MNRAS 456, 4407–4424 (2016) doi:10.1093/mnras/stv2995
Sulphur abundance determinations in star-forming regions – I. Ionization correction factor
O. L. Dors,1‹ E. Perez-Montero,´ 2‹ G. F. Hagele,¨ 3,4 M. V. Cardaci3,4 andA.C.Krabbe1 1Universidade do Vale do Para´ıba, Av. Shishima Hifumi, 2911, Cep12244-000, Sao˜ Jose´ dos Campos, SP, Brazil 2Instituto de Astrof´ısica de Andaluc´ıa (CSIC), PO Box 3004, E-18080 Granada, Spain 3Instituto de Astrof´ısica de La Plata (CONICET-UNLP), Argentina 4Facultad de Ciencias Astronomicas´ y Geof´ısicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina.
Accepted 2015 December 22. Received 2015 December 16; in original form 2015 September 21
ABSTRACT In this work, we used a grid of photoionization models combined with stellar population synthesis models to derive reliable ionization correction factors (ICFs) for the sulphur in star- Downloaded from forming regions. These models cover a large range of nebular parameters and yielding ionic abundances in consonance with those derived through optical and infrared observational data of star-forming regions. From our theoretical ICFs, we suggested an α value of 3.27 ± 0.01 in the classical Stasinska´ formulae. We compared the total sulphur abundance in the gas phase of a large sample of objects by using our theoretical ICF and other approaches. In average, the http://mnras.oxfordjournals.org/ differences between the determinations via the use of the different ICFs considered are similar to the uncertainties in the S/H estimations. Nevertheless, we noted that for some objects it could reach up to about 0.3 dex for the low-metallicity regime. Despite of the large scatter of the points, we found a trend of S/O ratio to decrease with the metallicity, independently of the ICF used to compute the sulphur total abundance. Key words: galaxies: abundances – galaxies: evolution – galaxies: formation – galaxies: gen-
eral – galaxies: ISM. by guest on March 7, 2016
+ 2+ λλ 1 INTRODUCTION the ions S and S , through the lines [S II] 6716, 31 and [S III] λλ9069, 9532 respectively, and by using an ICF to correct the un- The knowledge of the abundance of heavy elements (e.g. O, S, N, observed S3+, which produces forbidden lines at 10.51 μm. In the Ne) in the gas phase of star-forming regions play a key role in pioneer work, Stasinska´ (1978a) proposed an ICF for the sulphur studies of stellar nucleosynthesis, initial mass function (IMF) of based on both S+ and S2+ ions and given by stars and chemical evolution of galaxies. + α −1/α To derive the total abundance of a given element (X) in ionized + + O ICF(S + S2 ) = 1 − 1 − . (2) nebulae, after to estimate the electron temperature and electron O density of the gas phase, it is necessary to calculate the abundance α of all its ionization stages (see Osterbrock 1989). However, for Along decades, the value of have been largely discussed in the lit- the majority of the elements present in star-forming regions, only erature. For example, Stasinska´ (1978a), using the photoionization emission lines of some ionization stages can be measured. In these models of Stasinska´ (1978b), which assume the non-local thermo- cases, the use of ionization correction factors (ICFs) is necessary to dynamic equilibrium (NLTE) stellar atmosphere models of Mihalas α = derive the contribution of unobserved ions, as initially defined by (1972), suggested 3. French (1981), who used a sample of α = Peimbert & Costero (1969) H II regions and planetary nebulae, derived 2. Garnett (1989) combined spectroscopic data of H II regions containing the [S III] X/H +i = , λλ9069, 9532 emission lines (not considered by most of previous ICF(X ) + + (1) X i/H works) with photoionization models assuming different stellar at- where X+i is the ion whose ionic abundance can be calculated from mosphere models in order to estimate an ICF for the sulphur. From α its observed emission lines. this analysis, Garnett (1989) suggested that an intermediary value In particular, for sulphur, in the most of the cases the total abun- between 2 and 3 is correct. Vermeij & van der Hulst (2002), using dance is calculated by a direct determination of the abundance of the optical and infrared (IR) spectroscopic data of Vermeij et al. (2002), were able to derive directly an ICF for the sulphur and concluded that α = 3 is correct for O+/O > 0.2, being their re- E-mail: [email protected] (OLD); [email protected] (EP-M) sults less clear for higher ionization stages (see also Dennefeld &