Solar and Stellar Astrophysics
BSN-VIII: Detailed Photometric Modeling of Ten W UMa Contact Binaries and a Revised Empirical Period-Mass Relationship
Atila Poro, S. Javad Jafarzadeh, Kai Li et al. (+15 more)
This study continues our ongoing research on contact binary systems by presenting a detailed analysis of 10 targets. Ground-based observations from six different observatories were conducted and used together with TESS data for the analysis process. Photometric data from our observations were reduced with the recently developed AutoWISP pipeline, yielding high-quality light curves with reliable precision for analysis. An investigation of orbital period variations identifies long-term trends in six of the ten analyzed binaries, including three that also display cyclic variations. Four targets show essentially constant orbital periods. The secular trends are attributed to mass transfer. The cyclic modulations in three systems are caused by either
magnetic activity cycles or the Light-Travel Time Effect (LTTE) of a third body, while that in the remaining system is solely due to the LTTE. The BSN application was used to model the photometric light curves of the 10 target binaries. Iterative fitting and MCMC refinement provided robust estimates of the system parameters, while starspot modeling was applied for systems showing O'Connell-effect asymmetries. We refine the empirical orbital period-mass relationship for short-period contact binaries by analyzing a homogeneous
dataset of systems and deriving an updated primary-mass calibration based on the spectroscopic subset. Using this calibrated relation, the fundamental parameters of the studied systems were subsequently estimated.
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All authors Atila Poro, S. Javad Jafarzadeh, Kai Li, Jean-François Coliac, Sabrina Baudart, Meng Guo, Patrick Wullaert, David Valls-Gabaud, Anis Ben Lassoued, Daniel Verilhac, Laurent Corp, Emmanuel Foguenne, Hervé Lerat, Jean-Baptiste Marquette, Serge Vasseur, Anica Lekic, Fahri Alicavus, Neslihan Alan
Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc
Creation of an inflationary epoch from an Emergent Universe through quantum tunneling
Eduardo I. Guendelman, Ramon Herrera and Pedro Labraña
We consider that the very early Universe was well described by an Emergent Universe, whose geometry was that of a static and classically stable Einstein Universe, which is possible in certain Two Measures Theories (TMT). These solutions do not have a big bang, but rather they extend to arbitrarily early times. The TMT we consider are theories with spontaneously broken scale invariance and contain a dilaton field, crucial for the implementation of the scale invariance, that after the
symmetry breaking acquires a nontrivial effective potential, which we study in the Einstein frame. Using the conserved quantity associated with the dilaton, we recast the cosmological
evolution in terms of an effective potential $V(\dot{\phi})$, which exhibits a divergent barrier separating the static solution from the subsequent expanding
evolution. Quantizing the corresponding minisuperspace
Hamiltonian, we evaluate the tunneling probability in the WKB approximation. The tunneling action is well defined and finite once the branch of the square root is fixed by requiring a consistent semiclassical interpretation of the tunneling probability. We find that the barrier gives rise to a logarithmic contribution to the tunneling exponent, yielding a power-law rather than the usual exponential dependence of the tunneling probability, which for the parameter values considered is close to unity. The Universe thus emerges with a finite scale factor into a superinflationary phase that develops into slow-roll inflation. This
quantum creation of an inflationary Universe does not represent a
quantum creation of spacetime, which exists before the tunneling.
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High Energy Astrophysical Phenomena
Formation of black hole stars via star–black hole collisions
Yanlong Shi, Qingru Hu, Zhenghao Xu et al. (+2 more)
In dense stellar environments such as globular clusters and active galactic
nucleus (AGN) disks, stellar-mass black holes (sBHs) may frequently collide with massive
stars. We investigate this process using semi-analytic models, three-dimensional hydrodynamical simulations, and one-dimensional stellar
evolution calculations, focusing on collisions between sBHs and a $100\,M_\odot$ main-sequence
star. We find that gas drag retains the BH within the stellar envelope unless the impact velocity exceeds $\sim2\sqrt{G(M_\star+M_\bullet)/R_\star}$. The post-collision outcome depends primarily on the BH-to-
star mass ratio. For $M_\bullet\gtrsim30\,M_\odot$, the retained envelope is either quasi-spherical or disc-like, but remains dynamically unstable because of shock heating. In contrast, for $M_\bullet\lesssim10\,M_\odot$, the collision forms a “black hole
star” (BH*): a quasi-hydrostatic, extended stellar envelope surrounding the embedded BH. These results agree with our analytic prediction that BH* formation necessarily requires $M_\bullet\lesssim0.2\,M_\star$. Follow-up MESA calculations further show that, for these low-mass BHs, the shock-heated remnant thermally relaxes without triggering runaway expansion. We discuss several astrophysical implications of BH*s, including their
evolution, the possibility of gravitational-wave events from BH binaries assembled within a stellar envelope, and repeated
star–sBH collisions as a pathway for rapid BH growth in dense stellar systems. This mechanism may contribute to the formation of massive BHs in high-
redshift nuclear
star clusters and may be relevant to the origin of the “little red dots” discovered by JWST.
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All authors Yanlong Shi, Qingru Hu, Zhenghao Xu, Douglas N. C. Lin, Norman Murray
High Energy Astrophysical Phenomena
Dynamical formation of high-eccentricity compact binaries through BH–BH*/TZO collisions
Qingru Hu, Yanlong Shi, Douglas N. C. Lin et al. (+1 more)
The rapidly accumulating discoveries of binary stellar-mass black-hole (sBH) coalescences, detected by LIGO, have opened a new window into the formation and
evolution of compact binaries. In particular, residual orbital eccentricity may provide a distinctive signature of their formation channels. Here, we investigate a scenario in which high-eccentricity compact binaries form through the sequential capture of multiple sBHs by massive main-sequence
stars, using a combination of hydrodynamical and semianalytic few-body simulations. We find that sBHs with $M_\bullet\lesssim 0.2\,M_{\star}$ can be captured by massive
stars and settle into a quasi-hydrostatic black-hole
star (BH*) through gas dynamical friction. A subsequent encounter with a second sBH can then produce a compact binary embedded within the stellar envelope. Our hydrodynamical simulations show that through captures with small impact parameter, some binaries are born with high eccentricity ($e\gtrsim 0.5$), with its orbital frequency already entering the LISA band. Our semianalytic models further demonstrate that gas dynamical friction can pump the eccentricity to $e_{\rm 10\,Hz}>0.9$ before gravitational-wave emission eventually circularizes the binary during the final stage of coalescence. Once formed, the binary can merge quickly in $\sim 10$ hours. This channel may operate in dense stellar environments, such as
star clusters and active galactic
nucleus (AGN) disks. The same mechanism can also be applied to Thorne-Żytkow objects. A high-eccentricity binary in the LIGO band could therefore provide a distinctive signature of this formation scenario.
