HK Tauri

HK Tauri is a young binary star system in the constellation of Taurus about 434 light-years away, belonging to the Taurus Molecular Cloud.

HK Tauri

A light curve for HK Tauri, adapted from Roggero et al. (2021)[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Taurus
A
Right ascension	04h 31m 50.5716s[2]
Declination	+24° 24′ 17.775″[2]
Apparent magnitude (V)	15.10
B
Right ascension	04h 31m 50.6002s[3]
Declination	+24° 24′ 15.503″[3]
Characteristics
HK Tauri A
Evolutionary stage	pre-main-sequence star
Spectral type	M1.5[4]
Apparent magnitude (G)	14.106[2]
Variable type	T Tau
HK Tauri B
Spectral type	M2[4]
Apparent magnitude (G)	17.962[3]
Astrometry
HK Tauri A
Proper motion (μ)	RA: 5.076[2] mas/yr Dec.: −22.944[2] mas/yr
Parallax (π)	7.6247±0.0317 mas[2]
Distance	428 ± 2 ly (131.2 ± 0.5 pc)
HK Tauri B
Proper motion (μ)	RA: 2.668[3] mas/yr Dec.: −20.457[3] mas/yr
Parallax (π)	7.780±0.6322 mas[3]
Distance	420 ± 30 ly (130 ± 10 pc)
Position (relative to HK Tauri A)[4]
Component	HK Tauri B
Angular distance	2.32″
Position angle	170.4°
Projected separation	309 AU
Details
A
Mass	0.44+0.14 −0.11[4] M☉
Luminosity	0.56[5] L☉
Temperature	3680±150[6] K
B
Mass	0.37±0.2[4] M☉
Luminosity	0.42[5] L☉
Temperature	3550±150[6] K
Other designations
2MASS J04315056+2424180, WISE J043150.56+242417.6
HK Tauri A: Gaia EDR3 147847072275324416, Gaia DR2 147847072275324416
HK Tauri B: Gaia EDR3 147847072275766656, Gaia DR2 147847072274696704
Database references
SIMBAD	data

System

Artist's impressions of the two disks surrounding both stars

The two stars of the HK Tauri system are separated by 2.32″, equivalent to 309 AU at the distance of HK Tauri. The primary is a pre-main sequence star with a mass of 0.44 M_☉, while the secondary has a mass of 0.37 M_☉.^[4]

Properties

Both members of the binary are medium-mass objects still contracting towards the main sequence and accreting mass. Their ages are probably young (below 10 million years) but cannot be estimated with any accuracy because both stars are strongly obscured by the protoplanetary disks.^[6]

Protoplanetary system

HK Tauri, with the edge-on disk around the B component with Hubble

The companion star HK Tauri B is surrounded by a protoplanetary disk visible nearly edge-on. It contains water and carbon dioxide ices, along with gaseous carbon monoxide.^[7] The disk is unusually flat, with an aspect ratio of 4.4, while most young stars host disks with aspect ratios of about 3.^[8] The disk also contain relatively few large dust particles compared to fine dust, with a size distribution power-law slope of 4.2. The disk mass is relatively small, not larger than 0.0005M_☉, and dust distribution is asymmetric.^[9] The plane of the disk is not aligned with the orbit of the binary.^[10]

Multiple planets embedded in the disk of HK Tauri B have been suspected since 1993,^[11] although none were detected by 2020.^[12]

The HK Tauri A planetary system^[4]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
protoplanetary disk	0–28.7 AU				56.9±0.5°	—

The HK Tauri B planetary system^[4]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
protoplanetary disk	0–68.0 AU				83.2±0.2°	—