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Imke de Pater

Publications and source records attributed to Imke de Pater.

At least 37 records · Page 2Linked to original sources
arXiv2023

The Breakthrough Listen Search for Intelligent Life: Detection and Characterization of Anomalous Transits in Kepler Lightcurves

Never before has the detection and characterization of exoplanets via transit photometry been as promising and feasible as it is now, due to the increasing breadth and sensitivity of time domain optical surveys. Past works have made use of phase-folded stellar lightcurves in order to study the properties of exoplanet transits, because this provides the highest signal that a transit is present at a given period and ephemeris. Characterizing transits on an individual, rather than phase-folded, basis is much more challenging due to the often low signal-to-noise ratio (SNR) of lightcurves, missing data, and low sampling rates. However, by phase-folding a lightcurve we implicitly assume that all transits have the same expected properties, and lose all information about the nature and variability of the transits. We miss the natural variability in transit shapes, or even the deliberate or inadvertent modification of transit signals by an extraterrestrial civilization (for example, via laser emission or orbiting megastructures). In this work, we develop an algorithm to search stellar lightcurves for individual anomalous (in timing or depth) transits, and we report the results of that search for 218 confirmed transiting exoplanet systems from Kepler.

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arXiv2023

The Breakthrough Listen Search for Intelligent Life: Technosignature Search of 97 Nearby Galaxies

The Breakthrough Listen search for intelligent life is, to date, the most extensive technosignature search of nearby celestial objects. We present a radio technosignature search of the centers of 97 nearby galaxies, observed by Breakthrough Listen at the Robert C. Byrd Green Bank Telescope. We performed a narrowband Doppler drift search using the turboSETI pipeline with a minimum signal-to-noise parameter threshold of 10, across a drift rate range of $\pm$ 4 Hz\ $s^{-1}$, with a spectral resolution of 3 Hz and a time resolution of $\sim$ 18.25 s. We removed radio frequency interference by using an on-source/off-source cadence pattern of six observations and discarding signals with Doppler drift rates of 0. We assess factors affecting the sensitivity of the Breakthrough Listen data reduction and search pipeline using signal injection and recovery techniques and apply new methods for the investigation of the RFI environment. We present results in four frequency bands covering 1 -- 11 GHz, and place constraints on the presence of transmitters with equivalent isotropic radiated power on the order of $10^{26}$ W, corresponding to the theoretical power consumption of Kardashev Type II civilizations.

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arXiv2023

A Simultaneous Dual-site Technosignature Search Using International LOFAR Stations

The Search for Extraterrestrial Intelligence aims to find evidence of technosignatures, which can point toward the possible existence of technologically advanced extraterrestrial life. Radio signals similar to those engineered on Earth may be transmitted by other civilizations, motivating technosignature searches across the entire radio spectrum. In this endeavor, the low-frequency radio band has remained largely unexplored; with prior radio searches primarily above 1 GHz. In this survey at 110-190 MHz, observations of 1,631,198 targets from TESS and Gaia are reported. Observations took place simultaneously with two international stations (noninterferometric) of the Low Frequency Array in Ireland and Sweden. We can reject the presence of any Doppler drifting narrowband transmissions in the barycentric frame of reference, with equivalent isotropic radiated power of 10 17 W, for 0.4 million (or 1.3 million) stellar systems at 110 (or 190) MHz. This work demonstrates the effectiveness of using multisite simultaneous observations for rejecting anthropogenic signals in the search for technosignatures.

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arXiv2023

Latitudinal variations in methane abundance, aerosol opacity and aerosol scattering efficiency in Neptune's atmosphere determined from VLT/MUSE

