arXiv subjects

Eva Villaver

Publications and source records attributed to Eva Villaver.

At least 19 recordsLinked to original sources

arXiv2026

Joint Observations of PTPS Targets (JOTA). Combined spectroscopic and photometric analysis of 16 SB1 systems

Context. The Pennsylvania-Toru\'n Planet Search, which operates on a sample of $\sim$1000 northern-hemisphere stars, has been underway since 2004. Stars with radial velocity (RV) amplitudes exceeding 2 km s$^{-1}$ represent a backup subprogram sample dedicated to monitoring binary stars with multiple instruments. Aims. We used almost 21 years of combined RV measurements to search for Doppler signals consistent with stellar or brown dwarf companions and to produce a catalog of both known and previously unpublished binary stars in our planet-search sample. Methods. We analyzed the combined RV measurements, searching for stellar companions and obtaining orbital solutions for both known and new binary systems. We also searched for periods in available long-term photometric monitoring data from All Sky Automated Survey (ASAS), and applied Gaia astrometric data whenever available. Results. We report the results of long-term RV monitoring of 16 systems: new detections of low-mass companions to 11 stars and updated orbital elements based on combined sets of our new and literature data for five systems. For two objects (TYC 3318-00789-1 and TYC 3318-01538-1), we find evidence of activity or the influence of another unseen companion, possibly due to line-profile variations or unresolved spectral contamination. We present true masses for three systems in our sample: TYC 1931-1040-1, TYC 3451- 1449-1, and TYC 3667-1636-1. We obtained these masses using Gaia DR3 non-single star data.

astro-ph.SR↗

arXiv2026

A&A community survey on the future of scientific publishing: Credibility over speed, fairness over profit, human judgment over automation

(Abridged) Scientific publishing is undergoing major change, driven by a shift toward open access (OA), the rise of artificial intelligence (AI), and growing demands for transparency, reproducibility, and equity. At the same time, rapid growth in article output strains editors and reviewers and means that metrics and speed can eclipse quality and rigor. To better understand how the community is responding, Astronomy \& Astrophysics (A\&A) commissioned the {A\&A Survey on Trends and Challenges in Scientific Publishing}, which documents community opinion on journal choice, peer review, OA, research evaluation, and the role of AI, with the goal of informing future editorial policies and the wider conversation on sustainable, ethical, and equitable scientific communication. Distributed online in May 2025 to \SI{28787} A\&A authors and co-authors, the survey drew \SI{2944} responses from 69 countries by its closing date. The responses were clear. Journal quality and reputation are the most decisive factors in deciding where to publish, followed by cost. The principal worry about peer review is reviewer expertise and fairness rather than speed. Citation counts are still an important consideration, but many respondents want broader, more qualitative measures of impact. The majority prefers public or institutional funding for OA, and views on AI are polarized, with widespread acceptance of administrative and language assistance but firm opposition to autonomous decision-making or content generation. Integrity, credibility, and fairness are common themes in every section of the responses. Overall, the survey portrays an engaged community that values quality over speed, fairness over profit, and human oversight over automation, providing A\&A with clear insight into community preference and a solid framework for shaping future policies on OA, peer review, and the responsible integration of AI.

cs.DL↗

arXiv2026

An Ultra-Short Period Super-Earth and a Sub-Neptune Orbiting the K dwarf TOI-4311

We report the discovery and characterisation of the multi-planetary system around TOI-4311, a K dwarf kinematically between the Galactic thick disk and Hercules stream. TOI-4311 hosts an ultra-short-period super-Earth (P$\sim$0.99 d, $1.376\substack{+0.077\\-0.080}$ R$_\oplus$) and a longer period sub-Neptune (P$\sim$15 d, $2.47\substack{+0.12\\-0.11}$ R$_\oplus$) that was first detected in the TESS photometry. Using follow-up observations with CHEOPS and HARPS, we refine the planetary radius of both planets, derive the mass of planet b ($4.5\substack{+1.5\\-1.4}$ M$_\oplus$) and confirm the planetary nature of planet c. Intriguingly, a third periodic signal is clearly detected in our HARPS RVs that we cannot link to stellar activity. This signal could be attributed to a third planet (P$\sim$38 d, Msin(i)=$26.4\substack{+6.3\\-6.8}$ M$_\oplus$) in the system, however with the current photometric dataset we do not find a transit. Our dynamical analysis highlights that this potential outer planet would remain stable. Using the precise radius and mass for TOI-4311 b we model its interior structure and find that it is very dense given the host star's galactic kinematics and chemistry. Hence this system could challenge current formation theories and provide insights into planet formation across the galaxy.

