J/AJ/149/40 Variables in metal-rich clusters. IV. NGC 6496 (Abbas+, 2015)
Variable stars in metal-rich globular clusters. IV. Long-period variables in
NGC 6496.
Abbas M.A., Layden A.C., Guldenschuh K.A., Reichart D.E., Ivarsen K.M.,
Haislip J.B., Nysewander M.C., LaCluyze A.P., Welch D.L.
<Astron. J., 149, 40 (2015)>
=2015AJ....149...40A 2015AJ....149...40A
ADC_Keywords: Clusters, globular ; Photometry, VRI ; Stars, variable
Keywords: globular clusters: individual: NGC 6496 - stars: horizontal-branch -
stars: variables: general
Abstract:
We present VI-band photometry for stars in the metal-rich globular
cluster NGC 6496. Our time-series data were cadenced to search for
long-period variables (LPVs) over a span of nearly two years, and our
variability search yielded the discovery of 13 new variable stars, of
which 6 are LPVs, 2 are suspected LPVs, and 5 are short-period
eclipsing binaries. An additional star was found in the ASAS database,
and we clarify its type and period. We argue that all of the eclipsing
binaries are field stars, while five to six of the LPVs are members of
NGC 6496. We compare the period-luminosity distribution of these LPVs
with those of LPVs in the Large Magellanic Cloud and 47 Tucanae, and
with theoretical pulsation models. We also present a VI
color-magnitude diagram, display the evolutionary states of the
variables, and match isochrones to determine a reddening of
E(B-V)=0.21±0.02mag and apparent distance modulus of
15.60±0.15mag.
Description:
As part of the original survey for RR Lyrae stars in metal-rich
globular clusters, we obtained time-series images of for a 10' field
around NGC 6496 using the direct CCD camera on the 0.9m telescope at
Cerro Tololo Inter-American Observatory (CTIO) during two runs in May
and June of 1996 having one and seven usable nights, respectively. We
used filters matched to the CCD to reproduce the Johnson V and
Kron-Cousins I bandpasses. NGC 6496 is located at galactic coordinates
(l,b)=(348.03°, -10.01°), at a distance ∼11.3kpc from the
Sun.
Instrumental photometry of the CTIO 0.9m images was performed using
the DAOPHOT II (Stetson, 1987PASP...99..191S 1987PASP...99..191S) and ALLFRAME (Stetson,
1994PASP..106..250S 1994PASP..106..250S) packages following the procedures described in
(Baker et al., 2007AJ....133..139B 2007AJ....133..139B; Paper III). These packages were
used in a similar way to do photometry on the images from the 0.41m
Panchromatic Robotic Optical Monitoring and Polarimetry Telescope
number 4 (PROMPT4).
Additional observations were obtained using the 0.41m Panchromatic
Robotic Optical Monitoring and Polarimetry Telescope number 4
(PROMPT4). This telescope is equipped with a 1 k*1 k Apogee CCD
camera. On ∼63 nights that span almost two years (2009 February
through 2010 October) and that are each separated by one to two weeks,
we obtained images of NGC 6496 through standard V and I filters. On
each night, we obtained four 80s images in V, three 40s (long) images
in I, and three 10s (short) images in I.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 63 1675 Mean photometry of NGC 6496
table3.dat 78 14 Variable star coordinates and 2MASS photometry
table4.dat 67 800 Time series photometry
table5.dat 73 14 Variable star properties
--------------------------------------------------------------------------------
See also:
B/vsx : AAVSO International Variable Star Index VSX (Watson+, 2006-2014)
II/264 : ASAS Variable Stars in Southern hemisphere (Pojmanski+, 2002-2005)
VII/202 : Globular Clusters in the Milky Way (Harris, 1997)
J/AJ/145/165 : BVRI photometry in NGC 6496 (Fraga+, 2013)
J/AJ/117/1313 : NGC 6441 VI photometry (Layden+, 1999)
J/A+AS/109/1 : CCD photometry of globular clusters (Richtler, 1995)
J/ApJS/93/161 : CCD photometry of 6 globular clusters (Sarajedini+ 1994)
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 I4 --- Star [1/1678] Star identification number
6- 13 F8.3 pix Xpos X pixel coordinate (G1)
15- 22 F8.3 pix Ypos Y pixel coordinate (G1)
24- 29 F6.3 mag Vmag [10.2/19.2] Standard V band magnitude (1)
31- 36 F6.4 mag e_Vmag Uncertainty in Vmag
38- 43 F6.3 mag Imag [8.9/18.1] Standard weighted I band magnitude (1)
45- 50 F6.4 mag e_Imag Uncertainty in Imag
52- 56 F5.3 --- chi [0.786/2.393] Mean DAOPHOT "χ" value (2)
58- 63 F6.3 --- sharp [-3.429/2.062] Mean DAOPHOT "sharp" value (2)
--------------------------------------------------------------------------------
Note (1): Instrumental photometry of the Cerro Tololo Inter-American Observatory
(CTIO) 0.9m images was performed using the DAOPHOT II (Stetson,
1987PASP...99..191S 1987PASP...99..191S) and ALLFRAME (Stetson, 1994PASP..106..250S 1994PASP..106..250S) packages
following the procedures described in (Baker et al., 2007AJ....133..139B 2007AJ....133..139B;
Paper III).
