Environmental effects in and around forming high-mass clusters

John Bally, Ashley Barnes, Nate Bastian, Cara Battersby, Henrik Beuther, Crystal Brogan, Yanett Contreras, Joanna Corby, Jeremy Darling, Chris De Pree, Roberto Galván-Madrid, Guido Garay, Jonathan Henshaw, Todd Hunter, J. M. Diederik Kruijssen, Steven Longmore, Xing Lu, Fanyi Meng, Elisabeth A.C. Mills, Juergen Ott, Jaime E. Pineda, Álvaro Sánchez-Monge, Peter Schilke, Anika Schmiedeke, Daniel Walker, David Wilner, Leonardo Testi, Rowan Smith, Ke Wang, James Dale, Robert Loughnane, Erik Rosolowsky, Eric Koch, Ciriaco Goddi, Brett McGuire, Dick Plambeck, Melvyn Wright

Students: Justin Otter, Andreas Schwörer

This presentation can be found at https://keflavich.github.io/talks/ or, until September 22, 2019, at http://small.cat/cap

Caveat

I gave this talk as the very first one of the StarFormMapper 3 conference. Over the course of the conference, I learned that some of the results I thought were accepted and uncontroversial are, in fact, neither. There is still active discussion about whether the mass at a given size scale evolves during cluster formation, i.e., the static collapse model is still possibly viable.

How do bound clusters form?

The mass is pre-assembled in "starless" clumps, then collapses
- Combined with gas expulsion, favored by Banerjee & Kroupa (2014, 2015, 2017, 2018)
- Requires protoclusters to start more compact, since they expand with expulsion
The mass is assembled as stars form:
there is no starless phase, gas comes from larger scales
- Better supported by observational timescale arguments
- "Conveyor Belt" of Longmore+ 2014
Stars form in substructures, then merge into clusters
(e.g., Fujii+ 2012)
- However, Kuhn+ 2019 used GAIA to show young clusters are expanding and subclusters are moving away from each other

How do clusters form?

Growing consensus that they form by "conveyor belt": material flows in from large scales over several free-fall times.

Krumholz & McKee 2019 (theory), Krumholz, McKee, and Bland-Hawthorn ARAA 2019 (overview)
Walker+ 2016, Barnes+ 2019 for CMZ case studies
Urquhart+ 2018 for the Galaxy overview (talk this session)

Catalogs of forming clusters in the Galaxy constrain their formation path

Galactic plane surveys find few (~10s) of high-mass protoclusters
- Ginsburg+ 2012, Urquhart+ 2014a, b, 2018, Longmore+ 2014, 2017 Contreras+ 2017

Leroy+2018 resolved the starburst into protoclusters

These protoclusters account for up to 100% of the starburst

(moderately) less massive clusters were detectable but not detected in NGC 253:

The extreme starburst may preferentially produce high-mass clusters: theories say it has both a higher high-mass cutoff and low-mass cutoff to the mass function

The Galactic Central Molecular Zone is home to the most massive forming clusters

Image: NRAO, Ginsburg

Mills, Ginsburg+ 2018; Kruijssen & Longmore 2013

Cold Dust
Hot, ionized gas
Hot dust/PAHs

The Galactic Central Molecular Zone is home to the most massive forming clusters

Image: NRAO, Ginsburg

Ginsburg+ 2018a

The Galactic Central Molecular Zone is home to the most massive forming clusters

Image: NRAO, Ginsburg

Ginsburg+ 2018a

Cluster Formation Efficiency

What fraction of all stars form in bound clusters?

Not all do (e.g., Bressert+ 2010, Ward & Kruijssen 2018)
Varies with environment, increasing toward higher density
More stars formed in higher density regions in the early universe, so more in clusters

We can measure this locally, given an appropriate change in environment

Sgr B2: Most massive cloud + protoclusters

Protostar-counting observations

Tightly bound cluster:

$\sigma_{1D} \sim 9-12~\mathrm{km~s}^{-1}$

$\sigma_{1D} < v_{esc} \sim 14~\mathrm{km~s}^{-1}$ from RRL LOS velocities

Un- and clustered star formation in the same event (Ginsburg+ 2018)

In denser (parts of) galaxies, more stars form in clusters

Kruijssen 2012
Ginsburg & Kruijssen 2018

Γ is the fraction of stars forming in bound clusters

Galaxy averages

"Bound Cluster Fraction" is predicted higher the CMZ

Kruijssen 2012
Ginsburg & Kruijssen 2018

Γ is the fraction of stars forming in bound clusters

Galaxy averages

CMZ prediction

The "Bound Cluster Fraction" is higher in the CMZ

Kruijssen 2012
Ginsburg & Kruijssen 2018

Γ is the fraction of stars forming in bound clusters

Galaxy averages

CMZ prediction
Sgr B2 data

Sgr B2 N: Collapse

Collapse is morphologically obvious (Schwörer+ 2019)

There is evidence of accretion from larger scales

A closer look at Sgr B2 N

Schwörer+ 2019: Accretion along filaments

Several filaments have velocity gradients pointing in to the central region (~2000

$\mathrm{M}_\odot$ ) of Sgr B2 N: the central cluster (~0.1 pc) is accreting from ~half-pc scales
Protostars are also being accreted

How is star formation in high-mass clusters different?

