Essays on Gaia

I look here at two other interesting microlens systems discovered by means of the science alerts pipeline: Gaia19dke, a long-duration single-lens event but with multiple peaks, and Gaia19bld, an event in which rotating arc-like images have been spatially resolved for the first time.

AM CVn systems are a rare class of compact interacting binaries, distinguished from other cataclysmic variables by the absence of hydrogen in their spectra, and affected by the poorly modelled process of common envelope evolution. The 5th Gaia alerts object, Gaia14aae, is only the third known eclipsing AM CVn system, and the first in which the white dwarf is fully eclipsed.

Of nearly 6000 exoplanets known today, only two have been discovered so far by Gaia, both from transit photometry. Some 200 of the known planets are microlensing discoveries. A new Gaia microlens planet has recently been reported. Discovered from the Gaia science alerts, it has a number of interesting features that I will expand on here.

I introduced the topic of Gaia's Science Alerts in essay 36 (Sep 2021), when the archive contained some 5000 classified sources. Today, it lists more than 25,000 events (more than 10 a day), with 7000 assigned to 23 classes. I describe some of these interesting objects: including exotic supernovae, tidal disruption events, and Gaia's first microlensing planet.

YSOs represent the earliest stages of star formation, with the term embracing the protostar and subsequent pre-main-sequence phases. The variability analysis as part of Data Release 3 has identified 80,000 YSO candidates (40,000 new), while the Science Alerts pipeline is discovering more of the rare FU Ori and EXor accretion-driven outburst events.

Chromospheric activity originates as part of the cyclic regeneration of a star's large-scale magnetic field. Various spectral lines provide diagnostics of its physical conditions. Gaia DR3 provides the largest database to date of activity in the Ca II infrared triplet lines measured by Gaia's Radial Velocity Spectrometer.

I summarise the main approaches that are being used to search for stellar streams, the tidal remnants of galaxies captured by our own, then look more in details at the complex Cetus-Palca stream, an example of a dwarf galaxy that has undergone several periods of stripping, leaving behind debris at multiple locations in the Galaxy halo.

The Sagittarius dwarf galaxy is a nearby satellite of our own, discovered only 30 years ago. Tidally distorted, with dramatic and extensive preceding and trailing tidal streams, it is inferred to have completed several close orbits around the Milky Way. It provides a most striking example of galaxy disruption and ongoing accretion.

The gravitational capture by our own Milky Way of a satellite galaxy, some 10 Gyr ago, resulted in the so-called Gaia Sausage-Enceladus stream. The merger had a major effect on our Galaxy halo, where it accounts for more than 20 per cent of the halo's stellar population, and on the disk. I summarise our present knowledge.

The rotation curve of our Galaxy provides strong constraints on the mass distribution of its major structural components. New Gaia results suggest a strongly revised dependency within the solar circle, and a steepening Keplerian decline beyond 20 kpc, which may indicate the limits of our Galaxy's dark matter halo.

The detailed morphology of our Galaxy's central bar remains unclear. Estimates still vary concerning its axis ratio, its half-length, and its orientation with respect to the Galactic centre. Here I say more about the attempts and difficulties of elucidating its morphology, and include a surprising demonstration of Gaia's ability to image it.

Over the past 20 years, large N-body simulations, based on the Lambda CDM paradigm, have been developed to investigate how structure in the Universe has evolved over time. I will briefly outline these cosmological models, and show how Gaia is confirming many of their detailed predictions, and helping in their interpretation.

The origin of wide binary stars remains something of a mystery. In two earlier essays I looked at how Gaia detects them, and at suggestions that their orbits are inconsistent with Newtonian dynamics. Recent Gaia results throw new light on their formation, while researchers are still divided on the very long-period orbits.

Great progress has been made in understanding the structure of Galactic molecular clouds with the availability of the Gaia data, as a probe of both star distances and interstellar extinction. I will give some background, explain why their 3D structure is relevant, and illustrate how Gaia is contributing.

A recurrent theme in the Gaia literature is interstellar extinction. This is important in the determination of stellar parameters through spectral modelling, and is central in describing the distribution of gas and dust in the solar neighbourhood. I will summarise what has been measured with Gaia so far.

The number and variety of complex dynamical features in our Galaxy has been one of Gaia's greatest contributions to studies of its structure and evolution to date. Here I look at four more recent discoveries: one each related to the inner and outer halo, one to the central disk, and one to its cosmological evolution.

Data Release 3, in 2022, included a catalogue of metallicities, temperatures and gravities for 470 million sources. Since then, other community-generated catalogues of metallicities, temperatures and gravities have been made available. I will describe what is motivating these reductions, and how they are being used.

Some important contributions are being made by Gaia in furthering the use of the `tip of the red giant branch' as a robust distance indicator. These sorts of new insights are becoming ever more crucial in the context of the ongoing `Hubble tension' debate.

Gaia DR3 provides flux-calibrated low-resolution spectrophotometry for 220 million sources in the range 330-1050nm. Synthetic photometry can be obtained from them for any passband fully enclosed in this wavelength range. I will explain the principles, and the many applications which are being pursued.

An independent method of estimating the ages of nearby stellar associations makes use of their expanding space motions to yield what are referred to as dynamical or traceback ages. The difference between various methods is providing important clues about the cluster formation and gas dispersal processes.

As evolved stars cool and expand on the red giant or asymptotic giant branch, they become pulsationally unstable. Miras are AGB stars with large variability amplitude and long pulsation period. Here, I will look at some Gaia results regarding their use as distance indicators and probes of Galactic structure.

