Research
The general goal of my research is to characterise compact stars in binary systems. I search for potential gravitational wave sources that could serve as verification systems for the upcoming space-based detector LISA, and for potential supernova Ia progenitors that could help us better understand the mechanism behind these important standard candles, which led to the discovery of the expansion of the Universe. I am also particularly interested in extremely-low-mass white dwarfs and hot subdwarfs, whose existence can only be explained by binary evolution. Better characterisation of these systems can provide constraints to evolutionary and population synthesis models to help improve our understanding of the physics of binary evolution.
Below there is a summary of different research topics I am currently working or have worked on. You can check my complete and updated list of publications, as well as metrics, on my public ADS library.
Compact binaries in TESS
The Transiting Exoplanet Survey Satellite (TESS) has been gathering light curves for thousands of nearby stars. Although its primary goal is to find planets, the obtained light curves are also ideal to search for variability caused by a binary companion.
I have been taking advantage of TESS to study compact binaries. I am a co-lead of the TESS working group 8.4, which studies binary stars in which at least one of the components is a white dwarf or a subdwarf. Among my main findings is the star HD265435, which is a candidate supernova Ia progenitor and a gravitational wave source strong enough to be detected by LISA. You can read more about it in the Warwick press release.
Related works: arXiv:2308.00036, arXiv:2302.12507, arXiv:2207.02001, arXiv:2009.02968, arXiv:2107.09074
Extremely low-mass white dwarf stars
Extremely low-mass white dwarf stars (ELMs) are helium-core white dwarfs with mass below 0.3 solar masses, born either as a result of a common-envelope phase or after a stable Roche-lobe overflow episode in a multiple system.
As remnants of binary evolution, ELMs can shed light onto the poorly understood phase of common-envelope evolution and provide constraints to the physics of mass accretion. In addition, most known ELMs will merge in less than a Hubble time, contributing thus to the signal to be detected by upcoming space-based gravitational wave detectors. I have compiled a catalogue of 5762 ELM candidates selected from the data release 2 of Gaia, available here, which I am currently following up in order to identify potential LISA sources, and to compile an unbiased volume-limited sample to serve as benchmark for population synthesis models.
Related works: arXiv:1907.03766, arXiv:1805.04070, arXiv:1804.09059, arXiv:1801.00495.
Magnetic cataclysmic variables
Cataclysmic variables are systems in which a white dwarf accretes mass from a late-type main-sequence star via Roche lobe overflow. If the white dwarf is magnetic, the magnetic field can regulate the geometry and rate of accretion, causing distinct observational properties.
In particular, the white dwarf's spin period is revealed through photometric variations induced by spots, which result from accretion rate asymmetries caused by the magnetic field. Tracking the rate of spin change can provide insight into the competing effects of accretion and magnetic drag. I use the fast photometers HiPERCAM, ULTRACAM and ULTRASPEC to search for the spin period of the white dwarfs in cataclysmic variables. My most remarkable discovery was J024048.51+195226.9, the fastest spinning confirmed white dwarf, which was in a Warwick press release. It was also featured in many news articles, one of my favourite being the one by Phil Plait for SyFy Wire. There is also a YouTube video by SciShow Space.
Related works: arXiv:2306.09272, arXiv:2306.09732, arXiv:2208.08450, arXiv:2109.00553, arXiv:2108.11396.
Hot subdwarf stars
Hot subdwarfs are hydrogen-poor helium-burning stars, typically formed either by removing the envelope from a red giant, or by merging two helium-core white dwarfs.
Hot subdwarfs play an important role in our understanding of binary evolution, stellar atmospheres and interiors. They are key objects to study many peculiar events, ranging from star-planet interactions to type Ia supernova progenitors. My main contribution to the hot subdwarf field was using Gaia and TESS to show that they require binary interactions to be formed (see arXiv:2008.07522).
Related works: arXiv:2111.09873, arXiv:2106.03363, arXiv:2104.03867, arXiv:2008.07522, arXiv:1912.08338.
White dwarf stars
White dwarf stars are the most common end-product of stellar evolution, corresponding to the final observable evolutionary state of over 95 per cent of the stars in the Galaxy. They can show a range of different spectral types depending on their atmosphere composition, temperature, magnetic fields, and on the possible interaction with planetary debris.
White dwarfs play an important role in our understanding of the evolution of planets, stars, the Galaxy, and the Universe as a whole. I started working on white dwarfs as a first-year Physics undergraduate student, in 2009. Since then I have contributed to many spectral catalogues using the Sloan Digital Sky Survey (SDSS), which have lead to an eight-fold increse in the number of spectroscopically confirmed white dwarfs. I have also studied their magnetic fields and pulsations.
Related works: arXiv:2210.01608 arXiv:2108.10915, arXiv:2006.00965, arXiv:1909.05555, arXiv:1904.01626, arXiv:1712.07146, arXiv:1510.08409, arXiv:1411.4149, arXiv:1212.1222, arXiv:1211.5709.
Opacities for the H2+ quasi-molecule
During my masters in Astronomy, I developed theoretical work and was able to significantly improve line broadening models for the quasi-molecule H2+, a transient molecule formed in collisions between protons and hydrogen.
The energy levels of H2+ are different from the levels of neutral hydrogen, so that its spectra shows a distinct set of lines. These so-called satellite lines are especially prominent in UV spectra of blue stars, such as white dwarfs, blue horizontal branch stars (BHBs), and λ Bootis stars. The opacity tables are available on VizieR can be easily used with Synspec.
Related work: arXiv:1501.05609.
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