UBVRI CCD photometric study of the open clusters Basel 4 and NGC 7067

1 MB·English

by  R. K. S. Yadav

Mon.Not.R.Astron.Soc.000,1–14(2003) Printed2February2008 (MNLATEXstylefilev1.4)
UBV RI
CCD photometric study of the open clusters Basel
4 and NGC 7067
⋆ †
R. K. S. Yadav1 and Ram Sagar2
1Inter-UniversityCentre for Astronomy and Astrophysics, Ganeshkhind, Pune 411 007, India
2State Observatory, Manora Peak Nainital 263 129, India
4 Accepted ———.Received———;
0
0
2
ABSTRACT
n In this paper we present UBVRI CCD photometry in the region of two young open
a star clusters Basel 4 and NGC 7067 for the first time. Our sample consists of ∼ 4000
J stars down to V ∼ 21 mag. Stellar surface density profile indicates that radius of
6 Basel4 andNGC 7067areabout 1.8and3.0arcminrespectively.The (U−B)versus
1 (B−V) diagrams indicate that metallicity of NGC 7067 is solar while that of Basel
4 is Z ∼ 0.008.We estimate the mean value of E(B−V) = 0.45±0.05and 0.75±0.05
1 mag for Basel 4 and NGC 7067 respectively. The analysis of 2MASS JHK data in
v combination with the optical data in both the clusters yields E(J −K) = 0.30±0.20
0 mag and E(V −K) = 1.60±0.20 mag for Basel 4 while E(J −K) = 0.40±0.20 mag
3
and E(V −K) = 2.10±0.20mag for NGC 7067.Furthermore,colour excess diagrams
3
show a normal interstellar extinction law towards both the clusters.
1
0 Using the intrinsic colour-magnitude diagrams of the cluster members, we esti-
4 mated the distances of the clusters as 3.0±0.2 and 3.6±0.2Kpc for Basel 4 and NGC
0 7067 respectively. By fitting the proper metallicity isochrones to the bright cluster
/ members we estimated the age of the clusters as 200±50 and 100±25 Myr for Basel
h
4 and NGC 7067 respectively. The mass function slope which is derived by applying
p
- the corrections of field star contamination and data incompleteness are 1.55±0.25
o and 1.68±0.47 for Basel 4 and NGC 7067 respectively. The values of mass function
r slopes are thus not too different from the Salpeter’s (1955)value. Mass segregationis
t
s observedinboththeclusterswhichmaybeduetothedynamicalevolutionsorimprint
a
of star formation processes or both.
:
v
i
X Key words: Star clusters - individual: Basel 4 and NGC 7067 - star: Interstellar
r extinction, luminosity function, mass function, mass segregation- HR diagram.
a
1 INTRODUCTION is the question: how many stars of which masses formed or
exist in an ensemble of stars ? A function which described
Open clusters are ideal objects for the study of Galactic
thefrequencydistributionofstellarmassesiscalledthestel-
disk. The young open clusters are used to determine spiral
larmassfunction.Starclustersaresuitableobjectsformass
arm structure, to investigate the mechanisms of star for-
functiondeterminationasmembersformed(moreorless)at
mation and its recent history, and to constrain the initial
thesametimeandfromthesamecloud.Inadditiontothis,
luminosityandmassfunctioninaggregates ofstarsetc.For
study of mass segregation in open clusters provides a clue
such studies, it is important to know the basic parameters
about the spatial distribution of high and low mass stars
of the clusters. The Colour-Magnitude (CM) and Colour-
within the clusters. Generally, it is found that high mass
Colour (CC) diagrams of an open clusterare valuable tools
stars are concentrated towards the center of the clusters in
for obtaining their basic informations, such as its distance
comparison to lower mass stars. The cause of such kind of
andage,andforstudyingbothinterstellarextinctioninthe
distribution isstill notwell understood whetheritis dueto
direction of cluster and stellar evolution. An important as-
dynamical evolution or imprint of star formation itself.
