J/MNRAS/413/1275 VrI light curves of NGC6981 variables (Bramich+, 2011)
CCD time-series photometry of the globular cluster NGC 6981: variable star
census and physical parameter estimates.
Bramich D.M., Figuera Jaimes R., Giridhar S., Arellano Ferro A.
<Mon. Not. R. Astron. Soc., 413, 1275-1294 (2011)>
=2011MNRAS.413.1275B 2011MNRAS.413.1275B
ADC_Keywords: Clusters, globular ; Stars, variable ; Photometry, CCD
Keywords: stars: variables: general - stars: variables: RR Lyrae -
globular clusters: individual: NGC6981 - Galaxy: stellar content
Abstract:
We present the results from 10 nights of observations of the globular
cluster NGC 6981 (M72) in the V, R and I Johnson wavebands. We
employed the technique of difference image analysis to perform
precision differential photometry on the time-series images, which
enabled us to carry out a census of the understudied variable star
population of the cluster. We show that 20 suspected variables in the
literature are actually non-variable, and we confirm the variable
nature of another 29 variables while refining their ephemerides. We
also detect 11 new RR Lyrae variables and three new SX Phe variables,
bringing the total confirmed variable star count in NGC 6981 to 43. We
performed Fourier decomposition of the light curves for a subset of RR
Lyrae stars and used the Fourier parameters to estimate the
fundamental physical parameters of the stars using relations available
in the literature. Mean values of these physical parameters have
allowed us to estimate the physical parameters of the parent cluster.
We derive a metallicity of [Fe/H]ZW~-1.48±0.03 on the Zinn &
West scale (or [Fe/H]UVES~-1.38±0.03 on the new Carretta et al.
scale) for NGC 6981, and distances of ∼16.73±0.36 and
∼16.68±0.36kpc from analysis of the RR0 and RR1 stars separately. We
also confirm the Oosterhoff type I classification for the cluster, and
show that our colour-magnitude data are consistent with the age of
∼12.75±0.75Gyr derived by Dotter et al.
Description:
Time-series V, R, and I photometry for all the confirmed variables in
our field of view, except V27 and V35 which lie outside of our field
of view. We list standard and instrumental magnitudes and their
uncertainties corresponding to the variable star identification,
filter, and epoch of mid-exposure. For completeness, we also list the
reference flux, difference flux, and photometric scale factor, along
with the uncertainties on the reference and difference fluxes.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table23.dat 117 43 Celestial coordinates for all the confirmed
variables in our field of view, except V27 and
V35 which lie outside of our field of view and
details of all confirmed variables in NGC 6981
table4.dat 77 8114 CCD time-series V, R, and I photometry of
41 variables in NGC6981
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Byte-by-byte Description of file: table23.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- VName Variable Star ID (VN or VNN, NN≤56)
5- 6 I2 h RAh ? Right ascension (J2000.0) (1)
8- 9 I2 min RAm ? Right ascension (J2000.0) (1)
11- 15 F5.2 s RAs ? Right ascension (J2000.0) (1)
17 A1 --- DE- Declination sign (J2000.0) (1)
18- 19 I2 deg DEd ? Declination (J2000.0) (1)
21- 22 I2 arcmin DEm ? Declination (J2000.0) (1)
24- 27 F4.1 arcsec DEs ? Declination (J2000.0) (1)
29- 30 A2 --- K95 Kadla et al. (1995A&A...302..723K 1995A&A...302..723K) candidate
variables ID, <Cl* NGC 6981 KBP AN> in Simbad
32- 38 A7 --- VType Variable Type (RR0, RR0/RR1, RR1 or SX Phe)
39 A1 --- u_VType [abc] Uncertain variable type (2)
41- 45 A5 --- Blend Blend Guide: Inner or Outer (3)
47- 48 I2 --- Nmax ? Number of observed maxima (4)
49 A1 --- --- [,]
50- 51 I2 --- Nmin ? Number of observed minima (4)
53- 63 F11.3 d Tmax ? HJD epoch of maximum light
64 A1 --- n_Tmax [d] Uncertainty flag on Tmax (5)
66- 74 F9.7 d Per ? Period
75 A1 --- n_Per [efg] Note on Per (6)
77- 83 F7.5 d PerS ? Period from Shapley & Ritchie,
1920ApJ....52..232S 1920ApJ....52..232S (7)
84 A1 --- n_PerS [h] PerS does not match our data (8)
86- 94 F9.7 d PerR ? Period from Rosino (1953, Pub. Oss. Bologna,
6, 49 (No.2)) (7)
95 A1 --- n_PerR [h] PerR does not match our data (8)
97-104 F8.6 d PerN ? Period from Nobili (1957MmSAI..28..141N 1957MmSAI..28..141N) (7)
105 A1 --- n_PerN [h] PerN does not match our data (8)
107-116 F10.8 d PerD ? Period from Dickens & Flinn
(1972MNRAS.158...99D 1972MNRAS.158...99D) (7)
117 A1 --- n_PerD [h] PerD does not match our data (8)
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Note (1): The coordinates are from the V reference image,
which is the heliocentric Julian date ∼2453284.11d (J2004.7614)
Note (2): Note on VType as follows:
a = These variables are RR Lyrae stars, and we believe that they are
of the RR0 type, but we cannot confirm this (see Section 3.6)
b = the classification for this variable is most likely RR0
(see Section 3.5)
c = these variables are RR Lyrae stars, but we have been unable to
distinguish their subtype (see Section 3.6)
Note (3): A guide to the level of blending of the variable star PSF in the
reference images is provided, where "Inner" and "Outer" indicate that the
variable is inside or outside, respectively, of the highly crowded central
area of the cluster (the dividing line is placed at r=50"), and where "Blend"
indicates that the variable is blended with a star of similar or greater
brightness.
Note (4): Number Nmax and Nmin of light-curve maxima and minima, respectively,
that were observed in full during our observation runs.
Note (5): d: the epoch of maximum light is uncertain because we have not
observed the light-curve peak. The epoch reported here is that of the data
point closest to the suspected peak.
Note (6): Note as follows:
e = the period listed for V29 is the period P0 in equation (6). We detect
a secular period change for this star of β≈-1.38x10-8d/d
(see Section 3.5)
f = due to the poor phase coverage of our observations for this variable,
we have been unable to determine a reliable period (see Section 3.5)
g = the periods listed for the SX Phe variables are the periods associated
with the largest amplitude oscillation
Note (7): Note that most of the period estimates from previous authors fail
to phase our light curves properly, which highlights the importance of the
longer temporal baseline and much better photometric precision of our data.
Note (8): h: our photometric data are not well phased by these periods.
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- VName Variable star ID, as in table4
5 A1 --- Filter [VRI] Observation filter
7- 19 F13.5 d HJD Heliocentric Julian date at mid-exposure
21- 26 F6.3 mag magst ?=0.000 Standard magnitude
28- 33 F6.3 mag maginst Instrumental magnitude
35- 39 F5.3 mag e_mag Magnitude uncertainty (standard or instrumental)
41- 48 F8.3 s-1 Fref Reference flux (in ADU/s unit)
50- 54 F5.3 s-1 e_Fref Reference flux uncertainty (in ADU/s unit)
56- 63 F8.3 s-1 DFlux Difference flux (in ADU/s unit)
65- 70 F6.3 s-1 e_DFlux Difference flux uncertainty (in ADU/s unit)
72- 77 F6.4 --- p Photometric scale factor
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Acknowledgements:
Daniel Bramich, dbramich(at)eso.org
(End) Daniel Bramich [ESO, Germany], Patricia Vannier [CDS] 30-Mar-2012