4 Discussion

4.1 Molecular abundances

Depending on their formation processes, most molecular species observed in IRC+10216 are confined either to the inner envelope ($R\leq 6''$), or to a thin hollow shell of radius $\sim 15''$. This was discussed in particular by Guélin et al. ([1993]) and Guélin et al. ([1997]), who showed that the lines of the carbon chain radicals exhibit very similar spatial brightness distributions in spite of their different Einstein coefficients. Most metal-bearing species like NaCl, AlCl, KCl, AlF (Cernicharo & Guélin [1987]), as well as SiO and SiS belong to the first category, and the carbon chains, SiC, SiC₂ and MgNC to the second. Only CO and, to a lesser degree, HCN, are distributed throughout the envelope.

Though highly variable in the hot and dense inner envelope, the gas temperature and the gas density seem fairly constant in the 15'' radius hollow shell: $T_k \simeq 50$ K and $n_{\rm H_2}\sim 10^4$ cm^-3. Most species confined in this shell have thus a rather well defined rotational temperature and their radial column densities can be calculated with some accuracy. Our calculations, based on the present $\lambda\thinspace2$ mm data, as well as on our $\lambda\thinspace3$ mm and 1.3 mm data and the data of Kawaguchi et al. (1995), assume that the rotational levels within each vibrational state follow a Boltzman distribution with a rotational temperature $T_{\rm rot}$, and that the lines are optically thin (see Guélin et al. [1997]). Table 10 lists the rotation temperatures, radial column densities, and fractional abundances relative to H₂ for the species confined in the outer shell (and for which we observed enough lines to sample the partition function). For these calculations, the poorly known distance to IRC+10216 was assumed to be equal to 200 pc and the mass loss rate equal to 3 10^-5 $M_\odot$(Truong-Bach et al. [1991]). We note that the stellar mass loss rate derived from far-infrared ISO data for the same distance is twice smaller (Cernicharo et al. [1996]). Although the column density ratios depend neither of the distance, nor of the mass loss rate, the fractional abundances depend on the mass loss rate. Their uncertainties could thus be crudely estimated to be a factor of 2.

4.1.1 Oxygen-bearing compounds

The paucity of oxygen compounds is perhaps the most striking characteristic of this C-rich envelope. Only 3 O-bearing molecules are actually observed, CO, SiO and HCO⁺ (Lucas & Guélin [1990]), the former two trapping almost all the oxygen available in the gas phase. CO and SiO are formed in the stellar atmosphere and show centrally peaked distributions. HCO⁺ appears later and is only observed farther in the envelope (Lucas & Guélin [1998]).

4.1.2 Carbon chains and radicals

The large abundance of highly unsaturated carbon chains and radicals, most of which are linear, is the second characteristic feature of IRC+10216. Except for TMC 1, no other source in the sky shows such a wealth of long linear chains.

Because of the fairly high temperature, the lowest bending modes of these chains are excited and can give rise to relatively strong pure rotational lines.

Such is the case for the linear C₄H radical, which has a $^2\Sigma^+$ ground state and a low lying $^2\Pi$ electronic state. Renner-Teller interaction between these two states pushes down the lowest excited bending states, $1\nu_7$ and $2\nu_7$, to energies as low as 100-200 cm^-1 (Yamamoto et al. [1987]). Rotational transitions within these states are seen throughout the 2-mm window. The lines pertaining to the $1\nu_7(^2\Pi)$ and $2\nu_7(^2\Sigma$) states are remarkably strong with respect to the ground state lines, showing that theses excited states contain a significant fraction, $\simeq 1/7$ and 1/12 respectively, of the total C₄H column density.

4.1.3 Cyclic compounds

Discarding SiC₂, which has a T-shaped rather than cyclic structure, only three cyclic compounds were detected in our survey: c-C₃H, c-C₃H₂, and c-SiC₃, a rhombic-shaped molecule recently discovered by Apponi et al. ([1999]). The paucity of cyclic compounds is another characteristic feature of IRC+10216, whose IR spectrum does not show the ubiquitous unidentified infrared bands (Cernicharo et al. 1999, in preparation). The column density of c-C₃H is only a factor of $\simeq 2$ lower than that of its linear isomer. In comparison, the abundance ratio of l-C₃H₂over c-C₃H₂ is much smaller: $\leq 0.05$ (Cernicharo et al. [1991c]).

