Astronomical Measurements Research Articles

Radio Interferometric Observation of an Asteroid Occultation

Abstract The occultation of the radio galaxy 0141+268 by the asteroid (372) Palma on 2017 May 15 was observed using six antennas of the Very Long Baseline Array (VLBA). The shadow of Palma crossed the VLBA station at Brewster, Washington. Owing to the wavelength used, and the size and the distance of the asteroid, a diffraction pattern in the Fraunhofer regime was observed. The measurement retrieves both the amplitude and the phase of the diffracted electromagnetic wave. This is the first astronomical measurement of the phase shift caused by diffraction. The maximum phase shift is sensitive to the effective diameter of the asteroid. The bright spot at the shadow’s center, the so called Arago–Poisson spot, is clearly detected in the amplitude time-series, and its strength is a good indicator of the closest angular distance between the center of the asteroid and the radio source. A sample of random shapes constructed using a Markov chain Monte Carlo algorithm suggests that the silhouette of Palma deviates from a perfect circle by 26 ± 13%. The best-fitting random shapes resemble each other, and we suggest their average approximates the shape of the silhouette at the time of the occultation. The effective diameter obtained for Palma, 192.1 ± 4.8 km, is in excellent agreement with recent estimates from thermal modeling of mid-infrared photometry. Finally, our computations show that because of the high positional accuracy, a single radio interferometric occultation measurement can reduce the long-term ephemeris uncertainty by an order of magnitude.

Development of a Data Acquisition System for Kinetic Inductance Detectors: Wide Dynamic Range and High Sampling Rate for Astronomical Observation

Microwave kinetic inductance detectors have a variety of potential applications in astronomical observations. We built a data acquisition system for kinetic inductance detectors combining a dedicated analog board and a commercially available digital board to meet the requirements of astronomical measurements, such as observation of the cosmic microwave background. This paper reports the status of the development of the data acquisition system. We have already achieved simultaneous readout through 120 channels using a direct down-conversion method to decode the signal. A variety of software has been developed and tested using the functionalities of the system and actual detectors.

The Changes In Calendars In The Ancient World As A Tool To Teach The Development Of Astronomy

When teaching an introductory science survey course to college students learning astronomy at the Hebrew University of Jerusalem, Israel, we have devoted four hours to teaching the history of astronomy as a fruitful strategy to introduce important concepts surrounding the development of general scientific knowledge throughout history. In order to illustrate the impact of improved accuracy of astronomical measurements, we propose using the example the development of the calendars and, in particular, the widespread Hebrew calendars used throughout the adjacent Millennia of B.C. and A.C. The changes in the several determinations of the Hebrew calendar are demonstrated based on Babylonian and Jewish documents as well as works by al-Khwarizmi from the 9th century AD, found in the Khuda Bakhsh Oriental Library, in Patna India. Our experience suggests that the teaching of calendar development and evolutions demonstrates the interconnectedness between scientific endeavors and social-religious traditions.

A Binary Offset Effect in CCD Readout and Its Impact on Astronomical Data

We have discovered an anomalous behavior of CCD readout electronics that affects their use in many astronomical applications. An offset in the digitization of the CCD output voltage that depends on the binary encoding of one pixel is added to pixels that are read out one, two, and/or three pixels later. One result of this effect is the introduction of a differential offset in the background when comparing regions with and without flux from science targets. Conventional data reduction methods do not correct for this offset. We find this effect in 16 of 22 instruments investigated, covering a variety of telescopes and many different front-end electronics systems. The affected instruments include LRIS and DEIMOS on the Keck telescopes, WFC3 UVIS and STIS on HST, MegaCam on CFHT, SNIFS on the UH88 telescope, GMOS on the Gemini telescopes, HSC on Subaru, and FORS on VLT. The amplitude of the introduced offset is up to 4.5 ADU per pixel, and it is not directly proportional to the measured ADU level. We have developed a model that can be used to detect this “binary offset effect” in data, and correct for it. Understanding how data are affected and applying a correction for the effect is essential for precise astronomical measurements.

