Title Surveys of interstellar HCO+ absorption Pi R. Lucas Time 80 hrs 1. Name of program and authors Surveys of interstellar HCO+ absorption R. Lucas, H. Liszt 2. One short paragraph with science goal(s) Absorption measurement in front of background sources is the preferred method to detect unexcited interstellar molecules. Low density environments (diffuse and translucent clouds) are sites of a remarkable complex chemistry, despite their unprotected exposition to photodissociating radiation from stars. Relative abundances of the molecules are grossly similar to those of dark clouds, but with several notable differences. Moreover the small observed scatter in some of the abundance ratios clearly indicates that these low density regions are a choice laboratory for the study of the basic physical and chemical processes of the interstellar medium. We propose to do a systematic survey of the typically 900 point sources of flux larger than 0.2 Jy visible from Chajnantor. The available frequency coverage is 250 MHz if we use the highest velocity resolution (0.18 km/s). Alternately we may consider using only 0.37 km/s resolution and survey also the C2H molecule. This survey will provide a unique data base on diffuse/translucent molecular clouds, of importance comparable or larger than all existing optical interstellar absorption data. 3. Number of sources (e.g., 1 deep field of 4'x4', 50 YSO's, 300 T Tauri stars with disks, ...; do NOT list individual sources or your "pet object", except in special cases like LMC, Cen A, HDFS) 5 sources of About 900 sources with flux greater than 200 mJy 4. Coordinates: 4.1. Rough RA and DEC (e.g., 30 sources in Taurus, 30 in Oph, 20 in Cha, 30 in Lupus) Sources are everywhere. Indicate if there is significant clustering in a particular RA/DEC range (e.g. if objects in one particular RA range take 90% of the time) NO 4.2. Moving target: yes/no (e.g. comet, planet, ...) NO 4.3. Time critical: yes/no (e.g. SN, GRB, ...) NO 5. Spatial scales: 5.1. Angular resolution (arcsec): These are point sources. The angular resolution does not really matter. 5.2. Range of spatial scales/FOV (arcsec): (optional: indicate whether single-field, small mosaic, wide-field mosaic...) These are point sources. 5.3. Single dish total power data: yes/no NO 5.4. ACA: yes/no NO 5.5. Subarrays: yes/no NO 6. Frequencies: 6.1. Receiver band: Band 3, 6, 7, or 9 Band 3 6.2. Lines and Frequencies (GHz): (approximate; do NOT go into detail of correlator set-up but indicate whether multi-line or single line; apply redshift correction yourself; for multi-line observations in a single band requiring different frequency settings, indicate e.g. "3 frequency settings in Band 7" without specifying each frequency (or give dummies: 340., 350., 360. GHz). For projects of high-z sources with a range of redshifts, specify e.g. "6 frequency settings in Band 3". Apply redshift correction yourself) HCO+ (1-0) 89.19 GHz C2H (1-0) 87.28 GHz 6.3. Spectral resolution (km/s): 0.5 km/s 6.4. Bandwidth or spectral coverage (km/s or GHz): 250 MHz = 840 km/s is more than we really need. 7. Continuum flux density: 7.1. Typical value (Jy): (take average value of set of objects) (optional: provide range of fluxes for set of objects) Minimum 0.2 Jy, average around 0.3 Jy 7.2. Required continuum rms (Jy or K): .04 mJy is expected but not required. 7.3. Dynamic range within image: (from 7.1 and 7.2, but also indicate whether e.g. weak objects next to bright objects) 100, but no imaging is needed. 8. Line intensity: 8.1. Typical value (K or Jy): (take average value of set of objects) (optional: provide range of values for set of objects) typical 0.02 Jy (10% absorption). 8.2. Required rms per channel (K or Jy): 1% of flux density (.002 Jy for weakest sources) 8.3. Spectral dynamic range: 100 9. Polarization: yes/no (optional) no 9.1. Required Stokes total intensity only 9.2. Total polarized flux density (Jy) N/A 9.3. Required polarization rms and/or dynamic range N/A 9.4. Polarization fidelity N/A 10. Integration time for each observing mode/receiver setting (hr): total time 80 hr (600 s for 0.2Jy sources, 100s for 0.5Jy sources, 24s for 1Jy sources) 11. Total integration time for program (hr): total time 80 hr 12. Comments on observing strategy (e.g. line surveys, Target of Opportunity, Sun, ...): (optional) This is a survey. The observations are essentially self calibrated, so there is NO requirement on phase stability. The data are usable provided that the pointing accuracy is better than, say, 5" and the 3mm optical depth smaller than 0.5. This means the stringency is probably very low (close to 1). The sensitivity calculation is based on detecting features with typical line widths of 1.0 km/s. Higher velocity resolution is actually available and used to detect structure in stronger lines. ********************************************************************* Review Lee Mundy: OK. -------------------------------------------------- Review v2.0: ok