4.1.4 Silicon and metal-bearing species

The wealth of refractory Si-bearing and metal-bearing compounds is yet another characteristic of IRC+10216. The former includes SiO, SiS, SiC, SiC₂, SiC₃ and SiC₄. SiC₄, which is linear (Ohishi et al. [1989]), is too heavy to be observable at $\lambda 2$ mm. Although they are refractory, SiC and SiC₂ are mostly observed in the outer shell, whereas SiO and SiS are centrally peaked (Lucas et al. [1994]). SiC was first discovered in the course of this survey (Cernicharo et al. [1989]).

The diatomic metal compounds NaCl, KCl, AlCl, and AlF were also detected and identified in the course of this survey (Cernicharo & Guélin [1987]). They were the first metallic molecules ever observed outside planetary or stellar atmospheres. Such a wealth of metal-bearing ionic molecules was unexpected, as most abundant species in circumstellar envelopes have covalent bonds. It opens interesting prospects on circumstellar chemistry and has stimulated laboratory work on metallic radicals.

Two magnesium-bearing radicals are detected in IRC+10216: MgNC (Guélin et al. 1986; Kawaguchi et al. 1993) and MgCN (Ziurys et al. [1995]). Searches for MgCCH (Ziurys et al. in preparation), MgF and MgCl have yielded so far only upper limits.

The MgNC radical is observed in the same cold thin shell as the carbon-chain molecules and radicals, while the metal halides are concentrated in a hot region close to the star (Guélin et al. [1993]). Their line intensities peak at or below 2-mm wavelength except for AlF, the lightest of these species. The J= 15-14 line of AlF, observed at 494.2 GHz with the CSO telescope at the same angular resolution as the lines of this survey, has an intensity of 12 K km s^-1 and is 3 times stronger than the J=5-4 transition.

4.1.5 Phosphorus compounds

The CP radical (Guélin et al. [1990]) and PN (see Fig. 2) are the only P-bearing molecules observed in this source. Whereas CP is detected only in IRC+10216, PN is seen in several warm molecular clouds (Turner et al. [1990]). Besides the half-blended 140966 MHz line visible in Fig. 2, which coincides with the J = 3-2transition of PN (see Table 2), we have observed two weak lines at 93979 MHz and near 234935 MHz, which would correspond to the J = 2-1 and 5-4 transitions. The detection of PN in IRC+10216 is thus highly probable. Surprisingly, we failed to detect the stable HCP molecule, whose electron outer shell is analog to HCN (Turner et al. [1990]).

4.2 Isotopic species

As shown first by Wannier ([1980]) and later by Kahane et al. ([1988], [1992]), the elemental isotopic composition of IRC+10216 is markedly non-solar, despite its location in the solar neighbourhood. Most likely, the central star CWLeo has already reached the end of the AGB phase and expelled the bulk of its convective shell. What remains of the shell is highly enriched in processed material upheaved after the ultimate dredge-ups. The very low abundance of ¹⁵N, an element destroyed in equilibrium by the main CNO cycle, confirms that the gas in the outer circumstellar envelope is essentially formed of processed material. An analysis of the observed isotopic abundances of N, O and Mg in the light of stellar evolution calculations shows that CWLeo had a mass between 3 and 4 $M_\odot$ when it was on the main sequence (Guélin et al. [1995]; Forestini et al. [1997]).

As in our previous studies of the isotopic abundance ratios in IRC+10216 (Kahane et al. [1988], [1992]; Cernicharo et al. [1986c]; Cernicharo & Guélin [1987]; Cernicharo et al. [1991a]; Forestine et al. [1997]), we have made no attempt to model the line intensities: the isotopic ratios are simply the ratios of the integrated line intensities given in Table 2, corrected for a frequency factor of $\nu ^{-2}$. For a discussion of the different biases that may affect the derivation of elemental isotopic ratios from molecular lines observations, see Kahane et al. ([1988], [1992]).

Figure 3: The silicon isotopic abundance ratios in IRC+10216, derived from the SiS and SiC2 line intensities. Each triangle stands for a rotational transition. The ordinate is the intensity ratio observed for two different isotopomers, after a correction in $\nu ^{-2}$ had been applied (see Kahane et al. 1988). Abscissa denotes the energy (E/k) of the transition upper level

Figure 4: Same as Fig. 3 for sulfur

Figure 5: Same as Fig. 3 for carbon

Figure 6: The 12C/13C isotopic abundance ratios, derived from four CS transitions involving doubly substituted isotopomers. The ordinate is the intensity ratio divided by the square of the frequency ratio

Except for the magnesium-bearing species, the isotopic ratios analyzed here rely on larger sets of data than previously reported so that (i) the biases due to line opacities can be avoided or at least easily identified, (ii) the error bars on the average isotopic ratios are significantly reduced. In addition, using the C₄H doublets as secondary calibrators, we were able to check the intensities of a number of spectra and eventually to correct them with a reliability better than 10% rms. The results are reported in Table 11

