The Photoionization of a Star-forming Core in the Trifid Nebula
B. Lefloch, J. Cernicharo, L. F. Rodriguez, M. A. Miville-Deschênes, D. Cesarsky, A. Heras;
ApJ, 2002, 581, 335
ABSTRACT:We have carried out a comprehensive multiwavelength study of the bright-rimmed globule TC2 in the Trifid Nebula, using the IRAM 30 m telescope, the VLA
centimeter array, and the Infrared Space Observatory (ISO).
TC2 is one of the very
few globules to exhibit signs of active ongoing star formation while being
photoevaporated by the Lyman continuum flux of the exciting star of the nebula
(~1010 cm-2 s-1).
The globule consists of a cold
dense core of mass 27 Msolar surrounded by a lower density
envelope of molecular gas.
The impinging Lyman continuum photons induce the
propagation of an ionization front into the globule.
The evaporation of the
ionized gas forms a thin layer of density
ne=(1-2)×103 cm-3 around the globule, which could be mapped with the
VLA.
The globule is illuminated mainly on its rear side, by a far-ultraviolet
field of intensity G0~=1000.
It creates a photon-dominated
region (PDR) below the surface, which was mapped and characterized with the
ISOCAM circular variable filter and the Short Wavelength Spectrometer (SWS)
on board ISO.
The physical conditions derived from the analysis of the
far-infrared lines [O I] 63, 145 μm and [C II] 158 μm and the continuum emission
are in good agreement with some recent PDR models.
The emission of the
polycyclic aromatic hydrocarbon band at 6.2, 7.7, 8.6, and 11.3 μm is detected
over the whole globule.
The relative intensity variations observed
across the globule, in the PDR and the photoionized envelope, are consistent
with the changes in the ionization fraction.
In the head of TC2, we find a
second kinematic component, which is the signature of the radiatively driven
collapse undergone by the globule.
This component indicates that the PDR
propagates at low velocity inside the body of TC2.
The molecular emission
suggests that the star formation process was probably initiated a few times
105 years ago, in the large burst that led to the formation of the
nebula.
The globule has already evaporated half the mass of its
envelope.
However, the ionization timescale of the globule is long enough (~2 Myr) to let
the protostellar objects reach smoothly the ultimate stages of
protostellar evolution.
The impact of photoionization on the star formation
process appears limited.
KEYWORDS: ism: dust, extinction, ism: h ii regions, ism: globules, ism: individual: name: trifid nebula, ism: jets and outflows, stars: formation
CODE: lefloch2002