ArticlePDF Available

The Color-Magnitude Diagram of the Globular Cluster NGC 6362 and the Canonical Tilt of Horizontal Branches

Authors:

Abstract

In this paper we present new and accurate photometry for stars in the Galactic globular cluster NGC 6362. The color-magnitude diagram discloses two peculiarities in the distribution of stars: (1) a slightly tilted horizontal branch (HB) (Δ = 0.1 mag) and (2) a clump of stars near the red edge of the HB. We perform a detailed comparison with theoretical stellar models in both the HB and red giant branch (RGB) phases. It appears that in the moderately metal rich NGC 6362 the tilted HB can be explained as a natural product of canonical evolutionary theories, being a consequence of the minimum in the bolometric correction near 7500 K. We also investigate the effect of decreasing the efficiency of convective transport in stars climbing the RGB. Adopting Z = 0.002 and Y = 0.23, and performing a global fitting with the theoretical isochrones and zero-age HB, an age of 12 ± 1 Gyr is found, together with (m - M)V = 14.68 and E(B-V) = 0.08.
THE ASTROPHYSICAL JOURNAL, 527:230È235, 1999 December 10
1999. The American Astronomical Society. All rights reserved. Printed in U.S.A.(
THE COLOR-MAGNITUDE DIAGRAM OF THE GLOBULAR CLUSTER NGC 6362 AND THE
CANONICAL TILT OF HORIZONTAL BRANCHES
E. BROCATO1
Osservatorio Astronomico di Collurania, Via M. Maggini, I-64100 Teramo, Italy; brocato=astrte.te.astro.it
V. CASTELLANI2
Dipartimento di Fisica, di Pisa, Piazza Torricelli 2 I-56100 Pisa, Italy;Universita
`vittorio=astrte.te.astro.it
G. RAIMONDO1
Osservatorio Astronomico di Collurania, Via M. Maggini, I-64100 Teramo, Italy; raimondo=astrte.te.astro.it
AND
A. R. WALKER
Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories,3Casilla 603, La Serena, Chile; awalker=noao.edu
Received 1999 January 8; accepted 1999 July 21
ABSTRACT
In this paper we present new and accurate photometry for stars in the Galactic globular cluster NGC
6362. The color-magnitude diagram discloses two peculiarities in the distribution of stars : (1) a slightly
tilted horizontal branch (HB) mag) and (2) a clump of stars near the red edge of the HB. We(*V
tilt \ 0.1
perform a detailed comparison with theoretical stellar models in both the HB and red giant branch
(RGB) phases. It appears that in the moderately metal rich NGC 6362 the tilted HB can be explained
as a natural product of canonical evolutionary theories, being a consequence of the minimum in the
bolometric correction near 7500 K. We also investigate the e†ect of decreasing the efficiency of con-
vective transport in stars climbing the RGB. Adopting Z\0.002 and Y\0.23, and performing a global
Ðtting with the theoretical isochrones and zero-age HB, an age of 12 ^1 Gyr is found, together with
and E(B[V)\0.08.(m[M)V\14.68
Subject headings : globular clusters : individual (NGC 6362) È Hertzsprung-Russell diagram È
stars: evolution È stars : horizontal-branch È stars : interiors
1.INTRODUCTION
Galactic clusters have long been the stellar systems most
suitable for testing the predictions of stellar evolution theo-
ries over the main evolutionary phases of both H- and He-
burning structures. In more recent times, the
color-magnitude diagrams (CMDs) of Galactic globular
clusters (GCs) have become of critical importance in eluci-
dating the early phases of galactic evolution, as well as
providing severe constraints to the age of the universe. This
has stimulated much e†ort on both the theoretical (see, e.g.,
Cassisi et al. 1998 and references therein) and observational
fronts.
In a recent paper, Piotto et al. (1999) present a B,VCMD
for the GC NGC 6362 obtained with the Hubble Space
Telescope (HST ) Wide Field Planetary Camera 2, and com-
prising 4104 stars covering the center of the cluster. This
CMD is notable for showing a well-deÐned sequence of blue
straggler (BS) stars and a main sequence (MS) that extends
almost 2 mag deeper than that shown here, allowing a
careful study of the cluster luminosity function. However,
due to the small area covered by HST , the evolved star
sequences, such as the horizontal branch (HB), are not well
populated and have lower photometric accuracy than here;
thus, the two CMDs to a great extent complement each
other.
1Istituto Nazionale di Fisica Nucleare, LNGS, I-67100 LÏAquila, Italy.
2Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56100 Pisa,
Italy.
3Operated by the Association of Universities for Research in
Astronomy, under cooperative agreement with the National Science Foun-
dation.
In this paper we present a new and accurate CMD for
stars in the Galactic GC NGC 6362. In °2 we will discuss
the observational data and present CMDs for the cluster
stars. We Ðnd that NGC 6362 is a further representative of
the globular clusters showing a tilted HB. In °3 we discuss
this point, showing that (at least in this case) the tilted HB is
the natural product of canonical evolutionary theories,
without invoking the intervention of more sophisticated
mechanisms. In the same section we show that the observed
cluster CMD is well Ðtted by a 12 Gyr isochrone, i.e., the
same age already found on the basis of the same theoretical
scenario for much more metal poor clusters. A Ðnal dis-
cussion will close the paper.
