HP Lyrae (HP Lyr) is a variable star in the constellation Lyra, with a visual magnitude varying between 10.2 and 10.8. It is likely to be an RV Tauri variable, an unstable post-AGB star losing mass before becoming a white dwarf.

HP Lyrae

A blue band light curve for HP Lyrae, adapted from Wenzel (2013)[1]
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[5]
Mass	0.6[7] M☉
Radius	60[7] R☉
Luminosity	3,900±400[7] L☉
Surface gravity (log g)	1.0 cgs
Temperature	5,900[7] K
Metallicity [Fe/H]	−1.0 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] and 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 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 was 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 around 12,000 pc.[6] Using luminosities derived from a period-luminosity-colour relationship, together with interstellar extinctions, gives a distance 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 is crossing 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

• ASAS-3 information
• INTEGRAL-OMC catalogue