Tricolor simultaneous
photometric observations oftiny Near-Earth Objects (NEOs)
with Seimei/TriCCS

modiﬁedAugust 12th, 2021 @ Seimei Users Meeting 2021

Jin Beniyama (University of Tokyo)
Co: Shigeyuki SAKO, Ryou OHSAWA, Satoshi TAKITA,Seimei telescope@ Okayama Observatory, Kyoto University

Mamoru DOI, Yuu NIINO (University of Tokyo),
Kazuya MATSUBAYASHI, Kouji OHTA,
Keiichi MAEDA, and Miho KAWABATA (Kyoto University)
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Overview•

We conducted tricolor simultaneous photometric observations.
‣ Instrument‣ Mode‣ Target•: Seimei telescope / TriCCS camera (g + r + i or z): video observation at 1 fps: 5 tiny Near-Earth Objects (diameter < 100 m)

Rotational periods and spectral types were derived.
‣ Light curve -> rotational period(+ light curve of a reference star )Color-> spectral type
‣ We fully characterized 1 NEO.
‣ No spectral change was found in all 5 NEOsʼ light curves.

•Additional observations will be done in Semester 2021B.
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asteroid

Scientiﬁc motivation•Material transportation by asteroids is related tothe origin of the water and/or the life on the Earth.
Peslier+2020••

Near-Earth Objects (NEOs, perihelion distance q < 1.3 au)come from the main-belt (~ 10 Myr).
NEOs can deliver materials to the Earth.
NEOSun•a few Myr*Earthmain-belt

Spectroscopic or multi-band photometric observations
reveal the surface composition.
‣ Stony : S-type‣ Carbonaceous : C-type
From DeMeo+2009, Icarus, 202, 160, Fig.15

reﬂectance (0.45 ‒ 2.45 micron)3 / 13* Gladman+1997, Science, 277, 197

Space weathering and surface refreshing•

The surface property is altered by the space weathering: micro-meteorite impacts, solar wind, and cosmic rays.

•Asteroidsʼ surfaces can be refreshed byother mechanisms, such as close encountersweathering
refreshwith planets and rotational ﬁssion.

Binzel+2010, Nature let., 463, 331, Figure 1•

Both space weathering and its refresh are being discussed.
‣ Which weathering agents are dominant : micro-meteorite impact vs. solar wind?
‣ Is there any size dependence?

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Diﬃculties and goals•

Spectral type of small objects *Magnitude
Nathues+2006, Icarus, 175, 452, Fig. 2
X Day time

Small NEOs are hard to observe.
‣ limited observational window (hours ~ a few days)

Discovery

Light curveC‣ fast rotation ( < 1 minute)‣ large apparent motion etc.

Slarge•48 hours

Tiny~ 300 mThe numbers of tiny NEOs are~ 1mfrom Kiso
* Generated with data from Binzel+2019, Icarus, 324, 41,

not enough for statistical study.

Bus & Binzel 2002,ofIcarus,
158, 146,Binzel+2004,
170, 259,
Ephemerisa tinyNEO(D =Icarus,5 m)**Devogele+2019, AJ, 158, 196, Mommert+2016, AJ, 151, 98,** Generated
with the ephemerisof 2020
VH5 from PSS, 157, 82
Popescu+2019,
A&A 627, A124,
Perna+2018,
Minor Planet Center (https://minorplanetcenter.net/iau/MPEph/MPEph.html)

Goal of this study① Estimate spectral types of tiny NEOs (tricolor + ToO).
② Search for spectral changes in rotationally resolved colors.
(tricolor + ToO + time-resolved)
5 / 13Observation•Seimei telescope

Instrument : TriCCS‣ TriColor CMOS Camera and Spectrograph@ Kyoto University 3.8 m telescope (the Seimei telescope)‣ g + r + i/z (Pan-STARRS) tricolor imager‣ S/N=10, lim.mag ~ 18 @ g, r band, 1 sec exposure‣ ﬁeld of view 12.6 x 7.5 arcmin

TriCCS•

Targets‣ 5 NEOs (mean diameter ~ 50 m, soon after discovery ~ a few days)•

Observation Date & Time
‣ Feb 22-24th, Mar 8th, 2021
‣ 5 min ‒ 60 min / NEO
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Image from web page of Kazuya MATSUBAYASHI(http://www.kusastro.kyoto-u.ac.jp/~kazuya/p-triccs/overview/index.html)

