13707( 3) - 10-Jun-2015 19:37:50 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 13707 Version: 3 Check-in Time: 10-Jun-2015 19:37:50 STScI Edit Number: 1 Title Mapping the Extreme Horizontal Branch instability strip in omega Centauri ------------------------------------------------------------------------------------ Type Cycle GO 22 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Suzanna Randall European Southern Observatory - Germany Y CoI: Marilyn Latour Universitat Erlangen-Nurnberg N CoI: Dr. Pierre Chayer Space Telescope Science Institute N CoI: Dr. Thomas M. Brown Space Telescope Science Institute N CoI: Dr. Annalisa Calamida Space Telescope Science Institute N CoI: Prof. Gilles Fontaine Universite de Montreal N CoI: Dr. Wayne B. Landsman ADNET Systems, Inc N ------------------------------------------------------------------------------------ Abstract We request far-UV spectroscopy for the hottest and the coolest members of the recently discovered class of rapid subdwarf O pulsators on the Extreme Horizontal Branch (EHB) in omega Centauri. Our main aim is to derive reliable temperature estimates and thus map the location of the omega Cen EHB instability strip in the Teff-log g plane. Current estimates based on optical spectroscopy suggest the pulsators to be clustered around 50, 000 K, which is distinctly cooler than the red edge of the predicted instability strip. We suspect that this discrepancy is due, at least in part, to the known problem of the temperatures being significantly underestimated from optical spectroscopy (compared to the more realistic values from FUV spectroscopy) for very hot stars. Given the successful pulsational modeling of the cooler (~31, 000 K) rapid subdwarf B pulsators found among the Galactic field population, the inability to reproduce the omega Cen pulsators would point towards fundamental differences between the two EHB populations. On the other hand, if we are able to reconcile the predicted and observed instability regions for the omega Cen pulsators, this will open them up to exploitation via asteroseismology. Asteroseismology has proven to be a very powerful tool for determining the internal stellar structure in the subdwarf B pulsators, providing valuable constraints on competing evolutionary scenarios. In light of the currently raging debate as to the origin of EHB stars particularly in Globular Clusters, the prospect of applying asteroseismology to EHB stars in a Globular Cluster for the first time is highly enticing. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will obtain COS/G140L FUV spectroscopy for two known omega Cen pulsators (V1 and V5) with the main aim of pinning down their effective temperatures as well as metal abundances (primarily C, N, O). Each of the targets will be placed in the COS primary science aperture (no orientation restrictions) and requires one visit of 2 orbits each. Each of the two visits encompasses a FUV PEAKXD and PEAKD acquisition followed by four exposures on-source (FP-POS=1, 2, 3, 4). The availability of ACS 13707( 3) - 10-Jun-2015 19:37:50 - [ 2] photometry covering both fields implies a good astrometric accuracy (~0.2"), allowing us to skip the SEARCH step during acquisition. Given an orbital visibility of (2x) 57 minutes we can expect to get a total of 5336 s integration time on-source for each of our two targets (1209 s per exposure for FP-POS1, 2 and 1459 s for FP-POS3, 4). According to the COS ETC this yields a S/N~30-40 per spectral resolution element in the 1120-1550 A wavelength range. The resolving power (R~3000) is sufficient to resolve the lines and distinguish the metal lines arising from interstellar absorption (notably CIV at 1548, 1550 A) from the redshifted stellar lines. Since omega Cen has a well-measured velocity of 230 km/s, the stellar lines will be shifted by ~1 A in the wavelength range of interest, more than the ~0.5 A spectral resolution achieved. At B~18.5 (U~16.9) our targets are faint enough to observe in TIMETAG mode and pose no safety issues (the count rate over the entire detector is <600/s according to the COS ETC). The buffer time is larger than the exposure time for all exposures. Since our two pointings are in omega Cen, the BOT is not useful for determining whether the observations are safe. However, we carefully checked the surrounding area with the help of ACS/435W, WFPC2/F218W and UIT FUV data. All the surrounding stars falling in the COS apertures (~1") are main sequence or red giant stars with T < 5000 K and (B-R) > 1 that will have negligible FUV fluxes compared to our sdO targets. For V1, there are no objects brighter than U=16 within 15", and the UIT (lamda_eff=1615 A) data for omega Cen show no other FUV bright objects within 20" (see left panel of Fig. 5 in the proposal). For V5, there is one bright star with U~15.3 (m218_AB ~ 17.4) at a distance of ~6" from V5 (see the finding chart emailed to Charles Proffitt, the Contact Scientist - note that the chart provided in the right hand panel of Fig. 5 in the proposal is wrong). However, this star is significantly cooler than V5 (m218_AB-V = 1.85 vs m218_AB-V=-0.72 for V5) and is therefore expected to be fainter in the FUV. From doing simple counts statistics on the UIT FUV image, we estimate that this star has only 90% of the flux of V5 at 1615 A and is therefore not problematic from a safety point of view. There are no other FUV bright stars within 20". Our observations are not affected by the change in COS lifetime position, i.e. it would be o.k. to have one visit