Analysis of the Schmidt, Cohen & Margon (1980) Features in the Red Rectangle Nebula

The accurate measurement of intensities in stellar spectra, both relative and absolute, must be made in all observable spectral regions in order to obtain the basic data for the computation of model stellar atmospheres. The effects of interstellar and atmospheric extinction, absorption, and scattering in the instrument must be especially considered in the work of absolute spectrophotometry. Differential atmospheric extinction and instrumental absorption may be neglected if the various points on a line are measured relative to the continuum. The determination of the continuum, however, involves many problems. Sensitive photographic and photoelectric techniques record a stellar spectrum in order to allow the determination of the distribution of energy with wavelength. The low intensity level of most stellar sources has encouraged the development of photoelectric spectrophotometry, which in certain applications obtains better results than photographic methods. However, the present slow rate of scan, wavelength by wavelength, even with several photocells simultaneously in use, indicates that photographic procedures will undoubtedly be the most used for several years to come. The methods of calibration of the spectrograms, the use of the “densitometer” (generally called “microphotometer” by astronomers) and the correction of its reproduction of intensity data are the basic techniques applied to gain the fundamental data for different examples of stars and stellar systems. The determination of line profiles, both theoretically and by measurement, their importance to the theory of model atmospheres, the importance of high-dispersion studies of selected line profiles, and related procedures are further problems. The intensity distribution of the continuous spectrum for stars of different spectral types, the classification of stars by spectra, the theoretical interpretation of the measurements, the problems resulting from uncertainties in the determination of the continuum and stellar equivalent widths to approximately 10%—a permissible accuracy—completes the acquisition of data for the description of the atmosphere’s physical structure.

Effects of Particulate Complex Refractive Index and Particle Size Distribution Variations on Atmospheric Extinction and Absorption for Visible through Middle-Infrared Wavelengths

A comprehensive sensitivity study has been made using Mie theory to determine the effect of realistic variations in values of real and imaginary parts of the complex index of refraction on volume extinction and absorption coefficients for a wide range of log normal particle size distributions (defined by geometric mean radius r(g) and geometric standard deviation sigma(g)). Wavelengths lambda from the visible (0.55 microm) through the middle ir (10.6 microm) were considered. Extinction is independent of the complex index to within 20% for the majority of realistic particle size distributions, providing lambda < 2 microm. However, changes in extinction by up to an order of magnitude are caused by realistic variations in refractive indexes for 2 microm </= lambda </= 10.6 microm, with the real index being more important in affecting extinction than the imaginary index. Similar changes are caused by variations in particle size distribution for values of refractive indexes typical of atmospheric constituents. For bimodal size distributions representative of desert aerosols, values of the complex refractive index that result in minimum and maximum extinction coefficients are given. Absorption is generally less dependent on size distribution than is extinction and is not, in general, linear with the imaginary index, especially for broad particle distributions.

Radiative transfer modelling for a coupled ocean-atmosphere system near 685 nm indicates a sufficiently high fluorescence signal of the chlorophyll a transmitted to the top of the atmosphere. However, only the shortwave half is seen at the top of the atmosphere, the longwave half is fully masked by atmospheric oxygen and water vapour absorption. The impact of atmospheric aerosol extinction on the signal transmission is almost negligible. The Hα line of the sun, atmospheric water vapour absorption, and chlorophyll absorption near 670 nm influence radiative transfer in the shortwave tail of the fluorescence line making the search for a baseline in order to eliminate the background radiation a difficult task.

Noctilucent cloud particle size determination based on multi-wavelength all-sky analysis

Chapter 7 - Monitoring Visibility

A careful examination of Red Rectangle bands which have been considered as\ndiffuse interstellar bands (DIBs) in emission shows that a few are likely to be\nartifacts in the spectrum. Some others result from atmospheric extinction.\nConsequences for the Red Rectangle band/DIB associations are examined.\n I will also comment a striking resemblance between the DIB spectrum and the\nspectrum of NO2 in the 6150-6250A region. This suggests that some DIBs could be\nprovoked by atmospheric molecules.\n

Direct‐normal solar irradiance (DNSI), the energy in the solar spectrum incident in unit time at the Earth's surface on a unit area perpendicular to the direction to the Sun, depends only on atmospheric extinction of solar energy without regard to the details of the extinction, whether absorption or scattering. Here we report a set of closure experiments performed in north central Oklahoma in April 1996 under cloud‐free conditions, wherein measured atmospheric composition and aerosol optical thickness are input to a radiative transfer model, MODTRAN 3, to estimate DNSI, which is then compared with measured values obtained with normal incidence pyrhe‐liometers and absolute cavity radiometers. Uncertainty in aerosol optical thickness (AOT) dominates the uncertainty in DNSI calculation. AOT measured by an independently calibrated Sun photometer and a rotating shadow‐band radiometer agree to within the uncertainties of each measurement. For 36 independent comparisons the agreement between measured and model‐estimated values of DNSI falls within the combined uncertainties in the measurement (0.3–0.7%) and model calculation (1.8%), albeit with a slight average model underestimate (−0.18±0.94%) for a DNSI of 839 W m−2 this corresponds to −1.5±7.9 W m−2. The agreement is nearly independent of air mass and water‐vapor path abundance. These results thus establish the accuracy of the current knowledge of the solar spectrum, its integrated power, and the atmospheric extinction as a function of wavelength as represented in MODTRAN 3. An important consequence is that atmospheric absorption of short‐wave energy is accurately parametrized in the model to within the above uncertainties.

