DS Tucanae

DS Tucanae (HD 222259) is a binary star system 144 light years away in the constellation of Tucana. It has an apparent visual magnitude of 8.5,^[8] and is a RS Canum Venaticorum variable.^[5] The system is notable for being young as a member of the 45 Myr old Tucana-Horologium moving group^[10] and for the primary star hosting the confirmed exoplanet DS Tucanae Ab, discovered by THYME, using TESS.^{[11][8][12][13]}

DS Tucanae

DS Tucanae imaged with the NTT. DS Tucanae A is on the right and DS Tucanae B is on the left.
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Tucana
DS Tucanae A
Right ascension	23h 39m 39.48081s[1]
Declination	−69° 11′ 44.7077″[1]
Apparent magnitude (V)	8.47[2]
DS Tucanae B
Right ascension	23h 39m 39.26965s[3]
Declination	−69° 11′ 39.4936″[3]
Apparent magnitude (V)	9.84[4]
Characteristics
Spectral type	G6V+K3V[4]
Variable type	RS CVn[5]
Astrometry
A
Radial velocity (Rv)	7.20[6] km/s
Proper motion (μ)	RA: 79.529[1] mas/yr Dec.: −67.551[1] mas/yr
Parallax (π)	22.6367±0.0150 mas[1]
Distance	144.08 ± 0.10 ly (44.18 ± 0.03 pc)
B
Radial velocity (Rv)	5.32[7] km/s
Proper motion (μ)	RA: 78.133[3] mas/yr Dec.: −65.845[3] mas/yr
Parallax (π)	22.6524±0.0125 mas[3]
Distance	143.98 ± 0.08 ly (44.15 ± 0.02 pc)
Details
DS Tucanae A
Mass	1.01±0.06[8] M☉
Radius	0.964±0.029[8] R☉
Luminosity	0.725±0.013[8] L☉
Temperature	5,428±80[8] K
Rotational velocity (v sin i)	27.75+1.73 −1.54[9] km/s
Age	45±4[8] Myr
DS Tucanae B
Mass	0.84±0.06[8] M☉
Radius	0.864±0.036[8] R☉
Luminosity	0.327±0.010[8] L☉
Temperature	4,700±90[8] K
Age	45±4[8] Myr
Other designations
DS Tuc, HD 222259, CCDM J23397-6912AB, CD−69°2106, HIP 116748, WDS J23397-6912AB, TIC 410214986, TOI 200
Database references
SIMBAD	AB
	A
	B
	Ab

Stellar system

A light curve for DS Tucanae, plotted from TESS data^[14]

DS Tucanae is a visual binary.^[4] The binary consists of a G6V primary and a K3V secondary separated by 5″.^[15] Based on radial velocity measurements it was suggested that the secondary itself is a binary, but later studies could not find evidence for this claim.^[8]

Together with the red dwarf 2MASS J23321028–6926537, DS Tucanae might be a triple stellar system. This star shares a similar distance and similar proper motions. It is separated 113,000 astronomical units from DS Tuc.^[16]

Physical properties

High levels of magnetic activity, a strong 6708Å lithium line, and the position on the color-magnitude diagram, slightly above the main sequence, strongly support a young age of the system.^[12] The primary star is emitting a frequent and powerful (up to 5-8×10³⁴ ergs) X-ray flares.^[17]

Both components of the binary are main sequence stars. The primary has a mass very similar to the Sun, but slightly cooler and smaller, meaning it is only 72% as luminous as the Sun. The secondary is only 84% as massive as the Sun and only 33% as luminous.

Planetary system

The DS Tuc A planetary system^[8][12]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	<14.4[9] M🜨	0.18+0.13 −0.12	8.138268	—	88.73+0.18 −0.17[9]°	5.70±0.17 R🜨

DS Tuc Ab is one of the few transiting planets with an age smaller than 100 Myrs. Other examples are K2-33b, V1298 Tauri b and AU Microscopii b. Of these systems DS Tuc is the brightest and it is a good target for atmospheric characterization with JWST. The planet is a super-Neptune or sub-Saturn.^[8] The planet might be an inflated planet with an upper mass limit of 20 M_🜨.^[12] DS Tuc Ab will be observed by ESA's CHEOPS mission to characterize the planet.^[18]

The planet DS Tucanae Ab has a low orbital obliquity (λ = 2.93°+0.88°
−0.87° or λ = 12°±13°). This means that the orbital plane of this planet aligns with the stellar equator of the star. This is unusual for a short period planet. Many short period planets show high orbital obliquity, which was taken as a sign of the scattering of the planet into this short period orbit. It can also be interpreted as the formation of a planet in an inner disk with an axial tilt. But these previous measurements of orbital obliquity were made for giant planets around mature stars. DS Tucanae Ab is a relatively small young planet. This suggests that DS Tucanae Ab formed in a smooth disk that was not perturbed by the stellar companion DS Tucanae B.^[19][20] DS Tucanae Ab might therefore be a good target to study in-situ planet-formation of short-period planets.