Essays on Gaia

The origin of many hypervelocity stars, due to binary star disruption by a supermassive black hole, now seems rather secure. But the small class of hypervelocity white dwarfs, discovered with Gaia, presents more of a challenge. A variety of mechanism, which drive supernova generation and binary disruption, are being evaluated.

In addition to the use of stellar streams to detect dark matter sub-halos of intermediate mass, recent studies have shown that the motions of these predicted sub-halos will leave a kinematic imprint on the velocity field of nearby disk stars. They may explain part of the well-known Gaia phase-space spiral, or `Gaia snail'.

Our Galaxy halo is believed to be highly clumpy, with large numbers of high-velocity dark-matter sub-halos orbiting within it. They act as gravitational `bullets' that can punch through the relatively static stellar streams now known to exist within it, resulting in gaps along the stream's length. Searches and characterisation of these gaps are ongoing.

Using a new search method, borrowed from the field of high-energy particle physics, another unsupervised machine-learning algorithm for anomaly detection has discovered many more stellar streams. Amongst them are two complex clusters of streams, each covering some 30-40 degrees on the sky. Their nature is currently unknown.

More than 100 stellar streams have been discovered in our Galaxy's halo, mostly with Gaia. They are remnants of globular clusters or dwarf galaxies captured by our own over its multi-billion year history. A new study, using an interesting `spray model', has found 34 streams associated with the 156 known globular clusters of the Milky Way.

An important feature of spiral galaxies is the form of the light profiles in their outer disks, and how this compares with cosmological predictions in view of radial migration driven by bars or transient spiral arms. Gaia is contributing to an understanding of these effects in our own Galaxy, with interesting implications for the stellar age profile beyond the solar circle.

The first hints of extended tidal tails associated with open clusters pre-dated Gaia. But their occurrence, extent, and complexity has only become evident with the Gaia data. With more than 50 now known within 500 pc, their shapes encode a wealth of information about the clusters and their Galactic environment.

Binary systems offer an important testing ground of the idea that some stars may have engulfed one of their orbiting planets. Specifically, if one component is unusually rich in refractory elements, this might point to a past planetary engulfment. Gaia is making a number of contributions to this debate.

For a handful of stars, current models of stellar structure and evolution provide age estimates which sit uncomfortably close to our estimate of the age of the Universe itself. Larger statistical samples constructed from the accurate and homogeneous Gaia data are providing a more robust `cosmic clock'.

The Galaxy disk is no longer thought of as simply `flat'. In the 1950s it was shown to be warped. Gaia has revealed other dynamical and morphological structures, including the `phase-space spiral' and the Radcliffe Wave. Recent studies have revealed a much vaster distortion, the Great Wave.

The first hints that open clusters are sometimes found in pairs emerged in the 1970s. Today, binary clusters provide insights into cluster formation, star formation, and tidal interactions. New discoveries and detailed kinematic studies with Gaia DR3 are consolidating this picture.

Stefan Jordan, a member of the Gaia DPAC collaboration at the University of Heidelberg, recently used NotebookLM to synthesise my entire collection of essays to provide an overview of Gaia science divided into five subject areas. I reproduce this synthesis here.

Identifying stars which have escaped from open clusters since their birth is important in clarifying many details of the underlying star formation and evolution processes. A recent study combines the Gaia data with stellar rotational data from TESS, to more securely identify young escaping members of the Pleiades out to larger distances than has been possible using the Gaia data alone.

A recent study using Gaia DR3 has provided improved radii for 2573 exoplanet host stars and their associated 3556 exoplanets. Amongst the scientific findings is the confirmation of the `radius valley', first identified in the Kepler data, and attributed to the host star's X-ray/UV radiation. This results in sub-Neptunes losing their H/He envelopes and emerging as rocky super-Earths.

Multi-epoch photometry can identify the rotation period and spin-axis direction of solar system bodies. A gap in the period-diameter diagram of asteroids reveals a concentration of short-period `tumblers', those rotating in a non-principal axis state. A recent Gaia study provides a detailed model of the underlying physics: how they got into a tumbling state, and their distribution in the period-diameter diagram.

Four recent studies have exploited the Gaia colour-magnitude diagrams to study our Galaxy's evolutionary history. These are focused on (1) the star-formation history of the solar neighbourhood; (2) the evolution of the thin and thick disk populations; (3) the age and metallicity of GSE stars near the Sun; and (4)the age distribution of stars of the inner Milky Way.

A puzzling feature of the known hypervelocity stars is their particular distribution on the sky. A recent Gaia DR3 study provides an explanation: such stars originate both from the supermassive black hole at our Galaxy's centre, as well as from a supermassive black hole in the LMC, whose observed population is boosted by the LMC's orbital motion.

