JAC Style guide: template
 The Bipolar Outflow NGC 2264 G
Detailed observations in CO(2-1) have been obtained of the highly collimated bipolar outflow NGC2264G.
A complete map with 20 arcsec resolution shows rich structural detail in both the spatial morphology and
the velocity field of the outflow. Figure 1shows a
position-position image of 12CO(2->1) obtained with the JCMT. The top panel displays blue-shifted lobe
while the bottom panel shows the red lobe. Radial velocity is depicted with blue showing the lower velocity
gas and red for the most extreme velocity gas. The star in the center of each plot marks the position of the
recently discovered central source (Gómez et al, 1994) that is seen in NH3 and radio continuum with the
VLA. The ammonia is centered at 4.6 km/s, which we take to be the central velocity of the outflow,
perfectly consistent with observations of the large surrounding ambient cloud. This central source has
recently been detected at submillimeter wavelengths using the JCMT (Ward-Thompson, Eiroa, and Casali
1995) and found to have a luminosity of approximately 12 L(sun).
Figure 1.
The two lobes exhibit nearly identical extents in space and velocity although there are significant
differences in their structure. In particular at low velocities the red lobe shows a bow-shock like appearance
far from the central object. No such structure is apparent anywhere in the blue lobe. It is clear that there is
a systematic increase in flow velocity and collimation with distance from the central source.
Figure 2: Velocity profiles of each wing. A break in the power law slopes occurs at 28 kms-1
in both wings.
Both lobes show an elongated structure at the highest velocities, extending beyond the lower velocity gas
and unresolved in the direction perpendicular to the flow direction. These structures may be the molecular
component of the jet driving this outflow. Spatially integrated velocity profiles of the outflow gas in each
lobe are shown in Figure 2. These profiles, which trace mass of the flow if the gas is optically thin, appear
to be well described by a power-law. A line with slope 1.7, similar to what is seen in several other similar
outflows, is shown on each lobe's profile. However the highest velocity gas, that looks like a jet in the
position-position plots, requires a steeper power law slope (g » 4) with the identical index in both lobes.
Figure 3: Flow dynamical timescales for the flow determined from each observed
position in the outflow and binned in a histogram.
The systematic increase of velocity with distance shown in Figure 1 can be used to estimate a time scale.
This can be done for every point in the outflow by estimating the mass-weighted mean velocity at each point
<v> and the projected distance from the central source R. The time scale is then simply R
/ <v>. The frequency distribution of this timescale is shown in Figure 3. The positions in each
lobe are shown separately in solid lines for blue, dashed lines for the red lobe. Both distribution functions
are sharply peaked at a timescale corresponding to 3 x 10(4) years!
The mass, momentum, and luminosity of each lobe has been estimated and is shown in Table 1. The mass
and momentum estimates are probably the most robust and accurate quantities we have determined. The
momentum estimates indicate that that the driving engine of the flow is extremely efficient. In particular,
the energy required to drive the molecular outflow is found to be an appreciable fraction of the gravitational
accretion energy liberated in the process of forming the central protostellar object. The amount of
momentum injection required to account for our observations probably cannot be supplied by a typical
optical jet, unless it contains a substantial neutral component.
Mass and Energetics of the Outflow
Lobe Mass Momentum Mass Flux Thrust Lumin.
M(sun) M(sun) kms-1 (10-5)M(sun) yr-1 (10-4)M(sun) (L(sun))
kms-1 yr-1
Red 0.4 (0.5)a 6.0 (2)a 0.84 (1.2)a 1.5 (0.5)a 0.52
Blue 0.4 4.2 0.73 1.2 0.44
Total 0.8 (1.3)a 10.2 (12.2)a 1.67 (2.87)a 2.7 (3.2)a 0.96
a: Estimated mass and energetic parameters for the lowest velocity (6-12 kms-1) red-shifted gas in the
outflow and the totals in these flow quantities including this component. The contribution of this lowest
velocity red-shifted gas to the outflow mechanical luminosity is negligible and not included in the
Table.
References:
Gómez, J. F., Curiel, S., Torrelles, J. M., Rodríguez, L. F., Anglada, G., & Girart, J. M. 1994, ApJ,
436, 749.
Ward-Thompson, D., Eiroa, C., & Casali, M. 1995, MNRAS, 273, L25.
Michel Fich, University of Waterloo
& Charles J. Lada, Smithsonian Astrophysical Observatory
Last Modification Date 1996/04/08 - Last Modification Author: Graeme Watt (gdw)
|
