------------------------------------------------------------------------
 gcsiom.tex PASJ, Vol.53, 501-507 (2001)
Content-Type: text/plain; charset="iso-2022-jp" ; format="flowed"
Content-Transfer-Encoding: 7bit
Content-Length: 34193

% http://www.nro.nao.ac.jp/~amiya/papers/gcsiom_pasj53.pdf
% Paper: PASJ, 53, 501-507 (2001)

%% PASJ LaTeX file
\documentclass{pasj00}
\usepackage[dvips]{graphicx}

\begin{document}
\SetRunningHead{A.\ Miyazaki et al.}
{SiO Maser Survey in the Galactic Center Region}
\Received{2000/11/24}%{yyyy/mm/dd}
\Accepted{2001/03/09}%{yyyy/mm/dd}

\title{SiO Maser Survey in the Galactic Center Region \\
with a Multi-Beam Receiver}

%%% begin:list of authors
\author{%
Atsushi \textsc{Miyazaki},\altaffilmark{1,2}
Shuji \textsc{Deguchi},\altaffilmark{1}
Masato \textsc{Tsuboi},\altaffilmark{2}
Takashi \textsc{Kasuga},\altaffilmark{3}
and
Shuro \textsc{Takano},\altaffilmark{1}}
\altaffiltext{1}{
Nobeyama Radio Observatory,
\thanks{Nobeyama Radio Observatory (NRO)
is a branch of the National Astronomical Observatory,
an inter-university research institute operated
by the Ministry of Education, Science Sports and Culture.}
Minamimaki, Minamisaku, Nagano 384-1305}
\email{amiya@nro.nao.ac.jp}
\altaffiltext{2}{
Institute of Astrophysics and Planetary Science, Ibaraki University,
2-1-1 Bunkyou, Mito, Ibaraki 310-8512}
\altaffiltext{3}{
Department of System and Control Engineering, Hosei University,
3-7-2 Kajino-chou, Koganei, Tokyo 184-8584}
%%% end:list of authors

%% `\KeyWords{}' always has to be placed before `\maketitle'.
\KeyWords{Galaxy: center ---
Galaxy: kinematics and dynamics --- masers}

\maketitle

\begin{abstract}
We have made a wide-field survey of SiO masers
in the $7' \times 13'$ area around the Galactic center
with the Nobeyama 45-m radio telescope
using a $2 \times 3$ multi-beam SIS mixer receiver at 43~GHz.
We detected 9 SiO maser sources (6 of them are new);
newly detected 6 sources have been identified with
Miras or OH 1612~MHz maser sources.
The source surface number density, $\sim 360$ per square degree,
found in this survey is slightly smaller than
the number density of  OH 1612~MHz sources in the same area.
A least-squares analysis of the radial velocities
of the detected sources has revealed a rapid rotation
of the stars around the Galactic center.
This implies that most of the objects detected in this survey
belong to the nuclear disk near the Galactic center.
\end{abstract}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   Introduction
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Introduction}

The number density of stellar maser sources
is known to increase toward the Galactic center
(Lindqvist et al. 1992a).
These sources are mostly Asymptotic Giant Branch stars
under heavy mass loss,
where the SiO maser action occurs
in the circumstellar envelope very near the photosphere
(a radius of $\sim 10^{14}$ cm),
while the OH maser action occurs far ($\sim 10^{16}$ cm)
from the central star (Reid, Moran 1981).
Stellar maser sources have been used to study
the structure and dynamics of the stellar nuclear disk
in the Galactic center.
Though a number of SiO maser surveys of IRAS sources
have been made using the Nobeyama 45-m telescope
(Izumiura et al. 1995, 1999; Deguchi et al. 2000),
the area within \timeform{0.5D}
from the Galactic center was out of reach in these surveys
because of incompleteness of the IRAS catalog in this area
(Beichman et al. 1988).
Shiki et al. (1997) found 6 new SiO maser sources
toward the Sgr~B2 molecular cloud
($l \sim$ \timeform{0.7D} and $b \sim$ --\timeform{0.1})
with a non-biased mapping of a 0.154 square degree
by 40$''$ grid spacing.
The SiO source number density is about 40 per square degree
in this field, and is expected to be much larger
if we proceed toward the Galactic center.
In fact, Izumiura et al. (1998) found 15 SiO sources
in an approximately $40'' \times 120''$ area
around the Galactic center (Sgr~A$^*$)
with a moderately sensitive survey,
reaching to a source surface density of
$\sim 4 \times 10^{4}$ per square degree.

On the other hand,  the very sensitive OH 1612~MHz surveys
towards the Galactic center using the Very Large Array
yielded 254 detections within \timeform{0.5D}
from the Galactic center (Sjowerman et al. 1998;
also see Lindqivist et al. 1992b; Sevenster et al. 1997).
Therefore, we should find, at least,
a similar number of SiO sources in this area.
However, according to the past SiO maser studies
(Izumiura et al. 1999),  there is a minor difference
in the nature between OH and SiO sources;
the overlap of detected sources in both OH and SiO
is only one third of the SiO detected sources
in the well-studied area in both lines,
the reason for which has not been well understood.

