The expected Gaia revolution in asteroid science: Photometry and Spectroscopy

Transcription

1 A. Cellino (INAF- Torino Observatory) P. Tanga, D. Hestroffer, K. Muinonen, A. Dell Oro, L. Galluccio The expected Gaia revolution in asteroid science: Photometry and Spectroscopy

2 Although in situ exploration of single objects has opened new perspectives, the role of remote observations in asteroid science is still crucial.

3 The expected Gaia revolution in asteroid science: Photometry and Spectroscopy INAF --Osservatorio Astrofisico di Torino A. Cellino, EPSC 2014, Cascais Asteroid remote observations by GAIA will be a major step forward!

4 Expected Post-GAIA scenario in Asteroid science: The big Revolution INAF --Osservatorio Astrofisico di Torino The expected Gaia revolution in asteroid science: Photometry and Spectroscopy Huge improvement in orbital elements accuracy A. Cellino, EPSC 2014, Cascais Masses and average densities measured for ~ 100 objects. Sizes for ~1,000 objects inferred from signal analysis. Spin properties and general shapes for tens of thousands objects belonging to all taxonomic classes. Implications for evolution models Reflectance spectra from Blue to Red and new taxonomy of a big sample of the population. Abundance of unusual classes. Implications for the physical and dynamical evolution of main belt asteroids, of families, and for the origin of NEAs.

5 SMASSII Taxonomic Classes Reflectance spectra at visible wavelengths (based on modern CCD detectors) (Principal Components Analysis) Asteroid Spectroscopy

6 ... Classification later extended to include the near- IR.

7 The expected Gaia revolution in asteroid science: Photometry and Spectroscopy INAF --Osservatorio Astrofisico di Torino A. Cellino, EPSC 2014, Cascais The mineralogical interpretation of reflectance spectra

8 GAIA Spectro-Photometry capability Blue photometer: nm Red photometer: nm

9 Expected achievements Sampling of asteroid reflectance spectra from Blue to Red by means of BP/RP data. The reflectance spectrum of each object will come from averaging over tens of transits (checks for possible variations planned) This taxonomic classification will have two major advantages: 1. It will be homogeneous, being based on data collected by a single (and space-based!) instrument; 2. It will include for most objects the Blue region of the spectrum (as opposite to taxonomic classifications based on spectroscopic surveys from the ground carried out during the last decades). The Blue part of the reflectance spectrum is a very important diagnostic tool to distinguish among different sub-classes of primitive, low-albedo objects. As an example, this is true for the objects belonging to the old and interesting F class identified in the 80s based on UBVRI photometry.

10 The F taxonomic class was first introduced by Gradie and Tedesco (1982) and included low-albedo asteroids with a flat (that is why F) spectrum in the wavelength range of μm (Belskaya et al., 2005) Some F objects showed evidence of a cometary nature (Phaethon, Wilson- Harrington) TC3 was also an F-type! However, old F-class asteroids are now mostly included in the today larger B-class Peculiar polarimetric inversion angle of F-type Asteroids

11 Preparing a Gaia-based Taxonomy Starting from a limited number of asteroid spectra obtained at TNG, including objects belonging to the set of different taxonomic classes identified in the literature, and preferentially observed at phase angles comparable to those which will characterize Gaia transits: We did a convolution with BP/RP sensitivity curves. We have produced 100,000 random clone spectra. We are using these synthetic clones to test the taxonomy classification algorithm developed for Gaia. First results encouraging. Work still in progress. A paper will be submitted soon for publication

12 Example of the kind of reflectance spectra that we expect from Gaia, and of the random clones produced to test the Gaia taxonomy algorithm

13 The expected Gaia revolution in asteroid science: Photometry and Spectroscopy INAF --Osservatorio Astrofisico di Torino A. Cellino, EPSC 2014, Cascais GAIA diskintegrated photometry Sparse photometric data (no lightcurves) Good sampling of the variation of the aspect angle over five years of observations Simulation of Gaia observations of the asteroid (21) Lutetia

14 The inversion problem: The objects are assumed to be triaxial ellipsoids. A genetic algorithm is used to solve for the unknown spin period, spin axis direction, two axial ratios, rotational phase at t=0, and a phase-magnitude linear coefficient. The effectiveness of the inversion method tested by means of extensive numerical simulations: Triaxial ellipsoid shapes Complex shapes: digital shapes of (15) Eunomia, (6489) Golevka, (433) Eros, and other objects. Geometric and Hapke light scattering Photometric errors Different simulated orbits, spin periods and poles Application of the code to previous HIPPARCOS photometric data The results are generally encouraging: Photometry inversion for a number of the order of several tens of thousands asteroids could be possible

