Astronomy & Astrophysics

Lennart M. Böhm

PhD Researcher, European Southern Observatory

Predoctoral Fellow, Center for Astrophysics | Harvard & Smithsonian

I study how massive stars and stellar clusters assemble their mass – from parsec-scale gas flows to accretion onto individual protostellar cores – and how these flows interact with stellar feedback.

I study how the biggest, brightest stars and the star clusters around them keep growing – tracing the gas as it falls in from far outside the cluster all the way down onto the stars themselves, and seeing how that changes once the stars switch on and start blasting the surrounding gas with light.

I study how really, really big stars are born. They start as huge clouds of gas, and the gas slowly falls together to build the star, like water swirling down a drain. I want to know what happens to that gas once the star turns on and starts shining really brightly.

See the research CV

Filamentary accretion onto a massive hub... continuing even in the presence of stellar feedback?

Can accretion flows be traced continuously down to protostellar cores? And how do they survive the onset of ionising feedback?

Massive stars and the clusters they belong to assemble their mass through accretion flows that span enormous scales, from parsec-scale filaments down to individual protostellar cores. The first question is whether that flow stays kinematically connected across these scales, rather than fragmenting into disconnected clumps along the way. The second is what happens once the most massive stars switch on: their ionising radiation carves out H II regions and heats the surrounding gas, which should, in principle, choke off further accretion, yet young, still-forming clusters like W33 Main keep growing anyway. I combine Very Large Array NH₃ observations with VLA radio recombination-line and VLT/KMOS near-infrared data to trace the first question directly, and to test how the gas manages the second.

Does gas keep falling onto baby stars even after they start shining?

Stars form by pulling in surrounding gas, and the biggest stars form inside huge clouds that funnel gas inward from several light-years away, all the way down to the star itself. The first question is whether that inward flow stays smooth and connected the whole way, or breaks up into separate clumps along the way. The second is what happens once the heaviest stars switch on: they blast out light and particles that should blow the surrounding gas away. In theory, that should choke off the gas supply before a star gets especially big, yet we see stars out there dozens of times heavier than the Sun, already shining, as if they kept growing right through it. So does the gas supply actually get cut off, or does it keep sneaking through? I use radio telescopes to watch the gas flowing inward, and to see what the star's light is doing to it at the same time, in a young star cluster called W33 Main.

Can baby stars keep eating even after they start glowing?

Big stars grow by "eating" gas, like filling up a balloon. But once a star gets big enough, it starts glowing and blowing really hard, like a huge fan. You'd think that fan would blow all the leftover gas away before the star could eat any more. But somehow, some stars grow enormous anyway. So where does the extra gas come from? I use big radio dishes to watch the gas sneak in, in a star-forming cloud called W33 Main.

Read about the W33 / MACNIFY project →

Multi-wavelength composite of the W33 giant molecular cloud complex, spanning several parsecs, showing infrared, far-infrared, submillimetre and radio emission in false colour, with the W33 Main, A, and B regions outlined.

The same W33 composite with a zoomed inset over the Main region showing ammonia (NH3) emission contours, revealing a filamentary gas structure feeding the hub that is invisible in the dust and infrared bands alone.

The same W33 composite with a second, rotated inset over the Main region showing ionised-gas (radio continuum) emission, tracing the H II region carved out by the youngest massive stars.

Research themes

ALMA · VLA · APEX

Multi-scale accretion

W33 Main and the MACNIFY project: tracing gas flows from parsec-scale filaments into the feedback-active hub.

W33 Main: tracing gas as it falls from huge surrounding clouds down into a crowded, busy centre.

Watching gas fall into a busy, crowded star nursery called W33 Main.

VLA · VLT/KMOS

Stellar feedback

How ionising radiation and winds from newly formed massive stars reshape the gas that is still trying to feed them.

How light and wind from newly formed stars reshape the very gas that is still trying to feed them.

How a new star's light and wind push around the very gas still trying to feed it.

APEX · SEPIA660

Astrochemistry

Hydrogen chloride as a tracer of chlorine chemistry across the Galaxy – and, unexpectedly, of explosive molecular outflows.

Using one molecule, hydrogen chloride, as a chemical fingerprint across the Galaxy – and, unexpectedly, of violent stellar collisions.

Using one simple molecule as a clue-finder across the Galaxy – and, by surprise, for spotting star collisions.

APEX survey of interstellar HCl: ³⁵Cl/³⁷Cl isotopic ratios in dense cores and outflows

Lennart M. Böhm, Arshia M. Jacob, Friedrich Wyrowski, Karl M. Menten, Katharina Immer, Ashley T. Barnes

PublishedAstronomy & Astrophysics, 705, A802026

A systematic survey of hydrogen chloride (HCl) toward 28 high-mass star-forming regions, observed with the SEPIA660 receiver on the APEX 12 m telescope. H³⁵Cl is detected in every source, and the ³⁵Cl/³⁷Cl isotopic ratio is measured across a wide range of Galactocentric radii.

The ratios trace a gradient consistent with Galactic chemical-evolution models, in which the two chlorine isotopes are built up at different rates as the Galaxy ages and enriches. Several sources with strong molecular outflows show ratios that diverge from this trend, pointing to local chemical or excitation effects distinct from the Galactic-scale pattern – the starting point for the explosive-outflow follow-up described on the Research page.

I surveyed 28 massive star-forming regions for a molecule called hydrogen chloride, using a radio telescope in Chile, and found it in every single one. Comparing two slightly different versions of chlorine in each source traced a pattern that matches how the Galaxy's chemistry has built up over billions of years. A handful of sources broke that pattern – the first clue leading to the explosive-outflow story described on the Research page.

I looked for a certain kind of gas in 28 different star-forming clouds, and found it in every one. By comparing two slightly different versions of one of its atoms, I could tell how the Galaxy's chemistry has changed over a very, very long time. A few clouds looked strange – and that turned out to be a clue about stellar explosions, which you can read about on the Research page.

DOI arXiv:2511.21813 ADS / SciX PDF

MACNIFY I. Parsec-scale accretion in the W33 Main hub-filament system

Lennart M. Böhm, Katharina Immer, Ashley T. Barnes, Alexia Anguera-Gonzalez, Farideh Mazoochi, Álvaro Sánchez-Monge, James S. Urquhart, Taehwa Yoo, Qizhou Zhang

Submitted – in review2026

The first paper in the MACNIFY series investigates whether the parsec-scale filaments feeding the W33 Main hub-filament system remain kinematically connected into the central hub, where massive-star feedback is already active. Using Very Large Array NH₃ (1,1)–(7,7) observations complemented by Green Bank Telescope short-spacing data, the two dominant filaments are traced from a few parsecs out to the hub and characterised through their velocity structure and physical conditions.

The first paper in the MACNIFY series asks whether two long streams of gas feeding a young, massive star-forming region called W33 Main stay smoothly connected all the way into its crowded, feedback-active centre. Using a radio telescope that sees the glow of ammonia gas, together with a second telescope's wide-field data, the two main gas streams are traced from several light-years out down into the hub and their motion is mapped along the way.

This study watches two long rivers of gas, stretching for trillions of kilometres, flow into a busy star nursery called W33 Main. Using radio dishes that can see a particular glowing gas, it checks whether those rivers stay connected all the way to the crowded centre, or break apart before they get there.

All publications →