Discovery of extreme asymmetry in the debris disk surrounding HD 15115
Submitted to ApJ Letters December 18, 2006; Accepted April 04, 2007
Preprint typeset using LATEX style emulateapj v. 08/22/09
DISCOVERY OF EXTREME ASYMMETRY IN THE DEBRIS DISK SURROUNDING HD 15115
Paul Kalas
, Michael P. Fitzgerald
, James R. Graham
Submitted to ApJ Letters December 18, 2006; Accepted April 04, 2007
ABSTRACT
We report the first scattered light detection of a dusty debris disk surrounding the F2V star HD
15115 using the Hubble Space Telescope in the optical, and Keck adaptive optics in the near-infrared.
The most remarkable property of the HD 15115 disk relative to other debris disks is its extreme length
asymmetry. The east side of the disk is detected to ∼315 AU radius, whereas the west side of the disk
has radius >550 AU. We find a blue optical to near-infrared scattered light color relative to the star
that indicates grain scattering properties similar to the AU Mic debris disk. The existence of a large
debris disk surrounding HD 15115 adds further evidence for membership in the β Pic moving group,
which was previously argued based on kinematics alone. Here we hypothesize that the extreme disk
asymmetry is due to dynamical perturbations from HIP 12545, an M star 0.5◦ (0.38 pc) east of HD
15115 that shares a common proper motion vector, heliocentric distance, galactic space velocity, and
Subject headings: stars: individual(HD 15115) - circumstellar matter
1. INTRODUCTION
Volume-limited, far-infrared surveys of the solar neigh-
borhood suggest that ∼15% of main sequence stars have
excess thermal emission indicative of circumstellar grains
(Aumann 1985; Backman & Paresce 1993; Meyer et al.
2007). Direct imaging of dust scattered light reveals the
geometry of the grain population relative to the star,
which further elucidates the origin of dust. In some
cases, a circumstellar nebulosity may be amorphous with
asymmetric striated features produced when stellar ra-
diation pressure deflects interstellar dust (Kalas et al.
2002). In other cases, such as β Pictoris and Fomal-
haut, the geometry of dust is consistent with a circum-
stellar disk or belt related to the formation of planetesi-
mals (Smith & Terrile 1984; Kalas et al. 2005). Though
larger bodies such as comets and asteroids are not di-
rectly observed, they most likely exist as a reservoir for
injecting fresh debris into the system over the lifetime
of the star. Furthermore, circumstellar debris disks dis-
play significant structure and asymmetry that may be
linked, in principle, to dynamical perturbations from
a planetary system (Roques et al. 1994; Liou & Zook
1999; Moro-Mart́ın & Malhotra 2002). Unfortunately,
only ∼10% of stars with excess thermal emission have
detected scattered light disks due to the high contrast
between the host star and the low surface brightness neb-
ulosity at optical and near-infrared wavelengths. Fortu-
nately, the observational capabilities have improved in
recent years due to instrument upgrades on the Hubble
Space Telescope (HST) and the implementation of adap-
tive optics (AO) on large, ground-based telescopes.
Here we show new scattered light images of a debris
disk surrounding HD 15115, an F2 star at 45 pc (Table
1), first reported as a source of thermal excess emission
by Silverstone (2000). The spectral energy distribution
1 Astronomy Department and Radio Astronomy Laboratory, 601
Campbell Hall, Berkeley, CA 94720
2 National Science Foundation Center for Adaptive Optics, Uni-
versity of California, Santa Cruz, CA 95064
is consistent with a single temperature dust belt at ∼35
AU radius with an estimated dust mass of 0.047 M⊕
(Zuckerman & Song 2004; Williams & Andrews 2006).
Recently, Moór et al. (2006) identified HD 15115 as a
candidate for membership in the 12 Myr-old β Pic mov-
ing group (BPMG), based on new radial velocity mea-
surements that resulted in galactic kinematics similar to
those of the BPMG.
2. OBSERVATIONS & DATA ANALYSIS
We first detected the HD 15115 disk in scattered light
using the HST ACS High Resolution Camera (HRC) on
2006 July 17. We used the F606W broadband filter and
the 1.8′′ diameter occulting spot to artificially eclipse the
star. Three flatfielded frames of 700 seconds each from
standard pipeline processing of the HST data archive
were median combined for cosmic ray rejection. The
point spread function was then subtracted iteratively us-
ing five other stars of similar spectral type obtained from
the HST archive. The relative intensity scaling between
images was iteratively adjusted until the residual image
showed a mean radial profile equal to zero intensity per-
pendicular to the circumstellar disk. The images were
then corrected for geometric distortion, giving a 25 mas
pixel−1 scale.