Elmar K. Jessberger (1943–2017)

Elmar K. Jessberger (1943–2017)

A light carbon isotope composition for the Sun

Measurements by the Genesis mission have shown that solar wind oxygen is depleted in the rare isotopes, 17O and 18O, by approximately 80 and 100‰, respectively, relative to Earth’s oceans, with inferred photospheric values of about −60‰ for both isotopes. Direct astronomical measurements of CO absorption lines in the solar photosphere have previously yielded a wide range of O isotope ratios. Here, we reanalyze the line strengths for high-temperature rovibrational transitions in photospheric CO from ATMOS FTS data, and obtain an 18O depletion of δ18O = −50 ± 11‰ (1σ). From the same analysis we find a carbon isotope ratio of δ13C = −48 ± 7‰ (1σ) for the photosphere. This implies that the primary reservoirs of carbon on the terrestrial planets are enriched in 13C relative to the bulk material from which the solar system formed, possibly as a result of CO self-shielding or inheritance from the parent cloud.

On interstellar light polarization by diamagnetic silicate and carbon dust in the infrared

The motion of diamagnetic dust particles in interstellar magnetic fields is studied numerically with several different sets of parameters. Two types of behavior are observed, depending on the value of the critical number $R$, which is a function of the grain inertia, the magnetic susceptibility of the material and of the strength of rotation braking. If $R\leq10$, the grain ends up in a static state and perfectly aligned with the magnetic field, after a few braking times. If not, it goes on precessing and nutating about the field vector for a much longer time. Usual parameters are such that the first situation can hardly be observed. Fortunately, in the second and more likely situation, there remains a persistent partial alignment which is far from negligible, although it decreases as the field decreases and as $R$ increases. The solution of the complete equations of motion of grains in a field helps understanding the details of this behavior. One particular case of an ellipsoidal forsterite silicate grain is studied in detail and shown to polarize light in agreement with astronomical measurements of absolute polarization in the infrared. Phonons are shown to contribute to the progressive flattening of extinction and polarization towards long wavelengths. The measured dielectric properties of forsterite qualitatively fit the Serkowski peak in the visible.

Star Physics and Measurement Data

Astrophysics = the star physics was beginning its development without a supporting of measurement data, which could not be obtained then. Still astrophysics exists without this support, although now astronomers collected a lot of valuable information. This is the main difference of astrophysics from all other branches of physics, for which foundations are measurement data. The creation of the theory of stars, which is based on the astronomical measurements data, is one of the main goals of modern astrophysics. Below, the principal elements of star physics based on data of astronomical measurements are described. The theoretical description of a hot star interior is obtained. It explains the distribution of stars over their masses, mass-radius-temperature and mass-luminosity dependencies. All theoretical predictions are in a good agreement with the known measurement data, which confirms the validity of this consideration.

Temporal Variations of Telluric Water Vapor Absorption at Apache Point Observatory

Time-variable absorption by water vapor in Earth’s atmosphere presents an important source of systematic error for a wide range of ground-based astronomical measurements, particularly at near-infrared wavelengths. We present results from the first study on the temporal and spatial variability of water vapor absorption at Apache Point Observatory (APO). We analyze ∼400,000 high-resolution, near-infrared (H-band) spectra of hot stars collected as calibration data for the APO Galactic Evolution Experiment (APOGEE) survey. We fit for the optical depths of telluric water vapor absorption features in APOGEE spectra and convert these optical depths to Precipitable Water Vapor (PWV) using contemporaneous data from a GPS-based PWV monitoring station at APO. Based on simultaneous measurements obtained over a 3° field of view, we estimate that our PWV measurement precision is ±0.11 mm. We explore the statistics of PWV variations over a range of timescales from less than an hour to days. We find that the amplitude of PWV variations within an hour is less than 1 mm for most (96.5%) APOGEE field visits. By considering APOGEE observations that are close in time but separated by large distances on the sky, we find that PWV is homogeneous across the sky at a given epoch, with 90% of measurements taken up to 70° apart within 1.5 hr having ΔPWV < 1.0 mm. Our results can be used to help simulate the impact of water vapor absorption on upcoming surveys at continental observing sites like APO, and also to help plan for simultaneous water vapor metrology that may be carried out in support of upcoming photometric and spectroscopic surveys.

