Object 6: HD 98851
Podcast release date: 21 October 2019
Right ascension: 11:22:51.2
Constellation: Leo Minor
Corresponding Earth location: Slightly less than one-third of the way from Midway Island to Japan in the Pacific Ocean.
HD 98851 is a star located in the constellation of Leo Minor at a distance of about 492 light years or 151 parcsecs [1,2]. This is a rather complex and weird star, so it's going to take a while to describe it.
First, HD 98851 is classified as a chemically peculiar star, which means that it has a weird combination of elements in its atmosphere as measured through spectroscopy. Based on the overall spectrum and brightness of the star, it would be classified as an F-type red giant or subgiant star . Reg giant and subgiants stars are stars where the core has stopped fusing hydrogen into helium because it has filled up with helium but where the fusion of hydrogen is still taking place in a shell around that core. The classification as F-type means that the star is slightly hotter than the Sun . However, F-type stars commonly have small amounts of calcium in their outer atmospheres, and the conditions are usually just right for the calcium to absorb light at wavelengths of 3934 and 3968 Angstroms. This light absorption is not seen in the spectrum of HD 98851, so it is designated as chemically peculiar and given the spectral designation Am .
Second, HD 98851 is a type of variable star called a delta Scuti variable, and it's named after the brightest star in this class. The variability was only discovered back in 2000 [3,4,5]. In the last episode, I described a class of variable stars called RR Lyrae variables that are very old versions of the Sun that fuse helium into carbon in their cores, that fuse hydrogen into helium in shells around these cores, and that have unstable, pulsating outer atmospheres. The atmospheres of delta Scuti variable stars exhibit the same type of instability, but these types of stars are younger and could include stars that are powered by fusing hydrogen into helium in their cores like the Sun or stars where the cores have filled up with helium like red giants but where that helium is not being fused into carbon .
Even though the delta Scuti and RR Lyrae stars are very different ages and are powered by different fusion processes, both classes of stars apparently have the right conditions for the atmospheres to pulse periodically. At the beginning of the pulsation cycle, the outer atmosphere of one of these stars cools and contracts. This contraction causes an increase in the opacity of the stellar atmosphere, which then leads to an increase in temperature and brightness. This then causes the outer atmosphere to expand, but the expansion causes the gas to become less opaque and to cool, which then leads to the outer atmosphere contracting. This pulsation continues for a very long time .
One of the really surprising thing about HD 98851 is that it is both an Am star and a delta Scuti variable star. It is normal to find stars with similar brightnesses and colors as HD 98851 that are either Am chemically peculiar or delta Scuti variables, but it's peculiar to find an Am chemically peculiar delta Scuti variable . The other peculiar thing about this star is that the variations in its brightness could be described as having two different pulsation cycles, one with a period of about 76 minutes and one with a period of about 152 minutes . A few other delta Scuti variables exhibit the same type of pulsation, and it is not entirely clear why, but one of the possibilities is that HD 98851 and similar delta Scuti variables are in binary star systems where some type of interaction with the companion star modulates the pulsation [8,9]. If HD 98851 is in a binary star system, then this could be linked to why it is an Am star, as most other Am stars are also in binary star systems .
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 Gaia Collaboration et al., Gaia Data Release 2. Summary of the contents and survey properties, 2018, Astronomy & Astrophysics, 616, A1
 Joshi, Santosh et al., Discovery of unusual pulsations in the cool, evolved Am stars HD 98851 and HD 102480, 2003, Monthly Notices of the Royal Astronomical Society, 344, 431
 Zhou, A., Multiperiodicity of HD 98851, 2001, Communications in Asteroseismology, 140, 59
 Joshi, S. et al., Discovery of pulsations in HD 98851, 2001, Bulletin of the Astronomical Society of India, 29, 301
 Kurtz, D. W. and Martinez, P., Observing roAp Stars with WET: A Primer, 2000, Baltic Astronomy, 9, 253
 Matthew Templeton, Delta Scuti and the Delta Scuti variables, 2010, American Association of Variable Star Observers
 Arentoft, T. et al., Low-frequency variability and binarity of the delta Scuti star XX Pyx, 2001, Monthly Notices of the Royal Astronomical Society, 326, 192
 Liakos, Alexios and Niarchos, Panagiotis, Catalogue and properties of delta Scuti stars in binaries, 2017, Monthly Notices of the Royal Astronomical Society, 465, 1181