George's Random Astronomical Object

Object 36: HD 60435

Podcast release date: 14 December 2020

Right ascension: 07:30:57.0

Declination: -57:59:28

Epoch: J2000

Constellation: Carina

Corresponding Earth location: About 850 km from Antarctica in the direction of Western Australia

HD 60435, which is located at a distance of 800 light years (245 pc) [1,2], is representative of a group of stars called rapidly oscillating Ap stars [3,4]. This designation has a lot of different terms packed into it, so it's going to take me a while to explain everything.

An Ap star is a type of chemically peculiar star [5]. A chemically peculiar star is a type of star where the star's spectrum reveals the presence of an unusual combination of elements in the star's atmosphere. There are several different classes of chemically peculiar stars. The Ap stars specifically are stars a bit larger than the Sun and a bit hotter than the Sun, and they seem to have rather strong magnetic fields that can be quite variable [5]. In fact, Ap stars were the first stars other than the Sun in which magnetic field strengths were measured [5]. In terms of the chemical compositions of their atmospheres, they have abnormally high amounts of silicon, chromium, strontium, and europium among other things [5]. Unless you are really into chemistry, you probably haven't heard of strontium or europium before, although I hope you would have heard of silicon and chromium. Strontium is located just below calcium in the periodic table of elements and chemically acts like calcium, while europium is one of those obscure rare earth elements which occupies one of those extra rows of boxes at the bottom of the periodic table. One of it's common uses on Earth is in glow-in-the-dark paint [6]. The presence of these elements is probably related to various internal processes in the stars that dredge some specific heavy elements to the surface of the stars.

In the case of HD 60435 specifically, it was originally identified as chemically peculiar in the 1970s based on the presence of large amounts of strontium and europium in its atmosphere [7,8], but I didn't find any newer references that actually measured the amounts of these elements. More recent measurements of the chemical composition of the star were published in 2001, and these data indicate that HD 60435's atmosphere has a little more silicon than usual but a really abnormally large amount of chromium [9]. It also has really abnormal amounts of barium [9], which sits below calcium and strontium on the periodic table and behaves like calcium and strontium It also has extra amounts of weird rare earth elements like praseodymium, neodymium, samarium, ytterbium, and lutetium [9], some of which are elements that are known for being used on Earth in magnets [6] and some of which don't really seem to have any practical applications or impractical applications.

Rapidly oscillating Ap stars are an extra abnormal subset of Ap stars that seem to oscillate a lot, and these oscillations cause variations in the stars' brightnesses. HD 60435 is apparently one of the most notable examples of this class of stars. The rapid oscillations in the star's atmosphere were discovered in observations made in 1983 by D. W. Kurtz [3], and they are insanely fast and complicated. Based on the results from multiple sets of observations in the 1980s, this star's atmosphere has 17 different oscillation periods that range from as short as 12 minutes to as long as 24 minutes [4]. These pulsation periods also combine together to make it look like the star is pulsating on time periods as short as 4 minutes [4].

These oscillations look connected to the types of oscillations of another class of stars called Delta Scuti variable stars [10]. I discussed one of these stars in episode 6. These are stars that fuse hydrogen into helium in their cores like the Sun, or they may be stars with cores that have filled up with helium that are just beginning to evolve from Sun-like stars to red giants, but the outer atmospheres of these stars periodically expand and contract, which leads to periodic brightness variations [11,12]. At the beginning of one of these cycles for one of these stars, the atmosphere cools and contracts, which makes it slightly more opaque and slightly warmer. This causes the outer atmosphere to expand, but the atmosphere then turns more transparent and cools, which then leads to another cycle of contraction and expansion. Delta Scuti variable stars have similar colours, similar surface temperatures, and similar brightnesses to the rapidly oscillating Ap stars like HD 60435. The difference is that a typical pulsation period for a Delta Scuti variable star is a relatively more mellow 30 minutes to 7 hours [11,12], while for a rapidly oscillating Ap Star, it can be as short as a few minutes, as we've seen for HD 60435. Still, it's clear that both of these types of stars just happen to have the right conditions for their outer atmospheres to become unstable and to start pulsating. It's just that the rapidly oscillating Ap stars are more freakish.

References

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

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

[3] Kurtz, D. W., Two new rapidly oscillating AP stars : HD 60435 and HD 203932., 1984, Monthly Notices of the Royal Astronomical Society, 209, 841

[4] Matthews, Jaymie M. et al., The Rich p-Mode Spectrum of the Rapidly Oscillating Peculiar A Star HD 60435, 1987, Astrophysical Journal, 313, 782

[5] Preston, G. W., The chemically peculiar stars of the upper main sequence., 1974, Annual Reviews of Astronomy and Astrophysics, 12, 257

[6] Gray,Theodore, The Wooden Periodic Table Table, 2010

[7] Bidelman, W. P. and MacConnell, D. J., The brighter stars astrophysical interest in the southern sky., 1973, Astronomical Journal, 78, 687

[8] Houk, N. and Cowley, A. P., University of Michigan Catalogue of two-dimensional spectral types for the HD stars. Volume I. Declinations -90\_ to -53\_\textflorin0., 1975

[9] Shavrina, A. V. et al., Lithium on the surface of cool magnetic CP stars. II. Spectrum analysis of HD 83368 and HD 60435 with lithium spots, 2001, Astronomy & Astrophysics, 372, 571

[10] Balona, L. A. et al., High frequencies in TESS A-F main-sequence stars, 2019, Monthly Notices of the Royal Astronomical Society, 487, 2117

[11] Baglin, A. et al., Delta Scuti stars., 1973, Astronomy & Astrophysics, 23, 221

[12] Matthew Templeton, Delta Scuti and the Delta Scuti variables, 2010, American Association of Variable Star Observers

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Last update: 13 December 2020