The brighter star of the Pleione binary pair, component A, is a hot type B star 184 times more luminous than the Sun. It is classified as Be star with certain distinguishing traits: periodic phase changes and a complex circumstellar environment composed of two gaseous disks at different angles to each other. The primary star rotates rapidly, close to its breakup velocity, even faster than Achernar. Although some research on the companion star has been performed, stellar characteristics of the orbiting B component are not well known.
The location of the Pleiades (circled) in the constellation Taurus.
With an apparent magnitude of +5.05 in V, the star is rather difficult to make out with the naked eye, especially since its close neighbour Atlas is 3.7 times brighter and located less than 5 arcminutes away.[note 2] Beginning in October of each year, Pleione along with the rest of the cluster can be seen rising in the east in the early morning before dawn.[14] To see it after sunset, one will need to wait until December. By mid-February, the star is visible to virtually every inhabited region of the globe, with only those south of 66° unable to see it. Even in cities like Cape Town, South Africa, at the tip of the Africancontinent, the star rises almost 32° above the horizon. Due to its declination of roughly +24°, Pleione is circumpolar in the northern hemisphere at latitudes greater than 66° North. Once late April arrives, the cluster can be spotted briefly in the deepening twilight of the western horizon, soon to disappear with the other setting stars.[15]
There has been significant debate as to the star's actual distance from Earth. The debate revolves around the different methodologies to measure distance—parallax being the most central, but photometric and spectroscopic observations yielding valuable insights as well.[16][17] Before the Hipparcos mission, the estimated distance for the Pleiades star cluster was around 135 parsecs or 440 light years. When the Hipparcos Catalogue was published in 1997, the new parallax measurement indicated a much closer distance of about 119±1.0 pc (388±3.2 ly), triggering substantial controversy among astronomers.[16][18][19] The Hipparcos new reduction produced a broadly similar distance of 120±2 pc. If the Hipparcos estimate were accurate, some astronomers contend, then stars in the cluster would have to be fainter than Sun-like stars—a notion that would challenge some of the fundamental precepts of stellar structure. Interferometric measurements taken in 2004 by the Hubble Telescope's Fine Guidance Sensors and corroborated by studies from Caltech and NASA's Jet Propulsion Laboratory showed the original estimate of 135 pc or 440 ly to be the correct figure.[19] The Gaia EDR3 parallax is 7.24±0.1255 mas, indicating a distance around 138 pc. This is relatively imprecise for a Gaia result due to the brightness of the star, but still with a statistical margin of error similar to the Hipparcos results.[1]
Properties
In 1942 Otto Struve, one of the early researchers of Be Stars, stated that Pleione is "the most interesting member of the Pleiades cluster".[20] Like many of the stars in the cluster, Pleione is a blue-white B-typemain sequencedwarf star with a temperature of about 11,000 K. It has a bolometric luminosity of 184 L☉ assuming a distance of roughly 130 pc.[7] With a radius of 3.7 R☉ and mass that is 2.9 M☉, Pleione is considerably smaller than the brightest stars in the Pleiades. Alcyone for instance has a radius that is 10 R☉ with a luminosity 2,400 L☉, making it roughly 30 times more voluminous than Pleione and about 13 times brighter.[note 3]
Be star
Artist's impression of the Phi Persei system. Classical Be stars like Pleione and Phi Persei exhibit gaseous equatorial disks, likely caused by rapid rotation.[21]
