CyberSec Research

We report the discovery of the ultracool dwarf binary system J1250+0455AB, a low-mass (M$_\odot$$_\mathrm{tot} <$ 0.2 M$_\odot$) system in which the components straddle the M/L dwarf boundary. The binary was resolved through near-infrared adaptive optics imaging with LUCI1-SOUL on the Large Binocular Telescope, revealing a projected angular separation of 0.17 $\pm$ 0.015$\arcsec$, which, combined with a system distance of $71 \pm 5.8$\,pc, corresponds to a physical separation of 12.2 $\pm$ 1.5\,AU at a position angle of 84.8 $\pm$ 0.2{\deg}. We estimated the orbital period of J1250+0455AB to be 156 $\pm$ 8\,yr, the bolometric luminosities of the primary and secondary luminosities as $\log (L_\mathrm{bol} / L_\odot) = -3.45 \pm 0.04$ and $-3.58 \pm 0.04$, respectively, with the spectral types of M9 and L0 determined through binary template fitting and spectrophotometric relations. This binary system is part of a hierarchical triple with a separation of 10.44$\arcsec$ from its primary. We estimated the age of the system from the rotational period of the primary star as $0.56^{+0.07}_{-0.06}$ Gyr. Using evolutionary models, for each component we estimate the mass [0.079 $\pm$ 0.002\,M$_\odot$ / 0.072 $\pm$ 0.003\,M$_\odot$], effective temperature [2350 $\pm$ 38\,K / 2200 $\pm$ 43\,K], and radius [0.113 $\pm$ 0.003\,R$_\odot$ / 0.108 $\pm$ 0.002\,R$_\odot$]. Based on the system's binding energy, total mass, and separation, J1250+0455AB is predicted to be a highly stable system, remaining bound for $>$ 10\,Gyr. J1250+0455AB extends the growing population of UCD benchmark systems, providing a new system for refining evolutionary theories at the lowest stellar masses into the substellar regime.

Neutrino-cooled accretion disks can form in the aftermath of neutron-star mergers as well as during the collapse of rapidly rotating massive stars (collapsars) and the accretion-induced collapse of rapidly rotating white dwarfs. Due to Pauli blocking as electrons become degenerate at sufficiently high accretion rates $\dot{M}$, the resulting 'self-neutronization' of the dissociated accreting plasma makes these astrophysical systems promising sources of rapid neutron capture nucleosynthesis (the r-process). We present a one-dimensional general-relativistic, viscous-hydrodynamic model of neutrino-cooled accretion disks around black holes. With collapsars, super-collapsars and very massive star collapse in mind, we chart the composition of the accretion flow and systematically explore different radiatively efficient and inefficient accretion regimes with increasing $\dot M$, across a vast parameter space of $\dot{M}\sim 10^{-6}-10^6 M_\odot \,\text{s}^{-1}$, black hole masses of $M_\bullet\sim 1 - 10^4 M_\odot$ and dimensionless spins of $\chi_\bullet \in [0,1)$, as well as $\alpha$-viscosity values of $\alpha\sim 10^{-3}-1$. We show that these accretion regimes are separated by characteristic thresholds $\dot{M}_{\rm char}$ that follow power laws $\dot M_{\rm char}\propto M_{\bullet}^\alpha \alpha^\beta$ and that can be understood based on analytic approximations we derive. We find that outflows from such disks are promising sites of r-process nucleosynthesis up to $M_\bullet \lesssim 3000 M_\odot$. These give rise to lanthanide-bearing 'red' super-kilonovae transients mostly for $M_\bullet \lesssim 200-500 M_\odot$ and lanthanide suppressed 'blue' super-kilonovae for larger $M_\bullet$. Proton-rich outflows can develop specifically for large black hole masses ($M_\bullet \gtrsim 100 M_\odot$) in certain accretion regimes, which may give rise to proton-rich isotopes via the $\nu$p-process.

