The zero-age main sequence K-type star AB Dor, with an age of 25 to 125 Myr, is the most active young star in the solar neighbourhood. It is part of a quadruple system of young stars. The mass of AB Dor C, the closest and lowest mass companion, has been derived from astrometric observations (with the VLA and adaptive optics at the VLT) to 94+-3 times the mass of Jupiter. The low mass (close to the hydrogen burning limit) combined with the young age makes AB Dor C a unique calibration source for evolutionary tracks for very low-mass stars and brown dwarfs, provided that a precise age estimate can be derived for the system. We propose to use the HST planetary camera to obtain resolved component photometry of the M-type pre-main sequence star AB Dor Ba and Bb in order to derive individual spectral types and luminosities, which will enable us to age-date the AB Dor system to better than +-20 Myr. In addition, the observations will help to constrain the Ba/Bb orbit, and hence to derive dynamical mass estimates as well.

Seven exposures in six filters (F502N, F555W, F622W, F658N, F791W and F850LP) will be taken within a four-point dither pattern. Two exposures per dither position are taken with the F502N filter, once with a short exposure time in order not to saturate AB Dor A, and once with a long exposure time in order to achieve a good SNR for AB Dor Ba/Bb. Both AB Dor A and AB Dor Ba/Bb are included in all frames. Since a little bit of time remains in the second orbit after the dither loop has finished, we include also one exposure in the F439W filter, which can be used to estimate veiling effects of material accreting on the stars in the UV. Changed F658N filter to the correct WFPC2 H-alpha filter F656N, per the IS suggestion. COOL STARS 12 false 0.0 false false false false false false SUPPORTED STAR M V-IV This object was generated by the targetselector and retrieved from the SIMBAD database. However, due to the presence of AB Dor A, the coordinates of AB Dor B were only accurate down to an arcminute. The coordinates were therefore subsequently changed based on NACO images to more accurately represent the position of AB Dor B. J2000 05 28 44.0000 -65 27 0.00 J2000 WFPC2 Imaging No F502N WFPC2 Imaging No F622W WFPC2 Imaging No F791W WFPC2 Imaging No F555W WFPC2 Imaging No F658N WFPC2 Imaging No F850LP POS-TARG DITHER 0.559017 LINE 0.559017 false false 143.130096 4 26.56505 WFPC2-BOX 1 false (Tic-Visit :proposal-id 11132 :visit-id "01" :proposal-version -1 :trans-version "20070325" :date "13-Jun-2007 09:44:51.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 1299664 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11132 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11132 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2820 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11132 :visit-id "01" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2640 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) false 2 0