2.OBSERVATIONS AND DATA REDUCTION
NGC 6362 lies at a2000 \17h31m55s,d2000 \
[67¡02@52A. With Galactic coordinates b\(l\325¡.55,
and an estimated distance modulus of about 15[17¡.57)
mag (Fourcade 1974; Alcaino & Liller 1986), the cluster is
placed at the periphery of the Galactic bulge. Zinn & West
(1984) Ðnd the metallicity of NGC 6362 to be [Fe/H] \
[1.08 ^0.09, and Rutledge, Hesser, & Stetson (1997)
give [Fe/H] \[1.18 ^0.06 on the same scale, or
[Fe/H] \[0.99 ^0.03 on the scale of Carretta & Gratton
(1997), who themselves, based on only two stars, Ðnd
[Fe/H] \[0.96. Suntze†, Kinman, & Kraft (1991) give
[Fe/H] \[1.08. Thus, NGC 6362 is one of the most metal
rich clusters known with halo kinematics. Despite the high
metallicity, NGC 6362 has an extensive population of RR
Lyrae and many blue HB stars. Early reddening estimates
range between E(B[V)\0.08 (Fourcade 1974) and 0.12
(Harris & Racine 1979).
230
NGC 6362 AND CANONICAL TILT OF HORIZONTAL BRANCHES 231
As part of a program studying the RR Lyrae variables in
selected Galactic GCÏs, 315 CCD frames in the B,V, and I
passbands were obtained of a Ðeld centered on13@.6 ]13@.6
NGC 6362 during Ðve observing runs in 1993È1995, with
the CTIO 0.9 m telescope and Tektronix 2048 No. 3 CCD.
An analysis of the RR Lyrae observations will be given
elsewhere (A. R. Walker, in preparation). The clusters NGC
6981 (M72) and NGC 1851 were also observed as part of
the same program, and similar observing and reduction
techniques were followed for each, as described by Walker
(1998).
Photoelectric standards have been observed in the vicin-
ity of NGC 6362 by Alcaino (1970), in Vand Bonly, and in
UBV RI by Alcaino & Liller (1986). Many of these latter
stars have faint companions, and given the large CCD Ðeld
here, a new set of local standards was set up. The 21 stars
selected have Vmagnitudes between 14.6 and 16.8, and
were measured on 36 sets of frames on six fully photometric
nights on three di†erent observing runs. During each night
many standard stars were observed from Landolt (1992), in
order to determine color equations, extinction, and zero
points. Reductions for the primary and local standards fol-
lowed exactly the same procedures, to mitigate the intro-
duction of systematic errors. The rms scatter for the local
standards, looking at mean values for each of the six nights,
is typically very small, >0.01 mag, and thus the error in the
magnitude system for the local standards will be dominated
by the tie-in to the primary standards and should not be
signiÐcantly greater than ^0.01 mag.
Photometry for the stars on the 315 CCD frames utilized
the programs DAOPHOT and ALLSTAR (Stetson 1987,
1995), using scripts to efficiently process all the frames. At
the completion of the photometry, the lists of stars for each
frame were matched and cross-referenced using programs
DAOMATCH and DAOMASTER (Stetson 1995). Photo-
metry for the nonvariable stars was combined using the
following algorithm, remembering that there are up to 105
measurements in each color. If there were fewer than 20
measurements in each color, the star was rejected. For those
remaining, the measurements for each color were sorted,
and the median and the lower tenth percentile found.
The sorted lists were then clipped both below and above
the median using the di†erence between the median and the
lower tenth percentile, on the assumption that most
systematic measurement errors (cosmic rays, two stars
measured as one, etc.) bias the measured magnitude
brighter. The mean of the remaining measurements was
then calculated.
The resulting V,B[Vand I,V[ICMDs (Figs. 1 and 2)
show tightly deÐned sequences, superimposed on a large
Ðeld population. The RR Lyrae variables are plotted at
their mean magnitude values. The reddening and metallicity
can be derived from the CMD by a variety of methods
(A. R. Walker, in preparation). Mean values are
E(B[V)\0.05 ^0.02, rather smaller than the earlier esti-
mates, and [Fe/H] \[1.1 ^0.1, in good agreement with
values quoted above. This reddening value is in excellent
agreement with that obtained by Piotto et al. (1999),
E(B[V)\0.06 ^0.03, and indeed a careful comparison
shows that the two studies are in excellent photometric
agreement over the whole CMD, with di†erences of less
than 0.01 mag in Vand B[Vwhen Ðducials are compared.
Piotto et al. (1999) show that the center of NGC 6362
contains a extensive, narrow sequence of BS stars, but they
FIG. 1.ÈV,B[Vcolor-magnitude diagram
are not able to determine whether there is any di†erence in
radial distribution of these stars compared to other cluster
members. The BS stars are also apparent on our CMD, and
they are much more centrally concentrated than the other
cluster stars. We quantify this by comparing the number of
BS stars as a function of radial distance. For this compari-
son we use a CMD containing stars with a less stringent
error cuto† than the one used for accurately delineating the
CMD sequences, containing 8078 stars compared to 4054 ;
many of the extra stars are those measured with higher
errors in the crowded central regions of the cluster. We
count BS stars in the photometric box V\17È18,
B[V\0.25È0.40 and compare to a MS box V\18.5È
19.5, B[V\0.5È0.6, and for the inner two annuli, to a box
containing RGB stars between V\16.5 and 17.5. The BS
box is deliberately chosen to be well separated from the
FIG. 2.ÈI,V[Icolor-magnitude diagram
232 BROCATO ET AL. Vol. 527
cluster MS, so that it will not be contaminated by MS stars
with large photometric errors. The results are given in Table
1. These data should not be overinterpreted, since particu-
larly for the inner annulus the incompleteness factor is large
and uncertain. For this reason, we compare the BS counts
in the inner two annuli with samples both brighter (RGB)
and fainter (MS) than the BS sample. Corrections for Ðeld
star contamination are ascertained from counting stars at a
radius greater than 1000 pixels, excluding those that clearly
lie on the cluster MS and RGB. The Ðeld stars are sub-
tracted o† in the ““ corrected ÏÏ columns and are seen to be
very small. The results show that there are at least a factor
of 2 more BS stars than in the MS and RGB samples in the
innermost (0È200 pixel) annulus compared to the more
distant annuli. Similar selections using Piotto et al. (1999)
data provide for BS, MS, and RGB star counts of 12, 712,
and 61, respectively, with ratios BS/MS \0.017 and
BS/RGB \0.20, which can be compared to data in
Table 1. These values conÐrm our results for the bright part
of the diagram and show the large incompleteness
(crowding) a†ecting our MS counts in the inner annulus.