executed before the change and one after. ------------------------------------------------------------------------------------ 13707( 3) - 10-Jun-2015 19:37:50 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 OMEGA-CEN-13 STAR, SDO RA=13H27M11.7665S +/- 0.2", J2000 V = 230 V = 18.459 +/- 0.005 U=16.81, spectral 2712-473229 DEC=-47D32'28.97" +/- 0.2" type sdO Reference Frame: ICRS Comments: The target star should be the brightest object in the FOV in the FUV ------------------------------------------------------------------------------------------------------------------------------------ 2 OMEGA-CEN-13 STAR, SDO RA=13H27M14.9632S +/- 0.2", J2000 V = 230 V = 18.386 +/- 0.003 U=16.96, spectral 2715-473051 DEC=-47D30'50.85" +/- 0.2" type sdO Reference Frame: ICRS Comments: The target star should be the brightest object in the FOV in the FUV. There is another star with ~90% of the FUV flux of V5 about 6" to the North-East of V5, but that should fall outside of the COS field and not influence the observations. ------------------------------------------------------------------------------------------------------------------------------------ 13707( 3) - 10-Jun-2015 19:37:50 - [ 4] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: FUV spectroscopy of V1 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 OMEGA-CEN-1 COS/FUV ACQ/PEA PSA G140L 1105 1 6.5 S (6.5 GS ACQ SCENARIO BASE1B3 32712-47322 KXD S) 9 ------------------------------------------------------------------------------------------------------------------------------------ 2 OMEGA-CEN-1 COS/FUV ACQ/PEA PSA G140L 1105 NUM-POS=5, STEP-SIZE=0.9, 1 15.5 S 32712-47322 KD CENTER=FLUX-WT-FLR (15.5 S) 9 ------------------------------------------------------------------------------------------------------------------------------------ 3 OMEGA-CEN-1 COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=2400, FP-POS=1 1 1209 S 32712-47322 G (1209 S) 9 ------------------------------------------------------------------------------------------------------------------------------------ 4 OMEGA-CEN-1 COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=2400, FP-POS=2 1 1209 S 32712-47322 G (1209 S) 9 ------------------------------------------------------------------------------------------------------------------------------------ 5 OMEGA-CEN-1 COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=2400, FP-POS=3 1 1459 S 32712-47322 G (1459 S) 9 ------------------------------------------------------------------------------------------------------------------------------------ 6 OMEGA-CEN-1 COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=2400, FP-POS=4 1 1459 S 32712-47322 G (1459 S) 9 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ OMEGA-CEN-132712-47 1 COS/FUV ACQ/PEA PSA G140L 1105 none none none none 1 N/A 3229 KXD OMEGA-CEN-132712-47 2 COS/FUV ACQ/PEA PSA G140L 1105 none none none none 1 N/A 3229 KD OMEGA-CEN-132712-47 3 COS/FUV TIME-TA PSA G140L 1105 none none none none 1 N/A 3229 G OMEGA-CEN-132712-47 4 COS/FUV TIME-TA PSA G140L 1105 none none none none 1 N/A 3229 G OMEGA-CEN-132712-47 5 COS/FUV TIME-TA PSA G140L 1105 none none none none 2 N/A 3229 G OMEGA-CEN-132712-47 6 COS/FUV TIME-TA PSA G140L 1105 none none none none 2 N/A 3229 G 13707( 3) - 10-Jun-2015 19:37:50 - [ 5] Visit: 02 Visit Requirements: On Hold Comments: Additional Comments: FUV spectroscopy of V5 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 OMEGA-CEN-1 COS/FUV ACQ/PEA PSA G140L 1105 1 6.5 S (6.5 32715-47305 KXD S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 OMEGA-CEN-1 COS/FUV ACQ/PEA PSA G140L 1105 NUM-POS=5, STEP-SIZE=0.9, 1 15.5 S 32715-47305 KD CENTER=FLUX-WT-FLR (15.5 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 3 OMEGA-CEN-1 COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=2400, FP-POS=1 1 1209 S 32715-47305 G (1209 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 4 OMEGA-CEN-1 COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=2400, FP-POS=2 1 1209 S 32715-47305 G (1209 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 5 OMEGA-CEN-1 COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=2400, FP-POS=3 1 1459 S 32715-47305 G (1459 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 6 OMEGA-CEN-1 COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=2400, FP-POS=4 1 1459 S 32715-47305 G (1459 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ OMEGA-CEN-132715-47 1 COS/FUV ACQ/PEA PSA G140L 1105 none none none none 1 N/A 3051 KXD OMEGA-CEN-132715-47 2 COS/FUV ACQ/PEA PSA G140L 1105 none none none none 1 N/A 3051 KD OMEGA-CEN-132715-47 3 COS/FUV TIME-TA PSA G140L 1105 none none none none 1 N/A 3051 G OMEGA-CEN-132715-47 4 COS/FUV TIME-TA PSA G140L 1105 none none none none 1 N/A 3051 G OMEGA-CEN-132715-47 5 COS/FUV TIME-TA PSA G140L 1105 none none none none 2 N/A 3051 G OMEGA-CEN-132715-47 6 COS/FUV TIME-TA PSA G140L 1105 none none none none 2 N/A 3051 G 13707( 3) - 10-Jun-2015 19:37:50 - [ 6] Summary Form for Proposal 13707 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/PEAKD, ACQ/PEAKXD, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=2400, CENTER=FLUX-WT-FLR, FP-POS=1, FP-POS=2, FP-POS=3, FP-POS=4, NUM-POS=5, STEP-SIZE=0.9 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements GS ACQ SCENARIO BASE1B3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names OMEGA-CEN-132712-473229, OMEGA-CEN-132715-473051 ------------------------------------------------------------------------------------------------------------------------------------