10105( 5) - 10/22/04 11:59 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 10105 Version: 5 Check-in Date: 22-Oct-2004 15:58:32 1.Proposal Title: Accurate Mass Determination of the Ancient White Dwarf ER 8 Through Astrometric Microlensing ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle 4. Parallel Pointing Tolerance GO 13 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Dr. Kailash C. Sahu Space Telescope Science Institute CoI: Dr. Howard E. Bond Space Telescope Science Institute N CoI: Dr. Edmund Nelan Space Telescope Science Institute N CoI: Mr. Gaspar A. Bakos Smithsonian Institution Astrophysical N Observatory ------------------------------------------------------------------------------------ 6. Abstract We propose to determine the mass of the very cool white dwarf ER 8 through astrometric microlensing. We have predicted that ER 8 will pass very close to a 15th-mag background star in January 2006, with an impact parameter of less than 0.05 arcsec. As it passes in front, it will cause a deflection of the background star's image by >8 milliarcsec, an amount easily detectable with HST/FGS. The gravitational deflection angle depends only on the distances and relative positions of the stars, and on the mass of the white dwarf. Since the distances and positions can be determined precisely before the event, the astrometric measurement offers a unique and direct method to measure the mass of the white dwarf to high accuracy (<5%). Unlike all other stellar mass determinations, this technique works for single stars (but only if they are nearby and of sufficient mass). The mass of ER 8 is of special interest because it is a member of the Galactic halo, and appears to be the oldest known field white dwarf. This object can thus set a lower limit on the age of the Galactic halo, but since white-dwarf cooling rates depend on their masses, the mass is a necessary ingredient in the age determination. As a byproduct, we will obtain an accurate parallax for ER 8, and thus its luminosity and (from its effective temperature) its radius. Such quantities are at present rather poorly known for the coolest white dwarfs, and will provide strong constraints on white-dwarf physics. ------------------------------------------------------------------------------------ 10105( 5) - 10/22/04 11:59 - [ 2] Observations Description ------------------------ We need to know an accurate parallax and proper motion for ER 8, and an accurate location of the source star, in order to predict the precise circumstances of the event, including the date and impact parameter. The existing ground-based measurements do not have the ~1mas accuracy needed for these purposes, especially given the 6 mas/day proper motion of ER 8. Unfortunately ER 8, at 17th mag, is too faint for FGS observations. Therefore, we will use direct imaging with ACS to determine the astrometric parameters of ER 8, the source star, and the surrounding reference field. These determinations need to be done well in advance of the 2006 event, beginning in Cycle 12. During the actual event, we need to measure the gravitational deflection of the source to much better than 1 mas accuracy. Fortunately, at 15th mag, the source star's position (relative to surrounding reference stars) can be measured easily with the FGS to the required precision. These considerations dictate a four-phase observing program: (1) Prediction phase. In this phase, we will obtain the HST observations needed to predict the circumstances of the event (distance to the lens, impact parameter, and time of closest approach) with high accuracy. In Cycles 12 (2 visits) and 13 (1 visit), we will image ER 8, the background star, and the surrounding reference field with