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A. Evans

Publications and source records attributed to A. Evans.

At least 19 recordsLinked to original sources

The Multi-phase Biconical Outflow in the local IR-Luminous Merger IRASF01364-1042

We investigate the spatially-resolved ISM properties of the local ($z = 0.048$), IR-luminous ($L_{\rm IR} = 10^{11.87}$\,L$_\odot$), late-stage galaxy merger IRAS F01364-1042, combining multi-wavelength IFU observations from \textit{JWST/MIRI-MRS}, ALMA and Keck/KCWI. Using these datasets, we construct emission line maps of several key tracers of the ionized (e.g., [Ne\,II]\,12.8$\mu$m, [O\,III]$\lambda5007$), warm molecular (e.g., \ce{H2}\,0-0\,S(3)), and cold molecular gas (e.g., CO (J$=2-1$)), and perform detailed decomposition of spectra extracted in resolved regions across the areas of emission. We confirm the presence of a multi-phase galactic biconical outflow along the minor axis of a highly inclined rotating disk. The inferred outflow velocities are $\sim\,$500 - 600\,km\,s$^{-1}$ $\sim\,$350\,km\,s$^{-1}$, and $\sim\,$200 - 300\,km\,s$^{-1}$, in the ionized, warm and cold molecular phase, respectively, with corresponding mass outflow rates of $\sim 0.3 - 2.3$, $\sim 31$, and $\sim 38 - 240$\,M$_\odot\,$yr$^{-1}$. The cold molecular phase dominates both the total mass outflow rate and the associated kinetic energy ($\sim\,2 - 8 \times 10^{42}$\,erg\,s$^{-1}$). We confirm, for the first time, a dust-obscured AGN in IRAS\,F01364-1042, via detection of the [Ne\,V]\,14.3$\mu$m line. The low inferred AGN bolometric luminosity ($1.2 - 1.8 \times 10^{43}$\,erg\,s$^{-1}$) suggests that the nuclear starburst alone, with a star formation rate of $\sim 40 - 60$\,M$_\odot$\,yr$^{-1}$, can account for the energy required to drive the outflow, though a more active AGN phase in the recent past may have also played a role. Our work showcases the necessity of multi-wavelength observations for interpreting the gas dynamics in merger-driven dusty starbursts, and the capability of \textit{JWST/MIRI-MRS} to uncover obscured, low-luminosity AGN that may be common in these systems.

astro-ph.GA

Detection of a molecular hydrogen envelope around nova GK Persei

The eruption of Nova Persei 1901 (GK Per) occurred 125 yrs ago; remarkably it still holds major surprises. Using data from the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx), we find it has a bipolar molecular hydrogen shell. This shell, which has dimensions 18'x10', is co-spatial with the Halpha nebulosity surrounding the nova, which is purported to be an ancient planetary nebula (PN). The shell is detected most strongly in the 0--0 S(9) 4.6947 micron line. A filament of emission in the S(9) 4.6947 micron line is seen 45" SW of GKPer. This coincides, over much of its length, with the site of X-ray and non-thermal radio emission where the 1901 nova ejecta impinges on the ambient medium. We propose that the H_2 emission from the filament arises from the predicted neutral zone between the forward and reverse shocks. Since it is common for bipolar PNe to be accompanied by H_2 envelopes, it ostensibly suggests that the 18'x10' nebulosity is a conventional PN with a luminous, ionizing central source. We show this is not the case, and that the H$\alpha$ nebulosity may be surrounding gas belonging to pre-existing material that was ionized during the 1901 eruption. The ionized gas is presently undergoing recombination on a timescale of ~3000 years, explaining why the nebulosity is still visible.

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Near-infrared spectroscopy of RS Ophiuchi in 2021: the calm, the storm, and the abatement

We present near-infrared (NIR) observations of the 2021 eruption of the recurrent nova RS Ophiuchi. The dataset provides both pre- and post-eruption perspectives on the eruption, as well as NIR spectra at high cadence. The spectrum obtained in 2020 June (14.3 years after the 2006 eruption, and 428.1 days before the 2021 eruption), is that of the red giant secondary, on which are superimposed several emission lines which most likely arise in the red giant wind. Spectra obtained during the eruption consist of emission (including coronal) lines, superimposed on a bremsstrahlung continuum at 8900K. The temperature of the coronal gas is estimated to be $10^{6.0}$K on day 11.7, and $10^{5.9}$K on day 31.7. The high cadence observations, obtained on day 31.7 of the eruption, provide no conclusive evidence for rapid ($<\sim1$~minute) variations in the HeI 1.0833$\,\mu$m line. Data obtained about one year after the eruption show that there may have been changes in the spectral type of the secondary after the 2021 eruption.

