13 research outputs found
OH populations and temperatures from simultaneous spectroscopic observations of 25 bands
OH rotational temperatures are widely used to derive mesopause
temperatures and their variations. Since most data sets are only
based on a fixed set of lines of a single band, it is important to
know possible systematic uncertainties related to the choice of
lines. Therefore, a comprehensive study of as many OH bands as
possible is desirable. For this purpose, astronomical echelle
spectrographs at large telescopes are the most suitable instruments.
They offer a wide wavelength coverage, relatively high spectral
resolution, and high sensitivity. Moreover, since each ground-based
astronomical observation has an imprint of the Earth's atmosphere,
the data archives of large astronomical facilities are a treasure
for atmospheric studies. For our project, we used archival data of
the medium-resolution X-shooter echelle spectrograph operated by the
European Southern Observatory at Cerro Paranal in Chile. The
instrument can simultaneously observe all OH bands that are
accessible from ground. We reduced and analysed a set of 343
high-quality spectra taken between 2009 and 2013 to measure OH line
intensities and to derive rotational and vibrational temperatures of
25 bands between 0.58 and 2.24 μm. We studied the
influence of the selected line set, OH band, upper vibrational level
<i>v</i>′, and the molecular data on the derived level
populations and temperatures. The rotational temperature results
indicate differences by several degrees depending on the
selection. The temperatures for bands of even and odd <i>v</i>′ show
deviations which increase with <i>v</i>′. A study of the
temporal variations revealed that the nocturnal variability pattern
changes for <i>v</i>′ from 2 to 9. In particular, the spread of
temperatures tends to increase during the night, and the time of the
minimum temperature depends on <i>v</i>′. The vibrational temperatures
depend on the range of <i>v</i>′ used for their determination, since
the higher vibrational levels from 7 to 9 seem to be overpopulated
compared to the lower levels. The vibrational temperature tends to
increase during the night, while the intensity decreases. Our results
support the assumption that the OH emission altitude depends on
<i>v</i>′. Moreover, the emission layer appears to rise in the
course of the night, which makes the OH thermalisation less
efficient. The derived rotational temperatures and their change with
<i>v</i>′ seem to be significantly affected by non-equilibrium
populations
Measuring FeO variation using astronomical spectroscopic observations
Airglow emission lines of OH, O₂, O and Na are commonly used to probe the MLT (mesosphere–lower thermosphere) region of the atmosphere. Furthermore, molecules like electronically excited NO, NiO and FeO emit a (pseudo-) continuum. These continua are harder to investigate than atomic emission lines. So far, limb-sounding from space and a small number of ground-based low-to-medium resolution spectra have been used to measure FeO emission in the MLT. In this study the medium-to-high resolution echelle spectrograph X-shooter at the Very Large Telescope (VLT) in the Chilean Atacama Desert (24°37′ S, 70°24′ W; 2635 m) is used to study the FeO pseudo-continuum in the range from 0.5 to 0.72 µm based on 3662 spectra. Variations of the FeO spectrum itself, as well as the diurnal and seasonal behaviour of the FeO and Na emission intensities, are reported. These airglow emissions are linked by their common origin, meteoric ablation, and they share O₃ as a common reactant. Major differences are found in the main emission peak of the FeO airglow spectrum between 0.58 and 0.61 µm, compared with a theoretical spectrum. The FeO and Na airglow intensities exhibit a similar nocturnal variation and a semi-annual seasonal variation with equinoctial maxima. This is satisfactorily reproduced by a whole atmosphere chemistry climate model, if the quantum yields for the reactions of Fe and Na with O₃ are 13 ± 3 and 11 ± 2 % respectively. However, a comparison between the modelled O₃ in the upper mesosphere and measurements of O₃ made with the SABER satellite instrument suggests that these quantum yields may be a factor of -2 smaller
The effects of ram-pressure stripping on the internal kinematics of simulated spiral galaxies
We investigate the influence of ram-pressure stripping on the internal gas
kinematics of simulated spiral galaxies. Additional emphasis is put on the
question of how the resulting distortions of the gaseous disc are visible in
the rotation curve and/or the full 2D velocity field of galaxies at different
redshifts. A Milky-Way type disc galaxy is modelled in combined
N-body/hydrodynamic simulations with prescriptions for cooling, star formation,
stellar feedback, and galactic winds. This model galaxy moves through a
constant density and temperature gas, which has parameters similar to the
intra-cluster medium (ICM). Rotation curves (RCs) and 2D velocity fields of the
gas are extracted from these simulations in a way that follows the procedure
applied to observations of distant, small, and faint galaxies as closely as
possible. We find that the appearance of distortions of the gaseous disc due to
ram-pressure stripping depends on the direction of the acting ram pressure. In
the case of face-on ram pressure, the distortions mainly appear in the outer
parts of the galaxy in a very symmetric way. In contrast, in the case of
edge-on ram pressure we find stronger distortions. The 2D velocity field also
shows signatures of the interaction in the inner part of the disc. At angles
smaller than 45 degrees between the ICM wind direction and the disc, the
velocity field asymmetry increases significantly compared to larger angles.
