UBVI CCD Photometry of the Old Open Cluster NGC 1193
ABSTRACT We present UBVI photometry of the old open cluster NGC 1193. Color-magnitude diagrams (CMDs) of this cluster show a well defined main sequence and a sparse red giant branch. For the inner region of r<50 arcsec, three blue straggler candidates are newly found in addition to the objects Kaluzny (1988) already found. The color-color diagrams show that the reddening value toward NGC 1193 is E(B-V) =0.19 +/- 0.04. From the ultraviolet excess measurement, we derived the metallicity to be [Fe/H]=-0.45 +/- 0.12. A distance modulus of (m-M)_0 =13.3 +/- 0.15 is obtained from zero age main sequence fitting with the empirically calibrated Hyades isochrone of Pinsonneault et al. (2004). CMD comparison with the Padova isochrones by Bertelli et al. (1994) gives an age of log t =9.7 +/- 0.1.
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ABSTRACT: We present JHK near-infrared photometric study for the old open cluster (OC) Trumpler 5 (Tr 5), based on the 2MASS data. From the color-magnitude diagrams of Tr 5, we have located the position of the red giant clump (RGC) stars, and used the mean magnitude of the RGC stars in K-band to estimate the distance to Tr 5, d = 3.1 +/- 0.1 kpc ((m-M)_0 = 12.46 +/- 0.04). From fitting the theoretical isochrones of Padova group, we have estimated the reddening, metallicity, and age : E(B-V) = 0.64 +/- 0.05, [Fe/H] = -0.4 +/- 0.1 dex, and t =2.8 +/- 0.2 Gyr (log t=9.45 +/- 0.04), respectively. These parameters generally agree well with those obtained from the previous studies on Tr 5 and confirms that this cluster is an old OC with metallicity being metal-poorer than solar abundance, located in the anti-Galactic center region. Comment: JKAS (J. of the Korean Astron. Soc.) in press (2009 Dec issue), page numbers will be changedJournal of the Korean Astronomical Society 12/2009; · 0.91 Impact Factor
arXiv:0812.1774v1 [astro-ph] 9 Dec 2008
Journal of The Korean Astronomical Society
41: 1 ∼ 9, 2008 December
UBV I CCD Photometry of the Old Open Cluster NGC 1193
JAEMANN KYEONG1, SANG CHUL KIM1,2, DAVID HIRIART3AND EON−CHANG SUNG1
1Korea Astronomy & Space Science Institute, Daejeon 305-348, Korea
E-mail: jman, sckim, email@example.com
2Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada
3Instituto de Astronom´ ıa, Universidad Nacional Aut´ onoma de M´ exico, Campus Ensenada, Apdo. Postal 877,
22800 Ensenada, B.C., Mexico
(Received ???. ??, 2008; Accepted ???. ??, 2008)
We present UBV I photometry of the old open cluster NGC 1193.
(CMDs) of this cluster show a well defined main sequence and a sparse red giant branch. For the
inner region of r < 50′′, three blue straggler candidates are newly found in addition to the objects
Kaluzny (1988) already found. The color-color diagrams show that the reddening value toward NGC
1193 is E(B − V ) = 0.19 ± 0.04. From the ultraviolet excess measurement, we derived the metallicity
to be [Fe/H]= −0.45 ± 0.12. A distance modulus of (m − M)0= 13.3 ± 0.15 is obtained from zero age
main sequence fitting with the empirically calibrated Hyades isochrone of Pinsonneault et al. (2004).
CMD comparison with the Padova isochrones by Bertelli et al. (1994) gives an age of log t = 9.7±0.1.
Key words : open clusters and associations: individual (NGC 1193) – Galaxy: stellar content
Since the open clusters are composed of stars with
almost the same age and metallicity, they can be used
in testing the theoretical models for stellar evolution.
The relatively close open clusters can be good targets
for even small telescopes to get a good enough quality
photometric data (e.g.: Kyeong, Byun, & Sung 2001;
Kim & Sung 2003). In particular, the old open clusters
are very useful in testing the stellar isochrones because
they show clear evolutionary sequences.
There have been a few observational studies on NGC
1193, which has a condensed central region. One is the
BV photometry of Kaluzny (1988) which is used to
derive the basic parameters. The other is Friel & Janes
(1993)’s spectroscopic study of several stars of NGC
1193 to determine its metallicity.
