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Advances in Geosciences Vol. 25: Planetary Science (2010) Ed. Anil Bhardwaj c World Scientific Publishing Company


SOLAR SYSTEM SCIENCE WITH THE HYPER SUPRIME-CAM SURVEY F. YOSHIDA∗ , T. TERAI, S. URAKAWA, S. ABE, W.-H. IP, S. TAKAHASHI, T. ITO and HSC SOLAR SYSTEM SCIENCE GROUP ∗ National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan [email protected]

The Hyper Suprime-Cam (HSC) survey with the 8.2-m Subaru telescope is a multi-color photometric survey. The survey has been designed to provide a fundamental data set for various areas of cosmology-related researches in the next decade. However, it is also able to cover more general science purposes. In this paper, we describe briefly a current plan of the HSC survey and feasibility focusing on the Solar System science.

1. Introduction The Hyper Suprime-Cam (HSC) is a new mosaic CCD camera, which will be attached to the prime focus of the 8.2-m Subaru telescope at Mauna Kea in late 2011. The focal plane of the HSC consists of 116 CCDs array of 2K × 4K. The field of view (FOV) is 1.5 degree in diameter, which is six times larger than that of the current prime focus camera, SuprimeCam (FOV: 0.25 deg2 ). The pixel scale is 0.2 arcsec/pix. Minimum exposure time is 2 sec and minimum interval of exposure time is 20 sec including CCD readout and pointing change. At least, the g
, r
, i
, z
, Y-filters will be equipped (other filters are under discussion). Under current design of the filter folder, at least four filters can be loaded in the folder. The filter exchange time is about 10 min. The filter exchange should be done in the condition that telescope turns to the zenith. Exploiting its large FOV, the great sky condition on the Hawai island, and high CCD sensitivity, we will be able to obtain unprecedentedly remarkable observation data.

∗ Corresponding

author. 1

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The current plan of the HSC survey is based on 300 nights of Subaru telescope for 5 years, though the total number of nights is still under discussion. The HSC survey is designed to reach the limiting magnitude of at least one magnitude fainter than other on-going/planed wide field surveys in the HSC era to be conducted with 4-m class telescopes (e.g., the CFHT Legacy Survey and the Dark Energy Survey). The HSC survey plan has been discussed by several working groups over the last few years. The fundamental survey purposes and survey strategy have been mostly defined. There are six main sciences of the HCS survey as follows: (1) understanding the nature of dark energy, (2) exploring the large-scale structure, (3) studying the galaxy formation process, (4) understanding the nature of AGNs, (5) exploring the transient and variable objects (SNs, GRBs etc.), and (6) obtaining a better understanding of Solar System history. For optimizing survey design to various science cases, four survey categories were proposed (see Section 2).

2. Survey Categories The HSC survey is not a whole sky survey. This is because the Subaru telescope is not a dedicated survey telescope. Only 300 nights for 5 years will be available for the HSC survey. Therefore, we need well-organized survey strategy. The HSC survey plan was examined separately in the following four categories: wide survey, deep survey, ultra-deep survey, and solar system survey.

2.1. Wide survey The wide survey is a multi-band wide field imaging survey with five passbands (g
, r
, i
, z
, Y). Proposed survey area is between 1000 and 2000 deg2 . Table 1 shows the exposure time and survey depth with each band and the survey depth of the large synoptic survey telescope (LSST) survey1 was included for comparison. One can see that the HSC wide survey

Table 1.

Limiting magnitude at each band.

Band Ecp. Time (min) Survey depth (mag) Cf. LSST (Exp. 15sec, 5σ)

g

r

i

z

Y

15 26.5 25.0

20 26.4 24.7

15 25.8 24.0

20 24.9 23.3

25 23.7 22.1

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Candidate survey fields of the wide survey.

