arXiv subjects

A. Evans

Publications and source records attributed to A. Evans.

At least 37 records · Page 2Linked to original sources

arXiv2023

Infrared spectroscopy of the 2022 eruption of the recurrent nova U Sco

We present near-infrared spectroscopy of the 2022 eruption of the recurrent nova U Sco, over the period from 5.2 to 45.4 days after outburst. This is the most intensive infrared study of this nova. Our observations started early after the outburst and extended almost to the end of the ``Super Soft'' X-ray phase. A major find is the presence of coronal lines from day 9.41, one of the earliest appearances of these in any nova, classical or recurrent. The temperature of the coronal gas is $7\times10^5$ K. There is also evidence for the presence of much cooler ($\lesssim2.5\times10^4$ K) gas. Remarkable changes are seen in the HeI $1.083μ$m line, the strength of which declines, then recovers, in anti-correlation with the X-ray behaviour. We conclude that shock ionisation is the dominant excitation mechanism for the coronal line emission. There is evidence in the infrared spectra for the presence of black body emission at $\sim20000$ K, which we tentatively identify with the irradiated secondary, and for free-free/free-bound emission. For the previously determined binary inclination of $82.7$ degrees, the implied ejection velocities are as high as 22000 km s$^{-1}$. These velocities appear unprecedented in nova outflows, and are comparable to those seen in supernovae, thereby marking U Sco as a truly remarkable object.

astro-ph.SR↗

arXiv2022

Design and Performance of a Novel Low Energy Multi-Species Beamline for the ALPHA Antihydrogen Experiment

The ALPHA Collaboration, based at the CERN Antiproton Decelerator, has recently implemented a novel beamline for low-energy ($\lesssim$ 100 eV) positron and antiproton transport between cylindrical Penning traps that have strong axial magnetic fields. Here, we describe how a combination of semianalytical and numerical calculations were used to optimise the layout and design of this beamline. Using experimental measurements taken during the initial commissioning of the instrument, we evaluate its performance and validate the models used for its development. By combining data from a range of sources, we show that the beamline has a high transfer efficiency, and estimate that the percentage of particles captured in the experiments from each bunch is (78 $\pm$ 3)% for up to $10^{5}$ antiprotons, and (71 $\pm$ 5)% for bunches of up to $10^{7}$ positrons.

physics.acc-ph↗

arXiv2022

Rise and fall of silicate dust in RS Ophiuchi following the 2006 eruption

We present an analysis of archival Spitzer InfraRed Spectrograph (IRS) observations of the recurrent nova RS Ophiuchi obtained on several occasions, beginning about 7 months after the outburst in 2006. These data show atomic emission lines, absorption bands due to photospheric SiO, and the well known silicate dust features at $9.7\,μ$m and $18\,μ$m. The dust emission, arising in the wind of the secondary star, is fitted by Dusty models for mass-loss rates in the range $1.0-1.7\times10^{-7}$M$_{\odot}$yr$^{-1}$. The silicate features are similar in profile to those seen in circumstellar environments of isolated late-type stars and some dusty symbiotic binaries, although the longer wavelength feature peaks at $17\,μ$m,, instead of the usual $18\,μ$m, indicating peculiar grain properties. The dust features are variable, appearing stronger in 2006-2007 during outburst than in 2008-2009 when the system was in the quiescent state. This variability is attributed to changes in the ultraviolet output and the reformation of the accretion disk, although a decline in the mass-loss rate of the red giant secondary star could also play a role. Further observations, in the aftermath of the 2021 eruption, could provide a definitive conclusion.

astro-ph.SR↗

arXiv2022

The 2019 outburst of the 2005 classical nova V1047 Cen: a record breaking dwarf nova outburst or a new phenomenon?

