17303( 4) - 25-Apr-2023 20:20:06 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17303 Version: 4 Check-in Time: 25-Apr-2023 20:20:06 STScI Edit Number: 0 Title The primordial helium isotope ratio ------------------------------------------------------------------------------------ Type Cycle GO 30 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Ryan J. Cooke Durham Univ. Y CoI: Dr. Pasquier Noterdaeme CNRS, Institut d'Astrophysique de Paris N CoI: Prof. Max Pettini University of Cambridge N CoI: Dr. Louise Welsh University of Milano-Bicocca N CoI: Prof. David Weinberg The Ohio State University N CoI: Dr. Celine Peroux European Southern Observatory - Germany N CoI: Mr. James William Johnson The Ohio State University N ------------------------------------------------------------------------------------ Abstract The cosmic abundance of helium-3 (3He) is a key probe of cosmology and galactic chemical evolution. However, 3He is extremely difficult to measure. Recently, metastable 3He (3He I*) was detected for the first time in absorption against the light of an O star in the Orion Nebula. Motivated by their discovery, we searched the world's optical and near-infrared data archives for new examples of He I* absorbers. With this 1 orbit mid-cycle application, we propose to record HST/COS FUV data of the highest value target that we have only just identified - a young O star in the outer Milky Way (at a galactocentric radius of ~12.5 kpc) that intersects previously unidentified strong He I* absorption. Galactic chemical evolution models predict that the helium isotope ratio (3He/4He) in this distance regime is enhanced by just ~10 percent relative to a primordial composition. The key goal of these HST data is to measure the gas-phase metallicity of the cloud that gives rise to the near-infrared He I* absorption. Combining our new result with literature measurements, we will fit a linear relationship between metallicity and the 3He/4He ratio, and extrapolate this relationship to zero-metallicity; such an extrapolation will deliver our current best opportunity to deduce - for the first time - the primordial 3He/4He value. As an added bonus, with the same data we will: (1) compare absorption and emission line abundances of this outer Milky Way H II region; (2) search for evidence of a radial abundance gradient in the Milky Way using absorption line techniques; and (3) assess the prospects of measuring D/H with a future application. ------------------------------------------------------------------------------------ Observations Description ------------------------ Our aim is to measure the metallicity of the H II region PDS 241, which intersects strong metastable He I* absorption. The star that we will target is the brightest in the H II region. We will use COS/FUV (G130M with CENWAVE=1291) to measure the H I column density from Lya (1215A) and a column density of a volatile metal (N I, O I, or S II). We note that the H I column density of this sightline has never been measured, and it's FUV flux is not known. Based on the U band magnitude (U=12.5) and the extinction (E(B-V)=0.6) we have estimated that with one orbit, we will achieve a S/N=20 per resolution element. We will 17303( 4) - 25-Apr-2023 20:20:06 - [ 2] also use these data to assess the feasibility of measuring D I with future observations. ------------------------------------------------------------------------------------ 17303( 4) - 25-Apr-2023 20:20:06 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 RAFGL-5223 STAR, B0-B2 V-IV, BE, RA=07H08M38.7907S +/- 0.0174", J2000 V = 12.06 U=12.48, B=12.71 EMISSION LINE NEBULA, DEC=-04D19'4.85" +/- 0.0177" STAR FORMING REGION Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. The PI confirms the coordinates. Note that the target is a point source, but it resides in an H II region that is extended. The FUV and NUV magnitudes of the target are not known, so the buffer time is estimated based on a B0V star with E(B-V)=0.95, and U=12.5. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000.0 -7.668433420523167E-5 1.1399999999999999E-4 0.0001682 ------------------------------------------------------------------------------------------------------------------------------------ 17303( 4) - 25-Apr-2023 20:20:06 - [ 4] Visit: 01 (PDS241) Visit Requirements: On Hold Comments: Additional Comments: We only use FP-POS=3, as per the guidelines for G130M/1291. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 RAFGL-5223 COS/NUV ACQ/IMA BOA MIRRORA 1 44 S (44 GE S) ------------------------------------------------------------------------------------------------------------------------------------ 2 RAFGL-5223 COS/FUV TIME-TA PSA G130M 1291 FLASH=YES, FP-POS=3, 1 1107 S G BUFFER-TIME=2600, SEGMENT=BOTH (1107 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 RAFGL-5223 COS/FUV TIME-TA PSA G130M 1291 FLASH=YES, FP-POS=4, 1 1107 S G BUFFER-TIME=2600, SEGMENT=BOTH (1107 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ RAFGL-5223 1 COS/NUV ACQ/IMA BOA MIRRORA none none none none 1 N/A GE RAFGL-5223 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 N/A G RAFGL-5223 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 N/A G 17303( 4) - 25-Apr-2023 20:20:06 - [ 5] Summary Form for Proposal 17303 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures BOA, PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=2600, FLASH=YES, FP-POS=3, FP-POS=4, SEGMENT=BOTH ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G130M, MIRRORA ------------------------------------------------------------------------------------------------------------------------------------ Target Names RAFGL-5223 ------------------------------------------------------------------------------------------------------------------------------------