Spiral Structure in the Circumstellar Disk around AB Aurigae

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L53
The Astrophysical Journal, 605:L53–L56, 2004 April 10
? 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.
SPIRAL STRUCTURE IN THE CIRCUMSTELLAR DISK AROUND AB AURIGAE1
Misato Fukagawa,2,3Masahiko Hayashi,4,5Motohide Tamura,3,5Yoichi Itoh,6Saeko S. Hayashi,4,5Yumiko Oasa,6
Taku Takeuchi,6Jun-ichi Morino,4Koji Murakawa,4Shin Oya,4Takuya Yamashita,2,4Hiroshi Suto,4
Satoshi Mayama,7Takahiro Naoi,2Miki Ishii,3Tae-Soo Pyo,4Takayuki Nishikawa,8Naruhisa Takato,4
Tomonori Usuda,4Hiroyasu Ando,3,5Masanori Iye,3,5Shoken M. Miyama,3,5and Norio Kaifu3
Received 2004 January 29; accepted 2004 February 26; published 2004 March 15
ABSTRACT
We present a near-infrared image of the Herbig Ae star AB Aur obtained with the Coronagraphic Imager with
Adaptive Optics mounted on the Subaru Telescope. The image shows a circumstellar emission extending out to
a radius of AU, with a double spiral structure detected at
r p 580
decreases as , steeper than the radial profile of the optical emission possibly affected by the scattered
r
light from the envelope surrounding AB Aur. This result, together with the size of the infrared emission similar
to that of the
) disk, suggests that the spiral structure is indeed associated with the circumstellar
J p 1–0
disk but is not part of the extended envelope. We identified four major spiral arms, which are trailing if the
brighter southeastern part of the disk is the near side. The weak gravitational instability, maintained for millions
of years by continuous mass supply from the envelope, might explain the presence of the spiral structure at the
relatively late phase of the pre–main-sequence period.
Subject headings: planetary systems: protoplanetary disks— stars: individual (AB Aurigae)—
stars: pre–main-sequence
AU. The surface brightness
r p 200–450
?3.0?0.1
13CO (
1. INTRODUCTION
Protoplanetary disks are now believed to be ubiquitous, with
a remarkable diversity. Their investigation has come to be es-
sential for understanding the formation mechanismofplanetary
systems, since the diversity of disks could be relevant to the
diversity of extrasolar planets. We need a spatial resolution of
at least ∼0? .1 in order to resolve the disk morphology even for
nearby star-forming regions at
resolution imaging studies with the Hubble Space Telescope
(HST) revealed various disk morphologiesaroundyoungstellar
objects with several million years of age (e.g., Clampin et al.
2003; Grady et al. 2001). Such high resolution can be achieved
even from the ground by using adaptive optics (AO) on a large
telescope. In this Letter, we present a near-infrared(NIR)image
of AB Aur, a Herbig Ae star, at a resolution of 0? .1 obtained
with the Coronagraphic Imager with Adaptive Optics (CIAO;
Tamura et al. 2000) mounted on the 8.2 m Subaru Telescope
atop Mauna Kea.
AB Aur ( pc; A0 Ve, van den Ancker et al. 1997)
d p 144?17
is one of the best-studied Herbig Ae/Be stars, with a mass of
and an age of2.4 ? 0.2 M
4 ? 1
,
The circumstellar structure around AB Aur consists of two com-
ponents: a compact rotating disk and an extended (11000 AU)
pc. Recent high-
d ∼ 150
?23
Myr (deWarf et al. 2003).
1Based on data collected at the Subaru Telescope, which is operated by the
National Astronomical Observatory of Japan.
2University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan;
misato@optik.mtk.nao.ac.jp.
3National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka,
Tokyo 181-8588, Japan.
4Subaru Telescope, National Astronomical Observatory of Japan, 650 North
A‘ohoku Place, Hilo, HI 96720.
5School of Mathematical and Physical Science, Graduate University for
Advanced Studies (SOKENDAI), Hayama, Kanagawa 240-0193, Japan.
6Graduate School of Science and Technology, Kobe University, 1-1 Rok-
kodai, Nada, Kobe 657-8501, Japan.
7Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan.
8Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-
8601, Japan.
nebulosity or envelope surrounding the central disk. Mannings
& Sargent (1997) detected a rotating disk of 450 AU in radius
in the
) line. The disk mass was estimated to
J p 1–0
be 0.02from the millimeter continuum flux (Mannings &
M,
Sargent 1997; Henning et al. 1998). The extended reflection
nebulosity was observed in its optical image (e.g., Nakajima
& Golimowski 1995), indicating that AB Aur possesses abun-
dant envelope material despite its relatively old age. High-
resolution optical observations of AB Aur with the HST re-
vealed that the inner part of the circumstellar material, at a size
similar to that of the
(Grady et al. 1999), although the spiral pattern was not very
clearly visible possibly because the scattering emission from
the more extended material mingles with it. Thanks to the small
optical depth to the low-density extended envelope, the NIR
image presented here shows only the structure coincident with
the millimeter disk without contamination from the scattering
envelope.