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All authors Qingru Hu, Yanlong Shi, Douglas N. C. Lin, Norman Murray
Astrophysics of Galaxies
The Limited Lifetime of Self-Gravitating Accretion Disks In Galactic Centers
Isaac Shlosman
Globally self-gravitating
accretion disks in active galactic
nuclei (AGN) have been frequently discussed in the literature, but never observed. Few dozen mega-masering
accretion disks in AGN have been detected, which display a clear Keplerian rotation on scales of 0.1-1 pc, allowing to estimate their masses within the sphere of influence (SoI) of the central supermassive black holes (SMBHs) to be smaller by more than a factor of 10, compared with the parent SMBH masses. Furthermore, the stellar components, deep inside the SoI, are dwarfed by the SMBH masses of M_BH ~ few X 10^6 - few X 10^7 Mo. Theoretically, no limit exists on sizes and masses of gaseous disks in AGN, which in principle, can exceed masses of their central compact objects. We analyze instabilities which can operate in gaseous disks, i.e., fragmentation for a locally dominant self-
gravity and spontaneous breaking of axial
symmetry for the globally self-gravitating disks. We invoke the gas response to the latter instability which leads to gravitational collapse, leaving a negligible mass behind and dynamically stable remnants. Consequently, the characteristic timescale for globally self-gravitating disks to exist in AGN should not exceed few rotations, t_phi ~ 10^{3-4} yrs for M_BH ~ 10^{6-8} Mo. Disk rebuilding is expected to be > 10^7 yrs, meaning that probability of finding is < 10^{-3}, which explains their lack of detection. We conclude that observed sub-parsec disks in AGN, at least the Keplerian masering ones, can be remnants of globally self-gravitating
accretion disks, and the instability timescale can be related to the duty cycle of AGN, t_duty ~ 10t_phi ~ 10^{4-5} yr.
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Astrophysics of Galaxies
Measuring Extragalactic Microlens Masses and Motions in Strongly Lensed Quasar Systems with Intensity Interferometry
Abigail Moran and Ken Van Tilburg
The direct measurement of masses and transverse motions of individual
stars and compact objects in distant
galaxies remains an outstanding challenge, leaving extragalactic stellar populations constrained only through model-dependent population fits. We show that time-resolved intensity interferometry of strongly lensed
quasars, combined with flux-ratio photometry, can break the source-size, microlens-mass, and transverse-velocity degeneracies of conventional light-curve microlensing. Using the quadruply lensed
quasar B1422+231 as a fiducial system and a next-generation interferometer ($\sim$800\,m$^2$ collecting area, spectral resolving power of 20,000, and few-picosecond timing), we forecast the precision on the
quasar source size and on the mass, position, and transverse motion of an individual stellar-mass microlens in the lens
galaxy. A single epoch determines the
accretion disk size to $4\%$ in favorable geometries ($10\%$ for a typical detectable configuration) and the microlens mass to order unity for $m_l \gtrsim 0.1\,M_\odot$. An eight-year, 33-epoch campaign reaches $\lesssim1\%$ on the disk size, localizes the microlens' impact parameter to $\sim0.1$-$0.5,\mu\mathrm{as}$, and resolves its proper motion. Such observations would enable direct measurements of individual extragalactic compact-object masses and transverse motions at moderate
redshift. They also yield sub-$\mu\mathrm{as}\,\mathrm{yr}^{-2}$ sensitivity to light-centroid acceleration, which can help separate stellar microlensing from dark matter substructure
lensing, the subject of a companion paper.
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Earth and Planetary Astrophysics
Stellar Flares Rather than High $T_{int}$ As Possible Cause for the Missing Methane in Young Exoplanets: The Case Study of V1298~Tau\,b
Danica Adams, Pierre-Alexis Roy, Hilke Schlichting et al. (+1 more)
Recent observations of V1298~Tau\,b, a warm sub-Neptune (R$=9.85 \pm 0.3\,R_\oplus$, M$=13.1 \pm 5.3, M_\oplus$, T$_\mathrm{eq} = 670\,$K) orbiting a young (10-30\,Myr) K
star, show depleted methane and surprising abundances of CO$_2$ in its atmosphere. This makes the
planet an interesting target to study the "missing methane problem", where HST, Spitzer, and JWST have revealed many sub-Jovian
planets with effective temperatures cooler than 800\,K to be depleted in methane compared to
equilibrium chemistry predictions. On V1298~Tau\,b, an intrinsic temperature (T$_\mathrm{int}$) of 500\,K is required to explain the methane depletion if strong mixing from a warm interior is the reason for the missing methane, but this T$_\mathrm{int}$ is too large for a
planet of its age and size according to standard interior structure
evolution theories. Here, we show that stellar
flares can result in methane depletion in a warm
planet's atmosphere with nominal T$_\mathrm{int} \sim 100$\,K. During a stellar
flare, fast methane photolysis rates cannot be balanced by
diffusion from below into the upper atmospheric layers probed by observations. Young active
stars flare frequently, and we show that the timescale of
diffusion for the methane to recover from a
flare-induced depletion can be slow compared to the
flare frequency. Over time, the atmosphere reaches a new steady state set by the average
flare frequency and energy. This new steady state shows depleted methane in the 2\,mbar region, and abundant CO, CO${_2}$, and HCN, which results in synthetic transmission spectra that reproduce the main features of the recent JWST observations of V1298~Tau\,b.
