q_obj17.tex  Accepted in A&A (First results from ISO)

\documentstyle{l-aa}
\begin{document}

\headnote{\it Letter to the Editor: first results from ISO}
\thesaurus{1         % A&A Section 1: Letters
           (09.04.1;  % {\it (ISM:)} dust, extinction
            09.08.1;  % {\it (ISM:)} H\ts {\sc ii} regions
            09.13.2;  % ISM: molecules
            10.03.1;  % Galaxy: center
            13.09.4)} % Infrared: interstellar: lines
%
\title{ISOCAM CVF observations of the Quintuplet and Object\#17 
clusters near the Galactic Center
}

% \title{ISOCAM CVF observations of the Quintuplet and Object\#17 
% clusters near the Galactic Center
%  \thanks{
%  Based on observations with ISO, an ESA project with instruments 
%  funded by ESA Member States (especially the PI countries: France 
%  Germany, the Netherlands and the United Kingdom) and with the 
%  participation of ISAS and NASA.  
%  }
% }

\author{T. Nagata\inst{1} \and K. Kawara\inst{2,3} \and
        T. Onaka\inst{4} \and Y. Kitamura\inst{2} 
        \and H. Okuda\inst{2} }

\offprints{T. Nagata}

\institute{
Department of Physics, Nagoya University,
Nagoya 464-01, Japan
(nagata@zlab.phys.nagoya-u.ac.jp)
\and 
Institute of Space and Astronautical Science,
Yoshinodai, Sagamihara 229, Japan
\and
ISO Science Operations Centre, Astrophysics Division of 
ESA, Villafranca, 28080 Madrid, Spain
\and
Department of Astronomy, University of Tokyo,
Tokyo 113, Japan
}

\date{Received July 15, 1996; accepted August --, 1996}

\maketitle
\markboth{Quintuplet and Object\#17 near GC}{Nagata et al.}
% \maintitlerunninghead{Quintuplet and Object\#17 near GC}
% \authorrunninghead{Nagata et al.}

\begin{abstract}
Two fields near the Galactic Center, containing the Quintuplet and Object \#17
star-clusters, have been imaged with the ISOCAM circular variable filters 
(CVFs) in the wavelength range of 2.47 -- 3.08$\mu$m and 3.99 -- 9.09$\mu$m.  
Emission of [Ar II] 6.99$\mu$m is detected in the ``pistol-shaped'' H II 
region (G0.15-0.05) to the south of the Quintuplet cluster.  
Absorption probably due to O-H stretching vibration (2.8$\mu$m) 
is evident in the Quintuplet spectra.  
In addition, Quintuplet members and the central part of Object \#17 have 
CO$_2$(4.3$\mu$m) and CO (4.7$\mu$m) absorption.  
Abundant CO$_2$ might be present in these lines of sight.  

\keywords{
  dust, extinction -- 
  H\ts {\sc ii} regions -- 
  ISM: molecules -- 
  Galaxy: center -- 
  Infrared: interstellar: lines
}
\end{abstract}
%________________________________________________________________

\section{Introduction}
When studying interstellar extinction in the infrared, to find suitable 
background continuum sources is a difficult issue.  
The line of sight to the central parsec of the Galaxy is usually regarded 
as representing diffuse interstellar medium.  
The absorption features in the spectrum of Sgr A (\cite{wil79}) 
are thus thought to be characteristic of diffuse clouds.  
However, this region is very complicated.  
For example, many stars are late-type giants, 
but there are also H II regions and 
He emission line stars (\cite{kra91}).  
In addition, a molecular cloud component of extinction seems to exist 
along the line of sight to the Galactic Center 
because some infrared sources show 
deep H$_2$O ice absorption (IRS~19: \cite{wil82}) and solid CO 
absorption (IRS~12: \cite{mcf89}).  

We now know two other star clusters near the Galactic Center 
which may be used as background continua; 
one is the Quintuplet cluster at ($l=0\fdg 16, b=-0\fdg 06$), 
and the other is the Object \#17 cluster at ($l=0\fdg 12, b=0\fdg 02$).
The Quintuplet, first found in a polarimetric survey by Kobayashi et al. 
(1983), includes five very bright stars whose color temperatures are in 
the range of 600--900K (\cite{oku90}; \cite{nag90}).  
Object \#17 is a cluster of emission line stars (\cite{nag93}, 1995;
\cite{cot96}), which might be similar to the central parsec cluster.  
These star clusters are also intriguing in themselves.  

In this Letter, we report the spectroscopic observations of these two 
objects with the ISOCAM (\cite{kes96}; \cite{ces96}).  