13CO (
13CO disk, has a spiral band structure
2. OBSERVATIONS AND DATA REDUCTION
We carried out H-band imaging of AB Aur on 2004 January
8 and 11 using CIAO mounted on the Subaru Telescope. The
pixel scale was 21.33 ? 0.02
InSb ALADDIN II array. An occulting mask of 0? .6 di- 1024
ameter was employed so that the circumstellar emission could
be imaged even in the close vicinity of the star within 1?
(p144 AU) in radius, yet the halo of AB Aur was not saturated
with 1 s exposure. We used a circular Lyot stop to block out
the outer 20% of the pupil diameter.
We obtained two image data sets of AB Aur on January 8.
The total exposure time for the first data set was 8.9 minutes
with 81 frames, each of which was taken by co-adding 20
exposures of 0.33 s. The other data set was for 10.6 minutes
with 108 frames, each taken by co-adding six exposures of
1 s. SAO 57754 was observed for 5.2 minutes as a point-spread
function (PSF) reference star between the two AB Aur obser-
mas pixel?1on the 1024 #

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L54FUKAGAWA ET AL.Vol. 605
Fig. 1.—H-band image of the circumstellar structure around AB Aur after
a reference PSF was subtracted. The surface brightness is multiplied by the
distance squared from the center for display so that the fainter outskirts can
be viewed with a high contrast. Boxcar smoothing is applied with
Directions of the spider patterns are indicated by dashed lines. The inner area
of 1? .7 diameter (AU; filled circle) is photometrically unusable and is
r ! 120
masked. The field of view is. North is up, and east is to the left.
8 # 8
pixels.5 # 5
????
Fig. 2.—Azimuthally averaged radial profile of the surface brightness (filled
circles) after the assumed inclination of
show the dispersion of brightness over the azimuth of 360? and radial width
of 10 AU (0? .07). The dashed line indicates a power-law fit with an index of
?3.0 to the brightness over the radial range between 120 and 580 AU.
was corrected. Error bars
i p 30?
vations. The signal-to-noise ratio of the reference star PSF was
greater than 3 at , as was similar to that of the AB Aur
r ! 6
data. FS 111 was observed immediately before AB Aur and
was used as a photometric calibrator (Hawarden et al. 2001).
The sky was clear, and the natural seeing size was 0? .5. The
spatial resolution achieved with the AO system (Takami et al.
2004) was 0? .10 (FWHM), which was measured with the Lyot
stop in the optical path toward an unmasked star in the frames
of SAO 57754.
At the second observing run on January 11, the seeing size
and eventually the resolution with AO were a little worse and
variable, although the sky was clear. Hence we used 62 frames
that had resolutions similar to those obtained in the first run.
The total exposure time was 6.2 minutes for the 62 frames,
each taken by co-adding six exposures of 1 s. SAO 57393 was
observed immediately before and after AB Aur as a PSF ref-
erence, with the total exposure time of 9.0 minutes.
The obtained frames were calibrated in the standard manner
using IRAF: dark subtraction, flat-fielding with sky-flats, bad-
pixel substitution, and sky subtraction. A reference star PSF was
made by combining frames for either SAO 57754 or SAO 57393
depending on the observing date. We subtracted the reference
star PSF from the image of AB Aur in order to detect faint
structure buried in its halo. After shifting, rotating, and scaling
the PSF so that its peak position, spider pattern, and halo bright-
ness match those of each object frame, we made PSF-subtracted
frames and combined them to produce the final image (see Itoh
et al. 2002). We applied this subtraction method separately to
thedataobtainedonJanuary8and11,confirmingthattheimages
taken on both nights were consistent with each other even if the
reference stars and seeing conditions were different.
??
3. RESULTS
3.1. Scattered Light from the Disk
Figure 1 is the resultant H-band image after the PSF subtrac-
tion. We detected an extended emission seen from the edge of
the occulting mask ( AU) out to the radius of 580 AU
r ∼ 60
(p 4? .0), where the brightness drops to the detection limit of
0.3 mJy arcsec?2. Figure 2 shows an azimuthally averaged
radial brightness profile of the image deprojected with the as-
sumed inclination of 30? and major-axis position angle of 58?
