HD 146389 (also known as WASP-38), is a star with a yellow-white hue in the northern constellation of Hercules. The star was given the formal name Irena by the International Astronomical Union in January 2020.[8][9] It is invisible to the naked eye with an apparent visual magnitude of 9.4[2] The star is located at a distance of approximately 446 light years from the Sun based on parallax, but is drifting closer with a radial velocity of −9 km/s.[1] The star is known to host one exoplanet, designated WASP-38b or formally named 'Iztok'.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Hercules |
| Right ascension | 16h 15m 50.36526s[1] |
| Declination | 10° 01′ 57.2844″[1] |
| Apparent magnitude (V) | 9.447±0.024[2] |
| Characteristics | |
| Spectral type | F8[3] |
| B−V color index | 0.476 |
| J−H color index | 0.181 |
| J−K color index | 0.289 |
| Astrometry | |
| Radial velocity (Rv) | −9.06±0.53[1] km/s |
| Proper motion (μ) | RA: −31.073[1] mas/yr Dec.: −39.171[1] mas/yr |
| Parallax (π) | 7.3115 ± 0.0429 mas[1] |
| Distance | 446 ± 3 ly (136.8 ± 0.8 pc) |
| Details | |
| Mass | 1.203±0.036[4] M☉ |
| Radius | 1.331+0.030 −0.025[4] R☉ |
| Luminosity | 2.838±0.024[1] L☉ |
| Surface gravity (log g) | 4.25+0.012 −0.013[2] cgs |
| Temperature | 6,150±80[4] K |
| Metallicity [Fe/H] | 0.06[5] dex |
| Rotational velocity (v sin i) | 8.6±0.4[2] km/s |
| Age | 350 Myr[5] 400±500[6] Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
The stellar classification of HD 146389 is F8,[3] which is an F-type star of uncertain luminosity class. The age of the star is uncertain. It shows a low lithium abundance, which suggests an age of more than 5 billion years. However, the rotation rate indicates an age closer to one billion.[2] The study in 2015 utilizing Chandra X-ray Observatory, have failed to detect any X-ray emissions from the star during planetary eclipse, which may indicate an unusually low coronal activity or the presence of absorbing gas ring formed by atmosphere escaping planet WASP-38 b.[10] The star is 33% larger and 20% more massive than the Sun.[4] It is radiating nearly three[1] times the luminosity of the Sun at an effective temperature of 6,150 K.[4]
Planetary system
The "hot Jupiter" class planet WASP-38 b, later named 'Iztok', was discovered around HD 146389 in 2010.[2] The planet is losing significant amount of gas, estimated to 0.023 Earth masses per billion years.[3] In 2013, it was found the planetary orbit is surprisingly well aligned with the rotational axis of the parent star, despite the noticeable orbital eccentricity.[11][4]
A 2012 study, utilizing a Rossiter–McLaughlin effect, have determined the orbital plane of WASP-38b is poorly constrained but probably aligned with the equatorial plane of the star, misalignment equal to 15+33
−43°.[12]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b (Iztok) | 2.691±0.036 MJ | 0.07522+0.00074 −0.00075 | 6.871815+0.000045 −0.000042 | 0.0314+0.0046 −0.0041 | 89.69+0.3 −0.25° | 1.094+0.029 −0.028 RJ |