Spectral observations of Neptune made in 2019 with the MUSE instrument at the Very Large Telescope in Chile have been analysed to determine the spatial variation of aerosol scattering properties and methane abundance in Neptune's atmosphere. The darkening of the South Polar Wave (SPW) at $\sim$ 60$^\circ$S, and dark spots such as the Voyager 2 Great Dark Spot is concluded to be due to a spectrally-dependent darkening ($λ< 650$nm) of particles in a deep aerosol layer at $\sim$ 5 bar and presumed to be composed of a mixture of photochemically-generated haze and H$_2$S ice. We also note a regular latitudinal variation of reflectivity at wavelengths of very low methane absorption longer than $\sim$ 650 nm, with bright zones latitudinally separated by $\sim$ 25$^\circ$. This feature, similar to the spectral characteristics of a discrete deep bright spot DBS-2019 found in our data, is found to be consistent with a brightening of the particles in the same $\sim$5-bar aerosol layer at $λ> 650 $ nm. We find the properties of an overlying methane/haze aerosol layer at $\sim$ 2 bar are, to first-order, invariant with latitude, while variations in the opacity of an upper tropospheric haze layer reproduce the observed reflectivity at methane-absorbing wavelengths, with higher abundances found at the equator and also in a narrow `zone' at $80^\circ$S. Finally, we find the mean abundance of methane below its condensation level to be 6--7% at the equator reducing to $\sim$3% south of $\sim$25$^\circ$S, although the absolute abundances are model dependent.

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arXiv2023

Recovering simulated planet and disk signals using SCALES aperture masking

The Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES) instrument is a lenslet-based integral field spectrograph that will operate at 2 to 5 microns, imaging and characterizing colder (and thus older) planets than current high-contrast instruments. Its spatial resolution for distant science targets and/or close-in disks and companions could be improved via interferometric techniques such as sparse aperture masking. We introduce a nascent Python package, NRM-artist, that we use to design several SCALES masks to be non-redundant and to have uniform coverage in Fourier space. We generate high-fidelity mock SCALES data using the scalessim package for SCALES' low spectral resolution modes across its 2 to 5 micron bandpass. We include realistic noise from astrophysical and instrument sources, including Keck adaptive optics and Poisson noise. We inject planet and disk signals into the mock datasets and subsequently recover them to test the performance of SCALES sparse aperture masking and to determine the sensitivity of various mask designs to different science signals.

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arXiv2023

Simulating medium-spectral-resolution exoplanet characterization with SCALES angular/reference differential imaging

SCALES (Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy) is a 2 - 5 micron high-contrast lenslet-based integral field spectrograph (IFS) designed to characterize exoplanets and their atmospheres. The SCALES medium-spectral-resolution mode uses a lenslet subarray with a 0.34 x 0.36 arcsecond field of view which allows for exoplanet characterization at increased spectral resolution. We explore the sensitivity limitations of this mode by simulating planet detections in the presence of realistic noise sources. We use the SCALES simulator scalessim to generate high-fidelity mock observations of planets that include speckle noise from their host stars, as well as other atmospheric and instrumental noise effects. We employ both angular and reference differential imaging as methods of disentangling speckle noise from the injected planet signals. These simulations allow us to assess the feasibility of speckle deconvolution for SCALES medium resolution data, and to test whether one approach outperforms another based on planet angular separations and contrasts.

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arXiv2023

The Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES): driving science cases and expected outcomes

The Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES) is a $2-5~μ$m, high-contrast integral field spectrograph (IFS) currently being built for Keck Observatory. With both low ($R\lesssim250$) and medium ($R\sim3500-7000$) spectral resolution IFS modes, SCALES will detect and characterize significantly colder exoplanets than those accessible with near-infrared ($\sim1-2~μ$m) high-contrast spectrographs. This will lead to new progress in exoplanet atmospheric studies, including detailed characterization of benchmark systems that will advance the state of the art of atmospheric modeling. SCALES' unique modes, while designed specifically for direct exoplanet characterization, will enable a broader range of novel (exo)planetary observations as well as galactic and extragalactic studies. Here we present the science cases that drive the design of SCALES. We describe an end-to-end instrument simulator that we use to track requirements, and show simulations of expected science yields for each driving science case. We conclude with a discussion of preparations for early science when the instrument sees first light in $\sim2025$.

