Transcript harrold_kajubi_astro1

 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
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 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)
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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
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 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
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 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)
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 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
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 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
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 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
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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
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HR diagram with
distance increasing
from M13 core
(r in pix). Mees data
overplotted.
(Guarnieri et al, 1993)
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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
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-5
0
5
Compilation of M15
HR diagrams to
illustrate stellar
evolution. Mees data
overplotted.
(Durrell and Harris,
1993)
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 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.
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 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
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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.
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