arXiv subjects

H. F. Stevance

Publications and source records attributed to H. F. Stevance.

35 records · Page 2Linked to original sources

arXiv2022

Observations of the luminous red nova AT 2021biy in the nearby galaxy NGC 4631

We present an observational study of the luminous red nova (LRN) AT\,2021biy in the nearby galaxy NGC\,4631. The field of the object was routinely imaged during the pre-eruptive stage by synoptic surveys, but the transient was detected only at a few epochs from $\sim 231$\,days before maximum brightness. The LRN outburst was monitored with unprecedented cadence both photometrically and spectroscopically. AT\,2021biy shows a short-duration blue peak, with a bolometric luminosity of $\sim 1.6 \times 10^{41}$\,erg\,s$^{-1}$, followed by the longest plateau among LRNe to date, with a duration of 210\,days. A late-time hump in the light curve was also observed, possibly produced by a shell-shell collision. AT\,2021biy exhibits the typical spectral evolution of LRNe. Early-time spectra are characterised by a blue continuum and prominent H emission lines. Then, the continuum becomes redder, resembling that of a K-type star with a forest of metal absorption lines during the plateau phase. Finally, late-time spectra show a very red continuum ($T_{\mathrm{BB}} \approx 2050$ K) with molecular features (e.g., TiO) resembling those of M-type stars. Spectropolarimetric analysis indicates that AT\,2021biy has local dust properties similar to those of V838\,Mon in the Milky Way Galaxy. Inspection of archival {\it Hubble Space Telescope} data taken on 2003 August 3 reveals a $\sim 20$\,\msun\ progenitor candidate with log\,$(L/{\rm L}_{\odot}) = 5.0$\,dex and $T_{\rm{eff}} = 5900$\,K at solar metallicity. The above luminosity and colour match those of a luminous yellow supergiant. Most likely, this source is a close binary, with a 17--24\,\msun\ primary component.

astro-ph.SR↗

arXiv2022

Progenitor, environment, and modelling of the interacting transient, AT 2016jbu (Gaia16cfr)

We present the bolometric lightcurve, identification and analysis of the progenitor candidate, and preliminary modelling of AT2016jbu (Gaia16cfr). We find a progenitor consistent with a $\sim$22--25~$M_{\odot}$ yellow hypergiant surrounded by a dusty circumstellar shell, in agreement with what has been previously reported. We see evidence for significant photometric variability in the progenitor, as well as strong H$α$ emission consistent with pre-existing circumstellar material. The age of the environment as well as the resolved stellar population surrounding AT2016jbu, support a progenitor age of $>$10 Myr, consistent with a progenitor mass of $\sim$22~$M_{\odot}$. A joint analysis of the velocity evolution of AT2016jbu, and the photospheric radius inferred from the bolometric lightcurve shows the transient is consistent with two successive outbursts/explosions. The first outburst ejected material with velocity $\sim$650$kms^{-1}$, while the second, more energetic event, ejected material at $\sim$4500$kms^{-1}$. Whether the latter is the core-collapse of the progenitor remains uncertain. We place a limit on the ejected $^{56}$Ni mass of $<$0.016$M_{\odot}$. Using the BPASS code, we explore a wide range of possible progenitor systems, and find that the majority of these are in binaries, some of which are undergoing mass transfer or common envelope evolution immediately prior to explosion. Finally, we use the SNEC code to demonstrate that the low-energy explosion within some of these binary systems, together with sufficient CSM, can reproduce the overall morphology of the lightcurve of AT2016jbu.

astro-ph.HE↗

arXiv2022

Photometric and spectroscopic evolution of the interacting transient AT 2016jbu (Gaia16cfr)

