arXiv subjects

M. T. Rushton

Publications and source records attributed to M. T. Rushton.

At least 19 recordsLinked to original sources

arXiv2025

Infrared Spectroscopy of V838 Monocerotis in 2015 and 2022

We report medium-resolution $0.85-2.45\,μ$m spectroscopy obtained in 2015 and 2022 and high resolution $2.27-2.39\,μ$m and $4.59-4.77\,μ$m spectroscopy obtained in 2015 of V838 Monocerotis, along with modeling of the $0.85-2.45\,μ$ 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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arXiv2024

Uncovering the truth about M101, NGC 3938, and their significant others through radiative transfer

Solving the inverse problem in spiral galaxies, that allows the derivation of the spatial distribution of dust, gas and stars, together with their associated physical properties, directly from panchromatic imaging observations, is one of the main goals of this work. To this end we used radiative transfer models to decode the spatial and spectral distribution of the nearby face-on galaxies M101 and NGC 3938. In both cases we provide excellent fits to the surface-brightness distributions derived from GALEX, SDSS, 2MASS, Spitzer and Herschel imaging observations. Together with previous results from M33, NGC 628, M51 and the Milky Way, we obtain a small statistical sample of modelled nearby galaxies that we analyse in this work. We find that in all cases Milky Way-type dust with Draine-like optical properties provide consistent and successful solutions. We do not find any "submm excess", and no need for modified dust-grain properties. Intrinsic fundamental quantities like star-formation rates (SFR), specific SFR (sSFR), dust opacities and attenuations are derived as a function of position in the galaxy and overall trends are discussed. In the SFR surface density versus stellar mass surface density space we find a structurally resolved relation (SRR) for the morphological components of our galaxies, that is steeper than the main sequence (MS). Exception to this is for NGC 628, where the SRR is parallel to the MS.

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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.

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arXiv2022

Decoding NGC 628 with radiative transfer methods

We present an axi-symmetric model for the ultraviolet (UV)-to-submillimetre (submm) images of the nearly face-on spiral galaxy NGC 628. It was calculated using a radiative transfer (RT) code, accounting for the absorption and re-emission of starlight by dust in the interstellar medium of this galaxy. The code incorporates emission from Polycyclic Aromatic Hydrocarbons, anisotropic scattering and stochastic heating of the grains. This is the second successful modelling of a face-on spiral galaxy with RT methods, whereby the large-scale geometry of stars and dust is self-consistently determined. The solution was obtained by fitting azimuthally averaged profiles in the UV, optical and submm. The model predicts remarkably well all characteristics of the profiles, including the increase by a factor of 1.8 of the scale-length of the infrared emissivity between 70 and 500 $μ$m. We find that NGC 628 did not undergo an efficient inside-out disk growth, as predicted by semi-analytical hierarchical models for galaxy formation. We also find large amounts of dust grains at large radii, that could involve efficient transport mechanisms from the inner disk. Our results show that 71% of the dust emission in NGC 628 is powered by the young stellar populations, with the old stellar populations from the bulge contributing 65% to the heating of the dust in the central region ($R<0.5$ kpc). The derived star formation rate is $\rm SFR=2.00\pm0.15\,{\rm M}_{\odot}{\rm yr}^{-1}$..

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arXiv2017

A SOFIA FORCAST Grism Study of the Mineralogy of Dust in the Winds of Proto-planetary Nebulae: RV Tauri Stars and SRd Variables

We present a SOFIA FORCAST grism spectroscopic survey to examine the mineralogy of the circumstellar dust in a sample of post-asymptotic giant branch yellow supergiants that are believed to be the precursors of planetary nebulae. Our mineralogical model of each star indicates the presence of both carbon rich and oxygen rich dust species-contrary to simple dredge-up models-with a majority of the dust in the form of amorphous carbon and graphite. The oxygen rich dust is primarily in the form of amorphous silicates. The spectra do not exhibit any prominent crystalline silicate emission features. For most of the systems, our analysis suggests that the grains are relatively large and have undergone significant processing, supporting the hypothesis that the dust is confined to a Keplerian disk and that we are viewing the heavily processed, central regions of the disk from a nearly face-on orientation. These results help to determine the physical properties of the post-AGB circumstellar environment and to constrain models of post-AGB mass loss and planetary nebula formation.

