Download A Very Dense Low-Mass Molecular Condensation in Taurus

PASJ: Publ. Astron. Soc. Japan 51, 257-262 (1999)
A Very Dense Low-Mass Molecular Condensation in Taurus:
Evidence for the Moment of Protostellar Core Formation
Toshikazu ONISHI, Akira MIZUNO, and Yasuo FUKUI
Department of Astrophysics, Nagoya University, Chikusa-ku, Nagoya 464-8602
E-mail (TO): [email protected]
(Received 1998 November 10; accepted 1998 March 1)
Abstract
We present evidence for a protostellar condensation that is very close to the moment of the formation of
a protostellar core within a time scale of ~ 104 yr. This starless condensation, named MC 27 in complete
surveyed molecular condensations in Taurus, is ~ 0.1 pc in size and ~ 3M® in mass. It exhibits fairly
strong and narrow H 1 3 CO + emission of the J = 4-3, 3-2, and 1-0 transitions, as well as self-reversed
profiles of HCO+ J = 4-3 and 3-2. MC 27 has a density of - 106 cm" 3 within - 1000 AU at the center,
which is the highest value among the ~ 40 starless condensations in Taurus. MC 27 shows a sharply peaked
density distribution; the molecular intensity is well fitted by a power-law density distribution of r~~ 2 over
0.02 pc < r < 0.2 pc. A statistical analysis indicates a very short time scale of ~ 104 yr, which is consistent
with a free-fall time scale for a density of ~ 106 c m - 3 . These properties strongly suggest that MC 27 is in a
very early stage of star formation. A Monte Carlo simulation of the present profiles indicates that the infall
velocity at 2000-3000 AU should be 0.2-0.3 km s" 1 while it is less than 0.3 km s - 1 at < 1000 AU. This
derived infall velocity profile can be explained by a dynamical-collapse model of supercritical condensation
prior to formation of the first protostellar core by ~ 10 3 - 4 yr.
Key words: Interstellar: clouds — Interstellar: molecules — Line profiles — Stars: formation
1.
Introduction
Astronomers' efforts toward understanding of star formation have been sharply focused on detecting an extremely early stage of protostar formation in a dense
molecular condensation. Stars are formed in dense molecular condensations, that can be detected only in molecular line or dust continuum emission at millimeter or
sub-millimeter wavelengths (e.g., Benson, Myers 1989;
Fukui et al. 1991; Mizuno et al. 1994; Fukui, Yonekura
1997; Onishi et al. 1998). Mizuno et al. (1994) argued
that such condensations are collapsing in a time scale of
a few x 105 yr from the fact that starless cores are on average a factor of ~ 3 times smaller in size than those with
embedded stars.
Fairly young protostars formed in them have been observed as molecular outflows and are sometimes designated as Class I or Class 0 objects whose evolutionary
time scale is likely in the order of 105 yr (e.g., Lada 1987;
Fukui et al. 1989; Andre et al. 1993). In millimeter molecular emission, recent observations toward such objects
have revealed asymmetric self-absorbed line profiles that
are plausibly explained in terms of infall motion (e.g.,
Zhou et al. 1993), and high-resolution interferometric
observations suggest spatially resolved infall signatures
(e.g., Hayashi et al. 1993; Ohashi et al. 1997; Momose et
al. 1998).
On the other hand, a study of the moment of protostar formation whose time scale is < 104 yr has not yet
been done because of the difficulty of detecting such a
rare event. While theoretical studies of the collapse of
a molecular condensation predict how molecular gas begins to contract dynamically to form a protostar (Boss,
Yorke 1995), there has been no observational study of
such an extremely early stage of protostellar collapse to
constrain the model. Here, we report on the detection of
a low-mass protostellar condensation that is likely to be
very close to the moment of the formation of a protostellar core within a time scale of ~ 104 yr, and discuss the
astrophysical implications.
2.