pect for understanding star formation and stellar evolution
Inthelightofabovediscussions,weconductedUBVRI
CCD stellar photometry in two young open star clusters
⋆
E-mail:[email protected] Basel 4 and NGC 7067 aiming to investigate the cluster’s
† E-mail:[email protected] basic parameters (e.g. reddening, distance and age), mass
(cid:13)c 2003RAS
2 R. K. S. Yadav and Ram Sagar
Table 1.GeneralinformationabouttheclustersunderstudytakenfromDiasetal.(2002)
Cluster IAU OCL l b Trumpler Radius Distance E(B−V) log(age)
(deg) (deg) class (arcmin) (Kpc) (mag) (yrs)
Basel4 C0545+302 455 179.23 1.20 II1p 2.5 5.6 0.53 7.0
NGC7067 C2122+478 208 91.19 -1.67 II1p 1.5 1.3 0.85 7.5
function and mass segregation etc. The existing basic in- photometric measurements of fainter stars, 2 to 3 deep ex-
formations on both the clusters are given in Table 1. The posures were taken in each passband. Furthermore, obser-
plan of the paper is as follows. In Sec. 2 we summarize the vations were taken in 2×2 pixel binning mode to improve
previous studies of Basel 4 and NGC 7067, while Sec. 3 is the S/N ratio. An identification map of cluster and field
dedicated on theobservation and datareduction strategies. regions for both the clusters are shown in Fig 1. Besides
Sec.4dealswiththedeterminationofclustersbasicparam- them,anumberofstandardstarfieldwerealsoobservedfor
eters as well as detail study of interstellar extinction, mass calibration purposes. We observed M67 (open cluster) and
function and mass segregation in the clusters under study. field PG0231+051 of Landolt (1992) for calibrating Basel 4
Finally, Sec. 5 summarizes our findings. andNGC7067 respectively.The V mag rangeofstarsused
for calibration is 11−13 mag in M67 and 12−16 mag in
PG0231+051 whilethe(V −I)colourrangeis0.5−1.1mag
in M67 and −0.5−2.0 in PG0231+051. Thus, the standard
2 EARLIER INVESTIGATIONS stars in these fields provide a good magnitude and colour
coverage, essential to obtain reliable photometric transfor-
Basel 4: This cluster was studied by Svolopoulos (1965)
mations.ThestandardfieldarealsoobservedinUBVRI at
photographicallyfirstinRGUsystem.Accordingtohimthe
differentairmassestoobtainareliableestimateoftheatmo-
location ofthisclustercoincideswithspiralarm+IIIwhich
spheric extinction coefficients. For correcting the bias level
couldbeexpected−ifexistingatall−atasimilardistance.
totheimage,anumberofbiasframeweretakenduringthe
In any case, it is remarkable that typical representatives of
observations while for the flat field correction, a number of
the galactic disk population are located so far out in the
flat frames were taken on thetwilight sky in each filter.
directionofthegalacticanticenter.Heclassified thiscluster
asaIII2m.Inadditiontothis,healsoconcludedthatBasel
4 is 2×107 yrs old, and has total apparent diameter of 6′.4 3.2 Data Reductions
atadistanceof5.9.Toourknowledgenootherstudieshave
been carried out so far. The CCD images were processed using IRAF data reduc-
tion package. Then, for a given filter, frame of the same
NGC7067:ThisclusterwasfirststudiedbyBecker(1963). exposuretimewerecombinedintoone,toimprovethestatis-
ItisapooryoungopenclusterlyinginCygnusspiralarm.It tics of the faintest stars. Instrumental magnitudes were de-
wasagain revisitedbyBecker(1965) andindicatedthatthe rived through Point Spread Function (PSF) fitting using
earliest spectral typeof theclustermemberisb0.5. Healso DAOPHOT(Stetson1987) withinMIDAS.Duringthepro-
estimated the cluster angular diameter of 2′.1, which cor- cess ofdetermining PSF,weused several well isolated stars
responds to a linear diameter of 2.6 pc. Hassan (1973) also to construct a single PSF for the entire frame on each ex-
studiedthisclusterphotoelectrically andderivedadistance posure. The bright stars were measured on the frames with
ofabout4.4KpchavingE(B−V)=0.83 magandageless short exposure times, as they were saturated in the longer
than 107 years. Dias et al. (2002) mentioned a distance of exposureframes.
1.3 Kpc for this cluster. The distance determination to the For transforming the instrumental magnitude to the
cluster is thusquiteuncertain. standard magnitude, the photometric calibration equations
are as follows:
u=U+6.40±0.01−(0.03±0.02)(U −B)+0.59X
3 OPTICAL OBSERVATIONS AND DATA b=B+4.39±0.01−(0.02±0.01)(B−V)+0.36X
REDUCTIONS v=V +4.08±0.01−(0.01±0.01)(B−V)+0.23X
r=R+4.00±0.01−(0.02±0.01)(V −R)+0.18X
3.1 CCD Photometric observations
i=I+4.55±0.01−(0.01±0.02)(R−I)+0.13X
We used CCD imaging to obtain UBV Johnson and RI
Cousins photometry of the stars in the region of the open Where U,B,V,R and I are the standard magnitudes
clusters Basel 4 and NGC 7067 on 02/03 Jan 2000 and andu,b,v,randiaretheinstrumentalaperturemagnitudes
11/12 Oct 2001 respectively. The data were obtained us- normalised for 1 second of exposure time and X is the air-
ing 2K×2K CCD system at the f/13 Cassegrain focus of mass. We have ignored the second order colour correction
the 104-cm Sampurnanand telescope of the State Observa- terms as they are generally small in comparison to other
tory, Naini Tal. Log of CCD observations is given in Table errors present in the photometric data reduction. The er-
2. The 0′′.36/pixel plate scale resulted in a field of view rorsinzeropointandcolourcoefficientsareestimatedfrom
of 12′.3×12′.3. The read-out noise and gain of the CCD thedeviationofdatapointsfromthelinearrelationandare
are 5.3 e− and 10 e−/ADU respectively. For the accurate ∼ 0.01 mag. Using these transformation equation, we cal-
(cid:13)c 2003RAS,MNRAS000,1–14
UBVRI CCD photometric study of the open clusters Basel 4 and NGC 7067 3
1000 1000 Field region
E w
N N
800 800
C
600 600
C
400 400
200 200
200 400 600 800 1000 200 400 600 800 1000
X(in Pixels) X (in Pixels)
Figure 1. Identification maps for the cluster and field regions of Basel 4 and NGC 7067. The (X, Y) coordinates are in pixel units
corresponding to 0′′.72 onthe sky. Direction isindicated inthe corresponding map. Filledcircles of different sizes represent brightness
ofthestars.Smallestsizedenotes starsofV∼21mag.Opencircleshavingcentreat’C’inthemaprepresenttherespectiveclustersize.