The IMF should depend on density, feedback
(e.g., Jones & Bate 2018, $M_c \propto \rho^{-1/5}$ )
Feedback from one star affects many in clustered regions
(Rosen+ 2016, Ginsburg+ 2017)
Total star formation efficiency is higher (because it goes on for more $t_{ff}$ s?). SFE_ff may be higher?
(obs: Ginsburg+ 2016, Gouliermis & Hony 2015, sim: Grudić+ 2018 )
Collisions assemble the most massive stars?
(e.g., Fujii+ & PZ 2013, but see Moeckel & Clarke 2011)
- Interactions certainly affect disks (e.g., Wijnen+ 2017, Vincke+ 2016, Rosotti+ 2014)

Accreting massive young stars affect their environment

Ginsburg+ 2017

There is less direct evidence that lower-mass clusters (which generally contain less massive forming stars; Orion Source I is

$15\mathrm{M}_\odot$ and heats only its 100-AU disk to >100 K)

are affected by such thermal feedback, but they exhibit other effects...

How is star formation in high-mass clusters different?

The IMF should depend on density, feedback: WIP
Feedback from one star affects many in clustered regions: Evidence in W51, others WIP
Total star formation efficiency is higher (because it goes on for more $t_{ff}$ s?). SFE_ff may be higher? (hinted at in NGC 253, otherwise not seen in this talk)
Collisions assemble the most massive stars?
No evidence presented here - Sgr B2 and Orion are tantalizing places to look

Interactions certainly affect disks: Orion's disks are smaller than other SF regions'

Summary

Clusters gain mass as they form
More stars form in bound clusters in higher density regions

Within forming clusters, feedback from the most massive stars affects neighbors, suppressing fragmentation
Dynamical interactions are important in clusters, which means disk properties are different (they're smaller)

Future Directions

Complete census of spatial and mass distribution of protostars from the ALMA-IMF program
A direct connection between the protostellar and stellar populations with JWST imaging and spectroscopy to pierce the extinction layers

What is a high-mass cluster?

(see Krumholz, McKee, and Bland-Hawthorn 2019 for a complete review)

Gravitationally bound collection of stars that survives the loss of gas
Collection of coeval stars that 'fully samples' the IMF
Clusters where interactions are important

Portegies-Zwart 2016, Vincke+ 2016

Around $10^4~\mathrm{M}_\odot$ , $v_{esc} \gtrsim10$ km s $^{-1}$ , so ionization alone does not disrupt gas

Bressert+ 2012, Matzner & Jumper 2015, Krumholz & Matzner 2009 , Grudić+ 2018

Environmental effects in and around forming high-mass clusters

Caveat

How do bound clusters form?

How do clusters form?

YMCs start large, collapse to small

Simulation: Accretion from large scales

Catalogs of forming clusters in the Galaxy constrain their formation path

How does cluster formation depend on environment?

The extragalactic view of Galactic clusters

The Galactic Central Molecular Zone is home to the most massive forming clusters

The Galactic Central Molecular Zone is home to the most massive forming clusters

The Galactic Central Molecular Zone is home to the most massive forming clusters

Cluster Formation Efficiency

Sgr B2: Most massive cloud + protoclusters

In denser (parts of) galaxies, more stars form in clusters

"Bound Cluster Fraction" is predicted higher the CMZ

The "Bound Cluster Fraction" is higher in the CMZ

Sgr B2 N: Collapse

A closer look at Sgr B2 N

A closer look at Sgr B2 N

A closer look at Sgr B2 N

How is star formation in high-mass clusters different?

IMF development: ongoing work

Accreting massive young stars affect their environment

Accreting massive young stars affect their environment

More material is flowing in to be heated

And W49....

How is star formation in high-mass clusters different?

Summary

Future Directions

YMCs are the best local analogs
of proto-Globular Clusters

MSPs in GCs

W51: X-ray stars

W51: X-ray stars

W51: X-ray stars + Cores and UCHII regions

W51: Cores and UCHII regions

How do clusters die?

What is a high-mass cluster?