arXiv subjects

Eoin Farrell

Publications and source records attributed to Eoin Farrell.

24 records · Page 2Linked to original sources

arXiv2020

Coupling thermal evolution of planets and hydrodynamic atmospheric escape in MESA

The long-term evolution of hydrogen-dominated atmospheres of sub-Neptune-like planets is mostly controlled by two factors: a slow dissipation of the gravitational energy acquired at the formation (known as thermal evolution) and atmospheric mass loss. Here, we use MESA to self-consistently couple the thermal evolution model of lower atmospheres with a realistic hydrodynamical atmospheric evaporation prescription. To outline the main features of such coupling, we simulate planets with a range of core masses (5-20 Mearth) and initial atmospheric mass fractions (0.5-30%), orbiting a solar-like star at 0.1 au. In addition to our computed evolutionary tracks, we also study the stability of planetary atmospheres, showing that the atmospheres of light planets can be completely removed within 1 Gyr, and that compact atmospheres have a better survival rate. From a detailed comparison between our results and the output of the previous-generation models, we show that coupling between thermal evolution and atmospheric evaporation considerably affects the thermal state of atmospheres for low-mass planets and, consequently, changes the relationship between atmospheric mass fraction and planetary parameters. We, therefore, conclude that self-consistent consideration of the thermal evolution and atmospheric evaporation is of crucial importance for evolutionary modeling and a better characterization of planetary atmospheres. From our simulations, we derive an analytical expression between the planetary radius and atmospheric mass fraction at different ages. In particular, we find that, for a given observed planetary radius, the predicted atmospheric mass fraction changes as age^0.11.

astro-ph.EP↗

arXiv2020

Massive black holes regulated by luminous blue variable mass loss and magnetic fields

We investigate the effects of mass loss during the main-sequence (MS) and post-MS phases of massive star evolution on black hole (BH) birth masses. We compute solar metallicity Geneva stellar evolution models of an 85 $M_{\odot}$ star with mass-loss rate ($\dot{M}$) prescriptions for MS and post-MS phases and analyze under which conditions such models could lead to very massive BHs. Based on the observational constraints for $\dot{M}$ of luminous stars, we discuss two possible scenarios that could produce massive BHs at high metallicity. First, if a massive BH progenitor evolves from the observed population of massive MS stars known as WNh stars, we show that its average post-MS mass-loss rate has to be less than $1\,\times10^{-5}\,M_{\odot}$/yr. However, this is lower than the typical observed mass-loss rates of luminous blue variables (LBV). Second, a massive BH progenitor could evolve from a yet undetected population of $80-85$ $M_{\odot}$ stars with strong surface magnetic fields, which could quench mass loss during the evolution. In this case, the average mass-loss rate during the post-MS LBV phase has to be less than $5\,\times10^{-5}\,M_{\odot}$/yr to produce 70 $M_{\odot}$ BHs. We suggest that LBVs that explode as SNe have large envelopes and small cores that could be prone to explosion, possibly evolving from binary interaction (either mergers or mass gainers that do not fully mix). Conversely, LBVs that directly collapse to BHs could have evolve from massive single stars or binary-star mergers that fully mix, possessing large cores that would favor BH formation.

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arXiv2020

The possible disappearance of a massive star in the low metallicity galaxy PHL 293B

We investigate a suspected very massive star in one of the most metal-poor dwarf galaxies, PHL~293B. Excitingly, we find the sudden disappearance of the stellar signatures from our 2019 spectra, in particular the broad H lines with P~Cygni profiles that have been associated with a massive luminous blue variable (LBV) star. Such features are absent from our spectra obtained in 2019 with the ESPRESSO and X-shooter instruments of the ESO's VLT. We compute radiative transfer models using CMFGEN that fit the observed spectrum of the LBV and are consistent with ground-based and archival Hubble Space Telescope photometry. Our models show that during 2001--2011 the LBV had a luminosity $L_* = 2.5-3.5 \times 10^6 ~L_{\odot}$, a mass-loss rate $\dot{M} = 0.005-0.020 ~M_{\odot}$~yr$^{-1}$, a wind velocity of 1000~km~s$^{-1}$, and effective and stellar temperatures of $T_\mathrm{eff} = 6000-6800$~K and $T_\mathrm{*}=9500-15000$~K. These stellar properties indicate an eruptive state. We consider two main hypotheses for the absence of the broad emission components from the spectra obtained since 2011. One possibility is that we are seeing the end of an LBV eruption of a surviving star, with a mild drop in luminosity, a shift to hotter effective temperatures, and some dust obscuration. Alternatively, the LBV could have collapsed to a massive black hole without the production of a bright supernova.