Table 2.LogofCCDobservationsalongwithequatorialcoordinates fortheepoch2000.
Region Filter ExposureTime Date
(inseconds)
Basel4 U 1800×2,300×1 02/03Jan2000
α2000 =07h32m00s B 1200×2,180×1 ,,
δ2000 =+30d12′57′′ V 900×2, 120×1 ,,
R 240×3,60×1 ,,
I 240×3,60×1 ,,
NGC7067 U 1800×3,300×2 11/12Oct2001
α2000 =21h24m11s B 1200×3,240×2 ,,
δ2000 =+48d00′57′′ V 900×3, 180×2 ,,
R 600×3, 120×2 ,,
I 300×3,60×2 ,,
ibrated the CCD instrumental magnitudes. For generating and also from theauthors. The format of thetable is listed
the local standards, we selected many well isolated stars in in Table 4 for Basel 4 and NGC7067.
theobservedregionandusedtheDAOGROWprogramme
fortheconstructionofanaperturegrowthcurverequiredfor
determining thedifference between aperture and profile fit- 3.3 Comparison with previous photometric study
ting magnitudes. Table 3 gives the photometric errors as a
As mentioned in Sec. 2, only NGC 7067 has photoelectric
functionofmagnitude.Theinternalerrorsestimatedonthe
datagivenbyBecker(1965)andwecomparedourdatawith
S/N ratio of the stars as output of the ALLSTAR mainly
thisdata.Table5presentsaveragedifferences(inthesense:
produce thescatter in thevarious CC and CM diagrams of
our values minus those of other) along with their standard
the clusters. It can be noticed that the errors become large
deviations.Thedifference∆inV,(B−V)and(U−B)are
(≥ 0.1 mag) for stars fainter than V= 20 mag, so the mea-
plottedinFig2.Table6andFig.2indicatethatourV mag-
surements should be considered unreliable below this mag-
nitudes are systematically brighter by ∼ 0.05 mag without
nitude.Thefinalphotometricdata isavailable in electronic
any dependence on the stellar magnitude. The ∆(B−V)
form at the WEBDA site http://obswww.unige.ch/webda/
values show a good agreement with the photoelectric data
while ∆(U−B) show a decreasing trend with theV mag.
(cid:13)c 2003RAS,MNRAS000,1–14
4 R. K. S. Yadav and Ram Sagar
Table 3. Internal photometric errors in magnitude as a func-
tionofbrightness.σ isthestandarddeviationperobservationin
magnitude.
Magnituderange σU σB σV σR σI Basel 4
≤12.0 0.01 0.01 0.01 0.01 0.01
12.0-13.0 0.01 0.01 0.01 0.01 0.01 3.0
13.0-14.0 0.01 0.01 0.01 0.01 0.01
14.0-15.0 0.01 0.01 0.01 0.01 0.01
15.0-16.0 0.01 0.01 0.01 0.01 0.01
16.0-17.0 0.02 0.01 0.01 0.01 0.02
17.0-18.0 0.03 0.02 0.02 0.02 0.03 2.5
18.0-19.0 0.04 0.05 0.03 0.05 0.06
19.0-20.0 0.05 0.09 0.05 0.09 0.08
2.0
50 100 150 200 250
Radius (in pixel)
0.30
0.15
0.00
-0.15
-0.30
0.30
7.0
0.15
0.00
NGC 7067
-0.15
-0.30
6.0
0.30
0.15
0.00
-0.15
-0.30 5.0
13 14 15
V
4.0
Figure2.Comparisonofourphotometrywithphotoelectricdata
ofBecker (1965)forNGC7067.