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Evidence for the merger hypothesis in V4332 Sgr: a low $^{12}$C/$^{13}$C ratio and multiple outbursts

Following the detections of the first extragalactic ``Luminous Red Nova'' (LRN) M31 RV in 1989, and its first Galactic counterpart V4332~Sgr in 1994, there have been many discoveries of similar, or closely related, objects. They are important because they bridge the luminosity gap between the brightest novae and supernovae, a largely unexplored parameter space. The cause of eruptions in LRNe is still unclear, a stellar merger being the most favored mechanism. However, barring V1309~Sco, there has been no direct evidence for a merger in the other objects. Here we present strong evidence that V4332~Sgr was a merger event. High resolution infrared observations of the CO fundamental band show an unusually small $^{12}$C/$^{13}$C ratio of $3.5\pm1$. This indicates that a violent event had occurred, whose effects penetrated deep enough to allow CNO cycle processed $^{13}$C in the inner H burning shell to be brought to the surface. We rule out planetary ingestion, and propose that the eruption was due to a merger between V4332~Sgr and a companion star. It is shown that V4332~Sgr was likely surrounded by an edge-on disk before its eruption. If this disk was a flattened common envelope containing V4332~Sgr and a companion star, then a merger scenario would not be inconsistent. Furthermore, V4332~Sgr had multiple outbursts, previously unreported but an important piece of information, since multiple outbursts are a trait shared by many LRNe.

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Probing Heavily Obscured AGN in Major Galaxy Mergers Using the mm-X-ray Correlation

The study of heavily obscured supermassive black hole (SMBH) growth in late-stage galaxy mergers is challenging: column densities $N_{\mathrm{H}}>10^{24},\mathrm{cm}^{-2}$ can block most nuclear emission, leaving significant gaps in the SMBH growth census. Millimeter-wave continuum emission offers a potential window into this obscured phase, as it can trace Active Galactic Nuclei (AGN) activity through mechanisms less affected by dust extinction. In this work, we test whether the observed correlation between millimeter ($\sim200,\mathrm{GHz}$) and hard X-ray (14 - 150,keV) luminosities can be used to plausibly identify hidden AGN in local (Ultra)Luminous Infrared Galaxies (U)LIRGs, including systems hosting confirmed dual AGN. We identify three sources -- one confirmed AGN and two strong candidates -- presenting significant evidence of AGN activity. The confirmed dual AGN lie within $\sim3\sigma$ of the mm--X-ray correlation, suggesting this relation can be used to identify hidden pairs. By combining the position of each source relative to this correlation with independent star formation rate constraints, we propose a method to disentangle AGN and star formation contributions for sources with measured column densities. While our analysis is based on a small, heterogeneous local sample and relies on empirical scaling relations, these results indicate that millimeter continuum emission may provide a useful complementary diagnostic for obscured SMBH growth. ALMA observations at high angular resolutions are particularly valuable for this approach, while future facilities such as the ngVLA will be essential to test its robustness in larger and more distant samples.

astro-ph.GA

The [Fe XIII] Infrared 10747 Angstrom and 10798 Angstrom Lines in Novae

The forbidden lines of [Fe XIII] at 10,747 Angstrom and 10,798 Angsrtom are among the most prominent lines in the near-infrared spectrum of the solar corona. They have been used routinely, both outside and during eclipses, as sensitive probes of the electron density and polarization in the solar corona. Many novae pass through a coronal phase, wherein the highly ionized nova ejecta have physical conditions that are remarkably similar to those of the solar corona. Many of the coronal emission lines that are seen are common to the spectra of both the Sun and novae. Yet, it appears that no robust detection of the [Fe XIII] lines has been made in a nova. Here we report the detection of these two infrared [Fe XIII]lines in the spectrum of the recurrent nova V3890 Sgr, taken 23.43 and 31.35 days after its August 2019 outburst. From their line strengths, we derive values of 10^10 per cubic cm and 10^[8.5-9] per cubic cm for the electron density on the two. The decrease in density between epochs can be explained if the density decreased with a power law n ~ r**alpha with a alpha inferred to be -3. The average temperature of the coronal gas is estimated to be T = (2.51\pm0.06) x 10^6~K. We find that recurrent novae with giant secondaries, including T CrB whose eruption is imminent, are the most suitable sources for further detections of the [Fe XIII] lines. epochs.

