Deep 3 micron Photometry of Substellar Mass
Objects around a Massive Star in S106
Munechika Tanaka1(munechika@stu.kobe-u.ac.jp), Yumiko Oasa2, Yoichi Itoh1
1 Kobe University, 2 National Taiwan Normal University

Abstract
Many “accretion-aborted” low-mass and substellar mass YSOs could be born due to the UV radiation from a nearby massive star. To investigate the formation process of substellar
objects under the influence of the UV radiation from a massive star, we have carried out JHK photometric survey of the S106 region, which is one of the nearest massive star forming
regions. From this survey, hundreds of substellar YSO candidates have been identified, and there exists a number of faint red sources detected at HK-bands or only K-band in thewestern region of the massive star. To examine their characteristics, we have carried out a deep L’-band photometric survey of the west region in S106. From the JHKL’ survey, 64
YSOs were identified and the YSO fraction and the K-L’ excesses increase toward the west. This indicates that the central O-star evaporates the disks around nearby low-mass YSOs, orthat star formation propagates from the central O-star to the west.

ResultsIntroduction[Color-Color Diagram]Fig 5; JHKL’ color-color (CC) diagram.
The stars which have JHKL’ mag areplotted (77 objects).

Substellar mass objects may form from contracting cloud fragmentation by itself just like low-massstars (Masunaga 1999), or removal of accretion envelops from low mass contracting cores due tophoto-evaporation through a nearby O-type star (Kroupa & Bouvier 2003).

Intrinsic CTTS

Av=10

[Substellar Mass Object]
Brown Dwarf (0.013~0.08Msun)

NIR-excess

From JHKL’ and HKL’ CC-diagrams, 64
YSOs with NIR-excesses were identified.
(JHKL’: 50/77, HKL’: 14/15)
Comparison of the JHK CC-diagram with
JHKL’ one, demonstrate that majority(70%) of sources lie in the K-L’ excess
region.
Combined with their J0 mag and evolution
model (Baraffe et al. 2002), 24 substellar
YSO candidates were identified.
(J0 [mag] = Absolute J [mag] – AJ [mag])Å 0.08Msun

・Effective Temperature (Teff)： < 3000K
Æ Not start the fusion reaction due to low Teff・Shining by gravitational contraction energy

Free-Floating Planet (<0.013Msun)

0.013Msun•The mass corresponds to that of giant planets, but this object
does not orbit a star.
Fig 1; An evolutionary track of very lowÆ Not a brown dwarf and not a planet Æ new type object
mass objects (Burrows et al. 1997).

[Spatial Distribution of YSOs]

Luminosities of substellar mass objects decreases with ages (Fig 1).
An O-star evaporates the disks around nearby low-mass YSOs
evolution※● each size indicates the mass

General evolutiontimeIRS4

MassAccretion
YSO

Fig 6; Spatial distribution of YSOs.
Green and blue marks show YSOs
identified from the JHKL’ and HKL’
CC-diagrams, respectively. Circlesand stars show YSOs and substellar
YSO candidates, respectively.

Circumstellar materials

Evolution exposed to UV
Evaporate circumstellar materials

Fig 2; The illustration of the star formation
under the UV from a massive star.

UV from a massive star

The UV radiation generated by the Trapezium OB stars evaporates the disks aroundnearby low-mass stars, and low-mass stars may terminate their pre-main sequence
evolution (Robberto et al. 2004).
Æ The number of intermediate-mass stars could be deficient in the initial mass function
and many “accretion-aborted” low-mass and substellar YSOs could be born.
PurposeThis study is to investigate the formation process of substellar objects
under the influence of the UV radiation from a massive star.

[S106]

N

Fig 7; K-L’ excesses as a functionof the distance from IRS4.

There are no objects with large
excesses near (< 0.2 pc) IRS4 (redellipse).

[Previous Study]

Fig 3; Three-color composite image of S106
obtained with SUBARU/CISCO (J: blue, H:green, K: red). The field of view is ~ 5’x5’. The
red and green boxes correspond to the regions
observed at Subaru and at UKIRT, respectively.

L’-band (~3.8 um) Observations
We have carried out a deep L’-band photometric survey of the west
regions of S106 where low-mass and substellar YSOs are concentrated.

• Identify substellar YSOs
• Search the distribution of YSOs

SUBARU Observation

Analysis[K-L’ Excesses vs Distance from IRS4]EJHK photometry (Fig 3; 5’x5’)

Date: 2005/5/4
Instrument: IRCS
Exposure time: 0.096s, 0.118s
Integration time: 500s ~ 1000s
FOV: 1’x2’ (red box of Fig 3)
Seeing: ~0.5”This indicates that star formation propagates from the central O-star towest, or that the central O-star evaporates the disks around nearby lowmass YSOs.

IRS4（RA, Dec）=（20h27m, +37°21’）• S106 is one of the nearest massive star
forming regions (~ 600 pc).
• A central O-type star (IRS4) makes
prominent bipolar nebulae and an HII region.

•Each limiting magnitude exceeds 20 mag.
•About 600 YSO candidates were identified.
•There appears to be a substantial substellar
population, some members of which possibly haveplanetary masses at ages of ~ 1Myr.
Æ They are concentrated to the western regionof IRS4.

0.45pcThe YSOs are concentrated close to IRS4, and the substellar YSO candidates areconcentrated to the left region (near IRS4), too.

This also indicates that starformation propagates from thecentral O-star to west, or that thecentral O-star evaporates the disks
around nearby low-mass YSOs.

Conclusions
From this study, we propose the two mechanism of the formation of substellar
objects; substellar YSOs of S106 formed due to propagation from O-star to thewest, or due to disk-evaporation by the UV radiation of O-star.

Future Perspective
To search the mechanism of the formation of substellar
objects around IRS4, we have carried out a deep L’band imaging survey of the central and eastern 2’x2’
region (green box of Fig.3) of S106 (in analysis).

UKIRT ObservationFig 4; L’-band image of the S106. This region
corresponds to the red box of Fig 3. The field ofview is 1’x2’.

All data were reduced using Image Reduction and Analysis Facility (IRAF).
Fig 5 shows the image mosaic. Total 102 objects were detected with thelimiting magnitude of about 15.5 mag (10 sigma detection).

Date: 2008/8/2-3
Instrument: UIST
Exposure time: 0.175s
Integration time: 4.42h
FOV: 2’x2’ (green box of Fig. 3)
Seeing: ~0.5”

Fig 8; L’-band image obtained with UKIRT/UIST.
The field of view is ~ 2’x2’. The blue circle
corresponds to the position of IRS4.