We propose to use HDS to obtain high-resolution (R∼165,000) spectra of 40 close-in (a=0.2–1 au) double-lined spectroscopic binaries consisting of two late-F/early-G dwarfs with nearly identical masses selected from Gaia DR3. The binaries have large enough radial-velocity differences that enable the separate measurements of the projected spin velocity vsini for each component via forward modeling of the absorption line broadening. We will use the difference of vsini in each pair to probe the spin misalignment and test the hypothesis that these close “twins" originate from competitive accretion from their surrounding circumbinary disks, which should result in good spin alignment. Together with our S21B program that targeted twins in wider orbits (a=50–5000 au), this program serves as a critical test on the origin scenario of twin binaries, and provides a new constraint on the spin misalignment in generic solar-mass binaries. Because the twin population with the mass ratio >0.95 — as targeted in this proposal — is known to account for O(10%) of close binaries with a<∼ 10 au, the result provides an important observational constraint on one major mode of close binary formation.
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