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arXiv2020

SNAPSHOT: Connections between Internal and Surface Properties of Massive Stars

We introduce SNAPSHOT, a technique to systematically compute stellar structure models in hydrostatic and thermal equilibrium based on 3 structural properties - core mass $M_{\rm core}$, envelope mass $M_{\rm env}$ and core composition. This approach allows us to connect these properties of stellar interiors to the luminosity and effective temperature $T_{\rm eff}$ in a more systematic way than with stellar evolution models. For MS models, we derive an analytical relationship between $M_{\rm core}$, $M_{\rm total}$ and central H abundance that can be used in rapid stellar evolution algorithms. Core-He burning models with $M_{\rm core}/M_{\rm total}$ from 0.2 to 0.8 have convective envelopes, low $T_{\rm eff}$ and will appear as red supergiants. For a given $M_{\rm core}$, they exhibit a small variation in luminosity (0.02 dex) and $T_{\rm eff}$ ($\sim 400\,\mathrm{K}$) over a wide range of $M_{\rm env}$ ($\sim 2 - 20\,\mathrm{M}_{\odot}$). This means that it is not possible to derive red supergiant masses from luminosities and $T_{\rm eff}$ alone. We derive the following relationship between $M_{\rm core}$ and the total luminosity of a red supergiant during core He burning: $\log M_{\rm core} \simeq 0.44 \log L/L_{\odot} - 1.38$. At $M_{\rm core}$/$M_{\rm total} \approx 0.2$, our models exhibit a bi-stability and jump from a RSG to a BSG structure. Our models with $M_{\rm core}/M_{\rm total} > 0.8$, which correspond to stripped stars produced by mass loss or binary interaction, show that $T_{\rm eff}$ has a strong dependence on $M_{\rm env}$, $M_{\rm core}$ and the core composition. We find the mass of one of these stripped stars in a binary system, HD 45166, to be less than its dynamical mass. When a large observational sample of stripped stars becomes available, our results can be used to constrain their masses and the physics of binary interaction.

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arXiv2020

The Uncertain Masses of Progenitors of Core Collapse Supernovae and Direct Collapse Black Holes

We show that it is not possible to determine the final mass $M_{\rm fin}$ of a red supergiant (RSG) at the pre-supernova (SN) stage from its luminosity $L$ and effective temperature $T_{\rm eff}$ alone. Using a grid of stellar models, we demonstrate that for a given value of $L$ and $T_{\rm eff}$, a RSG can have a range of $M_{\rm fin}$ as wide as 3 to $45~\mathrm{M}_{\odot}$. While the probability distribution within these limits is not flat, any individual determination of $M_{\rm fin}$ for a RSG will be degenerate. This makes it difficult to determine its evolutionary history and to map $M_{\rm fin}$ to an initial mass. Single stars produce a narrower range that is difficult to accurately determine without making strong assumptions about mass loss, convection, and rotation. Binaries would produce a wider range of RSG $M_{\rm fin}$. However, the final Helium core mass M$_{\rm He-core}$ is well determined by the final luminosity and we find $\log (\mathrm{M}_{\rm He-core}/M_{\odot}) = 0.659 \log (L/\mathrm{L}_{\odot}) -2.630$ Using this relationship, we derive M$_{\rm He-core}$ for directly imaged SN progenitors and one failed SN candidate. The value of $M_{\rm fin}$ for stripped star progenitors of SNe IIb is better constrained by $L$ and $T_{\rm eff}$ due to the dependence of $T_{\rm eff}$ on the envelope mass $M_{\rm env}$ for $M_{\rm env} \lesssim 1~$M$_{\odot}$. Given the initial mass function, our results apply to the majority of progenitors of core collapse SNe, failed SNe and direct collapse black holes.

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arXiv2018

Impact of binary interaction on the evolution of blue supergiants

A large fraction of massive stars evolve in interacting binary systems, which dramatically modifies the outcome of stellar evolution. We investigated the properties of blue supergiants in binary systems and whether they are suitable for extragalactic distance determinations using the flux-weighted gravity luminosity relationship (FGLR). This is a relationship between the absolute bolometric magnitude $M_{\rm bol}$ and the spectroscopically determined flux-weighted gravity $g_{\rm F} = g/T^4_{\rm eff}$. We computed a grid of binary stellar evolution models with MESA and use the v2.1 BPASS models to examine whether they are compatible with the relatively small scatter shown by the observed relationship. Our models have initial primary masses of 9 - 30 $M_\odot$, initial orbital periods of 10 - 2511 days, mass ratio $q$ = 0.9, and metallicity $Z$ = 0.02. We find that the majority of primary stars that produce blue supergiant stages are consistent with the observed FGLR, with a small offset towards brighter bolometric magnitudes. In between 1\% - 24\% of cases, binary evolution may produce blue supergiants after a mass transfer episode, that lie below the observed FGLR. A small number of such stars have been found in extragalactic FGLR studies, suggesting that they may have evolved through binary interaction. Some models with shorter periods could resemble blue hypergiants and LBVs. We used CMFGEN radiative transfer models to investigate the effects of unresolved secondaries on diagnostics for $T_{\rm eff}$ and $g$, and the biases on the determination of interstellar reddening and $M_{\rm bol}$. We find that the effects are small and within the observed scatter, but could lead to a small overestimate of the luminosity, of $T_{\rm eff}$ and of $g$ for extreme cases. We conclude that the observed FGLR can, in principle, be well reproduced by close binary evolution models.

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