Observed NEOs

Orbit diagram of a NEO (Feb 24th, 2021)
Mars orbit

EarthEarth orbit

NEO orbit

Image generated by JPL HORIZONS (https://ssd.jpl.nasa.gov/horizons.cgi)
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3D video cube•

12.6 arcmin

TriCCS FoV
7.5 arcmin

Non-sidereal tracking
6 arcminr bandTarget asteroid
3 arcminMoving direction

3d (x, y, time) ﬁts(partial ﬁeld of view, 1 fps,100 frames, 20x speed)
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Data Reduction
0-1. bias & dark subtraction & ﬂat-ﬁelding
0-2. Split 3d (x, y, t) to 2d (x, y)(x, y) = (x1, y1)

t = t1

Asteroid(x, y) = (x2, y2)0-3. astrometry (WCS, astrometry.net*)t = t2

Reference stars

1. NEO location determination
predict location by interpolation of(x, y) = (x1, y1), (x2, y2) …

3. Periodic analysis
・Rotational periods are estimated by Lomb-Scargle method(Lomb 1976, Scargle1982, VanderPlas2018)

2. Aperture photometry

Brightnes

・circle aperture ( python, SEP ** )Δmradius = 2 ‒ 3 FWHM (~ 5 arcsec)

P・relative photometry
with Pan-STARRS catalog **** Lang+2010, AJ, 131, 1682** Bertin+1996, AA, 117, 393, Barbary+2015, 10.5281/zenodo.15669*** Chambers+2016, arXiv:1612.05560

yfit

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Timeyobs

Result 1. Light curve (NEO)(Feb 24th, 2021)

g-bandtexp : 10 sec (stack 1 sec x 10 frames)•The rotational period is not estimatedin observing time (1 hour).
This is consistent with the period ~ 1.8 hour
in a previous study*.

r-bandz-band

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1 hourResult 1. Light curve (NEO)(Mar 8th, 2021)(Feb 24th, 2021)

texp : 1 sec

texp : 10 sec (stack 1 sec x 10 frames)

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Result 1. Light curve (Reference)① 1 sec exp.

g, r, i = (15.8, 15.4, 15.3)~ 1.6 %, 1.2 %, 2.1 %g, r, z = (14.8, 14.4, 14.2)② 10 sec exp.
(stack 1 sec x 10 frames)~ 0.57 %, 0.94 %, 1.4 %normalized
FtargetFreference

reference

g-bandg-bandr-bandr-bandi-bandz-bandtargetNote: Ref. stars are moving at ~1 pix/sec due to the non-sidereal tracking.
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Result 2. Rotational period•

We derived the rotational periods of 2 NEOs :‣ (Mar 8th)rot. P = 404 ± 5 s‣ (Feb 24th)rot. P = 778 ± 6 s

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Result 3. Spectral type•(preliminary) Spectral types of 2 NEOs are estimated.

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Reference objects are from SDSS Moving Object Catalog,Ivezic+2002, Proceedings of SPIE Vol. 4836,
http://faculty.washington.edu/ivezic/sdssmoc/sdssmoc.html

Result 4. Color variation•(preliminary) No color variation is detected in all the 5 light curves.

(Feb 24th)(Feb 24th)More detailed
analysis are needed.
(c.f. color transformation)

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Summary•

We conducted tricolor simultaneous photometric observations.
‣ Instrument : Seimei telescope / TriCCS camera (g + r + i or z)‣ Mode‣ Target•••: video observation at 1 fps: 5 Near-Earth Objects (diameter < 100 m)

We estimated both the rotational periods and the spectral types.
No spectral change was foundin all the 5 light curves (preliminary).

Additional observations will be donein Semester 2021B.
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2021B ToO観測へ向けて••当日または翌日のToO依頼が見込まれます。
現地観測者や天文台の方にお願いする際はよろしくお願いします。
NEO観測のための手順書に従いコマンド入力をすると観測ができるように整備しました。
(8/11に整備予定)Magnitude
Day time
Discovery

Light curve

48 hours
from Kiso
Ephemeris of a tiny NEO (D = 5 m)•

データ解析用scriptを公開しています。
1. TriCCS Data Reduction (tdr) https://bitbucket.org/jin̲beniyama/triccs-data-reduction/src/master/
2. Moving Object Photometry (movphot) https://bitbucket.org/jin̲beniyama/movphot/src/master/【観測所への要望】
即日や翌日ToOの観測ですので、リモート観測を心待ちにしています。
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