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arXiv2017

Temporal resolution of a pre-maximum halt in a Classical Nova: V5589 Sgr observed with STEREO HI-1B

Classical novae show a rapid rise in optical brightness over a few hours. Until recently the rise phase, particularly the phenomenon of a pre-maximum halt, was observed sporadically. Solar observation satellites observing Coronal Mass Ejections enable us to observe the pre-maximum phase in unprecedented temporal resolution. We present observations of V5589 Sgr with STEREO HI-1B at a cadence of 40 min, the highest to date. We temporally resolve a pre-maximum halt for the first time, with two examples each rising over 40 min then declining within 80 min. Comparison with a grid of outburst models suggests this double peak, and the overall rise timescale, are consistent with a white dwarf mass, central temperature and accretion rate close to 1.0 solar mass, 5x10^7 K and 10^-10 solar masses per year respectively. The modelling formally predicts mass loss onset at JD 2456038.2391+/-0.0139, 12 hrs before optical maximum. The model assumes a main-sequence donor. Observational evidence is for a subgiant companion; meaning the accretion rate is under-estimated. Post-maximum we see erratic variations commonly associated with much slower novae. Estimating the decline rate difficult, but we place the time to decline two magnitudes as 2.1 < t_2(days) < 3.9 making V5589 Sgr a "very fast" nova. The brightest point defines "day 0" as JD 2456038.8224+/-0.0139, although at this high cadence the meaning of the observed maximum becomes difficult to define. We suggest that such erratic variability normally goes undetected in faster novae due to the low cadence of typical observations; implying erratic behaviour is not necessarily related to the rate of decline.

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arXiv2016

Rise and fall of the dust shell of the classical nova V339 Delphini

We present infrared spectroscopy of the classical nova V339 Delphini, obtained over a $\sim2$ year period. The infrared emission lines were initially symmetrical, with HWHM velocities of 525 km s$^{-1}$. In later ($t\gtrsim77$days, where $t$ is the time from outburst) spectra however, the lines displayed a distinct asymmetry, with a much stronger blue wing, possibly due to obscuration of the receding component by dust. Dust formation commenced at $\sim$ day 34.75 at a condensation temperature of $1480\pm20$K, consistent with graphitic carbon. Thereafter the dust temperature declined with time as $T_{\rm d}\propto{t}^{-0.346}$, also consistent with graphitic carbon. The mass of dust initally rose, as a result of an increase in grain size and/or number, peaked at $\sim$ day 100, and then declined precipitously. This decline was most likely caused by grain shattering due to electrostatic stress after the dust was exposed to X-radiation. An Appendix summarises Planck Means for carbon, and the determination of grain mass and radius for a carbon dust shell.

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arXiv2015

Modelling the spectral energy distribution of the red giant in RS Ophiuchi: evidence for irradiation

We present an analysis of optical and infrared spectra of the recurrent nova RS Oph obtained during between 2006 and 2009. The best fit to the optical spectrum for 2006 September 28 gives effective temperature Tef = 3900~K for log g = 2.0, while for log g = 0.0 we find Tef = 4700~K, and a comparison with template stellar spectra provides Tef $\sim$ 4500 K. The observed spectral energy distribution (SED), and the intensities of the emission lines, vary on short ($\sim 1$~day) time-scales, due to disc variability. We invoke a simple one-component model for the accretion disc, and a model with a hot boundary layer, with high ($\sim 3.9 \times 10^{-6}$ \Mdot) and low ($\sim 2 \times 10^{-8}$ \Mdot) accretion rates, respectively. Fits to the accretion disc-extracted infrared spectrum (2008 July 15) yield effective temperatures for the red giant of Tef = 3800 +/- 100~K (log g = 2.0) and Tef = 3700 +/- 100~K (log g = 0.0). Furthermore, using a more sophisticated approach, we reproduced the optical and infrared SEDs of the red giant in the RS Oph system with a two-component model atmosphere, in which 90\% of the surface has Tef = 3600 K and 10\% has Tef = 5000~K. Such structure could be due to irradiation of the red giant by the white dwarf.

