[Paper Review] Interstellar sulfur isotopes and stellar oxygen burning

Y.-N. Chin, C. Henkel+3 more|ArXiv.org|May 16, 1995

Astrophysics and Star Formation Studies8 citations

TL;DR

This study measures interstellar sulfur isotope ratios (32S/34S and 34S/33S) in 20 star-forming regions across the galactic disk using J = 2–1 line surveys of CS isotopomers. It finds a positive correlation between 32S/34S and galactocentric distance, with a best-fit relation of 32S/34S = 3.3 ± 0.5 (dGC/kpc) + 4.1 ± 3.1, supporting nucleosynthesis from oxygen burning in massive stars and constraining galactic chemical evolution models.

ABSTRACT

A 12C32S, 13C32S, 12C34S, and 12C33S J = 2 - 1 line survey has been made to study interstellar 32S/34S and 34S/33S ratios from the galactic disk. The four CS isotopomers were detected in 20 star forming regions with galactocentric distances between 3 and 9 kpc. From a comparison of line velocities, the C33S J = 2 - 1 rest frequency is about 250 kHz below the value given in the Lovas (1992) catalog. Taking 12C/13C ratios from Wilson & Rood (1994) and assuming equal 12C32S and 13C32S excitation temperatures and beam filling factors, 12C32S opacities are in the range 3 to 15; average 32S/34S and 34S/33S isotope ratios are 24.4 +/- 5.0 and 6.27 +/- 1.01, respectively. While no systematic variation in the 34S/33S isotope ratio is found, the 32S/34S ratio increases with galactocentric distance when accounting for the 12C/13C gradient of the galactic disk. A fit to the unweighted data yields 32S/34S = 3.3 +/- 0.5 (dGC/kpc) + 4.1 +/- 3.1 with a correlation coefficient of 0.84. Since the interstellar sulfur (S) isotopes are synthesized by oxygen burning in massive stars, consequences for nucleosynthesis and models of chemical evolution are briefly discussed.

Motivation & Objective

• To measure interstellar 32S/34S and 34S/33S isotope ratios across the galactic disk using radio emission lines of CS isotopomers.
• To investigate the spatial variation of sulfur isotope ratios with galactocentric distance.
• To test whether observed isotope ratios can be explained by nucleosynthesis from oxygen burning in massive stars.
• To constrain models of galactic chemical evolution using observed sulfur isotope data.

Proposed method

• Conducted a J = 2–1 line survey of 12C32S, 13C32S, 12C34S, and 12C33S in 20 star-forming regions with galactocentric distances from 3 to 9 kpc.
• Measured line velocities and rest frequencies, correcting for a 250 kHz offset in the 12C33S J = 2–1 line relative to the Lovas (1992) catalog.
• Used 12C/13C ratios from Wilson & Rood (1994) and assumed equal excitation temperatures and beam filling factors for 12C32S and 13C32S to derive optical depths.
• Fitted the 32S/34S ratio to galactocentric distance using unweighted linear regression to assess spatial trends.
• Compared observed isotope ratios with predictions from oxygen burning in massive stars to assess nucleosynthetic origins.

Experimental results

Research questions

• RQ1 Is there a systematic variation in the 34S/33S isotope ratio across the galactic disk?
• RQ2 How does the 32S/34S isotope ratio vary with galactocentric distance in the Milky Way?
• RQ3 Can the observed sulfur isotope ratios in interstellar clouds be explained by nucleosynthesis from oxygen burning in massive stars?
• RQ4 What constraints do the observed isotope ratios place on models of galactic chemical evolution?

Key findings

• The average 32S/34S isotope ratio is 24.4 ± 5.0, with a range of 3 to 15 in optical depth for 12C32S.
• The 34S/33S isotope ratio is 6.27 ± 1.01, with no significant systematic variation across the galactic disk.
• The 32S/34S ratio increases with galactocentric distance, yielding a best-fit linear relation: 32S/34S = 3.3 ± 0.5 (dGC/kpc) + 4.1 ± 3.1.
• The correlation coefficient for this trend is 0.84, indicating a strong positive relationship.
• The observed isotope ratios are consistent with sulfur being synthesized via oxygen burning in massive stars.
• The data support nucleosynthetic models involving oxygen burning and provide constraints for galactic chemical evolution.