arXiv subjects

Beatrice Bucciarelli

Publications and source records attributed to Beatrice Bucciarelli.

At least 19 recordsLinked to original sources

arXiv2025

The Problem of the Global Astrometric Sphere Reconstruction in Astrometry -- Issues and Approaches

In this contribution we give a brief account of the problem of the Global Astrometric Sphere Reconstruction in Astrometry, with particular reference to the Gaia and Gaia-like astrometric missions, namely those adopting a scanning strategy with observations in TDI mode. We sketch the design of the Gaia mission, the mathematical modelling that comes naturally from its observing strategy, and how the problem of the global sphere reconstruction translates into that of the solution of large, sparse, and overdetermined system of linearized equations. After a short description of the two approaches to this problem implemented in the Gaia data reduction pipelines, we list the main known problems of the current approaches, with specific reference to the calibration and the correlation issues. Finally, we suggest how an arc-based solution could help to alleviate some of these problems, how it would be possible to devise a mathematical model for such an observable despite the TDI observing mode, and the main difficulty that a parallel implementation of this model would have to solve.

astro-ph.IM↗

arXiv2025

Computational challenges of 21st century Global Astrometry

Major advancements in space science and detector technology brought about a revolution in global astrometry, the science of measuring distances and motions of stars in the Milky Way and in the local universe. From the first ESA astrometric mission HIPPARCOS of the early 80s to the current Gaia mission, the data volume and computational complexity of the full reduction process has increased by several orders of magnitude, requiring high-performance computing and data throughput. We review the principles and computational complexity of general global astrometric models that lead to the statistical treatment of an extra-large, highly non-linear estimation problem. Some numerical aspects of inspecting Gaia's proper motions to find cosmological signals at all scales are also addressed.

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arXiv2025

The Gaia AVU-GSR solver: a CPU + GPU parallel code toward Exascale systems

The solver module of the Astrometric Verification Unit - Global Sphere Reconstruction (AVU-GSR) pipeline aims to find the astrometric parameters of $\sim$$10^8$ stars in the Milky Way, besides the attitude and instrumental settings of the Gaia satellite and the parametrized post Newtonian parameter $γ$ with a resolution of 10-100 micro-arcseconds. To perform this task, the code solves a system of linear equations with the iterative Least Squares (LSQR) algorithm, where the coefficient matrix is large (10-50 TB) and sparse and the iterations stop when convergence is reached in the least squares sense. The two matrix-by-vector products performed at each LSQR step were GPU-ported, firstly with OpenACC and then with CUDA, resulting in a $\sim$$1.5$x and $\sim$$14$x speedup, respectively, over an original code version entirely parallelized on the CPU with MPI + OpenMP. The CUDA code was further optimized and then ported with programming frameworks portable across different GPU architectures, obtaining a further $\sim$$2$x acceleration factor. One critical section of the code consists in the computation of covariances, whose total number is $N_{\rm unk} \times (N_{\rm unk} - 1)/2$ and occupy $\sim$1 EB, being $N_{\rm unk}$$\sim$$5 \times 10^8$ the total number of unknowns. This "Big Data" issue cannot be faced with standard approaches: we defined an I/O-based pipeline made of two concurrently launched jobs, where one job, i.e., the LSQR, writes the files and the second job reads them, iteratively computes the covariances and deletes them. The pipeline does not present significant bottlenecks until a number of covariances elements equal to $\sim$$8 \times 10^6$. The code currently runs in production on Leonardo CINECA infrastructure.

