Optical Astrometry Research Articles

A millisecond pulsar in an extremely wide binary system

We report on 22 yrs of radio timing observations of the millisecond pulsar J1024$-$0719 by the telescopes participating in the European Pulsar Timing Array (EPTA). These observations reveal a significant second derivative of the pulsar spin frequency and confirm the discrepancy between the parallax and Shklovskii distances that has been reported earlier. We also present optical astrometry, photometry and spectroscopy of 2MASS J10243869$-$0719190. We find that it is a low-metallicity main-sequence star (K7V spectral type, $\mathrm{[M/H]}=-1.0$, $T_\mathrm{eff}=4050\pm50$ K) and that its position, proper motion and distance are consistent with those of PSR J1024$-$0719. We conclude that PSR J1024$-$0719 and 2MASS J10243869$-$0719190 form a common proper motion pair and are gravitationally bound. The gravitational interaction between the main-sequence star and the pulsar accounts for the spin frequency derivatives, which in turn resolves the distance discrepancy. Our observations suggest that the pulsar and main-sequence star are in an extremely wide ($P_\mathrm{b}>200$ yr) orbit. Combining the radial velocity of the companion and proper motion of the pulsar, we find that the binary system has a high spatial velocity of $384\pm45$ km s$^{-1}$ with respect to the local standard of rest and has a Galactic orbit consistent with halo objects. Since the observed main-sequence companion star cannot have recycled the pulsar to millisecond spin periods, an exotic formation scenario is required. We demonstrate that this extremely wide-orbit binary could have evolved from a triple system that underwent an asymmetric supernova explosion, though find that significant fine-tuning during the explosion is required. Finally, we discuss the implications of the long period orbit on the timing stability of PSR J1024$-$0719 in light of its inclusion in pulsar timing arrays.

The trajectory and atmospheric impact of asteroid 2014 AA

Near-Earth asteroid 2014 AA entered the Earth’s atmosphere on 2014 January 2, only 21 h after being discovered by the Catalina Sky Survey. In this paper we compute the trajectory of 2014 AA by combining the available optical astrometry, seven ground-based observations over 69 min, and the International Monitoring System detection of the atmospheric impact infrasonic airwaves in a least-squares orbit estimation filter. The combination of these two sources of observations results in a tremendous improvement in the orbit uncertainties. The impact time is 3:05 UT with a 1σ uncertainty of 6 min, while the impact location corresponds to a west longitude of 44.2° and a latitude of 13.1° with a 1σ uncertainty of 140 km. The minimum impact energy estimated from the infrasound data and the impact velocity result in an estimated minimum mass of 22.6 t. By propagating the trajectory of 2014 AA backwards we find that the only window for finding precovery observations is for the three days before its discovery.

Direct Detections of the Yarkovsky Effect: Status and Outlook

AbstractWe report the current results on a comprehensive scan of the near-Earth asteroid catalog for evidence of the Yarkovsky effect in the orbital motion of these bodies. While most objects do not have sufficient observational data to reveal such slight acceleration, we do identify 42 asteroids with a “valid” detection of the Yarkovsky effect, i.e., those with a signal at least 3 times greater than the formal uncertainty and a value compatible with the Yarkovsky mechanism.We also identify a special category of non-detection, which we refer to as “weak signal,” where the objects are of a size that would permit a clear detection if the Yarkovsky effect is maximized, and yet the orbit is clearly incompatible with such accelerations. The implication is that the Yarkovsky effect is reduced in these cases, presumably due to mid-range obliquity, but possibly also due to size, bulk density, thermal inertia, albedo, or spin rate markedly different from assumptions.Finally, there are a number of asteroids showing a significant signal for nongravitational acceleration, and yet with a magnitude too great to be attributed to the Yarkovsky effect. We term these “spurious detections” because most are due to erroneous optical astrometry, often involving a single isolated night from precovery observations. Some cases may be due to other nongravitational accelerations, such as outgassing, mass loss, or micro-meteoroid flux.

