Astrometry

• Expected astrometric (parallax) error in microarcsec for a G2 V star

PROPER MOTIONS
 

• Expected astrometric (proper motion) error in microarcsec/year for a G2 V star

UPDATE - 20/02/2009. The astrometric accuracies now quoted at the GAIA website here are not quite as good as in the table above, with the expected astrometric accuracy for a 15th mag star having decreased by up to a factor of 2. The figure below shows the execpted accuracies as a function of colour for a 15th mag star.

Radial velocities

• GIAI will measure radial velocities, probably from Ca II lines in the near IR (8700 Å) at 1Å/pixel.
• 100-150 individual spectra collected over circa 5 years.
• Radial velocities to V = 15 should be +/- 3 km/s.
• Limit for radial velocities is approx V=17 (+/- 10 km/s)
• Slitless grating spectrograph

Photometry

• Photometric measurements in broad band filters and low resolution spectroscopy in the optical region.

Note: the expected accuracies (Feb 2009) have been reduced by up to a factor of 2 from these targets (see figure above).

Current Status

• Accepted by ESA as Cornerstone mission, to fly end of 2011 (last update Feb 2009)

Science Goals

• Hipparcos' distance horizon is circa 100 pc (distances of 10% accuracy)

• GAIA's distance horizon will be typically 5 kpc with 10% accuracy, 10 kpc with 20% accuracy

• Primary goals will therefore be Galactic structure and the physical calibration of stellar luminosity of a huge range of stellar types

• Huge numbers of O+B stars, luminous giants, Cepheids, and RR Lyrae variables all of which are important for the Cosmic distance scale.

• First extensive network of distance measurements throughout a significant fraction of our Galaxy, including the galactic centre, spiral arms, the halo, and the bulge.

Massive stars

• No good parallaxes have ever been measured for O stars, which play an important role in Galactic evolution.  Large numbers will be within 1 kpc, so GAIA will calibrate their luminosities. Proper motions of O stars in the outer disk will constrain the Galaxy's rotation curve.

Novae and nova-like variables

• Distances are currently quite uncertain for this type of star. Many should be available brighter than 16th (some will occur during the mission and reach V=12) necessary to provide distances to better than 5 per cent.

Planetary nebulae

• PN are useful distance indicators since they can be found in both spiral and elliptical galaxies.

• Distances of PN within the Galaxy are presently extremely poorly known, with uncertainties of more than 50%.

• 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 candles.

• Many tens of planetary nebulae would be measurable down to V=16 mag.

Cepheids and RR Lyrae stars

• In addition to the importance of these stars to models of stellar structure and evolution, Cepheids and RR Lyrae form the cornerstone of the extragalactic distance scale.

• Some 55 Cepheids and 26 RR Lyrae stars are known to lie within about 1000 pc

• Parallaxes at the 20 microarcsecond level would yield distance estimates to better than 2 per cent for these objects. In turn, the details of the period-luminosity-colour relationship for these objects would be significantly improved.

Stars in Open Clusters

• Only two open clusters (Hyades and Coma Ber) lie within the 100 pc distance horizon yielding distances from Hipparcos (from individual objects within the clusters) to better than 10 per cent accuracy.

• Some 30 open clusters are considered to lie within about 500 pc, sufficient to provide individual distances to 1 per cent accuracy with parallaxes measured at the 20 microarcsecond level.

• Full 3-D velocities will be measurable for many individual cluster members.

• Cluster studies are probably most important for stellar physics studies (large population of co-eval stars).

• studies of their internal 3-D dynamical behaviour

• direct resolution of stellar evaporation into the field

• calibration of stellar luminosities and distances via the Wilson-Bappu effect

• calibration of the mass-luminosity relation for binary stars

• calibration of the main sequence luminosity as a function of age, helium content, and metallicity.

• mass segregation

• binary fractions

• tidal distortion and signatures for encounters with giant molecular clouds

Globular clusters

• Ages of globular clusters: cluster age determinations require calibrations of the main-sequence and, in particular, the turn-off point as a function of chemical composition.

• For absolute ages to be accurate to a billion years, the distance of the cluster must be determined with an accuracy of better than 3 per cent.

• 15 mag and 20 microarcsecond accuracy on the parallaxes includes 20 or more globular clusters (such as 47 Tuc, omega Cen, M3, M5, and M15) lying between 5 and 10 kpc, and would yield individual distances accurate to better than 10 to 20 per cent.

• Some 10 or more of the brightest stars per cluster would be observable, resulting in mean cluster distances at least a factor of three better than these individual accuracies.

• 3-D positions and 3-D velocities for most of the globular clusters  will allow their orbits to be determined --- dark matter in the Galaxy

• Within 47 Tucanae, for example, proper motions of 20 microarcseconds per year correspond to transverse velocities of 0.4 km per second.

