Solar and Stellar Astrophysics
On modelling the 2017 galactic cosmic ray depression
O.P.M. Aslam, D. MacTaggart, M.S. Potgieter et al. (+2 more)
In the second half of 2017, as Solar Cycle 24 approached its minimum, a pronounced and rigidity-dependent depression in the galactic
cosmic ray (GCR)
proton flux was observed using AMS-02. This event was driven by a sequence of strong coronal mass ejections (CMEs). In this work, we model the modulation of GCR
protons during this period using a steady-state Parker transport framework with parameters constrained by AMS-02 observations. Treating the event as a purely long-term modulation interval yields excellent agreement with the observed
proton intensities, but implies an inferred rigidity dependence of particle mean free paths that is inconsistent with basic
turbulence theory, corresponding to an effective inversion of the scattering regime. We argue that this behaviour arises from the ability of highly flexible transport parameterizations to reproduce integrated modulation signatures through correlated adjustments of
diffusion coefficients during transient events. To address this inconsistency, we relax the commonly adopted assumption of a time-independent ratio of perpendicular to parallel
diffusion coefficients and introduce a physically motivated reduction of this ratio during the CME-dominated interval. This modification preserves the quality of the spectral fits whilst restoring the expected rigidity ordering of mean free paths. Our results indicate that allowing the perpendicular-to-parallel
diffusion ratio to vary in time provides an effective means of constraining transport solutions during the 2017 depression and may help to resolve similar inconsistencies reported in other GCR modulation studies.
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All authors O.P.M. Aslam, D. MacTaggart, M.S. Potgieter, M.D. Ngobeni, I.I. Ramokgaba
Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE
Multi-Wavelength Identification of a Luminous Mid-Infrared Supernova Powered by Circumstellar Interaction with Binary-Driven Pre-supernova Mass Loss
Kohki Uno, Kishalay De, Geoffrey Mo et al. (+6 more)
Terminal mass-loss in massive
stars is commonly reconstructed from observations of circumstellar interaction in core-collapse
supernovae (CCSNe); however, such signatures can be easily missed depending on the phase when they appear. In this paper, we present a multi-wavelength study of SN\,2022yyz, a nearby CCSN selected by its luminous late-time mid-infrared (MIR) excess in NEOWISE at $> 500$ days after the discovery. Classified as a Type II SN at discovery, it reached a peak bolometric luminosity of $1.3^{+0.7}_{-0.4}\times10^{43}$ erg s$^{-1}$ before fading away with a light-curve drop at $\sim 300$ days. We present late-time HST/WFC3 ultraviolet (UV) imaging and Keck/LRIS spectroscopy at $\sim 1200$ days, which reveal clear signs of circumstellar-medium (CSM) interaction via a UV excess consistent with a strong MgII contribution and a flat-topped H$\alpha$ profile. We show that the bolometric light curve and the late-time UV luminosity can be explained using a semi-analytic CSM interaction model involving a broken power-law density profile: with $\sim 2.7~M_\odot$ of nearby, dense CSM within a radius of $\sim 10^{16}$ cm surrounded by wind-like outer CSM with a mass-loss rate of $\sim 10^{-4} {\rm ~M_\odot~yr}^{-1}$. The broken density profile indicates $\gtrsim 100\times$ mass-loss rate enhancement starting a few hundred years before explosion, pointing to a possible common-envelope ejection in a binary system. Despite being overlooked for follow-up at such proximity, our results demonstrate that IR photometric selection methods provide a powerful way of mapping terminal mass-loss in massive
stars - laying the groundwork for the Roman Space
Telescope surveys.
