\documentstyle[epsf,psfig,paasms4,pptwocol,pub]{preprint}
\def\et{{{et~al.\ }}}
\begin{document}

\title{HST OBSERVATIONS OF NGC 121: FIRST DETECTION OF BLUE\\
~\\
STRAGGLERS 
IN AN EXTRAGALACTIC GLOBULAR CLUSTER\/\thanks{Based on observations made 
with the NASA/ESA {\it Hubble Space Telescope\/}, obtained at the Space 
Telescope Science Institute.  ST~ScI is operated by the Association of 
Universities for Research in Astronomy, Inc.\ under NASA contract NAS5-26555}}

\author{Michael M.\ Shara and S.\ Michael Fall\\
\\
Space Telescope Science Institute\\
3700 San Martin Drive\\
Baltimore, MD 21218\\
\and
R.\ Michael Rich\\
\\
Department of Astronomy\\
Columbia University\\
New York, NY, 10027\\
\and
David Zurek\\
\\
Space Telescope Science Institute}
\tobe{1 November 1998}{The Astrophysical Journal}
\recacc{18 December 1997}{8 June 1998}
\url{1266}
\maketitle

\abstract{
We have resolved stars in the core of the old SMC globular cluster NGC~121
with images from the refurbished {\it Hubble Space Telescope\/} and WFPC2. 
Our photometry reaches 1.5~mag below the main sequence turnoff, with 
negligible field contamination. A blue straggler star (BSS) sequence of 
42 candidates is clearly seen, the first such detection in an extragalactic 
globular cluster. The BSS are more centrally concentrated than the subgiant, 
red giant, and horizontal branch stars. Various blending and completeness 
tests demonstrate that at least 23 of our candidates are genuine BSS.
The BSS extend about 1.8 magnitudes brighter than the main sequence turnoff, 
in accord with similar sequences in Galactic Globular clusters. One BSS
candidate is 2.4 magnitudes brighter than the turnoff, as luminous as the 
brightest BSS seen in Galactic globulars. The cluster red horizontal branch is 
clearly detected and well populated, with a modest blueward extension.
The relaxation times of the cluster stars and the BSS are such that most
BSS are in energy equipartition with the other stars. We find 
$\Delta V^{HB}_{TO}=3.3\pm 0.1$~mag, corresponding to a cluster age 
that is 2~Gyr younger than that of most Galactic globular clusters,
and of one of the oldest LMC globulars.}
\base

\section{INTRODUCTION}

A significant population of apparently young (1--5~Gyr) blue stars exists in 
the core of every old ($>10$~Gyr) Galactic globular cluster imaged by the 
Hubble Space Telescope (e.g., Paresce \et 1991; Sarajedini, 1994). These Blue 
Straggler Stars (BSS) are so named because they lie bluewards of the main 
sequence turnoff point, and because their evolution appears (at face value) to 
lag far behind that of cluster stars of similar mass. BSS may be the merger 
products of stellar collisions between unbound stars (Leonard and Livio 1995), 
or the result of close binary coalescence (Rasio and Shapiro, 1995), or both. 
The evolution and ultimate fate of BSS are as uncertain today as when they 
were discovered (Sandage 1953). 

NGC~121 is most comparable among the SMC clusters to Galactic globular 
clusters; it is the most luminous $(M_v=-7.7$; van den Bergh 1991) and oldest 
(7--14~Gyr; Stryker, Da~Costa, \& Mould  1985) cluster in the Small Magellanic 
Cloud (SMC). The red horizontal branch (RHB) has been long noted, as have
four RR~Lyrae variables (Thackeray, 1951; Graham, 1975; Walker \& Mack 1988). 
Given its spectrographically determined metallicity, [Fe/H]~$=-1.4\pm 0.1$ 
(Suntzeff \et 1986), it has characteristics of younger globular clusters (RHB, 
a marginal carbon star), while at the same time is the youngest known globular 
cluster to have RR~Lyraes (Stryker \et 1985). Careful subsequent searches of 
populous clusters such as 10~Gyr old Lindsay~1 (Olszewski, Schommer \& Aaronson 
1987) continue to support this finding. As emphasized by Stryker \et (1985), 
NGC~121 very likely marks the boundary between ``old'' population~II 
(containing RR~Lyrae but not carbon stars) and intermediate-age populations 
(containing carbon stars, but not RR~Lyraes). It is reasonable to expect that 
NGC~121 contains BSS, but by no means certain that it does. 