Ultra-metal-poor stars are those considered to have the lowest metal abundances of all stars known today, [Fe/H] < -4. They are believed to be the direct descendants of the earliest (Population III) stars which came into existence after the Big Bang. They are rare, with only some 50 examples known. Gaia is contributing to their characterisation... and has discovered another!

Recent simulations have shown that the three interstellar objects that have been found travelling through our solar system may be part of prominent streams of protoplanetary debris lost by their host stellar system hundreds of millions of years ago. Escaping from their host's Oort-type cloud, they move on Galactic orbits similar to that of their parent star.

Over the past decade, three interstellar objects have been found travelling through our solar system. They are believed to have been formed and ejected from the protoplanetary disks of extrasolar planetary systems far from the Sun. I detail how Gaia is assisting in understanding the origin and dynamical evolution of these remarkable discoveries.

Asteroid masses are primarily determined from mutual orbit perturbations, which can exploit asteroid-asteroid, asteroid-planet, and even asteroid-spacecraft perturbations. I review the knowledge of asteroid masses pre-Gaia, and show how the various Gaia data releases are allowing improved mass estimates for many more of these solar system objects.

The existence of supermassive black holes is implicit in various scientific areas being impacted by Gaia, including tidal disruption events which result from the tidal disruption of a star as it passes close to a supermassive black hole. The recent discovery of the highest-known luminosity transient events points to the disruption of intermediate-mass stars onto particularly massive black holes.

Actinide-boost stars are a subset of stars enhanced with r-process elements. They display an over-abundance of actinide elements, characterised by the thorium/europium [Th/Eu] ratio. The availability of the Gaia DR3 distances and proper motions is providing some remarkable new insights into the physical mechanisms underlying the r-process enhancements, and the origin of the actinide-boost stars.

A new determination of astrophysical parameters from a recalibration of the 220 million BP/RP spectra made available in Data Release 3 has extended the training set to emphasise pre-main sequence stars. An analysis of the resulting sky distribution of young stars has identified a new young association, Ophion, which appears to be in the process of its rapid and complete disruption.

The Beta Cephei variables are the most massive pulsating stars on the main sequence, and are an important class for asteroseismology investigations. Recent studies have shown that the sparsely sampled Gaia photometry is nevertheless exceptionally accurate for detecting their dominant and secondary frequencies. I look at how these studies are furthering our understanding of their fundamental properties, and their detailed physics.

Recent studies of interstellar extinction, based on Gaia DR3 distances and low-resolution BP/RP spectra, have provided a substantial advance in mapping the total-to-selective extinction for 130 million lines of sight throughout our Galaxy. Their analysis throws new light on the processes of accretion and coagulation that are responsible for the growth of dust grains in the interstellar medium.

The occurrence of multiple stellar populations in globular clusters has been described as one of the most puzzling open issues of stellar astrophysics. I describe the problem, and demonstrate how the combination of Gaia's astrometry, and the synthetic photometry derived from its low-resolution BP/RP spectra, is helping to identify its origin.

The class of ultra-faint dwarf galaxies, the first of which were discovered only in 2005, represent the faintest, oldest, most metal-poor, and most dark-matter-dominated end of the dwarf galaxy spectrum. Providing important tests of LambdaCDM cosmology, Gaia is helping to refine their membership, clarify their morphologies (including evidence for tidal disruption), and define their orbits and orbital histories.

Today, NASA's Exoplanet Archive lists almost 6000 confirmed exoplanets. There are only five astrometric discoveries amongst them, with one found from VLT-FORS, and one from radio VLBA. The first Gaia astrometric discovery, HIP 66074, was reported in 2023. Here I describe the two latest discoveries, Gaia 4b and Gaia 5b. And I explain why the numbers discovered with Gaia are still rather small.

Delta Scuti variables are a common class of pulsating star located at the intersection of the instability strip with the main sequence, showing a rich variety of pulsation behaviour. Gaia-enabled studies are being used to characterise their period-luminosity relation as a function of pulsation mode and metallicity. And a long-standing question being advanced by Gaia is why many of the stars in the Delta Scuti instability strip do not pulsate.

I describe the latest Gaia-based studies of the classical dwarf spheroidal galaxies of the Local Group. Gaia DR3 is being used to derive improved orbits and orbital histories, and these are shown to link to associated star formation bursts at they approach pericentre. There are new studies of their structure, internal kinematics, and bulk rotation, and of their associated globular cluster systems.

The Oosterhoff dichotomy is a long-standing problem in the understanding of Galactic globular clusters. It refers to the fact that clusters divide into two distinct groups according to the average period of their RR Lyrae stars pulsating in the fundamental mode. I outline the problem, and show how Gaia is advancing our knowledge, relating it to the ancient merger debris in our Galaxy's halo.