100 200 300 400
3.4 Near - IR data Radius (in pixels)
We have used the Two Micron All Sky Survey (2MASS)
J(1.25 µm), H(1.65 µm) and K (2.17 µm) data for both
s
the clusters Basel 4 and NGC 7067. 2MASS data is taken Figure 3. Radial density profile for Basel 4 and NGC 7067.
from observations with two highly - automated 1.3-m tele- The length of the errorbar denotes errors resulting from sam-
scopes one at Mt Hopkins, AZ, and second at Cerro Tolalo plingstatistics(= 1 whereNisthenumberofstarsusedinthe
√N
Inter-American Observatory (CTIO), Chile. The data are densityestimation at that point). Dotted curves represent fitted
complete up to 16.0 mag in J, 15.5 mag in H and 15.0 in profileandarrowsrepresenttheleveloffieldstardensities.
K . The uncertainty is 0.155 mag for a star of K ∼ 16.5
s s
mag. The K magnitudes are converted into K magnitude
s to Mermilliod (1995), Basel 4 and NGC 7067 has a diame-
following Persson et al. (1998). 2MASS data is available at
terof 5and3arcmin, soourstudycoverstheentirecluster
web site http://www.ipac.caltech.edu/2MASS/.
region.Toestimatetheclusterradius,wederivethesurface
stellardensitybyperformingstarcountsinconcentricrings
aroundtheestimatedcenterofthecluster,andthendividing
4 ANALYSIS OF THE DATA bytheir respective areas. The centerof the cluster is deter-
mined iteratively by calculating average X and Y position
4.1 Cluster radius and radial stellar surface
of thestars within 300 pixels from an eyeestimated center,
density
until they converged to a constant value. The pixel coor-
Forareliabledeterminationofclusterparametersitisessen- dinate of the cluster center obtained in this way are (600,
tialtoknowabouttheradialextentofthecluster.According 780) and(690, 700) forBasel 4andNGC7067 respectively.
(cid:13)c 2003RAS,MNRAS000,1–14
UBVRI CCD photometric study of the open clusters Basel 4 and NGC 7067 5
Table4.CCDrelative(X,Y)positionsandV,(U−B),(B−V),(V−R)and(V−I)photometricmagnitudes
offewstars,asasamplemeasuredintheregionofthecluster Basel4andNGC7067. Starsarenumbered
inincreasingorder ofX value. Thelastcolumn representthe photometric membershipinformationswhere
mandnmrepresentthememberandnon-memberstarsrespectively.
Star X Y V (U−B) (B−V) (V −R) (V −I) Mem
(pixel) (pixel) (mag) (mag) (mag) (mag) (mag)
Basel4
1 1.87 701.86 19.64 * * 0.69 1.61 nm
2 2.01 519.82 18.76 * * 0.40 1.17 nm
3 2.71 790.32 20.66 * * 0.48 1.52 nm
4 2.87 30.33 19.63 * * 0.58 1.21 nm
5 3.34 877.65 20.33 * * 0.33 1.34 nm
NGC7067
1 1.69 835.80 19.37 * * 0.68 1.61 nm
2 3.47 214.41 19.57 * * 0.83 1.93 nm
3 3.87 116.54 17.60 * * 1.24 2.91 nm
4 3.94 99.46 19.33 * * 0.88 1.87 nm
5 4.01 496.79 20.22 * * 0.78 1.70 nm
Table 5.Comparisonof ourphotometry withBecker (1965) forthecluster NGC7067. Thedifference(∆)
isalwaysinthesensepresentminuscomparisondata.Themeanandstandarddeviationsinmagnitudeare
basedonNstars.Fewdeviated pointsarenotincludedintheaveragedetermination.
Cluster Comparisondata V range <∆V > <∆(B−V)> <∆(U−B)>
Mean±σ(N) Mean±σ(N) Mean±σ(N)
NGC7067 Becker (1965) <14.0 0.06±0.03(4) −0.10±0.03(4) 0.07±0.14(4)
14.0−15.0 0.06±0.04(11) −0.02±0.02(10) −0.07±0.08(10)
15.0−16.0 −0.07±0.09(14) −0.006±0.04(10) 0.06±0.12(9)
There may be few tens of pixels error in locating the clus- mates is lower with the value listed in Table 1 for Basel 4
ter center. Center of the clusters are marked by ”C” in the whileinthecaseofNGC7067 ourestimated valueislarger
identification maps (Fig 1). The radial density profile and (see Table 1).
