Andrew Yee
November 10th 05, 03:07 AM
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/aa/pdf/press-releases/PRAA200511.pdf
IMAGE CAPTIONS:
[Fig. 1:
http://www.edpsciences.org/papers/aa/abs/press-releases/PRaa200511/model_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/abs/press-releases/PRaa200511/comb-fig_normal.gif
(238KB)]
Observations of the HII region RCW 79 at various wavelengths.
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/aa/pdf/press-releases/PRAA200511.pdf
IMAGE CAPTIONS:
[Fig. 1:
http://www.edpsciences.org/papers/aa/abs/press-releases/PRaa200511/model_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/abs/press-releases/PRaa200511/comb-fig_normal.gif
(238KB)]
Observations of the HII region RCW 79 at various wavelengths.