Spectral analysis of red scattered sunlight at sunrise

We analyze and fit visible spectra of a red horizon at sunrise. The shape of the spectra consist of a blue continuum followed by a red bump. The reddest spectra are well fitted by the product of a spectrum of extinguished sunlight (Rayleigh extinction + ozone absorption) and 1/lambda^4. The former is essentially the radiation field in the outer atmosphere, at the scattering volume location; the latter corresponds to Rayleigh scattering by the gas. Moving to higher altitudes, a second component, corresponding to the spectrum of a blue sky, must be added. The spectra we have obtained are similar to spectra of red nebulae, suggesting there may be other explanations than an emission process to the red color of some nebulae.Comment: 5 figure

Scattered starlight contamination in the spectrum of reddened stars

The comparison, undertaken in preceding papers, of the UV observations of nebulae and of reddened stars reveals contradictory aspects of interstellar extinction. The aim of this paper is to understand the implications hidden behind the apparent contradictions. The questions treated will be: how can small grains with an isotropic phase function make an appreciable contribution in the UV spectrum of a star? Why are small grains not observed in the spectrum of a nebula? How much of starlight can be scattered by large grains in the forward direction?Comment: Accepted for publication in New Astronomy. 15 pages, 1 figur

A review of the properties of the scattered starlight which contaminates the spectrum of reddened stars

When a star is observed behind an interstellar cloud of sufficient column density, we do not observe the direct light from the star, which is totally extinguished. Rather, we see only starlight scattered at small angles from the star. I use several papers published in New Astronomy to recount the different steps which permit understanding how, and under which conditions, scattered starlight can be more important than direct starlight in the spectrum of a reddened star. Associated problems -the fit of the extinction curve, the nature of the scatterers (small grains, or, atoms or molecules from the gas), the 2200A bump- are also, briefly, discussed.Comment: 9 pages, 1 Figur

Spectral analysis of the background in ground-based, long-slit spectroscopy

This paper examines the variations, because of atmospheric extinction, of broad-band visible spectra, obtained from long-slit spectroscopy, in the vicinity of some stars, nebulae, and one faint galaxy.Comment: 12 figure

Spectral analysis of extinguished sunlight

SAOZ (Systeme d'Analyse par Observation Zenitale) is a balloon born experiment which determines the column density of several molecular species from the visible spectrum of sunlight. We will use sequence of spectra collected during a sunset to discuss atmospheric extinction, and the nature of the radiation field in the atmosphere. The radiation field in the atmosphere is, from daylight to sunset, and with a clear sky, dominated by light coming from the direction of the sun. This light is composed of direct sunlight (extinguished by the gas), and of sunlight forward-scattered by aerosols. As the sun sets, aerosol scattering is first perceived towards the UV. It progressively replaces direct sunlight over all of the spectrum. Our analysis permits fixing the main parameters of each component of the radiation field at any time. The fits we find for the extinction of sunlight in the atmosphere must also apply to starlight. Thus, the present work can be used in astronomy to correct ground-based spectral observations for extinction in the atmosphere.Comment: 6 figure

Diffuse Galactic light at high Galactic latitude: nature and interpretation

The hypothesis of an extended red emission (ERE) in diffuse Galactic light (DGL) has been put forward in 1998 by Gordon, Witt and Friedmann who found that scattered starlight was not enough to explain the amount of DGL in the R band, in some high Galactic latitude directions. This paper re-investigates, for high Galactic latitudes, the brightnesses and colours of DGL, integrated star and galaxy light (ISGL), and of the total extrasolar light (ISGL+DGL) measured by Pioneer. Under the traditional assumption that DGL is forward scattering of background starlight by interstellar dust on the line of sight, ISGL and Pioneer have very close colours, as it is found by Gordon, Witt and Friedmann. Pioneer observations at high |b| thus accept an alternative and simple interpretation, with no involvement of ERE in DGL.Comment: 9 pages, 5 figure

Reddening law and interstellar dust properties along Magellanic sight-lines

This study establishes that SMC, LMC and Milky Way extinction curves obey the same extinction law which depends on the 2200A bump size and one parameter, and generalizes the Cardelli, Clayton and Mathis (1989) relationship. This suggests that extinction in all three galaxies is of the same nature. The role of linear reddening laws over all the visible/UV wavelength range, particularly important in the SMC but also present in the LMC and in the Milky Way, is also highlighted and discussed.Comment: accepted for publication in Astrophysics and Space Science. 16 pages, 12 figures. Some figures are colour plot

An analysis of spectra in the Red Rectangle nebula

This paper presents an analysis of a series of spectra in the Red Rectangle nebula. Only the reddest part of the spectra can safely be attributed to light from the nebula, and indicates Rayleigh scattering by the gas, in conformity with the large angles of scattering involved and the proximity of the star. In the blue, light from HD44179, refracted or scattered in the atmosphere, dominates the spectra. This paper questions the reliability of ground-based observations of extended objects in the blue.Comment: 25 figure

L'extinction interstellaire

Interstellar extinction, or the way interstellar dust extinguishes starlight and modifies its spectrum, is still today not fully understood. This paper proposes a synthesis of the observations and results obtained during the fourty past years on interstellar extinction at visible and ultraviolet wavelengths. The difficulties met by current interpretations of the spectrum of reddened stars are analyzed. In regard of this analysis, observation suggests a large homogeneity of the extinction properties of interstellar matter in the universe. Observations also suggest an alternative to present interstellar dust models.L'extinction interstellaire, c'est-à-dire la façon dont la matière interstellaire éteint la lumière des étoiles et modifie son spectre, possède des propriétés remarquables aujourd'hui encore mal comprises. Cet article propose une synthèse des observations et résultats obtenus ces quarante dernières années sur l'extinction interstellaire aux longueurs d'onde visibles et ultraviolettes. Il tente de déterminer les raisons des difficultés rencontrées par les interprétations courantes du spectre des étoiles rougies. Au regard de cette analyse, les observations indiquent une grande homogénéité des propriétés d'extinction du milieu interstellaire dans l'univers. Elles suggèrent aussi une alternative aux modèles de poussières interstellaires utilisés actuellement.Zagury Frédéric. L'extinction interstellaire. In: Bulletin de la Classe des sciences, tome 20, 2009. pp. 5-29

The unidentified infrared bands and the spectrum of atomic hydrogen

This letter compares the footprints on a spectral axis of the main unidentified infrared emission bands (UIBs) with the recombination lines of atomic hydrogen. The comparison shows striking correspondences, suggesting that atomic hydrogen may be involved in the production of UIBs