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Understanding Gaia : a mission to map the galaxy
This book is the first to provide a comprehensive, readily understandable report on the European Space Agency's Gaia mission that will meet the needs of a general audience. It takes the reader on an exciting journey of discovery, explaining how such a scientific satellite is made, presenting the scientific results available from Gaia to date, and examining how the collected data will be used and their likely scientific consequences. The Gaia mission will provide a complete and high-precision map of the positions, distances, and motions of the stars in our galaxy. It will revolutionize our knowledge on the origin and evolution of the Milky Way, on the effects of mysterious dark matter, and on the birth and evolution of stars and extrasolar planets. The Gaia satellite was launched in December 2013 and has a foreseen operational lifetime of five to six years, culminating in a final stellar catalogue in the early 2020s. This book will appeal to all who have an interest in the mission and the profound impact that it will have on astronomy.
Book
A New Distortion Solution for NIRC2 on the Keck II Telescope
We present a new geometric distortion model for the narrow-field mode of the near-infrared camera (NIRC2) fed by the adaptive optics system on the W. M. Keck II telescope. The adaptive optics system and NIRC2 camera were realigned on 2015 April 13. Observations of the crowded globular cluster, M53, were obtained before and after the realignment to characterize the geometric field distortion. The distorted NIRC2 positions of M53 stars were compared with precise astrometry of this cluster from Hubble Space Telescope observations. The resulting distortion map constructed just before the realignment is consistent with the previous solution derived using data from 2007 to 2009, indicating that the distortion has been stable to ∼0.5 mas. The distortion map changed significantly after a realignment of 4.5 mas (75%) rms, and the new distortion model for post-realignment observations have a total accuracy of ∼1.1 mas.
Journal Article
Probing the Astrometric Properties of Gaia EDR3 Quasars at the Faintest Magnitudes
Gaia early data release 3 (EDR3) provides about 1.6 million QSO-like objects. Measuring the systematic residuals of the quasars convincingly in Gaia EDR3 is paramount for further fundamental astronomy use. We aim to conduct an independent assessment of the astrometric quality of Gaia EDR3 utilizing a sizable and confirmed extragalactic sources catalog. By positional cross-matching the known quasars to EDR3, we obtain a list of 299,004 confirmed QSOs. Together with the quasars identified in EDR3, we calculated the generalized moving mean of proper motions and parallaxes. Using spherical harmonics, the parallaxes and the proper motions are expanded to investigate their astrometric features. The global parallax bias is ∼−21 μas and ∼−27 μas for the five-parameter solution and the six-parameter solution samples, respectively. The mean proper motion of the five-parameter solution QSO-like subset is consistent with zero, while the mean proper motion of the six-parameter solution subset is a few μas yr−1 biased. The typical rotation and glide vector for the five-parameter solution quasars are (+3, +2, +1) ± 0.5 μas yr−1 and (0, −4, −2) ± 0.5 μas yr−1, respectively. Small systematic dependence of the rotation on the Gaia color and the G magnitude exists. The parallaxes of EDR3 quasars show an rms amplitude of 9.9 μas spatial correlation with the angular of ≥18°. At the level of Gaia EDR3's uncertainties, the quadrupole components of the vectorial spherical harmonics show that the mass-energy density of the gravitational waves ΩGW is about 0.0073 h −2.
Journal Article
A Code for Robust Astrometric Solution of Astronomical Images
I present a software tool for solving the astrometry of astronomical images. The code puts emphasis on robustness against failures for correctly matching the sources in the image to a reference catalog, and on the stability of the solutions over the field of view (e.g., using orthogonal polynomials for the fitted transformation). The code was tested on over 5 × 104 images from various sources, including the Palomar Transient Factory (PTF) and the Zwicky Transient Facility (ZTF). The tested images equally represent low and high Galactic latitude fields and exhibit failure/bad-solution rate of 2 × 10−5. Running on PTF 60-s integration images, and using the GAIA-DR2 as a reference catalog, the typical two-axes-combined astrometric root-mean square (rms) is 14 mas at the bright end, presumably due to astrometric scintillation noise and systematic errors. I discuss the effects of seeing, airmass, and the order of the transformation on the astrometric accuracy. The software, available online, is developed in MATLAB as part of an astronomical image processing environment and it can be run also as a stand-alone code.
Journal Article
Lost in Space? Relativistic Interstellar Navigation using an Astrometric Star Catalog
The exploration of interstellar space will require autonomous navigation systems that do not rely on tracking from the Earth. Here I develop a method to determine the 3D position and 3D velocity of a spacecraft in deep space using a star catalog. As a spacecraft moves away from the Sun, the observed positions and velocities of the stars will change relative to those in a Earth-based catalog due to parallax and aberration. By measuring just the angular distances between pairs of stars, and comparing these to the catalog, we can infer the coordinates of the spacecraft via an iterative forward-modeling process. I perform simulations with existing star catalogs to demonstrate the method and to compute its performance. Using the 20 nearest stars and a modest angular distance measurement accuracy of 1″, the position and velocity of a spacecraft light years from the Sun moving at relativistic speeds can be determined to within 3 au and 2 km s − 1 respectively. These accuracies improve linearly with the measurement accuracy, e.g., with angles measured to 1 mas the navigation accuracy is 1000 times better. Performance can also be improved using more stars, or by including onboard measurements of the stars’ radial velocities, as these too are affected by the spacecraft’s position and motion.
