arXiv subjects

H. B. Hammel

Publications and source records attributed to H. B. Hammel.

At least 19 recordsLinked to original sources

arXiv2025

Spectral trends across the rings and inner moons of Uranus and Neptune from JWST NIRCam images

JWST NIRCam images provide low-resolution spectra of the rings and inner moons orbiting Uranus and Neptune. These data reveal systematic variations in spectral parameters like the strength of the strong OH absorption band around 3 microns and the spectral slopes at continuum wavelengths. Neptune's rings show an extremely weak 3-micron band, which is likely due to the small particle sizes in these dusty rings. Neptune's small inner moons also have weaker 3-micron bands and redder continua than Uranus' small inner moons, indicating that Neptune's moons have a lower water-ice fraction. There are also clear spectral trends across the inner Uranian system. The strength of the 3-micron band clearly increases with distance from Uranus, with the rings having a noticeably weaker 3-micron band than most of the small inner moons, which have a weaker 3-micron band than the larger moons like Miranda. While the rings and most of the small moons have neutral spectra between 1.4 microns and 2.1 microns, the outermost small moon Mab exhibits a blue spectral slope comparable to Miranda, indicating that Mab's surface may also be relatively water-ice rich. The next moon interior to Mab, Puck, exhibits a stronger 3-micron band and bluer continuum slope than any of the moons orbiting interior to it, perhaps indicating that it is being covered by water-ice-rich material derived from Mab via the mu ring. Finally, the small moon Rosalind has a redder spectral slope than its neighbors, possibly due to being coated with material from the dusty nu ring.

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arXiv2024

PEARLS: Discovery of Point-Source Features Within Galaxies in the North Ecliptic Pole Time Domain Field

The first public 0.9-4.4μm NIRCam images of the North Ecliptic Pole (NEP) Time Domain Field (TDF) uncovered galaxies displaying point-source features in their cores as seen in the longer wavelength filters. We visually identified a sample of 66 galaxies (~1 galaxy per arcmin2) with point-like cores and have modeled their two-dimensional light profiles with GalFit, identifying 16 galactic nuclei with measurable point-source components. GalFit suggests the visual sample is a mix of both compact stellar bulge and point-source galaxy cores. This core classification is complemented by spectral energy distribution (SED) modeling to infer the sample's active galactic nucleus (AGN) and host-galaxy parameters. For galaxies with measurable point-source components, the median fractional AGN contribution to their 0.1-30.0μm flux is 0.44, and 14/16 are color-classified AGN. We conclude that near-infrared point-source galaxy cores are signatures of AGN. In addition, we define an automated sample-selection criterion to identify these point-source features. These criteria can be used in other extant and future NIRCam images to streamline the search for galaxies with unresolved IR-luminous AGN. The James Webb Space Telescope's superb angular resolution and sensitivity at infrared wavelengths is resurrecting the morphological identification of AGN.

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arXiv2024

Unveiling the ice and gas nature of active centaur (2060) Chiron using the James Webb Space Telescope

(2060) Chiron is a large centaur that has been reported active on multiple occasions including during aphelion passage. Studies of Chirons coma during active periods have resulted in the detection of C(triple)N and CO outgassing. Significant work remains to be undertaken to comprehend the activation mechanisms on Chiron and the parent molecules of the gas phases detected. This work reports the study of the ices on Chirons surface and coma and seeks spectral indicators of volatiles associated with the activity. Additionally, we discuss how these detections could be related to the activation mechanism for Chiron and, potentially, other centaurs. In July 2023, the James Webb Space Telescope (JWST) observed Chiron when it was active near its aphelion. We present JWST/NIRSpec spectra from 0.97 to 5.27 microns with a resolving power of 1000, and compare them with laboratory data for identification of the spectral bands. We report the first detections on Chiron of absorption bands of several volatile ices, including CO2, CO, C2H6, C3H8, and C2H2. We also confirm the presence of water ice in its amorphous state. A key discovery arising from these data is the detection of fluorescence emissions of CH4, revealing the presence of a gas coma rich in this hyper-volatile molecule, which we also identify to be in non-local thermal equilibrium (nonLTE). CO2 gas emission is also detected in the fundamental stretching band at 4.27 microns. We argue that the presence of CH4 emission is the first proof of the desorption of CH4 due to a density phase transition of amorphous water ice at low temperature in agreement with the estimated temperature of Chiron during the JWST observations (61 K). Detection of photolytic and proton irradiation products of CH4 and CO2 on the surface, in the coma ice grains, or in the ring material is also detected via a forest of absorption features from 3.5 to 5.3 microns.

