ESA Science & Technology - Stellar Astrophysics with Gaia

Introduction

Gaia will provide distances of astonishing accuracy for all types of stars of all stellar populations, even the brightest, or those in the most rapid evolutionary phases which are very sparsely represented in the Solar neighbourhood. With the parallel determination of extinction/reddening and metallicities by the use of multi-band photometry and spectroscopy, this huge amount of basic data will provide an extended basis for reading in situ stellar and galactic evolution.

All parts of the Hertzsprung-Russell diagram will be comprehensively calibrated, including:

• all phases of stellar evolution, from pre-main sequence stars to white dwarfs and all existing transient phases;
• all possible masses, from brown dwarfs to the most massive O stars;
• all types of variable stars;
• all possible types of binary systems down to brown dwarf and planetary systems;
• all standard distance indicators (pulsating stars, cluster sequences, supergiants, central stars of planetary nebulae, etc.).

This extensive amount of data of extreme accuracy will stimulate a revolution in the exploration of stellar and Galactic formation and evolution, and the determination of the cosmic distance scale. In this section we present some highlights of the science case for Gaia:

Stellar Structure and Evolution

One of the triumphs of stellar evolution theory is a detailed understanding of the preferred location of stars in the physical Hertzsprung-Russell diagram. There are however a number of uncertainties associated with stellar evolution models, and hence in age estimates based on the models. Probably the least understood aspect of stellar modelling is the transport process of matter, angular momentum and magnetic fields at both macroscopic and microscopic levels. As a result surface properties (such as effective temperature, or colour) of the stellar models are uncertain. Understanding the dynamics of stellar interiors remains a key challenge for astronomy.

This decade will see the start of asteroseismology from space. For the first time we will have direct indicators of the physical status of stellar interiors. The data from asteroseismology experiments, combined with measurements of global parameters provided by Gaia, will provide a major step improvement in our understanding of stellar evolution. The key to success will be in building a complete and homogeneous sample covering a large variety of independent parameters. The stellar absolute luminosity is derived from the parallax and the apparent magnitude, corrected for extinction, which can be deduced from the Gaia photometric and spectroscopic data. These also provide the effective temperature, metallicity indicators, and the projected rotational velocity of the stars, v sin(i). Ages can then be inferred from the location of stars in the Hertzsprung-Russell diagram.

Masses can be directly measured only in special cases, where the gravitational interactions with other bodies are easily measurable. The large number of systems for which the mass will be measured by Gaia will be used to validate the modelling of stars for which mass is known, providing in turn indirect estimates of the mass of other stars through the mass-luminosity relation.

Luminosities through Distances

Gaia will provide distances to 0.1 per cent accuracy for 700,000 stars out to a few hundred parsecs and to 1 per cent accuracy for more than 20 million stars up to a few kpc. Distances to 10 per cent accuracy will reach beyond 10 kpc. This will provide an extensive network of distance measurements for all stellar types throughout a significant fraction of our Galaxy, including the Galactic center, spiral arms, the halo and the bulge, and (for the brightest stars) to the closest galaxies of the Local Group. The faint limiting magnitude of Gaia will allow investigations of white dwarfs and the bottom of the main sequence down to brown dwarfs.

Essentially every stellar type, even those in fast stages of evolution, will be sampled in large numbers. Accurate measurements for the luminosity of every type of star in the Hertzsprung-Russell diagram will be obtained, providing formidable constraints to stellar structure models and evolution theory.

Variability

The large-scale photometric survey carried out by Gaia will have a very significant intrinsic scientific value. The high photometric accuracy, the multi-colour simultaneous coverage and the large number (100 to 150) of observations per target spread out over five years, will result in a massive survey for variable stars of several different types.

Variability, in addition to being interesting per se, is an essential tool to identify certain types of stars which are common but still poorly studied, and which represent important stages of evolution. Intrinsically variable stars include several key distance calibrators, such as Cepheids and RR Lyrae stars. A complete sample of objects will allow determination of the frequency of peculiar objects, and will calibrate period-luminosity relationships across a wide range of stellar parameters.

A systematic variability search will also allow for the identification of stars in short-lived but key stages of stellar evolution, such as the Helium core flash and the helium shell thermal pulses and flashes.