In this paper, we describe the UBV I CCD obser-
vations of the old open cluster NGC 1193, which is
located at α=3h05m, δ=+44◦23′(J2000). Although
the bright star in the western side hamper the wide
field observation, we present accurate UBV I photom-
etry for this cluster and derive the basic parameters
such as reddening, metallicity, distance modulus and
age from the photometric data. The robust determina-
tion of reddening is significant for the estimation of
distance, and subsequently age and metallicity.
we estimate the reddening through two-color diagram
method (U − B,B − V ) and other cluster parameters
are then derived by various conventional methods.
Corresponding Author: J. Kyeong
Section II describes the observations and data re-
duction processes. The comparison with the previous
photometric data is presented in Section III. Color-
magnitude diagrams (CMDs) and blue straggler candi-
dates are explained in Section IV. Cluster parameters
such as the reddening, metallicity, distance modulus
and age are derived in Section V. Section VI summa-
rizes our results.
II.OBSERVATIONS AND DATA REDUC-
UBV I CCD images of the open cluster NGC 1193
were obtained on a photometric night, 2006 August 30,
using the 1.5m telescope (f/13.5) and SITe 1K CCD
at the Observatorio Astron´ omico Nacional in the Siera
San Pedro M´ atir(OAN-SPM) in Baja California, Mex-
ico. The pixel scale is 0.′′25/pixel with a total field of
view of 4.′3×4.′3. The gain and readout noise of the
CCD camera are 1.2 e−/ADU and 7.8 e−, respectively.
The median seeing was around 1′′. Specially, in order
to get the long exposures, we excluded the bright star
(V = 11.23 mag, K = 6.1 mag) on the west of NGC
1193. The journal of the observation of NGC 1193 is
given in Table 1.
Standard procedures using the IRAF∗CCDRED
package were followed. Object CCD frames were flat-
∗IRAF (Image Reduction and Analysis Facility) is distributed by
the National Optical Astronomy Observatory, which are oper-
ated by the Association of Universities for Research in Astron-
omy, Inc., under cooperative agreement with the National Science
– 1 –
2KYEONG ET AL.
Table 1. Journal of Observations for NGC 1193
600 × 3
300 × 3
200 × 3
60 × 3
2006 August 30
Fig. 1.—Synthetic field (4.′3×4.′3) of NGC 1193 as observed in V frame. North is at the top and east is to the left. All
the detected stars are plotted. The X- and Y-axes are in pixels and 1 pixel corresponds to 0.′′25. Circle and square sizes are
proportional to the brightnesses. The large cross means the cluster center determined by Kaluzny (1988). The filled squares
represent the blue straggler candidates (See the text), while their sizes are also proportional to their brightnesses.
Table 2. Derived standardization coefficients and its errors
OPEN CLUSTER NGC 11933
Fig. 2.—Residuals between standard and transformed magnitudes of standard stars, plotted against standard magnitudes.
∆ means standard magnitude minus transformed magnitude.
tened using twilight sky flats after overscan correc-
tion and bias subtraction. For each filter, the three
frames were median-combined to remove the cosmic
rays and to get the high signal-to-noise ratio. Instru-
mental magnitudes of stars were obtained by the point-
spread function (PSF) fitting packages of DAOPHOT
II and ALLSTAR (Stetson 1990) and stars of different
frames were matched by DAOMATCH/DAOMASTER
routines (Stetson 1992). For each frame, a representa-
tive model PSF was constructed by using a number of
unsaturated and relatively isolated stars. Due to the
small field of view of the CCD, it was not necessary
to allow the shape of the model PSF to vary quadrat-
ically with position in each frame. Finally, aperture
corrections were made using the DAOGROW program
(Stetson 1990), for which we used the same stars used
in the PSF construction.
The observed CCD field of the cluster is shown in
Fig. 1. Each star is represented by an open circle,
whose size is proportional to the brightnesses. We use
the value (XC, YC) =(605, 214) determined by Kaluzny
(1988) as the center of the cluster. Of course their co-
ordinates are converted by the transformation equation
between two coordinate systems.
Additionally, we observed 14 standard stars of four
Landolt (1992) regions (PG2113, SA112, SA92×2).
The color range of these standard stars is −0.2 <
(B − V ) < 1.4 which fully covers the color range of
the stars in NGC 1193. Standard stars were observed
at airmasses between 1.1 and 2.4.
With these standard star observations, the transfor-
mation equations between standard and instrumental
magnitudes are as follows:
u − k1U XU+ CU (U − B) + ZU
b − k1BXB+ CB(B − V ) + ZB
v − k1V XV + CV (B − V ) + ZV
i − k1I XI+ CI (V − I) + ZI
where small letters represent instrumental magnitudes
and the capital letters indicate standard magnitudes.