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arXiv2024

TREASUREHUNT: Transients and Variability Discovered with HST in the JWST North Ecliptic Pole Time Domain Field

The JWST North Ecliptic Pole (NEP) Time Domain Field (TDF) is a $>$14 arcmin diameter field optimized for multi-wavelength time-domain science with JWST. It has been observed across the electromagnetic spectrum both from the ground and from space, including with the Hubble Space Telescope (HST). As part of HST observations over 3 cycles (the "TREASUREHUNT" program), deep images were obtained with ACS/WFC in F435W and F606W that cover almost the entire JWST NEP TDF. Many of the individual pointings of these programs partially overlap, allowing an initial assessment of the potential of this field for time-domain science with HST and JWST. The cumulative area of overlapping pointings is ~88 arcmin$^2$, with time intervals between individual epochs that range between 1 day and 4$+$ years. To a depth of $m_{AB}$ $\simeq$ 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 AGN, where we find 0.42% of the general $z$ $<$ 6 field galaxy population to vary at the $~3σ$ level. Based on a 5-year timeframe, this translates into a random supernova areal density of up to ~0.07 transients per arcmin$^2$ (~245 deg$^{-2}$) per epoch, and a variable AGN areal density of ~1.25 variables per arcmin$^2$ (~4500 deg$^{-2}$) to these depths.

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arXiv2024

Water-Ice Dominated Spectra of Saturn's Rings and Small Moons from JWST

JWST measured the infrared spectra of Saturn's rings and several of its small moons (Epimetheus, Pandora, Telesto and Pallene) as part of Guaranteed Time Observation program 1247. The NIRSpec instrument obtained near-infrared spectra of the small moons between 0.6 and 5.3 microns, which are all dominated by water-ice absorption bands. The shapes of the water-ice bands for these moons suggests that their surfaces contain variable mixes of crystalline and amorphous ice or variable amounts of contaminants and/or sub-micron ice grains. The near-infrared spectrum of Saturn's A ring has exceptionally high signal-to-noise between 2.7 and 5 microns and is dominated by features due to highly crystalline water ice. The ring spectrum also confirms that the rings possess a 2-3% deep absorption at 4.13 microns due to deuterated water-ice previously seen by the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft. This spectrum also constrains the fundamental absorption bands of carbon dioxide and carbon monoxide and may contain evidence for a weak aliphatic hydrocarbon band. Meanwhile, the MIRI instrument obtained mid-infrared spectra of the rings between 4.9 and 27.9 microns, where the observed signal is a combination of reflected sunlight and thermal emission. This region shows a strong reflectance peak centered around 9.3 microns that can be attributed to crystalline water ice. Since both the near and mid-infrared spectra are dominated by highly crystalline water ice, they should provide a useful baseline for interpreting the spectra of other objects in the outer solar system with more complex compositions.

