Panorama of La Silla Observatory in the Chilean Atacama desert, with the SEST dish telescope

Hi!

I am a PhD candidate at the Max Planck Institute for Astronomy in Heidelberg and interested in observational astronomy, binary stars, and black holes.

In September 2023, I started my PhD at MPIA Heidelberg, where I am working with Professor Hans-Walter Rix 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.

Johanna Müller-Horn standing on a beach by the ocean

Key Interests

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

Header image: La Silla Observatory, Chile, with the Swedish-ESO Submillimetre Telescope (SEST).

Research

Artist's impression of a star orbiting a dormant black hole

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.

Artist's impression of a binary star in which one star pulls material from its companion

Discovery of a Galactic intermediate-mass stripped star binary

When stars in a massive binary interact, one of them can be stripped of its hydrogen envelope by its companion. This should leave behind many intermediate-mass stripped stars (2–8 solar masses), yet in the Milky Way they have been remarkably hard to find. In SDSS-V Milky Way Mapper data, we found an unambiguous example: a 5.94-day binary in which a rapidly rotating O star orbits a hot (~60,000 K), helium-rich stripped star of 3.2–5.8 solar masses. We seem to have caught it in a brief, inflated phase right after mass transfer, which makes it bright enough to spot. This makes it a benchmark for models of binary evolution and of stripped-envelope supernova progenitors.
Image Credit: ESO/M. Kornmesser/S.E. de Mink

Illustration of the binary system HIP 15429

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.

Hubble image of the globular cluster 47 Tucanae

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

ALMA image of the protoplanetary disk around HD 163296

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.

Diagram of the convolutional neural network architecture

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.

Publications

You can find my publications on ADS or arxiv.

First-author papers
2026
2025
2022
Co-author papers
2026
2025
2023

Talks

2026
  • September 2026
    Hunting for dormant black holes — and their impostors
    IMPRS-HD Summer School 2026
    @ Heidelberg University
    Invited talk
  • July 2026
    Galactic Emission-Line Stars and Stripped Stars from SDSS-V
    SDSS-V Collaboration Meeting
    @ KU Leuven
    Contributed talk
  • July 2026
    Galactic Emission-Line Stars and Stripped Stars from SDSS-V
    LIAC42 - To Be or not to Be: Understanding the star, the disk, and the stripped companions
    @ University of Liège
    Contributed talk
  • April 2026
    Hunting for Dormant Black Holes and their impostors
    ECR Workshop Multiple Systems Across Scales
    @ Heidelberg University
    Invited talk
  • March 2026
    Black Holes, Emission Line Objects, and Stripped Stars
    SDSS-V MWM SciFest
    @ Osservatorio Astrofisico di Arcetri, Firenze
    Contributed talk
  • March 2026
    Toward mapping the brown dwarf desert - A clean and well-understood sample of substellar companions from Gaia
    Preparing for Gaia’s Exoplanet Bonanza
    @ Ringberg Castle
    Contributed talk
2025
  • December 2025
    Hunting for Dormant Black Holes with Gaia DR3
    Group meeting Mapelli
    @ ITA Heidelberg
    Invited talk
  • December 2025
    Observational Tests of Formation Models for Black Holes in Binaries
    Group meeting Schneider/Röpke
    @ HITS Heidelberg
    Invited talk
  • November 2025
    Hunting for Dormant Black Holes with Gaia DR3
    The Lifecycle of Stellar Black Holes
    @ KITP Santa Barbara
    Contributed talk, Recording
  • November 2025
    Observational Tests of Formation Models for Black Holes in Binaries
    Stellar-Mass Black Holes at the Nexus of Optical, X-ray, and Gravitational Wave Surveys
    @ KITP Santa Barbara
    Invited talk, Recording
  • July 2025
    In Search of the Invisible: Hunting for Dormant Black Holes with Gaia DR3
    Binary Stars in the Space Era
    @ Keele University
    Contributed talk
  • June 2025
    Hunting for Dormant Black Holes in SDSS-V: A Search for Dark Companions to Massive Stars
    SDSS-V Collaboration Meeting
    @ MPIA Heidelberg
    Contributed talk
  • February 2025
    A dearth of short-period binaries in the globular cluster 47 Tuc
    Galaxy Coffee
    @ MPIA
    Contributed talk
2024
  • July 2024
    Searching for Black Holes in Binaries with Spectroscopic Surveys
    Liège International Astrophysical Colloquium
    @ University of Liège
    Contributed talk
  • February 2024
    Binary properties of the globular cluster 47 Tuc
    Cahill Tea Talk
    @ Caltech
    Invited talk
2023
  • September 2023
    Binary Properties of the Globular Cluster 47 Tuc (NGC 104)
    Two in a million - The interplay between binaries and star clusters
    @ ESO, Garching
    Contributed talk, slides
  • June 2023
    The Binary Population of NGC 104 - Searching for Stellar Remnants in Globular Clusters with MUSE
    The Renaissance of Stellar Black-Hole Detections in The Local Group
    @ Lorentz Center, Leiden
    Poster presentation

CV

Current Position

  • 2023 – present
    Doctoral Student

    Max Planck Institute for Astronomy, Heidelberg

    • Supervised by Prof. Hans-Walter Rix
    • Thesis topic: Hunting for dormant black holes in binary systems

Education & Experience

  • 2023 – present
    PhD Astronomy

    International Max Planck Research School for Astronomy & Cosmic Physics at the University of Heidelberg (IMPRS-HD)

  • 2021 – 2022
    Research Intern (EU Erasmus+ Scholarship)

    The Oskar Klein Centre, Stockholm, Sweden

  • 2021
    Research Intern (MPIA Summer Internship)

    Max Planck Institute for Astronomy, Heidelberg

  • 2020 – 2023
    MSc Physics

    Georg-August-Universität Göttingen, Germany

  • 2019
    Exchange student (EU Erasmus+ Scholarship)

    Università degli Studi di Padova, Italy

  • 2016 – 2020
    BSc Physics

    Georg-August-Universität Göttingen, Germany

Honours & Awards

  • 2024
    Dissertation Prize

    Dr. Berliner-Dr. Ungewitter-Stiftung, University of Göttingen

  • 2023 – 2027
    IMPRS Graduate Fellowship

    IMPRS for Astronomy & Cosmic Physics, Heidelberg

  • 2017 – 2023
    Scholarship of the German Academic Scholarship Foundation

    Studienstiftung des deutschen Volkes

Contact

  • mueller-horn@mpia.de
  • Room E115
    Max-Planck-Institut für Astronomie
    Königstuhl 17
    D-69117 Heidelberg, Germany