[Paper Review] The chemical composition of the Galactic regions M8 and M17. A revision based on deep VLT echelle spectrophotometry

J. García–Rojas, C. Esteban+4 more|arXiv (Cornell University)|Oct 3, 2006

Astrophysics and Star Formation Studies7 citations

TL;DR

This study presents deep VLT UVES echelle spectrophotometry of the Galactic H II regions M8 and M17, measuring 375 and 260 emission lines respectively, with a focus on ionic abundances from recombination lines (RLs) and collisionally excited lines (CELs). It confirms consistent temperature fluctuations ($t^2 = 0.040 \pm 0.004$ for M8, $0.033 \pm 0.005$ for M17) and reports the detection of deuterium Balmer lines (up to Dε) in M8, with intensities consistent with continuum fluorescence, resolving long-standing abundance discrepancies through improved atomic data and ionization correction factors (ICFs).

ABSTRACT

We present new echelle spectrophotometry of the Galactic H II regions M8 and M17. The data have been taken with the VLT UVES echelle spectrograph in the 3100 to 10400 angstroms range. We have measured the intensities of 375 and 260 emission lines in M8 and M17 respectively, increasing significatively the number of emission lines measured in previous spectrophotometric studies of these nebulae. Most of the detected lines are permitted lines. Electron temperatures and densities have been determined using different diagnostics. We have derived He+, C++, O+ and O++ ionic abundances from pure recombination lines. We have also derived abundances from collisionally excited lines for a large number of ions of different elements. Highly consistent estimations of t2 have been obtained by using different independent indicators, the values are moderate and very similar to those obtained in other Galactic H II regions. We report the detection of deuterium Balmer emission lines, up to D$ε$, in M8 and show that their intensities are consistent with continuum fluorescence as their main excitation mechanism.

Motivation & Objective

• To improve the accuracy of ionic and total elemental abundances in Galactic H II regions M8 and M17 using high-spectral-resolution, high-signal-to-noise VLT UVES data.
• To investigate the origin of the long-standing 'abundance discrepancy' problem, where ionic abundances from CELs are systematically lower than those from RLs.
• To test the robustness of abundance determinations by comparing results derived from recombination lines (RLs) and collisionally excited lines (CELs), using consistent atomic data and ionization correction factors (ICFs).
• To detect and characterize deuterium Balmer emission lines in M8 and assess their excitation mechanism.
• To evaluate the consistency of temperature fluctuation parameters ($t^2$) derived from multiple independent diagnostics in both nebulae.

Proposed method

• Acquired deep echelle spectrophotometry with the VLT UVES spectrograph across 3100–10400 Å, achieving high signal-to-noise ratios to detect faint permitted emission lines.
• Measured intensities of 375 lines in M8 and 260 in M17, predominantly permitted lines, enabling precise diagnostics of physical conditions.
• Determined electron temperatures and densities using multiple independent line ratio diagnostics, including [S II], [O II], [O III], and [Ne III] lines.
• Calculated ionic abundances from recombination lines (RLs) for He⁺, C⁺⁺, O⁺, and O⁺⁺, which are less sensitive to temperature structure than CELs.
• Computed ionic abundances from collisionally excited lines (CELs) for a wide range of elements, applying consistent and modern atomic data and ionization correction factor (ICF) schemes.
• Assessed the excitation mechanism of deuterium Balmer lines (Dα to Dε) in M8 by comparing observed intensities with predictions from continuum fluorescence models.

Experimental results

Research questions

• RQ1 What are the most accurate ionic and total elemental abundances in M8 and M17, as derived from high-resolution VLT UVES data?
• RQ2 To what extent do recombination line (RL) and collisionally excited line (CEL) abundance determinations agree in M8 and M17, and what does this imply for the abundance discrepancy problem?
• RQ3 What is the level of temperature fluctuations ($t^2$) in M8 and M17, and how consistent are the values derived from independent diagnostic methods?
• RQ4 Are deuterium Balmer emission lines detectable in M8 and M17, and is continuum fluorescence a viable excitation mechanism for these lines?
• RQ5 How do the abundances derived in this work compare with previous determinations when using the same atomic data and ICF schemes?

Key findings

• The study reports the detection of deuterium Balmer emission lines (up to Dε) in M8, with intensities consistent with continuum fluorescence as the dominant excitation mechanism.
• The temperature fluctuation parameter $t^2$ is measured as $0.040 \pm 0.004$ in M8 and $0.033 \pm 0.005$ in M17, showing excellent consistency across multiple independent diagnostic methods.
• Ionic abundances derived from recombination lines (RLs) for C⁺⁺ and O⁺⁺ in M8 and M17 are in very good agreement with previous works when recalculated with the same atomic data and ICF schemes.
• Total elemental abundances for M8 and M17 are found to be robust and well-established, with discrepancies among previous studies primarily attributed to differences in atomic data and ICF schemes rather than measurement errors.
• The abundance discrepancy between CEL- and RL-derived abundances persists but is now better constrained, with the $t^2$ values supporting the temperature fluctuation scenario as a key contributor.
• Recomputing previous abundance determinations using the same atomic data and ICF scheme as this work reduces discrepancies to within ~0.1 dex, confirming the reliability of the current results.