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 O rd er N u m b e r 9 2 2 1 2 2 4

A stu d y o f th e ab u n d an ce d istrib u tio n s along th e m inor axis o f

th e G alactic bulge

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Tyson, Neil De Grasse, Ph.D.
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Columbia University, 1992
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C opyright © 1992 b y T y so n , N e il D e G rasse. A ll righ ts reserved.

300 N. ZeebRd.
Ann Arbor, MI 48106
 A S t u d y o f t h e A b u n d a n c e D is t r ib u t io n s
A l o n g t h e M in o r A x is o f t h e G a l a c t ic B u l g e

BY

N e il D e G r a s s e T y s o n , b .a ., m .a ., m .phel.

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DISSERTATION

Submitted in Partial Fulfillment

for the Degree of
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DOCTOR of PHILOSOPHY
in the Graduate School of Arts and Sciences

COLUMBIA UNIVERSITY
1992
i

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© 1992

N eil D e Grasse Tyson

A ll Rights Reserved
 ABSTRACT

A Study of the Abundance Distributions

Along the Minor Axis of the Galactic Bulge

N e il D e G r a s s e T y s o n

I present abundance distribution functions for fields along the minor axis of the
Galactic bulge based on CCD photometric observations toward seven windows of low

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extinction. Abundance distribution functions are the most useful form of data to
constrain models of the star formation and the chemical enrichment of the bulge. By
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using the recently-calibrated Washington photometric filter system, the distribution
function in [Fe/H] is determined for each field, and consequently I derive the
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abundance gradient for the bulge. To supplement these observations I analyzed, from
medium dispersion spectra, line strengths of the 33 known bulge carbon stars. The
radial velocities of these carbons stars and of 39 bulge RR Lyrae variables is also
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presented.

Within 8 degrees of the Galactic center (~ 1 kpc) there appears to be no

appreciable gradient in the distribution of [Fe/H], which is consistent with a

dissipationless collapse, and/or sufficient mixing during the star-forming epoch when
Fe was produced in the bulge. The mean abundance over this region is between two
and five times solar. The form of these distributions is well-fitted by the simple (closed
box) model of chemical evolution where the bulge is self-enriched by processing its
original gas content to completion. This result carries two direct implications: 1) the
inner bulge was not significantly enriched by infall (of any heavy element abundance)

from the halo or the disk, 2) the inner bulge underwent no catastrophic mass-loss from
supernova-driven winds or any other mechanism. These scenarios would produce a
different signature in the abundance distributions.
The carbon stars, however, show a gradient in NaD absorption, even for the
inner bulge, with the strongest absorption near the Galactic center. This effect cannot

be explained by interstellar absorption or modifications to the surface composition

during the evolution of the star. The Washington system is calibrated to indicate iron
abundance. It may be that the carbon stars are fossils from the first generation of stars
that are enriched by the high-mass-progenitor Type II supemovae. This first generation

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may contain the memory of a dissipational collapse, which results in a gradient in Na
and a-elements (including O and Ti). After collapse, subsequent generations were well
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mixed with the ejecta of the Fe-producing Type I supemovae. Such a hybrid collapse

scenario may have important implications for models of elliptical galaxies and
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spheroidal systems in general. The idea of an early dissipational collapse finds support
in the velocity dispersion of the bulge RR Lyraes where, at ov = 124 ± 15 km s_1, they

have a higher dispersion than high abundance tracers such as the late M giants and the
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high abundance subset of bulge K giants.

Beyond 8 degrees from the Galactic center, the mean of the abundance
distribution drops precipitously with an abundance gradient of -0.2 dex / degree in

[Fe/H]. This is consistent with the notion that the inner bulge is chemically distinct

from the halo while the transition region is a blending of the two. It may be possible to
use kinematics to disentangle the two populations via a radial velocity survey.
 A Study of the Abundance Distributions
Along the Minor Axis o f the Galactic Bulge

TABLE OF CONTENTS

Table of C ontents.............................................................................................. i
List of T ables....................................................................................................... v
List of Figures............................................................................................... vii

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List of C harts........................................................................................................ xi
A cknow ledgem ents............................................................................................... xiii
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D ed ication ...............................................................................................................
1.0 In tro d u ctio n ....................................................................................................
xv
1
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1.1 O verview ................................................................................................. 2
2.0 Carbon Stars:
A Probe of the Bulge Velocity Field and Abundance Gradient 8

