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Astro: ALMA images planet-forming disk around HL Tau

Posted 12/5/2014 12:12 AM by Ka Chun Yu | Comments

141105 ALMA HL 03 560

The dusty protoplanetary disk around the young star HL Tau compared with the Solar System at the same scale. The gaps in the disk are thought to have been created by growing planets which are sweeping up gas and dust. The orbits of Mars, Jupiter, Saturn, Uranus, and Neptune are visible in the depiction to the right.

Radio astronomers at the Atacama Large Millimeter/submillimeter Array (ALMA), located on the 16,000 foot high Chajnantor plain of the Atacama Desert in Chile, released the above left image of the star and planet forming disk surrounding the protostar HL Tau last month. You can find more pictures and descriptions from press releases at the European Southern Observatory (ESO) and the National Radio Astronomy Observatory (NRAO).

HL Tau is one protostar amongst many in the Lynds 1551 (or L1551) dark cloud, a cloud of cold molecular gas about 3 light years across, and located about 450 light years away in the constellation of Taurus the Bull. As you can see in this sky map, it's just off of the bright red giant star Aldebaran which makes up one of the eyes of the Bull.

Eso1436h 560

When you zoom into the cloud, you will find a cluster of young protostars within the inner 10 arcminutes. These objects -- HL Tau, XZ Tau, LkHα, HH 30, L1551 NE, and L1551 IRS5 -- have been heavily studied.  See for instance this image taken at near-infrared wavelengths by Hayashi & Pyo (2009; Astrophysical Journal, 694, 582-592), which shows the jets of gas erupting from the young protostars, and the shocked emission from jets plowing into the dark molecular gas that these stars are embedded in.

Hayashipyo09 Fig1 560

An even higher resolution image was taken with the Hubble Space Telescope which shows more detail:

L1551 Eso Annotated 560

including the nearly edge-on disk and twin jets of HH 30. Below is an animation of the jets from HH 30 consisting of images taken from Hubble's Wide Field and Planetary Camera 2 over six years. The disk is the dark band running left-to-right, which obscures and divides the bright crescent-shaped emission above and below. These "bowls" are cavities excavated out of the parent molecular cloud by the jet over time.

Hs 2000 32 B Animated Gif

ALMA is an array of 66 telescopes observing at millimeter and submillimeter wavelengths. Each of the telescope dishes are 12 meters or 7 meters in diameter, and can be moved around to different locations at the site (note the triangular pad in the lower left corner of the picture below). The radiation recorded by each dish is combined together to produce the final image. The resolution (or detail that can be resolved) by the array is determined by the diameter of the array, with the more widespread the dishes, the better the resolution.  When the 66 antennae are at their maximum extent of 16 km (10 miles), the resolution of the array is slightly better than that of the Hubble Space Telescope.

 Alma Antennae 560

For comparison, I scaled the ALMA release image with the previous record holder for best-resolution image of the HL Tau disk, taken from the CARMA (Combined Array for Research in Millimeter-wave Astronomy) interferometer in California (Kwon et al. 2011, Astrophysical Journal, 741, 3):

HL Tau ALMA CARMA Comp 560

The white egg below the CARMA image represents the resolution of that image, which is relatively fat because the CARMA dishes can be placed at most 2 km apart. The orbit of Neptune, 60 astronomical units or AU in diameter, would fit easily inside the 100 AU bar. By comparison, ALMA can see details as small as 5 AU in the image on the left. These images clearly show how having longer "baselines" between the dishes in an interferometer can result in finer resolved detail.

No peer-reviewed paper was announced in parallel with this press release. The radio astronomers were merely showing off the capabilities of the telescope array. But you can be sure that theoretical astronomers will be tweaking their models of how stars and planets formed based on this and future imagery from ALMA. 


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