J/ApJS/244/12 A search for variable stars in M4 with K2 LCs (Wallace+, 2019)
A search for variable stars in the globular cluster M4 with K2.
Wallace J.J., Hartman J.D., Bakos G.A., Bhatti W.
<Astrophys. J. Suppl. Ser., 244, 12-12 (2019)>
=2019ApJS..244...12W 2019ApJS..244...12W (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, globular; Stars, variable; Photometry, G band
Keywords: Close binary stars; Detached binary stars; Semi-detached binary stars
Binary stars; Eclipses; Eclipsing binary stars; Globular star clusters
RR Lyrae variable stars; Variable stars; X-ray binary stars
W Ursae Majoris variable stars; Algol variable stars
Abstract:
We extract light curves for 4554 objects with 9<G<19 in the K2
superstamp observations of the globular cluster M4, including 3784
cluster members, and search for variability. Among cluster-member
objects, we detect 66 variables, of which 52 are new discoveries.
Among objects not belonging to the cluster, we detect 24 variables, of
which 20 are new discoveries. We additionally discover 57
cluster-member suspected variables, 10 cluster-non-member suspected
variables, and four variables with ambiguous cluster membership. Our
light curves reach sub-millimagnitude precision for the cluster
horizontal branch, permitting us to detect asteroseismic activity in
six horizontal branch stars outside the instability strip and one
inside the strip but with only ∼1mmag amplitude variability. Nineteen
additional stars along the red giant branch also have detected
asteroseismic variability. Several eclipsing binaries are found in the
cluster, including a 4.6 day detached eclipsing binary and an EW-class
eclipsing binary, as well as an EW with uncertain cluster membership
and three other candidate EWs. A 22 day detached eclipsing binary is
also found outside the cluster. We identify a candidate X-ray binary
that is a cluster member with quiescent and periodic ∼20mmag optical
variability. We also obtain high-precision light curves for 10 of the
previously known RR Lyrae variables in the cluster and identify one as
a candidate Blazhko variable with a Blazhko period in excess of 78d.
Description:
The images used are the 16 target pixel files (TPFs) that make up
the M4 superstamp from the Mikulski Archive for Space Telescopes;
taken by the Kepler space telescope during the K2 portion of its
mission, specifically during Campaign 2 of that mission, which
occurred in 2014. Each is 50 pixels by 50 pixels in dimension. These
files had the K2 EPIC ID numbers 200004370-200004385. See section 2
for explanations on the methods to extract and analyze data from the
images.
We extracted light curves for 4554 objects in the globular cluster M4
from the K2 superstamp data of the cluster which represent ∼78 days of
continuous observations in the final light curves. Our final
photometric precision is 0.2mmag for G∼12, 1mmag for G∼15, and 10mmag
for G∼18 objects, with M4's main-sequence turnoff being around G∼16-17.
Objects:
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RA (ICRS) DE Designation(s)
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16 23 35.22 -26 31 32.7 M4 = NGC 6121
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 97 4554 Stars examined
table2.dat 93 15 Results for variables from
Clement+ (2001AJ....122.2587C 2001AJ....122.2587C ; see V/150) and
Stetson+ (2014, J/PASP/126/521)
table3.dat 100 106 Newly discovered cluster variables
table4.dat 110 25 Newly discovered variables that are
indeterminable blends
table5.dat 110 32 Newly discovered variables that are
not cluster members
lcs/* . 4556 The individual light curves in CSV format and
the ReadMe file of Joshua J. Wallace to use them
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See also:
B/gcvs : General Catalogue of Variable Stars (Samus+, 2007-2017)
VII/202 : Globular Clusters in the Milky Way (Harris, 1997)
I/337 : Gaia DR1 (Gaia Collaboration, 2016)
V/150 : Updated cat. of variable stars in globular clusters (Clement+ 2017)
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
J/PAZh/22/847 : Identification and coordinates in M4 (Shokin+, 1996)
J/AJ/145/43 : CASE. V. Three eclipsing binaries in M4 (Kaluzny+, 2013)
J/PASP/126/521 : UBVRIJHK photometry of RR Lyrae in M4 (Stetson+, 2014)
J/MNRAS/442/2381 : Variable stars in M4 primary field (Nascimbeni+, 2014)
J/MNRAS/452/3561 : Kepler EB stars. K2 Campaign 0 (LaCourse+, 2015)
J/ApJ/808/11 : RR Lyrae stars in M4. II. Spitzer/IRAC phot. (Neeley+, 2015)
J/MNRAS/463/1780 : Exoplanet candidates in Praesepe (M 44) (Libralato+, 2016)
J/MNRAS/463/1831 : M67 Kepler/K2 variable stars (Nardiello+, 2016)
J/ApJS/222/14 : Planetary cand. from 1st yr K2 mission (Vanderburg+, 2016)
http://arks.princeton.edu/ark:/88435/dsp01h415pd368 : the associated data to
this paper
