arXiv subjects

Richard Ignace

Publications and source records attributed to Richard Ignace.

At least 19 recordsLinked to original sources

arXiv2026

Stellar winds of O-type stars traced by high ionization fine-structure emission lines with JWST/MIRI

We investigate the presence of high ionization fine-structure emission lines across a range of 22 OB-type stars observed with JWST-MIRI as part of calibration programmes and the WISCI and MEAD projects. MIR wind emission is detected in 4 late O-dwarfs (O8 V - O9 V), 1 early O-dwarf (O5 V) and there are tentative detections in an additional 3 stars (O8.5 II, O8.5 IV and O9 I). We measure the wind speeds and make estimates of lower limits on the mass-loss rates of 5 O-type stars from broad, flat-topped emission in the fine-structure line of [Ne V] 14.3micron. We find terminal wind speeds that are generally in agreement with empirical trends, but note that in some cases the wind speeds are surprisingly low. We highlight two main takeaways from this sample, which combine to establish an exciting new window into the winds of massive stars. First, a new diagnostic ability is gained from lines formed at much higher ionization, larger spatial extent, and longer wavelengths than typical wind diagnostics. Secondly, there is frequent incidence of MIR emission in O-type stars, even in the 'weak-wind' regime where wind emission is often not detected in the UV and optical.

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arXiv2025

Polarization from Rapidly Rotating Massive Stars

Stellar rotation has long been recognized as important to the evolution of stars, by virtue of the chemical mixing it can induce and how it interacts with binary mass transfer. Binary interaction and rapid rotation are both common in massive stars and involve processes of angular momentum distribution and transport. An important question is how this angular momentum transport leads to the creation of two important classes of rapidly rotating massive stars, Be stars defined by disklike emission lines, and Bn stars defined by rotationally broadened absorption lines. A related question is what limits this rotation places on how conservative the mass transfer can be. Central to addressing these issues is knowledge of how close to rotational break-up stars can get before they produce a disk. Here we calculate diagnostics of this rotational criticality using the continuum polarization arising from a combination of rotational stellar distortion (i.e., oblateness) and redistribution of stellar flux (i.e., gravity darkening), and compare polarizations produced in the von Zeipel approximation with the approach of Espinosa Lara & Rieutord (ELR). Both produce similar photospheric polarizations that rise significantly in the far ultraviolet (FUV) for B stars, with a stronger signal in the von Zeipel case. For early main-sequence and subgiant stars, it reaches a maximum of ~1% at 140 nm for stars rotating at 98% of critical, when seen edge-on. Rotational rates above 80% critical result in polarizations of several tenths of a percent, at high inclination. These predicted stable signal strengths indicate that determinations of near-critical rotations in B stars could be achieved with future spectropolarimetric instrumentation that can reach deep into the FUV, such as CASSTOR, the Polstar mission concept, or the POLLUX detector design.

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arXiv2025

Chandra/HETG and NuSTAR Observations of V750 Ara, a $\gamma\,$Cas-type Star

We present 197 ks HETG and 95 ks NuSTAR spectra of the $\gamma\,$Cas-type object V750 Ara. The high-resolution X-ray spectra show that the target is similar to other objects of this class. Data are interpreted under the assumption that the X-rays come from an accreting white dwarf, and our analysis implies an accretion rate of about $3\times10^{-11}M_\odot\mathrm{yr}^{-1}$. Emission lines are weak, and predominantly from hydrogen-like ions: Mg XII, Si XIV, and S XVI. H-like and He-like Fe are both present, but Fe K$\alpha$ fluorescence is weak, being significantly detected only in the NuSTAR spectrum, but was not obviously detected in the HETG dispersed or zeroth-order spectra. The flux was variable above a level expected by Poisson statistics. There were no significant changes in the spectral hardness, though we are limited by lack of soft signal below 1 keV. Emission lines of Mg and Si were strong enough to measure velocity offsets and widths which were found to be marginally inconsistent. The H-like Mg line is consistent with instrumental broadening only, but shows a 300 km/s blueshift. He-like Mg and H-like Si lines have no significant shift in velocity but are broadened by about 1000 km/s. This suggests either different physical origins or velocity structure differing with plasma temperature.

