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Register a new userhtof: A new open-source tool for analyzing Hipparcos, Gaia, and future astrometric missions
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We present htof, an open-source tool for interpreting and fitting the intermediate astrometric data (IAD) from both the 1997 and 2007 reductions of Hipparcos, the scanning-law of Gaia, and future missions such as the Nancy Grace Roman Space Telescope (NGRST). htof solves for the astrometric parameters of any system for any arbitrary combination of absolute astrometric missions. In preparation for later Gaia data releases, htof supports arbitrarily high-order astrometric solutions (e.g. five-, seven-, nine-parameter fits). Using htof, we find that the IAD of 6617 sources in Hipparcos 2007 might have been affected by a data corruption issue. htof integrates an ad-hoc correction that reconciles the IAD of these sources with their published catalog solutions. We developed htof to study masses and orbital parameters of sub-stellar companions, and we outline its implementation in one orbit fitting code (orvara, https://github.com/t-brandt/orvara). We use htof to predict a range of hypothetical additional planets in the $beta$~Pic system, which could be detected by coupling NGRST astrometry with Gaia and Hipparcos. htof is pip installable and available at https://github.com/gmbrandt/htof .
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We present the Exoplanet Simple Orbit Fitting Toolbox (ExoSOFT), a new, open-source suite to fit the orbital elements of planetary or stellar mass companions to any combination of radial velocity and astrometric data. To explore the parameter space of Keplerian models, ExoSOFT may be operated with its own multi-stage sampling approach, or interfaced with third-party tools such as emcee. In addition, ExoSOFT is packaged with a collection of post-processing tools to analyze and summarize the results. Although only a few systems have been observed with both the radial velocity and direct imaging techniques, this number will increase thanks to upcoming spacecraft and ground based surveys. Providing both forms of data enables simultaneous fitting that can help break degeneracies in the orbital elements that arise when only one data type is available. The dynamical mass estimates this approach can produce are important when investigating the formation mechanisms and subsequent evolution of substellar companions. ExoSOFT was verified through fitting to artificial data and was implemented using the Python and Cython programming languages; available for public download at https://github.com/kylemede/ExoSOFT under the GNU General Public License v3.
This paper presents a cross-calibrated catalog of Hipparcos and Gaia astrometry to enable their use in measuring changes in proper motion, i.e., accelerations in the plane of the sky. The final catalog adopts the reference frame of the second Gaia data release (DR2) and locally cross-calibrates both the scaled Hipparcos-Gaia DR2 positional differences and the Hipparcos proper motions themselves to this frame. This gives three nearly independent proper motion measurements per star, with the scaled positional difference usually being the most precise. We find that a linear combination of the two Hipparcos reductions is superior to either reduction on its own, and address error inflation for both Hipparcos and Gaia DR2. Our adopted error inflation is additive (in quadrature) for Hipparcos and multiplicative for Gaia. We provide the covariance matrices along with the central epochs of all measurements. Our final proper motion differences are accurately Gaussian with the appropriate variances, and are suitable for acceleration measurements and orbit fitting. The catalog is constructed with an eye toward completeness; it contains nearly 98% of the Hipparcos stars. It also includes a handful of spurious entries and a few stars with poor Hipparcos reductions that the user must vet by hand. Statistical distributions of accelerations derived from this catalog should be interpreted with caution.
We present a framework for the analysis of direct detection planet finding missions using space telescopes. This framework generates simulations of complete missions, with varying populations of planets, to produce ensembles of mission simulations, which are used to calculate distributions of mission science yields. We describe the components of a mission simulation, including the complete description of an arbitrary planetary system, the description of a planet finding instrument, and the modeling of a target system observation. These components are coupled with a decision modeling algorithm, which allows us to automatically generate mission timelines with simple mission rules that lead to an optimized science yield. Along with the details of our implementation of this algorithm, we discuss validation techniques and possible future refinements. We apply this analysis technique to four mission concepts whose common element is a 4m diameter telescope aperture: an internal pupil mapping coronagraph with two different inner working angles, an external occulter, and the THEIA XPC multiple distance occulter. The focus of this study is to determine the ability of each of these designs to achieve one of their most difficult mission goals - the detection and characterization of Earth-like planets in the habitable zone. We find that all four designs are capable of detecting on the order of 5 Earth-like planets within a 5 year mission, even if we assume that only 1 out of every 10 stars has such a planet. The designs do differ significantly in their ability to characterize the planets they find. Along with science yield, we also analyze fuel usage for the two occulter designs, and discuss the strengths and weaknesses of each of the mission concepts.
We present a cross-calibration of Hipparcos and Gaia EDR3 intended to identify astrometrically accelerating stars and to fit orbits to stars with faint, massive companions. The resulting catalog, the EDR3 edition of the Hipparcos-Gaia Catalog of Accelerations (HGCA), provides three proper motions with calibrated uncertainties on the EDR3 reference frame: the Hipparcos proper motion, the Gaia EDR3 proper motion, and the long-term proper motion given by the difference in position between Hipparcos and Gaia EDR3. Our approach is similar to that for the Gaia DR2 edition of the HGCA, but offers a factor of ~3 improvement in precision thanks to the longer time baseline and improved data processing of Gaia EDR3. We again find that a 60/40 mixture of the two Hipparcos reductions outperforms either reduction individually, and we find strong evidence for locally variable frame rotations between all pairs of proper motion measurements. The substantial global frame rotation seen in DR2 proper motions has been removed in EDR3. We also correct for color- and magnitude-dependent frame rotations at a level of up to ~50 $mu$as/yr in Gaia EDR3. We calibrate the Gaia EDR3 uncertainties using a sample of radial velocity standard stars without binary companions; we find an error inflation factor (a ratio of total to formal uncertainty) of 1.37. This is substantially lower than the position dependent factor of ~1.7 found for Gaia DR2 and reflects the improved data processing in EDR3. While the catalog should be used with caution, its proper motion residuals provide a powerful tool to measure the masses and orbits of faint, massive companions to nearby stars.
Instrumentation techniques in the field of direct imaging of exoplanets have greatly advanced over the last two decades. Two of the four NASA-commissioned large concept studies involve a high-contrast instrument for the imaging and spectral characterization of exo-Earths from space: LUVOIR and HabEx. This whitepaper describes the status of 8 optical testbeds in the US and France currently in operation to experimentally validate the necessary technologies to image exo-Earths from space. They explore two complementary axes of research: (i) coronagraph designs and manufacturing and (ii) active wavefront correction methods and technologies. Several instrument architectures are currently being analyzed in parallel to provide more degrees of freedom for designing the future coronagraphic instruments. The necessary level of performance has already been demonstrated in-laboratory for clear off-axis telescopes (HabEx-like) and important efforts are currently in development to reproduce this accomplishment on segmented and/or on-axis telescopes (LUVOIR-like) over the next two years.
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