17202( 4) - 11-Jan-2023 04:58:56 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17202 Version: 4 Check-in Time: 11-Jan-2023 04:58:56 STScI Edit Number: 0 Title Does MWC 656 host a black hole or stripped helium star? FUV spectroscopy will tell. ------------------------------------------------------------------------------------ Type Cycle GO 30 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Kareem El-Badry California Institute of Technology Y CoI: Prof. Charlie Conroy Harvard University N CoI: Dr. Julia Bodensteiner European Southern Observatory - Germany N CoI: Dr. Tomer Shenar Universiteit van Amsterdam N CoI: Dr. Thomas Rivinius European Southern Observatory - Germany N CoI: Dr. Douglas Russell Gies Georgia State University Research N Foundation CoI: Dr. Luqian Wang Yunnan Astronomical Observatory, CAS N CoI: Dr. Kevin Burdge Massachusetts Institute of Technology N CoI: Dr. Ylva Goetberg Carnegie Institution of Washington N ------------------------------------------------------------------------------------ Abstract We propose multi-epoch STIS FUV spectroscopy of the only Be star + black hole binary candidate, MWC 656. These observations will reveal the nature of the Be star's elusive companion: a black hole, or an intermediate-mass (~3 Msun) stripped helium star. Either scenario would be extraordinary: intermediate-mass stripped stars are the main progenitors of stripped-envelope supernovae, X-ray binaries, and merging neutron stars, yet they have remained elusive for decades; the black hole would be in the mass gap and would be the only known X-ray faint high-mass X-ray binary containing a black hole. Our proposed observations will unambiguously distinguish between these two scenarios, as the UV spectrum of a massive stripped star will be very different from that of a black hole accretion disk. FUV spectra will also constrain the spectral type, mass, and RV variability of the Be star more reliably than has been possible with optical data, which are strongly contaminated by the Be star's circumstellar disk. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose to obtain 3 orbits of STIS MAMA high dispersion UV spectroscopy of MWC 656, a bright (V=8.8, GALEX FUV ~ 9.0) binary system. The FUV magnitude is estimated based on the spectral type and optical/IR SED. The extinction is low (E(B-V) ~ 0.2), so the expected FUV magnitude is reasonably well-constrained, and is also constrained from the measured TD-1 UV flux. 3 separate single-orbit visits are required, with a typical inter-visit spacing of 15 days, to resolve the RV variations of both components over the course of the 60-day orbital period. The expected RV variations of the Be star are ~20 km/s between visits, while those of the companion are ~100 km/s. Both will be readily detectable with our proposed instrumentation, which will yield RVs precision of ~0.5 km/s for the companion, and ~3 km/s for the Be star. We will use the E140M grating and MAMA detector with a 0.2 X 0.2 slit, yielding 17202( 4) - 11-Jan-2023 04:58:56 - [ 2] coverage of the far-UV (1150-1700\, \AA) with a resolving power of R = 45, 800. This setup produces spectra with sufficient resolution to measure the radial velocity variations of both components of the binary and to record in detail the narrow lines of the helium star companion, if it exists. We plan to visit the target three times and rebin the spectral dispersion down by a factor of four, for a final R = 11, 450. (Observing directly with a lower-resolution grating would risk saturating the detector). This is the same observing setup and approach that was recently successfully used by Wang et al. 2021 to characterize sdO companions to Be stars with FUV flux ratios as low as 1%. MWC 656 has 55 minutes of orbital visibility in each orbit. After subtraction of 6 minutes for guide star acquisition, 6 minutes for STIS target acquisition, and 8 minutes for STIS science observations overhead, there remain 2000 seconds science exposure time in one orbit. Using the STIS exposure time calculator, we estimate that we will obtain a signal-to-noise ratio of 70 per resolution element at 1425\, \AA\, (before rebinning; $\approx 150$ after rebinning) in each of the three visits. This exceeds our SNR requirement (to detect a companion that contributes 1% of the FUV flux) by a factor of 4. Our primary goals are to constrain the effective temperature and radius of both components and to measure the relative amplitude of both components' RV shifts in order to determine the mass ratio. We will perform a cross correlation analysis with model spectra to detect the spectral signals of the Be star and hot companion in order to measure radial velocities for both; observing at multiple epochs is essential for determining the mass ratio. We will fit the composite spectra using a binary spectral model, inferring temperatures and flux ratios for both components. Given the known distance from the precise Gaia parallax, the component radii can be determined from fits of the spectral energy distribution, and masses can subsequently be inferred from evolutionary models. If the companion remains consistent with a BH, our analysis of the RVs and Be star UV spectrum will significantly improve constraints on both component masses, which we will compare to evolutionary tracks to constrain the system's formation history and expected future evolution. Characterization of additional spectral features from the BH accretion disk in the UV will also improve constraints on the time-averaged accretion