Note (2): "chi" and "sharp" image diagnostic described in Stetson
(1987PASP...99..191S 1987PASP...99..191S).
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1 A1 --- --- [V]
2- 3 I2 --- Var [1/13]? Variable star number (G2)
5 A1 --- f_Var [a] Indicates the two stars located outside
the PROMPT4 field of view (1)
7- 10 I4 --- Star [1/1678]? Star identification number (table 2)
12- 17 F6.1 pix Xpos ? X pixel coordinate (table 2)
19- 23 F5.1 pix Ypos ? Y pixel coordinate (table 2)
25- 28 F4.2 arcmin Dist [1.14/5.89]? Projected angular distance
(Rproj) from the cluster center
30- 31 I2 h RAh ? Hour of Right Ascension (J2000) (2)
33- 34 I2 min RAm ? Minute of Right Ascension (J2000) (2)
36- 40 F5.2 s RAs ? Second of Right Ascension (J2000) (2)
42 A1 --- DE- ? Sign of the Declination (J2000) (2)
43- 44 I2 deg DEd ? Degree of Declination (J2000) (2)
46- 47 I2 arcmin DEm ? Arcminute of Declination (J2000) (2)
49- 52 F4.1 arcsec DEs ? Arcsecond of Declination (J2000) (2)
54- 59 F6.3 mag Jmag [9.31/14.89]? 2MASS J band magnitude (2)
61- 65 F5.3 mag e_Jmag ? 2MASS 1σ uncertainty in Jmag (2)
67- 72 F6.3 mag Kmag [8.19/14.63]? 2MASS Ks band magnitude (2)
74- 78 F5.3 mag e_Kmag ? 2MASS 1σ uncertainty in Kmag (2)
--------------------------------------------------------------------------------
Note (1): PROMPT4=0.41m Panchromatic Robotic Optical Monitoring and Polarimetry
Telescope number 4.
Note (2): We matched the positions of these variable stars on our images with
images from the Two Micron All-Sky Survey to find rough coordinates,
then extracted precise equatorial coordinates and magnitudes in the J
and K passbands (and their uncertainties) from the 2MASS Point Source
Catalog (II/246). All thirteen stars were relatively uncrowded, and
their "phot_qual" flags in the 2MASS database were "AAA," indicating
the highest quality infrared photometry.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1 A1 --- --- [V]
2- 3 I2 --- Var [1/13] Variable star number (G2)
5- 13 F9.4 d JD Julian date of the observation (JD-2450000)
15- 19 F5.3 --- Phase [0/1]?=- Light curve phase (1)
21- 26 F6.3 mag Ilmag [10.66/15.73]?=- Long exposure I band
magnitude
28- 32 F5.3 mag e_Ilmag ?=- Error in Ilmag
34- 35 I2 --- o_Ilmag [0/10] Number of comparison stars used in Ilmag
37- 42 F6.3 mag Ismag [11.4/15.53]?=- Short exposure I band
magnitude (2)
44- 48 F5.3 mag e_Ismag ?=- Error in Ismag
50- 51 I2 --- o_Ismag [0/10] Number of comparison stars used in Ismag
53- 58 F6.3 mag Vmag [12.8/16.44]?=- The V band magnitude
60- 64 F5.3 mag e_Vmag ?=- Error in Vmag
66- 67 I2 --- o_Vmag Number of comparison stars used in Vmag
--------------------------------------------------------------------------------
Note (1): For stars with cyclic behavior. A blank is used for long-period
variable stars for which we did not make period-phased light curves.
Note (2): For observations obtained from 2009 February through 2010 October
using the 0.41m Panchromatic Robotic Optical Monitoring and Polarimetry
Telescope number 4 (PROMPT4). On each night, we obtained four 80s images
in V, three 40s (long) images in I, and three 10s (short) images in I.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table5.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1 A1 --- --- [V]
2- 3 I2 --- Var [1/13]? Variable star number (G2)
5- 8 A4 --- ASAS [ASAS] Star in ASAS survey (3)
10- 14 F5.2 mag <Vmag> [11.3/16.1] Mean V band magnitude
16- 20 F5.2 mag <Ilmag> [8.9/15.4] Mean (long exposure) I band magnitude
22- 26 F5.2 mag Vmax [11.1/15.9] Maximum V band magnitude
28- 32 F5.2 mag Vmin [11.7/16.5] Minimum V band magnitude
34- 38 F5.2 mag Imax [10.67/15.21]? Maximum I band magnitude
40- 44 F5.2 mag Imin [10.7/15.5]? Minimum I band magnitude
46- 48 I3 --- NV [9/490] Number of V-band images obtained
50 A1 --- f_Per A long secondary period of 340±10days was
found for V3
52- 60 F9.5 d Per [0.28974/740]? Derived period (4)
62- 69 F8.5 d e_Per ? 1σ uncertainty in Per
71- 72 A2 --- Type Variability type (5)
73 A1 --- u_Type [?] Uncertainty flag on Type
--------------------------------------------------------------------------------
Note (3): Previous searches for variable stars in NGC 6496 found no candidates.
Sometimes, field star searches for variable stars identify cluster
variables. We therefore searched International Variable Star Index
database (cat. B/vsx; http://www.aavso.org/vsx/) for objects within 8' of
the cluster center, but found only one object, ASAS 175901-4411.5, from
the All Sky Automated Survey (Pojmanski & Maciejewski 2004, cat. II/264).
See Section 5.3 for additional details about this star.
Note (4): We used several methods to estimate pulsation periods, including phase
dispersion minimization (PDM; Stellingwerf, 1978ApJ...224..953S 1978ApJ...224..953S), a
template fitting method (Layden et al. 1999, cat. J/AJ/117/1313), and for
the irregular pulsators, simply averaging the time intervals between peaks.
For the long-period variables V1-6, the periods listed are the mean from
the three different methods, and the uncertainty is their standard
deviation. For the short-period variables V9-V13, we used PDM and the
template fitting method to search for the periods of these variables.
Note (5): Variability type defined as follows:
EB = Beta Lyrae ellipsoidal eclipsing binary;
EW = W Ursae Majoris type contact binary;
Lb = Irregular pulsators;
SR = Semi-regular pulsator.
--------------------------------------------------------------------------------
Global notes:
Note (G1): These coordinates correspond to those in Figure 1 (north is up and
east is to the left; 10'=1024pix) and to the FITS-format image available
at http://physics.bgsu.edu/~layden/publ.htm, where additional data products
from this study are available.
Note (G2): V1-V6 are classified as long-period variables while V9-V13 are
short-period variables.
History:
From electronic version of the journal
References:
Layden et al., Paper I, 1999AJ....117.1313L 1999AJ....117.1313L, cat. J/AJ/117/1313
Layden et al., Paper II, 2003AJ....126..255L 2003AJ....126..255L
Baker et al., Paper III, 2007AJ....133..139B 2007AJ....133..139B
(End) Greg Schwarz [AAS], Sylvain Guehenneux [CDS] 19-Feb-2015