%
%__________________________________________________________________
\begin{figure}
\picplace{8cm}
\caption[]{The Quintuplet cluster at 5.05$\mu$m.
Contour levels are 0.2, 0.5, 1, 2, and 4 Jy/$1\farcs 5$ pixel.}
\end{figure}

\section{Observation and Reduction}
The Quintuplet was observed on February 24 and Object \#17 on 
February 23, 1996, with the ISOCAM circular variable filters (CVFs).  
ISOCAM consists of two optical channels: short wavelength (SW) and 
long wavelength (LW).  
In the SW observations, 
the CVF ($\lambda /\Delta \lambda = 41$) was rotated every 3 steps 
(0.075$\mu$m) 
from 2.474$\mu$m to 3.079$\mu$m and 
from 3.987$\mu$m to 5.122$\mu$m.
The field of view was $45\arcsec \times 45\arcsec$ ($1\farcs 5/{\rm pixel}$).
In the LW observations, 
the CVF ($\lambda /\Delta \lambda \sim 40$) was rotated every 3 steps 
from 5.079$\mu$m to 9.09$\mu$m.  
The field of view was $87\arcsec \times 87\arcsec$ ($3\arcsec /{\rm pixel}$).
The SW and LW CVF scans were made in the direction of increasing wavelengths, 
and six exposures were taken at each wavelength.  
The exposure time was 6 sec for SW and 5 sec for LW.  

We followed standard reduction procedures described in the ISO Data Users 
Manual by Siebenmorgen et al. (1995),
using the ISOCAM libraries of dark current and flat fields.  
IDL (Interactive Data Language) was used, 
and deglitching was made with a median filter with width 3.  
The memory effect due to the long time constant of the detector arrays 
was examined by comparing the first-exposure spectra with 
the last-exposure spectra, 
where the first and last-exposure spectra comprise only data taken in the 
first and last exposures, respectively, at each CVF wavelength.  
No systematic differences were found in the SW spectra, 
but there were systematic differences in the LW spectra.  
The shifts were largest in the region where the flux level changes rapidly 
(e. g., from 8.3 to 9.1$\mu$m), and the maximum shift is 10\% of the 
flux level.  
The reader should be reminded that no corrections for the memory effect 
were made, so that the LW spectra must have been smoothed by this effect 
to some degree.  
Also note that the spectra presented here are simple medians of data 
in every exposure.  

At the beginning of the LW CVF scan, pixels exposed to the bright Quintuplet 
sources were saturated.  
The data of these sources were missed, 
but as a trade-off, faint diffuse sources were measured with good 
signal-to-noise ratios.  

\begin{figure}
\picplace{8cm}
\caption[]{The Quintuplet field at 7.02$\mu$m.  
Adjacent continua at 6.86 and 7.19$\mu$m were subtracted.  
Black regions are the pixels saturated in the LW observation.  
Contour levels are 0.1, 0.2, 0.3, and 0.4 Jy/$3\arcsec$ pixel.}
\end{figure}

\begin{table}
\caption[]{Magnitudes of Quintuplet in 1986}
\begin{flushleft}
\begin{tabular}{lrrrrrrr}
\hline\noalign{\smallskip}
source    & $K$ & $L$ & $M$ & $N$ &  $Q$ & [7.8] & [8.7] \\
\noalign{\smallskip}
\hline\noalign{\smallskip}
GCS~4     &     & 3.4 & 1.9 & 0.9 &  0.4 &  0.7 & 1.0 \\
GCS~3-I   & 7.8 & 4.6 & 3.2 & 2.4 &  1.2 &  1.9 & 2.5 \\
GCS~3-II  & 6.3 & 2.7 & 1.2 & 0.1 & -0.6 & -0.4 & 0.2 \\
GCS~3-III & 9.4 & 4.8 & 2.7 & 0.4 & -1.4 &  0.6 & 0.9 \\
GCS~3-IV  & 7.8 & 4.3 & 2.6 & 0.7 & -0.8 &  0.6 & 1.0 \\
\noalign{\smallskip}
\hline
\end{tabular}
\end{flushleft}
\end{table}

The absolute fluxes have been calibrated against the ground-based 
observations in Table 1 (\cite{oku90}: since their table has 
typographical errors, the correct magnitudes are shown below.) 
for the Quintuplet region, 
and the observations with a $6\arcsec$ aperture (\cite{nag93}) 
for Object \#17.

