arXiv subjects

Jesús A. Toalá

Publications and source records attributed to Jesús A. Toalá.

At least 19 recordsLinked to original sources

arXiv2026

Nova shells in 3D: I. A catalog of integral field spectroscopic observations

Classical novae eject material during thermonuclear eruptions on accreting white dwarfs, forming expanding shells whose structures and kinematics provide insights into the eruption mechanism and binary interaction. However, their intrinsic three-dimensional (3D) structures remain poorly constrained. We present the first 3D catalog of nova shells based on integral field spectroscopy, enabling us to investigate their structures beyond projection effects. We compiled observations of 19 nova shells obtained with MUSE at the VLT, complemented by new MEGARA observations at the GTC and WEAVE observations at the WHT. We constructed 3D visualizations of selected emission lines to explore the diversity of shell morphologies and kinematics. The shells reveal a wide variety of structures, including ellipsoidal or spherical shells, rings, bipolar features, and clumpy or irregular components, many of which are not apparent in 2D projections. Approximately 60% of the sample exhibits predominantly ellipsoidal or spherical shapes, while about 30% shows ring-like or bipolar structures. Several objects display strong line-dependent morphological differences, with [O III] or [N II] tracing distinct kinematic components from those seen in H$α$. Compared with imaging studies, integral field spectroscopy reveals a larger fraction of bipolar or toroidal shells. The differences between emission lines highlight the diverse excitation mechanisms and physical conditions within the ejecta. Finally, comparing expansion velocities in the plane of the sky and along the line of sight suggests low intrinsic aspect ratios, consistent with estimates from projected shell shapes.

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arXiv2026

Accretion in Binary Systems with Slow Stellar Winds

Wind accretion in binary systems is commonly described using the Bondi-Hoyle-Lyttleton (BHL) formalism. However, its standard implementation fails in the slow-wind regime, where the wind velocity of the donor star ($v_\mathrm{w}$) is comparable to or smaller than the orbital velocity of the accretor ($v_\mathrm{o}$). Tejeda & Toalá recently proposed a geometrical correction to the BHL formalism that accounts for the wind aberration caused by the binary's orbital motion, which tilts the accretion cylinder and reduces its effective cross-section. Here we present a suite of smoothed particle hydrodynamic simulations performed with PHANTOM to test the geometric correction to the wind accretion in binary systems, with particular interest in systems operating in the slow-wind regime. We explore circular configurations and directly measure mass accretion efficiencies from the simulations. Our results confirm that the standard BHL prescription systematically overestimates accretion rates for $v_\mathrm{w}/v_\mathrm{o}<1$, while the geometrically corrected model agrees with the simulated efficiencies with remarkable accuracy. A key finding is that the velocity relevant for accretion estimates is not the value derived from the unperturbed stellar wind, but the local gas velocity measured upstream of the accretor. The gravitational potential of the accretor perturbs the flow, altering the effective relative velocity and modifying the accretion efficiency, particularly for compact orbits. These results provide strong numerical support for the geometrically corrected framework and establish a physically motivated basis for modeling wind-fed accretion in interacting binaries, including symbiotic systems.

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arXiv2026

Formation and X-ray emission from hot bubbles in planetary nebulae - III. The impact of [Wolf-Rayet]-type winds

We use radiation-hydrodynamical simulations to investigate the formation and synthetic X-ray emission of hot bubbles within planetary nebulae (PNe) driven by the powerful winds of H-deficient, [Wolf-Rayet]([WR])-type stars. Our models, based on {\sc mesa} stellar evolution tracks for 1--3 M$_{\odot}$ progenitors, adopt a recent mass-loss rate prescription for [WR] stars and incorporate the enhanced radiative cooling of their C-rich material, comparing the results against standard H-rich PN models. The enhanced mass-loss in the [WR] models leads to an accelerated post-AGB evolution and a subsequent delay in hot bubble formation compared to their H-rich counterparts, as suggested by a previous work. By computing synthetic X-ray spectra that account for the mixed H-rich and H-deficient gas phases, we find that models incorporating [WR] winds exhibit significantly higher X-ray luminosities ($L_\mathrm{X}$) than their H-rich counterparts, but the emissivity-weighted plasma temperature of the X-ray-emitting gas converge to values of $T_\mathrm{X} = [1-3] \times 10^{6}$~K, regardless of whether the system follows a [WR]-type or an H-rich post-AGB evolutionary path. Our results reinforce previous suggestions that mixing is a key mechanism in generating the observed soft X-ray emission even for PN hosting [WR] central stars.

