Deep 3.8 Micron Observations of the Trapezium Cluster

Charles J. Lada, August A. Muench, Elizabeth A. Lada, João F. Alves

We present deep 3.8 μm L' imaging observations of the Trapezium

cluster in Orion obtained with the ESO VLT. We use these observations to

(1) search for infrared excess emission and evidence for protoplanetary

disks associated with the faint, substellar population of this young

cluster and (2) investigate the nature and extent of a recently

discovered population of deeply embedded sources located in dense

molecular gas behind the cluster. We detected 38 L' sources with

substellar luminosities. In addition, we detected 24 L' sources that

were spectroscopically classified as substellar objects in previous

studies. Examining the infrared colors of all these sources, we

determine an infrared excess fraction of 50%+/-20% from the

JHKsL' colors for both the luminosity-selected and

spectroscopically selected substellar samples. This finding confirms the

presence of infrared excess, likely due to circumstellar disks, around a

significant fraction of the cluster's substellar population, consistent

with the indications of earlier observations obtained at shorter

(JHKs) wavelengths. Our deep L' imaging survey also provides

new information concerning the deeply embedded population of young

objects located in the molecular cloud behind the cluster and revealed

in an earlier L-band imaging survey of the region. In particular, our

present L' survey doubles the number of sources in the cluster region

known to possess extremely red K-L colors. These objects exhibit

K-L' colors indicative of deeply buried, possibly

protostellar, objects that likely mark the site of the most recent and

ongoing star formation in the region. We find the surface density

distribution of the deeply embedded population to follow that of the

background molecular ridge and to be highly structured, consisting of a

string of at least five significant subclusters. These subclusters may

represent the primordial building blocks out of which the cluster was

and perhaps still is being assembled. These observations may thus

provide insights into the early stages of cluster formation and appear

consistent with recent simulations that suggest that the Trapezium

cluster may have formed from numerous but small primordial subclusters.


Based on observations collected at the European Southern Observatory,

Chile [ESO Program 70.C-0471(A)].

Department of Astrophysics
External organisation(s)
Harvard-Smithsonian Center for Astrophysics, University of Florida, Gainesville
The Astronomical Journal
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Peer reviewed
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