is deeper than the LSST survey. The main focus of the wide survey is weak lensing-related sciences. However, it can provide a massive detection of small solar system bodies (SSSBs) because of its survey depth and large survey area. Therefore, the data from the wide survey will be very useful for Solar System science, especially for exploring slow-moving objects in outer Solar System, the so-called Planet X, eccentric large TNOs, inner-Oort cloud objects, etc. Figure 1 shows the candidate fields of wide survey. The gray regions show the survey fields of the large area survey (LAS) of the UKIRT Infrared Deep Sky Survey (UKIDSS), the VIKING-N (VISTA Kilo-degree INfrared Galaxy - North) survey field, and the SDSS strip-82 field (also the UKIDSS LAS survey field), the black regions show the Canada-FranceHawaii Telescope Legacy Survey (CFHTLS) wide fields including the XMMLSS (X-ray Multi-Mirror Mission-Large Scale Structure Survey) field and the UKIDSS-DXS (deep extragalactic survey) fields, and the hatched area show preferable regions for weak lensing survey. The fields between 1000 and 2000 deg2 will be selected from the candidate fields. The fields near the ecliptic plane will be surveyed in the category of the Solar System survey.

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2.2. Deep survey The deep survey is an intermediate survey between the wide and ultra-deep surveys. The proposed survey area is between 20 and 50 deg2 , which will be selected from high ecliptic latitude areas of the wide survey fields. Survey depth depends on each science case, but the average depth is 27.6 mag with the r
-band. This survey is also designed for multi-epoch imaging to search for transient and variable objects. In this survey category, we expect to detect SSSBs that have high orbital inclinations at high ecliptic latitude regions (>15 degrees). Since the collisional probability/speed of such objects seems to be different from those of low inclination objects, we may find different size distributions between populations having high orbital inclination and those having low ones. We propose to determine their orbits and measure their colors with at least g
, r
, i
-bands.

2.3. Ultra-deep survey Specialized deep observing sequences to obtain faintest galaxies are proposed. It is called the ultra-deep surveys. The survey area is only a few deg2 (1 or 2 FOV). The candidate fields are SXDS (Subaru/XMMNewton deep survey) − UDS (ultra deep survey) (at R.A. 2h 18m 00s , Dec. −05d 00m 00s ) + COSMOS (cosmic evolution survey) − UltraVista (at R.A. 10h 00m 28.6s , Dec. +02d12m 21s ). The survey depth is 27–28 mag for broad bands and 26–27 mag for narrow bands. Primary science goals are (1) understanding the early galaxy formation and cosmic reionization and (2) giving strong constraints on the equation of state of dark energy. However, the ultra-deep survey can also address various important issues. For Solar System studies, this survey can be applicable to lightcurve observations for obtaining spin period and shape distributions of various population of SSSBs, because it would take images at the same field with the time interval of several ten minutes during one night. Two candidate fields of the ultra-deep survey locate at −18 and −9 degrees in ecliptic latitude. Since the estimate detection number of SSSBs with R < 24.7 mag is ∼80 at −18 degrees and ∼200 at −9 degrees in ecliptic latitude, respectively, this survey can increase significantly the data of the current lightcurve catalog of SSSBs. We propose to determine their orbits and measure their colors with at least g
, r
-bands.

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2.4. Solar system survey The Solar System survey is an extension of the wide survey. We proposed that the area near ecliptic plane in candidate survey fields of the wide survey fields should be observed with optimized methods (proper observing date, exposure time, exposure sequences, etc.) for detection of SSSBs. We deal with various science cases from near earth asteroids (NEAs) to inner Oort cloud objects as follows: (1) exploring meteorite source from main belt asteroids (MBAs), (2) investigating dynamical classes of trans Neptunian objects (TNOs), (3) exploring evidence of giant planet migration, (4) investigating physical properties of TNOs, (5) exploring the so-called Planet X, eccentric large TNOs, inner-Oort cloud objects, (6) exploring origin of Jupiter family comets, and (7) investigating characteristics of fast rotators. These science cases are all related to understanding the formation and evolution of Solar System. For achieving our science purposes, the HSC Solar System survey should be designed to obtain data on colors, brightness, photometric variability, orbital elements, and other physical properties of each small bodies population. By measuring brightness and velocity of each moving object detected on three consecutive images with each interval of 20 min at near opposition, we can estimate its absolute magnitude and then convert to the diameter. This allows us to obtain the size distributions of each SSSB groups (such as NEAs, MBAs, Hildas, Trojans, Centaurs, and TNOs). As you know, the size distribution is a kind of fingerprint of each population of small bodies. Thus, it plays very important role for identification of population origin and it can be used to investigate relationship between different dynamical populations in the current Solar System.2 By using g
, r
, i
, z
-filters, color relationships among different populations and between different orbital groups within a single population can be obtained. As Nice model3−5 predicted, if outer planet’s migration, catastrophic mixing and capture of small bodies happened in succession, the current small bodies around outer planets could be of the same origin, namely outer planetesimals. If this conjecture is correct, they must show similar size distribution and colors. Therefore, establishing size distributions and color relationships among different populations, especially at the outer regions of the Solar System is very important to obtain a clue to the dynamical interaction in the early Solar System. For this purpose, orbital determination of each object detected by the HSC survey is required.

