1. YSO/PMS disk types, time-scales and evolution
Luke Thomas Maud – ESAC Trainee 2009
Bruno Merin – Herschel Science centre (Supervisor)‏
Hervé Bouy – Herschel Science centre
(Research Fellow)

2. Outline Formation Scenario The Sample Spectra Categorization
Mass and Age Estimates
Results
Conclusion and Possible Interpretation
The Future

3. YSO Formation Using IR we can probe clouds and ‘see’ YSOs inside:
Young stellar objects form in molecular clouds, opaque to optical light
Clouds collapse, cores form..
Angular momentum conservation generates a spinning disk as material falls inwards..
As the disks evolve they disperse/evaporate and the central star joins the main sequence...
It is possible that a planetary system is created.

4. Formation – Spectral Energy Distributions
Since the late 80s investigations have been undertaken to evaluate the evolution of YSOs
Adams, Lada and Shu 1987 developed the ‘classical’ classification system for YSOs
Using Spectral Energy Distributions (SEDs) one can see how the flux of the emission varies
Thus how the YSO evolves
This however is the ‘general’ model and classes are based upon a slope from 2 to 24 um wavelengths

5. The Sample
IC 348 Perseus
Rho Ophiuchus
NGC 1333 Perseus
Serpens Core
This project is a continuation of previous papers investigating the 5 close star forming regions from the Cores to Disks (c2d) Spitzer legacy program (Evans et al 2009)
The initial sample had photometry for 1024 YSO candidates with estimated ages of Myrs
Encompassing the timescales of disk dissipation previously observed for low mass stars and probing slightly different environments

6. The Sample
However after analysis, the C2D YSOs had unexplainable age attributes
The problem is due to the degeneracy involved in SED fitting
We created a new sample of objects that had full spectroscopic data, this means the Spectral types of the stars could be constrained
We include only stars that are Class II and III from (Greene et al. 1994)‏
II -1.6 ≤ α < -0.3, III α < -1.6
We now have a fully usable sample of 819 YSOs

7. Spectra Energy Distributions – (Spectra)
For all the targets in the sample we create SEDs
Thus we plot the wavelength of photometric data vs. the flux at that wavelength

8. Classification
In comparison to the basic Class II and III we see a more diverse range of SEDs:

9. Classification
So we define limits of classification with reference to the median CTTS and the fitted photospheres 9

10. Mass and Age Estimates
We get temperatures from the spectral types and luminosities from the fitted photospheres
These are passed to HR diagrams for plotting against PMS tracks computed be Baraffe et al (1998) and Siess et al (2000)
Sanity check that we are not doing anything ‘silly’ as the objects fit on the HR diagram.

11. Mass and Age Estimates We group the objects into 4 groups now
These 664 targets have excellent SED fits and have mass and age constraints from the HR fitting:
Resulting SEDs: Good (81%)
Over-Lum (5%) Under-Lum (5%) Strange (9%)
We group the objects into 4 groups now
Under Luminous – Sources below the tracks – Need to be further away
Over-Luminous – Sources above the tracks – Need to be closer
Strange – Objects with non-fitting and ‘weird’ SEDs – Would require extra analysis
Final – Objects used in the main results – 664

12. Results
The resultant mass and age spread is sensible, and within expected values for Low mass star forming regions:
We appear to represent stellar masses down to ~0.03Mo and up to ~ 3.5 Mo

13. M < 0.5 Mo, 0.5 < M < 1.5 Mo and M > 1.5Mo
Results
We divide the final group in Mass ranges of:
M < 0.5 Mo, 0.5 < M < 1.5 Mo and M > 1.5Mo
Separating the stars’ physical properties
Fully convective, radiative core/convective envelope and convective core/radiative envelope respectively
And Age ranges of:
1 – 5 Myrs, 5 – 10 Myrs and 10 – 20 Myrs
To cover the general disk dispersal time scales and evolution
We produce 9 pie charts rich with information

14. Conclusions - Interpretation
Speculation
As it appears that disks around lower mass stars last longer
more time for instabilities and coagulation of particles
potential home for rocky planets?
The most massive stars’ disks disperse fast
timescales preferential for large ‘Jupiter’ planets that form over short times in rich disks (we need disk mass)
Latest statistics (Udry et al. 2007) suggest low mass stars have low mass multiple planet systems while high mass show single migrated Jupiter mass ones (obviously still limited in technology)
Conclusions - Interpretation
The Spitzer data allows the new categorization of the evolutionary stages of disks around YSOs via SEDs and clearly detects ‘transitionals’
Largest statistical sample 3-5x bigger than any previous and covers a larger range of ages and environments
Initial stages all appear the same (fisher test) – NEW RESULT!
More massive stars remove there disks faster, none are present after 10 Myrs
Primordial class disks are still evident at ages above 10 Myrs for stars with M < 1.5Mo
Mass plays a role in evolution – Has been suggested but this is evidence!

15. Future
The age constrains are the current best with simple HR fitting, however do not account for accretion luminosity and a more robust method is required
(under-luminous if not accreting = older)
To calculate disk mass we require observations extending into the sub-mm
Herschel will provide this coverage and add a new perspective allowing comparisons of disk and stellar mass. One of the key parameters of the disk itself
Our method is applicable to the Gould belt clouds currently underway
With a much large sample
High resolution spectroscopy would help identify transitional disks
Binaries are ‘empty’ vs. planet harbouring disks
Alma and JWST will shed extra light on the data with increased sensitivity/resolution
Imaging will allow disk radii to be constrained

16. Thank You for your attention Any Questions Please

17. A Problem? - Scaling Note the ranges probed by the photometry (MIPS1)
F/A type at ~ 21AU
Solar type at ~ 7 AU
Low-mass at ~ 3 AU
The previous assumptions assume the disks scale with mass and are all comparable – however they may not scale directly (1:1)
Thus for are low mass sources in the pie charts classified using a different range in the SEDs??

18. Scaling
NO, due to the nature of SED with the logarithmic scale we see only a small shift of data points
This would therefore supports that stars and disk may scale together as one may expect (more mass, more gravity, more disk)

19. Additional Classification
To move allow another perspective independent of the larger fluctuation of age in HR fitting; we also present a model based upon the scheme of Cieza et al (2007)
Cieza use l turnoff and α excess – which describe the distance of the inner disk to the central star and the degree of opacity or thickness of the disk
These are grouped for the same masses as the pie charts

20. Additional Results
The ranges of the 3 mass groups appear to be comparable
Suggests the actual evolutionary sequence maybe the similar independent upon mass
Note the tight range of Primordial disk, with Transitional disks above α = 0 and Settled moving right and down
Again the aforementioned ‘scaling’ problem is dismissed as one would expect the turn-off ranges to be altered, but clearly they are similar