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arXiv2023

The MPI + CUDA Gaia AVU-GSR Parallel Solver Toward Next-generation Exascale Infrastructures

We ported to the GPU with CUDA the Astrometric Verification Unit-Global Sphere Reconstruction (AVU-GSR) Parallel Solver developed for the ESA Gaia mission, by optimizing a previous OpenACC porting of this application. The code aims to find, with a [10,100]$μ$as precision, the astrometric parameters of $\sim$$10^8$ stars, the attitude and instrumental settings of the Gaia satellite, and the global parameter $γ$ of the parametrized Post-Newtonian formalism, by solving a system of linear equations, $A\times x=b$, with the LSQR iterative algorithm. The coefficient matrix $A$ of the final Gaia dataset is large, with $\sim$$10^{11} \times 10^8$ elements, and sparse, reaching a size of $\sim$10-100 TB, typical for the Big Data analysis, which requires an efficient parallelization to obtain scientific results in reasonable timescales. The speedup of the CUDA code over the original AVU-GSR solver, parallelized on the CPU with MPI+OpenMP, increases with the system size and the number of resources, reaching a maximum of $\sim$14x, >9x over the OpenACC application. This result is obtained by comparing the two codes on the CINECA cluster Marconi100, with 4 V100 GPUs per node. After verifying the agreement between the solutions of a set of systems with different sizes computed with the CUDA and the OpenMP codes and that the solutions showed the required precision, the CUDA code was put in production on Marconi100, essential for an optimal AVU-GSR pipeline and the successive Gaia Data Releases. This analysis represents a first step to understand the (pre-)Exascale behavior of a class of applications that follow the same structure of this code. In the next months, we plan to run this code on the pre-Exascale platform Leonardo of CINECA, with 4 next-generation A200 GPUs per node, toward a porting on this infrastructure, where we expect to obtain even higher performances.

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arXiv2023

The messy death of a multiple star system and the resulting planetary nebula as observed by JWST

Planetary nebulae (PNe), the ejected envelopes of red giant stars, provide us with a history of the last, mass-losing phases of 90 percent of stars initially more massive than the Sun. Here, we analyse James Webb Space Telescope (JWST) Early Release Observation (ERO) images of the PN NGC3132. A structured, extended H2 halo surrounding an ionised central bubble is imprinted with spiral structures, likely shaped by a low-mass companion orbiting the central star at 40-60 AU. The images also reveal a mid-IR excess at the central star interpreted as a dusty disk, indicative of an interaction with another, closer companion. Including the previously known, A-type visual companion, the progenitor of the NGC3132 PN must have been at least a stellar quartet. The JWST images allow us to generate a model of the illumination, ionisation and hydrodynamics of the molecular halo, demonstrating the power of JWST to investigate complex stellar outflows. Further, new measurements of the A-type visual companion allow us to derive the value for the mass of the progenitor of a central star to date with excellent precision: 2.86+/-0.06 Mo. These results serve as path finders for future JWST observations of PNe providing unique insight into fundamental astrophysical processes including colliding winds, and binary star interactions, with implications for supernovae and gravitational wave systems.

astro-ph.SR↗

arXiv2022

The Gaia AVU-GSR parallel solver: preliminary studies of a LSQR-based application in perspective of exascale systems

The Gaia Astrometric Verification Unit-Global Sphere Reconstruction (AVU-GSR) Parallel Solver aims to find the astrometric parameters for $\sim$10$^8$ stars in the Milky Way, the attitude and the instrumental specifications of the Gaia satellite, and the global parameter $γ$ of the post Newtonian formalism. The code iteratively solves a system of linear equations, $\mathbf{A} \times \vec{x} = \vec{b}$, where the coefficient matrix $\mathbf{A}$ is large ($\sim$$10^{11} \times 10^8$ elements) and sparse. To solve this system of equations, the code exploits a hybrid implementation of the iterative PC-LSQR algorithm, where the computation related to different horizontal portions of the coefficient matrix is assigned to separate MPI processes. In the original code, each matrix portion is further parallelized over the OpenMP threads. To further improve the code performance, we ported the application to the GPU, replacing the OpenMP parallelization language with OpenACC. In this port, $\sim$95% of the data is copied from the host to the device at the beginning of the entire cycle of iterations, making the code $compute$ $bound$ rather than $data$$-$$transfer$ $bound$. The OpenACC code presents a speedup of $\sim$1.5 over the OpenMP version but further optimizations are in progress to obtain higher gains. The code runs on multiple GPUs and it was tested on the CINECA supercomputer Marconi100, in anticipation of a port to the pre-exascale system Leonardo, that will be installed at CINECA in 2022.

