9071( 2) - 02/19/01 23:59 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9071 Version: 2 Check-in Date: 19-Feb-2001 23:59:01 1.Proposal Title: Sakurai's Novalike Object: Real-Time Monitoring of a Stellar Thermal Pulse ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Howard Bond Space Telescope Science Institute CoI: Dr. Don Pollacco Queens University Belfast N ------------------------------------------------------------------------------------ 5. Abstract This is a continuation of a Cycle 7-9 program. Sakurai's Object (V4334 Sgr) presents a ``once-in-a-lifetime'' opportunity for real-time observations of a star undergoing a final helium thermal pulse. The star rose from obscurity to become an 11th- magnitude ``born-again'' hydrogen-deficient red giant in 1995-96, and currently it is undergoing episodes of atmospheric dust formation. If it follows the pattern that the similar object V605 Aql took early this century, it will soon begin evolving back to high temperature. During the subsequent few years, it will begin to (re)- ionize its large, faint, old planetary nebula as well as the new ejecta, and we should be able to witness the re-establishment and evolution of a fast stellar wind as the effective temperature increases. \looseness=-1 When the star does begin to heat up, we will initiate Target-of-Opportunity STIS observations to monitor the star's spectroscopic development in the UV at regular intervals, continuing over the next 3 Cycles. We will also use WFPC2 twice over the next 3 years to continue our monitoring of the expansion of the ejecta and to determine the star's proper motion. In combination with ground-based monitoring (optical, IR, and mm), we will thus produce the first detailed case study of a thermal pulse, as the star re- traces its evolution across the HR diagram from the AGB back to the hot planetary-nebula- nucleus phase. ------------------------------------------------------------------------------------ 9071( 2) - 02/19/01 23:59 - [ 2] Observations Description ------------------------ V4334 Sgr is monitored regularly both by amateur astronomers (visual and CCD photometry) and by professionals (optical spectroscopy, IR photometry and spectra). Once we learn that the expected transition of Sakurai's Object back toward high surface temperature has begun to occur, we will begin to obtain STIS UV spectra at the rate of twice a year. In addition, regardless of its behavior, we will obtain two sets of WFPC2 images. begincenter STIS Target-of- Opportunity Program endcenter The STIS observations will initially be with the G230L grating, covering the range 1570 < Lambda < 3180 A, allowing us to track the changes in features such as the Mg II Lambda 2800 doublet. Later, as the star heats up further, we will include spectra taken with G140L in the range 1150< Lambda < 1730, in order to track the evolution of C IV lambda 1550 and other wind-sensitive resonance lines. The evolution should be at nearly constant bolometric luminosity, but increasing temperature (and hence a steadily fainter V magnitude, due to the bolometric correction). Consider three points in the evolution back to higher temperature: (a) when the star has warmed to about 8, 000 K and has V=11; (b) when it has reached 15, 000 K and V=12.2; and (c) when it reaches 30, 000 K and V=14. The (foreground) interstellar reddening is E(B-V)~0.6, and we assume that a clear hole will have opened up in the dust shell in our direction. In each case we assume a 5400-sec exposure, obtained over two HST orbits, and the 52*0.2 arcsec slit. At point (a), the G230L grating will give us S/N~200 per resolution element at lambda2800. At point (b), the G140L grating will give S/N~50 per resolution element at Lambda1550. And at point (c), we would obtain S/N~55, again at Lambda1550. In order to suitably sample the warming rate we will observe twice a year, and we would want to maintain this coverage for at least 3 years. This leads to a request of 4 STIS orbits per Cycle, throughout our Long-Term program. begincenter WFPC2 Regular Program endcenter In addition, we want to continue our monitoring of Sakurai's star with WFPC2 direct imaging, which we have carried out to date at roughly yearly intervals. This monitoring does not need to be as frequent as in the past (since we did not find any light echoes or other transient features---Bond 2000). However, the anticipated diameter of the ejecta from the 1996 outburst at the end of Cycle 10, i.e., in mid-2002, is diam = 0sec point12 left(v_ expvphantomd/100 kms\vphantomp\right) left(2 kpcoverd\right) left(t- 1996\vphantomd/2002-1996\vphantomp\right), where the nominal distance of 2 kpc is taken from Duerbeck et al. (2000), and the nominal expansion velocity of the ejecta is that of V605 Aql (Pollacco et al.\ 1992). We are thus on the verge of being able to resolve the ejecta with HST, justifying WFPC2 observations in 2002. We would also want to obtain another direct image with WFPC2 (or one of its successors) during Cycle 12, when the predicted diameter will have grown to ~0sec point17. The direct imaging observations are not target-of-opportunity: they should be obtained regardless of the star's behavior. In addition to the probability of beginning to resolve the ejecta, the WFPC2 frames will provide information on the star's proper motion, a key indicator of its approximate distance. Each WFPC2 visit takes one orbit, one of them in Cycle 10, and the second in Cycle 12. Real Time Justification ----------------------- As noted above, the spectroscopic portion of this proposal is a Target-of- Opportunity program. Probability of occurrence of the onset of evolution back to high temperature. There is some theoretical guidance, in the sense that the time to evolve from hot PNN to born-again red giant should be approximately comparable to the time for the subsequent evolution back to high temperature (Iben et al. 1983, Fig. 1). Weak observational guidance for when V4334 Sgr will begin to retrace its evolution to higher temperatures comes from V605 Aql, where we know that it evolved from hot PNN to born-again red giant over less than 5 years, and was a hot nucleus again when the first modern observations were made about half a century later. The default guess is that the evolution of V4334 Sgr back to higher temperature might begin about 5 years after it reached its optical maximum, i.e., during the HST Cycle 10-12 interval. We also require that a hole in the dust envelope has opened up sufficiently for us to obtain the spectra. Although the opening of such a hole may seem unlikely, the V605 Aql post-maximum light curve showed several brightenings indicative of lower dust attenuation. Further support for this possibility is given by the HST images of V605 Aql (cited in the Scientific Justification) that show its dust shell to be rather patchy, as well as the knotty-appearing images of the other more evolved post-shell-flash objects (A 30 and A 78). The R CrB variables also show evidence of patchy, highly non-spherical dust ejection. We thus think the combination of the extremely high scientific return, together with the non-zero probability of being able to make the observations successfully, warrants acceptance of the STIS spectroscopy as a Long-Term (Cycles 10-12) Target-of-Opportunity program. (Note again that the WFPC2 observations should be obtained regardless of the star's behavior.) Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9071( 2) - 02/19/01 23:59 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SAKURAI- Star, nova-like, RA=17H 52M 32.73S +/- 1", DEC=- J2000 V=+115 V = 22 NOVALIKE-VAR planetary nebula 17D 41' 06.8" +/- 1" E(B-V) = 0.6 central star; ISM, planetary nebula Coordinate Source: OTHER_SOURCE Comments: Coordinates are taken from measurements of WFPC2 frames taken in August 1996. The target flux level refers to the central star, and is the approximate anticipated brightness at the time of the 2001 observation. Note that the star is highly variable and its brightness cannot be predicted precisely in advance, but it is monitored regularly from the ground so we will know its brightness in advance of any particular HST observation and can adjust exposure times accordingly. 9071( 2) - 02/19/01 23:59 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENTation 93.05D TO 93.05D BETWEEN 01-AUG-2001 AND 31-DEC-2001 SCHEDulability 30% On Hold Comments: Additional Comments: ORIENT requirement is added to ensure same orientation as images taken in August 1996 and other subsequent visits. Exposure times assume star will have V = 22 and may have to be modified somewhat based on actual brightness. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 SAKURAI-NOV WFPC2 IMAGE PC1-FIX F502N CR-SPLIT=0.5 1 700 S POS TARG -8.5, -8.5 ALIKE-VAR ------------------------------------------------------------------------------------------------------------------------------------ 2 SAKURAI-NOV WFPC2 IMAGE PC1-FIX F555W CR-SPLIT=0.5 1 700 S SAME POS AS 1 ALIKE-VAR ------------------------------------------------------------------------------------------------------------------------------------ 3 SAKURAI-NOV WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=0.5 1 160 S SAME POS AS 1 END ORBIT ALIKE-VAR ------------------------------------------------------------------------------------------------------------------------------------ 9071( 2) - 02/19/01 23:59 - [ 5] Visit: 02 Visit Priority: Visit Requirements: SCHEDulability 50% ON HOLD On Hold Comments: Waiting for target to begin evolution to high temperature. Additional Comments: Target of Opportunity observation. Exposure times and possibly wavelength coverage will be modified depending on actual brightness and temperature of target. The STIS/MAMA observations specified below assume a V mag of 11, effective temperature of 10, 000 K, and E(B-V)=0.6. Depending on the exact status of the object, we might use the CCD instead. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 SAKURAI-NOV STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 0.1 S ALIKE-VAR CD P Comments: ETC ID = 13047 ------------------------------------------------------------------------------------------------------------------------------------ 2 SAKURAI-NOV STIS/N ACCUM 52X0.2 G230L 2376 1 2200 S EXPAND END ORBIT ALIKE-VAR UV-MAM A Comments: ETC ID = 13103 ------------------------------------------------------------------------------------------------------------------------------------ 3 SAKURAI-NOV STIS/F ACCUM 52X0.2 G140L 1425 1 2800 S EXPAND END ORBIT ALIKE-VAR UV-MAM A Comments: ETC ID = 13120 ------------------------------------------------------------------------------------------------------------------------------------ 9071( 2) - 02/19/01 23:59 - [ 6] Visit: 03 Visit Priority: Visit Requirements: SCHEDulability 50% ON HOLD On Hold Comments: Waiting for target to begin evolution to high temperature. Additional Comments: Target of Opportunity observation. Exposure times and possibly wavelength coverage will be modified depending on actual brightness and temperature of target. The observations specified below assume the star has evolved to V mag of 12.5, effective temperature of 20, 000 K, and has E(B-V)=0.6. Depending on the exact status of the object, we might use the CCD instead. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 SAKURAI-NOV STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 0.2 S ALIKE-VAR CD P Comments: ETC ID = 13440 ------------------------------------------------------------------------------------------------------------------------------------ 2 SAKURAI-NOV STIS/N ACCUM 52X0.2 G230L 2376 1 2200 S EXPAND END ORBIT ALIKE-VAR UV-MAM A Comments: ETC ID = 13445 ------------------------------------------------------------------------------------------------------------------------------------ 3 SAKURAI-NOV STIS/F ACCUM 52X0.2 G140L 1425 1 2800 S EXPAND END ORBIT ALIKE-VAR UV-MAM A Comments: ETC ID = 13452 ------------------------------------------------------------------------------------------------------------------------------------ 9071( 2) - 02/19/01 23:59 - [ 7] Summary Form for Proposal 9071 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX F28X50LP 52X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 STIS/CCD STIS/NUV-MAMA STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=0.5 ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENTation 93.05D TO 93.05D BETWEEN 01-AUG-2001 AND 31-DEC-2001 SCHEDulability 30% POS TARG -8.5, -8.5 SAME POS AS 1 SAME POS AS 1 END ORBIT SCHEDulability 50% ON HOLD EXPAND END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F502N F555W F814W MIRROR G230L G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names SAKURAI-NOVALIKE-VAR ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2376 1425 ------------------------------------------------------------------------------------------------------------------------------------