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All authors Kohki Uno, Kishalay De, Geoffrey Mo, Viraj Karambelkar, Kaitlyn Shin, Christoffer Fremling, Jesper Sollerman, Daichi Hiramatsu, Jacob E. Jencson
Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP
Detecting White Dwarf Exoplanets in the Roman Era
Zifan Lin and Tansu Daylan
White dwarfs (WDs) provide an unparalleled opportunity for detecting and characterizing
exoplanets because of the large
planet-to-
star size ratio. However, only one transiting WD
exoplanet is confirmed to date, and occurrence rate constraints remain loose because the rapid transits and weak phase curve modulations of WD
exoplanets require high-precision, high-cadence photometry. The Galactic Bulge Time Domain Survey (GBTDS) of the Nancy Grace Roman Space
Telescope will provide exactly this by observing thousands of WDs at 12.1-minute cadence. The expected WD
exoplanet yield of this survey is not yet quantified. Here, we predict the GBTDS WD
exoplanet yield over its first three high-cadence seasons, through both transiting and non-transiting (phase curve) channels. We run Monte Carlo detection probability simulations over a grid of
planet radius, orbital period, and host properties for a Besançon synthetic population of WDs. Jupiter-sized
planets dominate the transit yield, with a period-averaged effective survey size of $\overline{N}_\mathrm{eff}\approx8$, requiring an underlying occurrence rate of $\overline{\eta}_1\approx12\%$ for one expected detection, followed by $\overline{N}_\mathrm{eff}\approx2.6$ ($\overline{\eta}_1\approx38\%$) for Neptune-sized and $\overline{N}_\mathrm{eff}\approx1.8$ ($\overline{\eta}_1\approx55\%$) for sub-Neptune-sized
planets. Source blending is the major observational challenge in the crowded GBTDS fields, removing $\approx95\%$ of the raw predicted yield. Phase curve provides marginal detections, adding at most $\approx4\%$ to the transit yield. A null detection by GBTDS would constrain $\eta_\mathrm{WD}$ to $< 28\%$ for Jupiter-sized
planets at $95\%$ confidence. GBTDS will deliver the first WD
exoplanet occurrence-rate constraints beyond the solar neighborhood and identify
prime targets for future characterization.
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Solar and Stellar Astrophysics
Cycle Variation in the Occurrence of Great Soft X-ray Solar Flares
V.N. Obridko, M.M. Katsova and D.D. Sokoloff
Powerful nonstationary processes such as
flares and coronal mass ejections are detected mainly near the solar cycle maxima. However, the analysis of the great
flares since 1975, when the GOES monitoring started, leads us to more precise conclusions. We have traced the latitudinal distribution of X-class
flares and their dependence on the phase of the cycle throughout four cycles. Besides, we compared directly their positions with the
magnetic field structure. It was found that the relatively weak X4–7
flares occur at relatively high latitudes (15 – 20 degrees), where two waves of activity converge, one moving toward the equator and the other, the wave of the following cycle, directed poleward. Such events are observed almost constantly 1 – 2 years before the cycle maximum. At the same time, the number of the powerful X-ray
flares increases sharply during the maximum phase, but the greatest X10
flares are observed at the beginning of the decline phase, where wave interactions still persist, and throughout the decline phase. Note that they are virtually absent during the cycle growth phase. Thus, we conclude that the greatest X-ray
flares begin to appear 1 – 2 years before the maximum number of sunspots in the overlapping phase, when different kinds of activity waves coexist on the Sun at relatively high latitudes, and, then, continue to appear at the boundary, which separates the wave of local fields and the poleward wave of the following cycle.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.IM
The future of high-resolution UV spectroscopy: Science with a UV Échelle spectrograph on the Habitable Worlds Observatory, or a dedicated mission
Annalisa De Cia, David Cont, Kevin Heng et al. (+39 more)
High-resolution UV spectroscopy serves a diversity of science cases, from small bodies to
planets,
stars, and
galaxies, but is currently limited to the Hubble Space
Telescope and bright targets. Major advances require increasing sensitivity by at least one order of magnitude. Here we present the UV science cases for PEGASUS (
Planets, Earths,
Galaxies, And
Stars UV Spectrograph), a UV Échelle high-resolution spectrograph concept, with $R = \lambda/\delta\lambda \sim 100\,000$ (full range 10 000-140 000) and covering 90–400 nm, with a foreseen extension to at least 800 nm. PEGASUS is ideally suited for the Habitable Worlds Observatory (HWO), enabling transformative science across the UV/optical wavelength ranges. PEGASUS will be unique in high sensitivity (effective area) and high spectral resolution – an uncharted territory – as well as robustness, thanks to the simplicity of its design. Its UV science cases include: I) Formation and
evolution of
planets and their habitability: properties of
exoplanets and atmospheres, protoplanetary disks, Solar System bodies; II) Stellar lives and deaths at their extremes: the first
stars and the origin of the elements, compact and massive
stars,
Supernovae; III) Gas and metals in the baryon cycle of
galaxies: the interstellar, circumgalactic, and intergalactic medium and their roles in
galaxy growth. These are essential for the Astro Decadal 2020 Survey, Voyage 2050, and HWO. While this paper focuses on high-impact science enabled by UV high-resolution spectroscopy, PEGASUS will extend into the optical regime and lower spectral resolution, making it a multi-purpose, widely used, workhorse spectrograph for HWO.