Observations
Our recent complete search for molecular gas condensations in Taurus has revealed ~ 40 dense molecular
condensations without any star-formation activity in the
H 1 3 CO + J - 1-0 (Onishi et al. 1998; Onishi et al. in
preparation). Early results of the survey have been published elsewhere (Mizuno et al. 1994).. We observed nine
H 1 3 CO + J = 1-0 starless condensations that have the
highest integrated intensities among these condensations
in the J = 4-3 and 3-2 lines of both HCO+ and H 13 CO+.
© Astronomical Society of Japan • Provided by the NASA Astrophysics Data System
T. Onishi, A. Mizuno, and Y. Fukui
258
The observations were made on 1996 August 26-29 using the 10.4-m telescope at the Caltech Submillimeter
Observatory (CSO). The beam sizes of the J = 4-3 and
J = 3-2 lines are ~ 24" and 32", respectively. The beam
efficiencies are 65% and 76%, respectively. The rms noise
temperatures per channel for the HCO + J = 4-3, 3-2,
H 13 CO+ J = 4-3, and 3-2 are - 0.1, - 0.1, ~ 0.08, and
~ 0.05 K, respectively.
3.
[Vol. 51,
g 26- 50'
26*40'
Results
4h 26m Os 4h 25m 30s 4h 25m 0s
1
3.1. Summary of the Present Search
We detected H 1 3 CO + J = 3-2 emission only for 4 condensations whose H 1 3 CO + J = 1-0 antenna temperature exceeds 1 K. On the other hand, we did not detect
H 1 3 CO + J — 4-3 emission toward any starless condensations. The intensity of these H 1 3 CO + lines appears to
be a good indicator of the molecular density, since the
line is excited only at a density of > 10 5 - 6 c m - 3 . The
intensity of the H 1 3 CO + J = 3-2 line is in fact found
to be well correlated with that of the H 1 3 CO + J = 1-0
line. A radiative-transfer model employing the Large Velocity Gradient (LVG) approximation (Goldreich, Kwan
1974) is used to estimate the density in the central
2000 AU of these condensations for the three optically
thin transitions of H 1 3 CO + . As a result, MC 27 (Molecular Condensation 27), which is the 27-th condensation
in our catalog, is found to have the highest density of
~ (8±1) x 105 c m - 3 , significantly larger than those of the
other starless condensations of < (1 —2) x 105 c m - 3 . This
result strongly suggests that MC 27 is the most evolved
starless condensation in Taurus. In order to pursue the
evolutional status of MC 27, we describe the detailed
characteristics of MC 27 and discuss the implications on
the process of star formation in the following.
3.2. Characteristics of MC 27
MC 27 is located in the middle of the Taurus cloud
complex, between the HLC 2 and L1495 regions. This
object was discovered in the early stage of a search for
molecular condensations in H 1 3 CO + J = 1-0 emission,
as listed in table 1 (no. 3) of Mizuno et al. (1994). It
has no far-infrared counterpart in the IRAS point-source
catalog. We also checked the IRAS images and confirmed
that the luminosity of the point source is less than 0.1 L®
if it exists. This indicates that a star is not yet formed, or
that the formed star is very young, emitting only a small
amount of radiation energy compared to that of a mature
protostar. This condensation is embedded in a C 1 8 0
core isolated from the other C 1 8 0 cores (Onishi et al.
1996, 1998), and this C 1 8 0 core is surrounded by nearly
uniform low-density gas of ~ 103 c m - 3 , as observed in
the 13 CO J = 1-0 emission (Mizuno et al. 1995).
Figure 1 shows the 13 CO, C 1 8 0, and H 13 CO+ J = 1-0
i
ic 18 o
. ' •
£
-\
0.3 pc
26' 45'
\
:
'
•
:
'
!