There is a considerable scattered-light component.  
It consists of real diffuse flux from background stars and some 
instrumental flux like ghosts.  
We can see an out-of-focus image of the primary mirror over the 
entire field at a very low flux level in the LW channel, 
but it is impossible to distinguish it from the real diffuse background.  
Therefore, we chose ``sky'' regions in the frame and examined their spectra.  
In Quintuplet, we chose a region to the west of GCS~3-IV as ``sky''.  
In Object \#17, we chose a region of low stellar density to the northwest of
the center in the figures of Nagata et al. (1995) and Figer (1995).  
As long as the sky levels are insignificant, 
we can securely regard the object signals as real.  

%
%______________________________________________________________

\begin{figure}
\picplace{12cm}
\caption[]{CVF spectra of Quintuplet sources. 
The fluxes in $6\arcsec \times 6\arcsec$ regions have been summed.  
For the ``pistol'' region, $21\arcsec \times 9\arcsec$ flux is 
plotted (In the short wavelength region little signal was 
detected) after multiplication by 2 for clarity.  
These source fluxes are after subtraction of the sky flux, 
which is the summed and normalized signal of
the region to the west of GCS~3-IV.  
The plotted sky flux is converted to that in 
a $6\arcsec \times 6\arcsec$ region.  
}
\end{figure}

\section{Results}
The Quintuplet cluster field is shown in Figs. 1 and 2.  
Fig. 1 is a continuum image at 5.05$\mu$m; 
the five brightest members and the sixth source (VR5-5 in \cite{mon94}) 
are clearly seen.  
In Fig. 3, the spectra of $6\arcsec \times 6\arcsec$ around the peaks of 
these sources after subtracting the ``sky'' flux are shown.  
Standard deviations of median-filtered data are shown as error bars to aid 
estimating fluctuations, but these are generally smaller than 
systematic errors such as the selection of the ``sky'' area.  
Similarly, the spectrum of central $6\arcsec \times 6\arcsec$ region of 
the Object \#17 cluster is shown in Fig. 4.  
This region includes emission line stars 7, 8, 10, and probably 11 
in Nagata et al. (1995).  

\begin{figure}
\picplace{7cm}
\caption[]{CVF spectra of the central 
$6\arcsec \times 6\arcsec$ region of Object \#17
after subtraction of the sky flux.  
The sky flux is the summed signal of
the region to the northwest (normalized to 
$6\arcsec \times 6\arcsec$).  
}
\end{figure}

Fig. 2 is a 7.02$\mu$m Quintuplet image after subtracting adjacent 
wavelengths.  
The ``pistol-shaped'' H II region (G0.15-0.05) to the south of the 
five stars is bright at this wavelength.  
We identify this to be the fine-structure [Ar II] line at 6.99$\mu$m.  
The line intensity from the $21\arcsec (\alpha) \times 9\arcsec (\delta)$ 
region around the peak is 4.8 (-14) W m$^{-2}$.  
The spectrum of this region is shown in Fig. 3.  

This line is most prominent in the 4 to 8$\mu$m spectrum of Sgr A 
(\cite{wil79}; \cite{les81}).  
Willner et al. (1979) use the radio continuum emission of 26 Jy at 5 GHz 
in a $0\farcm 6 \times 1\arcmin$ beam (\cite{eke75}), and predict the 
intensity of [Ar II] line from Sgr A.  
Their measured [Ar II] flux 6.4 (-13) W m$^{-2}$ is twice the predicted 
flux, and they suggest overabundance of argon in the Galactic Center.  
In the ``pistol'' region, Yusef-Zadeh, Morris, \& Gorkom (1989) measured 
radio recombination lines and made a simple model of optically thin ionized 
plasma.  
Their model uses a 5 GHz continuum flux of 0.6 Jy, and the resultant number of 
ionizing photons is 3.9 (48) s$^{-1}$.  
Moneti et al. (1994) found He I emission in the ``ridge'' source 
on the west of the GCS~3 cluster, and Br~$\gamma$ and He I emission in the 
``pistol'' region.  
In addition, Figer et al. (1995) have recently observed an emission-line 
star near the ``pistol'' region (Star 3, which is \#25 in \cite{nag93} and 
the ``serendipitous'' source in \cite{mon94}; 
this star appears as the lowest contour in Fig. 1), 
and concluded that this star is similar to luminous blue variables (LBVs).  
They suggest that it can produce 3.4 (48) s$^{-1}$ ionizing photons.  
Thus, the radio contimuum and the [Ar II] line fluxes 
from the ``pistol'' are probably 
consistent with the picture of optically thin H II region powered by 
the LBV candidate and/or the ``ridge'' source.  

In Fig. 3, we find a broad absorption feature at 2.8$\mu$m and dips 
at 4.3$\mu$m and at 4.7$\mu$m in all the objects detected with sufficient 
signal-to-noise ratios.  
Similar dips seem to be present in Fig. 4 at 4.3$\mu$m and 4.7$\mu$m.  