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arXiv2025

A Multiwavelength View of $\rho$ Oph I: Resolving the X-ray Source Between A and B

We present a multiwavelength analysis of the central stellar pair of $\rho$ Oph, components A and B. Using recent high-resolution \textit{Chandra X-ray Observatory} observations, we demonstrate with high confidence that the dominant X-ray source is $\rho$ Oph B, while $\rho$ Oph A is comparatively X-ray faint. This result contrasts with earlier \textit{XMM-Newton} observations, which, due to limited spatial resolutions, attributed the X-ray emission to $\rho$ Oph A. An analysis of $\rho$ Oph B's X-ray light curves and spectra reveals properties more consistent with a cool star than a hot star. We therefore propose that $\rho$ Oph B is an Algol-like binary system, consisting of a B-type primary and an active, X-ray-emitting GK-type companion.

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arXiv2025

X-ray Polarimetry of Accreting White Dwarfs: A Case Study of EX Hydrae

We present the first first X-ray polarization measurements of a white dwarf, the intermediate polar EX Hya. We measured significant polarization only in the 2 -- 3 keV energy band with a polarization degree of 8 percent at a $3\sigma$ significance. No significant polarization was detected above 3 keV, which we attribute to the higher energy bands having lower signal-to-noise. We found that the scattering surface detected by the IXPE is nearly perpendicular to the optical scattering plane, showing that the X-ray scattering surface is the WD and close to the base of the accretion column. Finally, we show how the polarization can be used to estimate the height of the accretion shock above the white dwarf's surface.

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arXiv2025

New Frontiers in the Study of Magnetic Massive Stars with the Habitable Worlds Observatory

High-mass stars are notable for several reasons: they are characterized by strong winds, which inject momentum and enriched material into their surroundings, and die spectacularly as supernovae, leaving behind compact remnants and heavy elements (such as those that make life on Earth possible). Despite their relative rarity, they play a disproportionate role in the evolution of the galaxies that host them, and likely also played a significant role in the early days of the Universe. A subset ($\sim$10\%) of these stars was also found to host magnetic fields on their surface. These fields impact their evolution, and may lead to exotic physics (e.g., heavy stellar-mass black holes, pair-instability supernovae, magnetars, etc.). However, the detection and measurement of magnetic fields is limited, due to current instrumentation, to nearby massive stars in the Milky Way. To truly understand how magnetism arises in massive stars, and what role it might have played in earlier stages of our Universe, we require next-generation hardware, such as the proposed near-infrared-to-ultraviolet spectropolarimeter Pollux, on the Habitable Worlds Observatory (HWO). In this contribution, we detail how Pollux @ HWO will enable new frontiers in the study of magnetic massive stars, delivering results that will profoundly impact the fields of stellar formation, stellar evolution, compact objects, and stellar feedback.

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arXiv2025

Methods to Test the Source of the Extreme Gas Motions in WS 35

We present theoretical arguments toward the plausibility of a stellar wind to explain the 16000 km s$^{-1}$ line broadening in the optical spectra of WS 35, the central star in the Pa 30 nebula. The wind model is discussed in the context of super-Eddington flows. We argue that WS 35 potentially occupies a new regime of wind driving theory as the first metal-only wind. While this framework provides a promising avenue for explaining the high speed flow, questions remain about the source's true nature. We further describe how future radio observations can provide an independent test of the spherical wind scenario. A magnetically channeled wind would likely produce a relatively flat and bright radio spectral energy distributions. By contrast a spherical wind should result in a thermal radio spectrum with a canonical continuum slope of $\nu^{0.6}$, and a brightness level consistent with the currently predicted mass-loss rate.

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arXiv2025

Variable Polarization of WR~31a: Binary Companion or Co-Rotating Interaction Region?

