Modelling Massive Star Formation

Rowan SmithZAH/ITA University of Heidelberg

Ian Bonnell, Henrik Beuther, Paul Clark, Simon Glover, Ralf Klessen, Steven Longmore, Amy Stutz, Rahul Shetty

Motivation

1

Observations: Environment

Massive stars usually form at the centre of dense star forming clumps.

Pre-stellar massive cores either extremely short lived or don’t exist

Motte et. al. 2007

Star forming clumps form at the hub points of filaments.

Peretto et. al. 2012, Myers 2009, Schneider et. al. 2012

Observations: Inflow

Peretto et al. 2013 found the mass in the central pc of a massive IRDC (SDC335) could be doubled in a million years.

Kirk et al 2013 found infall gradients of ~ 30 Msol Myr-1 along the southern filament of Serpens South

- radial contraction onto the filament at ~ 130 Msol Myr-1

Observations: FragmentationInterferometry observations usually (but not always) reveal substructure on core size scales i.e. less than 0.1 pc scale.

see Bontemps et al. 2012, Rodon et al. 2012, Duart-Cabral et al. 2014

Girart et al. 2013

2.1 mG

Fragmentation with an entrained magnetic field.

Palau et al. 2013 & 2014

18 massive dense ~0.1 pc cores

5 one dominant source, 9 many (>4) sources

low fragmentation = stronger magnetic field

The SPH SimulationLoosely based on Orion A

• 10 000 Msol

• Smooth Particle Hydrodynamics

• 15.5 million particles

• Barytropic equation of state

• Sink particles for star formation

• Heating from sinks

• Self gravity

• Decaying turbulence

• No magnetic fields

Equivalent to a massive star forming region. see also Bonnell et al. 2011

Massive Stars and Collapsing Gas

2

Collapsing Clumps

Filament collapsing along its axis

- evolves to a more compact state with less sub-structure

Clump Alphain column densityblue: 0.05 gcm-2 yellow: 5 gcm-2

2.4 x 105 yrs

Rotating massive protostellar core at the centre. But no obvious pre-stellar core at early times.

Interferometry Observations

Longmore et al. 2009 observed clumps of gas where massive stars were thought to be forming. Used maser emission and chemical tracers to estimate their relative ages.

YOUNG OLD

0.75 tdyn 1.0 tdyn 1.25 tdyn

Subsequent accretion

A guessing game- which one of these cores forms a massive star?

The positions at which accreted material passes through a shell of radius r = 0.1 pc around a sink over 20,000 yr.

Subsequent accretionAnswer:

thermal jeans mass

0.8 M1.9 M

2.7 M

11.5 M

Smith et al. 2011a

The Core Mass Function

Correspondence between cores and stars within the simulated massive cluster is only for the sample as a whole rather than for individual stars. Implies accretion from outside core.

Smith et. al. 2009a

There is a resemblance between the stellar IMF and core mass function e.g Alves et al. 2007 and many others

Fate

Red = p-cores

Solid blue = sinks

Yellow = mass which will be accreted by the most massive sink within 2.4 x 105 yrs.

Massive star is mainly built out of gas that initially comes from the surrounding clump. See also Wang et al. 2010

t= 1 tdyn

Clump Alpha

Smith et. al. 2009bThis is in contrast to core accretion models for massive star formation e.g. McKee & Tan 2003

Massive Starless Cores

Generally massive condensations exhibit some sub-structure consistent with the predictions of these simulations.

Caveats:

My simulations lack magnetic fields (see Myers et al. 2013)

It is important to see what such regions would look like in actual observations.

Tan et al. 2013Rodon et al. 2012

A Comment

2) Supersonic turbulence is not an isotropic pressure and so it cannot support a core without also inducing fragment in regions that have been compressed.

Krumholz et al. 2012

Competitive Accretion vs. Turbulent Cores -> Probably both wrong

1) What we see in the simulations (Smith+ 2009, Wang+ 2010) is not competitive accretion in the original Bondi-Hoyle sense. The gas and cores are well coupled. It is the global collapse of the cloud that feeds the proto-stars.

Accretion and Filaments

3

Velocity Map

Large scale collapse

Flow is not purely radial.

Multiple filaments form a hub.

(see Myers 2011, Smith et. al. 2011, Schneider et al. 2012, Kirk et al. 2013, Perretto et al. 2013/2014)

Subsequent accretion

thermal jeans mass

0.8 M1.9 M

2.7 M

11.5 M

Irregular Shapes

Cores situated in more filamentary enviroments are more massive at the end of the simulation.

Low mass sinks tend to form from more spherical cores.

Type Number Percentage

0 115 32.3%

1 103 28.9%

2 138 38.8%

Smith et. al. 2011a

New Arepo Simulations

A slight digression...

