TNOs are Cool: A survey of the Transneptunian region with Herschel Space Observatory

Th. G. Müller1 (PI), E. Lellouch2 (Co-PI), H. Böhnhardt3, (Co-PI), J. Stansberry4, (NASA-PI), C. Kiss5, P. Santos-Sanz2, E. Vilenius1,S. Protopapa3, R. Moreno2, M. Mueller6, A. Delsanti2,7, R. Duffard8, S. Fornasier2, O. Groussin7, A. W. Harris9, F. Henry2, J. Horner10,P. Lacerda11, T. Lim12, M. Mommert9, J. L. Ortiz8, M. Rengel3, A. Thirouin8, D. Trilling13, A. Barucci2, J. Crovisier2, A. Doressoundiram2,E. Dotto14, P. J. Gutierrez Buenestado8, O. R. Hainaut15, P. Hartogh3, D. Hestroffer2, M. Kidger16, L. Lara8, B. Swinyard12, N. Thomas17, A. Pal5, D. Jewitt18, A. Guilbert18

(1) Max Planck Institute for Extraterrestrial Physics, Germany, (2) Observatoire de Paris, France, (3) Max Planck Institute for Solar System Research, Germany, (4) The University of Arizona, USA, (5) Konkoly Observatory of the Hungarian Academy of Sciences, Hungary, (6) Observatoire de la Cote d'Azur, France, (7) Laboratoire d'Astrophysique de Marseille, France, (8) Instituto de Astrofisica de Andalucia CSIC, Spain, (9) Deutsches Zentrum für Luft- und Raumfahrt DLR, Germany, (10) Department of Physics and Astronomy, Science Laboratories, University of Durham, United Kingdom, (11) Newton Fellow of the Royal Society, Astrophysics Research Centre, Queen's University, Belfast, UK, (12) Space Science and Technology Department, Rutherford Appleton Laboratory, UK, (13) Northern Arizona University, Department of Physics & Astronomy, USA, (14) INAF-Osservatorio Astronomico di Roma, Italy, (15) European Southern Observatory, Germany, (16) Herschel Science Centre, ESA, ESAC, Spain, (17) Universität Bern, Switzerland, (18) Dept. Earth Space Sciences, UCLA, California, USA.

Abstract

2. Orbits of our TNO / Centaur sample

7. Outlook

References

[1] Müller, Th. G. et al., Earth, Moon, Planets, 105:209-219, 2009.[2] Poglitsch, A. et al., A&A, in press, 2010.[3] Griffin et al., A&A, in press, 2010.[4] Lebofsky, L. A. et al., Icarus, Vol. 68, 239, 1986.[5] Veeder, G. J. et al., AJ, Vol, 97, pp. 1211-1219, 1989.[6] Harris, A. W., Icarus, Vol. 131, pp. 291-301, 1998.[7] Lagerros, J. S. V., A&A, Vol. 310, 1011, 1996.[8] Rabinowitz et al., ApJ 693, 43, 2006.[9] Stansberry et al., astro-ph/0702538, 2007.

1. Introduction

4. Thermal modeling

5. Herschel observations

About 400 hours of observing time have been granted to the Herschel Open Time Key Programme “TNOs are Cool: A survey of the Transneptunian region” [1]. In this programme we are using photometric observing modes of the PACS [2] and SPIRE [3] instruments to obtain the far-infrared fluxes of 138 objects representing different dynamical classes (resonant, classical, scattered disk and detached TNOs as well as Centaurs) and including 25 binary systems. Correlations between size, albedo, color, composition and orbital parameters are diagnostic of evolution processes. While Spitzer has revealed a large albedo diversity in the TNO population the increased wavelength coverage and sensitivity of Herschel will enable profound advances in this field. The shape of the size distribution constrains the formation theories of accretion and collisional erosion, and it is also used in determining the total mass. The four prime scientific goals of this programme are: (i) to determine sizes and albedos, (ii) to measure the density of binary TNOs, (iii) to constrain surface properties, and (iv) to determine lightcurves of five objects by continuously observing them throughout an entire rotational period. Our first results using combined data from Herschel and Spitzer as well as visual magnitudes from our ground-based support programmes give effective diameters, geometric albedos and thermal inertias of Haumea, Orcus, Makemake, Typhon, 2003 AZ

84, 2001 YH

140, 1997 CS

29, 2006 SX

368 and 2005 TB

190. The

Haumea lightcurve observed with PACS shows positive correlation between the optical and thermal lightcurves indicating that it is due to shape effects.

Acknowledgements: Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Herschel data presented in this poster were analysed using “HIPE”, a joint development by the Herschel Science Ground Segment Consortium, consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS and SPIRE consortia.

More than one thousand Trans-Neptunian objects (TNO) have so far been discovered in our solar system. They are remnants of the planetesimal disk; the size distribution of large TNOs is assumed to have remained unchanged although their surface material may have changed its composition over time due to collisions, meteorite impacts and space weathering. Red color of objects is a consequence of space weathering; it also makes surfaces darker. Objects having experienced recent impacts are expected to be brighter and bluer due to excavated un-weathered material.