WR 31a (Hen 3-519) is likely a post-luminous blue variable (LBV) star that is evolving to become a classical Wolf-Rayet star. Multicolor (UBVR) photopolarimetric observations of WR 31a were obtained over nine nights in early 2007. The linear polarization data of WR 31a trace a "loop" structure in a Stokes Q-U diagram, which is similar in all four passbands. After mean subtraction, the four loops align to form a single overall pattern. Such loops can be expected to arise from binary systems. We test the binary hypothesis with two models. The data are fit for a strictly circular orbit to derive an orbital period of 16.7 d, requiring a high inclination perspective of $i\sim 80^\circ$. We also consider an elliptical orbit under simplifying assumptions, yielding a match for $i\sim 75^\circ$ with eccentricity $e\sim 0.5$ and a longer orbital period of about 70 d. The prevalence of binarity among massive stars is well-known; the prospect of detecting a binary companion during the post-LBV stage of WR 31a would add to an emerging narrative of diverse interactions between massive multiple components as a function of evolutionary stage. However, if the loop originates because of a co-rotating interaction region (CIR), then the rotation period could be 8.5 d or 17 d. This would give an estimated equatorial rotation speed of 95 or 190 km/s. Either of these is a significant fraction of the estimated critical speed of rotational break-up at 320 km/s (for an Eddington factor of $\Gamma=0$).

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arXiv2025

Spectropolarimetry for Discerning Geometry and Structure in Circumstellar Media of Hot Massive Stars

Spectropolarimetric techniques are a mainstay of astrophysical inquiry, ranging from Solar System objects to the Cosmic Background Radiation. This review highlights applications of stellar polarimetry for massive hot stars, particularly in the context of ultraviolet (UV) spaceborne missions. The prevalence of binarity in the massive star population and uncertainties regarding the degree of rotational criticality among hot stars raises important questions about stellar interactions, interior structure, and even the lifetimes of evolutionary phases. These uncertainties have consequences for stellar population synthesis calculations. Spectropolarimetry is a key tool for extracting information about stellar and binary geometries. We review methodologies involving electron scattering in circumstellar envelopes; gravity darkening from rapid rotation; spectral line effects including the (a) "line effect", (b) Ohman effect, and (c) Hanle effect; and the imprint of interstellar polarization on measurements. Finally, we describe the Polstar UV spectropolarimetric SMEX mission concept as one means for employing these diagnostics to clarify the state of high rotation and its impacts for massive stars.

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arXiv2025

X-Shooting ULLYSES: massive stars at low metallicity: XII. The clumped winds of O-type (super)giants in the Large Magellanic Cloud

Mass loss governs the evolution of massive stars and shapes the stellar surroundings. To quantify the impact of the stellar winds we need to know the exact mass-loss rates; however, empirical constraints on the rates are hampered by limited knowledge of their small-scale wind structure or 'wind clumping'. We aim to improve empirical constraints on the mass loss of massive stars by investigating the clumping properties of their winds, in particular the relation between stellar parameters and wind structure. We analyse the optical and ultraviolet spectra of 25 O-type (super)giants in the LMC, using the model atmosphere code Fastwind and a genetic algorithm. We derive stellar and wind parameters including detailed clumping properties, such as the amount of clumping, the density of the interclump medium, velocity-porosity of the medium, and wind turbulence. We obtain stellar and wind parameters for 24 of our sample stars and find that the winds are highly clumped, with an average clumping factor of $f_{\rm cl}=33\pm14$, an interclump density factor of $f_{\rm ic}=0.2\pm0.1$, and moderate to strong velocity-porosity effects. The scatter around the average values of the wind-structure parameters is large. With the exception of a significant, positive correlation between the interclump density factor and mass loss, we find no dependence of clumping parameters on either mass-loss rate or stellar properties. In the luminosity range that we investigate, the empirical and theoretical mass-loss rates both have a scatter of about 0.5~dex, or a factor 3. Within this uncertainty, the empirical rates and the theoretical predictions agree. The origin of the scatter of the empirical mass-loss rates requires further investigation. It is possible that our description of wind clumping is still not sufficient to capture effects of the structured wind; this could contribute to the scatter.

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arXiv2024

A Time-Dependent Spectral Analysis of $\gamma$ Cassiopeiae

We investigated the temporal and spectral features of $\gamma$ Cassiopeiae's X-ray emission within the context of the white dwarf accretion hypothesis. We find that the variabilities present in the X-ray data show two different signals, one primarily due to absorption and the other due to flickering like in non-magnetic cataclysmic variables. We then use this two-component insight to investigate previously un-reported simultaneous XMM and NuSTAR data. The model fitting results find white dwarf properties consistent with optical studies alongside a significant secondary, thermal source. We propose a secondary shock between the Be decretion disk and white dwarf accretion disk as the source. Finally, we analyzed a unique, low-count rate event of the XMM light curve as potential evidence for the white dwarf encountering Be decretion disk structures.

