ABSTRACT The yield of Earth-like planets will likely be a primary science metric for future space-based missions that will drive telescope aperture size. Maximizing the exoEarth candidate yield is ...therefore critical to minimizing the required aperture. Here we describe a method for exoEarth candidate yield maximization that simultaneously optimizes, for the first time, the targets chosen for observation, the number of visits to each target, the delay time between visits, and the exposure time of every observation. This code calculates both the detection time and multi-wavelength spectral characterization time required for planets. We also refine the astrophysical assumptions used as inputs to these calculations, relying on published estimates of planetary occurrence rates as well as theoretical and observational constraints on terrestrial planet sizes and classical habitable zones. Given these astrophysical assumptions, optimistic telescope and instrument assumptions, and our new completeness code that produces the highest yields to date, we suggest lower limits on the aperture size required to detect and characterize a statistically motivated sample of exoEarths.
We describe how environmental context can help determine whether oxygen (O
) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in-depth, ...interdisciplinary example of O
biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O
is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life. However, as a biosignature, O
faces two major challenges: (1) it was only present at high abundance for a relatively short period of Earth's history and (2) we now know of several potential planetary mechanisms that can generate abundant O
without life being present. Consequently, our ability to interpret both the presence and absence of O
in an exoplanetary spectrum relies on understanding the environmental context. Here we examine the coevolution of life with the early Earth's environment to identify how the interplay of sources and sinks may have suppressed O
release into the atmosphere for several billion years, producing a false negative for biologically generated O
. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. We review the most recent knowledge of false positives for O
, planetary processes that may generate abundant atmospheric O
without a biosphere. We provide examples of how future photometric, spectroscopic, and time-dependent observations of O
and other aspects of the planetary environment can be used to rule out false positives and thereby increase our confidence that any observed O
is indeed a biosignature. These insights will guide and inform the development of future exoplanet characterization missions. Key Words: Biosignatures-Oxygenic photosynthesis-Exoplanets-Planetary atmospheres. Astrobiology 18, 630-662.
Abstract
Scattered light imaging of protoplanetary disks provides key insights on the geometry and dust properties in the disk surface. Here, we present James Webb Space Telescope (JWST) 2–21
μ
m ...images of a 1000 au radius edge-on protoplanetary disk surrounding an 0.4
M
⊙
young star in Taurus, Two Micron All Sky Survey (2MASS) J04202144 + 2813491. These observations represent the longest wavelengths at which a protoplanetary disk is spatially resolved in scattered light. We combine these observations with Hubble Space Telescope optical images and Atacama Large Millimeter/submillimeter Array continuum and CO mapping. We find that the changes in the scattered light disk morphology are remarkably small across a factor of 30 in wavelength, indicating that dust in the disk surface layers is characterized by an almost gray opacity law. Using radiative transfer models, we conclude that grains up to ≳10
μ
m in size are fully coupled to the gas in this system, whereas grains ≳100
μ
m are strongly settled toward the midplane. Further analyses of these observations, and similar ones of other edge-on disks, will provide strong empirical constraints on disk dynamics and evolution and grain growth models. In addition, the 7.7 and 12.
μ
m JWST images reveal an X-shaped feature located above the warm molecular layer traced by CO line emission. The highest elevations at which this feature is detectable roughly match the maximal extent of the disk in visible wavelength scattered light as well as of an unusual kinematic signature in CO. We propose that these phenomena could be related to a disk wind entraining small dust grains.
Abstract
We present James Webb Space Telescope imaging from 2 to 21
μ
m of the edge-on protoplanetary disk around the embedded young star IRAS04302+2247. The structure of the source shows two ...reflection nebulae separated by a dark lane. The source extent is dominated by the extended filamentary envelope at ∼4.4
μ
m and shorter wavelengths, transitioning at 7.7
μ
m and longer wavelengths to more compact lobes of scattered light from the disk itself. The dark lane thickness does not vary significantly with wavelength, which we interpret as an indication for intermediate-sized (∼10
μ
m) grains in the upper layers of the disk. Intriguingly, we find that the brightest nebula of IRAS40302 switches side between 12.8 and 21
μ
m. We explore the effect of a tilted inner region on the general appearance of edge-on disks. We find that radiative transfer models of a disk including a tilted inner region can reproduce an inversion in the brightest nebula. In addition, for specific orientations, the model predicts strong lateral asymmetries, which can occur for more than half possible viewing azimuths. A large number of edge-on protoplanetary disks observed in scattered light show such lateral asymmetries (15/20), which suggests that a large fraction of protoplanetary disks might host a tilted inner region. Stellar spots may also induce lateral asymmetries, which are expected to vary over a significantly shorter timescale. Variability studies of edge-on disks would allow us to test the dominant scenario for the origin of these asymmetries.
We present here new observations of the eccentric debris ring surrounding the Gyr-old solar-type star HD 202628: at millimeter wavelengths with ALMA, at far-infrared wavelengths with Herschel, and in ...scattered light with the Hubble Space Telescope (HST). The ring inner edge is found to be consistent between ALMA and HST data. As radiation pressure affects small grains seen in scattered-light, the ring appears broader at optical than at millimeter wavelengths. The best fit to the ring seen with ALMA has inner and outer edges at 143.1 1.7 au and 165.5 1.4, respectively, and an inclination of 57 4 0.4 from face-on. The offset of the ring center of symmetry from the star allows us to quantify its eccentricity to be . This eccentric feature is also detected in low resolution Herschel/PACS observations, under the form of a pericenter-glow. Combining the infrared and millimeter photometry, we retrieve a disk grain size distribution index of ∼−3.4, and therefore exclude in situ formation of the inferred belt-shaping perturber, for which we provide new dynamical constraints. Finally, ALMA images show four point-like sources that exceed 100 Jy, one of them being just interior to the ring. Although the presence of a background object cannot be excluded, we cannot exclude either that this source is circumplanetary material surrounding the belt-shaper, in which case degeneracies between its mass and orbital parameters could be lifted, allowing us to fully characterize such a distant planet in this mass and age regime for the very first time.