We present a detailed study of the 2019 outburst of the cataclysmic variable V1047~Cen, which hosted a classical nova eruption in 2005. The peculiar outburst occurred 14 years after the classical nova event and lasted for more than 400 days, reaching an amplitude of around 6 magnitudes in the optical. Early spectral follow-up revealed what could be a dwarf nova (accretion disk instability) outburst. However, the outburst duration, high velocity ($>$2000\,km\,s$^{-1}$) features in the optical line profiles, luminous optical emission, and presence of prominent long-lasting radio emission together suggest a phenomenon more exotic and energetic than a dwarf nova outburst. The outburst amplitude, radiated energy, and spectral evolution are also not consistent with a classical nova eruption. There are similarities between V1047~Cen's 2019 outburst and those of classical symbiotic stars, but pre-2005 images of the field of V1047~Cen indicate that the system likely hosts a dwarf companion, implying a typical cataclysmic variable system. Based on our multi-wavelength observations, we suggest that the outburst may have started with a brightening of the disk due to enhanced mass transfer or disk instability, possibly leading to enhanced nuclear shell burning on the white dwarf, which was already experiencing some level of quasi-steady shell burning. This eventually led to the generation of a wind and/or bipolar, collimated outflows. The 2019 outburst of V1047~Cen appears to be unique, and nothing similar has been observed in a typical cataclysmic variable system before, hinting at a potentially new astrophysical phenomenon.

astro-ph.SR↗

arXiv2022

A 4~Kpc Molecular Gas Lane in Cygnus A

We present the discovery of a 4 kpc molecular gas lane in the Cygnus A host galaxy, using ALMA CO 2-1 observations. The gas lane is oriented roughly perpendicular to the projected radio jet axis. The CO emission generally follows the clumpy dust lanes seen in HST I-band images. The total molecular gas mass is $30\times 10^8$ M$_\odot$ for Milky Way type clouds, and $3.6 \times 10^8$ M$_\odot$ for starburst conditions. There is a velocity change from the northern to southern CO peaks of about $\pm 175$~km~s$^{-1}$, and an apparently smooth velocity gradient between the peaks, although the emission in the central region is weak. In the inner $\sim 0.5"$ projected distance from the radio core, comparison of the CO velocities to those observed for H$_2$ 2.1218 $μ$m emission shows higher velocities for the vibrationally excited warm molecular gas than the cooler CO 2-1 line emitting gas at similar projected radii. A possible explanation for these different projected velocities at a given radius is that the cooler CO gas is distributed in a clumpy ring at radius $\sim 1.5"$ to $2"$, while the warm H$_2$ 2.12$μ$m emitting gas is interior to this ring. Of course, the current data cannot rule-out a clumpy, amorphous molecular gas distribution linearly distributed perpendicular to the radio jet axis. We consider surface brightness properties on scales down to $\sim 265$~pc, and discuss the Cygnus A results in the context of other radio galaxies with CO emission.

astro-ph.GA↗

arXiv2022

Infrared spectroscopy of the 2019 eruption of the recurrent nova V3890 Sgr: separation into equatorial and polar winds revealed

We present infrared spectroscopy of the 2019 eruption of the recurrent nova V3890 Sgr, obtained over the period 5.1-46.3 days after the eruption. The spectrum of the red giant became more prominent as the flux declined, and by day 46.3 dominated the spectrum. Hydrogen and helium emission lines consisted of a narrow component superposed on a broad pedestal. The full width at half maximum of the narrow components declined with time $t$ as the eruption progressed, as ${t}^{-0.74}$, whereas those of the broad components remained essentially constant. Conversely, the line fluxes of the narrow components of Pa,$β$ remained roughly constant, while those of the broad components declined by a factor $\sim30$ over a period of $\lesssim25$~days. The behaviour of the broad components is consistent with them arising in unencumbered fast-flowing ejecta perpendicular to the binary plane, in material that was ejected in a short $\sim3.3$-day burst. The narrow components arise in material that encounters the accumulated circumstellar material. The outburst spectra were rich in coronal lines. There were two coronal line phases, one that originated in gas ionised by supersoft X-ray source, the other in shocked gas. From the relative fluxes of silicon and sulphur coronal lines on day 23.4 - when the emitting gas was shocked - we deduce that the temperature of the coronal gas was $9.3\times10^5$~K, and that the abundances are approximately solar.

astro-ph.SR↗

arXiv2022

Stringent limits on $^{28}$SiO maser emission from the recurrent nova T Coronae Borealis

