J/AJ/138/1310 Variable stars in NGC 2257. I. (Nemec+, 2009)
Variable stars in the Large Magellanic Cloud globular cluster NGC 2257
I. Results based on 2007-08 B,V photometry
Nemec J.M., Walker A., Jeon Y.-B
<Astron. J. 138, 1310 (2009)>
=2009AJ....138.1310N 2009AJ....138.1310N
ADC_Keywords: Magellanic Clouds ; Clusters, globular ; Stars, variable ;
Photometry, UBV
Keywords: globular clusters: individual (NGC 2257) - Magellanic Clouds -
stars: horizontal-branch - stars: Population II
Abstract:
The variable stars in the Large Magellanic Cloud star cluster NGC 2257
are reinvestigated using photometry (to ∼20th mag) of over 400 new B,
V CCD images taken with the CTIO 0.9m telescope on 14 nights in 2007
December and 2008 January. New period searches have been made using
two independent algorithms (CLEAN, Period04); the resultant periods of
most of the stars are consistent with the pulsation periods derived
previously, and where there are discrepancies these have been
resolved. For the B and V light curves, accurate Fourier coefficients
and parameters are given. Six new variable stars have been discovered
(V45-50), including a bright candidate long-period variable star
showing secondary oscillations (V45) and two anomalously bright RRc
stars (V48 and V50), which are shown to be brightened and reddened by
nearby red giant stars. Also discovered among the previously known
variable stars are three double-mode RR Lyrae stars (V8, V16, and V34)
and several Blazhko variables. Archival Hubble Space Telescope images
and the photometry by Johnson et al. (Cat. J/ApJ/527/199) have been
used to define better the properties of the most crowded variable
stars. The total number of cluster variable stars now stands at
forty-seven: 23 RRab stars, four of which show Blazhko amplitude
variations; 20 RRc stars, one showing clear Blazhko variations and
another showing possible Blazhko variations; the three RRd stars, all
having the dominant period ∼0.36 day and period ratios P1/P0 ∼0.7450;
and an LPV star located near the tip of the red giant branch. A
comparison of the RRd stars with those in other environments shows
them to be most similar to those in IC4499.
Description:
The present investigation is based on two new sets of Johnson B,V CCD
observations, taken in the 2007-2008 LMC observing season with the
0.9m (36inch) telescope at the Cerro Tololo Inter-American
Observatory. NGC 2257 was observed on seven consecutive nights in 2007
December, starting on December 13/14, and on seven consecutive nights
in 2008 January, starting on Jan 3/4.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 75 2086 Cross-identifications, radial distances, and
J2000.0 coordinates
table3.dat 50 1979 Magnitude-averaged mean magnitudes and colors
table4.dat 61 198 Sample photometry file (for V1=#409)
--------------------------------------------------------------------------------
See also:
J/ApJ/717/277 : Chemical composition of old LMC clusters (Mucciarelli+, 2010)
J/ApJ/527/199 : HST observations of old clusters in the LMC (Johnson+, 1999)
J/AJ/98/2086 : CCD photometry of NGC 2257 RR Lyrae (Walker 1989)
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1 A1 --- r_Name Star source (1)
3- 6 A4 --- Name Common star name from previous investigation
8- 11 I4 --- ID [1/2170] Internal identification number
13- 15 I3 arcsec r Projected distance from cluster center
17- 23 F7.2 pix Xpix X pixel coordinate from Dec. 2007 frame V7075
25- 31 F7.2 pix Ypix Y pixel coordinate from Dec. 2007 frame V7075
33 I1 h RAh Hour of Right Ascension (J2000)
35- 36 I2 min RAm Minute of Right Ascension (J2000)
38- 42 F5.2 s RAs Second of Right Ascension (J2000)
44 A1 --- DE- Sign of the Declination (J2000)
45- 46 I2 deg DEd Degree of Declination (J2000)
48- 49 I2 arcmin DEm Arcminute of Declination (J2000)
51- 54 F4.1 arcsec DEs Arcsecond of Declination (J2000)
56- 64 F9.6 deg RAdeg Right Ascension in decimal degrees (J2000)
66- 75 F10.6 deg DEdeg Declination in decimal degrees (J2000)
--------------------------------------------------------------------------------
Note (1): Flag as follows:
a = Variable stars;
b = Gascoigne (1966MNRAS.134...59G 1966MNRAS.134...59G) stars;
c = Walker (1972MNRAS.156..459W 1972MNRAS.156..459W) stars;
d = Penny (1981MNRAS.197..693P 1981MNRAS.197..693P; <Cl* NGC 2257 P AA> in Simbad) stars;
e = Alcaino & Alvarado (1988AJ.....95.1724A 1988AJ.....95.1724A; <Cl* NGC 2257 AA A> in Simbad)
stars;
f = Walker (1989, Cat. J/AJ/98/2086; <Cl* NGC 2257 W NNN> for Simbad) local
standard stars;
g = Stetson standard stars
(http://www3.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/community/STETSON/);
h = other measured stars.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 I4 --- ID [1/2170] Internal identification number
6- 13 F8.3 pix Xpix X pixel coordinate from Dec. 2007 frame V7075
15- 22 F8.3 pix Ypix Y pixel coordinate from Dec. 2007 frame V7075
24- 29 F6.3 mag Vmag The V band magnitude
31- 36 F6.3 mag e_Vmag Uncertainty in Vmag
38- 43 F6.3 mag B-V The (B-V) color index
45- 50 F6.3 mag e_B-V Uncertainty in B-V
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 12 F12.4 d HJD Heliocentric Julian Date
14- 19 F6.1 pix Xpix X pixel coordinate
21- 26 F6.1 pix Ypix Y pixel coordinate
28- 33 F6.3 mag Vmag Calibrated V band magnitude
35- 39 F5.3 mag e_Vmag Error in Vmag
41- 43 I3 --- Seq Identification number in frame
45- 61 A17 --- Frame The frame label (1)
--------------------------------------------------------------------------------
Note (1): Where the "dec" and "jan" suffixes distinguish the Dec 07 and
Jan 08 images.
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 23-Jan-2012