Searching for Gas Giant Planets on Solar System Scales: VLT NACO/APP
  Observations of the Debris Disk Host Stars HD172555 and HD115892
  Using the APP coronagraph of VLT/NACO we searched for planetary mass
companions around HD115892 and HD172555 in the thermal infrared at 4 micron.
Both objects harbor unusually luminous debris disks for their age and it has
been suggested that small dust grains were produced recently in transient
events (e.g., a collision) in these systems. Such a collision of planetesimals
or protoplanets could have been dynamically triggered by yet unseen companions.
We did not detect any companions in our images but derived the following
detection limits: For both objects we would have detected companions with
apparent magnitudes between ~13.2-14.1 mag at angular separations between 0.4-
1.0" at the 5-sigma level. For HD115892 we were sensitive to companions with
12.1 mag even at 0.3". Using theoretical models these magnitudes are converted
into mass limits. For HD115892 we would have detected objects with 10-15 M_Jup
at angular separations between 0.4-1.0" (7-18 AU). At 0.3" (~5.5 AU) the
detection limit was ~25 M_Jup. For HD172555 we reached detection limits between
2-3 M_Jup at separations between 0.5-1.0" (15-29 AU). At 0.4" (~11 AU) the
detection limit was ~4 M_Jup. Despite the non-detections our data demonstrate
the unprecedented contrast performance of NACO/APP in the thermal infrared at
very small inner working angles and we show that our observations are mostly
background limited at separation >0.5".