We conduct three-dimensional hydrodynamical simulations of jets launched into a dense shell, reproducing two rings in a bipolar structure that resemble the two dusty rings of the planetary nebula (PN) NGC 1514. The scenario we simulate assumes that a strong binary interaction enhanced the mass loss rate from the asymptotic giant branch (AGB) stellar progenitor of NGC 1514, and shortly thereafter, the main-sequence companion accreted mass from the AGB star, launching a pair of jets. We find that adiabatic flows, where radiative losses are negligible, produce prominent rings, as observed in the infrared in NGC 1514. In contrast, when radiative cooling is significant, the rings are thin and faint. Our results reinforce the prevailing notion that jets play a substantial role in shaping planetary nebulae (PNe). More generally, as the binary companion to the central star of NGC 1514 avoided common envelope evolution, our results suggest that jets play a major role in many binary systems experiencing stable mass transfer at high rates. This conclusion complements the view that jets play a significant role in unstable mass transfer, specifically in common envelope evolution. Studies of strongly interacting binary systems, whether stable or not, should include jets. If jets continue to be active after ring formation, the outcomes are circum-jet rings, as observed in some other PNe and core-collapse supernova remnants.

(abridged) We aim to derive a robust estimate of the most important parameters describing the physical nature of T CrB, trace the accretion history onto its white dwarf, and account for the unexpected delay in the occurrence of the new outburst: the SAP prior to 1946 was brighter, and it was followed by the nova eruption within 6 months from its conclusion. This time the 2015-2023 SAP has been fainter and two years past its conclusion no new eruption has yet taken place. During 2005-2025, a period covering SAP and the preceding quiescence, we collected a massive amount of photometric and spectroscopic observations that we have analyzed together with Swift UVOT data. Guided by the results of the orbital solution and in particular by the radiative modeling to which we subjected the whole set of available data, we found for T CrB a binary period of 227.5528 days, an inclination of 61 deg, and masses of 1.35 Msun and 0.93 Msun for the white dwarf and the M3III companion, respectively, making mass transfer dynamically stable. The red giant fills completely its Roche lobe, and at Vrot sin(i)=4.75 +-0.26 km/s it is rotating much slower that the 16 km/s co-rotation value. The ~20 deg azimuth of the hot spot, implied by the hump shaping the optical light curve in quiescence, fixes the outer radius of the disk to 58 Rsun, the same as the canonical value expected from disk theory. In quiescence the disk is cold and mostly neutral. SAP has been caused by an inside-out collapse of the disk, during which the mean accretion rate onto the WD has been ~28x larger than in quiescence. SAP ended in April 2023, but from May 2024 mass-flow has intensively resumed at disk inner radii while the collapse wave reached the outer portions of the disk; the consequent revamp in mass accretion could fill the gap inherited by the fainter 2015-2023 SAP and eventually lead the WD accreted shell to ignition.

We present a first-order analytical model for line-of-sight velocity residuals, defined as the difference between observed velocities and those predicted by a fiducial model, assuming a flared, nearly axisymmetric disk with the perturbations in disk surface height $\delta h(r)$, inclination $\delta i(r)$, and position angle $\delta\mathrm{PA}(r)$. Introducing projection-deprojection mapping between sky-plane and disk-frame coordinates, we demonstrate that the normalized velocity residuals exhibit Fourier components up to the third harmonic ($\sin3\phi$ and $\cos3\phi$). Moreover, we show that the radial profiles of $\delta h(r)$, $\delta i(r)$, and $\delta\mathrm{PA}(r)$ can be uniquely recovered from the data by solving a linear inverse problem. For comparison, we highlight factors that are not considered in previous models. We also outline how our framework can be extended beyond the first-order residuals and applied to additional observables, such as line intensities and widths.