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All authors Annalisa De Cia, David Cont, Kevin Heng, Boris Gänsicke, Frank Grupp, Ralf Bender, Helena Lamprecht, Frances H. Cashman, Hsiao-Wen Chen, Valentina D'Odorico, Kevin France, Andrew Fox, Jens Hoeijmakers, Masahiro Ikoma, Wynn Jacobson-Galán, Sean D. Johnson, Mansi Kasliwal, Shingo Kameda, Varsha Kulkarni, Carlo F. Manara, Antonija Oklopčić, Cyrielle Opitom, Lidia Oskinova, Anna F. Pala, Ian U. Roederer, P. Christian Schneider, Peter Scicluna, Daniel E. Welty, Sascha Zeegers, Abel de Burgos Sierra, Miriam García, Avery Kim, Christina Konstantopoulou, Roberto Maiolino, Daniel Pauli, Tanita Ramburuth-Hurt, Philipp Richter, Andreas A.C. Sander, Tomer Shenar, Sergio Simón-Díaz, O. Grace Telford, Anna Velichko
Solar and Stellar Astrophysics
Chromospheric and Transition Region Responses of activities at the base of Coronal Plumes
Biswanath Malaker, Vishal Upendran and Durgesh Tripathi
We present the identification of the co-spatial and co-temporal small scale jets – sometimes also named as jetlets – at the base of the coronal hole plume associated with chromospheric and transition region (TR) flows. We identified 19 jets with coordinated observations recorded on March 19, 2016 using the Solar Dynamics Observatory (SDO) and the Interface Region Imaging Spectrograph (IRIS). Our analysis of \ion{Si}{4} 1393.7~Å line suggests blueshifts mostly in the range of 10 to 33~km~s$^{-1}$, but few less than -10~km~s$^{-1}$, while analysis of co-spatial \ion{Mg}{2}~h~&~k lines show both blueshifts and
redshifts in the range of {–}9 to 17~km~s$^{-1}$. We observed highly asymmetric, enhanced intensity in \ion{Si}{4} line during these events. The upflows observed in transition region through \ion{Si}{4} is strongly correlated with chromospheric downflows observed in \ion{Mg}{2}. We interpret these results as a signature of interchange reconnections creating bi-directional flows – giving rise to upflows (downflows) in transition region (chromosphere).
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP
The ALMA View of the Edge-on Gomez's Hamburger System: A Highly-Dynamic, Asymmetric Protoplanetary Disk Reveals the Earliest Phases of Giant Planet Formation
Charles J. Law, Jensen Lawrence, Richard Teague et al. (+18 more)
Chemical tracers provide some of the strongest observational signatures of ongoing
planet formation and localized dynamical perturbations in protoplanetary disks. In particular, sulfur-bearing molecules are predicted to be enhanced in regions of shock heating, ice sublimation, and gravitational instability. Here, we present high-angular-resolution ($\approx$0.$^{\prime\prime}$2) Atacama Large Millimeter/submillimeter Array observations of $^{12}$CO J=3-2, $^{13}$CO J=3-2, CS J=7-6, and SO J$_{\rm N}$=8$_8$-7$_7$ toward the large, edge-on Gomez's Hamburger ('GoHam'; IRAS 18059-3211) disk. We detect a narrow, one-sided arc of SO emission that peaks near a previously-identified gas over-density, suggesting localized heating around an early-stage giant protoplanet or disk fragment. The edge-on geometry of GoHam enables us to place this chemical signature in the broader context of the disk gas and dust structure. To do so, we map the vertical distribution of molecular gas relative to millimeter- and (sub)-micron-sized dust, identify a pronounced north-south continuum asymmetry, and detect non-Keplerian $^{12}$CO and $^{13}$CO emission indicative of a disk wind. We also derive a dynamical stellar mass of 2.2 $\pm$ 0.5 M$_{\odot}$ and a revised dust-extinction-map-based distance of 139 $\pm$ 24 pc, which places GoHam in the outskirts of the Scorpius-Centaurus association. Together, these observations reveal a highly dynamic disk in which localized sulfur chemistry may trace one of the earliest observable stages of wide-separation giant
planet formation.
