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Star Formation in Crowds

Star formation changes with environment.

High-mass stars define the neighborhood.


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: Anna McLeod, Connor McClellan, Justin Otter, Natalie Butterfield, Terry Melo, Virginie Montes
Slides available at tinyurl.com/HMSF-Morelia2019

Star formation drives the evolution of the universe

Star Formation oversimplified

The star formation rate, i.e., how much gas turns to stars
L / M
The light per unit mass, i.e., how stars and stellar populations turn matter into light

High-mass stars produce light & heavy elements
low-mass stars live practically forever

Point color shows effective temperature, point size shows luminosity (left) and mass (right)

The stellar initial mass function (IMF)

Stars are randomly sampled from this distribution

Almost all of the light in star-forming galaxies is produced by high-mass stars

The stars form in and from gas

Most of what we know of star formation in detail comes from small local clouds

Most of what we know of star formation in detail comes from small local clouds

Cartoon of low-mass star formation

A molecular cloud fragments

The core forms a central protostar

The protostar heats its parent core and forms a disk

It drives an outflow and consumes or blows out its core

Eventually, you end with just a star-disk system

Cartoon of low-mass star formation

Most of what we know of star formation in detail comes from small local clouds

They contain only low-mass stars and do not represent star formation in general

Most stars form in denser regions

NGC 1333, an embedded low-mass cluster
Lada & Lada 2003: >70% in embedded clusters

Most stars form in denser regions

NGC 3603 is a high-mass (104 M) cluster
Lada & Lada 2003:
5-10% in bound clusters
in our Galaxy

Star formation drives the evolution of the universe

Most stars in most galaxies formed long ago

Galaxies were smaller & denser back then

Our own Galaxy's center, the CMZ, has denser gas than the Galactic average

Cold Dust
Hot, ionized gas
Hot dust/PAHs

Our own Galaxy's center, the CMZ, has denser gas than the Galactic average

Our own Galaxy's center, the CMZ, has denser gas than the Galactic average

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

Γ is the fraction of stars forming in bound clusters
Galaxy averages

"Bound Cluster Fraction" is predicted higher the CMZ

Γ is the fraction of stars forming in bound clusters
Galaxy averages
CMZ prediction

The "Bound Cluster Fraction" is higher in the CMZ

Γ is the fraction of stars forming in bound clusters
Galaxy averages
CMZ prediction
Sgr B2 data

Summary so far:

  • At higher gas density, more stars form in bound clusters
  • Galaxies were denser in the past
  • Most stars formed when galaxies were denser
  • Most stars formed in or near high-mass clusters, in regions unlike the "local neighborhood"

How is star formation in high-mass clusters different?

Cartoon of high- and low-mass star formation

Main difference: massive stars affect their surroundings

Classic HII region feedback:
O-stars clear out their environment

Accreting massive young stars affect their environment

Accreting massive young stars affect their environment

The characteristic fragmentation scale


The Jeans Mass MJ is the mass where gravity and thermal pressure are balanced.

MJ ∝ T3/2 ρ−1/2

The characteristic fragmentation scale is larger

Jeans Mass MJ ∝ T3/2 ρ−1/2

The cartoon in the context of HMSF

These high mass cores suppress low-mass star formation (LMSF) in their vicinity. They reduce or prevent LMSF in the cores of stellar clusters.

More extreme: 'cooperative accretion'

With enough high-mass stars forming concurrently, massive stars may prevent fragmentation entirely.
If they still have enough gravity to bind the gas, the remaining gas is forced onto the most massive gravitational sinks.

Top-heavier mass functions in high-mass clusters

Summary part two:

  • High mass stars heat their environment, suppressing Jeans fragmentation and preventing lower-mass stars from forming
  • The feedback from high-mass stars is exaggerated in denser regions
  • By cooking their surroundings, high-mass stars help themselves form, and they may help create their own siblings

Large scales again:
What governs the star formation rate?

Turbulent ISM models

Turbulent ISM models

Turbulent ISM models

Local cloud studies support the idea of a gas density threshold for star formation

Thresholds are used in simulations to say
"if gas reaches this density, turn it into stars"

The California molecular cloud with protostars

ALMA enables protostar counting in
distant, massive clouds

Sgr B2: the most massive & star-forming cloud in the Galaxy

Is there a threshold?

Is there a threshold?

A threshold separates Sgr B2 from The Brick

Summary Part 3: SF Thresholds & Star Counts

  • If star formation occurs above density thresholds, they are not universal.
  • Clouds within the CMZ appear to have a higher and consistent threshold, so an environmentally-dependent threshold is plausible.
  • We can rule out subclasses of turbulent theories and measure their defining parameters
  • (proto)Star-counting measurements of star formation are now possible throughout the Galaxy.

Salts in Orion

Orion Source I
a disk around a 15 M YSO

Salt: NaCl

Temperature?

Temperature?

A contrived model

Observing the Keplerian rotation profile of a disk is the most direct way to measure a protostar's mass

(we can only see the disk, not the star itself)

We can use salts to measure HMYSO masses

  • NaCl, KCl are only in the disk, not the outflow
    (water traces both)
  • NaCl is detected in at least one other HMYSO
    (Maud, Ginsburg+ in prep)
  • Salts are observable with ALMA, the JVLA, and the future ngVLA
  • Future projects will involve observing and modeling salt disks to measure HMYSO masses

Summary

Stars form differently in high-density environments
  • More stars form in clusters
  • More stars are affected by feedback from neighbors

The local neighborhood is not representative
  • ALMA enables detailed study of distant regions (Sgr B2, W51, W43 so far)
  • ALMA-IMF will expand the sample to match or exceed local clouds
Salt is a new tool to probe disks around high-mass stars

Salt backup slides

Possible future uses for these lines?

  • Metallicity measurement in deeply embedded star-forming environments? (at least of Na, K, Cl)
  • Disk kinematics of high-mass stars, which are otherwise unobservable (τ>1 at mm wavelengths)
  • Disk kinematic measurements at early stages?
  • Probe dust destruction (and/or formation?) in outflows, disks?
  • Probe radiation environment around HMYSOs?

Why do we see salt?

  • Previously, NaCl & KCl only in AGB* atmospheres,
    associated with dust formation
  • Most likely dust destruction here
    Dust destruction happens immediately as the outflow is launched?
  • What about excitation? We see vibrationally excited lines, which are not seen in AGB*s

We do not have a viable model to explain these temperatures

A strong non-blackbody radiation field from 25-40 µm may explain them.
Forsterite (MgSiO4) has some emission bands in that range. Maybe?