On the Stellar Population and Star-Forming History
of the Orion Nebula Cluster
Lynne A. Hillenbrand
Department of Astronomy
601 Campbell Hall
University of California
Berkeley, CA 94720
(lynne@astron.berkeley.edu)
We report on the first phase of a study of the stellar population
comprising the Orion Nebula Cluster (ONC).
Approximately 50% of the ~3500 stars identified to date
within ~2.5 pc of the namesake Trapezium stars are optically visible, and
in this paper we focus on that sample with I < 17.5 mag.
The large number and number density (n_peak > 10^4 pc^{-3}) of stars,
the wide range in stellar mass (~0.1-50 M_sun),
and the extreme youth (< 1-2 Myr) of the stellar population,
make the ONC the best site for investigating:
1) the detailed shape of a truly ``initial" mass spectrum;
2) the apparent age spread in a region thought
to have undergone triggered star formation;
3) the time sequence of star formation as a function of stellar mass; and
4) trends of all of the above with cluster radius.
Nearly 60% of the ~1600 optical stars have sufficient data
(spectroscopy and photometry) for placement on a theoretical HR diagram;
this subsample is unbiased with respect to
apparent brightness or cluster radius, complete down to ~1 M_sun,
and representative of the total optical sample below ~1 M_sun
for the age and extinction ranges characteristic of the cluster.
Comparison of the derived HR diagram with traditional pre-main sequence
evolutionary calculations shows a trend of increasing stellar age
with increasing stellar mass. To avoid the implication of
earlier characteristic formation times for higher-mass stars
than for lower-mass stars,
refinement of early evolutionary theory in a manner similar to
the birthline hypothesis of Palla & Stahler (1993), is required.
Subject to uncertainties in the tracks and isochrones,
we can still investigate stellar mass and age distributions in the ONC.
We find the ONC as a whole to be characterized by a mass spectrum which
is not grossly inconsistent with
``standard" stellar mass spectra. In particular,
although there are structural differences between the detailed ONC
mass spectrum and various models constructed from solar neighborhood data,
the observed mass spectrum appears to a peak at ~0.2 M_sun and to
fall off rapidly towards lower masses; several substellar objects are present.
The abundance of low-mass stars relative to high-mass stars
suggests that there is no bi-modal star formation mode; somewhat ironically,
the ONC probably contains fractionally more low-mass stars
than the solar neighborhood since the population not yet located
on the HR diagram is dominated by sub-solar-mass stars.
Nonetheless, the ONC mass spectrum is biased towards higher-mass stars
within the innermost cluster radii (r_projected < 0.3 pc).
We find the ONC as a whole to be characterized by a mean age of < 1 Myr and
an age spread which is probably less than 2 Myr, but also by a bias towards
younger stars at smaller projected cluster radii.
Although the most massive stars and the youngest stars
are found preferentially towards the center of the ONC
it does not follow that the most massive stars are the youngest stars.
A lower limit to the total cluster mass in stars is
M_stars ~ 900 M_sun (probably a factor of < 2 underestimate).
A lower limit to the recent low-mass star formation rate
is ~10^{-4} M_sun yr^{-1}.
All observational data in this study
as well as stellar parameters derived from them
are available in electronic format.
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