Global Astrometric Interferometer For Astrophysics Research Articles

Optical polarimetry study of the Lambda-Orionis star-forming region

We present an optical polarimetry study of the nearby star-forming region Lambda-Orionis to map the plane-of-the-sky magnetic field geometry to understand the magnetized evolution of the HII region and the associated small molecular clouds. We made multiwavelength polarization observations of 34 bright stars distributed across the region. We also present the R-band polarization measurements that focused on the small molecular clouds, bright-rimmed clouds (BRC), BRC 17, and BRC 18, which are located at the periphery of the HII region. The magnetic field lines exhibit a large-scale ordered orientation consistent with the Planck submillimeter polarization measurements. The magnetic field lines in the two BRCs are found to be roughly in north-south directions. However, a larger dispersion is noted in the orientation for BRC 17 compared to BRC 18. Using a structure-function analysis, we estimate the strength of the plane-of-the-sky component of the magnetic field as ∼28 μG for BRC 17 and ∼40 μG for BRC 18. The average dust grain size and the mean value of the total-to-selective extinction ratio (RV) in the HII region are found to be ∼0.51 ± 0.05 μm and ∼2.9 ± 0.3, respectively. The distance of the whole HII region is estimated as ∼392 ± 8 pc by combining astrometry information from Global Astrometric Interferometer for Astrophysics (Gaia) early data release 3 (EDR3) for young stellar objects associated with BRCs and confirmed members of the central cluster Collinder 69.

Galactic Stellar Populations in the Era of the Sloan Digital Sky Survey and Other Large Surveys

Studies of stellar populations, understood to mean collections of stars with common spatial, kinematic, chemical, and/or age distributions, have been reinvigorated during the past decade by the advent of large-area sky surveys such as the Sloan Digital Sky Survey, the Two-Micron All Sky Survey, the Radial Velocity Experiment, and others. We review recent analyses of these data that, together with theoretical and modeling advances, are revolutionizing our understanding of the nature of the Milky Way and galaxy formation and evolution in general. The formation of galaxies like the Milky Way was long thought to be a steady process leading to a smooth distribution of stars. However, the abundance of substructure in the multidimensional space of various observables, such as position, kinematics, and metallicity, is now proven beyond doubt and demonstrates the importance of mergers in the growth of galaxies. Unlike smooth models that involve simple components, the new data reviewed here clearly exhibit many irregular structures, such as the Sagittarius dwarf tidal stream and the Virgo and Pisces overdensities in the halo and the Monoceros stream closer to the Galactic plane. These recent developments have made it clear that the Milky Way is a complex and dynamic structure, one that is still being shaped by the merging of neighboring smaller galaxies. We also briefly discuss the next generation of wide-field sky surveys, such as SkyMapper, Panoramic Survey Telescope & Rapid Response System, Global Astrometric Interferometer for Astrophysics, and the Large Synoptic Survey Telescope, which will improve measurement precision manyfold and include billions of individual stars. The ultimate goal, development of a coherent and detailed story of the assembly and evolutionary history of the Milky Way and other large spirals like it, now appears well within reach.

The Long-Lost Siblings of the Sun

The article discusses what became of the family of stars that bore the sun. According to the author, astronomers once believed the sun was a solitary star, but evidence from meteorites and the arrangement of comets suggest that Earth's sun was born out of a cluster of stars. Topics include the use of the European Space Agency's (ESA) Global Astrometric Interferometer for Astrophysics (GAIA) to identify former cluster members and isotope measurements of iron (Fe) 60 in meteorites. INSETS: MEET THE NEIGHBORS;FAMILY BREAKUP.

