The planetary nebula IC 5148 and its ionized halo

Many round or nearly roundish Planetary Nebulae (PNe) show multiple shells and halo structures during their evolutionary stage near the maximum temperature of their central star. Controversial debate is currently ongoing if these structures are recombination halos, as suggested by hydrodynamic modelling efforts, or ionized material. Recently we discovered a halo with even somewhat unusual structures around the sparsely studied PN IC~5148 and present for the first time spectroscopy going out to the halo of such a PN.} resolution spectroscopy is used to derive dust chemistry and mineralogy. We investigate the spatial distribution of material and its ionization state from the center of the nebula up to the very outskirts of the halo. We obtained long-slit low resolution spectroscopy (FORS2@VLT) of the nebula in two position angles, which we used to investigate the nebular structure and its halo in the optical range from 450 to 880\,nm. In addition we used medium resolution spectra taken with X-SHOOTER@VLT ranging from 320 nm to 2.4 mu to derive atmospheric parameters for the central star. We obtained the distance and position in the Galaxy from various methods combined with GAIA DR2 data. We also applied Cloudy models to the nebula in order to derive physical parameters of the various regions. We obtained spatially resolved structures and detailed descriptions of the outrunning shock front and a set of unusual halo structures denoted to further shock. The halo structures appears clearly as hot ionized material. Furthermore we derived a reliable photometric value for the central star at a GAIA distance of D=1.3kpc. Considering the large distance $z=1.0$\,kpc from the galactic plane together to its non-circular motion in the galaxy and, a metallicity only slightly below that of typical disk PNe, most likely IC 5148 originates from a thick disk population star.Comment: 12 pages, 17 figures, accepted for publication in Astronomy & Astrophysic

Skycorr: A general tool for spectroscopic sky subtraction

Airglow emission lines, which dominate the optical-to-near-IR sky radiation, show strong, line-dependent variability on various time scales. Therefore, the subtraction of the sky background in the affected wavelength regime becomes a problem if plain sky spectra have to be taken at a different time as the astronomical data. A solution of this issue is the physically motivated scaling of the airglow lines in the plain sky data to fit the sky lines in the object spectrum. We have developed a corresponding instrument-independent approach based on one-dimensional spectra. Our code skycorr separates sky lines and sky/object continuum by an iterative approach involving a line finder and airglow line data. The sky lines are grouped according to their expected variability. The line groups in the sky data are then scaled to fit the sky in the science data. Required pixel-specific weights for overlapping groups are taken from a comprehensive airglow model. Deviations in the wavelength calibration are corrected by fitting Chebyshev polynomials and rebinning via asymmetric damped sinc kernels. The scaled sky lines and the sky continuum are subtracted separately. VLT X-Shooter data covering time intervals from two minutes to about one year were selected to illustrate the performance. Except for short time intervals of a few minutes, the sky line residuals were several times weaker than for sky subtraction without fitting. Further tests show that skycorr performs consistently better than the method of Davies (2007) developed for VLT SINFONI data.Comment: 17 pages, 18 figures, accepted for publication in A&

Flux calibration of medium-resolution spectra from 300 nm to 2500 nm: Model reference spectra and telluric correction

While the near-infrared wavelength regime is becoming more and more important for astrophysics there is a marked lack of spectrophotometric standard star data that would allow the flux calibration of such data. Furthermore, flux calibrating medium- to high-resolution \'echelle spectroscopy data is challenging even in the optical wavelength range, because the available flux standard data are often too coarsely sampled. We will provide standard star reference data that allow users to derive response curves from 300nm to 2500nm for spectroscopic data of medium to high resolution, including those taken with \'echelle spectrographs. In addition we describe a method to correct for moderate telluric absorption without the need of observing telluric standard stars. As reference data for the flux standard stars we use theoretical spectra derived from stellar model atmospheres. We verify that they provide an appropriate description of the observed standard star spectra by checking for residuals in line cores and line overlap regions in the ratios of observed (X-shooter) spectra to model spectra. The finally selected model spectra are then corrected for remaining mismatches and photometrically calibrated using independent observations. The correction of telluric absorption is performed with the help of telluric model spectra.We provide new, finely sampled reference spectra without telluric absorption for six southern flux standard stars that allow the users to flux calibrate their data from 300 nm to 2500 nm, and a method to correct for telluric absorption using atmospheric models.Comment: Reference spectra available at CDS. Published in A&A 568, A9, 201