In order to find more SiO sources in the Galactic center area,
we conducted a spatially non-biased SiO survey
using a newly built multi-beam receiver
with the Nobeyama 45-m radio telescope.
The new receiver consists of 6-beam ($2 \times 3$) SIS mixers
working in the 40~GHz band,
and can produce the spectra of two SiO maser lines
({\it J}=1--0 {\it v}=1 and 2)
simultaneously at 6 positions with 90$''$ separation in the sky.
With this new instrument,
we have mapped the $7' \times 13'$ area
toward the Galactic center with a 30$''$ grid.
We detected 9 SiO maser sources in this survey.
We present the results of the survey in this paper.
The position of the continuum source Sgr~A$^*$,
R.A.=\timeform{17h42m29.314s},
Decl.=--\timeform{28D59'18.3''} (B1950)
[ $l_{\rm SgrA^*}=-\timeform{0.0557D}$,
$b_{\rm SgrA^*}=-\timeform{0.0461D}$;
Rogers et al. 1994 ],
is regarded as the center of the Galaxy throughout this paper.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   Observations and results
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Observations and Results}

We conducted a spatially non-biased survey of SiO maser sources
in the Galactic center region with the 45-m radio telescope
of the Nobeyama Radio Observatory
during 2000 March 8--11 and April 15--19
in the SiO {\it J}=1--0 {\it v}=1 and 2 transitions
at 43.122~GHz and 42.821~GHz, respectively.
We used the 40~GHz-band 6-beam SIS mixer receiver
(hereafter S40M) for a wide-field survey
and the in-house single-beam SIS mixer receiver (hereafter S40)
for confirmation observations.
For SiO line observations,
we improved the S40M multi-beam receiver,
which was originally designed for continuum observations to detect
the Sunyaev-Ze\'ldovich effect toward clusters of galaxies
(on this receiver, see Ohno et al. 1998;  Tsuboi et al. 2000).
The system temperature of the S40M receiver
was 160--200~K (DSB).
The measured beam size and aperture efficiency for each beam
at 43~GHz with observations of 3C$\;$273 and Jupiter were
38$\,''$ and 0.59 for channel 1,
40$\,''$ and 0.52 for channel 2,
40$\,''$ and 0.56 for channel 3,
40$\,''$ and 0.49 for channel 4,
39$\,''$ and 0.54 for channel 5, and
40$\,''$ and 0.52 for channel 6, respectively.
The intensity characteristics and beam patterns
of the 6 (2$\times$3) beams in S40M
were measured by mapping the strong SiO maser lines from VX~Sgr
several times prior to the mapping observations.
The angular interval between each beam
was designed to be $\sim 90''$.
We have applied some corrections on the obtained positions
of the maser sources in the S40M observations.

Acousto-optical spectrometer arrays
of both high resolution (AOS-H) and wide band (AOS-W)
with 2048 channels each were used;
the bandwidths of AOS-H and AOS-W are
40~MHz and 250~MHz, respectively.
The velocity coverages of these spectrometers at 43~GHz
were about 280~km$\:$s$^{-1}$ and 1740~km$\:$s$^{-1}$
for AOS-H and AOS-W, respectively.
The SiO {\it J}=1--0 {\it v}=1 and 2 transitions
at 43.122~and 42.821~GHz
were observed simultaneously in this system.
We configured 6 AOS-W spectrometers
for observing the SiO {\it J}=1--0 {\it v}=1 transition,
and 6 AOS-H spectrometers
for observing the SiO {\it J}=1--0 {\it v}=2 transition.
A single AOS-H spectrometer
covered the velocity range of only $\pm$ 140~km$\:$s$^{-1}$.
These sources with a large velocity deviation,
which were often found near to the Galactic center, were covered by
the AOS-W spectrometer in the {\it J}=1--0 {\it v}=1 transition.
The pointing of the telescope was checked almost every one or two hours
using a nearby strong SiO maser source, VX~Sgr.
The pointing accuracy
was better than $\pm 5\,''$ as the peak-to-peak value.

We mapped a rectangular area,
$-\timeform{3.'75} \le \Delta\alpha \le +\timeform{3.'15}$,
$-6' \le \Delta\delta \le +7'$,
centered at
R.A.=\timeform{17h42m29s},
Decl.=--\timeform{28D59'18''} (B1950)
with $30''$ grid spacing.
The grid was taken along R.A. and Decl. directions.
The typical integration time spent per point was 2.1 minutes.
In total, we spent about 4 hours
for this wide-field mapping with the S40M receiver,
and obtained spectra at 405 positions.
% Figure 1
Figure 1 shows the surveyed positions (shown by crosses).
Approximately 25 candidates for SiO maser sources
were found in this S40M survey.
They were reobserved
using the single-beam 40~GHz-band SIS mixer receiver (S40)
with an image-rejection filter.
The beam size and the aperture efficiency of the S40 receiver
was 38$\,''$ and 0.58, respectively.
The conversion factor from the antenna temperature
to the flux density was 3.0~Jy$\:$K$^{-1}$.
In this case,  both the SiO  {\it J}=1--0 {\it v}=1 and 2 transitions
were observed simultaneously
using the 6 high-resolution spectrometers (AOS-H)
with a velocity resolution of 0.3~km$\:$s$^{-1}$.
Three AOS-H spectrometers were configured to cover
$\pm$ 350~km$\:$s$^{-1}$ in $V_{\rm LSR}$ in each SiO line.
Moreover, the 2 wide-band AOS-W spectrometers were
also used for a backup of the spectra in the same frequency range.
The line intensities are better calibrated by this old receiver
because it has an image-rejection filter,
and the efficiency, beam size, and other characteristics
are better known owing to its long use.