Pleione is a classical Be star, often referred to as an "active hot star".[22] Classical Be stars are B-type stars close to the main sequence with the "e" in the spectral type signifying that Pleione exhibits emission lines in its spectrum, rather than the absorption lines typical of B-type stars.[23] Emission lines usually indicate that a star is surrounded by gas. In the case of a Be star, the gas is typically in the form of an equatorial disk, resulting in electromagnetic radiation that emanates not just from the photosphere, but from the disk as well. The geometry and kinematics of this gaseous circumstellar environment are best explained by a Keplerian disk – one that is supported against gravity by rotation, rather than gas or radiation pressure.[24][25] Circumstellar disks like this are sometimes referred to as "decretion disks", because they consist of material being thrown off the star (as opposed to accretion disks which comprise material falling toward the star).[26]
Be Stars are fast rotators (>200 km/s), causing them to be highly oblate, with a substantial stellar wind and high mass loss rate.[22] Pleione's rotational velocity of 329 km/s is considerably faster than the 251 km/s of Achernar, a prototypical Be star.[9][27] Pleione revolves on its axis once every 11.8 hours, compared to 48.4 hours for Achernar.[note 4] For comparison, the Sun takes 25.3 days to rotate. Pleione is spinning so fast that it is close to the estimated breakup velocity for a B8V star of about 370–390 km/s, which is why it is losing so much mass.[28]
Artist's impression of Alpha Arae rotating with a velocity close to breakup and losing mass through a stellar wind emerging from the poles.[29]
Pleione is unusual because it alternates between three different phases: 1) normal B star, 2) Be star and 3) Be shell star. The cause is changes in the decretion disc, which appears, disappears, and reforms. Material in the disc is pulled back towards the star by gravity, but if it has enough energy it can escape into space, contributing to the stellar wind.[25] Sometimes, Be stars form multiple decretion discs simultaneously, producing complex circumstellar dynamics.[30]
As a result of such dynamics, Pleione exhibits prominent long-term photometric and spectroscopic variations encompassing a period of about 35 years.[30] During the 20th century, Pleione went through several phase changes: it was in a Be phase until 1903, a B phase (1905–1936), a B-shell phase (1938–1954), followed by another Be phase (1955–1972).[28] It then returned to the Be-shell phase in 1972, developing numerous shell absorption lines in its spectrum. At the same time, the star showed a decrease in brightness, beginning at the end of 1971. After reaching a minimum brightness in late 1973, the star gradually re-brightened. In 1989, Pleione entered a Be phase which lasted until the summer of 2005.[30]
These phase changes are ascribed to the evolution of a decretion disc that formed in 1972.[30]Polarimetric observations show the intrinsic polarization angle has changed, indicating a change in orientation of the disc axis.[32] Because Pleione has a stellar companion with a close orbit, the shift in the polarization angle has been attributed to the companion causing a precession (wobble) of the disk, with a precession period of roughly 81 years.[32]
Photometric and spectroscopic observations from 2005 to 2007 indicated that a new disc had formed around the equator – producing a two discs at different inclination angles (60° and 30°).[30][32] Such a misaligned double-disc structure had not been observed around other Be stars.[30][32]
Pleione was an Oceanidnymph of Mount Kyllene in Arkadia (southern Greece), one of the three thousand daughters of the TitansOceanus and Tethys.[36][37] The nymphs in Greek mythology were the spirits of nature; oceanids, spirits of the sea.[38] Though considered lesser divinities, they were still very much venerated as the protectors of the natural world. Each oceanid was thence a patroness of a particular body of water — be it ocean, river, lake, spring or even cloud — and by extension activities related thereto. The sea-nymph, Pleione, was the consort of Atlas, the Titan, and mother of the Hyas, Hyades and Pleiades.[39]
Etymology
When names were assigned to the stars in the Pleiades cluster, the bright pair of stars in the East of the cluster were named Atlas and Pleione, while the seven other bright stars were named after the mythological Pleiades (the 'Seven Sisters'). The term "Pleiades" was used by Valerius Flaccus to apply to the cluster as a whole, and Riccioli called the star Mater Pleione.[40]
