Searcharxiv⌕ Search

arXiv subjects

Frederic Zagury

Publications and source records attributed to Frederic Zagury.

At least 19 recordsLinked to original sources
arXiv2026

Extinction curves, extinction laws, and the failure of interstellar dust models

The interpretation of ultraviolet Galactic interstellar extinction curves is obscured today by accumulated assumptions, such as a purported link between the 2200 A bump and metallicity, that are not firmly supported by observations. In this paper I define extinction curves as the ratio F*/F0 of the near-infrared-to-ultraviolet spectrum of a reddened star to that of the same star without intervening material, rather than in terms of a magnitude difference, and revisit their observed properties. Special attention is given to the connection that Galactic extinction curves with a 2200 A bump retain with the ultraviolet extrapolation of the exponential extinction law defined by their near-infrared-to-optical segment. This connection leads to the classification of all extinction curves into three types. A graphical representation of these types together with their underlying exponential extinction laws demonstrates that interstellar extinction curves can be interpreted in two ways. Either they result from the mixing of distinct extinction laws associated with different particles, as traditionally assumed, or Galactic ultraviolet curves with a bump are not extinction laws proper but instead deviate from a universal exponential extinction law owing to an additional contribution from coherently forward-scattered starlight. Given the observational constraints on the interpretation of extinction curves, such as their dependence on just two parameters, and the fact that bump-like extinction curves are barely observed outside the Galaxy, the latter interpretation emerges as the only logically consistent one.

astro-ph.GA↗
arXiv2024

Inverse Raman scattering and the diffuse interstellar bands: an exploration of the systemic interconnections between spontaneous and inverse Raman scattering and extended red emission, Red Rectangle bands, and diffuse interstellar bands

First identified in 1964, inverse Raman scattering (IRS) is a nonlinear stimulated phenomenon that induces Raman scattered absorptions where Raman emissions would be expected. While IRS is less well-known than stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS), this study highlights its significance in analyzing the spectra of stars located in the distant background of HI 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 absorption features in the optical spectra of the background stars. I show that all known interstellar Raman scattered emission lines in the H-alpha 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 longstanding puzzle of the processes involved in producing these bands, and perhaps also explains the equally mysterious 2200A bump of ultraviolet extinction curves. This identification of DIBs as IRS HI absorptions sheds new light on the perplexing relationship between DIBs and the Red Rectangle nebula emission bands (RRBs). The conditions under which DIBs are detected highlight the importance of considering the physical relationship between the observer, the HI medium, and the direction of the illuminating radiation field (i.e., the geometry of the observation) in observations of HI interstellar matter. Observing in the direction of the radiation field or on its side determines whether IRS, yielding DIBs and the 2200A bump, or spontaneous Raman scattering at wide scattering angles, resulting in ERE, Raman scattered emission lines (including RRBs), and the unidentified infrared bands, will be observed.

astro-ph.GA↗
arXiv2022

Unidentified infrared bands do not correlate with C/O ratio in planetary nebulae

The concrete evidence adduced to support the widely held idea that unidentified infrared bands (UIBs) are enhanced in carbon-rich planetary nebulae (PNe) is a remarkable UIB 7.7 μm versus C/O ratio correlation plot for six PNe, obtained from air-born observations and published in 1986 by M. Cohen and coworkers. However, the space-born data presented by Cohen & Barlow in 2005 undercut this correlation, and I show that the larger dataset they provide disproves a specific link between UIBs and carbon abundance in PNe. It also follows from these data that interstellar UIB carriers cannot originate from the atmosphere of carbon-rich PNe.

astro-ph.GA↗
arXiv2021

Raman Scattering as the Key Link Between Unidentified Infrared Bands, Diffuse Interstellar Bands, and Extended Red Emission

This paper calls attention to the relevance of Raman scattering by atomic hydrogen to three optical and near/mid-infrared spectral features of HI clouds: extended red emission (ERE), diffuse interstellar bands (DIBs), and the unidentified infrared bands (UIBs). DIBs, ERE, and UIBs are observed predominantly at the edge of HI clouds, are manifestly related, and remain poorly understood. Their salient properties correspond to two major characteristics of HI Raman scattering: unusual line broadenings and a concentration of the Raman scattered ultraviolet continuum in the vicinity of hydrogen's optical and infrared transitions. Raman scattering by atomic hydrogen has now been detected in several object classes where the spectral features are observed, and I argue that it can account for all three features. I further identify three factors that condition observation of Raman scattering in HI clouds, and thus of DIBs, ERE, and UIBs: the hardness of the radiation field, interstellar dust extinction, and the geometry of the observation. The geometry determines whether complete forward scattering, yielding DIBs, or scattering at large angles, yielding ERE in the vicinity of Hα and UIBs in the infrared spectrum, will be observed. ERE results from Raman scattering of photons near Ly\b{eta} and UIBs from excitation of hydrogen atoms close to the ionization limit. DIBs, ERE, UIBs are thus different facets of the same interstellar phenomenon: Raman scattering by atomic hydrogen.

astro-ph.GA↗
arXiv2017

On the relationship between the continuum of interstellar extinction curves, the 2200 Å bump, and the diffuse interstellar bands

A previous article argued that the antagonism between sight-lines with and without a bump at 2200 Å disappears, and that the observed properties of interstellar extinction can be globally understood, if it is accepted that scattered starlight contaminates the observed spectrum of reddened stars. The present paper develops this new paradigm by providing a better understanding of the characteristics of this scattered light. It further examines the consequences of this revision of interstellar extinction theory for interpreting the 2200 Å bump and the diffuse interstellar bands (DIBs). Two implications are worth noting: (1) the effect of interstellar extinction on cosmic distance estimations needs to be reconsidered; (2) it is unnecessary to invoke hypothetical particles, such as Poly Aromatic Hydrogenated molecules (PAHs), to explain the peculiarities of interstellar extinction. Hydrogen, by far the most abundant constituent of interstellar clouds, should alone account for the three major features of interstellar extinction: the departure of the continuum from linearity, the bump, and the DIBs.