PROBING THE GALACTIC POTENTIAL WITH NEXT-GENERATION OBSERVATIONS OF DISK STARS

Our current knowledge of the rotation curve of the Milky Way is remarkably poor compared to other galaxies, limited by the combined effects of extinction and the lack of large samples of stars with good distance estimates and proper motions. Near-future surveys promise a dramatic improvement in the number and precision of astrometric, photometric, and spectroscopic measurements of stars in the Milky Way's disk. We examine the impact of such surveys on our understanding of the Galaxy by observing particle realizations of nonaxisymmetric disk distributions orbiting in an axisymmetric halo with appropriate errors and then attempting to recover the underlying potential using a Markov Chain Monte Carlo approach. We demonstrate that the azimuthally averaged gravitational force field in the Galactic plane—and hence, to a lesser extent, the Galactic mass distribution—can be tightly constrained over a large range of radii using a variety of types of surveys so long as the error distribution of the measurements of the parallax, proper motion, and radial velocity are well understood and the disk is surveyed globally. One advantage of our method is that the target stars can be selected nonrandomly in real or apparent-magnitude space to ensure just such a global sample without biasing the results. Assuming that we can always measure the line-of-sight velocity of a star with at least 1 km s–1 precision, we demonstrate that the force field can be determined to better than ~1% for Galactocentric radii in the range R = 4-20 kpc using either: (1) small samples (a few hundred stars) with very accurate trigonometric parallaxes and good proper-motion measurements (uncertainties δ p,tri 10 μas and δμ 100 μas yr–1 respectively); (2) modest samples (~1000 stars) with good indirect parallax estimates (e.g., uncertainty in photometric parallax δ p,phot~ 10%-20%) and good proper-motion measurements (δμ ~ 100 μas yr–1); or (3) large samples (~104 stars) with good indirect parallax estimates and lower accuracy proper-motion measurements (δμ~ 1 mas yr–1). We conclude that near-future surveys, like Space Interferometry Mission Lite, Global Astrometric Interferometer for Astrophysics, and VERA, will provide the first precise mapping of the gravitational force field in the region of the Galactic disk.

Can theLaser Interferometer Space AntennaResolve the Distance to the Large Magellanic Cloud?

The Laser Interferometer Space Antenna (LISA) is expected to detect N ~ 22 × 10±1 close white dwarf binaries in the Large Magellanic Cloud (LMC) through their gravitational radiation with signal-to-noise ratios greater than ~10 in observational durations of 3 yr or more. In addition to chirp mass, location on the sky, and other binary parameters, the distance to each binary is an independent parameter that can be extracted from an analysis of gravitational waves from these binaries. Using a sample of binaries, one can establish the mean distance to the LMC as well as the variance of this distance. Assuming no confusion noise at frequencies above 2 mHz, for data collected over 10 yr, LISA might determine the LMC distance to ~4.5(N/22)1/2(1 yr/Tobs)1/2% and the line-of-sight extent of the LMC to ~15(N/22)1/4%, relative to its distance, at the 1 σ confidence. For a 3 yr observational window, these estimates are degraded by a factor of ~2.5. In general, the estimates based on LISA are competitive with some of the proposed direct geometric techniques of measuring the LMC distance in the future with missions such as the Space Interferometry Mission and the Global Astrometric Interferometer for Astrophysics.

Discovery of an Unbound Hypervelocity Star in the Milky Way Halo

We have discovered a star, SDSS J090745.0+024507, leaving the Galaxy with a heliocentric radial velocity of 853 ± 12 km s-1, the largest velocity ever observed in the Milky Way halo. The star is either a hot blue horizontal-branch star or a B9 main-sequence star with a heliocentric distance of 39 or 71 kpc, respectively. Corrected for the solar reflex motion and to the local standard of rest, the Galactic rest-frame velocity is 709 km s-1. We suggest that this star is the first example of a hypervelocity star ejected from the Galactic center, as predicted by Hills and later discussed by Yu & Tremaine. The star's radial velocity vector points 174° from the Galactic center. The star has [Fe/H] ~ 0, consistent with a Galactic center origin, and a travel time of 80 Myr from the Galactic center, consistent with its stellar lifetime. If the star is indeed traveling from the Galactic center, it should have a proper motion of ~0.3 mas yr-1 observable with the Space Interferometry Mission or the Global Astrometric Interferometer for Astrophysics. Identifying additional hypervelocity stars throughout the halo will constrain the production rate history of hypervelocity stars at the Galactic center.

A Candidate M31/M32 Intergalactic Microlensing Event

We report the discovery of a microlensing candidate projected 2'54'' from the center of M32, on the side closest to M31. The blue color (R-I = 0.00 ? 0.14) of the source argues strongly that it lies in the disk of M31, while the proximity of the line of sight to M32 implies that this galaxy is the most likely host of the lens. If this interpretation is correct, it would confirm previous arguments that M32 lies in front of M31. If more events are discovered in this direction in a dedicated experiment, they could be used to measure the mass function of M32 up to an unknown scale factor. By combining microlensing observations of a binary-lens event with a measurement of the M31-M32 relative proper motion using the astrometric satellites Space Interferometry Mission or Global Astrometric Interferometer for Astrophysics, it will be possible to measure the physical separation of M31 and M32, the last of the six phase-space coordinates needed to assign M32 an orbit.