 

Table 10: Molecular column densities

Molecule	$T_{\rm rot}$	Col. Dens.t	Frac. abund.
	kelvin	cm-2	N(X)/N(H2)
C2H	20	5.0 1015	7.1 10-6
l-C3H	20	7.0 1013	1.0 10-8
C4H	35	3.0 1015	4.3 10-6
C5H	25	4.4 1013	6.3 10-8
C6H	35	5.5 1013	7.8 10-8
C7H	35	2.2 1012	3.1 10-9
C8H	52	1.0 1013	1.4 10-8
C3N	20	2.5 1014	3.5 10-7
C5N	35a	6.3 1012	9.0 10-9

^a Assumed.
^t Total column density across the source,
i.e. twice the radial column density.

 

 

Table 11: Elemental isotopic abundance ratios

Isotopic ratio	average value	solar value	comments
29Si/30Si	1.45 (0.13)	1.52
28Si/29Si	15.4 (1.1)	19.6	opacity
28Si/30Si	20.3 (2.0)	29.8	opacity
33S/34S	0.18 (0.1)	0.18
32S/34S	21.8 (2.6)	22.5
32S/33S	62 (8)	125	opacity
12C/13C	45 (3)	89

and Figs. 3-6, together with the solar sytem values. For each intensity ratio, the rms errorbar includes two statistically independant uncertainties: a 20% calibration uncertainty (reduced to 10% when the spectrum calibration could be checked with a C₄H line) and a "fit uncertainty'' (derived from a model line fit as explained in Sect. 2.2). The average isotopic ratios derived from several transitions are computed using weights inversely proportional to the individual error bars.

4.2.1 Chlorine

³⁷Cl, which is 3.1 times less abundant than ³⁵Cl in the solar system, is detected in three species: Na³⁷Cl, Al³⁷Cland K³⁷Cl. In our detection paper (Cernicharo & Guélin [1987]) we reported a ³⁵Cl/³⁷Clabundance ratio of 2.4 $\pm$ 1 for aluminium chlorine and of 2 $\pm$ 1 for sodium chlorine. From the data in Tables 2 and 6 we derive for NaCl, KCl and AlCl (two lines) an average ³⁵Cl/³⁷Clratio of 3.1 $\pm$ 0.6 which is fully consistent with the solar system ratio.

4.2.2 Magnesium and aluminium

MgNC is abundant enough that the 2-mm and 3-mm lines of the rare isotopomers ²⁵MgNC and ²⁶MgNC could be detected.

The ²⁴MgNC:²⁵MgNC:²⁶MgNC abundance ratios are found equal to 78: 11 $\pm$ 1: 11 $\pm$ 1 and are consistent with the solar system ²⁴Mg:²⁵Mg:²⁶Mg isotopic ratios (79.0:10.0:11.0) (Guélin et al. [1995]).

A line with a rest frequency of 234433 MHz has been tentatively identified by Guélin et al. (1995) as the J= 7-6 transition of ²⁶AlF. ²⁶Al is an element with a lifetime $t_{1/2} = 7 \thinspace10^5$ yr which is indirectly observed in the interstellar medium. Unfortunately, the present spectral survey is not sensitive enough support or infirm this identification through the observation of the J= 5-4 and/or J= 4-3 rotational transitions.

4.2.3 Silicon

Following the first identification of ²⁸SiC₂ (Thaddeus et al. [1984]; Gottlieb et al. [1989]) and that of its rare isotopomers ²⁹SiC₂, ³⁰SiC₂ and Si¹³CC (Cernicharo et al. [1986c], [1991a]), we have identified in our survey a total of 49 lines pertaining to these species. Similarly, we detected 18 lines arising from 7 isotopomers of SiS (²⁸Si³²S, ²⁹SiS, ³⁰SiS, Si³⁴S, Si³³S, and the doubly-substituted species ²⁹Si³⁴S and ³⁰Si³⁴S). We have thus significantly enlarged our database to derive silicon isotopic ratios.

Two of the silicon isotopes, ²⁹Si and ³⁰Si, have significantly smaller abundances than the main isotope, ²⁸Si (by factors about 20 and 30 respectively in the solar system), which causes the millimeter lines of the rare SiC₂ and SiS isotopomers to be optically thin (see Cernicharo et al. [1986c], and Kahane et al. [1988], [1992]). It is thus straightforward to derive the ²⁹Si/³⁰Si abundance ratio. All the abundance ratios derived from the integrated intensities of Tables 4 and 5 are plotted in Fig. 3a. Except for two abundance ratios which have large error bars, due to poor signal-to-noise ratio, the measurements show a small dispersion. This strenghthens the idea that the molecular lines intensity ratios faithfully reflect the silicon isotopic ratios. From 5 lines of SiCC, 2 lines of SiS and 1 line of SiO, we obtain an average ²⁹Si/³⁰Si abundance ratio of 1.45 $\pm$ 0.13, in good agreement with the solar system ratio of 1.52.