Physics of Stellar Interiors

The accurate and homogeneous astrometric and photometric data provided by Hipparcos has resulted in more precise characteristics of individual stars and open clusters and the consequent confirmation of certain aspects of internal structure theory. Further progress on several aspects of stellar modelling is required (for example, atmospheric modelling, transport processes, low-temperature opacities, etc). On the observational side, more numerous samples of rare objects, including distant objects as well as those undergoing rapid evolutionary effects, an increased number of more common objects with extremely accurate data including masses, and a census over all stellar populations are required. The following are some of the effects that would be probed in detail with the Gaia data.

The size of convective cores: Data from asteroseismology experiments combined with accurate estimates of global parameters from Gaia can probe the size of the convective cores. These play an important role in the evolution of intermediate mass stars and define the amount of nuclear material available to sustain the luminosity.

Internal diffusion of chemical elements: Diffusion of chemical elements in stellar radiative zones may have important consequences for stellar evolution, in particular for stellar ages when fresh helium is brought to the stellar cores. Diffusion may also modify the composition at the surface of stars during their life implying difficulties in linking the abundances of the elements presently observed to the initial abundances of the protostellar cloud.

The high-precision positions in the Hertzsprung-Russell diagram of stars of known surface abundances, provided by Hipparcos and by high resolution spectroscopy, has already shown that there exists a discrepancy between the observed positions and the positions predicted by the standard stellar models. The large sample of stars with accurate parameters provided by Gaia will help in addressing these discrepancies.

Outer convective zones: Most stellar models are still built by treating convection according to the classical parametric mixing-length theory. This works well for stars on the lower main sequence, but not for upper-main sequence stars or those in other evolutionary phases. The careful calibration of the Hertzsprung-Russell diagram allowed by Gaia for samples of different chemistries, ages etc., will greatly enhance our capabilities of dealing with non-local convective models for stellar interiors.

Stellar Ages, Galactic Evolution, and Age of the Universe

Precise stellar age determinations are required for a variety of topics related to dynamical studies, galactic structure and evolution, and cosmological time scales. The primary determination of ages relies on comparisons of stellar models or isochrones with the best available data, in particular luminosity, effective temperature and abundances, on individual stars or stellar groups. The principle of the method is general, but its application to different types of stars requires specific considerations.

A/F stars and galactic evolution: The determination of the age of relatively young objects, with ages ranging from several million to a few times 10⁹ years are relevant for Galactic evolution studies. The objects concerned, open clusters and field main sequence A or F type stars, lie in the Galactic disc. The determination of the age of the oldest objects in our Galaxy (in the halo) provides a lower limit to the age of the Universe. This estimate can be used to constrain cosmological models. The oldest objects in the Milky Way are the metal-poor stars located in the spherical halo, field Population II stars and globular cluster stars.

Star clusters: Hertzsprung-Russell diagrams of open and globular clusters have been exploited for decades to constrain internal structure models and stellar evolution theories. Open and globular clusters will remain unique tools because cluster members share the same age and chemical composition with masses spread all along the mass spectrum. Most clusters will contain hundreds to thousands of members observed by Gaia, and very clean Hertzsprung-Russell diagrams will be obtained by using the Gaia astrometry, photometry and radial velocity to discard non-members.

Helium abundance and chemical evolution of the Galaxy: The position of the zero-age main-sequence in the Hertzsprung-Russell diagram depends critically on the chemical composition of stars. A large sample of non-evolved low-mass stars with determined metallicities and accurate positions in the Hertzsprung-Russell diagram will be a key tool to discuss the helium abundance of these stars, and of the possible relation between the helium abundance and metallicity. Gaia will provide data for a large number of stars with spectral types K-M.

The oldest stars and the age of the Universe: A minimum age of the Universe can be estimated directly by determining the age of the oldest objects in our Galaxy. This estimate can be used to constrain cosmological models, as the expansion age of the Universe is a simple function of the Hubble constant, the average density of the Universe, and the cosmological constant. Currently the best estimate for the age of the oldest stars is based on the absolute magnitude of the main-sequence turn-off in globular clusters, requiring that the distance to the globular cluster be known.

