arXiv subjects

John Forbes

Publications and source records attributed to John Forbes.

6 recordsLinked to original sources

arXiv2025

Very Large Telescope Observations of Interstellar Comet 3I/ATLAS. II. From Quiescence to Glow: Dramatic Rise of Ni i Emission and Incipient CN Outgassing at Large Heliocentric Distances*

We report VLT spectroscopy of the interstellar comet 3I/ATLAS (C/2025~N1) from $r_{\rm h}\!\simeq\!4.4$ to $2.85$~au using X-shooter (300--550\,nm, $R\!\simeq\!3000$) and UVES (optical, $R\!\simeq\!35k-80k$). The coma is dust-dominated with a fairly constant red optical continuum slope ($\sim$21--22\%/1000\AA). We report detection of CN emission and also detect numerous Ni\,\textsc{ii}~lines while Fe\,\textsc{i}~remains undetected, potentially implying efficiently released gas-phase Ni. At $r_{\rm h}\!\simeq\!3.14$~au we derive $3\sigma$ limits of $Q({\rm OH})<{1.48\times10^{26}}\ {\rm s^{-1}}$, but find no indications for [O\,\textsc{i}], C$_2$, C$_3$ or NH$_2$. From our latest X-shooter measurements conducted on 2025-08-21 ($r_{\rm h} = 2.85$\,au) we measure production rates of $\log~Q(\mathrm{CN}) = {24.81\pm 0.01}$ molecules s$^{-1}$ and $\log~Q$(Ni) $= {23.30\pm0.07}$ atoms s$^{-1}$, and characterize their evolution as the comet approaches perihelion.~We observe a steep heliocentric-distance scaling for the production rates $Q(\mathrm{Ni}) \propto r_h^{{-7.7 \pm 1.0}}$ and for $Q(\mathrm{CN}) \propto r_h^{{-6.7 \pm 0.2}}$, and predict a Ni--CO$_{(2)}$ correlation if the Ni\,\textsc{ii}\ emission is driven by the carbonyl formation channel.~Energetic considerations of activation barriers show that this behavior is inconsistent with direct sublimation of canonical metal/sulfide phases and instead favors low--activation--energy release from dust, e.g.~photon-stimulated desorption or mild thermolysis of metalated organics or Ni-rich nanophases, possibly including Ni--carbonyl-like complexes.~These hypotheses are testable with future coordinated ground-based and space-based monitoring as 3I becomes more active during its continued passage through the solar system.

astro-ph.SR↗

arXiv2024

Non-linear scattering of plane gravitational and electromagnetic waves

Fully non-linear, plane-symmetric exact solutions of the Einstein equations describing the scattering of gravitational and electromagnetic waves have existed for many years. For these closed-form solutions to be found, idealized wave profiles such as the Dirac delta and Heaviside theta functions must be assumed. Although pathological in that future curvature singularities generically occur, these exact solutions give useful insights into the non-linear features of the scattering process. Only a limited number of exact solutions exist and this leaves many other physically-motivated scattering situations without a non-linear description. The aim of this paper is to shed light on these unexplored cases. This is achieved through numerical solutions of the Friedrich-Nagy initial boundary value problem for the Einstein equations coupled to the source-free Maxwell equations in plane symmetry. Interesting non-linear scattering effects are presented for a variety of electromagnetic and gravitational wave profiles not currently described by an exact solution. Implications for electromagnetic wave observations are investigated through analyzing the time-delay and frequency shift imparted on the electromagnetic wave through the non-linear scattering with a gravitational wave of a given strain. Simple arguments supported by the numerical solutions suggest dramatic effects on the radiation may be observable.

gr-qc↗

arXiv2023

The star formation history of the Sco-Cen association. Coherent star formation patterns in space and time

