Generation Space Telescope Research Articles

Contactless active mirror for space telescopes

The SPLATT project is a technological research activity funded by INAF, in the field of primary active mirrors for next generation space telescopes. The goal is to demonstrate that active mirrors with contactless actuators are insensitive to disturbance from the telescope and can therefore be key-elements in the reduction of complexity and cost.
 Keywords: space telescopes, active mirror, active optics, LUVOIR.

Analytical Lightcurves for Transiting Exomoons

Transit photometry has remained the most effective technique to detect and characterize exoplanets. As thousands of exoplanets have been discovered in the last few decades, the possibility of the existence of exomoons in some of these systems can not be avoided. However, the detection of exomoons has still remained elusive, owing to their much smaller expected size compared to their planetary host. With the advent of next generation space telescopes and large ground based telescopes, the possibility to detect them is imminent in near future. In such scenario, a comprehensive analytical formalism to model the transit lightcurves of a transiting exomoon hosting system will be necessary to confirm their detection and study their physical and dynamical characteristics. Here, I present such an analytical formulation, that can be used to simulate the transit lightcurves for an exoplanetary system hosting transiting exomoons. This formalism uses the physical and orbital properties of the three-body star-planet-moon system, to solve their orbital dynamics, and model the transit lightcurves for every possible physical scenarios. In this report, various aspects of this analytical formalism has been discussed, and the model lightcurves for a few of the practical scenarios have been presented.

Commentary: Astronomy from space after the JWST

Commentary: Astronomy from space after the <i>JWST</i>

The next batch of ‘great observatories’

Astronomers have turned their eye towards the future following the US National Academies’ latest decadal survey of astronomy and astrophysics, which recommended a new generation of space telescopes. Keith Cooper explores their prospects, and the lessons learned from the troubled development of the James Webb Space Telescope.

Impact of Mg ii interstellar medium absorption on near-ultraviolet exoplanet transit measurements

ABSTRACT Ultraviolet (UV) transmission spectroscopy probes atmospheric escape, which has a significant impact on planetary atmospheric evolution. If unaccounted for, interstellar medium absorption (ISM) at the position of specific UV lines might bias transit depth measurements, and thus potentially affect the (non-)detection of features in transmission spectra. Ultimately, this is connected to the so called ‘resolution-linked bias’ effect. We present a parametric study quantifying the impact of unresolved or unconsidered ISM absorption in transit depth measurements at the position of the Mg ii h&amp;k resonance lines (i.e. 2802.705 Å and 2795.528 Å, respectively) in the near-ultraviolet spectral range. We consider main-sequence stars of different spectral types and vary the shape and amount of chromospheric emission, ISM absorption, and planetary absorption, as well as their relative velocities. We also evaluate the role played by integration bin and spectral resolution. We present an open-source tool enabling one to quantify the impact of unresolved or unconsidered Mg ii ISM absorption in transit depth measurements. We further apply this tool to a few already or soon to be observed systems. On average, we find that ignoring ISM absorption leads to biases in the Mg ii transit depth measurements comparable to the uncertainties obtained from the observations published to date. However, considering the bias induced by ISM absorption might become necessary when analysing observations obtained with the next generation space telescopes with UV coverage (e.g. LUVOIR, HABEX), which will provide transmission spectra with significantly smaller uncertainties compared to what obtained with current facilities (e.g. HST).

Heterogeneous Physical Chemistry in the Atmospheres of Earth, Mars, and Venus: Perspectives for Rocky Exoplanets

With recent progress of observational astronomy, the next generation of space telescopes (JWST, ARIEL, OST, HabEx, LUVOIR) will be able to determine the composition of exoplanetary atmospheres in the next decades. The discovery of rocky exoplanets, as potentially favorable harbors for life, finds a particularly strong echo in the scientific community and in the general audience. The interaction of the surface of a planetary body with its atmosphere is key to understanding the composition of the latter and hence to determine whether the planet may host life or not. In an effort to describe surface–atmosphere interactions on rocky planets and the heterogeneous reactions occurring on solid surfaces, in particular mineral dust grains, in such systems it is necessary to develop a systematic approach to this family of reactions. Such an approach is proposed in this Perspective on three very distinct examples of rocky planets: Earth, Mars, and Venus. These bodies have experienced very different evolutions, although they likely started from similar initial conditions; the pressure and temperature of their atmospheres cover a broad range of values, offering an invaluable set of planet-size laboratories to study the impact of physical and chemical parameters on the evolution of atmospheres. Systems that should be investigated in priority with relevance for Earth, Mars, and Venus are discussed. It is shown that a better knowledge of mineral dust supporting heterogeneous reactivity is crucial and that the impact of environmental parameters on these reactions needs to be carefully investigated. Current and future missions to Mars and Venus will require such work to better model and understand the observations. In particular, a strong effort is needed to study Venus, which has only few dedicated laboratory setups although it will be the target of exploration missions in the next decades. These studies will pave the way to build robust models of exoplanetary atmospheres, which will be crucial to account for the observations of their composition by space telescopes in the near future.