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arXiv2025

Magnetic fields in planetary nebulae detected through non-thermal radio continuum emission

Context. Planetary nebulae are shells ejected by low- and intermediate-mass stars. The slow wind ejected by the asymptotic giant branch star is compressed by a fast stellar wind to produce an expanding gaseous shell surrounding a hot bubble. The shell is a source of thermal radio emission which shows a spectral index between -0.1 and 2. Only two planetary nebulae are known to show non-thermal radio emission indicating magnetic fields and non-thermal electrons. Aims. The aim of this paper is verification of presence of magnetic fields of planetary nebulae. Magnetic fields can have a significant influence on shaping planetary nebulae. Methods. We observed a sample of northern planetary nebulae in radio continuum at 144 MHz with the Low Frequency Array. We combined our observations with archival observations at higher frequencies. Results. The spectral indices in 30 planetary nebulae were below -0.1, indicating non-thermal radio emission. The majority of this sample consists of bipolar planetary nebulae, which are known to originate from binary central stars. Most of the nebulae have sizes larger than 20 arcsec. Magnetic fields and nonthermal emission may be common in smaller planetary nebulae, but can be suppressed by thermal emission. Our results suggest that different mechanisms can be responsible for the origin of magnetic fields and non-thermal emission in planetary nebulae.

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arXiv2025

The Influence of the Accretion Disc Structure on X-ray Spectral States in Symbiotic Binaries

Symbiotic stars are binary systems where a white dwarf (WD) accretes material from the wind of an evolved, late-type companion. X-ray-emitting symbiotic systems are classified into $α$, $β$, $δ$, and $β/δ$ types, attributed to distinct physical mechanisms such as thermonuclear burning, wind interactions, and accretion-driven boundary layers. We present synthetic X-ray spectra derived from hydrodynamics simulations using the PHANTOM code, coupled with radiative-transfer calculations from SKIRT. We reproduce all X-ray spectral types by exploring different density structure of the accretion disc, the viewing angle, the plasma temperature of the boundary layer, and/or the presence of extended emission. The synthetic X-ray spectra consist of both absorbed and reflected components. In systems with massive, high-column density discs and viewing angles close to edge-on, the reflected continuum can dominate the X-ray emission. This effect is less pronounced in systems with low-mass, lower-column density discs. We explore i) systems going from $δ$ to $β$ states, ii) $δ$-types that become $β/δ$ sources, iii) the variability of the three Fe emission lines in the 6.0-7.0 energy range, and iv) the possible physical processes behind the $α$ sources. The observations from iconic symbiotic systems are discussed in line of the present models. Our framework offers predictive power for future X-ray monitoring and provides a path toward connecting accretion disc physics with observed spectral states in symbiotic binaries with accreting WDs.

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arXiv2025

Tiānguān ($ζ$ Tau) as a binary system consisting of a Be-star and an accreting White Dwarf: opening a gate to understanding enigmatic $γ$ Cas analogues

The analogues of $γ$ Cassiopea are binary early type Be stars which are X-ray bright with hard thermal spectra. The nature of companions in these stars and mechanisms of their X-ray emission remain enigmatic. Among the proposed ideas is the presence of an accretion disc around a white dwarf (WD) companion to the Be star donor. We use radiative transfer models including reflection physics in order to calculate the synthetic spectra of such systems, and assume that the hottest plasma is thermal and is located in the accretion disc boundary layer. The models are used to analyse the XMM-Newton observations of the $γ$ Cas analogue $ζ$ Tau (a.k.a. Tiānguān). Comparisons with X-ray-emitting symbiotic systems, particularly $δ$- and $β/δ$-type systems, support the idea that the hard X-ray emission in $ζ$ Tau is best explained by a WD accreting material expelled from the Be star. The plasma temperature and luminosity of the boundary layer associated with the accretion disc are used to estimate a mass accretion rate of $\dot{M}_\mathrm{acc} \approx 4\times 10^{-10}$ M$_\odot$ yr$^{-1}$, implying a nova recurrence time above 10$^{5}$ yr. Our analysis advances the understanding the production of hard X-ray emission in $γ$ Cas analogues, further supporting the idea of accreting WDs as companions of Be-stars in these systems.