We reconstruct the star formation history of the Sco-Cen OB association using a novel high-resolution age map of the region. We develop an approach to produce robust ages for Sco-Cen's recently identified 37 stellar clusters using the SigMA algorithm. The Sco-Cen star formation timeline reveals four periods of enhanced star formation activity, or bursts, remarkably separated by about 5 Myr. Of these, the second burst, which occurred about 15 million years ago, is by far the dominant, and most of Sco-Cen's stars and clusters were in place by the end of this burst. The formation of stars and clusters in Sco-Cen is correlated but not linearly, implying that more stars were formed per cluster during the peak of the star formation rate. Most of the clusters that are large enough to have supernova precursors were formed during the 15 Myr period. Star and cluster formation activity has been continuously declining since then. We have clear evidence that Sco-Cen formed from the inside out and contains 100-pc long chains of contiguous clusters exhibiting well-defined age gradients, from massive older clusters to smaller young clusters. These observables suggest an important role for feedback in forming about half of Sco-Cen stars, although follow-up work is needed to quantify this statement. Finally, we confirm that the Upper-Sco age controversy discussed in the literature during the last decades is solved: the nine clusters previously lumped together as Upper-Sco, a benchmark region for planet formation studies, exhibit a wide range of ages from 3 to 19 Myr.

astro-ph.SR↗

arXiv2023

The Earliest Stage of Galactic Star Formation

Using a recently-developed technique to estimate gas temperatures ($T_\textrm{SF}$) in star-forming regions from large photometric surveys, we propose a diagram, analogous to the Hertzsprung-Russell diagram for individual stars, to probe the evolution of individual galaxies. On this $T_\textrm{SF}$-sSFR (specific star formation rate) diagram, a small fraction of star-forming galaxies appear to be dominated by different feedback mechanisms than typical star-forming galaxies. These galaxies generically have younger stellar populations, lower stellar masses and increase in relative abundance towards higher redshifts, so we argue that these objects are in an earlier stage of galactic star formation. Further, Hubble observations find that these "core-forming" galaxies also exhibit distinct morphology, and that tracks on the $T_\textrm{SF}$-sSFR diagram are also a morphological sequence. Thus, unlike starburst phases which can be triggered environmentally, these earliest, core-forming galaxies, appear to be a stage that typical galaxies go through early in their star formation history. We therefore argue that most galaxies first go through a core formation stage, then subsequently disk formation, and finally become quiescent.

astro-ph.GA↗

arXiv2012

Evolving Gravitationally Unstable Disks Over Cosmic Time: Implications For Thick Disk Formation

Observations of disk galaxies at z~2 have demonstrated that turbulence driven by gravitational instability can dominate the energetics of the disk. We present a 1D simulation code, which we have made publicly available, that economically evolves these galaxies from z~2 to z~0 on a single CPU in a matter of minutes, tracking column density, metallicity, and velocity dispersions of gaseous and multiple stellar components. We include an H$_2$ regulated star formation law and the effects of stellar heating by transient spiral structure. We use this code to demonstrate a possible explanation for the existence of a thin and thick disk stellar population and the age-velocity dispersion correlation of stars in the solar neighborhood: the high velocity dispersion of gas in disks at z~2 decreases along with the cosmological accretion rate, while at lower redshift, the dynamically colder gas forms the low velocity dispersion stars of the thin disk.

astro-ph.GA↗

arXiv2009

Lyman-alpha transfer in primordial hydrogen recombination

Cosmological constraints from the cosmic microwave background (CMB) anisotropies rely on accurate theoretical calculations of the cosmic recombination history. Recent work has emphasized the importance of radiative transfer calculations due to the high optical depth in the HI Lyman lines. Transfer in the Lyman-alpha line is dominated by true emission and absorption, Hubble expansion, and resonant scattering. Resonant scattering causes photons to diffuse in frequency due to random kicks from the thermal velocities of hydrogen atoms, and also to drift toward lower frequencies due to energy loss via atomic recoil. Past analyses of Lyman-alpha transfer during the recombination era have either considered a subset of these processes, ignored time dependence, or incorrectly assumed identical emission and absorption profiles. We present here a fully time-dependent radiative transfer calculation of the Lyman-alpha line including all of these processes, and compare it to previous results that ignored the resonant scattering. We find a faster recombination due to recoil enhancement of the Lyman-alpha escape rate, leading to a reduction in the free electron density of 0.45% at z=900. This results in an increase in the small-scale CMB power spectrum that is negligible for the current data but will be a 0.9 sigma correction for Planck. We discuss the reasons why we find a smaller correction than some other recent computations.

astro-ph.CO↗