\ ' ' ',
,
4hf 26m 0s 4h 25m 30s 4h
S
O
i •
25J
26' 46'
0.05 pc
Right Ascension(1950)
Fig. 1. Contour maps of 1 3 C O J = 1-0, C l s O J = 1-0,
and H 1 3 C O + J = 1-0 total intensity of MC 27. The
first two were obtained with the Nagoya 4-m telescope and the last was obtained with the Nobeyama
45-m telescope. The position of the center of the
H 1 3 C O + map is indicated by crosses. The position
is a(1950) = 4 h 25 m 34P5, 5(1950) = 26°45 , 4 , / .
integrated intensity maps of MC 27 (Mizuno et al. 1994;
Mizuno et al. 1995; Onishi et al. 1996, 1998). These
maps indicate that MC 27 has a nearly circularly symmetric shape from the low-density region of 103 c m - 3 up
to the high-density region of 105 c m - 3 . The integrated
intensity distribution of C 1 8 0 and H 1 3 CO + J = 1-0
can be fitted by a single power-law ranging in the form
of TV - r-o.9±o.2 a n d N _ r -o.8±o.i j respectively, for
0.02 < r < 0.2 pc after correcting for beam dilution. If we
take into account its symmetric shape, it is likely that the
density distribution follows a power law of ~ r-(1-8-1.9) ^
whereas the index values of a typical starless condensation are ~ -(1-1.5) (Ward-Thompson et al. 1994; Tafalla
et al. 1999). MC 27 therefore shows the steepest density
© Astronomical Society of Japan • Provided by the NASA Astrophysics Data System
No. 2]
Evidence for the Moment of Protostellar Core Formation
2.5
259
I I I I I I I I I I I I
[I I I I I I I
2.0 E- HCO +
E_ (J =3-2)
1.5 t1.0
0.5
0.0
1.0
0.5
0.0
1.0
5
0.5
10
Ukms 1 )
0.0
13
+
0.5 L H C O
0.0
(J=3-2)
13
0.5
Fig. 3. Profile maps of HCO+ J = 3-2 (solid) and
H 1 3 CO+ J = 3-2 (dashed) of MC 27. The observed
grid spacing is 30 , corresponding to 4000 AU. The
H 1 3 CO+ J = 3-2 profiles are scaled by 1.67. The
broken lines drawn vertically indicate Vjsr = 5.9 and
7.2 km s - 1 .
I
A
i i ^ X a " •'Ppgq
+
.H CO
(J =4-3)
<U_
0.0
IIMMIMI.II
6
8
Velocity (km s" )
Fig. 2. Spectra of the center of MC 27 taken with the
10.4-m telescope at the Caltech Submillimeter Observatory (CSO), except for H 1 3 C O + J = 1-0 with
the 45-m telescope. The names of the line are indicated in the figure. The dashed lines are modelfitted spectra by a Monte Carlo simulation. The fitted model has a constant infall velocity of 0.2 km s - 1
only within 3000 AU from the center.
distribution among starless condensations.
Figure 2 shows the spectra of HCO+ J = 3-2, 4-3
and H 1 3 CO + J = 1-0, 3-2, 4-3 obtained toward the
center of MC 27. The broad and asymmetric profiles of
the HCO + J = 3-2 and 4-3 lines indicate a large optical depth and self-absorption due to the foreground gas.
This self-absorption is likely to be caused by dense gas
of n(H 2 ) ~ 105 c m - 3 , physically associated with MC 27.
The HCO + J = 3-2 line has apparent wing-like velocity components at Visr < 5.8 and Visr ^ 7 . 4 km s _ 1 . The
profile maps of HCO + J = 3-2 in figure 3 show that the
wing-velocity components are seen only toward the center. On the other hand, the J = 1-0 and 3-2 spectra
of less-abundant H 1 3 CO + have single and narrow profiles of line width Av ~ 0.6 km s _ 1 , comparable to that
of the others without stars, while the J = 4-3 emission
is not detected with the present sensitivity. The profile maps of H 1 3 CO + J = 3-2 in figure 3 show that the
high-density region is very compact within < 15" of the
H 13 CO+ J - 1-0 peak, i.e., < 2000 AU.