We present the results from a high cadence, multi-wavelength observation campaign of AT 2016jbu (aka Gaia16cfr), an interacting transient. This dataset complements the current literature by adding higher cadence as well as extended coverage of the lightcurve evolution and late-time spectroscopic evolution. Photometric coverage reveals that AT 2016jbu underwent significant photometric variability followed by two luminous events, the latter of which reached an absolute magnitude of M$_V\sim$-18.5 mag. This is similar to the transient SN 2009ip whose nature is still debated. Spectra are dominated by narrow emission lines and show a blue continuum during the peak of the second event. AT 2016jbu shows signatures of a complex, non-homogeneous circumstellar material (CSM). We see slowly evolving asymmetric hydrogen line profiles, with velocities of 500km$s^{-1}$ seen in narrow emission features from a slow moving CSM, and up to 10,000km$s^{-1}$ seen in broad absorption from some high velocity material. Late-time spectra ($\sim$+1 year) show a lack of forbidden emission lines expected from a core-collapse supernova and are dominated by strong emission from H, He i and Ca ii. Strong asymmetric emission features, a bumpy lightcurve, and continually evolving spectra suggest an inhibit nebular phase. We compare the evolution of H$α$ among SN 2009ip-like transients and find possible evidence for orientation angle effects. The light-curve evolution of AT 2016jbu suggests similar, but not identical, circumstellar environments to other SN 2009ip-like transients.

astro-ph.SR↗

arXiv2022

To be or not to be a black hole: detailed binary population models as a sanity check

We use the self-consistent stellar populations in the Binary Population A Spectral Synthesis (BPASS) models to assess whether NGC1850-BH1 is a black hole. Using search criteria based on reported physical properties in the literature we purposefully search for suitable systems with a black hole (or compact object) companion: we do not find any. Good matches to the observations are found in models where the bright component is a stripped star and the companion is natively (meaning we did not impose this in our search) 1 to 2.3 magnitudes fainter than the primary in the optical bands. This alone can explain the lack of detection of the companion in the MUSE spectra without the need to invoke rapid rotation, although the conservative mass transfer exhibited by these particular models is likely to lead to rapidly rotating companions which could further smear their spectroscopic signatures. We advise that future claims of unseen black holes in binary systems would benefit from exploring detailed binary evolution models of stellar populations as a sanity check

astro-ph.SR↗

arXiv2021

Estimating Transient Rates from Cosmological Simulations and BPASS

The detection rate of electromagnetic (EM) and gravitational wave (GW) transients is growing exponentially. As the accuracy of the transient rates will significantly improve over the coming decades, so will our understanding of their evolution through cosmic history. To this end, we present predicted rates for EM and GW transients over the age of the Universe using Binary Population and Spectral Synthesis (BPASS) results combined with four cosmic star formation histories (SFH). These include a widely used empirical SFH of Madau & Dickinson and those from three cosmological simulations: MilliMillennium, EAGLE and IllustrisTNG. We find that the choice of SFH significantly changes our predictions: transients with short delay times are most affected by the star formation rate, while long-delay time events tend to depend on the metallicity evolution of star formation. Importantly we find that the cosmological simulations have very different metallicity evolution that cannot be reproduced by the widely used metallicity model of Langer & Norman, which impacts the binary black hole merger and stripped-envelope supernovae rates in the local Universe most acutely. We recommend against using simple prescriptions for the metallicity evolution of the Universe when predicting the rates of events that can have long delay times and that are sensitive to metallicity evolution.

astro-ph.CO↗

arXiv2021

Binary pathways to SLSNe-I: SN 2017gci

Some hydrogen poor superluminous supernovae (SLSNe) exhibit bumps in the tails of their light-curves associated with hydrogen features in their late time spectra. Here we use the explosion parameters of one such SLSN -- SN 2017gci -- to search the stellar models of the Binary Population And Spectral Synthesis (BPASS) code for potential progenitors. We find good matches for a 30 $M_{\odot}$ progenitor star in a binary system and no matches from single star models. Common envelope and mass transfer after the giant branch, combined with increased mass loss from strong stellar winds immediately before death, allow the progenitor to lose its hydrogen envelope decades before the explosion. This results in a hydrogen poor SLSN and allows for delayed interaction of the ejecta with the lost stellar material

astro-ph.SR↗

arXiv2021

Using Artificial Intelligence to Shed Light on the Star of Biscuits: The Jaffa Cake

Before Brexit, one of the greatest causes of arguments amongst British families was the question of the nature of Jaffa Cakes. Some argue that their size and host environment (the biscuit aisle) should make them a biscuit in their own right. Others consider that their physical properties (e.g. they harden rather than soften on becoming stale) suggest that they are in fact cake. In order to finally put this debate to rest, we re-purpose technologies used to classify transient events. We train two classifiers (a Random Forest and a Support Vector Machine) on 100 recipes of traditional cakes and biscuits. Our classifiers have 95 percent and 91 percent accuracy respectively. Finally we feed two Jaffa Cake recipes to the algorithms and find that Jaffa Cakes are, without a doubt, cakes. Finally, we suggest a new theory as to why some believe Jaffa Cakes are biscuits.

