Can Stellar Winds Account for Temperature Fluctuations in H II

A&A 379, 1017–1023 (2001) Astronomy DOI: 10.1051/0004-6361:20011364 & c ESO 2001 Astrophysics Can stellar winds account for temperature fluctuations in H II regions? The case of NGC 2363 V. Luridiana1,2,M.Cerviño3, and L. Binette2 1 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany 2 Instituto de Astronom´ıa, Universidad Nacional Autónoma de México, Ap. Postal 70-264, 04510 México D.F., Mexico 3 Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße, 85748 Garching b. München, Germany Received 13 August 2001 / Accepted 25 September 2001 Abstract. We compare the rate of kinetic energy injected by stellar winds into the extragalactic Hiiregion NGC 2363 to the luminosity needed to feed the observed temperature fluctuations. The kinetic luminosity associated to the winds is estimated by means of two different evolutionary synthesis codes, one of which takes into account the statistical fluctuations expected in the Initial Mass Function. We find that, even in the most favorable conditions considered by our model, such luminosity is much smaller than the luminosity needed to account for the observed temperature fluctuations. The assumptions underlying our study are emphasized as possible sources of uncertainty affecting our results. Key words. Hiiregions – ISM: individual (NGC 2366, NGC 2363) – stars: clusters 1. Introduction For a long time, the only way temperature fluctuations were accounted for in theoretical models was ac- The presence of temperature fluctuations in photoionized knowledging the impossibility to reproduce the observed regions has been a matter of debate since the pioneer- intensity of the most affected lines; see, e.g., Luridiana ing work by Peimbert (1967). Although little doubt on et al. (1999) for the case of NGC 2363, and Stasińska & their relevance in real nebulae seems possible on observa- Schaerer (1999) for the case of IZw18. Recently, Binette tional grounds (see the review by Peimbert 1995), their & Luridiana (2000) developed a model to quantify the ef- inclusion in a theoretical framework is still controversial, fect of temperature fluctuations on the ionization and en- given both the incompleteness of the present theoretical ergy balance of nebulae. In the present work, we use their scenario, and the technical difficulties implied by their in- schema to investigate whether the kinetic energy provided clusion in photoionization models. Plain photoionization by stellar winds could feed the temperature fluctuations theory predicts temperature fluctuations to be very small, observed in NGC 2363. Throughout the paper, by “stellar mainly due to the steep dependence of the cooling rate winds” we generically refer to all those phenomena involv- on temperature, which implies that their lifetime would ing the ejection of mass from stars into the interstellar be quite short; yet, the existence of a mechanism steadily medium (ISM). Given the age range considered, the only providing the energy required to feed them is not excluded contributors to stellar winds actually included in the cal- by this line of reasoning. The importance of the question culations are hot stars and Wolf-Rayet stars (WRs). can be barely overrated given that, if temperature fluctu- The structure of the paper is as follows: in Sect. 2 we ations turn out to be as big as observations suggest, the summarize the relevant properties of NGC 2363. Section 3 present determinations of chemical abundances should be describes the model used to represent temperature fluctu- rescaled by as much as +0.5 dex. ations in nebulae. In Sect. 4 we estimate the kinetic luminosity of the stellar cluster in NGC 2363, and apply the Send offprint requests to: V. Luridiana, method of Sect. 3 to determine whether stellar winds could e-mail: [email protected] be responsible for the temperature fluctuations observed Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20011364 1018 V. Luridiana et al.: Temperature fluctuations in NGC 2363 in NGC 2363. Finally, in Sect. 5 we analyze several pos- respectively, V is the observed volume, and the average sible sources of uncertainty, and in Sect. 6 we summarize temperature T is given by: R 0 the main conclusions. T (r)N (r)N (r)dV V R e e i T0 = · (2) V Ne(r)Ni(r)dV 2. General properties of NGC 2363 Since the temperature of a model Hiiregion is not NGC 2363 is a giant extragalactic Hiiregion, located in spatially constant, temperature fluctuations appear and the south-west end of the irregular galaxy NGC 2366. It evolve following the evolution of the ionization field even in is one of the brightest extragalactic Hiiregions known, the simplest case of a static, pure photoionization nebula; and an ideal subject for our study, since plenty of data in particular, Pérez (1997) showed that large temperature are available, both