In this way, we confirmed the 9 detections of SiO maser sources
in both the {\it J=}1--0 {\it v=}1 and 2 lines.
Figure 1 shows the positions of the detected sources
in the Galactic center region (open circles)
overlaid on the observed grids (cross).
% Figure 2
The spectra of the detected SiO maser sources
are shown in figure 2.
% Table 1
The list of detected sources and the line parameters
are given in table 1.
The spectra of sources \#1--\#8 in figure 2
were taken using high-resolution spectrometers
(AOS-H; 0.3~km$\:$s$^{-1}$ resolution).
Because the SiO line intensities of source \#9  were not strong,
we have shown the spectra
which were taken using wide-band spectrometers (AOS-W)
in both transitions,
resulting in a slightly higher signal-to-noise ratio in figure 2.
The line parameters shown in table 1
were derived using the high-resolution spectrometers (AOS-H).

% Table 2
Furthermore, in order to obtain accurate positions of
the detected SiO maser sources,
we made five-point mapping observations for several sources.
In addition, $5 \times 5$ points mapping around the Galactic center
with 30$''$ grid spacing
was made using the S40 receiver.
%%%%%%%%%%
\begin{itemize}
\item For sources \#1--\#3,
we made 5-point mapping observations by $20''$ separation
(a half beam) around the SiO maser candidate.
By a Gaussian fitting with the peak intensity at each position,
we obtained accurate positions at the epoch of 1950
and position errors, which are given in table 2.
% Figure 3
An example of the 5-point mapping is shown in figure 3,
illustrating the case for source \#3.
We obtained the positions of sources to an accuracy
of about a few arcseconds with this 5-point mapping
(as long as a sufficiently high S/N was available).

\item For sources \#4--\#6,
we were unable to make any 5-point mapping
because of the faintness of the signal.
The position errors,  $\sim 20''$, shown
in columns 5 and 6 in table 2
corresponds to half of the diagonal separation
of the observation grid on the survey.
Because these sources were detected only at one position
in the grid, they were located not far from the beam center.

\item For sources \#7--\#9,
we performed $5 \times 5$ point mapping
in ($\Delta l, \Delta b$) = ($\pm1', \pm1'$) around Sgr~A$^*$
with $30''$ grid spacing with the S40 receiver.
We detected these 3 sources and obtained accurate positions
by a 2-dimensional Gaussian fitting.
However, the errors in the position of source \#9
are still inferred to be somewhat large due to low S/N of the lines.
We note that the spectra for source \#9 in figure 2
involves the SiO lines from sources \#7 and \#8
at $V_{\rm LSR}=$ 71 and --28~km$\:$s$^{-1}$, respectively,
because separations from \#9 to \#7 and \#8 are small.

\end{itemize}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   Discussion
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Discussion}

The line parameters and positions
of the detected SiO maser sources in the surveyed area
are summarized in tables 1 and 2, respectively.
The line intensities of these sources
($T_{\rm a}^{*} \sim$ 0.3--1.0 K)
are comparable with the intensities
of the relatively bright SiO sources found in the bulge SiO survey
(Izumiura et al. 1998; Deguchi et al. 2000).
Because of the shallowness of the present survey,
it maybe suspected that the detected SiO sources
are not actually the Galactic center objects,
but the foreground objects relatively near to the Sun.
However, based on the following analysis, we show that
these sources are certainly objects located near the Galactic center.

\subsection{Cross Reference Check}
%%%%%%%%%%%%%%%%%%%%%%%%%%

We checked the positions of the detected sources
with those of known SiO/OH sources in the same region
(Izumiura et al. 1999; Menten et al. 1997; Lindqvist et al. 1991;
Sjowerman et al. 1998).
The results are shown in the last column of table 2.
The radial velocities of the detected SiO masers coincide
well with those of known sources within a few km$\:$s$^{-1}$.
Catalogs of the near and middle-infrared sources in this region
became available recently (Glass et al. 2001; Egan et al. 1999).
Glass et al. (2001) surveyed the long-period
and semi-regular variables
within 12$'$ radius from the Galactic center
in the near-infrared $JHK$ band.
These stars (Miras) are the mass-losing AGB stars
which often accompany OH/SiO masers.
The position of any detected SiO maser sources coincide
with those of Mira stars in Glass et al. (2001).
The results, which we have check the positions,
also are shown in the last column of table 2.
We also checked the position coincidence of the infrared sources
in the MSX catalog (Midcourse Space Experiment; Egan et al. 1999).
However, no MSX source was found
within 40$''$ radius from the SiO maser source,
except for \#7 and \#8, which are well-known infrared objects.