Inferences from observations clearly show that mixing in stars extends beyond the convective boundaries defined by mixing length theory. This triggered the proposal of a variety of prescriptions to include additional mixing in stellar models. These prescriptions typically introduce free parameters to set the extent of the additional mixing and may also introduce numerical parameters. In the case of exponential overshooting, one must decide the threshold at which the exponential decay of the mixing coefficient can be treated as zero. Using the MESA stellar evolution code, I explore the effect of varying this parameter on asteroseismic models of main-sequence stars with growing convective cores. From this, I conclude that overshoot_D_min should be set to $10^{-2}$ cm$^2$/s or lower for these stars. The default value in MESA is four orders of magnitude higher than this recommendation, which results in discontinuous evolution.

The formation mechanisms of merging binary black holes (BBHs) observed by the LIGO-Virgo-KAGRA collaboration remain uncertain. Detectable eccentricity provides a powerful diagnostic for distinguishing between different formation channels, but resolving their eccentricity distributions requires the detection of a large number of eccentric mergers. Future gravitational wave detectors such as the Einstein Telescope and Cosmic Explorer will detect tens of thousands of BBH mergers out to redshifts $z \ge 10$, making it critical to understand the redshift-dependent evolution of eccentricity distributions. We simulate this evolution for two key channels: dynamical assembly in globular clusters (GCs), which leads to rapid, eccentric mergers; and hierarchical triples in the field, where three-body dynamics can induce eccentricity in the inner binary. When considering all BBH mergers, the GC channel dominates overall, consistent with previous studies. However, when focusing on mergers with detectable eccentricity in next-generation detectors, we find that hierarchical triples dominate the eccentric merger rate at $0\le z \le 4$, with GC mergers becoming competitive at higher redshifts. Across all model variations, eccentric mergers in the local Universe ($z\lesssim 1$) have significant contributions from field triples, challenging the common view that such systems primarily form in dense environments. We show that, regardless of cluster and stellar evolution uncertainties, hierarchical triples contribute at least 30 per cent of eccentric mergers across a large range of redshifts.

The estimation of CME arrival time strongly depends on the CME propagation models in interplanetary space and the geometrical aspects of the CME model. We conducted ensemble simulations of CMEs propagation with various elliptical cone shapes to study the relation between the CME speed and the optimum cone shape. We numerically searched for the best elliptical aspect ratio of the elliptical cone for each CME in our CME-ICME pair data. We found that the fast CMEs tend to have a higher elliptical aspect ratio (more circular) than the slower CMEs (flattened). Our results suggest that a fast CME gives a stronger push to all directions, which results in a more circular shape of the leading-edge. We believe that this velocity-dependent behavior is related to the different Lorentz force strengths during the early expansion of a CME.

We present near-infrared JHKs and narrow-band H2(1-0) photometric observations of the W51A region, obtained with GTC EMIR, aiming to characterize its young stellar population and provide mass estimates for individual cluster members and the proto-clusters. Our observations reveal over 3000 new sources, out of which 88 are located in the proto-clusters, W51 IRS2 and W51 Main. The average extinction (AV), measured from the J-H color, of sources is 19 AV in W51 IRS2 and 14 AV in W51 Main. We document 17 new instances of H2 emission in the region by utilizing observations from the H2(1-0) narrow-band filter. Despite limited completeness, we estimated masses for each cluster member and estimated the total cluster mass to be in the range of 900-4700 solar masses for W51 IRS2 and 500-2700 solar masses for W51 Main, using an assumed age range of 1-3 Myr. We measured the initial mass function (IMF) in the proto-clusters assuming a range of ages from 1-3 Myr and found that the IMF slopes for both proto-clusters are consistent with the Salpeter IMF in the mass range greater than or equal to 8 solar masses within 1 to 2 sigma.

