Brown Dwarfs in the Trapezium Region of the Orion Molecular Cloud Near-IR Images Obtained with HST/NICMOS Glenn Schneider, Steward Observatory, 933. N. Cherry Avenue, University of Arizona, Tucson, Arizona 85750 USA

BROWN DWARFS IN ORION'S TRAPEZIUM CLUSTER

The nearby Trapezium star cluster associated with the Orion star-forming molecular cloud (at a distance of about 1500 light years) has been the object of much scrutiny since the earliest days of telescopic observations.  Named for the trapezoid of blue-hot OB stars which dominate this cluster of over 300 very young stars and proto-stellar objects, the Trapezium cluster makes an ideal laboratory for studying the properties of neo-natal stellar and sub-stellar objects.  The cluster lies on the "front" side of the molecular cloud (as viewed from the Earth), the womb from which these newly-formed objects have emerged, and within a cavity in that cloud carved out by the ionizing radiation of the dominant Trapezium stars so easily seen even in the smallest back-yard telescopes.  The proximity of the cluster to the Earth, and the fortuitous viewing geometry which leaves many of its members relatively unobscured even at visible wavelengths, have made the Orion Nebula's Trapezium cluster not only one of the most well- known astronomical objects, but one of the most intensely observed and studied regions in our galaxy.

In 1994 and 1995 O'dell and collaborators observed the Trapezium cluster with the Hubble Space Telescope's Wide-Field and Planetary Camera 2 (WFPC-2) and discovered a new class of objects -- young proto-stars enshrouded in their disk-like birth cocoons seen in silhouette against the back-light of the nebula (STScI-PRC95-45).  More recently, the Near Infrared Camera and Multi-Object Spectrometer's (NICMOS) Instrument Definition Team obtained new images of the central region of the cluster, one light-year on a side, in two near infra-red "colors" revealing more deeply the multitude of young stars and brown dwarfs born within approximately the last million years. The higher resolution WFPC-2 images were taken in very narrow regions of the shorter-wavelength optical spectrum where the light from ionized and excited atoms provide detailed information on the structure and properties of the proto-planetary sillouhette disks, and the large-scale shock-fronts which pervade the region.  The broader-band, and longer-wavelength NICMOS images probe more deeply into the nebulosity, imaging individual sub-stellar objects with masses as small as 1% that of the Sun.

Taken as part of a survey of three young clusters conducted under a NICMOS guaranteed time observing program by Erik Young (U. Arizona, PI, GTO/7217), an analysis of the new Trapezium cluster images prepared by Glenn Schneider (U. Arizona) and the NICMOS IDT, are discussed by Kevin Luhman (Harvard-Smithsonian CfA) and collaborators in a paper published in the 20 September 2000 issue if the Astrophysical Journal. The investigators are seeking to learn the form and universality (or lack thereof) of "The Initial Mass Function {IMF} of Stars and Brown Dwarfs in Young Clusters".  The IMF describes at what frequency objects of different masses will arise as they form from their parent molecular clouds.  How dependent is the IMF on the properties of the cloud itself, and of the co-spatial stellar population? The NICMOS observations gives insights into these questions for both stars and brown dwarfs.

Compared with the other clusters studied of comparable ages, Luhman and his associates find the IMFs for low-mass stars similar over a wide range of stellar densities.  The IMF peaks with stars about 80% as massive as the sun, declines with both higher and lower masses, but flattens out in the regime of the brown dwarfs.  If the masses were determined, for example, by interactions between the stars, the form of the IMF would depend upon the space density of the stars.  This, however, is not seen in these data.

Brown dwarfs are objects which are not sufficiently massive to sustain nuclear-burning of hydrogen in their interiors.  Though the more massive ones "burn" deuterium in their early evolution, they are destined to turn-off and cool with time like a dying ember.  Brown dwarfs occupy a niche in the astronomical bestiary between low-mass stars and planets.  The NICMOS Trapezium images reveal a population of about 50 young brown dwarfs, to the sensitivity limits of the data of about 10 Jupiters (assuming the average age of the cluster). The relative abundance of brown dwarfs is unexpected from some models of both of how molecular clouds break up, and how material will coalesce until checked by the winds from the newly forming proto or sub-stellar objects.

Recently, many old isolated "field" brown dwarfs have been discovered, from large ground-based surveys such as the 2-micron All-Sky and Sloan Digital Sky surveys, and subsequently confirmed spectroscopically.  Indeed, it has been suggested that brown dwarfs, once considered somewhat exotic objects, may be as common as stars.  And, a number of brown dwarf companions to young stars have been strongly suggested by NICMOS coronagraphic observations.  Are the formation processes for these fundamentally different (eg., do companion brown dwarfs form in a process more like planets than stars in clusters like the Trapezium)?  This remains to be answered.

IMAGE COMPOSITION

As a quick reference, the five downloadable images are shown below (at 1/4 size) in the panel which illustrates how they were composited.  Click anywhere on the panel to bring up the full size image.

CLICK HERE or an explanation of what the images are and how they were made.

DOWNLOADABLE IMAGES

Here, you can find five images of the Trapezium star forming region in Orion, in a variety of file formats.  All images have been oriented so that North is 1.4° clockwise from vertical (with East essentially to the "left").  As the detector was oriented with North very close to vertical, I choose not have the images degraded by re-sampled interpolation from a 1.4° rotation to put north exactly up.  Note: The PostScript images have been rescaled from an original 700x701 pixel size to fit on a standard 8-1/2"x11" page with normal margins. Also, the TIFF images may not be readable by all TIFF readers, but they are in Adobe Photoshop.

FIGURE CAPTION: (for the NICMOS F110W+F160W image.)

The rich star-forming region of the Trapezium cluster in the Orion Nebula was imaged with the Near Infrared Camera and Multiobject Spectrometer (NICMOS) on January 12, 1998.  Camera 3, NICMOS's "wide field" imager which provides a field-of-view of 52" on a side (about 3% of the apparent diameter of the moon) was used with two near-infrared filters to obtain information on the colors of the stars at two wavelengths (1.1 and 1.6 microns).  The Trapezium field in this image is 140" on a side, covering an area of 1 square light-year.  The image was made from a "mosaic" of nine separate, but adjoining images, and were later combined.  The bright nebulosity of the region is very effectively penetrated in the near infrared and reveals stars and brown dwarfs which are both obscured and intrinsically faint in the visible regions of the spectrum.  This composite F110W (1.1 micron) and F160W (1.6 micron) image captures more than 300 stellar and sub- stellar objects, as faint as 17th magnitude at the longer (redder) wavelength.  Here, objects which are blue are hotter and less deeply embedded, and cooler (intrinsically lower mass) objects or those more deeply embedded in the cloud are red.  These images were taken during the NICMOS "Camera 3 campaign" when the HST secondary mirror was moved to take advantage of the best theoretical performance of the Camera 3 optical system.

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News/photo release by the Space Telescope Science Institute.

ASTRONOMY PICTURE OF THE DAY for August 30, 2000.

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