A giant molecular cloud associated with the eta Carinae nebula was fully mapped in CO with the Columbia Millimeter-Wave Telescope at Cerro Tololo. The cloud comples has a mass of roughly 700,000 solar mass and extends about 140 pc along the Galactic plane, with the giant Carina HII region situated at one end of the complex. Clear evidence of interaction between the HII region and the molecular cloud is found in the relative motions of the ionized gas, the molecular gas, and the dust; simple energy and momentum considerations suggest that the HII region is responsible for the observed motion of a cloud fragment. The molecular cloud complex appears to be the parent material of the entire Car OB1 Association which, in addition to the young clusters in the Carine nebula, includes the generally older cluster NGC 3325, NGC 3293, and IC 2581. The overall star formation efficiency in the cloud complex is estimated to be approximately 0.02

Cohen, R. S.

Grabelsky, D. A.

Thaddeus, P.

English

NASA Technical Reports Server

N87- 24272
Star Formation in Carina OBI: Observations of a Giant Molecular Cloud
Associated with the N Carinae Nebula
D.A. Grabelsky, 1,2 R.S. Cohen, I and P. Thaddeusl, 3
iColumbia University, Department of Astronomy, New York, NY
2Northwestern University, Dept. of Physics and Astronomy, Evanston, IL
3Goddard Institute for Space Studies, 2880 Broadway, New York, NY
ABSTRACT. A giant molecular cloud associated with the N Carinae nebula has
been fully mapped in CO with the Columbia Millimeter-Wave Telescope at Cerro
Tololo. The cloud complex has a mass of roughly 7 x 105 M 0 and extends about
140 pc along the Galactic plane, with the giant Carina HII region situated at
one end of the complex. Clear evidence of interaction between the HII region
and the molecular cloud is found in the relative motions of the ionized gas,
the molecular gas, and the dust; simple energy and momentum considerations
suggest that the HII region is responsible for the observed motion of a cloud
fragment. The molecular cloud complex appears to be the parent material of the
entire Car OBI Association which, in addition to the young clusters in the
Carina nebula, includes the generally older clusters NGC 3324, NGC 3293, and
IC 2581. We estimate the overall star formation efficiency in the cloud
complex to be -0.02.
I. INTRODUCTION
The N Carinae nebula (NGC 3372) is among the brightest giant HII regions in
the Galaxy, and the young clusters that supply the ionizing radiation -- Tr 14,
Tr 16, and Cr 228 -- comprise one of the largest known Galactic concentrations
of massive stars: 16 main sequence 0 stars, including 5 classified 03V
(Walborn 1971, 1973). Interstellar absorption lines observed toward many of
the member stars display velocities ranging over 550 km s "I (Walborn 1982), and
it is generally agreed that the Carina nebula is pumping a lot of energy into
the surrounding medium. Within about 2o along the Galactic plane lie the three
somewhat older and less spectacular clusters NGC 3324, NGC 3293, and IC 2581,
which, together with the clusters in the Carina nebula, make up the Carina OBI
Association.
Because it lies in the Southern skies, CO observations of the region in and
around the Carina nebula were, until recently, limited either to single
detections (Gillespie et al. 1977; White and Phillips 1983) or to small region
mapping (de Graauw et ai. 1981). In 1983 we installed the Columbia Southern
Millimeter-Wave Telescope at Cerro Tololo and carried out the first well-
sampled CO survey of the entire Southern Milky Way. As part of that survey, we
mapped a large molecular cloud complex that is the parent material of the Car
OBI Association. In this paper we present the observations of the N Carinae
molecular cloud complex, point out the interesting dynamical relationship
between the molecular cloud and the HII region, and give an estimate of the
overall star formation efficiency in the molecular cloud.
2. OBSERVATIONS
The CO observations were made between 1983 January and November at Cerro
CarolJ. Lonsda_ Persson(E_
S_rFormationin Galax_s 67
https://ntrs.nasa.gov/search.jsp?R=19870014839 2020-03-20T10:58:32+00:00Z