New Arepo Simulations

Suite of small scale simulations:

• 104 solar mass turbulent clouds

• Chemistry, gas self-shielding, heating and cooling, self-gravity.

• Jeans length always refined by at least 16 cells.

Plummer-like ProfilesFor super-critical non-isothermal filaments, when we fit with a Plummer-like profile as done in Arzoumanian et al. 2011

• Power law profiles are flat p~2 without magnetic fields .

• No systematic variation in filament properties with initial turbulence type (i.e solenoidal, compressive, mix).

now available on the arXiv 1407.6716

Smith et al. 2014b submitted

Filament Comparison

A11 - Arzoumanian et al. 2011

J12a - Juvela et al. 2012a

Filaments in Planck Cold Cores: Juvela et al. 2012a

The simulated filaments are very similar to observed filaments.

But no constant filament width of 0.1 pc

see also Hennemann et al. 2012

Filament Formation• The filaments seen in column

density are actually made up of a network of sub-filaments as in Hacar et al. 2013.

• The filament forms from smaller clumpy filaments being collected together by gravitational collapse.

• Sub-filament size consistent with the Jeans radius in 12K n=105 cm-3 gas.

Synthetic Observations

5

Observations

Fuller et. al. 2005

Chen et. al. 2010

see talk by Chang Won Lee yesterday for the low mass case

Massive Star Line Profiles

Optically thick line profiles often show a characteristic broad peak with a small red shoulder.

HCN F(2-1) HCO+

Smith et al. 2013

Post-process the massive star forming regions with radmc-3d

Line of sight

Superposition of large scale collapse motion, with smaller scale local core collapse within the massive star forming region.

Supersonic infall as proposed by Motte et. al. 2007 from observations of Cygnus X. See also Schneider et. al. 2010

Multiple density peaks (cores) along the line of sight.

Linewidths due to collapse not supportive turbulence, rotation, or outflows.

Comparision to Observations

Csengeri et. al. 2011

Red fit from their model.

Similar wide profiles with a small shoulder in observations.

Optically Thin Profiles

Multiple components in the optically thin lines. This has the potential to be diagnostic.

N2H+ (1-0) isolated hyperfine component observed over 0.06pc HWFM beam

Beuther et al. 2013

Optically Thin Profiles

This also becomes more apparent when observed with a narrow beam - implications for ALMA

N2H+ (1-0) isolated hyperfine component observed over 0.06pc HWFM beam

black = HCO+ (1-0)

red = N2H+ (1-0) *4

Future Work

5

Galactic Scale Arepo SimulationsSub-pc resolution study of the formation of molecular gas in a spiral galaxy.

Smith et al. 2014a

Galactic Scale ICs

These are the ideal initial conditions to revisit previous molecular clouds simulations and make observational predictions for low and high mass star-forming cores.

Conclusions

6

Conclusions

1. Massive stars in these simulations are fed primarily from gas from the clump rather than the core (defined in 3D using the gravitational potential).

2. Filamentary flows can feed massive protostellar cores through gravitational collapse.

3. Filament profiles in new hydro simulations with Arepo have p~2 profiles and a non-constant width.

4. Synthetic observations of the simulated massive star forming regions often have little self absorption. In optically thin dense gas tracers there are multiple line components when observed with a narrow beam.

5. Using Arepo and galactic scale simulations we are now revisiting this problem with more accurate methods and better initial conditions.

Sink HeatingBasic fit to MC models

Robitaille et. al. 2006

- a: 0.33 M < 10- a: 1.1 M > 10- q: -0.4 to -0.5

Overestimates feedback• Spherical symmetric• Isolated• Underestimates column

densities• Ignores cluster

structure, discs etc

CO emissionFilamentary inter-arm clouds may be the observable parts of much larger structures.

Density & Accretion

Accreted gas has a lower density and hence a longer free fall time.

- needs a long free fall time to reach the central sink without fragmentin on the way.

see also Wang et al. 2010

Is a core sufficient?

Smith et. al. 2009a

There is a resemblance between the stellar IMF and core mass function e.g Alves et al. 2007 and many others

Potential wells in in Smith et al. 2008b resemble the stellar IMF.

Note that these are potential wells not observational cores.

Angular momentum

Smith et al. 2011

• The angular momentum vector of the material accreted onto the core is not coherent.

• This will encourage fragmentation in the cores and may change the orientation of jets and outflows over time.

Global Collapse

3

Collapsing Clumps

Clump Betain column densityblue: 0.05 gcm-2 yellow: 5 gcm-2

Region formed by converging shocks

- evolves to a more compact state with enhanced densities

2.4 x 105 yrs

Comparison to Low Mass Cores

Compared to low mass cores massive star forming region line profiles are:

- Brighter and with larger linewidths

- Less variable with viewing angles

- Only weak self absorption signatures, broad blue peaks and small red shoulders

- Have multiple line components in optically thin dense gas tracers

Smith et al. 2013