A Low Mass H 2 Component to the AU Microscopii Circumstellar Disk Kevin France – CITA/U Toronto Aki Roberge – GSFC Roxana Lupu – JHU Seth Redfield – U Texas/Hubble Paul Feldman – JHU

Circumstellar Material 850  m dust emission Vega F disk Aumann et al. 84 Heap et al. 00 Holland et al. 98 Fomalhaut Vega  Pic Optical scattered light

Dust in Circumstellar Disks J-band coronagraphy Liu et al. 04  Pic Fomalhaut Kalas et al. 05 Beuzit/Mouillet et al. 97 AU Mic

H 2 H 2 and Dust in Circumstellar Disks Thi et al. 01 (  m) Roberge et al. 00  Pic CO AB Aur  Pic Gas in circumstellar disks has been a subject of debate. Gas to form planets vs. dissipation timescale Roberge et al. 01, Lecavelier des Etangs 01

Debris disk: AU Microscopii M1 V flare star d=10 pc Disk discovered in scattered light 850  m SCUBA Polarization map Kalas et al. 04 Liu et al. 04 Graham et al. 07 Krist et al., 05 ACS

Graham et al. 07 Roberge et al., 05 ACS Presence of dust well-established, detection of gas has been elusive CO and H 2 non- detections From CO: N(H 2 ) < 6.3 x cm -2 UV Absorption: N(H 2 ) < 1.7 x cm -2 Liu et al., 04 Roberge et al., 05 AU Mic

Far-UV Fluorescent H 2 Emission Lines in AU Mic Detected by FUSE

Weak H 2 emission lines detected (2 – 3  ) Upper level populated by coincidence with O VI 1032 [C – X, (1 – 1), Q(3) ] From H 2 Emission: N(H 2 ) = 1.9 x x cm -2

From H 2 Emission: N(H 2 ) = 1.9 x – 2.8 x cm -2 T(H 2 ) = 800 – 2000 K from OVI and not Ly  excitation N(H 2 )

Spitzer Prediction Data + (H 2 x 10 5 ) (  m) F Spitzer Data Consistent with all observed H 2 in the disk IR detection unlikely

Discussion N(H 2 ) / Disk Origin Required absorption column consistent with Roberge et al non-detection T(H 2 ) / Disk Origin IF T(H 2 )>2000K, Ly  -fluorescence should dominate the disk emission spectrum Stellar regions all > 2000K Photoelectric heating/grain- formation heating in the disk can produce T(H 2 )~1000K H 2 Mass – 4x10 -4 – 6x10 -6 M  Gas-to-Dust – (4x10 -2 – 5x10 -4 ) : 1 typical ISM ≈ 100:1

Discussion Comparison with  Pic (A5V): Approx coeval, ~12Myr M(  Pic) ~ 3.6 M(AU Mic) Disks have dust mass  stellar mass M D (  Pic) ~ 3.4 M D (AU Mic) Upper Limit on  Pic gas-to-dust: <3:1 Why are fluorescent H 2 lines not seen? 1) Stellar Activity - OVI line strength 2) Far-UV Continuum -  Pic extends to ≤ 1100 Å, while AU Mic has no flux in this region : contrast for weak lines

Conclusions H 2 Emission measured in the AU Mic circumstellar disk Gas is warm: T(H 2 ) = 800 – 2000 K Gas Mass: 4x10 -4 – 6x10 -6 M  Disk Gas-to-Dust Ratio: (4x10 -2 – 5x10 -4 ) : 1 IR detection will be challenging

H 2 H 2 and Dust in Circumstellar Media (  m) Pascussi 06 here Pascucci et al. 06 Thi et al. 01 (  m)