D. A. GRABELSKY ET AL.
Tololo with the Columbia Southern Millimeter-Wave Telescope (Grabelsky 1985;
Bronfman 1986). The telescope has a beamwidth of 8.8' at 2.6 mm, and its
pointing accuracy is better than I' The detection system consists of a liquid
nitrogen cooled superheterodyne receiver with a single-sideband noise
temperature of -380 K, and a filterbank spectrometer with 256 channels, each
1.3 km s -I wide at 2.6 mm for a total bandwidth of 333 km s -I. Spectra were
calibrated against a blackbody reference following the standard chopper wheel
technique (Kutner 1978), and only linear baselines were subtracted.
Integrations times were typically 5 minutes, yielding an rms noise per channel
of -0.14 K.
The region in and around the Carina nebula was covered in a survey of
molecular clouds in the Carina spiral arm (Grabelsky 1985). Observations were
space every 1/8 o in 2 and b in the range 270 o _ 2 _ 300 o , Ibl _ 1 °, with
latitude extensions as needed to cover all nonlocal emission. The spectrometer
bandwidth, centered at v = 0, was more than adequate to cover all Galactic
plane emission within the longitude range of the survey.
3. IDENTIFICATION OF THE CLOUD COMPLEX
A mosaic of ESO J plates showing the N Carinae nebula and its surroundings
is displayed in Figure i; the bright gas between i = 287 o and 288 o is the
Carina HII region. Although the young clusters in the Carina nebula are washed
out in this photograph, the other Car OBI clusters can be identified: IC 2581
(2,b = 284.7°,0.1°), NGC 3293 (285.9°,0.1°), and NGC 3324 (286.2o,-0.20).
Various published distances place these clusters, as well as the Carina nebula,
at 2.7 kpc. The overlaid contours in Figure 1 show CO emission integrated over
velocity from -50 to -9 km s -I. Along the Galactic plane a sequence of
molecular clouds from 2,b = 285o,0 o to 288°,-1 o is evident. Each molecular
cloud in the apparent grouping is physically related to one or more of the Car
OBI clusters, thus placing each cloud at the same distance and establishing the
identity of the molecular cloud complex. In each case the evidence for the
association of a cluster with a molecular cloud in the complex is the spatial
proximity and similar velocities; cluster velocities were determined either
from member stars or from associated HII regions. In the case of the Carina
nebula, discussed in the next section, there is, additionally, clear evidence
for direct contact between the HII region and the molecular cloud.
With the now widely-used assumption that integrated CO line temperature,
denoted W(CO), is proportional to N(H2) , the H 2 column density, and adopting
the _ ray calibration of Bloemen e_ ai. (1986) of the W(CO)-N(H2) relation, we
obtained a mass of 6.7 x 105 M O for the entire molecular cloud complex. We
call this the CO mass to indicate that it is based on W(CO). This value is
within about a factor of two of the virial mass derived by assuming that the
observed bulk motions in the complex support the cloud against gravity.
4. INTERACTION BETWEEN THE CLOUD AND THE HII REGION
One of the prominent features of the Carina nebula is the dark dust lane
that crosses the face of the bright gas. The generally good spatial
correlation between the CO and the dust suggests that a portion of the
molecular cloud must lie in front of the HII region, while the coincidence of
the strong CO peak at 2,b = 287.5o,-0.50 with the bright gas indicates that
68
STAR FORMATION IN CARINA OB1 
-35 -30 -25 -20  -15 -10 -5  0 
LSR RADIAL VELOCITY (km 5-1) 
FIGURE 1. Mosaic of 
ESO J plates showing 
the Q Carinae nebula 
and its surroundings. 
The overlaid contours 
show CO emission 
integrated over 
velocity from -50 to 
-9 km s-l. The 
contour interval is 
5 K km s-l. 
good correlation 
between the CO and 
the dust lanes in the 
vicinity of the 
bright gas. This 
dust represents a 
fragment of the 
molecular cloud that 
is being driven out 
by the HI1 region. 
Note the 
FIGURE 2. Longitude-velocity map 
of the Q Carinae molecular cloud 
complex. The CO emission is 
integrated in latitude from b = -lo 
to O o ;  the contour interval is 
0.125 K-deg. 
the emission is -19 km s-l, but 
