\documentstyle[twoside,epsfig,CJK]{carticle}
%\documentclass[twoside]{carticle}
%\usepackage{epsfig}
\pagestyle{empty}
\headsep=0.6cm %%%%%%%

\setcounter{page}{80}

%\input cchead.sty
%\input Vatola.sty

\def\thefootnoot{}

\textwidth=140mm \textheight=210mm \topmargin=0pt
\oddsidemargin=0pt \evensidemargin=0pt \footskip 0.2 truecm

%%...................
%\vspace*{1\baselineskip}
\doublerulesep=0pt
%%..................
% \renewcommand{\baselinestretch}{1.4} %%cc
\parindent=7.5mm

\setlength{\parskip}{0 true pt}
    \def\hang{\hangindent\parindent}
    \def\RE{\par\hang\textindent}
    \catcode`@=11
%%%双页
    \def\@evenhead{\underline{\parbox{\textwidth}
    {\small\rm\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
%%%单页
    \def\@oddhead{\underline{\parbox{\textwidth}
    {\small\rm  suppl.\hfill S. Luo: The Radio Identification of New
    Planetary Nebulae  %%（作者：题目名称）
    \vspace{1.5mm}
    \hfill\thepage}}} %%%%%%%
    \normalsize\rm
\zihao{5}\songti

  %  \begin{CJK}{GB}{song}
    \begin{document}


%%篇首
    \thispagestyle{empty}
    \vspace*{-1.5 true cm}
    \small\rm
    \zihao{6}\songti
    \noindent
    \\%%%[-0.5 true mm]
    Vol.44~ Suppl.
    \hfill\hspace{8mm} %%% if ``October/January'' then \hspace{8/10 mm} July
    \small ACTA ASTRONOMICA SINICA
    \hfill
      Feb.,~2003 \\[-1.5 true mm] %%January
    \rule {\textwidth}{0.12 true mm}\\[-3.2 true mm]
    \rule {\textwidth}{0.12 true mm}
%%%%%%%

\input{../newcom.ctx}

\renewcommand{\baselinestretch}{1}

\setcounter{footnote}{1}

\vs\vs\vs \bc {\Large\bf The Radio Identification of New
    Planetary Nebulae    %%（文题）
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize Shaoguang Luo}  %%（本文作者）

\vs ({\footnotesize{\it Astronomy Department, School of Physics,
Peking University,
Beijing }~ 100871})\\
(E-mail: lsg@bac.pku.edu.cn) %%（本文作者单位）
\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
Using 1.4\,GHz NRAO VLA Sky Survey (NVSS) data base, over 200
newly identified planetary nebulae with J2000 declination
$b\geq-40^{\circ}$ have been found to be radio sources at 20\,cm.
%A discussion about the optical depth at radio frequencies has
%also been given.
It is shown that the radio emission at 20\,cm is optical thick for
most of the planetary nebulae. This effect has to be taken into
account when using radio flux and H$_\alpha$ flux to derive the
optical extinction of the sources.

 %\vn
 %\no{\bf Key words}~~  pulsar-mode  change-spectrum
\end{quotation}

%%%% 正文
 \para{\large\bf{1}~~ Introduction}

\vs\vs
 Most planetary nebulae are weak radio sources.  Thermal radio
emission is an useful tracer of the ionization degree of a nebula.
Because the radio emission is not affected by dust grains, it is also used
to get the interstellar extinction by comparing the radio flux and optical
flux of H Balmer lines. For these and other reasons, there have been
several large-scale radio surveys of planetary nebulae before 1990
(See Milne \& Aller 1982, Zijlstra, Pattasch, \& Bignell 1989;
Aaquist \& Kwork 1990).

A survey covered nearly all of the sky north of $Dec\geq-40^{\circ}$
was taken by Condon et al. (1998) at 1.4 GHz using VLA (NVSS).
Using this data  base, 702 radio sources have been identified
(Condon \& Kaplan 1998; Condon et al., 1999) for
885 planetary nebulae listed in the Strasbourg-ESO
Catalogue of Galactic Planetary Nebulae (Acker et al. 1992,
hereafter GPN92).