Derivation of the ²⁸Si/²⁹Si and ²⁸Si/³⁰Si isotopic ratios are more difficult since most of the ²⁸Si-bearing molecular lines are somewhat optically thick, leading to an underestimate of the true isotopic ratio. A way to avoid this effect is to use doubly-substituted intensity ratios, such as ²⁸Si³⁴S/²⁹Si³⁴S. However, the rarer isotopic lines become so weak that the ratio is affected by poor signal-to-noise ratios and the risk of confusion with weak unrelated lines increases significantly. All the ²⁸Si/²⁹Si and ²⁸Si/³⁰Si abundances ratios derived from the integrated intensities of Table 4 are plotted in Figs. 3b and 3c. The average ratios are ²⁸Si/²⁹Si = 15.4 $\pm$ 1.1 and ²⁸Si/³⁰Si = 20.3 $\pm$ 2.0, i.e. respectively 1.3 and 1.5 times smaller than the solar system value. Such discrepancies are easily explained by opacity of the main isotopic lines, computed to be between 0.5 to 0.9. The analysis of the sulfur isotopomers of SiS confirm that the ²⁸SiS rotational lines, like the C³²S ones (see below), are indeed optically thick.

We thus conclude that all the silicon isotopic ratios in the circumstellar envelope of IRC+10216 are very probably close to the solar system values.

4.2.4 Sulfur

A similar study of the sulfur isotopic ratios can be made with our data. From one line of CS and two lines of SiS, we derive for the rare sulfur isotopes an isotopic ratio ³³S/³⁴S = 0.18 $\pm$ 0.01, identical to the solar system value (see Fig. 4a).

The ³²S/³⁴S isotopic ratio can be determined from the optically thin lines of the rare isotopomers ¹³CS, ²⁹SiS and ³⁰SiS (Fig. 4b). They provide an average value of 21.8 $\pm$ 2.6. This value appears significantly larger than the average value of 11.7 $\pm$ 1.7 derived from the optically thick lines of CS and SiS (see Fig. 4c) and quite consistent with the solar system value of 22.5. If, as is very likely, the discrepancy between both averages is due to optical depth effects on the strong CS and SiS lines, a mean opacity of about 1.4 is implied.

The ³²S/³³S isotopic ratio cannot be derived from doubly-substituted lines which are below our detection level. Using the single substituted species ²⁸Si³²S and ²⁸Si³³S, we find an average value ³²S/³³S $\simeq$ 62 $\pm$ 8 (see Fig. 3d), a factor of 2 smaller than the solar value of 125. This is just what is expected if the lines of the main isotope have opacities of the order of 1.6 (in good agreement with the estimate derived from the ³²S/³⁴S ratio).

As for the silicon, we conclude that all the sulfur isotopic ratios in the circumstellar envelope of IRC+10216 are compatible with the solar system values.

4.2.5 Carbon

We have detected 14 different ¹³C-bearing molecular species in our 2-mm survey, including the four ¹³C isotopomers of C₄H and the three isotopomers of C₃N. The averaged ¹²C/¹³C isotopic ratio is 45 $\pm$ 12 for C₄H and 35 $\pm$ 15 for C₃N. These ratios are mainly affected by the low line intensities of the ¹³C species. As will be discussed in detail in a forthcoming paper (Kahane et al. 1999, in preparation) the linear carbon chains HC₃N and to a lesser extent C₄H and C₃N could show, in addition to a moderate opacity of the main isotopomer lines, isotopic fractionation so that they cannot be easily used to derive the elemental ¹²C/¹³C isotopic ratio. We will thus focus here on the SiCC and CS lines.

The six transitions observed both for SiCC and for its ¹³C-bearing isotope in our 2-mm survey yield an average isotopic ratio of 37 $\pm$ 5 (see Fig. 4a), significantly smaller than the solar value of 89, but somewhat smaller than the value of 47 derived by Kahane et al. ([1988]) from optically thin lines. If attributed to the opacity of the main lines, this discrepancy leads to an optical depth of 0.5, in very good agreement with the estimates derived from the silicon isotopomer analysis.

