Triggered Star Formation by Massive Stars in Star-forming Regions Wen-Ping Chen & Hsu-Tai Lee NCU/Astronomy BATC Workshop Weihai NGC6823 by BATC

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 Solar System Formation in a Nutshell

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.

Radiation-Driven Implosion (RDI) shapes nearby clouds. An O star forms in the GMC. Sequential star formation takes place. 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.

Strom et al dwarfs giants reddening

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) Taurus50%1.5 Myr (Kenyon & Hartmann 1995) CMa R150%1.5 Myr (Soares & Bica, 2002) Trapezium cluster60%~1 Myr IC 34820%5 – 7 Myr (Lada & Lada 1995) η Chamaeleon27%9 Myr (Lyo 2003)

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) All stars in a star cluster lose their disks

ISO N-band excess vs. age for stellar samples of varying ages (Mamajek et al. 2004, ApJ, 612, 496) Circumstellar dust disks become optically thin at N band by ~20 Myr

Fig. 3: Low-resolution spectrum of a Herbig Ae/Be star shows the prominent λ6563 H α in emission. Fig. 4: Low-resolution spectrum of a T Tauri star shows the prominent λ6563 H α in emission. Our young star sample is selected from 2MASS star catalog with (1) good S/N, (2) YSO colors, (3) stellar “shapes”.

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 (J limit ~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.

NGC6823 by BATC Next: NGC 6823 with 2MASS and B Next: NGC 6823 with 2MASS and BATC H-alpha and [S II]