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arXiv2025

CGCS 6306, another X-ray-emitting asymptotic giant branch star confirmed to be a symbiotic binary

A number of asymptotic giant branch (AGB) stars are known to exhibit UV excess and/or X-ray emission. These have been considered signposts of a hot white dwarf (WD) companion in a symbiotic system (SySt), but AGB stars are so bright that they easily outshine these companions hampering their detection at optical wavelengths. A recent multi-wavelength investigation on the X-ray-emitting AGB (X-AGB) star Y Gem has confirmed the presence of a WD companion and, thus, its SySt nature. Our goal is to explore the true nature of another X-AGB star, namely CGCS 6306, to investigate whether some objects from this group may in fact be unnoticed symbiotic systems with AGB donors. Optical spectra and photometric data, together with X-ray observations, have been analyzed to investigate the properties of the stellar components and accretion process in CGCS 6306. CGCS 6306 is a carbon Mira with a pulsation period of 362 days. Its optical spectrum exhibits the typical saw-shaped features of molecular absorptions in addition to H I and He I recombination and [O I] and [O III] forbidden emission lines. The H$α$ line profile is broad, which can be interpreted as evidence for an accretion disk. The X-ray spectrum is hard, typical of highly-extincted hot plasma emission, and the X-ray luminosity is $\approx10^{32}$ erg s$^{-1}$. The detection of high-excitation optical emission lines and the X-ray properties of CGCS 6306 confirm the presence of a WD companion, making it a bona-fide $δ$-type X-SySt. Its X-ray luminosity is comparable to that of Y Gem, the other X-AGB confirmed to be a SySt, which was found to exhibit a high accretion rate. The lack of suitable information on the UV and blue optical properties of CGCS 6306, however, precludes a definitive estimate of the accretion rate in this system. Since CGCS 6306 is a carbon Mira, it adds to the small group of Galactic carbon SySts.

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arXiv2025

Geometric correction for wind accretion in binary systems

The Bondi-Hoyle-Lyttleton (BHL) accretion model is widely used to describe how a compact object accretes material from a companion's stellar wind in binary systems. However, its standard implementation becomes inaccurate when the wind velocity ($v_\mathrm{w}$) is comparable to or less than the orbital velocity ($v_\mathrm{o}$), predicting non-physical accretion efficiencies above unity. This limits its applicability to systems with low wind-to-orbital velocity ratios ($w= v_\mathrm{w} / v_\mathrm{o} \leq 1$), such as symbiotic systems. We revisit the implementation of the BHL model and introduce a geometric correction factor that accounts for the varying orientation of the accretion cylinder relative to the wind direction. This correction ensures physically plausible accretion efficiencies ($η\leq 1$) for all $w$ in circular orbits. Our new implementation naturally predicts the flattening of the accretion efficiency observed in numerical simulations for $w < 1$, without the need for ad hoc adjustments. We also peer into the implications of our prescription for the less-explored case of eccentric orbits, highlighting the key role of the geometric correction factor in shaping the accretion process. We compare our predictions with numerical simulations, finding good agreement for a wide range of parameters. Applications to the symbiotic star R~Aqr and the X-ray binary LS 5039 are presented. This improved implementation offers a more accurate description of wind accretion in binary systems, with implications for stellar evolution, population synthesis, and observational data interpretation.

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arXiv2024

Non-thermal radio emission in Sakurai's Object

The very late thermal pulse (VLTP) affects the evolution of $\sim$20\% of 1--8\,$\mathrm M_\odot$ stars, repeating the last phases of the red giant within a few years and leading to the formation of a new, but hydrogen-poor nebula within the old planetary nebula (PN). The strong dust formation in the latter obscures the optical and near-infrared radiation of the star. We aimed to determine the reheating timescale of the central star in Sakurai's object, which is an important constraint for the poorly understood VLTP evolution. We observed the radio continuum emission of Sakurai's object for almost 20 years from 2004 to 2023. Continuous, multi-frequency observations proved to be essential to distinguish between phases dominated by photoionization and shock ionization. The flux density fluctuates by more than a factor 40 within months to years. The spectral index remained negative between 2006 and 2017 and is close to zero since 2019. The emission region is barely resolved since 2021. Non-thermal radio emission observed from 2004 to 2017 traces shocks induced by wind interactions due to discrete mass-loss events. Thermal emission dominates during the period 2019--2023 and may indicate photoionization of the nebula by the central star.

