Radio Observations of Ultracompact H II Regions

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Radio Observations of Ultracompact H II Regions

• Luis F. Rodríguez
• CRyA, UNAM

The Ultracompact H II (UCHII) Regions are small
(radius lt 1017 cm), dense (electron density gt 104
cm-3) volumes of gas ionized by deeply embedded,
young OB stars. I will present recent
developments since the review by Churchwell
(2002, ARAA, 40, 27).
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Morphology
Cometary
plus spherical, irregular, and unresolved
morphologies (Churchwell 2002)
Shell-like
Core-halo
Bipolar
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Summary

• Morphology and electron density structure.
• Time change in UCHIIs (expansion, flux density
  change, and proper motion).
• Conclusions.

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DePree et al. (2005) studied with the VLA more
than 100 UCHII regions in Sgr B2 and W49A.
Similar morphologies than those found in the
Galactic plane surveys of Wood Churchwell
(1989) and Kurtz et al. (1994). However, DePree
et al. note much larger percentage of shell-likes
in their sample and propose elimination of
core-halo morphology on the grouds that most
UCHII regions show associated extended emission.
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It is unclear if the much larger percentage of
shell-like UCHIIs in the DePree et al. (2005)
sample is due to a real difference in the two
extreme star-forming regions studied, or to
differences in observational techniques (i. e.
snapshots in large samples vs. more detailed
imaging of selected regions, also interferometers
can filter spatial scales).
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NGC 6334
DSS2 Red
NGC 6334E
NGC 6334A
G351.200.70
G351.020.65
Carral et al. (2002) also find high incidence of
shell-like UCHIIs in NGC 6334
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Electron density structure
It is frequent for ultracompact and hypercompact
H II regions to exhibit a continuum spectrum that
is a power law with Sn proportional to n1
Franco et al. (2000)
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These angular size measurements as a function of
frequency are difficult to achieve, but have been
made for the components A and B of G34.260.15 by
Avalos et al. (2005).
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The free-free continuum for these components does
go as n1, the angular size goes as n-0.1, close
to flat.
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What is the explanation?

• It is possible that the UCHIIs and HCHIIs do not
  have a power law dependence of electron density
  with radius, but that they are hierarchically
  clumped structures, as proposed by Ignace
  Churchwell (2004). In this case one gets the
  power law spectral index for the continuum energy
  distribution, without a strong dependence of
  angular size with frequency.

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Changing Ultracompact H II Regions

• We usually think of the UCHIIs as objects steady
  in time. Of course, they are evolving and this
  has been detected in
• Expansion
• Flux density variation
• Proper motion

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VLA observations of G5.89-0.39 by Acord et al.
(1998) with Dt 5 years
This subtraction technique has been used for many
PNe but for only a few UCHIIs. Interpretation of
data implies expansion velocity of about 35 km
s-1.
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Kawamura Masson (1998) used VLA data with time
separation of 9 years to measure expansion in
W3(OH). Interpretation of data implies expansion
velocity of 3 5 km s-1.
The only two UCHIIs for which expansion is
measured (G5.89-0.39 and W3(OH)), show quite
different expansion velocities.
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Franco-Hernández Rodríguez (2004) studied VLA
data of NGC 7538 IRS1 taken over 12 years. No
expansion was detected, but the lobes show a
clear decrease in flux density of 20-30 over the
years
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These subtraction techniques (based on
cross-calibration of the data of different
epochs) are very reliable, as this null test
shows
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Finally, there is at least one clear case of a
moving UCHII region
In the radio, the BN object in the Orion BN/KL
region is detected as an UCHII region ionized by
a B-type star. Since 1995, Plambeck et al.
reported large proper motions (tens of km s-1) to
the NW.
BN Object VLA 7 mm
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In a recent analysis of the data, Tan (2004)
proposed that the BN object was ejected some
4,000 years ago by interactions in a multiple
system located at q1C Ori, the brightest star of
the Orion Trapezium.
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However, an analysis of VLA data taken over the
last two decades suggests that the radio source I
(apparently a thermal jet), is also moving in the
sky, receding from a point between it and the BN
object.
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The Radio Source I is also moving in the sky.
Radio Source I VLA 7 mm
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BN moves to the NW at 27-1 km s-1.
I moves to the SE at 12-2 km s-1.
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The data suggest that some 500 years ago, a
multiple stellar system, formed at least by BN
and I had a close encounter and the stars were
expeled in antiparallel directions BN or I have
to be close binary systems for this scenario to
work
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Encounters in multiple stellar systems can lead
to the formation of close binaries or even
mergers with eruptive outflows (Bally Zinnecker
2005).
Reipurth (2000)
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Indeed, around the BN/KL region there is the well
known outflow with an age of about 1000 years. It
is possible that the outflow and the ejection of
BN and I were result of the same
phenomenon. Energy in outflow is of order
4X1047 ergs, perhaps produced by formation of
close binary or merger.
H2 image with NH3 contours (Shuping et al. 2004
Wilson et al. 2000)
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Conclusions

• UCHII regions are understood only basically, many
  aspects remain (origin of morphology, density
  structure)
• Time variation is a new parameter that is being
  included in the analysis and that will help study
  these sources and their environment.