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All authors Danica Adams, Pierre-Alexis Roy, Hilke Schlichting, Björn Benneke
Astrophysics of Galaxies
Dust and PAHs in late-stage galaxy evolution: Imprints of TP-AGB dust injection, grain growth and AGN feedback in high-z quiescent galaxies with JWST and ALMA
D. Donevski, A. Nanni, K. E. Whitaker et al. (+9 more)
A major unknown in late-stage
galaxy evolution is what regulates the cold interstellar medium (ISM) after quenching, a question central to interpreting molecular gas, dust, and stellar content in quiescent
galaxies (QGs) now probed by ALMA and JWST to $z\sim7$. We present the first semi-analytic model that follows the coupled post-quenching
evolution of dust, cold gas, and polycyclic aromatic hydrocarbons (PAHs), using flexible
star-formation histories and a framework tracking small and large carbonaceous and silicate grains. At $z\sim1$, we find that QGs of similar mass ($M_\star\sim8\times10^{10} M_\odot$), stellar-population age ($\sim2$ Gyr), and cold gas fractions ($f_{\rm gas}\sim1$-$10\%$), span $2$-$3$ dex in $M_{\rm dust}/M_\star$ and $M_{\rm dust}/M_{\rm gas}$, ranging from
star-forming-like ratios to highly depleted dust states. The diversity arises from delayed dust injection by thermally pulsing asymptotic giant branch (TP-AGB)
stars and ISM grain growth, which sustain dust enrichment for up to $\sim2.5$ Gyr after quenching. Without these channels, the pre-quenching $M_{\rm dust}$ falls below $10\%$ of its initial value within $\lesssim0.5$-$1$ Gyr, and twice as fast when AGN feedback is active. The imprint of post-quenching dust processing persists in substantial reservoirs of small carbonaceous grains, with PAH fractions of $\sim2$-$3\%$, even if the cold-dust budget falls below typical ALMA continuum detection limits ($M_{\rm dust}/M_\star\lesssim10^{-4}$). Such signatures may remain detectable with JWST/MIRI at $\mu$Jy depths, probing chemically enriched dust phases in otherwise ALMA-faint
galaxies. Altogether, dust and PAHs provide independent probes of distinct stages of ISM
evolution in QGs, rather than simply tracing the residual cold ISM of the preceding
star-forming phase.
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All authors D. Donevski, A. Nanni, K. E. Whitaker, A. W. S. Man, A. Faisst, A. Lapi, I. García-Bernete, M. Romano, T. Petrushevska, G. Gururajan, G. Lorenzon, D. Narayanan
High Energy Astrophysical Phenomena
Exploring the capabilities of combined modelling strategies for supernova with relativistic jets using Redback
L. Cotter, N. Sarin and A. Martin-Carrillo
Events in which a
supernova (SN) is associated with a relativistic jet present multi-component light curves. In the past, the analysis of such complex light curves was carried out largely by investigating each component individually due to limitations in modelling and inference. Such decoupled analyses are susceptible to biased parameter estimation due to cross-contamination between the emission of the relativistic jet and the SN. Additionally, differences in adopted methodologies make it difficult to compare physical parameters across populations. In this work, we investigate three common modelling strategies adopted for events where a Gamma-ray Burst (GRB) is associated with an SN and compare them with a joint model and inference approach. We assess the ability of each approach to fit and reproduce the true injection parameters of 1000 simulated GRB-SN
datasets using Redback. We split these
datasets into three common GRB-SN light curve morphologies: one where the SN is dominant, one where the GRB is dominant, and one where the SN and GRB have relatively equal contributions to the light curve. We also investigate the behaviour of each modelling technique when the jet break of the GRB afterglow occurs at early times and at late times. We find that treating each component independently with an individual model leads to significant bias in the estimated parameters even though the resulting best fit to the data may appear acceptable. We further demonstrate that afterglow subtraction is unreliable for GRB-SN modelling as uncertainties in the afterglow fit are not fully propagated into the resulting afterglow-subtracted light curve. Instead, our results show that jointly modelling the relativistic jet and SN components provides the most reliable modelling approach, particularly when multi-band X-ray and
radio observations are available.
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Cosmology and Nongalactic Astrophysics
TDCOSMO. XXVII. JWST-based Lens Models and H$_0$ Measurement of WFI2033, HE0435, and PG1115
D. M. Williams, T. Treu, D. P. Johnson et al. (+13 more)
Time-delay cosmography offers a one-step, distance-ladder-independent route to the Hubble-Lemaitre constant, H_0. We present new cosmography-grade lens models of three quadruply imaged
quasars based on JWST-NIRCam/F115W imaging (WFI2033-4723, HE0435-1223, PG1115+080). We use the STARRED modeling technique, introduced in our previous analysis of WFI2033-4723, to reconstruct the complex JWST-NIRCam Point Spread Function at high fidelity. We combine NIRCam-based lens models with improved external convergence estimates, published time delays, and aperture-integrated stellar velocity dispersions from JWST NIRSpec to infer H_0. The analysis was carried out blindly for HE0435-1223 and PG1115+080, while it was not blind for WFI2033-4723, as we build upon the previous published model. For comparison with previous HST-based work, we limit our analysis to the case of no internal mass-sheet degeneracy ($\lambda_{\rm int}=1$). We quantify the impact of improved imaging, single-aperture kinematics, and environment measurements on central values and uncertainties. Within flat $\Lambda$CDM, assuming a uniform prior on $\Omega_{\rm m}$, we find H_0 = 71.8$_{-7.0}^{+9.2}$ $\lambda_{\rm int}$ km s$^{-1}$ Mpc$^{-1}$ for PG1115+080, 74.2$_{-4.2}^{+4.2}$ $\lambda_{\rm int}$ km s$^{-1}$ Mpc$^{-1}$ for HE0435-1223, and 73.4$_{-4.4}^{+3.4}$ $\lambda_{\rm int}$ km s$^{-1}$ Mpc$^{-1}$ for WFI2033-4723. Combining the three lenses yields H_0 = 73.5$_{-2.8}^{+2.7}$ $\lambda_{\rm int}$ km s$^{-1}$ Mpc$^{-1}$, consistent with HST-based results (73.6$_{-2.6}^{+2.6}$ $\lambda_{\rm int}$ km s$^{-1}$ Mpc$^{-1}$), but with reduced scatter between the three systems. These models will be incorporated in the TDCOSMO-2026 milestone paper with free $\lambda_{\rm int}$ in a hierarchical fashion. We close by outlining how 16 forthcoming JWST NIRCam targets will further tighten uncertainties toward percent-level precision on H_0.