(see below), showing that the surface brightness decreases as
with the radius r from 120 to 580 AU. The power-law
r
dependence revealed in this study is steeper than that of
for the optical nebulosity (Grady et al. 1999). The steeper slope
in the NIR suggests that the detected light originates mainly
from the disk itself without being significantly contaminated
by the scattering emission in the envelope, which shows a
shallower slope (Grady et al. 1999). This is also justified by
the fact that the NIR scattering emission has a size similar to
that of the
We integrated the scattered light over the radial range of
120 AU580 AU and calculated the ratio of the scattered
≤ r ≤
to total fluxes as
F
/F
p (1.2 ? 0.2) # 10
disktotal
total fluxmag of AB Aur (Hillenbrand 1992). The
H p 5.1
H-band flux ratio is comparable to those of
sured over similar, or even inner, radial ranges of opticallythick
disks around other young stars with similar ages (HD 100546,
Augereau et al. 2001; TW Hya, Weinberger et al. 2002; GM
Aur, Schneider et al. 2003). The large scattered light flux is
qualitatively accounted for if the disk around AB Aur is flared
to receive sufficient light from the central starandalargeamount
of dust particles contributing to the scattering at 1.6 mm are
present at the disk surface (e.g., Whitney & Hartmann 1992).
This is consistent with a flared disk geometry suggested by
model fitting to the mid- and far-infrared spectral energy dis-
tribution (SED) of AB Aur (Dominik et al. 2003).
Assuming that the emission comes from a tilted disk with a
circularly symmetric brightness distribution and applying an
ellipse isophoto fitting at the radii between
we derived the inclination and position angle of the major axis
as and P.A. p
i p 30? ? 5?
58? ? 5?
ence of spiral arms (see § 3.2) in this radial range does not
?3.0?0.1
?2
r
13CO disk (Mannings & Sargent 1997).
, adopting the
?2
mea-
?2
(2–4) # 10
and 1? .8,
r p 1? .4
, respectively. The pres-

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No. 1, 2004SPIRAL STRUCTURE IN DISK AROUND AB AUR L55
Fig. 3.—Same as Fig. 1, but the image is deprojected with an assumed
inclination of 30? to show the “face-on” view of the AB Aur disk. Some of
the major features are identified.
significantly affect the derivation of the geometry of the NIR
disk, because the fitting applied to inner and outer regions gave
consistent results. It should be noted, however, thatanyintrinsic
brightness distribution asymmetry that makes the northwestern
part darker, as may be the current case caused by an anisotropic
scattering phase function, does affect the inclination. The in-
clination of 30? should be taken as an upper limit in such a
case.
The derived inclination agrees well with the recent NIR in-
terferometric measurements for the inner (
(Eisner et al. 2003; Millan-Gabet, Schloerb, & Traub 2001)
and with the constraint (less than 45?) obtained by the optical
imaging with the HST/Space Telescope Imaging Spectrograph
(STIS; Grady et al. 1999). It is, however, significantly smaller
than 76? estimated from the
& Sargent (1997). The position angle of the major axis is also
different from that of the millimeter measurement(P.A. p 79?)
by 20?. Lower spatial resolutions of millimeter observations
may have caused such discrepancies, and higher resolution im-
agings of the thermal emission arenecessaryforobtainingmore
precise constraints on the disk geometry. On the other hand,
the STIS optical image (Grady et al. 1999) lacks a distinguish-
able axis, not showing any clear ellipticity. Because the image
shows a nebulosity much more extended and more circularly
distributed than the NIR image, the optical flux may be dom-
inated by scattering from the region with a large-scale height
especially at large radii. The STIS wedge occults the region
exactly along the derived major axis, which also makes it dif-
ficult to identify a distinguishable axis in the image.
AU) disk
r ∼ 0.5
13CO observations by Mannings
3.2. The Spiral Structure
TheH-bandimageinFigure1showsaremarkablespiralpattern
AU. The spiral pattern coincides with the spiral
r p 200–450
bandstructureseenintheopticalimagetakenwiththeHST(Grady
et al. 1999). The new image has, however, revealed the entire
spiral pattern located in the inner part (200 AU
In addition, it clearly shows the spirals with a high contrast
because the scattered light from the surrounding envelope is
negligible; the spiral pattern is associated with thecircumstellar
disk but not with the envelope. This is indeed the first case in
which a spiral pattern, not a ring or a circular gap, has been
detected in the NIR around a young star, although it was de-
tected in the optical image of HD 100546 (Grady et al. 2001)
as well as AB Aur.
The southeastern part is brighter, which suggests that this
part is the near side of the disk if we assume that forward
scattering is dominated as is the case of Mie scattering. The
observed winding direction of the spiral pattern projected on
the disk is S-wise, not Z-wise. These results, combined with
the velocity field of the disk in which the northeastern part is
blueshifted and the southwestern part is redshifted (Mannings
& Sargent 1997), mean that the arms are trailing.