The resulting optical images revealed a needle-like fea-
ture projecting westward from the star to the edge of
the field, but with almost no counterpart to the east.
Given the high degree of asymmetry that could conceiv-
ably arise from instrumental scattering, we endeavored
to confirm the disk using the Keck II telescope with
AO on 2006 October 07 and 2007 January 26. Utilizing
the near-infrared camera NIRC2, a 0.4′′ radius occulting
spot and a 10 mas pixel scale, we confirmed the exis-
tence of the disk in J (1.2 µm), H (1.6 µm), and K′ (2.2
µm). PSF subtraction is accomplished by allowing the
sky to rotate relative to the detector, thereby separat-
ing the stellar PSF from the disk. The observing pro-
cedure and data reduction procedure are fully described
in Fitzgerald et al. (2007). Due to poor observing condi-
http://arxiv.org/abs/0704.0645v1
2 Kalas, Fitzgerald, Graham.
tions in October, including intermittent cirrus clouds, we
used only the best fraction of data by visually selecting
frames of relatively constant intensity and PSF sharp-
ness. The resulting effective integration times are 450 s,
980 s, and 600 s for J, H, and K′, respectively. Stan-
dard star observations were obtained under similar, non-
photometric conditions and processed in a similar man-
ner. In January 2007 we re-observed HD 15115 (1930
s cumulative integration time) and two standard stars
under photometric conditions from Keck using the same
instrumentation with the H broadband filter. However,
the observations were made after meridian transit and
the limited rotation of the sky relative to the instrumen-
tal PSF causes disk emission to be included in the PSF
estimate, resulting in disk self-subtraction at small radii.
Our analysis of the 2007 January data therefore yields a
detection of the west ansa in the region 1.3′′ − 3.3′′ ra-
dius. The photometry in this second epoch agrees well
with that of the first epoch (on average, the 2007 Jan-
uary disk photometry is 0.13 mag arcsec−2 fainter than
2006 October), suggesting that our frame selection tech-
nique for the first epoch of cloudy conditions effectively
filtered out non-photometric data.
3. RESULTS
Fig. 1 shows the PSF-subtracted images of HD 15115
with HST and with Keck. The west side of the disk in
the optical HST data has PA = 278◦.5 ± 0.5 and is de-
tected from the edge of the occulting spot at 1.5′′ (67
AU) radius to the edge of the field at 12.38′′ (554 AU)
radius. The east midplane is detected as far as ∼7′′ (315
AU) radius. At this radius the east midplane begins to
intersect the outer portion of the coronagraph’s 3.0′′ oc-
culting spot. Further east, past the spot and to the edge
of the field, no nebulosity is detected 9.0′′−14.9′′ radius.
The appearance of the disk is more symmetric in the
2006 October Keck data, which show the disk between
0.7′′ (31 AU) - 2.5′′ (112 AU).
Optical surface brightness contours (Fig. 2) re-
veal a sharp midplane morphology for the west ex-
tension that indicates an edge-on orientation to the
line of sight. The west midplane is qualitatively simi-
lar to that of β Pic’s northeast midplane, including a
characteristic width asymmetry (Kalas & Jewitt 1995;
Golimowski et al. 2006). The northern side of the west
midplane is more vertically extended than the southern
side. For example, the full-width at half-maximum across
the disk midplane at 2′′ radius is 0.19′′ ± 0.10′′ in both
the optical and NIR data. However, the vertical cuts are
not symmetric about the midplane when measuring the
half-width at quarter-maximum (HWQM). The HWQM
north of the west midplane is 1.6 ± 0.1 times greater
than that of the HWQM south of the west midplane.
This width asymmetry is confirmed in the Keck data.
If the width asymmetry is found to be in the opposite
direction in the opposite midplane, then Kalas & Jewitt
(1995) refer to such a feature as the butterfly asymmetry.
