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What sets stellar masses in the Galaxy's richest environments
- Postdocs: Miriam Garcia Santa Maria (2024-2025), Allison Towner (2020-2023)
- PhD: Desmond Jeff (2025), Theo Richardson (2025; postdoc), Alyssa Bulatek (2026), Nazar Budaiev (2026), Savannah Gramze (2027), Taehwa Yoo (2028)
- Undergrad: Derod Deal, Aden Dawson
- Supported by NSF 2008101, 2206511, CAREER 2142300, STSCI 1905, 2221, 3523, 5365, 6151, Astropy
Collaborators:
John Bally, Ashley Barnes, Cara Battersby,
Roberto Galván-Madrid,
Jonathan Henshaw, J. M. Diederik
Kruijssen, Steven Longmore, Xing Lu, Fanyi Meng, Elisabeth A.C.
Mills, Juergen Ott, Justin Otter, Álvaro Sánchez-Monge,
Peter Schilke, Daniel Walker, Erik Rosolowsky, Eric Koch, Ciriaco
Goddi, Brett McGuire, Dick Plambeck, Melvyn Wright,
Johan van der Walt,
Henrik Beuther, Kei Tanaka, Yichen Zhang,
the ALMA-IMF team (Timea Csengeri, Fabien Louvet,
Nichol Cunningham, Frederique Motte, Patricio Sanhueza, Thomas
Nony, Yohan Pouteau, Melisse Bonfand, Fernando Olguin, Sylvain Bontemps, and
many others), and the ACES team
Slides available at
https://keflavich.github.io/talks/colloquium_sep2025_texas.html or from my webpage →talks
I'm here all week, try the ...
FWHM = 160 km s-1 [CS & SO], mm-only object. ???
Star formation drives the evolution of the universe
Star Formation oversimplified
Ṁ
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 sampled from this distribution
(but it's not universal)
High-mass stars populate the stellar graveyard
Almost all of the light in star-forming galaxies is produced by high-mass stars
The stars form in and from gas (as traced by dust)
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 isolated 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 runs from or blows out) its core
Eventually, there is just a star-disk system
Cartoon of isolated low-mass star formation
The isolated cartoon is wrong in several ways
Cores fragment.
Massive stars cook their neighbors in hot cores.
Cores continue to grow. Protostars are flung out.
Cores continue to grow. Protostars are flung out.
Cartoon → theory
What do we want from a star formation theory?
As observers, we want it to
- Predict the appearance of young stellar objects (YSOs)
- SED & morphological models
- Predict how common each type (or phase) of YSO is
- Accretion history, lifetime
- ... and thereby let us infer the mass distribution and history of star formation from an observed sample
- IMF, SFH
It's not there yet.
What do we want from a star formation theory?
More broadly, we want it to predict
- $L_*(t, \vec{x})$
- $M_*(t, \vec{x})$
- $\dot{M}_*(t, \vec{x})$
The path to better mass measurement:
YSO modeling → luminosity functions
NSF 2008101:
"How are stellar masses set?"
"How are stellar masses set?"
Theo Richardson
The path to better mass measurement:
YSO modeling → luminosity functions
Theo Richardson
The path to better mass measurement:
YSO modeling → luminosity functions
- Models now predict mm fluxes better
- Masses & column densities calculated for each model
- Unstable/unlikely objects flagged
- Links from YSO radiative transfer models to accretion history models
- Deep dive into the definition of 'YSO Stage'
- [soon] Predict full populations as a function of age, accretion history, efficiency
The path to better mass measurement:
YSO modeling → luminosity functions
We can track and compare "mass" - "luminosity" histories for different accretion models
$S_{3~\mathrm{mm}}$: Envelope Mass
$S_{100~\mathrm{\mu m}}$: Total Luminosity
$\dot{M}$
The path to better mass measurement:
YSO modeling → luminosity functions
...and finally make YSO population models (the "Protostar Luminosity Function", PLF, but observable)
$\dot{M}$
How much material is in the envelope?
$M_{\rm env}$ is measurable, but it's not part of the models.
Classic idea is 30% core→star efficiency
This is wrong for many reasons we'll explore.
*: there are other uncertainties I'm glossing over for now (variability, dust opacity, etc)
Class vs Stage
Class is observationally defined.
Stage is theoretical.