Digital zenith telescope prototype of China

Classical optical astronomy measures the direction of the local plumb line on the stellar background, and is influenced by variations of the earth gravity field. Therefore, classical optical astrometry can be used to measure and study variations of plumb line. This endows classical optical astrometric technologies special significance for the interdisciplinary researches of astronomy and geoscience, and makes them irreplaceable by technologies like very long baseline interferometry, satellite laser ranging. However, classical astrometric instruments have a major drawback of low efficiency, such as the low automaticity, more operating observers, and even operative error which makes it very difficult to reduce the random observation error in some visual instruments. To overcome this drawback, we have successfully developed the digital zenith telescope prototype (DZT-1) with the aperture of 200 mm, using charge coupled device and other advanced technologies and new equipments. As proved by test observations, DZT-1 can observe thousands of stars over one night, which significantly reduces the random observation error and increases the observation accuracy. In addition, DZT-1 has a high degree of automation, even allowing unattended observation by remote control. Besides, DZT-1 has a small-size and is easy to move, making it convenient for mobile measurement of the deflection of the vertical. In summary, DZT can be widely used in the geoscience and astronomy fields.

Orbit and bulk density of the OSIRIS-REx target Asteroid (101955) Bennu

The target asteroid of the OSIRIS-REx asteroid sample return mission, (101955) Bennu (formerly 1999 RQ36), is a half-kilometer near-Earth asteroid with an extraordinarily well constrained orbit. An extensive data set of optical astrometry from 1999 to 2013 and high-quality radar delay measurements to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect, with a mean semimajor axis drift rate da/dt=(-19.0±0.1)×10-4au/Myr or 284±1.5m/year. The accuracy of this result depends critically on the fidelity of the observational and dynamical model. As an example, neglecting the relativistic perturbations of the Earth during close approaches affects the orbit with 3σ significance in da/dt.The orbital deviations from purely gravitational dynamics allow us to deduce the acceleration of the Yarkovsky effect, while the known physical characterization of Bennu allows us to independently model the force due to thermal emissions. The combination of these two analyses yields a bulk density of ρ=1260±70kg/m3, which indicates a macroporosity in the range 40±10% for the bulk densities of likely analog meteorites, suggesting a rubble-pile internal structure. The associated mass estimate is (7.8±0.9)×1010kg and GM=5.2±0.6m3/s2.Bennu’s Earth close approaches are deterministic over the interval 1654–2135, beyond which the predictions are statistical in nature. In particular, the 2135 close approach is likely within the lunar distance and leads to strong scattering and numerous potential impacts in subsequent years, from 2175 to 2196. The highest individual impact probability is 9.5×10-5 in 2196, and the cumulative impact probability is 3.7×10-4, leading to a cumulative Palermo Scale of −1.70.

The runaway binary LP 400−22 is leaving the Galaxy

We present optical spectroscopy, astrometry, radio, and X-ray observations of the runaway binary LP 400-22. We refine the orbital parameters of the system based on our new radial velocity observations. Our parallax data indicate that LP 400-22 is significantly more distant (3 sigma lower limit of 840 pc) than initially predicted. LP 400-22 has a tangential velocity in excess of 830 km/s; it is unbound to the Galaxy. Our radio and X-ray observations fail to detect a recycled millisecond pulsar companion, indicating that LP 400-22 is a double white dwarf system. This essentially rules out a supernova runaway ejection mechanism. Based on its orbit, a Galactic center origin is also unlikely. However, its orbit intersects the locations of several globular clusters; dynamical interactions between LP 400-22 and other binary stars or a central black hole in a dense cluster could explain the origin of this unusual binary.

