8670( 2) - 09/28/00 10:30 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8670 Version: 2 Check-in Date: 03-Mar-2000 09:30:19 1.Proposal Title: The Role of Polycyclic Aromatic Hydrocarbons in Ultraviolet Extinction ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Geoffrey Clayton Louisiana State University & Agricultural & Medical College CoI: Prof. P. G. Martin University of Toronto N CoI: Dr. F. Salama NASA Ames Research Center N CoI: Prof. T. P. Snow University of Colorado N CoI: Prof. D. C. B. Whittet Rensselaer Polytechnic Institute N CoI: Prof. A. N. Witt University of Toledo N CoI: Dr. M. J. Wolff Space Science Institute N CoI: Dr. L. J. Allamandola NASA Ames Research Center N CoI: Dr. K. D. Gordon University of Arizona N ------------------------------------------------------------------------------------ 5. Abstract We propose to use the STIS on HST to search for structure in the ultraviolet interstellar extinction curve, with particular emphasis on a search for absorption features produced by polycyclic aromatic hydrocarbons (PAHs). The existence of these molecules in the interstellar medium has been postulated to explain the infrared emission features seen in the 3-13 micron spectra of numerous sources. However, these features have stubbornly resisted satisfactory identification. We propose to obtain high S/N UV spectra of stars which are significantly more reddened than those observed in previous studies. These data, which can now be obtained through the high sensitivity of STIS and HST, will allow us to estimate the abundances of PAHs in the ISM, to make identifications of individual PAH molecules and to put strong limits on the role of PAHs in UV extinction. The observational program will be guided by our ongoing laboratory studies of neutral and ionized PAHs at the low temperatures characteristic of the interstellar environment. PAHs are of importance because of their ubiquity and high abundance inferred from the infrared data and also because they may link the molecular and dust phases of the interstellar medium. A search for ultraviolet absorptions due to PAHs could be a definitive test of this hypothesis and, if successful, can identify the particular PAH species present; this is not possible from infrared data alone. ------------------------------------------------------------------------------------ 8670( 2) - 09/28/00 10:30 - [ 2] Observations Description ------------------------ We estimate the strength of the UV absorption by starting with the estimated abundance of PAH molecules per hydrogen nucleus and scaling the optical depth of the PAH absorption to the visual extinction. Assuming that 6% of the carbon is in the form of PAHs, and taking 20 as a typical number of C atoms/PAH molecule, we find the abundance of PAHs per hydrogen nucleus to be 10^-6. Because several PAH species will be present, the abundance of any particular species is unlikely to be greater than 10^-7 per hydrogen atom. Only the small PAHs under study here are likely to be this abundant, because of the large number of isomeric forms available to molecules containing greater numbers of atoms. The absorption cross-section of a single naphthalene molecule averaged over the strong 2120 Angstrom band is about 4 x 10^-16 cm^2. The strong bands of the other small PAH species have comparable strength. With N(PAH) = 10^- 7N_H, A(V) = 5 x 10^-22N_H, and taking 2.5 x 10^-16 cm^2 as a typical band strength, we find that the average optical depth in a strong PAH band would be: Tau = (2.5 x 10^-16)(10^-7)(A(V)/5 x 10^- 22) = 0.05 A(V). Thus a typical strong band due to an individual PAH produces >10% absorption in a star with substantial extinction (A(V) greater than 2). The peak absorption in a band may be greater than the adopted average value, which leads to a higher optical depth per unit visual extinction. Earlier attempts to detect such features with IUE and Copernicus failed because the S/N was too low in the stellar spectra and the dust column was too small (Snow et al. 1977; Seab & Snow 1985). Table 1 Program Stars Star Sp.T E(B-V) Comment PAH Abs. (%)---------------------------- ----------------- HD 229059 B1.5Ia 1.73 Cyg OB2 27 HD 194279 B1.5Ia 1.32 Cyg OB2 20 HD 283809 B3V 1.46 Taurus 23 HD 151805 B1Ia 0.35 comparison HD 51038 B3V 0.04 comparison We will use the STIS MAMAs to obtain the full UV (1100-3100 Angstrom) spectra of three reddened stars and two comparison stars. See Table 1. The two-pixel signal-to-noise ratio (S/N) target for these observations is >30 for the reddened stars and 100 for the comparison stars. We wish to achieve this target S/N at 1500 and 2175 A. The PAH absorption features which are the subject of this search are very broad (> 50 A) and therefore binning can be done to increase the S/N. For analysis, the S/N will be