Stellar Astrometry Research Articles

Direct-imaging Discovery of a Substellar Companion Orbiting the Accelerating Variable Star HIP 39017

We present the direct-imaging discovery of a substellar companion (a massive planet or low-mass brown dwarf) to the young, γ Doradus (γ Dor)-type variable star HIP 39017 (HD 65526). The companion’s SCExAO/CHARIS JHK (1.1–2.4 μm) spectrum and Keck/NIRC2 L′ photometry indicate that it is an L/T transition object. A comparison of the JHK+L ′ spectrum to several atmospheric model grids finds a significantly better fit to cloudy models than cloudless models. Orbit modeling with relative astrometry and precision stellar astrometry from Hipparcos and Gaia yields a semimajor axis of 23.8−6.1+8.7 au, a dynamical companion mass of 30−12+31 M J, and a mass ratio of ∼1.9%, properties most consistent with low-mass brown dwarfs. However, its mass estimated from luminosity models is a lower ∼13.8 M J due to an estimated young age (≲115 Myr); using a weighted posterior distribution informed by conservative mass constraints from luminosity evolutionary models yields a lower dynamical mass of 23.6−7.4+9.1 M J and a mass ratio of ∼1.4%. Analysis of the host star’s multifrequency γ Dor-type pulsations, astrometric monitoring of HIP 39017 b, and Gaia Data Release 4 astrometry of the star will clarify the system age and better constrain the mass and orbit of the companion. This discovery further reinforces the improved efficiency of targeted direct-imaging campaigns informed by long-baseline, precision stellar astrometry.

Near-Earth Object Observations using Synthetic Tracking

Synthetic tracking (ST) has emerged as a potent technique for observing fast-moving near-Earth objects (NEOs), offering enhanced detection sensitivity and astrometric accuracy by avoiding trailing loss. This approach also empowers small telescopes to use prolonged integration times to achieve high sensitivity for NEO surveys and follow-up observations. In this study, we present the outcomes of ST observations conducted with Pomona College’s 1 m telescope at the Table Mountain Facility and JPL’s robotic telescopes at the Sierra Remote Observatory. The results showcase astrometric accuracy statistics comparable to stellar astrometry, irrespective of an object’s rate of motion, and the capability to detect faint asteroids beyond 20.5th magnitude using 11 inch telescopes. Furthermore, we detail the technical aspects of data processing, including the correction of differential chromatic refraction in the atmosphere and accurate timing for image stacking, which contribute to achieving precise astrometry. We also provide compelling examples that showcase the robustness of ST even when asteroids closely approach stars or bright satellites cause disturbances. Moreover, we illustrate the proficiency of ST in recovering NEO candidates with highly uncertain ephemerides. As a glimpse of the potential of NEO surveys utilizing small robotic telescopes with ST, we present significant statistics from our NEO survey conducted for testing purposes. These findings underscore the promise and effectiveness of ST as a powerful tool for observing fast-moving NEOs, offering valuable insights into their trajectories and characteristics. Overall, the adoption of ST stands to revolutionize fast-moving NEO observations for planetary defense and studying these celestial bodies.

Didactic Intervention for the Teaching of Stellar Astrometry in Field Educational Contexts

Didactic Intervention for the Teaching of Stellar Astrometry in Field Educational Contexts

AIROPA IV: Validating point spread function reconstruction on various science cases

We present an analysis of six independent on-sky datasets taken with the Keck-II/NIRC2 instrument. Using the off-axis point spread function (PSF) reconstruction software AIROPA, we extract stellar astrometry, photometry, and other fitting metrics to characterize the performance of this package. We test the effectiveness of AIROPA to reconstruct the PSF across the field of view in varying atmospheric conditions, number and location of PSF reference stars, stellar crowding, and telescope position angle (PA). We compare the astrometric precision and fitting residuals between a static PSF model and a spatially varying PSF model that incorporates instrumental aberrations and atmospheric turbulence during exposures. Most of the fitting residuals we measure show little to no improvement in the variable-PSF mode over the single-PSF mode. For one of the data sets, we find photometric performance is significantly improved (by ∼10 × ) by measuring the trend seen in photometry as a function of off-axis location. For nearly all other metrics we find comparable astrometric and photometric precision across both PSF modes, with a ∼13 % smaller astrometric uncertainty in variable-PSF mode in the best case. We largely confirm that the spatially variable PSF does not significantly improve the astrometric and other PSF fitting residuals over the static PSF for on-sky observations. We attribute this to unaccounted instrumental aberrations that are not characterized through afternoon adaptive optics (AO) bench calibrations.