Recently a lot of efforts have been put to
find new planetary nebulae.
Kohoutek (2001)  have edited a new PN catalogue including 1510 PNe updated
to 1999.  Parker et al. (1998) have undertaken a high resolution, narrow-band
$H_\alpha$ survey of the Southern Galactic Plane.
In the CDROM version released in 2001 (Parker and Phillipps, 2001), 1214
PNe are included.
We have compared these two catalog with the GPN92 and found
that 919 enters of $Dec\geq-40^{\circ}$ are new,  569
from Parker's catalogue and 382 from Kohoutek's catalogue
(32 listed in both catalog). Besides, some new observations of
planetary nebulae have been carried out recently by
Cappellaro et al.(2001), Kohoutek (2002), Van de Steene \& Jacoby (2001).
After comparing these data with  Parker's and Kohoutek's catalog
as well as GPN92, 67 sources are found to be new.

If these new  planetary nebulae were identified with radio data,
the number of known radio planetary nebulae would be doubled.
We are trying to find the radio correspondents
of these new optical discoveries using the NVSS data. Though the
detected radio emission did not necessary excludes the contaminations by
coincident objects, it increases the reliable radio sources as a
planetary nebulae significantly.
Besides, the radio data would also give some information
about the optical extinction of sources if their fluxes of H Balmer
lines are available.

\newpage\vspace*{1mm}\para{\large\bf{2}~~The Radio Identification of Newly Found
Planetary Nebulae}

\vs\vs The radio identification was made by comparing the optical
positions of new planetary nebular candidates and those in NVSS
data base. Usually, when the position difference between optical
and radio source is less than 20'', they are identified to be the
same source. Over 200 NVSS sources have been identified to be
associated with newly discovered planetary nebular candidates. For
about 80 per cent of the identified sources, the position
differences between optical and radio centers are less than 10''.
For about 30 sources these differences are somewhat larger than
20'' but less than 45''. They are probably the same source because
of the complex in structure or relatively diffuse in emission. The
optical and radio positions as well as fluxes at 20\,cm for 36
sources are listed in Table 1. A complete catalogue of newly
identified radio sources would be published elsewhere.

\bc{\small\bf

 Table 1~~   The radio identification of planetary
nebulae
}\\[2mm]