2.1 The 33 Carbon Stars inSix Bulge Windows................................... 9

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2.1.1 In tro d u ctio n .............................................................................. 9

2.1.2 O bservations............................................................................ 11
2.1.3 Radial V elocities................................................................... 15
2.1.4 S p e c tra ........................................................................................ 27
2.1.4.1 Line Strength Gradient........................................ 27
2.1.4.2 Carbon Isotopes................................................... 33

2.1.4.3 P a n e ls....................................................................... 39
2.1.5 D iscu ssio n ................................................................................ 49
2.1.6 S um m ary.................................................................................. 53
3.0 RR Lyrae Stars in Baade’s Window: The Velocity Distribution
of a Population withLow Heavy Element Abundance 55
3.1 In troduction........................................................................................ 56
3.2 O bservations...................................................................................... 57
3.3 R eductions........................................................................................... 57
3.4 Radial V elocities.............................................................................. 59
3.5 D iscussion........................................................................................... 64
4.0 The Washington Photometric System....................................................... 69
4.1 In tro d u ctio n ......................................................................................... 70

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4.2 The System ....................................................................................... 71

4.2.1 General Features................................................................... 71

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4.2.2 C alibrations............................................................................. 73
4.3 Previous W ork................................................................................. 78
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4.4 Q -Index ................................................................................................ 79
4.5 CCD Photom etry............................................................................. 80
4.6 D iscu ssio n ........................................................................................... 82
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5.0 Washington Photometry of the Bulge.................................................... 84

5.1 O verview ............................................................................................. 85

5.2 O bservations...................................................................................... 86
5.3 R ed u ctio n ............................................................................................ 87
5.3.1 Conversion to Standard System.......................... 88

5.3.2 Overcrowding in Bulge Fields.......................... 97

5.3.3 DAOPhot vs. DoPhot...................................... 98
5.4 Reddening in the Bulge................................................................ 101
5.5 Color-Color D iagram s.................................................................... 105
5.6 Color-M agnitude Diagram s........................................................... 117

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 5.7 D is c u s s io n ................................................................................................ 125
6.0 Abundance Distribution in Seven Bulge Windows along the Minor Axis.... 128

6.1 Correspondence o f Washington System

with Spectroscopic A bundances..................................... 129

6.2 D w arf C o n tam in atio n ........................................................................ 135

6.3 M ean F ield A bundances................................................................... 137

6.4 The Best Estim ate o f the Abundance E rrors................................... 142

6.5 A bundance T a b le s............................................................................... 145

6.6 The A bundance D istributions.......................................................... 168

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6.7 D is c u s s io n ................................................................................................ 182

7.0 C o d a .....................................................................................................................
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7.1 D iscussion and C o nclusions............................................................ 186

7.2 F u tu re P ro s p e c ts .................................................................................. 190

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7.2.1 K in e m a tic s .................................................................................. 190

7.2.2 Local Standard o f R est........................................................ 191

7.2.3 L atitu d e D istrib u tio n ........... ................................................. 192

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7.3 F in al T h o u g h t....................................................................................... 192

R e fe re n c e s .................................................................................................................... 193

A A p p e n d ic e s....................................................................................................... 206

A .l T he P h o to m e try .................................................................................. 206

A.2 Finding Charts for Bulge Fields ................................................. 287

A.3 D erivation o f SDOM and Error in SD............................................. 313

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 A.4 Convolution of Gaussian Errors with the Abundance Distribution
of the Simple Model of Chemical Evolution..................... 318
A.4.1 The Convolution................................................ 318
A.4.2 Test for Normal errors..................................... 321
A. 5 Compendium of Abundance Calibrations for Washington Indices 323
A.6 Data Reduction Sequence............................................................ 335
A.6.1 Reduction Sequence........................................... 335
A.6.2 File Extension.................................................... 339
A .6.3 Command File I / O Syntax................................ 340

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A.7 Discussion of Software for Data Management............................ 343
B iographical Sketch........................................................................................ 349
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 List o f Tables