Richardson et al 2025 takes a deep dive into the definition of Class 0/I and Stage 0/I
Stage 0 is when the star has <50% of its final mass (most of the mass is still in the "core")
Stage I is when the star has >50% of its final mass.
Class 0 is a poor proxy for Stage 0.
Stage is theoretical.
Richardson et al 2025 takes a deep dive into the definition of Class 0/I and Stage 0/I
Stage 0 is when the star has <50% of its final mass (most of the mass is still in the "core")
Stage I is when the star has >50% of its final mass.
Class 0 is a poor proxy for Stage 0.
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 dense regions
Lada & Lada 2003:
5-10% in bound clusters
in our Galaxy
5-10% in bound clusters
in our Galaxy
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 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 CMZs
Γ is the fraction of stars forming in bound clusters
Galaxy averages
CMZ prediction
Sgr B2 data
Sgr B2 data
The "Bound Cluster Fraction" was higher in the past
Proto-cluster regions in our Galaxy are like the early universe
but we see DETAILS in our Galaxy
W51e → W51 IRS2 → Wd2 → Wd1
→
→
→
Are top-heavy IMFs limited to clusters?
NGC 3603 & Westerlund 1
SSCs are common in starburst nuclei and drive galactic outflows
NGC 253 protoclusters (Leroy+2018)
NGC 4945 protoclusters (Emig+2020)
Star formation drives the evolution of the universe
Most stars in most galaxies formed long ago
Galaxies were smaller & denser back then
STSCI; Pappovich, Ferguson, Faber, Labbe
From stars: high-mass clusters have top-heavier IMFs
In gas, we see the process in action
Dark regions are the cold sites where new stars will be born
Infrared shows warm gas and dust and young stars
Millimeter [ALMA] shows forming and future stars
Our Galaxy
Gaia star colors via ESA
Our Galaxy
2MASS via IPAC
Our Galaxy
Planck + HI4PI via lambda.gsfc.nasa.gov
Our Galaxy
HI: "Diffuse" gas
HI4PI
Our Galaxy
Dust
Planck
Our Galaxy
CO: molecular gas
Planck
Our Galaxy
Nearby star-forming regions form few or no massive stars
The Inner Galaxy is where most stars form
The Inner Galaxy is where most stars form
ALMA-IMF: 15 high-mass star-forming regions
Orange layer shows ATLASGAL 870$\mu$m dust:the dense gas where stars form
ALMA-IMF: 15 high-mass star-forming regions
Orange layer shows ATLASGAL 870$\mu$m dust:the dense gas where stars form ALMA-IMF targets massive protoclusters ($10^3-10^4 \mathrm{~M}_\odot$) 2-6 kpc from the sun
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
1mm Dust
[ALMA]
870 μm Dust
[APEX/ATLASGAL]
ALMA-IMF: CMF measurement & YSO counting
- Continuum data paper
- Survey overview (Motte+ 2022)
- Cores in W43 (Nony+ 2023)
- Shallow CMF in W43-MM2/3 (Pouteau+ 2022),
evolution (Pouteau+ 2023)
- CMF slope α ≲ 1
- 8 Hot Cores in W43 (Brouillet+ 2022) & ~60 more (Bonfand+ 2024)
- Line Data paper (Cunningham+ 2023)
- Single-dish combination (Díaz-González+ 2023)
- SiO Outflow catalog (Towner+ 2023)
- 320 SiO outflows cataloged
- Catalog paper (Louvet+ 2024)
- ~1000 cores cataloged, CMF steeper than IMF
- Gas infall kinematics (Álvarez-Gutiérrez+ 2024), Sandoval-Garrido+ 2025, Koley+ 2025
- Dust temperature / column maps with PPMAP (Dell Ova+ 2024)
- H41α estimates of free-free emission (Galván-Madrid+ 2024)
(these papers are all now submitted or accepted, but this is a screenshot that includes everyone's photos...)
ALMA-IMF gas flows → Panta-Rei
Gas flows in N2H+ filaments (Álvarez-Gutiérrez+ 2024,
Sandoval-Garrido+ 2025
)
Collapse is slower on larger scales, but fast enough to matter.
Panta-Rei will explore this for a larger sample.
Panta-Rei will explore this for a larger sample.
ALMA-IMF: Continuum Data → core catalogs
Pouteau+ 2022 W43-MM2/3
Context: Recent CMF measurements
However, a core is not a core.