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All authors D. M. Williams, T. Treu, D. P. Johnson, P. Mozumdar, S. Knabel, S. Birrer, C. D. Fassnacht, A. Galan, A. J. Shajib, K. C. Wong, M. Cappellari, F. Courbin, T. Morishita, V. Motta, D. Sluse, M. Stiavelli
Astrophysics of Galaxies
Resolved Dust in $z\approx1$ Galaxies with JWST/MIRI MRS: Survey Description and First View on PAHs, Mid-IR Atomic Emission, and Warm H$_2$
Wuji Wang, Andreas L. Faisst, Thomas S.-Y. Lai et al. (+16 more)
Dust is a key component of
galaxies that regulates their thermal balance and, consequently,
star formation and the build-up of stellar mass. Polycyclic aromatic hydrocarbons (PAHs) are responsible for reprocessing radiative energy of the
galaxies thus tracing dust
evolution. Using JWST/MIRI MRS, we present the first resolved view of the PAHs and mid-infrared emission lines in a sample of eight $z\approx1$
galaxies. A key novelty is our ability to directly map PAH emission near the end of
cosmic noon at JWST's limits. The sample is selected to be on the
star-forming main-sequence with stellar masses $M_{\star}=10^{10.6-11.2}\,M_{\odot}$ and infrared luminosities $L_{\rm IR}=10^{11.5-11.9}\,L_{\odot}$. Two of them contain active galactic
nuclei (AGN), and two are interacting systems. We detect and quantify primary PAH emissions from 3.3$\,\rm \mu m$ to 11.3$\,\rm\mu m$ throughout the
galaxies, alongside atomic fine structure lines (Ar, Ne, and Fe), Br$\alpha$, and H$_{2}$ rotational transitions. Through PAH ratio diagnostics and comparison to theoretical models, we qualitatively probe the physical properties of PAH molecules, i.e., size and charge. The AGN and mergers in our sample exhibit a higher fraction of neutral PAHs, possibly related to high radiation intensity and/or shocks, as suggested by increased atomic and H$_{2}$ line ratios. Leveraging the IFU data, we find that the grain sizes of the centrally located PAHs tend to be larger and less ionized than those in the outskirts of the
galaxies. Finally, we compare our results to observations of PAHs in local and similar-
redshift galaxies, revealing a potential evolutionary trend when controlling $L_{\rm IR}$.
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All authors Wuji Wang, Andreas L. Faisst, Thomas S.-Y. Lai, Kyle Finner, Jed McKinney, Andrew J. Battisti, Irene Shivaei, Leindert A. Boogaard, Daizhong Liu, Yu-Heng Lin, Lun-Jun Liu, Georgios Magdis, Vincenzo Mainieri, Alexander Rodriguez, John D. Silverman, Zhaoxuan Liu, Minju M. Lee, Ghassem Gozaliasl, Maximilien Franco
Cosmology and Nongalactic Astrophysics
Discovering Substellar Dark Matter Halos with Astrometric Weak Lensing of Multiply Imaged Quasars
Ken Van Tilburg and David E. Kaplan
We propose time-domain astrometric weak
lensing of multiply imaged
quasars as a probe of substellar dark matter (DM)
halos. In $\Lambda$CDM, each macro-image's light traverses many microhalos, producing a
stochastic centroid motion with a calculable red power spectrum.
Halos with crossing times longer than the survey impart a relative angular acceleration between image pairs. The response peaks at subhalo scale radii 0.005-2 pc and masses $3\times10^{-6}$-$10^2\,M_\odot$, 12-14 orders of magnitude below the smallest detected DM structures, down to the damping cutoffs of a 100 GeV thermal relic. We forecast the sensitivity of ten-year campaigns with 300 epochs for two
benchmark systems: the
galaxy-lensed quadruple B1422+231 at a 0.1 $\mu$as per-epoch precision forecast for extended-path intensity correlation, and the cluster-lensed triple SDSS J1029+2623 at 1 $\mu$as. The angular-acceleration channel reaches the standard $\Lambda$CDM microhalo population with a variance signal-to-noise ratio of order ten for the
galaxy lens (order unity after stellar microlensing subtraction) and 0.35 for the cluster under the optimistic assumption that 10% of the projected mass density at the image positions resides in surviving microhalos. A positive detection would: raise universal lower bounds on the DM particle mass to $\gtrsim400$ keV for
fermions and $\gtrsim10^{-12}$ eV for
bosons; constrain the DM kinetic decoupling temperature to $\gtrsim 10$ MeV; and be sensitive to the running of the spectral tilt at the 0.01 level over 15-20 e-folds of the primordial
curvature power spectrum. A robust null result across well-characterized lenses would constrain microhalo survival and/or imply violations of the above inequalities. These signatures motivate differential astrometric observations with extreme 0.1-1 $\mu$as light-centroiding precision on multiply imaged
quasars. [Abridged]
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Cosmology and Nongalactic Astrophysics · Cross-listed from physics.ins-det
A microwave SQUID multiplexing concept for macro-cryogenic calorimeter arrays
N. Ferreiro Iachellini, P. Szypryt, L. Canonica et al. (+10 more)
Massive cryogenic calorimeters read out by transition-edge sensors (TES) can reach eV-scale baseline resolution, but the single-channel dc-SQUID readout conventionally used in rare-event searches, with one amplifier chain and several wires per detector, limits practical arrays to a few tens of channels. We propose to apply microwave SQUID multiplexing ($\mu$MUX), developed for fast X-ray and
neutrino-mass microcalorimeters, to massive BGO (Bi$_4$Ge$_3$O$_{12}$) calorimeters operated at $\simeq 20$~mK; sapphire and TeO$_2$ absorbers are covered by the same framework. Using the established thermal and noise model for massive TES calorimeters, we derive a noise model for the multiplexed readout, including the HEMT, two-level-system and SQUID contributions, and its scaling with the multiplexing factor $N_{\rm mux}$. Since the slow thermal signals require only about a kHz of sampling per channel, the limiting requirement is not bandwidth but an input-coil sensitivity of $( 0.1$ to $1)\,\mu$A$/\Phi_0$, a factor of $10$ to $30$ beyond current $\mu$MUX devices, matched to the detector current noise of ($9$ to $10$)~pA$/\sqrt{\rm Hz}$, which is independent of the absorber mass. With a total flux noise of $\simeq$$1.2\,\mu\Phi_0/\sqrt{\rm Hz}$, the readout degrades the baseline resolution by less than $2\%$ up to $N_{\rm mux}=1000$, negligible compared with the TES-limited resolution. These results are implemented in a single-tower design: 52 BGO
crystals of 100~g each and one HEMT amplifier. The tower is the unit of a multi-tower array of several kilograms aimed at CE$\nu$NS and dark matter searches.
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All authors N. Ferreiro Iachellini, P. Szypryt, L. Canonica, W. B. Doriese, M. Durkin, A. Giachero, J. A. B. Mates, A. Nucciotti, L. Pattavina, S. Quitadamo, C. Shiu, J. N. Ullom, M. R. Vissers
High Energy Astrophysical Phenomena
A first-principles binary neutron star merger model of GW170817, GRB170817A, and AT2017gfo
Kenta Kiuchi, Sho Fujibayashi, Kota Hayashi et al. (+6 more)
The multimessenger observation of the binary
neutron star merger event GW170817, associated with its electromagnetic counterparts GRB170817A and AT2017gfo, marked a milestone in astrophysics, yet its unified physical explanation remains elusive. We conduct an end-to-end simulation based on a first-principles general-relativistic magnetohydrodynamics
neutrino-radiation transfer merger simulation, followed by nucleosynthesis calculations and
photon radiative transfer to generate kilonova light curves. We show that the large-scale dynamo simultaneously produces a relativistic jet with an isotropic-equivalent luminosity of $\sim 10^{51}~{\rm erg~s^{-1}}$ and $\approx 0.08M_\odot$ of
neutron-rich ejecta, reproducing the GRB170817A afterglow and the AT2017gfo kilonova light curves. Our results establish a unified first-principles framework for interpreting binary
neutron star mergers across gravitational wave, gamma-ray burst, and kilonova observations.