the corresponding Poisson error bars are depicted in Fig 3. As the observed area is much larger than the cluster
A clear radius density gradient present in Basel 4 confirms area,wehaveconsideredstarshavingmorethan2.5and1.6
theexistence ofclustering. In NGC7067 thedensityprofile clusterradiusasfieldstarsfortheclusterBasel4andNGC
decreases with radius slowly up to the limit of the covered 7067 respectively (see Fig 1). The areas of the field regions
region.FollowingKaluzny(1992),wedescribetheρ(r)ofan are 4.2×105 and 3.1×105 pixel2 for the cluster Basel 4 and
open cluster as: NGC7067respectively.ForBasel4,theclosestboundaryof
f the field region is about 4′.5 away from the cluster center
ρ(r)∝ 1+(r0/r )2, in the west direction while for NGC 7067, it is about 5′.0
c
awayinthenorthdirection.Forthefurtheranalysiswehave
where the cluster core radius r is the radial distance at
c considered thestars laying within cluster radius.
which the value of ρ(r) becomes half of the central density
f . We fit this function to the observed data points in the
0
clusterBasel 4anduseχ2 minimization techniquetodeter-
4.2 Apparent CM diagrams of the cluster regions
mine r and other constants. As can be seen in Fig 3, the
c
fittingofthefunctionissatisfactoryforBasel4.Suchfitting The apparent CM diagrams of Basel 4 and NGC 7067 for
could not be done in NGC 7067 due to large errors in the thestars present within thecluster radiusare shown in Fig
values of ρ(r). In the cluster Basel 4, values of ρ(r) flatter 4. The detection in U filter is not as deep as in BVRI be-
atlargerradiiindicatingprobablytheleveloffieldstarden- cause of thelow quantumefficiency of theCCD detectorin
sity in the cluster direction shown by arrow in Fig 3. The U region.So,therearelargenumberofstarswithoutanyU
field star density thus obtained is 2.1×10−3 per pixel2 for measurements. A well defined cluster MS contaminated by
Basel 4. Inthecase of NGC7067, thefield stardensity has field stars is clearly visible in the CM diagrams of Basel 4
been estimated as 0.3×10−3 per pixel2 based on the last whileinNGC7067theMSisnotsowellpopulatedbecause
two data points of its radial density profile in Fig 3. The ofthepoornessofthecluster.IntheclusterNGC7067, the
radiusatwhichthevalueofρbecomesapproximatelyequal stars on the red side of the MS appear to form a sequence
to the field star density has been considered as the cluster paralleltotheMSandthatcanbeascribedduetoGalactic
radius. The values determined in this way are 1′.8 and 3′.0 disk field stars. The field star contamination increases with
for Basel 4and NGC 7067 respectively.Present radiusesti- decreasing brightness. The cluster sequence fainter than V
(cid:13)c 2003RAS,MNRAS000,1–14
6 R. K. S. Yadav and Ram Sagar
12.0
14.0
16.0
18.0
20.0
0 1 1 2 1 2 1 2
(U-B) (B-V) (V-I) (V-I)
Figure 4.TheV,(U−B);V,(B−V)andV,(V −I)diagramsforthestarsobservedbyusinBasel4andNGC7067clusterregions
andV,(V −I)CMdiagrams ofthecorresponding fieldregions. Solidlinesrepresentthe blueandredenvelope of thecluster MS.The
redenvelopeisdeterminedbyshiftingblueenvelope verticallywith0.80mag.
∼ 16 mag in Basel 4 and V ∼ 18 mag in NGC 7067 have selected members by defining the binary sequence. It has
larger scatter. This may be due to photometric errors as been defined by shifting the blue envelope with 0.8 mag
well as field star contamination. It is difficult to separate vertically which is shown in the CM diagrams of the clus-
field stars from the cluster members. For the separation of ters.Somestars,inspiteoftheirambiguouspositionscould
cluster members from thefield stars, precise proper motion not definitively be rejected as likely cluster members. From
and/or radial velocity measurements of these stars are re- the V, (V −I) diagram of the field region, statistically ex-
quired. In the absence of such data for these clusters, we pected numberof field starsamong thephotometric cluster
(cid:13)c 2003RAS,MNRAS000,1–14
UBVRI CCD photometric study of the open clusters Basel 4 and NGC 7067 7
Table 6. Frequency distribution of the stars in the V, (V −I) diagram of the cluster and field regions.
NB, NS and NR denote the number of stars in a magnitude bin blueward, along and redward of the
clustersequencerespectively.Thenumberofstarsinthefieldregionsarecorrectedforareadifferences.NC
(difference between the NS value of cluster and field regions) denotes the statistically expected number of
clustermembersinthemagnitudebin.
