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 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

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.

Publications

You can find my publications on ADS or the arxiv.

First-author papers

  • Müller-Horn, J. et al. "Dormant BH candidates from Gaia DR3 summary diagnostics", Submitted to A&A: ADS, arxiv.

  • Müller-Horn, J. et al. "HIP 15429: A newborn Be star on an eccentric binary orbit", A&A, Volume 701, A9, 2025: ADS, arxiv.

  • Müller-Horn, J. et al. "Binary properties of the globular cluster 47 Tuc (NGC 104). A dearth of short-period binaries", A&A, Volume 693, A161, 2025: ADS, arxiv.

  • Müller-Horn, J. et al. "Emerging population of gap-opening planets around type-A stars. Long-term evolution of the forming planets around HD 163296", A&A, Volume 663, A163, 2022: ADS, arxiv.

    • Co-author papers

  • Nagarajan, P. [and 11 others including Müller-Horn, J.] "A Spectroscopic Search for Dormant Black Holes in Low-Metallicity Binaries", PASP, Volume 137, 2025: ADS, arxiv.

  • Li, J. [and 8 others including Müller-Horn, J.] "Millions of Main-Sequence Binary Stars from Gaia BP/RP Spectra", Submitted to A&A: ADS, arxiv.

  • Saracino, S. [and 10 others including Müller-Horn, J.] "Studying binary systems in Omega Centauri with MUSE - II. Observational constraints on the orbital period distribution", MNRAS, Volume 538, 2025: ADS, arxiv.

  • Li, J. [and 7 others including Müller-Horn, J.] "Identification of 30,000 White Dwarf-Main Sequence binaries candidates from Gaia DR3 BP/RP(XP) low-resolution spectra", ApJS, Volume 279, 2025: ADS, arxiv.

  • Saracino, S. [and 9 others including Müller-Horn, J.] "A closer look at the binary content of NGC 1850", MNRAS, Volume 526, 2023: ADS, arxiv.

    • Talks and Conferences

  • Binary Stars in the Space Era @ Keele University (07/2025)
    "In Search of the Invisible: Hunting for Dormant Black Holes with Gaia DR3"
    Contributed talk

  • SDSS-V Collaboration Meeting @ MPIA Heidelberg (06/2025)
    "Hunting for Dormant Black Holes in SDSS-V: A Search for Dark Companions to Massive Stars"
    Contributed talk

  • Liège International Astrophysical Colloquium @ University of Liège (07/2024)
    "Searching for Black Holes in Binaries with Spectroscopic Surveys"
    Contributed talk

  • Two in a million - The interplay between binaries and star clusters @ ESO, Garching (09/2023)
    "Binary Properties of the Globular Cluster 47 Tuc (NGC 104)"
    Contributed talk, slides available via Zenodo

  • The Renaissance of Stellar Black-Hole Detections in The Local Group @ Lorentz Center, Leiden (06/2023)
    "The Binary Population of NGC 104 - Searching for Stellar Remnants in Globular Clusters with MUSE" Poster presentation

    • Contact

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