A survey for occultation astrometry of main belt: expected astrometric performances

Context.Occultations of stars by asteroids are an efficient method to study the properties of minor bodies, and can be exploited as tools to derive very precise asteroid astrometry relative to the target star. With the availability of stellar astrometry thanks to the ESA missionGaia, the frequency of good predictions and the quality of the astrometry have been strongly enhanced.Aims.Our goal is to evaluate the astrometric performance of a systematic exploitation of stellar occultations, with a homogeneous data set and a given instrument setup. As a reference instrument, we adopt the example of a robotic 50 cm telescope, which is under construction at the Observatoire de la Côte d’Azur. We focus in particular on single-chord occultations.Methods.We created a data set of simulated light curves, that are modelled by a Bayesian approach. To build the final statistics, we considered a list of predicted events over a long time span, and stellar astrometry fromGaiadata release 2.Results.We derive an acceptable range of observability of the events, with clear indications of the expected errors in terms of timing uncertainties. By converting the distribution of such errors to astrometric uncertainties, we show that the precision on a single chord can reach levels equivalent to the performance ofGaia(sub-milli-arcseconds). The errors on the asteroid position are dominated by the uncertainty on the position of the occultation chord with respect to the barycentre of the object.Conclusions.The limiting factor in the use of occultation astrometry is not the light curve uncertainty, but our knowledge of the asteroid's shape and size. This conclusion is valid in a wide range of flux drops and magnitudes of the occulted star. The currently increasing knowledge of the shape, spin properties, and size, must be used to mitigate this source of error.

Radial Velocity Discovery of an Eccentric Jovian World Orbiting at 18 au

Abstract Based on two decades of radial velocity (RV) observations using Keck/High Resolution Echelle Spectrometer (HIRES) and McDonald/Tull, and more recent observations using the Automated Planet Finder, we found that the nearby star HR 5183 (HD 120066) hosts a 3 minimum mass planet with an orbital period of yr. The orbit is highly eccentric (e ≃ 0.84), shuttling the planet from within the orbit of Jupiter to beyond the orbit of Neptune. Our careful survey design enabled high cadence observations before, during, and after the planet’s periastron passage, yielding precise orbital parameter constraints. We searched for stellar or planetary companions that could have excited the planet’s eccentricity, but found no candidates, potentially implying that the perturber was ejected from the system. We did identify a bound stellar companion more than 15,000 au from the primary, but reasoned that it is currently too widely separated to have an appreciable effect on HR 5183 b. Because HR 5183 b’s wide orbit takes it more than 30 au (1″) from its star, we also explored the potential of complimentary studies with direct imaging or stellar astrometry. We found that a Gaia detection is very likely, and that imaging at 10 μm is a promising avenue. This discovery highlights the value of long-baseline RV surveys for discovering and characterizing long-period, eccentric Jovian planets. This population may offer important insights into the dynamical evolution of planetary systems containing multiple massive planets.

Prospects for Backtracing 1I/‘Oumuamua and Future Interstellar Objects

Abstract 1I/‘Oumuamua is the first of likely many small bodies of extrasolar origin to be found in the solar system. These interstellar objects (ISOs) are hypothesized to have formed in extrasolar planetary systems prior to being ejected into interstellar space and subsequently arriving at the solar system. This paper discusses necessary considerations for tracing ISOs back to their parent stars via trajectory analysis and places approximate limits on doing so. Results indicate that the capability to backtrace ISOs beyond the immediate solar neighborhood is presently constrained by the quality of stellar astrometry, a factor poised for significant improvement with upcoming Gaia data releases. Nonetheless, prospects for linking 1I or any other ISO to their respective parent stars appear unfavorable on an individual basis due to gravitational scattering from random stellar encounters, which limit traceability to the past few tens of millions of years. These results, however, do not preclude the possibility of occasional success, particularly after considering the potential for observational bias favoring the discovery of younger ISOs, together with the anticipated rise in the ISO discovery rate under forthcoming surveys.

A star passing close to the solar system.