≠
ALMA-IMF "Core" Mass CDF
The CMF in protoclusters is shallower than the IMF
(i.e., more massive objects than expected)
(i.e., more massive objects than expected)
Many! Differ in: resolution, algorithm, selection.
Nearby surveys are only-starless; more distant are starless + protostellar
| Publication | Distance | Ncores | Mmin | Mmax | Resoln [au] | Slope | "stage" | Figure |
|---|---|---|---|---|---|---|---|---|
| Louvet+ 2024 | 2-6 | 330 | 1.64 | 200 | 2000 | 0.97 | HM | |
| Zhang+ 2024 | 0.4 | 927 | 0.3 | 20 | 8000 | 1.4 | LM |  |
| Armante+ 2024 | 2.4 | 80 | 0.03 | 13.2 | 2000 | 1.44 | HII |  |
| Cheng+2024 | 4.5 | 183 | 0.5 | 10 | 3000 | 1.15 | HM |  |
| Pouteau+ 2023 | 6 | 205 | 0.8 | 70 | 2500 | 0.93 | HM |  |
| Li+ 2023 | 0.7 | 570 | 4 | 200 | 15000 | 1.35 | LM |  |
| Suárez+ 2021 | 7.1 | 40 | 2.0 | 59 | 1000 | 1.11 | HII |  |
| Könyves+ 2020 | 0.4 | 292 | 0.2 | 20 | 8000 | 1.33 | LM |  |
| ... | ||||||||
Assuming the classic model, we end up with a shallow IMF
...but the naïve version doesn't work
≠
Cores change states
How does the CMF become the IMF?
Many cores are protostellar - so there's a mass correction
Yoo+ acc.: W51
Budaiev+ 2024: Sgr B2
Fragmentation summary:
We aimed to measure the CMF with ALMA.
Is star formation different in other environments?
Is star formation different in other environments?
Is star formation different in other environments?
Beyond here are bonus slides, best accessed via the index
(there are some repeats)
Comparison of Sgr B2 (one cloud, 200 pc2) with
My story of high-mass star formation:
Theo Richardson's models turn this story into predictions
Theo Richardson
My story of high-mass star formation:
No massive starless cores in ALMAGAL or ALMA-IMF
My story of high-mass star formation:
My story of high-mass star formation:
0.25-0.5 M⊙ accreted
We don't know the frequency of these events; they are likely bright transients, but most often heavily dust-obscured
My story of high-mass star formation:
Many stars in clusters form from cooked gas.
Cooking may suppress further fragmentation.
Desmond Jeff+ 2024
Ten hot cores in Sgr B2 DS
outside the massive clusters
The Brick isn't forming many stars
The Brick is icy
The Brick is icy
Building new tools: Spectral Line Survey of The Brick
Salt backup slides
What's next for salts?
(VLA 22A-022, 23A-016)
Deep VLA observations of low-J lines in Orion
Classic HII region feedback:
The Jeans Mass MJ is the mass where gravity and thermal pressure are balanced.
MJ ∝ T3/2 ρ−1/2
Large scales again:
ALMA enables protostar counting in
Brighter cores are systematically warmer, therefore less massive for a fixed flux
however, the correction depends on the underlying distribution
however, the correction depends on the underlying distribution
How does the CMF become the IMF?
Cores fragment as they form YSOs
2000 AU → 200 AU
(no YSO, 1 YSO, >1 YSO) = (25%, 30%, 45%) W51-E, (53%, 39%, 8%) IRS2
Taehwa Yoo+: fragmentation
toward W51
toward W51
Cores
Fragments
(no YSO, 1 YSO, >1 YSO) = (25%, 30%, 45%) W51-E, (53%, 39%, 8%) IRS2
Taehwa Yoo+: fragmentation
toward W51
toward W51
20% of YSOs in W51 lie within the $>100$ K contour
Cores
Fragments
(no YSO, 1 YSO, >1 YSO) = (25%, 30%, 45%) W51-E, (53%, 39%, 8%) IRS2
Taehwa Yoo+: fragmentation
toward W51
toward W51
5000 AU → 500 AU
(no YSO, 1 YSO, >1 YSO) = (17%, 37%, 45%) Sgr B2
("cores" are bigger)
Budaiev+ 2024: fragmentation in Sgr B2
Trend: More massive cores fragment more.
Suggestive Trend: More massive cores make more massive single stars
Yoo+ acc.: W51
Budaiev+ 2024: Sgr B2
The number of detectable fragments goes up, but the mass gets more
concentrated into the most massive object.