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arXiv2022

Astrometric precision tests on TESS data

Background. Astrometry at or below the micro-arcsec level with an imaging telescope assumes that the uncertainty on the location of an unresolved source can be an arbitrarily small fraction of the detector pixel, given a sufficient photon budget. Aim. This paper investigates the geometric limiting precision, in terms of CCD pixel fraction, achieved by a large set of star field images, selected among the publicly available science data of the TESS mission. Method. The statistics of the distance between selected bright stars ($G \simeq 5\,mag$), in pixel units, is evaluated, using the position estimate provided in the TESS light curve files. Results. The dispersion of coordinate differences appears to be affected by long term variation and noisy periods, at the level of $0.01$ pixel. The residuals with respect to low-pass filtered data (tracing the secular evolution), which are interpreted as the experimental astrometric noise, reach the level of a few milli-pixel or below, down to $1/5,900$ pixel. Saturated images are present, evidencing that the astrometric precision is mostly preserved across the CCD columns, whereas it features a graceful degradation in the along column direction. The cumulative performance of the image set is a few micro-pixel across columns, or a few 10 micro-pixel along columns. Conclusions. The idea of astrometric precision down to a small fraction of a CCD pixel, given sufficient signal to noise ratio, is confirmed by real data from an in-flight science instrument to the $10^{-6}$ pixel level. Implications for future high precision astrometry missions are briefly discussed.

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arXiv2021

Progress on the Astrometric Gravitation Probe design

The Astrometric Gravitation Probe mission is a modern version of the 1919 Dyson-Eddington-Davidson experiment, based on a space-borne telescope with a permanent built-in eclipse, provided by a coronagraphic system. The expected improvement on experimental bounds to General Relativity and competing gravitation theories is by at least two orders of magnitude. The measurement principle is reviewed, in particular the principle of Fizeau-like combination of a set of individual inverted coronagraphs simultaneously feeding a common high resolution telescope. Also, the payload has a dual field of view property, in order to support simultaneous observations of stellar fields either very close, or far away, from the Sun, i.e. fields affected by either high or low light bending. We discuss a set of solutions introduced in the optical design to improve on technical feasibility and robustness of the optical performance against perturbations, in particular induced by manufacturing and alignment tolerances, and launch stresses.

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arXiv2021

The ASTRA project: a doorway to future astrometry

Astrometric Science and Technology Roadmap for Astrophysics (ASTRA) is a bilateral cooperation between China and Italy with the goal of consolidating astrometric measurement concepts and technologies. In particular, the objectives include critical analysis of the Gaia methodology and performance, as well as principle demonstration experiments aimed at future innovative astrometric applications requiring high precision over large angular separations (one to 180 degrees). Such measurement technologies will be the building blocks for future instrumentation focused on the "great questions" of modern cosmology, like General Relativity validity (including Dark Matter and Dark Energy behavior), formation and evolution of structure like proto-galaxies, and planetary systems formation in bio compatibles environments. We describe three principle demonstration tests designed to address some of the potential showstoppers for high astrometric precision experiments. The three tests are focused on the key concepts of multiple fields telescopes, astrometric metrology and very fine sub-pixel precision (goal: <1/2000 pixel) in white light.

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arXiv2019

The population of Galactic planetary nebulae: a study of distance scales and central stars based on the second GAIA release

We matched the astrometry of central stars (CSs) of spectroscopically-confirmed Galactic planetary nebulae (PNe) with DR2 Gaia parallaxes ($p$), finding 430 targets in common with $p>0$ and $|σ_{\rm p}/p|<1$. A catalog of PNe whose CSs have DR2 Gaia parallaxes is presented in Table 1. We compared DR2 parallaxes with those in the literature, finding a good correlation between the two samples. We used PNe parallaxes to calibrate the Galactic PN distance scale. Restricting the sample to objects with 20$\%$ parallax accuracy, we derive the distance scale ${\rm log}(R_{\rm pc})=-(0.226\pm0.0155)\times{\rm log}(S_{\rm Hβ})-(3.920\pm0.215)$, which represents a notable improvement with respect to previous ones. We found that the ionized mass vs. optical thickness distance scale for Galactic PNe is not as well constrained by the Gaia calibrators, but gives important insight on the nature of the PNe, and is essential to define the domain for our distance scale application. We placed the CSs whose distance has been determined directly by parallax on the HR diagram, and found that their location on the post-AGB H-burning evolutionary tracks is typical for post-AGB stars.