Radar imaging and physical characterization of near-Earth Asteroid (162421) 2000 ET70

We observed near-Earth Asteroid (162421) 2000 ET70 using the Arecibo and Goldstone radar systems over a period of 12days during its close approach to the Earth in February 2012. We obtained continuous wave spectra and range-Doppler images with range resolutions as fine as 15m. Inversion of the radar images yields a detailed shape model with an effective spatial resolution of 100m. The asteroid has overall dimensions of 2.6km×2.2km×2.1km (5% uncertainties) and a surface rich with kilometer-scale ridges and concavities. This size, combined with absolute magnitude measurements, implies an extremely low albedo (∼2%). It is a principal axis rotator and spins in a retrograde manner with a sidereal spin period of 8.96±0.01h. In terms of gravitational slopes evaluated at scales of 100m, the surface seems mostly relaxed with over 99% of the surface having slopes less than 30°, but there are some outcrops at the north pole that may have steeper slopes. Our precise measurements of the range and velocity of the asteroid, combined with optical astrometry, enables reliable trajectory predictions for this potentially hazardous asteroid in the interval 460–2813.

Celestial reference frames at multiple radio wavelengths

AbstractIn 1997 the IAU adopted the International Celestial Reference Frame (ICRF) built from S/X VLBI data. In response to IAU resolutions encouraging the extension of the ICRF to additional frequency bands, VLBI frames have been made at 24, 32, and 43 GHz. Meanwhile, the 8.4 GHz work has been greatly improved with the 2009 release of the ICRF-2. This paper discusses the motivations for extending the ICRF to these higher frequency radio bands. Results to date will be summarized including evidence that the high frequency frames are rapidly approaching the accuracy of the 8.4 GHz ICRF-2. We discuss current limiting errors and prospects for the future accuracy of radio reference frames. We note that comparison of multiple radio frames is characterizing the frequency dependent systematic noise floor from extended source morphology and core shift. Finally, given Gaia's potential for high accuracy optical astrometry, we have simulated the precision of a radio-optical frame tie to be ~10–15 μas (1-σ, per component).

DETECTION OF SEMIMAJOR AXIS DRIFTS IN 54 NEAR-EARTH ASTEROIDS: NEW MEASUREMENTS OF THE YARKOVSKY EFFECT

We have identified and quantified semi-major axis drifts in Near-Earth Asteroids (NEAs) by performing orbital fits to optical and radar astrometry of all numbered NEAs. We focus on a subset of 54 NEAs that exhibit some of the most reliable and strongest drift rates. Our selection criteria include a Yarkovsky sensitivity metric that quantifies the detectability of semi-major axis drift in any given data set, a signal-to-noise metric, and orbital coverage requirements. In 42 cases, the observed drifts (~10^-3 AU/Myr) agree well with numerical estimates of Yarkovsky drifts. This agreement suggests that the Yarkovsky effect is the dominant non-gravitational process affecting these orbits, and allows us to derive constraints on asteroid physical properties. In 12 cases, the drifts exceed nominal Yarkovsky predictions, which could be due to inaccuracies in our knowledge of physical properties, faulty astrometry, or modeling errors. If these high rates cannot be ruled out by further observations or improvements in modeling, they would be indicative of the presence of an additional non-gravitational force, such as that resulting from a loss of mass of order a kilogram per second. We define the Yarkovsky efficiency f_Y as the ratio of the change in orbital energy to incident solar radiation energy, and we find that typical Yarkovsky efficiencies are ~10^-5.

MASSES OF NEUTRON STARS IN HIGH-MASS X-RAY BINARIES WITH OPTICAL ASTROMETRY

Determining the type of matter that is inside a neutron star (NS) has been a long-standing goal of astrophysics. Despite this, most of the NS equations of state (EOS) that predict maximum masses in the range 1.4-2.8 solar masses are still viable. Most of the precise NS mass measurements that have been made to date show values close to 1.4 solar masses, but a reliable measurement of an over-massive NS would constrain the EOS possibilities. Here, we investigate how optical astrometry at the microarcsecond level can be used to map out the orbits of High-Mass X-ray Binaries (HMXBs), leading to tight constraints on NS masses. While previous studies by Unwin and co-workers and Tomsick and co-workers discuss the fact that the future Space Interferometry Mission should be capable of making such measurements, the current work describes detailed simulations for 6 HMXB systems, including predicted constraints on all orbital parameters. We find that the direct NS masses can be measured to an accuracy of 2.5% (1-sigma) in the best case (X Per), to 6.5% for Vela X-1, and to 10% for two other HMXBs.

