Laboratory of Stellar Nuleosynthesis:
Isotope Ratios in the Magellanic Cloud
Laboratory of Stellar Nuleosynthesis:
Isotope Ratios in the Magellanic Cloud
-
Y.-N. Chin
-
Institute of Astronomy & Astrophysics,
Academia Sinica
- P.O. Box 1-87 Nankang, 11529 Taipei, Taiwan
Paper appears in the proceeding of IAU Symposium 190 ``New View of
the Magellanic Clouds'' held in Victoria, Canada 12 - 17 July 1998,
edited You-hua Chu, Nicholas B. Suntzeff, James E. Hesser,
and David A. Bohlender, pp. 279 - 280.
You can click here (PostScript file of 38070
byes), or here (gzip-compressed PostScript
file of 16678 byes) for the preprint.
Abstract.
In radio astronomy, interstellar isotope ratios have been measured
for more than two decades towards different parts of the Milky Way and
central regions of some ``star-burst galaxies.
While signals are often too weak to detect rare isotopic species
in relatively distant extragalactic sources,
our Galaxy only provides an environment with limited metallicity range.
Obviously, this constraint can be removed by observing isotopic species
in the Magellanic Clouds, located only 50 -- 60 kpc away from us.
We thus observed isotope ratios of hydrogen, carbon, nitrogen, oxygen,
and sulfur and the results are listed in Table 1.
Introduction
It is clear that most of the elements (except of a few which were formed
soon after the Big Bang) are synthesized in the interior of stars.
As the source of stellar energy, nuclear reactions lead to the
formation of a great variety of elements and isotopes.
Although there are many reactions and therefore many parameters
involved when trying to model stellar interiors,
the synthesis of isotopes can be simplified by introducing a
concept discriminating between ``primary'' and ``secondary'' nuclei.
While the ``primary'' species are synthesized and released by massive stars
or by an initial stellar generation, the synthesis of ``secondary'' nuclei
either takes place in low mass stars or in stars
that are formed at later times and that possess certain ``seed'' nuclei.
As a consequence, the primary species are released earlier
into the interstellar medium than the secondary species.
Even this approach may be complex,
since some nuclei are synthesized in more than one reaction scheme,
possible involving both ``primary'' and ``secondary'' formation.
Nevertheless, we can observe isotope abundance ratios
as a function of metallicity.
This allows us to determine, how ``primary'' (or ``secondary'')
a given species is with respect to another, thus providing important
constraints on stellar nucleosynthesis and ``chemical'' evolution.
Observations
The observations were carried out during several periods
between 1997 and 1998 using the 15-m Swedish-ESO Submillimetre Telescope
(SEST) at La Silla, Chile.
The 12C/13C ratio has been derived from the
line intensity ratio of HCN to H13CN.
The opacity of the HCN(1--0) line has been determined by a comparison
of intensities of its hyperfine components.
Similarly, the 14N/15N ratio has been obtained
by additional observations of HC15N.
The D/H ratio has been obtained from the line intensity ratio of
H13CO+ and DCO+, as well as
H13CN and DCN, by applying the 12C/13C
ratio derived before.
Fractionation is estimated applying a detailed astrochemical model.
The oxygen 18O/17O isotope ratio has derived from
the intensities of the respective isotopic species of the CO molecule
and applying the 12C/13C ratio from HCN observation.
Similar to the oxygen isotope ratio, the sulfur isotope ratios
have been calculated by observing CS isotopic species and
applying the 12C/13C ratio.
Discussion
In Table 1 we present a brief summary of isotope abundance ratios obtained
in the Magellanic Clouds, our Galaxy, and nearby star-burst nuclei.
While the [H/D] ratio is related to cosmology,
other ratios are suitable tracers of stellar nucleosynthesis.
The nitrogen and oxygen ratios determined in the LMC
(which represents a less evolved environment) suggest some corrections
to the stellar nucleosynthesis model.
For instance, the very low 14N/15N ratio indicates that
15N has to be synthesized by massive stars.
This is consistent with the recent massive star models
which take the effects of rotation on the stellar structure
and nucleosynthesis into account (Langer et al. 1998).
Table 1: Isotope abundance ratios in various astronomical
environment§
| Isotope | Magellanic
Clouds | Galactic
Center
| Inner
Disk | Local
ISM
| Solar
System ¤ | Starburst
Nuclei
|
|---|
| 12C/13C
| 62 ± 5 | 25 ± 5 | 40 ± 10
| 70 ± 10 | 90 | ~ 50
|
| 14N/15N
| 114 ± 14 | 900 ± 200 | 375 ± 60
| 450 ± 60 | 272 | >> 100
|
| 16O/18O
| 2000 | 250 ± 30 | 300 ± 30
| 540 ± 30 | 499 | 150 -- 200
|
| 18O/17O
| 1.8 ± 0.4 | 3.5 ± 0.2 | 3.6 ± 0.3
| 3.6 ± 0.3 | 5.3 | > 8
|
| 32S/34S
| 18 ± 6 | -- | 24 ± 5
| 24 ± 5 | 22.6 | --
|
| 34S/33S
| 3.0 ± 0.7 | -- | 6 ± 1
| 6 ± 1 | 5.6 | --
|
- § While isotope abundance ratios of hydrogen, carbon, and oxygen
are summarized by Wilson & Rood (1994), those of sulfur
are measured by Chin et al. (1996).
- ¤ The values of the Solar System represent the isotope
abundance ratios in the LISM 4.5 × 109 years ago.
References
- Chin, Y.-N., Henkel, C., Whiteoak, J.B., Langer, N.,
Churchwell, E.B. 1996, A&A, 305, 960
- Langer, N., Heger, A., García-Segura, G. 1998,
Rev. Modern Astron., Vol. 11, in press
- Wilson, T.L., Rood, T.R. 1994, ARAA, 32, 191
Any suggestion or comments please
e-mail to einmann@asiaa.sinica.edu.tw.
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