Essays on Gaia

The large number of quasars measured by Gaia contributes fundamentally to the determination of the quasi-inertial reference frame. Today, Gaia's quasar survey is also being applied to two topical observational questions in Lambda CDM cosmology: the kinematic dipole anomaly, and the S8 tension.

In this last of five essays on ESA's October 2023 'Focused Product Release' topics, I look at a new search for strongly lensed quasars in the data interval used for Data Release 3. Exploiting Gaia's 0.18 arcsec angular resolution, this new analysis found 450 previously known systems but with 86 newly detected components, and 381 new lensed candidates.

In this fourth of five essays on ESA's October 2023 'Focused Product Release' topics, I look at improved modelling of the two diffuse interstellar bands (DIBs) present within the wavelength range of Gaia's Radial Velocity Spectrometer instrument (845-872 nm). Six million RVS spectra have been used to map their spatial distribution out to 4000 parsec.

In this third of five essays on ESA's October 2023 'Focused Product Release' topics, I look at the significant improvements in orbit determination for the 157,000 asteroids provided in DR3, but now exploiting the 66-month time interval that will form the basis of Data Release 4 in 2025. This is mainly due to the observations now extending beyond a typical orbital period.

In this second of five essays on ESA's October 2023 'Focused Product Release' topics, I look at the application of the newly available radial velocity time-series measurements (from Gaia itself) to the class of long-period variables. The time series data provide a powerful complement to the epoch photometry in identifying and characterising LPVs.

In essay 40 I described the first Gaia-based estimates of the distance to the globular cluster Omega Cen. Gaia DR3 contained 321,698 cluster sources. A series of special observations, and part of ESA's October 2023 'Focused Product Release', results in a further 526,587 sources in its core region. I also summarise other results on Omega Cen to date.

A growing number of accreted stellar 'streams' are being identified in our Galaxy's halo, from 20 in 2016 to nearly a hundred today, now mostly coming from Gaia. Some are attributed to captured dwarf galaxies, others to disrupted globular clusters. I described some of the early discoveries in essays 15 and 71, and bring the subject more up-to-date here.

Ten years on from the launch of Gaia, on 19 December 2013, I provide a picture of the tasks involved in the operations of the Gaia satellite. I will start with a brief background to some of the top-level requirements that influenced the operational design and implementation, and continue with a first-hand account by the Gaia spacecraft operations manager, David Milligan.

In essay 4 (25 January 2021), I gave a historical perspective on the early days of the Hipparcos mission and the 'push to space'. Here, I resume this historical and somewhat personal account describing the journey from the early concepts of Hipparcos to its launch in 1989. I describe how the various teams were organised, and some of the challenges that we faced.

Gaia represents a major advance in understanding our Galaxy's disk and halo kinematics. Here, I will look at the specific problem of determining the 'solar motion'. I explain what it is, why it is important, and how it is determined. As well as the more classical approach, methods are being developed to measure all three components with respect to the halo.

In essay 42, I described some new features in the Hertzsprung-Russell diagram, including a narrow gap in the M dwarf sequence, first reported in the DR2 data. Here I look at further details of this interesting feature. As Baraffe & Chabrier wrote in 2018: "Just a small gap in a colour-magnitude diagram could provide a deep insight into the interior structure of low-mass stars."

The Hyades is the nearest open cluster. Even so, distance uncertainties have limited the definition of its main sequence, and hence its ability to constrain evolutionary models. It has nevertheless been used as the basic observational material for various fundamental relationships in astrophysics. Gaia DR3 astrometry and photometry is transforming its understanding.

Convection represents one of the dominant sources of uncertainty in current stellar evolutionary models, propagating through to substantial uncertainties in ages and, in turn, understanding of the chemical evolution of the Galaxy. I outline the `mixing length theory' of convection, and Gaia's contribution to characterising the associated `mixing length parameter'.

I look at some other recent results on the non-radial pulsators in the SPB and Gamma Dor classes. These stars show periodic variations as a result of the usual restoring force of gravity (or buoyancy), as well as due to Coriolis forces resulting from stellar rotation. Many astrophysical results from Gaia compare favourably with the much denser sampling from Kepler.

I have discussed Gaia variables in several essays, focusing on Cepheids (43), on RR~Lyrae (45), their detection and classification (61), their distribution across the Hertzsprung--Russell diagram (62), and in the context of citizen science (132). Here I look at some new insights into the non-radial g-mode pulsators in the SPB and Gamma Dor class.

In essay 54, in January 2022, I gave links to some of the videos, animations, visualisations, and `fly-throughs' made to illustrate the science that Gaia is addressing. DR3 was released in June 2022, and many new explanatory videos have been made since. This is an introduction to some of the many video animations and illustrations that now exist.