HST/WFPC2 observations of the LMC pulsar PSR B0540–69

The study of the younger, and brighter, pulsars is important to understand the optical emission properties of isolated neutron stars. PSRB0540-69, the second brightest (V~22) optical pulsar, is obviously a very interesting target for these investigations. The aim of this work is threefold: constraining the pulsar proper motion and its velocity on the plane of the sky through optical astrometry, obtaining a more precise characterisation of the pulsar optical spectral energy distribution (SED) through a consistent set of multi-band, high-resolution, imaging photometry observations, measuring the pulsar optical phase-averaged linear polarisation, for which only a preliminary and uncertain measurement was obtained so far from ground-based observations. We performed high-resolution observations of PSRB0540-69 with the WFPC2 aboard the HST, in both direct imaging and polarimetry modes. From multi-epoch astrometry we set a 3sigma upper limit of 1 mas/yr on the pulsar proper motion, implying a transverse velocity <250 km/s at the 50 kpc LMC distance. Moreover, we determined the pulsar absolute position with an unprecedented accuracy of 70 mas. From multi-band photometry we characterised the pulsar power-law spectrum and we derived the most accurate measurement of the spectral index (0.70+/-0.07) which indicates a spectral turnover between the optical and X-ray bands. Finally, from polarimetry we obtained a new measurement of the pulsar phase-averaged polarisation degree (16+/-4%),consistent with magnetosphere models depending on the actual intrinsic polarisation degree and depolarisation factor, and we found that the polarisation vector (22+/-12deg position angle) is possibly aligned with the semi-major axis of the pulsar-wind nebula and with the apparent proper motion direction of its bright emission knot.

New solution of Earth Orientation Parameters in 20th century

AbstractWe present a new solution of Earth Orientation Parameters, based on optical astrometry and catalog EOC-4.

SIM Lite astrometric observatory for detection of Earth-like planets

AbstractThe SIM Lite Astrometric Observatory is an optical astrometry mission for detection of Earth-like planets and investigation of dark matter, galaxy assembly, black holes etc. SIM Lite is a minimum system that performs the full NRC recommenced SIM science programs. This paper summarize the latest status and progresse of the SIM Lite space mission.

RADIO INTERFEROMETRIC PLANET SEARCH. I. FIRST CONSTRAINTS ON PLANETARY COMPANIONS FOR NEARBY, LOW-MASS STARS FROM RADIO ASTROMETRY

Radio astrometry of nearby, low-mass stars has the potential to be a powerful tool for the discovery and characterization of planetary companions. We present a Very Large Array survey of 172 active M dwarfs at distances of less than 10 pc. Twenty nine stars were detected with flux densities greater than 100 microJy. We observed 7 of these stars with the Very Long Baseline Array at milliarcsecond resolution in three separate epochs. With a detection threshold of 500 microJy in images of sensitivity 1 sigma ~ 100 microJy, we detected three stars three times (GJ 65B, GJ896A, GJ 4247), one star twice (GJ 285), and one star once (GJ 803). Two stars were undetected (GJ 412B and GJ 1224). For the four stars detected in multiple epochs, residuals from the optically-determined proper motions have an rms deviation of ~0.2 milliarcseconds, consistent with statistical noise limits. Combined with previous optical astrometry, these residuals provide acceleration upper limits that allow us to exclude planetary companions more massive than 3-6 M_Jup at a distance of ~1 AU with a 99% confidence level.