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All authors Kenta Kiuchi, Sho Fujibayashi, Kota Hayashi, Kyohei Kawaguchi, Quentin Pognan, Alexis Reboul-Salze, Yuichiro Sekiguchi, Masaru Shibata, Shinya Wanajo
Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Detecting Axion-Like Particles With Coiled Optical Fibers I: Silica Fibers
Gia Dvali, Sebastian Zell and Tongxuan Zhang
We propose a new approach to
axion-like particle (ALP) searches based on long, coiled optical fibers in an external
magnetic field. We develop the theoretical framework required to describe
photon-ALP conversion in this geometry by incorporating transverse boundary conditions and fiber bending. For solid silica fibers with refractive index considerably larger than unity, we show that the leading signal is a phase shift of the
photon, with negligible loss due to ALP production. This setup has the potential to set new constraints in the regime of large ALP mass. We further identify parameter regions in which boundary effects become important, in particular for hollow-core fibers, where signals due to ALPs can be significantly enhanced.
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Solar and Stellar Astrophysics
CHARA Array Observations of the Evolved Components in Symbiotic Star Systems
Thomas Martin Gaudin, Ryan Norris, Magdalena Otulakowska-Hypka et al. (+30 more)
The nature of the mechanisms that drive mass transfer in symbiotic
stars remains an area of active research in stellar astronomy. Constraining the role that both stellar winds and Roche-lobe overflow play in this process is crucial to improving our understanding of these binaries and connecting them to important transient events such as recurrent novae and Type Ia
supernovae. The high-resolution capabilities of an optical interferometer can resolve the
geometric structure of the red giant in symbiotic
stars and help answer this question. This work presents the results of an optical interferometric study using the Center for High Angular Resolution Astronomy (CHARA) Array for the purpose of measuring the angular diameter of and imaging the cool giant in four symbiotic and related systems. Here we report H band observations collected with MIRC-X. Model fitting and image reconstruction are used to test for Roche-lobe-filling geometries. Near-simultaneous infrared spectroscopy taken using the NASA InfraRed
Telescope Facility (IRTF) is used to determine the fundamental stellar parameters of the cool giant in each system. The parametric fits reported here favor circularly symmetric disk models over elongated geometries, while imaging suggests the presence of surface features on three of these
stars. We find that the three systems with constrained orbits have inferred time-averaged filling factors below unity.
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All authors Thomas Martin Gaudin, Ryan Norris, Magdalena Otulakowska-Hypka, Rachael M. Roettenbacher, Nirupam Roy, Yesenia Beltran, Cameron Caruso, Cody Gustafson, Mason Earick, Andrew Kotowski, Rebecca Proni, Jacob Sandusky, Fabien Baron, Michelle J. Creech-Eakman, John D. Monnier, Stefan Kraus, Narsireddy Anugu, Jean-Baptiste Le Bouquin, Sorabh Chhabra, Isabelle Codron, Calire Davies, Jacob Ennis, Tyler Gardner, Mayra Gutierrez, Noura Ibrahim, Cyprien Lanthermann, Benjamin R. Setterholm, Christopher D. Farrington, Olli Majoinen, Norm Vargas, Douglas R Gies, Theo ten Brummelaar, Gail H. Schaefer
Cosmology and Nongalactic Astrophysics
The Atacama Cosmology Telescope: Constraints on the anisotropic screening and birefringence effects with DR6
Darby M. Kramer, Alexander van Engelen, Frank J. Qu et al. (+14 more)
While we have learned much from recent surveys of the mm-wave sky, many secondary anisotropies of the CMB remain under-explored. We consider two of these here: the anisotropic screening induced by Thomson scattering of CMB
photons off free electrons, and
cosmic birefringence, the rotation of CMB polarization due to parity-violating physics beyond the standard model. To measure these effects, we apply state-of-the-art
estimators for statistical anisotropy to the ACT Data Release 6 CMB data and construct noisy maps and power spectra of both effects. For anisotropic screening, we find no significant detection with the baseline data, and place an upper limit on its power spectrum that is two orders of magnitude above predictions from current reionization models. We obtain evidence for some foreground contamination at one of our frequencies that is consistent with expectations from extragalactic simulations. For our anisotropic birefringence analysis, we also find no detection and place an upper limit on the signal that is consistent with previous analyses. We report a corresponding upper limit on the Chern-Simons coupling term $g_{a \gamma} < 4.9 \times 10^{-2} / H_\mathrm{I}$, with $H_\mathrm{I}$ the inflationary Hubble scale. These analyses pave the way toward obtaining improved constraints with the next generation of ground-based CMB surveys.
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All authors Darby M. Kramer, Alexander van Engelen, Frank J. Qu, Christopher Cain, Irene Abril-Cabezas, J. Richard Bond, Vera Gluscevic, Carlos Hervías-Caimapo, Niall MacCrann, Mathew S. Madhavacheril, Jeff McMahon, Toshiya Namikawa, Bruce Partridge, Emmanuel Schaan, Neelima Sehgal, Cristóbal Sifón, Ed Wollack
Solar and Stellar Astrophysics
The JWST/NIRCam Scattered Light Disks GTO 2780 program: panchromatic coronagraphic imaging of the HD 10647, HD 32297, HD 61005, HD 107146, and HD 181327 debris disk systems
András Gáspár, Jarron M. Leisenring, Schuyler Grace Wolff et al. (+13 more)
Debris disks, composed of rocks, boulders, planetesimals, and the dust produced in their collisions, present the most readily observable components of mature planetary systems. They also serve as valuable diagnostic tools, enabling studies of planetary dynamical interactions and mineral compositions. Observed from optical to
radio wavelengths, each band reveals unique information about the dust populations. Optical and near-infrared observations are specifically sensitive to light scattered off the surfaces of the micron-sized particles. Here, we present results from the JWST/NIRCam GTO program 2780, designed to observe five disk systems previously identified to be exceptionally bright at optical wavelengths (HD 10647, HD 32297, HD 61005, HD 107146, and HD 181327) with six filters using the NIRCam coronagraphs. The NIRCam data complement previous shorter-wavelength images of these same systems. They reveal scattered light from the disks and from the extended
halos of tiny grains under the influence of radiative forces, at high resolution and signal to noise. All the systems show evidence for water ice, although it can have differing radial distributions and tends to show stronger signatures in the
halos. In the two cases we could analyze, the scattering phase function in the disks resembles the behavior of dust in the Solar System with evidence for enhanced forward scattering in the
halos, consistent with the latter being composed of tiny grains. MIRI images for two systems are more centrally concentrated than the shorter wavelength ones, suggesting a role for dragged-in larger grains.
