8320( 3) - 10/11/99 11:25 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8320 Version: 3 Check-in Date: 17-Feb-1999 10:35:50 1.Proposal Title: The H_2/CO ratio in the Large Magellanic Cloud ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Dominik Bomans Ruhr-Universitat Bochum CoI: Philipp Richter Sternwarte der Universitaet Bonn N CoI: Prof. Dr. Klaas S. de Boer Sternwarte der Universitaet Bonn N ------------------------------------------------------------------------------------ 5. Abstract Little is known about real abundance ratios of the two most abundant molecules H_2 and CO in the interstellar matter of the Large Magellanic Cloud. Because of their abundance, both molecules play a crucial role in the physics of molecular gas. The only possibility to observe the homoeopolar molecule H_2 in its cool phase is the measurement of its far UV resonance lines with absorption spectroscopy. So far, of 4 observed lines of sight to the LMC, only the one to LH 10:3120 showed a significant amount of the H_2 molecule at LMC velocities. The CO molecule also has resonance lines in the far UV, but due to its much smaller amount and the relatively poor signal to noise ratio in the ORFEUS spectrum of LH 10:3120, the presence of CO could not be established from those measurements. With its high resolution spectrograph (STIS), the Hubble Space Telescope provides the possibility to measure several strong CO absorption bands near 1500 Angstrom due to the LMC gas. We propose to observe LH 10:3120 and make the direct determination of the H_2/CO ratio for the LMC possible for the very first time. ------------------------------------------------------------------------------------ 8320( 3) - 10/11/99 11:25 - [ 2] Observations Description ------------------------ The target star LH10:3120 is of spectral type O5Vf, has V=12.80 and E(B-V)=0.17 mag (Parker et al. 1992). The ORFEUS spectra is flux calibrated and gives flux of 1.5 10^-12 erg cm^-2 s^-1 A^-1 at 1200 Angstrom. These data were used as input for the STIS exposure time calculator to derive the necessary exposure time below. The HST spectrograph (STIS) with E140M grating covers a wavelength range of the far UV between 1150 Angstrom and 1730 Angstrom at a spectral resolution of 45800. In this spectral range several strong CO bands are located : the A-X, 4-0 band at 1412 Angstrom; the A- X, 3-0 band at 1447 Angstrom; the A-X, 2-0 band at 1477 Angstrom\ and the A-X, 1-0 band at 1509 Angstrom. All these bands have nearly the same oscillator strengths. The transition energies from the different rotational states (given by J) are nearly the same in each band, so that the concerning absorption lines are very close together. An example for this is the A-X, 2-0 band at 1477 Angstrom: the absorption lines from R(1), R(0), and Q(1) lie in a wavelength range of 0.09 Angstrom and are therefore just resolved with the E140M grating. Unfortunately, the oscillator strengths of the individual CO transitions for the ground state (J=0) are not higher than f=0.35, so that the individual lines only become visible, if the signal-to-noise ratio exceeds a value of 50. However, is not necessary to have the individual transitions resolved to determine the total CO amount. Assuming a rotational temperature of 5 K for the CO molecule, the absorption strengths for J=0 and J=1 are nearly equal. (Absorption from transition with J=2 only contributes with 10 therefore can sum over the individual oscillator strengths for J=0, 1 and derive mean f-values of ~eq 0.06 for the individual bands. Calculating the values for log f Lambda, we can estimate the the expected equivalent widths for the CO bands for a column density of N(CO)=10^12.0 cm^-2 by using the empirical curve of growth for the H_2 for the LMC gas towards LH 10:3120 (de Boer et al. 1998). This calculation adopts the same velocity dispersion for the CO gas as for the H_2 gas. The resulting equivalent widths for the CO bands then turn out to be ~eq 5.0 milliAngstrom, equivalent to a relative absorption of 3 percent from the continuum. With an effective signal- to-noise ratio of about 45 the presented CO bands should be visible in the surrounding noise and can be measured in a reasonable degree of accuracy. As explained above, we can work with absorption bands. The ORFEUS data also show, that there is only one molecular cloud at LMC velocities