simulation tools for these so-called edge absorbers. In this
article, a validated finite element simulation model is presented, which is
able to predict the effect of an edge absorber on the acoustic field. With this
model, the interaction mechanisms between room and absorber can be studied by
high-resolved acoustic field visualizations in both room and absorber. The
finite element model is validated against transfer function data computed from
impulse response measurements in a reverberation chamber in style of ISO 354.
The absorber made of Basotect is modeled using the
Johnson-Champoux-Allard-Lafarge model, which is fitted to impedance tube
measurements using the four-microphone transfer matrix method. It is shown that
the finite element simulation model is able to predict the influence of
different edge absorber configurations on the measured transfer functions to a
high degree of accuracy. The evaluated third-octave band error exhibits
deviations of 3.25dB to 4.11dB computed from third-octave band averaged
spectra.Comment: 20 pages, 16 figures, 3 tables, Preprint submitted to Acta Acustic
First interstellar detection of OH+
The Atacama Pathfinder Experiment (APEX) 12m telescope was used to observe
the N=1-0, J=0-1 ground state transitions of OH+ at 909.1588 GHz with the
CHAMP+ heterodyne array receiver. Two blended hyperfine structure transitions
were detected in absorption against the strong continuum source Sagittarius
B2(M) and in several pixels offset by 18". Both, absorption from Galactic
center gas as well as absorption from diffuse clouds in intervening spiral arms
in a velocity range from -116 to 38.5 km/s is observed. The total OH+ column
density of absorbing gas is 2.4 \times 10^15 cm-2. A column density local to
Sgr B2(M) of 2.6 \times 10^14 cm-2 is found. On the intervening line-of-sight
the column density per unit velocity interval are in the range from 1 to 40
\times 10^12 cm-2/(km/s). OH+ is found to be on average more abundant than
other hydrides such as SH+ and CH+. Abundance ratios of OH and atomic oxygen to
OH+ are found to be in the range of 10^1-2 and 10^3-4, respectively. The
detected absorption of a continuous velocity range on the line-of-sight shows
OH+ to be an abundant component of diffuse clouds.Comment: 5 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
Molecular absorption lines toward star-forming regions : a comparative study of HCO+, HNC, HCN, and CN
Aims. The comparative study of several molecular species at the origin of the
gas phase chemistry in the diffuse interstellar medium (ISM) is a key input in
unraveling the coupled chemical and dynamical evolution of the ISM. Methods.
The lowest rotational lines of HCO+, HCN, HNC, and CN were observed at the
IRAM-30m telescope in absorption against the \lambda 3 mm and \lambda 1.3 mm
continuum emission of massive star-forming regions in the Galactic plane. The
absorption lines probe the gas over kiloparsecs along these lines of sight. The
excitation temperatures of HCO+ are inferred from the comparison of the
absorptions in the two lowest transitions. The spectra of all molecular species
on the same line of sight are decomposed into Gaussian velocity components.
Most appear in all the spectra of a given line of sight. For each component, we
derived the central opacity, the velocity dispersion, and computed the
molecular column density. We compared our results to the predictions of
UV-dominated chemical models of photodissociation regions (PDR models) and to
those of non-equilibrium models in which the chemistry is driven by the
dissipation of turbulent energy (TDR models). Results. The molecular column
densities of all the velocity components span up to two orders of magnitude.
Those of CN, HCN, and HNC are linearly correlated with each other with mean
ratios N(HCN)/N(HNC) = 4.8 1.3 and N(CN)/N(HNC) = 34 12, and more
loosely correlated with those of HCO+, N(HNC)/N(HCO+) = 0.5 0.3,
N(HCN)/N(HCO+) = 1.9 0.9, and N(CN)/N(HCO+) = 18 9. These ratios
are similar to those inferred from observations of high Galactic latitude lines
of sight, suggesting that the gas sampled by absorption lines in the Galactic
plane has the same chemical properties as that in the Solar neighbourhood. The
FWHM of the Gaussian velocity components span the range 0.3 to 3 km s-1 and
those of the HCO+ lines are found to be 30% broader than those of CN-bearing
molecules. The PDR models fail to reproduce simultaneously the observed
abundances of the CN-bearing species and HCO+, even for high-density material
(100 cm-3 < nH < 104 cm-3). The TDR models, in turn, are able to reproduce the
observed abundances and abundance ratios of all the analysed molecules for the
moderate gas densities (30 cm-3 < nH < 200 cm-3) and the turbulent energy
observed in the diffuse interstellar medium. Conclusions. Intermittent
turbulent dissipation appears to be a promising driver of the gas phase
chemistry of the diffuse and translucent gas throughout the Galaxy. The details
of the dissipation mechanisms still need to be investigated
A cyclic bipolar wind in the interacting binary V393 Scorpii
V393 Scorpii is a Double Periodic Variable characterized by a relatively
stable non-orbital photometric cycle of 253 days. Mennickent et al. argue for
the presence of a massive optically thick disc around the more massive B-type
component and describe the evolutionary stage of the system. In this paper we
analyze the behavior of the main spectroscopic optical lines during the long
non-orbital photometric cycle. We study the radial velocity of the donor
determining their orbital elements and find a small but significant orbital
eccentricity (e = 0.04). The donor spectral features are modeled and removed
from the spectrum at every observing epoch using the light-curve model given by
Mennickent et al. We find that the line emission is larger during eclipses and
mostly comes from a bipolar wind. We find that the long cycle is explained in
terms of a modulation of the wind strength; the wind has a larger line and
continuum emissivity on the high state. We report the discovery of highly
variable chromospheric emission in the donor, as revealed by Doppler maps of
the emission lines MgII 4481 and CI 6588. We discuss notable and some novel
spectroscopic features like discrete absorption components, especially visible
at blue-depressed OI 7773 absorption wings during the second half-cycle, Balmer
double emission with V/R-curves showing "Z-type" and "S-type" excursions around
secondary and main eclipse, respectively, and H_beta emission wings extending
up to +- 2000 km/s. We discuss possible causes for these phenomena and for
their modulations with the long cycle.Comment: 19 pages, 22 figures, accepted for publication in MNRA
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