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All authors András Gáspár, Jarron M. Leisenring, Schuyler Grace Wolff, Kellen Lawson, George H. Rieke, Dingshan Deng, Marcia J. Rieke, Antranik A. Sefilian, Charles Beichman, Joshua B. Lovell, John Krist, Marie Ygouf, Jorge Llop-Sayson, Geoffrey Bryden, Christopher C. Stark, Christine H. Chen
Earth and Planetary Astrophysics
An Equation of State for Supercritical Silicate-Hydrogen Mixtures at Sub-Neptune Interior Conditions
Sarah P. Marcum, Lars Stixrude and Edward D. Young
Many sub-Neptunes are expected to contain long-lived molten silicate interiors beneath dense H2-rich envelopes. At pressures and temperatures near the atmosphere-interior boundary, MgSiO3 and H2 may become fully miscible, forming a supercritical silicate-hydrogen fluid. Here we use density functional theory molecular dynamics simulations to construct a self-consistent equation of state for supercritical MgSiO3H4, corresponding to 3.86 wt% hydrogen expressed as equivalent H2. We fit the simulation results with a Helmholtz free-energy formulation that yields density,
entropy, heat capacity, thermal expansivity, bulk modulus, and Gruneisen parameter from a single thermodynamic surface. We find that the hydrogen-bearing fluid is lower in density than dry MgSiO3
liquid and deviates significantly from ideal specific-volume mixing between MgSiO3 and H2. The resulting excess volume varies with pressure, indicating that the interaction between hydrogen and the silicate framework evolves with compression. Structural analysis shows increasing Si-H coordination and decreasing persistent H-H bonding at high pressure, consistent with a transition away from molecular H2-like bonding toward a more strongly coupled silicate-hydrogen fluid. We incorporate the MgSiO3H4 equation of state into a composition-dependent MgSiO3-H lookup table and apply it to representative sub-Neptune structure models. The models show that hydrogen partitioning between the atmosphere and condensed interior affects the planetary adiabat relative to the MgSiO3 liquidus. These results demonstrate that supercritical silicate-hydrogen fluids have distinct thermodynamic and structural properties that must be accounted for when modeling sub-Neptune interiors.
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Astrophysics of Galaxies
Map of the Interstellar Continuum FUV Field within 800 Parsecs
David Lomeli, Kedron Silsbee, Alexei Ivlev et al. (+1 more)
The far-ultraviolet (FUV) field, consisting of
photons with energies between 6 and 13.6 electron volts, plays an important role in the chemical and thermodynamical
evolution of the interstellar medium. FUV continuum radiation is produced by bright
stars in the
galaxy and absorbed by interstellar dust. We used newly available maps of the dust distribution, in conjunction with catalogs of stellar positions and spectra, to create a map of the stellar FUV field within 800 parsecs of the Earth. The map shows a total FUV intensity in the solar neighborhood in line with existing measurements (0.7 times the Draine field), but substantial point-to-point fluctuations exist. Near the Galactic midplane, the FUV intensity decreases with increasing gas density up to a density of approximately 10 hydrogen atoms per cm$^3$, due to absorption of light by dust. Between 10 and 50 hydrogen atoms per cm$^3$ the UV field increases again, due to a positive correlation between high density gas and young hot
stars. We find that the spectrum is generally harder where the UV field is higher, and softer in regions where dust extinction is more important. The resulting map, as well as our catalog of derived stellar properties, are both available for download for those wishing to study a nearby region of the ISM.
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All authors David Lomeli, Kedron Silsbee, Alexei Ivlev, Marta Obolentseva
Astrophysics of Galaxies
LightCube: A Parsec-Resolution 3D Model of the Local Far-Ultraviolet Interstellar Radiation Field
Hannah V. Bish, J.E.G. Peek, Catherine Zucker et al. (+5 more)
We present a three-dimensional model of the local interstellar radiation field (ISRF) in the ultraviolet (UV). Using UV flux measurements from the TD1 catalog and stellar distances from Gaia and Hipparcos, we construct a catalog of
stars that we expect to dominate the UV flux in the nearby
Galaxy. We use the radiative transfer code DIRTY to model the propagation of
photons from these
stars through a 3D dust map, including the effects of scattering and absorption. The result is LightCube, a model of the ultraviolet ISRF out to 1.25 kpc from the Sun, with a maximum linear resolution of 1 pc. We model the ISRF in the four TD1 bands (1565 A, 1965 A, 2365 A, and 2740 A), as well as a single value for the full FUV range, and calculate the ISRF at the Sun to be 5.65$\times$10$^{-14}$ erg cm$^{-3}$ from 912 A to 2000 A. The modeled ISRF is quite variable, with more than an order of magnitude variation seen in dense regions, and about half that in lower density regions. By comparing LightCube to an estimate of the 3D distribution of total-to-selective extinction ratio, $R_V$, we find a positive correlation between UV flux and $R_V$ in regions of low UV radiation.
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All authors Hannah V. Bish, J.E.G. Peek, Catherine Zucker, Karl D. Gordon, Claire E. Murray, Susan E. Clark, Mathieu Willems, Erika Hamden
Cosmology and Nongalactic Astrophysics · Cross-listed from hep-th
Quantum Selection of Classical Histories
Omer Guleryuz
Can a finite
quantum episode populate a classical future that the same prepared state never reaches deterministically? We show that it can, using inflation as a cosmological laboratory. In our supergravity realization, two classical continuations already exist, but the prepared state reaches only one without noise. A stable
entropy direction temporarily becomes light without a tachyonic instability, allowing mode-derived phase-space fluctuations to access the competing continuation. Finite-horizon first passage assigns it a \(0.33\)–\(0.40\) conditional weight across three nearby CMB-normalized realizations, while suppressing the
stochastic source restores its exact deterministic zero. The
entropy sector then becomes heavy again, and the selected histories continue classically. A finite
quantum episode can thus leave a lasting redistribution among classical futures.