There are indications that the third known eruption of the recurrent nova T CrB is imminent, and multi-wavelength observations prior to the eruption are important to characterise the system before it erupts. T CrB is known to display the SiO fundamental vibrational feature at 8$\,μ$m. When the anticipated eruption occurs, it is possible that the shock produced when the ejected material runs into the wind of the red giant in the system may be traced using SiO maser emission. We have used the 100m Effelsberg Radio Telescope to search for $^{28}$SiO emission in the $\upsilon=1$, $\upsilon=2$, $J=1\rightarrow0$ transitions, at 43.122 GHz and 42.820~GHz respectively, while the system is in quiescence. We find no evidence for such emission.

astro-ph.SR↗

arXiv2022

The recurrent nova V3890~Sgr: a near-infrared and optical study of the red giant component and its environment

We present an analysis of the red giant component of the recurrent nova V3890 Sgr, using data obtained before and after its 2019 eruption. Its effective temperature is $T_{\rm eff}=3050\pm$200 K for $\log{g}=0.7$, although there are modest changes in $T_{\rm eff}$. There is an overabundance of both carbon ($0.20\pm0.05$~dex) and sodium ($1.0\pm0.3$~dex) relative to their solar values, possibly the result of ejecta from the 1990 nova eruption being entrained into the red giant photosphere. We find $^{12}$C/$^{13}$C $=25\pm2$, a value similar to that found in red giants in other recurrent novae. The interpretation of the quiescent spectrum in the 5--38$\,mu$m region requires the presence of photospheric SiO absorption and cool ($\sim400$~K) dust in the red giant environment. The spectrum in the region of the Na{\sc i} D lines is complex, and includes at least six interstellar components, together with likely evidence for interaction between ejecta from the 2019 eruption and material accumulated in the plane of the binary. Three recurrent novae with giant secondaries have been shown to have environments with different dust content, but photospheres with similar $^{12}$C/$^{13}$C ratios. The SiO fundamental bands most likely have a photospheric origin in the all three stars.

astro-ph.SR↗

arXiv2022

V4334 Sgr (Sakurai's Object): still churning out the dust

We present a $0.8-2.5\,μ$m spectrum of the Very Late Thermal Pulse object V4334 Sgr (Sakurai's Object), obtained in 2020 September. The spectrum displays a continuum that rises strongly to longer wavelengths, and is considerably brighter than the most recent published spectrum obtained seven years earlier. At the longer wavelengths the continuum is well fitted by a blackbody with a temperature of $624\pm8$ K. However, there is excess continuum at the shortest wavelengths that we interpret as being due to hot dust that has very recently formed in an environment with C/O $\simeq2.5$. Other possible sources for this excess continuum are discussed - such as the stellar photosphere dimly seen through the dust shell, and light scattered off the inner wall of the dust torus - but these interpretations seem unlikely. Numerous emission lines are present, including those of HeI, CI, [CI], and OI. Our observations confirm that emission in the HeI 1.083$\,μ$m and [CI] 0.9827/0.9852$\,μ$m lines is spatially extended. The [CI] line fluxes suggest that the electron density increased by an order of magnitude between 2013 and 2020, and that these two lines may soon disappear from the spectrum. The flux ratio of the 1.083$\,μ$m and 2.058$\,μ$m HeI lines is consistent with the previously-assumed interstellar extinction. The stellar photosphere remains elusive, and the central star may not be as hot as suggested by current evolutionary models.

astro-ph.SR↗

arXiv2021

The Infrared Evolution of Dust in V838 Monocerotis

Luminous Red Variables (LRVs) are most likely eruptions that are the outcome of stellar mergers. V838 Mon is one of the best-studied members of this class, representing an archetype for stellar mergers resulting from B-type stars. As result of the merger event, nova-like eruptions occur driving mass-loss from the system. As the gas cools considerable circumstellar dust is formed. V838 Mon erupted in 2002 and is undergoing very dynamic changes in its dust composition, geometry, and infrared luminosity providing a real-time laboratory to validate mineralogical condensation sequences in stellar mergers and evolutionary scenarios. We discuss recent NASA Stratospheric Observatory for Infrared Astronomy SOFIA 5 to 38 micron observations combined with archival NASA Spitzer spectra that document the temporal evolution of the freshly formed (within the last 20 yrs) circumstellar material in the environs of V838 Mon. Changes in the 10 micron spectral region are strong evidence that we are witnessing a classical dust condensation sequence expected to occur in oxygen-rich environments where alumina formation is followed by that of silicates at the temperature cools.

