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    {\small\rm\zihao{6}\songti\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
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    {\small\rm \zihao{6}\songti suppl.\hfill Y. Wu: Studies for High-mass Star Formation  %%�����ߣ���Ŀ���ƣ�
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\zihao{5}\songti

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    \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
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\vs\vs\vs \bc {\Large\bf Studies for High-mass Star Formation with
\\[2mm] Millimeter
and Centimeter Emissions   %%�����⣩
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize Yuefang Wu}  %%���������ߣ�

\vs
(\footnotesize  \pka)\\
({\footnotesize  {\it CAS-PKU Joint Beijing Astrophysics Center,
Beijing}~ 100871})

\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
Studies of high-mass star formation are less advanced than those
of low-mass stars, though they started earlier. Many unexpected
discoveries from centimeter(cm)-millimeter(mm) emission
observations have been made on the characteristics of high-mass
forming regions. Ultra compact (UC) HII regions have been examined
adequately. With the advancement of the understanding of low-mass
star formation and the development of detection technology,
remarkable progress has been obtained in the examination of
dynamic processes and the searches for early evolutionary phases.

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

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

\vs\vs Studies of star formation is advancing rapidly since radio
emission was employed widely in astronomy and since the mm
emission lines were found in the 60\,s of the past century. But
such advancement is mainly on low-mass star formation (van der Tak
et al. 2000). Abundant objects with various observation
characteristics such as Bok globules, T Tauris, Fu Oris and a
large number of infrared sources with different SED classes have
been observed. Meanwhile a single evolution route was established
and a clear picture of low-mass star formation has been
obtained(Shu et al. 1985). For high-mass star formation our
knowledge is less though studies actually began earlier than those
of low-mass ones mainly because of the relatively small population
of high-mass stars, long distances and high complexity of their
regions and the rapidity of their evolutionary processes. As radio
and mm technologies are developing, attention is dedicated to
high-mass star formation and new progress is being made.

\para{\large\bf{2}~~ Early discoveries and high-mass star formation regions}

\vs\vs Four decades ago, interstellar molecules were found by the
means of radio emission. Then, star formation studies began a new
era and developed rapidly. Many discoveries with cm-mm emission
lines were unexpected and many concerned with high-mass star
formation. We present some examples in Table 1. The first column
presents the name of object or phenomenon discovered for the first
time; Column 2 and 3 list the year and the authors; Column 4 lists
the lines. The last column presents the regions.

One can see from Table 1 that these discoveries were made with
cm-mm emission, including continuum and lines, thermal and
non-thermal, all in massive star formation regions. These
phenomena indicate that the corresponding regions are dense and
rather cold. For example, in the NH$_3$ emission regions, the
kinetic temperature is 23K and the column density is
2$\times$10$^{16}$\,cm$^{-2}$(Cheung et al. 1968). CO maps of
these regions are large and complex (Habing $\&$ Israel 1979 and
the reference there). Massive stars are formed in clusters. More
distant than low-mass star forming regions. For example, Orion
where there are high-mass stars being formed is about 500\,pc,
while low-mass star formation regions T Tauri are 140\,pc. Most of
the high-mass star formation regions are farther away from us than
Orion. The distribution of different mass molecular outflows also
shows the distance difference between high-mass and low-mass young
stellar objects  (Wei \& Wu in this proceeding).

\begin{center}
{\small\bf Table1.  Examples of early discoveries}\\[-0.5mm]
\end{center}
\small
\begin{tabular}{rrrrrrllllr}
\hline
\hline\\[-0.5mm]
{\parbox[t]{40mm}{\centering Object or phenomenon
\\discovered}}& {\parbox[t]{7mm}{\centering
year}}& {\parbox[t]{30mm}{\centering Authors}} &
{\parbox[t]{25mm}{\centering Emission}} &


