A search for debris disks

based on the AKARI mid-infrared all-sky survey
Hideaki FUJIWARA1 (fujiwara@astron.s.u-tokyo.ac.jp),
Daisuke ISHIHARA1,2, Takuya YAMASHITA3,Takashi ONAKA1, Misato FUKAGAWA4,Hiroshi MURAKAMI2, Hirokazu KATAZA2,
Takanori HIRAO2, Takafumi OOTSUBO2,
Takao NAKAGAWA2, and Keigo Enya2(1) University of Tokyo, (2) ISAS/JAXA, (3) Hiroshima University,(4) Osaka University

9 µm all-sky image by AKARI/IRC. Zodiacal light is subtracted.

ABSTRACT

1. Debris Disk

2. AKARI Observations

After the IRAS observations, main-sequencestars that have circumstellar debris disks
and thus show infrared excess have been
discovered. Since debris disk is thought tobe the final stage of planet formation, it isvery important to property and evolution ofdebris disks statistically. The AKARI is thefirst Japanese infrared astronomical satellite
dedicated for infrared astronomy. The midinfrared survey was carried out with the 9 µm
and 18 µm bands using the Infrared Camera
with higher sensitivity and spatial resolution
than IRAS. We are carrying on the unbiased
survey of debris disk candidates that show
mid-infrared excess by using the AKARI/IRC
mid-infrared all-sky survey data. So far, wehave seven new debris disk candidates thatshow large 18 µm excess. More systematicsurvey is also ongoing based on Hipparcosand 2MASS catalog. In this presentation, weshow the initial results of the debris disk
survey.

Infrared Excess

AKARI

“Vega-like stars”
Main-sequence stars with infrared excesses(λ>25µm)
Infrared Excess
Thermal emission from circumstellar dust disks

Japanese Infrared SatelliteLaunch: 2006/2/22
Lhe: ~2007/8/27
Now: Warm mission (NIR)
IRC (InfraRed Camera)Near-/Mid-Infraerd
FIS (Far-Infrared Surveyor)
Far-Infrared

Origin of Dust
Dust around Vega-like stars are NOT primordial
protoplanetary dust
Timescale of blow-out mechanism << AgeSecondary generated dustCollision of planetesimal?
“Debris Dust” or “Debris Disk”
Final stage of planet formation?

Comparison with Solar System
KBO analog -> Cold Dust -> FIR Excess

IRC Mid-Infrared All-Sky Survey
λ=9µm (S9W) & 18µm (L18W)
More than 90% of the sky is observed at least twice
Higher sensitivity and spatial resolution than IRASChannelS9W

L18W

Wavelength
Detection Limit

6-12 µm
50 mJy14-26 µm
120 mJy

Saturation

36 Jy

96 Jy

Spatial Resolution

<9.4”

<9.4”

* Parametersof AKARI/IRC
MIR All-Skysurvey* FIR (60 & 100 mm) Excess is
examined by IRAS observations

AKARI MIR all-Sky Survey is very powerful tostudy dust generated by collision of “asteroids” !

Asteroid analog -> Warm Dust -> MIR Excess

3. Results of the Pilot Survey

4. Ground-based Follow-up Observations

StrategyMerits

Take main-sequence stars and their position from Tycho-2 spectral type catalog (Wright et al. 2003)
Look for nearby Infrared counterparts in AKARI MIR images and get their MIR fluxes
Take Ks-[L18W] colors as MIR excess indicators for the 18µm-detected sourcesFind stars which show significant 18µm excesses based on the Ks-[L18W] colors (Ks-[L18W] > 0 ->Excess)Remove contaminated or mis-identified sources by comparing the AKARI images with DSS and2MASS images

Follow-up observations of the newly-discovered debris diskcandidates were made with Subaru/COMICS and Gemini/T-ReCS
High spatial resolution
Elimination of contaminations and mis-identification
Spatial distribution of debris dust
Multi-band photometry

Identified 18µm Excess Sources

Observations
Gemini/T-ReCSJun-Jul 2008
λ=8.8, 9.7, 10.4, 11.7, 12.4µm
Diffraction limit 0.3” @ 11.7µm
Properties of Sample 2
SpT=A0V, d=100pc

flux (Jy)101

flux (Jy)14 stars are identified as debris disk candidates that have large 18µm excesses(larger than 50% of photosphere level)

Case of Sample 25

photosphere

2MASSIRC
T-ReCS
0.01

Flux Density (Jy)1

Sample 2

Sample 1

Sample 3

Sample 9

Sample 81

Sample 10

10100

Sample 4

Sample 11

Wavelength (micron)Sample 5

Sample 6

Sample 12

Sample 7

Sample 13

1

Sample 1410100Statistics
Spectral Detect Excess Freq
Typeat 18um at 18um (%)
A
217
41.8
F

331

72.1

G

204

31.5

K

153

00.0

M

23

00.0

Total

910

141.5

Excess Frequency = 14/910 ~ 1.5%
Lower value than Spitzer’s results
A stars ~ 31% (Su+ 2006)
FGK stars ~ 6% (Beichman+2006)
Due to the difference in detection threshold?
Spitzer can detect fainter excesses (~10% of photosphere)Spectral type dependency?
Early-type: higher frequency
Late-type: Lower frequency
Due to the difference in stellar lifetimei

1015
20
wavelength (µm)

0.1Results

10

wavelength (µm)

SED
AKARI/IRC <-consistent-> Gemini/T-ReCS
IR excess is certainly associated with the starIR excess at λ > 11µm
Max Td ~300K, Inner Radius ~ a few AU
Feature at ~11µm -> Crystalline Forsterite?
Radial ProfileSpatially unresolved at 11.7µm
Disk size < 0.3” ~ 30AU at 100pc
Harmonic with the inner radiusderived from the SED

Green: Target
Red: PSF Standard

This work is based on observations with AKARI, a JAXA project with theparticipation of ESA. H.F. have been financially supported by the Japan Societyof Promotion of Science (JSPS). This work is supported in part by a Grant-inAid for Scientific Research on Priority Areas from the Ministry of Education,
Culture, Sports, Science and Technology of Japan and Grants-in-Aid for
Scientific Research from JSPS.

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