\fns \tabcolsep 2mm
\begin{tabular}{lcrrrrrr}
\hline\\[-3.6mm]\hline
 & \multicolumn{2}{c}{Optical Position(J2000)} &\multicolumn{2}{c} {Radio Position(J2000)} &$S_{\rm {20}}$ &$S_{\rm {6}}$&Note \\
Name    & RA  &         DEC  &   RA   &      DEC     & (mJy) &(mJy)& \\
 \hline
G124+02.1   &  01 02 24.76 &   65 46 35.9 &  01 02 23.18 &   65 46 33.7 &  7.4 &  &  2\\
G129-02.1   &  01 42 34.11 &   60 09 06.5 &  01 42 34.63 &   60 09 00.8 & 27.7 &  &  2\\
G193-04.1   &  05 57 08.97 &   15 25 28.7 &  05 57 08.42 &   15 25 53.1 &  6.5  &  &  2\\
G212.9-03.7 &  06 38 09.80 &  -01 42 26.0 &  06 38 09.77 &  -01 42 19.8 &  4.3  &  &  1 \\
G222.8-04.2 &  06 54 13.50 &  -10 45 39.0 &  06 54 13.26 &  -10 45 33.8 &  2.7  &  &  1,2,3 \\
G217.3-01.3 &  06 54 32.60 &  -04 31 45.0 &  06 54 32.44 &  -04 31 22.6 & 76.7  & 204& 1,5\\
G217.3-00.0 &  06 59 16.00 &  -03 59 37.0 &  06 59 15.68 &  -03 59 36.8 & 53.0  &  &  1 \\
G223.6-02.2 &  07 03 17.30 &  -10 35 06.0 &  07 03 17.58 &  -10 35 05.7 & 45.8  &  &  1 \\
G222.6-01.6 &  07 03 27.50 &  -09 23 04.0 &  07 03 28.20 &  -09 22 31.1 &  2.8  &  &  1 \\
G225-02.1   &  07 06 44.82 &  -11 45 21.8 &  07 06 44.96 &  -11 45 18.9 & 12.4  &  &  2\\
G234.2-01.4 &  07 26 16.30 &  -19 36 18.0 &  07 26 15.82 &  -19 36 37.9 &  2.8  &  &  1 \\
G235.0-01.4 &  07 27 46.80 &  -20 19 11.0 &  07 27 48.32 &  -20 19 45.8 &  4.0  &  &  1 \\
G235.7-01.2 &  07 30 06.10 &  -20 44 59.0 &  07 30 06.24 &  -20 44 48.1 &  5.3  &  &  1 \\
G247.2-07.2 &  07 30 58.30 &  -33 43 05.0 &  07 30 57.17 &  -33 43 35.3 &  3.5  &  &  1 \\
G234.7-00.2 &  07 31 48.80 &  -19 27 34.0 &  07 31 49.20 &  -19 27 35.2 &  9.8  &  &  1 \\
G239.3-02.7 &  07 31 59.80 &  -24 39 07.0 &  07 32 00.35 &  -24 39 16.4 &  4.0  &  &  1 \\
G237.2-01.2 &  07 33 11.40 &  -22 07 39.0 &  07 33 12.79 &  -22 07 31.5 &  9.1  &  &  1 \\
G237.3-01.2 &  07 33 19.70 &  -22 11 06.0 &  07 33 19.81 &  -22 10 57.3 & 19.9  &  &  1 \\
G237.2-01.0 &  07 33 57.10 &  -22 00 04.0 &  07 33 57.02 &  -22 00 05.3 & 20.1  &  &  1 \\
G237.5+01.8 &  07 45 34.80 &  -20 51 43.0 &  07 45 34.83 &  -20 51 39.4 & 13.7  &  &  1 \\
G248.3-03.6 &  07 48 31.90 &  -32 58 19.0 &  07 48 32.08 &  -32 58 21.3 & 23.5  &  &  1 \\
G245.3-00.2 &  07 54 47.70 &  -28 36 43.0 &  07 54 46.27 &  -28 36 59.3 &  7.7  &  &  1 \\
G250.5-03.4 &  07 54 55.00 &  -34 44 13.0 &  07 54 55.83 &  -34 44 08.5 & 21.2  &  &  1 \\
G251-03.1   &  07 57 06.62 &  -36 06 51.6 &  07 57 06.69 &  -36 06 40.6 &  2.6 &  &  2\\
G246.0+01.2 &  08 02 35.20 &  -28 28 04.0 &  08 02 35.86 &  -28 27 56.3 &  3.2  &  &  1 \\
G250.6-00.6 &  08 06 24.50 &  -33 17 35.0 &  08 06 24.47 &  -33 17 34.2 &  5.5  &  &  1 \\
G255-03.1   &  08 06 28.45 &  -38 53 25.1 &  08 06 27.75 &  -38 53 21.1 &  2.8 &  &  2\\
G251.7+01.6 &  08 18 48.20 &  -32 58 37.0 &  08 18 50.96 &  -32 58 37.5 &  5.8  &  &  1 \\
G259+00.1   &  08 37 07.98 &  -39 26 28.5 &  08 37 10.30 &  -39 26 03.3 &  6.6 &  &  2\\
G255.9+03.9 &  08 39 31.90 &  -35 00 24.0 &  08 39 34.02 &  -35 00 57.6 &  2.7  &  &  1 \\
G342+15.1   &  15 58 46.81 &  -32 00 19.4 & 15 58 47.16 &  -32 00 15.5 &  4.4 &  &  2\\
G347.4+01.8 &  17 04 16.30 &  -38 19 59.0 & 17 04 16.89 &  -38 19 57.8 &  4.6  &  &  1 \\
G356.4+08.6 &  17 04 17.70 &  -27 03 27.0 & 17 04 17.78 &  -27 03 26.3 &  5.9  &  &  1 \\
G349.6+03.1 &  17 06 01.50 &  -35 44 34.0 & 17 06 01.23 &  -35 44 37.8 &  3.6  &  &  1 \\
G011+17.1   &  17 06 54.94 &  -09 47 03.9 & 17 06 54.76 &  -09 46 51.5 &  3.4 &  &  2\\
     K523   &  17 08 34.20 &  -35 48 07.7 & 17 08 34.36 &  -35 48 03.2 & 25.3 &  & 4\\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}

 \parbox[tb]{13cm} {\hspace{2.5mm}\footnotesize Note: 1. Parker et al.,2001; 2. Kohoutek,
2001; 3. Condon et al.,1999; 4. Kohoutek, 2002; 5. \\
                Wright et al., 1996 (PMN).}
\ec

 The fraction of sources with radio emission is only about one
fourth of the new planetary nebular candidates. This is much less
than that derived by Condon \& Kaplan (1998) and Condon et al.
(1999) for GPN92 sources. This is understandable because a large
fraction of new discoveries in optical are low surface brightness
as pointed out by Parker et al. (2001).

For these new planetary nebulae with radio emission at 20\,cm, we
have tried to find the corresponding radio flux at 5GHz from PMN
(Wright et al., 1996) and  GB6 (Gregory et al., 1996) catalog, as
well as the survey taken by Becker et al. (Becker et al., 1994)
for Galactic Plane. Only about 20 sources have been detected at
5GHz. The small detection ratio at 5\,GHz may be caused partly by
different sky coverage and low sensitivity.

