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Massive stars vs. nebular abundances in the Orion nebula

The search of consistency between nebular and massive star abundances has been a longstanding problem. I briefly review what has been done regarding to this topic, also presenting a recent study focused on the Orion nebula: the O and Si stellar abundances resulting from a detailed and fully consistent spectroscopic analysis of the group of B stars associated with the Orion nebula are compared with the most recent nebular gas-phase results.


Introduction
Photospheres of OB stars are representative of the interstellar material from which they were born due to their relative youth. The evolutionary characteristics of blue massive stars imply that these objects and the associated ionized nebulae -H ii regions -must share the same chemical composition † Traditionally, chemical abundance studies in spiral and irregular galaxies have been based on the emission line spectra of H ii regions. This is logical, since H ii regions are luminous and have high surface brightness (in the emission lines) compared to individual stars in galaxies. Therefore, it is relatively easy to obtain high quality spectroscopic observational data, even with small and medium size telescopes. This has made it possible both the detailed study of individual nebulae (viz. Esteban et al. 2004), and the determination of radial gradients in the Milky way (viz. Shaver et al. 1983, Afflerbach et al. 1997) and other spiral galaxies (see the invited talk by F. Bresolin in these proceedings), imposing observational constraints to the chemical evolution models of these galaxies (see review by L. Carigi in these proceedings).
Although this is a commonly used methodology, it is not without some difficulties and problems. For example, it is known that the optical recombination lines (orl) in ionized nebulae indicate higher abundances than collisionally excited lines (cel); temperature fluctuations, density condensations and abundance inhomogeneities have been proposed to solve the orl/cel problem, however none of these explanations are completely satisfactory (see Esteban 2002 for a review). I refer to F. Bresolin talk (this proceedings) for a review of the use and limitations of strong line methods in the determination of nebular abundances. Stasińska (2005) has recently shown that for metal rich nebulae, the derived abundances based on a direct measurements of the electron temperature (T e ) may deviate systematically from the real ones. Finally, one must keep in mind other sources of uncertainties in the nebular abundance determination such as the atomic data, reddening corrections and the possible depletion of elements onto dust. † There are certain cases in which this in not completely fulfilled: (1) Strong stellar mass-loss may expose underlaying layers already contaminated on the surface of the star; (2) Several studies of OBA-type stars have found observational evidences of stellar surface contamination by products from the CNO-bicycle; rotating models by Maeder & Meynet (2000) predict that mixing of nuclear processed material at the surface will increase with stellar mass, age, initial rotational velocity and decreasing metallicity; (3) Certain elements in the nebular material can be depleted onto dust grains; in this case the gas-phase abundances, derived through a spectroscopic study of the H ii region, could be somewhat lower than the stellar ones.
Among the stellar objects, blue massive stars can be easily identified at large distances due to their high luminosities. Therefore, massive stars offer a unique opportunity to study present-day chemical abundances in spiral and irregular galaxies as an alternative method to classical H ii region studies. However, the amount of energy released by these stars is so large that it produces dramatic effects on the star itself: these stellar objects show mass outflows during their whole lifetime (so called stellar winds) and their atmospheres departure from lte conditions, two facts that make their modeling quite complex. It has not been until very recently that the development of massive star model atmospheres and the growth of the computational efficiency has allowed a reliable abundance analysis of these objects. Nowadays, the later is feasible; however, one must take into account that there are some effects that can affect the final results and must be carefully treated: (1) hypothesis on the stellar atmosphere modeling (lte vs. nlte, plane-parallel vs. spherical models, inclusion of line-blanketing effects), (2) atomic models and atomic data; (3) establishment of the stellar parameters and microturbulence.
Both nebular and stellar methodologies are now working in tandem to progress in our knowledge of the metallicity content of irregular and spiral galaxies (from the Milky Way to far beyond the Local Group). Since they sample similar spatial and temporal distributions, these objects offers us a unique framework in which the reliability of the derived abundances in both methodologies can be tested.