Optical verification experiments of sub-scale starshades

Starshades are a leading technology to enable the detection and spectroscopic characterization of Earth-like exoplanets. In this paper we report on optical experiments of sub-scale starshades that advance critical starlight suppression technologies in preparation for the next generation of space telescopes. These experiments were conducted at the Princeton starshade testbed, an 80 m long enclosure testing 1/1000th scale starshades at a flight-like Fresnel number. We demonstrate 1e-10 contrast at the starshade's geometric inner working angle across 10% of the visible spectrum, with an average contrast at the inner working angle of 2.0e-10 and contrast floor of 2e-11. In addition to these high contrast demonstrations, we validate diffraction models to better than 35% accuracy through tests of intentionally flawed starshades. Overall, this suite of experiments reveals a deviation from scalar diffraction theory due to light propagating through narrow gaps between the starshade petals. We provide a model that accurately captures this effect at contrast levels below 1e-10. The results of these experiments demonstrate that there are no optical impediments to building a starshade that provides sufficient contrast to detect Earth-like exoplanets. This work also sets an upper limit on the effect of unknowns in the diffraction model used to predict starshade performance and set tolerances on the starshade manufacture.

Wavefront sensing and control in space-based coronagraph instruments using Zernike’s phase-contrast method

Future space telescopes with coronagraph instruments will use a wavefront sensor (WFS) to measure and correct for phase errors and stabilize the stellar intensity in high-contrast images. The HabEx and LUVOIR mission concepts baseline a Zernike wavefront sensor (ZWFS), which uses Zernike's phase contrast method to convert phase in the pupil into intensity at the WFS detector. In preparation for these potential future missions, we experimentally demonstrate a ZWFS in a coronagraph instrument on the Decadal Survey Testbed in the High Contrast Imaging Testbed facility at NASA's Jet Propulsion Laboratory. We validate that the ZWFS can measure low- and mid-spatial frequency aberrations up to the control limit of the deformable mirror, with surface height sensitivity as small as 1 pm, using a configuration similar to the HabEx and LUVOIR concepts. Furthermore, we demonstrate closed-loop control, resolving an individual DM actuator, with residuals consistent with theoretical models. In addition, we predict the expected performance of a ZWFS on future space telescopes using natural starlight from a variety of spectral types. The most challenging scenarios require ~1 hr of integration time to achieve picometer sensitivity. This timescale may be drastically reduced by using internal or external laser sources for sensing purposes. The experimental results and theoretical predictions presented here advance the WFS technology in the context of the next generation of space telescopes with coronagraph instruments.

Phase-curve Pollution of Exoplanet Transit Depths

Abstract The next generation of space telescopes will enable transformative science to understand the nature and origin of exoplanets. In particular, transit spectroscopy will reveal the chemical composition of the exoplanet atmospheres with unprecedented detail thanks to precise measurements of the visible-to-infrared transit depths down to 10 parts per million. Such a level of instrumental precision raises the challenge to obtain even more precise astrophysical models so as not to significantly influence the interpretation of the observed data. We must therefore critically revisit some of the commonly accepted assumptions that were adequate for analyzing past and current observations. A common approximation in the analysis of exoplanetary primary transits is that the planet does not contribute to the recorded flux, so-called dark planet hypothesis. In this paper, we investigate the impact of the dark planet hypothesis on the parameters obtained from the analysis of transits with particular attention to the transit depth. We develop mathematical formulae and release new software to estimate the magnitude of the potential bias. These tools will be useful in the preparation of observing proposals, as well as within the scientific consortia of the James Webb Space Telescope (JWST) and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) missions. We probe the accuracy of the mathematical formulae through the analysis of synthetic observations with the JWST Mid-InfraRed Instrument. We find that self-blending from nightside emission attenuates the transit depth by &gt;3σ for some of the known exoplanet systems, in agreement with previous work. An additional unreported effect caused by the nightside rotating into view can also impart a significant effect, but in the opposite direction (increasing the transit depth); this effect can largely be removed with conventional detrending practices, at the expense of a slight increase in noise, and mixing astrophysical variations and instrumental drifts.

