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J/AJ/98/2086 CCD photometry of NGC 2257 RR Lyrae (Walker 1989)
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CCD photometry of the RR Lyrae variables in the LMC cluster NGC 2257
and the adjacent field
Walker A.R.
<Astron. J. 98, 2086 (1989)>
=1989AJ.....98.2086W (SIMBAD/NED BibCode)
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ADC_Keywords: Stars, variable ; Photometry, CCD
Abstract:
Extensive B and V band CCD photometry for 33 cluster and nine field RR
Lyrae variables near the old LMC cluster NGC 2257 provide accurate
mean magnitudes for the stars, and hence distances for the cluster and
field populations. The mean magnitudes for the two groups differ by
0.17mag. This is interpreted as a distance effect rather than a
difference in metal abundance or evolutionary state. The field stars
thus average 3.7kpc more distant from us than cluster stars, and
4.1kpc more distant than the plane of the LMC as defined by the de
Vaucouleurs and Freeman incline plane model. In addition the field
stars occupy less than 9kpc in depth; however, the data are not
inconsistent with the LMC field RR Lyrae, taken as a whole, forming a
spherical rather than a planar system. From the period-amplitude
diagram, both the cluster and field variables have metal abundance of
[Fe/H]=-1.8+/-0.1. There is little evidence for any gross spread in
metal abundance among the field variables, although periods for two
short-period and, presumably, metal-rich RRab stars are confirmed. The
new light curves extend previous work on period changes for the
cluster variables and have allowed revised periods and rates of period
change to be calculated. Two possible double-mode (RRd) field
variables were found to have incorrect periods and are actually RRabb
stars. Photometry for 462 stars, reaching to V~21 and sampling the
cluster to its very center, have been used to prepare a
color-magnitude of the evolved stars in NGC 2257. This shows a richly
populated blue horizontal branch, the width of which measurements of
the dispersion of absolute magnitudes of RR Lyrae in galactic globular
clusters by Sandage. The asymptotic giant branch is early separated
from the red giant branch allowing accurate numbers of stars in the
various stages of evolution to be counted. These counts are compared
with those for galactic globular clusters, with the result that NGC
2257 appears to be indistinguishable from its local counterparts such
as M3.
Objects:
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RA (2000) DE Designation(s)
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06 30.4 -64 19 NGC 2257
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table6 27 462 Photometry for the NGC 2257 non-variables
table9 71 33 Derived properties of the NGC 2257 variables
table11 65 9 Derived properties of the field variables
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Byte-by-byte Description of file: table6
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Bytes Format Units Label Explanations
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1- 3 I3 --- Seq Star sequential number
5- 9 F5.1 arcsec Xoff X offset from star 78 (Alcaino I) (+ is West) (1)
11- 15 F5.1 arcsec Yoff Y offset from star 78 (Alcaino I) (+ is North) (1)
17- 21 F5.2 mag Vmag V magnitude
23- 27 F5.2 mag B-V B-V colour index
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Note (1): Alcaino I in Alcaino & Alvarado, 1988AJ.....95.1724A.
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Byte-by-byte Description of file: table9 table11
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Bytes Format Units Label Explanations
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1- 5 A5 --- Star Star number
7- 15 F9.7 d Per Period (1)
17- 18 A2 --- Type Variable type
20- 25 F6.3 mag <Vmag> ? Intensity mean V magnitude
26 A1 --- u_<Vmag> Uncertainty flag on <Vmag>
28- 33 F6.3 mag <Bmag> ? Intensity mean B magnitude
34 A1 --- u_<Bmag> Uncertainty flag on <Bmag>
36- 40 F5.3 mag <Bmag>-<Vmag> ? <Bmag> minus <Vmag>
41 A1 --- u_<Bmag>-<Vmag> Uncertainty flag on <Bmag>-<Vmag>
43- 47 F5.3 mag <B-V> ? B-V magnitude mean
48 A1 --- u_<B-V> Uncertainty flag on <B-V>
50- 53 F4.2 mag AV ? V amplitude
54 A1 --- u_AV Uncertainty flag on AV
56- 59 F4.2 mag AB ? B amplitude
60 A1 --- u_AB Uncertainty flag on AB
62- 65 F4.2 --- Rise ? Rise time to maximum (in phase units)
66 A1 --- u_Rise Uncertainty flag on Rise
68- 71 F4.2 --- Phase ? Phase at the maximum light
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Note (1): The period given here is that adopted by NHH (Nemec et al.,
1985ApJS...57..329), and corresponds to the epoch given by these
authors (Tables V, VI of Nemec et al., 1985ApJS...57..329). The phase
at maximum light in February 1988 (column Phase) corresponds to this
ephemeris. See text for revised periods and period change rates
for many of these stars.
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History: Prepared via OCR at CDS.
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(End) Marie-Jose Wagner, Patricia Bauer [CDS] 29-Apr-1997