3.COMPARISON WITH THEORETICAL PREDICTIONS
Inspection of the CMDs reveals that in the V,B[V
plane the HB of NGC 6362 does not show a continuously
decreasing Vmagnitude when moving from the cool toward
the hot portion of the branch, and indeed the HB is bright-
est around B[V\0.2. An additional very interesting
feature, seen in both the V,B[Vand I,V[Idiagrams,
occurs at the red boundary of the HB, where a clump of
stars extends over some tenths of a magnitude in brightness
at near constant color.
Figures 3 and 4 show the observed distribution of HB
stars in the V,B[Vand I,V[ICMDs. According to data
in Figure 3, it is apparent that NGC 6362 appears as a
representative of globular clusters with a tilted HB, to be
added to NGC 1851 (Walker 1998) and perhaps NGC 6229
(Borissova et al. 1997, 1999) and the extreme cases of NGC
6388 and NGC 6441 (Piotto et al. 1997; Rich et al. 1997 ;
Layden et al. 1999). This feature, appearing in the B[V
color, is not seen in the I,V[Idiagram of NGC 6362
(Fig. 4).
In the case of NGC 6362 (and also in NGC 1851) the tilt
is of the order of mag, where is a measure-*V
tilt \ 0.1 *V
tilt
ment of the luminosity increase occurring from the red side
to the blue, i.e., the maximum luminosity distance of the HB
from a straight line drawn at the lower level of the red HB,
while in the two clusters NGC 6441 and NGC 6388 it is
mag. The matter has been recently discussed by*V
tilt D0.5
Sweigart & Catelan (1998), who suggest that such a severely
tilted HB should be regarded as evidence for the occurrence
FIG. 3.ÈObserved distribution of HB stars in the V,B[Vplane,
plotted together with the theoretical ZAHBs calculated for three di†erent
values of the mixing length. Symbols mark the location of stars with mass
0.80, 0.75, 0.70, 0.67, 0.65, 0.64, 0.63, 0.62, 0.61, and 0.60 M_.
of some unusual mechanism, such as rotation or di†erential
He enrichment of the external layers. However, one should
note that the occurrence of a tilted HB is not ignored by
available theoretical predictions concerning canonical HB
sequences, as already presented in the current literature.
For example, by looking, e.g., at Figures 8 and 9 in Castel-
lani, Chieffi, & Pulone (1991), one Ðnds that in the V,B[V
diagram, a very metal poor HB (Z\0.0001) has a lumi-
nosity that regularly slopes down when the temperature is
increasing, while by Z\0.001 the HB starts to tilt up.
Thus, metal-rich GCs are theoretically expected to have a
tilted HB.
The reason for the canonical tilt appears to be rather
obvious: whereas theoretical luminosities decrease when
the temperature of zero-age HB (ZAHB) stars is increasing,
the bolometric corrections have a minimum around Te\
7500 K. As a consequence, the bolometric correction (BC)
tends to push toward a brighter Vmagnitude around this
temperature. At lower metallicities, the resulting HB magni-
tude is dominated by the slope of the theoretical luminosity-
temperature relation, and theory predicts that the V
magnitude will continuously increase from the red to the
blue side of the branch. However, for higher metallicities the
theoretical slope decreases, and a minimum Vappears
TABLE 1
NUMBER OF STARS IN THE SELECTED EVOLUTIONARY PHASES
Annulus Radii
(pixels) BS MS RGB BScorr MScorr RGBcorr BScorr/MScorr BScorr/RGBcorr
0È200.......... 12 230 63 11.9 229 62 0.052 0.19
200È300 ....... 4 336 46 3.85 334 46 0.012 0.08
300È400....... 4 456 36 3.8 454 36 0.008 0.10
400È500 ....... 1 428 26 0.75 425 26 0.002 0.03
[1000 ........ 1 88 10 0 77 5 ... ...
NOTE.ÈSubscript ““ corrÏÏ indicates the correction for the Ðeld star contamination.
No. 1, 1999 NGC 6362 AND CANONICAL TILT OF HORIZONTAL BRANCHES 233
FIG. 4.ÈAs in Fig. 3, but in the I,V[Iplane. Symbols mark the
location of stars with mass 0.80, 0.75, 0.70, 0.67, 0.65, 0.64, 0.63, 0.62, and
0.61 M_.
within the HB. This is not the case for the Imagnitude,
since in that case the BC is much less dependent on the
stellar temperature.
To examine this problem in more detail, we used our
updated evolutionary code (see Cassisi et al. 1998) to
compute a set of evolutionary sequences, taking into
account element di†usion and for an original chemical com-
position that should be suitable for NGC 6362, namely,
Z\0.002, Y\0.23. Figure 5 shows theoretical predictions
concerning the ZAHB luminosity, as derived for three dif-
ferent assumptions about the mixing length parameter,
FIG. 5.ÈTheoretical ZAHBs as derived for the labeled values of the
mixing length. Symbols mark the location of stars with mass 0.80, 0.75,
070, 0.67, 0.65, 0.64, 0.63, 0.62, 0.61, 0.60, 0.58, 0.56, and 0.55 M_.
which governs the efficiency of the external convection that
a†ects the envelopes of the cooler stars. Figure 3 shows the
same theoretical result, but translated into the V,B[V
diagram, adopting model atmospheres from Castelli,
Gratton, & Kurucz (1997) and best Ðtted to the observation
with the labeled value of the cluster reddening and distance
modulus. In all cases, one Ðnds that theoretical predictions
appear to be in reasonable agreement with observations
and, in particular, that the theory shows a minimum V
magnitude at B[V^0.3. The assumption about the effi-
ciency of superadiabatic convection plays a relevant role in
governing the predicted location of the HB red portion.