A curious property of many collections of numbers, including naturally occurring data, is that the leading digits are not uniformly distributed, but are skewed toward smaller values. As Benford's law, it has been considered in many contexts, including identifying suspicious accounting. I consider it here in the context of the Gaia DR2 parallaxes.

Included in the DR3 data release are some 180,000 single-lined spectroscopic binaries, a colossal increase in numbers enabling the study of many aspects of short-period binaries. I look here at some new insights into orbit circularisation, where the Gaia data point to the process operating most efficiently in the pre-main sequence evolutionary phase.

Gaia is transforming the study of open clusters. Pre-2016, some 3000 clusters had been identified. But Gaia has shown that more than half of these are unreal, being simply asterisms. In their place, from nearly 25,000 new discoveries reported with Gaia (some duplicated), there are today nearly 14,000 unique clusters known in our Galaxy.

The ESA-Gaia `Image of the Week', on the first anniversary of the 34-month Data Release~3 (DR3), 13 June 2023, was a remarkable multi-dimensional map of the Milky Way, produced by the many members of the Data Processing and Analysis Consortium (DPAC). I'm showing these 10 splendid sky plots again here.

Gaia's contribution to understanding the distribution of dark matter follows from studies of the stellar halo population, Galactic kinematics, and stellar streams. Studies have also begun to place specific constraints on the properties of dark matter, for example if it is comprised of axions. I provide an overview of the various approaches.

White dwarfs have featured in a number of my previous essays. Gaia is providing well-defined samples in the solar neighbourhood, with more than 260,000 from Gaia DR2. I look here at the recent discovery of the new type of `white dwarf pulsars'. With just two members of this exclusive class, Gaia is contributing to their understanding,

Cataclysmic variables are interacting binaries containing a white dwarf accreting from a donor star. Their space density places strong constraints on models of their formation and evolution, and has revealed major discrepancies between observations and theory. New and improved space densities are becoming available with Gaia.

How common are quadruple star systems? How is Gaia discovering and characterising new systems, and what do they tell us about the star-formation process? One specific quadruple system is providing insights into the cooling and crystallisation of white dwarf interiors, and another is providing hints about the origin of type Ia supernovae.

Gaia has confirmed the existence of 'twin binaries' with components of equal brightness and presumably equal mass. This has stimulated new investigations into their enigmatic origin, the possible implications for models of star formation, and perhaps some consequences for the origin of our own solar system

Gaia is revolutionising the study of stellar occultations because its dense grid of accurate star positions allows greatly improved prospects of predicting such occultation events. I mention some recent results on outer solar system bodies, and explain how the technique is also being used to measure the angular diameters of stars

I describe three areas in which Gaia is contributing to the field of gravitational wave research: in helping to identify `verification binaries' for the LISA gravitational wave space mission, in searches for the electromagnetic counterparts of gravitational wave sources, and in providing constraints on the stochastic gravitational wave background.

How common are triple star systems? Why are they important to study, why they are so difficult to discover, and what can they tell us about the star formation process? Gaia is providing an unprecedented census of wide triple systems by virtue of its discovery depth and uniformity. Amongst them is the first resolved triple white dwarf system.

Binary and multiple stars occur in a bewildering variety of configurations, and range from the very compact to wide, long-period systems. Here I look at a uniform sample of 1.3 million resolved binaries constructed from Gaia EDR3. It includes white dwarf–white dwarf binaries, main sequence–white dwarf binaries, and many `twin' binaries.

Ellipsoidal variables are close binaries with orbit inclinations too small to yield eclipses, but with components nonetheless distorted by their mutual gravitation as revealed by their light curves. More than 60,000 have been identified in Gaia DR3. Amongst them, various searches for dormant black hole or neutron star secondaries are ongoing.

In essay 61, I looked at the identification and classification of variable stars from Gaia DR2, and some of the statistics of the resulting 550,000 variable stars. Here, I update the statistics for the 10 million variable stars identified in DR3. And I describe Gaia-Vari, the first Citizen Science contributions to Gaia which are assisting with the classification process.

Gaia is making some fascinating contributions to the known occurrence of white dwarf--white dwarf binary systems, and on the observational consequences of the eventual merger of the two white dwarfs. The story involves gravitational waves, the unusual properties of `ultra-massive' white dwarfs, and the precursors of (some) type~Ia supernovae.

The 'present-day mass function' describes the distribution of actual masses in any given stellar population. From it can be inferred the more fundamental 'initial mass function', intimately related to the original star formation processes. Its precise form, and whether it is universal, have been difficult to establish in the absence of accurate distances.

In essay 33 I described Gaia's census of stars within 100 pc, the GCNS. A recent result, also from EDR3, is the Fifth Catalogue of Nearby Stars, the CNS3. With 5931 objects out to 25 pc, it covers a smaller volume than GCNS, but it is much more complete. It comprises 4946 main-sequence, 20 red giants, 264 white dwarfs, and 701 brown dwarfs.