Accretion shocks are thought to play a crucial role in the early stages of star and planet formation, but their direct observational evidence remains elusive, particularly regarding the molecular tracers of these processes. In this work, we searched for features of accretion shocks by observing the emission of SO and SO$_2$ using ALMA in Band 6 towards nearby Class I protostars. We analyze the SO and SO$_2$ emission from Oph IRS 63, DK Cha, and L1527, which have different disk inclination angles, ranging from nearly face-on to edge-on. SO emission is found to be concentrated in rings at the centrifugal barriers of the infalling envelopes. These rings are projected onto the plane of the sky as ellipses or parallel slabs, depending on the inclination angles. Spiral-like streamers with SO emission are also common, with warm ($T_{\rm ex} > 50$ K) and even hot ($T_{\rm ex} \gtrsim 100$ K) spots or segments of SO$_2$ observed near the centrifugal barriers. Inspired by these findings, we present a model that consistently explains the accretion shock traced by SO and SO$_2$, where the shock occurs primarily in two regions: (1) the centrifugal barriers, and (2) the surface of the disk-like inner envelope outside the centrifugal barrier. The outer envelope gains angular momentum through outflows, causing it to fall onto the midplane at or outside the centrifugal barrier, leading to a disk-like inner envelope that is pressure-confined by the accretion shock and moves in a rotating-and-infalling motion. We classify the streamers into two types--those in the midplane and those off the midplane. These streamers interact with the inner envelopes in different ways, resulting in different patterns of shocked regions. We suggest that the shock-related chemistry at the surfaces of the disk and the disk-like inner envelope warrants further special attention.

Context: SN 2024ggi is a Type II supernova (SN) discovered in the nearby galaxy NGC 3621 (D $\approx6.7\pm0.d$ Mpc) on 2024 April 03.21 UT. Its proximity enabled a detailed investigation of the SN's properties and its progenitor star. This work focuses on the optical evolution of SN 2024ggi at the nebular phase. Aims: We investigate the progenitor properties and possible asymmetries in the ejecta by studying the nebular phase evolution between days 287 and 400 after the explosion. Methods: We present optical photometry and spectroscopy of SN 2024ggi during the nebular phase, obtained with the Las Campanas and Gemini South Observatories. Four nebular spectra were taken at 287, 288, 360, and 396 days post-explosion, supplemented by late-time $uBVgri$-band photometry spanning $320-400$ days. The analysis of the nebular emission features is performed to probe ejecta asymmetries. Based on the [O I] flux and [O I]/[Ca II] ratio, and comparisons with spectra models from the literature, we arrive to an estimate of the progenitor mass. Additionally, we construct the bolometric light curve from optical photometry and near-infrared data to derive the synthesized nickel mass. Results: Our analysis suggests a progenitor zero-age-main-sequence mass between $12-15 M_\odot$. The late-time bolometric light curve is consistent with a synthesized $^{56}$Ni mass of $0.05-0.06 M_\odot$. The line profiles exhibit only minor changes over the observed period and suggest a roughly symmetrical ejecta, with a possible clump of oxygen-rich material moving towards the observer. No signatures of circumstellar material interaction are detected up to 400 days after the explosion.

We propose a model for dust formation in Type II supernovae (SNe) interacting with confined circumstellar material (CSM), motivated by recent time-domain surveys that have revealed a substantial fraction of SN progenitors to be surrounded by CSM ejected shortly before core-collapse. We simulate the pre-SN mass eruption and the resulting confined CSM using the open-source code CHIPS, and follow the subsequent evolution of the SN ejecta and its interaction with the CSM. We show that a cold dense shell (CDS) is formed at the radiative shock under a wide range of conditions and later undergoes rapid adiabatic cooling during free expansion, leading to efficient dust condensation. The resulting dust mass ranges from $\sim10^{-3}\,M_\odot$ to $0.1\,M_\odot$, depending on the mass and spatial extent of the CSM. We further calculate the infrared (IR) emission from the newly formed dust and find broad consistency with observations of SN~1998S. Notably, the IR light curve exhibits a rapid rise within $\lesssim10\,{\rm d}$, closely resembling that of kilonovae (KNe). This suggests that dust emission powered by confined CSM interaction may be also discovered in KN searches. Moreover, the high-density environment of the CDS may allow dust grains to grow to larger sizes, enhancing their survivability against destruction by reverse shocks propagating from the interstellar medium at later times.