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All authors Charles J. Law, Jensen Lawrence, Richard Teague, Zhe-Yu Daniel Lin, Romane Le Gal, Kristina Monsch, Gordian Edenhofer, Hans Baehr, Edwin A. Bergin, L. Ilsedore Cleeves, Anne Dutrey, Coralie Foucher, Stéphane Guilloteau, Thomas J. Haworth, Thomas Henning, John D. Ilee, Marija R. Jankovic, Kamber R. Schwarz, Ilaria Pascucci, Dmitry Semenov, Ke Zhang
Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA
Overmassive No More: The Case for Little Red Dots Hosting Black Hole Seeds as Massive as Single Supermassive Stars
Wendy Q. Sun, Rohan P. Naidu, Hanpu Liu et al. (+17 more)
Little Red Dots (LRDs) display singular properties unlike any known class of AGN or
galaxies, motivating novel mass
estimators for their central engines. Inspired by their similarities to stellar phenomena, here we interpret the LRD continuum as being produced by a pseudo-photosphere. We fit tailored stellar atmosphere models to host-subtracted LRD central engines ("black hole
stars," BH*s) represented by stacks of $117$ objects. Typical BH* continuum spectra are well fit by models in a narrow range of temperatures ($T_{\rm eff}\approx4200-4800$ K), with bolometric luminosities $\approx10^{43-45}$ erg s$^{-1}$, implying pseudo-photospheric radii $\approx700-2000$ au. Based on these parameters, we explore four different approaches to deriving BH* masses: 1) using the surface
gravity from atmosphere models; 2) appealing to the resemblance to super-Eddington phenomena; 3) approximating the escape velocity from the outflowing material; and 4) exploiting the lack of variability to bound the dynamical time. For the typical BH*, all of these methods yield remarkably consistent masses of $\approx10^{4-5}\,M_\odot$, implying a highly super-Eddington luminosity of $L_{\rm{bol}}/L_{\rm{Edd}}\sim5-50$. These mass estimates place BH*s within the scatter of the local scaling relation between black hole mass and host
galaxy stellar mass, providing a self-consistent alternative to "overmassive" black holes that lie $2-3$ dex above it. Crucially, our derived masses are consistent with BH*s arising from single supermassive
stars (SMSs), whose masses cannot exceed $\approx10^{5-6}\,M_\odot$ due to general relativistic instabilities. Furthermore, for our derived $L_{\rm bol}/L_{\rm Edd}$, the sharp cutoff of the LRD luminosity function matches the maximum theoretical mass of an SMS. With LRDs, we may therefore be directly observing the birth of heavy black hole seeds.
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All authors Wendy Q. Sun, Rohan P. Naidu, Hanpu Liu, Anna de Graaff, Jenny E. Greene, Jorryt Matthee, Chris Ashall, John Chisholm, Anna-Christina Eilers, Qinyue Fei, Kasper E. Heintz, Daichi Hiramatsu, Vasily Kokorev, Joel Leja, Zhaoran Liu, Priyamvada Natarajan, Pascal A. Oesch, Robert A. Simcoe, Alberto Torralba, Andrea Weibel
Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA
The Ashes of Supermassive Stars: Globular Cluster-like Aluminum Enhancement in Little Red Dots
V. Kokorev, J. Chisholm, R. P. Naidu et al. (+16 more)
The relative abundances of elements in
galaxies serve as fossil records of the physical conditions and processes by which they were forged. While the Big Bang produced only the lightest elements, subsequent stellar nucleosynthesis imprinted characteristic abundance patterns onto the surrounding gas, set initially by the temperatures reached inside
stars and subsequently shaped by how the processed material was mixed and released. Globular clusters - dense, ancient groups of
stars - provide a striking unique example. Some contain
stars depleted in magnesium and enriched in aluminum, showing that they formed from gas exposed to exceptionally hot hydrogen burning. The
stars responsible remain unknown. Little Red Dots may provide this missing engine. These compact, luminous objects formed at
cosmic epochs similar to those associated with globular-cluster formation and are enshrouded by dense gas whose chemical composition can be measured with the James Webb Space
Telescope. Here, using deep spectroscopy from the SPURS program, we show that this abundance pattern characterizes the LRD central engine: magnesium-depleted and aluminum-enhanced gas with a
metallicity only 1% that of the Sun. This pattern is not produced by ordinary massive
stars at these
redshifts and cannot be mimicked by ionization, gas geometry or dust. Instead, it is reproduced by hot hydrogen burning in fully convective supermassive
stars, with the measured abundances implying masses of at least 10,000 solar masses - approximately 100 times larger than any
star observed in the present-day Universe. Little Red Dots may therefore reveal supermassive
stars during their brief lives or in the immediate aftermath of their direct collapse, simultaneously identifying the long-sought source of globular cluster abundance anomalies and a formation pathway for massive black hole seeds.
