October 20, 2005 Nearly Resolved Debris Disks STScI, Baltimore Hervé Beust Laboratoire d’Astrophysique de Grenoble, France FOST group The origin of the spiral structure in the HD debris disk : Flyby of differential precession ?

October 20, 2005 Nearly Resolved Debris Disks STScI, Baltimore [HST/STIS, Visible] Mouillet et al. (2001) central gap gap 2 rings An optically thin, ~5 Myrs old dust disk (L dust /L star = ) The disk is resolved en in scattered light (visible & IR) The disk is highly structured : Brightness asymmetries, rings, gaps and spiral arms. The debris disk around HD Question : What is the origin of these structures ? Dynamical perturbations ? By the outer companions or by massive planets ? [HST/ACS, Visible] (deprojected images ) Clampin et al. (2003) M4 M2 B9.5

October 20, 2005 Nearly Resolved Debris Disks STScI, Baltimore The origin of the spiral structure The outer spiral structure is probably due to tidal interaction with the companions. Is it created by tidal interaction (Quillen et al. 2005), possibly in a single flyby (Ardila et al. 2005), or by secular perturbations (differential precession of the orbits, Augereau & Papaloizou 2004) ? Actually, if the companions are bound to the primary, we have both, but at different timescales

October 20, 2005 Nearly Resolved Debris Disks STScI, Baltimore A dynamical simulation showing both spirals A simulation with bound companions (e=0.7), using the HJS symplectic integrator for multiple systems (Beust 2003) A first spiral structure appears after the first periastron passage (~flyby) The successive periastron passages destroy it A second, m=1 structure appears after ~1 Myr and lasts several Myrs. It is due to differential precession of the orbits. The first spiral is trailing with respect to the rotation sense of the disk, while the second is leading. We do not explain the inner structure of the disk (central gap)  we need to add a planet...\films\spiral1.avi..\films\spiral2.avi

October 20, 2005 Nearly Resolved Debris Disks STScI, Baltimore Leading or trailing ? If we are able to determine the rotation sense of the disk, we can state where the spiral arms are trailing or leading. CO (Augereau et al. 2005) and IR observations (Goto et al. 2005) reveal that the south edge of the disk is approaching us, while the north edge is receding. Is this enough to fix the rotation sense ?

October 20, 2005 Nearly Resolved Debris Disks STScI, Baltimore Leading or trailing ? Observing the velocity of the gas is not enough. There are still two possible solutions. We need to state which side of the disk is in front of the observed image. TrailingLeading

October 20, 2005 Nearly Resolved Debris Disks STScI, Baltimore Forward scattering ? The east side of the disk is brighter than the west side. If we assume that forward scattering is more efficient than backwards scattering, this side is in front of the image In this case, the spiral structure is trailing. N   Brighter side Front of the image ?

October 20, 2005 Nearly Resolved Debris Disks STScI, Baltimore A flyby ? If forward scattering dominates : 1)The spiral structure is trailing 2)It is compatible with a flyby of the companions. Another argument : The heliocentric velocities –Weinberger et al (1999) : –Augereau et al. (2005) : A : -7.6±0.2 km.s -1 –Conclusion : velocity (B+C ) / A ≥ 5 km.s -1 –But : evasion velocity (A+B+C 1000 AU ≈ 2.5 km.s -1  B and C seem to be unbound to A ! This is a strong indication for a flyby. HD AHD BHD C -6±5 km.s ±1 km.s ±1.1 km.s -1

October 20, 2005 Nearly Resolved Debris Disks STScI, Baltimore However… The periastron passage must have occurred ~1000 yr ago. Are we very lucky ? The inner spiral is one-armed (m=1) and tightly wound  more compatible with the differential precession process. The age of the system (~5 Myrs) is compatible with the timescale of the differential precession. In any case, we need a Jovian planet at ~130 AU in the disk