With this cross checking, we found that 3 sources (\#7--\#9)
are previously known SiO sources,
and 6 source are new detections in SiO maser lines.
Among these 6 new detections in SiO masers,
4 (\#2, \#4--\#6) have been identified with known
OH 1612~MHz maser sources (Sjowerman et al. 1998);
the positions coincide with these OH maser sources within 30$''$
and the radial velocities coincide within a few km$\:$s$^{-1}$.

Individually interesting objects are noted:
%%%%%%%%%%
\begin{itemize}
%
\item Source \#1 is located within $7''$
from Mira star ``1--72'' (``2--101'') of Glass et al. (2001),
The $K$ magnitude and period of this Mira star ``1--72''
are 9.1 mag and 577 days.

\item Source \#2 is located within $6''$
from Mira star ``3--266'' (``16--49'') of Glass et al. (2001),
which is a possible near-infrared counterpart
of the OH maser source, OH$\;$359.937--0.010
(Sjowerman et al. 1998).
The $K$ magnitude and period of this Mira star ``3--266''
are 9.9 mag and 524 days.

\item Source \#3 is located within a few arcsecond
from Mira star ``2--18'' (``3--7655'') of Glass et al. (2001),
The $K$ magnitude and period of this Mira star ``2--18''
are 9.0 mag and 667 days.

\item Source \#4 is located within $13''$
from Mira star ``2--1'' of Glass et al. (2001),
which is a possible near-infrared counterpart
of the OH maser source, OH$\;$359.998--0.005
(Sjowerman et al. 1998).
Although the position of the Mira star ``2--10''
is close to source \#4 ($\sim 6''$),
the radial velocities of the SiO source
coincide with that of OH$\;$359.998--0.005.
The $K$ magnitude and period of this Mira star ``2--1''
are 7.9 mag and 545 days.

\item Source \#5 is located near the OH maser source,
OH$\;$359.918--0.055 (Sjowerman et al. 1998).
This source has a very high velocity
($\sim$ --309~km$\:$s$^{-1}$).

\item Source \#6 is located near the OH maser source,
OH$\;$359.931-0.063
(Sjowerman et al. 1998; Winnberg et al. 1985).
A previous SiO maser search was negative
(Lindqvist et al. 1991).
The line intensity of the {\it J}=1--0 {\it v}=1 transition
was much weaker than that of
the {\it J}=1--0 {\it v}=2 transition.

\item Source \#7 is located within $7''$ from
the OH maser source, OH$\;$359.954--0.041
(Sjowerman et al. 1998; Winnberget at al. 1985).
The detection of SiO masers has been reported
in Lindqvist et al. (1991) and Izumiura et al. (1998).

\item Source \#8 is the infrared source IRS$\;$10EE
(Menten et al. 1997).
The OH 1612~MHz maser has been detected,
OH$\;$359.946--0.048 (Sjowerman et al. 1998).
The time variation in the infrared $K$-band has been reported
by Tamura et al. (1996) and  also by Wood et al. (1998),
assigning a source name, LWHM65, with $K=$7.8 mag
and a period of 736 days.
Note that the peak SiO line intensities of this source
in the present observations were approximately
a factor of 5 stronger than those found in 1997 May
by Izumiura et al. (1998).

\item Source \#9 is located near the OH maser source,
OH$\;$359.956--0.050 (Sjowerman et al. 1998).
This SiO maser source probably coincides with
the source SiO$\;$359.955--0.046 No.2  reported
by Izumiura et al. (1998)
with a radial velocity of $\sim$ 51~km$\:$s$^{-1}$.

\end{itemize}

\subsection{Velocity Distribution}
%%%%%%%%%%%%%%%%%%%%%%%%%%

% Figure 4
Figure 4 shows a longitude--velocity ($l-V$) diagram
for the detected SiO maser sources (shown as open circles),
which is overlaid on the $l-V$ diagram
of the CS {\it J}=1--0 emission line (taken from Tsuboi et al. 1999).
The radial velocities of the detected SiO maser sources
spread over a range between  --310~km$\:$s$^{-1}$
and 80~km$\:$s$^{-1}$.
Sources \#7 and \#9  are located
toward the 50~km$\:$s$^{-1}$ molecular cloud
(M$-0.02-0.07$; see Mezger et al. 1996).
No high-velocity source
with $|V_{\rm LSR}| \ge$ 100~km$\:$s$^{-1}$ was found,
except for source \#5.
This might be caused by an incompleteness
due to the survey velocity ranges;
the velocity coverage of the survey observations with S40M
were different between the $v$ = 1 and 2 transitions
(due to a restriction of the available spectrometers,
as mentioned in section 2).
For the $v$ = 2 transitions,
it was only $|V_{\rm LSR}| \le$ 140~km$\:$s$^{-1}$,
while it was $|V_{\rm LSR}| \le$ 870~km$\:$s$^{-1}$
for the $v$ = 1 transition.
The survey sensitivity outside of $\pm$ 140~km$\:$s$^{-1}$ was
by a factor of a few worse than the range inside.

