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Star Formation in Crowds

YSOs in protoclusters


  • Postdoc: Allison Towner
  • PhD: Desmond Jeff, Theo Richardson, Alyssa Bulatek, Nazar Budaiev
  • REU: Justin Otter, Danielle Bovie, Josh Machado
  • Undergrad: Madeline Hall, Michael Fero, Derod Deal, Parker Ormonde

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 photons & 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

Otter+, submitted soon

FOV: 0.07 pc (16000 AU)
72 YSOs
One "hot core"

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

The "Bound Cluster Fraction" was higher in the past

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

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

The "Bound Cluster Fraction" is predicted higher in 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

How do we learn about clustering? The IMF?

  • Count objects.
  • Cores are sometimes countable.
  • Protostars are countable.

YSO counting has taught us a lot in local clouds

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

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

3mm Luminosity Function

What are the sources?

At this sensitivity, all are M>8 YSOs
Desmond Jeff: Hot Cores and YSOs in Sgr B2 DS
  • Are there appropriate numbers of low-mass YSOs?
  • Is the IMF 'normal'?

MYSO counts let us investigate thresholds

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"
Compare YSO counts in Sgr B2 and the CMC

Is there a threshold?

Is there a threshold?

A threshold separates Sgr B2 from The Brick

From YSO counts to the IMF?

How do we measure the CMF if the cores all have YSOs in them?
Some CMF measurements in clusters have given us tantalizing hints of CMF variation...

YSO modeling → luminosity functions

NSF 2008101: "How are stellar masses set?"
Theo Richardson

Top-heavier IMFs are seen in high-mass clusters,
CMFs in protoclusters

ALMA-IMF is the next step in CMF measurement & YSO counting

  • Continuum data paper in prep (Ginsburg+, with big data reduction team: Roberto Galvan-Madrid, Nichol Cunningham, Timea Csengeri, Patricio Sanhueza, Fernando Olguin, Thomas Nony, Jordan Molet, Ana Lopez, Yohan Pouteau, Andrez Guzman, Manuel Fernandez, Melisse Bonfand)
    • Self-calibration (10-500% dynamic range improvement)
    • Mosaicing
    • Continuum selection
    • Method comparison
  • Survey overview paper in prep (Motte+)
  • Catalogue paper in prep (Louvet+)

ALMA-IMF data highlights

How do we measure masses?


On the top end, mass measurement is difficult:
  • cores are optically thick
  • cores are confused & blended
  • the measured luminosity can be the sum of whole (proto)clusters

Dynamical mass measurements are the gold standard.

Sometimes, we can't measure dynamical masses

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 two other HMYSOs
    (Tanaka+ 2020, Maud+ 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

W51 e2e: Too optically thick at 1mm to measure disk

CS v=0 J=1-0 and v=0 J=2-1 masers may trace the disk?

CS maser conditions

van der Walt+ 2020
  • Top: CS J=1-0, Bottom: CS J=2-1
  • Red: Consistent w/W51e2e observations
  • Masers do not coexist; require different specific CS column
    (N2-1=1015.6, N1-01016.1 cm-2)
  • Require high abundance (XCS > 10-5)
  • Hot (300-500 K), moderate-density (n~105 cm-3): Disk surface? Or outflow cavity wall?

Looking forward:

  • PASHION: Paschen Alpha Survey of Hydrogen Ions
  • JWST: Deep Paα, Brα, and broadband imaging

PASHION: Paschen Alpha survey of the Galaxy

Team:
  • John Bally (CU)
  • Elizabeth Lada, Steve Eikenberry (UF)
    • Students: Alyssa Bulatek, Michael Fero, Nazar Budaiev
  • Lockheed Martin (Alison Nordt, Gopal Vasudevan)
  • Tony Hull (UNM)
  • York Space Systems

PASHION: H2RG with Lockheed electronics, three narrow-band filters, 2.5" resolution, 25' FOV

A 24 cm dedicated survey telescope will be the most sensitive Galactic plane survey of ionized gas


These are fiducial numbers for a 1-year mission performing a 100 square degree blind survey. An extended mission may be possible.

PASHION, and JWST, recombination line science

Accretion onto YSOs
HII regions

Assuming typical AV~2 per kpc

Summary


Most stars form in regions unlike the solar neighborhood
  • Greater clustering, higher SF thresholds in denser clouds
  • ALMA-IMF will expand the sample to match or exceed local clouds

We have, and are building more, tools to measure MYSO masses
  • Salt is a new tool to probe disks around high-mass stars
  • CS masers may track YSO disks; they require rare conditions

Things I did not talk about today:
  • MUSTANG Galactic Plane Survey
    • HCHII regions are ~1/3 as abundant as UCHII
  • Spectral line survey (B34567) of The Brick [WIP]
  • Feedback, YSOs in W51
    • Massive (200-300 M), hot (200-600K) cores suppress fragmentation
    • Multidirectional accretion flows
  • Accretion flows, outflows in Sgr B2
  • Turbulence in Sgr B2 [WIP: student Madeline Hall]
  • Gas temperatures in W51 [WIP: student Josh Machado]
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?

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

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

Feedback affects dense gas

ALMA + VLA + GBT together give multiple temperature probes on multiple scales.
High-mass protoclusters are filled with gas warmed by feedback.
Ginsburg+ 2017, Machado+ in prep

YSO disk counts in W51

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.

Ammonia Masers

Large scales again:
What governs the star formation rate?

Turbulent ISM models

Turbulent ISM models

Turbulent ISM models

Measuring Line Profiles

SCOUSE uses pyspeckit for manual fits. Gausspy+ is machine-learning trained. We're exploring more automated approaches.