TW Hydrae

Starbox short
name = TW Hydrae
epoch = J2000.0
constell = Hydra
ra = RA|11|01|52
dec = DEC|-34|42|17
appmag_v = 11.1
absmag_v =
dist_ly = 176.245
dist_pc = 54 ± 5
spectral = K8V
names = GSC 07208-00347, HIP 53911
Simbad = TW+Hya

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 similar in mass to 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 space 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

David Wilner, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, began examining 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 swaths of pebbles extending outward for at least one billion miles. The pebbles could be the stuff that future planets will be made of. The planet formation process, according to common 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. [cite journal
author = Wilner, D. J.; D'Alessio, P.; Calvet, N.; Claussen, M. J.; Hartmann, L.
year = 2005
journal = The Astrophysical Journal
title = Toward Planetesimals in the Disk around TW Hydrae: 3.5 Centimeter Dust Emission
volume = 626
issue = 2
pages = L109–L112
url = http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2005ApJ...626L.109W&db_key=AST&nosetcookie=1
doi = 10.1086/431757]

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.

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°cite journal|journal=Nature|volume= 451|pages= 38–41|month=3 January| year=2008| doi=10.1038/nature06426| title=A young massive planet in a star–disk system| first=J.| last= Setiawan| coauthors= Th. Henning, R. Launhardt, A. Müller, P. Weise & M. Kürster| url=http://www.nature.com/nature/journal/v451/n7174/abs/nature06426.html| format=abstract] ), it has a true mass of 9.8±3.3 Jupiter masses. [cite news| url=http://space.newscientist.com/article/dn13135-first-planet-discovered-around-a-youthful-star.html | title=First planet discovered around a youthful star| date=02 January 2008| publisher= NewScientist.com news service| first= Maggie| last= McKee| accessdate=2008-01-02] However if the inclination is similar to the inner part of the dust disk (4.3±1.0°cite arXiv|eprint=0805.3314|class=astro-ph|title=Spectro-astrometric imaging of molecular gas within protoplanetary disk gaps|author=Pontoppidan, Klaus M. "et al."|year=2008|accessdate=2008-09-10] ), the mass would be 16±|5|3 Jupiter masses, making it a brown dwarf. Since the star itself is so young, it was presumed this is the youngest extrasolar planet yet discovered, and essentially still in formation. [cite web|url=http://www.mpia.de/Public/menu_q2e.php?Aktuelles/PR/2008/PR080103/PR_080103_en.html|title=A young extrasolar planet in its cosmic nursery: Astronomers from Heidelberg discover planet in a dusty disk around a newborn star|date=2008-01-02|publisher=Max Planck Institute for Astronomy|accessdate=2008-01-03]

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". [cite arXiv|author=Huelamo, N. "et al."|eprint=0808.2386|version=v1|class=astro-ph|title=TW Hydrae: evidence of stellar spots instead of a Hot Jupiter|year=2008|accessdate=2008-10-02] Similar wavelength-dependent radial velocity variations, also caused by starspots have been detected on other T Tauri stars. [cite arXiv|author=Prato, L. "et al."|title=A Young Planet Search in Visible and IR Light: DN Tau, V836 Tau, and V827 Tau|eprint=0809.3599|class=astro-ph|version=v1|year=2008|accessdate=2008-10-02]

References

External links

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