8714( 2) - 04/19/00 18:16 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8714 Version: 2 Check-in Date: 03-Mar-2000 13:15:52 1.Proposal Title: The ^12C/^13C abundance ratio in NGC 3242 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Francesco Palla Osservatorio Astrofisico di Arcetri CoI: Dr. Daniele Galli Osservatorio Astrofisico di Arcetri N CoI: Dr. Alessandro Marconi Osservatorio Astrofisico di Arcetri N CoI: Dr. Letizia Stanghellini Space Telescope Science Institute N CoI: Dr. Monica Tosi Osservatorio Astronomico di Bologna N ------------------------------------------------------------------------------------ 5. Abstract We propose to carry out high quality spectroscopic observations of ^12C and ^13C in the ionized gas of the planetary nebula (PN) NGC 3242 by using the C iii multiplet near <~m 1908 Angstrom. Our goal is to determine the ^12C/^13C ratio in this object, following the method successfully pionereed by Clegg et al. (1997). NGC 3242 is the only PN with a measured abundance of ^3He, an isotope of cosmological interest. The observed ^3He abundance is in agreement with the predictions of standard stellar evolution models for a ~ 1 M_\odot star. However, low-mass stars cannot produce ^3He at the level indicated by NGC 3242 and by standard models otherwise the resulting ^3He abundance in the Galaxy would be a factor ~ 20--100 higher than observed in the local ISM, in H ii regions, and in meteorites. The only solution to this problem appears to rely on a non-standard mixing process that preferentially destroys ^3He during the RGB/AGB phases. A consequence of such mixing mechanism is the reduction by a factor ~ 5 of the ^12C/^13C ratio in the stellar envelope. Thus, an independent measurement of the ^12C/^13C ratio, combined with the available information on the ^3He abundance, will make possible to perform a crucial test on theories of low-mass stellar nucleosynthesis. Only STIS on HST can perform the required UV observations of the C iii multiplet and measure the carbon isotopic ratio in this unique PN. ------------------------------------------------------------------------------------ 8714( 2) - 04/19/00 18:16 - [ 2] Observations Description ------------------------ The PN NGC 3242 has an outer diameter in the optical light of 25 arcsec. This is also clearly seen in the IUE slitless spectrum where the nebulosity extends for less than fifteen IUE pixels outward of the strong stellar continuum. We will observe NGC 3242 with STIS, through the G230M NUV-MAMA grating and the prime tilt centered at <~m 1933 Angstrom. Since we aim at detecting and resolving the C iii multiplet near <~m 1908 Angstrom, we choose the G230M grism whose range well encompasses the spectral region of interest, and whose pixel scale (0.09 Angstrom per pixel) is adequate for our observations. In order to evaluate the exposure time for our source, we use the following procedure: (1) NGC 3918 has a <~m 1909.6 Angstrom line flux of ~ 2.5* 10^-14 ergs cm^-2 s^-1. It was detected by Clegg Etal (1997) with the GHRS, by using the large aperture (projected area of 1.74 arcsec^2 after COSTAR). Since the area of NGC 3918 is larger than the aperture, assuming uniform emission, the line surface brightness in the <~m 1909.6 Angstrom line is approximately 1.5* 10^-14 ergs cm^-2 s^-1 arcsec^-2. Note that the central star is not bright in the emission lines, but only in the underlying continuum. (2) Comparing the IUE low-res spectra of NGC 3242 and NGC 3918, we find that F(NGC 3242)/F(NGC 3918) ~eq 1 in the spectral region around 1900 Angstrom. With the above constraints, we can calculate the expected surface brightness for NGC 3242 to be approximately 1.5* 10^-14 ergs cm^-2 s^-1 arcsec^-2 in the line of ^13C iii at <~m 1909.6 Angstrom. (3) Using G230M with the 52* 0.1 arcsec slit, we can achieve a spectral resolution of 0.36 Angstrom (~25 km s^-1) which is adequate to resolve the ^13C iii line at <~m 1909.6 Angstrom from the stronger ^12C iii line at <~m 1908.7 Angstrom. By feeding the estimated surface brightness for NGC 3242 to the STIS exposure time calculator available on the Web with an exposure time of about 13000 seconds, we achieve a S/N~2 per resolution element (i.e. 3 pixels in the dispersion direction and 2 pixels in the spatial direction). The big advantage of using STIS is that we can sum along the slit over about 1000 pixels (corresponding to ~500 resolution elements). Thus, in the final spectrum averaged over the total slit length, the signal-to-noise ratio of the weak ^13C iii line is expected to reach S/N~40. Taking into account that our target is visible on average 53 min per orbit, and that the total time for guide star acquisition, target acquisition and instrument overheads is 18 min in the first orbit and 6 min in the following ones, we need a total of 5 orbits to reach our goal. Due to the spatial extent of NGC 3242, we do not need any ACQ/PEAK. The global count rate is below the bright limit for the NUV-MAMA detector. As a team we plan to be able to derive as much information as possible from the requested observations, since we have experience both in the analysis of PNs' spectrophotometric data from ground based telescopes and from HST, on the derivation of chemical abundances and on the galactic evolution modeling which will be necessary to interpret the resulting values. Real Time Justification ----------------------- No supporting/coordinated observations are planned. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8714( 2) - 04/19/00 18:16 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC3242 ISM, PLANETARY NEBULA RA=10H 24M 46.1S +/- 1", DEC=- J2000 V = 12.7+/-0.5 18D 38' 32.6" +/- 1", PLATE- SURF-LINE(1908.7)=1+/-0.5e-12 ID=04UU SURF-LINE(1906.7)=1+/-0.5e-12 Coordinate Source: HST_IMAGE Comments: Broad Band magnitude is for the central star of the PN. Line surface brightnesses are the maximum expected for the CIII] doublet. 2 NGC3242- ISM, PLANETARY NEBULA RA-OFF=0.14S+/- 0.02S, DEC- J2000 SURF-LINE(1908.7)=1+/-0.5e-12 OFFSET OFF=0.6"+/- 0.05", FROM 1, PLATE SURF-LINE(1906.7)=1+/-0.5e-12 -ID=04UU V = 12.7+/-0.5 Coordinate Source: HST_IMAGE Comments: Line surface brightnesses are the maximum expected for the CIII] doublet. 8714( 2) - 04/19/00 18:16 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 27D TO 39D ORIENT 207D TO 219D On Hold Comments: Additional Comments: ORIENT was chosen to have STIS slit at PA between -18 and -6 deg. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC3242 STIS/C ACQ F28X50O MIRROR 1 10 S CD III ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC3242-OFF STIS/N ACCUM 52X0.1 G230M 1933 1 35 M EXPAND SET UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC3242-OFF STIS/N ACCUM 52X0.1 G230M 1933 1 45 M EXPAND SET UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC3242-OFF STIS/N ACCUM 52X0.1 G230M 1933 1 45 M EXPAND SET UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC3242-OFF STIS/N ACCUM 52X0.1 G230M 1933 1 45 M EXPAND SET UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC3242-OFF STIS/N ACCUM 52X0.1 G230M 1933 1 45 M EXPAND SET UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8714( 2) - 04/19/00 18:16 - [ 5] Summary Form for Proposal 8714 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50OIII 52X0.1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 27D TO 39D ORIENT 207D TO 219D EXPAND ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G230M ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC3242 NGC3242-OFFSET ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1933 ------------------------------------------------------------------------------------------------------------------------------------