Ry Cutter
Estimates of the lifetimes and inferred accretion rates of debris discs around polluted white dwarfs are often inconsistent with the limits predicted by the effect of shielded Poynting-Robertson drag. Moreover, many cool polluted white dwarfs do not show any observational evidence of accompanying discs. This discrepancy can be explained by decreasing the lifetime of the debris discs; this solution, by proxy, also produces accretion rates greater than those predicted by Poynting-Robertson drag alone. In this poster, I will demonstrate the role of a magnetic field on tidally disrupted, diamagnetic, planetesimal material, and its effect on the formation, evolution, and accretion rate of a debris disc. The results show that magnetic field strengths greater than ~10kG decrease the time needed for circularisation and disc lifetimes by several orders of magnitude and increase the associated accretion rates likewise. I suggest most white dwarfs present some form of magnetism below the detectable limit and that magnetic fields can account for a portion of polluted white dwarfs with missing debris discs. I show that diamagnetic drag can account for the higher accretion rate estimates that cannot be predicted solely by Poynting-Robertson drag and find a dependence on magnetic field strength, orbital distance, and particle sizes on disc lifetime and accretion rate.
[Opening slide, stencil art showing a frog falling off a stick]
Slide 2.
Diamagnetism is a property found in most planetary material. The idea being that a repulsive force is felt by a material when exposed to a magnetic field. In Berry(1997), physicists were able to demonstrate this repulsion could overcome gravity by levitating a small frog. In this poster, I will apply the same physics that levitates the frog to tidally disrupted material around a magnetic white dwarf.
[Example of a levitating frog with diamagnetic repulsion from Berry(2017)]
Magnetism in White Dwarfs was first observed in 1970 and the first confirmed polluted White Dwarf was found in the 1987. The composition of polluted White Dwarf atmospheres infers the presence of debris discs that are Carbonaceous and Silicate rich, both of which are diamagnetic. Given that White Dwarfs can hold magnetic field strengths over hundreds of Mega-Gauss, the dynamics of diamagnetic debris passing through these fields needs to be modelled.
[Equation: Diamagnetic Force equals Volume over Radius of Field Curvature multiplied by Field Strength (scaled by distance) squared divided by eight pi]
Slide 3.
When a rocky body passes too close (usually a solar radius) to its host star, the object is torn apart by tidal forces. This leaves a stream of debris which will eventually become a disc. By applying varying magnetic field strengths to the debris, the lifetime of the disc can be significantly reduced.
[A plot showing predicted disc lifetime as a function of magnetic field strength. Showing closer orbits and stronger magnetic fields will have shorter disc lifetimes.]
This is important, because shorter disc lifetimes mean higher accretion rates. Presently, many of the accretion rates observed in white dwarfs are beyond the allowable range predicted by current theories.
[A figure showing accretion rate as a function of temperature. A scatterplot of observed rates in white dwarfs is over-plotted on the maximum rate given for poyting-robertson drag. Showing approximately fifty percent of white dwarfs with accretion rates beyond the limits predicted by the theory.]
Slide 4.
By making some assumptions about the disc mass, the accretion rate of a disc exposed to magnetic field can be estimated. Reverse engineering the relationship between lifetime, field strength, and inferred accretion rates, a distribution of magnetic field strengths is created. Potentially revealing a whole population of magnetic white dwarfs that have gone unnoticed due to detection limits.
[A plot of predicted accretion rate for different sized dust grains against magnetic field strength. This shows that the new diamagnetic model encompasses the observed accretion rates better than the previous theory. Furthermore, the rates for centimetre sized particles have a good fit with observed accreting magnetic white dwarfs.]
[The model distribution of magnetic fields in white dwarfs from the theory is given with the observed distribution. Again, there is a good overlap between observed accreting white dwarfs and the theory, but the observed distribution peaks at much higher field strengths.]
This study shows that when looking at accretion of planetesimals around white dwarfs, the influence of a magnetic field needs to be considered. Furthermore, this work highlights the possibility that most white dwarfs exhibit some level of magnetism; a large portion of which have fields strengths below detectability.
More Information:
Arxiv Link: https://arxiv.org/abs/2009.03444
Kawka, et al, A&A, 538, A13 (2012)
Ferrario, et al, Space Science Reviews , 191, 1-4, 111 (2015)
Contact Us:
r.cutter@warwick.ac.uk
mah63@leicester.ac.uk