{\parbox[t]{10mm}{\centering
Regions}}\\[1mm]
\hline\\
{\parbox[t]{40mm}{\centering molecule OH\\the first radio molecule\\found in ISM}}&{\parbox[t]{7mm}{\centering 1963}}&{\parbox[t]{30mm}{\centering Weinreb et al.}}&{\parbox[t]{25mm}{\centering 1665/67MHz\\18cm absorption}}&{\parbox[t]{10mm}{\centering Cas A}}\\[1mm]
\hline\\
{\parbox[t]{40mm}{\centering Ammonia molecule NH$_3$\\The first polyatomic molecule\\found in ISM}}&{\parbox[t]{7mm}{\centering 1968}}&{\parbox[t]{30mm}{\centering Cheung et al.}}&{\parbox[t]{25mm}{\centering 23.69GHz\\1.3 cm emission lines}}&{\parbox[t]{10mm}{\centering Galactic center}}\\[1mm]
\hline\\
{\parbox[t]{40mm}{\centering H$_2$CO \\The first organic molecule\\found in ISM}}&{\parbox[t]{7mm}{\centering 1969}}&{\parbox[t]{30mm}{\centering Synder et al.}}&{\parbox[t]{25mm}{\centering 4829.7MHz\\6cm lines}}&{\parbox[t]{10mm}{\centering radio sources}}\\[1mm]
\hline\\
\normalsize {\parbox[t]{40mm}{\centering OH maser\\ The first
molecule maser\\found}}&{\parbox[t]{7mm}{\centering
1965}}&{\parbox[t]{30mm}{\centering Weaver et al.}}&{\parbox[t]{25mm}{\centering 1665MHz\\non-thermal emission \\18cm lines}}&{\parbox[t]{10mm}{\centering W$_3$(OH)}}\\[1mm]
\hline\\
{\parbox[t]{40mm}{\centering CO \\The most abundant molecule(except H$_2$)}}&{\parbox[t]{7mm}{\centering 1970}}&{\parbox[t]{30mm}{\centering Wilson et al.}}&{\parbox[t]{25mm}{\centering 115GHz\\2.6 mm lines}}&{\parbox[t]{10mm}{\centering Orion}}\\[1mm]
\hline\\
{\parbox[t]{40mm}{\centering CO molecular
outflows}}&{\parbox[t]{7mm}{\centering 1976}}&
{\parbox[t]{30mm}{\centering Zuckerman $\&$ Palmer 1975
\\Kwan $\&$ Scoville 1976 \\Zuckerman et al. 1976
}}&{\parbox[t]{25mm}{\centering 115GHz\\2.6 mm lines}}&{\parbox[t]{10mm}{\centering Orion}}\\[1mm]
\hline\\[2.0mm]
\end{tabular}

\para{\large\bf{3}~~ UCHII regions and high-mass YSO evolutionary time}

\vs\vs
 A discovery was made also from studies in
centimeter emission. Ryle and Downes (1967) obtained the E. M. =
5$\times$10$^7$\,cm$^{-6}$pc in DR21, a 6cm emission source.
Mezger et al. (1967) also got E. M. =10$^8$\,cm$^{-6}$pc in W49A.
These require ionization of one or more O type stars (Habing $\&$
Israel 1979). A new class of Galactic source HII regions were also
found with infrared emission. Actually, the newly formed stars are
bright. The surrounding dust absorbs UV or optical emission and
re-emits at longer wavelengths. The SED of UCHII regions is shown
in Fig.1 (Churchwell 1999).
% insert figure 11.ps here

\begin{figure}[h]
%\center {\includegraphics [bb=0 0 408 232,scale=0.8]{11.eps}}
\center {\includegraphics [scale=0.7]{f1.eps}}

   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,1~~ A schematic of the
spectral energy distribution of a UCHII region from radio to the
near infrared. (Churchwell,E., 1999)}