• 3-D velocities should be reconstructible at the level of a few km/s leading to models of the velocity distribution function, orbital families of stars and mass models of the cluster

• In addition, spectroscopic binaries have been detected in globular clusters with amplitudes of tens of km per second and periods of years, corresponding to separations of order 1 mas. These should be imaged directly by GAIA.

• Parallaxes and annual proper motions at the level of 50 microsecond or better would provide distances and orbital data necessary to clarify the formation and evolution of these binary systems, and their role in the formation of the milli-second pulsars now known to exist in the cores of globular clusters.

Stellar diameters

• Many nearby giant stars will be resolved --- measure radius and hence fundamental astrophysics

Detection of Planetary Systems and Brown Dwarfs

• For a 1 solar mass primary, at a distance of (say) 10 pc, the perturbation caused by a Jupiter-sized planet  has an amplitude of 500 microarcsecond and a period of 5 years, while the effect of an the Earth is a one-year period with 0.3 microarcsecond amplitude.

• GAIA specs sufficient to detect Jupiter-mass planets (at the 3 sigma level) out to 30pc, i.e. there are several thousand potential target stars, all of which can be monitored for possible companions.

• for 2 microarcsecond astrometry, which may be feasible for bright stars (V<10 mag), then the detection horizon for Jupiter-mass planets is pushed beyond 100 pc.  Screening all 50 million stars down to the survey limit of V=15 to 16 mag for possible signatures of planetary and brown dwarf companions will provide a complete census of such bodies to well-defined detection limits.

• find several 1000 free floating young brown dwarfs within 15 pc

Star formation

• luminosities of pre-main sequence stars

• kinematics of star forming regions

• detection of rapid evolutionary phases

• background light in emission line regions is a challange

• Any high proper motion HII regions --- formed around 10⁶ solar mass black hole dark matter?

White dwarfs

• detection of some 300,000 disk white dwarfs and some 3000 halo white dwarfs in 6-D space

• reconstruction of the star formation history of the milky way

• calibration of white dwarf cooling sequences

• age of the disk/halo from WD LF

• link to supernaove type Ia --- how many "loaded guns"?

Metal-poor stars and primordial nucleosynthesis

• Light element abundances in halo stars are important constraints on the production of these elements in the big bang.

• Large number of halo stars within 1 kpc will be accurate enough to determine surface gravities and light elemental abundances

• GAIA will recover millions of halo stars within 10 kpc with full 3-D positions and velocities.

Visual and astrometric binaries

• GAIA will resolve binaries with an apparent separation exceeding 1-2 mas and a moderate magnitude difference leading to absolute orbits and individual component masses.

• This will establish with very high precision the stellar luminosity to mass function.

• As many as half of the stars seen could be "binaries" and need special analysis of the photocenters

Interacting binary systems

• Many interacting binaries accessible with parallaxes in the 10 to 20 microarcsecond range.

• The evolutionary history of interacting binary systems, and the origin of Type I supernovae, millisecond pulsars, low mass x-ray binaries, and globular cluster x-ray sources is intimately bound up with the behavior of compact binaries with mass transfer and loss.

• accurate knowledge of the stellar masses and orbital separation can be derived from astrometric measurements (yielding the orbital separation and the orbital inclination) combined with estimates of the mass function determined by radial velocity measurements.

• address specific questions about accretion rates, precursors, mass distributions, and kinematic behaviour

• studies of black hole candidates (Galactic black hole candidates have bright secondaries  --- 9 mag in the case of Cyg X-1, and 12 mag or fainter in the case of V404 Cyg) and wide orbits (with orbital periods of about 6 days), which should yield definitive black hole masses by determining orbital separation and inclination.

The fraction of variable stars in the Hipparcos CMD. Stars in blue regions are rarely variable, whereas stars in the reddest regions are almost all variable. The code at the bottom of the plot shows the colour which indicates the fraction of stars which are variable.

Extragalactic Astrophysics and General Relativity

Proper motions of the Magellanic Clouds

• Different explanations for the dynamical behaviour of the LMC/SMC, in particular whether these systems are gravitationally bound to our own galaxy, implies systematic proper motions of below around 1 mas per year, very much at the limit of the Hipparcos capabilities. Large numbers of stars measured in the LMC/SMC at the level of 50 microarcsecond or better, would clarify their dynamical relationship with our own Galaxy.

Local Group Galaxies

Direct detection of the proper motion and rotation curves of local group galaxies Internal kinematics of nearby dwarf galaxies Galaxy orbits to give cosmological histories

Galaxies and active galactic nuclei

• Further out, the nuclei of active galaxies are sufficiently pointlike that their absolute proper motions may be measurable. At a redshift of z=0.03, a transverse velocity of 1000 km per second corresponds to a proper motion of 3 microsecond per year.

