IR-excesses around nearby Lambda Boo stars are caused by debris disks rather than ISM bow waves

Abstract

Lambda Boo stars are predominately A-type stars with solar abundant C, N, O, and S, but up to 2 dex underabundances of refractory elements. The stars' unusual surface abundances could be due to a selective accretion of volatile gas over dust. It has been proposed that there is a correlation between the Lambda Boo phenomenon and IR-excesses which are the result of a debris disk or interstellar medium (ISM) interaction providing the accreting material. We observe 70 or 100 and 160 $\mu$m excess emission around 9 confirmed Lambda Boo stars with the Herschel Space Observatory, to differentiate whether the dust emission is from a debris disk or an ISM bow wave. We find that 3/9 stars observed host well resolved debris disks. While the remaining 6/9 are not resolved, they are inconsistent with an ISM bow wave based on the dust emission being more compact for its temperature and predicted bow wave models produce hotter emission than what is observed. We find the incidence of bright IR-excesses around Lambda Boo stars is higher than normal A-stars. To explain this given our observations, we explore Poynting-Robertson (PR) drag as a mechanism of accretion from a debris disk but find it insufficient. As an alternative, we propose the correlation is due to higher dynamical activity in the disks currently underway. Large impacts of planetesimals or a higher influx of comets could provide enough volatile gas for accretion. Further study on the transport of circumstellar material in relation to the abundance anomalies are required to explain the phenomenon through external accretion.

19 pages, 8 figures, 8 tables, Accepted to MNRAS

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Cited by in corpus (12)

• Debris Disks: Structure, Composition, and Variability
• Gaia's view of the $\lambda$ Boo star puzzle
• Exocomets: A spectroscopic survey
• On the Chemical Abundance of HR 8799 and the Planet c
• Stellar Photospheric Abundances as a Probe of Disks and Planets
• The Faintest WISE Debris Disks: Enhanced Methods for Detection and Verification
• Characterization of debris disks observed with SPHERE
• A catalog of collected debris disks: properties, classifications and correlations between disks and stars/planets
• Abundance analysis of stars hosting gas-rich debris disks
• Discovery of lambda Boo stars in open clusters
• Discovery of the $\lambda$ Bootis phenomenon in an early B-type star
• The search for gas in debris discs: ALMA detection of CO gas in HD 36546