Radiative Transfer Models of Dusty YSOs
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Transcript of Radiative Transfer Models of Dusty YSOs
Radiative Transfer Models Radiative Transfer Models of Dusty YSOsof Dusty YSOs
Barbara Whitney (Space Science Institute), Tom Robitaille & Kenny Wood (St. Andrews University), Jon Bjorkman (U. Toledo), Remy
Indebetouw (U Va), Ed Churchwell (UW)
Outline
• Background and Motivation– Large Volumes of mid-IR data now available from
Spitzer Space Telescope, ground-based observatories and future space-based
• e.g., the GLIMPSE survey of the inner Galactic Plane
– Unanswered questions
• 2-D Models• 3-D Models (high mass)• Model Grid & Fitter• Answers to questions? A few, maybe
Canonical View of Low-Mass Star Formation
Dark cloud cores
• Free-fall times short, yet star formation efficiency low (Zuckerman & Evans 1974)
• Conditions for support/collapse– Magnetic fields/Ambipolar
diffusion (Shu 1977; Mouschovias 1976; Nakano 1976)
– Supersonic turbulence/local collapse (Mac Low & Klessen 2004)
t < 104 yrs
(Shu, Adams & Lizano 1987; Lada 1987)
Collapse -- Class 0
SED: T~30 K
t ~105 yrs
(Shu, Adams & Lizano 1987; Lada 1987)
Late Collapse -- Class I
SED slope, > 0, for2 < < 22 m
t ~106-107 yrs
Accretion Disk Stage -- Class II
SED slope, 0 > > -2
2 < < 22 m
t > 107 yrs
Debris or no Disk -- Class III
SED slope, < -22 < < 22 m
Massive Star Formation -- Competing theories
Analogous to low-mass (McKee & Tan 2003)
Mergers in dense clusters (Bonnell & Bate 2002)
0.5 pc 5 pc
Disk formation, collimated outlfows
Disk disruptionless collimated flows
Questions
• What are the global properties of star formation in the Galaxy? (GLIMPSE)– Star formation rate and efficiency– Timescales for evolution
• How do massive stars form?– Do they form planets?– Do low-mass stars in the vicinity of
massive stars form planets?
• What supports clouds against collapse?
Galactic Legacy Infrared Mid-Plane Survey Extraordinaire
• One of five Spitzer Legacy programs– No proprietary period +
enhanced data products
• 4 wavelength bands: 3.6, 4.5, 5.8, 8 mnew project, MIPSGAL, will get 24, 70, 160 !(PI: Sean Carey)
• b=[-1,+1], |l|=10-65GLIMPSE II: |l|<10 !
• Angular resolution <2”PI: Ed Churchwellwww.astro.wisc.edu/glimpse
GLIMPSE Data Products*
• GLIMPSE Point Source Catalog– Highly reliable (>99.5%) -- 31 million sources– Magnitude limits in 4 bands: 14.2, 14.1, 11.9, 9.5
• GLIMPSE Point Source Archive– Less reliable but more complete -- 48 million
sources– Magnitude limits: 14.5, 14.0, 13.0, 11.5
• Cleaned mosaic images– 1.1x0.8 degrees (0.6” pixels)– 3x2 degrees (1.2” pixels)– Southern hemisphere available in Dec. (all Spitzer
“BCD” images and mosaiced AORs are available)
*Available at http://www.astro.wisc.edu/glimpse/glimpsedata.html
Example of cluster formation?
tens of pc
Class 0 Source?
324.72+0.34
1-2-4 J-H-K
320.23-0.29
Ch 1,2,4
2MASS
332.73-0.61
317.35+0.01-2-4
3x2 deg
Radiative Transfer Models
• Monte Carlo method• 3-D spherical polar grid• Calculates radiative equilibrium of dust
(Bjorkman & Wood 2001)• Non-isotropic scattering + polarization• Output: images + SEDs (+ polarization)• Not included: PAHs, stochastic heating
of small grains, optically thick gas emission
(Whitney et al. 2003a,b, 2004)
2-D YSO Model Geometry• Rotationally-flattened infalling envelope
(Ulrich 1976)• Flared disk• Partially evacuated outflow cavity
AV through Envelope & Disk
Edge-on Pole-on
Low-Mass Protostar:
IRAS 04302+2247
L=0.5 Lsun
NIR 3-color (Padgett et al. 1999)
2-D RT models
Spitzer IRAC predictions
J-H-K [3.6]-]-[4.5]-[8.0] [24]-[70]-[160]
LateClass 0
Class I
(Whitney et al. 2003b)
IRAS 04368+2557
2MASS J-H-K Spitzer IRAC [3.6]-[4.5]-[8.0]
Low-mass
Analog?
Massive protostars
L*=40000T*=4000M*=17.5M=10-4
Md=1
Embedded Massive YSO
i Av
0 6
60 53
90 3e4
.
Embedded Low-Mass YSO
i Av
0 6
60 50
90 4e6
L*=1.1T*=4000M*=1M=10-5
Md=0.05
.
