Confident detection of doubly ionized thorium in the extreme Ap star CPD-62° 2717


Despite the Universe containing primordial thorium (Th) of sufficient abundance to appear in stellar spectra, detection of Th has to date been tentative and based on just a few weak and blended lines. Here, we present convincing evidence not only for the first Th detection in a magnetic chemically peculiar Ap star but also for the first detection of Th iii in a stellar spectrum. CPD-62° 2717 was initially recognized as a highly magnetized Ap star due to resolved magnetically split lines captured in H-band spectra from the SDSS/APOGEE survey. The star was subsequently pinpointed as extraordinarily peculiar when careful inspection of the H-band line content revealed the presence of five lines of Th iii, none of which are detected in the other ∼1500 APOGEE-observed Ap stars. Follow-up with the VLT + UVES confirmed a similarly peculiar optical spectrum featuring dozens of Th iii lines, among other peculiarities. Unlike past claims of Th detection, and owing to high-resolution observations of the strong (∼8–12 kG) magnetic field of CPD-62° 2717, the detection of Th iii can in this case be supported by matches between the observed and theoretical magnetic splitting patterns. Comparison of CPD-62° 2717 to stars for which Th overabundances have been previously reported (e.g. Przybylski’s Star) indicates that only for CPD-62° 2717 is the Th detection certain. Along with the focus on Th iii, we use time series measurements of the magnetic field modulus to constrain the rotation period of CPD-62° 2717 to ∼4.8 yr, thus establishing it as a new example of a superslowly rotating Ap star.


This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society 2023 Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.


stars, chemically peculiar stars, magnetic fields, CPD-62◦ 2717


S Drew Chojnowski, Swetlana Hubrig, David L Nidever, Ewa Niemczura, Jonathan Labadie-Bartz, Gautier Mathys, Sten Hasselquist, Confident detection of doubly ionized thorium in the extreme Ap star CPD-62° 2717, Monthly Notices of the Royal Astronomical Society, Volume 522, Issue 4, July 2023, Pages 5931–5945,
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