- The Ionization of Accretion Flows in High Mass Star Formation: W51e2
Previous observations show that the hypercompact HII region W51e2 is
surrounded by a massive molecular accretion flow centered on the HII region.
New observations of the H53a radio recombination line made with
the VLA at 0.45 arc second angular resolution show a velocity gradient
in the ionized gas within the HII region of > 500 kms/pc
comparable to the velocity gradient seen in the molecular accretion flow.
New CO line observations made with the SMA at arc second angular resolution
detect a molecular bipolar outflow immediately around the W51e2 HII region
and extending along the axis of rotation of the molecular flow.
These observations are consistent with an evolutionary phase for high
mass star formation in which a newly formed massive star first begins to
ionize its surroundings including its own accretion flow.
Eric Keto & Pamela Klaassen, 2008, arXiv 0804.0514
- The Early Evolution of Massive Stars: Radio Recombination Line Spectra
Velocity shifts and differential broadening
of radio recombination lines
are used to estimate the densities and velocities of the ionized gas
in several hypercompact and ultracompact
These small HII regions
are thought to be at their earliest evolutionary phase and associated with
the youngest massive stars.
The observations suggest that these HII regions are
characterized by high densities, supersonic flows and steep density
gradients, consistent with accretion and outflows
that would be associated with the formation of massive stars.
Eric Keto, Qizhou Zhang & Stanley Kurtz, 2008, ApJ, 672, 423
- The Formation of Massive Stars: Accretion, Disks and the Development of Hypercompact HII Regions
The hypothesis that massive stars form by accretion can be investigated
by simple analytical calculations that describe the effect that the formation of
a massive star has on its own accretion flow.
Within a very simple accretion model that includes angular momentum,
that of gas flow on ballistic trajectories around a star,
the increasing ionization of
a massive star growing by accretion produces a three-stage evolutionary sequence.
The ionization first forms
a small quasi-spherical
HII region gravitationally trapped within the accretion flow. At this stage the flow of
ionized gas is
entirely inward. As the ionization increases,
the HII region transitions to a bipolar
an initially narrow region of outflow about the bipolar axis and accretion elsewhere.
At higher rates of ionization,
the opening angle of the outflow region progressively increases.
Eventually, in the third stage, the accretion is confined
to a thin region about an equatorial disk.
Throughout this early evolution, the HII region is of hypercompact
to ultracompact size depending on the mass of the enclosed star or stars.
These small HII regions whose dynamics are
dominated by stellar gravitation and accretion are
than compact and larger HII regions whose dynamics are dominated by
the thermal pressure of the ionized gas.
Eric Keto, 2007, ApJ, 666, 976
- Observations on the Formation of Massive Stars by Accretion
Observations of the H66a recombination line from the ionized gas in the
cluster of newly formed massive stars, G10.6--0.4, show that most of the
continuum emission derives from the dense gas in
an ionized accretion flow that forms an ionized disk or torus around
a group of stars in the center of the cluster.
The inward motion observed in the accretion flow suggests that
despite the equivalent luminosity and ionizing radiation of several
O stars, neither radiation
pressure nor thermal pressure has reversed the accretion flow.
The observations indicate why
the radiation pressure of the stars and the thermal pressure of the HII
region are not effective
in reversing the accretion flow. The observed rate of the accretion flow,
10^-3 M/yr, is sufficient to form massive stars within
the time scale imposed by their short main sequence lifetimes.
A simple model of disk accretion relates quenched HII regions, trapped
hypercompact HII regions, and photo-evaporating disks in an evolutionary
Eric Keto & Kenneth Wood, 2006, ApJ, 637, 850
- On the Evolution of Ultracomapact HII Regions
The classical model for the pressure-driven expansion of HII
is re-evaluated to include the gravitational
force of the star responsible for the HII region.
The model shows that the gravitational
attraction of the star maintains a steep density gradient and
accretion flow within the ionized gas and prevents the HII region
from expanding hydrodynamically unless the
radius of ionization equilibrium is beyond the
radius where the sound
speed of the ionized gas approximates the
Once past this critical radius the
HII region will expand rapidly and the accretion flow
through the HII region is quickly reduced.
However, in contrast to
the model without gravity where the velocity of the ionized gas is
everywhere outward, in the model with gravity, the velocity
within the HII region is always inward.
The model implies that newly formed massive stars within dense
molecular cores may initially form very small HII regions that
at first evolve slowly through an increase in ionizing flux as
would be caused by an increase in the mass or number of stars
through continuing accretion through the HII region.
Eric Keto, 2002, ApJ, 580, 980