in the spectrum of LH 10:3120. We therefore can lower the spectral resolution provided by the E140M grating in order to boost the S/N. We propose to observe in normal mode, but bin the data in spectral direction to a resolution of ~15000. Because MAMA has no readout noise, this observational procedure maintains the full information content of the data and also enable us to improve S/N (at the cost of spectral resolution) in the region of the CO bands. We therefore can reach our goals with a spectrum with S/N of 30 at the nominal resolution of the E140M grating, increasing the S/N to 30 * sqrt3 ~eq 50 for the binned data. Using the STIS exposure time calculator we determined the minimal necessary exposure time of LH 10:3120 to be 5800s = 96min. According the the STIS Instrument Handbook V2.0 this S/N can be reached without employing special dithering techniques, which we therefore did not implement in our exposure time estimates. At the declination of LH 10:3120, the orbital visibility lasts 57 min per orbit. During the first orbit there are the following overhead times: guide star acquisition: 6 min, target acquisition: 6 min, and target pick-up: 10 min. Thus, 33 min is available for science exposure during the first orbit. During the second orbit, 5 min re-acquisition and 3 min STIS overhead are needed, leaving 49 min of on-source integration time. This equals to 82 min after 2 orbits, well short of the minimum requirement of 96 min. This leads us to a grand total of 3 orbits for our program. No specific roll angle or other special requirements are needed for this program. Real Time Justification ----------------------- None. The supporting observations (high dispersion Far UV spectrum) has been performed already during the ORFEUS II mission (Nov/Dec. 1996). Our spectrum of LH10:3120 was taken with the Tuebingen Echelle Spectrograph and covers the spectral range from 900 to 1350 Angstrom\ at a spectral resolution of 10000. The data are presented by de Boer et al. (1998). Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8320( 3) - 10/11/99 11:25 - [ 3] Data Distribution Paper Products: NO Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8320( 3) - 10/11/99 11:25 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 LH10-3120 EXT-STAR, OF RA= 4H 56M 46.88S +/- 0.1S, DEC= J2000 V=+280 V = 12.80+/-0.02 -66D 24' 46.8" +/- 0.5", PLATE- B-V = -0.07+/-0.05 ID=06AF E(B-V) = 0.3+/-0.1 F-CONT(1300)=1.6+/-0.2e-12 Comments: Brightest star in a tight group 8320( 3) - 10/11/99 11:25 - [ 5] Visit: 01 Visit Priority: Visit Requirements: PCS MODE Fine 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 LH10-3120 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 0.1 S ONBOARD ACQuisition FOR CD P 2-5 ------------------------------------------------------------------------------------------------------------------------------------ 2 LH10-3120 STIS/C ACQ/PEA 0.2X0.0 MIRROR 1 0.3 S ONBOARD ACQuisition FOR CD K 6 3-5 ------------------------------------------------------------------------------------------------------------------------------------ 3 LH10-3120 STIS/F ACCUM 0.2X0.0 E140M 1425 1 2300.0 UV-MAM 6 A ------------------------------------------------------------------------------------------------------------------------------------ 4 LH10-3120 STIS/F ACCUM 0.2X0.0 E140M 1425 1 3000.0 SAME POSition AS 3 UV-MAM 6 A ------------------------------------------------------------------------------------------------------------------------------------ 5 LH10-3120 STIS/F ACCUM 0.2X0.0 E140M 1425 1 3000.0 SAME POSition AS 3 UV-MAM 6 A ------------------------------------------------------------------------------------------------------------------------------------ 8320( 3) - 10/11/99 11:25 - [ 6] Summary Form for Proposal 8320 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 0.2X0.06 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE Fine ONBOARD ACQuisition FOR 2-5 ONBOARD ACQuisition FOR 3-5 SAME POSition AS 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR E140M ------------------------------------------------------------------------------------------------------------------------------------ Target Names LH10-3120 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 ------------------------------------------------------------------------------------------------------------------------------------