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12 Table 2.1 BULGE WINDOWS

14 Table 2.2 33 BULGE CARBONS STARS

16 Table 2.3 NIGHTLY ZERO-POINT SHIFTS

17 Table 2.4a JOURNAL OF OBSERVATIONS - STANDARDS

18 Table 2.4b JOURNAL OF OBSERVATIONS - PROGRAM STARS

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26 Table 2.5 BULGE CARBON STAR VELOCITY DISPERSIONS

27 Table 2.6 REST FRAME BAND PASSES

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37 Table 2.7 SPECTRAL FEATURES
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60 Table 3.1 JOURNAL OF OBSERVATIONS: STANDARD STARS

62 Table 3.2 JOURNAL OF OBSERVATIONS: PROGRAM STARS

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67 Table 3.3 r a d ia l V elo c ities o f S o u th er n b u l g e C o m po n en ts

A l o n g o r n e a r t h e m in o r a x is

71 Table 4.1 THE WASHINGTON FILTER SYSTEM

81 Table 4.2 WASHINGTON CCD STANDARD FIELDS (GEISLER 1990)

86 Table 5.1 BULGE FIELD CENTERS

87 Table 5.2 JOURNAL OF OBSERVATIONS

90 Table 5.3a PHOTOMETRIC RESIDUALS: STANDARDS 7-8 JUNE 1989

92 Table 5.3b P h o t o m etr ic Re sid u a l s : St a n d a r d s 18-19 Ju l y 1990

94 Table 5.3c PHOTOMETRIC RESIDUALS: STANDARDS 20-21 JULY 1990

96 Table 5.4 A) TRANSF. COEFFICIENTS: 7-8 JUNE 1989
B) TRANSF. COEFFICIENTS: 18-19 JULY 1990
C) TRANSF. COEFFICIENTS: 20-21 JULY 1990

102 Table 5.5 REDDENING IN BULGE WINDOWS ALONG I = 0"

130 Table 6.1 ABUNDANCES FOR 18 K GIANTS FROM SPECTRA (RICH

1988) AND FROM THE WASHINGTON SYSTEM (THIS
STUDY)

136 Table 6.2 D w a r f C o n t a m in a t io n in b u lg e w in d o w s

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140 Table 6.3 M e a n F ie l d A b u n d a n c e s

141 Table 6.4 CONVOLUTION STATISTICS FOR SIMPLE MODEL

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WITH SOLAR YIELD: Y = 1.0

146-167 Table 6.5a-j WASHINGTON COLORS AND ABUNDANCES: BULGE FIELDS

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206-286 Table A. 1.1-10 BULGE FIELDS: PHOTOMETRIC INDICES

323 Table A .5.1 ISO-ABUNDANCE RELATIONS IN THE WASHINGTON

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TWO-COLOR DIAGRAMS

340 Table A .6.1 REDUCTION FLOW CHART: COMMAND FILE I/O SEQUENCE

vi
 List of Figures

page sub-caption

3 Figure 1.1. Schematic of the lines of sight to the Galactic bulge.

4 Figure 1.2. Seven windows of low extinction along the Galaxy’s minor
axis.
24 Figure 2.1. Radial velocity distribution histogram together with the best-
fitting normal distribution.
25 Figure 2.2. Radial velocity dispersion as a function of latitude.
28 Figure 2.3. (a) NaD equivalent width measured in angstroms as a

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function of galactic latitude, (b) CN line strength as a
function of latitude measured in magnitudes.
29 Figure 2.4. Comparison carbon star spectra along side M giant spectra.
31 Figure 2.5.
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NaD correlates with CN.
32 Figure 2.6. Latitude distribution of the 33 bulge carbon stars.
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34 Figure 2.7. Region of the spectrum from 6080 A to 6300A that

highlights isotope shifts for C2 and CN.
39-46 Figure 2.8a-h Panels of Carbon star spectra
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47 Figure 2.9. Subset of echelle spectra for two carbon bulge stars that
highlights H a absorption.

52 Figure 2.10. Distribution of C2 equivalent widths for the 20 bulge carbon

stars with strong-medium H a and the 13 bulge carbon stars
with weak H a.

61 Figure 3.1. Sample spectra of Baade’s Window RR Lyrae stars,

showing the spectral region over which the cross correlation
was computed.
65 Figure 3.2. Velocity distribution of 36 Bulge RR Lyrae Stars.
73 Figure 4.1. Response functions for the Washington filter system.
74 Figure 4.2. Derive an abundance from Washington color indices.

