TW Hydrae

TW Hydrae is a T Tauri star approximately 196 light-years away^[1] in the constellation of Hydra (the Sea Serpent). TW Hydrae is about 80% of the mass of the Sun, but is only about 5-10 million years old. The star appears to be accreting from a protoplanetary disk of dust and gas, oriented face-on to Earth, which has been resolved in images from the ALMA observatory. TW Hydrae is accompanied by about twenty other low-mass stars with similar ages and spatial motions, comprising the "TW Hydrae association" or TWA, one of the closest regions of recent "fossil" star-formation to the Sun.

TW Hydrae

Inner region of TW Hydrae protoplanetary disc Credit: S. Andrews, B. Saxton, ALMA (see description)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Hydra
Right ascension	11h 01m 51.9054s[1]
Declination	−34° 42′ 17.0316″[1]>
Apparent magnitude (V)	11.27 ± 0.09[2]
Characteristics
Evolutionary stage	Pre-main-sequence
Spectral type	K6[2]
U−B color index	-0.33[3]
B−V color index	0.67[2]
J−H color index	0.659[2]
J−K color index	0.92[2]
Variable type	T Tauri
Astrometry
Radial velocity (Rv)	13.40 ± 0.8[2] km/s
Proper motion (μ)	RA: −68.389 ± 0.054[1] mas/yr Dec.: −14.016 ± 0.059[1] mas/yr
Parallax (π)	16.6428±0.0416 mas[1]
Distance	196.0 ± 0.5 ly (60.1 ± 0.2 pc)
Details
Mass	0.8[4] M☉
Radius	1.11[5] R☉
Luminosity (bolometric)	0.28[note 1] L☉
Temperature	4,000[5] K
Age	8[5] Myr
Other designations
TWA 1, TW Hya, CD−34° 7151, HIP 53911
Database references
SIMBAD	data

Stellar characteristics

A broadband optical light curve plotted from MOST microsatellite data published by Rucinski et al. (2008)^[6]

TW Hydrae is a pre-main-sequence star that is approximately 80% the mass of and 111% the radius of the Sun. It has a temperature of 4000 K and is about 8 million years old. In comparison, the Sun is about 4.6 billion years old^[7] and has a temperature of 5778 K.^[8] The star's luminosity is 28% (0.28x) that of the Sun, equivalent to that of a main-sequence star of spectral type ~K2. However, the spectral class is K6.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 11.27. It is too dim to be seen with the naked eye.

Planetary system

The TW Hydrae planetary system^[9][10]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b (unconfirmed)	23.72 M🜨	22	—	—	—	~4.25 R🜨
c (unconfirmed)	4 M🜨	42	—	—	—	—
Protoplanetary disk	1–30? AU				—	—

The star is known to host one likely exoplanet, TW Hydrae b.

Protoplanetary disk

Previously disproven protoplanet

In December 2007, a team led by Johny Setiawan of the Max Planck Institute for Astronomy in Heidelberg, Germany announced discovery of a planet orbiting TW Hydrae, dubbed "TW Hydrae b" with a minimum mass around 1.2 Jupiter masses, a period of 3.56 days, and an orbital radius of 0.04 astronomical units (inside the inner rim of the protoplanetary disk). Assuming it orbits in the same plane as the outer part of the dust disk (inclination 7±1°^[11]), it has a true mass of 9.8±3.3 Jupiter masses.^[11][12] However, if the inclination is similar to the inner part of the dust disk (4.3±1.0°^[13]), the mass would be 16⁺⁵
₋₃ Jupiter masses, making it a brown dwarf.^[13] Since the star itself is so young, it was presumed this is the youngest extrasolar planet yet discovered, and essentially still in formation.^[14]

In 2008 a team of Spanish researchers concluded that the planet does not exist: the radial velocity variations were not consistent when observed at different wavelengths, which would not occur if the origin of the radial velocity variations was caused by an orbiting planet. Instead, the data was better modelled by starspots on TW Hydrae's surface passing in and out of view as the star rotates. "Results support the spot scenario rather than the presence of a hot Jupiter around TW Hya".^[15] Similar wavelength-dependent radial velocity variations, also caused by starspots, have been detected on other T Tauri stars.^[16]

New study of more distant planet

In 2016, ALMA found evidence that a possible Neptune-like planet was forming in its disk, at a distance of around 22 AU.^[17]

Outflow of an embedded protoplanet

In 2024 observations with ALMA showed sulfur monoxide representing an outflow from an embedded protoplanet. The position of the brightest emission coincides with a planet-carved dust gap at 42 au. Previously this gap was associated with the formation of a super-earth and modelling of the outflow velocity, the researchers estimate a mass of about 4 earth-masses. The mass accretion of this embedded protoplanet is constrained to between 3 x10⁻⁷ and 10⁻⁵ M_J/year.^[10]

Detection of methanol

In 2016, methanol, one of the building blocks for life, was detected in the star's protoplanetary disk.^{[18][19][20][21][22][23][24][25]}

Gallery

• 
  Shadow moving counterclockwise around a gas and dust disc.^[26]
• 
  Simulation based on Hubble observations.
• 
  TW Hydrae protoplanetary disc.
• 
  This illustration is based on NASA/ESA Hubble Space Telescope images of a gas and dust discs encircling the young star TW Hydrae.
• This artist’s impression video shows the protoplanetary disc around TW Hydrae. The organic molecule methyl alcohol (methanol) has been found in this disc.

Notes

1. From ${\displaystyle {\begin{smallmatrix}L=4\pi R^{2}\sigma T_{\rm {eff}}^{4}\end{smallmatrix}}}$, where ${\displaystyle {\begin{smallmatrix}L\end{smallmatrix}}}$ is the luminosity, ${\displaystyle {\begin{smallmatrix}R\end{smallmatrix}}}$ is the radius, ${\displaystyle {\begin{smallmatrix}T_{\rm {eff}}\end{smallmatrix}}}$ is the effective surface temperature and ${\displaystyle {\begin{smallmatrix}\sigma \end{smallmatrix}}}$ is the Stefan–Boltzmann constant.