astro-ph.IM↗

arXiv2020

A systematic aging method I: HII regions D118 and D119 in NGC 300

Accurately determining the age of H\,{ii} regions and the stars they host is as important as it is challenging. Historically the most popular method has been isochrone fitting to Hertzsprung-Russell Diagrams or Colour-Magnitude Diagrams. Here we introduce a different method for age determination using BPASS and hoki. We infer the most likely ages of the regions D118 and D119 NGC 300 to be log(age/years)=6.86$^{0.05}_{0.06}$ and we also deduce stellar mass and number counts by comparison with the BPASS models. We compare how our binary and single star models perform and find that the latter are unable to predict 20 per cent ($\pm$ 10 per cent) of our sample. We also discuss how results obtained from isochrone fitting would differ. We conclude that ages could be underestimated by ~0.2 dex and that the limitations of the isochrone method is not solely due to the lack of binary stars. We propose that the method presented here is more reliable and more widely applicable since it can be used on smaller samples. Alongside this study, we release new hoki features to allow easy implementation of this method.

astro-ph.SR↗

arXiv2020

Weighing in on black hole binaries with BPASS: LB-1 does not contain a 70M$_{\odot}$ black hole

The recent identification of a candidate very massive 70 M(Sun) black hole is at odds with our current understanding of stellar winds and pair-instability supernovae. We investigate alternate explanations for this system by searching the BPASS v2.2 stellar and population synthesis models for those that match the observed properties of the system. We find binary evolution models that match the LB-1 system, at the reported Gaia distance, with more moderate black hole masses of 4 to 7 M(Sun). We also examine the suggestion that the binary motion may have led to an incorrect distance determination by Gaia. We find that the Gaia distance is accurate and that the binary system is consistent with the observation at this distance. Consequently it is highly improbable that the black hole in this system has the extreme mass originally suggested. Instead, it is more likely to be representative of the typical black hole binary population expected in our Galaxy.

astro-ph.SR↗

arXiv2020

Evaluating the impact of binary parameter uncertainty on stellar population properties

Binary stars have been shown to have a substantial impact on the integrated light of stellar populations, particularly at low metallicity and early ages - conditions prevalent in the distant Universe. But the fraction of stars in stellar multiples as a function of mass, their likely initial periods and distribution of mass ratios are all known empirically from observations only in the local Universe. Each has associated uncertainties. We explore the impact of these uncertainties in binary parameters on the properties of integrated stellar populations, considering which properties and timescales are most susceptible to uncertainty introduced by binary fractions and whether observations of the integrated light might be sufficient to determine binary parameters. We conclude that the effects of uncertainty in the empirical binary parameter distributions are likely smaller than those introduced by metallicity and stellar population age uncertainties for observational data. We identify emission in the He II 1640 Angstrom emission line and continuum colour in the ultraviolet-optical as potential indicators of a high mass binary presence, although poorly constrained metallicity, dust extinction and degeneracies in plausible star formation history are likely to swamp any measurable signal.

astro-ph.GA↗

arXiv2020

The shape of SN 1993J re-analyzed

SN 1993J is one of the best studied Type IIb supernovae. Spectropolarimetric data analyses were published over two decades ago at a time when the field of supernova spectropolarimetry was in its infancy. Here we present a new analysis of the spectropolarimetric data of SN 1993J and an improved estimate of its interstellar polarization (ISP) as well as a critical review of ISP removal techniques employed in the field. The polarization of SN 1993J is found to show significant alignment on the $q-u$ plane, suggesting the presence of a dominant axis and therefore of continuum polarization. We also see strong line polarization features, including $\mathrm{Hβ}$, He\,{\sc i} $λ5876$, $\mathrm{Hα}$, He\,{\sc i} $λ6678$, He\,{\sc i} $λ7065$, and high velocity (HV) components of He\,{\sc i} $λ5876$ and $\mathrm{Hα}$. SN 1993J is therefore the second example of a stripped envelope supernova, alongside iPTF13bvn, with prominent HV helium polarization features, and the first to show a likely HV \halpha contribution. Overall, we determine that the observed features can be interpreted as the superposition of anisotropically distributed line forming regions over ellipsoidal ejecta. We cannot exclude the possibility of an off-axis energy source within the ejecta. These data demonstrate the rich structures that are inaccessible if solely considering the flux spectra but can be probed by spectropolarimetric observations. In future studies, the new ISP corrected data can be used in conjunction with 3D radiative transfer models to better map the geometry of the ejecta of SN 1993J.