Stellar Astrometry![Figure][1] Many new comets could be produced by the passage of the small star Gliese 710 through the Oort Cloud. PHOTO: WILLIAM ATTARD MCCARTHY/ALAMY STOCK PHOTO The Gaia astrometry satellite has greatly improved measurements of the position and motion of nearby stars. Berski and Dybczynski searched the Gaia data for the star that will pass closest to the Sun. They found that Gliese 710, a small faint star, will pass within 13,000 ± 6,000 astronomical units (one astronomical unit is roughly the distance from Earth to the Sun). That will be well within the Oort Cloud, a collection of small bodies on the outskirts of our solar system. Some of those bodies will be knocked inward, generating large numbers of new comets. Earth is safe for now, though; the encounter will happen in 1.3 million years. Astron. Astrophys. 595 , L10 (2016). [1]: pending:yes

Testing the validity of the ray-tracing code GYOTO

In the next few years, the near-infrared interferometer GRAVITY will be able to observe the Galactic center. Astrometric data will be obtained with an anticipated accuracy of 10 $\mu$as. To analyze these future data, we have developed a code called GYOTO to compute orbits and images. We want to assess the validity and accuracy of GYOTO in a variety of contexts, in particular for stellar astrometry in the Galactic center. Furthermore, we want to tackle and complete a study made on the astrometric displacements that are due to lensing effects of a star of the central parsec with GYOTO. We first validate GYOTO in the weak-deflection limit (WDL) by studying primary caustics and primary critical curves obtained for a Kerr black hole. We compare GYOTO results to available analytical approximations and estimate GYOTO errors using an intrinsic estimator. In the strong-deflection limit (SDL), we choose to compare null geodesics computed by GYOTO and the ray-tracing code named Geokerr. Finally, we use GYOTO to estimate the apparent displacements of a star for different angles from Sagittarius A* (Sgr A*). We have demonstrated that GYOTO is accurate to a very high level, orders of magnitude better than the GRAVITY requirements. GYOTO is also valid in weak- and strong-deflection regimes and for very long integrations. At the astrometric precision that GRAVITY is aiming for, lensing effects must always be taken into account when fitting stellar orbits in the central parsec of the Galaxy.

The Point Spread Function Variations inside Wide-field Astonomical Images

The Point Spread Function (PSF) of the astronomical imaging system is usually approximated by a Gaussian or Moffat function. For simplification, the astronomical imaging system is considered to be time and space invariant. This means that invariable PSF within an exposed image is assumed. If real wide-field imaging systems are considered, this presumption is not fulfilled. In real systems, stronger optical aberrations are expected (especially coma) at greater distances from the center of the captured image. This impacts the efficiency of stellar astrometry and photometry algorithms, so it is necessary to know the PSF variation. In this paper, we perform the first step toward assigning PSF changes: we study the dependence of the Moffat function fitting parameters (FWHM and the atmospheric scattering coefficient ) on the position of a stellar object.

Astrometric Detection of Terrestrial Planets in the Habitable Zones of Nearby Stars with SIM PlanetQuest

ABSTRACTSIM PlanetQuest (formerly the Space Interferometry Mission) is a space‐borne Michelson interferometer for precision stellar astrometry, with a 9 m baseline, currently slated for launch in 2016. One of the principal science goals is the astrometric detection and orbital characterization of terrestrial planets in the habitable zones of nearby stars. Differential astrometry of the target star against a set of reference stars lying within 1° will allow measurement of the target star’s reflex motion with astrometric accuracy of 1 μas in a single measurement. The purpose of the present paper is to quantitatively assess SIM’s capability for detection (as opposed to characterization by orbital determination) of terrestrial planets in the habitable zones of nearby stars. Note that the orbital periods of these planets are generally shorter than the 5 year SIM mission. We formulate a “joint periodogram” as a tool for planet detection from astrometric data. For adequately sampled orbits (i.e., five or more observations per period over a sampling time span longer than the orbital period), we find that the joint periodogram is more sensitive than the χ2 test for the null hypothesis. In our analysis of the problem, we use Monte Carlo simulations of orbit detection, together with realistic observing scenarios, actual target and reference star lists, realistic estimates of SIM instrument performance, and plausible distributions of planetary system parameters.

Upgrades to the Flagstaff Astrometric Scanning Transit Telescope: A Fully Automated Telescope for Astrometry

