1. Formation of stellar systems: The evolution of SED (low mass star formation) Class 0 –The core is cold, 20-30K Class I –An infrared excess appears Class II –Classical T Tauri Star (CTTS) –The peak shifts as a disk forms Class III –Weak-lined T Tauri Star (WTTS) –The disk dissipates André, 2002, EAS(vol. 3)

2. Formation of stellar systems: 5. Class II and Class III (Definitions) Class II Based on the slope (as previous slides), –Classical T Tauri Star (CTTS) –After envelope infall has ceased, dusty disk produce IR emission. SED is much broader than a single blackbody Class III Based on the slope (as previous slides), –Weak-lined T Tauri Star (WTTS) –The disk dust is dissipated or coagulated. In the SED, emission from the central star and only small contribution from the disk are detectable - Disk Property 153 disks in the Taurus- Auriga star formation region (Andrews & Williams, 2008) Based

3. Formation of stellar systems: CTTS (Class II) Classical T Tauri Star T Tauri Star with strong Hα emission line - much brighter than other stars of similar T in IR –Dust in disk absorb light from central star and reradiate @ IR –‘reprocessing’ or ‘irradiated’( or ‘passive’) disk –Disk accretion produces jets and winds Figure (GM Aur star) –Scattered light (top left), Model (top right), and SED (below) –Model for the stellar photosphere emission (dark line) Model combining the stellar emission and the disk excess emission (white line) Watson et al., ‘Multi-wavelength imaging of YSO disks...’ Scattered light Model SED

4. Formation of stellar systems: Property of the disk Flat disk? Or Flared disk? –‘Flared disk’ : generally become proportionately thicker with increasing radius Milla-Gabet et al., ‘The circumstellar environments of..’ Lee Hartmann, 2000, Accretion Processes in Star Formation c.f) Spectral Index (s) is from

5. Class I Class II Class III Weak-lined T Tauri Star : T Tauri Star which has no strong optical excess emission @ NIR –Emission from the central star is only detectable –No jets or massive outflows –Solar-type magnetic activity with low-mass, pre-main- sequence star –Nominal definition : Young star with –Narrow Hα emission line Formation of stellar systems: WTTS (Class III) Lee Hartmann, 2000, Accretion Processes in Star Formationis

6. Formation of stellar systems: CTTS & WTTS (1) Hα emission at 656.2 nm Figure : Hα profiles (as a function of the velocity shift from line center) Wide velocity width (± 200 km/s) is contributed to wind expansion –DF Tau : wide and strong Hα profile  CTTS –DI Tau : narrow and weak Hα profile  WTTS Lee Hartmann, 2000, Accretion Processes in Star Formation CTTS WTTS

7. Formation of stellar systems: CTTS & WTTS (2) WTTS nominal definition Young star with W λ (H α ) < 10 Å Redding-corrected (K-L) colors (flux ratios of 3.5 ㎛ emission to 2.25 ㎛ emissions) –(K-L) < 0.3 W λ (H α ) < 10 Å  WTTS –Otherwise,  CTTS W(Hα) < 10 Å (K-L) < 0.3 Lee Hartmann, 2000, Accretion Processes in Star Formationis

8. Formation of stellar systems: Observations (1) Aurora Sicilia-Aguilar, ‘Disk Evolution at the Ages of Planet Formation’ (doctoral dissertation) Two young clusters in Cep OB2 association –Tr 37 (red) is placed at the edge of bubble (Cep OB2) containing bright O6 star HD 206267 o ~165 low-mass candidate o The age of the cluster members around 4 Myr –NGC 7160 (blue) lies near the center of a bubble. o ~50 low-mass candidates o The age of the cluster members around 10 Myr Tr 37 (Red =24 ㎛, Green=8.0 ㎛, Blue=3.6 ㎛ )

9. Formation of stellar systems: Observations (2) Tr 37 members (upper) and NGC 7160 members (below) –WTTS (green) –CTTS (red) –HD 206267 (blue) in Tr37 –High- and intermediate-mass stars (open stars) –Filled symbols : confirmed or those where Li absorption is detected –Open figures : Li abs. is not detectable due to poor signal- to-noise Tr 37 NGC 7160

10. Formation of stellar systems: Observations (3) SEDs of low-mass stars in Tr 37 (avg 4 Myr)  about 40-45% of the members in Tr 37 (total ~165) have accreting, circumstellar disks –black dotted line : similar spectral type derived from Kenyon & Hartmann (1995) –magenta dashed line : the median disk emission in Taurus –light blue line : the median disk emission in Tr 37 ex. CTTS according to Hα, even if it shows no sign of a disk ex. WTTS according to Hα ex. CTTS disk @ 5.8 um and longer = presence of outer disk

11. Formation of stellar systems: Observations (4) SEDs of low-mass stars in NGC 7160 (about 10 Myr) (according to the same procedures followed for Tr 37)  only 1 sample shows indications of active accretion (CTTS). –black dotted line : similar spectral type derived from Kenyon & Hartmann (1995) –magenta dashed line : the median disk emission in Taurus –light blue line : the median disk emission in Tr 37 ex. WTTS according to Hα ex. CTTS according to H α

12. Formation of stellar systems: Observations (5) Evolution of protoplanetary disks –Theoretical isochrones from Siess et al. (2000) for 1, 10 and 100 Myr are shown together –CTTS (red circle) –WTTS (green triangle)  Significant difference in the apparent age scatter between Tr 37 (dispersion) and NGC 7160 (mostly located along the 10 Myr) Hot Cold Tr 37 NGC 7160

13. Formation of stellar systems: Observations (6) Low-mass SED from the ages 1 to 10 Myr. –Taurus (1-2 Myr) - red –Tr 37 globule (1 Myr) - cyan –Tr 37 (avg ~4.5 Myr) - violet –NGC 7160 (10 Myr) – pink –TW Hya (10 Myr) - black –The steep line slopes of the SED are shown in NGC 7160 and TW Hya (these are comparably older than others) 1-2 Myr1 Myr 4.5 Myr 10 Myr

15. Formation of stellar systems: CTTS & WTTS (1) Hα emission at 656.2 nm Hα line profiles (R m =‘magnetospheric radius) Wide velocities ( ± 200 km/s) due to wind expansion Figure –DF Tau : wide and strong Hα emission and blueshifted absorption (charactering mass ejection)  CTTS –DI Tau : narrow and weak Hα emission  WTTS Lee Hartmann, 2000, Accretion Processes in Star Formation CTTS WTTS

16. Observations (3) Identification –Hα emission @ 656.2 nm : Strong Hα emission is produced in the accreting  CTTS Weaker Hα emission is produced in the chromosphere of non-accreting  WTTS Therefore, Hα emission from accretion processes, the large velocities of the accreting material (that can be up to ~200 km/s) produce significant velocity wings and, in general, the equivalent width (EW) of the line is larger than what is observed in WTTS –Li absorption @ 670.7 nm : Very good Indicators of youth. It was detectable in about 90% of the low-mass, young stars in the clusters.