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Stair systems, which provide the primary means of egress in and out of a building during and after an earthquake, play a critical role in seismic resiliency. Often, these systems are detailed with fixed flight-tolanding connections. However, previous experimental studies and field reconnaissance have shown that such configurations are prone to severe damage. To mitigate this damage to both the stair systems and the supporting structural members, as well as to avoid the ensuing potential to stiffen the lateral force resisting system of the building, stair connections are often detailed to release drift demands. Referred to as driftrelease, these approaches generally focus on releasing inter-story drift at the flight-to-landing connections. Findings from a recent shake table test of a 10-story prefabricated stair system within a mass timber building showed that stair systems with drift-release connections result in a seismically resilient egress solution. However, due to the free movement of stair flights, handrail connections received damage at multiple levels. In a complementary follow-on study, a 9-story stair system with refined connection details was integrated into a 10-story cold-formed steel (CFS10) building recently tested at NHERI@UC San Diego. Numerical analyses have indicated that stair systems with fixed flight-to-landing connections can influence the dynamic characteristics of the supporting structure due to strut action. As such, stairs in the CFS10 building system were designed to allow for easy modification between fixed and drift-release connections by installing or removing locking bolts at the flight to mid-landing interfaces. In the CFS10 building, white noise tests were conducted in each orthogonal direction under two configurations: first, with the stair connections fixed, and then with the locking bolts removed. This study aims to determine the impact of stair connection detailing on the modal characteristics of the building. In addition, complementary numerical simulations were carried out for both fixed and drift-release stair configurations. Based on both experimental and numerical studies, it is found that, under low-amplitude excitation, the impact of stair flexibility on the building vibration period is not significant because the drift-release connection is not appreciably engaged under small lateral loading. However, the impact of stair strut action under large-amplitude excitation still needs to be investigated. 

This study investigates uncertainty quantification using probabilistic deep learning models to detect out-of-distribution events. Deep learning models often perform poorly when exposed to data outside their training distribution, leading to unreliable predictions. This research evaluates probabilistic approaches to quantify prediction uncertainty and improve model robustness. Results demonstrate the effectiveness of uncertainty-aware deep learning methods in identifying anomalous or out-of-distribution conditions, improving reliability in real-world deployment scenarios. 

Context. Over 20 exocometary belts host detectable circumstellar gas, mostly in the form of CO. Two competing theories for its origin have emerged, positing that the gas is either primordial or secondary. Primordial gas survives from the belt’s parent protoplanetary disc and is therefore H₂-rich. Secondary gas is outgassed in situ by exocomets and is relatively H₂-poor. Discriminating between these scenarios has not been possible for belts that host unexpectedly large quantities of CO. Aims. We aim to break this gas origin dichotomy through direct measurement of H₂column densities in two edge-on, CO-rich exocometary belts around ∼15 Myr-old A-type stars, constraining the CO/H₂ratio and CO gas lifetimes. Observing edge-on belts enables rovibrational absorption spectroscopy against the stellar background. Methods. We present near-IR CRIRES+ spectra of HD 110058 and HD 131488, which provide the first direct probe of H₂gas in CO-rich exocometary belts. We targeted the H₂(v=1-0 S(0)) line at 2223.3 nm and the¹²COv= 2 → 0 rovibrational lines in the range 2333.8-2335.5 nm and derived constraints on column densities along the line of sight to the stars. Results. We detect¹²CO strongly, but not H₂, in the CRIRES+ spectra. This allows us to place 3σ lower limits on the CO/H₂ratios of >1.35 × 10⁻³and >3.09 × 10⁻⁵for HD 110058 and HD 131488, respectively. These constraints demonstrate that, at least for HD 110058, the exocometary gas is compositionally distinct and significantly H₂-poor compared to the <10⁻⁴CO/H₂ratios typical of protoplanetary discs. For HD 131488, we further compared the CO photodissociation timescale to the age of the system through simple shielding arguments, and find that we cannot formally rule out a primordial origin; however, we suggest that a more realistic model of CO survival likely supports a secondary origin for this system as well. Overall, a high CO/H₂ratio for HD 110058 indicates that the gas in this CO-rich belt is most likely not primordial in composition, supporting the presence of exocometary gas. 