At the high-altitude station on Terskol Peak (central Caucasus, 3100 m) by the Main Astronomical Observatory of the National Academy of Sciences of the Ukraine in 1992, spectral measurements of the solar disk-center intensity for the near IR region were performed. These measurements are a continuation of the solar absolute spectral energy distribution investigation programme. Data published earlier (Burlov-Vasiljev, Gurtovenko, and Matvejev, 1995a) are expanded now in the long-wave spectral region up to 1070 nm. The measurements were made with the specialized solar telescope SEF-1. The method of comparison of the solar disk-center brightness with the brightness of the calibrated region of a standard ribbon tungsten lamp was used. The atmospheric extinction was taken into account with Bouger’s ‘long’ method accompanied by the parallel-independent control of atmospheric stability. The uncertainty of the absolute solar disk-center intensity values is estimated to be 2% in regions free from the strong telluric absorption of atmosphere oxygen and water vapour. In these regions an additional reduction was carried out, which was derived from the synthetic atmospheric absorption spectra computed on the basis of the molecular parameter data and the standard model of the Earth’s atmosphere. The 1-nm integrals of the disk-center radiance in the wavelength range λλ650–1070 nm, which are established on 5-day measurements in March-October 1992, are given. With the help of the solar disk-darkening coefficients, the solar flux values at 1 AU are available. The measured 1-nm integrals were used for the high-resolution solar spectral atlas calibration in order to locate the solar continuum in absolute units. A comparison is made of the data obtained with the data by Neckel and Labs (1984) and data of some other authors.

First identified in 1964, inverse Raman scattering (IRS) is a nonlinear stimulated phenomenon that induces Raman-scattered absorptions where Raman emissions would be expected. This paper highlights the significance of IRS in analyzing the spectra of stars located in the distant background of H I interstellar clouds. Specifically, ultraviolet emission lines Raman scattered by atomic hydrogen, typically observed in emission at wide scattering angles in the optical spectra of symbiotic stars and nebulae, should appear as IRS absorptions in the optical spectra of the background stars. I show that all known interstellar Raman-scattered emission lines in the Hα wavelength region are detected in absorption as diffuse interstellar bands (DIBs) in the spectra of reddened stars, and conclude that IRS by atomic hydrogen resolves the long-standing DIB puzzle. This sheds new light on the perplexing relationship between DIBs and the Red Rectangle nebula emission bands (RRBs). The conditions under which either DIBs or RRBs are detected emphasize the importance of considering the physical relationship between the observer, the H I medium, and the direction of the illuminating radiation field (i.e., the geometry of the observation) in observations of H I interstellar matter. Observing in the direction of the radiation field or on its side determines whether IRS, yielding DIBs and the 2200 Å bump, or spontaneous Raman scattering at wide scattering angles, resulting in extended red emission, Raman-scattered emission lines (including RRBs), and unidentified infrared bands, will be observed.

Context. Medium-resolution echelle spectra of the Red Square Nebula surrounding the star MWC 922 are presented. The spectra have been obtained in 2010 and 2012 using the X-shooter spectrograph mounted on the Very Large Telescope (VLT) in Paranal, Chile. The spectrum covers a wavelength range between 300 nm−2.5 μ m and shows that the nebula is rich in emission lines. Aims. We aim to identify the emission lines and use them as a tool to determine the physical and chemical characteristics of the nebula. The emission lines are also used to put constraints on the structure of the nebula and on the nature of the central stars.Methods. We analyzed and identified emission lines that indicated that the Red Square Nebula consists of a low density bipolar outflow, eminent in the broad emission component seen in [Fe ii], as well as in P Cygni line profiles indicative of fast outflowing material. The narrow component in the [Fe ii] lines is most likely formed in the photosphere of a surrounding disk. Some of the emission lines show a pronounced double peaked profile, such as Ca ii, indicating an accretion disk in Keplerian rotation around the central star. [O i] emission lines are formed in the neutral atomic zone separating the ionized disk photosphere from the molecular gas in the interior of the disk, which is prominent in molecular CO emission in the near-IR. [N ii] and [S ii] emission clearly originates in a low density but fairly hot (7 000−10 000 K) nebular environment. H i recombination lines trace the extended nebula as well as the photosphere of the disk.Results. These findings put constraints on the evolution of the central objects in MWC 922. The Red Square shows strong similarities to the Red Rectangle Nebula, both in morphology and in its mid-IR spectroscopic characteristics. As for the Red Rectangle, the observed morphology of the nebula reflects mass-loss in a binary system. Specifically, we attribute the biconical morphology and the associated rung-like structure to the action of intermittent jets blown by the accreting companion in a dense shell, which has been created by the primary. We stress, though, that despite the morphological similarities, these two objects represent very different classes of stellar objects.