Finding Black Holes with Microlensing

The MACHO and OGLE collaborations have argued that the three longest duration bulge microlensing events are likely caused by nearby black holes, given the small velocities measured with microlensing parallax and nondetection of the lenses. However, these events may be due to lensing by more numerous lower mass stars at greater distances. We find a posteriori probabilities of 76%, 16%, and 4% that the three longest events are black holes, assuming a Salpeter initial mass function (IMF) and a 40 M☉ cutoff for neutron star progenitors; the numbers depend strongly on the assumed mass function but favor a black hole for the longest event for most standard IMFs. The longest events (>600 days) have an a priori ~26% probability of being black holes for a standard mass function. We propose a new technique for measuring the lens mass function using the mass distribution of long events measured with the Advanced Camera for Surveys on the Hubble Space Telescope, the Very Large Telescope Interferometer, the Space Interferometry Mission, or the Global Astrometric Interferometer for Astrophysics.

Four New δ Scuti Pulsators from a Variability Survey of 131 Stars

ABSTRACTIn a photometric variability survey of 131 stars with B≲14 mag, we have found four new δ Scuti stars. We were sensitive to oscillation amplitudes as low as a few millimagnitudes (mmag). The detection rate of short‐period (P<0.1 days) pulsating variable stars, which may be relevant for planned large‐variability surveys such as the Global Astrometric Interferometer for Astrophysics, the Panoramic Optical Imager, and even the Large‐Aperture Synoptic Survey Telescope, was therefore 3%. All four new variable stars have low pulsation amplitude (tens of mmag), and one has a very short period (0.0198 days). This star is one of the fastest known δ Scuti pulsators. The short period of this variable star makes it observationally tractable, and it may therefore be a particularly good candidate for asteroseismological studies. All four new variable stars will add to the cadre of low‐amplitude and relatively short period δ Scuti stars that are potentially useful for learning about the structure of stars on or near the main sequence, slightly more massive than the Sun.

Sub-nanometric optics stabilization in view of the GAIA astrometric mission

The global astrometric interferometer for astrophysics (GAIA) of the European Space Agency aims to determine the position of about 10 9 stars with an accuracy better than 10 μarcsec. This implies stabilizing the most critical mirror movements of the GAIA telescope at the picometer level, a target requiring the development of appropriate sub-nanometric technology in the field of dimensional metrology and active mirror mechanisms. An Italian team has designed an actively stabilized telescope meeting the stabilization target and has developed and demonstrated the relevant technology through a laboratory prototype. The paper focuses on the design and simulated results of the GAIA optics control system and briefly reports the relevant ground experiments.

Sub-Nanometric Optics Stabilization In View Of Gaia Astrometric Mission

The Global Astrometric Interferometer for Astrophysics (GAIA) mission of the European Space Agency aims to determine the position of one billion stars with an accuracy better than 10 micro-arcseconds. This implies stabilizing the most critical mirror movements of the GAIA telescope at the picometer level (10-12m), an unprecedented target requiring the development of appropriate sub-nanometric technology in the field of dimensional metrology and active mirror mechanisms. A team lead by Alenia Aerospazio has produced a design of the actively stabilized GAIA telescope that meets the stabilization target and has developed and demonstrated the relevant sub-nanometric technology through a laboratory prototype. The paper focuses on the design and on the simulated results of the GAIA active optics control system, employing the metrology and the mirror mechanisms tested by on-ground experiments.

Are there sextuplet and octuplet image systems?

We study gravitational lensing by the family of scale-free galaxies with flat rotation curves. The models are defined by a shape function, which prescribes the radius of the isophote as a function of the position angle from the major axis. The critical curves are analytic, while the caustic network is reducible to a simple quadrature. The cusps are always located at the turning points of the shape function. We show that the models with exactly elliptic isophotes never admit butterfly or swallowtail cusps and so have at most four (or five) images. Higher order imaging is brought about by deviations of the isophotes from pure ellipses, such as pointedness caused by embedded discs or boxiness caused by recent merging. The criteria for the onset of sextuple and octuple imaging can be calculated analytically in terms of the ellipticity e and the fourth-order Fourier coefficients (a4 and b4) used by observers to parametrize the isophote shapes. The six or eight images are arranged roughly in a circle, which appears as an incomplete Einstein ring if inadequately resolved. Using data on the shapes of elliptical galaxies and merger remnants, we estimate that ∼1 per cent of all multiply imaged quasars may be sextuplet systems or higher. Forthcoming satellites such as the Global Astrometric Interferometer for Astrophysics (GAIA) will provide data sets of ∼4000 multiply imaged systems, and so ∼40 will show sextuple imaging or higher.