astro-ph.EP↗

arXiv2026

A homogeneous three-dimensional view of Molecular Cloud kinematics out to 2.5 kpc. Using Young Stellar Objects and Open Clusters as complementary tracers

Understanding the large-scale dynamics of molecular clouds (MCs) is crucial for constraining the processes that govern star formation and the structure and evolution of the Galaxy. While gas tracers have traditionally been used to map MC kinematics, stellar tracers such as young stellar objects (YSOs) and open clusters (OCs) provide a complementary approach that enables direct comparisons between the stellar and gaseous components. We aim to validate OCs as complementary tracers by testing whether they retain the same bulk kinematic imprint as YSOs, and to reconstruct the three-dimensional (3D) motions of the main MC complexes within 2.5 kpc of the Sun using YSOs and young OCs as tracers. Using Gaia DR3 astrometry together with complementary spectroscopic surveys for radial velocities, we compiled a unified sample of 24,732 stellar tracers. We applied robust clustering in proper motion space to identify co-moving YSOs and derived cloud-averaged motions via Monte Carlo sampling. These were compared with the kinematics of OCs younger than 30 Myr. Finally, we performed orbital integrations in a realistic Galactic potential to trace the past evolution of the clouds and quantify their expansion and rotation. We derive homogeneous 3D kinematics for 15 MC complexes within 2.5 kpc. YSOs and OCs exhibit strongly consistent kinematics, with a median spatial velocity offset of $\simeq 2$ km s$^{-1}$, confirming that both populations trace the bulk motion of their parent clouds. The resulting cloud kinematics show a median peculiar velocity of $\simeq 8.7$ km s$^{-1}$ with respect to Galactic rotation. We trace back the Solar System's voyage through the Orion cloud and the common origin of Lupus, Ophiuchus, and Corona Australis in Sco-Cen. Internally, we detect significant expansion in Orion and Ophiuchus ($5\sigma$) and coherent rotation in at least seven complexes.

astro-ph.GA↗

arXiv2025

Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: A Low Surface Brightness Science Case

The Extragalactic Low Surface Brightness (LSB, $\mu_V\gtrsim 27$ mag/arcsec$^2$) Universe represents a crucial, yet largely unseen, frontier in modern astrophysics. This faint realm holds the keys to completing our understanding of galaxy evolution, hierarchical assembly, and even the fundamental nature of dark matter. Our current theoretical models are inherently incomplete, largely mirroring the properties of the brightest, most easily observed objects. To overcome this critical bias and unlock the secrets of this realm, a transformative leap in observational capability is required. A 30 to 40m class telescope, leveraging unprecedented sensitivity and spatial resolution, especially with adaptive optics, is the essential tool to fundamentally probe these faint, low-density stellar regimes. This white paper details the transformative LSB science that such a facility, strategically positioned in the Northern Hemisphere (NH) to access crucial nearby structures and rich environments, can achieve.

astro-ph.IM↗

arXiv2025

Planetary-mass companions to a retired B star BD+37 3172 and a retired F star BD+42 2315

Radial velocity searches may lead to detection of exoplanets at large orbital separations only if long-enough time-series of data are available. Therefore publication of precise measurements collected in the past is very valuable even if not successfully completed with a definitive detection. Here we present 309 precise ($\sigma$RV$\approx$5-7 m s$^{-1}$) multi-epoch radial velocities for 28 stars observed with the Hobby-Eberly Telescope and its High Resolution Spectrograph between 2004 and 2013. Based on the observations gathered we present a low mass companion ($m_{p}\sin{i}$ = 10.6 $M_{J}$ in 1887.76 $\pm$ 0.01 d, 4.65 au orbit with $e$ = 0.59 $\pm$ 0.01) to a K giant BD+37 3172 ($M$=3.75$\pm$ 0.86 M$_{\odot}$), and a planetary mass companion (m$_{p}\sin{i}$=0.55 M$_{J}$ in 123.05$\pm$0.04 day, 0.55 au orbit with e=0.73$\pm$0.03) to a K giant BD+42 2315 ($M$=1.38$\pm$ 0.30 M$_{\odot}$). We also present two preliminary detections of new spectroscopic binaries: BD+56 2957 (K5) and HD 236555 (G5).