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arXiv2013

STEREO/HI and Optical Observations of the Classical Nova V5583 Sagittarii

The classical nova V5583 Sgr (Nova Sagittarii 2009 No 3) has been observed during the rise phase and shortly after by NASA's STEREO/HI instruments, with later optical spectroscopy obtained with the R-C Spectrograph at CTIO, Chile. The time of peak in the STEREO passband has been constrained to within 4 hours, as a result of the high cadence data obtained by STEREO/HI. The optical spectra show the nova evolving from the permitted to the nebular phases. The neon abundance in the ejecta is [Ne/O] > +1:0, which suggests that V5583 Sgr was most likely a neon nova.

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arXiv2013

Modelling the recurrent nova U Scorpii in quiescence

VLT and SALT spectroscopy of U Sco were obtained $\sim$18 and $\sim$30 months after the 2010 outburst. From these spectra the accretion disc is shown to take at least 18 months to become fully reformed. The spectral class of the companion is constrained to be F8$^{+5}_{-6}$\,IV-V at the 95% confidence level when the irradiated face of the companion is visible.

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arXiv2012

Infrared observations of the recurrent nova T Pyxidis: ancient dust basks in the warm glow of the 2011 outburst

We present Spitzer Space Telescope and Herschel Space Observatory infrared observations of the recurrent nova T Pyx during its 2011 eruption, complemented by ground-base optical-infrared photometry. We find that the eruption has heated dust in the pre-existing nebulosity associated with T Pyx. This is most likely interstellar dust swept up by T Pyx - either during previous eruptions or by a wind - rather than the accumulation of dust produced during eruptions.

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arXiv2011

Solid-phase C_60 in the peculiar binary XX Oph?

We present infrared spectra of the binary XX Oph obtained with the Infrared Spectrograph on the Spitzer Space Telescope. The data show some evidence for the presence of solid C_60 - the first detection of C_60 in the solid phase - together with the well-known ``Unidentified Infrared'' emission features. We suggest that, in the case of XX Oph, the C_60 is located close to the hot component, and that in general it is preferentially excited by stars having effective temperatures in the range 15,000-30,000K. C_60 may be common in circumstellar environments, but un-noticed in the absence of a suitable exciting source.

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arXiv2011

The Helium Abundance in the Ejecta of U Scorpii

U Scorpii is a recurrent nova which has been observed in outburst on 10 occasions, most recently in 2010. We present near-infrared and optical spectroscopy of the 2010 outburst of U Sco. The reddening of U Sco is found to be $E(B-V) = 0.14\pm0.12$, consistent with previous determinations, from simultaneous optical and near-IR observations. The spectra show the evolution of the line widths and profiles to be consistent with previous outbursts. Velocities are found to be up to 14000\,kms$^{-1}$ in broad components and up to 1800\,kms$^{-1}$ in narrow line components, which become visible around day 8 due to changes in the optical depth. From the spectra we derive a helium abundance of $N$(He)/$N$(H)$ = 0.073\pm0.031$ from the most reliable lines available; this is lower than most other estimates and indicates that the secondary is not helium-rich, as previous studies have suggested.

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arXiv2010

Near-infrared studies of the 2010 outburst of the recurrent nova U Scorpii

We present near-IR observations of the 2010 outburst of U Sco. JHK photometry is presented on ten consecutive days starting from 0.59 days after outburst. Such photometry can gainfully be integrated into a larger database of other multi-wavelength data which aim to comprehensively study the evolution of U Sco. Early near-IR spectra, starting from 0.56 days after outburst, are presented and their general characteristics discussed. Early in the eruption, we see very broad wings in several spectral lines, with tails extending up to ~10000km/s along the line of sight; it is unexpected to have a nova with ejection velocities equal to those usually thought to be exclusive to supernovae. From recombination analysis, we estimate an upper limit of 10^-4.64[+0.92.-0.74]Msun for the ejected mass.