Massive Star+Disk
i Av
0 0
60 0.1
90 3e3
L*=40000T*=30000M*=17.5Md=0.1
Low-Mass Star + Disk
i Av
0 0
60 0.1
90 3e5
L*=40000T*=4000M*=17.5Md=0.01
Effect of Bipolar Cavity on Colors
• Models without cavities (e.g., 1-D) will underestimate evolutionary stage!
Near-IR IRAC
No cavity
cavity
Massive Stars: The need for 2-D, 3-D models
>100 m: no<100 m: yes
(van der Tak et al. 2000)
3-D models
• Motivation– UCHII regions: Previous 1-D models of
mid-IR spectra can’t fit full SED: give too deep 10 m absorption for a given FIR flux, and too steeply rising SED in NIR/MIR (Faison et al. 1998, van der Tak et al. 2000)
Model Ingredients
• O star in a molecular cloud (massive stars heat up large volumes)
• Use fractal ISM structure, D=2.6 (Elmegreen 1997)
• Average radial density profile is varied from r0 to r-2.5
• Smooth-to-clumpy ratio is varied from 3% to 100%
(Indebetouw et al. 2005)
Indebetouw et al. (2005)
IRAC MIPS
3-D clumpy modelsNIR
Clumpy model SEDs
Average Smooth (1-D) model
200 sightlines from 1 source (grey lines)
Fits to Data: G5.89-0.39
Best smooth modelBest clumpy modelGrey lines show other sight lines
Mid-IR data: Faison et al. (1998)
G5.89 Model parameters
Tstar 41000 K
L 2.54x105
Rin 0.0001 pc
Rout 2.5 pc
Menv 50000
Av_ave 131
Smooth/Clumpy 10%
Radial density ave~r0
Fractal dimension 2.6
Color-color plots
Smooth model
200 sightlines from 1 clumpy model
All the UCHII Observations
Grey lines: G5.89 best model
Mid-IR data: Faison et al. (1998)
3-D Model summary
• UCHII regions may be O-B stars still embedded in their natal molecular clouds but not surrounded by infalling envelopes.
• Bolometric flux of clumpy models varies by a factor of 2 lower and higher than the true luminosity depending of viewing angle
(Indebetouw et al. 2005)
2-D/3-D Model grid + Data fitter
• Large Grid of YSO Models (20,000) x 10 inclinations = 200,000 SEDs!6 weeks of cpu time on about 50 processors
• Linear Regression Fitter to find best model to fit an observed SED– Models are convolved with any broadband filter of
interest– First tries to find good fit from a grid of stellar
atmosphere files– Simultaneously fits foreground AV
– Can process the GLIMPSE survey in about a week
(Robitaille et al. 2005)
Grid Creation• Sample stellar mass and age (logarithmically)• calculate T* and R* from evolutionary tracks (Bernasconi & Maeder
1996; Siess et al. 2000)
Grid Parameters
198,680 SEDs
Relating Observed Class to Model “Stage”
Class Spectral Index (2-20 m)
I >0
II -2 - 0
III <-2
Stage Envelope Infall rate (Msun/yr/
M*1/2)
Disk mass
(M*)
I >2x10-6
II <2x10-6 >1x10-7
III 0 <1x10-7
Synthetic cluster Color-color plots -- IRAC
• D=4 kpc (RCW 49)
• GLIMPSE low/high sensitivity limits
• “Stage I”• Stage II• Stage III• allstars
Reddening line
• Classification spectral index was defined over wavelength range of 2-22 m (Lada 1987).
• What happens for 2-I?
Class vs Stage
Motivation for Fitter
• Fit as many datapoints as available simultaneously
• Unbiased (except for grid choices) -- shows all fits to a given dataset– Estimates uncertainties
• Estimates foreground AV
(Robitaille et al. 2005)
Fitter results on a single source
GLIMPSE Empty Field
• 99.6% of sources fit with stellar atmospheres
• 0.4% evolved stars, bad data or YSOs?
RCW 49
RCW 49
• 96.6% of sources fit with stellar atmospheres
• 3% well-fit with YSO models
Class I source
IC348 Mass histogram
• “Known” IMF (using prior information on stellar parameters)
• Data from Lada et al. (2005)
IC348 Mass histogram
• Based on Model Fitter Only
RCW 49 Synthetic Mass histogram
• Sampled masses from grid using Salpeter IMF (flatter slope below 0.5 Msun)
• Sampled ages using Taurus ratios (Kenyon & Hartmann 1995)
• Apply GLIMPSE sensitivy limits
RCW 49 FittedMass
histogram
• Use model fitter to determine masses
Applications of Grid & Fitter
• Study Global properties of star formation in Galaxy– Star formation rate, lifetimes of
evolutionary states, IMF– A high star formation efficiency argues for
turbulent cloud support (vs. magnetic)
• Search for disks around massive stars– Adds further credence to accretion model
for high-mass star formation– Disks form planets
…applications
• Study low-mass star formation in vicinity of high-mass– May be more common mode of star formation
(Hester & Desch 2004)– Disk lifetimes, sizes
• 3-D extinction map• Galactic structure
– 80% of stars are K giants– Fitter can distinguish gravity (I.e., giants/MS)
Future Work
• Radiative Transfer– Add PAHs, stochastic heating of small grains
• Grid and fitter will be publicly available in 2006
• RT codes available at http://gemelli.spacescience.org/~bwhitney/codes