J/AJ/169/154 HST magnitudes of NGC 205 RR Lyrae variables (Sarajedini, 2025)
Color-magnitude diagram of NGC 205 and its RR Lyrae variables.
Sarajedini A.
<Astron. J., 169, 154 (2025)>
=2025AJ....169..154S 2025AJ....169..154S
ADC_Keywords: Stars, variable; Photometry, VRI; Colors
Keywords: RR Lyrae variable stars ; CCD photometry ; Dwarf elliptical galaxies
Abstract:
We present the deepest color-magnitude diagram (CMD) for NGC 205 in
the F606W (∼V) and F814W (∼I) filters. Our archival Hubble Space
Telescope/Advanced Camera for Surveys field is centered ∼3.8'
northwest of the galaxy's center. The CMD shows the canonical features
of a predominantly intermediate-to-old age stellar population. Among
other features, the CMD reveals a prominent red clump as well as a
blue horizontal branch. Notably absent is any significant signature of
a young main sequence typically produced by a stellar population with
an age less than a few Gyr. From the magnitude of the red-giant-branch
tip, we derive a distance modulus of (m-M)o=24.66±0.05. When
compared with α-enhanced theoretical isochrones, the color
distribution of the red giant branch stars yields a peak metallicity
value of [Fe/H]=-0.51±0.10. Our photometry also reveals a sample of
479 RR Lyrae variable stars (443 ab-type and 36 c-type). Utilizing
equations that relate the periods and amplitudes to the metallicities
of these stars, we find a mean <[Fe/H]≥-1.45±0.02 on the Zinn &
West scale for the purely old stellar population in NGC 205. The
quoted uncertainty represents the standard error of the mean abundance
value is consistent with that of the six genuine old globular clusters
in NGC 205.
Description:
The NGC 205 observations in the present work consist of archival
imaging data taken by the Advanced Camera for Surveys Wide Field
Channel (ACS/WFC) onboard HST in 2020 October by program GO-15902
(PI:Weisz) downloaded and pipeline calibrated through the Barbara A.
Mikulski Archive for Space Telescopes (MAST). The data described here
may be obtained from the MAST archive.
These consist of 11 fully pipeline-calibrated individual exposures
(FLC images) taken in each of the F606W (∼V) and F814W (∼I) filters
for a total of 22 frames spanning an observing window of approximately
three days in Heliocentric Julian Date (HJD). Specifically, the
observations span the range of 2459136.0<HJD<2459139.0. Among these
images, there are two short exposure frames (1x60s in F606W and 1x60s
in F814W) that were used in the reduction procedure but not in the
variable star analysis because of their limited photometric depth.
Stellar photometry was performed on the WFC1 and WFC2 images.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 70 479 NGC 205 RR Lyrae variables
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See also:
J/A+A/384/371 : Kinematical data on early-type galaxies. VI (Simien+, 2002)
J/AJ/125/3037 : RI photometry of NGC 205 carbon stars (Demers+, 2003)
J/AJ/127/2738 : ACS VI photometry of M31 halo RR Lyrae (Brown+, 2004)
J/AJ/128/224 : VI photometry in M33 outer regions (Tiede+, 2004)
J/AJ/130/1693 : HST-ACS observations of 5 globular clusters (Brown+, 2005)
J/AJ/132/1361 : RR Lyrae variables in M33 (Sarajedini+, 2006)
J/AJ/138/184 : RR Lyrae var. in 2 fields in M31 spheroid (Sarajedini+, 2009)
J/ApJ/724/799 : RR Lyrae variables in M33. II. (Yang+, 2010)
J/MNRAS/455/2163 : BI light curves of DDO210 pulsating var. (Ordonez+, 2016)
J/MNRAS/468/870 : RR Lyrae variables in M33 (Tanakul+, 2017)
http://archive.stsci.edu/doi/resolve/resolve.html?doi=10.17909/797f-f657 : RRL
data (Sarajedini, 2024)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Seq [1/479] Sequential identifier in table
5- 6 I2 h RAh Hour of Right Ascension (J2000)
8- 9 I2 min RAm Minute of Right Ascension (J2000)
11- 15 F5.2 s RAs Second of Right Ascension (J2000)
17- 18 I2 deg DEd Degree of Declination (J2000)
20- 21 I2 arcmin DEm Arcminute of Declination (J2000)
23- 26 F4.1 arcsec DEs Arcsecond of Declination (J2000)
28- 33 F6.4 d Per [0.27/0.71] Period (1)
35- 39 F5.3 mag Amp-F606W [0.28/1.43] Amplitude, F606W (1)
41- 46 F6.3 mag F606W [24.8/25.9] Mean F606W Vega magnitude (1)
48- 52 F5.3 mag Amp-F814W [0.1/1.3] Amplitude, F814W (1)
54- 59 F6.3 mag F814W [24.3/25.5] Mean F814W Vega magnitude (1)
61- 65 F5.3 mag Color [0.35/0.73] Mean Vega Color, F606W-F814W (1)
67- 70 A4 --- Type RR Lyrae, ab-type or c-type (1)
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Note (1): Values derived using the PIKAIA genetic algorithm,
Charbonneau (1995ApJS..101..309C 1995ApJS..101..309C); Yang+2010 (J/ApJ/724/799).
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History:
From electronic version of the journal
(End) Prepared by [AAS], Robin Leichtnam [CDS] 05-Jan-2026