- N(YSO) > N(core) [maybe obvious, but now empirical]
- More massive cores contain more fragments
- ...but more massive fragments dominate their cores more
- 20% of YSOs reside in hot cores: exposed to hot core chemistry even if they didn't form there
Several different scenarios: mix of mechanisms
Protostars exist within most cores; the PMF is the useful observable
JWST peers into Super Star Clusters
W51e → W51 IRS2 → Wd2 → Wd1
→
→
→
Super Star Cluster formation in the Galactic Center
Super Star Cluster formation in the Galactic Center
Is star formation different in other environments?
In the Galactic Center?
Is star formation different in other environments?
In the Galactic Center?
Is star formation different in other environments?
In the Galactic Center?
The CMZ
$\sim10^8$ M$_\odot$ of gas in $\sim200$ pc, 10% of Galactic star formation
$>\frac{1}{3}$ of CMZ SF is in bound clusters (3-8$\times$ local)
$\sim$50% of CMZ SF occurs in the Sgr B2 cloud.
The Central Molecular Zone of the Galaxy represents one extreme of star forming conditions in the Galaxy
Sgr B2 with JWST/
NIRCam
MIRI
on MEERKAT: Ionized gas shows layers of the cloud
MIRI reveals deeply embedded gas
MIRI 25 micron shows the outflow:
first IR light from within Sgr B2 N
first IR light from within Sgr B2 N
Sgr B2 N Accretion + Outflow
JWST has found new HII regions:
the SFR is higher than estimated from radio
the SFR is higher than estimated from radio
(remember Sgr B2 already accounts for $\sim$50% of the CMZ SFR)
R=770
G=(480-[410-405])
B=Brα
G=(480-[410-405])
B=Brα
■ New HII regions
There is an overall asymmetry in the star formation seen both in JWST (HII regions) and ALMA (embedded YSOs)
JWST shows the transition is sharp
What ALMA sees, JWST doesn't: 3/700 point source matches
(HII regions, outflow cavities do match)
(HII regions, outflow cavities do match)
What does this mean in context?
TBD, but it hints at an ongoing compression event
TBD, but it hints at an ongoing compression event
The CMZ: ACES
The first complete survey of the CMZ with 2.4" resolution between ~2 microns and 10 cm. (previous best was ~15")
https://sites.google.com/view/aces-cmz/home
https://sites.google.com/view/aces-cmz/home
First five data papers submitted July 2025.
Six early result papers out: https://sites.google.com/view/aces-cmz/publications
Six early result papers out: https://sites.google.com/view/aces-cmz/publications
Ask if you want to know more about the MUBLO
MIRI image taken 2025-09-02
F187N F182M F150W
F210M F187N F182M
F212N F210M F187W
F300M F212N F210M
F360M F300M F212N
F410M F405N F360M
F466N F410M F300M
F466N F410M F405N
F480M F410M F360M
F480M F466N F410M
Galactic Plane Massive Clusters: W51
480
405
187
182
162
140
360
335
210
480
410
405
770
560
480
335/2100
480-360
410-405
PAH Hot dust? Scattered?
PAH Hot dust? Scattered?
Colors show physical mechanism
Interesting feature highlight reel
Sharp dust filaments
The most extreme massive stars still forming (IRS2E saturates at all long wavelengths - it's a massive star with a hot disk)
Giant proplyds? EGGs?
The HII region is criss-crossed by overdensities and dust lanes
A sharp bar analogous to the Orion Bar
The extinction-producing layers seen at short wavelengths are not the ALMA star-forming gas: this hints at edge-on filamentary (or planar) structures
A gigantic bow-shock-like bubble, far from the cluster
Objects classified as YSOs - they are massive stars lighting up leftover dust
Symmetric HII region
Symmetric HII region - but more embedded
Paintbrush-stroke filaments
HII regions with PAHs
20/200 ALMA sources have JWST matches
20/200 ALMA sources have JWST matches
Probably disks (hot dust) with small but still significant envelopes
Probably disks (hot dust) with small but still significant envelopes
JWST - ALMA mismatches:
In high-mass star-forming regions, JWST detects only later-stage objects
This is different from local clouds: the more massive cloud is responsible.
Consequence: YSO counts have environmentally different meanings
→ we measure (recent) star formation history w/JWST
"stage" here means "relative amount of leftover envelope"
This is different from local clouds: the more massive cloud is responsible.
