J/AJ/108/555 BVI photometry in globular cluster M68 (Walker, 1994)
BVI CCD photometry of galactic globular clusters. II. M68
Walker A.R.
<Astron. J. 108, 555 (1994)>
=1994AJ....108..555W 1994AJ....108..555W
ADC_Keywords: Clusters, globular; Photometry; Colors
Abstract:
A total of 363 CCD frames in the B, V, and I passbands have been used
to study the color-magnitude diagram (CMD) and the variable-star
population of the metal-poor galactic globular cluster M68 (NGC 4590,
C1236-264). Light curves have been prepared for 40 RR Lyrae variables,
five of which are new discoveries, and for two probable SX Phe stars.
The RR Lyraes have =15.64±0.01, and via several methods
E(B-V)=0.07±0.01. [Fe/H]=-2.1 is adopted. From a comparison
with Sandage's data for M3 (NGC 5272, C1339+286), a period shift
of Delta(logP)/Delta([Fe/H])=-0.11±0.02 is derived, and the
mass-to-light parameter is found to be =1.90. RR Lyrae masses are
derived from Fourier fits to the light curves, from the RRd variables,
and by comparison with HB (horizontal branch) evolutionary models
calculated by Dorman. The light-curve structure agrees qualitatively
with recent theoretical models. From short exposures, CMDs have been
prepared that are complete from the tip of the red giant branch (RGB)
to below the level of the HB, while from the long exposures a CMD to
V∼23 is provided for 7298 stars more than 158 arcsec distant from the
cluster center. The HB is skewed to the blue with the eight bluest
stars clearly separated from the remainder of the HB stars. This HB
gap is at the identical color to that in M15 (NGC 7078, C2127+119),
but M68 lacks the extensive population of the very blue HB stars
present in M15. The total number of HB stars is deficient with respect
to the RGB population, with the ratio of RGB to HB stars R=0.99. There
are several likely blue stragglers identified on CMDs of inner annuli,
but near to the cluster center photometric errors scatter stars off
the main sequence (MS) into this region. The deep CMDs are in
excellent agreement with existing CCD CMDs for M68, with the MS
turnoff Delta(V)=3.42±0.10mag below the HB. Matching the M68 CMD to
that for M15 in the subgiant-MS turnoff region shows that the age of
M68 is identical to that for M15 to within ±0.4Gyr.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table4.dat 37 4680 Photometry of the RR Lyrae variables
table12.dat 41 562 Photometry of the brighter M68 stars
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Byte-by-byte Description of files: table4.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- Name Star name
5- 15 F11.3 d HJD Heliocentric Julian day (1)
18- 23 F6.3 mag Vmag ?=99.999 V band magnitude
25- 30 F6.3 mag Bmag ?=99.999 B band magnitude
32- 37 F6.3 mag Imag ?=99.999 I band magnitude
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Note (1): Heliocentric Julian dates correspond to the start of the
V-band exposures. The midpoints of the V, B, and I exposures are,
respectively, 2.5, 11.0, and 18.5 min later than this HJD
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Byte-by-byte Description of file: table12.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Name Star number
5-10 F6.1 pix Xpos X coordinate
12-17 F6.1 pix Ypos Y coordinate
20-25 F6.3 mag Vmag V magnitude
28-33 F6.3 mag B-V B-V color
36-41 F6.3 mag V-I ? V-I color
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Origin: AAS CD-ROM series, Volume 3, 1995
Addresses:
Walker A.R.
Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
(End) Lee Brotzman [ADS] 10-Oct-199, Patricia Bauer [CDS] 16-Feb-1995