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arXiv2023

Thermal properties of the leading hemisphere of Callisto inferred from ALMA observations

We present a thermal observation of Callisto's leading hemisphere obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) at 0.87 mm (343 GHz). The angular resolution achieved for this observation was $\sim$$0.16^{\prime\prime}$, which for Callisto at the time of this observation ($D\sim 1.05^{\prime\prime}$) was equivalent to $\sim$6 elements across the surface. Our disk-integrated brightness temperature of 116 $\pm$ 5 K (8.03 $\pm$ 0.40 Jy) is consistent with prior disk-integrated observations. Global surface properties were derived from the observation using a thermophysical model (de Kleer et al. 2021) constrained by spacecraft data. We find that models parameterized by two thermal inertia components more accurately fit the data than single thermal inertia models. Our best-fit global parameters adopt a lower thermal inertia of 15-50 $\text{J}\:\text{m}^{-2}\:\text{K}^{-1}\:\text{s}^{-1/2}$ and a higher thermal inertia component of 1200-2000 $\text{J}\:\text{m}^{-2}\:\text{K}^{-1}\:\text{s}^{-1/2}$, with retrieved millimeter emissivities of 0.89-0.91. We identify several thermally anomalous regions, including spots $\sim$3 K colder than model predictions co-located with the Valhalla impact basin and a complex of craters in the southern hemisphere; this indicates the presence of materials possessing either a higher thermal inertia or a lower emissivity. A warm region confined to the mid-latitudes in these leading hemisphere data may be indicative of regolith property changes due to exogenic sculpting.

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arXiv2023

Spectral determination of the colour and vertical structure of dark spots in Neptune's atmosphere

Previous observations of dark vortices in Neptune's atmosphere, such as Voyager-2's Great Dark Spot, have been made in only a few, broad-wavelength channels, which has hampered efforts to pinpoint their pressure level and what makes them dark. Here, we present Very Large Telescope (Chile) MUSE spectrometer observations of Hubble Space Telescope's NDS-2018 dark spot, made in 2019. These medium-resolution 475 - 933 nm reflection spectra allow us to show that dark spots are caused by a darkening at short wavelengths (< 700 nm) of a deep ~5-bar aerosol layer, which we suggest is the H$_2$S condensation layer. A deep bright spot, named DBS-2019, is also visible on the edge of NDS-2018, whose spectral signature is consistent with a brightening of the same 5-bar layer at longer wavelengths (> 700 nm). This bright feature is much deeper than previously studied dark spot companion clouds and may be connected with the circulation that generates and sustains such spots.

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arXiv2023

Keck Near-Infrared Detections of Mab and Perdita

We report the first near-infrared detection of Uranus's tiny moon Mab, the presumed source of the blue and diffuse $μ$ ring, using the NIRC2 instrument at Keck Observatory. The detection was permitted by an updated shift-and-stack procedure allowing us to integrate on Mab as it moved across the detector in 23 separate exposures taken over $\sim$2 hours, as well as the very low (0.02$^{\circ}$) phase angle at the time of observation. At this phase angle, Mab has an integrated I/F of 24 $\pm$ 3 km$^2$ at 1.6 $μ$m and $\lesssim$37 km$^2$ at 2.1 $μ$m. Comparing these values with Mab's visible reflectance as derived by HST reveals that Mab is spectrally blue; its (0.5 $μ$m)/(1.6 $μ$m) color is more consistent with Miranda's value than Puck's value. Mab is therefore more likely a $\sim$6-km radius body with a Miranda-like surface than a 12-km radius body with a Puck-like surface, in agreement with prior work based on infrared upper limits, but we caution that a Puck-like color is only ruled out at the 2$σ$ level. We also report the first infrared photometry of Perdita, finding an integrated I/F of 31 $\pm$ 3 km$^2$ at 1.6 $μ$m.

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arXiv2023

On Detecting Interstellar Scintillation in Narrowband Radio SETI

To date, the search for radio technosignatures has focused on sky location as a primary discriminant between technosignature candidates and anthropogenic radio frequency interference (RFI). In this work, we investigate the possibility of searching for technosignatures by identifying the presence and nature of intensity scintillations arising from the turbulent, ionized plasma of the interstellar medium (ISM). Past works have detailed how interstellar scattering can both enhance and diminish the detectability of narrowband radio signals. We use the NE2001 Galactic free electron density model to estimate scintillation timescales to which narrowband signal searches would be sensitive, and discuss ways in which we might practically detect strong intensity scintillations in detected signals. We further analyze the RFI environment of the Robert C. Byrd Green Bank Telescope (GBT) with the proposed methodology and comment on the feasibility of using scintillation as a filter for technosignature candidates.