    \end{figure}

UCHII regions usually have electron densities of
10$^5$\,cm$^{-3}$, E. M. of $~$10$^8$\,cm$^{-6}$pc, life time of
$~$10$^5$\,yrs, and sizes of $~$10$^{16}$\,cm. Their SED was
explained with various models (Churchwell 1999). For the
morphologies of UCHII regions, Wood and Churchwell (1989) divided
them into cometary(20\%), core-haro (16\%), shell-like(4\%),
irregular or multiply peaked (17\%), spherical or unresolved
(43\%). UCHII regions also received much theoretical attention
(Churchwell, 1999). Fig.2 is the in-falling model, explaining the
observation characteristics, whose density and pressure increase
at the I-front of the HII region (Churchwell 1999 and the
reference there).

UCHII regions were thought as a sign post of high-mass star
forming processes. But the stars inside are already on the main
sequence while still surrounded by the materials of the parent
cloud. The Young-Stellar-Object (YSO)-shell system means that the
in-fall materials of the cloud are really slower than the K-H
contract. The vicinity of the star was destroyed by ionization,
which makes it difficult to catch the star's early evolutionary
characteristics.

\para{\large\bf{4}~~ A Current Bloom}

\vs\vs Efforts were made to resolve the forming regions to see
whether high-mass stars have the same formation procedures as
low-mass stars. A number of typical and relatively isolated
sources were found, which are good examples with different
evolutionary characteristics. IRAS 23385+6053 was identified as a
massive class 0 object (Molinari et al. 1998a). A bipolar outflow was also
found in this source (Yang $\&$ Wu, 2000). In W51, G192.16-3.82
and IRAS 20126+4104, in-fall and/or disc were detected (Zhang et
al. 1998; Shepherd $\&$ Kurtz 1999; Cesaroni et al. 1997).
Recently, IRAS 00117+6412 and IRAS 02461+6147 were found as
isolated sources (Shi et al. \& Zhao et al. in this proceeding ).
Particularly, high velocity molecular outflows are found
frequently near massive YSOs. In a sample of 39 sources Zhang et
al. $($2001$)$ measured 35 outflows. Among 26 high-mass star
formation regions, Beuther et al.(2002a) found 21 bipolar
outflows. More than 350 molecular outflows have been found till
June of 2002(Wu et al. 2002a), about $\sim$100 of them are
associated with infrared source with the bolometric luminosity
greater than 10$^3$ L$\odot$. They may be high-mass YSOs.
Considering the smaller population of high-mass stars, the
detection rate of outflows in high-mass star formation regions is
similar to that in low-mass ones. These should be evidence that
high-mass star formation also proceeds through accreting rather
than coalescing. Wang (2002) recently suggested a route of massive
star formation.

Searching for early evolutionary phase is another focus in the
current studies of massive star formation. Hot cores are observed.
They are dense, $\sim$10$^6$\,cm$^{-3}$ and hot, with temperature
of $\sim$100-200\,K (van der Tak et al. 2000; Cesaroni et al.
1992). They are close to HII regions, but do not overlap with HII
regions. Fig.3 is the contours of CH$_3$OH J=12-11 integrated
intensity superpost on the 3.6\,cm continuum images (Wilner et al.
2001 and the reference there). The hot core lifetime is most
likely to be shorter than, but comparable to that of the UCHII
regions. CH$_3$OH J=12-11 and dust images show that hot cores may
be produced by young stellar sources precursors to O-type stars
(Wilner et al. 2001).