The butterfly asymmetry is evident in the morphology of
β Pic, that Golimowski et al. (2006) recently related to
the presence of a second disk midplane tilted relative to
the main disk midplane. However, our detection of HD
15115’s east midplane has insufficient signal to noise to
confirm the presence of a width asymmetry here.
We note that none of the surface brightness profiles
show evidence for significant flattening inward toward
the star (Fig. 3). All four surface brightness profiles
are well-represented by a single power law decrease with
radius. If there is an inner dust depletion, then it resides
within 40 AU radius. This constraint is consistent with
model fits of the spectral energy distribution that place
the dominant emitting dust component at∼35 AU radius
(Zuckerman & Song 2004; Williams & Andrews 2006).
The color of the disk may be estimated in the 2.0′′ −
3.3′′ region where the H-band and V-band data overlap
(Fig. 3). At face value, ΣV − ΣH ≈ −0.6 mag arcsec
at 2′′ radius, increasing to −1.9 mag arcsec−2 at 3.3′′
radius for the West disk extension. The east ansa has
similar blue scattering at 2′′ radius, but the V-band sur-
face brightness profile is steeper in the east than in the
west, giving a roughly constant blue color with increasing
radius in the east.
In a future paper we will present a detailed model of
dust scattering and thermal emission properties, which
requires a more complicated treatment of the obvious
disk asymmetry. However, for isotropically scattering
grains in an edge-on disk, an analytic approach shows
that the grain number density distribution as a function
of radius within the disk midplane follows a power-law
with index equivalent to one minus the sky-projected ra-
dial midplane power-law index. From the Keck data in
Fig. 3, we estimate that the disk number density dis-
tribution decreases with disk radius as r−3 in the inner
region up to ∼3.3′′ radius for both sides of the disk. At
> 3.3′′ radius, the optical data show that this profile
continues for the east extension, but that the disk num-
ber density profiles flattens for the west extension, as
described in Fig. 3. A precise measurement of the color
and polarization of the disk scattered light is necessary
to further constrain the grain size distribution, the corre-
sponding scattering phase function and albedo, and the
effect on the disk number density profile.
4. DISCUSSION
Asymmetric disk structure is evident in the majority of
debris disks, and most authors invoke planetary pertur-
bations as the likely origin. Secular perturbations may
offset the center of global disk symmetry from the lo-
cation of the star, though this effect may also be pro-
duced by an external perturber (Wyatt et al. 1999). The
edge-on debris disk surrounding β Pic displays a vari-
ety of radial and vertical asymmetries on large scales
(Kalas & Jewitt 1995) that may be most relevant to the
study of HD 15115. In the deepest optical images of the
β Pic disk, the northeast and southwest disk midplanes
are traced to 1835 AU and 1450 AU, respectively, giving
a ratio of 1.27 (Larwood & Kalas 2001). In the case of
HD 15115 the corresponding ratio is >1.75. This ratio
is a lower limit given that the 550 AU extent of the west
midplane is limited only by our field of view.
A single stellar flyby, or a periodic flyby by a bound
companion on an eccentric orbit, has been studied as
a potential mechanism for producing β Pic’s large-scale
asymmetry (Larwood & Kalas 2001). However, in a
kinematic study of Hipparcos-detected stars with pub-
lished radial velocities, Kalas et al. (2001) did not find
any perturbers that approached closer than 0.6 pc of β
Pic, though the sample was estimated as only 20% com-
plete. In the case of HD 15115, Moór et al. (2006) noted
Extreme Debris Disk Asymmetries 3
that another β Pic moving group member, HIP 12545, is
located relatively nearby in sky position.
Table 1 summarizes the observed properties of both
stars. Their projected separation is 0.51◦, which trans-
lates to 0.38 pc at a mean heliocentric distance of 43
pc. Within the uncertainties, the proper motion vectors,
the (U, V ) galactic space motions, and heliocentric dis-
tance are identical. Furthermore, the eastward location
of HIP 12545 is in the direction of the truncated side of
the HD 15115 debris disk. This geometry is consistent
with the dynamical simulation of a disk disrupted by a
stellar flyby in Larwood & Kalas (2001). Specifically, in
their Fig. 18, the long end of a highly perturbed disk
is located in the direction of periastron. The perturber
follows a parabolic trajectory such that in a later epoch
it is located in the direction opposite of periastron, or
in the direction of the truncated side of the disk. Peri-
astron in these models is ∼700 AU, with an initial disk
radius of ∼500 AU. Overall, the ensemble of evidence fa-
vors further consideration of HD 15115 and HIP 12545 as
a possible wide-separation multiple system with a highly
eccentric orbit (e > 0.95).