astro-ph.SR

Newborn jet in the symbiotic system R Aquarii

R Aquarii (R Aqr) is a well-known symbiotic binary that has attracted renewed interest during its recent periastron passage, an event that occurs only once every about 40 years. This passage marks the first to be observed with modern, state-of-the-art instruments. We investigate the inner, sub-arcsecond active region of R Aqr during this recent periastron passage, with the goal of gaining insight into the jet-launching mechanisms at work in this system. We analyse Ha speckle interferometric images obtained one month apart using Fourier techniques. These are complemented by high-resolution optical spectra in the same emission line. Our speckle imaging reveals a newborn two-sided jet orientated in the north-south direction. Its proper motion, 66 +- 19 mas per year, confirms that it was launched around 2020 Jan 7, at the onset of the periastron passage. Further analysis of the elongated central structure reveals a knot in the southern counterpart of the jet, moving away from the binary with a 27 +- 17 mas per year at a position angle of 187 degrees, and an ejection time around 2019 Oct 28. This interpretation is further supported by our high-resolution spectroscopic data. In addition, we update the expansion parallax distance of R Aqr to 260 pc.

astro-ph.SR

Infrared Spectroscopy of V838 Monocerotis in 2015 and 2022

We report medium-resolution $0.85-2.45\,\mu$m spectroscopy obtained in 2015 and 2022 and high resolution $2.27-2.39\,\mu$m and $4.59-4.77\,\mu$m spectroscopy obtained in 2015 of V838 Monocerotis, along with modeling of the $0.85-2.45\,\mu$ spectrum. V838 Mon underwent a series of eruptions and extreme brightenings in 2002, which are thought to have occured as a result of a stellar merger. The new spectra and modelling of them reveal a disturbed red giant photosphere that is probably continuing to contract and ejecta that are cooling and continuing to disperse at velocities up to 200kms$^{-1}$.

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Transition radiation in dusty novae with non-thermal radio emission

Transition radiation is produced when a relativistic charged particle enters or leaves a solid medium. The electrons that produce synchrotron radiation may interact with the dust in circumstellar environments, leading to the emission of transition radiation. We explore the production of transition radiation in dusty novae that also display synchrotron radiation emission. Transition radiation is emitted in the hard ultra-violet/X-ray range. We suggest that, even when the transition radiation is not itself directly observable, it may have a role in determining the ionisation balance of, and grain heating in, nova ejecta. Furthermore, it may be important in other dusty environments (such as supernova remnants) with non-thermal radio emission.

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Near-Infrared Spectroscopy of the Recurrent Nova M31N 2008-12a

Near infrared (NIR) 0.9--2.5$\mu$m spectra of the remarkable recurrent nova M31N 2008-12a were obtained on days 6.3 and 10.3 after discovery of its 2024 outburst, and are the first NIR spectra of this object. The only prominent line seen in the spectra is that of HeI 1.083$\mu$m, on day 6.3. Apart from this HeI line, there are only two other weak emission features: one at 1.0786$\mu$m, suggested to be the [FeXIII] 1.075$\mu$m coronal line, and one unidentified feature at 1.0969$\mu$m. The observed full width at half maximum of the HeI line on day 6.3 (1350 km s$^{-1}$) is consistent with the behaviour of optical HeI lines during earlier eruptions of this RN, which show that the nova ejecta decelerate as they interact with the secondary's wind. The HeI 1.083$\mu$m line faded rapidly, and was absent in the day 10.3 spectrum, along with any other emission lines. We use the relative strengths of optical He and H lines in previous eruptions to estimate the expected strengths of the HeI 1.083$\mu$m line and of other infrared (including coronal) lines at 6.3 days after eruption. Our findings are consistent with the infrared spectra we observed during the 2024 eruption. We apply our analysis to account for the relative weakness of NIR coronal emission in known Galactic recurrent novae with giant secondaries.