The silicon isotopic ratios being very probably close to solar, we may also use the optically thin doubly-substituted species to derive the ¹²C/¹³C ratio. In Fig. 4b, we have plotted the five intensity ratios derived from Si¹³CC and ³⁰SiCC. They correspond to an average value of 0.60 $\pm$ 0.06 for the double isotopic ratio (¹³C/¹²C)(²⁸Si/²⁰Si). Assuming a solar (²⁸Si/³⁰Si) ratio, it leads to a ¹²C/¹³C isotopic ratio of 50 $\pm$ 7, in good agreement with our previous estimate. Similarly, the observations of ²⁹SiCC allow us to derive an average (¹²C/¹³C)(²⁹Si/²⁸Si) ratio of 2.1 $\pm$ 0.2. With a solar (²⁸Si/²⁹Si) ratio, it corresponds to a ¹²C/¹³C isotopic ratio of 41 $\pm$ 4, also in agreement with the previous values.

With the four rare isotopes of CS (see Table 9) we may also derive ¹²C/¹³C ratios free of opacity effects. These ratios are obtained directly from ¹²C³⁴S/¹³C³⁴S, and using solar values for the sulfur isotopic ratios for the other pairs of lines. They have a small dispersion (see Fig. 5) and one calculates an average value of 46 $\pm$ 6, also very close to the previous estimates.

In conclusion, when averaging the three ¹²C/¹³C ratios derived from optically thin lines of SiCC and CS, we obtain a value of 45 $\pm$ 3, in very good agreement with the value given in Kahane et al. ([1988]) and Cernicharo et al. ([1991a]).

A sensitive search for ¹⁴CO (Forestini et al. [1997]), carried out at 1.3 and 2.6 mm, yielded only low upper limits to the abundance of this species and to the ¹⁴C:¹²C isotopic abundance ratio ( $< 2 \thinspace10^{-5}$), despite an earlier claim by Wright ([1994]) for its detection.

4.3 Unidentified lines

The list of U-lines is given in Table 3. Some of these lines could be arising from vibrationally excited states of well known molecules, just as we see many lines pertaining to the $\nu_7=1$ state of HC₃N (see Table 8), the $\nu_7=1$, 2 states of C₄H (see Table 7), the v=1, 2, 3 states of SiS (see Table 4), the v=1 of CS (see Table 9), and the $\nu_3=1$ state of SiC₂(see Table 5). Potential candidates could be the low-lying bending states of C₅H, C₆H, MgNC and NaCN whose spectroscopic constants are poorly known. However, taking into account the intensity of the ground state lines of these species, the vibrationally excited states could only account for the weak U lines.

The stronger U-lines must arise from new, not yet identified, molecular species. No harmonic relation exists between any subset of them. They could arise either from asymmetric top molecular species, or from species with non-zero electronic angular momentum. Surprisingly, only few asymmetric top molecules have been so far identified in IRC+10216. This probably results from the small number of asymmetric top radicals studied in the laboratory and from the difficulty to identify ab initio such species in IRC+10216's spectrum. We are currently searching for characteristic spectroscopic patterns at 3 mm and 1 mm in order to allow the identification of the carriers.

4.4 Other molecular species

Many other molecules and radicals of astrophysical interest, such as MgH, MgO, CCN, CCO, KCN, H₂CS, CH₃OH, H₂CO, HNCO, Si₂H₂, H₂CSi, H₂C₂Si, have strong lines within the frequency range covered by our survey, but have not been detected. It is difficult, however, to set significant upper limits to their abundances, as long as their spatial distributions and rotation temperatures remain unknown.

4.5 Vibrationally excited species

Many lines of the mm-wave spectrum of IRC+10216 correspond to rotational transitions inside vibrationally excited states. As we have seen (see also Guélin et al. [1987b]; Yamamoto et al. [1987]), the $1\nu_7(^2\Pi)$, $2\nu_7(^2\Sigma$), and $2\nu_7(^2\Delta$) bending states of the ground $^2\Sigma^+$ electronic state of C₄H give rise to an impressive number of lines in the 2-mm and 3-mm bands. Other such lines arise from C₃H (Guélin et al. [1998]), HCN, HC₃N, CS (Turner [1987a]), SiS (Guélin et al. [1987b]; Turner [1987b]), SiC₂(Forestini et al. [1997]) and MgNC (Guélin et al. [1997]). It is worth noting that despite sensitive searches, no vibrationally excited lines have been detected so far for C₂H, C₃N (Murakami et al. [1989]), SiC (Mollaaghababa et al. [1990]). There is a weak feature (0.03 K) at the frequency of the v=1 J= 2-1 line of CO; unfortunately the J= 1-0 line is blended with C₄H. Observations of higher J lines are needed to confirm the detection of vibrationally excited carbon monoxide in IRC+10216.