I described Gaia's early contributions to the study of RR Lyrae stars in an earlier essay at the end of 2021. Here I bring the topic up-to-date with the results from DR3. Based on its 270,000 classified objects, applications include further constraints on the period-luminosity relation, various studies of Galactic structure, and improved inputs to stellar evolutionary models.

I described Gaia's early contributions to the study of Cepheids in an earlier essay at the end of 2021. Here I bring the topic up-to-date with the results from DR3. Based on its 15,000 classified Cepheids, important applications have been improved assignment to their membership of open clusters, updates of the period-luminosity relation, and various studies of Galactic structure.

I am in the process of converting my written Gaia essays into audio format using the powerful technology of generative AI. I am using Google's NotebookLM to create an audio discussion-type dialogue between two avatars. The results, placed on my YouTube channel (www.youtube.com/@GaiaEssays) are comprehensive, accurate, didactic, and engaging.

A remarkable subset of the binary star ellipsoidal variables are the so-called heartbeat stars, named in view the resemblance of their light curves to an electrocardiogram. I outline the discovery journey, the physics underlying the phenomenon, and the role that Gaia can play in their understanding.

Distances can be determined to Local Group disk galaxies using the `rotational parallax'. For a system in uniform bulk rotation, equating the rotation amplitude from proper motions to that from radial velocities fixes the distance. I describe the cosmological relevance, and the current status for M31 and M33.

A curious and unexpected discovery are the thousands of filamentary stellar structures in the solar neighbourhood, referred to as strings (or snakes), and multiple orbitally coherent clusters referred to as pearls. The strings appear to be coherent in terms of their spatial and kinematic distribution, and age.

Gaia results support a hierarchical picture of star formation, in which stars are formed across a continuous density distribution throughout molecular clouds, and in which OB associations are formed in situ as relatively large-scale and gravitationally-unbound structures.

The substantial improvement in the definition of the colour–magnitude diagrams of nearby open clusters is leading to improved definition of their main sequence, the detailed structure of the extended turnoff region, the origin of blue stragglers, and the evolutionary history of white dwarfs.

Gaia's improved membership and astrometry of open clusters is providing detailed insights into their Galactic orbits, their bulk rotation and expansion, the tidal tails formed by their escaping members, and their binary population evolution and mass segregation.

The ages of open cluster are generally obtained by comparing their colour–magnitude diagrams to theoretical isochrones. I look at our present understanding of their ages, and how other dating methods (based on lithium depletion, gyrochronology, dynamical traceback, and asteroseismology) compare.

Of the 3000 open clusters catalogued pre-Gaia, nearly two thirds have now been shown to be chance overdensities, while Gaia's census today lists around 14,000. I describe how open clusters are distributed throughout our Galaxy, how they relate to the spiral arms, and the interesting class of binary clusters.

The biggest stars have radii some 1500 times that of the Sun. Although extremely luminous, their detection poses a number of challenges, and their large distances make them difficult to characterise. I explain their importance for understanding stellar evolution, and the ways in which Gaia is contributing.

I explain how a global offset of the Gaia parallaxes arises. And I describe the investigations carried out to identify and characterise the nature of such a `zero-point' offset, both for Gaia DR2 and (E)DR3. In practice, the offset is a non-trivial function of magnitude, colour, and sky coordinate.

Compared to the few hundred stellar diameters directly measured with an accuracy of 1-2 percent, Gaia's distances and stellar parameters are providing diameters, of somewhat lower accuracy, for some 500 million stars. One consequence is for the improved radius estimates for several thousand transiting exoplanets.

I review Gaia's contribution to supermassive and intermediate-mass black holes, and focus on the latest discoveries of black holes in wide, non-interacting binary systems, through the astrometric motion of their companion star. The three systems Gaia~BH1, BH2, and BH3 provide some deep insights into their formation and evolution.

I continue the theme of my previous essay, and look how Gaia is contributing to the calibration and pointing of the Hubble and James Webb Space Telescopes, and ESA's satellite missions operated at ESOC. I look also at just a few examples from ground-based telescopes, including SAAO, SDSS, and WHT.

Gaia is making substantial contributions to the improved calibration and pointing of ground- and space-based telescopes. Here, I look first at what is now largely consigned to history - how the first space astrometry positions from Hipparcos were applied to the calibration of photographic plates and meridian circles.

Gaia has recently identified 55 massive runaway stars in the young cluster R136 in the LMC, increasing the known number by an order of magnitude. They fall into two groups, an isotropic sample ejected soon after the cluster's birth, and 16 ejected in a preferentially northern direction. I discuss the interpretation, and the cosmological significance.