Object 38: HD 220140

Podcast release date: 11 January 2021

Right ascension: 23:19:26.6


Epoch: ICRS

Constellation: Cepheus

Corresponding Earth location: About 175 km east of Northeast Greenland in the Greenland Sea

To begin with, HD 220140 (also known as V368 Cep) is the brighter red dwarf in a star system that also contains a fainter red dwarf called HD 220140B [1]. Both are smaller, redder versions of the Sun that are separated by a distance of about 4000 Astronomical Units (where one Astronomical Unit is equivalent to the distance from the Earth to the Sun) and that orbit each other about once every 2650 years [2]. The distance to the star system is about 62 light years, or less than 20 parsecs [3,4]. This isn't quite close enough to get the two stars onto a list of the top fifty closest stars to Earth, but it's still fairly close. (Also, if I could have found a good copyright-free Star Wars sound effect to use here, I would have done so.)

HD 220140 seemed like a relatively ordinary star at first, and it was ignored for a very long time. However, when astronomers started using X-ray telescopes in the 1980s, HD 220140 was identified as a strong source of low-frequency X-ray emission [5]. Stars like the Sun are not normally strong X-ray sources, so astronomers spent more time looking at HD 220140 to try to understand what was going on.

The first thought was that HD 220140 was a type of eclipsing binary star system with a really short orbital period of a few days [5]. I didn't really find a good justification for why it should be an eclipsing binary star system other than the X-ray emission looked a little similar to the X-ray emission from another binary star system . Now as I stated earlier, HD 220140 is in a binary star system, but the two stars are not eclipsing or otherwise close enough to interact in a way that would produce X-ray emission. It seemed possible that HD 220140 had another companion star and that this companion was so close that the two stars were eclipsing each other, but if this was the case, we would expect to see some variations in the Doppler shifting of the light from the star system as the two stars orbit each other. Since no such Doppler shifting was actually seen after repeated observations were made of the star, the eclipsing binary star hypothesis was eliminated [6,7].

The more plausible explanation is that HD 220140 is a star with a lot of sunspot activity on its surface [6,7,8,9]. Sunspots on our Sun are cool spots created by locations with very strong magnetic fields poking out of the surface, and they tend to be associated with solar flares and solar prominences, but sunspots on our Sun don't cover that much of the Sun's surface. Sunspots (or starspots) on HD 220140 seem to cover much broader regions on that star, and the magnetic fields are much stronger. This would lead to much brighter solar flares that would produce the type of low-frequency X-rays that astronomers see from the star. As I mentioned earlier, the star is fairly close to Earth, and this probably helps to make it easier to detect the X-ray emission from the star's flares. Variations in the brightness of HD 220140 caused by the starspots moving from one side of the star to the other indicate that the star rotates on its axis about once every 2 days and 18 hours [7,8,9]. For comparison, the Sun rotates on its axis once every 27 days or so, so HD 220140 seems to be spinning really fast.

Interestingly, quite a few astronomers have worked on measuring the age of HD 220140, and they think that this star is between 20 and 50 million years old [9]. This is young in astronomical terms; our Sun has an age between 4 and 5 billion years old. Stars younger than HD 220140 are called T Tauri stars after a star in Taurus, and they are still surrounded by the disks of gas and dust that they formed from. HD 220140 is slightly past this phase, and it no longer has a disk of gas and dust, so it is called a naked young post T Tauri star or sometimes just a naked young star [9,10]. This demonstrates that some astronomers will use double entendres whenever possible, probably as clickbait for their science journal papers.

The other notable thing about HD 220140 as well as its companion, HD 220140B, is that it they are about 0.2 light years away from another red dwarf called LSPM J2322+7847 [1]. This is actually fairly close in interstellar terms. All three of the stars are similar distances from the Earth, they look like they are travelling in the same direction in the sky, and they look like they are similar ages, so it is likely that these three stars formed together at the same time in the same nebula.


[1] Makarov, V. V. et al., The Nearby Young Visual Binary HIP 115147 and Its Common Proper Motion Companion LSPM J2322+7847, 2007, Astrophysical Journal Letters, 668, L155

[2] Tokovinin, Andrei, The Updated Multiple Star Catalog, 2018, Astrophysical Journal Supplement Series, 235, 6

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

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

[5] Pravdo, S. H. et al., The identification of H 2311+77 with HD 220140, a probable RS CVn star., 1985, Monthly Notices of the Royal Astronomical Society, 215, 11P

[6] Bianchi, L. et al., The nature of HD 220140 from optical and IUE observations., 1991, Astronomy & Astrophysics, 245, 604

[7] Mantegazza, L. et al., Photospheric activity of HD 220140, the optical counterpart of the X-ray source H 2311+77., 1992, Astronomy & Astrophysics, 256, 459

[8] Chugainov, P. F. et al., V368 Cep - a Post T Tau Spotted Single Star, 1991, Information Bulletin on Variable Stars, 3623, 1

[9] Kahanpää, J. et al., Time series analysis of V 368 Cephei photometry, 1999, Astronomy & Astrophysics, 350, 513

[10] Nations, H. L. et al., The Bright Stellar X-Ray Source HD 220140: A Nearby Naked T Tauri Star?, 1990, in Bulletin of the American Astronomical Society, 22, 1254


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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