astro-ph.SR↗

arXiv2025

An Ultra-Short Period Super-Earth and Sub-Neptune Spanning the Radius Valley Orbiting the Kinematic Thick Disk Star TOI-2345

A crucial chemical link between stars and their orbiting exoplanets is thought to exist. If universal, this connection could affect the formation and evolution of all planets. Therefore, this potential vital link needs testing by characterising exoplanets around chemically-diverse stars. We present the discovery of two planets orbiting the metal-poor, kinematic thick-disk K-dwarf TOI-2345. TOI-2345 b is a super-Earth with a period of 1.05 days and TOI-2345 c is a sub-Neptune with a period of 21 days. In addition to the target being observed in 4 TESS sectors, we obtained 5 CHEOPS visits and 26 radial velocities from HARPS. By conducting a joint analysis of all the data, we find TOI-2345 b to have a radius of $1.504\substack{+0.047\\-0.044}$ R$_\oplus$ and a mass of $3.49\pm0.85$ M$_\oplus$; and TOI-2345 c to have a radius of $2.451\substack{+0.045\\-0.046}$ R$_\oplus$ and a mass of $7.27\substack{+2.27\\-2.45}$ M$_\oplus$. To explore chemical links between these planets and their host star, we model their interior structures newly accounting for devolatised stellar abundances. TOI-2345 adds to the limited sample of well characterised planetary systems around thick disk stars. This system challenges theories of formation and populations of planets around thick disk stars with its Ultra-Short Period super-Earth and the wide period distribution of these two planets spanning the radius valley.

astro-ph.EP↗

arXiv2025

A Kinematic History of Stellar Encounters with Beta Pictoris

Beta Pictoris is an A-type star hosting a complex planetary system with two massive gas giants and a prominent debris disk. Variable absorption lines in its stellar spectrum have been interpreted as signatures of exocomets (comet-like bodies transiting the star). Stellar flybys can gravitationally perturb objects in the outer comet reservoir, altering their orbits and potentially injecting them into the inner system, thereby triggering exocomet showers. We aim to assess the contribution of stellar flybys to the observed exocomet activity by reconstructing the stellar encounter history of beta Pictoris in the past and future. We used Gaia DR3 data, supplemented with radial velocities from complementary spectroscopic surveys, to compile a catalogue of stars currently within 80 pc of beta Pictoris. Their orbits were integrated backward and forward in time in an axisymmetric Galactic potential (Gala package) to identify encounters within 2 pc of the system. We identified 99 416 stars within 80 pc of beta Pictoris at present with resolved kinematics. Among these, 49 stars (including the eight components of five binaries) encounter beta Pictoris within 2 pc between -1.5 Myr and +2 Myr. For four of the binaries, the centre-of-mass trajectories also pass within 2 pc. We estimate the sample to be more than 60 % complete within 0.5 Myr of the present. Despite beta Pictoris being the eponym of its famous moving group, none of the identified encounters involved its moving group members; all are unrelated field stars. We find no encounter capable of shaping observed disc structures, although stellar flybys may contribute to the long-term evolution of a potential Oort Cloud. Our catalogue constitutes the most complete reconstruction of the beta Pictoris encounter history to date and provides a robust foundation for future dynamical simulations.

