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Astron. Astrophys. 327, 909-920 (1997)

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3. Model parameters inferred from Circinus

3.1. A high ionization parameter

The strengths of the extremely high excitation lines, such as [S IX ] and [Si IX ], observed by OSMM in the Circinus Galaxy could not be reproduced using the ionization parameter [FORMULA] =0.04 adopted in Paper I. As a first step, we determined that values as high as [FORMULA] =0.5 are needed to account for the relative strength of [S IX ]1.25 [FORMULA] m and [Si IX ]3.935 [FORMULA] m. Adopting [FORMULA] =0.5, the second step consisted of determining the thickness of the high excitation MB clouds. This was obtained by simply requiring that the model fit the relative strengths of all three infrared silicon lines. These span a wide range in excitation yet are exclusively produced within the MB component. In our calculations, [Si VI ] effectively sets the thickness of the MB slab since too much [Si VI ] is generated if the MB component is excessively thick. In short, the Si species observed in the ratio: {[Si VI ]1.963 [FORMULA] m : [Si VII ]2.483 [FORMULA] m : [Si IX ]3.935 [FORMULA] m} = { [FORMULA] } were successfully reproduced with a slab thickness such that 35% of the ionizing photons are absorbed by the MB slab. This number is very similar to that favored in Paper I.

3.2. The density

To select the density, we also proceed from observations. As indicated by Mo96, the [Ne V ] 24.3 [FORMULA] m/14.3 [FORMULA] m line ratio provides an excellent density diagnostic for the high excitation gas. The observed value of 1.5 indicates a density [FORMULA] cm-3 for the [Ne V ] emitting gas. This ratio is reproduced in our model with [FORMULA] =0.5 by adopting a density at the irradiated cloud surface of [FORMULA] cm-3. As a result of the increasing density with increasing distance into the MB slab, reaching [FORMULA] cm-3 at the back of the cloud, the [Ne V ] emitting region has the appropriate average density of [FORMULA] cm-3.

3.3. Solar metallicity and dust-free gas

We assume the gas is dust-free and of solar metallicity (Anders & Grevesse 1989). An absence of dust in the MB cloud can be justified on the grounds that the strength of [Ca VIII ]2.321 [FORMULA] m is perfectly consistent with the solar Ca/H abundance ratio, therefore implying that depletion into dust grains is negligible (see OSMM). It is worth noting that OSMM did not detect any [Ca II ] [FORMULA] 7291 while its strength is predicted to exceed observed neighboring lines of [Ar III ] [FORMULA] 7135 and [O II ] [FORMULA] 7325 (cf. Table 1). This suggests that dust is probably present in the low excitation IB component (Villar-Martín & Binette 1996).


Table 1. MB and IB line ratios relative to H [FORMULA] (=1.00)

3.4. Line ratios from the high excitation MB cloud

As described in the above subsections, we adopt [FORMULA] =0.5, and [FORMULA] cm-3 for our MB cloud which absorbs [FORMULA] 35% of the incident ionizing radiation. The calculated UV, optical and infrared line strengths are presented in Table 1 and are identified as model H (third column, heading R [FORMULA]). For comparison, we include in Table 1 two other models, M and L, which differ only in their lower ionization parameters of [FORMULA] and 0.02, respectively. The discussion of these and of their corresponding low excitation IB components (heading R [FORMULA] in Table 1) is postponed to Sect. 5.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998