Consequence: YSO counts have environmentally different meanings
→ we measure (recent) star formation history w/JWST
"stage" here means "relative amount of leftover envelope"
- YSO model grid is testing accretion models
- Sgr B2 is forming & fragmenting YSOs - asymmetrically
- W51 fragments are dominated by most massive
The Galactic Center Dust ridge...
...has a foreground cloud in front
We measure ice via stellar absorption
(but come to tomorrow's seminar for the rest of this)
(but come to tomorrow's seminar for the rest of this)
- YSO model grid is testing accretion models
- Sgr B2 is forming & fragmenting YSOs - asymmetrically
- W51 fragments are dominated by most massive
- JWST measures ice abundance across the galaxy
- We can measure metallicity with ice
Star Formation oversimplified
Ṁ
L / M
The CMZ gas is constantly replenished, as seen in "Extreme Velocity Features"
How does gas flow into the Galactic Center?
We think the ring-like feature formed in this simulation is a good model for the CMZSormani+ in prep
The CMZ:
CMZOOM (SMA survey) & early ALMA surveys started to map high-mass star formation
Battersby+ 2020,
Hatchfield+ 2020,
Callanan+ 2023,
Hatchfield+ 2024
The proto-Super Star Cluster Sgr B2 dominates SF in the CMZ
Two $>10^4$ M$_\odot$ clusters within $<3$ pc; $10^6$ M$_\odot$ of gas in $\sim10$ pcZoom in to Sgr B2
The proto-Super Star Cluster Sgr B2 dominates SF in the CMZ
Two $>10^4$ M$_\odot$ clusters within $<3$ pc; $10^6$ M$_\odot$ of gas in $\sim10$ pcThe proto-Super Star Cluster Sgr B2 dominates SF in the CMZ
Two $>10^4$ M$_\odot$ clusters within $<3$ pc; $10^6$ M$_\odot$ of gas in $\sim10$ pcComparison of Sgr B2 (one cloud, 200 pc2) with
ALMA-IMF (15 SFRs, 53 pc2)
3mm catalog → Simplistic mass inferences
At this sensitivity, all are M>8⊙ YSOs
Mtot = 8 M⊙
∫0∞ M N(M) dM
∫8∞ M N(M) dM
∫8∞ M N(M) dM
= 96 M ⊙
The "Bound Cluster Fraction" is higher in CMZs
Γ is the fraction of stars forming in bound clusters
Galaxy averages
CMZ prediction
Sgr B2 data
Sgr B2 data
Higher CMF + top-heavy cluster IMF = top-heavy CMZ IMF
The CMZ: ACES
The first complete survey of the CMZ with 2.4" resolution (previous best was ~15")
between ~2 microns and 10 cm.
between ~2 microns and 10 cm.
Results forthcoming! Continuum, line data papers, catalogs, kinematic analyses, filament identifications all in prep....
... but the first result is unrelated (?) to star formation:
Summary
Dynamical interactions are common where stars form
Most stars form in regions unlike the solar neighborhood
Weird things abound
and they are more common in clusters. Environment matters!
Most stars form in regions unlike the solar neighborhood
- Shallower Core Mass Function in richer SF regions
- The Galactic Center forms more stars in clusters
Weird things abound
- Gas-phase salt is prominent in disks around high-mass stars
- There's an extremely broad linewidth, compact source in the Galactic Center
Astrochemistry:
Hot Cores
Ices
Line Surveys
Hot Cores
Ices
Line Surveys
F200W F182M F115W
F212N F200W F182M
F356W F212N F200W
F410M F356W F212N
F444W F410M F356W
F466N F444W F410M
keflavich.github.io/talks/EPOS_2024.html for more about the Brick
Hot Cores
Hot cores are chemically rich sites of high-mass star formation.
Bounded at ~100 K
Bounded at ~100 K
Hot Cores
Hot cores are chemically rich sites of high-mass star formation.
Accretion is ongoing
Accretion is ongoing
all MYSOs make hot cores
Hot cores:
- 76 HCs in ALMA-IMF sample from CH3OCHO (Bonfand+ 2024; left, Brouillet+ 2022)
- ~10% of ALMA-IMF continuum cores are within hot cores
- 60 from ALMA-ATOMS (Qin+ 2022)
- > 15 in Sgr B2 (Jeff+ 2024, Bonfand+ 2017)
-
They start as normal-ish looking seeds:
there is no massive prestellar core.