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>&prime;, 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>&prime; show deviations which increase with <i>v</i>&prime;. A study of the temporal variations revealed that the nocturnal variability pattern changes for <i>v</i>&prime; 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>&prime;. The vibrational temperatures depend on the range of <i>v</i>&prime; 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>&prime;. 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>&prime; seem to be significantly affected by non-equilibrium populations

Observations and Modeling of Potassium Emission in the Terrestrial Nightglow

The ablation of cosmic dust entering the atmosphere causes the formation of an atomic potassium (K) layer in the mesopause region. It can be studied via resonance fluorescence from the K(D1) line at 769.9 nm, stimulated by sunlight or a laser. In addition, the faint emission from a chemiluminescent cycle involving ozone and oxygen atoms has been observed with a nocturnal mean intensity of about 1 Rayleigh. In this study, the K nightglow is investigated in much greater detail, using 2,299 high‐resolution spectra taken with the astronomical echelle spectrograph Ultraviolet and Visual Echelle Spectrograph at Cerro Paranal in Chile (24.6°S) between 2000 and 2014. The seasonal variation is dominated by a maximum in June. During the night, the highest intensities are found close to sunrise. Moreover, there is a clear negative correlation with solar activity. These variations are very different from those of the well‐studied sodium (Na) nightglow. The K nightglow at Cerro Paranal was also simulated with the Whole Atmosphere Community Climate Model including K chemistry. The observed and modeled climatologies do not match well, largely because of unreliable Whole Atmosphere Community Climate Model ozone densities. Satellite‐based profile retrievals for ozone and temperature from Sounding of the Atmosphere using Broadband Emission Radiometry and K from Optical Spectrograph and Infrared Imaging System were then used to simulate the K nightglow and to derive the quantum yield of the K(D) emission with respect to the reaction of K with ozone. Considering that the obscured K(D2) line is expected on theoretical grounds to be 1.67 times brighter than K(D1), we find about 30% for this quantum yield, which is much higher than for Na(D) emission

Molecfit: A general tool for telluric absorption correction II. Quantitative evaluation on ESO-VLT X-Shooter spectra

Context: Absorption by molecules in the Earth's atmosphere strongly affects ground-based astronomical observations. The resulting absorption line strength and shape depend on the highly variable physical state of the atmosphere, i.e. pressure, temperature, and mixing ratio of the different molecules involved. Usually, supplementary observations of so-called telluric standard stars (TSS) are needed to correct for this effect, which is expensive in terms of telescope time. We have developed the software package molecfit to provide synthetic transmission spectra based on parameters obtained by fitting narrow ranges of the observed spectra of scientific objects. These spectra are calculated by means of the radiative transfer code LBLRTM and an atmospheric model. In this way, the telluric absorption correction for suitable objects can be performed without any additional calibration observations of TSS. Aims: We evaluate the quality of the telluric absorption correction using molecfit with a set of archival ESO-VLT X-Shooter visible and near-infrared spectra. Methods: Thanks to the wavelength coverage from the U to the K band, X-Shooter is well suited to investigate the quality of the telluric absorption correction with respect to the observing conditions, the instrumental set-up, input parameters of the code, the signal-to-noise of the input spectrum, and the atmospheric profiles. These investigations are based on two figures of merit, I_off and I_res, that describe the systematic offsets and the remaining small-scale residuals of the corrections. We also compare the quality of the telluric absorption correction achieved with moelcfit to the classical method based on a telluric standard star. (Abridged)Comment: Acc. by A&A; Software available via ESO: http://www.eso.org/sci/software/pipelines/skytools