increased by binning the data to 10 A resolution. The declination of the stars gives a visibility of 54 minutes per orbit. For the reddened star HD 229059, the exposure times to achieve our S/N target are 1225 and 8100 seconds for the FUV and NUV, respectively. Accounting for guide star acquisitions, STIS acquisition, and 2 readouts of the MAMAs requires 4 orbits for HD 229059. HD 283809 is similar to HD 229059 and will also require 4 orbits. HD 194279 is significantly brighter, enough so that we will be required to use the 31x0.05NDB slit. Our target S/N can be achieved for both the NUV and FUV in one orbit with all the overheads quoted for HD 229059 including a STIS peak-up. The comparison stars HD 151805 and 51038 are relatively unreddened stars and, as such, we will have to use the 31x0.05NDB and 31x0.05NDC slits. Even using these slits, we will be able to achieve a S/N of 100 in one orbit for the NUV and FUV. To account for the effects of using neutral density slits for two of our stars, we will use the existing observations of GD153 (GO# 7096, Bohlin) to calibrate our observations. All exposure time and signal-to- noise calculations used the STIS WWW exposure time tool. The unreddened standards have been selected to match the reddened stars within one half a spectral subtype (B1.5 versus B1). This minimizes the noise due to mismatch between the stellar features of the reddened and unreddened stars. The noise due to a mismatch of 1 spectral subtype is of similar strength to that due to photometric errors (Cardelli, Sembach, & Mathis 1992). We estimate that the maximum uncertainty in the extinction curves due to optical photometry and mismatch uncertainties to be approximately 2% due to large reddening of our sightlines (E(B-V) > 1). This is substantially smaller than the expected PAH feature strength of >20% Estimated absorptions for the three reddened stars are given in Table 1. Real Time Justification ----------------------- None Unlike many other HST proposals in which supporting astronomical observations from ground-based observatories or other satellites might be described in this section, we report instead on supporting observations made in the laboratory. Thus our program provides a unique opportunity to use the HST in concert with ``ground truth'' data obtained through experiments conducted here on the earth. We are currently engaged in a unique laboratory program to obtain UV spectra of isolated PAH species - neutral and ionized - at the low temperatures characteristic of the ISM (e.g., Salama et al. 1995, 1996; Chillier et al. 1999). We also have a laboratory program to study the gas phase chemistry of PAH ions and related molecules which will help constrain and also further explore the consequences of the results of the proposed HST program. PAHs are expected to be partially ionized in the interstellar medium. Small PAHs (less than 50 C atoms) which are the most relevant for our proposal have small electron recombination rates and can only be found in the neutral and positively ionized forms. Larger PAHs (above 50 C atoms) approach classical (grain-like) recombination rates and are predominantly neutral with a small fraction in anionic form. Recent findings indicate that these large PAHs may also be important in the ISM. We have thus measured the absorption spectra of PAHs ranging from naphthalene (C_10H_8) up to dicoronene (C_48H_20), including hydrogenated derivatives in the neutral and ionized forms, extending to wavelengths as short as 1800 A. These data constitute a representative dataset, for the UV and visible spectra of more than 25 small PAHs. At the time that the observations proposed here would be scheduled, we expect to have laboratory data on additional PAH molecules (larger PAHs and PAHs including 5-membered rings). Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8670( 2) - 09/28/00 10:30 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD229059 STAR, B0-B2 III-I RA=20H 21M 15.36S +/- 0.0", J2000 V=-65 V = 8.70+/-0.05 DEC=+37D 24' 31.2" +/- 0.0" B-V = 1.53+/-0.05 E(B-V) = 1.73+/-0.1 Coordinate Source: HIPPARCOS/TYCHO_CATALOGUE 2 HD194279 STAR, B0-B2 III-I RA=20H 23M 18.17S +/- 0.0", J2000 V=-31 V = 7.05+/-0.02 DEC=+40D 45' 32.6" +/- 0.0" B-V = 1.02+/-0.02 E(B-V) = 1.22+/-0.1 Coordinate Source: HIPPARCOS/TYCHO_CATALOGUE 3 HD283809 STAR, B3-B5 V-IV RA=04H 41M 24.72S +/- 0.0", J2000 V = 10.72+/-0.05 DEC=+25D 54' 48.6" +/- 0.0" B-V = 1.42+/-0.05 E(B-V) = 1.62+/-0.1 Coordinate Source: HIPPARCOS/TYCHO_CATALOGUE 4 HD151805 STAR, B0-B2 III-I RA=16H 51M 35.71S +/- 0.0", DEC= J2000 V = 8.91+/-0.02 -41D 46' 35.5" +/- 0.0" B-V = 0.15+/-0.02 E(B-V) = 0.37+/-0.05 Coordinate Source: HIPPARCOS/TYCHO_CATALOGUE 5 HD51038 STAR, B3-B5 V-IV RA=06H 54M 42.58S +/- 0.0", DEC= J2000 V = 9.12+/-0.05 -24D 55' 28.8" +/- 0.0" B-V = -0.15+/-0.05 E(B-V) = 0.05+/-0.1 Coordinate Source: HIPPARCOS/TYCHO_CATALOGUE 8670( 2) - 09/28/00 10:30 - [ 4] 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 HD229059 STIS/C ACQ F28X50O MIRROR ACQTYPE=POINT 1 1.0 S CD II ------------------------------------------------------------------------------------------------------------------------------------ 2 HD229059 STIS/F ACCUM 52X2 G140L 1425 2 2500 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8670( 2) - 09/28/00 10:30 - [ 5] Visit: 02 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 HD229059 STIS/C ACQ F28X50O MIRROR ACQTYPE=POINT 1 1.0 S CD II ------------------------------------------------------------------------------------------------------------------------------------ 2 HD229059 STIS/N ACCUM 52X2 G230L 2376 2 2500 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8670( 2) - 09/28/00 10:30 - [ 6] Visit: 03 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 HD194279 STIS/C ACQ F28X50O MIRROR ACQTYPE=POINT 1 0.2 S CD II ------------------------------------------------------------------------------------------------------------------------------------ 2 HD194279 STIS/C ACQ/PEA 31X0.05 MIRROR 1 0.2 S CD K NDB ------------------------------------------------------------------------------------------------------------------------------------ 3 HD194279 STIS/F ACCUM 31X0.05 G140L 1425 1 850 S UV-MAM NDB A ------------------------------------------------------------------------------------------------------------------------------------ 4 HD194279 STIS/N ACCUM 31X0.05 G230L 2376 1 850 S UV-MAM NDB A ------------------------------------------------------------------------------------------------------------------------------------ 8670( 2) - 09/28/00 10:30 - [ 7] Visit: 04 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 HD283809 STIS/C ACQ F28X50O MIRROR ACQTYPE=POINT 1 5.0 S CD II ------------------------------------------------------------------------------------------------------------------------------------ 2 HD283809 STIS/F ACCUM 52X2 G140L 1425 2 2500 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8670( 2) - 09/28/00 10:30 - [ 8] Visit: 05 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 HD283809 STIS/C ACQ F28X50O MIRROR ACQTYPE=POINT 1 5.0 S CD II ------------------------------------------------------------------------------------------------------------------------------------ 2 HD283809 STIS/N ACCUM 52X2 G230L 2376 2 2500 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8670( 2) - 09/28/00 10:30 - [ 9] Visit: 06 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 HD151805 STIS/C ACQ F28X50O MIRROR ACQTYPE=POINT 1 0.2 S CD II ------------------------------------------------------------------------------------------------------------------------------------ 2 HD151805 STIS/C ACQ/PEA 31X0.05 MIRROR 1 0.2 S CD K NDB ------------------------------------------------------------------------------------------------------------------------------------ 3 HD151805 STIS/F ACCUM 31X0.05 G140L 1425 1 850 S UV-MAM NDB A ------------------------------------------------------------------------------------------------------------------------------------ 4 HD151805 STIS/N ACCUM 31X0.05 G230L 2376 1 850 S UV-MAM NDB A ------------------------------------------------------------------------------------------------------------------------------------ 8670( 2) - 09/28/00 10:30 - [ 10] Visit: 07 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 HD51038 STIS/C ACQ F28X50O MIRROR ACQTYPE=POINT 1 0.2 S CD II ------------------------------------------------------------------------------------------------------------------------------------ 2 HD51038 STIS/C ACQ/PEA 31X0.05 MIRROR 1 0.2 S CD K NDC ------------------------------------------------------------------------------------------------------------------------------------ 3 HD51038 STIS/F ACCUM 31X0.05 G140L 1425 1 800 S UV-MAM NDC A ------------------------------------------------------------------------------------------------------------------------------------ 4 HD51038 STIS/N ACCUM 31X0.05 G230L 2376 1 800 S UV-MAM NDC A ------------------------------------------------------------------------------------------------------------------------------------ 8670( 2) - 09/28/00 10:30 - [ 11] Summary Form for Proposal 8670 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50OII 52X2 31X0.05NDB 31X0.05NDC ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ACQ/PEAK ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140L G230L ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD229059 HD194279 HD283809 HD151805 HD51038 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 2376 ------------------------------------------------------------------------------------------------------------------------------------