astro-ph.SR↗

arXiv2021

Near-Infrared and Optical Observations of Type Ic SN2020oi and broad-lined Ic SN2020bvc: Carbon Monoxide, Dust and High-Velocity Supernova Ejecta

We present near-infrared and optical observations of the Type Ic Supernova (SN) 2020oi in the galaxy M100 and the broad-lined Type Ic SN2020bvc in UGC 9379, using Gemini, LCO, SOAR, and other ground-based telescopes. The near-IR spectrum of SN2020oi at day 63 since the explosion shows strong CO emissions and a rising K-band continuum, which is the first unambiguous dust detection from a Type Ic SN. Non-LTE CO modeling shows that CO is still optically thick, and that the lower limit to the CO mass is 0.001 Msun. The dust temperature is 810 K, and the dust mass is ~10^(-5) Msun. We explore the possibilities that the dust is freshly formed in the ejecta, heated dust in the pre-existing circumstellar medium, and an infrared echo. The light curves of SN2020oi are consistent with a STELLA model with canonical explosion energy, 0.07 Msun Ni mass, and 0.7 Msun ejecta mass. A model of high explosion energy of ~10^(52) erg, 0.4 Msun Ni mass, 6.5 Msun ejecta mass with the circumstellar matter, reproduces the double-peaked light curves of SN2020bvc. We observe temporal changes of absorption features of the IR Ca~II triplet, S~I at 1.043 micron, and Fe~II at 5169 Angstrom. The blue-shifted lines indicate high velocities, up to 60,000 km/s for SN2020bvc and 20,000 km/s for SN2020oi, and the expansion velocity rapidly declines before the optical maximum. We present spectral signatures and diagnostics of CO and SiO molecular bands between 1.4 and 10 microns.

astro-ph.SR↗

arXiv2020

Infrared spectroscopy of CK Vulpeculae: revealing a remarkably powerful blast from the past

CK Vulpeculae, which erupted in AD 1670-71, was long considered to be a nova outburst; however, recent observations have required that alternative scenarios be considered. Long slit infrared spectroscopy of a forbidden line of iron reported here has revealed high line-of-sight velocities ($\sim\pm900$~km~s$^{-1}$) of the ansae at the tips of the bipolar lobes imaged in H$α$ in 2010. The deprojected velocities of the tips are approximately $\pm2130$~km~s$^{-1}$ assuming the previously derived inclination angle of $65^\circ$ for the axis of cylindrical symmetry of the bipolar nebula. Such high velocities are in stark contrast to previous reports of much lower expansion velocities in CK~Vul. Based on the deprojected velocities of the tips and their angular expansion measured over a 10-year baseline, we derive a revised estimate, with estimated uncertainties, of $3.2^{+0.9}_{-0.6}$~kpc for the distance to CK~Vul. This implies that the absolute visual magnitude at the peak of the 1670 explosion was $M_V = -12.4^{+1.3}_{-2.4}$, indicating that the 1670 event was far more luminous than previous estimates and brighter than any classical nova or any Galactic stellar merger. We propose that CK~Vul belongs to the class of Intermediate Luminosity Optical Transients (ILOTs), objects which bridge the luminosity gap between novae and supernovae. While eruptions in lower luminosity ILOTs are attributed to merger events, the origin of the highly luminous ILOT outbursts is currently not known.

astro-ph.SR↗

arXiv2020

Isotopic ratios in the red giant component of the recurrent nova T Coronae Borealis

