Here are the main projects I work/have been working on recently, most of them are centered on different evolutionary stages of massive binary stars.

  • Predicting the binary black hole content in the Milky Way

With Shea Garrison-Kimmel and the FIRE collaboration.


Stellar surface density (left), binary black hole surface density (middle, from Lamberts et al, 2018) and binary white dwarf density (right, from Blunt, Lamberts, in prep) based on a cosmological simulation of the Milky Way. All images have the same dynamic range and are normalized to the maximum in a given image.

At Caltech I became part of the FIRE collaboration, and got access to high resolution cosmological simulations of different types of galaxies. These simulations provide a realistic view of the star formation over cosmic history and are ideal to determine the formation of progenitors of gravitational wave events. I postprocessed the “Latte” simulation (Wetzel et al, 2016), which is similar to the Milky Way. I uniquely combined it with a binary evolution model to determine the distribution of binary black holes, and their merger history. This work highlights that the binaries preferentially “live” in the outskirts of the galaxy (halo, tidal streams and satellites) due to their low metallicity. I have been working with graduate student Sarah Blunt for a similar model for white dwarfs, which will the dominant source for LISA.

  • Coupling between interferometric results and numerical simulations to study the colliding wind binary Gamma Velorum

With Florentin Millour , Olivier Chesneau (Observatoire de la Côte d’Azur, France), Adriane Liermann (Leibniz-Institut fur Astrophysik Potsdam (AIP))

Gamma Velorum is the closest WR star and is a prime target to study the final phase of massive stars.  The colliding wind region, resulting from the interaction with a O-type companion star, provides constraints on the mass loss of the system. Near-infrared interferometric observations can provide unique constraints on the spatial extension and structure of the interaction region. To fully benefit from the unique data obtained with the AMBER instrument at VLTI, we created mock emission maps and visibility curves based on 3D hydrodynamical simulations I performed. This work is a direct application of the study of colliding stellar winds I performed for my PhD thesis (Lamberts+2011, 2012).

Combining observations with mock data, we can confirm the detection of the colliding wind region and estimate its flux (Lamberts+2017, MNRAS, 468, 2655)


Volume rendering of the density in the wind collision region. The large red spot is the WR star, the tiny dot the O star.


Volume rendering of the wind collision region (top left), resulting continuum emission map in the near infrared (top right) and comparison of the resulting visibility curve (bottom, black line) with the observational data (bottom, colored points) (Lamberts+2017)

  • Reproducing X-ray flares in GRB afterglows with a stratified ejecta

With Frédéric Daigne (Institut d’Astrophysique de Paris)

Long gamma-ray bursts result from the final explosion of very massive stars. While the prompt gamma-ray emission is  directly associated with the central engine, the afterglow emission results from the interaction of the ejecta with the surrounding medium.  The Swift satellite revealed strong variability in the early X-ray afterglow, including the presence of flares. Because of the wide range of physical scales between the development of internal shocks and the deceleration of the ejecta, I have developed a 1D spherical grid with moving coordinates for my relativistic hydro code. My simulations show many interactions between the reverse shock and the internal shocks, which gives flares in the resulting light curves (Lamberts, Daigne, 2018, MNRAS, 474, 2813).


  • Joining galaxy formation with massive stellar evolution to determine where GW 150914 probably formed.

With Shea Garrison-Kimmel, Drew Clausen and Philip Hopkins (TAPIR, Caltech)

This work was stirred by the very exciting announcement of the detection of gravitational waves generated by the mergers of two stellar black holes by the LIGO detector.  Given the large mass of the black holes, the progenitor stars were formed in  a low metallicity environment. I immediately asked myself the simple question ” where are these low metallicity environments?” and there seemed to be no obvious answer.   This analytic work combines Drew’s binary population synthesis models with a model for the amount of low metallicity star formation through cosmic history.


Black hole binary merger rate as a function of host galaxy mass and progenitor metallicity (left) and formation time (right) showing a bimodal distribution (Lamberts+2016, MNRAS, 463, 31L).

We find that most mergers come from massive galaxies with stars formed around the peak of cosmic star formation but have a significant contribution from later star formation in dwarf galaxies.


  • Impact of TeV blazar heating on the intergalactic medium and the formation of large scale structure.

With Philip Chang (CGCA, University of Wisconsin-Milwaukee), Christoph Pfrommer (HITS), Avery Broderick (Perimeter Institute), Ewald Puchwein (Cambridge), Mohamad Shalaby (PI)

The gamma-rays emitted by certain supermassive black holes interact with the extragalactic background light to form electron/positron pairs. In the standard hypothesis, these pairs lose their energy through inverse Compton interactions with the CMB. Broderick, Chang and Pfrommer (2012) proposed an alternate model where the pairs lose their energy to the surrounding medium through plasma instabilities.   This heats the intergalactic medium and has potential impact on later large scale structure formation.

I joined this collaboration during my first postdoc, and became experienced in large scale structure formation . Motivated by observational constraints, I lead the effort to develop


Temperature-density distribution of the intergalactic medium at z=2. The wide range of temperatures at low density (left) contrasts with the homogeneous heating model (grey lines). (Lamberts+2015)

an inhomogeneous model for blazar heating, taking into account clustering of sources. I found that this can lead to a wide range of temperature for low density gas. I am currently modeling the resulting Ly alpha forest in order to compare with observational data sets (Lamberts+2019, in prep).

Besides this main contribution to the collaboration, I have participated in papers related to the plasma physics of the electron/positron beam as, the gamma-ray signature expected from the TeV blazars and their constraints on large scale magnetic fields.

  • Structure and stability of colliding stellar winds and gamma-ray binaries.

With Guillaume Dubus (IPAG,France), Sébastien Fromang (SAP/CEA, France)

Gamma-ray binaries are composed of a massive star and a fast rotating pulsar, which emits a highly relativistic wind. The collision between the stellar and pulsar wind creates a shocked structure where particles are accelerated and eventually yield gamma-ray emission.  These systems display orbital variability at all wavelengths, which is impossible to reproduce with one-zone models. To fully understand these systems, it is necessary to couple relativistic hydrodynamics  with a refined model for non-thermal particles and their emission.


To model these systems, I have developed a relativistic extension to the AMR code Ramses (Teyssier, 2002), which is publicly available with the RAMSES code ( and described in Lamberts+2013.

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