4.
Analysis and Discussion
The asymmetric profile with a brighter blue peak of
the HCO + profiles is qualitatively explicable by an infall
motion in a collapsing spherical cloud (Zhou et al. 1993).
In order to test quantitatively how the infall model is applicable to MC 27, we carried out a model fitting using
Monte Carlo simulations (Bernes 1979). In the fitting, we
assumed a spherical model of uniform HCO + abundance
and kinetic temperature. The radius was assumed to be
2', corresponding to 16000 AU, and the sphere was divided into 60 shells at regular intervals. Because MC 27
does not include protostars of luminosity > O.IL®, the
assumption concerning the temperature seems to be plausible. We also assume a density gradient of n(H.2) ~ r~2->
based on the observed gradient of the column density
and a density of 106 c m - 3 at 1000 AU. Under these assumptions, we calculated the HCO + and H 1 3 CO + profiles smoothed to the beam size for a variety of velocity
© Astronomical Society of Japan • Provided by the NASA Astrophysics Data System
T. Onishi, A. Mizuno, and Y. Fukui
260
-n
1
1
1
1
1
r
Distance from center (AU)
Fig. 4. Derived parameters for the infall velocity. The
shaded area indicates the area where a solution exists. The two solid lines are the velocity profile of
inside-out collapse of T = 10 K, and the ages are
5 x 10 4 yr and 1.5 x 10 5 yr. The two dashed lines
give an example of run-away collapse just before formation of the first protostellar core. They describe
the radiation hydrodynamic model of protostellar
collapse. These lines indicate ~ 1.4 x 10 4 yr and
~ 2.0 x 10 3 yr before formation of the first core.
The thick solid line indicates the density distribution which we assumed. This density distribution is
also the same as that used in Masunaga's simulations (Masunaga et al. 1998), at > 1000 AU.
distributions for more than 50 cases.
As a result, it is found that molecular gas at 20003000 AU from the center should have an infall velocity of 0.2-0.3 km s - 1 in order to explain the asymmetric double-peaked profiles of HCO + . If the infall
motion is localized only around 1000 AU, the present
beam smoothes out the asymmetry, and only a symmetric double-peaked profile is expected. H 1 3 CO + lines
can be used to constrain the velocity of the gas of
n ~ 106 cm" 3 located at < 1000 AU, where the H 1 3 CO +
emission arises. It is found that the gas at ~ 1000 AU
must have an infall velocity lower than 0.3 km s _ 1 because the linewidths of the H 1 3 CO + lines are as small as
~ 0.6 km s _ 1 . These constraints are shown in figure 4,
and the best-fit profiles are superposed by the dashed
lines in figure 2. The fitting seems to be reasonably good,
except for the wing component of the HCO + J = 3-2,
suggesting that the assumption of n ~ r~2 is fairly good.
The wing feature is asymmetric, significantly detected in
the HCO + J = 3-2 line, but not so much in the J = 4-3
line.
In principle, the wing feature can be explained by either outflow or infall. LVG calculations indicate that the
density of the wing is estimated to be < 5 x 104 c m - 3 ,
[Vol. 51,
somewhat lower than that in the center of H 1 3 C O + condensation, in order to fit the average intensities of the
wings. This suggests that there are two components
within ~ 1500 AU from the center; one is gas of high density (~ 106 c m - 3 ) with a small velocity (< 0.3 km s _ 1 );
the other has a lower density ( < 5 x 104 c m - 3 ) with a
higher velocity (~ 1 km s _ 1 ) . If the wing is due to infall, the density of the wing must be much higher than
the present value. Moreover, the present model cannot
reproduce this wing as infall, because the large infall velocity o f - l k m s " 1 inside ~ 2000 AU requires H 13 CO+
spectra with a larger line width. Therefore, this wing is
probably due to outflow. The asymmetry of the wing is
also consistent with this.