&amp;lt;p&amp;gt;Absorbing aerosols affect the climate system (radiative forcing, cloud formation, precipitation and more) by strongly absorbing solar radiation, particularly at ultraviolet and visible wavelengths. The environmental impacts of an absorbing aerosol layer are influenced by its single scattering albedo (SSA), the albedo of the underlying surface, and also by the atmospheric residence time and column concentration of the aerosols.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Black-carbon (BC), the collective term used for strongly absorbing, carbonaceous aerosols, emitted by incomplete combustion of fossil fuel, biofuel and biomass, is a significant contributor to atmospheric absorption and probably a main-driver in inter-model differences and large uncertainties in estimating the aerosol radiative forcing due to aerosol-radiation interaction (RFari). Estimates of BC direct radiative forcing suggest a positive effect of +0.71 Wm&amp;lt;sup&amp;gt;-2&amp;lt;/sup&amp;gt; (Bond and Bergstrom (2006)) with large uncertainties [+0.08, +1.27] Wm&amp;lt;sup&amp;gt;-2&amp;lt;/sup&amp;gt;. These uncertainties result from poor estimates of BC atmospheric burden (emissions and removal rates) and its radiative properties. The uncertainty in the burden is due to the uncertainty in emissions (7.5 [2, 29] Tg yr&amp;lt;sup&amp;gt;-1&amp;lt;/sup&amp;gt;) and lifetime (removal rates). In comparison with the available observations, global climate models (GCMs) tend to under-predict absorption near source (e.g. at AERONET stations), and over-predict concentrations in remote regions (e.g. as measured by aircraft campaigns). This may be due to GCM&amp;amp;#8217;s weak emissions at the source, but longer lifetime of aerosols in the atmosphere.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;This study aims to address the parametric uncertainty of GCMs and constrain the direct radiative forcing using a perturbed parameter ensemble (PPE) and a collection of observations, from remote sensing to in-situ measurements. Total atmospheric aerosol extinction is quantified using satellite observations that provide aerosol optical depth (AOD), while the SSA is constrained by the use of high-temporal resolution aerosol absorption optical depth (AAOD) measured with AERONET sun-photometers (for near-source columnar information of aerosol absorption) and airborne black-carbon in-situ measurements collected and synthesised in the Global Aerosol Synthesis and Science Project (GASSP) (for properties of long-range transported aerosols). Measurements from the airborne campaigns ATOM and HIPPO are valuable for constraining aerosol absorption in remote areas, while CLARIFY and ORACLES, that were employed over Southeast Atlantic, are considered in our study for near source observations of biomass burning aerosols transported over the bright surface of stratocumulus clouds.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Using the PPE to explore the uncertainties in the aerosol absorption as well as the dominant emission and removal processes, and by comparing with a variety of observations we have confidence to better constrain the aerosol direct radiative forcing.&amp;lt;/p&amp;gt;

The high-resolution CIO absorption signature in the region of 308.1 nm has a very low absorption fraction, of the order of 6 x 10(-5), and linewidths comparable with those of the solar background spectrum. Because of the need for reliable absorption measurements of the abundance of this species, which is important in ozone photochemistry, a Fourier-analysis-based technique for the deconvolution of atmospheric solar absorption spectra in this region has been developed. The technique utilizes the regularity of the CIO spectrum and results in a significant reduction in the minimum signal-to-noise required for the retrieval of CIO abundances from absorption spectra.

Sprites are a potential thermal infrared radiation source in the stratosphere and mesosphere through molecular vibrational excitation. We developed a plasma‐chemical model to compute the vibrational kinetics induced by a sprite streamer in the 40‐ to 70‐km altitude range until several tens of seconds after the visible flash is over. Then, we computed the consecutive time‐dependent thermal infrared spectra that could be observed from the stratosphere (from a balloon platform), high troposphere (from an aircraft), and low troposphere (aircraft or altitude observatory) using a nonlocal thermodynamic equilibrium radiative transfer model. Our simulations predict a strong production of CO2 in the (001) vibrational level which lasts at least 40 s before falling to background concentrations. This leads to enhanced emissions in the long‐wavelength infrared, around 1,000 cm−1, and midwavelength infrared, around 2,300 cm−1. The maximum sprite infrared signatures (sprite spectra minus background spectra) reach several 10−7 W/sr/cm2/cm−1 after propagation through the mesosphere and stratosphere, to an observer located at 20–40 km of altitude. This maximum signal is about 1 order of magnitude lower if propagated until the troposphere. From the two spectral bands, the 1,000‐cm−1 one could be detected more easily than the 2,300‐cm−1 one, which is more affected by atmospheric absorption (CO2 self‐trapping at all altitudes and H2O mostly in the troposphere). With a sufficiently sensitive instrumentation, mounted in an open stratospheric balloon platform for example, the 1,000‐cm−1 band could be detected from 20–40 km of altitude.

Fine structure of sprites and proposed global observatiions