arXiv subjects

Thomas Greene

Publications and source records attributed to Thomas Greene.

31 records · Page 2Linked to original sources

arXiv2022

Design of SCALES: A 2-5 Micron Coronagraphic Integral Field Spectrograph for Keck Observatory

We present the design of SCALES (Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy) a new 2-5 micron coronagraphic integral field spectrograph under construction for Keck Observatory. SCALES enables low-resolution (R~50) spectroscopy, as well as medium-resolution (R~4,000) spectroscopy with the goal of discovering and characterizing cold exoplanets that are brightest in the thermal infrared. Additionally, SCALES has a 12x12" field-of-view imager that will be used for general adaptive optics science at Keck. We present SCALES's specifications, its science case, its overall design, and simulations of its expected performance. Additionally, we present progress on procuring, fabricating and testing long lead-time components.

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arXiv2019

High-resolution Infrared Spectrograph for Exoplanet Characterization with the Keck and Thirty Meter Telescopes

HISPEC (High-resolution Infrared Spectrograph for Exoplanet Characterization) is a proposed diffraction-limited spectrograph for the W.M. Keck Observatory, and a pathfinder for the MODHIS facility project (Multi-Object Diffraction-limited High-resolution Infrared Spectrograph) on the Thirty Meter Telescope. HISPEC/MODHIS builds on diffraction-limited spectrograph designs which rely on adaptively corrected single-mode fiber feeds. Seeing-limited high-resolution spectrographs, by virtue of the conservation of beam etendue, grow in volume following a D^3 power law (D is the telescope diameter), and are subject to daunting challenges associated with their large size. Diffraction-limited spectrographs fed by single mode fibers are decoupled from the telescope input, and are orders of magnitude more compact and have intrinsically stable line spread functions. Their efficiency is directly proportional to the performance of the adaptive optics (AO) system. AO technologies have matured rapidly over the past two decades and are baselined for future extremely large telescopes. HISPEC/MODHIS will take R>100,000 spectra of a few objects in a 10" field-of-view sampled at the diffraction limit (~10-50 mas), simultaneously from 0.95 to 2.4 microns (y-K). The scientific scope ranges from exoplanet infrared precision radial velocities, spectroscopy of transiting, close-in, and directly imaged exoplanets (atmospheric composition and dynamics, RM effect, spin measurements, Doppler imaging), brown dwarf characterization, stellar physics/chemistry, proto-planetary disk kinematics/composition, Solar system, extragalactic science, and cosmology. HISPEC/MODHIS features a compact, cost-effective design optimized to fully exploit the existing Keck-AO and future TMT-NFIRAOS infrastructures and boost the scientific reach of Keck Observatory and TMT soon after first light.

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arXiv2019

Imaging Cool Giant Planets in Reflected Light: Science Investigations and Synergy with Habitable Planets

Planned astronomical observatories of the 2020s will be capable of obtaining reflected light photometry and spectroscopy of cool extrasolar giant planets. Here we explain that such data are valuable both for understanding the origin and evolution of giant planets as a whole and for preparing for the interpretation of similar datasets from potentially habitable extrasolar terrestrial planets in the decades to follow.

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arXiv2019

Characterizing Transiting Exoplanets with JWST Guaranteed Time and ERS Observations

We highlight how guaranteed time observations (GTOs) and early release science (ERS) will advance understanding of exoplanet atmospheres and provide a glimpse into what transiting exoplanet science will be done with JWST during its first year of operations. These observations of 27 transiting planets will deliver significant insights into the compositions, chemistry, clouds, and thermal profiles of warm-to-hot gas-dominated planets well beyond what we have learned from HST, Spitzer, and other observatories to date. These data and insights will in turn inform our understanding of planet formation, atmospheric transport and climate, and relationships between various properties. Some insight will likely be gained into rocky planet atmospheres as well. JWST will be the most important mission for characterizing exoplanet atmospheres in the 2020s, and this should be considered in assessing exoplanet science for the 2020s and 2030s and future facilities.

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arXiv2018

TDEM final report: Enhanced direct imaging exoplanet detection with astrometry mass determination