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All authors V. Kokorev, J. Chisholm, R. P. Naidu, M. Gieles, S. Finkelstein, D. Berg, H. Akins, A. Taylor, S. Fujimoto, L. J. Furtak, J. Greene, A. de Graaff, K. Hawkins, T. Hsiao, D. Nandal, J. Matthee, S. Monty, P. Rinaldi, M. Boylan-Kolchin
Solar and Stellar Astrophysics
Exploring pulsational instabilities in the O-type supergiant HD 152249
Subharthi Dasgupta, Abhay Pratap Yadav, Sugyan Parida et al. (+1 more)
Variability in O-type supergiants is a well known phenomena although their origin is not fully understood. In very massive, luminous
stars, strange mode pulsations have been suggested to play a role in the variability. Here, we present a preliminary theoretical study of O-type supergiant models corresponding to the observed parameters of the
star HD 152249, which shows line profile variability. Non-adiabatic linear stability analysis with respect to radial perturbations is carried out for the considered models of this
star. Instabilities with growth rates of the order of dynamical timescale are found, with characteristics suggestive of strange modes. Non-linear simulations for selected models indicate that the instabilities could lead to significant envelope inflation, finite amplitude pulsation and may contribute to enhanced mass loss.
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All authors Subharthi Dasgupta, Abhay Pratap Yadav, Sugyan Parida, Santosh Joshi
Solar and Stellar Astrophysics · Cross-listed from physics.plasm-ph
On the modeling of oblique firehose instabilities in regularized Kappa plasmas using ALPS
D. L. Schröder, M. Lazar, H. Fichtner et al. (+2 more)
In-situ measurements in space
plasmas indicate that the velocity distributions of charged particles are not in thermal
equilibrium, deviating from a standard Maxwellian mainly due to anisotropies and suprathermal populations which enhance high-energy tails. Although the Standard ąppa}-Distribution (SKD) is well established in modeling these non-
equilibrium distributions, its application is often viewed controversially due to certain unphysical implications, in particular divergent velocity moments. To address these issues, the Regularized ąppa}-Distribution (RKD) was introduced. Such advanced, in general anisotropic RKDs are invoked here for the first time to investigate oblique firehose instabilities, including those induced by the temperature anisotropy of electrons and
protons, the dominant species in space
plasmas. In weakly collisional
plasmas, both of these instabilities are expected to play significant roles in the self-regulation of the macroscopic properties of space
plasmas (e.g. the expanding solar wind) reported by observations. Is not yet possible to resort to a general dispersion tensor related to RKD
plasmas (whose derivation is still a challenge), instead the Arbitrary Linear
Plasma Solver (ALPS) is exploited here. The unstable solutions obtained for the already established Maxwellian & SKDs successfully validate the capability of ALPS. For RKDs, the instabilities confirm the stimulating effect of suprathermal populations, with lower ąppa} values generally enhancing the firehose growth rates. For the oblique electron firehose instability, RKDs substantially modify the competition between periodic & aperiodic branches only at intermediate angles & can sustain significantly increased growth rates of especially aperiodic modes at highly oblique propagation beyond both the Maxwellian unstable regime & the one that is accessible with SKDs.
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All authors D. L. Schröder, M. Lazar, H. Fichtner, K.G. Klein, D. Verscharen
Solar and Stellar Astrophysics
Open Clusters as Laboratories for Cool Star Evolution: Highlights from the Cool Stars 23 Splinter Session
Deepak Chahal, Beatrice Caccherano, Edward Gillen et al. (+16 more)
Open clusters remain among the most powerful laboratories for calibrating how fundamental stellar properties such as rotation,
magnetic activity, surface chemistry, and internal structure evolve over the pre-main-sequence to main-sequence and post-main-sequence stellar lifetime. Because cluster members share a common age, galactic environment, and initial composition, they provide empirical anchors upon which age-dating techniques such as gyrochronology, activity-age relations, and chemical clocks are built. This paper summarizes the Cool
Stars 23 splinter session - Open Clusters as Laboratories for Cool
Star Evolution, held on 15 June 2026 in Tokyo, Japan. The session comprised one invited review, ten contributed talks, and ten poster pop-up presentations, organized around three themes: the
evolution of rotation,
magnetic activity, and chemical abundances (Li-depletion, [C/N], [Y/Mg], and
neutron-capture elements). We close with a summary of the open questions identified in the panel discussion and the observational and theoretical work that new surveys (e.g. Gaia DR4, 4MOST, WEAVE) and missions (e.g. PLATO, Roman) will enable over the coming years. Together, these efforts will help assess the current state of the field and shape its future directions.
