George's Random Astronomical Object

Object 94: WD 2211-495

Podcast release date: 20 March 2023

Right ascension: 22:14:11.9

Declination: -49:19:27

Epoch: J2000

Constellation: Grus

Corresponding Earth location: About 870 km northeast of the island of South Georgia in the Atlantic Ocean

This episode's coordinates point to a location in the constellation Grus, which is a constellation in the southern part of the sky that was created in the 1500s by the Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman [1]. The constellation is named after a crane, as in the bird and not the mechanical object, although it looks more like the mechanical object than the bird. Unlike many of the southern constellations that I have derided on this podcast, this constellation actually has relatively bright stars in it and it actually vaguely looks like something, so it's much better than something like the constellations Antlia, Pictor, or Microscopium.

In any case, the specific object that this episode's coordinates point to is WD 2211-495, a white dwarf located 191.8 light years (58.8 pc) from Earth [2, 3]. Interestingly, this star was discovered in the early 1990s by an X-ray telescope named ROSAT that was also equipped with an ultraviolet camera [4]. The star managed to elude astronomers until that point in time because it is relatively faint in the visible part of the electromagnetic spectrum. However, WD 2211-495 has a surface temperature of 62000 K [5], and as a consequence of this, it is quite bright at ultraviolet wavelengths. So, when ROSAT began making maps of the entire sky at X-ray and ultraviolet wavelengths in the 1990s, WD 2211-495 stood out very prominently. As an additional note, this star's temperature is not only hot compared to Earth standards but also compared to most other stars, although it isn't that unusual compared to other white dwarfs.

As a brief review, a white dwarf like WD 2211-495 is the dead core left over from when a star about the size of the Sun reach the end of its life. A Sun-like star will start out its life fusing hydrogen into helium in its core, but once its core fills up with helium, it will expand into a red giant, with the fusion of hydrogen into helium continuing around the inert helium core until the star reaches the point where the fusion of helium into carbon and oxygen is triggered. Then, several other things happen, and in a while, the star's core fills up with carbon and oxygen, but a Sun-sized star doesn't have the mass to trigger the fusion of those elements, so the carbon and oxygen just sits there and does nothing. Eventually, the outer gas layers of the star get blown away, leaving the inert core, which is the white dwarf. That white dwarf might retain thin layers of hydrogen and helium after that, or it could be just carbon and/or oxygen.

Occasionally, though, white dwarfs are found with outer atmospheres that contain other heavy elements, and WD 2211-495 is one of those stars [6, 7]. Among other things, the outer atmosphere of WD 2211-495 contains silicon, aluminum, iron, and nickel. Even though the star would not have been able to create these elements in its core before it became a white dwarf, it might have formed from an interstellar gas cloud that contained these things. However, the general expectation is that the heavier elements in white dwarfs should sink into their centers, with the lighter elements lying on top. We should never see the heavier elements if they've all sunk into the star's core.

This led to the question of where did the heavy elements in the outer atmosphere of WD 2211-495 come from? One possibility was that, because WD 2211-495 is hot, the heavy elements within the star's core were able to absorb a lot of extra radiation within the star which caused those atoms to get hot and levitate to the star's surface. The other possibility is that something like a gravitationally-disrupted planet or some asteroids or something else with lots of heavy elements is falling onto the surface of the white dwarf.

It was not immediately clear which of these scenarios applies to WD 2211-495. However, an analysis of data from the Hubble Space Telescope published by a collaboration led by Simon Preval found that the amount of heavy elements in the outer atmosphere of WD 2211-495 as well as a couple of similar hot white dwarfs was simply too high to be explained by the levitation of elements from the center [7]. It therefore looks more likely that stuff has been falling onto the surfaces of the hot white dwarfs over time, which implies that these objects may have some sort of planetary systems.

WD 2211-495 continues to attract a lot of attention because it's so hot, or, to rephrase that slightly, because it's a hot white dwarf, and because it has so many heavy elements in its outer atmosphere. You can expect to hear more about WD 2211-495 in the future.


[1] Ridpath, Ian, Star tales, 1988

[2] Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1

[3] Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533

[4] Pounds, K. A. et al., The ROSAT Wide Field Camera all-sky survey of extreme-ultraviolet sources. I. The bright source catalogue., 1993, Monthly Notices of the Royal Astronomical Society, 260, 77

[5] Barstow, M. A. et al., Heavy-element abundance patterns in hot DA white dwarfs, 2003, Monthly Notices of the Royal Astronomical Society, 341, 870

[6] Holberg, J. B. et al., The Discovery of NI V in the Photospheres of the Hot DA White Dwarfs RE 2214-492 and G191-B2B, 1994, Astrophysical Journal Letters, 425, L105

[7] Preval, Simon P. et al., A far-UV survey of three hot, metal-polluted white dwarf stars: WD0455-282, WD0621-376, and WD2211-495, 2019, Monthly Notices of the Royal Astronomical Society, 487, 3470

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Last update: 20 March 2023