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Physics Astrophysics Solar and Stellar Astrophysics

astro-ph.SR Watch Solar and Stellar Astrophysics

Wednesday 14 October · still arriving

Solar and Stellar Astrophysics

Dynamics of a switchback-like magnetic deflection induced by a propagating coronal jet

Jade Touresse, Etienne Pariat, Clara Froment et al. (+1 more)

Parker Solar Probe (PSP) revealed the ubiquity of switchbacks (SBs), sharp, localised magnetic deflections in the nascent solar wind, renewing interest in their origin. One proposed mechanism involves solar jets: collimated, impulsive features driven by magnetic reconnection that launch Alfvénic torsional wavefronts into the heliosphere. We test whether realistic spacecraft sampling of a propagating solar jet produces in-situ signatures consistent with PSP observations. We also examine the 3D magnetic topology of the resulting structures and how deflection amplitudes evolve with the ambient plasma environment. We perform a 3D magnetohydrodynamic simulation of a solar jet in the ambient solar wind, embedding a virtual spacecraft trajectory based on PSP's actual 22nd-perihelion orbit to generate synthetic measurements. The simulation reproduces SB-like signatures with durations and profiles consistent with PSP observations, confirming earlier results from simplified trajectories. We visualise the fully 3D magnetic topology of a jet-driven SB region, identifying two field-line families: one with a single deflection and another with two sequential deflections along the same field line. Both are outward-propagating Alfvénic structures. Deflection amplitudes depend strongly on background plasma $\beta$, increasing substantially in low-$\beta$ regions and evolving more moderately where $\beta \sim 1$. These results show that solar jets can produce Alfvénic, SB-like structures along realistic spacecraft trajectories. Their topology is inherently 3D and structurally diverse, and their evolution depends on ambient plasma $\beta$. Complete magnetic reversals would require longer residence times in low-$\beta$ environments. Our findings support a solar origin for a fraction of SBs and constrain interpretations of their underlying magnetic structure.
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All authors Jade Touresse, Etienne Pariat, Clara Froment, Peter F. Wyper

Solar and Stellar Astrophysics

Modelling the 6.7 GHz Methanol Flaring in G358.93-0.03 Using the Maxwell-Bloch Framework

Ningyan Fang, Toktam Rashidi, Yoshinori Yonekura et al. (+4 more)

We model selected 6.7 GHz methanol flare profiles in G358.93-0.03 using the one-dimensional Maxwell-Bloch framework. The 2019 non-periodic flare, associated with an accretion burst, produced complex single-dish light curves characterized by sharp rises, multiple peaks, secondary structure, and extended decays. For six representative velocity channels, we model each light curve as a sum of scaled intensity responses from effective emitting systems driven by phenomenological pump pulses of fixed width but with component-dependent background rates, amplitudes, and arrival times. The inferred initial inverted column densities correspond to characteristic superradiance timescales $T_{R,0} \simeq 0.3$-$1.4~{\rm d}$, which are shorter than the adopted dephasing and relaxation timescales, $T_2 = 6.9~{\rm d}$ and $T_1 = 144.4~{\rm d}$. The fitted components therefore lie in the fast-transient superradiance regime. Existent VLBI observations motivate the possibility that a single-dish velocity channel blends emission from multiple unresolved flaring regions, although these observations are not used as direct fitting constraints. Although the decomposition is not unique, it provides a physically motivated description of the complex flare morphology within a unified Maxwell-Bloch framework.
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All authors Ningyan Fang, Toktam Rashidi, Yoshinori Yonekura, Vahid Anari, Ross A. Burns, Martin Houde, Fereshteh Rajabi

Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

Probing Planet Formation within 10 au: Dust Substructures in Extremely Compact Protoplanetary Disks

Pietro Curone, Gijs D. Mulders, Nienke van der Marel et al. (+1 more)

ALMA high-resolution surveys have so far focused mainly on large, bright protoplanetary disks, where substructures are routinely detected, leaving compact disks poorly characterized. Yet, the survival of millimeter emission in Class II sources implies that radial drift must have been halted by pressure traps, previously undetected due to limited angular resolution. We present ALMA Band 7 (0.9 mm) continuum observations of 11 of the smallest ($R_{68} < 10$ au) Class II disks in Lupus, reaching $0.02''$ (2 au) resolution, the highest currently available with ALMA at this wavelength. We detect substructures in 4 disks, including a clear inner cavity of radius 1.6 au, and hints of substructure in 3 more. Two disks also host faint outer rings beyond their bright inner core. Combining these results with demographic information on Class II disks observed with ALMA, we examine how the substructure detection rate varies with disk size, restricting to disks whose diameter is resolved by at least 3 resolution elements. We find that nearly all disks ($\gtrsim$95%) larger than 30 au show substructures, compared to a still substantial $\sim$50% between 3 and 30 au, with the lower rate likely affected by optical depth. The planet masses required to open gaps within 5 au fall in the super-Earth to mini-Neptune regime. These results demonstrate that substructures are ubiquitous across the disk population, and that planet formation conditions can be probed down to a few au from the host star.
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All authors Pietro Curone, Gijs D. Mulders, Nienke van der Marel, Laura M. Pérez

Solar and Stellar Astrophysics

HD 120678: a classical Be star with a tilting heartbeat disk

Dietrich Baade, Jonathan Labadie-Bartz and Thomas Rivinius

Classical Oe/Be/Ae stars possess Keplerian circum-equatorial decretion disks which produce not just the eponymous emission lines but also the bewildering diversity of photometric variations on timescales above a few days. The variability is due to varying amounts and geometric distributions of the circumstellar gas. The ASAS-3 V-band light curve of HD 120678 between March 2005 and November 2008 resembled an inverse P Cygni profile, which may have been a one-off event. The peak-to-valley amplitude reached ~0.9 mag. Before and after this photometric event, the Balmer emission lines and their variability matched those of classical Be stars with an intermediate to low inclination angle. At the sharp minimum of the light curve, Gamen et al. (2012) observed shell-type profiles implying an edge-on (high-inclination) view. For the historical activity phases of gamma Cas and 59 Cyg, such a Be => Be-shell => Be sequence has been successfully interpreted as rotations of the disk plane. HD 120678 and a few other Be stars add a not previously recognized phenomenon to the plethora of Be-star variabilities, namely heartbeat light curves. Analogously to the distortion of heartbeat stars, the variability is due to heartbeat disks gravitationally tilted by an additional star as modeled by Marr et al. (2022) for Pleione. While the typical amplitudes of heartbeat stars are only a few 0.01 mag, heartbeat disks can explain the ten times larger amplitudes observed in Be stars.
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Solar and Stellar Astrophysics · Cross-listed from nucl-ex