Distribution and properties of fragments and debris from the split Comet 73P/Schwassmann-Wachmann 3 as revealed by Spitzer Space Telescope

During 2006 March–2007 January, we used the IRAC and MIPS instruments on the Spitzer Space Telescope to study the infrared emission from the ensemble of fragments, meteoroids, and dust tails in the more than 3° wide 73P/Schwassmann-Wachmann 3 debris field. We also investigated contemporaneous ground-based and HST observations. In 2006 May, 55 fragments were detected in the Spitzer image. The wide spread of fragments along the comet’s orbit indicates they were formed from the 1995 splitting event. While the number of major fragments in the Spitzer image is similar to that seen from the ground by optical observers, the correspondence between the fragments with optical astrometry and those seen in the Spitzer images cannot be readily established, due either to strong non-gravitational terms, astrometric uncertainties, or transience of the fragments’ outgassing. The Spitzer data resolve the structure of the dust comae at a resolution of ∼1000 km, and they reveal the infrared emission due to large (mm to cm size) particles in a continuous dust trail that closely follows the projected orbit. We detect fluorescence from outflowing CO 2 gas from the largest fragments (B and C), and we measure the CO 2 : H 2 O proportion (1:10 and 1:20, respectively). We use three dimensionless parameters to explain dynamics of the solid particles: the rocket parameter α is the reaction force from day-side sublimation divided by solar gravity, the radiation pressure parameter β is the force due to solar radiation pressure divided by solar gravity, and the ejection velocity parameter ν is the particle ejection speed divided by the orbital speed of the comet at the time of ejection. The major fragments have ν > α > β and are dominated by the kinetic energy imparted to them by the fragmentation process. The small, ephemeral fragments seen by HST in the tails of the major fragments have α > ν > β and are dominated by rocket forces (until they become devolatilized). The meteoroids along the projected orbit seen by Spitzer have β ∼ ν ≫ α and are dominated by radiation pressure and ejection velocity, though both influences are much less than gravity. Dust in the fragments’ tails has β ≫ ( ν + α ) and is dominated by radiation pressure.

400 years of astrometry: from Tycho Brahe to Hipparcos

Galileo Galilei’s use of the newly invented telescope for astronomical observation resulted immediately in epochal discoveries about the physical nature of celestial bodies, but the advantage for astrometry came much later. The quadrant and sextant were pre-telescopic instruments for measurement of large angles between stars, improved by Tycho Brahe in the years 1570–1590. Fitted with telescopic sights after 1660, such instruments were quite successful, especially in the hands of John Flamsteed. The meridian circle was a new type of astrometric instrument, already invented and used by Ole Romer in about 1705, but it took a hundred years before it could fully take over. The centuries-long evolution of techniques is reviewed, including the use of photoelectric astrometry and space technology in the first astrometry satellite, Hipparcos, launched by ESA in 1989. Hipparcos made accurate measurement of large angles a million times more efficiently than could be done in about 1950 from the ground, and it will soon be followed by Gaia which is expected to be another one million times more efficient for optical astrometry.

New evidence of precision premium for Galilean satellites from CCD imaging

After a CCD image of the four Galilean satellites of Jupiter is obtained by a long focal length telescope, we can compare the theoretical positions of these satellites with their pixel positions so as to obtain the calibration parameters of the CCD field of view. In theory, when two of the four satellites have small enough separation, their relative positional measurement will have a good accuracy since the error existing in the solved calibration parameters has a direct proportional effect on the separation of the two satellites. The 347 CCD images taken by 1-m long focal length telescope at Yunnan Observatory in 2002–2005 are used to perform the experimental test. After we improve the centroid algorithm for the satellites and our former halo-removal technique, the results show that the positional measurement of two small-separation satellites has an external precision as good as 0.01–0.03 arcsec. This precision has comparability as that from rarely occurring mutual events of the Galilean satellites. This experiment confirms the finding of the “precision premium” firstly presented by Pascu [1994. An appraisal of the USNO program of photographic astrometry of bright planetary satellites. In: Morrison, L.V., Gilmore, G.F., (Eds.), Galatic and Solar System Optical Astrometry, pp. 304–311] using photographic observations. We believe that this type of observations, besides mutual event observations, might also be used to improve our knowledge of the orbital motions of the Galilean satellites because of its much more opportunities.