Sensitivity of LEKID for space applications between 80 GHz and 600 GHz

We report the design, fabrication, and testing of lumped element kinetic inductance detectors (LEKID) showing performance in line with the requirements of the next generation space telescopes operating in the spectral range from 80 GHz to 600 GHz. This range is of particular interest for cosmic microwave background studies. For this purpose we designed and fabricated 100 pixel arrays covering five distinct bands. These wafers were measured via multiplexing, in which a full array is read out using a single pair of lines. We adopted a custom cold black body installed in front of the detectors and regulated at temperatures between 1 K and 20 K. In this paper, we describe in the main design considerations, fabrication processes, testing and data analysis.

High-resolution Transmission Spectroscopy of Four Hot Inflated Gas Giant Exoplanets

Abstract The technique of transmission spectroscopy allows us to constrain the chemical composition of the atmospheres of transiting exoplanets. It relies on very high signal-to-noise spectroscopic (or spectrophotometric) observations and is thus most suited for bright exoplanet host stars. In the era of the Transiting Exoplanet Survey Satellite, Next Generation Space Telescope, and PLAnetary Transits and Oscillations of stars (PLATO), more and more suitable targets, even for mid-sized telescopes, are discovered. Furthermore, a wealth of archival data is available that could become a basis for long-term monitoring of exo-atmospheres. We analyzed archival High Accuracy Radial velocity Planet Searcher (HARPS) spectroscopic time series of four host stars to transiting bloated gas exoplanets, namely WASP-76b, WASP-127b, WASP-166b, and KELT-11b, searching for traces of sodium (sodium doublet), hydrogen (Hα, Hβ), and lithium (670.8 nm). The archival data sets include spectroscopic time series taken during transits. Comparing in- and out-of-transit spectra we can filter out the stellar lines and investigate the absorption from the planet. Simultaneously, the stellar activity is monitored using the Mg i and Ca i lines. We detect sodium in the atmosphere of WASP-76b at a 7–9σ level. Furthermore, we report also at a 4–8σ level of significance the detection of sodium in the atmosphere of WASP-127b, confirming earlier results based on low-resolution spectroscopy. The data show no sodium nor any other atom at high confidence levels for WASP-166b nor KELT-11b, hinting at the presence of thick high clouds.

A Thousand Earths: A Very Large Aperture, Ultralight Space Telescope Array for Atmospheric Biosignature Surveys

An outstanding, multi-disciplinary goal of modern science is the study of the diversity of potentially Earth-like planets and the search for life in them. This goal requires a bold new generation of space telescopes, but even the most ambitious designs yet hope to characterize several dozen potentially habitable planets. Such a sample may be too small to truly understand the complexity of exo-earths. We describe here a notional concept for a novel space observatory designed to characterize 1,000 transiting exo-earth candidates. The Nautilus concept is based on an array of inflatable spacecraft carrying very large diameter (8.5m), very low-weight, multi-order diffractive optical elements (MODE lenses) as light-collecting elements. The mirrors typical to current space telescopes are replaced by MODE lenses with a 10 times lighter areal density that are 100 times less sensitive to misalignments, enabling light-weight structure. MODE lenses can be cost-effectively replicated through molding. The Nautilus mission concept has a potential to greatly reduce fabrication and launch costs, and mission risks compared to the current space telescope paradigm through replicated components and identical, light-weight unit telescopes. Nautilus is designed to survey transiting exo-earths for biosignatures up to a distance of 300 pc, enabling a rigorous statistical exploration of the frequency and properties of life-bearing planets and the diversity of exo-earths.

Imaging and Characterization of Extrasolar Planets with the Next Generation of Space Telescopes

The study and characterization of the exoplanets’ atmospheres and composition is in its infancy. The large facilities that will make feasible to image an exo-Earth are currently under study. This contribution to the special issue on “detection and characterization of extrasolar planets” is a summary on the current status of the design studies to build large space-based facilities working in the 100–3000 nm range for this purpose. The three basic designs: Fresnel imagers, starshades, and coronagraphs on large space telescopes are described. An outline of the pros and cons for each design is provided. The relevance of transmission spectroscopy to characterize exoplanets atmospheres is pointed out.

