Detection of the [C I] λ8727 emission line

Low-ionization structures in NGC 7009

¹ Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, GR 15236 Penteli, Greece
Corresponding author; stavrosakras@gmail.com
² Instituto de Física e Química, Universidade Federal de Itajubá, Av. BPS 1303, Pinheirinho, 37500-903, Itajubá, MG, Brazil
³ European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
⁴ Observatório do Valongo, Universidade Federal do Rio de Janeiro, Ladeira Pedro Antonio 43, 20080-090, Rio de Janeiro, Brazil
⁵ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain

Received: 11 April 2024
Accepted: 14 July 2024

Abstract

We report the first spatially resolved detection of the near-infrared [C I] λ8727 emission from the outer pair of low-ionization structures in the planetary nebula NGC 7009 from data obtained by the Multi Unit Spectroscopic Explorer integral field unit. This atomic carbon emission marks the transition zone between ionized and neutral gas and for the first time offers direct evidence that LISs are photodominated regions. The outer LIS pair exhibits intense [C I] λ8727 emission, but He I λ8733 is absent. Conversely, the inner pair of knots shows both lines, likely due to the host nebula emission. Furthermore, the [C I] λ8727 line is absent in the host nebula emission, but He I λ8733 is present. Although the origin of the [C I] λ8727 line is still debated, its detection supports the scenario of photoevaporated dense molecular clumps.