Johanna Müller-Horn

In September 2023, I started my PhD at MPIA Heidelberg, where I am working with Professors Hans-Walter Rix and Kareem El-Badry on the search for dormant black holes. Based on high-resolution, multi-epoch spectroscopy, I search for and characterize binary star systems with possible compact companions. I am interested in uncovering the formation pathways of these systems and, more broadly, in constraining the properties of the black hole population in the Milky Way.

Before that, I did my undergraduate studies at the University of Göttingen and have spent two semesters abroad at the Universities of Padova and Stockholm. In my Master's thesis, I studied spectroscopic binaries in globular clusters, working with Professors Stefan Dreizler and Sebastian Kamann and the MUSE consortium.

Key Interests

• Binary star systems
• Spectroscopy
• Black holes and compact objects
• Population synthesis
• Globular clusters
• Bayesian Statistics

Image Credit: 47 Tuc by NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration Acknowledgment: J. Mack (STScI) and G. Piotto (University of Padova, Italy)

Research

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Hunting for dormant black holes in the Milky Way

While population synthesis models anticipate billions of stellar-mass BHs in the Milky Way, only a few have been identified, mostly in X-ray-emitting, accreting binary systems. My research instead concentrates on dormant BHs in detached binary systems. These BHs induce distinct variations in their companion's radial velocity, flux, and light-centroid position, offering an indirect detection method.
Image Credit: ESO/Calçada, L.

Stripped stars and black hole impostors

The binary system HIP 15429 was initially proposed to host a dormant black hole based on its high mass function and single-lined spectrum. However, our detailed spectroscopic analysis reveals it is actually a post-interaction binary with two non-degenerate stars—a bloated stripped star and a rapidly rotating Be star. This system joins a growing class of "black hole impostors" that can mimic the observational signatures of genuine black hole binaries. Intriguingly, HIP 15429's remarkably high eccentricity challenges standard tidal evolution models, suggesting either inefficient tidal dissipation or external influences such as a tertiary companion.

Binary properties of the globular cluster 47 Tuc (NGC 104)

Observations of binary stars in globular clusters are essential to understand cluster dynamics, shed light on the formation of peculiar stellar objects and to probe the elusive populations of dark stellar remnants. Using multi-epoch data from the VLT/MUSE integral field spectrograph, I have analysed the binary population of 47 Tuc, one of the oldest and most massive Galactic globular clusters. Results include the identification of several hundred binary candidates based on radial velocity variability, a new estimate of the total binary fraction in the cluster and constraints on the number of black holes in binary systems in 47 Tuc.
Image Credit: ESA/NASA/Hubble

Gap-opening planets around type-A stars

During a summer internship @MPIA Heidelberg, I worked on theoretical planet formation models under the supervision of Dr. Gabriele Pichierri and Dr. Bertram Bitsch. I investigated whether and under what circumstances the presumed gap-opening planets in protoplanetary disks around young stars could represent progenitors of the discovered exoplanet population. More specifically, I simulated the evolution of the embedded planets during the lifetime of the disk and analysed the dependence of the final planetary masses and orbital parameters on the disk viscosity.
Image Credit: DSHARP survey, Andrews, S. et al.

Cosmological parameter inference from the distribution of dark matter halos

In 2021, I worked on a cosmological project at the Oskar Klein Centre in Stockholm with Dr. Jens Jasche as part of an ERASMUS internship. My work there focused on inferring cosmological parameters from the large scale density distribution of galaxies using convolutional neural networks (CNN). For the project, I worked with a subset of Quijote N-body simulations from which I generated training data. The CNN achieved significant improvement in prediction accuracy compared to related work, especially regarding Omega matter.

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Publications

You can find my publications on ADS or arxiv.

Talks & Events

Contact

• mueller-horn@mpia.de
• Room 323
  Max-Planck-Institut für Astronomie
  Königstuhl 17
  D-69117 Heidelberg, Germany