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 5 A5 --- ID Identifier (G1)
7- 9 A3 --- DR Gaia data realease (2)
11- 29 I19 --- Gaia Gaia identifier within the release (2)
31- 32 I2 h RAh [16] Hour of Right Ascension (J2000) (3)
34- 35 I2 min RAm [22/24] Minute of Right Ascension (J2000) (3)
37- 41 F5.2 s RAs Second of Right Ascension (J2000) (3)
43 A1 --- DE- [-] Sign of the Declination (J2000) (3)
44- 45 I2 deg DEd [26] Degree of Declination (J2000) (3)
47- 48 I2 arcmin DEm [20/30] Arcminute of Declination (J2000) (3)
50- 53 F4.1 arcsec DEs Arcsecond of Declination (J2000) (3)
55- 59 F5.2 mag Gmag [9.4/21] Gaia magnitude (4)
61- 65 F5.2 mag GBPmag [9.7/20.5]? Gaia blue passband magnitude (4)
67- 71 F5.2 mag GRPmag [9/19.5]? Gaia red passband magnitude (4)
73- 76 I4 --- N [701/3773] Number of points in light curve (5)
78- 84 F7.2 mmag RMS [0.1/1261] Light curve RMS (6)
86- 90 F5.3 --- Mm [0/1]? Membership probability as calculated by
Wallace (2018RNAAS...2d.213W 2018RNAAS...2d.213W)
92- 97 A6 --- n_Mm Note when membership is null (7)
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Note (2): Taken from DR1 (I/337) or DR2 (I/345) as indicated. The DR2 ID was
preferentially used and only 11 objects in this table have their DR1 IDs
quoted.
Note (3): Data taken from Gaia DR1 (Lindegren+ 2016A&A...595A...4L 2016A&A...595A...4L ; see I/337)
or DR2 (Lindegren+ 2018A&A...616A...2L 2018A&A...616A...2L ; see I/345) as indicated in
the DR column (see Note #2).
Note (4): Taken from either Gaia DR1 (van Leeuwen+ 2017A&A...599A..32V 2017A&A...599A..32V ;
see I/337) or DR2 (Riello+ 2018A&A...616A...3R 2018A&A...616A...3R ; see I/345) as
indicated in the Gaia column (see table Note #2). Please note that G
had a different definition between DR1 and DR2 (Evans+ 2018A&A...616A...4E 2018A&A...616A...4E).
GBPmag and GRPmag are taken only from Gaia DR2 and were not included
in Gaia DR1, nor are they available for all Gaia DR2 sources.
Note (5): Raw light curves are used for objects with identifiers beginning
with "V" and final light curves for all others. Raw light curves can
include data from cadences 1-49 and so may have more points than the
maximum of 3724 for the final light curves.
Note (6): With sigma clipping (3σ, iterated three times). Raw light
curves are used for objects with identifiers beginning with "V" and
final light curves for all others.
Note (7): Note as follows:
N. DR2 = this object was not matched to a Gaia DR2 source;
N. D. = this object lacked proper motion data in Gaia DR2 and
its membership probability could not be calculated;
Dup. = this DR1 source was matched to multiple DR2 sources.
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Byte-by-byte Description of file: table[2345].dat
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Bytes Format Units Label Explanations
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1 A1 --- n_ID [VSAM] Note on object (1)
3- 7 A5 --- ID Identifier (G1)
9 A1 --- f_ID [ghijklmuv] Flag on ID (1)
11- 12 I2 h RAh [16] Gaia DR2 Hour of Right Ascension (J2000)
14- 15 I2 min RAm [22/24] Minute of Right Ascension (J2000)
17- 21 F5.2 s RAs Second of Right Ascension (J2000)
23 A1 --- DE- [-] Gaia DR2 Sign of the Declination (J2000)
24- 25 I2 deg DEd [26] Gaia DR2 Degree of Declination (J2000)
27- 28 I2 arcmin DEm [20/30] Arcminute of Declination (J2000)
30- 33 F4.1 arcsec DEs Arcsecond of Declination (J2000)
35- 39 F5.2 mag Gmag [9.4/21] Gaia DR2 G-band magnitude
41 A1 --- u_Per [~] Uncertainty flag on Per
42- 50 F9.6 d Per [0.04/34]? Period of the variability
52- 53 I2 d Per2 [30]? Upper range of variability
(only for Table 2)
55 A1 --- f_Per [apdebt] Flag on Per (1)
57- 63 F7.2 10-4d e_Per [0.03/8000]? Uncertainty of Per (see Section 2.8)
65 A1 --- u_Amp [~] Uncertainty flag on Amp
66- 71 F6.2 mmag Amp [-40/990]? Amplitude of the variability
(see Section 2.8) (2)
73 A1 --- f_Amp [no] Flag on Amp (1)
75 A1 --- u_Epoch [~] Uncertainty flag on Epoch
76- 82 F7.2 d Epoch ? Epoch of the minimum of variability,
expressed in KBJD (BJD-2454833) (3)
84- 88 A5 --- Meth Method (only for Tables 3 and 5) (4)
90-100 A11 --- Type Classification based on the GCVS variability
types (5)
102 A1 --- f_Type [qr] Flag on Type (1)
104-108 F5.3 --- Mm [0/1]? Cluster membership probability from Table 1
(only for Tables 4 and 5)
110 A1 --- f_Mm [fcs] Flag on Mm (1)
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Note (1): Flag as follows:
V = Variable
S = Suspected variable (see Appendix B for the reasons of marking the stars
suspected variables rather than definite variables)
A = blended stars All cluster members
M = blended stars Mixed between cluster members and non- or ambiguous members
a = The trapezoid model struggled to fit well, and the calculated
uncertainty on the period was unrealistically small, and so we decided
to not report it.