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arXiv2024

The X-ray rise and fall of the Recurrent Symbiotic System T CrB

We present the analysis of publicly available NuSTAR, Suzaku and XMM-Newton observations of the symbiotic recurrent nova T CrB covering the 2006.77-2022.66 yr period. The X-ray spectra are analysed by adopting a model that includes a reflection component produced by the presence of a disk that mimics the accretion disk and the immediate surrounding medium. Our best-fit model requires this disk to have a radius of 1 AU, effective thickness of 0.1 AU, averaged column density 10$^{25}$ cm$^{-2}$ and orientation of 50$^{\circ}$ with respect to the line of sight. This disk is about a factor of two larger than recent estimations for the accretion disk and its presence contributes significantly via reflection to the total X-ray flux detected from T CrB, which naturally produces the emission of the 6.4 keV Fe line. Our analysis suggests that the temperature of the boundary layer evolved from 14.8 keV in the steady-state phase (before 2016), to 2.8 keV in the 2017.24 epoch, to finally stabilise to about $\sim$8 keV in the subsequent epochs. These variations in the plasma temperature of the boundary layer are attributed to the evolution of the mass accretion rate ($\dot{M}_\mathrm{acc}$), which is estimated to have an averaged value of $\dot{M}_\mathrm{acc}$=2.6$\times10^{-8}$ M$_\odot$ yr$^{-1}$ for the current active phase. The presence of emission lines in the XMM-Newton RGS spectrum of 2017.24 prevents from adopting a black body emission model to fit the soft X-ray range. Instead, we use plasma emission models that suggest the presence of adiabatically-shocked gas produced by gas velocities of 110-200 km s$^{-1}$, very likely tracing jet-like ejections similar to what is found in other symbiotic systems. The analysis of X-ray and optical data together show that T CrB has a similar evolution as black hole binaries, accreting neutron stars and AGN in the hardness-intensity diagram.

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arXiv2024

Peering into the Wolf-Rayet phenomenon through [WO] and [WC] stars

Spectroscopic observations have shown for decades that the Wolf-Rayet (WR) phenomenon is ubiquitous among stars with different initial masses. Although much effort to understand the winds from massive WR stars has been presented in the literature, not much has been done for such type of stars in the low-mass range. Here we present an attempt to understand the winds from [WR]-type stars using results from spectral analyses with the full non-LTE stellar atmosphere code PoWR. These results are put into context with the properties of massive WR stars. We found that WC+[WC] stars and WO+[WO] stars create independent sequences in the mass-loss rate ($\dot{M}$) and modified wind momentum ($D_\mathrm{mom}$) versus luminosity ($L$) diagrams. Our analysis indicates that even when the winds of WR and [WR] stars become optically thin, there is no breakdown of the general mass-loss trend, contrary to the observed ``weak wind phenomenon'' in OB stars. We report that all WR-type stars studied here broadly define single sequences in the wind efficiency ($η$) versus transformed mass-loss rate ($\dot{M}_\mathrm{t}$), the $\dot{M}_\mathrm{t}$-$T_\mathrm{eff}$ diagram, and the $(L, T_\mathrm{eff}, \dot{M})$ space, which suggest these to be fundamental properties of the WR phenomenon (regardless of the mass range), at least for WR-type stars of the O and C sequences. Our analytical estimations could drive computations of future stellar evolution models for WR-type stars.

