HP Lyrae
Variable star in the constellation Lyra
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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.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Lyra |
| Right ascension | 19h 21m 39.066s[2] |
| Declination | +39° 56′ 08.05″[2] |
| Apparent magnitude (V) | 10.43[3] (10.2 - 10.8[4]) |
| Characteristics | |
| Spectral type | A2-F2 Iab[4] |
| U−B color index | +0.1 - +0.5[4] |
| B−V color index | +0.3 - +0.7[4] |
| Variable type | RV Tau[5] |
| Astrometry | |
| Radial velocity (Rv) | −107[5] km/s |
| Proper motion (μ) | RA: −2.485±0.038[6] mas/yr Dec.: −5.500±0.047[6] mas/yr |
| Parallax (π) | 0.0822±0.0242 mas[6] |
| Distance | 6,700±380[7] pc |
| Absolute magnitude (MV) | −4.5[4] |
| Orbit[7] | |
| Period (P) | 1,631 days |
| Eccentricity (e) | 0.17 |
| Argument of periastron (ω) (secondary) | 5.5° |
| Semi-amplitude (K1) (primary) | 7.7 km/s |
| Details | |
| Mass | 0.6[7] M☉ |
| Radius | 60[7] R☉ |
| Luminosity | 3,900±400[7] L☉ |
| Surface gravity (log g) | 1.0[5] cgs |
| Temperature | 5,900[7] K |
| Metallicity [Fe/H] | −1.0[5] dex |
| B | |
| Mass | 0.5 - 0.6[7] M☉ |
| Other designations | |
| HP Lyrae, TYC 3138-54-1, ASAS J192139+3956.1, IRAS 19199+3950, 2MASS J19213906+3956080, AAVSO 1918+39 | |
| Database references | |
| SIMBAD | data |
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]
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]
Properties
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]