A gravitational-wave standard siren measurement of the Hubble constant.

On 17 August 2017, the Advanced LIGO and Virgo detectors observed the gravitational-wave event GW170817-a strong signal from the merger of a binary neutron-star system. Less than two seconds after the merger, a γ-ray burst (GRB 170817A) was detected within a region of the sky consistent with the LIGO-Virgo-derived location of the gravitational-wave source. This sky region was subsequently observed by optical astronomy facilities, resulting in the identification of an optical transient signal within about ten arcseconds of the galaxy NGC 4993. This detection of GW170817 in both gravitational waves and electromagnetic waves represents the first 'multi-messenger' astronomical observation. Such observations enable GW170817 to be used as a 'standard siren' (meaning that the absolute distance to the source can be determined directly from the gravitational-wave measurements) to measure the Hubble constant. This quantity represents the local expansion rate of the Universe, sets the overall scale of the Universe and is of fundamental importance to cosmology. Here we report a measurement of the Hubble constant that combines the distance to the source inferred purely from the gravitational-wave signal with the recession velocity inferred from measurements of the redshift using the electromagnetic data. In contrast to previous measurements, ours does not require the use of a cosmic 'distance ladder': the gravitational-wave analysis can be used to estimate the luminosity distance out to cosmological scales directly, without the use of intermediate astronomical distance measurements. We determine the Hubble constant to be about 70 kilometres per second per megaparsec. This value is consistent with existing measurements, while being completely independent of them. Additional standard siren measurements from future gravitational-wave sources will enable the Hubble constant to be constrained to high precision.

Simulation and Testing of a Linear Array of Modified Four-Square Feed Antennas for the Tianlai Cylindrical Radio Telescope

A wide bandwidth, dual polarized, modified four-square antenna is presented as a feed antenna for radio astronomical measurements. A linear array of these antennas is used as a line-feed for cylindrical reflectors for Tianlai, a radio interferometer designed for 21[Formula: see text]cm intensity mapping. Simulations of the feed antenna beam patterns and scattering parameters are compared to experimental results at multiple frequencies across the 650–1420[Formula: see text]MHz range. Simulations of the beam patterns of the combined feed array/reflector are presented as well.

Projection, Scale, and Accuracy in the 1721 Kangxi Maps

The Kangxi Emperor employed Jesuit brothers (1708–18) to produce maps of the provinces of China using a combination of Western and Chinese survey methods. The maps were completed by 1721. They were sent back to Europe and became the basis for maps of China produced by Jean Baptiste Bourguignon d'Anville in 1735. The main changes from traditional Chinese mapping were to use latitude and longitude as primary coordinates, map them using a spherical projection, and use astronomical measurements of latitude and longitude to establish baselines. Changes in latitude and longitude were found using traditional metric survey and relationships between distance north–south and latitude and distance east–west and longitude to convert to degrees. A digitized facsimile of the 1721 map series is available from the US Library of Congress. In this article, the digitized images were used to reconstruct the parameters of the sinusoidal projection, establish scale, and re-project and mosaic the maps into various forms for presentation. The accuracy of five of the province maps is discussed in detail. It is found that poor astronomical measurements of longitude are the most serious issue for the maps, but that apart from areas distorted by the poor longitude estimates, the accuracy was commensurate with that of European land maps of the time.