Basel4 NGC7067
V range Clusterregion Fieldregion Clusterregion Fieldregion
N N N N N N N N N N N N N N
B S R B S R C B S R B S R C
12-13 0 2 0 0 0 0 2 0 1 2 0 0 0 1
13-14 0 2 0 0 0 0 2 0 2 3 1 1 4 1
14-15 0 4 3 0 0 1 4 0 4 11 0 0 6 4
15-16 0 8 5 0 2 3 6 0 3 8 0 1 4 2
16-17 0 11 4 1 5 3 6 0 12 8 0 4 16 8
17-18 0 16 8 4 7 4 9 0 17 11 1 7 19 10
18-19 2 17 5 7 9 2 8 5 35 20 2 17 36 18
19-20 3 22 3 9 9 1 13 18 44 19 10 23 61 21
members has been given in Table 6. The frequency distri- Table7.AcomparisonofthecolourexcessratioswithE(B−V)
bution of the field star contamination in different part of forbothstarclusterswiththecorrespondingvaluesforthenormal
the CM diagram can be estimated from the Table 6. It is interstellarextinctionlawgivenbyCardellietal.(1989).
thus clear that all photometric probable members can not
beclustermembersandnon-membersshouldbesubtracted Objects E(U−B) E(V−R) E(V−I)
inthestudiesofclusterMFetc.However,probablemembers E(B−V) E(B−V) E(B−V)
located within a cluster radius from its center can be used Normalinterstellar 0.72 0.65 1.25
todeterminetheclusterparameters, astheyhaverelatively Basel4 0.71±0.05 0.68±0.04 1.33±0.10
less field star contamination and this has been done in the NGC7067 0.69±0.04 0.56±0.02 1.33±0.08
sections to follow.
agreefairly wellwithvaluesestimatedearlierbyothers(see
Table 1).
We investigate the nature of interstellar extinction law
4.3 Interstellar extinction towards the clusters
towards the clusters, by considering the stars having spec-
Fig5showsthe(U−B)versus(B−V)diagramsfordeter- tral typeearlier than A0. This has been selected from their
mining the interstellar extinction using the probable clus- position in the (U −B) versus (B−V) and apparent CM
ter members. We fit the intrinsic zero-age main-sequence diagramswhich revealsthatbrightstarswithV<16.0mag
(ZAMS)givenbySchmidt-Kaler(1982)validforstarsoflu- and (B−V)<0.60 mag in Basel 4 and with V< 16.5 mag
minosityclassVtotheMSstarsofspectraltypeearlierthan and(B−V)<0.75maginNGC7067areneededstars.The
A0assumingtheslopeofreddeningE(U−B)/E(B−V)as numberofsuchstarsare11and12inBasel4andNGC7067
0.72. respectively. The intrinsic colours for these stars have been
In the cluster Basel 4, ZAMS given by Schmidt-Kaler determinedusingUBV photometricQ-method(cf.Johnson
(1982) is not fitting well for the stars of spectral type A, F & Morgan 1953; Sagar & Joshi 1979) and the calibrations
and G. Excess in (U −B) colour is clearly visible for the given by Caldwell et al. (1993) for (U −B)0, (V −R)0 and
starsof(B−V)>0.50mag.Thisindicatesthatthecluster (V−I)0with(B−V)0.Themeanvaluesofthecolourexcess
ismetaldeficient.TheUVexcessδ(U−B)determinedwith ratios derived in this way are listed in Table 7 for both the
respect to Hyades MS turns out to be ∼ 0.1 mag. Using clusters.Theyindicatethatthelawofinterstellarextinction
the [Fe/H] versus δ(U −B) relation of Carney (1979) we in the direction of the clusters underdiscussion is normal.
estimated [Fe/H] ∼ −0.35 which correspond to Z ∼ 0.008.
To estimating the reddening in the direction of this cluster
4.3.1 Interstellar extinction in near - IR
wethereforefittedtheZAMSgivenbySchaereretal.(1993)
forZ =0.008 whichis shown byshort dash linesin thetwo By using the optical and infrared data, we estimated the
colourdiagramofBasel4.TheZAMSofZ=0.008fitsnicely interstellar extinction for both clusters under study. There
and provide the reddening E(B−V)=0.45±0.05 for this are65and44commonstarsintheclusterBasel4andNGC
cluster which is in agreement with the earlier findings (see 7067withintheclusterradiusrespectively.Fig.6showsthe
Table 1). (J−K)vs(V −K)diagramsandfitaZAMSformetallicity
UnlikeBasel4,intheclusterNGC7067,ZAMSgivenby Z = 0.008 taken from Schaerer et al. (1993) in the cluster
Schmidt-Kaler(1982)forthesolarmetallicityfitsbothearly Basel 4 and Z = 0.02 taken from Schaller et al. (1992) in
andlatetypestars.ThefittedvaluesofE(B−V)varyfrom the cluster NGC 7067. This gives E(J −K) = 0.30±0.20
0.70to0.80mag.ThemeanvalueisE(B−V)=0.75±0.05 magandE(V −K)=1.60±0.20magfortheclusterBasel4
mag. Our mean reddening estimate for the imaged region andE(J−K)=0.40±0.20magandE(V −K)=2.10±0.20
(cid:13)c 2003RAS,MNRAS000,1–14
8 R. K. S. Yadav and Ram Sagar
1.2
0.0
Basel 4
0.2
0.8
0.4
0.6 0.4
Basel 4 NGC 7067
0.8 E(B-V)=0.45
2 3 4 2 3
(V-K)
1.0
0.50 0.75 1.00 1.25 Figure6.The(J−K)versus(V−K)colour-colourdiagramof
(B-V) all the stars which are common in V and JHK data within the
cluster radius for the cluster Basel 4 and NGC 7067. The solid
line is a ZAMS fitted for the marked values of colour excesses.