\bc{\small\bf

 Table 2~~   The optical depth distribution at 1.4\,GHz and 5\,GHz
             for Siodmiak's sample
}\\[2mm]

\fns \tabcolsep 4mm
\begin{tabular}{rrrrrrrr}
\hline\\[-3.6mm]\hline
 $\tau$ &$ < 0.1$ &$ \le 0.2$ &$ \le 0.4$  &$ \le  0.6$ &$ \le 1.0$
 &$ \le 2.0$  & $ > 2.0$\\
\hline
 20cm & 52 &    19  &  28 &   27 & 40&  41&  57\\
 5cm&189 &   36 &   23  &  12  & 4  & 0  & 0\\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec

\bc{\small\bf

 Table 3~~  The optical depths at 1.4\,GHz and 5\,GHz
}\\[2mm]

\fns \tabcolsep 4mm
\begin{tabular}{rrrrrr}
\hline\\[-3.6mm]\hline
 Name &$S_{20\,cm}$ & $S_{6\,cm}$ & $S_{3\,cm}$   &
$\tau_{1.4}$ & $\tau_{5}$ \\
\hline
 Van 2 &   3.5 &   4.3 &  4.2   &   0.768 &   0.053 \\
 Van 3 &   5.5 &  12.4 & 10.0   &   2.585 &   0.178 \\
 Van 5 &   6.5 &  11.3 & 10.5   &   1.718 &   0.119 \\
 Van16 &  18.2 &  27.0 & 24.7   &   1.260 &   0.087 \\
 Van17 &   6.8 &   9.4 &  8.2   &   1.070 &   0.074 \\
 Van24 &   6.7 &  16.0 & 14.1   &   2.802 &   0.193 \\
 Van31 &  27.2 &  11.5 & $>3.0$  &    &    \\
 Van34 &   3.6 &   1.7 & $>1.5$  &    &    \\
 Van36 &  23.6 &  31.1 & 31.1   &   0.945 &   0.065 \\
 Van37 &  57.4 &  16.0 & $>5.5$  &    &    \\
 Van38 &   2.6 &   3.0 &  2.0   &   0.614 &   0.042 \\
 Van41 &   8.8 &  16.7 & 17.6   &   1.992 &   0.138 \\
 Van42 &   9.8 &  12.9 & 15.5   &   0.942 &   0.065 \\
 Van46 &  11.7 &   3.8 & $>1.8$  &    &   \\
 Van52 &   3.1 &  24.0 & 22.8   &  13.526 &   0.934 \\
 Van54 &  73.0 &  82.3 & 75.2   &   0.559 &   0.039 \\
 Van55 &   4.3 &   9.2 & 10.0   &   2.396 &   0.165 \\
 Van56 &   5.7 &  14.1 & 12.2   &   2.940 &   0.203 \\
 Van58 &  20.3 &  29.5 & 21.3   &   1.204 &   0.083 \\
 Van64 &  10.0 &  28.3 & 25.6   &   3.511 &   0.242 \\
 Van66 &  46.2 &  65.3 & 60.9   &   1.129 &   0.078 \\
 Van70 &  11.4 &  13.0 & 11.5   &   0.586 &   0.040 \\
 Van73 &   4.0 &  13.7 & 13.0   &   4.469 &   0.308 \\
 Van75 &   2.5 &  22.5 & 19.0   &  17.713 &   1.223 \\
 Van81 &   2.7 &  20.8 & 20.8   &  13.417 &   0.926 \\
 Van85 &   2.9 &   4.4 &        &   1.323 &   0.091 \\
 Van86 &   4.1 &   8.5 & $>2.5 $ &  2.287 &   0.158 \\
 Van97 &   7.8 &  11.5 & 11.8   &   1.243 &   0.086 \\
 Van98 &   4.5 &  21.8 & 24.0   &   6.911 &   0.477 \\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec

\para{\large\bf{3}~~The Optical Depth at Radio Wavelengths}

\vs\vs The optical extinction of a planetary nebula $C_\beta$ at
$H_\beta$ can be derived by comparing the radio flux and $H_\beta$
flux when the optical depth at observation frequency is much
smaller than 1. The error caused by this approximation is still
smaller than 10 per cent even the optical depth $\tau \sim 0.2$ at
radio frequency. However, at low frequency, the influence of
optical depth can not be ignored, especially for those source with
small angular radius.  Condon \& Kaplan (1998) have discussed this
problem and pointed that the radio flux observed at low frequency
should be corrected for by a factor of  ${\tau/(1-e^{-\tau}}$)
when the radio flux was used to derive the optical extinction. In
principle if the optical depth of a source at any frequency can be
determined independently, the expected $H_\beta$ flux can be
derived since both optical depth at radio frequency and $H_\beta$
flux have a same dependence on the emission  measure $E$ (here $E
=\int {n_e}^2dl$ ) if  the contribution of ionized He on the
electron  density $n_e$ can be ignored and the electron
temperature can be determined. From the observed flux and angular
radius of the source at some frequency, or from the fluxes
observed at two frequencies, the optical depth at any radio
frequency can be estimated. However, it is important to ensure
that the signals at each observation frequency  are from the same
space range when using the observation fluxes at these two
frequencies to determine the optical depths. Siodmiak \& Tylenda
(2002) have analysed radio fluxes of 264 planetary nebulae at
1.4\,GHz and 5\,GHz. Using these data, we calculated the optical
depths at both observation frequencies. The results were listed in
Table 2. It is remarkable that the optical depths are larger than
1 for about half the sources and less than 0.1 for about 20 per
cent of the sources at 1.4\,GHz. On the other hand, the optical
depths of 225 sources are less than 0.2 and larger than 0.6 for
only 16 sources at 5\,GHz.  This result implies that for most of
the sources optical thin approximation at 5\,GHz can be applied,
but at 1.4\,GHz only for a small number of the sources.

Van de Steene \& Jacoby (2001) have observed 64 newly identified
galactic bulge planetary nebulae at 5GHz and 10\,GHz using
Australia Telescope Compact Array (ATCA). For 29 of these 64
sources the radio fluxes at 1.4GHz were  found in NVSS data base.
Since the fluxes at 3\,cm is less well determined than that at
6\,cm as pointed by author, we only use their fluxes at 6\,cm and
fluxes at 20\,cm from NVSS to estimate  the optical depths. The
optical depth of 25 sources were derived and the results were
listed in Table 3. It is shown that for 18 sources, the optical
depths  is less than 0.2 at 6\,cm, but none is less than 0.5 at
1.4\,GHz for all 25 sources.  For another 4 source with flux at
1.4\,GHz two times larger than that at 5\,GHz, we noted that the
fluxes at 10\,GHz are lower than the detection limit of the ATCA
and therefore the radio emission may be intrinsically the
nonthermal-origin. Therefore, we can conclude that for most of the
planetary nebulae, they are optical thick at 20\,cm. A caution
must be paid when using the radio fluxes at this frequency to
derive the optical extinction.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

 \nref Aaquist O.B., Kwok S., A\&AS, 1990, 84, 229
\nref Acker A., Ochsenbein F., Stenholm R., et al., The
Strasbourg-ESO Catalogue of Galactic Planetary
           Nebulae (Garching: ESO)
\nref Becker R.H., White R.L., Helfand D.J., et al., ApJS, 1994,
91, 347 \nref Cappellaro E., Sabbadin F., Benetti S., et al.,
A\&A, 2001, 377, 1035 \nref Condon J.J., Cotton W.D., Greisen
E.W., et al., AJ, 1998, 115, 1693 (NVSS) \nref Condon J.J., Kaplan
D.J., ApJS, 1998, 117, 361 \nref Condon J.J., Kaplan D.J., Terzian
Y., ApJS, 1999, 123, 219 \nref Gregory P.C., Scott W.K., Douglas
K., et al., ApJS, 1996, 103, 427 (GB6) \nref Kohoutek L., A\&A,
2001, 378, 843 \nref Kohoutek L., AN, 2002, 323, 57 \nref Milne
D.K., Aller L.H., A\&AS, 1982, 50, 209
 \nref Parker Q.A., 1hillips S., A\&G, 1998, 39, 10
\nref Parker Q.A., Hartley M., Russeil D., et al.,
 2002, in the Edinburgh/AAO/Strasbourg Catalogue of
           Galactic Planetary Nebulae, Preliminary version 1.0, November 2001
\nref Siodmiak N., Tylenda R., A\&A, 2001, 373, 1032 \nref Van de
Streene G.C., Jacoby G.H., A\&A, 2001, 373, 536 \nref Wright A.E.,
Griffith M.R., Hunt A.J., et al., Astrophys. J. Suppl. Ser. 1996,
103, 145 (PMN) \nref Zijlstra A.A., Pottasch S.R., Bignell C.,
A\&AS, 1989, 79, 329

%\end{CJK}
\end{document}