Reflectance model for densely packed media: Estimates of the surface properties of the high-albedo satellites of Saturn

Interpretation of photometric and polarimetric observations of atmosphereless celestial bodies faces the problems connected with both the insufficient accuracy and level of details in groundbased observations and the current state of the theory of the multiple scattering of light. In application to sparse media, where the electromagnetic waves, propagating between the scatterers, can be considered as spherical (the socalled far-field approximation), this theory is rather well developed for both the diffuse and coherent components of the scattered radiation. In this paper, we show that this theory can be also successfully applied to the measurements of polarization of light scattered by densely packed, though nonabsorbing or weakly absorbing, media. For this purpose, we calculated the models for a semi-infinite layer of the medium composed of randomly oriented clusters of spherical particles and compared them with the data of laboratory and astronomical measurements. The potential of the present approach is illustrated by an example of the interpretation of the polarization measurements of the ice satellites of Saturn—Rhea and Enceladus—which allowed some properties of the surface of these celestial bodies to be estimated. In particular, the ratio of the surface area that makes no contribution to the negative polarization of light reflected at small phase angles to the area producing the negative polarization branch was found. Under the assumption of the same albedo of these areas, this ratio turned out to be 3.31–3.66 and 1.7–3.8 for Rhea and Enceladus, respectively. For Enceladus, it is difficult to obtain a sufficiently narrow range of the estimated parameters, since the number of measurement points in the phase dependence of polarization of this satellite is small. For the surface of Rhea, the estimated packing density of particles, participating in the opposition effects, is approximately 15%, while their smallest size is of the order of the wavelength of visible light.

Legal Time of the Republic of Colombia and its international traceability using the Cesium Atomic Clock – Time and Frequency National Standard

Around the world, there are different providers of timestamp (mobile, radio or television operators, satellites of the GPS network, astronomical measurements, etc.), however, the source of the legal time for a country is either the national metrology institute or another designated laboratory. This activity requires a time standard based on an atomic time scale. The International Bureau of Weights and Measures (BIPM) calculates a weighted average of the time kept in more than 60 nations and produces a single international time scale, called Coordinated Universal Time (UTC). This article presents the current time scale that generates Legal Time for the Republic of Colombia produced by the Instituto Nacional de Metrología (INM) using the time and frequency national standard, a cesium atomic oscillator. It also illustrates how important it is for the academic, scientific and industrial communities, as well as the general public, to be synchronized with this time scale, which is traceable to the International System (SI) of units, through international comparisons that are made in real time.

GaiaNIR – A future all-sky astrometry mission

AbstractWith the launch of Gaia in December 2013, Europe entered a new era of space astrometry following in the footsteps of the very successful Hipparcos mission. A weakness of Gaia is that it only operates at optical wavelengths. However, much of the Galactic centre and the spiral arm regions are obscured by interstellar extinction. An obvious improvement on Gaia is to include the Near-Infra-Red (NIR) which requires the use of new types of detectors. Additionally, to scan the entire sky and measure global absolute parallaxes the spacecraft must have a constant rotation resulting in a moving image that must be compensated for by, for example, operating the detectors in Time Delayed Integration (TDI) mode. If these technical issues can be solved a new Gaia-like mission separated by a 20 year interval would give; 1) NIR all-sky astrometry and photometry to penetrate the obscured regions and to observe intrinsically red objects with almost diffraction limited resolution; 2) improved proper motions with fourteen times smaller errors than from Gaia alone opening up new science cases, such as long period exoplanets and accurate halo measurements; 3) allow the slowly degrading accuracy of the Gaia reference frame, which will be the basis for future astronomical measurements, to be reset.

Jian-Liang Lin; Hong-Sen Yan. Decoding the Mechanisms of Antikythera Astronomical Device. x + 281 pp., figs., bibls., apps., index. Heidelberg: Springer, 2016. $109 (cloth).

Jian-Liang Lin; Hong-Sen Yan. Decoding the Mechanisms of Antikythera Astronomical Device. x + 281 pp., figs., bibls., apps., index. Heidelberg: Springer, 2016. $109 (cloth).

