238 research outputs found
The star formation history of Trumpler 14 and Trumpler 16
H-R diagrams are presented for the very young galactic clusters Trumpler 14 and Trumpler 16, which are the two most populous clusters in the region of vigorous star formation surrounding η Carinae. Point spread function photometry of UBV CCD images is presented down to V~19 for over 560 stars in Tr 16 and 290 stars in Tr 14. We have also obtained similar data for a local background field. After determining cluster membership through proper motions from a previous work, we find that the reddening of cluster members is significantly lower than that of the local background stars. Thus, we are able to use individual reddenings to identify likely members at far deeper levels than possible with proper motions. This work has revealed a significant population of pre-main-sequence (PMS) stars in both clusters. The location of the PMS stars in the H-R diagram indicates that the theoretical ``stellar birthline'' of Palla & Stahler follows the locus of stars far better than that of Beech & Mitalas. Comparison with both pre- and post-main-sequence isochrones also reveals that although intermediate-mass stars have been forming continuously over the last 10 Myr, the high-mass stars formed within the last 3 Myr. There is no evidence that the formation of the intermediate-mass stars was truncated by the formation of the high-mass stars
Can photo-evaporation trigger planetesimal formation?
We propose that UV radiation can stimulate the formation of planetesimals in
externally-illuminated protoplanetary disks. We present a numerical model of
disk evolution including vertical sedimentation and photo-evaporation by an
external O or B star. As solid material grows and settles toward the disk
midplane, the outer layers of the disk become dust depleted. When such a disk
is exposed to UV radiation, heating drives photo-evaporative mass-loss from its
surface, generating a dust-depleted outflow. The dust:gas surface density ratio
in the disk interior grows until dust in the disk midplane becomes
gravitationally unstable. Thus, UV radiation fields may induce the rapid
formation of planetesimals in disks where sedimentation has occurred.Comment: 4 pages, 1 figure. Revised and accepted by ApJ
A Keck High Resolution Spectroscopic Study of the Orion Nebula Proplyds
We present the results of spectroscopy of four bright proplyds in the Orion
Nebula obtained at a velocity resolution of 6 km/s. After careful isolation of
the proplyd spectra from the confusing nebular radiation, the emission line
profiles are compared with those predicted by realistic dynamic/photoionization
models of the objects. The spectral line widths show a clear correlation with
ionization potential, which is consistent with the free expansion of a
transonic, ionization-stratified, photoevaporating flow. Fitting models of such
a flow simultaneously to our spectra and HST emission line imaging provides
direct measurements of the proplyd size, ionized density and outflow velocity.
These measurements confirm that the ionization front in the proplyds is
approximately D-critical and provide the most accurate and robust estimate to
date of the proplyd mass loss rate. Values of 0.7E-6 to 1.5E-6 Msun/year are
found for our spectroscopic sample, although extrapolating our results to a
larger sample of proplyds implies that 0.4E-6 Msun/year is more typical of the
proplyds as a whole. In view of the reported limits on the masses of the
circumstellar disks within the proplyds, the length of time that they can have
been exposed to ionizing radiation should not greatly exceed 10,000 years - a
factor of 30 less than the mean age of the proplyd stars. We review the various
mechanisms that have been proposed to explain this situation, and conclude that
none can plausibly work unless the disk masses are revised upwards by a
substantial amount.Comment: 23 pages, 8 figures, uses emulateapj.sty, accepted for publication in
The Astronomical Journal (scheduled November 1999
Global variation of the dust-to-gas ratio in evolving protoplanetary discs
Recent theories suggest planetesimal formation via streaming and/or
gravitational instabilities may be triggered by localized enhancements in the
dust-to-gas ratio, and one hypothesis is that sufficient enhancements may be
produced in the pile-up of small solid particles inspiralling under aerodynamic
drag from the large mass reservoir in the outer disc. Studies of particle
pile-up in static gas discs have provided partial support for this hypothesis.
Here, we study the radial and temporal evolution of the dust-to-gas ratio in
turbulent discs, that evolve under the action of viscosity and
photoevaporation. We find that particle pile-ups do not generically occur
within evolving discs, particularly if the introduction of large grains is
restricted to the inner, dense regions of a disc. Instead, radial drift results
in depletion of solids from the outer disc, while the inner disc maintains a
dust-to-gas ratio that is within a factor of ~2 of the initial value. We
attribute this result to the short time-scales for turbulent diffusion and
radial advection (with the mean gas flow) in the inner disc. We show that the
qualitative evolution of the dust-to-gas ratio depends only weakly upon the
parameters of the disc model (the disc mass, size, viscosity, and value of the
Schmidt number), and discuss the implications for planetesimal formation via
collective instabilities. Our results suggest that in discs where there is a
significant level of midplane turbulence and accretion, planetesimal formation
would need to be possible in the absence of large-scale enhancements. Instead,
trapping and concentration of particles within local turbulent structures may
be required as a first stage of planetesimal formation.Comment: Accepted by Monthly Notices of the Royal Astronomical Society on the
8th of March 2012. 20 pages. 16 figure
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Absolute Calibration of Image Plate for electrons at energy between 100 keV and 4 MeV
The authors measured the absolute response of image plate (Fuji BAS SR2040) for electrons at energies between 100 keV to 4 MeV using an electron spectrometer. The electron source was produced from a short pulse laser irradiated on the solid density targets. This paper presents the calibration results of image plate Photon Stimulated Luminescence PSL per electrons at this energy range. The Monte Carlo radiation transport code MCNPX results are also presented for three representative incident angles onto the image plates and corresponding electron energies depositions at these angles. These provide a complete set of tools that allows extraction of the absolute calibration to other spectrometer setting at this electron energy range
Rapid planetesimal formation in turbulent circumstellar discs
The initial stages of planet formation in circumstellar gas discs proceed via
dust grains that collide and build up larger and larger bodies (Safronov 1969).
How this process continues from metre-sized boulders to kilometre-scale
planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick
together poorly (Benz 2000), and spiral into the protostar in a few hundred
orbits due to a head wind from the slower rotating gas (Weidenschilling 1977).
Gravitational collapse of the solid component has been suggested to overcome
this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even
low levels of turbulence, however, inhibit sedimentation of solids to a
sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al.
2007), but turbulence must be present to explain observed gas accretion in
protostellar discs (Hartmann 1998). Here we report the discovery of efficient
gravitational collapse of boulders in locally overdense regions in the
midplane. The boulders concentrate initially in transient high pressures in the
turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are
augmented a further order of magnitude by a streaming instability (Youdin &
Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven
by the relative flow of gas and solids. We find that gravitationally bound
clusters form with masses comparable to dwarf planets and containing a
distribution of boulder sizes. Gravitational collapse happens much faster than
radial drift, offering a possible path to planetesimal formation in accreting
circumstellar discs.Comment: To appear in Nature (30 August 2007 issue). 18 pages (in referee
mode), 3 figures. Supplementary Information can be found at 0708.389
Applied Plasma Research
Contains research objectives, summary of research and reports on four research projects.National Science Foundation (Grant GK-28282X1)National Science Foundation (Grant GK-33843
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