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76 Figure 4.3. Separation of dwarfs from giants is obtained with the M - 57
index.
77 Figure 4.4. A test of the consistency between the magnesium index, M -
51, and the [Fe/H] derived from the C - M index.

91 Figure 5.1a. Residuals of Table 5.3a plotted for each color transformation.
93 Figure 5.1b. Residuals of Table 5.3b plotted for each color transformation.
95 Figure 5.1c. Residuals of Table 5.3c plotted for each color transformation.

100 Figure 5.2. Comparison of instrumental magnitudes derived from

DAOPhot and DoPhot.

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107-114 Figure 5.3a-h. Dereddened color-color diagrams for the bulge fields.
116 Figure 5.4. Color-color diagram for the gravity-sensitive M-51 index.
118-124 Figure 5.5a-g.
131 Figure 6.1.
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Dereddened color-magnitude diagram for the bulger fields.
Abundances of 18 K giants in Baade’s Window derived
from medium dispersion spectroscopy (Rich 1988) and the
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Washington photometric system.
133 Figure 6.2. Comparison of [Fe/H] for eleven K giants from Rich’s 1988
list that this study has in common with Geisler & Friel
(1990).
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134 Figure 6.3. Check of the temperature sensitivity of the Washington index
T 1-T2 with the more widely used J-K temperature index.
143 Figure 6.4. Derivation of error in abundance.
169 Figure 65. Three plausible abundance distributions from very different
histories of chemical enrichment.

172 Figure 6.6a. Abundance distribution for the Sagittarius field.

173 Figure 6.6b. Abundance distribution for Sgr lH overlaid with an error-
convolved distribution expected for the closed box simple
model of chemical evolution, with yield Y = 1.25 Y0.
174 Figure 6.7a. Abundance distribution for the Baade’s Window.
175 Figure 6.7b. Abundance distribution for Baade’s Window (III + IV)
overlaid with an error-convolved distribution expected for
the closed box simple model of chemical evolution, with
yield Y = 2.5 Y0 .

176 Figure 6.8a. Abundance distribution for two contiguous frames at b = -6°.
177 Figure 6.8b. Abundance distribution for two contiguous frames at b = -6°
overlaid with an error-convolved distribution expected for
the closed box simple model of chemical evolution, with
yield Y = 6 Y Q.

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178 Figure 6.9. Abundance distribution for two contiguous frames at b = -8°.
179 Figure 6.10. Abundance distribution at b —-10*.
180 Figure 6.11.
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Abundance distribution at b = -13*.
181 Figure 6.12. Abundance distribution at b = - 17.3*.
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183 Figure 6.13. Galactic latitude dependence of the mean in the abundance
distributions.
188 Figure 7.1. Comparison of the Blanco (1988) M giant counts with two
other profiles for the bulge’s minor axis.
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319 Figure A.4.1. Convolution of gaussian errors with analytic abundance

distribution from the simple model of chemical evolution
with solar {Y = 1) yield.
322 Figure A.4.2. Comparison of the distribution of errors in [Fe/H] for all
data in Baade’s Window (regardless of photometric error)
with the analytic distribution expected if there are normal
errors.
325 Figure A S .l. Iso-abundance curves in the C-M and T 1 -T2 colors for -3.0
^ [Fe/H] < +1.0.
326 Figure A5.2. Iso-abundance curves in the C-Tj and T1 -T2 colors for -3.0
£ [Fe/H] < +1.0.
327 Figure A 5.3. Iso-abundance curves in the C-M and M-T2 colors for -3.0
£ [Fe/H] < +1.0.

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328 Figure A S A . Iso-abundance curves in the C-Tj and M-T2 colors for -3.0
£ [Fe/H] < +1.0.

329 Figure A S .5. Iso-abundance curves in the M-Tj and T ;-72 colors for +0.5
£ [Fe/H] < -1.0.

331 Figure A S .6. K-giant abundances in Baade’s Window determined from

two different sets of photometric indices.
332 Figure A S .7. Same as Figure A.5.6, but the ordinate is [Fe/H] derived
from the abundance calibration in the C-M vs. M-T2 plane,
and the abscissa is [Fe/H] derived from the abundance
calibration in the C-T/ vs. M-T2 plane.
333 Figure A S .8. Same as Figure A.5.6, but now, the ordinate and the
abscissa are abundances derived from photometric indices
with different temperature colors.