Thermal emission of an airless body depends primarily on its size and albedo. Surface emissivity, roughness and porosity also influence the shape of the SED. The albedo and absolute reflectance are important in constraining the surface composition (without absolute reflectance the results from spectroscopy are semi-quantitative); this requires the combination of Herschel and ground-based support observations at optical wavelengths.

The fluxes of TNOs, with temperatures in the range 20-50 K, have their maxima in the PACS wavelengths (55 to 210 m). Our flux estimates of the 138 targets at the PACS and SPIRE (194 to 672 m) wavelengths range from a few mJy to 400 mJy.

Thermal and thermophysical models (STM [4], FRM/ILM [5], NEATM [6], TPM [7]) provide sizes and albedos, and they also give indications on the surface properties. As input we use the far-infrared fluxes together with the magnitude in visible wavelengths.

Lightcurves of the five objects in our lightcurve sub-programme are influenced by two major factors: albedo features on the surface and the shape of the object. In the case of shape effects, the mean flux and amplitude are diagnostic of the distribution of temperatures on the object, thereby constraining the spin vector and the thermal inertia. Large TNOs (radius >100 km) may have the primordial distribution of angular momenta whereas smaller objects have had their spins, shapes and sizes collisionally altered.

Within our ``TNOs are Cool`` programme we will observe about 140 TNOs and the results are expected to provide a benchmark for understanding the solar system debris disk, and extra-solar ones as well. We will observe 25 binary TNOs as well as the lightcurves of Varuna, Haumea, 2003 VS2, 2004 TY364, and 2003 AZ84 for over a rotational period. By September 2010 we have Herschel observations for about 60 targets, including the most prominent dwarf planets Pluto, Eris, Haumea and Makemake.

Herschel / PACS observations produce a map with a spatial resolution of 1“-2“ per pixel and 50“ diameter area useful for photometry, whereas Herschel / SPIRE produces a map of 5' diameter with a resolution of 6“-14“ per pixel. Based on our experience on observing TNOs with Herschel we conclude that a follow-on observation at a different sky background while the target is still within the map of the first observation is a useful strategy at wavelengths higher than 100 m in order to distinguish the target from the background. A typical on-source time per target is about 0.5 h, except for lightcurve targets which are observed several hours to cover a rotational period.

The total mass of a binary system is known from its orbit, and the mass ratio from the flux ratio between the components. Without Herschel the diameter D, and thus volume and density, would be derived from the D=D(H,p

v) relation, where H is the absolute

magnitude in the IAU (H,G) system and pv is the geometric

albedo. Both of these parameters may contain large uncertainties. Herschel provides the effective diameter needed to derive more accurate bulk density.

Model calculations of the semi-empirical Standard Thermal Model (STM) for objects with diameters 100 km and 1000 km (smooth, spherical, slowly rotating body) with thermal beaming, shape and thermal conductivity effects taken into account through the beaming parameter .

Model calculations of the Thermophysical Model (TPM) compared to an average Spitzer TNO at 40 AU to demonstrate the influence of surface properties on the thermal flux. The thermal inertia causes major uncertainties at wavelengths below the emission peak while the unknown emissivities affect the sub-mm range where also the influence of extreme surface conditions is seen (dashed lines).

Thermophysical temperature calculation for Haumea (shape model [8]) as seen from Herschel with thermal properties based on Spitzer data [9]. Based on this model thermal lightcurves can be predicted and the effects of surface properties, spin vector orientation and wavelength can be studied. The middle figure shows Haumea's thermal lightcurve at 100 m with a spin vector perpendicular to the ecliptic plane, and the right figure shows the same with spin axis at 45o angle.

Herschel/SPIRE observations of the dwarf planet Makemake. This map is a positive/negative combination of two observations following each other after 43 hours, each of the maps was a combination of two cross scan maps composed of 12 parallel scans. The flux at 250 m is (9.5+/-3.1) mJy.

Haumea lightcurve observed with PACS 100 and 160 m channels. The thermal lightcurve is correlated with the visible lightcurve with a factor of variation 2 while the visible lightcurve has a factor of variation 1.3.

• TPM with thermal inertia as a free parameter or NEATM with a fitted beaming parameter explain surface temperature distribution better than “canonical” models.

• Single albedo solutions work well in most cases, one exception is Makemake with a two-terrain model.

• First results published in three papers:

Müller, Lellouch, Stansberry et al., A&A, 518, L146, 2010“TNOs are Cool”: A survey of the Trans-Neptunian region I. Results from the Herschel Science Demonstration Phase (SDP)

Lellouch, Kiss, Santos-Sanz et al., A&A, 518, L147, 2010“TNOs are Cool”: A survey of the Trans-Neptunian region II. The thermal lightcurve of (136108) Haumea

Lim, Stansberry, Müller et al., A&A, 518, L148, 2010“TNOs are Cool”: A survey of the Trans-Neptunian region III. Thermophysical properties of 90482 Orcus and 136472 Makemake

6. Results

3. Improved bulk density of binaries