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arXiv2024

The Polstar UV Spectropolarimetry Mission

The Polstar small explorer concept is for an ultraviolet (UV) spectropolarimetry space telescope mission with a focus on massive star astrophysics. The instrument waveband will be from 115 nm - 286 nm for spectroscopy and 122 nm - 286 nm for polarimetry. All 4 Stokes parameters, IQUV, will be measured at a resolving power of R=20,000 (15 km/s velocity resolution). The telescope aperture will be 40 cm with an effective area of about 22 cm^2 at a reference wavelength of 150 nm. The thrust of the science goals will be to determine the astrophysics of angular momentum exchange and transport, and consequences for massive star properties and evolution. This includes the effects of rapid to critical rotation for individual stars (magnetic and non-magnetic), and the effects of mass transfer for massive binaries, including identification of stripped core stars. If selected by the NASA/SMEX program, Polstar would launch around 2031 and observe ~300 stars to achieve science goals. The mission will include a Guest Observer program to advance discovery in other areas of astrophysics.

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arXiv2024

Chandra HETG X-ray Spectra and Variability of $\pi$ Aqr, a $\gamma$ Cas-type Be star

High-resolution X-ray spectra of $\pi\,$Aqr, a $\gamma\,$Cas-type star, obtained with the Chandra/HETG grating spectrometer, revealed emission lines of H-like ions of Mg, Si, S, and Fe, a strong, hard continuum, and a lack of He-like ions, indicating the presence of very hot thermal plasma. The X-ray light curve showed significant fluctuations, with coherent variability at period of about 3400 seconds in one observation. The hardness ratio was relatively constant except for one observation in which the spectrum was much harder and more absorbed. We interpret the X-ray emission as arising from accretion onto the secondary, which is likely a magnetic white dwarf, an intermediate polar system.

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arXiv2023

Using CHIRON Spectroscopy to Test the Hypothesis of a Precessing Orbit for the WN4 star EZ CMa

The bright WN4 star EZ CMa exhibits a 3.77 day periodicity in photometry, spectroscopy, and polarimetry but the variations in the measurements are not strictly phase-locked, exhibiting changes in reference times, amplitudes, and the shape of the variability happening over times as short as a few weeks. Recently, 137 days of contiguous, variable photometry from BRITE-Constellation was interpreted as caused either by large-scale dense wind structures modulated by rotation, or by a fast-precessing binary having a slightly shorter 3.626 day orbital period and a fast apsidal motion rate of $1315^\circ\,\text{yr}^{-1}$. We aim at testing the latter hypothesis through analysis of spectroscopy and focus on the N\,{\sc v} $\lambda\,4945$ line. We derive an orbital solution for the system and reject the 3.626 day period to represent the variations in the radial velocities of EZ CMa. An orbital solution with an orbital period of 3.77 days was obtained but at the cost of an extremely high and thus improbable apsidal motion rate. Our best orbital solution yields a period of $3.751\pm0.001$\,days with no apsidal motion. We place our results in the context of other variability studies and system properties. While we cannot fully reject the precessing binary model, we find that the corotating interaction region (CIR) hypothesis is better supported by these and other data through qualitative models of CIRs.

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arXiv2023

Survey of X-rays from Massive Stars Observed at High Spectral Resolution with Chandra

Identifying trends between observational data and the range of physical parameters of massive stars is a critical step to the still-elusive full understanding of the source, structure, and evolution of X-ray emission from the stellar winds, requiring a substantial sample size and systematic analysis methods. The \emph{Chandra} data archive as of 2022 contains 37 high resolution spectra of O, B, and WR stars, observed with the \emph{Chandra}/HETGS and of sufficient quality to fit the continua and emission line profiles. Using a systematic approach to the data analysis, we explore morphological trends in the line profiles (i.e., O, Ne, Mg, Si) and find that the centroid offsets of resolved lines versus wavelength can be separated in three empirically-defined groups based on the amount of line broadening and centroid offset. Using \ion{Fe}{17} (15.01 \AA, 17.05 \AA) and \ion{Ne}{10} $\alpha$ (12.13 \AA) lines which are prevalent among the sample stars, we find a well-correlated linear trend of increasing Full Width Half Maximum (FWHM) with faster wind terminal velocity. The H-like/He-like total line flux ratio for strong lines displays different trends with spectral class depending on ion species. Some of the sources in our sample have peculiar properties (e.g., magnetic and $\gamma$ Cas-analogue stars) and we find that these sources stand out as outliers from more regular trends. Finally, our spectral analysis is presented summarily in terms of X-ray spectral energy distributions in specific luminosity for each source, plus tables of line identifications and fluxes.

