X-ray signals of the destruction of the white dwarf planet

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David Trevascus is a graduate honors student in astrophysics at Monash University in Melbourne (applying for PhD positions). In their year of honor they explored planetary systems around white dwarfs and performed hydrodynamic simulations of eccentric gas disks. You are currently working as a research assistant on the processing of X-ray image data and in your free time you enjoy playing new board games and running a self-made Dungeons and Dragons campaign.

Title: A white dwarf accreting planetary material determined from X-ray observations

Authors: Tim Cunningham, Peter J. Wheatley, Pier-Emmanuel Tremblay, Boris T. Gänsicke, George W. King, Odette Toloza, and Dimitri Veas

Institution of first author: Department of Physics, University of Warwick, Coventry, CV4 7AL, UK

Status: Published in Nature (Closed Access); Available on archive

A white dwarf is the last stage of life for a low-mass star like our sun. After our sun has burned off all the hydrogen and helium in its core, it leaves behind a sluggish ball of carbon and oxygen. We have observed Thousands of white dwarfs from surveys of the night sky, but we know much less about what happens to the planets around these dead stars. Is there a chance that one day we will be able to see the remains of an Earth-like planet orbiting a white dwarf?

A phenomenon known as “metal pollution” indicates the existence of planets around white dwarf stars. White dwarfs are covered with a thin outer layer of leftover hydrogen and helium known as photosphere. All heavier elements (metals) present in the photosphere will sink out of this layer relatively quickly due to the strong gravitational forces of the white dwarf. So it is surprising that we find this when observing the chemical spectra of white dwarfs 25-50% of them have metals contaminating their outer layers. The commonly accepted explanation for this pollution is the accretion of planets (and other, smaller bodies) onto the surfaces of these white dwarfs. Today’s paper describes the first known evidence of roentgen Emissions caused by this type of white dwarf accretion G29-38.

Why would growing material emit X-rays? Well, it’s about what happens when the orbiting material hits the white dwarf. In the process grow on the white dwarf, the orbiting material loses a lot of kinetic energy very quickly. This is done by heating to high temperatures and generating high-energy radiation (such as X-rays) that transports the energy away.

We have already detected X-ray emissions from accreting white dwarfs, but previous discoveries have all come from binary star systems where the accreting material used to belong to the other star. The main differences for this detection were that the photons were concentrated at lower energies and that the overall X-ray luminosity of the event was lower. This is due to the lower mass and hence lower accretion rates of planetary material as opposed to stellar material.

To determine the rate of accretion of material on the white dwarf, we first need to know the X-ray luminosity of the accretion event. This requires that we take the grand total (i.e. the integral) of x-ray flux over different photon energies.

The X-ray flux of this event was measured with the ACIS-S detector on the Chandra X-ray Observatory. This detector is the most sensitive photon energies in the range 1.0 – 6.5 keV. A significant fraction of the electrons detected were at lower energies (less than 0.5 keV) where the detector is less sensitive.

The authors of this article addressed this problem by simulating the X-ray flux generated by this accretion event. Their model takes into account the effective temperature of the white dwarf’s photosphere as well as the composition and temperature distribution of the growing material. By fitting these models to the observed photon energy distribution, the authors were able to determine the total X-ray flux and hence the luminosity of the emission event.

From the X-ray luminosity, the authors of this work were able to determine the accretion rate of planetary material on the white dwarf (since the two are directly proportional). They measured an accretion rate of 1.63109 grams per second. This is the first direct measurement of the accretion rate of planetary material on a white dwarf from X-ray observations.

Previous measurements of accretion rates of planetary material on a white dwarf have depended on what is known as a “steady state” model. This model assumes that the abundance of metals in the photosphere remains roughly constant over time as they accumulate on the white dwarf and then diffuse into its core.

The authors of this paper took this opportunity to compare their new independent measurement of the accretion rate with stationary measurements. They note that measured steady-state accretion rates are about an order of magnitude slower than their observations. However, they note that the steady-state accretion rates do not account for additional mixing of stellar material between layers of the white dwarf found in 3D convection models (as opposed to 1D models) – a phenomenon known as convective overshoot. Accounting for the convective overshoot leads to a rough agreement between the two measurements of the accretion rate.

Figure 1: Comparison between measured accretion rates (including uncertainties) with respect to white dwarf photosphere temperature. The open diamond and circle data points indicate the accretion rates measured from X-ray emission, using different compositions (Earthmass vs. photosphere) and different temperature distributions (isothermal vs. cooling flux) of the accretion material in the X-ray flux modelling. The blue band indicates the 68% confidence interval of the X-ray accretion rate. The solid lines show the steady-state accretion rates with (green) and without (red) convective overshoot. The filled blue and orange circles show previously measured growth rates for G29-38. Figure 3 from the paper.

This method of measuring the instantaneous rate of accretion of planetary material onto polluted white dwarfs should help us answer a number of unanswered questions about how metal pollution from white dwarfs occurs. We know from infrared observations that many polluted white dwarfs also harbor a dusty debris disk of planetary material (similar to the asteroid or Kuiper belt in our own solar system). We do not yet fully understand the mechanism by which this material is deposited on the white dwarf’s surface. We also don’t understand the variability we’re seeing in the infrared radiation emitted by these disks over time.

Because of the uncertainties in estimating X-ray flux at lower wavelengths, the authors of this paper admit that their accretion rate measurement provides a lower bound on the true accretion rate of planetary material. However, the authors point out that future X-ray telescopes, e.g Advanced Telescope for High-ENergy Astrophysics (ATHENA) be able to better study X-ray emissions from white dwarf planetary systems.

Astrobite edited by Sumeet Kulkarni

Credit for selected images: NOIRLab/NSF/AURA/J. by Silva

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