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Instrumentation and Methods for Astrophysics
The MICADO first light imager for the ELT: first on-bench performance tests of the SCAO hybrid LQG+Integrator controller
N. Levraud, F. Ferreira, A. Sevin et al. (+6 more)
The Extremely Large Telecope (ELT) is expected to begin scientific operations in 2030, with MICADO as its first-light imager. The Single Conjugate Adaptive Optics (SCAO) system for MICADO is currently being assembled and tested at the Observatoire de Paris. MICADO's baseline AO control strategy uses a hybrid approach: a Linear Quadratic Gaussian (LQG) regulator for low-order modes, optimised to adapt to the disturbance caused by vibrations and ELT windshake, and an integral action controller for higher-oder modes. Despite the non linearity of the Pyramid Wavefront Sensor (PyWFS), the so-called "optical gains", LQG control has proven to be highly effective in simulation. It has been shown that the LQG regulator is resilient to optical gain variations in the loop or to inaccuracies in their estimation. Increasing the number of modes controlled by the LQG regulator would therefore make the controller less sensitive to the optical gains, which are notoriously difficult to estimate during operation. A first hybrid controller, with only tip-tilt controlled with LQG control, has been validated using COMPASS, the end-to-end, GPU-accelerated, AO simulation platform. We will present the first experimental validation of this hybrid controller on the MICADO-SCAO bench. We will also present results obtained by increasing the number of modes controlled with LQG control and the work carried out to go towards an autonomous LQG regulator.
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All authors N. Levraud, F. Ferreira, A. Sevin, E. Gendron, N. Galland, H.-F. Raynaud, Y. Clénet, R. Davies, C. Kulcsár
Cosmology and Nongalactic Astrophysics
Near universality of nonlinear transverse and radial velocity responses in spherical collapse with arbitrary radial profiles
Valerio Marra
The nonlinear relation between density and expansion is usually formulated for a homogeneous spherical top hat, which has a single local Hubble rate. A smooth spherical profile instead expands differently along ($H_{\parallel}$) and across ($H_{\perp}$) the radial direction-a direct signature of radial inhomogeneity. We show that, for growing-mode pressureless matter with a cosmological constant, the complete shellwise response is nevertheless fixed by one top-hat function. Given the local density contrast $\delta(t,r)$ and the enclosed contrast $\Delta(t,r)$, a transverse response and its derivative determine $H_{\perp}$ and $H_{\parallel}$. The linear and second-order limits are algebraic maps whose only dynamical input is the usual growth rate $f$. An exact equal-age construction supplies the nonlinear response without integrating an
evolution equation. We check that it reconstructs full $\Lambda$LTB profiles to numerical precision. We also provide a derivative-aware, cosmology-independent three-term symbolic fit that requires only $f$, $\delta$, and $\Delta$. Across a representative set of matter-
curvature-
redshift combinations and for a shell located in the compensated transition, the maximum relative errors are $0.3\%$ and $0.7\%$ in the transverse and radial responses, respectively. This compact formulation separates the production of a density profile from its expansion response and makes the effect of radial gradients explicit.
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Astrophysics of Galaxies
Non-isothermal vertical distribution functions for the Milky Way
Maria Djurić and Ralph Schönrich
The vertical structure of our
Galaxy has commonly been assumed to follow a pseudo-isothermal distribution. However, there is no a priori reason to expect this form to arise from scattering by giant molecular clouds (GMCs), since GMCs are confined to a narrow layer around the Galactic midplane and therefore do not randomise
stars uniformly in vertical phase space. We first present a simple statistical argument to derive a new vertical distribution function (DF), in the limiting case of a razor-thin distribution of GMCs, which admits an extra factor of ${J_z}^{-\alpha}$, where $\alpha>0$ and varies for different potentials. In light of this, we revisit the
diffusion coefficients for vertical heating from the mechanics of two-body scattering, in a more realistic, extended distribution of GMCs. The resulting drift and
diffusion coefficients are strongly non-linear in $v_z$, particularly at low relative velocities, where the effective range of weak, local encounters can vanish. We determine stationary solutions, and also numerically solve the orbit-averaged Fokker–Planck equation with time-dependent coefficients. We find, in both cases, a vertical action distribution that deviates from that of the pseudo-isothermal case, and propose a new, family of vertical DFs that generalise the pseudo-isothermal form. We find the two primary
agents of non-isothermality are: (i) a GMC distribution confined near the midplane, and (ii) the fact that the Coulomb logarithm, $\Lambda$ is strongly velocity-dependent at small velocities, and cannot be treated as a constant, contrary to the commonly adopted assumption.
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Astrophysics of Galaxies
$N$-body simulations of the open cluster population in the Milky Way and the impact of GMC encounters
Timmi G. Jørgensen and Ross P. Church
We investigate the population of open clusters in the Solar Neighbourhood and model the effect of encounters with giant molecular clouds (GMCs) over the last 1 Gyr. We combine a Galactic model with $N$-body simulations of 20692 unique clusters in the mass range $[50-24000]$ $\mathrm{M}_{\odot}$. Each cluster is simulated twice: with and without tidal forces from the GMCs. We find that an initial cluster mass function truncated at $\sim 9000$ $\mathrm{M}_{\odot}$ best reproduces the observed mass function
evolution. For the age function, the observations show a decline in clusters for ages older than $\sim 1$ Myr, whereas our simulated clusters show a decline after $\sim 50$ Myr. The observed early disruption suggests that some clusters form supervirial, whereas our simulated clusters are created in virial
equilibrium. Low-mass ($<600 \, \mathrm{M}_{\odot}$) clusters are most sensitive to GMC encounters, which accelerate their disruption in the first $\sim 200$ Myr. After $\sim 500$ Myr, the impact of GMCs becomes irrelevant since the clusters will have been destroyed regardless of whether they experience GMC encounters or not. The survival of intermediate-mass ($600-6000\, \mathrm{M}_{\odot}$) clusters is significantly reduced by GMCs at ages up to 1 Gyr. High-mass ($>6000 \, \mathrm{M}_{\odot}$) clusters survive to 1 Gyr with minimal disruption with or without GMCs. We find that clusters that have had strong GMC encounters within the last $20$ Myr should have tidal tails that are randomly orientated with respect to the Galactic centre.