V Sge is a peculiar, highly luminous long-period (12.34h) binary star that can display a super-soft X-ray emitting component when in the faint phase of its V~ 10-13mag variability range. Apparently undergoing Eddington-limited accretion from its more massive secondary, it is in a very rare, short-lived evolutionary phase towards the double degenerate channel. Its complex and highly variable optical emission features, from Balmer and Heii to high-ionisation lines, including strong fluorescence features, have been challenging to interpret, especially given the absence of any absorption lines associated with photospheric features from either stellar component. With the detailed properties of V Sge, especially the donor, still controversial, we undertook a VLT/X-Shooter campaign over three months in 2023, obtaining high S/N, high resolution spectra that revealed multiple components in both high- and low-ionisation lines. This allows us to track V Sge's principal emitting regions via Doppler tomography, obtaining new insights into high accretion-rate dynamics. In particular, we identify a stationary, double-peaked emission core which we interpret as a circumbinary ring, analogous to SS433. This enables us to derive limits on the system masses. Furthermore, we find very broad emission-line wings whose mean velocity can vary over hundreds of kilometres per second on timescales of decades, yet ``flip'' between states in <1 week. We show that the super-soft X-ray source interpretation is able to account for these and other observational attributes significantly better than the hot binary model, concluding that V Sge could be one of the brightest known Galactic super-soft sources.

Runaway stars are defined as stars that depart from their birth clusters at high peculiar velocities. There are two main mechanisms for the formation of runaway stars, i.e., the binary-supernova scenario (BSS) and the dynamical ejection scenario (DES). Investigating the binary fraction of runaway stars is an important step in further exploring the relative significance of the two mechanisms. We analyzed the binary fraction of 203 Galactic B-type runaway stars identified in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope Data Release 8 database. Our analysis of radial velocity variations in the runaway star sample reveals an observed spectroscopic binary fraction of $5.4\%\pm 1.6\%$, representing the proportion of objects that exhibit statistically significant variations in radial velocity with amplitudes larger than $\rm 16~km~s^{-1}$. We employed a Monte Carlo method to correct for observational biases and determined an intrinsic binary fraction of $27\%\pm 8\%$. The period and mass ratio distributions that best reproduce the observation are $f(P)\propto P^{-5.7}$ for $1\leq P\leq 1000$ days, and $f(q)\propto q^{-3.6}$ for $0.1\leq q\leq 1.0$, indicating a preference for binaries with shorter periods and less massive companions compared to a uniform distribution. The intrinsic binary fraction, in combination with previous studies on the binary fractions of runaway stars formed by the BSS and the DES, implies that both scenarios contribute comparably to the formation of Galactic B-type runaway stars, where the ratio of the BSS to the DES is 0.86.

When dense cores in molecular clouds or filamentary structures collapse and form protostars, they may undergo fragmentation and form binary or multiple systems. In this paper, we investigated the key mechanisms influencing fragmentation by comparing the physical conditions of fragmented and unfragmented dense cores (~0.1 pc) in Orion A. Utilizing archival submillimeter continuum data from the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/Submillimeter Array survey of Class 0 and I protostars at a 0.''1 resolution, we identified 38 dense cores hosting single protostars and 15 cores hosting binary or multiple systems. We measured the dense cores properties with the Herschel dust temperature, Nobeyama 45m N$_2$H$^+$ J=1-0, and JCMT polarization data. Our results reveal that the dense cores hosting binary/multiple systems exhibit significantly higher density and Mach number compared to those hosting single protostars, while there are no correlations between the occurrence of fragmentation and the energy ratios of turbulence and magnetic field to gravity. Our results suggest that the higher density and supersonic turbulence of the dense cores can lead to local collapse and fragmentation to form binary/multiple systems, while the magnetic field has limited influence on fragmentation in the dense cores in Orion A.