near the longitude of the HI1 
region (1 = 287.5O) a cloud 
fragment with a velocity of about 
-26 km is seen. This fragment 
or filament corresponds to the 
dust lane in front of the Carina 
nebula, and its appearance as a 
blue-shifted feature (relative to 
the mean cloud velocity) indicates 
that it is moving out from the 
main body of the cloud. The CO 
mass of the filament is -2 x l o 4  Mg 
and its velocity relative to the 
mean cloud velocity is 7 km s-l. 
The HI1 region is evidently 
driving the filament outward. 
The mean velocity of 
much of the molecular cloud is located behind the HI1 region. 
velocity map integrated from b = -lo to O o  is shown in Figure 2. 
velocity of the cloud complex is -19 km s - l ,  quite close to the mean HI1 region 
A longitude- 
The mean 
69 
D. A. GRABELSKY E T AL.
velocity of -20 km s-l; but at the longitude of the Carina HII region there a
CO "filament" with a velocity of about -26 km s-I This blue-shifted filament
corresponds to the dust lane and the HII region may be driving the filament
outward with a relative velocity of -7 km s -I. The spatial correlation of the
blue-shifted CO filament and the dust lane is confirmed in spatial maps which
sample narrow velocity "slices" through the cloud (not shown here).
To check whether the HII region could be the source of the filament's
motion, we considered two possible mechanisms for driving the filament out from
the main body of the cloud: a stellar wind-driven bubble, and the rocket
effect. Following Castor et ai. (1975), the velocity of the swept-up shell
around a wind-driven bubble can be expressed vs = 2.7 x 103 (M Vw 2 no'l) I/5
t-2/5 km s"I. With the assumption that the CO filament is a piece of swept-up
shell and reasonable values in the Carina nebula for the stellar mass loss rate
(M), wind velocity, (Vw) , ambient cloud density (no), and age (t), a stellar
wind-driven bubble can easily account for the observed motion of the filament.
Similarly, in the Carina nebula there is enough ionizing radiation to make the
rocket effect of ionized gas streaming off the filament a plausible mechanism
for accelerating the filament. Evidently, the HII region is severely
disrupting the cloud.
5. STAR FORMATION EFFICIENCY
The star formation efficiency, SFE, is the ratio of stellar mass to total
stellar plus molecular cloud masses. For the cloud mass we used the CO mass
found above (§ 3.). To estimate the stellar mass we simply tallied up the mass
in all the observed stars with spectral types earlier than B0.5, and used the
IMF of Miller and Scalo (1979) to determine the total mass in spectral types
BO.5 and later. The IMF was calibrated to the number of observed BO.5V stars.
Our assumption that a reasonable total mass in early type stars can be attained
by simply counting them is based on the unusually large number of early type
stars observed in the Carina nebula. Using this method we obtained SFE = 0.02.
This result is in good agreement with SFE determined by Myers et a2. (1986) for
about 50 inner-Galaxy giant molecular clouds.
REFERENCES
Bloemen, J.B.G.M. et a2. 1986, Astr. Ap., 154, 25.
Bronfman, L. 1986, Ph.D. thesis, Columbia University.
Castor J., McCray, R., and Weaver, R. 1975, Ap. d. (Letters), 200, LI07.
de Graauw, T., Lidholm, S., Fitton, B., Beckman, J. Israel, F.P.,
Nieuwenhuijzen, H., and Vermue, J. 1981, Astr. Ap., 102, 257.
Gillespie, A.R., Huggins, P.J., Sollner, T.C.L.G., Phillips, T.G.,
Gardner, F.F., and Knowles, S.H. 1977, Astr. Ap., 60, 221.
Grabelsky, D.A. 1985, Ph.D. thesis, Columbia University.
Kutner, M.L. 1978, Ap. Letters, 19, 81.
Miller, G.E., and Scalo, J.M. 1979, Ap. d. Supp1., 41, 513.
Myers, P.C., Dame, T.M., Thaddeus, P., Cohen, R.S., Silverberg, R.F.,
Dwek, E., and Hauser, M.G. 1986, Ap. d., 301, 398.
Walborn, N.R. 1971, Ap. d. (Letters), 167, L31.
Walborn, N.R. 1973, Ap. d., 179, 517.
Walborn, N.R. 1982, Ap. d. Suppl., 148, 145.
White. G.J., and Phillips, J.P. 1983, M.N.R.A.S., 202, 255.
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Star formation in Carina OB1: Observations of a giant molecular cloud associated with the eta Carinae Nebula

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