As discussed above, MC 27 is unique among the
starless condensations in Taurus. Especially, it has
the largest density of ~ 106 c m - 3 , significantly higher
than that of the other present starless condensations,
~ 105 c m - 3 , typical of the low-mass star-forming regions. This suggests that MC 27 is the most evolved
starless condensation in terms of gravitational collapse.
The free-fall time scale at 106 c m - 3 is quite short, a few
x 104 yr, indicating that the evolutionary stage of MC 27
is very short-lived. Assuming a constant star-formation
rate during the last several x 106 yr in Taurus (Kenyon
et al. 1994), the time scale of MC 27 is estimated to be ~
104 yr from the statistics; the number of YSOs younger
than 106 yr is estimated to be ~ 100 (Kenyon, Hartmann
1995). The upper limit for the luminosity, ~ 0.1 L®
(Kenyon, Hartmann 1995), can be used to further constrain the evolutionary stage. We shall assume a steady
mass-accretion rate given by M = 0.975c 3 /G (Shu 1977),
where cs is the effective sound speed, 2 x 10_6M® y r - 1
for 10 K molecular gas. The accretion luminosity is calculated as L a c c = GM+M/R*, where M* is the stellar
mass and R* is the stellar radius. The luminosity being
< 0.1 L® implies a stellar mass of < 0.01 M® if we adopt
a stellar radius of 1.5 R® from the numerical simulation
by Stahler (1988) for stellar mass of < 0.3 Afe. The age
of a 0.01 M® protostar under a constant mass-accretion
rate is then calculated to be ~ 104 yr, consistent with
that of MC 27 derived above. Therefore, even if MC 27
has formed a star in it, the star must be younger than
~ 104 yr. Such a low-luminosity protostellar object in the
early stage of star formation has been studied in previous theoretical work (e.g., Boss, Yorke 1995). According
to Boss and Yorke (1995), a luminosity < 0.1 L® corresponds to a stage younger than ~ 2 x 104 yr, which they
call Class —I. At this stage, the first protostellar core
consisting of molecular hydrogen may have been formed
or, alternatively, the second (final) protostellar core may
have been just formed.
More detailed constraints on the evolutionary stage of
MC 27 may be obtained through model fitting of the
line profiles. First, we show a comparison of the in-
© Astronomical Society of Japan • Provided by the NASA Astrophysics Data System
No. 2]
Evidence for the Moment of Protostellar Core Formation
fall motion with the existing theoretical models. One of
the most fundamental models is inside-out collapse (Shu
1977); another is run-away collapse (e.g., Larson 1969;
Penston 1969). These two models have been well studied theoretically to describe the collapsing process of a
dense condensation. Our result needs an infall velocity
of 0.2-0.3 km s _ 1 at 2000-3000 AU in order to reproduce the asymmetric absorption profiles. When the infall
velocity reaches that value at 2000-3000 AU in inside-out
collapse, the infall velocity at 1000 AU must exceed at
least 0.5 km s _ 1 (figure 4), which is inconsistent with the
present result. When the infall velocity at 1000 AU is
below 0.3 km s" 1 , the velocity at 2000-3000 AU of the
inside-out collapse model is too small. In the run-away
collapse model of Larson (1969) and Penston (1969), the
model cloud has an infinite size, an infall velocity of
~ 0.6 km s _ 1 , and large mass-infall rates of 5 x 10~ 5
to 1 x 10~~4 M® y r - 1 , which are not consistent with the
present values, either. However, the existence of infall
motion at 2000-3000 AU indicates that the dynamical
motion should exist even before the formation of a protostellar core, because the age of MC 27 is estimated to
be quite young, < 104 yr. Recent numerical simulations
of dynamical collapse with a finite boundary show a similar velocity distribution to the present results, as shown
in figure 4 (e.g., Masunaga et al. 1998).
