Essays on Gaia

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.

In just two days from today, on 15 January 2025, the Gaia satellite will cease its science operations. More than 10 years of spectacular astrometric (and related) observations of unprecedented accuracy, number, and magnitude range, will come to an end. I'll provide some details.

The challenges in searching for extraterrestrial intelligence are compounded by the multiple dimensions of potential search space. The problem can be simplified if the task is considered of mutual interest to both parties. In that case, appeal can be made to `game theory' to establish more efficient search strategies.

I will end this fourth year of my weekly essays on Gaia with some of the positive words that have appeared in the scientific literature about the mission’s advances. I hope they make enjoyable reading, especially for those involved in Hipparcos or Gaia.

In my previous essay, I gave an overview of the many scientific areas being impacted by Gaia, divided into five categories: solar system; the photometry and RVS spectra; stellar physics; Galaxy structure and dynamics; and cosmology. Here, I will say more on some of the highlights to date.

I give a concise ‘run through’ of the scientific areas impacted by Gaia so far. I divide this into five categories: solar system; applications of the photometry and radial velocity spectrometer (RVS) spectra; stellar physics; Galaxy structure and dynamics; and local group and cosmology.

Tidal disruption events, or TDEs, result from a star passing sufficiently close to a supermassive black hole that tidal forces overcome its self-gravity. Predicted 50 years ago, some 100 such events are now known. The Gaia science alerts system has discovered 25, and I will describe what is known about the two brightest, Gaia19bvo and Gaia19eks.

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.