Scale as Nostalgic Form: Patricio Guzmán's &lt;em&gt;Nostalgia for the Light&lt;/em&gt; (2011)

In the winter of 2014, the Millennium Scientific Initiative Nucleus on Protoplanetary Disks and the Anilla Cultural Museum of Contemporary Art in Santiago, Chile, organized an interdisciplinary initiative-Art, Astronomy, Technology and Society (AATS)-that sought to engage the general public in the sciences through creative projects and collaborations.1 The project launched a creative incubator to develop small-scale creative projects in collaboration with local astronomers and held an Astronomy Day consisting of a series of lectures and workshops that connected the driving questions of astronomical investigation to contemporary artistic production. The initiative culminated with an exciting exhibition that married data visualization and contemporary art installation. The collaboration yielded new media visualizations of different celestial phenomena in mixed media that made the cosmic sensuously graspable. Whether a manipulable computer program of exoplanetary formation or a fiberoptic light sculpture programmed to the captured sound waves of radio telescopes, the artistic practices on display attempted to reduce scale, convert dates, and emplace the artist-spectator, suggesting that to comprehend the cosmic one must first apprehend the microcosmic. Astronomy as visual culture foregrounds how comprehension necessitates apprehension and opens onto a series of questions about the forensic value of images and the political and epistemological stakes of their use in memory debates in postdictatorship Chile. I explore these questions by turning to Patricio Guzman's 2011 Nostalgia de la luz (Nostalgia for the Light) in order to articulate astronomy to both history and cinema and to suggest a different framework for thinking the image within the historiographical practices in the region.The opening AATS lecture series yielded a dialogue between astrophysicists and artists in which the former attempted to make the abstract concrete and the latter attempted to make the concrete abstract.2 The installation work by Marcelo Arce, Astronomical Unit, overlaid sound wave patterns recorded by radio telescopes to shift the hues illuminated in an amorphous fiberoptic mass.3 The installation's suggestive title is borrowed from astronomy, defined by Arce's astrophysicist interlocutor Hector Canovas as a unit of length roughly the distance from Earth to the sun. In his remarks, Canovas establishes two scalar factors: the astronomical unit as a unit of distance and the cosmic epoch as a unit of time. The astronomical unit is a metric for astronomical measurements, originally defined through telemetric readings of photon reflections off celestial objects.4 More important for our purposes, the astronomical unit is a scalar factor for astronomical imaging. Canovas's images all bear a linear scale. The only means to make the image legible is to provide a scalar factor that systemizes the coordinates of the image, and the only way to make the image intelligible is to use terrestrial intervals as a reference. And yet, the astronomical unit still exceeds our grasp, so Canovas provides yet another scalar conversion, both spatial and temporal. He first explains that the distance from Earth to the moon can be scaled to the distance from Valparaiso to Vina del Mar, the distance from Earth to the Sun to the distance from Valparaiso to Quito, and finally the distance from Earth to another planet to over twenty thousand circumnavigations of the world. The cosmic epoch functions as a unit of time determined by the relative intensities of photons received. Canovas again reduces this temporal scale through reference to our terrestrial clock time: light travels mere seconds in the first case, four days in the second case, and over one million years in the final trajectory. He makes the human body and its units of length and time the yardstick through which to make these epochs comprehensible.Arce's installation was part of the final exhibition, which was conceived and curated under the signposts of totality, myth, concealment, and discovery in order to interpellate the spectator. …

