9153( 5) - 06/14/02 10:14 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9153 Version: 5 Check-in Date: 14-Jun-2002 10:13:30 1.Proposal Title: The Black Hole (?) in BG Gem ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Scott Kenyon Smithsonian Institution Astrophysical Observatory CoI: Dr Priscilla Benson Whitin Observatory, Wellesley College N CoI: Dr Jennifer Sokoloski Department of Physics and Astronomy, Y University of Southampton CoI: Dr Kim McLeod Whitin Observatory, Wellesley College N ------------------------------------------------------------------------------------ 5. Abstract We propose to acquire low resolution ultraviolet spectra of the long period black hole binary candidate BG Geminorum. STIS spectra allow an unambiguous test of the nature of the 4.5 M(sun) primary star, which makes no measurable contribution to the optical spectrum. Detection of a strong ultraviolet continuum or ultraviolet absorption lines from Si II and Si III (among others) would indicate a B-type primary; failure to detect these features or detection of (broad) high ionization emission lines would favor a black hole primary. If it contains a black hole, BG Gem would be the longest period black hole binary known (by a factor of ten), challenging current theories of black hole formation. It would also be the only eclipsing binary known to contain a black hole, which would provide a unique laboratory for testing accretion disk models for black hole binary systems. ------------------------------------------------------------------------------------ 9153( 5) - 06/14/02 10:14 - [ 2] Observations Description ------------------------ To achieve our goals, we plan STIS observations using the MAMA detector and UV gratings G140L (far-ultraviolet; FUV) and G230L (near-ultraviolet; NUV). Both settings are needed to make a proper measurement of the UV continuum and to search unambiguously for any absorption line expected from a B-type star. As noted above, FUV spectra will allow us to search for absorption lines of O-B stars; NUV spectra include absorption features found in B-A stars. To isolate these features from other emission line sources, we plan observations at two orbital quadratures, two primary minima, and one secondary minimum. Observations at two quadratures and at two primary minima will test for system stability; optical observations show no evidence for significant variations over many orbital cycles. Observations at two primary minima are also needed to attain good S/N. As a first approximation, the spectrum of the primary star is beginequation F_1 = F_q - F_p . endequation The spectrum of the secondary star is beginequation F_2 = F_q - F_s . endequation In these expressions, F_q is the spectrum at quadrature, F_p is the spectrum at primary minimum, and F_s is the spectrum at secondary minimum. These equations are not precisely correct, because they assume that each component emits the same spectrum in all directions. The emission is probably anisotropic to some degree due to illumination of the secondary by the primary, but this effect should be much less than ~ 5\ We expect a small continuum flux from the secondary star, less than 5\ We expect modest low ionization emission lines (such as Mg II) often observed in red giants. Quantifying this emission from the secondary star will allow us to separate emission lines associated with the secondary from emission lines associated with the primary and thus make a better assessment of the nature of the primary star. The quadrature spectra will also yield an accurate measurement of the UV luminosity from the disk. Deriving this luminosity will place good limits on the accretion rate, because we know the ratio of stellar mass to stellar radius, M_star/R_\star, for a B-type star and a black hole. To estimate exposure times, we adopt the observed 4000 Angstrom flux of the primary, F_Lambda ~ 4 * 10^-15 erg cm^-2 s^-1 AA^-1; A_V = 1.65 mag; and assume a spectrum F_Lambda \propto lambda^-2.3. This spectrum is appropriate for the steady-state accretion disk that fits the optical spectrum of the primary and is slightly steeper than the UV continuum slope for a B5 V star. According to the online exposure time calculator, we