15 For ideal triaxial ellipsoid shapes, tests performed so far indicate that we can expect between 1% and 3% of bad solutions, for photometric errors up to about 0.03 mag. Some more when photometric errors are high, 0.03 mag. We can also expect about twice as much of non-inverted cases. For complex shapes, the fraction of wrong solutions, without introducing solution filtering, could reach 20%. Residuals larger than average photometric errors can be used to remove most wrong solutions, at the cost of removing also a number of fairly good solutions. Hipparcos data suggest that when one has at disposal only a few, poor quality data, the treatment of light scattering becomes negligible.

16

17 We expect to find solutions (poles, periods and axial ratios) for a significant fraction of the asteroid population. General application: Spin properties of a much bigger and heterogeneous sample as an important constraint to model the evolution of the Main Belt population. Among other possible applications: test of the existence of possible preferential alignments of the spin axes among family members. Koronis Family (Slivan et al. 2003)

18 The expected Gaia revolution in asteroid science: Photometry and Spectroscopy INAF --Osservatorio Astrofisico di Torino A. Cellino, EPSC 2014, Cascais Number of expected good Gaia transits per object as a function of diameter Main Belt Asteroids: the angular sizes of objects larger than km will be measured at least once with a nominal accuracy equal or better than 10%. This corresponds to ~ 1000 objects The size that will be determined by signal processing corresponds to the largest axis a of the derived triaxial ellipsoid shape, the b/a and c/a axial ratios being given by photometry inversion

19 In a nutshell... Mass (astrometry + binaries) Size + Shape Bulk Density Bulk Density Taxonomy Interpretation of taxonomy in terms of composition and internal structure The Gaia revolution in asteroid science

20 So what? What else we do when we have a lot of new reflectance spectra, spin properties, and a new taxonomy?

21 The expected Gaia revolution in asteroid science: Photometry and Spectroscopy INAF --Osservatorio Astrofisico di Torino A. Cellino, EPSC 2014, Cascais Physics of collisional disruption phenomena Asteroid Collisional Evolution Asteroid inventory and size distribution Asteroid Dynamical Evolution Asteroid Families Formation of Binary Systems Asteroid interiors Meteorite showers Origin of near-earth Asteroids

22 Taxonomy and Dynamical Families Spectroscopy and Taxonomy can be particularly important when dealing with families that mutually overlap in the proper element space. moreover, it will be extremely important to see whether the frequency of strange, peculiar spectroscopic features will increase when new families will be analyzed. If this will be the case, we will have probably to modify some of the general ideas we have on the meaning of taxonomic classes, and the importance of space weathering and collisionally induced thermal metamorphism effects. (From Cellino et al., Asteroids III book)

23 Asteroid families tend to be spectrally homogeneous. Unusual taxonomic classes are usual among families! (e.g., F, L, K-type) Apart from the special case of Vesta, no definitive evidence of a differentiated parent body has been found so far.

24 The spectral homogeneity of asteroid families is always very useful to identify family interlopers. Interlopers can be misleading when determining the size distribution and the kinematical properties of the families. Eos family, Vokrouhlicky et al., 2006

25 The old (Cellino et al., 2001) inferences about the Nysa-Polana clan now fully confirmed (Milani et al., 2014). In this plot we make use of the albedo, but the spectra give similar results

26 After Gaia : the occultation revival Today poor predictability for objects <50 km bright Hipparcos/Tycho stars favoured ~0.1 events/object/year Current practical limit: 100 km at 10% accuracy After Gaia (100 X orbit improvement): Uncertainty smaller than the asteroid at >20 km 1-m automated telescope(s): Single site: events/yr for any object of ~20 km Network: completeness of diameters > 20 km in a few yr Derivation of projected shape Discovery of binary systems

27 The expected Gaia revolution in asteroid science: Photometry and Spectroscopy INAF --Osservatorio Astrofisico di Torino A. Cellino, EPSC 2014, Cascais Thank you! Picture taken here in Helsinki many years ago at the beginning of this adventure

28 Photometry Stray light science impact (G2V star) Some tenths of mag loss for achieving 0.01 mag single transit accuracy Since the stray-light effect is both time-dependent and FOV position-dependent, for each object there will be some clean transits and some dirty transits Mitigation Background modelling software for all instruments already adapted to stray light Longer windows for astrometric data (direct measurements of background) On-board software updates for end