Ground-based resolution is not sufficient to probe the cores of extragalactic 
globular clusters, and until the refurbishment of the Hubble Space Telescope, 
we lacked the tools to determine their stellar content. In this {\it Letter\/} 
we report the first detection of a blue straggler population in a globular 
cluster outside of the Milky Way, NGC~121 in the Small Magellanic Cloud. (An 
earlier search for Blue Stragglers in the old LMC globular, Hodge~11, (Mighell 
\et 1996) was inconclusive due to the extremely high surface density of stars 
in the core of this cluster). We also offer evidence that NGC~121 is younger 
than most Galactic globular clusters, confirming the result of Stryker \et 
(1985). In \S 2, we describe our observations and in \S 3 we present the 
color-magnitude diagram, the blue straggler population, luminosities and 
inferred BSS ages and masses. The cluster structure and dynamics, and comparison 
with the distribution of BSS stars are given in \S 4, while a detailed series 
of blending and completeness checks are described in \S 5. The age determination 
of the cluster is discussed in \S 6, and our results are summarized in \S 7.

\section{ OBSERVATIONS}

NGC~121 was imaged on 26~January 1994 using WFPC2; exposures of 300s with the 
F555W~(V) filter and 600s with the F450W (wide~B) filter were obtained.  The 
cluster core was placed on the PC chip (Figure~1). Since the data were obtained 
prior to the May 1994 cooldown of WFPC2, there are numerous persistent hot 
pixels (Holtzman \et 1995a) that occur in both exposures. We used CCDCAP 
(Mighell \& Rich 1995, Mighell \et 1996) to do the photometry described below. 
Blue straggler completeness tests were carried out with DAOPHOT/ALLFRAME 
(Stetson, 1987; Stetson, 1994)). Instrumental magnitudes in F450W and F555W 
were transformed to Johnson B, V as prescribed in Holtzman \et 1995b. 
\begin{figure*}
\centerline{\epsfxsize=5in\epsfbox{pp1266f1.ps}}
\caption{\label{figi}
Planetary Camera image of NGC~121, obtained through the filter F555W~(V).  
North and East as indicated in the image.}
\end{figure*}

\section{THE COLOR-MAGNITUDE DIAGRAM}

Figure~2 shows our color-magnitude diagram for the core of NGC~121, obtained 
from the Planetary Camera images.  The main sequence turnoff has been reached, 
and both the red giant branch (RGB) and red horizontal branch (RHB) are 
well defined. While the RHB is densely populated (285~stars) and well 
separated from the RGB, a handful of bluer HB~stars are present.  
This is in accord with the presence of RR~Lyrae variables in NGC~121.  
\begin{figure*}
\centerline{\epsfxsize=5in\epsfbox{pp1266f2.eps}}
\caption{\label{figii}
Color-magnitude diagram of NGC~121, based on Planetary Camera data only.
The red horizontal branch and turnoff populations are evident.
Notice that the blue straggler sequence is clearly defined up to 
1.8~mag brighter than the turnoff. VandenBerg isochrones are at ages of
(from top) 8, 10, 12, 14, 16 and 18 Gyr, with [Fe/H]~$= -1.40$. The Yale 
isochrones are at ages (from the top) of 1, 2, 3, 4 and 5~Gyr, 
with [Fe/H]~$= -1.30$. }
\end{figure*} 

\subsection{Blue Stragglers} 

Forty two blue straggler candidates are highlighted in Figure~2; they are the 
first BSS ever detected in an extragalactic globular cluster. They lie above 
the main sequence turnoff, and are well separated from the turnoff population. 
The positions (in the F450W Planetary Camera image), V~magnitude, color and 
formal uncertainties in color and magnitude of these 42 stars are listed in 
Table~1. (A detailed discussion of the reality and completeness of the sequence 
is deferred to \S 5).The photometry of all cluster stars may be obtained from 
MMS in electronic form. 
   