[ALMA-IMF, ALMA-ATOMS, ALMAGAL all confirm this] - They accrete for some time, during which their core must be replenished
- They have a disk most of this time, but it is not a "classic" T Tauri disk, it's small and constantly changing directions: accretion is chaotic
- Massive YSOs spend most of their accretion mass (maybe time) luminous, cooking their surroundings: all MYSOs make hot cores
Theo Richardson
Including source count and flux predictions spanning whole clusters
Richardson+ in prep
See Josh Peltonen's talk next...
-
They start as normal-ish looking seeds:
there is no massive prestellar core.
[ALMA-IMF, ALMA-ATOMS, ALMAGAL all confirm this] - They accrete for some time, during which their core must be replenished
- They have a disk most of this time, but it is not a "classic" T Tauri disk, it's small and constantly changing directions: accretion is chaotic
- Massive YSOs spend most of their accretion mass (maybe time) luminous, cooking their surroundings: all MYSOs make hot cores
(stuff above $\sim10 \mathrm{~M}_\odot$ is protostellar)
-
They start as normal-ish looking seeds: there is no massive prestellar core.
[ALMA-IMF, ALMA-ATOMS, ALMAGAL all confirm this] - They accrete for some time, during which their core must be replenished
- They have a disk most of this time, but it is not a "classic" T Tauri disk, it's small and constantly changing directions: accretion is chaotic
- Massive YSOs spend most of their accretion mass (maybe time) luminous, cooking their surroundings: all MYSOs make hot cores
Massive, hot cores exist
MYSOs do have cores, those cores must grow in parallel
~200 $\mathrm{~M}_\odot$ hot core
~200 $\mathrm{~M}_\odot$ hot core
-
They start as normal-ish looking seeds: there is no massive prestellar core.
[ALMA-IMF, ALMA-ATOMS, ALMAGAL all confirm this] - They accrete for some time, during which their core must be replenished
- They have a disk most of this time, but it is not a "classic" T Tauri disk, it's small and constantly changing directions: accretion is chaotic
- Massive YSOs spend most of their accretion mass (maybe time) luminous, cooking their surroundings: all MYSOs make hot cores
Chaotic accretion in another ~200 M$_\odot$ hot core
The outflow (& disk) around W51 North changed direction by ~50 deg
in < 100 years.
0.25-0.5 M⊙ accreted
We don't know the frequency of these events; they are likely bright transients, but most often heavily dust-obscured
DIHCA: Digging into the Interior of Hot Cores with ALMA
Olguin+ 2025/in prep: kinematic mass measurements of 31 objects with $M>8 M_\odot$
The 'disks' are messy.
The 'disks' are messy.
-
They start as normal-ish looking seeds: there is no massive prestellar core.
[ALMA-IMF, ALMA-ATOMS, ALMAGAL all confirm this] - They accrete for some time, during which their core must be replenished
- They have a disk most of this time, but it is not a "classic" T Tauri disk, it's small and constantly changing directions: accretion is chaotic
- Massive YSOs spend most of their accretion mass (maybe time) luminous, cooking their surroundings: all MYSOs make hot cores
- Massive YSOs spend most of their accretion mass (maybe time) luminous, cooking their surroundings: all MYSOs make hot cores
Many stars in clusters form from cooked gas.
Cooking may suppress further fragmentation.
Hot cores in the Galactic center: Distributed MYSOs
Desmond Jeff+ 2024
Ten hot cores in Sgr B2 DS
outside the massive clusters
TG ~ 200-500 K
M ~ 200 - 2900 M⊙
(proto-O-stars / clusters)
~5% of cores are hot cores
M ~ 200 - 2900 M⊙
(proto-O-stars / clusters)
~5% of cores are hot cores
Sgr B2 DS: More massive cores than the Disk
Hot Cores
Hot cores are chemically rich sites of high-mass star formation.
They are only found in the more distant disk & CMZ regions
They are only found in the more distant disk & CMZ regions
Hot Cores
Hot cores are chemically rich sites of high-mass star formation.
They are only found in the more distant disk & CMZ regions
They are only found in the more distant disk & CMZ regions
Alyssa Bulatek: Spectral Line Survey of The Brick
- Dasar line at 107 GHz
- CH3OH only in absorption
- Occurs at density $n<10^6 \mathrm{cm}^{-3}$
Some notes on Data Scale
Brinary disks
The SrcI disk has gas-phase salt (NaCl, KCl) and water (H2O).
