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Non-linear Effects of Atmospheric Extinction on Observations in Astronomy

Frédéric Zagury

Institut Louis de Broglie

Paris - France

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Contents

I. Modifications of the night-sky spectrum in the vicinity of a star

I.1 The night-sky spectrumI.2 Diffracted and scattered starlightI.3 Intermediate distances

II. The Red Horizon

III. Analysis of the spectrum of the Red Rectangle nebula

III.1 Broad-band analysisIII.2 Fine structure: the Red Rectangle bands

• Diffuse Interstellar Bands

• Conclusion

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I. Modifications of the night-sky

spectrum in the vicinity of a star

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5700-6000 Å region

Night-sky spectrum at Mt Wilson, USA

I.1 The night-sky spectrum

> a few arc-minute (3 10-4 rd)

Massey et al., 1990, PASP, 112, 566 Massey & Foltz, 2000, PASP, 102, 1046

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Night skies at La Silla-Paranal and Mount Wilson

Spectrum of the night sky at Mount Wilson (USA)

Spectrum of the night sky at La Silla-Paranal (Chili)

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I.2 Diffracted and scattered starlight

< a few arcsecond (5 10-6 rd)

5’’

10’’

10’’

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I.3 Spectra at intermediate distances from the star

( ~ a few 10’’) 10’’

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The Red Rectangle nebula

30 ’’

HST image of the RR nebula with the WFPC2 camera

930 ’’

10’’

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II The Red Horizon

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Why is the horizon red?

Spectrum of a red horizon at sunrise

(Observed spectrum)/(spectrum of the sun)

Fit: Cte x exp(-2.3 1020 oz) x exp(-b/4)/4

Rayleigh extinction

ozone

7000 6000 50005500 Å4700

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III. Analysis of the spectrum of the

Red Rectangle nebula

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The spectrum of the Red Rectangle nebula

= 5’’30 ’’

> 10’’

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Spectrum of the Red Rectangle nebula close to HD44179

( = 5’’)

nebula

Diffracted light from HD44179

30 ’’

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Spectrum of the Red Rectangle nebula away from HD44179

( > 10’’)

nebula

Diffracted light from HD44179

Nebula ( = 14’’)

Background (x 6)Scattered starlight (1/ x HD44179)

30 ’’

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Red slope of the Red Rectangle nebula spectrum

Nebula/(non reddened A0 star)1/4

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Ground-based spectrum of galaxy UGC 11917

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Atmosphere

Telescope Telescope

stargalaxy, nebula

ozoneRayleigh

ozoneRayleigh

Non-linearity of atmospheric extinction

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Fine structure of the Red Rectangle spectrum:

the Red Rectangle bands

Spectrum of the nebula

Background x 7

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Comparison of the Red Rectangle and background spectra in the 5800 Å region

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Comparison of the Red Rectangle and the background spectra in the 6100 Å region

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Comparison of the Red Rectangle and the background spectra in the 6200 Å region

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Temptative identification of the carriers for the atmospheric absorption in the Red Rectangle nebula - 1

Sun occultation (SAOZ experiment)

Red Rectangle spectrum

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Temptative identification of the carriers for the atmospheric absorption in the Red Rectangle nebula - 2

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Temptative identification of the carriers for the atmospheric absorption in the Red Rectangle nebula - 3

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Temptative identification of the carriers for the atmospheric absorption in the Red Rectangle nebula - 3

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IV. Diffuse Interstellar Bands (DIBS)

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Telescope

Interstellar cloud

From Jenniskens & Desert, 1994, A&ASS, 106, 39

Star

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The 6200 Å Diffuse Interstellar Band

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Atmosphere

Interstellar cloud

Telescope

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V. CONCLUSION

1. Consequences of the observed non-linearity of atmospheric extinction

- Need for a review of the data reduction process used in Astronomy

-The atmosphere may impose the most serious limits to future deep observations of the Universe from Earth

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CONCLUSION

2. Synergies between Astronomy and studies of the atmosphere

3. DIBs and Red Rectangle bands:

A need for observations with HST