Most Galactic globular clusters contain one or more BSS with $M_v \approx2.0$, 
which corresponds to the onset of the bulk of our detected BSS at 
V~$\approx21.2$.
The distribution in luminosity of the brighter BSS in NGC~121 is in accord with 
that of Galactic globular BSS. A single candidate BSS is seen at V~$=20.6$, 
brighter by 0.6~magnitudes than all other BSS. The most luminous BSS in 
Galactic globular clusters have $M_v=1.5$, although such objects are rare
(Sarajedini 1993; Fusi Pecci, Ferraro and Cacciari 1993). Our most luminous 
BSS (at V~$=20.6$) matches this brightness almost exactly. There is a small 
probability (discussed in \S 5) that this star is actually a blend of two blue 
stragglers. 

The extent in luminosity of the NGC~121 BSS sequence is of physical interest. 
BSS sequences longer than about 2.5~magnitudes can only be produced when the 
brightest BSS has more than twice the turnoff mass. This, in turn, rules out 
binary coalescence (at least for the brightest BSS) or simple single-star 
collisions as the source of the BSS; a three star interaction must be invoked.
An accurate turnoff magnitude is required to determine the sequence length; 
this is done with isochrone fitting (described below). The best estimate MSTO 
brightness is V~$=23.0$.  The BSS sequence is then  23.0--21.2~= 1.8~magnitudes 
long (excluding the brightest BSS), in excellent accord with the Galactic 
globular cluster sequences, and consistent with either a binary coalescence or 
collisional stellar merger scenario.

The first direct measurement of the mass of a blue straggler in the core of a 
globular cluster (BSS--19 in 47~Tuc) yields a value of $1.7\pm 0.4$~M$_{\odot}$ 
(Shara, Saffer and Livio 1997). This is in excellent agreement with the mass 
deduced from the star's luminosity (by assuming that BSS--19 is a zero age main 
sequence star). It supports the idea that the mass (and possibly age) of a BSS 
can be measured from the star's position in its cluster CMD. We have fitted 
isochrones from the Yale group (Demarque \et 1996) with ages of 1, 2, 3, 4 and
5~Gyr to the CMD in Figure~1. These isochrones clearly demonstrate that no 
single age can be assigned to all the Blue Stragglers; the youngest are 
reasonably fit by the 2~Gyr isochrone, while the oldest are about 5~Gyr old. 
These ages correspond to a mass range of about 1.4--2~M$_{\odot}$, consistent 
with their central concentration in the cluster. 

\section{NGC 121 STRUCTURE AND DYNAMICS}

Our images allow us to determine the structural parameters of NGC~121, which 
are needed to interpret our stellar population results. Azimuthally averaging 
the surface brightness profiles of Figure~1, we find that the half-maximum 
surface brightness occurs at $r=10.4\pm 0.5$~arcsec (in the F450W image) and at 
$r=11.4\pm 0.5$~arcsec in the F555W image. We adopt the mean of these, 
$r_c = 10.9\pm 0.5$~arcsec, as the core radius of NGC~121. This corresponds to a 
physical radius of $r_c = 3.2$~pc at the SMC modulus of 18.93 (van den Bergh, 
1991, and Sonneborn \et 1997). The tidal radius of NGC~121 (determined from the 
ST~ScI digital sky survey images of NGC~121, and taken here as the geometric 
mean of the limiting radii along the major and minor axes of the cluster) is 
$r_t = 143$~arcsec or 42~pc. The median radius (three dimensional) is then 
$r_h \approx 28$~arcsec or 8.2~pc. 

We have plotted the cumulative radial distribution (shown in Figure~3) for BSS, 
as well as for main sequence turn-off (MSTO), subgiant, horizontal branch (HB), 
and asymptotic giant branch (AGB) stars to demonstrate that the BSS are true 
cluster members, and to check for population gradients. The data strongly 
suggest that the core population is enhanced in blue stragglers, and this is 
borne out by the K-S test. The BS stars in the center of NGC~121 are more 
centrally concentrated than the MSTO, HB, subgiant and RGB stars at the 99.5, 
97.5, 95 and 94 percentile confidence levels, respectively. These results are 
very similar to those determined in Galactic globular clusters with BSS 
populations. We find that the BSS core radius is 7.2~arcsec, in contrast with 
the core radius of 10.9~arcsec for other classes of stars. Just as remarkable 
is that all the BSS are concentrated within 18~arcsec of the cluster center, 
while other cluster members extend 8 times further out.
\begin{figure*}
\centerline{\psfig{figure=pp1266f3.eps,width=5in,angle=-90}}
\caption{\label{figiii}
Normalized cumulative radial distributions of AGB, HB, RGB, subgiant,
MSTO and Blue Straggler stars in the core of NGC~121. The apparent strong 
central concentration of the BSS is confirmed by the K-S test (see text).}
\end{figure*}