First light – II. Emission line extinction, population III stars, and X-ray binaries

We produce synthetic spectra and observations for metal-free stellar populations and high mass X-ray binaries in the Renaissance Simulations at a redshift of 15. We extend our methodology from the first paper in the series by modelling the production and extinction of emission lines throughout a dusty and metal-enriched interstellar and circum-galactic media extracted from the simulation, using a Monte Carlo calculation. To capture the impact of high-energy photons, we include all frequencies from hard X-ray to far infrared with enough frequency resolution to discern line emission and absorption profiles. The most common lines in our sample in order of their rate of occurrence are Ly$\alpha$, the C IV $\lambda\lambda1548,1551$ doublet, H-$\alpha$, and the Ca II $\lambda\lambda\lambda8498,8542,8662$ triplet. The best scenario for a direct observation of a metal-free stellar population is a merger between two Population III galaxies. In mergers between metal-enriched and metal-free stellar populations, some characteristics may be inferred indirectly. Single Population III galaxies are too dim to be observed photometrically at $z = 15$. Ly$\alpha$ emission is discernible by $JWST$ as an increase in $\rm{J_{200w} - J_{277w}}$ colour off the intrinsic stellar tracks. Observations of metal-free stars will be difficult, though not impossible, with the next generation of space telescopes.

A NEW CONCEPT FOR SPECTROPHOTOMETRY OF EXOPLANETS WITH SPACE-BORNE TELESCOPES

ABSTRACT We propose a new concept for the spectral characterization of transiting exoplanets with future space-based telescopes. This concept, called densified pupil spectroscopy, allows us to perform high, stable spectrophotometry against telescope pointing jitter and deformation of the primary mirror. This densified pupil spectrometer comprises the following three roles: division of a pupil into a number of sub-pupils, densification of each sub-pupil, and acquisition of the spectrum of each sub-pupil with a conventional spectrometer. Focusing on the fact that the divided and densified sub-pupil can be treated as a point source, we discovered that a simplified spectrometer allows us to acquire the spectra of the densified sub-pupils on the detector plane−an optical conjugate with the primary mirror−by putting the divided and densified sub-pupils on the entrance slit of the spectrometer. The acquired multiple spectra are not principally moved on the detector against low-order aberrations such as the telescope pointing jitter and any deformation of the primary mirror. The reliability of the observation result is also increased by statistically treating them. Our numerical calculations show that because this method suppresses the instrumental systematic errors down to 10 ppm under telescopes with modest pointing accuracy, next generation space telescopes with more than 2.5 m diameter potentially provide opportunities to characterize temperate super-Earths around nearby late-type stars through the transmission spectroscopy and secondary eclipse.

CLUSTER CANDIDATES AROUND LOW-POWER RADIO GALAXIES ATz∼ 1-2 IN COSMOS

We search for high redshift ($z\sim$1-2) galaxy clusters using low luminosity radio galaxies (FR~I) as beacons and our newly developed Poisson Probability Method (PPM) based on photometric redshift information and galaxy number counts. We use a sample of 32 FR~Is within the Cosmic Evolution Survey (COSMOS) field from Chiaberge et al. (2009) catalog. We derive a reliable subsample of 21 {\it bona fide} Low Luminosity Radio Galaxies (LLRGs) and a subsample of 11 High Luminosity Radio Galaxies (HLRGs), on the basis of photometric redshift information and NRAO VLA Sky Survey (NVSS) radio fluxes. The LLRGs are selected to have 1.4~GHz rest frame luminosities lower than the fiducial FR~I/FR~II divide. This also allows us to estimate the comoving space density of sources with $L_{1.4}\simeq 10^{32.3}\,\hbox{erg}\,\hbox{s}^{-1}\,\hbox{Hz}^{-1}$ at $z\simeq 1.1$, which strengthens the case for a strong cosmological evolution of these sources. In the fields of the LLRGs and HLRGs we find evidence that 14 and 8 of them reside in rich groups or galaxy clusters, respectively. Thus, overdensities are found around $\sim70\%$ of the FR~Is, independently of the considered subsample. This rate is in agreement with the fraction found for low redshift FR~Is and it is significantly higher than that of FR~IIs at all redshifts. Although our method is primarily introduced for the COSMOS survey, it may be applied to both present and future wide field surveys such as SDSS Stripe 82, LSST, and Euclid. Furthermore, cluster candidates found with our method are excellent targets for next generation space telescopes such as JWST.

