Assistant Professor at the University of Tokyo
When did the first stars form? How can we use metal-poor stars as time machine to the early Universe? Can artificial intelligence help us to discover new science? Why are the first supermassive black holes so heavy? And how can we make research more inclusive? These are some of the major questions that I try to answer together with my colleagues. You can find more about my research under Publications. We have hosted the conference Stellar Archaeology as a Time Machine to the First Stars in Tokyo. Presentation files are available online.
Our groups offers several interesting research projects for undergraduate and graduate students: comparison of supernova nucleosynthetic yields, anthropic principle of star formation, gravitational waves from the first stars, finding the first supernovae with next-generation telescopes. The University of Tokyo also offers two fellowships for international students in physics and astronomy: GSGC program, MEXT Fellowship. We are very happy to assist you with the application. Please contact me for further details.
I explore how machine learning can help in the classification and hence understanding of extremely metal-poor stars. Based on theoretical supernova yields of Ishigaki+18, I train decision trees to distinguish the chemical signatures of mono- and multi-enriched stars. In this first proof-of-concept, the trained network can correctly classify 70% stars of a blind sample. Further optimisation and the inclusion of observation biases will improve this method and yield the chemical abundances with the highest information gain to understand star formation in the early Universe.
Formation of carbon-enhanced metal-poor stars as a consequence of inhomogeneous metal mixinghttps://arxiv.org/abs/1812.01820
We present a novel scenario for the formation of carbon-enhanced metal-poor (CEMP) stars. Carbon enhancement at low stellar metallicities is usually considered a consequence of faint or other exotic supernovae. An analytical estimate of cooling times in low-metallicity gas demonstrates a natural bias, which favours the formation of CEMP stars as a consequence of inhomogeneous metal mixing: carbon-rich gas has a shorter cooling time and can form stars prior to a potential nearby pocket of carbon-normal gas, in which star formation is then suppressed due to energetic photons from the carbon-enhanced protostars.
Illustration of how a C-normal SN ([C/Fe] < 0.7) can trigger the formation of CEMP stars ([C/Fe] > 0.7). The crucial quantities are the separation of the two clumps, d, and their carbonicity difference, ∆[C/H].
Descendants of the first stars: the distinct chemical signature of second-generation starshttp://arxiv.org/abs/1801.05044, http://arxiv.org/abs/1810.04713
We have developed a new diagnostic to distinguish mono- from multi-enriched second-generation stars based on their chemical fingerprint. These results will help to classify spectroscopic observations of extremely metal-poor stars and thereby allow us to infer the masses and multiplicities of the first stars.
Probability of mono-enrichment for magnesium over carbon vs. the metallicity. There are regions of the parameter space in our model with a probability of almost 100% for finding second-generation stars that formed from gas that was enriched by only one previous supernova.
Probability distributions for mono-enrichment for seven recently observed extremely metal-poor stars from the TOPoS survey. The black stars mark the probability for the observationally derived abundances and the blue contours illustrates the probability density for values within 0.5dex of this derived abundance. Only one out of seven stars is more likely to be mono-enriched.
Gravitational wave signals from the first massive black hole seedshttp://arxiv.org/abs/1805.06901
We investigate the gravitational wave signature from supermassive stars that form as binaries at high redshift. In our optimistic model we find up to 0.6 detections per year that can be clearly attributed to such progenitors. Also a non-detection of these binary black hole mergers at z>15 over the lifetime of LISA can provide upper limits on the abundance and binary fraction of supermassive stars.
Rates for the merging of black hole binary from supermassive stellar binaries as a function of redshift. Only our optimistic scenario with Jc = 30J21 and binarity of 100% can produce a population of BHB mergers at z>15 that are clearly distinguishable from other channels of binary black hole formation.
Active Galactic Nuclei outflows in galaxy discshttp://arxiv.org/abs/1707.03826
Galactic outflows, driven by active galactic nuclei (AGN), play a crucial role in galaxy formation and in the self-regulated growth of supermassive black holes. With a novel 2D analytical model for AGN-driven outflows in a gaseous disc we demonstrate the main improvements, compared to existing 1D solutions. We find significant differences for the outflow dynamics and wind efficiency (see Figure). The recovery time of gas in the disc plane is remarkably short, of the order 1Myr. This indicates that AGN-driven winds cannot suppress BH growth for long. Published in MNRAS, 2018, 476, 2288.
Time evolution of the position of the wind shock front in polar coordinates for our fiducial model. Top: 2D solution with the galactic disc in the horizontal plane. Bottom: 1D spherical solution. The concentric grey rings indicate the radius in log(r/pc) from 10pc to 100kpc. Whereas in the 1D model all the gas is ejected out of the halo, only 10% of the galactic gas is ejected perpendicular to the disc plane in our improved 2D model.
Gravitational Waves from the Remnants of the First Stars
Initiated by the groundbreaking observation of the first gravitational wave signal in 2015, we studied the detectability of the first stars by the merger of their remnant black holes. We developed a cosmologically representative dark matter merger tree and coupled it to a model for stellar binary evolution. The overall contribution from the first stars is small, but individual events can still be detected with aLIGO and depending on the initial mass function of the first stars, we will see several black hole mergers in the next years that originate from the first stars. Turning this result around, we can use the upcoming detections of gravitational waves to learn more about the masses and binarity of the first stars. Our catalogues of Pop III binaries are publicly available here. Published in MNRAS, 2016, 460, L74
Expected number of BH-BH merger detections per year as a function of the total binary mass for the current aLIGO sensitivity (top) and final design sensitivity (bottom). The mass range of GW150914 is indicated by the grey area. With sufficient detections around Mtot ~300Msun, we could discriminate different Pop III IMFs based on their GW fingerprint.
© 2019 Tilman Hartwig