Cataclysmic variable (CVs) are close interacting binaries in which a white dwarf accretes materials from a low mass main sequence companion. CVs can experience nova eruptions due to low mass transfer rates. In the standard theory of CV evolution, the ejected materials during nova eruptions are assumed to leave the system in the form of fast, isotropic, optically thick winds, which predicts that novae only result in positive variation (expansion) of orbital period (i.e. positive $\Delta P$). In addition, the angular momentum losses (magnetic braking and gravitational radiation) only predicts a steady long-term decay in the orbital period of CVs, i.e. $\dot P$ is negative. Interestingly, an observation lasting over 30 years reveals positive and negative values for both $\Delta P$ and $\dot P$ in CVs, strongly conflicting with the standard evolutionary patterns. However, it cannot be excluded that these observations originate from short-term phenomena caused by nova eruptions because of a short timescale of observations. In this paper, we model the effect of instantaneous nova eruptions on the evolution of CVs, considering three mechanisms associated with mass loss in nova eruptions, including fast wind, Frank jet and binary-driven mass loss. By assuming that the observed $\Delta P$ and $\dot P$ are dominated by short-term phenomena, our results show that the binary-driven mass loss can explain almost all of the observations of normal CVs. However, the Frank jet may be needed for some of long-period CVs with evolved companions.

While significant advances have been made in photometric classification ahead of the millions of transient events and hundreds of supernovae (SNe) each night that the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will discover, classifying SNe spectroscopically remains the best way to determine most subtypes of SNe. Traditional spectrum classification tools use template matching techniques (Blondin & Tonry 2007) and require significant human supervision. Two deep learning spectral classifiers, DASH (Muthukrishna et al. 2019) and SNIascore (Fremling et al. 2021) define the state of the art, but SNIascore is a binary classifier devoted to maximizing the purity of the SN Ia-norm sample, while DASH is no longer maintained and the original work suffers from contamination of multi-epoch spectra in the training and test sets. We have explored several neural network architectures in order to create a new automated method for classifying SN subtypes, settling on an attention-based model we call ABC-SN. We benchmark our results against an updated version of DASH, thus providing the community with an up-to-date general purpose SN classifier. Our dataset includes ten different SN subtypes including subtypes of SN Ia, core collapse and interacting SNe. We find that ABC-SN outperforms DASH, and we discuss the possibility that modern SN spectra datasets contain label noise which limit the performance of all classifiers.

We analysed the cooling of white dwarfs in the globular cluster 47 Tucanae using deep observations from the Hubble Space Telescope that resolve the white dwarf cooling sequence to late enough cooling times that the white dwarf core has begun to crystallise and the envelope has become convectively coupled to the core. At such late cooling times, both the state of matter assumed for ions in the treatment of element diffusion and the thickness of the outer H envelope become important considerations for modelling white dwarf cooling. Using the stellar evolution software Modules for Experiments in Stellar Astrophysics (MESA), we created a suite of white dwarf cooling models for different treatments of element diffusion, as well as different values of the white dwarf mass and H envelope thickness parameters. Three different diffusion scenarios were considered: i) the standard MESA implementation, which implicitly uses an ideal gas approximation for the ions, ii) a custom modified implementation that accounts for non-ideal gas effects, and iii) no diffusion. An unbinned likelihood analysis was performed to compare these cooling models to the observations. This work both constrains the values of parameters important for modelling white dwarf cooling and tests the implementation of element diffusion in MESA to late cooling times. We find that models with thicker H envelopes are preferred and that the standard MESA diffusion treatment produces a best-fitting model that well reproduces the cumulative white dwarf luminosity functions of the observations.