Young star TW Hydrae: the closest protoplanetary disk


Images (see links to credits): NASA/HST (left) and ESO/ALMA (right) images of the pole-on disk. A planet with between 6 and 28 Earth masses may be forming in the gap seen in the disk in the Hubble image, around 80 AU from TW Hydrae itself (the central hole in this image is a result of coronagraphic imaging, where the light from the central star is blocked out within the instrument itself). TW Hydrae is a K8 dwarf of 0.55 solar masses, rather more sun-like than the dim red M dwarfs, has an age less than 10 Myr, and lies 54 pc distant. The orbit of Neptune is just a little larger than the central hole in the ALMA image, whose radius is ~ 30 AU. This hole is real, and the inner edge marks the “snow-line” beyond which molecules, CO in this case, condense into solid ices. For a given ice – carbon monoxide, methane, water, carbon dioxide – the distance from the star where this occurs will depend on the temperature at which the bulk material becomes solid. In this case a rare radical, fragile diazenylium, is easily observed and traces CO:

Instead of looking for the snow — as it cannot be observed directly — they searched for a molecule known as diazenylium (N2H+), which shines brightly in the millimeter portion of the spectrum, and so is a perfect target for a telescope such as ALMA. The fragile molecule is easily destroyed in the presence of carbon monoxide gas, so would only appear in detectable amounts in regions where carbon monoxide had become snow and could no longer destroy it. In essence, the key to finding carbon monoxide snow lies in finding diazenylium.

TW Hydrae is the nearest accreting T Tauri star to the Sun. The T Tauri phase in the evolution of a protostar roughly corresponds to the appearance of the star in visible light, such that it can be placed on the Hertzsprung-Russell diagram. At earlier epochs, the protostar is embedded within the material from which it is forming, and it can only be observed in the infrared. Astronomers classify protostars in classes, Class 0 being deeply embedded and the T Tauri phase roughly coincident with Class III. T Tauri stars show emission lines in their spectra, which indicate the presence of a disk. At earlier stages, and depending on the changing amount of obscuration by circumstellar material as the star evolves (veiling) and the optical depth of this material, absorption lines from the photosphere itself may or may not be observable. If they are, the star may be classified by spectral type on the HR diagram, allowing comparison of its temperature and radius with theoretical models at different ages.

As I have noted before on this blog, recent Herschel observations of TW Hydrae, in the far-infrared, have been able to confirm that the disk is massive, indicating 50 Jupiter masses of potential planet-forming material. Even more recent observations at these wavelengths have revealed the debris disks around a sizeable fraction of nearby FGK stars. These disks are the remnant material left behind after planetary system formation, consist of cold dust which emits in the far IR, as well as grains and likely larger bodies, analogous to the Kuiper belt in the solar system.

Update: Inner system dust in 5 – 20 Myr young A stars


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