So it's brine.
(blame Adam Leroy for this term)
IRAS16547A/B (Tanaka+ 2020)
have (unresolved) salt water disks
Brine lines measure dynamical mass
NaCl v=1 J=18-17
Stack of v=[0,1] Ju=[18,17]
SrcI
15 M⊙
30 M⊙
40 M⊙
What have we learned about brinaries?
- Neither common nor rare: 10 known so far, >23 HMYSO candidates examined
- Y: SrcI, G17, IRAS16547, NGC6334I, G351.77, W33A
- Ginsburg+ 2019, Tanaka+ 2020, Ginsburg+ 2022
- N: I16523, I18089, G11, G5, NGC6334IN, S255IR NIRS3, G333.23-0.06, I18162
- Ginsburg+ 2022
- Y: SrcI, G17, IRAS16547, NGC6334I, G351.77, W33A
- Coincide w/line-poor sources
- Not hot cores; little mass reservoir?
- Trace reasonably symmetric disks (in the well-resolved cases)
- there is some ambiguity b/w disk & outflow in one case
Compare: G17 vs G11
G17: Brinary
Hot (ionizing) photosphere. Circular disk.
G11: Not-Brinary
Molecule rich, kinematically messy & extended
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?
Deep VLA observations of low-J lines in Orion
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?
M = 24-10+12M⊙
if the masers trace a disk
if the masers trace a 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?
How is star formation in high-mass clusters different?
- Feedback from one star affects many in clustered regions
- IMF depends on density, feedback, global conditions (e.g., Jones & Bate 2018, Narayanan & Dave 2012)
- Total star formation efficiency is higher.
- Collisions assemble the most massive stars?
(e.g., Fujii+ & PZ 2013, but see Moeckel & Clarke 2011)
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
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.
If they still have enough gravity to bind the gas, the remaining gas is forced onto the most massive gravitational sinks.
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. ALMA enables protostar counting in
distant, massive clouds
Sgr B2: the most massive & star-forming cloud in the Galaxy
How do we learn about clustering? The IMF?
- Count objects:
- Cores are (sometimes) countable
- Protostars are countable
YSO 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"
"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
Walker+ 2021
3mm Luminosity Function
What are the sources?
At this sensitivity, all are M>8⊙ YSOs
Mass measurements: Optically thin, isothermal dust
TD estimated with PPMAP fits to Herschel data
(~6" resolution)
Simple models assuming TD ~ f(M) don't change CMF much
We can do better with YSO models and TG measurements
From YSO counts to the IMF?
How do we measure the CMF if the cores all have YSOs in them?
Mapping accretion histories
(left: IS, right: TC)
onto the Robitaille 2017 model grid
Key addition:
Envelope mass!
(left: IS, right: TC)
onto the Robitaille 2017 model grid
Key addition:
Envelope mass!
SPICY-ALMA-IMF:
ALMA-IMF:
ALMA-IMF:
Allison Towner
Nazar has cataloged 100s of new masers in Sgr B2 (VLA 18A-229)
A quick look at what's coming: Nazar has cataloged 100s of new masers in Sgr B2 (VLA 18A-229)
VLA 19A-154
Orion Source I
FOV: 0.07 pc (16000 AU)
Likely the only gas-rich protocluster with a complete YSO census:
0.25-0.5 M⊙ accreted
We don't know the frequency of these events; they are likely bright transients, but most often heavily dust-obscured
Questions in Star Formation
Richardson-enhanced Robitaille+ 2017 model grid
fits including ALMA data
UG team:
Sydney Petz
Brice Tingle
Morgan Himes
Brice Tingle
Morgan Himes
Our Galaxy's center, the CMZ, has denser gas than the Galactic average
Cold Dust
Hot, ionized gas
Hot dust/PAHs
Hot, ionized gas
Hot dust/PAHs
ALMA-IMF:
The CMF is shallow (top-heavy) in HMSFRs
ALMA-IMF:
The CMF is shallow, and steepens with time?
Louvet+, subm
ALMA-IMF Line Data: 320 cataloged SiO Outflows
Allison Towner
Resolved, structured SiO outflows
5-60% of SiO at low-velocities:
Relic outflows?