$^{16}$O(p,$\gamma$)$^{17}$F Reaction and the C and O Isotopic Ratios in Red Giant Branch Stars

D. Robb, P. McGlynn, M. Aliotta et al. (+55 more)

The $^{16}$O/$^{17}$O isotopic ratio is a widely used tracer of nucleosynthesis and mixing processes in red giant stars. Its value depends on the rates of the proton-capture reactions responsible for the production and destruction of $^{16}$O and $^{17}$O among which the $^{16}$O(p,$\gamma$)$^{17}$F reaction plays a central role, as it involves both isotopes. This work reports on a new measurement of the $^{16}$O(p,$\gamma$)$^{17}$F cross section carried out at the Laboratory for Underground Nuclear Astrophysics (LUNA) over a center-of-mass energy range of 200 - 380 keV, a region previously unexplored by experimental studies. Two complementary techniques were employed: prompt gamma-ray detection and activation. The results obtained with the two techniques are consistent within experimental uncertainties. From the measured cross sections and existing literature data, we evaluate a new thermonuclear reaction rate, which is then used to demonstrate how simultaneous measurements of stellar carbon and oxygen isotopic ratios constrain the initial mass and the extent of the mixing events in red giant branch stars. In particular, we demonstrate how the $^{12}$C/$^{13}$C abundance ratio is primarily sensitive to the temperature T$_{\textrm{max}}$ attained during mixing, whereas the $^{16}$O/$^{17}$O abundance ratio is highly sensitive to the initial stellar mass M$_i$. Therefore, simultaneous measurements of these isotopic ratios can disentangle and place stringent constraints on M$_i$ and T$_{\textrm{max}}$, and the remaining uncertainty is now dominated by stellar abundance measurements rather than nuclear-physics inputs.
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All authors D. Robb, P. McGlynn, M. Aliotta, C. Ananna, F. Barile, L. Barbieri, D. Bemmerer, A. Best, R. Biasissi, A. Boeltzig, R. Bonnell, C. Broggini, C.G. Bruno, A. Caciolli, M. Campostrini, F. Casaburo, F. Cavanna, G.F. Ciani, T. Chillery, P. Colombetti, A. Compagnucci, P. Corvisiero, L. Csedreki, T. Davinson, R. Depalo, A. Di Leva, Z. Elekes, F. Ferraro, A. Formicola, Zs. Fülöp, G. Gervino, R. M. Gesuè, G. Gosta, Gy. Gyürky, A. Guglielmetti, C. Gustavino, C. Hebborn, G. Imbriani, M. Junker, M. Lugaro, J. Marsh, E. Masha, R. Menegazzo, D. Mercogliano, V. Paticchio, D. Piatti, P. Prati, D. Rapagnani, V. Rigato, M. Rossi, R. Sariyal, R.S. Sidhu, J. Skowronski, O. Straniero, T. Szücs, S. Turkat, M. Vagnoni, S. Zavatarelli

Solar and Stellar Astrophysics

Revisiting the Optical F-corona from the Artemis II Total Solar Eclipse with RAW Data: How Far Can Consumer JPEG Images Be Trusted for Astronomical Photometry?

Kohji Tsumura and Ko Arimatsu

We reanalyze the total solar eclipse images taken with a consumer camera during the Artemis II lunar flyby, using the RAW data as ground truth for our previous analysis of the released JPEG image. Calibrated with field stars, the RAW data show that the lens vignetting and the variation of the pixel solid angle reduce the signal of diffuse light by up to $\sim$70% at the edge of the field, and that the signal on the dark side of the Moon is most likely foreground light rather than an instrumental offset. Between 10 degrees and 15 degrees elongation, the radial profiles of the F-corona follow power laws with indices of $-2.0$ to $-2.3$ (east), $-2.5$ (west), and $-2.4$ to $-2.7$ (north and south), consistent with space-based observations, and the surface brightness along the ecliptic is $\sim$26 MJy sr$^{-1}$ ($\pm 7\%$) at 10 degrees. The isophotes have flattening indices of $\sim$0.6-0.7 and superellipse indices of $\sim$1.5-1.6. Comparison with the ZodiSURF zodiacal light model supports a radial dust density index of $\alpha \simeq 1.3$ down to $\sim$0.2 au and suggests that the vertical distribution of the dust broadens toward the Sun. The JPEG image retains nearly the full vignetting, despite its metadata, and its tone response is not a single power law; together with the zero-point treatment, these effects account for the steep and asymmetric profiles reported previously, while the flattened morphology and $\alpha \simeq 1.3$ are confirmed. We identify quantities robust in JPEG images and show that the tone response can be reconstructed from field stars.
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Solar and Stellar Astrophysics

HD 152384: A 5 Myr old debris disk system showing a gaseous ring of refractory elements at the dust sublimation radius

Mario van den Ancker, Daniela Iglesias, Olja Panić et al. (+2 more)

Context: Circumstellar gas around young stars provides key insights into planet formation and disk evolution. HD 152384, a young (5 Myr) A0 V star in Sco-Cen, shows hybrid-disk traits: a modest infrared excess from small dust grains and atomic emission lines dominated by refractory elements, but its disk structure and origin remain unclear. Aims: We characterise the kinematics and excitation of the circumstellar gas around HD 152384 and constrain the physical properties of its emitting region. Methods: We use new high-resolution optical and near-infrared VLT/UVES and VLT/CRIRES spectra to re-examine the circumstellar environment of HD 152384, previously suggested to host a broad, gas-rich disk. Results: The observations reveal 74 narrow, double-peaked emission lines of Fe, Mg, Si, and Ca, with similar profiles and a 103 km/s peak separation. Keplerian modelling places the gas in a geometrically thin ring at 0.09-0.12 au, close to the dust sublimation radius (0.10-0.14 au for a sublimation temperature of about 1500 K). The Fe I emission is optically thick, whereas the optically thin Mg I lines imply 2.2 x 10^22 kg (3.6 x 10^-3 Earth masses) of magnesium gas in the ring. Conclusions: The dust and gas properties are consistent with a secondary origin involving collisions of rocky planetesimals. We propose that small collision-produced dust grains spiral inward and sublimate near the dust sublimation radius, accumulating atomic gas there. HD 152384 thus represents a young analogue of gas-bearing debris disks such as Beta Pictoris and gaseous debris disks around metal-polluted white dwarfs, offering a new window on the early violent processing of rocky planetesimals.
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All authors Mario van den Ancker, Daniela Iglesias, Olja Panić, Monika Petr-Gotzens, Miguel Vioque

Solar and Stellar Astrophysics

Flares and flux ropes: an entangled story

Rony Keppens, Yang Guo and Wenzhi Ruan

Solar flares serve as standard candles for all magnetic-reconnection-based energy releases, and their consequences into the heliosphere can be modeled from first principles. This review presents the latest developments in modeling solar flares and their associated flux rope expulsions with state-of-the-art multidimensional magnetohydrodynamic (MHD) descriptions. Observationally established ingredients beyond a single-fluid, thermal plasma evolution have been incorporated to varying degrees, and we highlight self-consistent efforts realized in "standard flare" setups.
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Solar and Stellar Astrophysics

Tracking a solar energetic eruption and associated energetic particles from Sun to spacecraft

D. E. Morosan, M. Baumgartner-Steinleitner, K. Bhandari et al. (+17 more)

Context. Solar energetic particles (SEPs) in the heliosphere are produced by flaring processes on the Sun or shocks driven by coronal mass ejections (CMEs). Electrons, in particular, are regularly detected remotely as electromagnetic radiation (X-rays or radio emission), which they generate through various processes, as well as in situ by spacecraft monitoring the Sun and the heliosphere. Aims. We aim to combine remote-sensing and in situ observations of an energetic solar eruption to link energetic processes in the solar corona to energetic particles observed in situ in the heliosphere. This will be done using the tools developed in the Horizon Europe SOLER project. Methods. Here, we investigate a solar energetic eruption at the Sun together with its associated signatures of accelerated electrons observed at radio and X-ray wavelengths and combine these remote observations to in situ particles observed by spacecraft on 10 March 2024. These observations are combined with a magneto-hydrodynamic (MHD) model of the solar corona in order to make the remote to in situ connections. Results. We observe a high-energy SEP event at multiple spacecraft (>25 MeV protons and 1 MeV electrons), despite the event being associated with a slow white-light CME. Remote observations of the Sun show that the event was associated with a fast large scale coronal wave (1400 km/s) and also multiple regions where electrons are accelerated around this large scale wave observed as type II radio bursts. In addition, we observe a strong coronal dimming away from the main flaring region, providing evidence of field lines getting opened to interplanetary space, dislocated from the flare site.
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All authors D. E. Morosan, M. Baumgartner-Steinleitner, K. Bhandari, N. Dresing, G. Farwa, J. A. J. Mitchell, C. Palmroos, A. Razquin, J. Zhang, C. Briand, K. Dissauer, A. Fedeli, J. Gieseler, E. K. J. Kilpua, S. Normo, J. Pomoell, D. J. Price, A. M. Veronig, A. Warmuth, R. Vainio

Solar and Stellar Astrophysics

Stationary Resistive MHD Solutions Near an Isolated Three-Dimensional Magnetic Null

Kuljeet Singh Saddal, Dieter H. Nickeler, Rodrigo Meneses Pacheco et al. (+1 more)

Magnetic null points define the magnetic field topology in stellar coronae, planetary magnetospheres, and astrospheres, and are commonly associated with magnetic reconnection. We investigate the local structure of stationary three-dimensional (3D) resistive, incompressible magnetohydrodynamic (MHD) flows in the vicinity of an isolated magnetic null and an isolated velocity stagnation point, assuming that the two nulls are colocated. The governing MHD equations are solved self-consistently using a first-order local expansion of the magnetic and velocity fields about the null. For any given magnetic-null configuration, the equations admit a two-parameter family of stationary solutions, comprising multiple solution branches with distinct velocity topologies. Along these families, the velocity-null polarity can change at bifurcation points where the flow becomes effectively 2.5D. A single magnetic null can therefore support positive-radial, negative-radial, and spiral stagnation flows, with qualitatively different inflow, outflow, and rotational properties. The associated pressure and resistivity distributions are likewise determined self-consistently and vary across the solution family, demonstrating that they are coupled to the flow topology rather than prescribed independently. The generic solutions do not require the velocity and magnetic separatrices to coincide, allowing plasma to cross the magnetic fan separatrix and providing a local reconnective flow. These results demonstrate that, in the presence of nonuniform resistivity, the magnetic topology alone does not uniquely determine the associated stationary flow, pressure, or resistivity structure around a 3D magnetic null, revealing the nonlinear feedback between all macroscopic variables and resistivity. The constant-resistivity field-aligned and spiral field-crossing solutions identified previously are recovered as special cases.
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All authors Kuljeet Singh Saddal, Dieter H. Nickeler, Rodrigo Meneses Pacheco, Michaela Kraus

Solar and Stellar Astrophysics

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

Janis B. Rodríguez-González, Edgar Santamaría, Martin A. Guerrero et al. (+1 more)

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${\alpha}$. 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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All authors Janis B. Rodríguez-González, Edgar Santamaría, Martin A. Guerrero, Jesús A. Toalá

Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

On-sky performance and prospects of near-infrared CMOS detectors in exoplanet detection: first year of SPIRIT

Clàudia Janó-Muñoz, Matthew J. Hooton, Peter P. Pedersen et al. (+8 more)

Ultracool dwarfs are promising targets for detecting and characterising temperate terrestrial exoplanets. Their flux peaks in the near-infrared, beyond the wavelength cut-off of the silicon-based CCDs used by ground-based surveys dedicated to observing them, such as SPECULOOS. We present the on-sky performance of SPIRIT (SPeculoos InfraRed Imager for Transits), an InGaAs CMOS detector on the Callisto telescope at the SPECULOOS Southern Observatory, during its first year of operation, from mid-2022 to 2023. We use 328 visits of 73 targets to optimise our CMOS-specific reduction process, including our approach to glow effects and bad-pixel handling. We find the measured precision to be consistent with our theoretical noise model, achieving sub-part-per-thousand precision under good observing conditions across targets spanning $J$-band magnitudes of 10–13. We further illustrate SPIRIT's capabilities with two case studies: a simultaneous SPIRIT $zYJ$ and CCD $I + z$ observation of a TRAPPIST-1\,h transit, and monitoring observations of the fast-rotating dwarf LP~944-20. Our results confirm that InGaAs infrared CMOS detectors are a cost-effective and competitive technology for ground-based transit photometry of cool stars, and that they show promising prospects to become a core part of the next generation of instruments.
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All authors Clàudia Janó-Muñoz, Matthew J. Hooton, Peter P. Pedersen, Didier Queloz, Khalid Barkaoui, Yasmin T. Davis, Michaël Gillon, Benjamin V. Rackham, Mathilde Timmermans, Amaury H. M. J. Triaud, Sebastián Zúñiga-Fernández

Solar and Stellar Astrophysics

High-Frequency Transverse Waves in Chromospheric Fibrils Revealed by DKIST

Qifan Dong, Xiaoli Yan, Zhike Xue et al. (+4 more)

Chromospheric fibrils are ubiquitous dynamical structures that can support magnetohydrodynamic waves and may contribute to energy transport in the solar atmosphere. Using high-resolution observations from the Visible Broadband Imager (VBI) onboard the Daniel K. Inouye Solar Telescope (DKIST), we investigate transverse oscillations of H$\alpha$ fibrils. Our observations encompass both quiet-Sun and active regions. The results reveal that high-frequency transverse oscillations are highly prevalent in fibrils across different magnetic environments. The detected oscillations exhibit typical periods of 56 $\pm$ 49 s, displacement amplitudes of 37 $\pm$ 25 km, velocity amplitudes of 6 $\pm$ 4 km\,s$^{-1}$, and phase velocities of 21 $\pm$ 13 km\,s$^{-1}$. Under typical chromospheric conditions, the estimated wave energy flux ranges from 20 to 200 W\,m$^{-2}$. The characteristics of these transverse oscillations are consistent with those of Alfvénic kink-mode waves. Our results reveal the existence of the high-frequency, small-amplitude transverse waves in chromospheric fibrils and providing new constraints on their contribution to the chromospheric and coronal heating.
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All authors Qifan Dong, Xiaoli Yan, Zhike Xue, Liheng Yang, Jincheng Wang, Yadan Duan, Zhe Xu

Solar and Stellar Astrophysics

Evidence for Suppressed Non-thermal Broadening in a Flare Ribbon from Stereoscopic Observations

Ziwen Huang, L. P. Chitta and L. Teriaca

Flare ribbons mark the footpoints of newly reconnected magnetic loops and are commonly associated with strong plasma flows and enhanced non-thermal broadening of transition-region lines due to impulsive energy deposition. However, the mechanisms governing energy transport into flare ribbons remain uncertain, particularly under different magnetic field configurations. We investigate the energy deposition process at a flare ribbon near a sunspot by determining the physical nature of its small-scale downflows. We study small-scale downflows in a remote ribbon during the M7.7-class event on 30 September 2024, using coordinated observations from SDO, IRIS, and Solar Orbiter. We use stereoscopic imaging from Solar Orbiter and IRIS to detect plane-of-sky motions on small scales (200 km). We conduct spectroscopic analysis of IRIS Si IV profiles, complemented by C II and Mg II diagnostics. Apparent downflow speeds measured from Solar Orbiter imaging (limb view) are 85-130 km/s, while IRIS images (disk view) reveal plane-of-sky speeds of 70-85 km/s. In contrast, IRIS Si IV profiles at the brightest ribbon pixels are well approximated by a single Gaussian, with minimal Doppler shifts and unexpectedly small non-thermal widths (10 km/s). Peak Si IV emission also coincides with a deeper Mg II central reversal, while electron densities are one to two orders of magnitude higher than typical quiet-Sun values. The absence of significant line-of-sight velocities and non-thermal broadening differs from typical flare-ribbon behavior. Our results are consistent with a scenario in which plasma motions are guided by highly inclined (nearly horizontal) magnetic fields in chromospheric canopy and are not accompanied by strong turbulence or wave motions. This study suggests that flare ribbons can exhibit a wide diversity of lower-atmospheric responses and emphasizes the critical role of magnetic field geometry.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Constraint on particle mass of dark matter based on XTE J1814-338 observation and DANS model

Lin Wang and Dehua Wen

The small observed radius of neutron star XTE J1814-338 (with mass of $1.21_{-0.05}^{+0.05}$ $M_{\odot}$ and radius of $7_{-0.4}^{+0.4}$ $\rm{km}$) poses a great challenge to the traditional neutron star model composed of normal nuclear matter. The dark matter admixed neutron star (DANS) model provides a clue to solve this challenge. By employing the self-interacting fermionic dark matter model, we investigate the effects of dark matter parameters including the particle mass, self-interaction strength, and mass fraction of dark matter on the properties of DANSs. Our results indicate that the joint constraints from the observation of XTE J1814-338 and the astrophysical limits on the dark matter self-interaction cross section derived from observations of different colliding galaxy clusters can provide effective constraints on the particle mass of dark matter. Specifically, to simultaneously satisfy the small radius of XTE J1814-338 and the self-interaction limit, the particle mass of self-interacting fermionic dark matter is constrained to the range from sub-GeV to a few GeV.
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Solar and Stellar Astrophysics

Post-merger rotational evolution of magnetised single hot subdwarfs results in slow rotation due to magnetic braking

Facundo D. Moyano, Zhengwei Liu, Zhanwen Han et al. (+3 more)

Single hot subdwarfs are believed to form through mergers of helium-core white dwarfs, which can lead to large-scale magnetic fields generated during the merger, and may be observable at the surface once they evolve into the stable core-helium burning phase. As a post-merger product, the orbital angular momentum of the binary system that forms it, could lead to a rapidly rotating hot subdwarf. But previous models were not able to explain the rather slow rotational velocities of observed single hot subdwarfs. Motivated by recent detections of surface magnetic fields in hot subdwarfs in the range 200 - 500 kiloGauss, we explore in this work the role of large-scale magnetic fields on their rotational evolution via rotating stellar evolution models. We find that large-scale stable magnetic fields can extract angular momentum efficiently from the star via coupling to stellar winds, producing slowly rotating single hot subdwarfs and thus providing a possible explanation to both the long rotation periods provided by asteroseismology and low spectroscopic rotational velocities. This hints magnetic braking could be an important component of the rotational properties of low-mass stellar mergers.
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All authors Facundo D. Moyano, Zhengwei Liu, Zhanwen Han, Xuefei Chen, Tiara Battich, Philipp Podsiadlowski

Solar and Stellar Astrophysics

Magnetic Reconnection Governs the Entire Lifecycle of Solar Eruptive Magnetic Flux Rope

Xinkai Bian, Chaowei Jiang, Qingjun Liu et al. (+3 more)

While magnetic flux ropes (MFRs) are widely accepted to exist prior to many solar eruptions, the physical mechanism responsible for triggering and subsequently driving their eruptions remains debated. Classical models commonly attribute eruption onset to ideal magnetohydrodynamic instabilities, particularly the torus instability, while magnetic reconnection is often regarded as a secondary process. Using high-resolution three-dimensional MHD simulations, we investigate the eruption of an MFR that forms self-consistently through flux cancellation driven by converging motions and photospheric diffusion following an initial shearing phase. The pre-eruption MFR rises quasi-statically and reaches a height where the decay index exceeds the classical torus instability threshold, yet the system continues to evolve quasi-statically without eruption. Instead, eruption begins only when the current sheet beneath the MFR becomes sufficiently thin for magnetic reconnection to set in. Increasing numerical resolution systematically delays the eruption onset, whereas the decay index at eruption onset remains nearly unchanged, demonstrating that the onset is governed by reconnection rather than torus instability. During the eruption, magnetic reconnection continuously converts surrounding sheared arcades into newly reconnected twisted flux that merges with the pre-eruption MFR and continuously to build the erupting MFR. A component-resolved analysis of the Lorentz force work further shows that the newly reconnection flux contributes predominantly to the main acceleration phase. These results demonstrate that magnetic reconnection plays a central role throughout the entire lifecycle of the eruptive MFR: it builds the MFR, directly triggers its eruption, and provides the dominant driving of the eruption.
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All authors Xinkai Bian, Chaowei Jiang, Qingjun Liu, Xueshang Feng, Pingbing Zuo, Yi Wang

Tuesday 13 October · still arriving

Solar and Stellar Astrophysics

Transients and Stellar Activity as Revealed by Ultraviolet Diagnostic Tracers

Richard de Grijs, Martin A. Barstow, Deepak Chahal et al. (+5 more)

Ultraviolet (UV) observations offer unique insights into transient and variable astrophysical phenomena, from the Sun and nearby stars to supernovae and star-forming galaxies. Here, we summarise and review the diagnostic power of the UV spectrum for probing the physical conditions, dynamics and energetics of these events. We trace the historical development of UV astronomy from early satellite missions to the current landscape shaped by the Hubble Space Telescope, the GALaxy Evolution eXplorer, the Neil Gehrels Swift Observatory and AstroSat/UVIT. Particular emphasis is placed on resonance lines such as Mg II $h$ and $k$, C II, Si IV, C IV and N V, which probe the (solar and) stellar chromosphere and transition region, revealing magnetic heating, shocks and mass loss across stellar types. We examine UV diagnostics of stellar flares, pre-main-sequence activity and rotation–activity correlations, linking them to exoplanetary irradiation and habitability. We then explore the UV view of supernova shock breakout and early-time emission, as well as the far-UV mapping of young stellar populations and hierarchical star formation in the Magellanic Clouds. Together, these aspects illustrate how the UV connects small-scale plasma processes to galactic-scale star formation. Despite remarkable progress, UV transient astronomy remains limited by instrument availability and cadence. Forthcoming missions promise to transform this field by capturing the rapid, luminous phenomena that define the dynamic Universe.
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All authors Richard de Grijs, Martin A. Barstow, Deepak Chahal, Fabio Favata, Xiaoting Fu, Ana I. Gómez de Castro, Sipra Hota, Firoza Sutaria

Solar and Stellar Astrophysics

Twisted Pseudodisk and Asymmetric Mass Accretion on the Circumstellar Disk II: Parameter Survey

Masahiro N. Machida, Shingo Hirano and Shantanu Basu

Using three-dimensional magnetohydrodynamic simulations accounting for ohmic dissipation, we investigate the long-term evolution of the infalling envelope for $\sim 100,000$\,yr after protostar formation. The primary control parameter is the initial misalignment angle $\theta_0$ between the rotation axis and the magnetic field, varied from $0^\circ$ to $90^\circ$. In all misaligned models ($\theta_0 \ne 0^\circ$), the pseudodisk (or infalling envelope) surrounding the circumstellar disk develops a highly distorted and complex morphology. Gas accretion from the envelope proceeds through discrete, channel-like flows, producing asymmetric streams rather than axisymmetric infall. Protostellar outflows are launched even in these complex environments, and inflow and outflow regions are strongly interwoven. Outflows transport angular momentum anisotropically, leading to time-dependent changes in disk orientation and outflow direction, which further enhance envelope asymmetry. We examine the relative orientation between gas flows and magnetic fields and find that, although statistical tendencies exist, accretion streams cannot be uniquely characterized as either parallel or perpendicular to the magnetic field, particularly in strongly misaligned cases. We also show that most infalling gas accretes at nearly the free-fall velocity, with only a minor fraction significantly slowed within the pseudodisk. Asymmetric accretion and stream-like structures persist beyond 10,000\,au, demonstrating that large-scale asymmetry can arise intrinsically. Our results show that complex streams and asymmetric envelopes naturally emerge from magnetized core physics, even in the absence of initial turbulence or external gas accretion.
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Solar and Stellar Astrophysics

Tracing Accretion and Inner-Disk Gas Evolution in the Orion Nebula via Br-alpha and CO

Konstantin V. Getman, Maria Claudia Ramirez-Tannus, Jenny Frediani et al. (+17 more)

We present a systematic spectroscopic study of hydrogen Brackett-alpha (Br-alpha) and carbon monoxide (CO) fundamental-band (4.3-5.2 micron) emission from a homogeneous sample of 0.2-1.5 Msun pre-main-sequence (PMS) stars in the Orion Nebula observed with JWST/NIRSpec-MOS as part of our Chandra-JWST project. Using 73 Br-alpha and 46 CO spectra, we investigate correlations between warm-gas emission, stellar properties, accretion, and environment with censored correlation and regression analyses accounting for multiple testing. Br-alpha luminosity correlates strongly with accretion luminosity (Lacc), even after excluding sources within 2' of Theta1 Ori C, the nebula's main ionizing O-type star, whose radiation may bias proplyd Lacc estimates. This supports Br-alpha as a robust empirical tracer of accretion in PMS stars. Br-alpha also correlates with visual extinction (Av) and more weakly with stellar bolometric luminosity and mass. CO fundamental-band luminosities likewise correlate positively with Lacc and Av, relationships that persist in a larger combined JWST/NIRSpec+MIRI sample spanning multiple star-forming regions. In contrast to Br-alpha, CO luminosity shows no significant dependence on basic stellar properties, suggesting that warm molecular gas is governed primarily by accretion and local disk conditions. The weaker correlations of the Spitzer/IRAC spectral slope with Lacc and Av suggest that warm inner-disk gas may evolve or disperse faster than optically thick dust. Overall, the CO results tentatively indicate declining warm-gas emission as disks evolve, modulated by accretion-powered thermal and ultraviolet excitation, although variations in emitting area inherited from natal core conditions may also contribute.
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All authors Konstantin V. Getman, Maria Claudia Ramirez-Tannus, Jenny Frediani, Kevin L. Luhman, Eric D. Feigelson, Bayron Portilla-Revelo, Christian Rab, Valentin Brunn, Jan Forbrich, Thomas J. Haworth, Rens Waters, Arjan Bik, Charles J. Law, Andrew J. Winter, Vladimir S. Airapetian, Abygail R. Waggoner, Michael A. Kuhn, L. Ilsedore Cleeves, Sergio A. Dzib, Marco Padovani

Solar and Stellar Astrophysics

Chandra-JWST Probing the Effects of Moderate X-Ray Superflares on Protoplanetary Disks

Konstantin V. Getman, Kevin L. Luhman, Bayron Portilla-Revelo et al. (+17 more)

Powerful X-ray flares from young stars may rapidly alter the physical and chemical conditions of their planet-forming disks, but direct observational tests remain scarce. We present a Chandra-JWST/NIRSpec study of infrared spectral variability associated with pre-main-sequence (PMS) X-ray flares in the Orion Nebula. Combining six Chandra/ACIS-I observations with three NIRSpec-MOS visits, we analyze two-epoch spectra of 10 X-ray Flare stars and 12 matched non-flaring Controls among 84 observed disk-bearing PMS stars. We investigate three diagnostics: narrow 2.8-4.3 micron features, HI Br-alpha, and the CO fundamental band. Flare stars show tentative evidence for relatively fewer variable narrow features at longer wavelengths than Controls, while feature incidence decreases strongly with projected distance from the dominant ionizing O star Theta1 Ori C. Most classifiable features appear in absorption. These properties suggest that at least some features may arise in externally irradiated disk atmospheres and/or associated photoevaporative flows, with their apparent epoch-to-epoch variability potentially reflecting spatial sampling of such locally structured material rather than intrinsic temporal changes. Br-alpha behaves differently: flare stars show systematically stronger variability and uniquely exhibit absorption-core profiles, suggesting control by magnetospheric accretion and stellar magnetic processes rather than disk irradiation. The CO fundamental band shows no significant flare-associated variability, consistent with thermochemical models predicting weak responses to moderate X-ray superflares. This pilot study demonstrates the potential of coordinated Chandra-JWST observations to probe how young stellar magnetic activity affects planet-forming environments, motivating future monitoring to build larger superflare samples and extend them into the rare X-ray mega-flare regime.
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All authors Konstantin V. Getman, Kevin L. Luhman, Bayron Portilla-Revelo, Christian Rab, Valentin Brunn, Eric D. Feigelson, Arjan Bik, Jan Forbrich, Jenny Frediani, Thomas J. Haworth, Charles J. Law, Marco Padovani, Maria Claudia Ramirez-Tannus, Andrew J. Winter, Vladimir S. Airapetian, L. Ilsedore Cleeves, Sergio A. Dzib, Michael A. Kuhn, Abygail R. Waggoner, Rens Waters

Solar and Stellar Astrophysics

A Distinct Type of Standstill in Ultra-Compact White Dwarf Binaries

Jan Kára, Liliana Rivera Sandoval, Iwona Kotko et al. (+3 more)

AM CVn stars are ultra-compact, semi-detached binaries consisting of a white dwarf primary and a hydrogen-poor secondary. In this work, we analyse the AM CVn-type systems MGAB-V270, ZTF18aaxuusk and ASASSN-19ct, which have similar properties and show unusual post-superoutburst behaviour. We focus on the occurrence of standstill-like phases. In MGAB-V270, superoutbursts are followed by a series of echo outbursts that evolves into a standstill-like phase before the system returns to quiescence. ZTF18aaxuus shows similar post-superoutburst activity, with one echo-outburst series also developing into a standstill-like phase. ASASSN-19ct exhibits comparable echo outbursts but no standstill-like phases. Periodicity analysis using TESS data shows that superhumps can persist during both echo outbursts and standstill-like phases. Although standstills have previously been observed in AM CVn stars, the standstill-like phases we observe appear to represent a distinct phenomenon associated with post-superoutburst activity. We compare our results to other AM CVns that may exhibit similar behaviour.
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All authors Jan Kára, Liliana Rivera Sandoval, Iwona Kotko, Jean-Marie Hameury, Luis E. Salazar Manzano, Ryan J. Oelkers

Solar and Stellar Astrophysics

Understanding the Complex Morphology of a Coronal Mass Ejection. III: How to Create a Severe Geomagnetic Storm

Cecilia Mac Cormack, Abril Sahade, Erika Palmerio et al. (+4 more)

On April 21, 2023, a complex filament eruption produced an Earth-directed coronal mass ejection (CME) that triggered the first severe geomagnetic storm of solar cycle 25. The primary objective of this work is to understand the physical origin of the severe impact of this CME on Earth. We explore the CME's complex morphology by simulating the eruption using the thermodynamic magnetohydrodynamic (MHD) model CORHEL-CME. We model the pre-eruptive structure as two intertwined magnetic flux ropes and validate the early evolution of the event by comparing synthetic observables generated from the model with extreme-ultraviolet and white-light observations. We then compare the simulated CME structure with in situ measurements. From our analysis, we conclude that the CME merged with a stream interaction region (SIR) before reaching Earth. This interaction compressed the CME sheath, producing a sustained negative $B_n$ component that, together with the associated enhancement in solar wind density and velocity, triggered the geomagnetic storm. The internal configuration of the magnetic flux rope, which also maintained a sustained southward component during its propagation toward Earth, intensified the storm to severe levels. This model-data comparison underscores the power of using MHD simulations, when validated by multi-viewpoint observations, to reveal hidden dynamical processes and improve the understanding and prediction of space weather impacts.
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All authors Cecilia Mac Cormack, Abril Sahade, Erika Palmerio, Evangelia Samara, Phillip Hess, Robin C. Colaninno, Teresa Nieves-Chinchilla

Solar and Stellar Astrophysics

Velocity-field estimation techniques for on-disk solar coronal bright fronts (CBFs) and shock-wave propagation

Oleg Stepanyuk and Kamen Kozarev

Coronal mass ejections (CMEs) are large expulsions of plasma and magnetic fields from the solar corona into the heliosphere. Coronal shock waves are driven by CMEs, and accurate Solar Energetic Particles (SEP) forecasting requires knowing when and where shocks form, how fast they propagate, and how they interact with the coronal magnetic field. By measuring the speed, geometry and temporal evolution of coronal bright fronts (CBFs), one can create more realistic predictions of SEP onset and intensity at Earth, thus velocity-field estimation techniques supply these critical parameters. High-resolution velocity field maps can provide even deeper insight. Instead of yielding only an overall speed of the front, techniques like FLCT and optical flow can show the spatial distribution of velocities across the entire wave dome or front. These analyses reveal which parts of the front are moving fastest and how the front might be deforming as it expands. Here we discuss a set of practical parameters for Lucas–Kanade, Horn–Schunck, FLCT, and Farneback algorithms, arrived at by combining analytic reasoning with empirical measurements on a single event—and demonstrate the potential of these techniques for on-disk shock-wave propagation during the X1.5-class event that took place 10 May 2022, processed by the wavelet-based Wavetrack technique. A better understanding of how to fine-tune these parameters could improve the precision of algorithms running in pipelines, and is especially valuable where per-event hand-tuning is impractical.
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Solar and Stellar Astrophysics

SPICY 30021: an embedded bona fide FUor with a mid-infrared precursor

Zhen Guo, Calum Morris, Vitor Fermiano et al. (+37 more)

Unstable mass accretion in young stellar objects (YSOs) often leads to observable outbursts, which play a key role in stellar mass assembly during pre-main-sequence evolution. The highest-amplitude, longest-duration events are classified as FUors. Until recently, only 50 FUor events had been confirmed. Many occur in embedded protostars, which are difficult to monitor in optical surveys. We present photometric and spectroscopic observations of SPICY 30021, an eruptive young star discovered in the mid-infrared (MIR) NEOWISE time series. We aim to construct its spectral energy distribution and characterise its eruptive nature. We compiled multi-wavelength photometric time series from NEOWISE, VVV and VVV-RENEW, and obtained photometric and spectroscopic observations during the outburst with SOAR, Magellan and VLT, including data from the ESO public survey VVVX-GalCen. We identified the outbursting source, separated it from nearby companions, and estimated the line-of-sight extinction from spectroscopic features. Stellar parameters were constrained using radial velocity, proper motions and stellar evolution models. SPICY 30021 is an optically obscured FUor with a total interstellar and circumstellar extinction of Av = 12.5 mag. Its spectrum closely resembles those of classical FUors. Multiple methods yield a distance of 1.7+_0.4 kpc. Its accretion luminosity is Lacc = 32 L_sol, moderate among FUors. Quiescent near-infrared photometry is consistent with a young 0.9 M_sol progenitor, although the mass depends on the adopted model and distance. We confirm SPICY 30021 as a bona fide FUor event. Its MIR precursor suggests an outside-in triggering mechanism.
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All authors Zhen Guo, Calum Morris, Vitor Fermiano, Carolina Lizana-Vidal, Aldo Aliaga, Javier Osses, Felipe Navarete, Philip Lucas, Roberto K. Saito, Adolfo Carvalho, Vardan Elbakyan, Carlos Contreras Peña, Koshvendra Singh, Min Fang, Philipp Weber, He Zhao, Tigran Yu. Magakian, Tigran A. Movsessian, Matias Gomez, Dante Minniti, Valentin Ivanov, Maria Gabriela Navarro, Sara Federle, Ilaria Petralia, Alonso Luna, Jesus Corral-Santana, Elisa R. Garro, Sean D. Points, Cesar Briceño, Zenghua Zhang, Radostin Kurtev, Jura Borissova, Jose G. Fernández-Trincado, Javier Alonso-García, Claudio Cáceres, Casmir O. Obasi, Daniela Galdeano, Laura Baravalle, Teresa Giannini, Zhikai Zhu

Solar and Stellar Astrophysics

First L-type substellar object discovered in the vicinity of the embedded cluster NGC 2023

Styliani Tsilia, Carlos Dominguez-Tagle, Maruša Žerjal et al. (+8 more)

We present the first substellar object in a star-forming region spectroscopically confirmed with Euclid, namely J054158.70-021031.4. This young and faint source is located close to the embedded open cluster NGC 2023. It has a spectral type of L3, a reddening of Av = 3 mag, proper motion consistent with the nearby cluster NGC 2024, and a magnitude of $\sim24.5$ in the Euclid optical band. At the distance of 403 pc, its inferred mass lies close to the planetary-mass boundary. We discovered the object by searching for ultracool dwarf candidates in the Euclid Early Release Observations. The search was based on photometric criteria, followed by the analysis of slitless 2D spectro-images in order to distinguish features that indicate the nature of these objects. This work demonstrates the potential of Euclid to unveil previously hidden populations of very low-mass objects, located in obscured regions where star formation is ongoing.
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All authors Styliani Tsilia, Carlos Dominguez-Tagle, Maruša Žerjal, Eduardo L. Martín, Hervé Bouy, Nafise Sedighi, Sara Muñoz Torres, Nikola Vitas, Khalid Barkaoui, Nicolas Lodieu, Jerry J.-Y. Zhang

