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Overview Globular Clusters (GC) M13 and M15 HR Diagrams Observational Tools and Photometry Mees Telescope with Charge-Coupled Device (CCD) Camera Photometry with ATV Hertzsprung-Russell (HR) Diagrams of M13 and M15 Giant branches detected Bounds on main sequence detection Sources of uncertainty – room for improvement C.E.K. Mees Observatory, Naples, NY Slide 2/16 Compact spherical shape, 20-100 pc (60-300 ly) diameter 104-106 stars of same age and composition Orbiting galactic core as satellites in spherical distribution; not confined to galactic disk Studied to judge intragalactic distances, Milky Way’s structure, stellar evolution (via HR diagram) GC distribution about Milky Way (Chaboyer) Slide 3/16 Characteristic Constellation Distance from Earth Apparent Magnitude (V band) Apparent Dimensions (V band) Radius Age Concentration Class (1 high – 12 low) M13 Hercules 7.7 kpc (25.1 klyr) 5.8 mag 15’ 26 pc (84 lyr) 1.4×1010 yrs Class 5 M15 Pegasus 10.3 kpc (33.6 klyr) 6.2 mag 18’ 27 pc (88 lyr) 1.3×1010 yrs Class 4* *core-collapsed, stars mass segregated M13, through Mees RGB combined, two coadded 5-min exposures 19 Sep 08 Slide 4/16 Measure of luminosity (e.g. magnitude) vs measure of temperature (e.g. color or spectral type) Also called color-magnitude diagrams Apparent magnitude (V vs B-V) incorporates distance Slide 5/16 Main sequence fitting Shift from apparent mag to absolute mag for spectral type gives cluster distance modulus Turn-off point (a “knee”) Hotter stars (i.e. earlier spectral type, higher luminosity) leave main sequence TP farther down main sequence indicates older cluster age (via luminosity-mass relation) Slide 6/16 C.E.K Mees observatory Located 40 mi south of UR 24” mechanized Cassegrain reflector telescope Plate scale: 25”/mm SBIG ST9XE CCD Camera 512×512 pix at 25 μm/pix → Plate scale on Mees: 0.5”/pix → Field of view: 4.3’×4.3’ SBIG CW8 filter wheel Filter Red Green (V = Visual) Blue CCD Camera with filter wheel Passband (nm) 612-670 488-574 312-508 Slide 7/16 M13: three 5-minute exposures per RGB filter (only two per filter were unobstructed) M15: two 10-minute exposures per RGB filter (only one per filter was unobstructed) Electronic noise corrections: bias, dark sky, dark exposure, flat field Extreme pixel correction: hot/cold pixel removal (satellite tracks, cosmic rays, broken pixels) Align images and add according to filter Flux calibration with standard star N E 30” M13, reduced 5-min exposure, green filter 19 Sep 08 Slide 8/16 ATV in Interactive Data Language (IDL) (screenshots at right) Interface for working with astronomical images Point-and-click aperture photometry but by hand for every star (~100 stars in view for G and B filters) Screen shots Other methods Starfinder in IDL – would not recognize stars MOPEX from Spitzer Science Center, CalTech – needs thorough modifications to work with nonSpitzer data Slide 9/16 0 2 V (mag) 4 6 8 10 12 14 -5 -4 -3 -2 -1 0 1 B-V (mag)* *large variance likely because of by hand photometry 2 3 4 Slide 10/16 HR diagram with distance increasing from M13 core (r in pix). Mees data overplotted. (Guarnieri et al, 1993) Slide 11/16 0 2 V (mag) 4 6 8 10 12 14 -3 -2 -1 0 1 B-V (mag)* *large variance likely because of by hand photometry 2 3 4 Slide 12/16 -5 0 5 Compilation of M15 HR diagrams to illustrate stellar evolution. Mees data overplotted. (Durrell and Harris, 1993) Slide 13/16 Very large uncertainty incurred through by-hand photometry (note B-V scales do not match) Spread resembles asymptotic (red) giant branches No evidence of main sequence Better photometry is needed to: Verify distance modulus from main sequence shift Verify cluster age from turn-off point From difference in magnitudes, 20 min exposure time per filter should show main sequence. We had 10 min due to obstructed views. Slide 14/16 By-hand photometry with ATV → imprecise flux integration Fix Starfinder or adapt MOPEX to Mees images Moon ¾ full → observations background limited Observe during new to quarter moon Inaccurate tracking → stars blur and become elliptical Install guide camera (a lot of work) Thermal noise → contributes to background Use CCD electric cooler Low effective integration time → low S/N for faint sources Plan for at least three 10-minute exposures per RGB filter Slide 15/16 Chaboyer, B. "Milky Way Globular Clusters." personal homepage, Dartmouth College. http://www.dartmouth.edu/~chaboyer/ mwgc.html (accessed 30 Sep 2008). Durrell, Patrick R., and William E. Harris. "A Colour-Magnitude Study of the Globular Cluster M15." The Astonomical Journal 105, no. 4 (Apr 1993): 14201440. Guarnieri, M. D., A. Bragaglia, and F. Fusi Pecci. "Colour Magnitude Diagram for the Globular Cluster M 13." Astron. Astrophys. Suppl. Ser. 102 (Dec 1993): 397-400. Yes, Mees can take good pics too. Conditions: 6:30am, 10% cloud cover, 0.001s blue filter exposure (!), very tired. Slide 16/16