astro-ph.SR↗

arXiv2019

Differential Astrometric Framework for the Jupiter Relativistic Experiment with Gaia

We employ differential astrometric methods to establish a small field reference frame stable at the micro-arcsecond ($μ$as) level on short timescales using high-cadence simulated observations taken by Gaia in February 2017 of a bright star close to the limb of Jupiter, as part of the relativistic experiment on Jupiter's quadrupole. We achieve sub$μ$as-level precision along scan through a suitable transformation of the field angles into a small-field tangent plane and a least-squares fit over several overlapping frames for estimating the plate and geometric calibration parameters with tens of reference stars that lie within $\sim$0.5 degs from the target star, assuming perfect knowledge of stellar proper motions and parallaxes. Furthermore, we study the effects of unmodeled astrometric parameters on the residuals and find that proper motions have a stronger effect than unmodeled parallaxes. For e.g., unmodeled Gaia DR2 proper motions introduce extra residuals of $\sim$23$μ$as (AL) and 69$μ$as (AC) versus the $\sim$5$μ$as (AL) and 17$μ$as (AC) due to unmodeled parallaxes. On the other hand, assuming catalog errors in the proper motions such as those from Gaia DR2 has a minimal impact on the stability introducing sub$μ$as and $μ$as level residuals in the along and across scanning direction, respectively. Finally, the effect of a coarse knowledge in the satellite velocity components (with time dependent errors of 10$μ$as) is capable of enlarging the size of the residuals to roughly 0.2 mas.

astro-ph.IM↗

arXiv2018

The Global sphere reconstruction (GSR) - Demonstrating an independent implementation of the astrometric core solution for Gaia

Context. The Gaia ESA mission will estimate the astrometric and physical data of more than one billion objects, providing the largest and most precise catalog of absolute astrometry in the history of Astronomy. The core of this process, the so-called global sphere reconstruction, is represented by the reduction of a subset of these objects which will be used to define the celestial reference frame. As the Hipparcos mission showed, and as is inherent to all kinds of absolute measurements, possible errors in the data reduction can hardly be identified from the catalog, thus potentially introducing systematic errors in all derived work. Aims. Following up on the lessons learned from Hipparcos, our aim is thus to develop an independent sphere reconstruction method that contributes to guarantee the quality of the astrometric results without fully reproducing the main processing chain. Methods. Indeed, given the unfeasibility of a complete replica of the data reduction pipeline, an astrometric verification unit (AVU) was instituted by the Gaia Data Processing and Analysis Consortium (DPAC). One of its jobs is to implement and operate an independent global sphere reconstruction (GSR), parallel to the baseline one (AGIS, namely Astrometric Global Iterative Solution) but limited to the primary stars and for validation purposes, to compare the two results, and to report on any significant differences. Results. Tests performed on simulated data show that GSR is able to reproduce at the sub-$μ$as level the results of the AGIS demonstration run presented in Lindegren et al. (2012). Conclusions. Further development is ongoing to improve on the treatment of real data and on the software modules that compare the AGIS and GSR solutions to identify possible discrepancies above the tolerance level set by the accuracy of the Gaia catalog.

