18118( 4) - 02-Oct-2025 12:49:35 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18118 Version: 4 Check-in Time: 02-Oct-2025 12:49:35 STScI Edit Number: 0 Title VFTS 812: A Quiescent Eccentric Black Hole Binary in the LMC? ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Mr. Kunal Deshmukh Institute of Astronomy, KU Leuven Y CoI: Prof. Hugues Sana Institute of Astronomy, KU Leuven N CoI: Dr. Tomer Shenar Tel Aviv University - Wise Observatory N CoI: Dr. Bethany Ann Ludwig Institute of Astronomy, KU Leuven N CoI: Dr. Reinhold Willcox Institute of Astronomy, KU Leuven N CoI: Mr. Olivier Verhamme Katholieke Universiteit Leuven N ------------------------------------------------------------------------------------ Abstract Understanding how black holes form in massive binary systems is a central question in modern astrophysics, with direct implications for the origin of gravitational-wave sources. VFTS 812, an eccentric 17-day period binary in the sub-solar metallicity environment of the Tarantula Nebula was identified as a prime candidate hosting an O4V star with a possible black hole companion. Current optical observations strongly suggest a non-luminous companion exceeding the typical mass threshold for black holes, but cannot definitively exclude a faint, non-degenerate star. Furthermore, the primary mass and atmospheric properties are not constrained enough to trace its past evolution. We propose HST UV observations to decisively determine if the unseen companion in VFTS 812 is indeed a black hole, clarify the physical properties of the O4V primary, and reveal signatures of past mass-transfer episodes. A UV spectroscopic analysis will precisely constrain the rotation and wind characteristics key to understanding the systems evolutionary history. Confirming the black hole companion candidate will provide critical insights into supernova natal kicks, the evolution of massive stars in low-metallicity environments, and the formation of merging binary black holes. ------------------------------------------------------------------------------------ Observations Description ------------------------ We aim to obtain UV spectra with the G160M (1533 & 1623) grating of the O+black-hole binary candidate VFTS 812 during two randomly timed visits. Each visit requested includes two orbits, one each for the two wavelength settings. The SNR in each wavelength setting is expected to reach 15-20 based on ETC calculations. ------------------------------------------------------------------------------------ 18118( 4) - 02-Oct-2025 12:49:35 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 2MASS-J05393 STAR, MAIN SEQUENCE O RA=05H39M36.1197S +/- 0.019", J2000 V = 14.319 +/- 0.075 Swift UVOT UVW2 = 611-6904591 DEC=-69D04'59.13" +/- 0.019" 15.14 +/- 0.01, Spectral type O4 V Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 3.1632775383112326E-4 6.57E-4 2.8999999999999998E-6 ------------------------------------------------------------------------------------------------------------------------------------ 18118( 4) - 02-Oct-2025 12:49:35 - [ 3] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 2MASS-J0539 COS/NUV ACQ/IMA PSA MIRRORB 1 2 S (2 S) 3611-690459 GE 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 2MASS-J0539 COS/FUV TIME-TA PSA G160M 1533 FP-POS=1, BUFFER-TIME=1248 1 450 S 3611-690459 G see below 1 (1102 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 2MASS-J0539 COS/FUV TIME-TA PSA G160M 1533 FP-POS=2, BUFFER-TIME=1248 1 450 S 3611-690459 G see below 1 (1054 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 2MASS-J0539 COS/FUV TIME-TA PSA G160M 1623 FP-POS=1, BUFFER-TIME=1859 1 450 S 3611-690459 G see below 1 (1183 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 2MASS-J0539 COS/FUV TIME-TA PSA G160M 1623 FP-POS=2, BUFFER-TIME=1859 1 498 S 3611-690459 G see below 1 (1307 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ 2MASS-J05393611-690 1 COS/NUV ACQ/IMA PSA MIRRORB none none none none 1 N/A 4591 GE 2MASS-J05393611-690 2 COS/FUV TIME-TA PSA G160M 1533 none none none none 1 1102.0 S 4591 G 2MASS-J05393611-690 3 COS/FUV TIME-TA PSA G160M 1533 none none none none 1 1054.0 S 4591 G 2MASS-J05393611-690 4 COS/FUV TIME-TA PSA G160M 1623 none none none none 2 1183.0 S 4591 G 2MASS-J05393611-690 5 COS/FUV TIME-TA PSA G160M 1623 none none none none 2 1307.0 S 4591 G 18118( 4) - 02-Oct-2025 12:49:35 - [ 4] Visit: 02 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 2MASS-J0539 COS/NUV ACQ/IMA PSA MIRRORB 1 2 S (2 S) 3611-690459 GE 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 2MASS-J0539 COS/FUV TIME-TA PSA G160M 1533 FP-POS=3, BUFFER-TIME=1248 1 450 S 3611-690459 G see below 1 (1102 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 2MASS-J0539 COS/FUV TIME-TA PSA G160M 1533 FP-POS=4, BUFFER-TIME=1248 1 450 S 3611-690459 G see below 1 (1054 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 2MASS-J0539 COS/FUV TIME-TA PSA G160M 1623 FP-POS=3, BUFFER-TIME=1859 1 450 S 3611-690459 G see below 1 (1183 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 2MASS-J0539 COS/FUV TIME-TA PSA G160M 1623 FP-POS=4, BUFFER-TIME=1859 1 498 S 3611-690459 G see below 1 (1307 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ 2MASS-J05393611-690 1 COS/NUV ACQ/IMA PSA MIRRORB none none none none 1 N/A 4591 GE 2MASS-J05393611-690 2 COS/FUV TIME-TA PSA G160M 1533 none none none none 1 1102.0 S 4591 G 2MASS-J05393611-690 3 COS/FUV TIME-TA PSA G160M 1533 none none none none 1 1054.0 S 4591 G 2MASS-J05393611-690 4 COS/FUV TIME-TA PSA G160M 1623 none none none none 2 1183.0 S 4591 G 2MASS-J05393611-690 5 COS/FUV TIME-TA PSA G160M 1623 none none none none 2 1307.0 S 4591 G 18118( 4) - 02-Oct-2025 12:49:35 - [ 5] Summary Form for Proposal 18118 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=1248, BUFFER-TIME=1859, FP-POS=1, FP-POS=2, FP-POS=3, FP-POS=4 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G160M, MIRRORB ------------------------------------------------------------------------------------------------------------------------------------ Target Names 2MASS-J05393611-6904591 ------------------------------------------------------------------------------------------------------------------------------------