reach S/N = 25 at 2500 Angstrom in ~ 600 sec with G230L and S/N = 25 at 1500 Angstrom in ~ 1500 sec with G140L using the 52 * 0sec point2 slit. Each of these settings yields a factor of two lower S/N at ~ 1700 Angstrom. With a small overhead to acquire this V = 13 star, we can achieve S/N >~ 12 from 1200 Angstrom to 3000 Angstrom with two exposures in a single orbit. The UV flux from the primary should decrease at primary minimum. We predict a factor of ten decline at ~ 2500 Angstrom and a factor of 20--30 decline at 1300 Angstrom. We thus expect S/N = 8 at 2500 Angstrom in ~ 600 sec with G230L and S/N = 5 at 1500 Angstrom in ~ 1500 sec with G140L. Two spectra acquired during successive primary minima should be sufficient to measure the eclipse depth and to test for system stability. begincenter begintabulartl c c c c multicolumn5c Table 1. Proposed Spectra , Orbital Phase & Orbital Geometry & G140L & G230L & Goals , 0.00 & primary minimum & SQRT( & SQRT( & primary emission lines , 0.25 & quadrature & SQRT( & SQRT( & primary absorption spectrum , 0.50 & secondary minimum & SQRT( & SQRT( & secondary emission lines , 0.75 & quadrature & SQRT( & SQRT( & primary absorption spectrum , 1.00 & primary minimum & SQRT( & SQRT( & primary emission lines , endtabular endcenter 2ex Table 1 summarizes our proposed spectra as a function of orbital phase. We measure phases relative to the first primary minimum to be observed by HST. Any of the four primary minima visible during an HST cycle is acceptable. We plan two spectra (one with G140L and one with G230L) in each of 5 orbits as listed in the Table. Real Time Justification ----------------------- Our program requires observations during specific orbital phases of an eclipsing binary. The orbital period is almost precisely 0.25 yr; there are four opportunities to observe a complete eclipse per year. BG Gem is close to the ecliptic; the star is difficult to observe during some phases. We do not require specific position angles for the slit, which minimizes scheduling constraints. We prefer that all of the observations be made over a single orbital cycle. Observations at quadratures should not be difficult to schedule, because each requires only one orbit in a ten day interval. Visits during the middle of primary and secondary eclipses require one orbit in a 4 day interval. Failure to schedule all observations in one orbital cycle of the binary would not affect our program significantly. Acquiring eclipse observations from different cycles might make it difficult to detect radiation from the primary unambiguously. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9153( 5) - 06/14/02 10:14 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 BG-GEM STAR, INTERACTING RA=6H 3M 30.8S +/- 0.3S, DEC=27D J2000 V = 28.0 V = 13.0 BINARY 41' 51.1" +/- 1.0", PLATE- B-V = 1.0 ID=01ZL E(B-V) = 0.5 Coordinate Source: GUIDE_STAR_CATALOG Comments: variable target 9153( 5) - 06/14/02 10:14 - [ 4] Visit: 01 Visit Priority: Visit Requirements: PERIOD 91.645 D AND ZERO-PHASE JD2452203.112300 GROUP 01, 02, 04, 05 WITHIN 200 D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 BG-GEM STIS/C ACQ F25ND3 MIRROR ACQTYPE=POINT 1 9.0 S PHASE 0.00 TO 0.02 CD Comments: First Primary Eclipse - Actual Phase = -0.01 to 0.01 ------------------------------------------------------------------------------------------------------------------------------------ 2 BG-GEM STIS/F ACCUM 52X0.2 G140L 1425 1 920.0 S PHASE 0.00 TO 0.02 UV-MAM A Comments: First Primary Eclipse - Actual Phase = -0.01 to 0.01 ------------------------------------------------------------------------------------------------------------------------------------ 3 BG-GEM STIS/N ACCUM 52X0.2 G230L 2376 1 400.0 S PHASE 0.00 TO 0.02 UV-MAM A Comments: First Primary Eclipse - Actual Phase = -0.01 to 0.01 ------------------------------------------------------------------------------------------------------------------------------------ 9153( 5) - 06/14/02 10:14 - [ 5] Visit: 02 Visit Priority: Visit Requirements: PERIOD 91.645 D AND ZERO-PHASE JD2452203.112300 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 BG-GEM STIS/C ACQ F25ND3 MIRROR ACQTYPE=POINT 1 9.0 S PHASE 0.24 TO 0.28 