If the heliocentric distances are in fact nearly equiva-
lent, then the projected sky separation approximates the
true separation. Kalas et al. (2001) discuss the Roche ra-
dius, at, of a star as containing the volume within which
the stellar potential dominates the Galactic tidal field.
Using their Eq. 2 and the stellar mass estimates in our
Table 1, we find at = 1.1 pc and at= 0.7 pc for HD 15115
and HIP 12545, respectively. Therefore, for a small body
gravitationally bound to HD 15115, the potential well
of HIP 12545 exerts a more significant perturbing force
than the Galactic tidal field at the current epoch. This is
not the case if we take the Hipparcos parallaxes at face
value. These give a line-of-sight separation between the
stars of ∼4 pc, and we derive a 3-D separation of 5.1±2.8
pc. We further calculate that closest approach will occur
∼1 Myr in the future. Therefore, improving the par-
allax measurements for both stars is a critical task for
future work that would examine their possible physical
association.
A prediction of the Larwood & Kalas (2001) model is
that the perturber may capture disk material, and dis-
play a tenuous and highly asymmetric tail of escaping
material pointing away from the mother disk. To test
the hypothesis that HD 15115 suffered a close encounter
with HIP 12545, high-contrast observations of HIP 12545
should reveal circumstellar nebulosity due to captured
material. Since this is captured material, the nebulosity
may not resemble a disk, and any tail should point away
from the mother disk (eastward).
To futher investigate this hypothesis, we examined
ACS/HRC coronagraphic observations of HIP 12545 ob-
tained by program GO-10487 (Principal Investigator
David Ardila). The observing technique is similar to
that described here for HD 15115. After PSF subtrac-
tion, we do not detect nebulosity in the vicinity of HIP
12545. Therefore the possibility that the extreme disk
asymmetry of HD 15115 is created by dynamical inter-
actions with HIP 12545 does not have further supporting
evidence at the present time.
Finally, we note that among the four debris disks
imaged in scattered light in the BPMG, the dust ap-
pears depleted for HD 15115. The values of dust opti-
cal depth in 10−4 units are given as 24.3±1.1, 4.9±0.4,
29.3±1.6 and 4.0±0.3 for β Pic (A5V), HD 15115 (F2V),
HD 181327 (F5.5V) and AU Mic (M2V), respectively
(Moór et al. 2006). The factor of ∼five smaller opti-
cal depth for HD 15115 compared to β Pic and HD
181327 suggests a different evolutionary path for the
disk. Though a stellar flyby is one possibility, mi-
gration and dynamical instabilities within a hypotheti-
cal planetary system may also play a role in the rapid
diminution of dust parent bodies around HD 15115 (e.g.
Morbidelli & Valsecchi 1997).
5. SUMMARY
Optical and near-infrared coronagraphic images of the
F2 star HD 15115 reveal a highly asymmetric debris disk
with an edge-on orientation. We describe the morpho-
logical and photometric properties of the disk, deferring
a detailed model of scattering and thermal emission of
grains to future work. The blue scattered light color may
indicate grain properties most similar to those of the AU
Mic debris disk, where ΣV − ΣH ≈ −1 mag arcsec
relative to the star (Fitzgerald et al. 2007), and less like
those of β Pic, which is predominantly red scattering
(Golimowski et al. 2006). A key follow-up measurement
would be polarization, which in the case of AU Mic re-
vealed highly porous macroscopic grains (Graham et al.
2007).
With outer optical radius >550 AU, HD 15115 pos-
sesses the second largest debris disk next to β Pic. How-
ever, the length asymmetry between its west and east
midplanes greatly exceeds that of β Pic and other disks.
HD 15115 is now the fourth debris disk discovered in
scattered light among the β Pic moving group mem-
bers. Future work should test our hypothesis that ex-
treme asymmetries are due to dynamical perturbations
from the nearby M star HIP 12545.
Acknowledgements: Support for GO-10896 was
provided by NASA through a grant from STScI under
NASA contract NAS5-26555.
