Seeking independence from the temperature dependence metal recombination lines

How reliable are the chemical abundances derived from the auroral-line method at high metallicity? Are the effects of temperature gradients measurable? There are several ways to carry out these tests. Using objects other than H ii regions to trace galactic abundance gradients is becoming feasible with high-signal-to-noise-ratio spectroscopy of planetary nebulae and luminous stars in nearby galaxies. As an example, the blue supergiant metallicities derived by Urbaneja et al. (2005) in M33 are in good agreement with the H ii-region abundances calculated by Vilchez et al. (1988) and Crockett et al. (2006). Detailed photoionization modeling also offers a possible route; however, as mentioned earlier, current discrepancies between empirical and model-based abundances are not fully understood. A further approach envisions the analysis of nebular lines that depend far less than the collisionally excited lines on the electron temperature, such as optical metal recombination lines and IR fine-structure lines. In both cases the line emissivity is only moderately dependent on Te, even though it can be a strong function of the gas density. For example, considering Equation (17.2) for fine-structure transitions, in which the excitation potentials are very small (in the range 0.01-0.04 eV, versus 2-3 eV for optical transitions), the exponential term becomes virtually unity, and only a moderate Te dependence is left.

The use of [O iii] A.A.52, 88 to derive abundances in extragalactic H ii regions is limited to only a few cases in the literature. Garnett et al. (2004) have used ISO spectra of region CCM10 in M51 to derive 12 + log(O/H) = 8.8, which compares well to 12 + log(O/H) = 8.6 obtained by Bresolin et al. (2004) on the basis of auroral-line analysis. Additional far-IR studies include those of H ii regions in the Milky Way by Peeters et al. (2002), Martin-Hernandez et al. (2002), and Rudolph et al. (2006). Alternatively, it is also possible to use the mid-IR, accessible to Spitzer, to study the [Ne ii] A12.8 |~im and [Ne iii] A15.6 lines, as done for M33 by Willner & Nelson-Patel (2002).

The emissivity of recombination lines from metals is also only weakly dependent on Te: €x ~ oeff(A) ~ Te-1. When measuring abundances relative to hydrogen, the Te dependence becomes negligible, since H recombination lines have a similar sensitivity to Te. The main drawback in their use for abundance studies is their weakness. Among the strongest metal recombination lines are the O ii multiplet at 4651Á, and C ii A4267. These lines are beautifully detected in the high-resolution spectra of some well-known H ii regions in the Milky Way, such as the Orion nebula (Esteban et al. 2004) and the Triffid nebula (García-Rojas et al. 2006). Detection in extragalactic nebulae can still be challenging, and results for only one relatively high-metallicity, namely 12 + log(O/H) = 8.5, Hii region, NGC5461 in M101 (Esteban et al. 2002), have been published so far.

In general, it is found that oxygen abundances obtained from recombination lines are 0.2-0.3 dex larger than those derived from collisionally excited lines under the assumption (tacitly implied in the discussion so far) that temperature fluctuations are absent in H ii regions. A similar result is obtained when comparing O/H derived from a combination of optical and far-IR [O iii] lines or from optical data alone (Jamet et al. 2005). This systematic difference can be explained by invoking the existence of temperature fluctuations in H ii regions, as argued by M. Peimbert and collaborators in a series of papers; see Peimbert et al. (2007) for a recent review. With mean-square temperature fluctuations t2 in the 0.02-0.06 range, one can reconcile the abundances from collisionally excited lines with those from recombination lines for the H ii regions for which both sets of emission lines have been analyzed.

Figure 17.3 shows the same diagram as Figure 17.2, with the addition of Galactic and extragalactic H ii regions for which a comparison between abundances obtained from collisionally excited lines and from metal recombination lines has been carried out so far. The sources can be found in Peimbert & Peimbert (2005) and Peimbert et al. (2007). These objects populate the diagram in its upper branch only down to log R23 — 0.5. Efforts to extend this sample to higher metallicities with high-resolution spectroscopy at 8-10-m telescopes are under way. Bresolin (2007) recently measured the C ii A4267 line, as well as the mean-square temperature fluctuation t2 = 0.06, in H1013, an H ii region in the inner, metal-rich parts of M101. The oxygen abundance derived within the scheme developed originally by Peimbert (1967) and Peimbert & Costero (1969) is 12 + log(O/H) = 8.9 (1.7 x Solar), about 0.3 dex larger than the value obtained without accounting for temperature fluctuations. These two values are represented by the open and solid square symbols in Figure 17.3.

The results obtained from recombination lines are in good agreement with the R23 calibration derived from photoionization models, as Figure 17.3 shows. This is a consequence of the fact that, even though photoionization models fail at reproducing the intensity of an auroral line like [O iii] A4363, calling for an as-yet-unidentified

log R23

Figure 17.3. The same as Figure 17.2, with the observational data points shown with small symbols. The larger symbols represent Galactic and extragalactic H 11 regions for which abundances have been derived both from collisionally excited lines (open symbols) and from metal recombination lines (full symbols); the sources are specified in Peimbert et al. (2007). Data for the H 11 region H1013 in M101 studied by Bresolin (2007) are shown by squares.

log R23

Figure 17.3. The same as Figure 17.2, with the observational data points shown with small symbols. The larger symbols represent Galactic and extragalactic H 11 regions for which abundances have been derived both from collisionally excited lines (open symbols) and from metal recombination lines (full symbols); the sources are specified in Peimbert et al. (2007). Data for the H 11 region H1013 in M101 studied by Bresolin (2007) are shown by squares.

heating source (Stasiñska & Schaerer 1999), the predictions for the strong lines [011] X3727 and [0111] H4959,5007 are less sensitive to the temperature structure of the nebulae. The virtual lack of dependence of the abundances obtained from metal recombination lines on Te makes them more suitable to provide an accurate calibration of strong-line methods, such as R23, than collisionally excited lines. The extension of the recombination-line measurements to the high-metallicity regime then becomes imperative.

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