The thin-solid straight line in figure 4 shows a least-squares fitting
for the 9 detected SiO sources in the surveyed region.
The least-squares fitting gives
\begin{equation}
V_{\rm LSR} = 1315(\pm 811)\,\Delta l_{\rm s}\,(^{\circ})
- 65.1(\pm 37.5)\;~{\rm km \: s^{-1}},
\end{equation}
where $\Delta l_{\rm s}~(=l_{\rm SiO} - l_{\rm SgrA^*})$
is the position offset from Sgr~A$^*$ along the Galactic longitude.
However, it may be peculiar
that source \#5 has on especially high velocity
($\sim$ --310~km$\:$s$^{-1}$),
although it is expected that there are a large number
of high-velocity maser sources in the Galactic center region.
A least-squares fitting (thick-solid line in figure 4) for SiO sources,
excluding source \#5, gives
\begin{equation}
V_{\rm LSR} = 490(\pm 487)\,\Delta l_{\rm s}\,(^{\circ})
- 19.7(\pm 23.2)\;~{\rm km \: s^{-1}}.
\end{equation}
On the other hand, a least-squares fitting
(thick-dashed lines in figure 4)
for the OH 1612~MHz maser sources in Sjouwermen et al. (1998)
in a region which is comparable to the present SiO survey region
($7\,'\times13\,'$ around the Galactic center),
excluding the counterpart (OH$\;$359.918--0.055)
of source \#5, gives
\begin{equation}
V_{\rm LSR} = 411(\pm 149)\,\Delta l_{\rm s}\,(^{\circ})
+ 7.1(\pm 6.9)\;~{\rm km \: s^{-1}}.
\end{equation}
The velocity gradient of the SiO maser source, excluding source \#5,
is consistent with that of the OH 1612~MHz maser sample.

The rotational motion of the SiO maser sample in the present survey
is appreciably larger than the rotational speed
found for the inner bulge IRAS/SiO maser sources within
$|l|<3^{\circ}$ and $|b|<3^{\circ}$
[ $22.7(\pm 7.9)\,l\,(^{\circ}) - 3.5(\pm 13.5)\;$km$\:$s$^{-1}$;
Deguchi et al. 2000 ].
The SiO sources detected in the present survey
show a characteristic of the rapidly rotating, stellar nuclear disk,
which was found by Lindquivist et al. (1992a)
and Sjowerman et al. (1998).
Therefore, we conclude
that most of the SiO maser sources found in the present survey
belong to the nuclear disk.


\subsection{Density of SiO Maser Sources}
%%%%%%%%%%%%%%%%%%%%%%%%%%

We detected 9 SiO maser source
in the region of $7\,'\times13\,'$ around the Galactic center.
This corresponds to the number density of the SiO maser sources,
$\sim$360 per square degree.
The 52 OH 1612~MHz maser sources (Sjouwermen et al. 1998)
found in the same region
gives a larger number density of  OH 1612~MHz sources,
$\sim$2000 per square degree.
This indicates that the present SiO survey was not deep enough.
These source densities can be compared
with the values obtained by previous surveys:
2.4 per square degree at the inner bulge
(86 SiO maser sources in $|l|<3^{\circ}$ and $|b|<3^{\circ}$;
Deguchi et al. 2000),
or $\sim$40 per square degree towards Sgr~B2
(6 SiO sources in 0.154 Square degree; Shiki et al. 1997).
Though the sensitivities of these surveys
are not the same as that of the present survey,
it is apparent that the SiO source density
is increasing toward the Galactic center,
as found in the OH 1612~MHz survey (Lindquivist et al. 1992a).
These source densities indicate that
the contamination probability of the inner bulge
or the foreground sources to the present sample is quite small;
a reasonable estimate involving the correction
for the IRAS color range of sources in the sample
by Deguchi et al. (2000) gives a contamination probability
of 0.1 source in a $7'\times13'$ area.
Therefore, we conclude, again, that most of the detected SiO sources
belong to the central nuclear disk.

The antenna temperature of faintest SiO maser sources
detected in the present survey
was $T_{\rm a}^* \approx$ 0.3~K.
We can easily improve the sensitivity by a factor of
a few by applying a longer integration time in the future
(but with compensation of the survey area).
It is inferred that the SiO source number density in the nuclear disk
will be several-times higher than
the value obtained in the present survey.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   Summary
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Summary}

We have detected 9 relatively strong SiO maser sources
in $7\,'\times13\,'$ mapping observations
of the Galactic center region with a 30$''$ grid
with a 40~GHz-band 6-beam SIS mixer receiver
installed on the Nobeyama 45-m radio telescope.
The source surface number density, $\sim 360$ per square degree,
found in this survey is slightly lower than
the number density of  OH 1612~MHz sources in the same area.
A least-squares fitting of the radial velocities
with the Galactic longitude resulted in a large rotational speed
around the Galactic center for this sample,
indicating that these maser sources belong to the central nuclear disk.
The contamination probability from the foreground source
was found to be small.
\par