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All authors Deepak Chahal, Beatrice Caccherano, Edward Gillen, Dario J. Fritzewski, Robin D. Jeffries, Kevin R. Covey, Andrew Boyle, Lyra Cao, Federica Chiti, Alexander Hughes, Leslie Moranta, Natalie R. Myers, Emily K. Pass, Phil Van-Lane, Fábio C. Wanderley, Mackenna L. Wood, Stephanie T. Douglas, David Montes, Loredana Prisinzano
Solar and Stellar Astrophysics
An analysis of four stellar rings
E. Paunzen, J. Florian, A. Gütl-Wallner et al. (+9 more)
About 50 years ago, one thousand ring-like structures (called stellar-rings) were discovered by Isserstedt (1968). They were believed to be groups of young
stars formed by shell-like triggered
star formation, which would make them excellent tracers of spiral arms, for example. Neglected for 40 years, we used highly accurate kinematic, astrometric, and photometric data to investigate the four most prominent stellar rings. The aim is to investigate if those structures are indeed physically related groups of
stars. We used proper motions and parallaxes from the Gaia DR2 to calculate distances and to search for common properties. Color-magnitude diagrams using BVJHKs measurements were investigated and isochrones fitted. None of the four stellar rings consists of a physically related group of young
stars. The location of
stars in the line-of-sight mimics a ring-like structure on the sky. The color-magnitude diagrams are typical for an integrated field population and not for a young
star cluster, for example. The currently available data are sufficient to analyze ring-like structures with a high statistical significance. This allows a new search for such structures in the Milky Way.
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All authors E. Paunzen, J. Florian, A. Gütl-Wallner, A. Herdin, E. Kralofsky, K. Leschinski, M. Mach, H.M. Maitzen, M. Prišegen, M. Rockenbauer, M. Rode-Paunzen, S. Wallner
Solar and Stellar Astrophysics
Now you see me, now you don't: Deep, long-lived obscuration events of white dwarfs by dust and debris from disintegrating planetesimals
J. van Roestel, S. Bhattacharjee, J. A. Guidry et al. (+22 more)
Transiting dust and debris around white dwarfs provide a direct view of planetary disruption. We report six previously non-variable white dwarfs that underwent single, deep obscuration events, including ZTF~J2327+0019, which fully disappeared for more than a year. These discoveries raise the number of confirmed debris-transit white dwarfs to 20. In addition, the systematic search of negative alerts from the Zwicky Transient Facility (ZTF), followed by photometric and spectroscopic observations, also identified three hot ($T\approx 55000$–$92000$ K) irregular variables as candidate debris hosts. The six confirmed events last weeks to years, reach transit depths of 25–100%, and show no recurrence over the 8-yr ZTF baseline. They are therefore much deeper and longer lived than the recurring, minute-scale transits typical of previously known systems. The observed reddening is consistent with dust extinction, and high-speed photometry reveals minute-scale structure in several systems, indicating rapidly moving material close to the white dwarf. ZTF~J2327+0019 also shows a simultaneous WISE infrared brightening, the first observed coincidence of infrared brightening and optical obscuration in a white dwarf. Two of the six hosts, one DAZ and one DBZ, are strongly metal enriched, with
accretion rates among the highest known for their spectral types and minimum accreted masses of $3.5\times 10^{19}$ and $2\times 10^{24}$ g. We interpret the one-off transit events as collisions between planetesimal remnants and pre-existing circumstellar debris that trigger collisional cascades: short-timescale variability traces dust-producing fragments crossing the
star, while slower changes trace the build-up and clearing of an optically thick dusty structure. In contrast, ZTF~J0436+5728 shows a coherent 5.910-h period, consistent with a rocky body shedding material near the tidal-disruption radius.