Recently Molinari et al. (1996, 1998b, 2002) and Zhang et al.
(2001) have investigated 163 IRAS sources. Their samples exclude
known HII regions and with flux density at 60\,$\mu$m $\geq$100Jy.
The IRAS colors are:
$$0.61\leq[60-25]\leq1.74$$
$$0.087\leq[100-60]\leq0.52$$

They divided the sources into a high group and a low group with
[25-12]$>$0.57 or $<$0.57 respectively. They have examined the two
group sources with cm$-$mm$-$submm emissions. Fig.4 shows their
observation chart. The left column lists the sources and results
for the high group. The right column lists similar facts for the
low group. The middle column lists the items of results. In
conclusion they obtained 11 sources which are without free-free
emission and detected with mm continuum emission (one of the 12
sources was detected at 3.6\,cm later). Their dust temperature
ranges from 24\,K to 45\,K, and their luminosity is $\sim$
10$^4$L$_{\odot}$. The outflow detection rate is 90$\%$ (Zhang et
al. 2001); the exponent of the dust emission vs frequency
power-law of these sources indicates that these sources may be
younger than those with radio continuum. These sources may be
precursors of UCHII regions.

% insert figure 22.ps here
\begin{figure}[th]
%\center {\includegraphics [bb=0 0 260 291,scale=1]{22.eps}}
\center {\includegraphics [scale=0.6]{f2.eps}}
\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize
Fig.\,2~~ A schematic illustrating the confinement of a UCHII
region by dense, ambient, molecular gas which provides the thermal
pressure required for confinement. (Churchwell,E., 1999)}
\end{figure}

Sridharan et al. (2002) and Buther et al. (2002b) have
investigated another group with 69 IRAS sources. The samples
satisfy the criteria of UCHII regions and have CS J=2-1 emission.
The samples are also without known HII regions and bright with
flux density of f$_{100}$ $>$500Jy and f$_{60}$ $>$90\,Jy. They
observed continuum and spectral lines at millimeter and radio
wavelengths, including 3.6\,cm, 1.2\,mm, CO J=2-1, NH$_3$, H$_2$O
and CH$_3$OH maser emissions. They found most of the sources are
prior to UCHII regions. Rotation temperature from NH$_3$ (1, 1)
(2, 2) is $\sim$20\,K. The dust temperature is $\sim$40\,K. The
analysis of radial intensity profiles indicates that the inner
regions are more quiescent. The outflow detection rate is 81\%
(Burther et al. 2002a) and the detected H$_2$O and CH$_3$OH masers
are 29 and 26 respectively. Those sources are similar to those of
Molinari et al. (2000). Plume et al. (1997, 1992), Zinchenko et
al. (1998) and Wu et al. (2001) also studied massive YSOs or
massive dense cores. Samples of all these surveys are bright and
without an upper limit of flux density of IRAS sources.
\begin{figure}[th]
%\center {\includegraphics [bb=0 0 361 490,scale=0.8]{33.eps}}
\center {\includegraphics [scale=1.5]{f3.ps}}
\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize
Fig.\,3~~ Images of CH$_3$CN J=12-11 integrated intensity
superposed on the 3.6\,cm continuum image. (Wilner et al. 2001 \&
the reference there. )}
\end{figure}
\begin{figure}[th]
\center {\includegraphics [bb=147 107 358 474,scale=1]{44.eps}}
\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize
Fig.\,4~~ Flow-chart for the selection of the sample of
intermediate-to-high mass candidate protostellar objects.
(Molinari, S. et al. 2000)}
\end{figure}

Then a question is arising: how to find a much earlier
evolutionary phase of the high-mass stars, which means earlier
than the precursors of UCHII regions as mentioned above?

