Spitzer's Deep Observations of Vega, Fomalhaut, and epsilon Eridani

High-contrast Imaging with Spitzer: Deep Observations of Vega, Fomalhaut, and epsilon Eridani

Authors:

Janson et al

Abstract:

Stars with debris disks are intriguing targets for direct imaging exoplanet searches, both due to previous detections of wide planets in debris disk systems, as well as commonly existing morphological features in the disks themselves that may be indicative of a planetary influence. Here we present observations of three of the most nearby young stars, that are also known to host massive debris disks: Vega, Fomalhaut, and eps Eri. The Spitzer Space Telescope is used at a range of orientation angles for each star, in order to supply a deep contrast through angular differential imaging combined with high-contrast algorithms. The observations provide the opportunity to probe substantially colder bound planets (120--330 K) than is possible with any other technique or instrument. For Vega, some apparently very red candidate point sources detected in the 4.5 micron image remain to be tested for common proper motion. The images are sensitive to ~2 Mjup companions at 150 AU in this system. The observations presented here represent the first search for planets around Vega using Spitzer. The upper 4.5 micron flux limit on Fomalhaut b could be further constrained relative to previous data. In the case of eps Eri, planets below both the effective temperature and the mass of Jupiter could be probed from 80 AU and outwards, although no such planets were found. The data sensitively probe the regions around the edges of the debris rings in the systems where planets can be expected to reside. These observations validate previous results showing that more than an order of magnitude improvement in performance in the contrast-limited regime can be acquired with respect to conventional methods by applying sophisticated high-contrast techniques to space-based telescopes, thanks to the high degree of PSF stability provided in this environment.

Vega's Dust Disk Created by Outwardly Migrating Exoplanets

Vega's hot dust from icy planetesimals scattered inward by an outward-migrating planetary system

Authors:

Raymond et al

Abstract:

Vega has been shown to host multiple dust populations, including both hot exo-zodiacal dust at sub-AU radii and a cold debris disk extending beyond 100 AU. We use dynamical simulations to show how Vega's hot dust can be created by long-range gravitational scattering of planetesimals from its cold outer regions. Planetesimals are scattered progressively inward by a system of 5-7 planets from 30-60 AU to very close-in. In successful simulations the outermost planets are typically Neptune-mass. The back-reaction of planetesimal scattering causes these planets to migrate outward and continually interact with fresh planetesimals, replenishing the source of scattered bodies. The most favorable cases for producing Vega's exo-zodi have negative radial mass gradients, with sub-Saturn- to Jupiter-mass inner planets at 5-10 AU and outer planets of 2.5 to 20 Earth masses. The mechanism fails if a Jupiter-sized planet exists beyond ~15 AU because the planet preferentially ejects planetesimals before they can reach the inner system. Direct-imaging planet searches can therefore directly test this mechanism.