We report the determination of abundances and isotopic ratios for C, O and Si in the photosphere of the red giant component of the recurrent nova T Coronae Borealis from new 2.284--2.402 $μ$m and 3.985--4.155 $μ$m spectroscopy. Abundances and isotopic ratios in the photosphere may be affected by (i) processes in the red giant interior which are brought to the surface during dredge-up, (ii) contamination of the red giant, either during the common envelope phase of the binary evolution or by material synthesised in recurrent nova eruptions, or a combination of the two. We find that the abundances of C, O and Si are reasonably consistent with the expected composition of a red giant after first dredge-up, as is the $^{16}$O/$^{17}$O ratio. The $^{28}$Si/$^{29}$Si ratio is found to be $8.6\pm3.0$, and that for $^{28}$Si/$^{30}$Si is $21.5\pm3.0$. The $^{12}$C/$^{13}$C ratio ($10\pm2$) is somewhat lower than expected for first dredge-up. The $^{16}$O/$^{18}$O ratio ($41\pm3$) is highly inconsistent with that expected either from red giant evolution ($\sim550$) or from contamination of the red giant by the products of a nova thermonuclear runaway. In particular the C and O isotopic ratios taken in combination are a puzzle. We urge confirmation of our results using spectroscopy at high resolution. We also encourage a thorough theoretical study of the effects on the secondary star in a recurrent nova system of contamination by ejecta having anomalous abundances and isotopic ratios.

astro-ph.SR↗

arXiv2020

The infrared view of dust and molecules around V4334 Sgr (Sakurai's Object): a 20-year retrospective

We present an analysis of the evolution of circumstellar dust and molecules in the environment of the very late thermal pulse object V4334 Sgr (Sakurai's Object) over a $\sim20$-year period, drawing on ground-, airborne- and space-based infrared photometry and spectroscopy. The dust emission, which started in 1997, resembles a blackbody that cooled from $\sim1200$K in 1998 August to $\sim180$K in 2016 July. The dust mass, assuming amorphous carbon, was $\sim5\times10^{-10}$M$_\odot$ in 1998 August, and we estimate that the total dust mass was $\sim2\times10^{-5}$M$_\odot$ by $\sim2016$. The appearance of a near infrared excess in 2008 suggests a new episode of (or renewed) mass loss began then. We infer lower limits on the bolometric luminosity of the embedded star from that of the dust shell, which rose to $\sim16000$L$_\odot$ before declining to $\sim3000$L$_\odot$. There is evidence for weak 6-7$μ$m absorption, which we attribute to hydrogenated amorphous carbon formed in material ejected by Sakurai's Object during a mass ejection phase that preceded the 1997 event. We detect small hydrocarbon and other molecules in the spectra, and trace the column densities in hydrogen cyanide (HCN) and acetylene (C$_2$H$_2$). We use the former to determine the $^{12}$C/$^{13}$C ratio to be $6.4\pm0.7$, 14 times smaller than the Solar System value.

astro-ph.SR↗

arXiv2019

Infrared spectroscopy of the recent outburst in V1047 Cen (Nova Centauri 2005)

Fourteen years after its eruption as a classical nova (CN), V1047 Cen (Nova Cen 2005) began an unusual re-brightening in 2019 April. The amplitude of the brightening suggests that this is a dwarf nova (DN) eruption in a CN system. Very few CNe have had DN eruptions within decades of the main CN outburst. The 14 years separating the CN and DN eruptions of V1047 Cen is the shortest of all instances recorded thus far. Explaining this rapid succession of CN and DN outbursts in V1047 Cen may be challenging within the framework of standard theories for DN outbursts. Following a CN eruption, the mass accretion rate is believed to remain high $(\dot{M}\sim10^{-8}$M$_\odot$yr$^{-1})$ for a few centuries, due to the irradiation of the secondary star by the still-hot surface of the white dwarf. Thus a DN eruption is not expected to occur during this high mass accretion phase as DN outbursts, which result from thermal instabilities in the accretion disk, and arise during a regime of low mass accretion rate $(\dot{M}\sim10^{-10}$M$_\odot$yr$^{-1})$. Here we present near-infrared spectroscopy to show that the present outburst is most likely a DN eruption, and discuss the possible reasons for its early occurrence. Even if the present re-brightening is later shown to be due to a cause other than a DN outburst, the present study provides invaluable documentation of this unusual event.