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arXiv2018

Identifying quasars with astrometric and mid-infrared methods from APOP and ALLWISE

Context. Quasars are spatially stationary, and they are essential objects in astrometry when defining reference frames. However, the census of quasars is far from complete. Mid-infared colors can be used to find quasar candidates because AGNs show a peculiar appearance in mid-infrared color, but these methods are incapable of separating quasars from AGNs. Aims. The aim of our study is to use astrometric and mid-infrared methods to select quasars and get a reliable quasar candidates catalog. Methods. We used a near-zero proper motion criterion in conjuction with WISE (all-sky Wide-field Infrared Survey Explorer) [W1-W2] color to select quasar candidates. The [W1-W2] color criterion is defined by the linear boundary of two samples: LAMOST DR5 quasars, which serve as the quasar sample, and LAMOST DR5 stars and galaxies, which serve as the non-quasar sample. The contamination and completeness are evaluated. Results. We present a catalog of 662 753 quasar candidates, with a completeness of about 75% and a reliability of 77.2%.

astro-ph.GA↗

arXiv2018

Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant

We present HST photometry of a selected sample of 50 long-period, low-extinction Milky Way Cepheids measured on the same WFC3 F555W, F814W, and F160W-band photometric system as extragalactic Cepheids in SN Ia hosts. These bright Cepheids were observed with the WFC3 spatial scanning mode in the optical and near-infrared to mitigate saturation and reduce pixel-to-pixel calibration errors to reach a mean photometric error of 5 millimags per observation. We use the new Gaia DR2 parallaxes and HST photometry to simultaneously constrain the cosmic distance scale and to measure the DR2 parallax zeropoint offset appropriate for Cepheids. We find a value for the zeropoint offset of -46 +/- 13 muas or +/- 6 muas for a fixed distance scale, higher than found from quasars, as expected, for these brighter and redder sources. The precision of the distance scale from DR2 has been reduced by a factor of 2.5 due to the need to independently determine the parallax offset. The best fit distance scale is 1.006 +/- 0.033, relative to the scale from Riess et al 2016 with H0=73.24 km/s/Mpc used to predict the parallaxes photometrically, and is inconsistent with the scale needed to match the Planck 2016 CMB data combined with LCDM at the 2.9 sigma confidence level (99.6%). At 96.5% confidence we find that the formal DR2 errors may be underestimated as indicated. We identify additional error associated with the use of augmented Cepheid samples utilizing ground-based photometry and discuss their likely origins. Including the DR2 parallaxes with all prior distance ladder data raises the current tension between the late and early Universe route to the Hubble constant to 3.8 sigma (99.99 %). With the final expected precision from Gaia, the sample of 50 Cepheids with HST photometry will limit to 0.5% the contribution of the first rung of the distance ladder to the uncertainty in the Hubble constant.

astro-ph.CO↗

arXiv2017

Application of Time Transfer Functions to Gaia's global astrometry - Validation on DPAC simulated Gaia-like observations

A key objective of the ESA Gaia satellite is the realization of a quasi-inertial reference frame at visual wavelengths by means of global astrometric techniques. This requires an accurate mathematical and numerical modeling of relativistic light propagation, as well as double-blind-like procedures for the internal validation of the results, before they are released to the scientific community at large. Aim of this work is to specialize the Time Transfer Functions (TTF) formalism to the case of the Gaia observer and prove its applicability to the task of Global Sphere Reconstruction (GSR), in anticipation of its inclusion in the GSR system, already featuring the suite of RAMOD models, as an additional semi-external validation of the forthcoming Gaia baseline astrometric solutions. We extend the current GSR framework and software infrastructure (GSR2) to include TTF relativistic observation equations compatible with Gaia's operations. We use simulated data generated by the Gaia Data Reduction and Analysis Consortium (DPAC) to obtain different least-squares estimations of the full stellar spheres and gauge results. These are compared to analogous solutions obtained with the current RAMOD model in GSR2 and to the catalog generated with GREM, the model baselined for Gaia and used to generate the DPAC synthetic data. Linearized least-squares TTF solutions are based on spheres of about 132,000 primary stars uniformly distributed on the sky and simulated observations spanning the entire 5-yr range of Gaia's nominal operational lifetime. The statistical properties of the results compare well with those of GREM. Finally, comparisons to RAMOD@GSR2 solutions confirmed the known lower accuracy of that model and allowed us to establish firm limits on the quality of the linearization point outside of which an iteration for non-linearity is required for its proper convergence.

