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Asteroseismology probes the internal structures of stars by using their natural pulsation frequencies(1). It relies on identifying sequences of pulsation modes that can be compared with theoretical models, which has been done successfully for many classes of pulsators, including low-mass solar-type stars(2), red giants(3), high-mass stars(4) and white dwarfs(5). However, a large group of pulsating stars of intermediate mass-the so-called delta Scuti stars-have rich pulsation spectra for which systematic mode identification has not hitherto been possible(6,7). This arises because only a seemingly random subset of possible modes are excited and because rapid rotation tends to spoil regular patterns(8-10). Here we report the detection of remarkably regular sequences of high-frequency pulsation modes in 60 intermediate-mass main-sequence stars, which enables definitive mode identification. The space motions of some of these stars indicate that they are members of known associations of young stars, as confirmed by modelling of their pulsation spectra.

The pulsation spectra of intermediate-mass stars (so-called delta Scuti stars) have been challenging to analyse, but new observations of 60 such stars reveal remarkably regular sequences of high-frequency pulsation modes.

Following its flyby and first imaging of the Pluto-Charon binary, the New Horizons spacecraft visited the Kuiper belt object (KBO) 2014 MU69 (also known as (486958) Arrokoth). The imaging showed MU69 to be a contact binary that rotates at a low spin period (15.92 hours), is made of two individual lobes connected by a narrow neck and has a high obliquity (about 98 degrees)(1), properties that are similar to those of other KBO contact binaries inferred through photometric observations(2). However, all scenarios suggested so far for the origins of such configurations(3-5) have failed to reproduce these properties and their probable frequent occurrence in the Kuiper belt. Here we show that semi-secular perturbations(6,7) operating on only ultrawide KBO binaries close to their stability limit can robustly lead to gentle, slow binary mergers at arbitrarily high obliquities but low rotational velocities, reproducing the characteristics of MU69 and other similar oblique contact binaries. Using N-body simulations, we find that approximately 15 per cent of all ultrawide binaries with a cosine-uniform inclination distribution(5,9) are likely to merge through this process. Moreover, we find that such mergers are sufficiently gentle to deform the shape of the KBO only slightly. The semi-secular contact binary formation channel not only explains the observed properties of MU69, but may also apply to other Kuiper belt or asteroid belt binaries and in the Solar System and extra-solar moon systems.

The high obliquity and low rotation period of the Kuiper belt object (2014) MU69 and other similar contact binaries is successfully reproduced from the collision and post-collision characteristics of initially wide binaries.