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arXiv2023

PEARLS: JWST counterparts of micro-Jy radio sources in the Time Domain Field

The Time Domain Field (TDF) near the North Ecliptic Pole in JWST's continuous-viewing zone will become a premier "blank field" for extragalactic science. JWST/NIRCam data in a 16 arcmin$^2$ portion of the TDF identify 4.4 $μ$m counterparts for 62 of 63 3 GHz sources with S(3 GHz) > 5 μJy. The one unidentified radio source may be a lobe of a nearby Seyfert galaxy, or it may be an infrared-faint radio source. The bulk properties of the radio-host galaxies are consistent with those found by previous work: redshifts range from 0.14 to 4.4 with a median redshift of 1.33. The radio emission arises primarily from star formation in $\sim 2/3$ of the sample and from an active galactic nucleus in $\sim 1/3$, but just over half the sample shows evidence for an AGN either in the spectral energy distribution or by radio excess. All but three counterparts are brighter than magnitude 23 AB at 4.4 $μ$m, and the exquisite resolution of JWST identifies correct counterparts for sources for which observations with lower angular resolution would mis-identify a nearby bright source as the counterpart when the correct one is faint and red. Up to 11% of counterparts might have been unidentified or misidentified absent NIRCam observations.

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arXiv2023

JWST molecular mapping and characterization of Enceladus' water plume feeding its torus

Enceladus is a prime target in the search for life in our solar system, having an active plume likely connected to a large liquid water subsurface ocean. Using the sensitive NIRSpec instrument onboard JWST, we searched for organic compounds and characterized the plume's composition and structure. The observations directly sample the fluorescence emissions of H2O and reveal an extraordinarily extensive plume (up to 10,000 km or 40 Enceladus radii) at cryogenic temperatures (25 K) embedded in a large bath of emission originating from Enceladus' torus. Intriguingly, the observed outgassing rate (300 kg/s) is similar to that derived from close-up observations with Cassini 15 years ago, and the torus density is consistent with previous spatially unresolved measurements with Herschel 13 years ago, suggesting that the vigor of gas eruption from Enceladus has been relatively stable over decadal timescales. This level of activity is sufficient to maintain a derived column density of 4.5x1017 m-2 for the embedding equatorial torus, and establishes Enceladus as the prime source of water across the Saturnian system. We performed searches for several non-water gases (CO2, CO, CH4, C2H6, CH3OH), but none were identified in the spectra. On the surface of the trailing hemisphere, we observe strong H2O ice features, including its crystalline form, yet we do not recover CO2, CO nor NH3 ice signatures from these observations. As we prepare to send new spacecraft into the outer solar system, these observations demonstrate the unique ability of JWST in providing critical support to the exploration of distant icy bodies and cryovolcanic plumes.

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arXiv2019

Combined Emerging Capabilities for Near-Earth Objects (NEOs)

Assess the joint capabilities of emerging telescopes for near-Earth objects (NEOs) survey and characterization, and what they will add to the current capabilities or replace. NASA telescopes in prime mission, in development, or under study, and requested for this assessment, include: - The Transiting Exoplanet Survey Satellite (TESS) - The James Webb Space Telescope (JWST) - The Wide Field Infrared Survey Telescope (WFIRST) - The Near-Earth Object Camera (NEOCam). Also requested for this assessment is the Large Synoptic Survey Telescope (LSST), an 8.4-meter ground-based telescope in development by the National Science Foundation and Department of Energy (DOE), with the capability to discover and catalogue NEOs.

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arXiv2019

Comparing key compositional indicators in Jupiter with those in extra-solar giant planets

Spectroscopic transiting observations of the atmospheres of hot Jupiters around other stars, first with Hubble Space Telescope and then Spitzer, opened the door to compositional studies of exoplanets. The James Webb Space Telescope will provide such a profound improvement in signal-to-noise ratio that it will enable detailed analysis of molecular abundances, including but not limited to determining abundances of all the major carbon- and oxygen-bearing species in hot Jupiter atmospheres. This will allow determination of the carbon-to-oxygen ratio, an essential number for planet formation models and a motivating goal of the Juno mission currently around Jupiter

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arXiv2017

Neptune long-lived atmospheric features in 2013-2015 from small (28-cm) to large (10-m) telescopes