We recently have surveyed 22 IRAS sources with $^{13}$CO and
C$^{18}$O $J=1-0$ lines (Wu et al. 2002b). The sources are red,
with $[$25$-$12${]}>$0.7 and $[$60$-$12${]>}$1.4. They are not
known HII regions and not associated with low-mass sources. An
upper limit of IRAS flux density was set: f$_{100}$ $<$500Jy. All
the sources were detected with the pair line emissions. The column
densities of C$^{18}$O are $\geq$ 10$^{15}$ cm$^{-2}$. The
excitement of C$^{18}$O is $\sim13K$. We examined the sources with
MSX data. The middle infrared emission of these sources are very
weak. The MSX and IRAS energy distributions show that the emission
peak of these sources falls in far infrared or longer wavelength.
All these facts show that these sources are cold, dense and young.
We judge whether they are massive or not from line widths and
bolometric luminosity. According to Myers et al. (1983) and Wu et
al. (2001), the widths of $^{13}$CO lines are $\sim$1.3\,km/s and
$\sim$3.0\,km/s for low-mass dense cores and massive dense cores
respectively. For our sources there are 11 with with $^{13}$CO
line width $\geq$3.0\,km/s. According to Beichman et al.
$($1986$)$ the bolometric luminosity of low-mass sources are
usually $<$50L$\odot$. Among the 11 sources with the line widts
$\geq$3.0\,km/s, there are 7 with bolometric luminosity
$>$50L$\odot$, which may be high-mass or intermediate-mass YSOs.
Of course, mapping with some dense molecular probes, mm and sub-mm
continuum emission observations will be helpful for further
identification of these sources.

Recently, Yang et al. (2002) surveyed 1331 cold IRAS sources with
CO J=1-0 line, which may contain very young stellar objects. Li \&
Goldsmith (2000) have detected molecular clumps with temperature
inside lower than that of outside, showing these regions to be
at very early evolution phase.
\newpage
\para{\large\bf{5}~~ A Bright Future}

\vs\vs Recently, unresolved problems of high-mass star formation
draw much attention, such as initial conditions, formation of ISM
fragmentations, formation of clusters, formation dynamic process,
driving mechanics of outflows, as well as roles and evolution of
discs. These make the studies of high-mass star formation a real
hot topic in the astrophysical frontier. With the availability of
new and fine equipments such as SMA, SIRTF, Herschel and ALMA, the
mysteries of high-mass star formation are going to be revealed.
With the domestic cm-mm equipments and international opening
equipments we have much to explore in this field.

\vs\vn{\bf ACKNOWLEDGEMENTS}~~ I thank Wentao Yu, Jarken Esimbek
and Ming Zhao for their assistance in the preparation of the talk
and the proceeding paper. The project is supported by the
G1999075405 NKBRSF, the NSFC Grant 10128306, 10133020 and
10203003.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

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\nref  Mezger P.G., Schraml J., Terzian Y., ApJ, 1967, 150, 807
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%\end{CJK}
\end{document}

\bc{\small\bf Table 1~~ Examples of early discoveries
}\\[2mm]

\fns \tabcolsep 1mm
\begin{tabular}{ccccc}
\hline\\[-3.6mm]\hline
 Object or phenomenon       & year &  Authors    & Emission    &Regions\\
 discovered                 &      &             &             &\\
 molecule OH                &1963  & Weinreb et al. & 1665/67\,MHz&  Cas A\\
 The first radio molecule   &      &             &18\,cm
 absorption&\\
 found in ISM               &      &             &             &\\
  Ammonia molecule NH$_3$   &1968 &  Cheung et al. &  23.69\,GHz&Galactic \\
  The first polyatomic      &     &               &1.3 cm emission
  lines&center\\
  molecule found in ISM     &     &              &
  &\\
 H$_2$CO The first organic& 1969& Synder et al. &4829.7\,MHz
 &radio\\
 molecule found in ISM   &      &               &6\,cm lines &
 sources\\
 OH maser The first&1965 &  Weaver et al. &  1665\,MHz
 non-thermal&W$_3$(OH)\\
molecule maser found &   &      &emission 18\,cm lines&\\
CO The most abundant&1970 &  Wilson et al. & 115\,GHz&   Orion\\
 molecule(except H$_2$) & &     & 2.6\,mm lines\\
  CO molecular outflows & 1976 &Zuckerman \& Palmer 1975&
  115GHz&Orion\\
                       &       &Kwan\&Scoville 1976&2.6\,mm lines&\\
             &&          Zuckerman et al.
                       1976&&\\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec