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All authors J. van Roestel, S. Bhattacharjee, J. A. Guidry, I. Caiazzo, T. Cunningham, J. J. Hermes, S. Torres, Z. Vanderbosch, A. J. Drake, M. Hollands, B. T. Gänsicke, K. De, T. Prince, A. C. Rodriguez, L. B. Ould Rouis, A. A. Mahabal, E. C. Bellm, M. W. Coughlin, M. J. Graham, D. Hale, G. Helou, J. C. Jaimes, M. Kasliwal, R. R. Laher, R. L. Reed
Solar and Stellar Astrophysics
The role of specific entropy in the onset of the luminosity bump
S. Hekker
During their
evolution,
stars follow a distinct path in luminosity–temperature space. Low-mass
stars that have exhausted hydrogen in their core follow the so-called red-giant branch which is predominantly in the direction of increasing luminosity and decreasing surface temperature. The luminosity bump is a temporary decrease in luminosity against this otherwise increasing trend. The bump is present in both observations and stellar
evolution models. However, with canonical physics included in the models, the bump in computed evolutionary tracks appears at higher luminosities than the observed bump. To understand why the bump appears later in the models than in observations, the physics of the bump needs to be unravelled. The end point of the bump is well-understood; however, the onset of the bump is still an enigma. Here, we report on the physical origin of the onset of the luminosity bump. We show that the difference in specific
entropy at the mean molecular weight discontinuity decreases due to the discontinuity moving in. This decrease is attributed to the decrease in the ratio of the temperature to the pressure at smaller radii. Upon reaching a critical value the specific
entropy difference at the mean molecular weight discontinuity is reduced sufficiently to reduce the specific
entropy in the convective envelope. The latter is a key signature of the bump. Hence the
evolution of the specific
entropy at the mean molecular weight discontinuity provides a viable description for the onset of the bump.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP
XUE. ProDiMo models of internally and externally irradiated planet-forming disks around 0.3-4.0 solar mass stars (The IRIS project I)
Jenny Frediani, Konstantin V. Getman, Peter Woitke et al. (+18 more)
Most
stars and
planets form in massive
star-forming regions, where disks are exposed to external far-ultraviolet (FUV) radiation from nearby O- and B-type
stars. The combined effects of stellar irradiation and external FUV fields on terrestrial
planet-forming regions (< 10 au) across stellar masses remain unclear. We investigate how internal UV and X-ray irradiation and external FUV fields affect mid-infrared (mid-IR) gas emission and the atmospheric carbon-to-oxygen (C/O) ratios inferred in T Tauri and Herbig Ae/Be disks. We compute disk structures with ProDiMo and synthetic spectra with FLiTs, convolved to a representative JWST/MIRI-MRS resolution (R$\sim$2680). (1) We present the Internal and external irRadIation of diSks (IRIS) grid: four model sets spanning stellar masses of 0.3-4.0 solar masses, including stellar X-ray
flares and an external FUV field of 1e4 G0 (Habing units). (2) We predict increasing flux densities with stellar mass for key atomic and molecular mid-IR tracers. (3) External FUV irradiation enhances CH3+ and H2 emission, whereas FUV-induced disk truncation yields inner disk chemistry resembling that of disks irradiated only by their host
stars. (4) Mid-IR H2O, CO2, and C2H2 line ratios imply carbon-rich compositions (C/O$\sim$1-10) in the warm emitting layers of T Tauri and Herbig Ae/Be disks, primarily reflecting stellar irradiation, with little sensitivity to external FUV irradiation. The IRIS grid provides a flexible framework for interpreting JWST and future Extremely Large
Telescope (ELT) infrared disk observations across a broad range of stellar properties and irradiation conditions. Future models should include FUV-driven photoevaporative winds, X-ray radiative transfer, and time-dependent X-ray irradiation.
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All authors Jenny Frediani, Konstantin V. Getman, Peter Woitke, Bayron Portilla-Revelo, Eric D. Feigelson, Christian Rab, Arjan Bik, María Claudia Ramírez-Tannus, Rens Waters, Thomas Henning, Inga Kamp, Germán Chaparro, Sebastián Hernández, Simón Rodríguez, Pablo Cuartas-Restrepo, Andrew Winter, Thomas J. Haworth, Thomas Preibisch, Sierk E. van Terwisga, Veronica Roccatagliata, Alexis Brandeker