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Hydrodynamic Predictions for the Next Outburst of T Coronae Borealis: It will be the Brightest Classical or Recurrent Nova Ever Observed in X-rays

T Coronae Borealis (TCrB) is a recurrent nova (RN) with recorded outbursts in 1866, and 1946 and possible outbursts in 1217 and 1787. It is predicted to explode again in 2025 or 2026 based on multiple observational studies. The system consists of a massive (M$_{wd}$ $\gtrsim$ 1.35 M$_\odot$) white dwarf (WD) and a red giant (M3-M4 III). We have performed 1-D hydrodynamic simulations with NOVA to predict the behavior of the next outburst. These simulations consist of a range of mass accretion rates onto $\sim$1.35 M$_\odot$ WDs, designed to bound the conditions necessary to achieve ignition of an explosion after an $\approx$80 year inter-outburst period. We have used both carbon-oxygen and oxygen-neon initial compositions, in order to include the possible ejecta abundances to be measured in the observations of the next outburst. As the WD in the TCrB system is observed to be massive, theoretical predictions reported here imply that the WD is growing in mass as a consequence of the TNR. Therefore, the secular evolution of the WD may allow it to approach the Chandrasekhar limit and either explode as a Type Ia supernova or undergo accretion induced collapse, depending on its underlying composition. We have followed the evolution of just the WD, after removing the ejected matter from the surface layers. Our intent is to illuminate the mystery of the unique, second, maximum in the two well observed outbursts and we have found conditions that bracket the predictions.

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Near-infrared spectroscopy of the LMC recurrent nova LMCN 1968-12a

We have obtained near-infrared ($0.80-2.45\mu$m) spectra of the recurrent nova LMCN 1968-12a on two occasions during its 2024 August eruption. This is the first near-infrared spectroscopy of an extragalactic nova. The initial spectrum, on day 8.48, caught the nova in the coronal phase, with the [SiX] $1.43\mu$m line being extremely strong. This line had a luminosity of $\sim95$L$_\odot$, and is clearly a very powerful coolant. Its presence, together with the absence of [SiIX] 1.56$\mu$m, implies a coronal temperature $\gtrsim3\times10^6$K, possibly amongst the highest recorded coronal temperature in a nova eruption. With the exception of the [SiX] line, the near-infrared spectra are remarkable for being devoid of metal lines. We suggest that this is due, in part, to the exceptionally high temperature of the coronal gas, causing ions, whose emission lines would normally appear in the near-infrared spectrum, to be collisionally ionised to higher stages.

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A dusty rain falls on the nova V959 Monocerotis

We present archival and ground-based infrared observations of the gamma-ray-emitting nova V959 Mon, covering the period 100-4205 days after the 2012 eruption. We use these data to determine that the secondary in the nova system is a G5 main sequence star. Data from the NEOWISE survey reveal a significant increase in the emission at 3.4 microns and 4.6 microns at late (>~600 days) times, which we interpret as emission by dust. Other interpretations are considered but cannot be reconciled with the data. The presence of such late dust emission, and in particular its variation with time, are unprecedented in the context of novae. The behaviour of the dust emission suggests a qualitative interpretation in which ejecta from the 2012 eruption encounter denser pre-eruption circumbinary material, giving rise to Rayleigh-Taylor instabilities that cause clumps of dust-bearing material to fall back towards the central binary, the dust undergoing destruction by chemisputtering as it does so. The observed rise in the dust temperature, the decline in the nova-dust distance and in the dust mass, are consistent with this interpretation. Not all novae are expected to show this behaviour, but inspection of resources such as NEOWISE might reveal other novae post-eruption that do.

