Voyager 2 observations of NGC 7023 - Dust scattering shortward of 1600 A

Witt, A. N.; Petersohn, J. K.; Holberg, J. B.; Murthy, Jayant; Dring, A. R.; Henry, R. C.

doi:10.1086/172788

Cited by 27 publications

(39 citation statements)

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“…Reflection nebulae are bright and often heated by a single star. By assuming a simple scattering geometry estimates of Λ are given for the reflection nebula NGC 2023 (Burgh et al 2002) and NGC 7023 (Witt et al 1982;Witt et al 1993). They are shown in Fig.…”

Section: Extinctionmentioning

confidence: 99%

Dust in the diffuse interstellar medium

Siebenmorgen

Вощинников

Bagnulo

2014

A&A

112

123

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We present a model for the diffuse interstellar dust that explains the observed wavelength-dependence of extinction, emission, and the linear and circular polarisation of light. The model is set up with a small number of parameters. It consists of a mixture of amorphous carbon and silicate grains with sizes from the molecular domain of 0.5 up to about 500 nm. Dust grains with radii larger than 6 nm are spheroids. Spheroidal dust particles have a factor 1.5-3 greater absorption cross section in the far-infrared than spherical grains of the same volume do. Mass estimates derived from submillimetre observations that ignore this effect are overestimated by the same amount. In the presence of a magnetic field, spheroids may be partly aligned and polarise light. We find that polarisation spectra help to determine the upper particle radius of the otherwise rather unconstrained dust size distribution. Stochastically heated small grains of graphite, silicates, and polycyclic aromatic hydrocarbons (PAHs) are included. We tabulate parameters for PAH emission bands in various environments. They show a trend with the hardness of the radiation field that can be explained by the ionisation state or hydrogenation coverage of the molecules. For each dust component its relative weight is specified so that absolute element abundances are not direct input parameters. The model is compared to the average properties of the Milky Way, which seem to represent dust in the solar neighbourhood. It is then applied to specific sight lines towards four particular stars, with one of them located in the reflection nebula NGC 2023. For these sight lines, we present ultra-high signal-to-noise linear and circular spectro-polarimetric observations obtained with FORS at the VLT. Using prolate rather than oblate grains gives a better fit to observed spectra; the axial ratio of the spheroids is typically two and aligned silicates are the dominant contributors to the polarisation.

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Section: Extinctionmentioning

confidence: 99%

Dust in the diffuse interstellar medium

Siebenmorgen

Вощинников

Bagnulo

2014

A&A

112

123

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“…While the data at wavelengths longer than 1600 Å are in good agreement with the dust properties adopted in the widely used molecular dissociation and ionization rates calculated by Roberge et al (1991) and , at shorter wavelengths there are differences. In particular, rather than remaining constant, the albedo decreases from 0.6 at 1600 Å to 0.4 at 1100 Å (Witt et al, 1992(Witt et al, , 1993Murthy et al, 1993). The scattering phase function is not revised by these new measurements.…”

Section: A the Penetration Of Far-ultraviolet Radiationmentioning

confidence: 99%

Photodissociation regions in the interstellar medium of galaxies

Hollenbach

Tielens²

1999

Rev. Mod. Phys.

751

712

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The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eVϽh Ͻ13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the interstellar medium in which the heating and chemistry are regulated by far ultraviolet photons are termed PhotoDissociation Regions (PDRs). These regions are the origin of most of the non-stellar infrared (IR) and the millimeter and submillimeter CO emission from galaxies. The importance of PDRs has become increasingly apparent with advances in IR and submillimeter astronomy. The IR emission from PDRs includes fine structure lines of C, C ϩ , and O; rovibrational lines of H 2 ; rotational lines of CO; broad mid-IR features of polycyclic aromatic hydrocarbons; and a luminous underlying IR continuum from interstellar dust. The transition of H to H 2 and C ϩ to CO occurs within PDRs. Comparison of observations with theoretical models of PDRs enables one to determine the density and temperature structure, the elemental abundances, the level of ionization, and the radiation field. PDR models have been applied to interstellar clouds near massive stars, planetary nebulae, red giant outflows, photoevaporating planetary disks around newly formed stars, diffuse clouds, the neutral intercloud medium, and molecular clouds in the interstellar radiation field-in summary, much of the interstellar medium in galaxies. Theoretical PDR models explain the observed correlations of the [CII] 158 m with the CO Jϭ1 -0 emission, the CO Jϭ1 -0 luminosity with the interstellar molecular mass, and the [CII] 158 m plus [OI] 63 m luminosity with the IR continuum luminosity. On a more global scale, PDR models predict the existence of two stable neutral phases of the interstellar medium, elucidate the formation and destruction of star-forming molecular clouds, and suggest radiation-induced feedback mechanisms that may regulate star formation rates and the column density of gas through giant molecular clouds. [S0034-6861(99)01001-6] CONTENTS

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“…Grains are observed through their thermal emission in the mid /far-IR (Schnee et al 2005 and references therein) and in the attenuation and scattering of ultraviolet and visible photons (Cardelli et al 1989;Witt et al 1993;Burgh et al 2002;Draine 2003). IRAS observations of PDRs find that grain emission is spatially correlated with near-IR H 2 emission ( Luhman & Jaffe 1996).…”

Section: Aefs H 2 and Dust Emission In Photodissociation Regionsmentioning

confidence: 99%

A Cometary Bow Shock and Mid‐Infrared Emission Variations Revealed inSpitzerObservations of HD 34078 and IC 405

France

McCandliss

Lupu

2007

ApJ

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We present new infrared observations of the emission /reflection nebula IC 405 obtained with the Spitzer Space Telescope. Infrared images in the four IRAC bands (3.6, 4.5, 5.8, and 8.0 m) and two MIPS bands (24 and 70 m) are complemented by IRS spectroscopy (5Y30 m) of two nebular filaments. The IRAC (8.0 m) and MIPS imaging shows evidence of a bow shock associated with the runaway O9.5 V star, HD 34078, created by the interaction between the star and nebular material. The ratio of emission at 24 to 70 m is higher in the immediate vicinity of HD 34078 than in the outer filaments, providing evidence for elevated dust temperatures (T d k 90 K) in the shock region. The nebular imaging reveals that the morphology is band dependent, with varying contributions from aromatic emission features, H 2 , and dust emission. Nebular spectroscopy is used to quantify these contributions, showing several aromatic emission bands between 6Y14 m, the S(5), S(3), S(2), and S(1) pure rotational emission lines of H 2 , and atomic fine-structure lines of Ne, S, and Ar. The low-dispersion spectra provide constraints on the ionization state of the large molecules responsible for the aromatic infrared features. H 2 rotational temperatures of the two bright nebular filaments are determined from the observed line strengths. An average T (H 2 ) $ 400 K is inferred, with evidence for additional nonuniform excitation by UV photons in the intense radiation field of HD 34078. The photoexcitation hypothesis is supported by direct measurement of the far-UV H 2 fluorescence spectrum , obtained with FUSE.

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Voyager 2 observations of NGC 7023 - Dust scattering shortward of 1600 A

Cited by 27 publications

References 0 publications

Dust in the diffuse interstellar medium

Dust in the diffuse interstellar medium

Photodissociation regions in the interstellar medium of galaxies

A Cometary Bow Shock and Mid‐Infrared Emission Variations Revealed inSpitzerObservations of HD 34078 and IC 405

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