Searching for infrared excesses around white dwarfs

Abstract

Many white dwarf stars (WDs) have been found to be 'polluted,' meaning they contain elements heavier than hydrogen and helium in their atmospheres. One proposed external mechanism for atmospheric pollution of cooler WDs is the accretion of rocky bodies. Disintegration of rocky bodies (formerly planets, asteroids etc.) would leave a circumstellar (CS) disk of debris that would then accrete onto the WD, 'polluting' the atmosphere. When heated, CS disks emit excess infrared (IR) light. WDs with IR excesses suggestive of a CS disk are known as dusty WDs. Statistical studies are still needed to determine how numerous dusty, polluted WDs are, along with trends and correlations regarding the rate of planetary accretion, the lifetimes of CS disks around WDs, and the structure and evolution of WD CS disks. A sample of 39 WD stars were initially identified as containing IR excesses based on WISE data as part of the WISE InfraRed Excesses around Degenerates (WIRED) Survey (Debes et al. 2011). Data for these along with 27 additional targets was acquired by the FourStar Infrared Camera Array. Archival optical and IR data from WISE, Spitzer, 2MASS, DENIS, SDSS, PanSTARRS, and UKIDSS were used to create spectral energy distributions (SEDs) to search for excesses. FourStar data are much higher spatial resolution than the WISE data initially used to select the WIRED candidates, and thus can indicate if an apparent excess is due to a nearby companion vs. a CS disk. Seventeen WDs look to have CS disks, 7 of which are newly identified, ten of which are known to be metal-rich. Ten additional targets have an excess, but these are likely due to a BD companion, 7 of which are new detections, 1 of which also has an excess from source confusion. Eleven targets had excesses resulting from source confusion, including 3 that are still considered disk candidates and 1 WD+BD candidate. Thirty targets were determined to not contain an excess, one of which in literature was thought to contain an excess, two of which are metal-rich. Two targets were determined not to be WDs after all.