arXiv subjects

Andrew Allan

Publications and source records attributed to Andrew Allan.

5 recordsLinked to original sources

arXiv2024

Magnetic Field Evolution of Hot Exoplanets

Numerical simulations have shown that the strength of planetary magnetic fields depends on the convective energy flux emerging from planetary interiors. Here we model the interior structure of gas giant planets using \texttt{MESA}, to determine the convective energy flux that can drive the generation of magnetic field. This flux is then incorporated in the Christensen et al. dynamo formalism to estimate the maximum dipolar magnetic field $B^\mathrm{(max)}_\mathrm{dip}$ of our simulated planets. First, we explore how the surface field of intensely irradiated hot Jupiters ($\sim 300 M_\oplus$) and hot Neptunes ($\sim 20 M_\oplus$) evolve as they age. Assuming an orbital separation of 0.1 au, for the hot Jupiters, we find that $B^\mathrm{(max)}_\mathrm{dip}$ evolves from 240 G at 500 Myr to 120 G at 5~Gyr. For hot Neptunes, the magnetic field evolves from 11 G at young ages and dies out at $\gtrsim$ 2 Gyr. Furthermore, we also investigate the effects of atmospheric mass fraction, atmospheric evaporation, orbital separations $\alpha$ and additional planetary masses on the derived $B^\mathrm{(max)}_\mathrm{dip}$. We found that $B^\mathrm{(max)}_\mathrm{dip}$ increases with $\alpha$ for very close-in planets and plateaus out after that. Higher atmospheric mass fractions lead in general to stronger surface fields, because they allow for more extensive dynamo regions and stronger convection.

astro-ph.EP↗

arXiv2023

Characterising a World Within the Hot Neptune Desert: Transit Observations of LTT 9779 b with HST WFC3

We present an atmospheric analysis of LTT 9779 b, a rare planet situated in the hot Neptune desert, that has been observed with HST WFC3 G102 and G141. The combined transmission spectrum, which covers 0.8 - 1.6 $\mu$m, shows a gradual increase in transit depth with wavelength. Our preferred atmospheric model shows evidence for H$_{\rm 2}$O, CO$_{\rm 2}$ and FeH with a significance of 3.1 $\sigma$, 2.4 $\sigma$ and 2.1 $\sigma$, respectively. In an attempt to constrain the rate of atmospheric escape for this planet, we search for the 1.083 $\mu$m Helium line in the G102 data but find no evidence of excess absorption that would indicate an escaping atmosphere using this tracer. We refine the orbital ephemerides of LTT 9779 b using our HST data and observations from TESS, searching for evidence of orbital decay or apsidal precession, which is not found. The phase-curve observation of LTT 9779 b with JWST NIRISS should provide deeper insights into the atmosphere of this planet and the expected atmospheric escape might be detected with further observations concentrated on other tracers such as Lyman $\alpha$.

astro-ph.EP↗

arXiv2023

A Low-Mass Helium Star Progenitor Model for the Type Ibn SN 2020nxt

A growing number of supernovae (SNe) are now known to exhibit evidence for significant interaction with a dense, pre-existing, circumstellar medium (CSM). SNe Ibn comprise one such class that can be characterised by both rapidly evolving light curves and persistent narrow He I lines. The origin of such a dense CSM in these systems remains a pressing question, specifically concerning the progenitor system and mass-loss mechanism. In this paper, we present multi-wavelength data of the Type Ibn SN 2020nxt, including $HST$/STIS ultraviolet spectra. We fit the data with recently updated CMFGEN models designed to handle configurations for SNe Ibn. The UV coverage yields strong constraints on the energetics and, when combined with the CMFGEN models, offer new insight on potential progenitor systems. We find the most successful model is a $\lesssim4 {\rm M}_\odot$ helium star that lost its $\sim 1\,{\rm M}_\odot$ He-rich envelope in the years preceding core collapse. We also consider viable alternatives, such as a He white dwarf merger. Ultimately, we conclude at least some SNe Ibn do not arise from single, massive ($>30 {\rm M}_\odot$) Wolf-Rayet-like stars.

astro-ph.HE↗

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.

astro-ph.SR↗

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.

astro-ph.SR↗