astro-ph.SR↗

arXiv2025

Low-mass companions to nine stars

We present an independent spectroscopic and radial velocity analysis for nine stars from the Pennsylvania-Toru\'n Planet Search. For BD+24 4697, we present an updated true companion's mass (0.16$\pm$0.02 \, M$_{\odot}$), as well as evidence of stellar activity. For BD+54 1640 and BD+65 1241 we present true masses of companions, $m = 0.15 \pm 0.04\,M_\odot$ and $m = 0.091 \pm 0.005\,M_\odot$, respectively. For BD+63 974 and BD+69 935 we find low mass companions with $m \sin i = 0.046 \pm 0.001\,M_\odot$ and $m \sin i = 0.090 \pm 0.005\,M_\odot$. For BD+52 1281, BD+54 1382, TYC 2704-2680-1, and TYC 3525-02043-1 we present evidence of low-mass companions with $m \sin i$ of 0.115 $\pm 0.006\,M_\odot$, 0.083 $\pm 0.007\,M_\odot$, 0.279 $\pm 0.009\,M_\odot$, and $0.064 \pm 0.006\,M_\odot$, respectively. Consequently, BD+54 1382, BD+63 974, BD+65 1241, BD+69 935 and TYC 3525-02043-1 appear to be Brown Dwarf host candidates.

astro-ph.SR↗

arXiv2025

Finding White Dwarfs' Hidden Companions using an Unsupervised Machine Learning Technique

White dwarfs (WD) with main-sequence (MS) companions are crucial probes of stellar evolution. However, due to the significant difference in their luminosities, the WD is often outshined by the MS star. The aim of this work is to find hidden companions in Gaia's sample of WD candidates. Our methodology involves applying an unsupervised machine learning algorithm for dimensionality reduction and clustering, known as Self-Organizing Map (SOM), to Gaia BP/RP (XP) spectra. This strategy allows us to naturally separate WDMS binaries from single WDs from the detection of subtle red flux excesses in the XP spectra that are indicative of low-mass MS companions. We validate our approach using confirmed WDMS binaries from the SDSS and LAMOST surveys, achieving a precision of $\sim 90\%$. We demonstrated that the luminosity of the faint companions in the missed systems is $\sim 50$ times lower than that of their WD primaries. Applying our SOM to 90,667 sources, we identify 993 WDMS candidates, 506 of which have not been previously reported in the literature. If confirmed, our sample will increase the known WDMS binaries by $20\%$. Additionally, we use the Virtual Observatory Spectral Energy Distribution Analyzer (VOSA) tool to refine and parameterize a ``golden sample'' of 136 WDMS binaries through multi-wavelength photometry and a two-body Spectral Energy Distribution fitting. These high-confidence WDMS binaries are composed by low-mass WDs ($\sim 0.42 M_{\odot}$), with cool MS companions ($\sim 2800$ K). Finally, 13 systems exhibit periodic variability consistent with eclipsing binaries, making them prime targets for further follow-up observations.

astro-ph.SR↗

arXiv2024

In-situ observations of resident space objects with the CHEOPS space telescope

The CHaracterising ExOPlanet Satellite (CHEOPS) is a partnership between the European Space Agency and Switzerland with important contributions by 10 additional ESA member States. It is the first S-class mission in the ESA Science Programme. CHEOPS has been flying on a Sun-synchronous low Earth orbit since December 2019, collecting millions of short-exposure images in the visible domain to study exoplanet properties. A small yet increasing fraction of CHEOPS images show linear trails caused by resident space objects crossing the instrument field of view. To characterize the population of satellites and orbital debris observed by CHEOPS, all and every science images acquired over the past 3 years have been scanned with a Hough transform algorithm to identify the characteristic linear features that these objects cause on the images. Thousands of trails have been detected. This statistically significant sample shows interesting trends and features such as an increased occurrence rate over the past years as well as the fingerprint of the Starlink constellation. The cross-matching of individual trails with catalogued objects is underway as we aim to measure their distance at the time of observation and deduce the apparent magnitude of the detected objects. As space agencies and private companies are developing new space-based surveillance and tracking activities to catalogue and characterize the distribution of small debris, the CHEOPS experience is timely and relevant. With the first CHEOPS mission extension currently running until the end of 2026, and a possible second extension until the end of 2029, the longer time coverage will make our dataset even more valuable to the community, especially for characterizing objects with recurrent crossings.

