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 0.9 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 and HRC 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. Due the the non-recovery of HRC on SM4, the last visit was changed to STIS.

We propose observing HD 93129 at three different epochs using ACS/HRC and FGS. Due the the non-recovery of HRC on SM4, the last visit was changed to STIS. HOT STARS 12 false 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 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 POS-TARG DITHER 0.025 LINE false false 2 90.0 STIS-ALONG-SLIT 1 false POS-TARG DITHER 0.035 LINE false false 2 45 LINE 2 false The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. 1256222365000 239.00000000000003 (Tic-Visit :proposal-id 11784 :visit-id "01" :proposal-version -1 :trans-version "20080807" :date "12-Jun-2009 10:04:32.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 313296 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11784 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11784 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2640 :al-overhead-at-end 0 :internal-dur-at-end 401 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3517 :pcs-gap 0 ) false 1 0 The scheduling data was generated by CASM 17.5. CASM was run in Phase II mode. (Tic-Visit :proposal-id 11784 :visit-id "51" :proposal-version -1 :trans-version "20090915" :date "17-Nov-2009 10:11:26.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 313296 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11784 :visit-id "51" :obset-id "51" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11784 :visit-id "51" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2640 :al-overhead-at-end 0 :internal-dur-at-end 401 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3517 :pcs-gap 0 ) true 1 0