astro-ph.SR↗

arXiv2019

The January 2016 eruption of recurrent nova LMC 1968

We present a comprehensive review of all observations of the eclipsing recurrent Nova LMC 1968 in the Large Magellanic Cloud which was previously observed in eruption in 1968, 1990, 2002, 2010, and most recently in 2016. We derive a probable recurrence time of $6.2 \pm 1.2$ years and provide the ephemerides of the eclipse. In the ultraviolet-optical-IR photometry the light curve shows high variability right from the first observation around two days after eruption. Therefore no colour changes can be substantiated. Outburst spectra from 2016 and 1990 are very similar and are dominated by H and He lines longward of 2000 Angstrom. Interstellar reddening is found to be E(B-V) = $0.07\pm0.01$. The super soft X-ray luminosity is lower than the Eddington luminosity and the X-ray spectra suggest the mass of the WD is larger than 1.3 M$_\odot$. Eclipses in the light curve suggest that the system is at high orbital inclination. On day four after the eruption a recombination wave was observed in Fe II ultraviolet absorption lines. Narrow line components are seen after day 6 and explained as being due to reionisation of ejecta from a previous eruption. The UV spectrum varies with orbital phase, in particular a component of the He II 1640 Angstrom emission line, which leads us to propose that early-on the inner WD Roche lobe might be filled with a bound opaque medium prior to the re-formation of an accretion disk. Both this medium and the ejecta can cause the delay in the appearance of the soft X-ray source.

astro-ph.SR↗

arXiv2019

Gas phase SiO in the circumstellar environment of the recurrent nova T Coronae Borealis

We report the discovery of the diatomic molecule SiO in the gas phase in the environment of the recurrent nova T Coronae Borealis. While some of the SiO is photospheric, a substantial portion must arise in the wind from the red giant component of T CrB. A simple fit to the SiO feature, assuming local thermodynamic equilibrium, suggests a SiO column density of 2.8x10^17 /cm2 and temperature ~1000K; the SiO column density is similar to that present in the winds of field red giants. A search for SiO maser emission is encouraged both before and after the next anticipated eruption. We find that the 12C/13C ratio in the red giant is <9, with a best fit value of ~5, a factor ~18 times lower than the solar value of 89. We find no convincing evidence for the presence of dust in the environment of T CrB, which we attribute to the destructive effects on nucleation sites of hard X-ray emission. When the next eruption of T CrB occurs, the ejected material will shock the wind, producing X-ray and coronal line emission, as is the case for the recurrent nova RS Oph. T CrB is also a good candidate for very high energy gamma-ray emission, as first observed during the 2010 outburst of V407 Cyg. We include in the paper a wide variety of infrared spectroscopic and photometric data.

astro-ph.SR↗

arXiv2018

Early formation of carbon monoxide in the Centaurus A supernova SN 2016adj

We present near-infrared spectroscopy of the NGC 5128 supernova SN 2016adj in the first 2 months following discovery. We report the detection of first overtone carbon monoxide emission at $\sim58.2$ d after discovery, one of the earliest detections of CO in an erupting supernova. We model the CO emission to derive the CO mass, temperature and velocity, assuming both pure $^{12}$CO and a composition that includes $^{13}$CO; the case for the latter is the isotopic analyses of meteoritic grains, which suggest that core collapse supernovae can synthesise significant amounts of $^{13}$C. Our models show that, while the CO data are adequately explained by pure $^{12}$CO, they do not preclude the presence of $^{13}$CO, to a limit of $^{12}$C/$^{13}$C $> 3$, the first constraint on the $^{12}$C/$^{13}$C ratio determined from near-infrared observations. We estimate the reddening to the object, and the effective temperature from the energy distribution at outburst. We discuss whether the ejecta of SN 2016adj may be carbon-rich, what the infrared data tell us about the classification of this supernova, and what implications the early formation of CO in supernovae may have for CO formation in supernovae in general.

astro-ph.SR↗