arXiv subjects

Anthony E. Lynas-Gray

Publications and source records attributed to Anthony E. Lynas-Gray.

4 recordsLinked to original sources

arXiv2026

Detection of C3 in Titan with VLT-ESPRESSO

Titan is regarded as a natural laboratory in the Solar System for studying atmospheric photochemistry and the abiotic production of organic molecules on cold small exoplanets. Since the end of the Cassini-Huygens mission, telescope observations have enabled new detections of increasingly complex carbon-based molecules at infrared and sub-millimetre wavelengths, while the optical regime has been largely overlooked. Following a recent tentative detection of the 405 nm absorption band of C3 in Titan in archived optical VLT UVES spectra at resolving power R = 60000, this work reports an eight sigma detection of the C3 405 nm absorption band in Titan using dedicated ultra high resolution VLT ESPRESSO observations at R = 190000, the highest spectral resolution optical observations of Titan to date. The VLT ESPRESSO spectrum is compared to model spectra of Titan with varying C3 abundances. A chi squared analysis is used to assess the agreement between non solar spectral features and C3 absorption as the C3 abundance is varied, and a Bayesian Markov Chain Monte Carlo fit between model and observed spectra is performed. The chi squared analysis yields an eight sigma detection of C3, consistent with a C3 column density of approximately 1.5E13 cm-2, while the MCMC fit retrieves a C3 column density of 1.47E13 cm-2 at five sigma. These values are consistent with the order of magnitude predicted by photochemical models, which reach parts per million levels in the Titan mesosphere. This work demonstrates the usefulness of instruments and techniques originally developed for exoplanet research when applied to Solar System targets.

astro-ph.EP↗

arXiv2021

ExoMol line lists -- XLIV. IR and UV line list for silicon monoxide (SiO)

A new silicon monoxide ($^{28}$Si$^{16}$O) line list covering infrared, visible and ultraviolet regions called SiOUVenIR is presented. This line list extends the infrared EBJT ExoMol line list by including vibronic transitions to the $A\,{}^{1}Π$ and $E\,{}^{1}Σ^{+}$ electronic states. Strong perturbations to the $A\,{}^{1}Π$ band system are accurately modelled through the treatment of 6 dark electronic states: $C\,{}^{1}Σ^{-}$, $D\,{}^{1}Δ$, $a\,{}^{3}Σ^{+}$, $b\,{}^{3}Π$, $e\,{}^{3}Σ^{-}$ and $d\,{}^{3}Δ$. Along with the $X\,{}^{1}Σ^{+}$ ground state, these 9 electronic states were used to build a comprehensive spectroscopic model of SiO using a combination of empirical and ab initio curves, including the potential energy (PE), spin-orbit (SO), electronic angular momentum (EAM) and (transition) dipole moment curves. The ab initio PE and coupling curves, computed at the multireference configuration interaction (MRCI) level of theory, were refined by fitting their analytical representations to 2617 experimentally derived SiO energy levels determined from 97 vibronic bands belonging to the $X$-$X$, $E$-$X$ and $A$-$X$ electronic systems through the MARVEL procedure. 112 observed forbidden transitions from the $C$-$X$, $D$-$X$, $e$-$X$, and $d$-$X$ bands were assigned using our predictions, and these could be fed back into the MARVEL procedure. The SiOUVenIR line list was computed using published ab initio transition dipole moments for the $E$-$X$ and $A$-$X$ bands; the line list is suitable for temperatures up to 10,000 K and for wavelengths longer than 140 nm. SiOUVenIR is available from www.exomol.com and the CDS database.

astro-ph.EP↗

arXiv2008

Hot Subdwarfs in Binaries as the Source of the Far-UV Excess in Elliptical Galaxies

The excess of far-ultraviolet (far-UV) radiation in elliptical galaxies has remained one of their most enduring puzzles. In contrast, the origin of old blue stars in the Milky Way, hot subdwarfs, is now reasonably well understood: they are hot stars that have lost their hydrogen envelopes by various binary interactions. Here, we review the main evolutionary channels that produce hot subdwarfs in the Galaxy and present the results of binary population synthesis simulations that reproduce the main properties of the Galactic hot-subdwarf population. Applying the same model to elliptical galaxies, we show how this model can explain the main observational properties of the far-UV excess, including the far-UV spectrum, without the need to invoke ad hoc physical processes. The model implies that the UV excess is not a sign of age, as has been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population.

astro-ph↗

arXiv2007

Binary Stars as the Source of the Far-UV Excess in Elliptical Galaxies

The discovery of an excess of light in the far-ultraviolet (UV) spectrum in elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. This has major implications for understanding the evolution of the UV excess and of elliptical galaxies in general. In particular, it implies that the UV excess is not a sign of age, as had been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population.

astro-ph↗