Thus, the tilt observed in NGC 6362 must be regarded as a
natural expectation from canonical theories.
Turning now to the clump of stars at the cool end of the
red HB, we Ðrst note that Fusi Pecci et al. (1992) predict
that this location is close to where blue straggler descen-
dants should be found, which is relevant since NGC 6362
appears to be fairly rich in blue stragglers. However, the
Piotto et al. (1999) data do not show an excess of stars
clumped near the red HB, which would be expected given
the large number (D30) of BS stars, thus apparently ruling
out this possibility. In addition, this feature disappears in
the Ðeld population (i.e., for stars at radius greater than
1000 pixels); thus, it is not due to a chance superimposition
of Ðeld stars. Therefore, we suggest that either we are facing
evolutionary e†ects (see, e.g., the synthetic HB of Catelan et
al. 1998 and Brocato et al. 1999) on a clump of red ZAHB
stars with mixing length parameter a\1.0, or the ZAHB
follows the a\1.6 predictions but with a disagreement
between theory and observations of the order of 0.05 mag.
By relying again on Castelli et al. (1997) model atmo-
spheres and adopting the above reported values for the
cluster reddening and distance modulus, one can translate
the theoretical results onto the observational I,V[I
diagram. We adopted E(V[I)\1.25E(B[V) from Bessell
& Brett (1988), and thus dI\1.75E(B[V). The Ðtting, as
shown in Figure 4, appears quite satisfactory.
The theoretical scenario has Ðnally been used to derive
cluster isochrones for a suitable range of ages. According to
the discussion given in Brocato et al. (1998), the cluster
reddening and distance modulus can be Ðrmly established
by Ðtting the HB. Following the quoted procedure, we
derive and E(B[V)\0.08(m[M)V\14.68 ^0.05
^0.02, in good agreement with De Santis & Cassisi (1999).
The reddening is slightly higher than the value obtained in
°2, but the observational and Ðtted values do agree within
their estimated uncertainties. Figures 6 and 7 show that
with the already given values of DMs, E(B[V), and
E(V[I), stars at the cluster turno† and along the subgiant
branch appear very nicely Ðtted by the 12 Gyr isochrone,
with a\2.3.
The mismatch between theory and observation in the
upper portion of the RGB may be due to the adopted color
temperature relation being incorrect, as discussed by
Lejeune, Cuisinier, & Buser (1997). An alternative explana-
tion is that the efficiency of convective transport is progres-
sively decreasing with respect to the predictions of
evolution with a constant value for the mixing length, in the
sense that a lower value of the mixing length is needed in
order to Ðt the observations. To explore this possibility, we
computed evolutionary models in which we progressively
varied the aparameter, i.e., the mixing length, as stars start
to climb the RGB. We parameterize the[log (L/L_)º0.5]
234 BROCATO ET AL. Vol. 527
FIG. 6.ÈFit of the NGC 6362 data to the theoretical isochrone aged 12
Gyr.
mixing length as a linear function of the stellar luminosity:
where is the usual value and bisa\a0[blog (L/L_), a0
the decreasing rate that needs to be calibrated. The best Ðts
are obtained by adopting a decreasing rate of b\0.20, as
shown in Figures 8 and 9, where we also plot the isochrones
derived with b\0.10 and b\0.15.
4.FINAL REMARKS
In this paper we have shown that the occurrence of tilted
HBs, at least up to mag, is a natural prediction of*V
tilt \ 0.1
current evolutionary theories for old GCÏs with moderate to
large metallicities. The Ðtting between theory and obser-
vations of stars in NGC 6362 appears quite satisfactory,
predicting a cluster age of about 12 Gyr. Such an age is
similar to the ages derived adopting the same theoretical
scenario for other Galactic globular clusters with much
lower metallicity, such as M68 (Z\0.0004, t^11 Gyr;
Cassisi et al. 1999), so that no evidence can be found here to
FIG. 7.ÈFit of the NGC 6362 data to the theoretical isochrone aged 12
Gyr.
FIG. 8.ÈE†ect of di†erent decreasing rates of the aparameter. The thin
solid line corresponds to a constant value (a\2.3), as in Fig. 6; dotted line
corresponds to b\0.10, and long-dashed line to b\0.15. The best Ðt is
performed with b\0.20 (thick solid line).
FIG. 9.ÈAs in Fig. 8, but in the I,V[Iplane. The thin solid line
corresponds to a constant value (a\2.3), as in Fig. 7; the dotted line
corresponds to b\0.10, and the long dashed line to b\0.15. The best Ðt
is performed with b\0.20 (thick solid line).
support a possible correlation between age and metallicity.
Note also that similar ages have also been measured for old
globular clusters in the Large Magellanic Cloud (Brocato et
al. 1996; Olsen et al. 1998).
However, the explanation for the clumping of stars at the
red boundary of the HB remains an open question, for
which we have only suggested some possible alternative
explanations.
We thank Frank Valdes for advice on combining mea-
surements. G. R. acknowledges the Amm. Provinciale di
Teramo for a fellowship.
No. 1, 1999 NGC 6362 AND CANONICAL TILT OF HORIZONTAL BRANCHES 235
REFERENCES
Alcaino, G. 1970, ApJ, 159, 325
Alcaino, G., & Liller, W. 1986, AJ, 91, 303
Bessell, M. S., & Brett, J. M. 1988, PASP, 100, 1134
Borissova, J., Catelan, M., Ferraro, F. R., Spassova, N., Buonanno, R.,
Iannicola, G., Richtler, T., & Sweigart, A. V. 1999, A&A, 343, 813
Borissova, J., Catelan, M., Spassova, N., & Sweigart, A. V. 1997, AJ, 113,
692
Brocato, E., Castellani, V., Ferraro, F. R., Piersimoni, A. M., & Testa, V.
