Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

---

Measurement of the C/C Ratio in Planetary Nebulae

J. R. Walsh
Space Telescope European Coordinating Facility, European Southern Observatory, Germany

R. E. S. Clegg
Particle Physics and Astronomy Research Council, Swindon, UK

P. J. Storey
Department of Physics and Astronomy, University College London, UK

L. Neale
Department of Physics and Astronomy, University College London, UK

Abstract:

The C/C isotope ratio from the ionized gas has been measured in three planetary nebulae with the GHRS using a new method based on transitions caused by hyperfine effects. The C III] multiplet near 1908Å has a transition at 1909.6Å completely forbidden in C, but allowed as a result of the non-zero nuclear spin of C. The C III] line was observed with the GHRS and Echelle-B and a positive detection of the C line was made in the Galactic C-rich PN NGC 3918; a marginal detection of the C line was made in SMC N2, which is also C rich. In NGC 3918 the derived C/C ratio is 15 3 and 20 in SMC N2. A tentative detection of the C line was also made in the dense Type 1 PN LMC N122. For the two C-rich objects, the C isotope ratio is lower than the typical range found in carbon stars, suggesting that C is mixed to the surface during the late stages of the AGB.

Measuring Isotopic Abundances in PN

Abundances of the light elements and isotopes of C, O, N and Mg give evidence for nuclear processing on the main sequence and the asymptotic giant branch (AGB) of low-intermediate mass stars (M6M). Stellar evolution models predict changes in abundances in three episodes:
1st dredge-up: during the ascent of the giant branch CN material is brought to the surface;
2nd dredge-up: for higher mass stars ((M3M) CNO-cycled material is brought to the surface;
3rd dredge-up: on the Asymptotic Giant Branch (AGB) He-burning products + s-process elements are mixed to the surface.

The velocity field of planetary nebulae (PN) can easily obscure nuclear mass (isotopic) and nuclear moment (hyperfine) shifts making observation difficult or impossible in the optical-IR. However, at mm wavelengths the shifts of hyperfine transitions are large enough to separate lines, but the line emission of the more abundant isotopic species is often optically thick. However, for P - S transitions of Be-like and Mg-like ions, a finite nuclear spin allows an electric dipole transition for the J=0-0 transition, which is completely forbidden in the absence of nuclear spin.

The well-known C P - S J=1-0 and J=2-0 transitions at 1906.7 and 1908.7Å, respectively, also have a J=0-0 transition at a wavelength of 1909.6Å, which is strictly forbidden for C. However, the finite nuclear spin of the C atom (I=1/2) allows an F= electric dipole transition. This transition is well separated from the P - S line, thus, facilitating detection of C. The transition probability of the C transition has been calculated. In order to use the line strength of the 1909.6Å line relative to the 1908.7Å line to deduce the C/C ratio, the electron collision strengths of the S - P and P - P and the electron density are required. The electron density can be calculated from the ratio of the P - S J=1-0 and J=2-0 lines (Keenan et al. 1992). The closeness in wavelength of the C and C lines make any correction for extinction negligible. A full description of the method, computation of the transition probabilities and presentation of observational results will appear in the near future (Clegg et al. 1996).

GHRS Observations

The GHRS with Echelle-B and detector 2 (resolution 0.018Å per diode) was employed to study the C III] transitions in three PN. The large science aperture was employed to ensure sufficient flux in the weak C line. One high surface brightness Galactic PN was selected (NGC 3918) which is C rich and two PN in the Magellanic Clouds to investigate any effects of lowered metallicity on the C/C ratio. One of the Magellanic Cloud PN (LMC N122) was a type 1 PN with high He and N abundances, considered to arise from a high mass progenitor and affected by the 2nd dredge-up.

 
Figure: The observed line profiles of the C III] 1906.7 and 1908.7Å lines from NGC 3918 are shown together with a two-component Gaussian fit to each line. The positions of the split components of the C 1909.6Å line are arrowed. The zero point of the velocity scale corresponds to the rest velocity of the P - S transition. The residuals on the fit (crosses) and the statistical errors (bars) on the data points are shown above the spectrum plot.

Results

A positive detection of the C 1909.6Å line was made in NGC 3918, a marginal detection in SMC N2 and a possible detection in LMC N122. Figure 1 shows a multi-component Gaussian fit to the P - S and P - S line profiles for NGC 3918. Each of the lines is split into two components by the expansion velocity of the nebula (21 kms). The positions of the detected split components of the C line are arrowed. The profiles are clearly more complex than a two-Gaussian representation and there is weak emission on the positive and negative sides of both strong lines. Figure 2 shows the spectrum in the region of the C line. The histogram is the data and the line the fit; the quality of the detection of the C F= line is clear. The derived C/C ratio is 153. For SMC N2 the C line is much weaker and a 2 detection was made: the C/C ratio was measured as 21.

 
Figure: An expanded section of Figure 1 showing the split C III] 1909.6Å lines and the fit by two Gaussians whose separation is the same as that of the split C components. The quality of the detection of this weak line is evident.

Conclusions

A new method has been proven to measure C/C in the ionized gas of nebulae. This method is independent of the mm wave measurements of carbon isotope transitions and does not require any modelling of the radiative transfer of the line emission. It is limited to nebulae with high surface brightness C III] emission (high excitation and low extinction) and moderate electron density. The C/C ratio measured is much lower than the solar value (C/C = 89) and ISM value (C/C 70) giving direct evidence of the CNO processing that has occurred in the PN central star subsequent to ejection of the nebula. The values are lower than those measured for carbon stars, the likely progenitors of C-rich PN. C-rich AGB stars have C/C 30--70 (e.g., Busso et al. 1995). It is suggested that further mixing of C to the surface occurs in the late stages of AGB evolution.

References:

Busso, M., Lambert, D. L., Beglio, L., Gallino, R., Raiteri, C. M., & Smith, V. V. 1995, ApJ, 446, 775

Clegg, R. E. S., Storey, P. J., Walsh, J. R., & Neale, L., 1996, in prep

Keenan, F. P., Feibelmann, W. A., & Berrington, K. A. 1992, ApJ, 389, 443