A. Moór¹, A. Juhász², Á. Kóspál^3,10, P. Ábrahám¹, D. Apai⁴, T. Csengeri⁵, C. Grady^6,7, Th. Henning⁸, A. M. Hughes⁹, Cs. Kiss¹, I. Pascucci⁴, M. Schmalzl², and K. Gabányi¹

¹Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, P.O. Box 67, H-1525 Budapest, Hungary
²Leiden Observatory, Leiden University, Niels Bohrweg 2, NL-2333-CA Leiden, The Netherlands
³Research and Scientific Support Department, European Space Agency (ESA-ESTEC, SRE-SA), P.O. Box 299, 2200-AG Noordwijk, The Netherlands
⁴Department of Astronomy and Department of Planetary Sciences, The University of Arizona, Tucson, AZ 85721, USA
⁵Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁶NASA Goddard Space Flight Center, Code 667, Greenbelt, MD 20771, USA
⁷Eureka Scientific, 2452 Delmer Street, Suite 100, Oakland, CA 94602, USA
⁸Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁹Wesleyan University Department of Astronomy, Van Vleck Observatory, 96 Foss Hill Dr., Midletown, CT 06457, USA
¹⁰ESA fellow.

Circumstellar disks around stars older than 10 Myr are expected to be gas-poor. There are, however, two examples of old (30-40 Myr) debris-like disks containing a detectable amount of cold CO gas. Here we present Atacama Large Millimeter/Submillimeter Array (ALMA) and Herschel Space Observatory observations of one of these disks, around HD 21997, and study the distribution and origin of the dust and its connection to the gas. Our ALMA continuum images at 886 μm clearly resolve a broad ring of emission within a diameter of ~4.”5, adding HD 21997 to the dozen debris disks resolved at (sub)millimeter wavelengths. Modeling the morphology of the ALMA image with a radiative transfer code suggests inner and outer radii of ~55 and ~150 AU, and a dust mass of 0.09 M _⊕. Our data and modeling hints at an extended cold outskirt of the ring. Comparison with the morphology of the CO gas in the disk reveals an inner dust-free hole where gas nevertheless can be detected. Based on dust grain lifetimes, we propose that the dust content of this gaseous disk is of secondary origin and is produced by planetesimals. Since the gas component is probably primordial, HD 21997 is one of the first known examples of a hybrid circumstellar disk, a thus-far little studied late phase of circumstellar disk evolution.

Reference
Moór A, Juhász A, Kóspál Á, Ábrahám P, Apai D, Csengeri T, Grady C, Henning Th, Hughes AM, Kiss Cs, Pascucci I, Schmalzl M, and Gabányi K (in press) ALMA Continuum Observations of a 30 Myr Old Gaseous Debris Disk around HD 21997. The Astrophysical Journal – Letters
[doi:10.1088/2041-8205/777/2/L25]

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