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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
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 (M
6M
).
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).
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.
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 15
3. 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.
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.
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