Searching for Dust around Hyper Metal-Poor Stars

TL;DR

This study investigates mid-infrared fluxes of ultra-metal-poor stars to determine the presence of circumstellar dust, finding most stars lack detectable disks, but one star shows a possible dust signature.

Contribution

First mid-infrared survey of hyper metal-poor stars to constrain circumstellar dust presence and its relation to low metallicity.

Findings

Most stars show no mid-IR excess, ruling out warm debris disks.

One star, HE0107-5240, shows a tentative 10-micron excess suggesting possible dust or binarity.

Abstract

We examine the mid-infrared fluxes and spectral energy distributions for metal-poor stars with iron abundances [Fe/H] $\lesssim-5$, as well as two CEMP-no stars, to eliminate the possibility that their low metallicities are related to the depletion of elements onto dust grains in the formation of a debris disk. Six out of seven stars examined here show no mid-IR excess. These non-detections rule out many types of circumstellar disks, e.g. a warm debris disk ($T\!\le\!290$ K), or debris disks with inner radii $\le1$ AU, such as those associated with the chemically peculiar post-AGB spectroscopic binaries and RV Tau variables. However, we cannot rule out cooler debris disks, nor those with lower flux ratios to their host stars due to, e.g. a smaller disk mass, a larger inner disk radius, an absence of small grains, or even a multicomponent structure, as often found with the chemically peculiar Lambda Bootis stars. The only exception is HE0107-5240, for which a small mid-IR excess near 10 microns is detected at the 2-$\sigma$ level; if the excess is real and associated with this star, it may indicate the presence of (recent) dust-gas winnowing or a binary system.