rate. If UV spectra instead reveal a stripped helium star companion, analysis of its spectrum will constrain its temperature and luminosity. Comparison of these to evolutionary tracks for stripped stars from Gotberg et al. (2018) will then similarly constrain the system's evolutionary history. ------------------------------------------------------------------------------------ 17202( 4) - 11-Jan-2023 04:58:56 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD-215227 STAR, B0-B2 III-I, BE, RA=22H42M57.2979S +/- 0.01", J2000 V = 8.81 +/- 0.1 TD-1 flux in 156.5 nm INTERACTING BINARY DEC=+44D43'18.21" +/- 0.01" band: 6.9e-12 erg/s/cm^2/Angstrom Reference Frame: ICRS Comments: The target is outside the GALEX footprint. However, we have two ways to extimate the UV flux: (i) Assuming a B1.5 III template with Teff = 20, 000 K, R = 8 Rsun, d = 2 kpc, and E(B-V) = 0.2 (parameters from Casares et al. 2014; distance from Gaia; extinction from Bayestar dust map), we estimate an FUV flux density ranging from (6-10)*10^(-11) erg/s/cm^2/AA over the 1150-1700 AA range. (ii) The target was observed by the TD-1 UV flux survey, which measured a flux density of (0.69 +/- 0.03)*10^(-11) erg/s/cm^2/AA at 1565 AA, which is in good agreement with the model assumed in (i). For the estimate in (i), the ETC predicts a global count rate of 35, 000 counts/s, well below the bright limit of 200, 000 counts/s. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2016.0 -3.263279253486386E-4 -0.003159 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 17202( 4) - 11-Jan-2023 04:58:56 - [ 4] Visit: 01 (first visit) Visit Requirements: Period 59.12 D AND ZERO-PHASE HJD2455722.2 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 HD-215227 STIS/CCD ACQ F25ND3 MIRROR ACQTYPE=POINT 1 1 S (1 S) PHASE 0.45 TO 0.55 ------------------------------------------------------------------------------------------------------------------------------------ 2 HD-215227 STIS/FUV ACCUM 0.2X0.2 E140M 1425 1 2000 S -MAMA (2000 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WAVE STIS/FUV ACCUM 0.2X0.2 E140M 1425 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ HD-215227 1 STIS/CCD ACQ F25ND3 MIRROR none none none none 1 N/A HD-215227 2 STIS/FUV ACCUM 0.2X0.2 E140M 1425 none none none none 1 N/A -MAMA WAVE 3 STIS/FUV ACCUM 0.2X0.2 E140M 1425 none none none none 1 N/A -MAMA 17202( 4) - 11-Jan-2023 04:58:56 - [ 5] Visit: 02 (second visit) Visit Requirements: Period 59.12 D AND ZERO-PHASE HJD2455722.2 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 HD-215227 STIS/CCD ACQ F25ND3 MIRROR ACQTYPE=POINT 1 1 S (1 S) PHASE 0.95 TO 0.05 ------------------------------------------------------------------------------------------------------------------------------------ 2 HD-215227 STIS/FUV ACCUM 0.2X0.2 E140M 1425 1 2000 S PHASE 0.95 TO 0.05 -MAMA (2000 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WAVE STIS/FUV ACCUM 0.2X0.2 E140M 1425 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ HD-215227 1 STIS/CCD ACQ F25ND3 MIRROR none none none none 1 N/A HD-215227 2 STIS/FUV ACCUM 0.2X0.2 E140M 1425 none none none none 1 N/A -MAMA WAVE 3 STIS/FUV ACCUM 0.2X0.2 E140M 1425 none none none none 1 N/A -MAMA 17202( 4) - 11-Jan-2023 04:58:56 - [ 6] Visit: 03 (third visit) Visit Requirements: Period 59.12 D AND ZERO-PHASE HJD2455722.2 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 HD-215227 STIS/CCD ACQ F25ND3 MIRROR ACQTYPE=POINT 1 1 S (1 S) PHASE 0.2 TO 0.3 ------------------------------------------------------------------------------------------------------------------------------------ 2 HD-215227 STIS/FUV ACCUM 0.2X0.2 E140M 1425 1 2000 S PHASE 0.2 TO 0.3 -MAMA (2000 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WAVE STIS/FUV ACCUM 0.2X0.2 E140M 1425 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ HD-215227 1 STIS/CCD ACQ F25ND3 MIRROR none none none none 1 N/A HD-215227 2 STIS/FUV ACCUM 0.2X0.2 E140M 1425 none none none none 1 N/A -MAMA WAVE 3 STIS/FUV ACCUM 0.2X0.2 E140M 1425 none none none none 1 N/A -MAMA 17202( 4) - 11-Jan-2023 04:58:56 - [ 7] Visit: 04 (third visit-repeat) Visit Requirements: Period 59.12 D AND ZERO-PHASE HJD2455722.2 On Hold Comments: Additional Comments: This is a repeat of the visit that failed due to an instrument error. 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 HD-215227 STIS/CCD ACQ F25ND3 MIRROR ACQTYPE=POINT 1 1 S (1 S) PHASE 0.2 TO 0.3 ------------------------------------------------------------------------------------------------------------------------------------ 2 HD-215227 STIS/FUV ACCUM 0.2X0.2 E140M 1425 1 2000 S -MAMA (2000 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WAVE STIS/FUV ACCUM 0.2X0.2 E140M 1425 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ HD-215227 1 STIS/CCD ACQ F25ND3 MIRROR none none none none 1 N/A HD-215227 2 STIS/FUV ACCUM 0.2X0.2 E140M 1425 none none none none 1 N/A -MAMA WAVE 3 STIS/FUV ACCUM 0.2X0.2 E140M 1425 none none none none 1 N/A -MAMA 17202( 4) - 11-Jan-2023 04:58:56 - [ 8] Summary Form for Proposal 17202 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 0.2X0.2, F25ND3 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PHASE 0.2 TO 0.3, PHASE 0.45 TO 0.55, PHASE 0.95 TO 0.05, Period 59.12 D AND ZERO-PHASE HJD2455722.2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements E140M, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD-215227, WAVE ------------------------------------------------------------------------------------------------------------------------------------