This Final Report (FR) presents the results of the Enhanced direct imaging exoplanet detection with astrometry mass determination project, which was executed in support of NASA's Exoplanet Exploration Program and the ROSES Technology Development for Exoplanet Missions (TDEM). The first milestone is concerned with a demonstration of medium fidelity astrometry accuracy and the second milestone demonstrates high-contrast imaging utilizing the same astrometry-capable optics. We have met milestone #1 with a comfortable margin. The average accuracy obtained over the three datasets is 5.75e-5 L/D, which is 4 times better than the milestone requirement, or equivalent to 2.5microarcsec on 2.4m telescope, or 1.5microarcsec for a 4m telescope, working in the visible band. These results show the potential of this technique to enable detection and measure masses of Earth-like planets around nearby stars, hence bringing a real benefit to the astronomy community. We also met milestone #2 and demonstrated that it is possible to achieve high-contrast imaging utilizing a coronagraph fed by a telescope equipped with a DP, enabling dual use of the telescope. We performed three different high-contrast imaging runs and met the milestone #2 of 5e-7 raw contrast for all of them. On average, we obtained 3.33e-7 raw contrast considering all data sets. This result is 35% better than the milestone #2 requirement. We validated the stability of the high-contrast region by averaging frames and subtracting the average from single frames, which resulted in contrast improvement of approximately one order of magnitude, reaching 2.72e-8 contrast. The main achievement of this work was the medium fidelity demonstration and feasibility validation of performing astrometry and direct imaging using the same instrument, significantly enhancing the expected scientific yield of dedicated exoplanet characterization missions.

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arXiv2018

The Implications of 3D Thermal Structure on 1D Atmospheric Retrieval

Using the atmospheric structure from a 3D global radiation-hydrodynamic simulation of HD 189733b and the open-source BART code, we investigate the difference between the secondary-eclipse temperature structure produced with a 3D simulation and the best-fit 1D retrieved model. Synthetic data are generated by integrating the 3D models over the Spitzer, HST, and JWST bandpasses, covering the wavelength range between 1 and 11 um. Using the data from different observing instruments, we present detailed comparisons between the temperature-pressure profiles recovered by BART and those from the 3D simulations. We calculate several averages of the 3D thermal structure and implement two temperature parameterizations to investigate different thermal profile shapes. To assess which part of the thermal structure is best constrained by the data, we generate contribution functions for both our theoretical model and each of our retrieved models. Our conclusions are strongly affected by the spectral resolution of the instruments included, their wavelength coverage, and the number of data points combined. We also see some limitations in each of the temperature parametrizations. The results show that our 1D retrieval is recovering a temperature and pressure profile that most closely matches the arithmetic average of the 3D thermal structure. When we use a higher resolution, more data points, and a parametrized temperature profile that allows more flexibility in the middle part of the atmosphere, we find a better match between the retrieved temperature and pressure profile and the arithmetic average.

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arXiv2017

PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST

As we approach the James Webb Space Telescope (JWST) era, several studies have emerged that aim to: 1) characterize how the instruments will perform and 2) determine what atmospheric spectral features could theoretically be detected using transmission and emission spectroscopy. To some degree, all these studies have relied on modeling of JWST's theoretical instrument noise. With under two years left until launch, it is imperative that the exoplanet community begins to digest and integrate these studies into their observing plans, as well as think about how to leverage the Hubble Space Telescope (HST) to optimize JWST observations. In order to encourage this and to allow all members of the community access to JWST & HST noise simulations, we present here an open-source Python package and online interface for creating observation simulations of all observatory-supported time-series spectroscopy modes. This noise simulator, called PandExo, relies on some aspects of Space Telescope Science Institute's Exposure Time Calculator, Pandeia. We describe PandExo and the formalism for computing noise sources for JWST. Then, we benchmark PandExo's performance against each instrument team's independently written noise simulator for JWST, and previous observations for HST. We find that \texttt{PandExo} is within 10% agreement for HST/WFC3 and for all JWST instruments.

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arXiv2015

Toward Direct Imaging of Low-Mass Gas-Giants with the $\textit{James Webb Space Telescope}$

In preparation for observations with the $\textit{James Webb Space Telescope}$ ($\textit{JWST}$), we have identified new members of the nearby, young M dwarf sample and compiled an up to date list of these stars. Here we summarize our efforts to identify young M dwarfs, describe the current sample, and detail its demographics in the context of direct planet imaging. We also describe our investigations of the unprecedented sensitivity of the $\textit{JWST}$ when imaging nearby, young M dwarfs. The $\textit{JWST}$ is the only near term facility capable of routinely pushing direct imaging capabilities around M dwarfs to sub-Jovian masses and will provide key insight into questions regarding low-mass gas-giant properties, frequency, formation, and architectures.

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arXiv2014

Near-IR Spectroscopic Monitoring of Class I Protostars: Variability of Accretion and Wind Indicators

We present the results of a program that monitored the near-IR spectroscopic variability of a sample of 19 embedded protostars. Spectra were taken on time intervals from 2 days to 3 years, over a wavelength range from 0.85 μm to 2.45 μm, for 4-9 epochs of observations per target. We found that the spectra of all targets are variable, and that every emission feature observed is also variable (although not for all targets). With one exception, there were no drastic changes in the continua of the spectra, nor did any line completely disappear, nor did any line appear that was not previously apparent. This analysis focuses on understanding the connection between accretion (traced by H Br $γ$ and CO) and the wind (traced by He I, [FeII], and sometimes H$_2$). For both accretion and wind tracers, the median variability was constant versus time interval between observations, however the maximum variability that we observed increased with time interval between observations. Extinction is observed to vary within the minimum sampling time of 2 days, suggesting extinguishing material within a few stellar radii at high disk latitudes. The variability of [FeII] and H$_2$ were correlated for most (but not all) of the 7 YSOs showing both features, and the amplitude of the variability depends on the veiling. Although the occurrence of CO and Br $γ$ emission are connected, their variability is uncorrelated, suggesting that these emissions originate in separate regions near the protostar (e.g., disk and wind).

