Spectroscopic observations of some metal-rich white dwarfs ( WDs ) , believed to be polluted by planetary material , reveal the presence of compact gaseous metallic disks orbiting them .
The observed variability of asymmetric , double-peaked emission line profiles in about half of such systems could be interpreted as the signature of precession of an eccentric gaseous debris disk .
The variability timescales — from decades down to 1.4 yr ( recently inferred for the debris disk around HE 1349–2305 ) — are in rough agreement with the rate of general relativistic ( GR ) precession in the test particle limit .
However , it has not been demonstrated that this mechanism can drive such a fast , coherent precession of a radially extended ( out to 1 R _ { \odot } ) gaseous disk mediated by internal stresses ( pressure ) .
Here we use the linear theory of eccentricity evolution in hydrodynamic disks to determine several key properties of eccentric modes in gaseous debris disks around WDs .
We find a critical dependence of both the precession period and radial eccentricity distribution of the modes on the inner disk radius , r _ { \mathrm { in } } .
For small inner radii , r _ { \mathrm { in } } \lesssim ( 0.2 - 0.4 ) R _ { \odot } , the modes are GR-driven , with periods of \approx 1 - 10 yr. For r _ { \mathrm { in } } \gtrsim ( 0.2 - 0.4 ) R _ { \odot } , the modes are pressure-dominated , with periods of \approx 3 - 20 yr .
Correspondence between the variability periods and inferred inner radii of the observed disks is in general agreement with this trend .
In particular , the short period of HE 1349–2305 is consistent with its small r _ { \mathrm { in } } .
Circum-WD debris disks may thus serve as natural laboratories for studying the evolution of eccentric gaseous disks .