Solar and Stellar Astrophysics

The mass functions of multiple stellar populations in 47 Tucanae

M. Scalco, J. Bruce, E. Vesperini et al. (+1 more)

We used a combination of James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) observations to investigate the mass functions (MFs) of the different stellar populations in the globular cluster (GC) 47 Tucanae (NGC 104) over a wide stellar-mass interval. We identified the multiple populations along the lower main sequence using only JWST photometry and, in particular, colour combinations sensitive to variations in oxygen abundance. We derived the MFs of the first- and second-population (1P and 2P) stars separately and found that in the low-mass regime, the present-day MF of the 1P population is significantly steeper than that of the 2P population, which appears nearly flat. We also present the results of a dynamical model showing that this behaviour requires a different initial mass function (IMF) for the 2P and the 1P populations in the 0.1-0.4 solar masses mass range, thus providing a new fundamental constraint for the study of the star formation of multiple populations in GCs.
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All authors M. Scalco, J. Bruce, E. Vesperini, L. R. Bedin

Solar and Stellar Astrophysics

JWST observations of 47 Tucanae: I. The low-mass main sequence down to the hydrogen-burning limit

M. Scalco, R. Gerasimov, L. R. Bedin et al. (+4 more)

We present the first results of a series of studies based on JWST observations of the globular cluster 47 Tucanae (47 Tuc, or NGC 104) obtained with NIRCam. By combining new data with earlier JWST observations, we derive high-precision proper motions (PMs) over a multi-year baseline (of ~3 years) and construct a clean sample of cluster members extending to very faint magnitudes (F444W~24.25 mag, corresponding to ~0.077 solar masses, slightly above the estimated hydrogen-burning limit of 47 Tuc, ~0.074 solar masses) along the main sequence (MS). We investigated the presence of the low-mass MS kink previously identified in 47 Tuc using the new F090W and F444W photometry. No significant signature of the feature is found using these filters, consistent with its proposed physical interpretation as a consequence of the transition in carbon chemistry toward a methane-dominated equilibrium in very low-mass stars. A comparison with theoretical isochrones indicates that the stellar population near the bottom of the MS is likely dominated by oxygen-poor stars, in agreement with previous findings. Using artificial-star tests, we quantify the completeness of our sample and show that the requirement of PM-based membership significantly limits the detection of the faintest objects, indicating that current JWST observations of 47 Tuc are not sufficiently deep to reliably trace the cluster's brown dwarf cooling sequence. The mass function (MF) of 47 Tuc shows a structured behaviour, with a break at ~0.15 solar masses not associated with the onset of convection, and a mild but systematic radial variation consistent with mass segregation. Our analysis indicates that the MF shape in this mass regime is more complex than a simple power-law description. This feature in the MF may point to a presently unrecognised mechanism affecting the formation or dynamical evolution of low-mass stars in globular clusters.
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All authors M. Scalco, R. Gerasimov, L. R. Bedin, E. Vesperini, M. Griggio, D. Nardiello, J. Anderson

Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

Thermodynamically consistent equation of state tables for use in giant planet and brown dwarf interior modeling

Emily Sandford and Yamila Miguel

Equation of state (EOS) tables used in giant planet modeling exhibit regions of strong thermodynamic inconsistency, or disobedience to the first law of thermodynamics. These generally result from smooth interpolation over physical discontinuities or between thermodynamic sub-domains governed by different physical models. The effect of thermodynamic inconsistency on structure models of planets evolved with these EOS has never been studied in detail, although it is understood to introduce spurious sources or sinks of energy or entropy in the models, leading to inaccuracies. Here, we present a method for improving the thermodynamic consistency of an existing EOS table, based on constructing a Helmholtz free energy $F(\rho,T)$ covering the EOS domain and deriving all the other necessary EOS quantities from it. This method produces EOS tables with roughly thousandfold better thermodynamic consistency across the EOS domain of interest, as quantified by three thermodynamic consistency metrics derived from the first law. We also present a controlled experiment comparing giant planet models evolved with MESA using the original EOS and the $F$-derived, thermodynamically consistent versions, and find that the models can differ by up to $5\%$ in radius at an age of 5 Gyr, and by up to $25\%$ in luminosity at earlier stages of evolution; thermodynamic consistency is thus an especially important consideration for characterising young directly imaged planets. We present the $F$-derived, thermodynamically consistent versions of the tables for download at \href{https://doi.org/10.5281/zenodo.21103561}{https://doi.org/10.5281/zenodo.21103561}, along with example MESA inlists to reproduce our evolution models.
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Solar and Stellar Astrophysics

Any colour you like: The diverse chromaticities of white dwarfs

Mark A. Hollands, Boris T. Gaensicke, Pier-Emmanuel Tremblay et al. (+1 more)

Main sequence stars have perceived colours ranging from orange to white to blue with increasing effective temperature. This is because their atmospheric opacities cause only minor deviations from blackbody colours when integrated over the broadband sensitivity curves of the human visual system. White dwarfs can have much denser atmospheres, reaching ~ 1 g/cm3 for the coolest known objects. If metals such as carbon, calcium, or sodium are present in these ultra-dense atmospheres, extreme broadening of atomic and molecular opacities can produce optical spectra that diverge significantly from blackbodies. We investigated how the combination of temperature and chemical composition can affect the perceived colours (chromaticity) of white dwarfs. While we found that most white dwarfs are similar in colour to blackbodies, we also demonstrate examples of known stars that would appear yellow, green, cyan, pink, or purple if viewed close enough for their stellar discs to be resolved. We examined the chromaticity distribution for the volume-limited 20 pc local sample of white dwarfs finding a median correlated colour temperature of ~= 6800 K, establishing that, on average, white dwarfs are indeed white in colour. Finally we show how white dwarf spectral models can be used to generate colour-accurate images of their stellar discs as they would appear from up close.
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All authors Mark A. Hollands, Boris T. Gaensicke, Pier-Emmanuel Tremblay, Detlev Koester