Gaia will improve the age estimate of the oldest stars for at least two reasons. The number of subdwarfs with accurate distances will considerably increase in each metallicity interval allowing us to apply the main-sequence fitting technique to derive the distance of an increased number of globular clusters of various chemical compositions. Furthermore, distances of a substantial number of field subgiants will be measured, improving the age determination of the field halo stars.

Isolated Brown Dwarfs

Gaia will detect a large number of isolated brown dwarfs. Luminosities of brown dwarfs fade rapidly to very faint absolute magnitudes, the lighter brown dwarfs being more sensitive to this effect. It is clear that brown dwarfs detected at G < 20 mag will be strongly biased towards very young objects and those in the upper mass interval. Young brown dwarfs are however visible at relatively large distances. Old brown dwarfs will only be visible if they are nearby.

Although most brown dwarfs will be at the faint end of the Gaia survey, their proximity ensures that the relative precision on parallaxes will typically be better than a few per cent. It is expected that the positioning of these objects in the Hertzsprung-Russell diagram will be excellent, allowing the determination of ages and masses by sequence fitting. This will give an accurate picture of the recent brown dwarf formation history, including their formation rate and mass function.

White Dwarfs

White dwarfs are well-studied objects and the physical processes that control their evolution are relatively well-understood. The mechanical structures of white dwarfs, which are largely supported by the pressure of the gas of degenerate electrons, are very well modelled, except for the outer layers. These layers control the output of energy and correct modelling is necessary to understand the evolution of white dwarfs. This demands precise spectrophotometric data. Accurate parallaxes, as provided by Gaia, will provide very tight new constraints on the models.

White dwarf luminosity function of the disc: White dwarfs can provide important information about the age of the Galactic disc by comparing their luminosity function at low luminosities, and especially the position of its cut-off, with theoretical predictions. Current observational uncertainties contribute up to 2 Gyr to the total error budget of the Galactic age. After Gaia, the age of the disc will be known with an accuracy of about 0.5 Gyr. An accurate luminosity function can not only provide a tight constraint to the Galactic age but also has the bonus of providing important information about the temporal variation of the star formation rate. Gaia would be able to distinguish among the white dwarf luminosity function of the thin and the thick disc from the kinematic properties and hence provide an unprecedentedly deep insight into Galactic history.

White dwarf luminosity function of the halo: The scarcity of bright halo white dwarfs and the lack of the good kinematical data necessary to distinguish halo white dwarfs from those of the disc have prevented up to now the construction of a good luminosity function for the halo. Gaia will completely change the situation since high-quality parallaxes and proper motions will result in accurate tangential velocities, thus allowing good discrimination of these two populations. A robust determination of the bright part of the halo luminosity function will narrow the range of allowed IMF's (Initial Mass Function) for the halo and will constrain the nature of the microlensing events observed in the direction of the Magellanic Clouds.

White dwarfs as laboratories for fundamental physics: White dwarfs are well suited to test any departure from standard physics, since even small changes in physical constants can result in prominent effects when the relevant time scales of white dwarf cooling are taken into account. In the case of a hypothetical change in the gravitational constant G, upper bounds derived from the white dwarf luminosity function are comparable to the upper bounds derived from the binary pulsar PSR 1913+16. This is a statistical upper limit, thus improvements in our knowledge of the white dwarf luminosity function will translate into a more stringent upper bound for changes in G.

Other Specific and Rare Stellar Types

Perhaps the most dramatic effect of Gaia's contribution to stellar astrophysics will be visible for the rarer stages in stellar evolution, for which Hipparcos has not been able to supply strong constraints on the luminosity, given their large average distances. Examples include Tc-rich S stars, the central stars of planetary nebula, and Population II stars, as well as many others. For all of these stellar types the few available Hipparcos parallaxes show important discrepancies with existing theoretical models or earlier calibrations.

Massive stars: Only a small fraction of stars in the Galaxy are more massive than 20 Solar masses. These stars, which spend most of their short lives as H-burning O-type stars, play an important role in galactic structure and evolution. Accurate knowledge of the luminosity of these stars is important for comparing masses derived from stellar evolutionary models with those derived from stellar atmosphere models, for determining initial mass functions, and for studying stellar evolution in the high luminosity/high mass region of the Hertzsprung-Russell diagram. Absolute magnitudes of O stars are at present poorly determined.