1996, MNRAS, 282, 614
Brocato, E., Castellani, V., Raimondo, G., & Walker, A. R. 1999, in Evolv-
ing Evolution, ed. T. Zanzu, V. Testa, & M. Bellazzini (Carloforte: Staz.
Astron. Cagliari), 69
Brocato, E., Castellani, V., Scotti, G. A., Saviane, I., Piotto, G., & Ferraro,
F. R. 1998, A&A, 335, 929
Carretta, E., & Gratton, R. G. 1997, A&AS, 121, 95
Cassisi, S., Castellani, V., DeglÏInnocenti, S., Salaris, M., & Weiss, A. 1999,
A&AS, 134, 103
Cassisi, S., Castellani, V., DeglÏInnocenti, S., & Weiss, A. 1998, A&AS, 129,
267
Castellani, V., Chieffi, A., & Pulone, L. 1991, ApJS, 76, 911
Castelli, F., Gratton, R. G., & Kurucz, R. L. 1997, A&A, 318, 841
Catelan, M., Borissova, J., Sweigart, A. V., & Spassova, N. 1998, ApJ, 494,
265
De Santis, R., & Cassisi, S. 1999, MNRAS, 308, 97
Fourcade, C. R. 1974, PASP, 91, 589
Fusi Pecci, F., Ferraro, F. R., Corsi, C. E., Cacciari, C., & Buonanno, R.
1992, AJ, 104, 1831
Harris, W. E., & Racine, R. 1979, ARA&A, 17, 241
Landolt, A. U. 1992, AJ, 104, 340
Layden, A. C., Ritter, L. A., Welch, D. L., & Webb, T. M. A. 1999, AJ, 117,
1313
Lejeune, T., Cuisinier, F., & Buser, R. 1997, A&AS, 125, 229
Olsen, K. A. G., Hodge, P. W., Mateo, M., Olszewski, E. W., Schommer,
R. A., Suntze†, N. B., & Walker, A. R. 1998, MNRAS, 300, 665
Piotto, G., et al. 1997, in Advances in Stellar Evolution, ed. R. T. Rood &
A. Renzini (Cambridge: Cambridge Univ. Press) 84
ÈÈÈ. 1999, AJ, 117, 264
Rich, R. M., et al. 1997, ApJ, 484, L25
Rutledge, G. A., Hesser, J. E., & Stetson, P. B. 1997, PASP, 109, 907
Stetson, P. B. 1987, PASP, 99, 191
ÈÈÈ. 1995, DAOPHOT II UserÏs Manual (Victoria, BC: Dominion
Astrophys. Obs., Herzberg Inst. Astrophys.)
Suntze†, N. B., Kinman, T. D., & Kraft, R. P. 1991, ApJ, 367, 528
Sweigart, A. V., & Catelan, M. 1998, ApJ, 501, L63
Walker, A. R. 1998, AJ, 116, 220
Zinn, R., & West, M. J. 1984, ApJS, 55, 45
... The basic properties of NGC 6362 appear to be relatively well determined. Most estimates of the foreground reddening fall in the range E B V 0.07 0.09   -( ) (e.g., see Schlegel et al. 1998;Brocato et al. 1999;Olech et al. 2001;Schlafly & Finkbeiner 2011; and the 2010 edition of the catalogue by Harris 1996). Similar good consistency has been found for the cluster metallicity over the years, with the majority of studies finding [Fe/H] values between −0.96 (CG97) and −1.15 (KI03), including the investigations by, e.g., Zinn & West (1984), CBG09, Mucciarelli et al. (2016), and Massari et al. ...
... However, it turns out that a single ZAHB locus cannot provide a satisfactory fit to the faintest HB stars across the entire color range that they occupy. The problem is that, as pointed out by Brocato et al. (1999), the HB of NGC 6362 has an odd HB morphology in that the faintest stars just to the blue of the instability strip are ∼0.1 mag brighter than the faintest of the red HB stars; i.e., there is a significant downward tilt of the HB in the direction from blue to red. Although Brocato et al.suggested that variations in the bolometric corrections with [Fe/H] and T eff may be responsible for this behavior, this speculation is not supported by our HB models. ...
Article
M55 (NGC 6809) and NGC 6362 are among the few globular clusters for which masses and radii have been derived to high precision for member binary stars. They also contain RR Lyrae variables, which, together with their non-variable horizontal-branch (HB) populations, provide tight constraints on the cluster reddenings and distance moduli through fits of stellar models to their pulsational and evolutionary properties. Reliable (m - M)V estimates yield M V and M bol values of comparable accuracy for binary stars, because the V-band bolometric corrections applicable to them have no more than a weak dependence on effective temperature () and [Fe/H]. Chemical abundances derived from the binary mass-M V relations are independent of determinations based on their spectra. The temperatures of the binaries, which are calculated directly from their luminosities and the measured radii, completely rule out the low scale that has been determined for metal-deficient stars in some recent spectroscopic and interferometric studies. If [α/Fe] = 0.4 and [O/Fe] = 0.5 ±0.1, we find that M55 has , [Fe/H] = -1.85 ±0.1, and an age of 12.9 ±0.8 Gyr, whereas NGC 6362 has , [Fe/H] = -0.90 ±0.1, and an age of 12.4 ±0.8 Gyr. The HB of NGC 6362 shows clear evidence for multiple stellar populations. Constraints from the RR Lyrae standard candle and from local subdwarfs (with Gaia DR2 parallaxes) are briefly discussed. © 2018. The American Astronomical Society. All rights reserved..