Journal Article
Optical superluminal motion measurement in the neutron-star merger GW170817
The afterglow of the binary neutron-star merger GW170817
1
gave evidence for a structured relativistic jet
2
–
6
and a link
3
,
7
,
8
between such mergers and short gamma-ray bursts. Superluminal motion, found using radio very long baseline interferometry
3
(VLBI), together with the afterglow light curve provided constraints on the viewing angle (14–28 degrees), the opening angle of the jet core (less than 5 degrees) and a modest limit on the initial Lorentz factor of the jet core (more than 4). Here we report on another superluminal motion measurement, at seven times the speed of light, leveraging Hubble Space Telescope precision astrometry and previous radio VLBI data for GW170817. We thereby obtain a measurement of the Lorentz factor of the wing of the structured jet, as well as substantially improved constraints on the viewing angle (19–25 degrees) and the initial Lorentz factor of the jet core (more than 40).
Optical superluminal motion in the binary neutron-star merger GW170817 is used to constrain the speed and morphology of the structured jet, and improve constraints on the inclination angle of the merging binary system.
Journal Article
An Algorithm for Coordinate Matching in World Coordinate Solutions
Algorithms for point source extraction and catalog-to-image coordinate matching for world coordinate solutions are presented. In particular the coordinate matching algorithm is lightweight, simple to understand, easy to code, and solves orders of magnitude more quickly than existing solutions to this common astrometric problem.
Journal Article
Astrometric Calibration and Performance of the Dark Energy Camera
We characterize the ability of the Dark Energy Camera (DECam) to perform relative astrometry across its 500 Mpix, 3-deg2 science field of view and across four years of operation. This is done using internal comparisons of ∼4 × 107 measurements of high signal-to-noise ratio stellar images obtained in repeat visits to fields of moderate stellar density, with the telescope dithered to move the sources around the array. An empirical astrometric model includes terms for optical distortions; stray electric fields in the CCD detectors; chromatic terms in the instrumental and atmospheric optics; shifts in CCD relative positions of up to 10 m when the DECam temperature cycles; and low-order distortions to each exposure from changes in atmospheric refraction and telescope alignment. Errors in this astrometric model are dominated by stochastic variations with typical amplitudes of 10-30 mas (in a 30 s exposure) and 5′-10′ coherence length, plausibly attributed to Kolmogorov-spectrum atmospheric turbulence. The size of these atmospheric distortions is not closely related to the seeing. Given an astrometric reference catalog at density 0.7 arcmin − 2 , e.g., from Gaia, the typical atmospheric distortions can be interpolated to 7 mas rms accuracy (for 30 s exposures) with 1 ′ coherence length in residual errors. Remaining detectable error contributors are 2-4 mas rms from unmodelled stray electric fields in the devices, and another 2-4 mas rms from focal plane shifts between camera thermal cycles. Thus the astrometric solution for a single DECam exposure is accurate to 3-6 mas ( 0.02 pixels, or 300 nm) on the focal plane, plus the stochastic atmospheric distortion.
Journal Article
Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua)
‘Oumuamua (1I/2017 U1) is the first known object of interstellar origin to have entered the Solar System on an unbound and hyperbolic trajectory with respect to the Sun
1
. Various physical observations collected during its visit to the Solar System showed that it has an unusually elongated shape and a tumbling rotation state
1
–
4
and that the physical properties of its surface resemble those of cometary nuclei
5
,
6
, even though it showed no evidence of cometary activity
1
,
5
,
7
. The motion of all celestial bodies is governed mostly by gravity, but the trajectories of comets can also be affected by non-gravitational forces due to cometary outgassing
8
. Because non-gravitational accelerations are at least three to four orders of magnitude weaker than gravitational acceleration, the detection of any deviation from a purely gravity-driven trajectory requires high-quality astrometry over a long arc. As a result, non-gravitational effects have been measured on only a limited subset of the small-body population
9
. Here we report the detection, at 30
σ
significance, of non-gravitational acceleration in the motion of ‘Oumuamua. We analyse imaging data from extensive observations by ground-based and orbiting facilities. This analysis rules out systematic biases and shows that all astrometric data can be described once a non-gravitational component representing a heliocentric radial acceleration proportional to
r
−2
or
r
−1
(where
r
is the heliocentric distance) is included in the model. After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects originating from ‘Oumuamua potentially being composed of several spatially separated bodies or having a pronounced offset between its photocentre and centre of mass, we find comet-like outgassing to be a physically viable explanation, provided that ‘Oumuamua has thermal properties similar to comets.
‘Oumuamua—the first known interstellar object to have entered the Solar System—is probably a comet, albeit with unusual dust and chemical properties owing to its origin in a distant solar system.
Journal Article
Most nearby young star clusters formed in three massive complexes
Efforts to unveil the structure of the local interstellar medium and its recent star-formation history have spanned the past 70 years (refs.
1
–
6
). Recent studies using precise data from space astrometry missions have revealed nearby, newly formed star clusters with connected origins
7
–
12
. Nonetheless, mapping young clusters across the entire sky back to their natal regions has been hindered by a lack of clusters with precise radial-velocity data. Here we show that 155 out of 272 (57%) high-quality young clusters
13
,
14
within 1 kiloparsec of the Sun arise from three distinct spatial volumes. This conclusion is based on the analysis of data from the third Gaia release
15
and other large-scale spectroscopic surveys. At present, dispersed throughout the solar neighbourhood, their past positions more than 30 million years ago reveal that these families of clusters each formed in one of three compact, massive star-forming complexes. One of these families includes all of the young clusters near the Sun—the Taurus and Scorpius–Centaurus star-forming complexes
16
,
17
. We estimate that more than 200 supernovae were produced from these families and argue that these clustered supernovae produced both the Local Bubble
18
and the largest nearby supershell GSH 238+00+09 (ref.
19
), both of which are clearly visible in modern three-dimensional dust maps
20
–
22
.
Analysis of data from Gaia Data Release 3 and other large spectroscopic surveys shows that nearly 60% of high-quality young clusters within 1 kpc of the Sun originated from just three distinct star-forming complexes.
Journal Article