Boyer et al. 2025, AAS journals, ... dust around ... stars in ... Sextans A

Discovery of SiC and iron dust around AGB stars in the very metal-poor Sextans A dwarf galaxy with JWST: Implications for dust production at high redshift

M.L. Boyer (STScI), G.C. Sloan (STScI, UNC), A. Nanni (Nat. Cen. for Nuclear Research in Poland, INAF-Teramo), E. Tarantino (STScI), I. McDonald (Univ. Manchester), S. Goldman (STScI), J.A.D.L. Blommaert (Vriji Univ.) F. Dell'Agli (INAF-Roma), M. Di Criscienzo (INAF-Roma), D.A. Garcia-Hernandez (Inst. Astrof. Canarias, Univ. Laguna) R.D. Gehrz (Univ. of Minnesota), M.A.T. Groenewegen (Royal Obs. Belgium), A. Javadi (IPM Tehran), O.C. Jones (UKATC Edinburgh) F. Kemper (ICE Barcelona, IEEC Barcelona), M. Marengo (Florida State), K.B.W. McQuinn (STScI, Rutgers), J.M. Oliveira (Keele Univ.), R. Sahai (JPL), E.D. Skillman (Univ. Minnesota), S. Srinavasan (UNAM), J.Th. van Loon (Keele Univ), D.R. Weisz (Univ. California Berkeley), P.A. Whitelock (SAAO)

2025, AAS journals, submitted

Low-resolution infrared spectroscopy from JWST confirms the presence of SiC and likely metallic iron dust around asymptotic giant branch (AGB) stars in the Sextans A dwarf galaxy, which has a metallicity ~1-7% Z_⊙. While metal-poor carbon-rich AGB stars are known to produce copious amounts of amorphous carbon dust owing to the dredge up of newly synthesized carbon to their atmospheres, this is the first time that Si- and Fe-bearing dust has been detected at this extreme metallicity. Of the six AGB stars observed, one is an intermediate-mass (~1.2-4 M_⊙) carbon star showing SiC dust, and another is an oxygen-rich M-type star with mass ~4-5 M_⊙ that is likely undergoing hot bottom burning. The infrared excess of the M-type star is strong, but featureless. We tested multiple possible dust species, and find that it is best fit with metallic iron dust. Assuming its dust-production rate stays constant over the final 2-3×10⁴ yr of its evolution, this star will produce ~0.9-3.7 times the iron dust mass predicted by models, with the range depending on the adopted stellar mass. The implications for dust-production in high-redshift galaxies are potentially significant, especially regarding the assumed dust species used in cosmic dust evolution models and the timescale of AGB dust formation. Stars on the upper end of the AGB mass range can begin producing dust as early as 30-50 Myr after they form, and they may therefore rival dust production by supernovae at high redshift.

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