Abstract Recent scaling theories for the eddy fluxes in the two-layer quasigeostrophic (QG) model assume a flat-bottom boundary. Here, we discuss an organizing principle for how rough topography (i.e., topography with length scales similar to or smaller than the eddy scale) modifies the fully developed state of baroclinic turbulence. In particular, we focus on random, homogeneous topography in the two-layer QG model on anfplane, forced by a zonal shear and dissipated by linear drag. We present a suite of numerical simulations using idealized monoscale topography, systematically modifying the topographic length and height scales and the strength of the drag. We outline the dependence of the eddy diffusivityD, barotropic eddy energyE, and eddy mixing length, on the two nondimensional control parameters:, controlling the strength of the drag, and, controlling the strength of topographic–advective interactions. Two distinct regimes are identified and quantitatively predicted by a regime transition parameterα, which depends on bothand. Onceαsurpasses ancritical value, all eddy scales are reduced below their flat-bottom values and become much less sensitive to the drag coefficient. Spectral energy budgets reveal that energy pathways are importantly reorganized in this regime compared to the flat-bottom limit. We show how this phenomenology extends to more realistic, multiscale topography and to three-layer QG simulations. 

Abstract In an effort to search for faint sources of emission over arbitrary timescales, we present a novel method for analyzing forced photometry light curves in difference imaging from optical surveys. Our method “ATLAS Clean,” or ATClean, utilizes the reported fluxes, uncertainties, and fits to the point-spread function (PSF) from difference images to quantify the statistical significance of individual measurements. We apply this method to control light curves across the image to determine whether any source of flux is present in the data for a range of specific timescales. From ATLASo-band imaging at the site of the Type II supernova (SN) 2023ixf in M101 from 2015–2023, we show that this method accurately reproduces the 3σflux limits produced from other, more computationally expensive methods. We derive limits for emission on timescales of 5 days and 80–300 days at the site of SN 2023ixf, which are 19.8 and 21.3 mag, respectively. The latter limits rule out variability for unextinguished red supergiants with initial masses >22M_⊙, comparable to the most luminous predictions for the SN 2023ixf progenitor system. We also compare our limits to short-timescale outbursts, similar to those expected for Type IIn SN progenitor stars or the Type II SN 2020tlf, and rule out outburst ejecta masses of >0.021M_⊙, much lower than the inferred mass of circumstellar matter around SN 2023ixf in the literature. In the future, these methods can be applied to any forced photometry on difference imaging from other surveys, such as Rubin optical imaging. 

ABSTRACT We present a search for luminous long-duration ambiguous nuclear transients (ANTs) similar to the unprecedented discovery of the extreme ambiguous event AT2021lwx with a $$\gt 150$$ d rise time and luminosity $$10^{45.7}$$ erg s$$^{-1}$$. We use the Lasair transient broker to search Zwicky Transient Facility (ZTF) data for transients lasting more than one year and exhibiting smooth declines. Our search returns 59 events, 7 of which we classify as ANTs assumed to be driven by accretion onto supermassive black holes. We propose the remaining 52 are stochastic variability from regular supermassive black hole accretion rather than distinct transients. We supplement the seven ANTs with three nuclear transients in ZTF that fail the light curve selection but have clear single flares and spectra that do not resemble typical active galactic nucleus. All of these 11 ANTs have a mid-infrared flare from an assumed dust echo, implying the ubiquity of dust around the black holes giving rise to ANTs. No events are more luminous than AT2021lwx, but one (ZTF19aamrjar) has twice the duration and a higher integrated energy release. On the other extreme, ZTF20abodaps reaches a luminosity close to AT2021lwx with a rise time $$\lt 20$$ d and that fades smoothly in $$\gt 600$$ d. We define a portion of rise-time versus flare amplitude space that selects ANTs with $$\sim 50$$ per cent purity against variable AGNs. We calculate a volumetric rate of $$\gtrsim 3\times 10^{-11}$$ Mpc$$^{-1}$$ yr$$^{-1}$$, consistent with the events being caused by tidal disruptions of intermediate and high-mass stars. 

Abstract The generation of broadband wave energy frequency spectra from narrowband wave forcing in geophysical flows remains a conundrum. In contrast to the long-standing view that nonlinear wave–wave interactions drive the spreading of wave energy in frequency space, recent work suggests that Doppler-shifting by geostrophic flows may be the primary agent. We investigate this possibility by ray tracing a large number of inertia–gravity wave packets through three-dimensional, geostrophically turbulent flows generated either by a quasigeostrophic (QG) simulation or by synthetic random processes. We find that, in all cases investigated, a broadband quasi-stationary inertia–gravity wave frequency spectrum forms, irrespective of the initial frequencies and wave vectors of the packets. The frequency spectrum is well represented by a power law. A possible theoretical explanation relies on the analogy between the kinematic stretching of passive tracer gradients and the refraction of wave vectors. Consistent with this hypothesis, the spectrum of eigenvalues of the background flow velocity gradients predicts a frequency spectrum that is nearly identical to that found by integration of the ray tracing equations.