Treasurehunt: Transients and variability discovered with HST in the JWST North Ecliptic Pole time-domain field

The James Webb Space Telescope (JWST) North Ecliptic Pole (NEP) Time-domain Field (TDF) is a >14' diameter field optimized for multiwavelength time-domain science with JWST. It has been observed across the electromagnetic spectrum both from the ground and from space, including with the Hubble Space Telescope (HST). As part of HST observations over three cycles (the "TREASUREHUNT" program), deep images were obtained with the Wide Field Camera on the Advanced Camera for Surveys in F435W and F606W that cover almost the entire JWST NEP TDF. Many of the individual pointings of these programs partially overlap, allowing an initial assessment of the potential of this field for time-domain science with HST and JWST. The cumulative area of overlapping pointings is ∼88 arcmin2, with time intervals between individual epochs that range between 1 day and 4+ yr. To a depth of mAB ≃ 29.5 mag (F606W), we present the discovery of 12 transients and 190 variable candidates. For the variable candidates, we demonstrate that Gaussian statistics are applicable and estimate that ∼80 are false positives. The majority of the transients will be supernovae, although at least two are likely quasars. Most variable candidates are active galactic nuclei (AGNs), where we find 0.42% of the general z ≲ 6 field galaxy population to vary at the ∼3σ level. Based on a 5 yr time frame, this translates into a random supernova areal density of up to ∼0.07 transients arcmin−2 (∼245 deg−2) per epoch and a variable AGN areal density of ∼1.25 variables arcmin−2 (∼4500 deg−2) to these depths

Numerical Investigation of Orographic Cloud and Vortex Dynamics on Ice Giant Planets

Farther out and smaller than Jupiter and Saturn, Neptune and Uranus form a pair of planets known as the ice giants. These ice giants share similar atmospheric conditions and chemistry, including strikingly bright methane cloud features. These features are typically the most visible phenomena in these atmospheres as observed from earth. Some of these clouds are orographic in nature, tracking the motions of large vortex features known as Dark Spots. In 1989, the Voyager II encounter with Neptune revealed such a pairing, with first Great Dark Spot (GDS-89), arguably the most dynamic large vortex feature in the outer solar system, matched with an orographic bright companion cloud. Later Hubble Space Telescope observations revealed two other Dark Spots on Neptune in the northern hemisphere, dubbed NGDS-32 and NGDS-15, the former of which appeared to have an orographic feature. All of the Neptune Dark Spots are transient, lasting months to a few years, and the most recent HST observations evidenced no GDS activity. In contrast, Uranus showed no vortex features during the Voyager II encounter in 1986, but in the summer of 2006 what is likely the first observed GDS on Uranus was spotted. There is some evidence that a bright, possibly orographic cloud feature tracked this vortex at certain times. This paper numerically investigates the source of these orographic features and their influence on the vortex dynamics through simulations with a general circulation model called EPIC. Understanding these features will provide increased insight into the planetary atmosphere environment on the ice giants, enabling comparative meteorology and enhancing our ability to design future missions to this planet

Effects of Hemispheric Circulation on Uranian Atmospheric Dynamics and Methane Depletion

The solar system is filled with meteorological phenomena. For example, geophysical vortices range from hurricanes to the Great Red Spot on Jupiter to the Dark Spots of Uranus and Neptune. These Ice Giant vortices have exhibited unusual dynamical behaviors, such as the shape oscillations and meridional drift of the Great Dark Spot, discovered and observed in 1989 by Voyager II. On the other hand, the Uranian Dark Spot exhibited little to no drift over a similar stretch of observation when it appeared shortly before the spring equinox on Uranus in 2006. Another phenomenon is regions of persistent clouds, common in the banding patterns of the gas giants. The bright companion of the Great Dark Spot is a different type of persistent cloud, arising orographically as the vortex moved through the atmosphere. The Uranian Dark Spot may also have had a similar, although intermittent, cloud companion. Another notable long-lived cloud feature called S34 or the Berg in the southern hemisphere of Uranus, which drifted equatorward covering approximately 30 degrees in latitude over the course of a few years as equinox approached, having previously spent several years in the vicinity of 34 degrees south latitude. While this motion resembles in some ways that of the Great Dark Spot and its Bright Companion, there was no visible vortex associated with the Berg\u27s cloud. A proposed cause of this unexpected drift is the development of a strong meridional, Hadley-cell circulation that caused the cloud (and a possible unseen companion vortex) to drift equatorward. This same circulation may account for observations that showed upper tropospheric methane gas (a primary cloud constituent on Uranus) in the southern hemisphere was preferentially accumulating near the equator while depleting near the south pole. This paper presents the first efforts to examine these phenomena by numerically modeling a full Uranian atmosphere. These simulations are designed to examine these changes in the Uranian atmosphere, probably related to the extreme seasonal change of this planet. While this research will improve our understanding of the Uranian atmosphere and the design of future missions to this system, it will also assist in understanding the similar dynamics on the other Ice Giant planet Neptune, and potentially with similar phenomena in Earth\u27s atmosphere like hurricane drift. © 2012 by by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc

Keck and VLT AO observations and models of the uranian rings during the 2007 ring plane crossings

We present observations of the uranian ring system at a wavelength of 2.2 μm, taken between 2003 and 2008 with NIRC2 on the W.M. Keck telescope in Hawaii, and on 15-17 August 2007 with NaCo on the Very Large Telescope (VLT) in Chile. Of particular interest are the data taken around the time of the uranian ring plane crossing with Earth on 16 August 2007, and with the Sun (equinox) on 7 December 2007. We model the data at the different viewing aspects with a Monte Carlo model to determine: (1) the normal optical depth τ0, the location, and the radial extent of the main rings, and (2) the parameter Aτ0 (A is the particle geometric albedo), the location, and the radial plus vertical extent of the dusty rings. Our main conclusions are: (i) The brightness of the ∊ ring is significantly enhanced at small phase and ring inclination angles; we suggest this extreme opposition effect to probably be dominated by a reduction in interparticle shadowing. (ii) A broad sheet of dust particles extends inwards from the λ ring almost to the planet itself. This dust sheet has a vertical extent of ∼140 km, and Aτ0 = 2.2 × 10−6. (iii) The dusty rings between ring 4 and the α ring and between the α and β rings are vertically extended with a thickness of ∼300 km. (iv) The ζ ring extends from ∼41,350 km almost all the way inwards to the planet. The main ζ ring, centered at ∼39,500 km from the planet, is characterized by Aτ0 = 3.7 × 10−6; this parameter decreases closer to the planet. The ζ ring has a full vertical extent of order 800-900 km, with a pronounced density enhancement in the mid-plane. (v) The ηc ring is optically thin and less than several tens of km in the vertical direction. This ring may be composed of macroscopic material, surrounded by clumps of dust