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HP Lyrae
Variable star in the constellation Lyra From Wikipedia, the free encyclopedia
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HP Lyrae (HP Lyr) is a variable star in the constellation Lyra, with a visual magnitude varying between 10.2 and 10.8. It will likely be an RV Tauri variable, an unstable post-AGB star losing mass before becoming a white dwarf.
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Discovery
HP Lyr was first reported to be variable in 1935 by Otto Morgenroth of the Sonneberg Observatory. The range was given as 9.5 - 10.5 and the variability type only as long-period.[8] In 1961, it was formally designated as a β Lyr eclipsing variable with two A type supergiants in a close orbit producing smooth continuous variations with alternating minima of different depths. The period was given as 140.75 days, covering two maxima, and both a deep primary minimum and a slightly less deep secondary minimum.[9]
In 2001 a request was made for observations of HP Lyr.[10] Shortly after it was reported that HP Lyr was likely to be an RV Tauri variable rather than an eclipsing binary.[11] This was confirmed with a more detailed study published in 2002. [4] Some authors still maintain that the spectral type and nature of variation mean HP Lyr is more likely to be an eclipsing variable.[12]
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Variability
HP Lyr varies by about 0.5 magnitude over a "halfperiod" of 68.4 days.[7] The formal period, defined for an RV Tauri variable as being from deep minimum to deep minimum is twice that length. Its spectrum changes from A2-3 at maximum to F2 at the deepest minima. The radial velocity changes are typical for the pulsations of an RV Tauri variable, but are not compatible with a binary orbit. The spectral type and colour indicated that it was likely to be the hottest known RV Tauri star.[4]
Until 1960, the period of HP Lyr was very consistent at 140.75 days. Since then it has been observed to reduce to below 140 days, probably quite suddenly. A survey of historic photography including the star showed that the period changed in 1962 or 1963, taking no more than four cycles to reach a new value of 138.66 days.[1]
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Properties
Summarize
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A 2005 study of the elemental abundances of RV Tauri stars calculated that HP Lyr had a temperature around 6,300 K and typical abundances for an RV Tauri variable. It also revealed that the abundances were altered by dust-gas separation in circumstellar material.[5] HP Lyr has been included in a catalog of confirmed post-AGB stars, highly evolved and on its way to becoming a white dwarf.[13] In 2017, the temperature was calculated to be 5,900 K, still one of the hottest known RV Tau variables.[7]
The distance is uncertain, although large. Gaia Data Release 2 contains a parallax indicating a distance of around 12,000 pc.[6] Using luminosities derived from a period-luminosity-colour relationship, together with interstellar extinctions, gives a distance of around 6,700 K. From the radius and effective temperature, the radius is calculated to be 60 R☉.[7]
HP Lyrae is a post-AGB star, one that has completed its evolution along the asymptotic giant branch (AGB) and is now rapidly shedding its outer layers prior to becoming a white dwarf. During this process it becomes hotter and crosses the instability strip which causes it to become unstable and pulsate.[7]
Binary
Many RV Tauri stars are found to be in binary systems, and HP Lyrae has an invisible companion in a 1,631 d orbit. Its properties are not known, but the mass is estimated to be a little under 0.6 M☉, leaving open the possibility that it is a white dwarf.[7]
References
External links
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