CD Comments: First Quadrature ------------------------------------------------------------------------------------------------------------------------------------ 2 BG-GEM STIS/F ACCUM 52X0.2 G140L 1425 1 920.0 S PHASE 0.24 TO 0.28 UV-MAM A Comments: First Quadrature ------------------------------------------------------------------------------------------------------------------------------------ 3 BG-GEM STIS/N ACCUM 52X0.2 G230L 2376 1 400.0 S PHASE 0.24 TO 0.28 UV-MAM A Comments: First Quadrature ------------------------------------------------------------------------------------------------------------------------------------ 9153( 5) - 06/14/02 10:14 - [ 6] Visit: 03 Visit Priority: Visit Requirements: PERIOD 91.645 D AND ZERO-PHASE JD2452203.112300 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 BG-GEM STIS/C ACQ F25ND3 MIRROR ACQTYPE=POINT 1 9.0 S PHASE 0.50 TO 0.52 CD Comments: Secondary Eclipse ------------------------------------------------------------------------------------------------------------------------------------ 2 BG-GEM STIS/F ACCUM 52X0.2 G140L 1425 1 920.0 S PHASE 0.50 TO 0.52 UV-MAM A Comments: Secondary Eclipse ------------------------------------------------------------------------------------------------------------------------------------ 3 BG-GEM STIS/N ACCUM 52X0.2 G230L 2376 1 400.0 S PHASE 0.50 TO 0.52 UV-MAM A Comments: Secondary Eclipse ------------------------------------------------------------------------------------------------------------------------------------ 9153( 5) - 06/14/02 10:14 - [ 7] Visit: 04 Visit Priority: Visit Requirements: PERIOD 91.645 D AND ZERO-PHASE JD2452203.112300 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 BG-GEM STIS/C ACQ F25ND3 MIRROR ACQTYPE=POINT 1 9.0 S PHASE 0.74 TO 0.78 CD Comments: Second Quadrature ------------------------------------------------------------------------------------------------------------------------------------ 2 BG-GEM STIS/F ACCUM 52X0.2 G140L 1425 1 920.0 S PHASE 0.74 TO 0.78 UV-MAM A Comments: Second Quadrature ------------------------------------------------------------------------------------------------------------------------------------ 3 BG-GEM STIS/N ACCUM 52X0.2 G230L 2376 1 400.0 S PHASE 0.74 TO 0.78 UV-MAM A Comments: Second Quadrature ------------------------------------------------------------------------------------------------------------------------------------ 9153( 5) - 06/14/02 10:14 - [ 8] Visit: 05 Visit Priority: Visit Requirements: PERIOD 91.645 D AND ZERO-PHASE JD2452203.112300 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 BG-GEM STIS/C ACQ F25ND3 MIRROR ACQTYPE=POINT 1 9.0 S PHASE 0.00 TO 0.02 CD Comments: Second Primary Eclipse - Actual Phase = -0.01 to 0.01 ------------------------------------------------------------------------------------------------------------------------------------ 2 BG-GEM STIS/F ACCUM 52X0.2 G140L 1425 1 920.0 S PHASE 0.00 TO 0.02 UV-MAM A Comments: Second Primary Eclipse - Actual Phase = -0.01 to 0.01 ------------------------------------------------------------------------------------------------------------------------------------ 3 BG-GEM STIS/N ACCUM 52X0.2 G230L 2376 1 400.0 S PHASE 0.00 TO 0.02 UV-MAM A Comments: Second Primary Eclipse - Actual Phase = -0.01 to 0.01 ------------------------------------------------------------------------------------------------------------------------------------ 9153( 5) - 06/14/02 10:14 - [ 9] Summary Form for Proposal 9153 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F25ND3 52X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PERIOD 91.645 D AND ZERO-PHASE JD2452203.112300 GROUP 01, 02, 04, 05 WITHIN 200 D PHASE 0.00 TO 0.02 PERIOD 91.645 D AND ZERO-PHASE JD2452203.112300 PHASE 0.24 TO 0.28 PHASE 0.50 TO 0.52 PHASE 0.74 TO 0.78 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140L G230L ------------------------------------------------------------------------------------------------------------------------------------ Target Names BG-GEM ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 2376 ------------------------------------------------------------------------------------------------------------------------------------