Infrared L-Band Observations of the Trapezium Cluster: A Census of Circumstellar Disks and Candidate Protostars

Charles J. Lada, August Muench, Karl E., Jr. Haisch, Elizabeth A. Lada, João F. Alves, Eric V. Tollestrup, S. P. Willner

We report the results of a sensitive near-infrared JHKL imaging survey

of the Trapezium cluster in Orion. We use the JHKL colors to obtain a

census of infrared excess stars in the cluster. Of (391) stars brighter

than 12th magnitude in the K and L bands, 80%+/-7% are found to exhibit

detectable infrared excess on the J-H, K-L color-color diagram.

Examination of a subsample of 285 of these stars with published spectral

types yields a slightly higher infrared excess fraction of 85%. We find

that 97% of the optical proplyds in the cluster exhibit excess in the

JHKL color-color diagram indicating that the most likely origin of the

observed infrared excesses is from circumstellar disks. We interpret

these results to indicate that the fraction of stars in the cluster with

circumstellar disks is between 80%-85%, confirming earlier published

suggestions of a high disk fraction for this young cluster. Moreover, we

find that the probability of finding an infrared excess around a star is

independent of stellar mass over essentially the entire range of the

stellar mass function down to the hydrogen burning limit. Consequently,

the vast majority of stars in the Trapezium cluster appear to have been

born with circumstellar disks and the potential to subsequently form

planetary systems, despite formation within the environment of a rich

and dense stellar cluster. We identify 78 stars in our sample

characterized by K-L colors suggestive of deeply embedded objects. The

spatial distribution of these objects differs from that of the rest of

the cluster members and is similar to that of the dense molecular cloud

ridge behind the cluster. About half of these objects are detected in

the short wavelength (J and H) bands, and these are found to be

characterized by extreme infrared excess. This suggests that many of

these sources could be protostellar in nature. If even a modest fraction

(i.e., ~50%) of these objects are protostars, then star formation could

be continuing in the molecular ridge at a rate comparable to that which

produced the foreground Trapezium cluster.

Department of Astrophysics
External organisation(s)
INAF - Osservatorio Astrofisico di Arcetri, Boston University, University of Florida, Gainesville, Harvard-Smithsonian Center for Astrophysics
The Astronomical Journal
Publication date
Peer reviewed
Portal url