the ACS/WFC. Observations will be made at the times of maximum parallax factor (July 2003, January 2004, July 2004; one orbit each). These observations will establish the proper motion and parallax of ER 8 and the location (and any parallax) of the source, all to a higher accuracy (<1 mas) than possible from the ground. (2) FGS baseline phase. In mid-Cycle 12, we will use FGS1r to obtain accurate baseline astrometry of the source and surrounding reference stars to establish their relative positions before the onset of any gravitational deflection (1 visit, 2 orbits). (3)Encounter phase. In mid-Cycle 13 (January 2005), when the deflection should first start to become detectable, we will obtain an FGS observation (2 orbits). Another FGS observation will be obtained at the beginning of Cycle 14 in July 2005 (2 orbits). Then, starting in late December 2005, we will begin our intensive FGS monitoring of the deflection event. This will consist of 1 FGS visit (2 orbits) every 3 days for a 9-day interval centered on the closest approach (total of 4 visits, 8 orbits). (4) Post-encounter phase. Our final FGS observation in Cycle 14 will be in June 2006 (2 orbits) when the deflection will be nearing its end. At this point we will also obtain a final ACS image (1 orbit) for further refinement of the event circumstances based on the longest time baseline. Therefore, our request is for 4 orbits in Cycle 12, 3 in Cycle 13, and 13 in Cycle 14. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 10105( 5) - 10/22/04 11:59 - [ 3] 10105( 5) - 10/22/04 11:59 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 ER8-FIELD EXT-STAR, F3-F9 RA=13H12M52.1000S +/- 0.3", J2000 V = 15.5+/-0.2 DEC=-47D28'1.00" +/- 0.3" Coordinate Source: IMAGE_TIED_TO_GSC_FRAME 2 ER8-SOURCE EXT-STAR, F3-F9 RA=13H12M55.1900S +/- 0.3", J2000 V = 15.5+/-0.2 DEC=-47D28'9.00" +/- 0.3" Coordinate Source: GUIDE_STAR_CATALOG 3 REF1 CALIBRATION, RA=13H12M44.5000S +/- 0.1", J2000 V = 12.72+/-0.5 ASTROMETRIC DEC=-47D28'19.50" +/- 0.1", PLATE-ID=03C4 Coordinate Source: GUIDE_STAR_CATALOG 4 REF2 CALIBRATION, RA=13H12M34.6000S +/- 0.1", J2000 V = 11.1+/-0.3 ASTROMETRIC DEC=-47D27'8.90" +/- 0.1", PLATE-ID=03C4 Coordinate Source: GUIDE_STAR_CATALOG 5 REF3 CALIBRATION, RA=13H12M57.5000S +/- 0.1", J2000 V = 15.5+/-0.5 ASTROMETRIC DEC=-47D27'54.90" +/- 0.1", PLATE-ID=05XT Coordinate Source: GUIDE_STAR_CATALOG 6 REF4 CALIBRATION, RA=13H13M1.0000S +/- 0.1", DEC=- J2000 V = 13.3+/-0.3 ASTROMETRIC 47D27'23.40" +/- 0.1", PLATE- ID=03C4 Coordinate Source: GUIDE_STAR_CATALOG 7 REF5 CALIBRATION, RA=13H13M10.5200S +/- 0.1", J2000 V = 13.6+/-0.3 ASTROMETRIC DEC=-47D27'22.80" +/- 0.1", PLATE-ID=05XT Coordinate Source: GUIDE_STAR_CATALOG 8 REF6 CALIBRATION, RA=13H13M13.9000S +/- 0.1", J2000 V = 14.9+/-0.3 ASTROMETRIC DEC=-47D29'14.96" +/- 0.1", PLATE-ID=05XT Coordinate Source: GUIDE_STAR_CATALOG 9 REF7 CALIBRATION, RA=13H12M43.7000S +/- 0.1", J2000 V = 14.73+/-0.3 ASTROMETRIC DEC=-47D29'9.20" +/- 0.1", PLATE-ID=03C4 Coordinate Source: GUIDE_STAR_CATALOG 10 REF8 CALIBRATION, RA=13H13M1.7000S +/- 0.1", DEC=- J2000 V = 14.3+/-0.3 ASTROMETRIC 47D26'57.80" +/- 0.1", PLATE- ID=05XT Coordinate Source: GUIDE_STAR_CATALOG 11 REF9 CALIBRATION, RA=13H13M3.8000S +/- 0.1", DEC=- J2000 V = 15.1+/-0.3 ASTROMETRIC 47D29'32.80" +/- 0.1", PLATE- ID=05XT Coordinate Source: GUIDE_STAR_CATALOG 12 ER8-HRC EXT-STAR, DA RA=13H12M55.4000S +/- 0.1", J2000 V = 17.0 DEC=-47D27'59.50" +/- 0.1" Coordinate Source: IMAGE_TIED_TO_GSC_FRAME 10105( 5) - 10/22/04 11:59 - [ 5] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-HRC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.15 Line_Spacing 0.098 Coordinate_Frame POS-TARG Pattern_Orient 19.89 Angle_Between_Sides 63.5 