We have recently used FGS and HRC observations to (a) resolve HD 93129A into two components with very similar optical/UV colors and a magnitude difference of 1.3 and to (b) detect their relative orbital motion over a span of 8 years. HD 93129Aa is the prototype O2 If* star, with an evolutionary mass near 100 M_Sun, while Ab is likely to be a very early O main-sequence star with a similar or only slightly smaller mass. Our HST astrometric measurements yield a total mass above 100 M_Sun, thus confirming the extremely high mass of the binary, and indicate that the system appears to be approaching periastron. We request new FGS observations to (a) calculate the mass ratio of the system by measuring the orbit of each of the components with respect to the nearby stars, (b) obtain the periastron epoch, and (c) start measuring the orbit in order to produce an estimate of the total mass. These measurements are crucial to shed light on the value of the stellar upper mass limit.

We propose observing HD 93129 at three different epochs using ACS/HRC and FGS. HOT STARS 12 false 0 0.0 0 false false false false false false SUPPORTED STAR OF SUPERGIANT O WIND O2 If* J2000 10 43 57.5000 -59 32 53.00 J2000 CALIBRATION ASTROMETRIC GS8626-01835 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC GS8626-02625 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC GS86260-1570 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC GS8626-02425 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC GS8626-02096 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC GS8626-02077 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC GS8626-00153 J2000 00 00 0.0000 +00 00 0.00 J2000 CALIBRATION ASTROMETRIC GS8626-00123 TR14-20 J2000 00 00 0.0000 +00 00 0.00 J2000 ACS/HRC Imaging No No F435W ACS/HRC Spectroscopic No No PR200L ACS/HRC Imaging No No F330W ACS/HRC Imaging No No F344N ACS/HRC Imaging No No F475W ACS/HRC Imaging No No F555W ACS/HRC Imaging No No F606W ACS/HRC Imaging No No F625W ACS/HRC Imaging No No F660N ACS/HRC Imaging No No F775W ACS/HRC Imaging No No F814W ACS/HRC Imaging No No F892N FGS TRANS No F5ND FGS POS No F5ND FGS POS No F5ND FGS POS No F5ND FGS POS No F5ND FGS POS No F5ND FGS POS No F583W FGS POS No F583W FGS POS No F583W FGS POS No F583W FGS POS No F583W (Tic-Visit :proposal-id 11294 :visit-id "02" :proposal-version -1 :trans-version "20070325" :date "08-May-2007 16:11:00.0" :trans-errors '(:WARNINGS 16 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "05" :si-bright-earth-limb-angle 15.5 :data-volume 0 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11294 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11294 :visit-id "02" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2832 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3222 :pcs-gap 0 ) true 1 0