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arXiv2023

Evolution of Neptune at Near-Infrared Wavelengths from 1994 through 2022

Using archival near-infrared observations from the Keck and Lick Observatories and the Hubble Space Telescope, we document the evolution of Neptune's cloud activity from 1994 to 2022. We calculate the fraction of Neptune's disk that contained clouds, as well as the average brightness of both cloud features and cloud-free background over the planet's disk. We observe cloud activity and brightness maxima during 2002 and 2015, and minima during 2007 and 2020, the latter of which is particularly deep. Neptune's lack of cloud activity in 2020 is characterized by a near-total loss of clouds at mid-latitudes and continued activity at the South Pole. We find that the periodic variations in Neptune's disk-averaged brightness in the near-infrared H (1.6 $μ$m), K (2.1 $μ$m), FWCH4P15 (893 nm), F953N (955 nm), FWCH4P15 (965 nm), and F845M (845 nm) bands are dominated by discrete cloud activity, rather than changes in the background haze. The clear positive correlation we find between cloud activity and Solar Lyman-Alpha (121.56 nm) irradiance lends support to the theory that the periodicity in Neptune's cloud activity results from photochemical cloud/haze production triggered by Solar ultraviolet emissions.

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arXiv2023

Drift Rates of Major Neptunian Features between 2018 and 2021

Using near-infrared observations of Neptune from the Keck and Lick Observatories, and the Hubble Space Telescope in combination with amateur datasets, we calculated the drift rates of prominent infrared-bright cloud features on Neptune between 2018 and 2021. These features had lifespans of $\sim 1$ day to $\geq$1 month and were located at mid-latitudes and near the south pole. Our observations permitted determination of drift rates via feature tracking. These drift rates were compared to three zonal wind profiles describing Neptune's atmosphere determined from features tracked in H band (1.6 $μm$), K' band (2.1 $μm$), and Voyager 2 data at visible wavelengths. Features near $-70 °$ measured in the F845M filter (845nm) were particularly consistent with the K' wind profile. The southern mid-latitudes hosted multiple features whose lifespans were $\geq$1 month, providing evidence that these latitudes are a region of high stability in Neptune's atmosphere. We also used HST F467M (467nm) data to analyze a dark, circumpolar wave at $- 60 °$ latitude observed on Neptune since the Voyager 2 era. Its drift rate in recent years (2019-2021) is $4.866 \pm 0.009 °$/day. This is consistent with previous measurements by Karkoschka (2011), which predict a $4.858 \pm 0.022 °$/day drift rate during these years. It also gained a complementary bright band just to the north.

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arXiv2023

A VERITAS/Breakthrough Listen Search for Optical Technosignatures

The Breakthrough Listen Initiative is conducting a program using multiple telescopes around the world to search for "technosignatures": artificial transmitters of extraterrestrial origin from beyond our solar system. The VERITAS Collaboration joined this program in 2018, and provides the capability to search for one particular technosignature: optical pulses of a few nanoseconds duration detectable over interstellar distances. We report here on the analysis and results of dedicated VERITAS observations of Breakthrough Listen targets conducted in 2019 and 2020 and of archival VERITAS data collected since 2012. Thirty hours of dedicated observations of 136 targets and 249 archival observations of 140 targets were analyzed and did not reveal any signals consistent with a technosignature. The results are used to place limits on the fraction of stars hosting transmitting civilizations. We also discuss the minimum-pulse sensitivity of our observations and present VERITAS observations of CALIOP: a space-based pulsed laser onboard the CALIPSO satellite. The detection of these pulses with VERITAS, using the analysis techniques developed for our technosignature search, allows a test of our analysis efficiency and serves as an important proof-of-principle.

