7732( 5) - 10/12/98 11:51 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7732 Version: 5 Check-in Date: 12-Oct-1998 11:51:02 1.Proposal Title: Extreme UV Stars in Omega Cen. CY 7 Extension ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO/CAR 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Robert W O'Connell University of Virginia CoI: Ben Dorman University of Virginia N CoI: Noella D'Cruz University of Virginia N CoI: Wayne Landsman Hughes/STX N CoI: Ralph Bohlin Space Telescope Science Institute N CoI: Theodore Stecher Goddard Space Flight Center N CoI: Robert Hill Hughes/STX N CoI: Jonathan Whitney University of Virginia N CoI: Robert Rood University of Virginia N ------------------------------------------------------------------------------------ 5. Abstract Far-ultraviolet imaging of the globular cluster Omega Centauri with the ASTRO-1 Ultraviolet Imaging Telescope has identified a large sample of hot stars in advanced evolutionary phases. Most of these appear to be normal blue horizontal branch (HB) stars, but some fall in extreme locations in the color- magnitude diagram. These include stars bluer than the nominal end of the HB, several of which have colors bluer than that of an infinitely hot blackbody, and very blue objects more luminous than HB stars which may be in the "AGB-Manque" phase of post-HB evolution. Here, we propose to obtain FOS spectra of a sample of these stars with the goal of clarifying their astrophysical status. Supplementary UV/optical WFPC2 frames will provide accurate positions and valuable high signal-to-noise photometry of the hot star population. ------------------------------------------------------------------------------------ 7732( 5) - 10/12/98 11:51 - [ 2] Observations Description ------------------------ Given the TAC allocation of 15 orbits, our program now will involve spectroscopy of 6 stars in Omega Cen. A typical observation will consist of two visits to each target. In the first visit, WFPC2 early acquisition images of the FOS targets are obtained in order to derive offset coordinates accurate to 0.3 arc-sec, suitable for FOS acquisition. We estimate that existing coordinates for the brightest target (UIT #151) are good enough for an FOS acquisition without a separate WFPC2 exposure. (The coordinates have been measured with respect to cluster stars with available ground-based astrometry from Dickens and tied to the local GSS reference stars.) In two cases, two targets will fall within a single WFPC2 frame. Thus, only 3 WFPC2 early acquisition visits will be scheduled. The telescope roll angle is restricted for the fields containing two sources; for each field there are two ranges of roll angle possible, each 90 degrees in width and separated by 180 degrees. During each imaging visit, a F160BW and a F555W exposure will be taken. The far -UV exposure (considerably longer than in V) is required to unambiguously identify the UV-bright target, since the available UIT coordinates are in most cases only good to ~1 arcsec. The V-band exposure is necessary to assess problems with field crowding (potentially severe in this very rich cluster) and to select an appropriate offset star for acquisition. These frames will also be used as part of the science analysis, providing accurate UV-visual color- magnitude-diagrams to analyze the population of very hot objects in Omega Cen. Exposure times were calculated from the WFPC2 Instrument Handbook, taking into account the reduced throughput of the F160W filter (Watson et al. 1994, preprint). The F160BW exposure time will be 1620 secs. The bright red giants and numerous main sequence stars will be suppressed on this exposure, and the time is determined by the average brightness of the targets. The images should yield S/N = 20 photometry for the faintest of the targets. All exposures will be CR-split (50/50). In the V-band, the target stars will be much fainter than the giants and main sequence objects. Stars near the tip of the cluster giant branch are 8-9 mags brighter than the target stars, posing a serious threat of CCD image bleeding. Therefore, the V-band exposures will be broken into four parts of 2 seconds each. These are short enough that the brightest giants should not produce bleeding. Most FOS visits consist of two orbits. Much of the first orbit for each object is used for target acquisition, since these will be done by offset from a nearby brighter source. We hope to use the 1'' circular aperture for most observations, since most targets appear to be relatively uncrowded on our existing images, and the smaller apertures require longer acquisition times. We cannot select the aperture until the WFPC2 frames are inspected, however. V magnitudes for the target stars, needed for acquisition, will be measured from the WFPC2 frames. Exposures will then be taken with the G160L and G400H gratings to obtain spectra at a signal-to-noise ratio of 20 per diode and resolutions of 6.60 and 2.95 Angstroms, respectively. For the two bluest stars in the sample, we will not take G400H exposures, since they would be prohibitively long. FOS confirmation images will also be taken to positively identify each target. 6/97 Update: FOS observations of all targets except UIT 1124 (the hottest source) were obtained during Cycles 5 and 6. The program was offered a 2-orbit extension using STIS during Cycle 7. We will devote both orbits to spectroscopy with the STIS First-Order MAMA G140L configuration. We will use the 52 x 0.5 arcsec-wide slit, which eliminates the need for a peakup exposure. A suitable offset star (V = 13.8) within 3 arcsec was identified using the early-acquisition WFPC2 F160 and F555 images. We have removed all other program targets and observations from the original Cycle 5 program from this file, leaving only one visit in the file. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- Supplementary observations: Broad-band far-UV images were taken of Omega Cen with UIT during the ASTRO-1 mission of December 1990 (AJ, 108, 1350, 1994). A Stromgren u CCD image was obtained at CTIO to match the UIT sources and produce a far-UV, Stromgren u color-magnitude diagram. Additional UIT UV images of Omega Cen and other clusters will be obtained during the ASTRO-2 mission. 6 sources which were bright enough have been observed with IUE, and several others are scheduled. Dickens, Hatzidimitriou, & Rood have acquired ground based spectra of more than 400 BHB stars which can be used for coordinated abundance studies. ------------------------------------------------------------------------------------ 7732( 5) - 10/12/98 11:51 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7732( 5) - 10/12/98 11:51 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC5139- STAR, SDO RA=13H 26M 51.040S +/- 0.08S, 2000 V = 17.67+/-0.2 UIT1124 DEC=-47D 29' 2.84" +/- 1.3", B-V = -0.4+/-0.2 PLATE-ID=00AK F-CONT(1620)=47+/-8e-16 F-CONT(3500)=57+/-30e-18 Comments: Very hot star. FUV contin flux from UIT. V flux from WFPC2-F555W. 2 NGC5139- STAR RA=13H 26M 51.117S +/- 0.08S, 2000 V = 13.81+/-0.1 UIT1124- DEC=-47D 29' 5.84" +/- 1", PLATE U = 15.97 OFFSET -ID=00AK Comments: Offset star, identified on WFPC2 F555W exposure. Color unknown. 7732( 5) - 10/12/98 11:51 - [ 5] Visit: 01 Visit Priority: Visit Requirements: 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 ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC5139-UIT STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 0.5 S ONBOARD ACQ FOR 10-20 1124-OFFSET CD P ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC5139-UIT STIS/F ACCUM 52X0.5 G140L 1425 1 2300 S 1124 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5139-UIT STIS/F ACCUM 52X0.5 G140L 1425 1 2600 S 1124 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 7732( 5) - 10/12/98 11:51 - [ 6] Summary Form for Proposal 7732 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.5 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO/CAR ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 10-20 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC5139-UIT1124-OFFSET NGC5139-UIT1124 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 ------------------------------------------------------------------------------------------------------------------------------------