J. Farihi, B. T. Gänsicke, P. R. Steele, J. Girven, M. R. Burleigh, E. Breedt, D. Koester
This paper reports follow-up photometric and spectroscopic observations,
including warm Spitzer IRAC photometry of seven white dwarfs from the SDSS with
apparent excess flux in UKIDSS K-band observations. Six of the science targets
were selected from 16,785 DA star candidates identified either
spectroscopically or photometrically within SDSS DR7, spatially
cross-correlated with HK detections in UKIDSS DR8. Thus the selection criteria
are completely independent of stellar mass, effective temperature above 8000 K,
and the presence (or absence) of atmospheric metals. The infrared fluxes of one
target are compatible with a spatially-unresolved late M or early L-type
companion, while three stars exhibit excess emissions consistent with warm
circumstellar dust. These latter targets have spectral energy distributions
similar to known dusty white dwarfs with high fractional infrared luminosities
(thus the K-band excesses). Optical spectroscopy reveals the stars with
disk-like excesses are polluted with heavy elements, denoting the ongoing
accretion of circumstellar material. One of the disks exhibits a gaseous
component - the fourth reported to date - and orbits a relatively cool star,
indicating the gas is produced via collisions as opposed to sublimation,
supporting the picture of a recent event. The resulting statistics yield a
lower limit of 0.8% for the fraction dust disks at DA-type white dwarfs with
cooling ages less than 1 Gyr. Two overall results are noteworthy: all stars
whose excess infrared emission is consistent with dust are metal-rich; and no
stars warmer than 25,000 K are found to have this type of excess, despite
sufficient sensitivity.
View original:
http://arxiv.org/abs/1112.5163
No comments:
Post a Comment