A specific application of the study of stellar streams is in placing constraints on the existence and nature of the numerous dark matter sub-halos (halos within halos) that are predicted, in standard Cold Dark Matter cosmology, to exist surrounding the Milky Way. The stream GD-1 shows some tantalising prospects.

Gaia presented a number of new and demanding requirements for its CCDs, which in turn required an extensive development program pre-launch. I will describe some of these, and expand on why the issue of radiation damage was so critical for Gaia, and what measures were taken to minimise the effects of the solar cycle.

The YORP effect is a second-order Yarkovsky-like effect, influencing the spin rate and spin axis orientation of sub-km sized irregular asteroids. Like the Yarkovsky force, its effects accumulate over long periods, resulting in some rather remarkable changes in the properties of the asteroid population, seen clearly in DR3.

The Yarkovsky effect is important in solar system dynamics. For a small rotating body illuminated by the Sun, re-radiated thermal emission lags behind the incident radiation, contributing a component of force in the direction of its orbital motion. I explain why it is important, and how Gaia is contributing to its understanding.

Photometry of minor solar system bodies provides information about their shape and rotation. Reflected sunlight encodes information about their composition and taxonomic classification, from which information about their origin and evolution can be deduced. DR3 gives reflectance spectra for more than 60,000 such bodies.

Masses are one of the most fundamental stellar properties, crucial in determining their structure and evolution. Yet ways of determining accurate masses are strictly limited and, even today, only a couple of hundred are known to better than 1-2 per cent. I summarise how Gaia is contributing.

The prominent bifurcation in the white dwarf colour-magnitude diagram, seen for the first time in Gaia DR2, was tentatively attributed to different evolutionary tracks for hydrogen- and helium-dominated atmospheres. Subsequent studies have focussed on trace amounts of `dredged-up' carbon being the cause.

Intermediate-mass black holes lie between the masses of stellar mass black holes, formed by single star collapse, and supermassive black holes, formed in the high-density environment of galaxy centres. Plausible formation mechanisms, but no definitive candidates, are known. I describe some insights from Gaia in the case of our nearest globular cluster, M4.

I look at how the existence of stellar mass black holes might affect the morphology and kinematics of the stellar streams that are now known to exist in the inner and outer halo of our Galaxy, and to what extent Gaia can help to distinguish between those that are rich in, or devoid of, stellar mass black holes.

The existence of stellar mass black holes has observable consequences on the dynamics of open clusters, which can in turn place useful constraints on their formation. I look at some early results for the Hyades open cluster, which suggest that the cluster should contain 2-3 stellar mass black holes in long-period binary systems.

A subset of astrometric binaries evaded identification by Hipparcos, but they become recognisable from the difference in the proper motion determined by Hipparcos and that measured by Gaia. Recent work on these `proper motion anomalies' has underlined their ubiquity, and their scientific importance.

In essay 114, I looked at advances being made in understanding our Galaxy's spiral arm structure. Here I will look at some remarkable insights into their kinematics. Gaia is providing confirmation of their `breathing modes' which, in turn, support some of the theoretical and numerical models being developed to understand their origin.

For Hipparcos and Gaia, the `basic angle' is the angle between the instrument's two viewing directions on the sky. For Hipparcos, the basic angle was 58 degrees. For Gaia it is 106.5 degrees. What is the reason for the two fields of view in the first place? How is the angle between them chosen? And why is it so different for Hipparcos and Gaia?

The Large and Small Magellanic Clouds are two of our nearest galaxy neighbours. The line-of-sight structure of the SMC is particularly complex, and the wealth of observational data has proven difficult to interpret. A recent study using Gaia DR3 suggests a radically new picture: that the SMC is composed of two distinct superimposed structures.

Metrication concerns the move from the historical use of feet and inches, of pounds and ounces, and of pints and gallons, to the coherent metric system in which units are inter-related. The UK's resistance to full adoption of the metric system, used by almost all other countries, has left society confused, and at an economic disadvantage.

Gaia Data Release 3 includes 33 million radial velocities, obtained from its Radial Velocity Spectrometer, ten times as many as previously known. DR4, in 2025, should include some 100 million. A recent study has derived 125 million from the low-resolution BP/RP spectra, albeit of much lower accuracy, with perhaps 1–2 billion coming available in the future.

S stars are cool luminous giants, lying between the O-rich M-type giants and the C-rich carbon stars on the AGB. With atmospheres enriched with s-process elements brought to the surface through the `third dredge-up', they have created around half the atomic nuclei heavier than iron. Gaia astrometry and photometry is advancing their understanding.

Carbon stars, which reside on the asymptotic giant branch, are an important phase of stellar evolution. Their very high luminosities makes them important for integrated light studies of galaxies, and they are being considered as potential standard distance indicators. Their large distances and complex physics provide numerous challenges for theoretical modelling.

In essay 22 (May 2021) I described some of the early insights that Gaia was providing in the field of hypervelocity stars, a rare and exotic type of star, racing through our Galaxy with velocities of 500-1000 km per second or more. Here, I bring the Gaia results up-to-date, describing the latest searches, and more recent insights into their origin.

Runaway stars are stars with such high space velocities that they must have been imparted by a particular formation process. Many insights into the favoured scenarios - binary-supernova and dynamical ejection - are being made as Gaia's astrometry yields their accurate distances and space motions.

I look at some recent advances in the numbers of blue stragglers identified by Gaia, and Gaia's contribution to the identification and understanding of the new class of sub-subgiant star. I discuss whether these offer a particularly interesting search sample for stars hosting primordial black holes in their centres.

Dual active galactic nuclei, we now know, provide an important probe of the physical processes that drive the in-spiralling of supermassive black hole pairs inside a single merged galaxy. Difficult to detect and characterise, Gaia is making a significant contribution to their discovery.

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.