1. Using ALMA to disentangle the Physics of Star Formation in our Galaxy
Doug Johnstone: NRC-HIA, U. Victoria
11/13/2018
AAS Calgary 2006

2. Why the Millimeter/Submillimeter
Molecular clouds are cold (T ~ 5-20 K)
Even heated dust near stars typically T < 1000 K
Dust peak emission at submillimeter
SED of dust emission yields dust temperature
Longer wavelength emission thin! - column density
Molecular lines abundant in millimeter/ submillimeter
mm - Ground state CO - traces all but the densest cool gas
mm - Ground state N2H+ - traces densest gas
Band 3 for ALMA (3mm) - being built in Canada
submm - higher energy lines - warmer medium
Need an excellent site above much of atmosphere
11/13/2018
AAS Calgary 2006

3. The Enormous Scales in Star Formation
Mass Range
Molecular cloud (M ~ Msun)
Stars (M ~ Msun)
Length Scale
Molecular cloud (R ~ cm)
Star-forming core (R ~ cm)
Disk (R ~ cm)
Stars (R ~ cm)
Additional scale issues
Angular momentum
Magnetic field strength
11/13/2018
AAS Calgary 2006

4. Motte, Andre, & Neri 1998
11/13/2018
AAS Calgary 2006

5. The Angular Resolution Problem
Distance
100 pc (low mass)
500 pc (Orion)
2500 pc (high mass) 1”
100 AU
500 AU
2500 AU
0.1”
10 AU
50 AU
250 AU
0.01”
1 AU
5 AU
25 AU
11/13/2018
AAS Calgary 2006

6. The Need For ALMA Baselines as long as 10 km
Angular resolution ~ 0.01”
Large collecting area ( m dishes)
Sensitivity to match resolution
Excellent sampling of UV plane
Superior image fidelity
Large bandwidth
Multiple lines
Sensitive continuum measurements
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AAS Calgary 2006

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AAS Calgary 2006

8. Antenna Configurations (max)
4 950 GHz
Site infrastructure (AOS/OSF) + inner array completed 2008
11/13/2018
AAS Calgary 2006

9. 50 Antenna ALMA UV Coverage
11/13/2018
AAS Calgary 2006

10. Sensitivity after a few hours angular resolution
11/13/2018
AAS Calgary 2006

11. Problem 1: Turbulence Clouds are not supported by thermal pressure
Jeans mass ~ several Msun
Support by non-thermal motions
Turbulence (possibly mediated by magnetic fields)
Turbulence must dissipate
Scale of dissipation ~ mean free path
For supersonic shocks: d ~ AU (current estimate ~100AU)
Large temperature/density variation across shock
Resolution is key to observing this scale
Desire also to observe diagnostics of warm gas
Need to find the energy emitted during dissipation process!
ALMA’s higher frequency bands well suited to this problem
11/13/2018
AAS Calgary 2006

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AAS Calgary 2006

13. Problem 2: The Domain of Gravity
Formation of a star requires loss of support
Gravity > thermal/non-thermal pressures
Quiescent Cores observed (pre-stellar?)
Definition: sufficient mass to be bound
Di Francesco et al PPV chapter
Size determined by Jeans length ~ 103 AU
Want to probe this region in detail
Power-law profile, flattened centres - initial conditions
Temperature and/or dust property variations - chemistry
Substructure in the core?
Multiple density peaks
11/13/2018
AAS Calgary 2006

14. Problem 2: The Domain of Gravity
What can be done with ALMA
Dust continuum measurements ( mm)
Temperature and dust properties
Molecular line observations
CO, N2H+, CS, HCO+ (freeze out, kinematics, infall)
Polarization measurements
Dust continuum
Molecular lines
11/13/2018
AAS Calgary 2006

15. L1544
90 mm
200 mm
850 mm
1300 mm
11/13/2018
AAS Calgary 2006

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AAS Calgary 2006

17. B68 C18O Optical Image Dust Emission
NICER Extinction Map
Radial Density Profile, with Critical Bonnor-Ebert Sphere Fit
11/13/2018
AAS Calgary 2006

18. Strong B, magnetic support implies: non-tangled (smooth) field lines
hourglass morphology
11/13/2018
AAS Calgary 2006
Shu, The Physical Universe (1982)

19. Problem 2: The Domain of Gravity
Massive star formation is a significant problem
Distance makes resolution more important
If anything the relevant scale is smaller
Jeans mass argument
Time scale for evolution shorter (cloud and star)
Gregarious regions - much more complex
Many sources clustered together
Internal and external heating sources
Need resolution and image fidelity
ALMA will be well suited to this problem
High angular resolution
Excellent image fidelity
Large range of molecular tracers (warm and cold gas/dust)
11/13/2018
AAS Calgary 2006

20. DR 21
Orion
11/13/2018
AAS Calgary 2006

21. Problem 3: Collapse to Protostar
Need to observe the infall motions
Optically thin and thick tracers
Need to separate the forming components
Envelope: temperature, density, chemistry
Disk: size, support, chemistry
Variety of molecules and molecular lines (+ continuum)
Evolution with time
Large surveys of protostars
Following up on Legacy surveys with Spitzer, Herschel, JCMT etc…
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AAS Calgary 2006

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AAS Calgary 2006

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AAS Calgary 2006

25. Gould’s Belt - Nearby Star Formation Sites
SCUBA2 Legacy Project: Mapping all star formation in Gould’s Belt, covering 700 square degrees, and sensitive to every Class O & I protostar and every L1544-like pre-stellar core within 0.5 kpc!
11/13/2018
AAS Calgary 2006

26. Problem 4: Outflows Outflows and jets are important
Determination of local star formation efficiency
Removal mechanism for angular momentum
May play a role in energizing the cloud
May play a role in stopping collapse
Interface between the jet and outflow is key
The jet is launched from the inner disk
The molecular outflow is somehow entrained
Observations require resolution/image fidelity
Many mol. lines - temperature variation/chemistry
11/13/2018
AAS Calgary 2006

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AAS Calgary 2006

29. Problem 5: Disks and Planets
Everyone is excited by protostellar disks
Ubiquitous around young stars
Age and environmental diagnostic
Significant reservoir of material
Chemical processing
Planet formation
Connection to the Origin of the Solar System
Uniqueness of life….
11/13/2018
AAS Calgary 2006

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AAS Calgary 2006

33. Problem 6: PDR Physics The interface between atomic and molecular gas
Surfaces of molecular clouds
Boundary between massive stars and cloud
The Orion Bar
Outer regions of pre- and proto-stellar envelopes
Interesting UV chemistry
Strength and hardness of the radiation field
Diagnostic of cloud conditions
11/13/2018
AAS Calgary 2006

34. Problem 7: Chemical Abundances
Absorption spectroscopy
Probes both gas phase and solids
History of the conditions in the ISM
Atomic/chemical inventory throughout Galaxy
Variations - star formation history/gas mixing…
Need bright background continuum sources
ALMA’s sensitivity is key
11/13/2018
AAS Calgary 2006

35. Where to go for more Information
North American ALMA Science Centre
Located in Charlottesville
Excellent web resources (
Talk to someone from the ANASAC
We are interested in your needs/concerns
Get a copy of the Science with ALMA brochure
Lists wavelength coverage, sensitivity, baselines…
Visit and look through the ALMA DRSP
A sampling of key projects for ALMA
Most Importantly: Start thinking about this NOW!
11/13/2018
AAS Calgary 2006