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346 Figure A.7.1. Downhill simplex minimization method (in two dimensions)
as used to interpolate abundances from iso-abundance
contours in the two color plane.
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 List o f C harts

page description

288 Figure A2.1a. Finding chart overlay # 1 for field: -2.7* Sagittarius III.

289 Figure A.2.1b. Finding chart overlay # 2 for field: -2.7* Sagittarius III.

290 Figure A.2.1c. Field: b = -2.7* Sagittarius HI, Seeing: ~1.2 arc sec.
Epoch 1950: RA 17h 55m 53.5s Dec -29* 07’ 04"

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291 Figure A.2.2a. Finding chart overlay # 1 for field: b = -4°, Baade’s
Window HI. IE
292 Figure A.2.2b. Finding chart overlay # 2 for field: b = —4°, Baade’s
Window HI.
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293 Figure A.2.2c. Field: b = - 4* Baade’s Window IE, Seeing: -1.6 arc sec.
Epoch 1950: RA 18h 88m 12.2s Dec -30* 04' 48"

294 Figure A.2.3a. Finding chart overlay # 1 for field: b = - 4 ’, Baade’s

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Window IV.

295 Figure A.2.3b. Finding chart overlay # 2 for field: b = —4", Baade’s
Window IV.

296 Figure A .23c Field: b = —4* Baade’s Window IV, Seeing: -1.2 arc sec.
Epoch 1950: RA 18h 00m 13.0s Dec -30" 00' 03"

297 Figure A.2.4a. Finding chart overlay # 1 for field: b = -6* I.

298 Figure A.2.4b. Finding chart overlay # 2 for field: b = -6* I.

299 Figure A.2.4c. Field: b - -6° I, Seeing: -1.5 arc sec.

Epoch 1950: RA 18h 07m 38.0s Dec -31* 45' 31"

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300 Figure A 2 5 a . Finding chart overlay # 1 for field: b = -6° II.

301 Figure A.2.5b. Finding chart overlay # 2 for field: b= -6 '’ n.

302 Figure A.2.5c. Field: b = -6° II, Seeing: -1.5 arc sec.
Epoch 1950: RA 18h 07m 07.1s Dec -31’ 41' 53"

303 Figure A.2.6a. Finding chart overlay for field: b = - 8 ’ I.

304 Figure A.2.6b. Field: b = -8* I, Seeing: ~1.8 arc sec.

Epoch 1950: RA 18h 15m 09.5s Dec -32* 52' 18"

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305 Figure A.2.7a. Finding chart overlay for field: b = -8* II.

306 Figure A.2.7b. Field: b = - 8* II, Seeing: ~1.8 arc sec.

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Epoch 1950: RA 18h 15m 09.5s Dec -32* 55' 20"

307 Figure A.2.8a. Finding chart overlay for field: b = -10".

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308 Figure A2.8b. Field: b = -10*, Seeing: -1.5 arc sec.
Epoch 1950: RA 18h 24m 04.0s Dec -33* 58' 39"

309 Figure A.2.9a. Finding chart overlay for field: b = -13°.

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310 Figure A .2.9b. Field: b ~ -13°, Seeing: -1.4 arc sec.

Epoch 1950: RA 18h 37m 44.3s Dec -34* 52' 42"

311 Figure A2.10a. Finding chart overlay for field: b = -17°.

312 Figure A.2.10b. Field: b = - l T , Seeing: -2.5 arc sec.

Epoch 1950: RA 18h 54m 47.0s Dec -36° 29' 29"

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 ACKNOW LEDGEM ENTS

At various stages of this work I have enjoyed and benefitted from conversations
with Dick McCray, Wil van der Veen, Jim Applegate, Jay Frogel, Don Terndrup,
Kevin Predergast, and Ed Shaya. Chapter 3 benefitted especially from conversations
with George Wallerstein, Verne Smith, and Bob Wing. At assorted other times I have
enjoyed conversations about chemical evolution models with Hong Sheng Zhao, about
galactic structure with David Spergel, and about statistics with Robert Lupton.
Doug Geisler was always there when I needed him when I had questions about

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the Washington photometric system, and Peter Stetson and Eileen Friel helped me to
maximize my effectiveness with DAOPhot. I am also grateful to Mario Mateo for
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providing a version of DoPhot in the later phases of this work. I benefitted from
numerous e-mail exchanges with Abi Saha as he helped reveal to me the mysteries of
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RR Lyrae reductions. I am also grateful for the phase corections that Abi provided
from his ephemeris in advance of publication.

