Free Astronomy Magazine September-October 2014

PLANETOLOGY their brightness is maximum at wave- lengths greater than those currently uti- lized to study them (thus beyond the near-infrared), and it is for this reason that Eric Jensen (Department of Physics & As- tronomy, Swarthmore, Pennsylvania) and Rachel Akeson (NASA Exoplanet Science In- stitute, IPAC/Caltech, Pasadena, California) decided to begin observations of a good candidate with the Atacama Large Millime- ter/submillimeter Array (ALMA) of ESO. The candidate is the HK Tauri system, a pair of young stars 525 light-years away, in the Taurus constellation. Listed as a variable star, it actually consists of two components, HK Tau A and HK Tau B, located 2.4 arcseconds away from each other (it is a projected separation), which –at that distance– correspond to 386 astro- nomical units (AU), or nearly 58 billion km. The age of the system is estimated to be 1-4 million years, age range within which planets form. Curiously, when a luminous contribution was identified in the system, later rightly attributed to a gas and dust disk (the second disk was yet to be discov- ered), the new source was named HK Tau C, as if it were a third stellar component. The southernmost of the two stars, HK Tau B, appears fainter because the gas and dust disk surrounding it presents itself nearly edgewise to us and obscures the starlight. This particular arrangement means that the disk, by absorbing and re-emitting a con- siderable amount of visible light, can be easily detected in diffused light, both in the visible and in the near-infrared domain. Also the northernmost star, HK Tau A, is surrounded at millimetric wavelengths by a protoplanetary disk, but as the latter does not block as effectively the starlight (it is not turned edgewise to us), it cannot be seen in diffused light, since blotted out by the dazzling brilliance of the star. That the two disks were misaligned with each other and with respect to the orbital plane of the stars was more than a suspi- cion (also statistically a good alignment is considered unlikely), but no one had ever been able to accurately measure the de- gree of misalignment, due to the impossi- bility to achieve the necessary spatial reso- T he most de- tailed picture of the binary sys- tem HK Tauri available today, obtained by com- bining images taken in visible light and in the infrared by Hub- ble Space Tele- scope with data produced by ALMA. [B. Saxton (NRAO/AUI/NSF); K. Stapelfeldt et al. (NASA/ESA Hubble)]