[Paper Review] An optical absorption study of the Helix Planetary Nebula: NaI and CaII lines, and a search for diffuse bands

N. Mauron, T. R. Kendall|ArXiv.org|Sep 1, 2004

Astrophysics and Star Formation Studies36 references3 citations

TL;DR

This study uses high-resolution VLT/UVES spectroscopy of 8 background stars to probe the circumstellar environment of the Helix Planetary Nebula (NGC 7293) for diffuse band (DIB) absorptions. Despite detecting strong Na I absorption in two lines of sight consistent with circumstellar origin, no DIBs were found, suggesting DIB carriers are not abundant in the neutral gas of this carbon-rich planetary nebula.

ABSTRACT

We present the results of an optical absorption study of NGC 7293, the Helix planetary nebula (PN), designed to search for diffuse band (DIB) absorptions perhaps arising in the carbon-rich circumstellar matter of the PN. We made high resolution spectra of 8 stars located close to and behind the Helix. Through a detailed analysis of the NaI and CaII lines, we find that two lines of sight situated close to the PN as mapped in CO and HI exhibit very strong NaI absorption. This absorption is due to the PN neutral or molecular material. No corresponding CaII line is observed in agreement with the very high Ca depletion often observed in PNs. No trace of any DIB features was found in these two circumstellar lines of sight (shortened abstract).

Motivation & Objective

• To search for diffuse band (DIB) absorptions in the circumstellar envelope of the carbon-rich Helix Planetary Nebula (NGC 7293).
• To determine whether DIB carriers are present in the neutral or molecular gas of planetary nebulae, particularly in regions with high carbon abundance.
• To use background stars as probes to isolate circumstellar absorption from interstellar and photospheric features.
• To investigate the spatial inhomogeneity of DIBs by observing multiple lines of sight across the nebula.
• To assess the potential role of planetary nebulae as sites for DIB carrier formation, given their carbon-rich environments.

Proposed method

• High-resolution (R ≈ 50,000) optical spectroscopy was performed using the VLT/UVES instrument on 8 background stars located angularly close to and behind the Helix Planetary Nebula.
• Stars were selected based on UBV and 2MASS photometry to estimate spectral types (mainly F-G dwarfs) and distances (200–1500 pc), ensuring several were located far beyond the nebula (210 pc).
• Spectral analysis focused on Na I D1 and D2 lines and Ca II K line to distinguish interstellar medium (ISM) absorption from circumstellar absorption, leveraging the fact that Ca II is strongly depleted in planetary nebulae.
• Equivalent widths of Na I and Ca II lines were measured and compared to expected ISM contributions derived from extinction maps and IS line data from three bright stars in the region.
• Diffuse band (DIB) features were searched for at known wavelengths (e.g., λ5780, λ6614), with upper limits on equivalent widths established based on signal-to-noise and spectral resolution.
• The absence of corresponding Ca II K absorption in strong Na I absorbers supports a circumstellar origin, as Ca is typically depleted in the cool, neutral phases of planetary nebulae.

Experimental results

Research questions

• RQ1 Are diffuse bands (DIBs) detectable in the circumstellar environment of the carbon-rich Helix Planetary Nebula?
• RQ2 Do the strong Na I absorptions observed in two lines of sight originate in the interstellar medium or in the nebula’s circumstellar material?
• RQ3 Is there evidence for DIB carriers in the neutral or molecular gas of planetary nebulae, particularly in regions with high carbon abundance?
• RQ4 How does the inhomogeneous structure of planetary nebulae affect the detectability of DIBs, and does multi-line-of-sight observation improve sensitivity?
• RQ5 Can the absence of Ca II K absorption in strong Na I absorbers be used as a diagnostic for circumstellar origin in planetary nebulae?

Key findings

• Two lines of sight (targets #4 and #5) exhibit strong Na I D2 absorption with equivalent widths exceeding 1000 mÅ, far above the expected ISM contribution of ~200 mÅ.
• The absence of corresponding Ca II K absorption in these two sightlines supports a circumstellar origin for the Na I absorption, consistent with strong Ca depletion in the cool, neutral phases of planetary nebulae.
• No diffuse band features were detected in any of the 8 sightlines, including the two strong circumstellar lines of sight, with upper limits on DIB equivalent widths of 5–20 mÅ for λ6614.
• The strong Na I absorption in the circumstellar lines of sight is inconsistent with ISM origin due to the high galactic latitude (b = -57°), where ISM column densities are low.
• The lack of DIB detection in the Helix PN, despite its carbon-rich environment, suggests that DIB carriers are not abundantly formed in such circumstellar regions.
• The circumstellar sightlines open new avenues for detecting molecular species such as C2 and CN, which are not observable at radio wavelengths, and for probing the total hydrogen column density in the nebula.