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arXiv2024

Reflection physics in X-ray-emitting Symbiotic Stars

X-ray-emitting symbiotic stars exhibit a variety of spectral shapes classified as $α$, $β$, $γ$, $δ$ and $β$/$δ$ types, which have been attributed to different phenomena such as thermonuclear burning on the surface of the white dwarf (WD) component, shocks between winds and jets with the red giant companion's extended atmosphere, the presence of heavily extinguished hot plasma from the inner region from an accretion disk and/or a combination of these. However, there is observational evidence that this classification scheme is not definite and, for example, some sources change from one type to another within months or years. In this work, it is proposed that a simple disk-like model can be used to explain the X-ray properties observed from reflection dominated symbiotic stars. For this purpose we use the Stellar Kinematics Including Radiative Transfer (SKIRT) code, which has been recently upgraded to include radiative transfer from X-ray photons. It is found that thee properties of the accretion disk (geometry and density) in combination with the viewing angle can be invoke to explain the spectral properties of $β$, $δ$ and $β/δ$ X-ray-emitting symbiotic stars. Spectral variations and type swaps observed for some X-ray-emitting sources can also be explained by variations in the disk properties.

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arXiv2023

A comprehensive hydrodynamical study of SB DEM L50: understanding off-centre SNe and soft X-ray luminosity

The superbubbles (SBs) carved in the interstellar medium by stellar winds and supernovae (SNe) are filled with hot ($T>$10$^{6}$~K) gas that produces soft X-ray emission (0.3-2.0\,keV). Models that assume a constant density medium and central SNe events fail to reproduce the soft X-ray luminosity that is observed in some SBs. We address this problem by generating models that trace the history of SNe in the SB and that produce off-centre SNe, that account for the missing soft X-ray emission. We test the models against archival, radio, optical, and X-ray observations of the SB DEM\,L50 located in the Large Magellanic Cloud. The soft X-ray properties of DEM L50, including its high luminosity, makes it a perfect candidate to test our models. Furthermore, the multiple wave-band observations of this object will help us to assess how well our models can reproduce other SB properties beside its soft X-ray properties. We find that a configuration where DEM L50 forms at the edge of a filament reproduces the observed soft X-ray luminosity, optical morphology, shell velocity, and swept-up mass of neutral gas. This configuration is supported by IR observations of the LMC. In addition, we find that off-centre SNe, which enhance soft X-ray emission, naturally occur for all of the initial ambient conditions we tested in our models. Finally, we show that an off-centre SN can explain the observed soft X-ray luminosity of DEM L50, and that the resulting luminosity is consistent with a plasma in non-equilibrium ionisation.

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arXiv2022

Spectral variability of the born-again ejecta in A 58

Born-again planetary nebulae (PNe) allow investigating stellar evolution, dust production, and nebular shocks in human timescales. Here we present an analysis of multi-epoch optical spectroscopic observations of the born-again PN A 58 around V605 Aql, which experienced a very late thermal pulse (VLTP) about a century ago. The H-deficient ejecta has experienced a considerable brightening in the time period considered, from 1996 to 2021, with notable changes also in many emission line ratios. Neither the reduction of the extinction caused by the dilution of the ejecta nor the increase of the ionizing photon flux from the central star seem capable to produce these spectral changes, which are instead attributed to shocks in the bipolar H-poor outflow dissociating molecular material and propagating through the outer nebula.

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arXiv2022

Catching the Butterfly and the Homunculus of $η$ Carinae with ALMA

The nature and origin of the molecular gas component located in the circumstellar vicinity of $η$ Carinae are still far from being completely understood. Here, we present Atacama Large Millimeter/Submillimeter Array (ALMA) CO(3$-$2) observations with a high angular resolution ($\sim$0.15$''$), and a great sensitivity that are employed to reveal the origin of this component in $η$ Carinae. These observations reveal much higher velocity ($-$300 to $+$270 km s$^{-1}$) blue and redshifted molecular thermal emission than previously reported, which we associate with the lobes of the Homunculus Nebula, and that delineates very well the innermost contours of the red- and blue-shifted lobes likely due by limb brightening. The inner contour of the redshifted emission was proposed to be a {\it disrupted torus}, but here we revealed that it is at least part of the molecular emission originated from the lobes and/or the expanding equatorial skirt. On the other hand, closer to systemic velocities ($\pm$100 km s$^{-1}$), the CO molecular gas traces an inner butterfly-shaped structure that is also revealed at NIR and MIR wavelengths as the region in which the shielded dust resides. The location and kinematics of the molecular component indicate that this material has formed after the different eruptions of $η$ Carinae.