Do stellar and nebular abundances agree?
Two type of studies can be addressed to investigate if there is a consistency of abundance results from stellar and nebular studies: the comparison of radial abundance gradients in spiral galaxies or mean abundances in irregular galaxies (global approach), and the comparison of absolute abundances in the same galactic region (local approach). Values found in the literature for the oxygen abundance gradient in the Galaxy range from -0.04 to -0.08 in the nebular case , and references therein), and from -0.03 to -0.07 in the stellar case (Gummersbach et al. 1998, Rolleston et al. 2000, Daflon & Cunha 2004. One could conclude that results from both methodologies are in agreement within the intrinsic uncertainties, however this may not be the case when comparing results from individual studies (see e.g Figure 1).
In the extragalactic case, Urbaneja et al. (2005b) found good agreement in the comparison of stellar and Hii region O abundances (based on direct determinations of the T e of the nebulae) in M33; however, other works in spiral galaxies has found that the stellar and nebular agreement tend to be very dependent on the calibrations used in the strong nebular methods (Trundle et al. 2002, Urbaneja et al. 2005a. Examples of the comparison between stellar and nebular abundances in the same galactic region or mean abundances in the Magellanic Clouds are found in Cunha & Lambert (1994), Korn et al. (2002) and Trundle et al. (2004). Although within the typical errors of the stellar and nebular analyses the abundances determined by both methodologies are fairly consistent, more work remains necessary. For example, until very recently, there has not been any consistent detailed comparison of stellar and nebular abundances in the same star forming region.
Within this aim Simón-Díaz et al (2006) selected the Orion nebula, a well studied and spatially resolved H ii region with a cluster of a few massive stars inside it (the Trapezium cluster). Below, I will present the results of a detailed and fully consistent spectroscopic analysis of three B0.5V stars associated with the Orion nebula for deriving their O and Si abundances. The resulting abundances are compared with the most recent nebular gas-phase results.

Massive stars and nebular abundances in Orion nebula
The stellar spectroscopic observational dataset consists of int@ ids 2.5m spectra of θ 1 Ori A, θ 1 Ori D, θ 2 Ori B, three B0.5V stars located inside the Orion nebula. Details on the spectroscopic analysis for the stellar parameters and oxygen abundance determination can be found in Simón-Díaz et al (2006). The analysis of silicon abundances were presented in Simón-Díaz (2005, thesis), and will be published soon. Both stellar parameters and abundances where determined by means of the stellar atmosphere code fastwind (Santolaya et al. 1997, Puls et al. 2005, a nlte code taking into account line blanketing and stellar wind effects. A detailed O and Si abundance analysis by multiplets was previously performed, using the slow rotator B0.2V star τ Sco, to correctly estimate the abundance uncertainties related to the stellar parameters, microturbulence and line-to-line abundance dispersion. Table 1 summarizes the results from the stellar abundance determination, along with the values found in the literature for the nebular gas-phase abundances. The most recent and complete analysis of the chemical composition of the Orion nebula has been presented by Esteban et al. (2004); these authors used a wide variety of collisionally excited and recombination lines from a very deep uves@ vlt 8.2m spectra for the abundance determination. Table 1 shows the oxygen gas-phase abundance proposed by these authors, along with their estimated gas+dust oxygen abundance, which takes into account a dust deplection factor of 0.08 dex (obtained by . For the nebular silicon abundance determination I refer to the works by Rubin et al. (1993) and Garnett et al. (1995); these authors estimated the Si abundances in the Orion nebula by using iue high-dispersion spectra.

Results and discussion in the context of the mru
The mean value of the stellar oxygen abundances derived by Simón-Díaz et al. (2006) is in agreement with the nebular gas-phase oxygen abundance proposed by , however the dust+gas nebular oxygen abundance is somewhat larger than the stellar value. These result may imply a lower oxygen dust depletion factor than previously considered for the Orion nebula.
I would like to finally remark the importance of this kind of comparisons for the study of the Metal Rich Universe. In high metallicity environments, nebular T e diagnostic lines are faint and hence hardly measurable, so strong line methods (which depend on calibrations) must be used. Moreover, even if very deep spectra are obtained, derived nebular abundances may be affected by biases (Stasińska 2005). These deficiencies in the nebular methodology can be solved by using massive stars, since the strength of metal lines in the stellar spectra increases with metallicity, and hence stellar abundance diagnostic lines are (even!) more clearly seen in those high metallicity environments.