The mass of the Andromeda galaxy

ABSTRA C T This paper argues that the Milky Way galaxy is probably the largest member of the Local Group. The evidence comes from estimates of the total mass of the Andromeda galaxy (M31) derived from the three-dimensional positions and radial velocities of its satellite galaxies, as well as the projected positions and radial velocities of its distant globular clusters and planetary nebulae. The available data set comprises 10 satellite galaxies, 17 distant globular clusters and nine halo planetary nebulae with radial velocities. We find that the halo of Andromeda has a mass of ,12:3 118 26 10 11 M(; together with a scalelength of ,90 kpc and a predominantly isotropic velocity distribution. For comparison, our earlier estimate for the Milky Way halo is 19 136 217 10 11 M(: Although the error bars are admittedly large, this suggests that the total mass of M31 is probably less than that of the Milky Way .W e verify the robustness of our results to changes in the modelling assumptions and to errors caused by the small size and incompleteness of the data set. Our surprising claim can be checked in several ways in the near future. The numbers of satellite galaxies, planetary nebulae and globular clusters with radial velocities can be increased by ground-based spectroscopy, while the proper motions of the companion galaxies and the unresolved cores of the globular clusters can be measured using the astrometric satellites Space Interferometry Mission (SIM) and Global Astrometric Interferometer for Astrophysics (GAIA). Using 100 globular clusters at projected radii 20 & R & 50 kpc with both radial velocities and proper motions, it will be possible to estimate the mass within 50 kpc to an accuracy of ,20 per cent. Measuring the proper motions of the companion galaxies with SIM and GAIA will reduce the uncertainty in the total mass caused by the small size of the data set to ,22 per cent.

The Future of Space Astrometry

AbstractThe future of space-based astrometric missions appears to be very promising. Three missions were described at IAU Colloquium 180. These are the Full-sky Astrometric Mapping Explorer (FAME), the Space Interferometry Mission (SIM) and the Global Astrometric Interferometer for Astrophysics (GAIA) missions. These missions will substantially improve the accuracy of global astrometric measurements made by the Hipparcos space mission by factors of 20 to 250. A brief summary of these projects is given.

Optical configuration for a micro-arcsecond astrometric interferometer in space

One of the ESA candidate cornerstone missions is dedicated to wide-angle astrometry and aims at dramatically increasing the accuracy of astrometric measurements and the limiting magnitude with respect to Hipparcos (ESA Symposium, Venice, 1997). This mission was initially designated by the acronym GAIA (Global Astrometric Interferometer for Astrophysics) and featured several Fizeau interferometers. One important drawback of the original configuration proposed by Lindegren & Perryman (1996) was the confusion effect. This effect, which is a superposition of the light coming from different objects, is due to recording the fringe signal in the exit pupil plane of the instrument, necessary to relax the pixel size in the considered configuration. In order to overcome the confusion effect while retaining the original Fizeau interferometer concept, we present an optical configuration allowing direct fringe detection, i.e. in the image plane. It features focal length of 30 m inducing technologically feasible pixel size in the bandwidth of interest, and meets the condition to fit the Ariane V launcher fairing.