Compared to distortions caused by tidal interactions, the effects of
ram-pressure stripping on the velocity field are relatively low in all cases
and difficult to observe at intermediate redshift in seeing-limited
observations. (abridged)Comment: 9 pages, 11 figures, accepted for publication in A&
The unusual Nova Cygni 2006 (V2362 Cyg)
Context: Optical nova lightcurves often have structures, such as rapid
declines and recoveries, due to nebular or dusty phases of the ejecta. Nova
Cygni 2006 (V2362 Cyg) underwent an unusual brightening after an early rapid
decline. The shape of the lightcurve can be compared to that of V1493 Aql, but
the whole event in that case was not as bright and only lasted a couple of
weeks. V2362 Cyg had a moderately fast decline of t_2 = 9.0 before
rebrightening, which lasted 250 days after maximum. Aims: We present an
analysis of our own spectroscopic investigations in combination with AAVSO
photometric data covering the whole rebrightening phase until the return to the
final decline. Methods: We used the medium resolution spectroscopy obtained in
ten nights over a period of 79 nights to investigate the change of the velocity
structure of the ejecta. The publicly available AAVSO photometry was used to
analyze the overall properties and the energy of the brightening. Results:
Although the behavior of the main outburst (velocity, outburst magnitude, and
decline timescales) resembles a ``normal'' classical nova, the shell clearly
underwent a second fast mass ejecting phase, causing the unusual properties.
The integrated flux during this event contributes ~ 40 % to the total radiation
energy of the outburst. The evolution of the H_alpha profile during the bump
event is obtained by subtracting the emission of the detached shells of the
main eruption by a simple optically thin model. A distance of D ~ 7.5
{+3.0}{-2.5} kpc and an interstellar extinction E(B-V) = 0.6 +/- 0.1 was also
derived.Comment: 4 pages, 4 Postscript figures, accepted for A&A Letter
On the influence of ram-pressure stripping on the star formation of simulated spiral galaxies
We investigate the influence of ram-pressure stripping on the star formation
and the mass distribution in simulated spiral galaxies. Special emphasis is put
on the question where the newly formed stars are located. The stripping radius
from the simulation is compared to analytical estimates. Disc galaxies are
modelled in combined N-body/hydrodynamic simulations (GADGET-2) with
prescriptions for cooling, star formation, stellar feedback, and galactic
winds. These model galaxies move through a constant density and temperature
gas, which has parameters comparable to the intra-cluster medium (ICM) in the
outskirts of a galaxy cluster (T=3 keV ~3.6x10^7 K and rho=10^-28 g/cm^3). With
this numerical setup we analyse the influence of ram-pressure stripping on the
star formation rate of the model galaxy. We find that the star formation rate
is significantly enhanced by the ram-pressure effect (up to a factor of 3).
Stars form in the compressed central region of the galaxy as well as in the
stripped gas behind the galaxy. Newly formed stars can be found up to hundred
kpc behind the disc, forming structures with sizes of roughly 1 kpc in diameter
and with masses of up to 10^7 M_sun. As they do not possess a dark matter halo
due to their formation history, we name them 'stripped baryonic dwarf'
galaxies. We also find that the analytical estimate for the stripping radius
from a Gunn & Gott (1972) criterion is in good agreement with the numerical
value from the simulation. Like in former investigations, edge-on systems lose
less gas than face-on systems and the resulting spatial distribution of the gas
and the newly formed stars is different.Comment: 8 pages, 7 figures, accepted for publication in A&
DIE KYBERNETISCHE FUNKTION DES DECKUNGSBEITRAGES
DIE KYBERNETISCHE FUNKTION DES DECKUNGSBEITRAGE
Thermal accommodation of payloads on German Spacelab Mission D1
SIGLETIB: MU 8069 (818-85 Oe) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman
Scattered moonlight observations with X-shooter implications for the aerosol properties at Cerro Paranal and the Eso sky background model
Estimating the sky background is critical for ground-based astronomical research. In the optical, scattered moonlight dominates the sky background, when the moon is above the horizon. The most uncertain component of a scattered moonlight model is the aerosol scattering. The current, official sky background model for Cerro Paranal uses an extrapolated aerosol extinction curve. With a set of X-shooter sky observations, we have tested the current sky background model as well as determined the aerosol extinction from the ultra-violet (UV) to near-infrared (NIR). To our knowledge, this is the first time that a scattered moonlight model has been used for this purpose. These observations were taken of blank sky, during three different lunar phases, and at six different angular distances from the moon for each lunar phase. Overall, the current model does reproduce the observations for average conditions quite well. Using a set of sky background models with varying aerosol distributions to compare with the observations, we found the most likely aerosol extinction curves, phase functions, and volume densities for the three nights of observations and compare them with the current model. While there are some degeneracies in the aerosol scattering properties, the extinction curves tend to flatten towards redder wavelengths and are overall less steep compared to the extrapolated curve used in the current model. Also, the current model had significantly less coarse particles compared to the favored volume densities from the X-shooter data. Having more coarse particles affects the phase function by being more peaked at small angular distances. For the three nights of sky observations, the aerosol size distributions differed, most likely reflecting the changes in atmospheric conditions and aerosol content, which is expected. In short, the current sky background model is in fair agreement with the observations, and we have determined better aerosol extinction curves and phase functions for Cerro Paranal. Using nighttime sky observations of scattered moonlight and a set of sky background models is a new method to probe the aerosol content of the atmosphere