%%%%%%%%%%%%%%%%%%%%%%%%%%
% acknowledgment
\vspace{1pc}\par

We thank students of Ibaraki and Hosei Universities
for the help during the observations.
We also thank Dr. L. O. Sjouwerman for the helpful comments.
This research was financially supported by
a Grant-in-Aid, No.1044061 (to M. T.), and No.12640243 (to S. D.),
from the Ministry of Education, Science, Sports, and Culture, Japan.
\par


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   References
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{thebibliography}{}
%% Journals(e.g. A\&A,ApJ,AJ,NMRAS,PASP ...)
\bibitem[]{}
Beichman, C. A., Neugebauer, G., Habing, H. J., Clegg, P. E.,
\& Chester, T. J.\ 1988, IRAS Catalogs and Atlases,
I. Explanatory Supplement, NASA RP-1190
(Washington: US Government Printing Office), pVII-22
\bibitem[]{}
Deguchi, S., Fujii, T., Izumiura, H., Kameya, O., Nakada, Y.,
Nakashima, J., Ootsubo, T., \& Ukita, N.\ 2000, \apjs, 128, 571
\bibitem[]{}
Egan, M. P., Price, S. D., Moshir, M., Cohen, M., Tedesco, E. F.,
Murdock, T.  L., Zweil, A., Burdick, S., et al.\
1999, Air Force Research Laboratory Technical Report
No. AFRL-VS-TR-1999-1522
(availabe at IPAC,  http://irsa.ipac.caltech.edu/applications/MSX/
  with the MSX catalog)
\bibitem[]{}
Glass, I. S., Matsumoto, S., Carter, B. S., \& Sekiguchi, K.\ 2001,
\mnras, 321, 77
\bibitem[]{}
Izumiura, H., Deguchi, S., Hashimoto, O., Nakada, Y., Onaka, T.,
Ono, T., Ukita, N., \& Yamamura,　I.\ 1995, \apj, 453, 837
\bibitem[]{}
Izumiura, H., Deguchi, S., \& Fujii, T.\ 1998, \apj, 494, L89
\bibitem[]{}
Izumiura, H., Deguchi, S., Fujii, T., Kameya, O., Matsumoto, S.,
Nakada, Y., Ootsubo, T., \& Ukita, N.\ 1999, \apjs, 125, 257
\bibitem[]{}
Lindqvist, M., Habing, H. J., \& Winnberg, A.\ 1992a,
\aap, 259, 118
\bibitem[]{}
Lindqvist, M., Winnberg, A., Habing, H. J., \& Matthews, H. E.\
1992b, \aaps, 92, 43
\bibitem[]{}
Lindqvist, M., Ukita, N., Winnberg, A., \& Johansson, L. E. B.\ 1991,
\aap, 250, 431
\bibitem[]{}
Menten, K. M., Reid, M. J., Eckart, A., \& Genzel, R.\ 1997,
\apj, 475, L111
\bibitem[]{}
Mezger, P. G., Duschl, W. J., \& Zylka, R.\ 1996, \aapr, 7, 289
\bibitem[]{}
Ohno, T., Miyazaki, A., Tsuboi, M., Kasuga, T., Noguchi, T.,
\& Sakamoto, A.\ 1998, Proc. of SPIE Conf.,
Millimeter and Submillimeter Waves and Applications, 3465, 453
\bibitem[]{}
Reid, M. J., \& Moran, J. M.\ 1981, \araa, 19, 231
\bibitem[]{}
Rogers, A. E. E., Doeleman, S., Wright, M. C. H., Bower, G. C.,
Backer, D. C., Padin, S., Phillips, J. A.,  Emerson, D. T., et al.\
1994, \apj, 434, L59
\bibitem[]{}
Sevenster, M. N., Chapman, J. M., Habing, H. J., Killeen, N. E. B.,
\& Lindqvist, M.\ 1997, \aaps, 122, 79
\bibitem[]{}
Shiki, S., Ohishi, M., \& Deguchi, S.\ 1997, \apj, 478, L206
\bibitem[]{}
Sjouwerman, L. O., van Langevelde, H. J., Winnberg, A.,
\& Habing, H .J.\ 1998, \aaps, 128, 35
\bibitem[]{}
Tamura, M., Werner, M. W., Becklin, E. E., \& Phinney, E. S.\ 1996,
\apj, 467, 645
\bibitem[]{}
Tsuboi, M., Kasuga, T., Ohno, T., \& Abe, Y.\ 2000,
Proc. of SPIE Conf., Astronomical Telescopes
and Instrumentation 2000, 4015, 278
\bibitem[]{}
Tsuboi, M., Handa, T., \& Ukita, N.\ 1999, \apjs, 120, 1
\bibitem[]{}
Winnberg, A., Baud, B., Matthews, H. E., Habing, H. J.,
\& Olnon, F. M.\ 1985, \apj, 291, L45
\bibitem[]{}
Wood, P. R., Habing, H. J., \& McGregor, P. J.\ 1998,
\aap, 336, 925
\end{thebibliography}