Article
We have derived accurate distances to Galactic globular clusters by combining data from the Gaia Early Data Release 3 (EDR3) with distances based on Hubble Space telescope (HST) data and literature based distances. We determine distances either directly from the Gaia EDR3 parallaxes, or kinematically by combining line-of-sight velocity dispersion profiles with Gaia EDR3 and HST based proper motion velocity dispersion profiles. We furthermore calculate cluster distances from fitting nearby subdwarfs, whose absolute luminosities we determine from their Gaia EDR3 parallaxes, to globular cluster main-sequences. We finally use HST based stellar number counts to determine distances. We find good agreement in the average distances derived from the different methods down to a level of about 2 per cent. Combining all available data, we are able to derive distances to 162 Galactic globular clusters, with the distances to about 20 nearby globular clusters determined with an accuracy of 1 per cent or better. We finally discuss the implications of our distances for the value of the local Hubble constant.
Preprint
Full-text available
The present work explores the origin of the formation of star clusters in our Galaxy and in Small Magellanic Cloud (SMC) through simulated H-R diagrams and compare those with observed star clusters. The simulation study produces synthetic H-R diagrams by Markov Chain Monte Carlo (MCMC) technique using star formation history (SFH), luminosity function (LF), abundance of heavy metal (Z) and a big library of isochrones as basic inputs and compares them with observed H-R diagrams of various star clusters. The distance based comparison between those two diagrams is carried out through two dimensional matching of points in Color-Magnitude Diagram (CMD) after optimal choice of bin size and appropriate distance function. It is found that a poor medium of heavy elements (Z = 0.0004), Gaia LF along with mixture of multiple Gaussian distributions of SFH may be the origin of formation of globular clusters (GCs). On the contrary, enriched medium (Z = 0.019) is favoured with Gaia LF along with double power law (i.e. unimodal) SFH. For SMC clusters, the choice of exponential LF and exponential SFH is a proper combination for poor medium whereas Gaia LF with Beta type SFH is preferred in an enriched medium for the formation of star clusters.
Article
Aims. The unusual transient ATLAS17aeu was serendipitously detected within the sky localisation of the gravitational wave trigger GW 170104. The importance of a possible association with gravitational waves coming from a binary black hole merger led to an extensive follow-up campaign, with the aim of assessing a possible connection with GW 170104. Methods. With several telescopes, we carried out both photometric and spectroscopic observations of ATLAS17aeu, for several epochs, between ∼3 and ∼230 days after the first detection. Results. We studied in detail the temporal and spectroscopic properties of ATLAS17aeu and its host galaxy. Although at low significance and not conclusive, we found similarities to the spectral features of a broad-line supernova superposed onto an otherwise typical long-GRB afterglow. Based on analysis of the optical light curve, spectrum, and host galaxy spectral energy distribution, we conclude that the redshift of the source is probably z ≃ 0.5 ± 0.2. Conclusions. While the redshift range we have determined is marginally compatible with that of the gravitational wave event, the presence of a supernova component and the consistency of this transient with the Ep – Eiso correlation support the conclusion that ATLAS17aeu was associated with the long gamma-ray burst GRB 170105A. This rules out the association of the GRB 170105A/ATLAS17aeu transient with the gravitational wave event GW 170104, which was due to a binary black hole merger.
Article
Full-text available
New time-series VI CCD photometry of the globular cluster NGC 6362 is studied with the aim of estimating the reddening, mean metallicity and distance of the cluster from its population of RR Lyrae stars. The Fourier decomposition of carefully selected single-mode RR Lyrae light curves, and the use of well-established semi-empirical calibrations and revised zero points, lead to the values of [Fe/H]UVES =1:066 +// 0:126 and -1:08 +/- 0:16 and the distance 7:93 +/- 0:32 and 8:02 +/- 0:15 kpc from the RRab and RRc stars respectively. The distribution of RR Lyrae stars in the horizontal branch shows a neat segregation of pulsating modes about the red edge of the first overtone instability strip, which is not necessarily expected in an OoI type cluster like NGC 6362. Four RRab stars are found likely advanced in their evolution towards the AGB. One new foreground SX Phe star, some 4 kpc in front of the cluster and projected onto the field of our images is reported. We comment on the heavy light contamination, by a very close neighbouring star, of the peculiar double-mode V37 variable, recently postulated as a non-typical RRc variable.
Article
Full-text available
We present an extenstive literature compilation of age, metallicity, and chemical abundance pattern information for the 41 Galactic globular clusters (GGCs) studied by Schiavon et al. (2005). Our compilation constitutes a notable improvement over previous similar work, particularly in terms of chemical abundances. Its primary purpose is to enable detailed evaluations of and refinements to stellar population synthesis models designed to recover the above information for unresolved stellar systems based on their integrated spectra. However, since the Schiavon sample spans a wide range of the known GGC parameter space, our compilation may also benefit investigations related to a variety of astrophysical endeavours, such as the early formation of the Milky Way, the chemical evolution of GGCs, and stellar evolution and nucleosynthesis. For instance, we confirm with our compiled data that the GGC system has a bimodal metallicity distribution and is uniformly enhanced in the alpha-elements. When paired with the ages of our clusters, we find evidence that supports a scenario whereby the Milky Way obtained its globular clusters through two channels, in situ formation and accretion of satellite galaxies. The distributions of C, N, O, and Na abundances and the dispersions thereof per cluster corroborate the known fact that all GGCs studied so far with respect to multiple stellar populations have been found to harbour them. Finally, using data on individual stars, we also confirm that the atmospheres of stars become progressively polluted by CN(O)-processed material after they leave the main sequence and uncover evidence which suggests the alpha-elements Mg and Ca may originate from more than one nucleosynthetic production site. [abridged]
Article
Full-text available
We use the pulsational properties of the RR Lyrae variables in the globular cluster NGC 1851 to obtain detailed constraints of the various sub-stellar populations present along its horizontal branch. On the basis of detailed synthetic horizontal branch modeling, we find that minor helium variations (Y~0.248-0.280) are able to reproduce the observed periods and amplitudes of the RR Lyrae variables, as well as the frequency of fundamental and first-overtone RR Lyrae stars. Comparison of number ratios amongst the blue and red horizontal branch components and the two observed subgiant branches also suggest that the RR Lyrae variables originated from the progeny of the bright subgiant branch. The RR Lyrae variables with a slightly enhanced helium (Y~0.270-0.280) have longer periods at a given amplitude, as is seen with Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with Y~0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI) variables. This correlation does suggest that the pulsational properties of RR Lyrae stars can be very useful for tracing the various sub-populations and can provide suitable constraints on the multiple population phenomenon. It appears of great interest to explore whether this conclusion can be generalized to other globular clusters hosting multiple populations.