Since 2013, observations of Neptune with small telescopes have resulted in several detections of long-lived bright atmospheric features that have also been observed by large telescopes such as Keck II or Hubble. The combination of both types of images allows the study of the long term evolution of major cloud systems in the planet. In 2013 and 2014 two bright features were present on the planet at southern mid latitudes. These may have merged in late 2014, possibly leading to the formation of a single bright feature observed during 2015 at the same latitude. This cloud system was first observed in January 2015 and nearly continuously from July to December 2015 in observations with telescopes in the 2 to 10 meter class and in images from amateur astronomers. These images show the bright spot as a compact feature at 40.1 deg South planetographic latitude well resolved from a nearby bright zonal band that extended from 42 deg South to 20 deg South. Tracking its motion from July to November 2015 suggests a longitudinal oscillation of 16 deg in amplitude with a 90 day period, typical of dark spots on Neptune and similar to the Great Red Spot oscillation in Jupiter. The limited time covered by high-resolution observations only covers one full oscillation and other interpretations of the changing motions could be possible. HST images in September 2015 show the presence of a dark spot at short wavelengths in the southern flank of the bright cloud observed throughout 2015.

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arXiv2017

Retrieving Neptune's aerosol properties from Keck OSIRIS observations. I. Dark regions

We present and analyze three-dimensional data cubes of Neptune from the OSIRIS integral-field spectrograph on the 10-m Keck telescope, from July 2009. These data have a spatial resolution of 0.035"/pixel and spectral resolution of R~3800 in the H and K broad bands. We focus our analysis on regions of Neptune's atmosphere that are near-infrared dark- that is, free of discrete bright cloud features. We use a forward model coupled to a Markov chain Monte Carlo algorithm to retrieve properties of Neptune's aerosol structure and methane profile above ~4 bar in these near-infrared dark regions. Using a set of high signal-to-noise spectra in a cloud-free band from 2-12N, we find that Neptune's cloud opacity is dominated by a compact, optically thick cloud layer with a base near 3 bar and composed of low albedo, forward scattering particles, with an assumed characteristic size of ~1$μ$m. Above this cloud, we require a vertically extended haze of smaller (~0.1 $μ$m) particles, which reaches from the upper troposphere (~0.6 bar) into the stratosphere. The particles in this haze are brighter and more isotropically scattering than those in the deep cloud. When we extend our analysis to 18 cloud-free locations from 20N to 87S, we observe that the optical depth in aerosols above 0.5 bar decreases by a factor of 2-3 or more at mid- and high-southern latitudes relative to low latitudes. We also consider Neptune's methane (CH$_4$) profile, and find that our retrievals indicate a strong preference for a low methane relative humidity at pressures where methane is expected to condense. Our preferred solution at most locations is for a methane relative humidity below 10% near the tropopause in addition to methane depletion down to 2.0-2.5 bar. We tentatively identify a trend of lower CH$_4$ columns above 2.5 bar at mid- and high-southern latitudes over low latitudes.

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arXiv2016

A Distant Mirror: Solar Oscillations Observed on Neptune by the Kepler K2 Mission

Starting in December 2014, Kepler K2 observed Neptune continuously for 49 days at a 1-minute cadence. The goals consisted of studying its atmospheric dynamics (Simon et al. 2016), detecting its global acoustic oscillations (Rowe et al., submitted), and those of the Sun, which we report on here. We present the first indirect detection of solar oscillations in intensity measurements. Beyond the remarkable technical performance, it indicates how Kepler would see a star like the Sun. The result from the global asteroseismic approach, which consists of measuring the oscillation frequency at maximum amplitude "nu_max" and the mean frequency separation between mode overtones "Delta nu", is surprising as the nu_max measured from Neptune photometry is larger than the accepted value. Compared to the usual reference nu_max_sun = 3100 muHz, the asteroseismic scaling relations therefore make the solar mass and radius appear larger by 13.8 +/- 5.8 % and 4.3 +/- 1.9 % respectively. The higher nu_max is caused by a combination of the value of nu_max_sun, being larger at the time of observations than the usual reference from SOHO/VIRGO/SPM data (3160 +/- 10 muHz), and the noise level of the K2 time series, being ten times larger than VIRGO's. The peak-bagging method provides more consistent results: despite a low signal-to-noise ratio (SNR), we model ten overtones for degrees l=0,1,2. We compare the K2 data with simultaneous SOHO/VIRGO/SPM photometry and BiSON velocity measurements. The individual frequencies, widths, and amplitudes mostly match those from VIRGO and BiSON within 1 sigma, except for the few peaks with lowest SNR.

