Transcript Triggered Star Formation by Massive Stars in Star

Triggered Star Formation
by
Massive Stars
in
Star-forming Regions
Wen-Ping Chen & Hsu-Tai Lee
NCU/Astronomy
BATC Workshop 2005.08.11 Weihai
NGC6823 by BATC
Solar System Formation in a Nutshell
The Sun and planets were
formed out of an interstellar
molecular cloud.
Cloud collapsed
→ central T 
→ nuclear fusion
 Sun
→ dust coagulated in circumstellar disk
→ planetesimals (asteroids)
 planets
If cloud massive enough
 fragmented to form a
star cluster
For a giant molecular cloud
 an OB association
But how does this come about?
(Why is the IMF in OB associations
similar to that in the field?)
Do massive and low-mass stars form
at the same time (and at the same place)?
Or, there are different modes of SF?
Which kind of stars, massive or low-mass stars, form first?
Triggered Star Formation by
Massive Stars
• A massive star can be a double-edged sword in
subsequent SF (1) it may disrupt and disperse
the cloud, hence prohibiting any further SF, or
(2) it could provide “just the right touch” to
induce the cloud to collapse which otherwise
may not occur.
• Our working hypothesis: Massive stars play a
constructive role in certain environments.
Bright-Rimmed Clouds and
Comet-shaped Clouds in
Orion OB1 and Lac OB1
associations
Bright-rimmed clouds in the
Orion star-forming region.
An O star forms in the GMC.
Sequential star formation
takes place.
Radiation-Driven Implosion
(RDI) shapes nearby clouds.
An OB association is formed.
Observational Diagnosis
A triggered star formation process has several imprints which
can be observationally diagnosed:
• The remnant cloud is extended toward, or pointing to, the
massive stars.
• The young stellar groupings in the region are roughly lined
up between the remnant cloud and the luminous star.
• Stars closer to the cloud, formed later in the sequence, are
younger in age, with the youngest stars at the interacting
region (i.e., bright rims of the cloud).
• There are no young stars within the BRC.
(3) and (4) are noticeably in contrast to the case of
spontaneous star formation which conceivably would not
leave such distinguishing sequential and positional
signposts.
reddening
giants
Strom et al. 1995
dwarfs
Ojha et al. 2004, ApJ, 616, 1042
Different PMS
populations (CTTSs,
WTTSs, HAEBEs)
occupy distinctly
different regions in
the NIR color-color
diagram (Lee et al.,
2005)
IR Excess and Age
Fraction of sources with IR excess
 Ophiuchus
50-70% < 1 Myr (Greene & Meyer 1995)
Taurus
50%
1.5 Myr
(Kenyon & Hartmann 1995)
CMa R1
50%
1.5 Myr (Soares & Bica, 2002)
Trapezium cluster 60%
~1 Myr
IC 348
20%
5 – 7 Myr (Lada & Lada 1995)
η Chamaeleon
27%
9 Myr (Lyo 2003)
All stars in a
star cluster lose
their disks
JHKL excess/disk
fraction as a
function of mean
cluster age. The
decline in the disk
fraction as a
function of age
suggests a disk
lifetime of 6 Myr.
(Haisch & E. Lada,
2001)
Circumstellar dust disks
become optically thin at
N band by ~20 Myr
ISO N-band excess vs. age for stellar samples of
varying ages (Mamajek et al. 2004, ApJ, 612, 496)
Our young star sample is selected from 2MASS
star catalog with (1) good S/N, (2) YSO colors,
(3) stellar “shapes”.
Fig. 3: Low-resolution spectrum
of a Herbig Ae/Be star shows the
prominent λ6563 Hαin emission.
Fig. 4: Low-resolution spectrum of
a T Tauri star shows the prominent
λ6563 Hαin emission.
Spectroscopy shows that our criteria of selecting
YSOs from the 2MASS database are very
effective. Out of 32 candidates, 24 were
confirmed to be bona fide young stars, with the
reaming 4 as M dwarfs and 4 as carbon stars.
Other related project: Herbig Ae/Be stars in
open clusters, e.g., NGC 1857; Wolf-Rayet stars
in OB associations.
CTTSs with forbidden lines are
by and large younger.
CTTSs physically closer to the BRCs are
systematically younger.
This is because they were
formed later in the sequence.
In general, extinction in BRCs is not large,
so any CTTSs could not escape from
2MASS detection (Jlimit~15 mag).
This is because the BRCs have been
engraved by the UV photons of O stars.
Indeed, we find
(1)The youngest CTTSs are located at the
interaction layers (bright rims).
(2) There are no CTTSs leading the
ionization fronts into the clouds.
(3) Within the clouds only class 0 and I
sources exist. They present the present
star-forming activities.
Conclusions
We find compelling evidence for the triggering process to
dominate star formation in BRCs, comet-shaped clouds
(both median-scaled) and superbubbles (large-scales)
that
•
The remnant cloud is extended toward, or pointing to,
the massive stars (currently in existence or already
exploded as a SN). (cloud morphology)
•
The young stellar groupings in the region are roughly
lined up between the remnant cloud and the luminous
star. (Star formation history)
•
Stars closer to the cloud, formed later in the sequence,
are younger in age, with the youngest stars at the
interacting region (i.e., bright rims of the cloud).
•
There are no young stars within the BRC.
Next: NGC
6823
with 2MASS
Next:
NGC
6823
with and BATC
H-alpha and
[S II]
2MASS
and
B
NGC6823 by BATC