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20 Years of Studying the HR 4796A Debris Ring

Twenty years ago, the Hubble Space Telescope (HST) turned its then recently installed Near InfraRed Camera and Multi-Object Spectrometer (NICMOS; an instrument designed and built under the direction of UA astronomers with Prof. R. Thompson as PI) on a 240 light-year distant, young (about 8 million years old) star known as HR 4796A. UA astronomer Glenn Schneider, using the instrument's coronagraphic camera (designed to reduce the glare of the star itself), then discovered a solar-system size bright ring of starlight-scattering debris particles surrounding the star, whose existence was suspected from prior measured thermal radiation. At the time, HR 4796A became only the second such starlight-scattering circumstellar debris disk imaged (Beta Pic is the first, as well as our own Sun's zodiacal light). The disk was thus understood as a tracer for planetary system formation and evolutionary processes in play. Since then, about 40 other such starlight-scattering debris systems have been coronagraphically imaged, but they - with HR 4796A being one of the best studied - have closely hoarded many of their secrets hidden still in the remaining blinding light of their host stars. 

Now, after two decades of intense scrutiny with the most advanced telescopes and instruments on Earth and in space, Dr. Schneider and his collaborators have revisited the HR 4796A system with a visible-light coronagraph in NICMOS's sister instrument, STIS  (Space Telescope Imaging Spectrograph), to produce the most deeply sensitive images of its circumstellar environment - with some unexpected surprises. The prior seen debris ring itself was revealed to be deeply embedded within a much larger, and very faint, starlight-scattering "halo" of inferred small particles extending from HR 4796A as far as 875 Earth-Sun distances (AU). These particles are likely created within the ring by planetesimal collisions, thus replenishing the environment, and blown outward by the star's own radiation pressure and/or the debris system's interaction with the local interstellar medium. The latter is betrayed in the new images by an enhancement in the brightness of the dust on the leading edge of the exoplanetary debris system, indicative of a bow shock as the system plows its way through interstellar space. The debris halo, additionally, is extremely asymmetric and appears truncated in the direction toward its much fainter red-dwarf companion, HR 4796B at about 580 AU, whose presence may also be influencing the distribution of the dust particles. You can see these features in the photo which can be enlarged HERE. Details of this investigation appear in a new paper in the February, 2018 Astronomical Journal (Schneider et al., AJ, 155, 77). Other coauthors with Steward connections are Andras Gaspar and Dean Hines.

The Space Telescope Science Institute press release can be found HERE.

(we thank Dr Glenn Schneider for providing the text.)


Courtesy Glenn Schneider and collaborators.