The past 20 years has seen the discovery a new class of dwarf galaxy, the ultra-faint dwarfs, of which some 60 are known today. They include some of the most dark matter-dominated systems known, with core-density profiles challenging predictions of structure formation. A striking discovery with Gaia has been Antlia II, similar in spatial extent to the LMC.

The structure and origin of the grouping of bright stars known as the Gould Belt has been studied for more than a century. Distances from Gaia rule out it being a physical ring-like structure. But the 3D arrangement of nearby molecular clouds has instead revealed a more complex and more mysterious Galactic-scale distribution of dense gas, the Radcliffe Wave.

It sounds like a simple enough question: how far is the Sun from our Galaxy's mid-plane? But studies over the past century have failed to reach a consensus answer, with estimates still ranging from as little as 5 parsec to as much as 20 parsec. And even though distances are central in deriving such a linear quantity, Gaia has so far failed to resolve the debate.

The Besançon Galaxy model is a stellar population synthesis model replicating many important features of our Galaxy. It played a key part in the preparation of the Gaia mission, and is now being used, by comparison with the latest data releases, as a probe of properties such as our Galaxy's stellar initial mass function, star formation rate, and gravitational potential.

Planetary nebulae are the dust and gas shells ejected during the late asymptotic giant branch phase for intermediate mass stars, the ejecta being ionised by the radiation from the hot, evolving central star. Their modelling has been constrained by poor knowledge of their distances. With Gaia DR3, the number with good distances has gone from some 40, to around 2000.

I am deviating from my usual topic of scientific results from Gaia, and offer some suggestions about one aspect of using the ADS (Astrophysics Data System) bibliography in Latex documents. Whether to use, for citations, the 19-character code created by ADS, or your own ad hoc code. If you struggle with the management of citations, do read!

I give a big picture of where we are in establishing the cosmic distance scale, noting the `tension' that exists between the `early' and `late Universe' estimates of the Hubble constant. This, in turn, seems to imply that there is a problem with the late Universe distance `ladder'. I point to the areas where we are expecting Gaia to contribute in the future.

I will look briefly at the big picture involved in modelling stellar structure and evolution. I will point to some of the areas of stellar physics where Gaia is providing new constraints, and which are contributing to a number of ongoing model refinements. Playing their part are advances in software infrastructure, computer processing power, and data storage.

Solar analogues are loosely defined as stars that are `very similar' to the Sun. Their identification provides the basis for a number of important applications across astronomy, including their value in determining the colours of our own Sun. Compared with some 100 candidates previously known, more than 5000 have already been identified in Gaia DR3.

Brown dwarfs are objects with masses below the hydrogen-burning limit. They are faint in the optical, with most isolated objects, even the nearest, close to Gaia's survey limit. Those in binary systems can be detected through the orbital motion of the brighter component. Many new brown dwarfs have already been discovered in Data Release 3.

Since the emergence of the Lambda CDM parameterisation of the standard `big bang' cosmological model in the late 1990s, various difficulties emerged in reconciling numerical simulations with observations at galaxy scales. Here I describe the `plane of satellites' problem, and how Gaia EDR3 data has been used to resolve it.

I have previously looked at some of the kinematic features that have been revealed, using the Gaia data, in the local stellar population. An important Gaia discovery has been rich structure particular prominent in plots of the vertical location in the solar neighbourhood versus the vertical velocity. I explain the nature of this `phase-space spiral', and theories as to its origin.

Following my previous essay, I look at two more of the two dozen or so kinematic groups known to exist in the solar neighbourhood: the Arcturus group and the HR 1614 group. Data in Gaia DR2 allow an unprecedented view of the local kinematic structure, and detailed studies suggest that both are `phase-space waves' that could have been cause by an ancient merger.

Stars in the local solar neighbourhood do not move at random, but instead include groups or streams of stars sharing a common bulk motion around the Galaxy. Their reality long debated, Gaia spectacularly confirms their existence. For the Hercules stream, accurate distances and motions confirm that it results from resonant motions driven by our Galaxy's rotating bar.

While it is well established that our Milky Way is a barred spiral galaxy, the detailed structure of its spiral arms is difficult to establish since our vantage point lies deep within the disk. Despite the ubiquity of spiral galaxies, even the driving mechanism of spiral arms remains unclear. The millions of stars with accurate astrometry in Gaia DR3 is casting new light on this long-standing question.

General relativity predicts that light escaping from a gravitational potential well, such as the surface of a star, will be redshifted, by some 636 km/s in the case of our Sun. The effect is most pronounced for white dwarfs. Gaia astrometry is furthering these studies, and allowing the effects to be characterised for main sequence stars in open clusters and wide binaries.