The 2.8$\mu$m absorption is probably due to O-H stretching vibration.  
This absorption feature is marginally present in the ground-based 
observations (\cite{oku90}), and it is now clearly shown thanks to 
the presence of the 2.5--2.8$\mu$m part of spectra.  
The O-H bending mode at 6.1$\mu$m, whose absorption intensity in H$_2$O 
is more than one magnitude weaker (\cite{tie96}), is not present in the 
spectra.  
The 2.8$\mu$m absorption seems to be distributed inhomogeneously in 
the direction of the Galactic Center because it is not seen in Object \#17.  
This is reminiscent of the 3$\mu$m absorption in the central parsec;
Galactic Center IRS~19 has a deep feature, 
but those for IRS~3 and IRS~7 are much shallower (\cite{mcf89}).  
We are not certain what material causes this 2.8$\mu$m absorption, but
possible candidates are: 1) hydrated silicate in diffuse interstellar
medium (\cite{kna82}, see also \cite{wad91}), 
2) water ice in molecular clouds, 
and 3) water vapor in the stellar atmosphere.  
Water ice is not a very likely candidate because the maximum absorption of 
water ice is usually at 3.0 -- 3.1$\mu$m (e. g., \cite{smi89}), 
and that for IRS~19 is at only slightly shortward of 
3.0$\mu$m (\cite{mcf89}).  
Water vapor is also unlikely because the Quintuplet spectra are smooth in the 
$K$ band without a trace of the CO overtone bands near 2.3$\mu$m and 
the H$_2$O band near 1.9$\mu$m (\cite{oku90}).  

The 4.7$\mu$m absorption is caused by CO.  
In Quintuplet, Okuda et al. (1990) detected the P and R branches of the 
gaseous CO v=0-1 band.  
They have derived the column density of CO to be 1 (18) cm$^{-2}$.  
Since Okuda et al. did not detect solid CO, 
the line of sight to the Quintuplet cluster is free of densest parts of 
molecular clouds where solid CO exists.  
This is consistent with the absence of (deep) water ice absorption.  

The 4.3$\mu$m absorption is due to strong CO$_2$ resonance.  
The absorption $\tau$ at 4.3$\mu$m in the CVF measurement of 
$\Delta \nu = 57 {\rm cm}^{-1}$ is $\sim 0.1$ for GCS~3-II and Object \#17,
and $\sim 0.04$ for other GCS~3 sources and GCS~4.  
We are uncertain whether the difference in $\tau$ between GCS~3-II 
and other Quintuplet members is significant.  
The absorption intensity of gaseous CO$_2$ 
(1 (-16) cm molecule$^{-1}$: \cite{gri62}) 
is not very different from those of solid CO$_2$ in various ices 
(7 (-17) -- 2 (-16) cm molecule$^{-1}$: \cite{san88}; \cite{san90}).  
Although CO$_2$ is unlikely to have appreciable abundance 
from gas chemistry (\cite{her86}), 
CO$_2$ can form on the grains and its high production rate 
influences the gas phase (\cite{dhe86}).  
Since CO is in the gas phase toward the Quintuplet, we tentatively adopt 
the gas absorption intensity of CO$_2$ and calculate its column density.  
If we use $\tau$ of 0.05, the column density of CO$_2$ is 3 (16) cm$^{-2}$.
This is 1/30 of the CO column density (\cite{oku90}), 
and it can be the second abundant molecule.  
It is necessary to locate this CO$_2$ and establish whether it is in 
the gas or solid phase.

\begin{acknowledgements}
We wish to thank the ISO team for realizing this wonderful satelite project.  
\end{acknowledgements}