Wolf-Rayet stars: These are a stage in the evolution of stars more massive than about 30 Solar masses which have been stripped of their H-rich envelopes by winds in previous evolutionary phases. Because of their short total lifetime of less than 4 million years, they are indicators of recent star formation, or of young clusters. Because of their distinctive optical spectra, dominated by strong emission lines, they can be easily distinguished with low resolution spectroscopy and narrow band filter photometry. Accurate distances measured by Gaia are important for several reasons: (i) the distance determinations of very young clusters by means of Wolf-Rayet stars makes it possible to trace the spiral arms across the Galaxy; (ii) evolutionary calculations show that the ratio of C-rich to N-rich Wolf-Rayets increases with increasing metallicity, so by measuring the distances of the stars and this C/N ratio as a function of location in the Galaxy, we can trace the metallicity as a function of Galactic location; (iii) because of their intrinsic brightness and their remarkable spectra, late N-rich type Wolf-Rayets can be observed in distant galaxies up to about 60 Mpc. This makes these stars very good candidates for measuring distances significantly beyond the Cepheid limit. This requires careful calibration of the MV-subtype relation for Wolf-Rayet stars. Trigonometric parallax measurements by Gaia will play a crucial role in this calibration.

Tc-rich S stars: These are interesting for stellar structure because they accurately time the occurrence of a dredge-up of core material to the surface. Current data, from Hipparcos, indicates that their derived core masses are lower than expected from models. Stringent constraints cannot be placed on the problem until Gaia quality parallaxes become available.

Long-period variables: These red giants are on a critical, short-lived stage of the evolution of intermediate mass stars leading to planetary nebulae and white dwarfs. They also strongly contribute to the chemical evolution of the Galaxy (mass-loss) and are a useful probe of galactic structure. They are promising distance indicators, as a complement to the Cepheids, which are some five times less numerous at the same luminosity.

Physics of Cepheids: Accurate distance of Cepheids throughout the Galaxy will allow derivation of period-luminosity relations at different metallicities. In the Hertzsprung-Russell diagram, the instability strip will be delineated precisely, which will constrain the structure of the outer convective zones, and the properties of turbulent convection. The presently available data is not large enough to allow statistical studies relevant for pulsation-convection coupling and time-dependent convection.

Novae and Nova-like variables: Distance determinations to novae are required to interpret the energetics of the outburst, and to place these objects more securely within the context of evolutionary models. Distance estimates can be made through modelling of the shell expansion velocity, but such applications are restricted to particular epochs after outburst, and also suffer from modelling uncertainties. Most Galactic novae are brighter than V = 12 mag at maximum, although measurements to V = 16 mag or fainter would also allow the determination of distances to Galactic novae observed over the last few decades. Related objects, such as dwarf novae, AM Her stars, symbiotic stars, and cataclysmic binaries could be studied, providing accurate luminosities needed to distinguish among alternate possible energy generation mechanisms. Many such nova-like variables would lie within the distance horizon and the magnitude limit necessary to provide distances to better than 5 per cent.

T Tauri stars: Low-mass pre-main sequence stars are of particular interest to stellar evolution, among other things because they are, if sufficiently young, in a fully convective stage of evolution. The Gaia parallaxes will completely remove the distance uncertainty in the determination of the fundamental stellar parameters for pre-main sequence stars. Gaia is the only tool to help observationally in exploring these fundamental problems of stellar evolution. Accurate distances will be the key in pinning down the ages of the rich pre-main sequence population available in the nearest kpc.

Pulsars: Optical identification of Gaia objects with radio pulsars would provide astrometric distances and proper motions of pulsars, the former required for determining accurate electron densities in the warm ionized part of the ISM. Pulsar distances would also provide distances to the cold neutral hydrogen, as needed for the construction of a kinematic model of the Milky Way disc gas. Of the currently known optical counterparts, only the Crab Pulsar will be accessible to Gaia. Gaia also offers the unique opportunity to discover fast-moving objects that may be radio-quiet pulsars (such as Geminga) should these objects exist and have escaped detection so far at other wavelengths.

