Infrared Emission by Dust Around lambda Bootis Stars: Debris Disks or Thermally Emitting Nebulae?

TL;DR

This paper models infrared excesses around lambda Bootis stars as due to either debris disks or heated interstellar dust, suggesting most nearby excess stars likely have debris disks, while more distant ones may be ISM heated dust.

Contribution

It introduces a model explaining infrared excesses as heating of interstellar dust, providing an alternative to debris disk explanations for lambda Bootis stars.

Findings

Infrared excesses can be explained by ISM dust heating or debris disks.

Most nearby lambda Bootis stars with IR excess likely have debris disks.

Distant stars' IR excesses may result from interstellar dust heating.

Abstract

We present a model that describes stellar infrared excesses due to heating of the interstellar (IS) dust by a hot star passing through a diffuse IS cloud. This model is applied to six lambda Bootis stars with infrared excesses. Plausible values for the IS medium (ISM) density and relative velocity between the cloud and the star yield fits to the excess emission. This result is consistent with the diffusion/accretion hypothesis that lambda Bootis stars (A- to F-type stars with large underabundances of Fe-peak elements) owe their characteristics to interactions with the ISM. This proposal invokes radiation pressure from the star to repel the IS dust and excavate a paraboloidal dust cavity in the IS cloud, while the metal-poor gas is accreted onto the stellar photosphere. However, the measurements of the infrared excesses can also be fit by planetary debris disk models. A more detailed consideration of the conditions to produce lambda Bootis characteristics indicates that the majority of infrared-excess stars within the Local Bubble probably have debris disks. Nevertheless, more distant stars may often have excesses due to heating of interstellar material such as in our model.