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arXiv2010

ISM Dust Grains and N-band Spectral Variability in the Spatially Resolved Subarcsecond Binary UY Aur

The 10 micron silicate feature is an essential diagnostic of dust-grain growth and planet formation in young circumstellar disks. The Spitzer Space Telescope has revolutionized the study of this feature, but due to its small (85cm) aperture, it cannot spatially resolve small/medium separation binaries (<3"; <420 AU) at the distances of the nearest star-forming regions (~140 pc). Large, 6-10m ground-based telescopes with mid-infrared instruments can resolve these systems. In this paper, we spatially resolve the 0.88" binary, UY Aur, with MMTAO/BLINC-MIRAC4 mid-infrared spectroscopy. We then compare our spectra to Spitzer/IRS (unresolved) spectroscopy, and resolved images from IRTF/MIRAC2, Keck/OSCIR and Gemini/Michelle, which were taken over the past decade. We find that UY Aur A has extremely pristine, ISM-like grains and that UY Aur B has an unusually shaped silicate feature, which is probably the result of blended emission and absorption from foreground extinction in its disk. We also find evidence for variability in both UY Aur A and UY Aur B by comparing synthetic photometry from our spectra with resolved imaging from previous epochs. The photometric variability of UY Aur A could be an indication that the silicate emission itself is variable, as was recently found in EX Lupi. Otherwise, the thermal continuum is variable, and either the ISM-like dust has never evolved, or it is being replenished, perhaps by UY Aur's circumbinary disk.

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arXiv2009

A Direct Measurement of Atmospheric Dispersion in N-band Spectra: Implications for Mid-IR Systems on ELTs

Adaptive optics will almost completely remove the effects of atmospheric turbulence at 10 microns on the Extremely Large Telescope (ELT) generation of telescopes. In this paper, we observationally confirm that the next most important limitation to image quality is atmospheric dispersion, rather than telescope diffraction. By using the 6.5 meter MMT with its unique mid-IR adaptive optics system, we measure atmospheric dispersion in the N-band with the newly commissioned spectroscopic mode on MIRAC4-BLINC. Our results indicate that atmospheric dispersion is generally linear in the N-band, although there is some residual curvature. We compare our measurements to theory, and make predictions for ELT Strehls and image FHWM with and without an atmospheric dispersion corrector (ADC). We find that for many mid-IR applications, an ADC will be necessary on ELTs.

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arXiv2006

Telescope to Observe Planetary Systems (TOPS): a high throughput 1.2-m visible telescope with a small inner working angle

The Telescope to Observe Planetary Systems (TOPS) is a proposed space mission to image in the visible (0.4-0.9 micron) planetary systems of nearby stars simultaneously in 16 spectral bands (resolution R~20). For the ~10 most favorable stars, it will have the sensitivity to discover 2 R_E rocky planets within habitable zones and characterize their surfaces or atmospheres through spectrophotometry. Many more massive planets and debris discs will be imaged and characterized for the first time. With a 1.2m visible telescope, the proposed mission achieves its power by exploiting the most efficient and robust coronagraphic and wavefront control techniques. The Phase-Induced Amplitude Apodization (PIAA) coronagraph used by TOPS allows planet detection at 2 lambda/d with nearly 100% throughput and preserves the telescope angular resolution. An efficient focal plane wavefront sensing scheme accurately measures wavefront aberrations which are fed back to the telescope active primary mirror. Fine wavefront control is also performed independently in each of 4 spectral channels, resulting in a system that is robust to wavefront chromaticity.

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arXiv2002

Spectroscopy of Brown Dwarf Candidates in the NGC 1333 Molecular Cloud

We present an analysis of low-resolution infrared spectra for 25 brown dwarf candidates in the NGC 1333 molecular cloud. Candidates were chosen on the basis of their association with the high column density cloud core, and near-infrared fluxes and colors. We compare the depths of water vapor absorption bands in our candidate objects with a grid of dwarf, subgiant, and giant standards to determine spectral types which are independent of reddening. These data are used to derive effective temperatures and bolometric luminosities which, when combined with theoretical tracks and isochrones for pre-main sequence objects, enable us to estimate masses and ages. Depending on the models considered, a total of 9 to 20 brown dwarfs are identified with a median of age of < 1 Myr.

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