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Therefore, we proposed to observe the same fields for two consecutive nights and one additional night within one week. The g
, r
, i
, z
color data can determine of asteroids taxonomic types having different albedo. Since we cannot obtain asteroids’ albedo from only optical observation such as the HSC survey, we use taxonomic types as albedo index for estimating asteroid diameter. This helps to estimate the diameter with less uncertainties. With the addition of Y-band data to g
, r
, i
, z
colors, we can distinguish the Q-type from S-type asteroids. From the study of meteorites’ spectra and spectroscopic observations of S-type and Q-type in the NEAs population, the Q-type asteroids seem to be fragments of S-type ones and have been regarded as ordinary chondrites. Though generally we have believed that the Q-type asteroids came from the main belt through usual supply route of NEAs, only two Q-type asteroids were discovered so far in the extremely young asteroid family, the Datura family, which was created 450 kyr ago.6 We think this is an observational bias. The Q-type is generally smaller than S-type, because they are fragments. If we can detect smaller asteroids in the main belt by using the HSC, we may find more Q-type asteroids there. If there is a Q-type cluster in the main belt, it can be a reservoir of meteorites to the Earth. Searching for meteorite reservoir is important to understand the supply rate of NEOs at the current Solar System. From the point-spread function (PSF) fitting of detected objects and the periodicity analyses of lightcurves, we may find binary (or multiple systems) asteroids. If we can measure the distance between primary and companion and determine the orbital period of the companion, we can calculate the mass of the primary. Since the volume of primary can be estimated from the brightness of objects, then we can determine the density of the primary based on its mass and volume. If we find very elongated shape asteroid from lightcurve observation, the asteroid might be a contact binary. In such case, if we employ Roche binary lightcurve simulations to construct a shape model of the asteroid, its density can be estimated.7 Undoubtedly, the densities are very important to understand dynamical and physical relationships between stony bodies and icy bodies. The lightcurve data also allow us to measure rotation periods and estimate approximate shapes (ratio of axes). These two properties are related to the collisional evolution of each population of small bodies. These properties of very small SSSBs detected only by the HSC would bring new insight on physical characteristics of asteroid bodies (internal structure) and collisional evolution of each population.

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We requested that minimum survey area should be ∼50 deg2 near the ecliptic and two observation modes are needed. Mode 1 is for multicolor observation. The selected 10 ∼ 17 points, along Jupiter’s orbit, with intervals of 20 degrees in longitude will be observed with five bands (g
, r
, i
, z
, Y) for ∼2 h at one night. For avoiding the brightness change due to asteroid rotation, the observation sequence should be like r
− g
− i
− r
− z
− z
− r
− Y − Y − r
− Y − Y − r
. Mode 2 is for lightcurve observation. In this mode, we will keep taking images of a single field during a whole night with r
, g
, or i
-filter. 2.4.1. Detection limit of SSSBs The detection limit of the Solar System bodies depends on their motion. Figure 2 shows the detection limit of each group with R-band, which was estimated based on our experience with Suprime-Cam. For MBAs, the detection limit is 24.4 mag with the R-band. For Jupiter Trojans, the limit is 24.7 mag, and for TNO, it is 26.0 mag. In the case that we took many images and stack them along TNO’s motion, the limit reaches to 28 mag (corresponding to 10 km size TNOs). Comparing HSC with Pan-STARRS and LSST, the HSC survey is deeper than Pan-STARRS in any case and for the objects beyond Jupiter, the HST is deeper than LSST survey. Therefore, we can say that the HSC survey is the most suitable tool to explore outer Solar System. Although the survey area for the Solar System science is not yet fixed, some preliminary estimates can be given to the detection number of each Solar System group, which are summarized in Table 2. These number are not so large compared with Pan-STARRS and LSST survey. However, since the

Fig. 2.