PHOTOCHEMISTRY IN TERRESTRIAL EXOPLANET ATMOSPHERES. II. H2S AND SO2PHOTOCHEMISTRY IN ANOXIC ATMOSPHERES

Sulfur gases are common components in the volcanic and biological emission on Earth, and are expected to be important input gases for atmospheres on terrestrial exoplanets. We study the atmospheric composition and the spectra of terrestrial exoplanets with sulfur compounds (i.e., H2S and SO2) emitted from their surfaces. We use a comprehensive one-dimensional photochemistry model and radiative transfer model to investigate the sulfur chemistry in atmospheres ranging from reducing to oxidizing. The most important finding is that both H2S and SO2 are chemically short-lived in virtually all types of atmospheres on terrestrial exoplanets, based on models of H2, N2, and CO2 atmospheres. This implies that direct detection of surface sulfur emission is unlikely, as their surface emission rates need to be extremely high (>1000 times Earth's volcanic sulfur emission) for these gases to build up to a detectable level. We also find that sulfur compounds emitted from the surface lead to photochemical formation of elemental sulfur and sulfuric acid in the atmosphere, which would condense to form aerosols if saturated. For terrestrial exoplanets in the habitable zone of Sun-like stars or M stars, Earth-like sulfur emission rates result in optically thick haze composed of elemental sulfur in reducing H2-dominated atmospheres for a wide range of particle diameters (0.1-1 micron), which is assumed as a free parameter in our simulations. In oxidized atmospheres composed of N2 and CO2, optically thick haze, composed of elemental sulfur aerosols (S8) or sulfuric acid aerosols (H2SO4), will form if the surface sulfur emission is 2 orders of magnitude more than the volcanic sulfur emission of Earth. Although direct detection of H2S and SO2 by their spectral features is unlikely, their emission might be inferred by observing aerosol-related features in reflected light with future generation space telescopes.

Gamma-ray optics for high-energy astrophysics

Gamma-ray astronomy presents an extraordinary scientific potential for the study of the most powerful sources and the most violent events in the Universe. Those extreme conditions occur generally at the endpoints of stellar lives, when the comparatively calm thermal evolution gives way to more violent non-thermal processes.Present telescopes in nuclear astrophysics make use of inelastic interaction processes based on geometrical optics or quantum optics, i.e. shadowcasting in modulating aperture systems, and particle tracking detectors respectively. After reviewing the above instrument concepts, we focus on recent developments in crystal diffraction optics. For the first time in gamma-ray astronomy, this type of optics permits to concentrate photons from a large collector onto a small detector, dramatically improving the sensitivity of next generation space telescopes.

Demonstration of high optical sensitivity in far-infrared hot-electron bolometer

We report on the measurement of a very low noise equivalent power of 3×10−19 W/Hz1/2 at 620 GHz in a superconducting antenna-coupled hot-electron bolometer. The sensing element is a micron-size titanium transition-edge sensor with NbTiN superconducting contacts fabricated on a sapphire substrate. The high sensitivity is due to the small device volume, low operating temperature, and weak electron-phonon coupling in titanium film. Measurements were done using a cryogenic black body emitter producing well-controlled femtowatt power levels. The achieved optical sensitivity is suitable for the low-background spectroscopy of molecular lines on next generation space telescopes.

Ultra-low-noise MoCu transition edge sensors for space applications

A study of ultra-low-noise MoCu transition edge sensors (TESs) has been performed in the context of realizing the highly sensitive far infrared imaging arrays needed for the next generation of space telescopes. More than 50 TESs, on four different chips, cut out of two different wafers were characterized. The TESs were in the form of 16-element arrays and were read out using superconducting quantum interference device (SQUID) time division multiplexing. The devices were fabricated on 200-nm-thick silicon nitride membranes, with leg widths and lengths covering the ranges of 1–4 μm and 160–960 μm, respectively. The apparent critical temperatures varied over 110–127 mK, but it is shown that much of the variation was due to differential loading by stray light, amounting to 2 ± 2 fW across the array. The measured thermal conductances to the heat bath spanned the range 0.12–1.1 pW/K, with the lowest values being typical of those needed for ultra-low-noise operation. We also studied the inherent variation in the conductances of 15 nominally identical TESs on the same chip and found a value of ±10%, which is higher than that seen on our high-conductance devices designed for ground-based operation. We measured and modeled the electrical input impedance of a subset of these TESs, and studied their step responses. The models, based on previously determined material parameters, are in excellent agreement with the measurements. Dark noise spectra were recorded and compared with the same electrothermal models using the same parameters as the dynamical simulations. The measured noise is reasonably well described by the sum of the contributions from phonon noise in the legs, Johnson noise in the bilayer, and SQUID readout noise. Dark noise equivalent powers as low as 4.2 × 10−19 W/Hz were measured. The NEP was higher than the theoretical limit by a factor of about 1.6.