The role of quasars

• By reaching 15 to 16 mag GAIA would include a number of quasars, and this in turn would allow a direct tie between the resulting reference system and an inertial reference frame, something which has not been possible (directly) in the case of Hipparcos.

• A few 1000 quasars should be measured --- microarcsecond optical maps of QSOs --- superluminal motion maps from several hundred images over 5 years?

Light-bending by the Sun and Earth

• The reduction of the Hipparcos data has necessitated the inclusion of stellar aberration up to terms in (v/c)², and the general relativistic treatment of light bending due to the gravitational field of the Sun (and Earth).

• Light bending by the Sun amounts to 4 mas even at 90° (i.e., for light arriving perpendicular to the ecliptic).

• The astrometric residuals may be tested for discrepancies with general relativity's predictions; in principle this provides a constraint on the post-Newtonian light-bending term, γ, which is equal to unity in general relativity.

• The GAIA measurements would provide a precision of about 1 part in 10⁶ or better in the determination of γ due to the Sun.

Higher-order gravitational effects

• At the level of accuracy expected from GAIA, even more subtle effects will start to become apparent, such as the quadrupole components of the gravitational fields of the Sun and the planets, and the `frame-dragging' effects of their motions and rotations.

MACHOS

Light curve detected by the MACHO experiment of a star in the LMC,
caused by microlensing by a dark body in the Galactic halo

• Light modulation effects due to gravitational lensing by MACHOs, and the possible metric perturbations due to gravitational waves, must also be considered.

Microlensing by a planet around a faint star,
microlensing a bright background star.

• Amongst a few hundred million stars within 1 kpc a significant microlensing rate is expected.

• Expect circa 1000 microlensing detections toward the bulge --- probes the stellar mass function at low mass

Dynamics and Evolution of the Galaxy

• Tens of millions of G and K dwarfs will be visible within 1 kpc with measurable [Fe/H], T_eff, log(g).

• Full 3-D space velocities at <1 km/s resolution

• discover and map all the nearby spiral arms

• considerable phase space structure should be resolvable i..e moving groups or star streams, motions of stars across spiral arms --- weigh the spiral arms

• Photometry will be used to measure  the [Fe/H] distribution of metallicity as a function of position in the Galactic disk --- constrain evolutionary models of the gas into stars

• complete census of the solar neighbourhood will give a superbly determined stellar luminosity function --- reducing to hugely the major uncertainty in converting star counts to models of Galactic structure

• star formation history via directly determined ages for millions of F and G type stars, within 2 kpc (!) of the sun

• role of giant molecular clouds and spiral waves in "heating" the Galactic disk (Age-velocity relation).

• determination of the outer disk rotation curve

Rotation curve of the Milky Way. The curve shows the circular rotation rate of stars and gas in the disk as a function of distance from the Galactic center. The "flatness" of this curve (rotation speed is the same at all radii) is a major piece of evidence for dark matter in the Galaxy.

• disk mass profile -- amount of matter in the disk within a radius of circa 1 kpc, amount of "graininess" in the matter distribution

• role of mergers in Galactic history

• balance between vertical kinetic energy of stellar populations and the disk potential

• bulge/bar kinematics and inner disk structure

• distance to the Galactic center

• kinematics of the outer disk warp

Galactic Halo

Simulation of a satellite being accreted by the Milky Way. It leaves behind a trail of stars which GAIA should be able to find, if they exist. Many such satellite accretions are expected during the early life of the Milky Way, and continue up to the present time.  Source: Johnston et.al. www.sns.ias.edu/~kvj/

• Satellite systems which have been accreted by the Galaxy probably leave behind tidal "streamers" in the halo. Finding and tracing these yields information about the accretion process and the dark matter distribution.

• the number and mass of these systems (especially in the outer halo) will test hierarchial models of the  formation of galaxies (e.g. CDM)

Pulsars

• some pulsars may be detectible, pulsar companions in general detectible --- yields distances and luminosities of pulsars

Cosmology

• GAIA will fill in the gap the "corellation function", between Microwave Background measurements and ground based galaxy surveys (like APM) by uniform detection of galaxies over wide areas

• Supernovae --- during the 5 year mission approx 50,000 supernovae will be detected out to z =0.10. --- accurate distances to large numbers of galaxies from light curves and measurement of deviations of the Hubble flow from redshifts (ground based followup probably needed)

Solar System Bodies

• Expect to detect 10⁵ - 10⁶ new asteroids

• Near Earth Objects may be detected (uncertain)

• ~2000 resolved asteroids? depends on movement during CCD crossing

Summary of tracers

Primary reference: GAIA website