From Cataclysmic Variables to AM CVn Systems: Modeling LISA Gravitational Wave Sources Using MESA
Authors
Pedersen, Casper Christian ; Larsen, Hans Christian Gjedsig
Term
4. term (FYS10)
Education
Publication year
2025
Submitted on
2025-05-30
Pages
72
Abstract
This thesis simulates AM CVn binaries—a rare class of very close binary star systems—assuming they form via the cataclysmic variable (CV) channel. The goal is to estimate the strength of their gravitational-wave signals (strain amplitudes) and assess whether the Laser Interferometer Space Antenna (LISA) will detect them. We outline the key physics that drives their evolution, including mass transfer between the stars, nova eruptions, magnetic braking (loss of angular momentum through a star’s magnetized wind), and gravitational-wave radiation. We adopt the convection and rotation boosted (CARB) magnetic braking prescription. Using MESA (Modules for Experiments in Stellar Astrophysics, version 24.08.1), we evolve model systems that include these effects. To evaluate the simulations, we compare with observed AM CVn systems and with evolved CV systems to test whether the assumed CV-to-AM CVn pathway can reproduce what is seen. We find that the CV formation channel can reproduce known AM CVn systems. The simulated binaries emit strong gravitational waves, and most would be detectable by LISA once it is operational, based on their strain amplitudes and LISA’s sensitivity curves. The CARB magnetic braking model has a strong impact on the evolution. It reduces the need for fine-tuning initial parameters but leads to mass transfer rates in the AM CVn phase that are higher than observed. Turning off magnetic braking for compact donors improves agreement but then produces too low mass transfer rates. This indicates that a different magnetic braking prescription for compact donors is needed to better match observations.
Dette speciale simulerer AM CVn-dobbeltsystemer—en sjælden type meget tætte stjernepar—under antagelse af, at de dannes via den kataklysmiske variabel (CV) dannelseskanal. Målet er at estimere styrken af deres gravitationsbølgesignaler (strain-amplituder) og vurdere, om Laser Interferometer Space Antenna (LISA) vil kunne opdage dem. Vi gennemgår den centrale fysik, der styrer deres udvikling, herunder masseoverførsel mellem stjernerne, nova-udbrud, magnetisk bremsning (tab af drejningsmoment via en stjernes magnetiske vind) og gravitationsbølgestråling. Vi anvender den convection and rotation boosted (CARB) opskrift for magnetisk bremsning. Vi bruger MESA (Modules for Experiments in Stellar Astrophysics, version 24.08.1) til at udvikle modelsystemer, der inkluderer disse effekter. For at evaluere modellerne sammenligner vi med observerede AM CVn-systemer og med udviklede CV-systemer for at teste, om den antagne CV-til-AM CVn-vej kan genskabe observationerne. Vi finder, at CV-dannelseskanalen kan genskabe kendte AM CVn-systemer. De simulerede dobbeltsystemer udsender stærke gravitationsbølger, og de fleste vil være detekterbare med LISA, når missionen er i drift, baseret på deres strain-amplituder og LISAs følsomhedskurver. CARB-modellen for magnetisk bremsning har en markant indvirkning på udviklingen. Den reducerer behovet for finjustering af begyndelsesparametre, men giver også masseoverførsler i AM CVn-fasen, som er højere end observeret. At slå magnetisk bremsning fra for kompakte donorer forbedrer overensstemmelsen, men giver for lave masseoverførsler. Dette peger på behovet for en anden opskrift for magnetisk bremsning for kompakte donorer for bedre at matche observationerne.
[This apstract has been rewritten with the help of AI based on the project's original abstract]
Keywords