astro-ph.SR↗

arXiv2020

Spectropolarimetry of the Type IIb SN 2008aq

We present optical spectroscopy and spectropolarimetry of the Type IIb SN 2008aq 16 days and 27 days post-explosion. The spectrum of SN 2008aq remained dominated by Halpha P Cygni profile at both epochs, but showed a significant increase in the strength of the helium features, which is characteristic of the transition undergone by supernovae between Type IIb and Type Ib. Comparison of the spectra of SN 2008aq to other Type IIb SNe (SN 1993J, SN 2011dh, and SN 2008ax) at similar epochs revealed that the helium lines in SN 2008aq are much weaker, suggesting that its progenitor was stripped to a lesser degree. SN 2008aq also showed significant levels of continuum polarisation at p_cont = 0.70 (+/- 0.22) % in the first epoch, increasing to p_cont = 1.21 (+/- 0.33) % by the second epoch. Moreover, the presence of loops in the q-u planes of Halpha and He I in the second epoch suggests a departure from axial symmetry.

astro-ph.SR↗

arXiv2019

Spectropolarimetry of stripped envelope core collapse supernovae and their progenitors

Over the past 30 years, spectropolarimetry has proven to be a great tool to probe the 3D geometry of core collapse supernovae (CCSNe). The number of high-quality multi-epoch spectropolarimetric data sets for CCSNe remains quite low, however, due to the challenging nature of such observations. In this Thesis we present and analyse the data of two Type IIb SNe (SN 2008aq - two epochs, SN 2011hs - seven epochs) and one Type Ic-bl (SN 2014ad - seven epochs). The latter is the most complete spectropolarimetric data set for a Type Ic-bl to date. We also provide a preliminary re-analysis of the spectropolarimetric data of SN 1993J. The new datasets demonstrate that the spectropolarimetry of stripped envelope supernovae is complex and varied, and although early studies of type IIb SNe showed similar behaviour to type IIP, it is now clear that this early trend is broken by a growing sample of SNe. Additionally, we identified potential signatures of a jet in SN 2014ad. Furthermore, a toy model to simulate spectropolarimetry from simple geometries (similar to Reilly et al. 2016) was created. A systematic approach to parameter space exploration was developped; this showed that unfortunately these models suffer extreme degeneracies and may not lead to accurate interpretations. Finally, spectropolarimetry can also be used to probe asymmetries caused by fast rotation in the potential progenitors of stripped envelope CCSNe. Although numerous Wolf-Rayet stars of type WN and WC have been observed with spectropolarimetry, no single WO stars have been studied to date. In our data of two Galactic WO stars, we found no line effect and could not sufficiently constrain the rotational velocity to exclude a collapsar scenario. Therefore, the absence of a line effect may not necessarily equate to a low rotational velocity.

astro-ph.SR↗

arXiv2019

The 3D shape of Type IIb SN 2011hs

We observed seven epochs of spectropolarimetry in optical wavelengths for the Type IIb SN 2011hs, ranging from -3 to +40 days with respect to V -band maximum. A high degree of interstellar polarization was detected (up to ~3 percent), with a peak lying blueward of 4500A. Similar behaviours have been seen in some Type Ia SNe, but had never been observed in a Type IIb. We find that it is most likely the result of a relative enhancement of small silicate grains in the vicinity of the SN. Significant intrinsic continuum polarization was recovered at -3 and +2 days (p = 0.55 +\- 0.12 percent and p = 0.75 +\- 0.11 percent, respectively). We discuss the change of the polarization angle across spectral lines and in the continuum as diagnostics for the 3D structure of the ejecta. We see a gradual rotation by about -50 degree in the continuum polarization angle between -2 and +18 days after V - band maximum. A similar rotation in He I λ5876, Hα and the Ca II infrared triplet seems to indicate a strong influence of the global geometry on the line polarization features. The differences in the evolution of their respective loops on the Stokes q - u plane suggest that line specific geometries are also being probed. Possible interpretations are discussed and placed in the context of literature. We find that the spectropolarimetry of SN 2011hs is most similar to that of SN 2011dh, albeit with notable differences.