It is instructive to estimate the relaxation time at several points within the 
cluster. This is given by
\begin{equation}
t_r = 
{{0.065v_m^{3}}\over
{\bar{m}\rho G^2 \ln\Lambda}}~~,
\end{equation}
where $v_m$ is the velocity dispersion in three dimensions, $\bar{m}$ is the 
mean stellar mass, $\rho$ is the mass density, $G$ is the gravitational 
constant,
and $\ln\Lambda$ is the Coulomb logarithm (see eq.~2--62 of Spitzer 1987). Near
the center of the cluster, the velocity distribution should be close to 
isotropic; thus, we expect $v_{m}(0) \approx \sqrt {3} v_{los}(0)$, where 
$v_{los}$ is the velocity dispersion along the line of sight. A recent 
measurement gives $v_{los}(0) = 5.1^{+1.7}_{-2.4}$~km~s$^{-1}$ (Dubath, Meylan 
and Mayor, 1997). The central density of the cluster is related to the velocity 
dispersion and core radius by $\rho(0)=3v_{m}(0)^{2}/4\pi Gr_c^2$. Adopting 
$\bar{m} = 0.5$~M$_{\odot}$, and $\ln\Lambda = 12$, we then estimate the central 
relaxation time as $t_{r}(0)\approx 0.9$~Gyr. Neglecting the weak radial variations of 
$v_m$ and $\bar{m}$, the relaxation time at a distance $r$ from the center of the cluster can 
be estimated from $t_r(r)\approx t_r(0)\rho(0)/\rho(r)$. All of the BSS 
candidates lie within $r = 1.7 r_c$. For a King (1962) model with $\log(r_t/r_c) 
= 1.12$, we have $\rho(1.7 r_c)\approx0.12\rho(0)$ and hence $t_r(1.7 
r_c)\approx8$~Gyr.

We show in \S 6 that the age of the cluster is about 10~Gyr. As discussed above, 
from the CMD, the ages of the BSS are 2--5~Gyr. Most of the BSS lie in the core 
of the cluster, where the condition $t_r\aplt$age is satisfied. Thus we expect 
them to be at or near energy equipartition with the other stars in the cluster. 
This probably explains why the BSS are more centrally concentrated than the 
other stars. Their greater masses would cause them to have a smaller velocity 
dispersion and hence to sink deeper into the potential well of the cluster.

\section{COMPLETENESS AND CROWDING SIMULATIONS}

Could blending of two or more faint turnoff stars produce many (or all) of the 
candidate BSS? To test for this possibility, we have constructed artificial
images of NGC~121 in the following way. Each of the 6892 stars detected on the 
Planetary Camera chip was removed by subtracting a PSF of that star at the 
appropriate position and magnitude. All the detected stars except the 42~BSS 
were then added back to the blank image, after being shifted randomly in each 
of X and Y by 0 to 20~pixels. This shift is small in comparison with the size 
of the PC field, but comparable to or larger than the nearest neighbour 
separation overb the field. This produced an artificial image with the same 
noise characteristics and stellar distribution as the original image. 
Analysis of these artificial images was carried out in the same way as that
of the real images. In ten trials, 
this technique generated CMDs with an average of $6.2\pm 1.2$ spurious BSS per 
trial. We conclude that 15\% of our BSS candidates may be blends, but that the 
large majority are bona fide BSS.

Could some real BSS be missed in the crowded core of the cluster? To test for 
completeness, we have used DAOPHOT (Stetson 1994) to insert artificial BSS up 
to four magnitudes brighter than the MS turnoff throughout the F450W and F555W  
Planetary Camera frames. We find that our 
detection efficiency is 75\% in the innermost 5~arcseconds of the cluster 
core, and 80--85\% further out, regardless of brightness. Our incompleteness
is entirely due to confusion when an artificial BSS falls too close to a bright 
star to be recovered.  DAOPHOT finds no additional BSS brighter than V~$=22.2$ 
possibly missed by CCDCAP. We conclude that we are unlikely to have missed a 
significant population of bright BSS in the inner core of NGC~121.

