Astron. Astrophys. 354, 1053-1061 (2000)

ISO SWS-LWS observations of the prototypical reflection nebula NGC 7023 ^*

A. Fuente ¹, J. Martn-Pintado ¹, N.J. Rodrguez-Fernández ¹, J. Cernicharo ² and M. Gerin <sup>3

1</sup> Observatorio Astronómico Nacional (IGN), Campus Universitario, Apdo. 1143, 28800 Alcalá de Henares (Madrid), Spain (fuente@oan.es)
² Instituto de Estructura de la Materia, Departamento de Fsica Molecular, CSIC, Serrano 121, 28006 Madrid, Spain
³ DEMIRM, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France

Received 22 September 1999 / Accepted 24 December 1999

Abstract

We present SWS and LWS ISO observations towards a strip across the photodissociation region (PDR) in the reflection nebula NGC 7023. SWS02 and LWS01 spectra have been taken towards the star and the brightest infrared filaments located NW and SW from the star (hereafter NW and SW PDRs). In addition, SWS02 spectra have been taken towards two intermediate positions (NW1 and SW1). This has provided important information about the extent and spatial distribution of the warm H₂ and of the atomic species in this prototypical reflection nebula.

Strong emission of the [SiII] 34.8 line is detected towards the star. While all the PDR tracers (the [CII] 157.7, [OI] 63.2 and 145.6, [HI] 21cm and the H₂ rotational lines) present a ring-like morphology with the peaks toward the NW and SW PDRs and a minimum around the star, the SiII emission is filling the hole of this ring with the peak towards the star. This morphology can only be explained if the SiII emission arise in the lowest extinction layers of the PDR (A_v 2 mag) and the HII region. At least 20% - 30% of the Si must be in gas phase in these layers. For A_v 2 mag, the Si is mainly in solid form ( Si = -1.3).

The NW and SW PDRs have very similar excitation conditions, high density filaments (n 10⁶ cm^-3) immersed in a more diffuse interfilament medium (n 10⁴ cm^-3). In both, the NW and SW PDRs, the intensities of the H₂ rotational lines can only be fitted by assuming an ortho-to-para-H₂ ratio lower than 3 in gas with rotation temperatures from 400 to 700 K. Therefore, there is a non-equilibrium OTP ratio in the region. Furthermore, the comparison between the OTP ratio derived from H₂ vibrational lines and the pure H₂ rotational lines, shows that the OTP ratio increases from 1.5 to 3 across the photodissociation region with larger values in the less shielded gas (A_v 0.7 mag). This behavior is interpreted as a consequence of an advancing photodissociation front.

We have not detected the OH, CH and CH₂ lines towards the observed positions. This is consistent with the weakness of these lines in other sources and can be explained as a consequence of the small beam filling factor of the dense gas in the LWS aperture. The CO J=1716 line has been tentatively detected towards the star.

Key words: stars: individual: HD 200775 stars: pre-main sequence ISM: abundances ISM: individual objects: NGC 7023 ISM: reflection nebulae infrared: ISM: lines and bands

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries France, Germany, the Netherlands and the United Kingdom) and with participation of ISAS and NASA

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Contents

• 1. Introduction
• 2. Observations
• 3. Results
  • 3.1. Recombination lines
  • 3.2. The CII and OI lines
  • 3.3. Other ionic and atomic fine structure lines: SiII
  • 3.4. H₂ rotational lines
  • 3.5. Molecular lines
• 4. Discussion
  • 4.1. SiII
  • 4.2. A gradient in the ortho-to-para-H₂ ratio
  • 4.3. Absence of molecular emission
• 5. Summary: a model for the nebula
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000
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