astro-ph.EP↗

arXiv2024

Identifying new high-confidence polluted white dwarf candidates using Gaia XP spectra and Self-Organizing Maps

The identification of new white dwarfs (WDs) polluted with heavy elements is important since they provide a valuable tool for inferring chemical properties of putative planetary systems accreting material on their surfaces. The Gaia space mission has provided us with an unprecedented amount of astrometric, photometric, and low resolution (XP) spectroscopic data for millions of newly discovered stellar sources, among them thousands of WDs. In order to find WDs among this data and to identify which ones have metals in their atmospheres, we propose a methodology based on an unsupervised artificial intelligence technique called Self-Organizing Maps (SOM). In our approach a nonlinear high-dimensional dataset is projected on a 2D grid map where similar elements fall into the same neuron. By applying this method, we obtained a clean sample of 66,337 WDs. We performed an automatic spectral classification analysis to them, obtaining 143 bona fide polluted WD candidates not previously classified in the literature. The majority of them are cool WDs and we identify in their XP spectra several metallic lines such as Ca, Mg, Na, Li, and K. The fact that we obtain similar precision metrics than those achieved with recent supervised techniques highlights the power of our unsupervised approach to mine the Gaia archives for hidden treasures to follow-up spectroscopically with higher resolution.

astro-ph.SR↗

arXiv2024

A search for Galactic post-asymptotic giant branch stars in Gaia DR3

Context. When low and intermediate-mass stars leave the Asymptotic Giant Branch (AGB) phase, and before they reach the Planetary Nebulae stage, they enter a very brief and rather puzzling stellar evolutionary stage named post-AGB. Aims. To provide a reliable catalogue of galactic post-AGB stars together with their physical and evolutionary properties obtained through Gaia DR3 astrometry and photometry. Methods. We started by identifying post-AGB stars or possible candidates from the bibliography with their Gaia DR3 counterpart sources. Using the available photometry, interstellar extinction, literature spectroscopically derived temperatures or spectral types and parallax-derived distances from Gaia DR3, we fitted their Spectral Energy Distributions and we estimated their luminosities and circumstellar extinctions. When compared to models, luminosity values allowed us to disclose objects that are likely post-AGB stars from other target types. Their position on the HR diagram allows direct comparison with updated post-AGB evolutionary tracks and an estimation of their masses and evolutionary ages. Results. We obtained a sample of 69 reliable post-AGB candidates that meet our classification criteria, providing their coordinates, distances, effective temperature, total extinction, luminosity, mass, and evolutionary age. In addition, similar data for other stellar objects in our initial compilation, such as supergiant stars or young stellar objects, is provided. Conclusions. We have filtered out the data that have the best precision in parallaxes and distances to obtain more accurate luminosities, which allows us to classify with confidence the objects of the sample among different stellar phases. This allows us to provide a small but reliable sample of post-AGB objects. Derived mean evolutionary time and average mass values are in agreement with theoretical expectations.

astro-ph.SR↗

arXiv2023

Planet formation around Intermediate-mass stars I: Different disc evolutionary pathways as a function of stellar mass

The study of protoplanetary disc evolution and planet formation has mainly concentrated on solar (and low) mass stars since they host the majority of the confirmed exoplanets. Nevertheless, the numerous planets found orbiting stars up to $\sim3M_\odot$ has sparked interest in understanding how they form and how their hosting discs evolve. Our goal is to improve our knowledge on the gas disc evolution around intermediate mass stars for future planet formation studies. We study the long-term evolution of protoplanetary discs affected by viscous accretion, X-ray and FUV photoevaporation from the central star around stars between $1 - 3M_\odot$ considering the effects of stellar evolution. We explore different values of the viscosity parameter and the initial mass of the disc. We find that the evolutionary pathway of disc dispersal depends on the stellar mass. Our simulations reveal four distinct evolutionary pathways for the gas disc not reported before that are a consequence of stellar evolution, and which will likely impact dust evolution and planet formation. As the stellar mass grows from 1 to $\sim2M_\odot$, the disc evolution changes from the conventional inside-out clearing to a homogeneous disc evolution scenario where both inner and outer discs, formed after photoevaporation opened a gap, vanish over a similar timescale. As the stellar mass continues to increase, reaching $\sim 3M_\odot$, we have identified a distinct pathway that we refer to as revenant disc evolution, where the inner and outer discs reconnect after the gap opened. For the largest masses, we observe outside-in disc dispersal, in which the outer disc dissipates first due to the strong FUV photoevaporation. Revenant disc evolution stands out as it is capable of extending the disc lifespan. Otherwise, the disc dispersal time scale decreases with increasing stellar mass except for low viscosity discs.