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arXiv2022

An XMM-Newton EPIC X-ray view of the Symbiotic Star R~Aquarii

We present the analysis of archival XMM-Newton European Photon Imaging Camera (EPIC) X-ray observations of the symbiotic star R Aquarii. We used the Extended Source Analysis Software (ESAS) package to disclose diffuse soft X-ray emission extending up to 2.2 arcmin ($\approx$0.27 pc) from this binary system. The depth of these XMM-Newton EPIC observations reveal in unprecedented detail the spatial distribution of this diffuse emission, with a bipolar morphology spatially correlated with the optical nebula. The extended X-ray emission shares the same dominant soft X-ray-emitting temperature as the clumps in the jet-like feature resolved by Chandra in the vicinity of the binary system. The harder component in the jet might suggest that the gas cools down, however, the possible presence of non-thermal emission produced by the presence of a magnetic field collimating the mass ejection can not be discarded. We propose that the ongoing precessing jet creates bipolar cavities filled with X-ray-emitting hot gas that feeds the more extended X-ray bubble as they get disrupted. These EPIC observations demonstrate that the jet feedback mechanism produced by an accreting disk around an evolved, low-mass star can blow hot bubbles, similar to those produced by jets arising from the nuclei of active galaxies.

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arXiv2022

First images of the molecular gas around a born-again star revealed by ALMA

Born-again stars allow probing stellar evolution in human timescales and provide the most promising path for the formation of hydrogen-deficient post-asymptotic giant branch objects, but their cold and molecular components remain poorly explored. Here we present ALMA observations of V605 Aql that unveil for the first time the spatio-kinematic distribution of the molecular material associated to a born-again star. Both the continuum and molecular line emission exhibit a clumpy ring-like structure with a total extent of $\approx$1$^{\prime\prime}$ in diameter. The bulk of the molecular emission is interpreted as being produced in a radially-expanding disk-like structure with an expansion velocity v$_{\rm exp}$$\sim$90 km s$^{-1}$ and an inclination $i$$\approx$60$^{\circ}$ with respect to the line-of-sight. The observations also reveal a compact high-velocity component, v$_{\rm exp}$$\sim$280 km s$^{-1}$, that is aligned perpendicularly to the expanding disk. This component is interpreted as a bipolar outflow with a kinematical age $τ$$\lesssim$20 yr, which could either be material that is currently being ejected from V605 Aql, or it is being dragged from the inner parts of the disk by a stellar wind. The dust mass of the disk is in the range $M_{\rm dust}$$\sim$0.2-8$\times$10$^{-3}$ M$_{\odot}$, depending on the dust absorption coefficient. The mass of the CO is $M_{\rm CO}$$\approx$1.1$\times10^{-5}$ $M_{\odot}$, which is more than three orders of magnitude larger than the mass of the other detected molecules. We estimate a $^{12}$C/$^{13}$C ratio of 5.6$\pm$0.6, which is consistent with the single stellar evolution scenario in which the star experienced a very late thermal pulse instead of a nova-like event as previously suggested.

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arXiv2020

A Radio Pinwheel Emanating from WR147

Wolf-Rayet (WR) stars are evolved massive stars, presumably on their way to becoming supernova. They are characterized by high luminosities and fast and dense stellar winds. We have detected signs of a radio continuum pinwheel associated with WR147, a nitrogen-rich WR star with spectral subtype WN8. These structures are known around a handful of late-type carbon-rich WR stars with massive companions were the dust formed at the zone where the two winds collide produces a plume of dense gas and dust that is carried out with the WR wind. As the binary system rotates, an Archimedean spiral detectable in the infrared is formed. The resulting pinwheel contains information on wind speeds, wind-momentum ratio and orbital parameters. However, WR147 is a WN star and the formation of dust is unlikely so a different emission mechanism must be at work. Our analysis of the data suggests that in this case the emission is dominantly of non-thermal nature (synchrotron), although we cannot exclude that some clumps could be brighter in free-free emission. It is possible that the pinwheels associated with WN stars will be detectable only as non-thermal emitters at radio wavelengths. From the characteristics of the pinwheel we estimate a period of 1.7 years for the binary system (the WN8 star and a companion yet undetected directly) responsible for the pinwheel.

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