X-ray Alerts from White Dwarf Planet Destruction

David Trevascus is a graduated honors scholar in astrophysics from Monash College in Melbourne (making use of for PhD positions). Of their honor 12 months, they researched planetary programs round white dwarfs, performing hydrodynamic simulations of eccentric fuel discs. They at present work as a analysis assistant processing x-ray picture information, and revel in enjoying new board video games and working a homebrew Dungeons and Dragons marketing campaign throughout their spare time.

Title: A white dwarf accreting planetary materials decided from X-ray observations

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

First Creator’s Establishment: Division of Physics, The College of Warwick, Coventry, CV4 7AL, UK

Standing: Revealed in Nature (closed entry); Out there on archive

A white dwarf is the ultimate stage of life for a low mass star like our Solar. After our solar burns away all of the hydrogen and helium in its core, it leaves behind an inert ball of carbon and oxygen. Now we have noticed 1000’s of white dwarfs via surveys of the night time sky, however we all know a lot much less about what occurs to the planets round these lifeless stars. Is there an opportunity that we’d sooner or later be capable to see the remnants of an Earth-like planet orbiting a white dwarf?

A phenomenon often known as “metallic air pollution” hints on the existence of planets round white dwarf stars. White dwarfs are coated in a skinny outer layer of leftover hydrogen and helium often known as the photosphere. Any heavier parts (metals) current within the photosphere will comparatively shortly sink out of this layer because of the sturdy gravitational forces of the white dwarf. Subsequently it’s stunning that, after we observe the chemical spectra of white dwarfs, we discover that 25-50% of them have metals polluting their outer layers. The widely accepted rationalization for this air pollution is the accretion of planets (and different, smaller our bodies) onto the surfaces of those white dwarfs. Right now’s paper describes the primary identified detection of X ray emissions attributable to one of these accretion from the white dwarf G29-38.

Why would accreting materials emit x-rays? Effectively, it is all about what occurs when the orbiting materials hits the white dwarf. Within the technique of accreting onto the white dwarf, the orbiting materials loses a whole lot of kinetic vitality in a short time. It does this by heating as much as excessive temperatures and producing excessive vitality radiation (ie x-rays) that carries away the vitality.

We have detected x-ray emissions from accreting white dwarfs earlier than, however earlier detections have all been from binary star programs, the place the accreting materials used to belong to the opposite star. The important thing variations for this detection had been that the photons had been concentrated at decrease energies and that the general x-ray luminosity of the occasion was decrease. This stems from the decrease mass, and due to this fact decrease accretion charges, of planetary materials versus stellar materials.

With the intention to decide the accretion fee of fabric onto the white dwarf, we first have to know the x-ray luminosity of the accretion occasion. This requires us to take the overall sum (ie the integral) of x-ray flux over completely different photon energies.

The x-ray flux from this occasion was measured by the ACIS-S detector on the Chandra X-ray Observatory. This detector is most delicate to photon energies within the 1.0 – 6.5 keV vary. A good portion of the electrons detected had been at decrease energies (lower than 0.5 keV) the place the detector is much less delicate.

The authors of this paper tackled this downside by simulating the x-ray flux created by this accretion occasion. Their mannequin takes into consideration the efficient temperature of the white dwarf’s photosphere, in addition to the composition and temperature distribution of the accreting materials. By becoming these fashions to the noticed photon vitality distribution the authors had been in a position to decide the overall x-ray flux and due to this fact the luminosity of the emission occasion.

From the x-ray luminosity, the authors of this paper had been in a position to decide the accretion fee of planetary materials onto the white dwarf (because the two are instantly proportional). They measured an accretion fee of 1.63109 grams per second. That is the primary direct measurement of the accretion fee of planetary materials onto a white dwarf from x-ray observations.

Earlier measurements of the accretion charges of planetary materials onto a white dwarf have been depending on what is called a “regular state” mannequin. This mannequin assumes the abundances of metals within the photosphere stay roughly fixed additional time, as they accrete onto the white dwarf after which diffuse into its core.

The authors of this paper have taken this chance to check their new impartial measurement of the accretion fee to steady-state measurements. They discover that the measured steady-state accretion charges are roughly an order of magnitude decrease than their observations. Nonetheless, they observe that the steady-state accretion charges do not account for extra mixing of stellar materials between layers of the white dwarf present in 3D convection fashions (versus 1D fashions) – a phenomenon often known as convective overshoot. Accounting for convective overshoot leads to a tough match between the 2 accretion fee measurements.

Determine 1: Comparability between measured accretion charges (together with uncertainties) in relation to the white dwarf photosphere temperature. The open diamond and round information factors point out the accretion charges as measured from the x-ray emission, utilizing completely different compositions (bulk earth vs photospheric) and completely different temperature distributions (isothermal vs cooling movement) of the accreting materials within the x-ray flux modeling . The blue band signifies the 68% confidence interval on the x-ray accretion fee. The stable traces point out the steady-state accretion charges with convective overshoot (inexperienced) and with out (purple). The stable blue and orange circles point out beforehand measured accretion charges for G29-38. Determine 3 from the paper.

This technique of measuring the instantaneous accretion fee of planetary materials onto polluted white dwarfs ought to assist us reply plenty of open questions on how white dwarf metallic air pollution happens. We all know from infrared observations that many polluted white dwarfs additionally host a dusty particles disc of planetary materials (just like the asteroid or Kuiper belts in our personal photo voltaic system). We don’t but absolutely perceive the mechanism via which this materials is deposited onto the white dwarf’s floor. Neither can we perceive the variability that we see within the infrared radiation emitted from these discs over time.

As a result of uncertainties in estimating x-ray flux at decrease wavelengths, the authors of this paper do admit that their measurement of the accretion fee is a decrease restrict for the true accretion fee of planetary materials. Nonetheless, the authors observe that future x-ray telescopes, such because the Superior Telescope for Excessive-ENergy Astrophysics (ATHENA) will be capable to higher research x-ray emissions from white dwarf planetary programs.

Astrobite edited by Sumeet Kulkarni

Featured picture credit score: NOIRLab/NSF/AURA/J. as a result of Silva

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