Effective emission heights of various OH lines from X‐shooter and SABER observations of a passing quasi‐2‐day wave

Chemiluminescent radiation of the vibrationally and rotationally excited hydroxyl (OH) radical, which dominates the nighttime near-infrared emission of the Earth's atmosphere in wide wavelength regions, is an important tracer of the chemical and dynamical state of the mesopause region between 80 and 100 km. As radiative lifetimes and rate coefficients for collision-related transitions depend on the OH energy level, line-dependent emission profiles are expected. However, except for some height differences for whole bands mostly revealed by satellite-based measurements, there is a lack of data for individual lines. We succeeded in deriving effective emission heights for 298 OH lines thanks to the joint observation of a strong quasi-2-day wave (Q2DW) in eight nights in 2017 with the medium-resolution spectrograph X-shooter at the Very Large Telescope at Cerro Paranal in Chile and the limb-sounding SABER radiometer on the TIMED satellite. Our fitting procedure revealed the most convincing results for a single wave with a period of about 44 hr and a vertical wavelength of about 32 km. The line-dependent as well as altitude-resolved phases of the Q2DW then resulted in effective heights which differ by up to 8 km and tend to increase with increasing vibrational and rotation excitation. The measured dependence of emission heights and wave amplitudes (which were strongest after midnight) on the line parameters implies the presence of a cold thermalized and a hot non-thermalized population for each vibrational level

Climatologies of various OH lines from about 90,000 X‐shooter Spectra

The nocturnal mesopause region of the Earth's atmosphere radiates chemiluminescent emission from various roto-vibrational bands of hydroxyl (OH), which is therefore a good tracer of the chemistry and dynamics at the emission altitudes. Intensity variations can, for example, be caused by the general circulation, gravity waves, tides, planetary waves, and the solar activity. While the basic OH response to the different dynamical influences has been studied quite frequently, detailed comparisons of the various individual lines are still rare. Such studies can improve our understanding of the OH-related variations as each line shows a different emission profile. We have therefore used about 90,000 spectra of the X-shooter spectrograph of the Very Large Telescope at Cerro Paranal in Chile in order to study 10 years of variations of 298 OH lines. The analysis focuses on climatologies of intensity, solar cycle effect (SCE), and residual variability (especially with respect to time scales of hours and about 2 days) for day of year and local time. For a better understanding of the resulting variability patterns and the line-specific differences, we applied decomposition techniques, studied the variability depending on time scale, and calculated correlations. As a result, the mixing of thermalized and nonthermalized OH level populations clearly influences the amplitude of the variations. Moreover, the local times of the variability features shift depending on the effective line emission height, which can mainly be explained by the propagation of the migrating diurnal tide. This behavior also contributes to remarkable differences in the effective SCE

Unifying W-Algebras

We show that quantum Casimir W-algebras truncate at degenerate values of the central charge c to a smaller algebra if the rank is high enough: Choosing a suitable parametrization of the central charge in terms of the rank of the underlying simple Lie algebra, the field content does not change with the rank of the Casimir algebra any more. This leads to identifications between the Casimir algebras themselves but also gives rise to new, `unifying' W-algebras. For example, the kth unitary minimal model of WA_n has a unifying W-algebra of type W(2,3,...,k^2 + 3 k + 1). These unifying W-algebras are non-freely generated on the quantum level and belong to a recently discovered class of W-algebras with infinitely, non-freely generated classical counterparts. Some of the identifications are indicated by level-rank-duality leading to a coset realization of these unifying W-algebras. Other unifying W-algebras are new, including e.g. algebras of type WD_{-n}. We point out that all unifying quantum W-algebras are finitely, but non-freely generated.Comment: 13 pages (plain TeX); BONN-TH-94-01, DFTT-15/9