In order to obtain deeper insight, we briefly discuss the
physical processes in the collapse. A pre-stellar molecular condensation can be divided into two regimes: one
is a magnetically subcritical condensation which is supported from gravitational contraction by magnetic fields;
the other is a magnetically supercritical one which is
dominated by gravity. Subcritical condensation requires
some dissipation mechanism of magnetic fields to contract. Ambipolar diffusion provides a dissipation mechanism, leading to the contraction of subcritical condensations through a relative drift of the charged particles
and neutral species in the gas (Nakano 1979; Shu et al.
1987; Lizano, Shu 1989). When the supporting force is
reduced to that which is comparable to the gravitational
force, dynamical collapse starts. The index of the density distribution of such a dynamically collapsing cloud
is estimated to be —2 (Lizano, Shu 1989), which is consistent with that of MC 27. A supercritical condensation
should contract dynamically on nearly the free-fall time
scale. The contraction can be described by isothermal
self-similar solutions, also having an index of density distribution of ~ -2 (Larson 1969; Penston 1969). In both
cases, after the final protostellar core is formed, insideout collapse occurs.
Infall motion has been recently claimed for another
starless condensation in L1544 based on asymmetric selfabsorbed profiles (Tafalla et al. 1998). The molecular gas
of LI544 is spatially more extended than that of MC 27.
The index of the density profile of LI544 has been de-
261
rived to be ~ —1.5 and the spatial distribution of the
LI544 molecular cloud is not circularly symmetric, especially in the J = 1-0 line of C 1 8 0 (Tafalla et al. 1998).
The spherically asymmetric distribution of the molecular
gas in LI544 makes any discussion of infall inconclusive,
since the self-absorption feature is easily affected by the
distribution of the surrounding low-density gas. Molecular line observations with higher-J transitions are necessary to estimate a density higher than ~ 105 c m - 3 , as
discussed above, while such information is not available
for LI544. These additional observations should reveal
the evolutionary status of LI544 more precisely.
We next discuss the wing feature of the HCO + J = 32. These wings are localized toward the center of MC 27,
at <, 1500 AU. The model fitting discussed above indicates that the origin is probably due to outflow. The estimated dynamical time scale for the wings is ~ 104 yr
or smaller, which is consistent with that of MC 27. If the
wing feature is really due to outflow, this result indicates
that the outflow phenomenon occurs at a very early stage
of protostellar collapse. Recent theoretical calculations
(e.g., Kudoh, Shibata 1997) indeed show that outflow can
begin at a very early stage of dynamical collapse after a
rotating disk is formed, and that the velocity of the protostellar jet is estimated to be around the Keplerian velocity. If a protostellar core of 0.01 M® is already formed
in MC 27, the inner gas can have a Keplerian velocity
of > 10 km s _ 1 at the inner edge of the disk, which can
be a cause of the outflow. High-resolution interferometric
observations in higher-J transitions can be used to probe
the inner structure of MC 27, thus allowing us to study
the origin and onset of the outflow.
Finally, we describe future studies needed to understand the process of dynamical collapse in more detail.
While the general properties of dynamical collapse in a
very early stage of star formation have been revealed for
MC 27, a detailed study on the density distribution and
velocity distribution of the inner region of < 1000 AU is
necessary. Such a study should reveal the evolutionary
stage of MC 27 more precisely, and can be compared with
the theory of formation of the first protostellar core. The
gas distribution may also reveal when the spherical symmetry breaks to form a disk. More sensitive far-infrared
and submillimeter continuum observations make it possible to check whether the protostellar core really exists.
The existence of the protostar can easily accelerate the
outflow. Moreover, the detection of other objects like
MC 27 is important to study the process of dynamical
collapse. More surveys of dense molecular condensations
are needed for the other star-forming regions, such as
Ophiuchus, Chamaeleon, and Lupus, since the number
of objects like MC 27 is only ~ 1/100 of the young stars
of age ~ 106 yr because of its very short time scale of
~ 104 yr. With these further studies, we can obtain a
better comprehensive knowledge of the most basic prob-
© Astronomical Society of Japan • Provided by the NASA Astrophysics Data System
262
T. Onishi, A. Mi: no, and Y. Fukui
lem, how a protostar is formed via contraction of molecular gas.