astro-ph.EP↗

arXiv2023

On the computation of interstellar extinction in photoionized nebulae

Ueta & Otsuka (2021) proposed a method, named as the "Proper Plasma Analysis Practice", to analyze spectroscopic data of ionized nebulae. The method is based on a coherent and simultaneous determination of the reddening correction and physical conditions in the nebulae. The same authors (Ueta & Otsuka 2022, UO22) reanalyzed the results of Galera-Rosillo et al. (2022, GR22) on nine of the brightest planetary nebulae in M31. They claim that, if standard values of the physical conditions are used to compute the extinction instead of their proposed method, extinction correction is underestimated by more than 50% and hence, ionic and elemental abundance determinations, especially the N/O ratio, are incorrect. Several tests were performed to assess the accuracy of the results of GR22, when determining: i) the extinction coefficient, ii) the electron temperature and density, and iii) the ionic abundances. In the latter case, N+ /H+ ionic abundance was recalculated using both H_alpha and H_beta as the reference H I emissivity. The analysis shows that the errors introduced by adopting standard values of the plasma conditions by GR22 are small, within their quoted uncertainties. On the other hand, the interstellar extinction in UO22 is found to be overestimated for five of the nine nebulae considered. This propagates into their analysis of the properties of the nebulae and their progenitors. The python notebook used to generate all the results presented in this paper are of public access on a Github repository. The results from GR22 are proven valid and the conclusions of the paper hold firmly. Although the PPAP is, in principle, a recommended practice, we insist that it is equally important to critically assess which H I lines are to be included in the determination of the interstellar extinction coefficient, and to assert that physical results are obtained for the undereddened line ratios.

astro-ph.GA↗

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

Evidence for the disruption of a planetary system during the formation of the Helix Nebula

The persistence of planetary systems after their host stars evolve into their post-main sequence phase is poorly constrained by observations. Many young white dwarf systems exhibit infrared excess emission and/or spectral absorption lines associated with a reservoir of dust (or planetesimals) and its accretion. However, most white dwarfs are too cool to sufficiently heat any circumstellar dust to detectable levels of emission. The Helix Nebula (NGC 7293) is a young, nearby planetary nebula; observations at mid- and far-infrared wavelengths revealed excess emission associated with its central white dwarf (WD 2226-210). The origin of this excess is ambiguous. It could be a remnant planetesimal belt, a cloud of comets, or the remnants of material shed during the post-asymptotic giant branch phase. Here we combine infrared (SOFIA, Spitzer, Herschel ) and millimetre (ALMA) observations of the system to determine the origin of this excess using multi-wavelength imaging and radiative transfer modelling. We find the data are incompatible with a compact remnant planetesimal belt or post-asymptotic giant branch disc, and conclude the dust most likely originates from deposition by a cometary cloud. The measured dust mass, and lifetime of the constituent grains, implies disruption of several thousand Hale-Bopp equivalent comets per year to fuel the observed excess emission around the Helix Nebula's white dwarf.

astro-ph.EP↗

arXiv2022

Theia : science cases and mission profiles for high precision astrometry in the future

High-precision astrometry well beyond the capacities of Gaia will provide a unique way to achieve astrophysical breakthroughs, in particular on the nature of dark matter, and a complete survey of nearby habitable exoplanets. In this contribution, we present the scientific cases that require a flexibly-pointing instrument capable of high astrometric accuracy and we review the best mission profiles that can achieve such observations with the current space technology as well as within the boundary conditions defined by space agencies. We also describe the way the differential astrometric measurement is made using reference stars within the field. We show that the ultimate accuracy can be met without drastic constrains on the telescope stability.

astro-ph.IM↗