The Flagstaff Astrometric Scanning Transit Telescope (FASTT) is a fully automated telescope that takes about 41,000 CCD frames of data a year for various research projects. All aspects of the telescope's operation have been automated (e.g., target selection, observing, reduction of data, and collation of results), and manpower needs are now under one person per year, mostly involved with routine maintenance and the dissemination of data. This paper describes the FASTT instrumental system, methods used with its automated operation, and the various FASTT research projects. Among the projects, astrometry is provided in support of various spacecraft missions, to predict occultation events, calculate dynamical masses for selective asteroids, and improve the ephemerides for thousands of asteroids, the planets Jupiter to Pluto, and 17 satellites of Jupiter through Neptune. Although most of the FASTT observing program involves the solar system, FASTT stellar astrometry was used to set up a number of astrometric calibration regions along the celestial equator, verify the Hipparcos link to the International Celestial Reference Frame, determine accurate positions for a large sample of radio stars, and investigate systematic errors in the FK5 star catalog. Furthermore, the FASTT produces accurate magnitudes that are being used to investigate the shapes of thousands of asteroids. By the end of year 2003, the FASTT will have produced over 190,000 positions of solar system objects in a program to provide a very large and homogeneous database for each object that will extend over many years and include positions accurate to ±47 to ±300 mas, depending on the magnitude of each observed object (3.5 < V < 17.5). Moreover, extensive efforts have been undertaken to improve the systematic accuracy of FASTT equatorial positions by applying corrections in the reductions for differential color refraction, distortions in the focal plane, and correcting for a positional error that is dependent on magnitude. The systematic accuracy of FASTT observations is now about ±20 mas in both right ascension and declination. FASTT data have contributed very significantly to recent successful spacecraft missions and to a dramatic improvement in the predictions made for occultation events.

Snapshot VLBI Observations of SiO v=1 and v=2 (J=1-0) Maser Sources

Abstract We have made snapshot VLBI observations of SiO masers of the $J= 1$−0 transition at two vibrational levels $v=1$ and $v=2$ using the Kashima–Nobeyama Interferometer (KNIFE). Out of 37 sources observed, we detected both of the lines from 16 sources with a typical detection limit of 10 Jy. Most of the SiO masers were significantly spatially resolved with a KNIFE baseline (200 km), indicating extended/complicated structures of the SiO masers with a typical angular size of a few milliarcseconds. Similar fringe-phase profiles along a radial velocity spanning larger than $1.0~\mathrm{km}~\mathrm{s}^{-1}$ between the two lines indicate a similar spatial structure along the velocity spacings. This tendency, shown from 13 SiO maser sources except Orion KL, supports spatial coincidence of the two lines found previously for M-type stars VY CMa and W Hya. The spatial coincidence should enable us to measure group-delays of the SiO masers for stellar astrometry.

Report of the IAU Working Group on ICRS

AbstractAfter the adoption of the ICRS and its primary materialization by the ICRF in January 1998, the last three years have witnessed a burst of interest in stellar astrometry stimulated by the availability of the ICRF and its optical counterpart. This paper reports on the activities carried out by the members of the IAU appointed working group on reference frames as well on other works directly related to the main goals of this working group.

Astrometry With The Yale Heliometer, 1882–1910

In 1880 Yale ordered a 6-inch heliometer from Repsold of Hamburg which was delivered on time for observations of the transit of Venus in 1882. Stellar astrometry, however, was not begun until W. L. Elkin came to Yale in 1884. Trained at Strassburg his Ph.D. dissertation dealt with the parallax of α Centauri (Elkin 1880). He then gained practical experience with Sir David Gill at the Cape. Elkin was the ideal man for the Yale heliometer and to secure his appointment H. A. Newton (1884) solicited subscriptions of $100 per year for three years from ten officers and friends of the observatory.

Speckle Interferometry in Astrometry

Speckle interferometry, which is characterised by diffraction limited resolution and enhanced accuracy, offers a wide range of possibilities for astrometry. Speckle applications are limited by the constraint of isoplanicity with the result that only differential rather than absolute astrometry can be carried out with the technique. Nevertheless, potentially important applications exist in solar system and stellar astrometry, but speckle interferometry has so far made significant contributions only in binary star astrometry.

Stellar radio astrometry. I - Precise positions of the radio emission of twenty stars. II - Precise optical positions of radio stars in the FK4 system. III - Preliminary comparison of the radio reference frame and the optical FK4-based reference frame by use of stellar radio emission

Stellar radio astrometry. I - Precise positions of the radio emission of twenty stars. II - Precise optical positions of radio stars in the FK4 system. III - Preliminary comparison of the radio reference frame and the optical FK4-based reference frame by use of stellar radio emission

Theoretical studies of the effects of grain noise on photographic stellar astrometry and photometry

A method of incorporating the effects of photographic emulsion grain noise into digital image centering algorithms is presented which improves the accuracy of the derived stellar positions and magnitudes. Theoretical formulae are then derived for the limiting error of the center, and the photometric parameters. For IIIa-J, this error is 0.2-0.3 mu for bright unsaturated images, which agrees quite well with measurements made with the Yale PDS microdensitometer. It is expected that, with further improvements in the positional accuracy of the PDS, it should be possible to reach the emulsion grain noise limit, providing that emulsion shifts or other large scale errors do not dominate. It is also shown that, with appropriate trimming, marginal distribution image centering algorithms can yield an accuracy only slightly poorer than that obtained with two-dimensional distributions.