There is some diversity of opinion as to the origin of the names Pleione and Pleiades. There are three possible derivations of note. Foremost is that both names come from the Greek word πλεῖν, (pr. ple'-ō), meaning "to sail".[40][41] This is particularly plausible given that ancient Greece was a seafaring culture and because of Pleione's mythical status as an Oceanid nymph. Pleione, as a result, is sometimes referred to as the "sailing queen" while her daughters the "sailing ones". Also, the appearance of these stars coincided with the sailing season in antiquity; sailors were well advised to set sail only when the Pleiades were visible at night, lest they meet with misfortune.[39]
Another derivation of the name is the Greek word Πλειόνη[37](pr. plêionê), meaning "more", "plenty", or "full"—a lexeme with many English derivatives like pleiotropy, pleomorphism, pleonasm, pleonexia, plethora and Pliocene. This meaning also coincides with the biblical Kīmāh and the Arabic word for the Pleiades — Al Thurayya.[40] In fact, Pleione may have been numbered amongst the Epimelides (nymphs of meadows and pastures) and presided over the multiplication of the animals, as her name means "to increase in number".[42]
Finally, the last comes from Peleiades (Ancient Greek: Πελειάδες, lit. 'doves'), a reference to the sisters' mythical transformation by Zeus into a flock of doves following their pursuit by Orion, the giant huntsman, across the heavens.[43]
Modern legacy
In the best-selling 1955 nature book published by Time-Life called The World We Live In, there is an artist's impression of Pleione entitled Purple Pleione.[44] The illustration is from the famed space artist Chesley Bonestell and carries the caption: "Purple Pleione, a star of the familiar Pleiades cluster, rotates so rapidly that it has flattened into a flying saucer and hurled forth a dark red ring of hydrogen. Where the excited gas crosses Pleione's equator, it obscures her violet light."
^The brightness ratio of Atlas versus Pleione is derived from the formula for apparent magnitude and is based on their respective visualmagnitudes: Atlas () at 3.62 and Pleione () at 5.05. Therefore:
^The relative size of Alcyone (VA) compared to Pleione (VP) is determined by comparing their volumes. It is assumed that the volume of each star is reasonably approximated by the formula for a sphere:
VA ≈ 4⁄3π × 103 ≈ 4,188.79 VSun
VP ≈ 4⁄3π × 3.23 ≈ 137.26 VSun
Therefore, the volume of Alcyone compared to Pleione ≈ 4,188.79 ÷ 137.26 ≈ 30.52
Alcyone's luminosity compared to Pleione is:
^The time it takes for Achernar (TA) and Pleione (TP) to rotate on its own axis is determined by taking the star's radius in solar units, multiplying by the Sun's radius in kilometers, then calculating the star's circumference at the equator and dividing by its speed of rotation per hour. Therefore:
TA = 10 R☉ × 696,000 km × 2 × π ÷ 251 km/s ÷ 3,600 ≈ 48.4 hrs
^ abcSamus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: B/gcvs. Bibcode:2009yCat....102025S.
^Taranova, O.; Shenavrin, V.; Nadjip, A. D. (2008). "Pleione (BU Tau): IR Fading of the Star in 1999 - 2007". Peremennye Zvezdy Prilozhenie. 8 (6): 6. Bibcode:2008PZP.....8....6T.
^Su, K. Y. L.; Rieke, G. H.; Stansberry, J. A.; Bryden, G.; Stapelfeldt, K. R.; Trilling, D. E.; Muzerolle, J.; Beichman, C. A.; Moro-Martin, A.; Hines, D. C.; Werner, M. W. (2006). "Debris Disk Evolution around a Stars". The Astrophysical Journal. 653 (1): 675–689. arXiv:astro-ph/0608563. Bibcode:2006ApJ...653..675S. doi:10.1086/508649. S2CID14116473.
^
Allen, J.; Boyd, P. (15 April 1997). "Finding Stellar Distances". Ask an Astrophysicist. NASA. Retrieved 2010-09-14. A straightforward summary of the different methods used by astronomers to measure stellar distances.
^
Atsma, A. J. (8 March 2010). "Pleione". Theoi Greek Mythology. Retrieved 2010-06-14.
^Hesiod (1914). "ll. 618–640". Works and Days. Translated by Evelyn-White, H. G. The Internet Sacred Text Archive. ISBN0-585-30250-2. Retrieved 2010-10-13.