Warm pixels on the PC CCD and/or SMC field stars could also produce spurious 
blue straggler candidates.  The strong central concentration of BSS in Figure~2 
argues against either of these possibilities being the dominant source of 
candidates. The images of our 42~candidates have been carefully examined to 
look for unusually bright pixels; four candidates are flagged in Table~1 as 
suspicious. To quantify these effects further, we have looked for BSS in the 
quarter of the WF3 CCD chip furthest from the cluster center. (This 
$400\times400$ pixel zone covers the same area on the sky imaged by the PC chip, 
and is centered 5 core radii from the cluster center. Only 281 stars are 
detected in this area, which has a mean star density 25 times lower than that 
of the PC chip). Nine stars in the WF image are found to lie in the same part 
of the color-magnitude diagram as the BSS. By (conservatively) estimating that 
nine of our 42 BSS candidates are due to field stars, four are warm pixels, and 
that six candidates are really blends, we conclude that we have located about 
23~genuine BSS in the core of NGC~121. In comparison, we note that in the larger 
(but more remote) field of Stryker \et (1985) there are only two stars brighter 
than the turnoff which fall in the BSS region of the CMD.
 
We cannot unambiguously distinguish BSS from turnoff stars within (roughly) one 
magnitude of the turnoff. Luminosity functions of BSS in Galactic globular 
clusters indicate that 40--50\% of all BSS lie within 1~magnitude of the MS 
turnoff (Sarajedini 1993; Fusi Pecci, Ferraro and Cacciari 1993). There are 
thus likely to be about 20 more relatively faint ($M_v = +3$ to +4) BSS in the 
core of NGC~121. Higher signal-to-noise ratio images, and/or UV images will be 
required to find them, and to confirm each of our candidates.

\section{ISOCHRONES, THE HORIZONTAL BRANCH, AND THE AGE OF\break NGC 121}

We do not know the exact distance to the SMC, and whether NGC~121 lies on the 
near or far side of this galaxy. To avoid the issue of the distance to the 
cluster, we derive its age using the difference in magnitudes between the 
horizontal branch and turnoff point.  Defining the turnoff point as the 
V~magnitude of the bluest point on the best-fit isochrone, we find $\Delta 
V^{HB}_{TO} =3.3 \pm 0.1$~mag, in good agreement with the value of 3.2~mag 
measured by Stryker \et (1985). Comparison with the adjacent Wide Field Camera 
CCD chips demonstrates that our cluster core sample has very little field 
contamination. (As noted above, the average stellar densities on the PC and
WF3 chips differ by a factor of 25.) Most significantly, our HST data clearly 
shows that $\Delta V^{HB}_{TO}$ for NGC~121 is 0.25~mag less than the mean 
value of 3.55 for the old Galactic halo globular clusters (Buonnano, Corsi, \& 
Fusi Pecci 1989; Renzini, 1991). Only a handful of halo clusters (e.g., Pal~5 
and Rup~106) exhibit such small values of $\Delta V^{HB}_{TO}$.
%\newpage

In order to convert $\Delta V^{HB}_{TO}$ to a relative age difference, one must 
rely on various theoretical calibrations.  Using Figure~3 of Ortolani \et 
(1995), 
we find that even for two extreme calibrations of HB luminosity as a function of 
[Fe/H], the formal age of NGC~121 is 3~Gyr younger than the old Galactic 
globular clusters.  Using Walker's (1992) relationship $\log t_{Gyr} = 
-0.045{\rm [Fe/H]} +0.37\Delta V^{HB}_{TO} -0.24$,  Mighell \& Rich's (1996) 
version of the above formula,  and the Chaboyer \et (1992) calibration yields 
essentially the same result: NGC~121 is 2~Gyr younger than most Galactic 
globular clusters, and the old LMC cluster Hodge~11 (Mighell \et 1996). 