\clearpage
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   Table
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%
% Table 1
\begin{table*}[t]
\small
\begin{center}
Table~1.\hspace{4pt}
Line parameter of the detected SiO maser sources. \\
\end{center}
\vspace{6pt}
\begin{tabular*}{\textwidth}{@{\hspace{\tabcolsep}
\extracolsep{\fill}}p{1pc}ccccccccc}
\hline\hline\\[-6pt]
&& \multicolumn{4}{c}{SiO {\it J}=1--0 {\it v}=1} &
\multicolumn{4}{c}{SiO {\it J}=1--0 {\it v}=2} \\
&&\multicolumn{4}{c}
{---------------------------------------------}&
\multicolumn{4}{c}
{---------------------------------------------}\\
No. & name &
$T_{\rm a, peak}^*$ & $V_{\rm LSR}$ & $I$ & rms &
$T_{\rm a, peak}^*$ & $V_{\rm LSR}$ & $I$ & rms \\
[4pt]\hline \\[-6pt]
& & (K) & (km$\:$s$^{-1}$) &
{\footnotesize (K~km$\:$s$^{-1}$)} & (K)
& (K) & (km$\:$s$^{-1}$) &
{\footnotesize (K~km$\:$s$^{-1}$)} & (K)\\
1 & 000.062--00.006 &
0.39 & 21.8 & 1.08 & 0.049 & 0.30 & 21.8 & 0.79 & 0.052 \\
2 & 359.939--00.010 &
0.52 & --73.7 & 1.11 & 0.084 & 0.52 & --73.5 & 1.11 & 0.082 \\
3 & 359.966+00.002 &
1.03 & --7.4 & 2.35 & 0.054 & 0.65 & --8.1 & 2.23 & 0.053 \\
4 & 000.002--00.005 &
0.31 & --6.6 & 1.13 & 0.057 & 0.32 & --4.8 & 1.32 & 0.060 \\
5 & 359.914--00.060 &
0.51 & --309.1 & 1.33 & 0.062 & $\sim$0.35 &
--308.6 & 0.77 & 0.073 \\
6 & 359.928--00.063 &
$\ltsim$0.18 & --92.3 & $\ltsim$0.48 & 0.068 &
0.46 & --92.9 & 1.34 & 0.070 \\
7 & 359.953--00.040 &
0.58 & 71.5 & 1.27 & 0.051 & 0.53 & 71.6 & 1.68 & 0.056 \\
8 & 359.944--00.047 &
0.50 & --27.3 & 1.22 & 0.055 & 0.77 & --27.3 & 1.72 & 0.059 \\
9 & 359.954--00.048 &
$\sim$0.26 & 52.1 & 0.67 & 0.069 &
$\sim$0.29 & 52.7 & 1.67 & 0.069 \\[4pt]
\hline
\end{tabular*}
\end{table*}


%%%%%%%%%%%%%%%%%%%%%%%%%%
% Table 2
\begin{table*}[t]
\small
\begin{center}
Table~2.\hspace{4pt}
Positions of the detected SiO maser sources
in the Galactic center. \\
\end{center}
\vspace{6pt}
\begin{tabular*}{\textwidth}{@{\hspace{\tabcolsep}
\extracolsep{\fill}}p{1pc}ccccccl}
\hline\hline\\[-6pt]
No. & name & R.A.$^{\ast}$ & Decl.$^{\ast}$ &
R.A. error & Decl. error & remark & possible identification \\
[4pt]\hline \\[-6pt]
& & (h\ \ m\ \ s) & ($\,^{\circ}$\ \ $'$\ \ $''\,$)
& ($\;''\;$) & ($\;''\;$) & & \\
1 & 000.062--00.006 &17 42 37.1 & --28 52 01 &
2.2 & 3.2 & new & 1-72$^{\dagger\dagger}$\\
2 & 359.939--00.010 & 17 42 20.2 & --28 58 27 &
3.3 & 2.2 & new &
OH$\;$359.937--0.010$^{\ast\ast}$, 3-266$^{\dagger\dagger}$\\
3 & 359.966+00.002 & 17 42 21.3 & --28 56 40 &
2.6 & 1.5 & new & 2-18$^{\dagger\dagger}$\\
4 & 000.002--00.005 & 17 42 28.0 & --28 55 03 &
$\sim$20$^{\dagger}$ & $\sim$20$^{\dagger}$ & new &
OH$\;$359.998--0.005$^{\ast\ast}$, 2-1$^{\dagger\dagger}$\\
5 & 359.914--00.060 & 17 42 28.0 & --29 01 18 &
$\sim$20$^{\dagger}$ & $\sim$20$^{\dagger}$ &
new  & OH$\;$359.918--0.055$^{\ast\ast}$\\
6 & 359.928--00.063 & 17 42 30.8 & --29 00 41 &
$\sim$20$^{\dagger}$ & $\sim$20$^{\dagger}$ &
new  & OH$\;$359.931--0.063$^{\ast\ast}$\\
7 & 359.953--00.040 & 17 42 29.1 & --28 58 42 &
$\ltsim$5$^{\ddagger}$ & $\ltsim$5$^{\ddagger}$ &
known$^{\S}$ & OH$\;$359.954--0.041$^{\ast\ast}$\\
8 & 359.944--00.047 & 17 42 29.5 & --28 59 19 &
$\ltsim$5$^{\ddagger}$ & $\ltsim$5$^{\ddagger}$ &
known$^{||}$ & OH$\;$359.946--0.048$^{\ast\ast}$ \\
9 & 359.954--00.048 & 17 42 31.0 & --28 58 52  &
$\ltsim$5$^{\ddagger}$ & $\ltsim$5$^{\ddagger}$ &
known$^{\sharp}$ & OH$\;$359.956--0.050$^{\ast\ast}$ \\[4pt]
\hline
\end{tabular*}
\vspace{3pt}\par\noindent
$^{\ast}$ Epoch of 1950.  \\
$^{\dagger}$ These position errors indicate grid spacing
of survey observation.  \\
$^{\ddagger}$ Typical errors of the positions
that was estimated from the $5 \times 5$ mapping
with 30$''$ grid spacing (source \#7-\#9).  \\
$^{\S}$ Lindqvist et al. (1991), Izumiura et al. (1998)  \\
$^{||}$ IRS 10EE; Menten et al. (1997), Izumiura et al. (1998)  \\
$^{\sharp}$ Izumiura et al. (1998)  \\
$^{\ast\ast}$ Sjowerman et al. (1998);
We had adopted the VLA source in this catalog
if the sources had been detected
overlapping in both the Very Large Array (VLA)
and the Australia Telescope Compact Array (ATCA) data sets.  \\
$^{\dagger\dagger}$ Glass et al. (2001)
\end{table*}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   Figure Captions
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\clearpage
%%%%%%%%%%%%%%%%%%%%%%%%%%
% Fig.1
\begin{figure}
   \begin{center}
%    \FigureFile(75mm,75mm){figure1.eps}
\includegraphics[width=75mm,height=75mm,keepaspectratio,clip]{figure1.eps}
   \end{center}
   \caption{
Surveyed area overlaid
on the CS {\it J}=1--0 integrated intensity map
in the velocity range of $V_{\rm LSR} = \pm200$~km$\:$s$^{-1}$
toward the Galactic center (Tsuboi et al. 1999).
The grid points of the observation are shown by crosses,
and the positions of the detected SiO maser sources
are shown in open circles.
}
\end{figure}