Article
Surface brightness fluctuations (SBFs) have been proved to be a very powerful technique to determine the distance and characterize the stellar content in extragalactic systems. Nevertheless, before facing the problem of stellar content in distant galaxies, we need to calibrate the method onto nearby well known systems. In this paper we analyze the properties at the J and Ks bands of a sample of 19 star clusters in the Large Magellanic Cloud, for which accurate near-infrared (NIR) resolved star photometry and integrated photometry are available. For the same sample, we derive the SBF measurements in the J and Ks bands. We use the multipurpose stellar population code SPoT (Stellar POpulations Tools) to simulate the color-magnitude diagram, stellar counts, integrated magnitudes, colors, and SBFs of each cluster. The present procedure allows us to estimate the age and metallicity of the clusters in a consistent way, and provides a new calibration of the empirical s-parameter. We take advantage of the high sensitivity of NIR SBFs to thermally pulsing asymptotic (TP-AGB) stars to test different mass-loss rates affecting the evolution of such stars. We argue that NIR-SBFs can contribute to the disentangling of the observable properties of TP-AGB stars, especially in galaxies, where a large number of these stars are present.
Article
Full-text available
We investigate the stellar content of the starburst dwarf galaxy IC10 using accurate and deep optical data collected with the Advanced Camera for Surveys and with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. The comparison between theory and observations indicates a clear change in age distribution when moving from the center toward the external regions. Moreover, empirical calibrators and evolutionary predictions suggest the presence of a spread in heavy element abundance of the order of one-half dex. The comparison between old and intermediate-age core He-burning models with a well defined overdensity in the color-magnitude diagram indicates the presence of both intermediate-age, red clump stars and of old, red horizontal branch stars.
Article
Full-text available
BV CCD photometry of the central (1.5arcmin x 2.0arcmin ) part of the mildly concentrated outer-halo globular cluster NGC 6229 is presented. The data reduction in such a crowded field was based on a wavelet transform analysis. Our larger dataset extends the previous results by Carney et al. (1991, AJ, 101, 1699) for the outer and less crowded fields of the cluster, and confirms that NGC 6229 has a peculiar color-magnitude diagram for its position in the Galaxy. In particular, NGC 6229's horizontal branch (HB) presents several interesting features, among which stand out: a well populated and very extended blue tail; a rather blue overall morphology, with (B-R)/(B+V+R) = 0.24+/- 0.02; a bimodal color distribution, resembling those found for NGC 1851 and NGC 2808; and gaps on the blue HB. NGC 6229 is the first bimodal-HB cluster to be identified in the Galactic outer halo. A low value of the R parameter is confirmed, suggestive of a low helium abundance or of the presence of a quite substantial population of extreme HB stars fainter than our photometric limit ( =~ 2.5 mag below the RR Lyrae level in V). Twelve new possible variable stars were found in the central part of the cluster. The morphology of the red giant branch (RGB) also seems to be peculiar. In particular, the RGB luminosity function ``bump" is not a prominent feature and has only been tentatively identified, on the basis of a comparison with a previously reported detection for M3 (NGC 5272). Finally, we compare the properties of NGC 6229 with those for other outer-halo globular clusters, and call attention to what appears to be a bimodal HB distribution for the outer-halo cluster population, where objects with very red or very blue HB types are much more frequently found than clusters with intermediate HB types.
Article
Full-text available
A grid of theoretical evolutionary models covering the horizontal-branch (HB) and the asymptotic-giant-branch (AGB) phases of globular cluster stars is presented. The computations were performed for a fixed amount of the original He and for metallicity values. For each value of the assumed metallicity, the evolutionary structure at the He flash was used to construct an initial set of He-burning models with the same mass of the central He core but with decreasing masses of the H-rich envelopes. Results are presented of the evolution of these models through the exhaustion of central He up to the minimum in luminosity marking the reignition of the H shell. The luminosity of the AGB clump and the number ratio of AGB to HB stars was found to remain fairly constant with the cluster metallicity. The parameter R is recalibrated, and it is suggested that there is a small increase in the adopted value of the original He in Galactic globulars and a correlation of helium and metals in 47 Tuc.
Article
Full-text available
We present new, deep (V, V−I) colour-magnitude diagrams (CMDs) for the three old, metal-poor Magellanic Cloud (MC) clusters: NGC 1786, 1841 and 2210. The photometry reaches V ≃ 24 mag, yielding the first insight into the turn-off region and the upper main sequence of these clusters. On the basis of these new data, we found that the three clusters have quite similar CMDs, which closely resemble that of the metal-poor Galactic globular M68. Our main conclusion is that the most metal-poor clusters in the MC have not only similar metallicity to, but are also of age comparable with the metal-poor globulars in the Galaxy. Considering the observational uncertainties we estimate the relative age difference uncertainty is ~ 20 per cent (i.e. 3 Gyr). This evidence suggests an evolutionary scenario in which the production of globular clusters started in a similar way in the two galaxies, whilst more metal-rich clusters show evidence of a different evolution, as proved by the occurrence in the Large Magellanic Cloud (LMC) of clusters which are actually intermediate between the two galactic Oosterhoff types.
