1. Induced Star Formation in the Orion and Monoceros Molecular Clouds Hsu-Tai Lee and Wen-Ping Chen National Central University, Institute of Astronomy

2. Outline Introduction Source of Data and Observations Individual Molecular Cloud Discussion

3. Introduction The Orion & Monoceros molecular clouds are 2 nearby active star-forming regions Orion OB1 association (450 pc) Mon R2 (830 pc) Orion-Eridanus bubble – a series of supernova explosions in the Orion OB1

4. Triggered Star Formation Elmegreen, B. G. 1998, ASP Conf. Ser., 148 Small Scale: Globule-Squeezing Intermediate Scale: Collect and Collapse Large Scale: Shells and Rings

5. Small scale triggering Direct squeezing of pre-existing clouds or globules by high pressure. This includes triggering in bright rims, proplyds, and small cometary globules. OB stars Molecular cloud Young stars

6. Intermediate scale triggering Compression of a nearby pre-existing cloud from one side, leading to a dense ridge of moving gas. OB stars Molecular cloud Young stars

7. Large scale triggering Accumulation of ambient gas into an expanding shell or ring partially surrounding the pressure source, with star formation in the shell or ring presumably triggered by gravitational collapse of swept- up gas.

8. Pre-Main Sequence Star Classical T Tauri star (CTTS) Strong IR and UV excess Strong emission line Weak-lined T Tauri star (WTTS) More evolved pre-main sequence star Weak emission line Strong soft X-ray source

9. CTTSWTTS

10. Source of Data 2MASS – J (1.25 μ m), H (1.65 μ m) and Ks (2.17 μ m) all sky survey CO image – Dame, T. M. et al.(2001, ApJ) IRAS 100 μ m – Schlegel, D. et al. ( 1998, ApJ) Hα image – Finkbeiner, D. P. (2003, ApJS)

11. Observations BAO 2.1m spectroscopic observations in early 2003 Low-resolution spectra have been observed for 32 PMS candidates 24 CTTSs, 4 M stars and 4 carbon stars Most of CTTSs are located at molecular cloud; M and carbon stars are not associated with molecular cloud

12. HαHα HβHβ

13. Maddalena et al. 1986

14. The Red dots are the CTTSs; blue dots are the O stars. CO imageHα image

15. Orion A & B Molecular Cloud

16. TheλOri is a O8III star, and it is surrounded by a ring molecular cloud (Duerr, Imhoff, & Lada, 1982, ApJ) IRAS 100μmHα image

17. IC 2118, LDN 1616 & LDN 1634 Hα imageIRAS 100μm LDN 1616 IC 2118LDN 1634

18. LDN 1652 Hα imageCO image

19. objectscalePressure source λOri regionSHigh mass star IC 2118 regionSHigh mass star Orion A & BISuperbubble LDN 1652L or ISupperbubble S – small scale triggering; I – intermediate scale triggering; L – large scale triggering Triggered star formation

20. Discussion I Our selection criterion is good to find CTTSs. For those bright-rimmed clouds associated with both strong Hα and IR emission accompany star formation High mass star interact with molecular cloud would trigger star formation Superbubble can also induce star formation hundreds pc away (LDN 1652)

21. Discussion II CTTSs outside the bright-rimmed cloud seem to be older than those inside the bright-rimmed cloud. Compared with WTTSs, CTTSs are more closer to recent star formation. Our CTTS candidates can be used to trace the nearby star-forming region (< 2 kpc).