Dashedlinesshowtheerrorbar.
stars of spectral type earlier than A0. We determined the
colour excesses by comparing the observed colours of the
stars with its intrinsic colours derived from the MKK spec-
tral type-luminosity class colour relation given by FitzGer-
ald (1970) for (U−V) and (B−V);by Johnson (1966) for
(V −R)and(V −I);andbyKoornneef(1983) for(V −J),
(V −H) and (V −K). For normalisation, we selected the
E(V −J)colourexcessduetoreasonsdescribedinYadav&
Sagar(2002). InFig. 7weplot thecolourexcessE(U−B),
E(B−V), E(V −R),E(V −I),E(V −H) and E(V −K)
against E(V −J). In this Fig, straight line represents the
leastsquarelinearfitstothedatapoints.Thevaluesofcor-
relation coefficient (r) and fit indicate that the data points
are well represented by linear relation. The slopes of these
straight lines as given in Table 8 represent reddeningdirec-
tionsintheformofcolourexcessratios.Forcomparison,the
colour excess ratios given by Cardelli et al. (1989) for the
Figure 5.The(U−B)versus(B−V)diagramsofthestarsin normalinterstellarmatterarealsolistedintheTable8.The
clusterregionforBasel4andNGC7067.Starsconsideredasnon present reddeningdirections agree well with those.
memberinFig.4areshownasopencircles.Thearrowrepresents In addition to this we have also estimated the value of
slope0.72anddirectionofthereddeningvector. Thesolidcurve Rtoknowaboutthenatureofinterstellarextinctionlawin
representsthelocusofSchmidt-Kaler’s(1982)ZAMSfittedfor the direction of clusters under study. We used the relation
the marked values of colour excesses. The curve shown by short R=1.1E(V −K)/E(B−V)(Whittet&Breda1980)which
dashlinesinBessel4istheZAMSgivenbySchaereretal.(1993) is generally used at longer wavelengths. The average values
forZ=0.008
of R = 3.51 ±0.30 (sd) and 3.04±0.22 (sd) for Basel 4 and
NGC 7067 respectively are not too different from the value
mag for the cluster NGC 7067. For both clusters the ratio 3.1 for normal extinction law.
E(J−K) ∼0.20±0.30 isin good agreement withthenormal
E(V K)
inter−stellarextinctionvalue0.19suggestedbyCardellietal. In the light of above analysis, we conclude that inter-
(1989).However,scatteringislargerduetotheerrorsizein stellar extinction law is normal towards both Basel 4 and
JHK data. NGC 7067 in agreement with ourearlier result.
4.3.2 The law of interstellar extinction 4.3.3 Near-IR excess Fluxes
Weusedthecolour excess ratio methoddescribed byJohn- Aninfraredexcessisproducedbythestarswhicharehaving
son(1968) forthestudyofinterstellarextinctionlaw inthe theirownenvelopeofgasanddust.Toinvestigatethenear-
direction of both clusters understudy.Forthis we used the IR flux in the stars of the clusters under study, we plotted
(cid:13)c 2003RAS,MNRAS000,1–14
UBVRI CCD photometric study of the open clusters Basel 4 and NGC 7067 9
2.20 Basel 4 NGC 7067
r=0.98 Basel 4 1.0 1.0
r=0.99 2.00
0.40
1.80 0.0 0.0
1.60 -1.0 -1.0
0.30
1.40
2.00
00..6600 r=0.98 r=0.99 1.0 1.0
1.80
00..5500 0.0 0.0
1.60
-1.0 -1.0
1.40
00..4400
1.2 1.4 1.6 1.6 1.8
0.40 r=0.99 r=0.99 E(V-J) in mag E(V-J) in mag
0.80
0.35 0.60 Figure 8. Plots of near-IR flux excess/deficiency in terms of
∆(V −H)and∆(V −K)againstthecolourexcessE(V −J)for
Basel 4 and NGC 7067. The horizontal dotted lines denote zero
1.2 1.4 1.6 1.2 1.4 1.6 excess.Theshortdashedlinesdenote theextent oftheexpected
E(V-J) errors.
