Detection of Submillimeter-wave [C I] Emission in Gaseous Debris Disks of 49 Ceti and β Pictoris Aya Higuchi, Nami Sakai (RIKEN), Aki Sato, Takashi Tsukagoshi, Munetake Momose (Ibaraki Unv.), Kazunari Iwasaki (Osaka Unv.), Hiroshi Kobayashi, Daisuke Ishihara, Hidehiro Kaneda (Nagoya Unv.) and Satoshi Yamamoto (UT) Evolutionary stages from protoplanetary disk to debris disk ?

• Previous CO survey
•   Zuckerman et al. (1995, IRAM), Dent et al. (2005, JCMT), Hales et al. (2014, APEX+ASTE), Moor et al. (2011, 2015, APEX+IRAM). • 3/70 objects - CO detection ●    49Ceti (Zuckerman et al. 1995, Dent et al. 2005) o HD 21997 (Moor et al. 2011) o HD131835 (Moor et al. 2015) CO(B-2) JCMT  Dent et al. (2005)

• Primordial
•  Remnant gas of protoplanetary disks (e.g., Kospal et al. 2013). •  Gas composition: ISM abundance (e.g., X(CO) = 10 -4 ) as in protoplanetary disks
• Secondary
•  Sublimation of dust grains (e.g., Kobayashi et al. 2008) or planetesimals (Lagrange et al. 1998), collision of comets or icy planetesimals (Zuckerman & Song 2012). • Gas composition • CO : main gas, only a small amount of H 2 is expected. • CO : photodissociation —> C, C+
Atacama Submillimeter Telescope Experiment  (ASTE) (Sep. - Oct. 2016) •[C I]: 492.161 GHz (Band 8 receiver) ●dv=1.1km/s 49 Ceti
β Pictoris ntensity [K
If there is a large amount of H 2 molecular gas,  C will easily return to CO. H2 reaction O reaction Photodissociation e reaction Higuchi et al. 2017 Chemical reaction of CO in the interstellar medium. CO is dissociated by ultraviolet radiation to become C and C + . If there are large amount of H 2 , C + will return to CO again.
The high C/CO ratios are likely attributed to a lack of H 2 molecule  ? H2 reaction O reaction Photodissociation e reaction Higuchi et al. 2017 Chemical reaction of CO in the interstellar medium. CO is dissociated by ultraviolet radiation to become C and C + . If there are large amount of H 2 , C + will return to CO again.

• Primordial
• Remnant gas of protoplanetary disks (e.g., Kospal et al. 2013). • Gas composition: ISM abundance (e.g., X(CO) = 10% (e.g. protoplanetary disks • Secondary •  Sublimation of dust grains (e.g., Kobayashi et al. 2008) or planetesimals (Lagrange et al. 1998), collision of comets or icy planetesimals (Zuckerman & Song 2012). • Gas composition •  CO : main gas, only a small amount of H 2 is expected. ° CO : photodissociation —>  C, C+
•  We have firstly detected [C I] emissions in the gaseous debris disks of 49 Ceti and β Pictoris with the ASTE. •  The line profiles of [C I] are found to resemble those of CO(J=3–2). • This result suggests that atomic carbon (C) coexists with CO in the debris disks and is likely formed by the photodissociation of CO. • The C/CO column density ratio is thus derived to be 54 ± 19 and 69 ± 42 for 49 Ceti and β Pictoris, respectively. • The high C/CO ratios are likely attributed to a lack of H 2 molecules. o This result implies a small number of H 2 molecules in the gas disk, i.e., there is an appreciable contribution of secondary gas from dust grains.
•  (Higuchi) ALMA observation with the high resolution and sensitivity to understand the spatial and velocity distribution of [C I] emission. • Understanding the spatial distribution of [C I] emission with ALMA .  Derivation of C/CO and Gas to Dust (G/D) ratio •  (Sato) ASTE observation for increasing the number of [C I] detection sources. •  (Quentin Kral/Mark Wyatt/Luca Matra) Collaboration with UK team for following up APEX result. (Iwasaki/Kobayashi) Modeling of the photodissociation chemical reactions in debris disks (PDR calculation) for giving a constraint on the amount of hydrogen molecule in debris disks