V1298 Tauri

V1298 Tauri is a young (23±4 Myr) weakly-lined T Tauri star^[5] that is part of the Taurus-Auriga association in the Taurus Molecular Cloud. Alternatively it is part of a proposed moving group, called Group 29 (or 93 Tau group) that is slightly older.^[6][7][3][8] The system has four transiting exoplanets, discovered with the Kepler space telescope in the K2 mission.^[5] One of the planets was discovered in August 2019^[3] and the other three were discovered in November 2019 by the same team.^[5]

V1298 Tauri

The planetary system V1298 Tauri Credit: Exoplanet Exploration Program and the Jet Propulsion Laboratory for NASA’s Astrophysics Division
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Taurus
Right ascension	04h 05m 19.59121s[1]
Declination	+20° 09′ 25.5635″[1]
Apparent magnitude (V)	10.31 - 10.43[2]
Characteristics
Spectral type	K0-K1.5[3]
Variable type	Irregular[2]
Astrometry
Proper motion (μ)	RA: 5.228 ± 0.131[1] mas/yr Dec.: -16.077 ± 0.048[1] mas/yr
Parallax (π)	9.2139±0.0593 mas[1]
Distance	354 ± 2 ly (108.5 ± 0.7 pc)
Details
Mass	1.095+0.049 −0.047[4] M☉
Radius	1.33+0.04 −0.03[4] R☉
Luminosity	0.934 ± 0.044[3] L☉
Temperature	4970 ± 120[3] K
Rotation	2.97+0.03 −0.04 d[4]
Age	23 ± 4[3] Myr
Other designations
K2-309, 2MASS J04051959+2009256, BD+19 656, EPIC 210818897, RX J0405.3+2009, 1SWASP J040519.59+200925.5
Database references
SIMBAD	data

Stellar characteristics

A light curve for V1298 Tauri, adapted from David et al. (2019)^[3]

V1298 Tauri has a spectral type of K0 - K1.5 and it has a mass of about 1.1 M_☉. The star appears in x-rays from ROSAT data and it does show strong lithium absorption lines, both signatures of youth and therefore it was a proposed member of Taurus-Auriga. On the other hand it does not show signs of accretion and it lacks infrared excess. Instead it shows H-alpha in absorption.^[3]

In 2007, Konstantin Nikolaevich Grankin et al. announced their discovery that V1298 Tauri is a variable star.^[9] It was given its variable star designation in 2011.^[10] The brightness of V1298 Tauri varies in an unpredictable way between a maximum visual magnitude of 10.31 and a minimum of 10.54.^[2] The light curve of the star shows quasi-periodic variability that was interpreted as stellar rotation and starspots. The light curve also showed several flares.^[3]

Based on Gaia DR2 data this star is part of a co-moving pair, together with HD 284154.^[6] The star is included in an analysis of the 93 Tau group, which finds an age of 35 ±5 Myrs.^[8]

Planetary system

V1298 Tauri has four confirmed planets of which planets c, d and b are near a 1:2:3 resonance (with periods of 8.25, 12.40 and 24.14 days). Planet e only shows a single transit in the K2 light curve and has a period larger than 36 days. Planet e might be in a low-order resonance (of 2:3, 3:5, 1:2, or 1:3) with planet b. The system is very young and might be a precursor of a compact multiplanet system. The 2:3 resonance suggests that some close-in planets may either form in resonances or evolve into them on timescales of less than 10 Myr. The planets in the system have a size between Neptune and Saturn. Only planet b has a size similar to Jupiter.^[5]

Models predict that the planets have a minimum core mass of 5 M_🜨 and are surrounded by a thick envelope that make up 20% of their mass. The total mass of planet c and d was predicted to be 2 - 28 M_🜨 and the total mass of planet d and b was predicted to be 9 - 120 M_🜨.^[5] In a follow-up paper the mass of V1298 Tauri b was constrained to <2.2 M_J.^[11] The planet c was suspected to be shedding mass due to intense irradiation by the host star, but hydrogen tail existence was refuted by 2021.^[12]

Orbits of the planets b and c are nearly coplanar and planet c is not inclined to the equatorial plane of the star, misalignment equals to 2⁺¹²
₋₄ degrees.^[13]

Planet b was observed with Hubble WFC3 and a transmission spectrum was produced. This observation found a clear primordial atmosphere and water vapor absorption. The mass was constrained from this observation to less than 23 earth-masses, making this planet one of the lowest density planet observed. The team retrieved a low metallicity for the atmosphere, challenging formation mechanisms. The planet will likely evolve into a sub-Neptune in the future.^[14] Comparison between the transmission spectrum of planet b and a newly obtained WFC3 transmission spectrum for planet c found that planet b has a large, haze-free envelope. For planet c hazes could not be ruled out. The masses were constrained for planet b to below 20 earth masses and for planet c to 17+13
−6 earth masses. An ongoing transit-timing variation study suggest that both planet b and c are in the mass range of super-Earth to sub-Neptune planets and will evolve into these types of planet.^[15]

Planet e could be a planet with a water-rich core and an substantial hydrogen envelope.^[16] Planet e was possibly detected by Kepler, TESS and CHEOPS in three transits with an orbital period of around 45 days. The transits have different depths, lengths and maybe has TTVs of a few hours. Alternatively CHEOPS could have detected a fifth planet.^[17]

The V 1298 Tauri planetary system^{[5][11][16][15]}

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
c	17+13 −6 M🜨	0.0825 ± 0.0013	8.24958 ± 0.00072	<0.43	88.49+0.92 −0.72°	0.499+0.032 −0.029 RJ
d	<41 M🜨	0.1083 ± 0.0017	12.4032 ± 0.0015	<0.21	89.04+0.65 −0.73°	0.572+0.040 −0.035 RJ
b	<20 M🜨	0.1688 ± 0.0026	24.1396 ± 0.0018	<0.29	89.00+0.46 −0.24°	0.916+0.052 −0.047 RJ
e	0.66 ± 0.26 MJ	0.308+0.182 −0.066	50.29±6.62[18]	<0.57	89.40+0.26 −0.18°	0.89±0.04[18] RJ

See also

• List of nearby stellar associations and moving groups
• List of exoplanets discovered in 2019
• Kepler-223 - the first confirmed system with four exoplanets in resonance
• K2-138 - a system with five exoplanets in a 3:2 resonance chain
• HD 110067 - a system with seven planets in orbital resonance