Predicting the Earth encounters of (99942) Apophis

Arecibo delay–Doppler measurements of (99942) Apophis in 2005 and 2006 resulted in a five standard-deviation trajectory correction to the optically predicted close approach distance to Earth in 2029. The radar measurements reduced the volume of the statistical uncertainty region entering the encounter to 7.3% of the pre-radar solution, but increased the trajectory uncertainty growth rate across the encounter by 800% due to the closer predicted approach to the Earth. A small estimated Earth impact probability remained for 2036. With standard-deviation plane-of-sky position uncertainties for 2007–2010 already less than 0.2 arcsec, the best near-term ground-based optical astrometry can only weakly affect the trajectory estimate. While the potential for impact in 2036 will likely be excluded in 2013 (if not 2011) using ground-based optical measurements, approximations within the Standard Dynamical Model (SDM) used to estimate and predict the trajectory from the current era are sufficient to obscure the difference between a predicted impact and a miss in 2036 by altering the dynamics leading into the 2029 encounter. Normal impact probability assessments based on the SDM become problematic without knowledge of the object's physical properties; impact could be excluded while the actual dynamics still permit it. Calibrated position uncertainty intervals are developed to compensate for this by characterizing the minimum and maximum effect of physical parameters on the trajectory. Uncertainty in accelerations related to solar radiation can cause between 82 and 4720 Earth-radii of trajectory change relative to the SDM by 2036. If an actionable hazard exists, alteration by 2–10% of Apophis' total absorption of solar radiation in 2018 could be sufficient to produce a six standard-deviation trajectory change by 2036 given physical characterization; even a 0.5% change could produce a trajectory shift of one Earth-radius by 2036 for all possible spin-poles and likely masses. Planetary ephemeris uncertainties are the next greatest source of systematic error, causing up to 23 Earth-radii of uncertainty. The SDM Earth point-mass assumption introduces an additional 2.9 Earth-radii of prediction error by 2036. Unmodeled asteroid perturbations produce as much as 2.3 Earth-radii of error. We find no future small-body encounters likely to yield an Apophis mass determination prior to 2029. However, asteroid (144898) 2004 VD17, itself having a statistical Earth impact in 2102, will probably encounter Apophis at 6.7 lunar distances in 2034, their uncertainty regions coming as close as 1.6 lunar distances near the center of both SDM probability distributions.

Solution of Earth orientation parameters in 20th century based on optical astrometry and new catalog EOC-3

AbstractA new star catalog has been derived recently, which is based on combination of Hipparcos/Tycho Catalogues with long lasting ground-based astrometric observations. In addition to ‘classical’ mean positions and proper motions of the stars, it contains also information on periodic motions of many stars that are due to double or multiple stellar systems. The catalog, called EOC-3, contains 4418 different objects, out of which 585 have significant periodic motions. This improved catalog is used as a reference frame in optical wavelength to derive a new series of Earth Orientation Parameters. To this end, almost five million observations of latitude/universal time variations made at 33 observatories are used. The new EOP series covers almost the entire 20th century (namely the interval 1899.7-1992.0).

A project of teaching ground-based astrometry

AbstractThe optical ground-based astrometry of solar system objects may have its accuracy strongly improved by using new methods for making observations and reductions of them. New photometric methods of observating the mutual phenomena occurring in the solar system, may provide astrometric data with a higher precision than the classical direct imaging. In order to help preparing observers for the future campaigns of observations (2008–2010) and to promote this kind of high-accuracy astrometry, we plan to organize a spring school in 2008 in Beijing, China, for PhD and post-doctoral students, and for interested young astronomers.