Circumstellar Disks in the Orion Nebula Cluster
Lynne A. Hillenbrand,
Stephen E. Strom,
Nuria Calvet,
K. Michael Merrill, Ian Gatley,
Russell B. Makidon, Michael R. Meyer, and Michael F. Skrutskie
We combine our previous optical spectroscopic and photometric analysis
of ~1600 stars located in the Orion Nebula Cluster (ONC), with our own
and published near-infrared photometric surveys of the region, in order to
investigate the evidence for and properties of circumstellar disks.
We use the near-infrared continuum excess as our primary disk diagnostic,
although we also study sources with Ca~II triplet emission and those designated
as ``proplyds.'' The measured near-infrared excess is influenced by
1) the presence or absence of a circumstellar disk;
2) the relative importance of disk accretion and inner disk holes;
3) the relative contrast between photospheric and disk emission; and
4) system inclination.
After attempting to understand the effects of these influences, we estimate
the frequency of circumstellar disks and discuss the evidence for trends
in the disk frequency
with stellar mass (over the mass range <0.1-50 M_sun),
stellar age (over the age range <0.1-2 Myr), and
projected cluster radius (over the radial range 0-3 pc).
We find that the fraction of stars which have retained their inner
(< 0.1 AU) circumstellar disks to the present time is at least 55%
and probably no more than 90%, averaged over the entire range
in stellar mass and stellar age represented in the ONC, and
over the entire area of our survey.
We find no trend in the disk fraction with stellar age,
at least not over the limited age range of the cluster.
We find that more massive stars are less likely to have disks,
consistent with a scenario in which the evolutionary timescales
are more rapid for disks surrounding more massive stars than for disks
surrounding less massive stars. We also find that the disk frequency begins
to decrease towards the lowest masses, although objects of all masses
(including those which appear to be substellar) can have disks.
We find that the disk frequency increases towards the cluster center.
We then argue using several lines of evidence that a large fraction
of the disks associated with stars in the ONC are accretion disks.
The observed trends with stellar age, with stellar mass, and with
projected cluster radius in the disk frequency may, in fact, be driven primarily
by trends in the disk accretion properties.
From the magnitude of the near-infrared excess above that expected from
pure-irradiation disks, we find an accretion-disk fraction
among the stars identified as having disks, of 61-88%.
In addition, approximately 20% of the stars
in our optical spectroscopic sample show broad
(several hundred kms^{-1} FWHM) Ca~II emission lines, features often
associated with accretion-disk/wind phenomena; another 50% of the sample
have Ca~II lines which (at our spectral resolution) are ``filled in,''
thus indicating an independently derived accretion disk frequency of ~70%.
Finally, we discuss the near-infrared and optical emission-line properties of
that portion of our sample which has been identified from HST imaging
as having dark silhouette or externally ionized structure. This sample,
which has been proposed in the literature to have accretion disks,
appears no different in terms of its stellar or circumstellar
properties from the rest of the ONC population. The only feature
distinguishing these objects from their ONC siblings thus may be their current
(but short-lived) proximity to the massive stars near the cluster center.
back to lynne's page