Dynamics of Centaur Chariklo and evolution of its rings

It follows from observations that the asteroid Chariklo has two outer rings. The purpose of this paper is constructing the equilibrium model of asteroid and developing the kinetic mechanism of evolution of its rings. We have specified for Chariklo the density \(\rho_{0} \approx 2.71~\mbox{g}/\mbox{cm}^{3}\), the mass \(M_{0} \approx 8.817 \times 10^{21}~\mbox{g}\), and the average radius \(R_{0} \approx 128.16~\mbox{km}\). Its rings are modeled by circular gravitating tori consisting of the small rock-ice particles that orbit the asteroid. The method does not imply the presence of hidden satellites close to the asteroid, and the equilibrium of the rings is determined by the small velocity dispersion and gravity of particles. The problem of expansion of the internal torus gravitational potential in series in powers of its geometrical parameter is solved. This enables the gravitational energy of Chariklo’s rings to be found and to express their masses in terms of the mass of the asteroid \(M_{0}\). We calculated the mass of the inner ring to be \(M_{r1} \approx 9.8 \times 10^{18}~\mbox{g}\), and its relation to the mass of the asteroid is \(\frac{M_{r1}}{M_{0}} \approx 0.001\); similarly, for the outer ring \(M_{r2} \approx 10^{18}~\mbox{g}\), and \(\frac{M_{r2}}{M_{0}} \approx 0.0001\). The mass ratio \(\frac{M_{r1}}{M_{r2}} \approx 10\) is typical for satellites of other asteroids and dwarf planets. The velocity dispersion of particles in rings \(\upsilon_{1} \approx 1~\mbox{m}/\mbox{s}\) and \(\upsilon_{2} \approx 45~\mbox{cm}/\mbox{s}\) is no greater than \(1 \div 2~\%\) of its rotational velocity \(\upsilon_{rot} \approx 40~\mbox{m}/\mbox{s}\). The particle of medium radius \(r_{p} = 25~\mbox{cm}\) has the mean free path \(\lambda \approx 7~\mbox{m}\), and its diffusion time from center line on the surface of the torus is \(\frac{T'_{1}}{T_{rot1}} \approx 13\) and \(\frac{T'_{2}}{T_{rot2}} \approx 5\). The dissipation rate equation for Chariklo’s rings is derived. From this equation a surprising result follows: the time of energy dissipation (the time of evolution of rings) is only \(T_{d1} \approx 10^{3} \div 10^{4}~\mbox{years}\), which by astronomical measures is a very short time scale. We adduce the arguments supporting the idea that these rings in near future can become Chariklo’s satellites, and that the transformation of the rings into satellites energetically is favored.

On the non-Gaussian errors in high-z supernovae type Ia data

The nature of random errors in any data set is Gaussian, which is a well established fact according to the Central Limit Theorem. Supernovae type Ia data have played a crucial role in major discoveries in cosmology. Unlike in laboratory experiments, astronomical measurements cannot be performed in controlled situations. Thus, errors in astronomical data can be more severe in terms of systematics and non-Gaussianity compared to those of laboratory experiments. In this paper, we use the Kolmogorov-Smirnov statistic to test non-Gaussianity in high-z supernovae data. We apply this statistic to four data sets, i.e., Gold data (2004), Gold data (2007), the Union2 catalog and the Union2.1 data set for our analysis. Our results show that in all four data sets the errors are consistent with a Gaussian distribution.

Everything we’d like to do with LSST data, but we don’t know (yet) how

AbstractThe Large Synoptic Survey Telescope (LSST), the next-generation optical imaging survey sited at Cerro Pachon in Chile, will provide an unprecedented database of astronomical measurements. The LSST design, with an 8.4m (6.7m effective) primary mirror, a 9.6 sq. deg. field of view, and a 3.2 Gigapixel camera, will allow about 10,000 sq. deg. of sky to be covered twice per night, every three to four nights on average, with typical 5-sigma depth for point sources of r=24.5 (AB). With over 800 observations in ugrizy bands over a 10-year period, these data will enable a deep stack reaching r=27.5 (about 5 magnitudes deeper than SDSS) and faint time-domain astronomy. The measured properties of newly discovered and known astrometric and photometric transients will be publicly reported within 60 sec after observation. The vast database of about 30 trillion observations of 40 billion objects will be mined for the unexpected and used for precision experiments in astrophysics. In addition to a brief introduction to LSST, we discuss a number of astro-statistical challenges that need to be overcome to extract maximum information and science results from LSST dataset.