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type ACS-HRC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.198 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 44.28 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 10105( 5) - 10/22/04 11:59 - [ 6] Visit: 33 Visit Priority: Visit Requirements: PCS MODE FINE; SCHED 30%; ORIENT 286.0D TO 286.0 D; BETWEEN 01-JAN- 2005:00:00:00 AND 05-JAN-2005:00:00:00 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| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 ER8-HRC ACS/HRC ACCUM HRC F606W CR-SPLIT=NO,GAIN=2 1 15 S PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 ER8-HRC ACS/HRC ACCUM HRC G800L GAIN=2,CR-SPLIT=2 1 800 S PATTERN 2 2 ------------------------------------------------------------------------------------------------------------------------------------ 3 ER8-HRC ACS/HRC ACCUM HRC G800L GAIN=2,CR-SPLIT=2 1 740 S PATTERN 2 3 ------------------------------------------------------------------------------------------------------------------------------------ 4 ER8-HRC ACS/HRC ACCUM HRC F606W CR-SPLIT=NO,GAIN=2 1 150 S PATTERN 1 4 ------------------------------------------------------------------------------------------------------------------------------------ 5 ER8-FIELD ACS/WFC ACCUM WFCENTER F606W CR-SPLIT=2 1 678 S ------------------------------------------------------------------------------------------------------------------------------------ 6 ER8-FIELD ACS/WFC ACCUM WFCENTER F606W CR-SPLIT=3 1 60 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 | ------------------------------------------------------------------------------------------------------------------------------------ ER8-HRC 1 ACS/HRC ACCUM HRC F606W 1 none none none 1 N/A ER8-HRC 1 ACS/HRC ACCUM HRC F606W 2 none none none 1 N/A ER8-HRC 1 ACS/HRC ACCUM HRC F606W 3 none none none 1 N/A ER8-HRC 1 ACS/HRC ACCUM HRC F606W 4 none none none 1 N/A ER8-HRC 2 ACS/HRC ACCUM HRC G800L 1 none none 1 1 N/A ER8-HRC 2 ACS/HRC ACCUM HRC G800L 1 none none 2 1 N/A ER8-HRC 2 ACS/HRC ACCUM HRC G800L 2 none none 1 1 N/A ER8-HRC 2 ACS/HRC ACCUM HRC G800L 2 none none 2 1 N/A ER8-HRC 3 ACS/HRC ACCUM HRC G800L 1 none none 1 2 N/A ER8-HRC 3 ACS/HRC ACCUM HRC G800L 1 none none 2 2 N/A ER8-HRC 3 ACS/HRC ACCUM HRC G800L 2 none none 1 2 N/A ER8-HRC 3 ACS/HRC ACCUM HRC G800L 2 none none 2 2 N/A ER8-HRC 4 ACS/HRC ACCUM HRC F606W 1 none none none 2 N/A ER8-HRC 4 ACS/HRC ACCUM HRC F606W 2 none none none 2 N/A ER8-HRC 4 ACS/HRC ACCUM HRC F606W 3 none none none 2 N/A ER8-HRC 4 ACS/HRC ACCUM HRC F606W 4 none none none 2 N/A ER8-FIELD 5 ACS/WFC ACCUM WFCENTER F606W none none none 1 3 N/A ER8-FIELD 5 ACS/WFC ACCUM WFCENTER F606W none none none 2 3 N/A ER8-FIELD 6 ACS/WFC ACCUM WFCENTER F606W none none none 1 3 N/A ER8-FIELD 6 ACS/WFC ACCUM WFCENTER F606W none none none 2 3 N/A ER8-FIELD 6 ACS/WFC ACCUM WFCENTER F606W none none none 3 3 N/A 10105( 5) - 10/22/04 11:59 - [ 7] Summary Form for Proposal 10105 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures HRC WFCENTER ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/HRC ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO GAIN=2 CR-SPLIT=2 CR-SPLIT=3 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE FINE; SCHED 30%; ORIENT 286.0D TO 286.0 D; BETWEEN 01-JAN-2005 00 00 00 AND 05-JAN-2005 00 00 00 PATTERN 1 1 PATTERN 2 2 PATTERN 2 3 PATTERN 1 4 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W G800L ------------------------------------------------------------------------------------------------------------------------------------ Target Names ER8-HRC ER8-FIELD ------------------------------------------------------------------------------------------------------------------------------------