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Gemini Survey Uncovers Plethora of Trapezium Treasures

June 15, 2005

Using the Flamingos-I near-infrared imager at Gemini South, a UK/Japanese team of astronomers has imaged an outer region of the Trapezium Cluster near the center of the Orion Nebula, and identified 396 sources. Of these, 138 are brown dwarf candidates and 33 of those are candidate planetary mass objects. This could have an impact on our understanding of the initial mass function (IMF) for very low-mass bodies, especially in a hostile star forming environment (see sidebar at right).

Figure 1. Color composite image from near-infrared J, H and K-band images including two of three fields observed in this Gemini South/Flamingos-I survey of the edge of the Trapezium Cluster at the core of the Orion Nebula. This edge field was selected to avoid saturation of the core region due to the sensitivity/depth of these observations. Indicated on this image are the two bipolar nebulae (“blow-ups” right) discovered with this data. Also indicated (left of center) is a grouping of six stars that is representative of a tendency for both stars and brown dwarfs to form in small groups of between four to eight sources. 

Flamingos-I was a visitor instrument built by the University of Florida.

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The observations were made in J, H, and K bands (Figure 1) during observing runs in 2001 and 2002. Three fields were imaged for one hour each in K and H bands and two hours in J. The depth of these data allowed the identification of a variety of new low-mass objects, and led to comparisons with other independent surveys of adjacent regions, resulting in statistically significant conclusions.

The survey’s results are expected to provide a wealth of targets for future research in this star-forming region due to its depth and resolution. An ongoing study by the team is a spectroscopic follow-up with Gemini of the planetary mass objects (PMOs) of M < 13 MJup.  Due to an increase in background contamination at this luminosity, the present study puts the upper limit of 3-13 MJup PMOs at 13% and likely at <10% of the cluster population (assuming an age of one million years for cluster members).

Figure 2. Mass distribution of low-mass objects (at an assumed age of 1 Myr) in the surveyed area of the Trapezium region. The IMF peaks at 0.2 Msun, and declines through the brown dwarf regime.  The increase at Log(M/Msun) = - 2.4 is due to background contamination.

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In addition to the identification of two new bipolar nebulae with bright central sources in the region, the study also found multiple trails and filaments that appear to be associated with low luminosity point sources. Others have no apparent point-source associations.

Another result of the survey analysis is that binary pairs of brown dwarfs and very low-mass stars (M<0.1Msun) are less likely than more massive stars to have wide separations (>150 AU) between them and  any binary companions. When this is combined with an independent Subaru sample (of a different region in the Trapezium cluster), this relationship is confirmed to a 96% confidence level. Furthermore, a statistically very significant excess was found for both stars and brown dwarf pairs with small separations (< 6 arcseconds or 2,600 AU), which is consistent with the "ejected stellar embryo" hypothesis for brown dwarf formation. (See previous Gemini study of very low mass stars by Laird Close et al.)

For more details, see the article “A Deep Survey of Brown Dwarfs in Orion with Gemini,” by P.W. Lucas, P.F. Roche and M. Tamura in Monthly Notices of the Royal Astronomical Society, April 27, 2005.