For her research project "The formation of dormant black hole binaries: a key to mass transfer and stellar dynamics (DoBlack)", the German Research Foundation (DFG) has granted Prof. Michela Mapelli, astrophysicist at the Center for Astronomy of Heidelberg University (ZAH), around half a million euros. DoBlack will investigate the formation of dormant black holes which are part of a binary star system with a companion star. They are called dormant because they show no signs of mass transfer, which is usually detectable by the emission of X-rays. DoBlack will study the formation of dormant BHs via numerical simulations of binary evolution and star-cluster dynamics.
For several decades, a dozen X-ray Black Hole (BH) binary systems with dynamical mass measurement have been the main observational benchmark for theoretical models of black hole formation. However, in 2015, the LIGO interferometers captured the merger of two BHs for the very first time via gravitational-waves. Since then, the number of gravitational-wave event candidates has been growing rapidly and now approaches the 100 mark. The fourth observing run of the LIGO, Virgo and KAGRA interferometers started in May 2023. It has led to more than 160 new significant event candidates and made Tthe zoo of black hole (BH) binaries rich has never been so rich.before.
In the last few years, a further family of BH binaries called "dormant Black Holes" has attracted growing interest. These X-ray silent BHs that are members of a binary system with a companion star.
While BHs detected with gravitational wave interferometers are the tip of the iceberg of the population with extreme orbital properties, dormant binaries are more representative of the black hole population in the local Universe. Hitherto, the observed sample consists of half a dozen systems plus several candidates. Two of them are located in the globular cluster NGC3201and thus represent an excellent test for dynamical interactions of black holes in stellar clusters. The system VFTS243 in the Large Magellanic Cloud comprises a 9 solar mass black hole that orbits a massive star of 25 solar masses.
But great expectations cast their shadows ahead: the largest potential for constraining dormant BHs is hidden in data provided by the Gaia astrometric satellite. Theoretical models suggest that the Gaia data release 4 (DR4) will reveal several hundreds to thousands of dormant BHs.
To date, Gaia data have already triggered the detection of three such systems. The orbital period of the first two of them, Gaia BH1 and BH2 (186 and 1300 days, respectively) is a puzzle for astrophysical models: too tight to be explained with a non-interacting system and too large for a binary star that evolved via unstable mass transfer. Conversely, Gaia BH3 is the first BH with mass above 30 solar masses detected via classical optical observations. The low metallicity of its companion star is the very first observational confirmation that massive stellar BHs originate from the collapse of metal-poor stars, as predicted by models. Moreover, the association of Gaia BH3 with the ED-2 stellar stream originating from a disrupted globular cluster hints for a possible dynamical formation of this system in a stellar cluster, also investigated for Gaia BH1 and BH2.
These new observational findings ideally complement research activities of Heidelberg astrophysicist Prof. Dr. Michela Mapelli. Her "DEMOBLACK" project (full title: Demography of black hole binaries in the era of gravitational-wave astronomy) already aims to shed light on the formation channels of binary black holes by means of innovative numerical models via two scenarios: the formation of binary black holes from the evolution of massive binary stars and the dynamical pairing of black holes in dense star clusters.
"DoBlack is a natural and exciting extension in understanding the formation of the new class of BH binaries", resumes Mapelli. The goal of DoBlack is to model the formation of dormant black holes (BHs) with new binary evolution models, star cluster simulations, and a comprehensive exploration of the parameter space. “We aim to reconstruct the most likely formation pathways of dormant black holes and provide a key to explain the seemingly impossible orbital period of Gaia BH1 and BH2”, says Michela Mapelli. “We will explore the evolution of such binary systems with cutting-edge dynamical simulations and with the joint effort of binary evolution and population synthesis calculations.”
DoBlack-related research will begin in summer 2025 and will produce models that can be compared against DR4 data, which are expected to be published about mid-2026.
FURTHER INFORMATION
Homepage of Prof. Dr Michela Mapelli: demoblack.com
DFG (German Research Foundation): https://www.dfg.de/en
More about Gaia (ESA): https://www.cosmos.esa.int/web/gaia
SCIENTIFIC CONTACT
Prof. Dr. Michela Mapelli
Centre for Astronomy of Heidelberg University (ZAH)
Institute for Theoretical Astrophysics (ITA)
mapelli@uni-heidelberg.de
CONTACT FOR THE MEDIA
Dr. Guido Thimm
Centre for Astronomy of Heidelberg University (ZAH)
thimm@uni-heidelberg.de