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arXiv2023

Evidence of a Polar Cyclone on Uranus from VLA Observations

We present observations of Uranus in northern spring with the VLA from 0.7 cm to 5 cm. These observations reveal details in thermal emission from Uranus' north pole at 10s of bars, including a dark collar near 80N and a bright spot at the polar center. The bright central spot resembles observations of polar emission on Saturn and Neptune at shallower pressures. We constrain the variations in temperature and NH3/H2S abundances which could explain these features. We find that the brightness temperature of the polar spot can be recreated through 5 K temperature gradients and/or 10x depletion of NH3 or H2S vapor between 10-20 bars, both consistent with the presence of a cyclonic polar vortex. The contrast of the polar spot may have increased since 2015, which would suggest seasonal evolution of Uranus' polar circulation at depth.

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arXiv2023

Io's Optical Aurorae in Jupiter's Shadow

Decline and recovery timescales surrounding eclipse are indicative of the controlling physical processes in Io's atmosphere. Recent studies have established that the majority of Io's molecular atmosphere, SO2 and SO, condenses during its passage through Jupiter's shadow. The eclipse response of Io's atomic atmosphere is less certain, having been characterized solely by ultraviolet aurorae. Here we explore the response of optical aurorae for the first time. We find oxygen to be indifferent to the changing illumination, with [O I] brightness merely tracking the plasma density at Io's position in the torus. In shadow, line ratios confirm sparse SO2 coverage relative to O, since their collisions would otherwise quench the emission. Io's sodium aurora mostly disappears in eclipse and e-folding timescales, for decline and recovery differ sharply: ~10 minutes at ingress and nearly 2 hr at egress. Only ion chemistry can produce such a disparity; Io's molecular ionosphere is weaker at egress due to rapid recombination. Interruption of a NaCl+ photochemical pathway best explains Na behavior surrounding eclipse, implying that the role of electron impact ionization is minor relative to photons. Auroral emission is also evident from potassium, confirming K as the major source of far red emissions seen with spacecraft imaging at Jupiter. In all cases, direct electron impact on atomic gas is sufficient to explain the brightness without invoking significant dissociative excitation of molecules. Surprisingly, the nonresponse of O and rapid depletion of Na is opposite the temporal behavior of their SO2 and NaCl parent molecules during Io's eclipse phase.

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arXiv2023

A deep-learning search for technosignatures of 820 nearby stars

The goal of the Search for Extraterrestrial Intelligence (SETI) is to quantify the prevalence of technological life beyond Earth via their "technosignatures". One theorized technosignature is narrowband Doppler drifting radio signals. The principal challenge in conducting SETI in the radio domain is developing a generalized technique to reject human radio frequency interference (RFI). Here, we present the most comprehensive deep-learning based technosignature search to date, returning 8 promising ETI signals of interest for re-observation as part of the Breakthrough Listen initiative. The search comprises 820 unique targets observed with the Robert C. Byrd Green Bank Telescope, totaling over 480, hr of on-sky data. We implement a novel beta-Convolutional Variational Autoencoder to identify technosignature candidates in a semi-unsupervised manner while keeping the false positive rate manageably low. This new approach presents itself as a leading solution in accelerating SETI and other transient research into the age of data-driven astronomy.

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arXiv2022

NaCl & KCl in Io's Atmosphere

We present the first comprehensive study of NaCl and KCl gases in Io's atmosphere in order to investigate their characteristics, and to infer properties of Io's volcanoes and subsurface magma chambers. In this work, we compile all past spectral line observations of NaCl and KCl in Io's atmosphere from the Atacama Large Millimeter/submillimeter Array (ALMA) and use atmospheric models to constrain the physical properties of the gases on several dates between 2012 and 2018. NaCl and KCl appear to be largely spatially confined and for observations with high spectral resolution, the temperatures are high (~500-1000 K), implying a volcanic origin. The ratio of NaCl:KCl was found to be ~5-6 in June 2015 and ~3.5-10 in June 2016, which is consistent with predictions based on observations of Io's extended atmosphere, and less than half the Na:K ratio in chondrites. Assuming these gases are volcanic in origin, these ratios imply a magma temperature of ~1300 K, such that the magma will preferentially outgas KCl over NaCl.

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