astro-ph.GA↗
arXiv2012

Coherent forward scattering of starlight by a cloud of atomic hydrogen

Theory predicts that a plane wave scattered by a thin slab of gas yields, in the forward direction and under specific circumstances, a larger irradiance than would be observed in the absence of the gas. This enhanced Rayleigh scattering depends on the size of the Fresnel zones at the slab location, as seen from the observer's position, and results from the coherence of the scattering. On astronomical scales the exceptional size of Fresnel zones (~1500km) has particular relevance when considering forward-scattered starlight by an interstellar cloud of atomic hydrogen.

astro-ph.GA↗
arXiv2012

The 2200 A bump and the UV extinction curve

The 2200 A bump is a major figure of interstellar extinction. Extinction curves with no bump however exist and are, with no exception, linear from the near-infrared down to 2500 A at least, often over all the visible-UV spectrum. The duality linear versus bump-like extinction curves can be used to re-investigate the relationship between the bump and the continuum of interstellar extinction, and answer questions as why do we observe two different kinds of extinction (linear or with a bump) in interstellar clouds? How are they related? How does the existence of two different extinction laws fits with the requirement that extinction curves depend exclusively on the reddening E(B-V) and on a single additional parameter? What is this free parameter? It will be found that (1) interstellar dust models, which suppose the existence of three different types of particles, each contributing to the extinction in a specific wavelength range, fail to account for the observations; (2) the 2200 A bump is very unlikely to be absorption by some yet unidentified molecule; (3) the true law of interstellar extinction must be linear from the visible to the far-UV, and is the same for all directions, including other galaxies. In extinction curves with a bump the excess of starlight (or the lack of extinction) observed at wavelengths less than lambda=4000 A is due to a large contribution of light scattered by hydrogen on the line of sight. Although counter-intuitive this contribution is predicted by theory. The free parameter of interstellar extinction is related to distances between the observer, the cloud on the line of sight, and the star behind it (the parameter is likely to be the ratio of the distances from the cloud to the star and to the observer). The continuum of the extinction curve or the bump contain no information concerning the chemical composition of interstellar cloud.

astro-ph.GA↗
arXiv2012

The Extinction Curve in the Visible and the Value of Rv. Part II: Addendum to AN 333, 160

This paper corrects and completes a previous study of the shape of the extinction curve in the visible and the value of Rv. A continuous visible/infrared extinction law proportional to 1/λ^p with p close to 1 ({\pm}0.4) is indistinguishable from a perfectly linear law (p = 1) in the visible within observational precision, but the shape of the curve in the infrared can be substantially modified. Values of p slightly larger than 1 would account for the increase of extinction (compared to the p = 1 law) reported for λ > 1μm and deeply affect the value of Rv. In the absence of gray extinction Rv must be 4.04 if p = 1. It becomes 3.14 for p = 1.25, 3.00 for p = 1.30, and 2.76 for p = 1.40. Values of p near 1.3 are also attributed to extinction by atmospheric aerosols, which indicates that both phenomena may be governed by similar particle size distributions. A power extinction law may harmonize visible and infrared data into a single, continuous, and universal, interstellar extinction law.

astro-ph.GA↗
arXiv2012

The extinction curve in the visible and the value of Rv

This article discusses the interstellar extinction curve in the visible and the value of Rv. It is concluded that the visible extinction curve is likely to be linear in the visible, and that indirect estimates of Rv from tentative determinations of Av, infrared, or UV observations are questionable. There is currently no evidence of any variation of Rv with direction. If Rv is close to 3, as it has been inferred from mid-infrared data, starlight in the visible is extinguished by a factor F/F_0=(2.5exp{-2micron/lambda})^{E(B-V)} in the visible. But if the visible wavelength range alone is considered, 4 appears as its most natural and probable value, and F/F_0= exp{-2E(B-V)/lambda}.

astro-ph.GA↗
arXiv2009

On the relationship between Red Rectangle and diffuse interstellar bands

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

astro-ph.GA↗
arXiv2009

Analysis of the Schmidt, Cohen & Margon (1980) features in the Red Rectangle nebula

This study investigates the relationship between atmospheric extinction and the spectrum of the Red Rectangle nebula on scales of a few to a few tens of Ang. It is found that the fine structure of the nebula's continuum short-ward of 6700A is similar to background spectra, and is thus determined either by atmospheric absorption or by light from HD44179 scattered in the earth atmosphere.

astro-ph.GA↗
arXiv2007

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.

astro-ph↗
arXiv2007

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.

astro-ph↗
arXiv2006

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.

astro-ph↗
arXiv2006

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.

astro-ph↗
arXiv2006

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.

astro-ph↗
arXiv2004

Some implications of the introduction of scattered starlight in the spectrum of reddened stars

This paper presents new investigations on coherent scattering in the forward direction (orders of magnitude; conservation of energy; dependence of scattered light on geometry and wavelength), and on how scattered light contamination in the spectrum of reddened stars is possibly related to as yet unexplained observations (the diminution of the 2200 A bump when the obscuring material is close to the star, the difference between Hipparcos and photometric distances). This paper then goes on to discuss the fit of the extinction curve, a possible role of extinction by the gas in the far-UV, and the reasons of the inadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.

astro-ph↗