Relic outflows?
ALMA-IMF Line Data: CH3CN, CH3CCH
Temperature measurements with per-pixel rotation diagrams
Jeff+ in prep (CH3OH in CMZ), Wyrowski+ in prep (CH3CN in ALMA-IMF)
Hot cores in ALMA-IMF: From rare objects to a population
Cores with line forests
TD>50 K
TG ≳100K
TD>50 K
TG ≳100K
Hot core overview:
- 9 HCs in W43-MM1 (Brouillet+ 2022)
- ~60-70 HCs in ALMA-IMF sample from CH3OCHO (Bonfand+ 2024; left)
- ~10% of continuum cores are within hot cores
- CH3CN temperature maps (Wyrowski+ in prep)
ALMA-IMF Key Results summary
- Rich, science-ready data (Ginsburg+ 2022, Cunningham+ 2023)
- CMF is top-heavy in HMSFRs (Pouteau+ 2022, Louvet+ 2024)
- Core fragmentation is not 1-to-1 [WIP] (Budaiev+ (CMZ), Yoo+ (W51), Louvet+ (W43), ...)
- 10% of $M\gtrsim1\mathrm{~M}_\odot$ cores are hot cores (Brouillet+ 2022, Bonfand+ 2024, Wyrowski+)
- Outflow feedback builds over time, sets initial conditions for many cores (Nony+ 2022, Towner+ 2023)
What's next?
Nazar has cataloged 100s of new masers in Sgr B2 (VLA 18A-229)
Ammonia Masers:
Derod Deal
VLA 19A-154
the gas & dust where stars form: Orion
the gas & dust where stars form: Orion
the gas & dust where stars form: Orion
the gas & dust where stars form: Orion
The Integral-Shaped filament has $\sim10^4$ M$_\odot$ of gas over $\sim$10 pc Kong+ 2018 CARMA-Orion surveythe gas & dust where stars form: Orion
The Orion Molecular Cloud is the closest (d$\sim400$ pc) site of high-mass star formationthe gas & dust where stars form: Orion
BOOM! 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)
Salt is a tool to weigh HMYSOs
Keplerian orbits measure mass
More Brinary disks
Ginsburg+ 2023: New sample. Miriam Garcia Santa-Maria is following up
Briny chemistry:
Salts, SiS, SiO, and PN are seen together [but limited spectral coverage]
The Orion Molecular Cloud Core is a dense cluster
FOV: 0.07 pc (16000 AU)
Likely the only gas-rich protocluster with a complete YSO census:
ALMA (Otter), VLA (Ballering, Wright), Chandra, JWST....
N*OMC(Otter+ 2021) = 1.6 x 105 pc-3
N*ONC(Otter+ 2021) = 0.6 x 105 pc-3
N*ONC(Hillenbrand+ 1998) = 0.2 x 105 pc-3
Most stars form in denser regions
NGC 1333, an embedded low-mass cluster
Lada & Lada 2003: >70% in embedded clusters
500 years ago, four or more stars were in a multiple system
The old picture involved source n, but it's not moving fast enough
Clusters are sites of interactions & collisions
The BN/I/x interaction is the poster case of accretion ended by dynamical interaction.
...even though Orion is only a medium-mass proto-open-cluster
Interactions are (probably) more common in high-mass clusters
The outflow (& disk) around W51 North changed direction by ~50 deg
in < 100 years.
0.25-0.5 M⊙ accreted
We don't know the frequency of these events; they are likely bright transients, but most often heavily dust-obscured
Big Questions in Star Formation
I'll highlight incremental progress toward answering some of these
spiced with a few intriguing discoveries
- How does the IMF vary, and what controls its variation?
- What controls the star formation rate in galaxies?
- When and how do planets form?
- How does stellar clustering affect each of these?
- What initiates star formation?
- What Processes Govern Protostellar Evolution and Determine the Stellar Mass and Rotation Rate?
- How do Massive Stars Form?
- What is Lifetime of Molecular Clouds (and the Duration of Star Formation)?
- What Physics Determines the Initial Mass Function?
- What Physics Regulates the Rate & Efficiency?
- What Role Does Environment Play in Star and Planet Formation?
- What Physics Regulates Star Formation in Clusters?
- How homogeneous are clusters?
- What is the role of feedback (outflow, radiation, wind, etc) in star formation?
- What is the role of disks in star and planet formation?
- How do multiple systems form?