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Extreme Debris Disk Asymmetries 5
Fig. 1.— False-color, log scale images of the HD 15115 disk as originally discovered using the ACS/HRC F606W (λc= 591 nm, ∆λ =
234 nm) [LEFT] and confirmed in H-band with Keck II adaptive optics [RIGHT; 2006 October 26 data]. North is up, east is left and the
scale bars span 5′′. In the HST image we use gray fields over the occulting bar and 3.0′′ occulting spot located to the left of HD 15115,
as well as a gray disk covering PSF-subtraction artifacts surrounding HD 15115 itself. If the HD 15115 disk were a symmetric structure,
then the east side of the disk would have been detected within the rectangular box, shown below the ACS/HRC occulting finger. The NIR
data [RIGHT] show a more symmetric disk within 2′′ radius, with asymmetry becoming more apparent beyond 2′′ radius. Due to poor
observing conditions, the field is contaminated by residual noise due to the diffraction pattern of the telescope (e.g. at 2 o’clock and 7
o’clock relative to HD 15115). However, whereas the residual diffraction pattern noise of the telescope rotates relative to the sky orientation
over a series of exposures, the image of the disk remains fixed and it is confirmed as real.
Fig. 2.— Surface brightness isocontours for the HD 15115 debris disk converted from F606W to Johnson V-band (derived using
STSDAS/SYNPHOT with a Kurucz model atmosphere and the appropriate observatory parameters). The disk was rotate by 8◦ clockwise
such that the midplane lies along a horizontal line. The bottom frame is the east extension, transposed across the vertical axis, and the
gray region marks the area occupied by the ACS/HRC occulting finger and 3.0′′ occulting spot. The left edge of the frame corresponds
to 2′′ radius from the star. The innermost contour (bold) is 19.0 mag arcsec−2 and the outermost contour represents 23.0 mag arcsec−2,
with a contour interval of 0.5 mag arcsec−2.
6 Kalas, Fitzgerald, Graham.
Fig. 3.— Radial surface brightness (mag arcsec−2) distribution along the west and east midplanes of HD 15115. We plot the difference
between the measured disk surface brightness and the stellar magnitudes of H=5.86 and V=6.80. Disk photometry was extracted from
boxes 0.25′′ × 0.25′′ centered on the midplane. We plot a representative sample of error bars that gives the standard deviation of the
background residuals as a function of radius. The aperture corrections derived from point source photometry are 0.48 and 0.57 for the
H-band data in the 2006 October and 2007 January observations, respectively, and 0.70 for the V band data. The H-band radial profiles
between 0.7′′ and 2.3′′ radius may be described by power-laws with indices -3.7 and -4.4 for the west and east disk extensions, respectively.
In the V band, the east midplane profile may be fit by a power-law with index -4.0 between 2.0′′ and 6.0′′ radius. Thus, our data do not
show a significant color gradient as a function of radius for the east ansa. The west midplane profile in V band may be fit by a single
power-law with index -3.0 between 2.0′′ and 10.0′′ radius. This profile is significantly shallower than the H band profile, resulting in a blue
color gradient as a function of radius for the west ansa.
Extreme Debris Disk Asymmetries 7
TABLE 1
Stellar Properties
HD 15115 HIP 12545 Ref.
Spectral Type F2 M0 Hipparcos
mV (mag) 6.79 10.28 Hipparcos
Mass (M⊙) 1.6 0.5 Astrophys. Quant.
Distance (pc) 44.78+2.22
−2.01
40.54+4.38
−3.61
Hipparcos
RA (ICRS) 02 26 16.2447 02 41 25.89 Hipparcos
DEC (ICRS) +06 17 33.188 +05 59 18.41 Hipparcos
µα(mas/yr) 86.09 ± 1.09 82.32± 4.46 Hipparcos
µδ (mas/yr) −50.13 ± 0.71 −55.13± 2.45 Hipparcos
µα(mas/yr) 87.1 ± 1.2 82.3± 4.3 Tycho-2
µδ (mas/yr) −50.9± 1.2 −55.1± 2.7 Tycho-2
U (km / s) −13.2± 1.9 −14.0± 0.5 a
V (km / s) −17.8± 1.2 −16.7± 0.9 a
W (km / s) −6.0± 2.3 −10.0± 0.5 a
a Galactic kinematics for HD 15115 and HIP 12545 from Moór et al. (2006)
and Song et al. (2003), respectively