An estimate of the actual age of NGC~121 (as opposed to age differences)
requires knowledge of the distance, reddening, and stellar evolution parameters, 
with all the uncertainties that this implies. The most recent globular cluster 
isochrones available with a wide range of [Fe/H] have been kindly supplied by 
D.~VandenBerg. An SMC apparent distance modulus of 18.93 (Sonneborn, 1997; van 
den Bergh 1992) and reddening $E_{B-V}$ of 0.04 (Djorgovski 1993) have been used 
in fitting the VandenBerg isochrones to the observed NGC~121 CMD. The best fit 
is 
obtained with [Fe/H]~$=-1.40\pm 0.1$, consistent with the spectrographically
determined metallicity (Suntzeff \et 1986). A set of VandenBerg isochrones 
(at 8, 10, 12, 14, 16 and 18~Gyr) are superposed on our CMD (Figure~2). The best 
fit is obtained with the 10~Gyr isochrone, and we adopt this as the cluster age, 
with an uncertainty of about 2~Gyr. This value is in agreement with the less 
precise age (7--14 Gyr) determined by Stryker \et (1985) from groundbased data.
Our value of 10~Gyr seems to be about 4~Gyr younger than the commonly accepted 
age of the Galactic globular clusters. However, we note that recent Hipparcos 
subdwarf parallaxes yield an age of $11.8_{-2.5}^{+2.1}$~Gyr for nine Galactic 
globular clusters (Gratton \et 1997), consistent with our determination of the 
relative youth of NGC~121. These isochrones also fix the MSTO at V~$=23.0\pm 
0.1$. 

\section{SUMMARY}

Stars are resolved at the center of the old SMC globular cluster NGC~121 in 
Hubble Space Telescope Planetary Camera images. We present the B, V 
color-magnitude diagram of the cluster population.

1. The main sequence turnoff is clearly detected, as are the red horizontal
branch and red giant branch populations. The cluster core radius is 
$10.9 \pm 0.5$ arcseconds, corresponding to 3.2 parsecs in physical size.

2. A sequence of 42 blue straggler candidates is found up to 1.8~mag brighter 
than the turnoff, in accord with many Galactic globular BSS sequences. The 
sequence length is consistent with either a binary coalescence or two-star
collisional merger origin for the BSS. These are the first blue stragglers 
found in an extragalactic globular cluster. 

3. The BSS are more centrally concentrated than the cluster subgiants, red 
giants, horizontal branch, or MSTO stars. A few blends and some hot pixels 
probably mimic BSS, but simulations and comparisons with a nearby field 
indicate that at least half of the candidates are genuine BSS stars. A 
comparable number of fainter BSS may be present, but we cannot distinguish them 
from MSTO stars.

4. The central relaxation time of NGC~121 is about 0.9~Gyr and the relaxation 
time at the radius encompassing all of the BSS is about 8~Gyr. Most BSS are thus 
likely to be in energy equipartition with the other stars in the cluster. The 
smaller core radius of the BSS relative to the MSTO stars implies that the BSS 
are more massive than the turnoff stars, consistent with estimates from the 
color-magnitude diagram.   
 
5.  We find $\Delta V^{HB}_{TO}=3.3\pm 0.1$~mag, corresponding to an age for 
NGC~121, the oldest SMC globular cluster, that is significantly younger (by 
2~Gyr)than the ages of most Galactic globular clusters, and of 
Hodge~11, one of the oldest LMC clusters (Mighell \et 1996).

\acknow
MMS and RMR acknowledge extensive and helpful inputs from Ken Mighell. DZ 
acknowledges helpful conversations with, and access to isochrones of Don 
VandenBerg. This research is based on observations made with the NASA/ESA 
{\it Hubble Space Telescope\/} obtained at the Space Telescope Science
Institute.  ST~ScI is operated by the Association of Universities for
Research in Astronomy, Inc.\ under NASA contract NAS 5-26555.  Support for 
this work was provided by NASA through grant No.\ GO-4546 from ST~ScI.


%\newpage
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\onecolumn
%\documentstyle[12pt]{article}
%\begin{document}