observational and theoretical. Two dis- fluctuations arise during the WR phase, as a consequence tinctive knots can be clearly distinguished in the region: of the hardening of the spectrum. Following the definition by Ferland (1996), we will refer to such “structural” tem- throughout this paper, we will always refer to the bright- 2 2 est, youngest knot, often called “knot A”. perature fluctuations as tstr.Typicaltstr values for photoionization models of chemically and spatially homoge- NGC 2363 has been observed by several groups (e.g., neous model nebulae are in the range 0.00 ≤ t2 ≤ 0.02. Peimbert et al. 1986; González-Delgado et al. 1994; Izotov str The best-fit photoionization model by Luridiana et al. et al. 1997), and modeled by Luridiana et al. (1999) and (1999) yields a value t2 ∼ 0.009. Drissen et al. (2000) among others, while the temperature- str On the observational side, González-Delgado et al. fluctuation parameter has been determined by González- (1994) find for knot A in NGC 2363 the value t2 =0.064 Delgado et al. (1994) in both knots. obs by comparing the Paschen temperature, T (Pa), to the In the present work, we will assume that both the stel- e [O iii] temperature, obtained from the λ4363/λ5007 ratio. lar cluster and the gas nebula of NGC 2363 are well de- Even if in this case the formal error on t2 is quite large scribed by the best-fit model of Luridiana et al. (1999), obs (we calculated σ(t2 )=0.045, mainly due to the large which is a spherical, hollow, radiation-bounded nebula, obs error on T (Pa)), the differences between the model pre- consisting of two concentric shells of different densities, e dictions and the observations seem to imply that an extra- ionized by a young cluster undergoing an extended burst heating mechanism, other than photoionization, is at work of star formation. The properties of the model relevant for 2 in most objects, producing an additional textra such that this work will be described throughout the text. 2 2 2 tobs = tstr + textra. In the case of NGC 2363, we find 2 textra =0.055 0.045. Note that such value still leaves the 3. The energy implications of temperature door open to the possibility, small but not negligible, that 2 textra =0.00; however, we assume this not to be the case. fluctuations 2 In this respect, it is interesting to note that tobs =0.098 To quantify the energy injection rate needed to fuel tem- in knot B of the same region (González-Delgado et al. perature fluctuations in a nebula, we follow the model 1994). Given the presumed age and metallicity of this re- proposed by Binette & Luridiana (2000) and Binette et al. gion (Luridiana et al. 1999), and assuming the same filling 2 (2001). The first of these two papers contains a detailed factor as in knot A, we can roughly estimate tstr =0.02 2 2 − 2 description of the method, while we refer to the second for (see, e.g., Pérez 1997), yielding textra = tobs tstr =0.078, 2 an alternative representation of the results. The model de- with an associated error of about σ(tobs)=0.019. picts temperature fluctuations as a collection of hot spots, More generally, even though the errors might be com- 2 arising above a uniform equilibrium temperature floor, patible with textra = 0 in individual cases of ionized re- 2 and fed by an unknown heating agent. This representa- gions with tobs > 0, this is certainly not the case when tion allows one to compute the average thermal properties large samples are considered. As an example, galactic Hii of the nebula in the temperature-fluctuation regime, but regions and planetary nebulae collectively show higher without having to compute real localized fluctuations, as values, typically around 0.04 (e.g., Peimbert et al. 1995; described below. Peimbert 1995, and references therein). The presence of temperature fluctuations inside NGC 2363 bears important cosmological consequences, as 3.1. Definition of temperature fluctuations it affects the determination of the chemical abundances, The amplitude of temperature fluctuations in a neb- lowering the extrapolated primordial helium abundance 2 ula is measured by the parameter t , defined as follows Yp (e.g., Peimbert et al. 2001a, and references therein). In 2 (Peimbert 1967): our case, tobs =0.064 implies a downward change in the R helium abundance of NGC 2363 of order 2%, the exact fig- 2 (Te(r) − T0) Ne(r)Ni(r)dV ure depending on which He lines the abundance determi- t2 = V R (1) 2 nation is based on (see also González-Delgado et al. 1994). T0 V Ne(r)Ni(r)dV It is important to note, however, that temperature fluctu- where Te(r) is the local electron temperature, Ne(r)and ations are not the only factor affecting a proper chemical Ni(r) are the local values of the electron and ionic density abundance determination, other being, e.g, the ionization V.

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