Stellar physics at sub-nanoradian angular resolution

TL;DR

This paper discusses the potential of extremely high angular resolution optical interferometry, reaching sub-nanoradian levels, to observe detailed stellar features such as starspots, transients, and surface variations on various types of stars.

Contribution

It explores the scientific possibilities enabled by future km-scale optical interferometers with unprecedented angular resolution, surpassing current capabilities.

Findings

Current interferometers achieve nanoradian resolution, resolving features on large stars.

Future km-scale arrays could resolve white dwarf surfaces in detail.

High-resolution imaging could reveal stellar surface phenomena and transient events.

Abstract

Many stars -- if they could be imaged with enough angular resolution -- would exhibit features expected from theory but not possible to extract from spectra. We may group these by increasing complexity as follows. First, smooth variations in brightness across the surface, resembling solar limb darkening but much more prominent and involving more processes in stars with fast spin or external tides. Next, there are periodic features: not only oscillations, but also convective cells and starspots, which appear to transit across a star as its spins, and exoplanets that really do transit across the star. Then, there are transients like flares. Current optical interferometers provide synthetic apertures of a few hundred metres and angular resolutions down to about nanoradian ($\simeq 0.2\,$milliarcsecond), enough to resolve some of the above features on the nearest upper main-sequence stars, giants and supergiants. Ongoing projects aims to km-scale synthetic apertures, enough to measure the radius of the nearest white dwarf. In this White Paper we briefly discuss what could be observed with synthetic apertures over $\sim20\,$km -- resolving detail on white dwarfs at the level currently possible on supergiants.