astro-ph.SR↗

arXiv2018

SPLOT: a Snapshot survey for Polarised Light in Optical Transients

We present SPLOT, a small scale pilot survey to test the potential of snapshot (single epoch) linear imaging polarimetry as a supplementary tool to traditional transient follow-up. Transients exist in a vast volume of observational parameter space and polarimetry has the potential to highlight sources of scientific interest and add value to near real-time transient survey streams. We observed a sample of $\sim 50$ randomly selected optical transients with the EFOSC2 and SofI instruments, on the 3.6m New Technology Telescope (NTT) to test the feasibility of the survey. Our sample contained a number of interesting individual sources: a variety of supernovae, X-ray binaries, a tidal disruption event, blazar outbursts, and, by design, numerous transients of unknown nature. We discuss the results, both for the individual sources and the survey in detail. We provide an overview on the success and limitations of SPLOT and also describe a novel calibration method for removing instrumental polarisation effects from Nasymth-mounted telescopes. We find that a SPLOT-like survey would be a benefit to the large scale future transient survey streams such as LSST. The polarimetric measurements have added scientific value to a significant number of the sources and, most importantly, has shown the potential to highlight unclassified transient sources of scientific interest for further study.

astro-ph.HE↗

arXiv2018

Probing the rotational velocity of Galactic WO stars with spectropolarimetry

Oxygen sequence Wolf-Rayet stars (WO) are thought to be the final evolution phase of some high mass stars, as such they may be the progenitors of type Ic SNe as well as potential progenitors of broad-lined Ic and long gamma-ray bursts. We present the first spectropolarimetric observations of the Galactic WO stars WR93b and WR102 obtained with FORS1 on the VLT. We find no sign of a line effect, which could be expected if these stars were rapid rotators. We also place constraints on the amplitude of a potentially undetected line effect. This allows us to derive upper limits on the possible intrinsic continuum polarisation, and find P$_{\rm cont}$ < 0.077 percent and P$_{\rm cont}$ < 0.057 percent for WR93b and WR102, respectively. Furthermore, we derive upper limits on the rotation of our WO stars by considering our results in the context of the wind compression effect. We estimate that for an edge-on case the rotational velocity of WR93b is v$_{\rm rot}$ < 324 km/s while for WR102 v$_{\rm rot}$ < 234 km/s. These correspond to values of v$_{\rm rot}$/v$_{\rm crit}$ <19 percent and <10 percent, respectively, and values of log(j)<18.0 cm$^2$/s for WR93b and <17.6 cm^2 /s for WR102. The upper limits found on v$_{\rm rot}$/v$_{\rm crit}$ and log(j) for our WO stars are therefore similar to the estimates calculated for Galactic WR stars that do show a line effect. Therefore, although the presence of a line effect in single WR stars is indicative of fast rotation, the absence of a line effect does not rule out significant rotation, even when considering the edge-on scenario.

astro-ph.SR↗

arXiv2017

The evolution of the 3D shape of the broad-lined type Ic SN 2014ad

We present optical spectropolarimetry and spectroscopy of the broad-lined Type Ic (Ic-bl) SN 2014ad. Our spectropolarimetric observations cover 7 epochs, from -2 days to 66 days after V-band maximum, and the spectroscopic data were acquired from -2 days to +107 days. The photospheric velocity estimates showed ejecta speeds similar to those of SN 1998bw and other SNe associated with GRBs. The spectropolarimetric data revealed aspherical outer ejecta and a nearly spherical interior. The polarisation associated with O I λ7774 and the Ca II infrared triplet suggests a clumpy and highly asymmetrical distribution of these two species within the ejecta. Furthermore it was shown that the two line forming regions must have been spatially distinct and oxygen was found to have higher velocities than calcium. Another oxygen line-forming region was also identified much closer to the core of the explosion and distributed in a spherical shell. It is difficult to reconcile the geometry of the deeper ejecta with a jet driven explosion, but the high ejecta velocities of SN 2014ad are typical of those observed in SNe Ic-bl with GRBs, and the behaviour of the oxygen and calcium line-forming regions is consistent with fully jet-driven models. The metallicity of the host galaxy of SN 2014ad was also calculated and compared to that of the hosts of other SNe Ic-bl with and without GRBs, but due to the overlap in the two populations no conclusion could be drawn.

astro-ph.HE↗