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arXiv2015

High S/N Keck and Gemini AO imaging of Uranus during 2012-2014: New cloud patterns, increasing activity, and improved wind measurements

We imaged Uranus in the near infrared from 2012 into 2014, using the Keck/NIRC2 camera and Gemini/NIRI camera, both with adaptive optics. We obtained exceptional signal to noise ratios by averaging 8-16 individual exposures in a planet-fixed coordinate system. noise-reduced images revealed many low-contrast discrete features and large scale cloud patterns not seen before, including scalloped waveforms just south of the equator. In all three years numerous small (600-700 km wide) and mainly bright discrete features were seen within the north polar region (north of about 55°N). Over 850 wind measurements were made, the vast majority of which were in the northern hemisphere. These revealed an extended region of solid body rotation between 62°N and at least 83°N, at a rate of 4.08$\pm0.015$°/h westward relative to the planet's interior (radio) rotation of 20.88°/h westward. Near-equatorial speeds measured with high accuracy give different results for waves and small discrete features, with eastward drift rates of 0.4°/h and 0.1°/h respectively. The region of polar solid body rotation is a close match to the region of small-scale polar cloud features, suggesting a dynamical relationship. While winds at high southern latitudes (50°S - 90°S) are unconstrained by groundbased observations, a recent reanalysis of 1986 Voyager 2 observations by Karkoschka (2015, Icarus 250, 294-307) has revealed an extremely large north-south asymmetry in this region, which might be seasonal. Greatly increased activity was seen in 2014, including the brightest ever feature seen in K' images (de Pater et al. 2015, Icarus 252, 121-128). Over the 2012-2014 period we identified six persistent discrete features. Three were tracked for more than two years, two more for more than one year, and one for at least 5 months and continuing.

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arXiv2014

The continued optical to mid-IR evolution of V838 Monocerotis

The eruptive variable V838 Monocerotis gained notoriety in 2002 when it brightened nine magnitudes in a series of three outbursts and then rapidly evolved into an extremely cool supergiant. We present optical, near-IR, and mid-IR spectroscopic and photometric observations of V838 Monocerotis obtained between 2008 and 2012 at the Apache Point Observatory 3.5m, NASA IRTF 3m, and Gemini South 8m telescopes. We contemporaneously analyze the optical & IR spectroscopic properties of V838 Monocerotis to arrive at a revised spectral type L3 supergiant and effective temperature Teff~2000--2200 K. Because there are no existing optical observational data for L supergiants in the optical, we speculate that V838 Monocerotis may represent the prototype for L supergiants in this wavelength regime. We find a low level of Halpha emission present in the system, consistent with interaction between V838 Monocerotis and its B3V binary; however, we cannot rule out a stellar collision as the genesis event, which could result in the observed Halpha activity. Based upon a two-component blackbody fit to all wavelengths of our data, we conclude that, as of 2009, a shell of ejecta surrounded V838 Monocerotis at a radius of R=263+/-10 AU with a temperature of T=285+/-2 K. This result is consistent with IR interferometric observations from the same era and predictions from the Lynch et al. model of the expanding system, which provides a simple framework for understanding this complicated system.

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arXiv2014

Instrumental Methods for Professional and Amateur Collaborations in Planetary Astronomy

Amateur contributions to professional publications have increased exponentially over the last decades in the field of Planetary Astronomy. Here we review the different domains of the field in which collaborations between professional and amateur astronomers are effective and regularly lead to scientific publications. We discuss the instruments, detectors, softwares and methodologies typically used by amateur astronomers to collect the scientific data in the different domains of interest. Amateur contributions to the monitoring of planets and interplanetary matter, characterization of asteroids and comets, as well as the determination of the physical properties of Kuiper Belt Objects and exoplanets are discussed.

