0 Assigned ID: 3687 ----- Attempting Submission 1 (Fri Mar 25 23:11:06 GMT 2022) ----- Visit 03 failed and I submitted a successful HOPR. I have now added a repeat of that visit. -1006473488 ----- Attempting Submission 1 (Fri Jul 01 06:51:44 GMT 2022) ----- Your proposal could not be saved. Submission aborted. ----- Attempting Submission 2 (Fri Dec 09 22:48:59 GMT 2022) ----- ----- Attempting Submission 3 (Wed Jan 11 02:41:56 GMT 2023) ----- Does MWC 656 host a black hole or stripped helium star? FUV spectroscopy will tell. 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. 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 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. Kareem El-Badry (PI) is an expert on binary stars and stellar evolution. He has worked extensively on binaries containing candidate stellar mass black holes and stripped stars. He is leading searches for detached compact objects orbiting main-sequence stars with TESS and SDSS-V and has extensive experience with spectroscopic data. Luqian Wang and Doug Gies are experts on Be + stripped star binaries. They led a successful HST program obtaining FUV STIS spectroscopy (very similar to the observations proposed here) for a sample of a dozen Be + sdO binary candidates. Tomer Shenar and Julia Bodensteiner are experts in optical and spectroscopy of hot stars. Thomas Rivinius is a world expert in observational studies of Be stars. Ylva Goetberg is an expert developer of evolutionary models for stripped stars and tools for interpretation of their spectra. Kevin Burdge is an expert in photometric variability in binaries. Charlie Conroy is an expert in stellar evolution and spectroscopy and has extensive experience using HST. Stellar Physics and Stellar Types Galaxies 6 false false true false false false false SUPPORTED STAR B0-B2 III-I BE INTERACTING BINARY 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. J2000 J2000 TD-1 flux in 156.5 nm band: 6.9e-12 erg/s/cm^2/Angstrom STIS/FUV-MAMA Spectroscopic No No E140M 1425 STIS/FUV-MAMA Spectroscopic No No E140M 1425 STIS/FUV-MAMA Spectroscopic No No E140M 1425 /private/var/folders/gf/4vj_39zn6jx3x1g817gdn5r00000gn/T/OPVisit18362856601979223042.xml The scheduling data was generated by CASM 30.3. 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CASM was run in Phase II mode. 1670419310000 50.97 (Tic-Visit :proposal-id 17202 :visit-id "02" :proposal-version -1 :trans-version "20220630" :date "10-Jan-2023 21:39:51.0" :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 104864 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17202 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 17202 :visit-id "02" :obset-id "A2" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17202 :visit-id "02" :orbit-number 1 :pcs-time 393 :si-vis-utilization 2845 :al-overhead-at-end 194 :internal-dur-at-end 319 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3238 :pcs-gap 0 ) true 1 0 /private/var/folders/gf/4vj_39zn6jx3x1g817gdn5r00000gn/T/OPVisit3093773941758853289.xml The scheduling data was generated by CASM 30.3. CASM was run in Phase II mode. 1671776821000 59.45027787693999 (Tic-Visit :proposal-id 17202 :visit-id "03" :proposal-version -1 :trans-version "20220630" :date "10-Jan-2023 21:39:52.0" :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 104864 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17202 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 17202 :visit-id "03" :obset-id "A3" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17202 :visit-id "03" :orbit-number 1 :pcs-time 393 :si-vis-utilization 2845 :al-overhead-at-end 194 :internal-dur-at-end 319 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3238 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 30.3. CASM was run in Phase II mode. /Users/kareem/Downloads/OP-cache/phaseII_resubmit-v04.xml This is a repeat of the visit that failed due to an instrument error. (Tic-Visit :proposal-id 17202 :visit-id "04" :proposal-version -1 :trans-version "20220630" :date "10-Jan-2023 20:45:45.0" :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 104848 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17202 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17202 :visit-id "04" :orbit-number 1 :pcs-time 393 :si-vis-utilization 2493 :al-overhead-at-end 194 :internal-dur-at-end 319 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3238 :pcs-gap 0 ) true 1 0