Direct Detection of Planets Orbiting Large Angular Diameter Stars: Sensitivity of an Internally-Occulting Space-Based Coronagraph

TL;DR

This paper evaluates the sensitivity of an internally-occulting space-based coronagraph for detecting planets around large angular diameter stars, highlighting its potential for studying evolved stars and nearby systems like Alpha Centauri.

Contribution

It quantifies how stellar size affects coronagraph contrast performance and explores mask designs to optimize observations of diverse stellar targets.

Findings

Coronagraph contrast degrades with increasing stellar angular diameter.

Band-limited image masks can mitigate contrast loss for large stars.

Applications include studying planets around evolved stars and nearby systems like Alpha Centauri.

Abstract

High-contrast imaging observations of large angular diameter stars enable complementary science questions to be addressed compared to the baseline goals of proposed missions like the Terrestrial Planet Finder-Coronagraph, New World's Observer, and others. Such targets however present a practical problem in that finite stellar size results in unwanted starlight reaching the detector, which degrades contrast. In this paper, we quantify the sensitivity, in terms of contrast, of an internally-occulting, space-based coronagraph as a function of stellar angular diameter, from unresolved dwarfs to the largest evolved stars. Our calculations show that an assortment of band-limited image masks can accommodate a diverse set of observations to help maximize mission scientific return. We discuss two applications based on the results: the spectro-photometric study of planets already discovered with the radial velocity technique to orbit evolved stars, which we elucidate with the example of Pollux b, and the direct detection of planets orbiting our closest neighbor, Alpha Centauri, whose primary component is on the main-sequence but subtends an appreciable angle on the sky. It is recommended that similar trade studies be performed with other promising internal, external, and hybrid occulter designs for comparison, as there is relevance to a host of interesting topics in planetary science and related fields.