\clearpage
%%%%%%%%%%%%%%%%%%%%%%%%%%
% Fig.2
\begin{figure*}
   \begin{center}
%    \FigureFile(170mm,200mm){figure2.eps}
\includegraphics[width=170mm,height=200mm,keepaspectratio,clip]{figure2.eps}
   \end{center}
   \caption{
SiO {\it J}=1--0 {\it v}=1 and 2 spectra for the detected sources
in the Galactic center region.
The spectra for sources \#1--\#8
were taken by high-resolution spectrometers.
The spectra for source \#9
were taken by wide-band spectrometers.
The spectra for source \#9
involves the SiO lines from sources \#7 and \#8
at $V_{\rm LSR}=$ 71 and $-28$ km$\:$s$^{-1}$, respectively,
because separations from \#9 to \#7 and \#8 are small.
}
\end{figure*}

\clearpage
%%%%%%%%%%%%%%%%%%%%%%%%%%
% Fig.3
\begin{figure*}
   \begin{center}
%    \FigureFile(170mm,75mm){figure3.eps}
\includegraphics[width=170mm,height=75mm,keepaspectratio,clip]{figure3.eps}
   \end{center}
   \caption{
Illustration of the 5 point mapping for the case of source \#3.
The center position is at
R.A.=$17^{\rm h}42^{\rm m}22^{\rm s}$,
Decl.=$-28^{\circ}56'48''$ (B1950).
The source position was found to be
$\Delta\alpha=-10.\hspace{-2pt}''6$,
$\Delta\delta=+8.\hspace{-2pt}''3$
shifted from the center.
}
\end{figure*}

\clearpage
%%%%%%%%%%%%%%%%%%%%%%%%%%
% Fig.4
\begin{figure}
   \begin{center}
%    \FigureFile(75mm,200mm){figure4.eps}
\includegraphics[width=75mm,height=200mm,keepaspectratio,clip]{figure4.eps}
   \end{center}
   \caption{
Longitude--velocity ($l-V$) diagram
for the detected SiO sources (open circles),
which was overlaid on the $l-V$ diagram
averaged in the range of $-11' \le b \le +5'$
of the CS $J=$1--0 emission line (Tsuboi et al. 1999).
A least-squares fitting for all SiO sources
and those excluding the extreme source \#5
are shown by the thin- and thick-solid line, respectively.
The triangles show the positions
of the OH 1612~MHz masers (Sjouwermen et al. 1998);
the filled triangles and open triangles indicate the sources
that have positions located
in and outside of the present survey area, respectively.
The thin- and thick-dashed lines show
least-squares fitting for the OH maser samples
including and excluding the counterpart (OH$\;$359.918--0.055)
of source \#5 and the sources outside the present survey area,
respectively.
}
\end{figure}


\end{document}