How do massive stars form? (Forwarded)

EDP Sciences
Les Ulis, France

Contact persons:

Science:
Dr. Annie Zavagno
Laboratoire d'Astrophysique de Marseille
2, place Le Verrier
13248 Marseille, Cedex 4
Phone: +33 (0)4 95 04 41 55

Press office:
Dr. Jennifer Martin
Journal Astronomy & Astrophysics
61, avenue de l'Observatoire
75014 Paris, France
Phone: +33 1 43 29 05 41

Released: November 8th, 2005

How do massive stars form?

A case of triggered high-mass stars formation

"Triggered massive-star formation on the borders of Galactic HII
regions. II. Evidence for the collect and collapse process around RCW
79", by Zavagno et al.

To be published in Astronomy & Astrophysics.

This press release is issued as a collaboration with the Observatoire
Astronomique de Marseille Provence and the Centre National de la
Recherche Scientifique.

The most complete picture of a "triggered" star-forming region will be
published in an upcoming issue of the journal Astronomy & Astrophysics.
Combining observational data from various wavelengths, the team led by
A. Zavagno and L. Deharveng found that the different structures in the
whole region are morphologically related, and these morphological
comparisons help in building a complete view of how stars form there.

In an upcoming issue, the journal Astronomy & Astrophysics will publish
the most complete picture of a "triggered" star-forming region. Induced
(or "triggered") star formation is one of the processes that are
supposed to lead to the formation of massive stars. Massive stars play a
key role in the chemical and dynamical evolution of galaxies. The way
massive stars form is still much debated among the astronomers'
community: it is currently one of the hottest astrophysical topics. Do
they form by accretion as low-mass stars do or do they need the
environment of a dense cluster to form through the merging of low mass
protostars?

In this framework, the team led by Annie Zavagno and Lise Deharveng
(from the Laboratoire d'Astrophysique de Marseille, France) selected
regions where several generations of massive stars are likely to be
formed. Stars more massive than 8 solar masses, once formed, emit
intense UV photons that ionize the surrounding gas. The region filled
with ionized hydrogen is called an HII region. Theory suggests that the
expansion of the HII region can trigger massive star formation: after
the HII region has formed, it expands continuously because the
temperature inside the region is much higher than in the cold
environment that surrounds it. During the expansion, a dense layer of
gas and dust is collected around the HII region, after which
gravitational instabilities in the layer cause it to fragment into dense
clumps, which then go on to collapse into new stars. The fragments are
massive and thus form massive objects (stars or clusters). The
successive steps of this process, called collect and collapse process,
are shown in Figure 1.

To characterize this process, the team selected the Galactic HII region
RCW 79, located 14000 light-years from the Earth. They combined
observational data obtained at different wavelengths, as well as from
various origins (space, ground-based telescopes, and archived
observations), to probe different parts of the region, as illustrated in
Figure 2.

The orange image was obtained in the infrared range with NASA's Spitzer
Space Telescope: it depicts with high precision the dust shell that
surrounds the HII region RCW 79. The blue part of the image corresponds
to the H-alpha emission line that probes the ionized hydrogen
(observations from the SuperCOSMOS Sky Survey): clearly, the shell is
filled in by ionized hydrogen. The team then obtained their own set of
observations to elucidate the complete picture of the star-forming
region. The yellow contours correspond to observations obtained at
millimeter wavelengths with the ESO Swedish Submillimetre Telescope
(SEST). These contours depict cold dust condensations in the shell
structure. The team has identified the newly-formed stars associated
with these condensations, using mid-infrared Spitzer observations from
the GLIMPSE survey. They find that second-generation massive stars (with
mass higher than 8 solar masses) are associated with the main
condensations. One of these condensations was observed at near-infrared
wavelengths with the ESO-New Technology Telescope (see insert in Figure
2). It includes a massive star that is evolved enough to emit
high-energy photons and to give rise to a compact HII region. This
compact HII region is thus a second-generation HII region.

The locations of all the structures that were picked out at various
wavelengths agree very well with the predictions of the collect and
collapse process. The conclusions drawn by the team largely rely on the
morphological relations between these structures. The combined picture
of RCW 79 they obtained is therefore a straightforward illustration of
the triggered massive-star formation process that now occurs in this
region. These observations show that the collect and collapse process is
the main triggering agent of massive star formation observed on the
borders of this region.

[1] The team is made of A. Zavagno, L. Deharveng (France), F. Comeron
(Germany), J. Brand, F. Massy (Italy), J. Caplan, and D. Russeil (France).

Triggered massive-star formation on the borders of Galactic HII regions.
II. Evidence for the collect and collapse process around RCW 79
By A. Zavagno, L. Deharveng, F. Comeron, J. Brand, F. Massy, J. Caplan,
and D. Russeil.
To be published in Astronomy & Astrophysics (DOI number:
10.1051/0004-6361:20053952)

Full article available in PDF format,

http://www.edpsciences.org/articles/...PRAA200511.pdf

IMAGE CAPTIONS:

[Fig. 1:
http://www.edpsciences.org/papers/aa...del_normal.gif
(46KB)]
The collect and collapse process: a way of triggering the formation of
massive stars.

[Fig. 2:
http://www.edpsciences.org/papers/aa...fig_normal.gif
(238KB)]
Observations of the HII region RCW 79 at various wavelengths.