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arXiv2017

The Gaia Data Release 1 parallaxes and the distance scale of Galactic planetary nebulae

In this paper we gauge the potentiality of Gaia in the distance scale calibration of planetary nebulae (PNe) by assessing the impact of DR1 parallaxes of central stars of Galactic PNe (CSPNe) against known physical relations. For selected PNe targets with state-of-the-art data on angular sizes and fluxes, we derive the distance-dependent parameters of the classical distance scales, i.e., physical radii and ionized masses, from DR1 parallaxes; we propagate the uncertainties in the estimated quantities and evaluate their statistical properties in the presence of large relative parallax errors; we populate the statistical distance scale diagrams with this sample and discuss its significance in light of existing data and current calibrations. We glean from DR1 parallaxes 8 CSPNe with S/N$>$1. We show that this set of potential calibrators doubles the number of extant trigonometric parallaxes (from HST and ground-based), and increases by two orders of magnitude the domain of physical parameters probed previously. We then use the combined sample of suitable trigonometric parallaxes to fit the physical-radius-to-surface-brightness relation. This distance scale calibration, although preliminary, appears solid on statistical grounds, and suggestive of new PNe physics. With the tenfold improvement in PNe number statistics and astrometric accuracy expected from future Gaia releases the new distance scale, already very intriguing, will be definitively constrained.

astro-ph.SR↗

arXiv2017

Tests of the Galactic planetary nebula distance scale with the initial Gaia parallax distances of their central stars

We used the "primary dataset" of Gaia Data Release 1 (DR1) to search for parallax measurements of central stars (CSs) of Galactic planetary nebulae (PNe), to determine PN distances. We found that a trigonometric parallax is available for 16 CSs, seven of which with relative uncertainty below 80%. The limited comparison of these trigonometric distances to other reliable individual determinations discloses good correlation between the two sets, with the Gaia parallax distances being lower by a factor of ~0.1 dex in the logarithmic distances. We tested with the Gaia parallaxes the most popular Galactic PN distance scales, namely, the physical radius vs. surface brightness, and the ionized mass vs. inverse optical thickness scales. While the number of available calibrators may still be too low, and their relative uncertainties too high, to derive a working distance scale, we were able to assess the current sample and to reveal the very promising potential of the future Gaia releases for a recalibration of the distance scale of Galactic PNe.

astro-ph.SR↗

arXiv2016

A test of Gaia Data Release 1 parallaxes: implications for the local distance scale

We present a comparison of Gaia Data Release 1 (DR1) parallaxes with photometric parallaxes for a sample of 212 Galactic Cepheids at a median distance of 2~kpc, and explore their implications on the distance scale and the local value of the Hubble constant H_0. The Cepheid distances are estimated from a recent calibration of the near-infrared Period-Luminosity P-L relation. The comparison is carried out in parallax space, where the DR1 parallax errors, with a median value of half the median parallax, are expected to be well-behaved. With the exception of one outlier, the DR1 parallaxes are in remarkably good global agreement with the predictions, and the published errors may be conservatively overestimated by about 20%. The parallaxes of 9 Cepheids brighter than G = 6 may be systematically underestimated, trigonometric parallaxes measured with the HST FGS for three of these objects confirm this trend. If interpreted as an independent calibration of the Cepheid luminosities and assumed to be otherwise free of systematic uncertainties, DR1 parallaxes would imply a decrease of 0.3% in the current estimate of the local Hubble constant, well within their statistical uncertainty, and corresponding to a value 2.5 sigma (3.5 sigma if the errors are scaled) higher than the value inferred from Planck CMB data used in conjunction with Lambda-CDM. We also test for a zeropoint error in Gaia parallaxes and find none to a precision of ~20 muas. We caution however that with this early release, the complete systematic properties of the measurements may not be fully understood at the statistical level of the Cepheid sample mean, a level an order of magnitude below the individual uncertainties. The early results from DR1 demonstrate again the enormous impact that the full mission will likely have on fundamental questions in astrophysics and cosmology.

astro-ph.IM↗