Figure 3 shows a deprojected image of Figure 1 with some
of the features identified. We can see inner and outer spiral
arms especially at the eastern half of the disk where the pres-
ence of a dark lane makes the double-arm structure evident.
The inner arm located at
r ∼ 230
northeast is the brightest. The outer arm running from the south
to northeast is traced at
r ∼ 330
structure. On the western side of the disk, a fainter arm is seen
atAU, and another outer arm at
r ∼ 260
southwest is largely open to the south.
at
300 AU).
? r ?
AU running from the east to
AU with a branch and a knotty
AU in the
r ∼ 440
4. DISCUSSION
What is the mechanism to excite and maintain the spiral
structure in the disk of AB Aur, a single star with the age of
4 Myr? Theoretical calculations show that a forming planet lo-
cated in a disk opens a gap (Takeuchi, Miyama, & Lin 1996)
that is often associated with a spiral structure extending inward
and outward into the disk from the planet (e.g., Bate et al. 2003).
If the dark lane atAU is a gap where an unseen com-
r ∼ 300
panion is located, its mass must be less than 10MJ, which is
estimated from the evolutionary tracks given by Burrows et al.
(1997) and Allard et al. (2001) in order to be consistent with
the detection limit ofmag in the interarm region. The
H ∼ 16.5
main structure that we may observe in a disk, however, would
beacirculargapbutnotaspiralstructuresuchastheonerevealed
in this study if there is an unseen companion, because matter is
cleared away in the gap while the spiral pattern is merely the
density fluctuation of matter. It is therefore not probable that the
spiral structure is produced by an unseen companion.
On the other hand, the gravitational instability may excite
the spiral structure without any gap in a disk. According to
theoretical studies (e.g., Nelson et al. 1998), spiral structure is
produced and sustained if a circumstellar disk hastheminimum
Q-value of 1.52.0, where
? Q ?
Q parameter with, Q, and S being the sound speed, angular
cS
velocity, and surface density, respectively. Because Q is min-
imized at the outer edge of a disk for the standard model
(D’Alessio et al. 1998), we evaluate Q at the outermost arm
radiusAU in order to see if the gravitationalinstability
r p 450
occurs. Taking the disk mass of 0.02
1997; Henning et al. 1998) and the radial dependenceofsurface
density as , as observed for several T Tauri disks
S ∝ r
(Kitamura et al. 2002), with
c p 0.23
Miroshnichenko et al. 1999) and the assumed Kepler rotation,
we obtain at
Q ∼ 17
mass of 0.15, as derived if we use the opacity in Pollack
M,
is Toomre’s
Q p c Q/(pGS)
S
(Mannings & Sargent
M,
?0.5
km s?1( K;
T p 15
S
AU. If we take a larger disk
r p 450

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L56FUKAGAWA ET AL.Vol. 605
et al. (1994) for smaller dust particles (Bouwman et al. 2000),
we obtain . Although the value of Q has a large uncer-
Q ∼ 2
tainty as estimated above, the smaller value of Q may allow
the spiral structure to be excited in the disk.
The presence of spiral structure around the star with the age
of 4 Myr suggests that the structure is maintained for ∼106yr,
much longer than the dynamical timescale of 104yr at r p
AU. The present mass accretion rates from disks onto stars450
are derived to be ∼10?8M,yr?1from UV spectra of Herbig
Ae/Be stars, including AB Aur (Miroshnichenko et al. 1999).
This small value, if constant, may allow the disk to keep suf-
ficient mass for instability for ∼106yr. In addition, AB Aur
could have the accretion rate of ∼10?8M,yr?1from its halo,
as Miroshnichenko et al. (1999) estimated from their SED fit-
ting. The mass supply from the envelope of AB Aur may thus
be sufficient to replenish the disk to sustain the spiral structure.
Another single Herbig Ae/Be star HD 100546 also shows a
spiral disk with a surrounding envelope (Grady et al. 2001),
giving support to the possibility that the mass supply from
envelopes contributes to the spiral instability.
We thank the referee Carol A. Grady for her comments,
which helped us to improve this Letter. We appreciate the sup-
port from the Subaru Telescope staff, especially from Sumiko
Harasawa for making our observations successful. We are also
gratefultoTaishi Nakamoto,MunetakeMomose,KeiichiWada,
Yoshihiko Saito, Akihiko Ibukiyama, and Shigeru Ida for val-
uable discussions. Yoichi Itoh is supported by the Sumitomo
foundation and the Itoh science foundation.
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