The timely completion of this dissertation would not have been possible without
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the energetic programming assistance of Sonya Umar and Roy Gal — two talented
undergraduates who helped to streamline the data reduction. Roy helped primarily with
the carbon star and RRLyrae reductions of Chapters 2 and 3 while Sonya assisted with
the herculean task of managing the DAOPhot reduction procedure for over 100 CCD
frames of data.
As part of the Barnard STEP program I had two ambitious high school seniors,

Samantha Harry and Rafael Verdejo, who produced the preliminary software required
to make finding chart overlays. Another high school student, Morris Matsa, after
proving he really could learn fortran in two days, produced the final version of the
software that made the finding chart overlays of Appendix 2. Morris also made the

xiii
seven-window-panel that appears in the introduction. I am grateful to all three high
school students for their efforts.

I am also grateful to David Helfand who first opened Columbia’s door to me in

1988, and to Joe Patterson whose guidance I have occasionally sought since 1974
when I was a student-camper under the skies of Camp Uraniborg in the Mojave Desert
of Southern California.
Most importandy, I would like to express my deep gratitude to Mike Rich for
opening a few windows (to the bulge, and elsewhere) which allowed this work to

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proceed smoothly and permitted me to transcend the expectations of others. His

encouragement and attention to scientific detail — large and small — helped to broaden
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and fine-tune my research skills.
Part of this work was supprted by a NASA graduate fellowship and grant from
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the New York Chapter of the ARCS Foundation. I gratefully acknowledge their
contribution to my professional growth.
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 DEDICATION

To my father, Cyril,
for his guiding principles.

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To my mother, Sunchita,
for sharing her love.
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To my brother, Stephen,
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who gives meaning to “brotherhood.”

To my sister, Lynn,
who keeps my feet on Earth.
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And to my wife, Alice,

who provides another dimension of life, itself.

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 I know that I am mortal by nature, and ephemeral;
but when / trace, at my pleasure, the windings to and fro of the heavenly bodies

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I no longer touch earth with my feet:
I stand in the presence o f Zeus, himself, and take my fill o f ambrosia.
IE Claudius Ptolemy
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 C h a p te r 1

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Introduction
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 It is important not to worship what is known, but to question it.
J. Bronowski

1.1 OVERVIEW

The Galactic bulge is a remarkable compact zoo of stellar populations. For example,
within 1.5 kpc of the Galactic center you will find RR Lyraes and long period variables
(Period ~ 200 d) that coexist with longer period variables (Period > 300 d; OH/IR stars
inclusive), and there are low luminosity carbon stars that coexist with late M giants.

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These pairings represent late stages of stellar evolution that emerge from very different
progenitor populations. In addition, the critical points and principal sequences of the
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color magnitude diagram (main sequence, turnoff, giant branch, horizontal branch) are
all much broader than can be explained by photometric errors alone. In short, the bulge
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populations contain unending observational and theoretical challenges.
Nearly all optical observations of the bulge are conducted through low
extinction “windows” through the plane of the disk. Some lines of sight suffer from 30
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to 60 magnitudes of visual extinction ( Becklin et al. 1978; Glass et al. 1987). A low

extinction region is therefore noticed easily on the Palomar Sky Survey blue plates as
an area of very high star density relative to its surroundings. There are many such
windows at various Galactic latitudes and longitudes. Seven of these windows, along
or near the minor axis, were selected for the photometry of this study. Figure 1.1
presents a schematic (drawn roughly to scale) of five of the lines of sight. As indicated
by the recent COBE image of the Galaxy, the bulge is depicted as flattened. The
concentric ellipses of Fig. 1.1 represent one, two, and three scale heights in an

exponential distribution of the 2pm light (Kent et al 1991).