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arXiv2010

The peculiar dust shell of Nova DZ Cru (2003)

We present Spitzer Space Telescope observations of the "peculiar variable" DZ Cru, identified by Rushton et al. (2008, MNRAS, 386, 289) as a classical nova. A dust shell, on which are superimposed a number of features, is prominent in the 5-35micron range some 4 years after eruption. We suggest that the dust in DZ Cru is primarily hydrogenated amorphous carbon in which aliphatic bands currently predominate, and which may either become predominantly aromatic as the dust is photo-processed by ultraviolet radiation from the stellar remnant, or more likely completely destroyed.

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arXiv2010

First Overtone CO Bands in the Giant Component of RS Ophiuchi: the 12C/13C Ratio in 2008

We present an analysis of a high resolution (R = 40000) infrared spectrum of the RS Oph secondary around the first overtone CO bands, obtained in 2008 May on the Gemini South 8 m. The 12CO and 13CO bands are well-resolved, and we compute synthetic spectra to determine the 12C/13C ratio. We find 12C/13C = 16 +/- 3, consistent with the interpretation of the secondary as red giant which has evolved beyond the first drege-up phase of evolution.

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arXiv2009

Changes in the red giant and dusty environment of the recurrent nova RS Ophiuchi following the 2006 eruption

We present near infrared spectroscopy of the recurrent nova RS Oph obtained on several occasions after its latest outburst in 2006 February. The 1-5 mircon spectra are dominated by the red giant, but the H I, He I, and coronal lines present during the eruption are present in all our observations. From the fits of the computed infrared spectral energy distributions to the observed fluxes we find T_eff=4200+/-200,K for the red giant. The first overtone CO bands at 2.3 micron, formed in the atmosphere of the red giant, are variable. The spectra clearly exhibit an infrared excess due to dust emission longward of 5 micron; we estimate an effective temperature for the emitting dust shell of 500K, and find that the dust emission is also variable, being beyond the limit of detection in 2007. Most likely, the secondary star in RS Oph is intrinsically variable.

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arXiv2009

Double radio peak and non-thermal collimated ejecta in RS Ophiuchi following the 2006 outburst

We report MERLIN, VLA, OCRA-p, VLBA, Effelsberg and GMRT observations beginning 4.5 days after the discovery of RS Ophiuchi undergoing its 2006 recurrent nova outburst. Observations over the first 9 weeks are included, enabling us to follow spectral development throughout the three phases of the remnant development. We see dramatic brightening on days 4 to 7 at 6 GHz and an accompanying increase in other bands, particularly 1.46 GHz, consistent with transition from the initial "free expansion" phase to the adiabatic expansion phase. This is complete by day 13 when the flux density at 5 GHz is apparently declining from an unexpectedly early maximum (compared with expectations from observations of the 1985 outburst). The flux density recovered to a second peak by approximately day 40, consistent with behaviour observed in 1985. At all times the spectral index is consistent with mixed non-thermal and thermal emission. The spectral indices are consistent with a non-thermal component at lower frequencies on all dates, and the spectral index changes show that the two components are clearly variable. The estimated extent of the emission at 22 GHz on day 59 is consistent with the extended east and west features seen at 1.7 GHz with the VLBA on day 63 being entirely non-thermal. We suggest a two-component model, consisting of a decelerating shell seen in mixed thermal and non-thermal emission plus faster bipolar ejecta generating the non-thermal emission, as seen in contemporaneous VLBA observations. Our estimated ejecta mass of 4+/-2x10^{-7} M_\odot is consistent with a WD mass of 1.4 M_\odot. It may be that this ejecta mass estimate is a lower limit, in which case a lower WD mass would be consistent with the data.

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