\begin{thebibliography}{}

\bibitem[Cesarsky et al. 1996]{ces96} 
 Cesarsky, C. J., et al.
 1996, A\&A, this volume
\bibitem[Cotera et al. 1996]{cot96} 
 Cotera, A. S., Erickson, E. F., Colgan, W. J., Simpson, J. P., Allen, D. A., 
 \& Burton, M. G.
 1996, ApJ, 461, 750
\bibitem[d'Hendecourt et al. 1986]{dhe86} 
 d'Hendecourt, L. B., Allamandola, L. J., Grim, R. J. A., \&
 Greenberg, J. M.
 1986, A\&A, 158, 119
\bibitem[Ekers et al. 1975]{eke75} 
 Ekers, R. D., Goss, W. M., Schwarz, U., J., Downes, D., \& 
 Rogstad, D. H.
 1975, A\&A, 43, 159
\bibitem[Figer 1995]{figa95} 
 Figer, D. F.
 1995, Ph. D. Thesis, University of California at Los Angeles
\bibitem[Figer et al. 1995]{figb95} 
 Figer, D. F., McLean, I. S., \& Morris, M.
 1995, ApJ, 447, L29
\bibitem[Gribov \& Smirnov 1962]{gri62} 
 Gribov, L. A., \& Smirnov, V. N.
 1962, Sov. Phys. Usp., 4, 919
\bibitem[Herbst \& Leung 1986]{her86} 
 Herbst, E., \& Leung, C M.
 1986, MNRAS, 222, 689
\bibitem[Kessler et al. 1996]{kes96} 
 Kessler, M. F., et al.
 1996, A\&A, this volume
\bibitem[Knacke et al. 1982]{kna82} 
 Knacke, R. F., McCorkle, S., Puetter, R. C., Erickson, E. F., 
 \& Kratschmer, W.
 1982, ApJ, 260, 141
\bibitem[Kobayashi et al. 1983]{kob83} 
 Kobayashi, Y., Okuda, H., Sato, S., Jugaku, J., \& Dyck, H. M.
 1983, PASJ, 35, 101
\bibitem[Krabbe et al. 1991]{kra91} 
 Krabbe, A., Genzel, R., Drapatz, S., \& Rotaciuc, V.
 1991, ApJ, 382, L19
\bibitem[Lester et al. 1981]{les81} 
 Lester, D. F., Bregman, J. D., Witteborn, F. C., Rank, D. M., 
 \& Dinerstein, H. L.
 1981, ApJ, 248, 524
\bibitem[McFadzean et al. 1989]{mcf89} 
 McFadzean, A. D., Whittet, D. C. B., Longmore, A. J., Bode, M. F.,
 \& Adamson, A. J.
 1989, MNRAS, 241, 873
\bibitem[Moneti et al. 1994]{mon94} 
 Moneti, A., Glass, I. S., \& Moorwood, A. F. M.
 1994, MNRAS, 268, 194
\bibitem[Nagata et al. 1990]{nag90} 
 Nagata, T., Woodward, C. E., Shure, M., Pipher, J. L., \& Okuda, H.
 1990, ApJ, 351, 83
\bibitem[Nagata et al. 1993]{nag93}
 Nagata, T., Hyland, A. R., Straw, S. M., Sato, S., \& Kawara, K.
 1993, ApJ, 406, 501
\bibitem[Nagata et al. 1995]{nag95} 
 Nagata, T., Woodward, C. E., Shure, \& Kobayashi. N.
 1995, AJ, 109, 1676
\bibitem[Okuda et al. 1990]{oku90} 
 Okuda, H., Shibai, H., Nakagawa, T., Matsuhara, H., Kobayashi, Y.,
 Kaifu, N., Nagata, T., Gatley, I., \& Geballe, T. R.
 1990, ApJ, 351, 89
\bibitem[Sandford et al. 1988]{san88} 
 Sandford, S. A., Allamandola, L. J., Tielens, A. G. G. M., \& Valero, G. J.
 1988, ApJ, 329, 498
\bibitem[Sandford \& Allamandola 1990]{san90} 
 Sandford, S. A. \&  Allamandola, L. J.
 1990, ApJ, 355, 357
\bibitem[Smith et al. 1989]{smi89} 
 Smith, R. G., Sellgren, K., \& Tokunaga, A. T.
 1989, ApJ, 344, 413
\bibitem[Tielens et al. 1996]{tie96} 
 Tielens, A. G. G. M., Wooden, D. H., Allamandola, L. J., Bregman, J., 
 \& Witteborn, F. C.
 1996, ApJ, 461, 210
\bibitem[Wada et al. 1991]{wad91} 
 Wada, S., Sakata, A., \& Tokunaga, A. T.
 1991, ApJ, 375, L17
\bibitem[Willner et al. 1979]{wil79} 
 Willner, S. P., Russel, R. W., Puetter, R. C., Soifer, B. T.,
 \& Harvey, P. M.
 1979, ApJ, 229, L65
\bibitem[Willner \& Pipher 1982]{wil82} 
 Willner, S. P., \& Pipher, J. L.
 1982, in: The Galactic center, AIP Conf. Proc. No.83, 
 eds.\ G. R. Reigler, R. D. Blandford, 
 AIP, New York, p.\ 77
\bibitem[Yusef-Zadeh et al. 1989]{yus89} 
 Yusef-Zadeh, Morris, \& Gorkom
 1989, in: The Center of the Galaxy, IAU Symp. 136,
 ed.\ M. Morris,
 Kluwer, Dordrecht, p.\ 275

\end{thebibliography}

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