Protostars -- Oral Presentation
Spectacular Spitzer images of the Trifid Nebula: Protostars in a
young, massive-star-forming region
Jeonghee Rho, Spitzer Science Center/Caltech
W. T. Reach (SSC/Caltech), B. Lefloch (LAOG) and G. Fazio (CFA)
Spitzer IRAC and MIPS images of the Trifid Nebula (M20) reveal
its spectacular appearance in infrared light, demonstrating its special
evolutionary stage: recently-formed massive protostars and numerous
young stars, including a single O star that illuminates the surrounding
molecular cloud from which it formed and unveiling large-scale, filamentary
dark clouds. The hot dust grains show contrasting infrared colors in
shells, arcs, bow-shocks and dark cores. Multiple protostars, previously
defined as Class 0 from dust continuum and molecular outflow observations,
are revealed in the infrared within the cold dust continuum peaks TC3
and TC4. The cold dust continuum cores of TC1 and TC2 contain only one
protostar each; the newly-discovered infrared protostar in TC2 is the
driving source of the HH399 jet. The Spitzer color-color diagram allowed
us to identify ~150 young stellar objects (YSO) and classify them into
different evolutionary stages, and also revealed a new class of YSO
which are bright at 24$\mu$m but with spectral energy distribution
peaking at 5-8$\mu$m; we name these sources ``Hot excess'' YSO. Despite
of expectation that Class 0 sources would be ``starless'' cores, the
Spitzer images, with unprecedented sensitivity, uncover mid-infrared
emission from these Class 0 protostars. The mid-infrared detections of
Class 0 protostars show that the emission escapes the dense, cold
envelope of young protostars; the mid-infrared emission cannot arise
from the same location as the mm-wave emission, and instead must arise
from a much smaller region with less intervening extinction to the
central accretion. The presence of multiple protostars within the cold
cores of Class 0 objects implies that clustering occurs at this early
stage of star formation. The most massive stars are located at the
center of the cluster and are formed simultaneously with low-mass stars.
The angular and mass distributions of protostars within the dust cores
imply that these early protostars are competing for materials and
the clustering is consistent with competitive accretion. We also compare
statistics and characteristics of the sources detected at infrared
wavelengths by Spitzer with the discrete X-ray sources detected by
Chandra in the Trifid Nebula. By generating spectral energy distributions
of these discrete X-ray sources using data from Spitzer observations (as
well as complementary near-infrared and millimeter data), we can classify
the detected young stellar objects into their evolutionary stages. We
demonstrate the advantage of our multi-wavelength complementary study in
understanding a complete population of young stars in a star-forming
region and the evolution of protostars and disks.