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arXiv2014

Relating jet structure to photometric variability: the Herbig Ae star HD 163296

Herbig Ae/Be stars are intermediate-mass pre-main sequence stars surrounded by circumstellar dust disks. Some are observed to produce jets, whose appearance as a sequence of shock fronts (knots) suggests a past episodic outflow variability. This "jet fossil record" can be used to reconstruct the outflow history. We present the first optical to near-infrared (NIR) VLT/X-shooter spectra of the jet from the Herbig Ae star HD 163296. We determine physical conditions in the knots, as well as their kinematic "launch epochs". Knots are formed simultaneously on either side of the disk, with a regular interval of ~16 yr. The velocity dispersion versus jet velocity and the energy input are comparable in both lobes. However, the mass loss rate, velocity, and shock conditions are asymmetric. We find Mjet/Macc ~ 0.01-0.1, consistent with magneto-centrifugal jet launching models. No evidence for dust is found in the high-velocity jet, suggesting it is launched within the sublimation radius (<0.5 au). The jet inclination measured from proper motions and radial velocities confirms it is perpendicular to the disk. A tentative relation is found between the structure of the jet and the photometric variability of the source. Episodes of NIR brightening were previously detected and attributed to a dusty disk wind. We report for the first time significant optical fadings lasting from a few days up to a year, coinciding with the NIR brightenings. These are likely caused by dust lifted high above the disk plane; this supports the disk wind scenario. The disk wind is launched at a larger radius than the high-velocity atomic jet, although their outflow variability may have a common origin. No significant relation between outflow and accretion variability could be established. Our findings confirm that this source undergoes periodic ejection events, which may be coupled with dust ejections above the disk plane.

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arXiv2010

Triplicity and Physical Characteristics of Asteroid (216) Kleopatra

To take full advantage of the September 2008 opposition passage of the M-type asteroid (216) Kleopatra, we have used near-infrared adaptive optics (AO) imaging with the W.M. Keck II telescope to capture unprecedented high resolution images of this unusual asteroid. Our AO observations with the W.M. Keck II telescope, combined with Spitzer/IRS spectroscopic observations and past stellar occultations, confirm the value of its IRAS radiometric radius of 67.5 km as well as its dog-bone shape suggested by earlier radar observations. Our Keck AO observations revealed the presence of two small satellites in orbit about Kleopatra (see Marchis et al., 2008). Accurate measurements of the satellite orbits over a full month enabled us to determine the total mass of the system to be 4.64+/-0.02 10^18 Kg. This translates into a bulk density of 3.6 +/-0.4 g/cm3, which implies a macroscopic porosity for Kleopatra of ~ 30-50%, typical of a rubble-pile asteroid. From these physical characteristics we measured its specific angular momentum, very close to that of a spinning equilibrium dumbbell.

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arXiv2010

First Earth-based Detection of a Superbolide on Jupiter

Cosmic collisions on planets cause detectable optical flashes that range from terrestrial shooting stars to bright fireballs. On June 3, 2010 a bolide in Jupiter's atmosphere was simultaneously observed from the Earth by two amateur astronomers observing Jupiter in red and blue wavelengths. The bolide appeared as a flash of 2 s duration in video recording data of the planet. The analysis of the light curve of the observations results in an estimated energy of the impact of 0.9-4.0x10^{15} J which corresponds to a colliding body of 8-13 m diameter assuming a mean density of 2 g cm^{-3}. Images acquired a few days later by the Hubble Space Telescope and other large ground-based facilities did not show any signature of aerosol debris, temperature or chemical composition anomaly, confirming that the body was small and destroyed in Jupiter's upper atmosphere. Several collisions of this size may happen on Jupiter on a yearly basis. A systematic study of the impact rate and size of these bolides can enable an empirical determination of the flux of meteoroids in Jupiter with implications for the populations of small bodies in the outer Solar System and may allow a better quantification of the threat of impacting bodies to Earth. The serendipitous recording of this optical flash opens a new window in the observation of Jupiter with small telescopes.

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