Cosmic Distance Scale

Gaia will provide accurate distance and proper motions for such huge numbers of each category of stellar distance indicators that the analysis methods can be drastically changed. The sampling of open and globular clusters in age, metal, oxygen or helium content will be complete all over the Galaxy. Parallel improvement in the transformation between the observational and the theoretical Hertzsprung-Russell diagram will be required to take full benefit of these accuracies in terms of stellar evolution and age determination; photometric and/or spectroscopic data should allow the determination of the bolometric magnitude and of the effective temperature from the observed magnitudes and colours.

For pulsating variables, the sampling versus period, populations, colours and metal content will be as good as possible as excellent distance determinations will be obtained for all observable galactic stars, and a first reliable estimation of the intrinsic dispersion of the period-luminosity relations will be possible. A first check of the universality of these relations will be possible, directly for LMC Cepheids, or using Gaia mean distances for the closest galaxies of the Local Group, at least LMC, SMC and Sagittarius. Parallel improvement in the determination of metallicity, and of interstellar extinction/reddening will be required.

Cepheids and RR Lyrae stars: In addition to the importance of these stars to models of stellar structure and evolution, Cepheids and RR Lyrae stars form the cornerstone of the extragalactic distance scale. The parallax accuracy from Gaia will be better than 1 per cent up to 3 kpc, better than 4 per cent for all Galactic Cepheids, 8 per cent for Cepheids in Sagittarius, and between 10 and 30 per cent for Cepheids in the LMC. The details of the period-luminosity-colour relationship will be significantly improved.

Planetary nebulae: The central stars of planetary nebulae have masses in a very narrow range, and thus provide the possibility of being good distance indicators. Because of their rarity, and therefore their typical distances, and their nebulosity, no satisfactory method yet exists for their distance estimation. Parallax measurements of the central stars would lead to significant advance in the understanding of the formation and evolution of the shells, the status of the central stars, and the role of these objects as standard distance indicators. Tens of planetary nebulae would be measurable down to V = 16 mag. Accurate trigonometric parallaxes for many hundreds of planetary nebulae are required to calibrate the planetary nebula luminosity function, which is used extensively as an extragalactic indicator.

Blue supergiants: Recent advances in the theory of stellar atmospheres and winds have revealed a new way to determine luminosities of blue supergiants in other galaxies by means of the wind-momentum luminosity relationship with an accuracy rivaling that of the period luminosity relationship for Cepheids. The basis for the successful application of this method is its galactic calibration using blue supergiants with accurate distances in the Solar neighbourhood. Accurate trigonometric parallaxes for blue supergiants of extreme brightness are not available so far. With distances larger than 400 pc they were out of the reach of Hipparcos. Gaia, on the other hand, will allow measurement of trigonometric parallaxes for dozens of supergiants of extreme absolute magnitude. This will yield a solid foundation of the wind-momentum luminosity relationship method as a new and superior way to determine extragalactic distances.

Globular clusters: Mean distances to better than 1 per cent will be obtained for about 110 of the 140 globular clusters of our Galaxy. Combined with their space motions, this will allow derivation of their orbits, which in turn constrains the mass of the Milky Way. The Gaia mean distances also calibrate, as a function of metallicity, the distances of external globular clusters derived via main sequence fitting of Hubble Space Telescope colour-magnitude diagrams. The luminosity function of the ensemble of Galactic globular clusters calibrates the method that uses the entire globular cluster luminosity function of a galaxy as a distance indicator.

Nearby galaxies: Direct mean distances to the closest galaxies of the Local Group will be within reach, and, for example, the controversy between LMC distances determined from Cepheids (mainly located in the bar) and RR Lyrae stars (mainly in globular clusters) will be resolved. A sufficiently large number of Cepheids and RR Lyrae will be observed by Gaia to obtain mean distances without the use of intermediate objects or indirect methods.

Masses from Microlensing

Gravitational microlensing allows measurement of masses of astronomical objects with an accuracy of a few per cent. Astrometric microlensing has two advantages over photometric microlensing - first, the astrometric cross-section is substantially larger than the photometric, and second, the degeneracy with regard to the mass of the lens is removed. Gaia can probe this very different regime. The expected number of astrometric microlensing events detected by Gaia will be a few hundred. The main microlensing objective of Gaia is to determine the present-day mass function of the disc and bulge. The faint end of the mass function is very poorly determined and Gaia can hope to make a very real contribution here.