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arXiv2023

The RS Oph outburst of 2021 monitored in X-rays with NICER

The 2021 outburst of the symbiotic recurrent nova RS Oph was monitored with the Neutron Star Interior Composition Explorer Mission (NICER) in the 0.2-12 keV range from day one after the optical maximum, until day 88, producing an unprecedented, detailed view of the outburst development. The X-ray flux preceding the supersoft X-ray phase peaked almost 5 days after optical maximum and originated only in shocked ejecta for 21 to 25 days. The emission was thermal; in the first 5 days only a non-collisional-ionization equilibrium model fits the spectrum, and a transition to equilibrium occurred between days 6 and 12. The ratio of peak X-rays flux measured in the NICER range to that measured with Fermi in the 60 MeV-500 GeV range was about 0.1, and the ratio to the peak flux measured with H.E.S.S. in the 250 GeV-2.5 TeV range was about 100. The central supersoft X-ray source (SSS), namely the shell hydrogen burning white dwarf (WD), became visible in the fourth week, initially with short flares. A huge increase in flux occurred on day 41, but the SSS flux remained variable. A quasi-periodic oscillation every ~35 s was always observed during the SSS phase, with variations in amplitude and a period drift that appeared to decrease in the end. The SSS has characteristics of a WD of mass >1 M(solar). Thermonuclear burning switched off shortly after day 75, earlier than in 2006 outburst. We discuss implications for the nova physics.

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arXiv2023

A Multi-Year Photopolarimetric Study of the Semi-Regular Variable V CVn and Identification of Analogue Sources

The semi-regular variable star V Canum Venaticorum (V CVn) is well-known for its unusual linear polarization position angle (PA). Decades of observing V CVn reveal a nearly constant PA spanning hundreds of pulsation cycles. This phenomenon has persisted through variability that has ranged by 2 magnitudes in optical brightness and through variability in the polarization amplitude over 0.3% and 6.9%. Additionally, the polarization fraction of V CVn varies inversely with brightness. This paper presents polarization measurements obtained over three pulsation cycles. We find that the polarization maximum does not always occur precisely at the same time as the brightness minimum. Instead, we observe a small lead or lag in relation to the brightness minimum, spanning a period of a few days up to three weeks. Furthermore, the PA sometimes exhibits a non-negligible rotation, especially at lower polarization levels. To elucidate the unusual optical behavior of V CVn, we present a list of literature sources that also exhibit polarization variability with a roughly fixed PA. We find this correlation occurs in stars with high tangential space velocities, i.e., "runaway" stars, suggesting that the long-term constant PA is related to how the circumstellar gas is shaped by the star's high-speed motion through the interstellar medium.

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arXiv2022

Constraints on Clumps in the Representative Wind of the WN8 Wolf-Rayet star HD96548 = WR40 with Simultaneous Broadband Light and Linear-Polarisation Variability

We report precision ground-based broadband optical intensity and linear-polarisation light-curves for the sky's brightest WN8 star, WR40. WN8 stars are notorious for their high level of variability, stemming from stochastic clumps in their strong winds that are slower and less hot than the winds of most other Wolf-Rayet (WR) stars. We confirm previous results that many WR stars display an amplitude of variability that is an order-of-magnitude higher in photometry than in polarimetry. For the first time, the unique nature of near simultaneity of our photometric and polarimetric observations of WR40 allows us to check whether the two types of variability show correlated behaviour, of which we find none. Assuming simple temporal functions for the brightness and polarisation of individual clumps, a model for simulated light curves is found to reproduce the properties of the observations, specifically the absence of correlation between photometric and polarimetric variations, the ratio of standard deviations for photometric and polarimetric variability, and the ratio of the average intrinsic polarisation relative to its standard deviation. Mapping the solution for time variability to a spatial coordinate suggests that the wind clumps of WR40 are free-free emitting in addition to light scattering.

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