We t h a n k S. Inutsuka and H. Masunaga for fruitful
discussion on the many aspects of the theoretical study;
A. Kawamura for d a t a reduction, discussion, and reading of the manuscript; K. Tachihara for helping observations and reading of the manuscript; a n d Y. Yonekura for
reading the manuscript. We also t h a n k the staff of the
CSO for their invaluable assistance. T h e research of T.
O. is supported by the Research Fellowships of the J a p a n
Society for the P r o m o t i o n of Science.
References
Andre P., Ward-Thompson D., Barsony M. 1993, ApJ 406,
122
Benson P.J., Myers P C . 1989, ApJS 71, 89
Bernes C. 1979, A&A 73, 67
Boss A . P , Yorke H.W. 1995, ApJ 439, L55
Fukui Y., Iwata T., Takaba H., Mizuno A., Ogawa H.,
Kawabata K., Sugitani K. 1989, Nature 342, 161
Fukui Y., Mizuno A., Nagahama T., Imaoka K., Ogawa H.
1991, Mem. Soc. Astron. Ital. 62, 801
Fukui Y., Yonekura Y. 1997, in IAU Symp. 179, New Horizons
from Multi-Wavelength Sky Surveys, ed B.J. McLean,
D.A. Golombek, J.J.E. Hayes, H.E. Payneet (Kluwer, Dordrecht) pi 65
Goldreich P., Kwan J. 1974, ApJ 189, 441
Hayashi M., Ohashi N., Miyama S.M. 1993, ApJ 418, L71
Kenyon S.J., Gomez M., Marzke R.O., Hartmann L. 1994, AJ
108, 251
Kenyon S.J., Hartmann L. 1995, ApJS 101, 117
Kudoh T., Shibata K. 1997, ApJ 476, 632
Lada C.J. 1987, in IAU Symp. 115, Star Forming Regions, ed
M. Peimbert, J. Jugaku (D. Reidel, Dordrecht) pi
Larson R.B. 1969, MNRAS 145, 271
Lizano S., Shu F.H. 1989, ApJ 342, 834
Masunaga H., Miyama S.M., Inutsuka S. 1998, ApJ 495, 346
Mizuno A., Onishi T., Hayashi M., Ohashi N., Sunada K.,
Hasegawa T., Fukui Y. 1994, Nature 368, 719
Mizuno A., Onishi T., Yonekura Y., Nagahama T., Ogawa
H., Fukui Y. 1995, ApJ 445, L161
Momose M., Ohashi N., Kawabe R., Nakano T., Hayashi M.
1998, ApJ 504, 314
Nakano T. 1979, PASJ 31, 697
Ohashi N., Hayashi M., Ho P.T.P, Momose M. 1997, ApJ
475, 211
Onishi T., Mizuno A., Kawamura A., Ogawa H., Fukui Y.
1996, ApJ 465, 815
Onishi T., Mizuno A., Kawamura A., Ogawa H., Fukui Y.
1998, ApJ 502, 296
Penston M.V. 1969, MNRAS 144, 425
Shu F.H. 1977, ApJ 214, 488
Shu F.H., Adams F.C., Lizano S. 1987, ARA&A 25, 23
Stahler S.W. 1988, ApJ 332, 804
Tafalla M., Mardones D., Myers P C . , Caselli P., Bachiller R.,
Benson P.J. 1998, ApJ 504, 900
Ward-Thompson D., Scott P.F., Hills R.E., Andre P. 1994,
MNRAS 268, 276
Zhou S., Evans N.J. II, Kompe C., Walmsley C M . 1993, ApJ
404, 232
© Astronomical Society of Japan • Provided by the NASA Astrophysics Data System