X is the airmass of each filter. All coefficients and its
errors are derived from our own standard star observa-
tions are also given in Table 2.
Fig. 2 shows the standardization residuals for UBV I
filters. We inspected the particular trends like UT term
but didn’t find any trends in the residuals. In particu-
lar, there was no dependency on the secondary extinc-
tion coefficient in U and B band, so only the primary
extinction coefficients are used in the standardization.
III.COMPARISON WITH THE PREVIOUS
Fig. 3 shows the comparison of the photometries
between this study and the BV photometry Kaluzny
4 KYEONG ET AL.
12 1314 151617
0 0.51 1.5
Fig. 3.—Comparison of our photometry with that of Kaluzny (1988), where ∆ means Kaluzny minus this study. For the
case of ∆(B − V ) (upper panel), a linear fitting is overlaid to show the trend clearly.
(1988). For the comparison, we have transformed the
X- and Y-coordinates between ours and Kaluzny’s us-
ing the position of several bright stars.
Fig. 3 shows that the differences of B and V magni-
tudes of stars with V < 17 between the two datasets are
negligible within the standardization residuals. On the
other hand, for the case of ∆(B − V ) (upper panel),
a trend is shown. In particular, there are two max-
imum differences of ∆(B − V ) = −0.02 at the blue
color (B−V = 0.30) and ∆(B−V ) = +0.040 at the red
color (B−V = 1.3). This trend means that these color
differences make the bluer (than the turn-off) stars
bluer and make the red giant stars redder in Kaluzny
(1988)’s CMDs. Therefore, the age of NGC 1193 de-
rived by Kaluzny (1988) using the morphological age
index (MAI) method or isochrone fitting method ap-
pears to be somewhat young.
The photometric data of Kaluzny (1988) are slightly
undersampled (with the relatively large pixel scale of
0.′′86/px compared to the seeing of ∼ 1.′′5), and the sys-
tematic errors of their data are as large as 0.06 mag
which is resulted from the uncertainties in their stan-
dard transformations and those in their aperture cor-
rections. The source of this trend in ∆(B − V ) is not
IV. COLOR MAGNITUDE DIAGRAMS AND
BLUE STRAGGLER CANDIDATES
331 stars are detected in all the UBV I optical bands.
This small number of stars is mainly due to the scarcity
of detected stars in the U filter. In Fig. 4 we present
CMDs in various passbands. But the CMDs of NGC
1193 show the distinguishable features usually seen in
the CMDs of old open clusters (e.g., NGC 2243, NGC
188, etc): well defined main sequence (MS), sub giant
branch and red giant branch.
Ahumada & Lapasset (1995) define the blue strag-
gler region as 0.0 < (B − V )0< 0.8 and 1.0 < MV <
5.0, located above the zero age main sequence (ZAMS)
line. They pointed out that the blue stragglers seems
to be concentrated in the central region of the clusters
and the degree of concentration is stronger in the case
of old open clusters. Adopting the distance to NGC
1193 derived in this study (see §V), (m−M)0= 13.32,
MV range for the blue stragglers in NGC 1193 becomes
14.3 − 18.3. We identified the 15 blue straggler candi-
dates already known in literatures†including the five
stars discovered by Kaluzny (1988) and identified three
new objects by applying the Kaluzny (1988) and Ahu-
mada & Lapasset (1995)’s criterion. Fig. 5 shows these
blue straggler candidates in CMDs. The UBV I mag-
nitudes of these blue stragglers are given in Table 3.
V. PHYSICAL PARAMETERS OF NGC 1193
Reddening is an important basic parameter of a star
clusters, since it can significantly affect the determina-
tion of other physical parameters. Several reddening
values for NGC 1193 are reported.
estimated the reddening to be E(B − V ) = 0.12 by
isochrone fitting. Noriega-Mendoza & Ruelas-Mayorga
(1997) determined the reddening and metallicity values
straggler candidates in NGC 1193, while one is outside of the
field of view of our CCD frame.
OPEN CLUSTER NGC 11935
Fig. 4.—Color-magnitude diagrams of NGC 1193.
Fig. 5.—Blue straggler candidates of NGC 1193 are denoted as larger filled symbols on the optical-band CMDs. Filled
squares are three blue straggler candidates newly found in this study and filled circles are the candidates already known in
the literatures, while small open circles are all other stars. Solid lines denote the ZAMS line by Bertelli et al.(1994) with
[Fe/H]= −0.61, and log t = 7.8.