Abstract
The exoplanetary system of HR 8799 is one of the rare systems in which multiple planets have been directly imaged. Its architecture is strikingly similar to that of the solar system, with ...the four imaged giant planets surrounding a warm dust belt analogous to the Asteroid Belt, and themselves being surrounded by a cold dust belt analog to the Kuiper Belt. Previous observations of this cold belt with ALMA in Band 6 (1.3 mm) revealed its inner edge, but analyses of the data differ on its precise location. It was therefore unclear whether or not the outermost planet HR 8799 b was dynamically sculpting it. We present here new ALMA observations of this debris disk in Band 7 (340 GHz, 880
μ
m). These are the most detailed observations of this disk obtained so far, with a resolution of 1″ (40 au) and sensitivity of 9.8
μ
Jy beam
−1
, which allowed us to recover the disk structure with high confidence. In order to constrain the disk morphology, we fit its emission using radiative transfer models combined with a Markov Chain Monte Carlo procedure. We find that this disk cannot be adequately represented by a single power law with sharp edges. It exhibits a smoothly rising inner edge and smoothly falling outer edge, with a peak in between, as expected from a disk that contains a high-eccentricity component, hence confirming previous findings. Whether this excited population and inner edge shape stem from the presence of an additional planet remains, however, an open question.
Abstract
We describe a joint high-contrast imaging survey for planets at the Keck and Very Large Telescope of the last large sample of debris disks identified by the
Spitzer Space Telescope
. No new ...substellar companions were discovered in our survey of 30
Spitzer
-selected targets. We combine our observations with data from four published surveys to place constraints on the frequency of planets around 130 debris disk single stars, the largest sample to date. For a control sample, we assembled contrast curves from several published surveys targeting 277 stars that do not show infrared excesses. We assumed a double power-law distribution in mass and semimajor axis (SMA) of the form
, where we adopted power-law values and logarithmically flat values for the mass and SMA of planets. We find that the frequency of giant planets with masses 5–20
M
Jup
and separations 10–1000 au around stars with debris disks is 6.27% (68% confidence interval 3.68%–9.76%), compared to 0.73% (68% confidence interval 0.20%–1.80%) for the control sample of stars without disks. These distributions differ at the 88% confidence level, tentatively suggesting distinctness of these samples.
A ring-shaped debris disk around the G2V star HD 202628 (d = 24.4 pc) was imaged in scattered light at visible wavelengths using the coronagraphic mode of the Space Telescope Imaging Spectrograph on ...the Hubble Space Telescope. The ring is inclined by ~64degrees from face-on, based on the apparent major/minor axis ratio, with the major axis aligned along P.A. = 130degrees. It has inner and outer radii (> 50% maximum surface brightness) of 139 AU and 193 AU in the northwest ansae and 161 AU and 223 AU in the southeast ( Delta r/r approximate 0.4). The maximum visible radial extent is ~254 AU. With mean surface brightness of V approximate 24 mag arcsec super(-2), this is the faintest debris disk observed to date in reflected light. The center of the ring appears offset from the star by ~28 AU (deprojected). An ellipse fit to the inner edge has an eccentricity of 0.18 and a = 158 AU. This offset, along with the relatively sharp inner edge of the ring, suggests the influence of a planetary-mass companion. There is a strong similarity with the debris ring around Fomalhaut, though HD 202628 is a more mature star with an estimated age of about 2 Gyr. We also provide surface brightness limits for nine other stars in our study with strong Spitzer excesses around which no debris disks were detected in scattered light (HD 377, HD 7590, HD 38858, HD 45184, HD 73350, HD 135599, HD 145229, HD 187897, and HD 201219).
Proplyds in the flame nebula NGC 2024 Haworth, Thomas J; Kim, Jinyoung S; Winter, Andrew J ...
Monthly notices of the Royal Astronomical Society,
03/2021, Letnik:
501, Številka:
3
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
A recent survey of the inner 0.35 × 0.35 pc of the NGC 2024 star-forming region revealed two distinct millimetre continuum disc populations that appear to be spatially segregated by the ...boundary of a dense cloud. The eastern (and more embedded) population is ∼0.2–0.5 Myr old, with an ALMA mm continuum disc detection rate of about $45\,$ per cent. However, this drops to only $\sim 15\,$ per cent in the 1-Myr western population. When these distinct populations were presented, it was suggested that the two main UV sources, IRS 1 (a B0.5V star in the western region) and IRS 2b (an O8V star in the eastern region, but embedded) have both been evaporating the discs in the depleted western population. In this paper, we report the firm discovery in archival HST data of four proplyds and four further candidate proplyds in NGC 2024, confirming that external photoevaporation of discs is occurring. However, the locations of these proplyds changes the picture. Only three of them are in the depleted western population and their evaporation is dominated by IRS 1, with no obvious impact from IRS 2b. The other five proplyds are in the younger eastern region and being evaporated by IRS 2b. We propose that both populations are subject to significant external photoevaporation, which happens throughout the region wherever discs are not sufficiently shielded by the interstellar medium. The external photoevaporation and severe depletion of mm grains in the 0.2–0.5 Myr eastern part of NGC 2024 would be in competition even with very early planet formation.
PDF