%\scriptsize{
\begin{table}[t] %t=top b=bottom h=here p=next page
\begin{centering}
\vbox{\caption{\bf NGC 121 Blue Stragglers}
\label{sdbpars}
\vspace{\baselineskip}
\begin{tabular}{cccccccl}
\hline\hline
\vspace{3pt}
\strut  Number & X & Y & V & $\sigma_V$ & B-V & $\sigma_{B-V}$ & Comment\\ 
\strut & (pixels) & (pixels) & (mag) & (mag) & (mag) & (mag) & \\ 
\hline
1 & 223.59 & 105.71 & 21.480 & 0.022 & 0.162 & 0.030 & \\
2 & 544.65 & 108.04 & 22.102 & 0.032 & 0.202 & 0.043 & \\
3 & \en86.01 & 199.56 & 21.769 & 0.027 & 0.244 & 0.037 & \\
4 & 636.32 & 242.59 & 21.839 & 0.028 & 0.244 & 0.039 & \\
5 & 683.43 & 246.01 & 22.034 & 0.031 & 0.306 & 0.042 & Hot Pixel(?) \\
6 & 436.34 & 262.27 & 20.629 & 0.017 & 0.144 & 0.024 &\\
7 & 143.91 & 265.50 & 21.460 & 0.023 & 0.218 & 0.032 &\\
8 & 116.88 & 270.13 & 22.064 & 0.032 & 0.244 & 0.043 &\\
9 & 400.05 & 270.07 & 21.926 & 0.029 & 0.251 & 0.039 &\\
10 & 237.56 & 273.15 & 21.686 & 0.025 & 0.239 & 0.035 & Hot Pixel(?) \\
11 & 736.87 & 285.00 & 21.419 & 0.023 & 0.415 & 0.033 &\\
12 & 280.31 & 288.82 & 21.710 & 0.030 & 0.243 & 0.040 &\\
13 & 456.58 & 309.76 & 21.734 & 0.027 & 0.298 & 0.037 &\\
14 & 244.50 & 332.42 & 21.421 & 0.023 & 0.173 & 0.032 &\\
15 & 517.84 & 334.57 & 21.530 & 0.026 & 0.138 & 0.035 &\\
16 & 316.61 & 338.61 & 22.134 & 0.034 & 0.306 & 0.047 &\\
17 & 223.04 & 340.54 & 21.705 & 0.028 & 0.279 & 0.039 &\\
18 & 700.36 & 342.01 & 21.959 & 0.028 & 0.196 & 0.038 &\\
19 & 398.30 & 347.96 & 22.158 & 0.038 & 0.213 & 0.052 &\\
20 & 253.74 & 370.15 & 22.087 & 0.033 & 0.203 & 0.044 &\\
21 & 132.48 & 386.29 & 21.769 & 0.029 & 0.295 & 0.039 &\\
22 & \en65.87 & 417.88 & 21.319 & 0.026 & 0.191 & 0.035 &\\
23 & 532.38 & 426.69 & 21.330 & 0.022 & 0.409 & 0.032 &\\
24 & 309.93 & 437.47 & 21.565 & 0.026 & 0.343 & 0.036 &\\
25 & 352.30 & 438.89 & 21.368 & 0.031 & 0.095 & 0.040 &\\
26 & 524.53 & 439.61 & 21.352 & 0.027 & 0.164 & 0.035 &\\
27 & 358.01 & 453.54 & 21.958 & 0.044 & 0.238 & 0.056 &\\
28 & 423.11 & 466.54 & 21.352 & 0.028 & 0.131 & 0.037 &\\
29 & 511.53 & 482.61 & 22.018 & 0.036 & 0.257 & 0.049 & Hot Pixel(?) \\
30 & 545.47 & 493.85 & 22.024 & 0.034 & 0.188 & 0.044 &\\
31 & 461.31 & 498.79 & 21.462 & 0.033 & 0.400 & 0.043 &\\
32 & 527.34 & 503.87 & 21.250 & 0.024 & 0.327 & 0.034 &\\
33 & 315.84 & 508.09 & 22.109 & 0.032 & 0.280 & 0.043 &\\
34 & 353.06 & 514.31 & 21.834 & 0.028 & 0.261 & 0.038 &\\
35 & 262.52 & 534.60 & 21.771 & 0.028 & 0.192 & 0.037 &\\
36 & 400.34 & 535.94 & 21.317 & 0.022 & 0.251 & 0.031 & Hot Pixel(?) \\
37 & 408.75 & 536.88 & 21.920 & 0.034 & 0.320 & 0.047 &\\
38 & 523.93 & 559.27 & 21.841 & 0.029 & 0.344 & 0.040 &\\
39 & 776.71 & 577.85 & 21.495 & 0.023 & 0.177 & 0.032 &\\
40 & 420.60 & 580.19 & 22.229 & 0.034 & 0.224 & 0.047 &\\
41 & 240.06 & 603.09 & 21.298 & 0.021 & 0.288 & 0.030 &\\
42 & 549.46 & 686.28 & 21.492 & 0.025 & 0.074 & 0.034 &\\
\hline
\end{tabular}
}
\end{centering}
\end{table}

\end{document}