Limiting magnitudes of the Solar System objects to be observed by HSC.

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Table 2. Detection numbers and sky number densities estimated from observations by Suprime-Cam and proposed survey areas. The numbers in parentheses are related to Mode 2 observation. Number density/FOV

Estimated detection number

Planning survey area

NEAs β = 0◦ Inner MBAs β = 0◦

1.5 180

100 12000

100 deg2

MBAs β = 0◦ JTs β = 0◦ TNOs β = 0◦

450 0–75∗ 50∗∗

15000 (900) 700 (300) 1800 (100)

50 deg2

3

>200

100 deg2

Group

TNOs high β(>15◦ ) ∗ The

detection number depends on the longitude of survey field from L4 or L5 point. We estimated the number based on a simulation using known L4 Jupiter Trojans (see Fig. 3 in Ref. [8]). ∗∗ The detection number based on a deep sky survey of EKBOs (Edgeworth-Kuiper Belt objects) by Yamamoto et al.9 Other detection number were estimated from Refs. [10–14].

survey depth of HSC survey is deeper than them, the HSC survey would give a significant contribution for investigating the faint end of physical properties of each Solar System group.

3. Summary Hopefully, in a couple of years beginning with 2013, the HSC Solar System survey can provide data on colors, photometric variability, and other physical properties on faintest small bodies throughout the Solar System. The HSC data will be combined with comprehensive catalogs provided from LSST and Pan-STARRS and then will give us one of the ultimate clues to comprehend the structure and history of our Solar System through a remarkably new and interesting set of observation data of small bodies. The data from HSC, combined with past, ongoing and future missions/programs (HST, Spitzer, Herschel, NEOCam, etc.) and further ground-based observations are also of special interest for the community. However, readers have to be aware that the HSC survey plan is still under intense investigations, and discussions to make the entire plan better and more specific are on-going. The survey area and detection number may shrink a little bit, although the detection limit and achieving science cases are not changed. We will have to be prepared to accept and interpret the

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impending deluge of data provided by the HSC Solar System surveys and other surveys.

References 1. R. L. Jones, S. R. Chesley, A. J. Connolly, A. W. Harris, Z. Ivezic, Z. Knezevic, J. Kubica, A. Milani and D. E. Trilling, Earth Moon Planet 105 (2009) 101. 2. R. G. Strom, R. Malhotra, T. Ito, F. Yoshida and D. A. Kring, Science 309 (2005) 1847. 3. K. Tsiganis, R. Gomes, A. Morbidelli and H. F. Levison, Nature 435 (2005) 459. 4. A. Morbidelli, H. F. Levison, K. Tsiganis and R. Gomes, Nature 435 (2005) 462. 5. R. Gomes, H. F. Levison, K. Tsiganis and A. Morbidelli, Nature 435 (2005) 466. 6. T. Mothe-Diniz and D. Nesvorny, A&A 486 (2008) L9. 7. S. Takahashi and W. H. Ip, Publ. Astron. Soc. Jpn. 56 (2004) 1099. 8. T. Nakamura and F. Yoshida, Publ. Astron. Soc. Jpn. 60 (2008) 293. 9. N. Yamamoto, D. Kinoshita, T. Fuse, J. Watanabe and K. Kawabata, Publ. Astron. Soc. Jpn. 60 (2008) 285. 10. F. Yoshida, T. Nakamura, J. Watanabe, D. Kinoshita, N. Yamamoto and T. Fuse, Publ. Astron. Soc. Jpn. 55 (2003) 701. 11. F. Yoshida and T. Nakamura, Adv. Space Res. 33 (2004) 1543. 12. F. Yoshida and T. Nakamura, Astron. J. 130 (2005) 2900. 13. F. Yoshida and T. Nakamura, Planet. Space Sci. 55 (2007) 1113. 14. F. Yoshida and T. Nakamura, Publ. Astron. Soc. Jpn. 60 (2008) 297. 15. S. Miyazaki, Y. Komiyama, M. Sekiguchi, S. Okamura, M. Doi, H. Furusawa, M. Hamabe, K. Imi, M. Kimura, F. Nakata, N. Okada, M. Ouchi, K. Shimasaku, M. Yagi and N. Yasuda, Publ. Astron. Soc. Jpn. 54 (2002) 833.

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