HD 177765

HD 177765 is a white-hued star in the southern constellation of Sagittarius. With an apparent magnitude of 9.15, it is too faint to be seen by the naked eye from Earth, but is dimly visible using binoculars.^[9] It is located at a distance of 1,284 light-years (394 parsecs) according to Gaia EDR3 parallax measurements.

HD 177765

Location of HD 177765 (circled)
Observation data Epoch J2000.0[1]      Equinox J2000.0[1]
Constellation	Sagittarius
Right ascension	19h 07m 09.77940s
Declination	−26° 19′ 54.5064″
Apparent magnitude (V)	9.15[2]
Characteristics
Evolutionary stage	A5 SrEuCr[3]
B−V color index	0.45[2]
J−H color index	0.071[4]
J−K color index	0.055[4]
Astrometry
Proper motion (μ)	RA: -2.063[1] mas/yr Dec.: −4.425[1] mas/yr
Parallax (π)	2.5411±0.0186 mas[1]
Distance	1,284 ± 9 ly (394 ± 3 pc)
Absolute magnitude (MV)	1.55,[5] 1.166[6]
Details[5]
Mass	1.81, 2.2[7] M☉
Radius	2.57 R☉
Luminosity	18.9, 26.9,[6] ~32[7] L☉
Surface gravity (log g)	3.79 cgs
Temperature	7420, 7002,[6] 8000[7] K
Rotational velocity (v sin i)	2.50[8] km/s
Age	955 Myr
Other designations
CD−26° 13816, CPD−26° 6650, Gaia DR2 6763969142066777344, HD 177765, SAO 187692, PPM 269324, EPIC 214503319, TIC 465996299, TYC 6882-1808-1, GSC 06882-01808, 2MASS J19070978-2619543, Renson 49550[4]
Database references
SIMBAD	data

Description

The star is classified as a rapidly oscillating Ap star (roAp star). It shows super-solar abundances of chromium and strontium as well as many rare-earth elements such as europium and cerium, but is depleted of carbon and nickel.^[8] In 2012, the star was found to pulsate with a low radial velocity amplitude of 7–150 m/s and a period of 23.6 minutes, the latter being the longest out of any known roAp star at the time.^[7] Two additional pulsation frequencies were detected in 2016.^[10]

The precise stellar parameters vary from publication to publication, but the star is considered to be part of a group of evolved roAp stars with long pulsation periods, alongside β CrB A and HD 116114. The existence of this group implies a systematic shift of rare-earth emission line anomalies as roAp stars age.^[7]