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Earth and Planetary Astrophysics
Dust torques for realistic dust size distributions
V. Roatti, G. Picogna and F. Marzari
Previous studies have shown that a population of dust particles with a fixed Stokes number can exert a substantial torque on a low-mass
planet embedded in a protoplanetary disk, modifying its migration rate. We aim to characterize the dust torque on a low-mass
planet for a realistic distribution of dust grain sizes. We performed 2D hydrodynamical simulations of
planet-disk interactions using the PLUTO code, with the addition of Lagrangian superparticles representing dust dynamics. We apply an energy-based criterion to exclude the particles that are gravitationally bound to the
planet, to prevent circumplanetary flow to contaminate the torque measurements. We find that the dust torque is dominated by the largest grains in the size distribution and is highly sensitive to the maximum grain size. For typical disk conditions, the torque becomes positive for marginally coupled particles ($\mathrm{St} \gtrsim 10^{-2}$) and can exceed the gas torque in the presence of cm-sized pebbles, leading to outward migration of low-mass
planets. Unlike previous studies, the turbulent dust
diffusion has a negligible influence on the torque over the explored range of $\alpha = 10^{-4}$ to $3\times 10^{-3}$. The dominant contribution arises from within the planetary Hill sphere, highlighting the need for high spatial resolution and accurate integration of particle trajectories. We derive a scaling law for the dust torque as a function of the maximum grain size and the planetary mass, suitable for implementation in population synthesis models.
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Solar and Stellar Astrophysics
Panchromatic Spectra of Nearby Low-mass and Sun-like Stars with Directly Imageable Habitable Zones
Sarah Peacock, Breanna A. Binder, Edward W. Schwieterman et al. (+4 more)
Characterizing the high-energy radiation environments of
stars plays a critical role in determining which systems'
planets are capable of retaining atmospheres and sustaining habitable conditions. X-ray through ultraviolet (UV) radiation drives atmospheric photochemistry, heating, and escape, making accurate characterization of stellar high-energy emission essential for both interpreting future
exoplanet observations and identifying the most promising targets for life detection. We construct panchromatic spectral energy distributions (SEDs) spanning the X-ray through
radio for 12 nearby low-mass and Sun-like
stars with directly imageable habitable zones that are prioritized targets for the Habitable Worlds Observatory (HWO) and Extremely Large
Telescopes (ELTs). These SEDs are generated using forward stellar atmosphere models guided and constrained by available archival X-ray and UV observations. We find that many
stars in this sample exhibit elevated high-energy radiation environments relative to the modern Sun, with habitable zone X-ray and extreme UV (XUV) fluxes frequently exceeding solar values by 1-2 orders of magnitude. The elevated emission likely reflects a combination of sample selection effects, differences in stellar age and rotation, and intrinsic
magnetic variability, with multi-epoch observations demonstrating that variability alone can significantly alter inferred radiation environments. These results highlight high-energy radiation as an important discriminator in identifying the most promising habitable
planet hosts and demonstrate the need for expanded X-ray and UV observations to complete the stellar characterization necessary for HWO target prioritization.
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All authors Sarah Peacock, Breanna A. Binder, Edward W. Schwieterman, Margaret C. Turnbull, Stephen R. Kane, Katherine Garcia-Sage, Alison Farrish
Astrophysics of Galaxies
Exploring the survivability of higher-order multiple protostellar systems – The case of VLA1623
N. M. Murillo and A. Pérez-Villegas
Higher-order protostellar systems ($\geq$3 components) are commonly observed in the early stages of low-mass
star formation. A persistent question in
star formation and
evolution is whether these higher-order protostellar systems survive as gravitationally bound systems or dissolve into binaries over time. We explore this question with a case study of the embedded quadruple protostellar system VLA1623 and its observational constraints, assuming that the components A1, A2, B, and W are gravitationally bound. Using $N$-body simulations, we run a grid of models considering
gravity, mass
accretion onto protostars, the presence of the protostellar cloud core, and its dispersal. The simulations are integrated over a period of 8 Myr to take into account the
evolution from the protostellar phase (Class 0 and I, 1 Myr), through the pre-main sequence (Class II and III, 2 – 3 Myr), and into the main sequence (4 Myr). Our results show that VLA1623 has a probability of 30% to remain as a gravitationally-bound quadruple system up to 8 Myr from the current state. There is also a 25–30% probability of VLA1623 dissolving into a triple system. This suggests that the dissolution of quadruple protostellar systems contributes to the formation of stable triple (proto)stellar systems. Components A1 and A2 are the most likely to be ejected, while W has a lower probability of being ejected from the system. The stability of VLA1623 depends on a combination of component mass ratios wrt the primary, separations, and eccentricities.
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Astrophysics of Galaxies
Revisiting the enigmatic sixth star cluster in the Fornax dwarf spheroidal galaxy
Chiara Crociati, Annette M. N. Ferguson, Raffaele Pascale et al. (+7 more)
The Fornax dwarf spheroidal (dSph)
galaxy is one of only two Local Group dSphs that host a population of globular
star clusters (GCs), the present-day properties of which have often been used to investigate the nature of the dark matter. An additional overdensity of
stars called Fornax 6, lying at a projected distance of $\sim$0.3 kpc from the
galaxy centre, was recently identified as a sixth GC residing in Fornax. However, this conclusion was based on shallow, low-resolution photometric observations and a limited number of spectra. Here, we reinvestigate the nature of Fornax 6 by analysing MUSE/Wide-Field-Mode observations alongside deep GMOS-S imagery. Using a sample of 132 spectra of red giant and horizontal branch
stars, we confirm the Fornax 6 overdensity as a distinct chemo-dynamical component with respect to the surrounding field population. Specifically, we identify 43 likely members associated with the cluster, from which we measure $\rm [Fe/H] =-0.61\pm0.03\,$dex, $v_{\rm los} = 50.9^{+0.8}_{-0.7}\,\rm km \,s^{-1}$, $\sigma_v = 3.2^{+1.4}_{-1.5}\,\rm km \,s^{-1}$. The main-sequence turn-off, observed here for the first time, strongly suggests that these
stars are coeval and well described by a $\sim$3 Gyr old isochrone. The cluster is characterised by an irregular morphology, a large half-light radius ($R_{h}=8.8^{+1.1}_{-1.3}\,$pc), a small flattening ($e=0.14^{+0.12}_{-0.10}$), and low luminosity ($M_{V} = -5.0 \pm 0.4$). Our improved characterisation of Fornax 6 supports its classification as a genuine low-mass cluster likely undergoing tidal disruption, making it the youngest and most metal-rich member of Fornax's unique GC system.
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All authors Chiara Crociati, Annette M. N. Ferguson, Raffaele Pascale, Sebastian Kamann, Silvia Martocchia, Jess M. Howell, Pete B. Kuzma, Dougal Mackey, Sakurako Okamoto, Masato Onodera