Article
Full-text available
Available data on blue stragglers and their parent clusters is reviewed, and it is argued that some of the very red Horizontal Branch (HB) stars always present in the CMDs of Galactic globular clusters containing blue stragglers are in fact blue straggler progeny. Hence, the red HB extreme could be controlled by environment-induced effects via the complex phenomena related to blue straggler formation and dynamical and intrinsic evolution. A grouping of clusters is made on the basis of their detected blue straggler population and HB morphology.
Article
We compare our compilation of the Wcalcium index for 71 Galactic globular clusters to the widely used Zinn and West (1984 ApJS, 55, 45) (Fe/H) scale and to Carretta and Gratton's (1997 A&AS, 121, 95) scale from high-dispersion spectra analyzed with Kurucz (1992, private communication) model atmospheres. We find our calcium ranking to be tightly correlated with each comparison set, in a non-linear and a linear fashion, respectively. By combining our calcium index information with the Zinn and West ranking, we are able to rank the globular clusters in our sample with a typical precision of ±0.05 dex for (Fe/H)ZW84 � < 0.5; for clusters more metal rich than this, the ranking is less precise. The significant differences between these metallicity scales raise important questions about our understanding of Galactic formation and chemical enrichment processes. Furthermore, in spite of the apparent improvement in metallicity ranking for the Galactic globular clusters that results from our addition of information from the Ca II triplet lines to the potpourri of other metallicity indicators, caution - perhaps considerable - may be advisable when using Was a surrogate for
Article
Spectrograms (121 A mm `)have been obtained of 60 galactic globular clusters and of three globular clusters in the Small Magellanic Cloud. The radial velocities measured from these spectrograms by the cross-correlation technique have an accuracy of +20 km and are in good agreement with most previous measurements. They support Shawl, Hesser, and Meyer's conclusion that some of Mayall's measurements for southern clusters are substantially in error. The pseudoequivalent widths of the hydrogen lines H , Hy, and H and the Ca ii K line (K), the Ci band (Ci) and the Mg I b lines (Mg) have been measured. Most galactic globular clusters have very similar H line strengths, but the lines of four clusters (NOC 6535, 6642, 6717, and 7099 [M30]) are unusually strong. This peculiarity of NCiC 7099 had been noted earlier by Rabin, who speculated that it is related to the color gradient in the cluster. Our spectroscopic observations of two regions offset from the center of NCiC 7099 confirm Rabin's suggestion. It is not known whether the other strong-lined clusters possess similar color and H line gradients or are more homogeneous. The K, Ci, and Mg pseudo-equivalent widths correlate very well with each other and' with the photometric integrated light index Q39. The relationships between Q39 and these widths have been used to transform them into equivalent values of Q39. For the 14 clusters that had not been included in Paper I, values of E( B - V) have been estimated from their apparent B - V colors and the relationship between Q39 and (B-V)0. The metallicity ranking provided by Q39 has been compared with ones in the literature that are based on observations of individual cluster stars (i.e., the analyses of high-dispersion echelle spectrograms of red giants, measures of giant-branch effective temperature, S measurements of RR Lyrae variables, and spectrophotometry or photometry of red giants). These comparisons, which involve 50 clusters, indicate that Q39 is able to rank clusters with a precision of +0.15 in [Fe/H], which is close to that attained by the best of these other methods. It appears that Q39 is not affected much by the variation in horizontal-branch morphology that exists among clusters of similar metallicity. For 121 galactic globular clusters, the metallicities derived from observations of individual stars, the integrated light, or both, have been placed on Cohen's new metallicity scale. The values obtained for the most metal-rich and metal-poor clusters by extrapolating the scale are [Fe/H] = +0.24 and -2.58, respectively. Subject headings: clusters: globular - galaxies: Magellanic Clouds - galaxies: Milky Way - radial velocities - stars: abundances
Article
UBVRI photoelectric observations have been made on the Johnson-Kron-Cousins photometric system of 526 stars centered on the celestial equator. The program stars within a 298 number subset have sufficient measures so that they are capable of providing, for telescopes of intermediate and large size in both hemispheres, an internally consistent homogeneous broadband standard photometric system around the sky. The stars average 29 measures each on 19 nights. The majority of the stars in this paper fall in the magnitude range 11.5-16.0, and in the color range -0.3 to +2.3.
Article
The tasks of the DAOPHOT program, developed to exploit the capability of photometrically linear image detectors to perform stellar photometry in crowded fields, are discussed. Raw CCD images are prepared prior to analysis, and following the obtaining of an initial star list with the FIND program, synthetic aperture photometry is performed on the detected objects with the PHOT routine. A local sky brightness and a magnitude are computed for each star in each of the specified stellar apertures, and for crowded fields, the empirical point-spread function must then be obtained for each data frame. The GROUP routine divides the star list for a given frame into optimum subgroups, and then the NSTAR routine is used to obtain photometry for all the stars in the frame by means of least- squares profile fits. The process is illustrated with images of stars in a crowded field, and shortcomings and possible improvements of the program are considered.
Article
Metal abundances are derived for 60 newly discovered RR Lyraes in three Lick Astrographic Survey fields at three Galactic altitudes (l,b): (86 deg, +35 deg), (52 deg, +35 deg), and (18 deg, +25 deg). In addition, Delta S values were obtained for five nearby field RR Lyraes which have no previous records, and improved Delta S values were obtained for 24 nearby RR Lyraes. These values, in conjunction with data from previous Lick surveys and those obtained by Saha and Oke (1985), were used to study the nature of the Galactic halo in the galactocentric distance range 4-30 kpc, based on 171 Lyraes. It was found that the Galactic halo undergoes a change in the behavior of the Fe/H ratio at the position of the solar circle.