# TW Hydrae

TW Hydrae is a T Tauri star approximately 196 light-years away[1] in the constellation of Hydra (the Sea Serpent). The star is the closest such star to the Solar System. 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 face-on protoplanetary disk of dust and gas, 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.

Observation data Epoch J2000.0      Equinox J2000.0 Constellation Inner region of TW Hydrae protoplanetary discCredit: S. Andrews, B. Saxton, ALMA (see description) Hydra 11h 01m 51.9054s[1] −34° 42′ 17.0316″[1]> 11.27 ± 0.09[2] Pre-main-sequence K6[2] -0.33[3] 0.67[2] 0.659[2] 0.92[2] T Tauri 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[1] mas Distance 196.0 ± 0.5 ly (60.1 ± 0.2 pc) 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 TWA 1, TW Hya, CD−34° 7151, HIP 53911 SIMBAD data

## Stellar characteristics

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[6] and has a temperature of 5778 K.[7] The star's luminosity is 28% that of the Sun.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 11.7. It is too dim to be seen with the naked eye.

## Planetary system

The TW Hydrae planetary system[8]
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
b 23.72 M 22 ~4.25 R
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°[9]), it has a true mass of 9.8±3.3 Jupiter masses.[9][10] However, if the inclination is similar to the inner part of the dust disk (4.3±1.0°[11]), the mass would be 16+5
−3
Jupiter masses, making it a brown dwarf.[11] Since the star itself is so young, it was presumed this is the youngest extrasolar planet yet discovered, and essentially still in formation.[12]

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".[13] Similar wavelength-dependent radial velocity variations, also caused by starspots, have been detected on other T Tauri stars.[14]

##### 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.[15]

#### Detection of methanol

In 2016, methanol, one of the building blocks for life, was detected in the star's protoplanetary disk.[16]

## 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.

## References

1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
2. "V* TW Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2014-01-02.
3. ^ Mermilliod, J.C. (1991), Homogeneous Means in the UBV System, Institut d'Astronomie, Universite de Lausanne, Bibcode:2006yCat.2168....0M.Vizier catalog entry
4. ^ Chunhua, Qi; et al. (August 2013). "Imaging of the CO Snow Line in a Solar Nebula Analog". Science. 341 (6146): 630–632. arXiv:1307.7439. Bibcode:2013Sci...341..630Q. doi:10.1126/science.1239560. PMID 23868917.
5. ^ a b c Rhee, J.H.; et al. (May 2007), "Characterization of dusty debris disks: the IRAS and Hipparcos catalogs", The Astrophysical Journal, 660 (2): 1556–1571, arXiv:astro-ph/0609555, Bibcode:2007ApJ...660.1556R, doi:10.1086/509912.Vizier catalog entry
6. ^ Fraser Cain (16 September 2008). "How Old is the Sun?". Universe Today. Retrieved 19 February 2011.
7. ^ Fraser Cain (15 September 2008). "Temperature of the Sun". Universe Today. Retrieved 19 February 2011.
8. ^ Tsukagoshi, Takashi; Nomura, Hideko; Muto, Takayuki; Kawabe, Ryohei; Ishimoto, Daiki; Kanagawa, Kazuhiro D.; Okuzumi, Satoshi; Ida, Shigeru; Walsh, Catherine; Millar, Tom J. (2016). "A Gap with a Deficit of Large Grains in the protoplanetary disk around TW Hya". The Astrophysical Journal. 829: L35. arXiv:1605.00289. Bibcode:2016ApJ...829L..35T. doi:10.3847/2041-8205/829/2/L35.
9. ^ a b Setiawan, J.; Henning, Th.; Launhardt, R.; Müller, A.; Weise, P.; Kürster, M. (3 January 2008). "A young massive planet in a star–disk system". Nature. 451 (7174): 38–41. Bibcode:2008Natur.451...38S. doi:10.1038/nature06426. PMID 18172492.
10. ^ McKee, Maggie (2 January 2008). "First planet discovered around a youthful star". NewScientist.com news service. Retrieved 2008-01-02.
11. ^ a b Pontoppidan, Klaus M.; et al. (2008). "Spectro-astrometric imaging of molecular gas within protoplanetary disk gaps". The Astrophysical Journal. 684 (2): 1323–1329. arXiv:0805.3314. Bibcode:2008ApJ...684.1323P. doi:10.1086/590400.
12. ^
13. ^ Huelamo, N.; et al. (2008). "TW Hydrae: evidence of stellar spots instead of a Hot Jupiter". Astronomy and Astrophysics. 489 (2): L9–L13. arXiv:0808.2386. Bibcode:2008A&A...489L...9H. doi:10.1051/0004-6361:200810596.
14. ^ Prato, L.; et al. (2008). "A Young Planet Search in Visible and IR Light: DN Tau, V836 Tau, and V827 Tau". The Astrophysical Journal. 687 (2): L103–L106. arXiv:0809.3599. Bibcode:2008ApJ...687L.103P. doi:10.1086/593201.
15. ^ https://www.independent.co.uk/news/science/giant-planet-born-in-stars-dust-cloud-tw-hydrae-a7307931.html
16. ^ http://www.space.com/33193-organic-molecule-planet-forming-disk.html
17. ^ "Shadow on TW Hydrae's disc". www.spacetelescope.org. Retrieved 12 January 2017.