b = Period very close to a systematic period, but this object was kept as a
variable owing to the strength of the signal.
c = Probable photometric cluster member.
d = The best period found for this object in our period search was 1.500days,
which may be a modulation of the ∼0.5day period.
e = The best period found for this object was 0.38587days. We were unable to
get a good fit on the ∼0.5day period, but this object does have visible
variability when folded on this period and has an image location very
close to the other stars in this blended group. It may be that there is
more than one variable in this group.
f = No proper motion data available; probable photometric cluster member.
g = Both W3883 and W3894 have similar periods and epochs as W4084, but
are ∼66 pixels away.
h = Not a cluster member; significantly blended with V19 and unable to
determine its own variability.
i = Not a cluster member.
j = Six other stars observed with same variability; this was chosen as
variable since it was the most robust detection (see paper for details).
k = These two stars (W689 and W1154) are 27 pixels apart but have consistent
periods and, based on our analysis, may phase with each other.
l = Slightly blended with V8. This detected variability is not a
(sub)harmonic of that variability, so we are confident this belongs to
the star itself.
m = Slightly blended with V10. This detected variability is not a
(sub)harmonic of that variability, so we are confident this belongs to
the star itself.
n = The trapezoid model appeared to fail to fit the full amplitude of the
signal. Actual amplitude may be ∼2-3 times larger.
o = Epoch and possibly amplitude may be inaccurate, owing to PDM being
employed to fold these transits and a harmonic fit being used to
determine epoch and amplitude.
p = Single event
q = Classified as DSCUT by Yao & Tong (1989IBVS.3334....1Y 1989IBVS.3334....1Y), but we do not
observe the same variability they report, and we think a GDOR
classification is more likely to be correct.
r = Classification from Yao+ 2006Ap&SS.302..241Y 2006Ap&SS.302..241Y
s = Lacked proper motion data to calculate membership probability.
t = Blended with V19; this period appears in the data only after
removing V19's blended signal.
u = Blended with W3825, which our code also marked as a variable; however,
using a small aperture to evaluate differences in local flux amplitudes
revealed this star to be the source of the variability.
v = Eclipse was not identified by our main period-finding pipeline but was
noticed in our by-eye vetting.
Note (2): A negative amplitude means that the light curve shows a box-like
signal that is a brightening, rather than the more common
eclipse-based dimmings for such signals.
Note (3): The epoch of the minimum of the variability, expressed in KBJD
(BJD-2454833.0). See Section 2.8 and Appendix B for details on how
this is measured. Significant differences in epochs between blended
objects are due to differences in the fitted harmonics for each case;
these objects do phase up.
Note (4): Method used for determining amplitude and epoch as follows:
Harm. = a harmonic fit was used
Trap. = a trapezoid fit was used.
Note (5): Classification based on the GCVS Variability Types, fourth edition
(Samus+ 2017AZh....94...87S 2017AZh....94...87S ; see B/gcvs) where possible.
Additional designations used: "mmRR," millimagnitude RR Lyrae; "mh,"
multiharmonic variability; "shortperiod," sinusoidal variability of
<0.1 day period; "xrb," a likely X-ray binary, but not classified as
"X" since we do not know of variability in the X-ray emission.
Classifications are not attempted for the suspected variables.
Explanations regarding why these are reported as suspected instead of
discovered variables can befound in Appendix B.
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Global notes:
Note (G1): Those prepended with "V" are previously identified variables from the
catalog of Clement+ (2001AJ....122.2587C 2001AJ....122.2587C), June 2016 edition (see V/150),
not marked as constant; those prepended with "SC" are candidate
variables from Stetson+ (2014, J/PASP/126/521); those prepended with
"W" are additional Gaia DR1 sources examined in this work with a
sequential running number between 1 to 4540; except W1873 which is not
included in this table.
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History:
From electronic version of the journal for the tables.
Light curves downloaded from:
http://arks.princeton.edu/ark:/88435/dsp01h415pd368
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 12-Feb-2020