Dynamical Evolution of Open Star Clusters

Stars are not formed independently, but instead they form in clusters. The influence of the initial mass function (IMF) on the evolution of open star clusters is analyzed using numerical integrations of N-body systems by the code NBODY5 (S. J. Aarseth, in Multiple Time Scales, ed. J. U. Brackbill & B. I. Cohen [New York: Academic, 1985], p. 377), which include tidal effects, mass loss due to stellar evolution, the realistic fraction of primordial binaries, and the formation of multiple systems. Five different IMFs (E. E. Salpeter, ApJ, 121, 161 [1955]; L. G. Taff, AJ, 79, 11 [1974]; G. E. Miller & J. M. Scalo, ApJS, 41, 513 [1979]; P. Kroupa, C. A. Tout, & G. Gilmore, MNRAS, 262, 545 [1993]; J. M. Scalo, Fundam. Cosmic Phys., 11, 1 [1986]) are used for generating stellar masses. The results confirm significant differences with singlemass models and allow us to distinguish between the standard power-law models and modern ones. An approximate analytic expression for the escape rate is derived in order to fit the data obtained. When stellar evolution is included, the results show that for all the IMF’s studied, the evolution of the cluster is slowed down and the initial core collapse loses importance because of an expansion of the inner regions of the cluster. We find that the total disruption time is very IMF dependent because of different numbers of massive stars and also depends on the richness of the cluster. A differential behavior is found between poor and rich systems with respect to mass loss. Poor systems disrupt earlier than homologous ones without mass loss; the opposite is found for rich systems. The transition population is about N 300. The binary escape rate seems preferentially due to close encounters in poor clusters, but it seems mainly exponential for populated clusters. It suggests that ejection is the main mechanism for binary escape in poor clusters and evaporation is the dominant one for rich clusters. The formation and evolution of hierarchical systems is also considered in detail. Some questions concerning multicomponent clusters, such as the preferential evaporation of light stars, are reviewed. The global results are compared with the observational data of actual clusters from The Database for Stars in Open Clusters: BDA (J.-C. Mermilliod, in ASP Conf. Ser. 90, The Origins, Evolution, and Destinies of Binary Stars in Clusters, ed. E. F. Milone & J.-C. Mermilliod [San Francisco: ASP, 1996]). The likely observational properties of the final stages of the evolution of open clusters are also investigated, and the results are compared with the available observational data (L. O. Loden, ApS R. P. Stefanik, J. R. Caruso, G. Torres, S. Jha, & D. W. Latham, Baltic Astron., 6, 137 [1997]). It is found that they depend on the membership of the cluster, the abundance of primordial binaries, and the initial mass function. The final cluster remnant is very rich in binaries and hierarchical systems. Remnants of poorly populated clusters are relatively easy to identify because they contain earlytype stars. Remnants of rich open clusters are difficult to detect and might exist in large numbers. The detection of rich open clusters’ remnants (OCRs) is a great challenge for the largest available telescopes because they contain only faint stars. It is possible that some of the oldest known open clusters are in fact remnants of densely populated ( ) clusters. The N ≈ 40,000 existence of a large number of OCRs may be relevant for dark matter in the Galactic disk. This hypothesis could be tested by using the capabilities of the proposed global astrometry mission Global Astrometric Interferometer for Astrophysics (GAIA).

Drift Scanning from Ground and in Space

The drift scanning technique offers, in principle, a simple and efficientmethod for imaging wide regions of the sky with intrinsic high astrometricaccuracy. Its performance is analyzed in two distinct environments, namelyfrom a small class ground based telescope and a spaceborne interferometer. Inparticular, the study is referred to the proposed ESA cornerstone missionGAIA (Global Astrometric Interferometer for Astrophysics).

Optical design, modelling and tolerancing of a Fizeau interferometer dedicated to astrometry

Space-based interferometers dedicated to wide-angle astrometry would dramatically increase the accuracy of angular measurements fundamental to a wide range of astrophysical problems. The proposed Global Astrometric Interferometer for Astrophysics (GAIA), a continuously rotating instrument comprising two or three interferometers, will reach the as level on more than 35 million objects. The necessary wide field-of-view for such a precision could be obtained with a Fizeau interferometer. We designed and modelled a 2.6 m baseline interferometer with two 40 cm apertures, and overall dimensions compatible with the size of the Ariane V payload shroud. It has a ~ 1 degree diffraction limited field-of-view. The response of the optical system to small perturbations on each optical element is given in terms of fringe visibility, which is shown to depend mainly on sub-aperture spot separation. The robustness of the design to thermal, mechanical and manufacturing errors is discussed.

HIPPARCOS Data on Pulsating Stars

AbstractThe preliminary astrometric and photometric results obtained from the first 30 months of HIPPARCOS data are presented. Examples of the results obtained for the various categories of pulsating variables included in the HIPPARCOS programme are given. The perspectives opened by GAIA (Global Astrometric Interferometer for Astrophysics), the concept for an astrometric interferometer submitted in September 1994 to the ESA Horizon 2000+ survey committee, are briefly exposed.