∆(V −H) and ∆(V −K) against E(V −J) in Fig 8. The
differences between the observed colour excess in (V −H)
and(V −K)basedonspectralclassificationandthederived
colour excess from E(V −J) assuming normal extinction
0.5
r=0.86 NGC 7067 law are calculated. The differences can be considered sta-
r=0.78 2.2 tistically significant only if their absolute values are larger
than ∼0.5 mag. The short dashed lines in Fig 8 represent
0.4 2.0 theextentofexpectederrors.Observationaluncertaintiesin
JHK magnitudes, inaccuracies in estimation of E(V −J)
1.8 and errors in spectral classification may play major role in
thedeterminationofdifferences.AninspectionofFig8leads
0.8 r=0.81 r=0.85 that the absolute values of ∆(V −H) and ∆(V −K) are
2.0 close tozero of all themembers. This indicate that there is
0.7
nosignature of near-IR excess fluxes.
0.6
1.8
4.4 Distance to the clusters
r=0.96 r=0.93
TheZAMSfittingprocedurewasemployedtoderivethedis-
0.5 1.0 tancesoftheclusters.Fig9showstheintrinsicCMdiagrams
for Basel 4 and NGC 7067 which is plotted by considering
the probable cluster members. For converting apparent V
0.4 0.8 magnitudeand(U−B),(B−V),(V−R)and(V−I)colours
into intrinsic one, we used average values of E(B−V) and
1.6 1.8 1.6 1.8
following relations for E(U −B) (cf. Kamp 1974; Sagar &
E(V-J)
Joshi1979),A andE(V −I)(Walker1987) andE(V −R)
v
(Alcal´a et al. 1988).
Figure7.TheplotofE(U−B),E(B−V),E(V −R)E(V−I),
E(V −H)and (V −K)againstE(V −J)forBasel 4andNGC E(U−B) = [X + 0.05E(B−V)]E(B−V)
7067.Solidlineineachdiagramrepresentsleastsquarelinearfit
tothedatapoints.Thevaluesofcorrelationcoefficientsareshown where X = 0.62 − 0.3(B−V)0 for (B−V)0 < −0.09
inthediagram. and X = 0.66 + 0.08(B−V) for(B−V) > −0.09
0 0
A =[3.06+0.25(B−V) +0.05E(B−V)]E(B−V);
v 0
and E(V −R) = [E1 + E2E(B−V)]E(B−V)
where E1 = 0.6316 + 0.0713(B−V)
0
(cid:13)c 2003RAS,MNRAS000,1–14
10 R. K. S. Yadav and Ram Sagar
12.0
14.0
16.0
18.0
-1 0 1 0 1 0 0.5 0 1
Figure 9. The V0, (U −B)0; V0, (B−V)0; V0, (V −R)0 and V0, (V −I)0 diagrams for stars of the Basel 4 and NGC 7067. The
continuouscurves,ZAMSfittedtotheMSanddottedcurvesaretheisochronesforZ=0.008starsoflog(age)=8.2,8.3and8.4forBasel
4andZ=0.02starsoflog(age)=7.9, 8.0and8.1forNGC7067.
and E2 = 0.0362 + 0.0078(B−V) ; as 12.4±0.2 and 12.8±0.2 mag for Basel 4 and NGC 7067
0
respectively. The corresponding distances are 3.0±0.2 Kpc
E(V − I) = 1.25[1 + 0.06(B − V) + 0.014E(B − and 3.6±0.2 Kpc. The fact that we were able to find faint
0
V)]E(B−V) probableclustermembersallowustogetabetterdefinition
of the cluster lower main sequence which in turn improves
The ZAMS for Z = 0.008 is plotted in V , (U −B) ; the estimation of the distances. The distance of the cluster
0 0
V , (B−V) ; V , (V −R) and V , (V −I) diagrams of Basel 4 is estimated as 3.0±0.2 Kpc, which is smaller than
0 0 0 0 0 0
Basel4aretakenfromSchaereretal.(1993).InV ,(U−B) the value 5.6 Kpc given by Svolopoulos (1965) based on
0 0
andV ,(B−V) diagramsofNGC7067,wefittedthesolar RGU photographic photometry. We derived 3.6±0.2 Kpc
0 0
distance for the cluster NGC 7067, which is also the value
metallicityZAMSgivenbySchmidt-Kaler(1982)whilethat
given by Walker (1985) was fitted in V , (V −I) diagram. given by Lyng˚a (1987). The distance of this cluster is 1.3
0 0
For V , (V −R) diagram, we have calculated (V −R) Kpc given in the catalogue of Dias et al. (2002), which is
0 0 0
using its relation with (B −V) given by Caldwell et al. much smaller than thevalue estimated by us.
0
(1993). The good fitting of the ZAMS to the intrinsic CM
diagrams was achieved for a distance modulus (m−M)
0
(cid:13)c 2003RAS,MNRAS000,1–14