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A Photoionization model for the Infrared Coronal Line Emission in the Classical Nova V1716 Scorpii

A near-infrared spectrum of nova V1716 Scorpii (PNV J17224490-4137160), a recent bright (V_max = 7.3 mag), Fermi-LAT detected gamma-ray source, was modeled using the photoionization code CLOUDY. Abundances were estimated for He, C, N, O, Si, Al, Mg, Fe, Ne, S, Ca, and P. Notably, P (a factor of 120) and N (a factor of 248) are highly overabundant. It was necessary to assume the ejecta consist of two components (with a cylindrical geometry): a dense component from which the bulk of the H, He, and neutral O~I and N emission arises and a more diffuse component from which most of the coronal lines arise. Some of the coronal lines are found to originate from both the dense and diffuse components. The mass of the ejecta, including neutral and ionized gas, is ~ 4.19e-4 solar masses. Our analysis indicates that in the case of V1716 Sco (which has a carbon-oxygen white dwarf), a fraction of 25% white dwarf matter rather than 50% is favored for the mixing between white dwarf and the accreted envelope before the outburst. This mixing ratio is like that found for Oxygen-Neon novae where a 25% mixing fraction is also indicated. Helium hydride -- the first molecule to form after the Big Bang -- may have formed in the ejecta of V1716 Sco based on photoionization modeling. This prediction suggests that novae may be potential formation sites of this important molecular ion.

astro-ph.SR

Pre-solar grains from novae and "Born-again giants"

We review the properties of dust formed during classical nova eruptions and the Very Late Thermal Pulses (VLTPs) that occur during the later stages of post-Asymptotic Giant Branch evolution of low-mass stars. In both cases, carbon and hydrocarbon dust is produced. Novae may also produce silicate dust, contrary to the usual paradigm about the C:O ratio and dust composition. Despite the expectation that these dust sources are not expected to make significant contributions to the Galactic dust population, there is a significant body of evidence that grains from both stellar sources have been identified in recovered meteoritic and cometary material, and that certain infrared spectral signatures seen in comets are common to novae, VLTPs and pre-solar grains.

astro-ph.EP

Snowflakes in a furnace: formation of CO and dust in a recurrent nova eruption

We report the detection of carbon monoxide (CO) and dust, formed under hostile conditions, in recurrent nova V745 Sco about 8.7 days after its 2014 outburst. The formation of molecules or dust has not been recorded previously in the ejecta of a recurrent nova. The mass and temperature of the CO and dust are estimated to be T(CO) = 2250 +/- 250 K, M(CO) = (1 to 5) E-8 solar masses, and T(dust) = 1000 +/- 50 K, M(dust) approximately E-8 to E-9 solar masses respectively. At the time of their detection, the shocked gas was at a high temperature of approximately E+7 K as evidenced by the presence of coronal lines. The ejecta were simultaneously irradiated by a large flux of soft X-ray radiation from the central white dwarf. Molecules and dust are not expected to form and survive in such harsh conditions; they are like snowflakes in a furnace. However, it has been posited in other studies that, as the nova ejecta plow through the red giant's wind, a region exists between the forward and reverse shocks that is cool, dense and clumpy where the dust and CO could likely form. We speculate that this site may also be a region of particle acceleration, thereby contributing to the generation of gamma-rays.

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V445 Puppis -- Dustier than a Thousand Novae

V445 Puppis, the only known Galactic helium nova, is a unique test-bed to verify supernova (SN) theories in the single degenerate channel that involve a white dwarf (WD) accreting matter from a helium-rich donor. An estimate of the mass of the helium shell on the WD is crucial to deciding whether or not it will undergo a SN detonation. In this context, this study estimates the dust and ejecta masses in the 2000 November eruption of V445 Pup. Subsequent to its outburst, the star became cocooned in a dust envelope. An analysis of the spectral energy distribution (SED) of the dust using infrared data shows that V445 Pup produced at least $10^{-3}$ solar masses of dust which is unprecedented for a classical or recurrent nova. The SED can be explained by a combination of a cold dust component at 105 +/- 10 K, mass (1.9 +/- 0.8) $\times 10^{-3}$ solar masses, and a warm dust component at 255 +/- 10 K, mass (2.2 +/- 1.2) $\times 10^{-5}$ solar masses. For a conservative choice of the gas-to-dust mass ratio in the range 10--100, the mass of the ejecta is 0.01--0.1 solar masses. Such a high mass range raises the question: why did V445 Pup not detonate as a Type 1a SN as is predicted in certain double-detonation sub-Chandrasekhar supernovae formalisms? We re-examine the nature of V445 Pup and discuss its role as a potential SN progenitor.

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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