# TW Hydrae

TW Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Hydra
Right ascension 11h 01m 52s[1]
Declination −34° 42′ 17″[1]
Apparent magnitude (V) 11.27 ± 0.09[1]
Characteristics
Evolutionary stage Pre-main-sequence
Spectral type K6V:e[1]
U−B color index -0.33[2]
B−V color index 0.67[1]
J−H color index 0.659[1]
J−K color index 0.92[1]
Variable type T Tauri
Astrometry
Radial velocity (Rv) 13.40 ± 0.8[1] km/s
Proper motion (μ) RA: -66.19 ± 1.85[1] mas/yr
Dec.: -13.90 ± 1.47[1] mas/yr
Parallax (π) 18.62 ± 2.14[1] mas
Distance approx. 180 ly
(approx. 54 pc)
Details
Mass 0.8[3] M
Luminosity (bolometric) 0.28[note 1] L
Temperature 4,000[4] K
Age 8[4] Myr
Other designations
Database references

TW Hydrae is an orange dwarf star approximately 176 light-years away in the constellation of Hydra (the Sea Serpent). The star is the closest T Tauri 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 Hubble Space Telescope. 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.

## Planetary system

### Protoplanetary disk

Artist’s impression of snow lines around TW Hydrae.[5]

David Wilner, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, began examining TW Hydrae in the late 1990s, enabled by the new capabilities of telescopes. In 2005 he discovered that the gaseous disk surrounding TW Hydrae holds vast swathes of pebbles extending outward for at least one billion miles. The planet formation process, according to core-accretion theory, begins when dust grains in a disk collide and accrete to form larger and larger clumps. Eventually, after millions of years of colliding and combining, the clumps form planets.

Wilner and his colleagues used the National Science Foundation (NSF)-funded Very Large Array (VLA) radio telescope to measure the strength of radio waves emitted by TW Hydrae. Based on the relationship between wavelength and particle size, they determined the grainy materials surrounding the star to be centimeter-sized.

One of the collaborators, Mark Claussen of the National Radio Astronomy Observatory, thought the strong and variable emissions detected from TW Hydrae in previous X-ray detections indicated magnetic activity common in young stars. Claussen thought that if they monitored TW Hydrae at radio wavelengths for a period of a few months, they could determine if the emissions might be strong enough to image at a much higher resolution with the NSF-funded Very Long Baseline Array and study this activity. To their surprise, they found that the radio emissions did not vary significantly.

He decided to revisit the VLA. The observatory's twenty-seven operating antennae are spread throughout the plains of San Augustin, N.M., and arranged in one of four configurations that are changed every few months. Wilner found the pebbles using a larger configuration and higher angular resolution of the VLA. He enlisted the help of Nuria Calvet, a colleague at the Center for Astrophysics, who created a computer model of the disk surrounding TW Hydrae using previously published data.[6]

Recently, Wilner collaborated with his graduate student, Meredith Hughes, and several other colleagues to identify a hole in TW Hydrae's dusty disk. Wilner says that the hole was probably created when a Jupiter-sized planet cleared that gap of much of its rocky material. This latest research was accepted for publication by the Astrophysical Journal in April 2007.

Astronomers using the Herschel Space Observatory have discovered the equivalent of thousands of times the amount of water on earth in a planet-forming ring around the star. The water was in the form of relatively cold water vapor.[7][8] The results were published in the October 21, 2011 issue of the journal Science.[9]

### 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 thet dust disk (inclination 7±1°[10]), it has a true mass of 9.8±3.3 Jupiter masses.[10][11] However if the inclination is similar to the inner part of the dust disk (4.3±1.0°[12]), the mass would be 16+5
−3
Jupiter masses, making it a brown dwarf.[12] Since the star itself is so young, it was presumed this is the youngest extrasolar planet yet discovered, and essentially still in formation.[13]

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".[14] Similar wavelength-dependent radial velocity variations, also caused by starspots, have been detected on other T Tauri stars.[15]

Based on scalings to previous hydrodynamic simulations of gap-opening criteria for embedded proto-planets, a planetary companion creating the gap could have a mass between 6–28 M (orbital radius of 80 AU).[16]

## Notes

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

## References

1. "V* TW Hya -- T Tau-type Star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2014-01-02.
2. ^ Mermilliod, J.C. (1991), Homogeneous Means in the UBV System, Institut d'Astronomie, Universite de Lausanne, Bibcode:2006yCat.2168....0M.Vizier catalog entry
3. ^ 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.
4. ^ 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
5. ^ "Snow in an Infant Planetary System". ESO Press Release. Retrieved 21 July 2013.
6. ^ Wilner, D. J.; D'Alessio, P.; Calvet, N.; Claussen, M. J.; Hartmann, L. (2005). "Toward Planetesimals in the Disk around TW Hydrae: 3.5 Centimeter Dust Emission". The Astrophysical Journal 626 (2): L109–L112. arXiv:astro-ph/0506644. Bibcode:2005ApJ...626L.109W. doi:10.1086/431757.
7. ^ Herschel Finds Oceans of Water in Disk of Nearby Star
8. ^ Herschel Finds Oceans of Water in Disk of Nearby Star
9. ^ Watery Disks by Rachel Akeson Science 21 October 2011: Vol. 334 no. 6054 pp. 316-317 DOI: 10.1126/science.1213752
10. ^ a b Setiawan, J.; Th. Henning, R. Launhardt, A. Müller, P. Weise & M. Kürster (3 January 2008). "A young massive planet in a star–disk system" (abstract). Nature 451 (7174): 38–41. Bibcode:2008Natur.451...38S. doi:10.1038/nature06426. PMID 18172492.
11. ^ McKee, Maggie (2 January 2008). "First planet discovered around a youthful star". NewScientist.com news service. Retrieved 2008-01-02.
12. ^ 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.
13. ^
14. ^ 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. Retrieved 2008-10-02.
15. ^ 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.
16. ^ The 0.5-2.22-micron Scattered Light Spectrum of the Disk Around TW Hya: Detection of a Partially Filled Disk Gap at 80 AU