A&A 492, 731-734 (2008)
DOI: 10.1051/0004-6361:200810723
K. Murakawa1 - S. Oya2 - T.-S. Pyo2 - M. Ishii2
1 - Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69, 53121 Bonn, Germany
2 -
Subaru Telescope, National Astronomical Observatory of Japan,
650 North A'ohoku place, Hilo, HI 96720, USA
Received 31 July 2008 / Accepted 15 October 2008
Abstract
We present the -band high-resolution polarimetric images of the low-mass
proto-stellar object HL Tau using the adaptive optics-equipped
CIAO instrument on the Subaru telescope. Our polarization images show
a butterfly-shaped polarization disk with an
extension. In the nebula, where polarization vectors are centro-symmetrically
aligned, the polarization is as high as
,
,
and
.
On the other hand, low polarizations of P<3% in the
J, H, and K bands and a low color excess ratio of
EJ-H/EH-K=1.1compared to the standard cloud value of 1.75 are detected towards the central
star. We estimated the upper limit of the grain sizes
to be
0.4
m in the nebula and
0.7
m in the line of sight towards
the central star. Our high-resolution polarimetric data, which spatially
resolves the polarization disk, provides us with important information about
grain growth in the region close to the central star.
Key words: polarization - ISM: dust, extinction
The disk is a key component of young stellar objects because the physical
structure is characterized by the star-forming process, and it is a field
where grain growth progresses. However, it is difficult to directly detect
the images and investigate the physical properties because of small angular
sizes; e.g. a 0
7 radius for a typical T Tauri disk radius of 100 AU at
a distance of 140 pc. High-resolution techniques such as adaptive optics (AO),
speckle interferometry, and infrared interferometry achieve sub-0
1 angular
resolutions which can, in theory, resolve typical T Tauri disks if objects are
in nearby star-forming regions. It is also known that NIR imaging polarimetry
is a powerful technique to derive dust properties and geometry structures via
dust scattered light. For example, bipolar nebulae seen nearly edge-on show
a centro-symmetric polarization vector pattern in the lobes on a large scale
and a polarization disk in the equatorial plane
(Perrin et al. 2004; Kenyon et al. 1993; Whitney et al. 1997). These signatures are explained
with a model consisting of a disk/torus and infalling envelope
(Bastien & Ménard 1988; Fischer et al. 1996; Whitney & Hartmann 1993). Furthermore, our recent experiments have
demonstrated that multiwavelength, high-resolution imaging polarimetry
allows us to also derive multiple grain populations; e.g., large grains in the
equatorial disk region and small grains in the surrounding nebula
(Murakawa et al. 2008a,c,b). In this paper, we present
-band high-resolution polarimetric images of the low mass protostar
HL Tau and discuss the polarization properties and the grain sizes in the disk
and envelope.
We reduced the observed data as described in our previous papers
(Murakawa et al. 2004,2005). After subtraction of the dark frame and
flat-fielding, we obtained images of the Stokes IQU parameters, the polarized
intensity (PI), the degree of linear polarization (P), the polarization
orientation (), as well as their error images. We found
signal-to-noise ratios (SNs) of the Stokes I images of SNJ = 30-300,
SNH = 30-300, and SNK = 20-200 per pixel and the errors of linear
polarization of PJ<3%, PH<3%, and PK<5% per pixel within
from the central star. The accuracy of the polarization
orientation per pixel is
in the lobes. The estimated PSF sizes are
in the J band and
in the H and K bands.
The second panels in Fig. 1 show the color index images of J-H(left), H-K (middle), and
(J-H)/(H-K) (right). We see some spots with low
J-H values (0.7) near the central star, which approximately trace
the C-shaped feature, and a high value of 7.6 mag towards the central
star in the J-H image. We also find a different color pattern between the
east and west sides of the nebula, which is split by a long arc-like feature
extending from the north to the east, crossing near the central star. The east
side has a J-H of
4 mag, an H-K of
2 mag, and a
(J-H)/(H-K)of 2-3.5. On the other hand, lower
mag, higher
mag, and
lower
(J-H)/(H-K) of 0-1 are detected on the western side. The H-K color
index pattern can be explained by a stronger extinction in the southwest lobe
than in the northeastern lobe because the northeastern side of the HL Tau
system is tilted towards the observer (Mundy et al. 1996; Hayashi et al. 1993). In contrast,
the J-H color index pattern does not agree with the simple extinction model
above. One reason could be due to a complicated optical depth structure; i.e.
the northeastern part is optically thin in the H and K bands, but it
becomes optically thick in the J band. The color information towards the
central star indicates the reddening by dust. If we assume the intrinsic color
of the central star to be
and
,
which is obtained from an estimated stellar
temperature of
4000 K (e.g. Men'shchikov et al. 1999), the color excess values and
their ratio are estimated to be
EJ-H=7.3,
EH-K=6.4, and
EJ-H/EH-K=1.1. The color index ratio
EJ-H/EH-K is significantly
lower than an estimated average value of 1.75 for the Taurus dark cloud
(Whittet et al. 2007), suggesting larger grain sizes close to the central star
compared to the ones in the parent molecular cloud.
![]() |
Figure 1:
J, H, and K band polarimetric data of HL Tau obtained using
CIAO on the Subaru telescope. The field of view is
![]() |
Open with DEXTER |
In the polarized intensity images (the third row), the northern arc, the C-shaped feature, and the southwestern lobe show high polarized intensity and the southwestern feature is split by the dark lane, which corresponds to the long arc detected in the J-H magnitude image. This dark lane is due to an optically thick disk and the long arc appears due to this disk, where scattering occurs several times, resulting in low polarization.
The bottom panels in Fig. 1 show the degree of polarization images.
The polarization vector lines are centro-symmetrically aligned in the outer
part of the nebulosity. The degree of polarization there reaches
,
,
and
.
A polarization disk lies across the central
star at a position angle of
.
This is consistent with previous
results of imaging polarimetry (Weintraub et al. 1995; Lucas et al. 2004; Gledhill & Scarrott 1989).
The polarization disk shows a butterfly shape with low polarizations of
,
,
and PK=1.0% applying a 0
3 diameter
aperture. These values are lower than the results of PH,K = 3-4% using
the UKIRT and
using the Subaru presented by Lucas et al. (2004).
The higher polarizations in their results are probably due to larger PSF sizes
(0
4 for both the UKIRT and Subaru data). Namely, scattered polarized
fluxes around the central star contaminate the polarization measurement more at
a larger PSF size. Therefore, our higher resolution data (0
1-0
2)
would show more realistic polarization values. The extension of the polarization
disk slightly depends on the wavelength. The angular size is a
in
the direction perpendicular to the disk plane and
,
,
and
parallel to the disk plane in the J, H, and K bands,
respectively, if the region of the polarization lower than 5% is measured.
The threshold polarization to determine the polarization disk is not defined.
Instead, a polarization disk is regarded as the lower polarization
region, which resulted in multiple scattering and the regime is roughly
a region of an optical depth higher than 1.0. In many observational results,
the extension of a polarization disk is larger than the estimated dust disk
size. In fact, previous millimeter and centimeter radio interferometric images
estimated a 100 AU radius or an even smaller size of 65 AU for HL Tau's
(Keplerian accretion) disk (e.g. Greaves et al. 2008; Wilner et al. 1996; Mundy et al. 1996, and references
therein), resulting in an expected extention
(diameter) of 0
9 to 1
4 at a distance of 140 pc (Kenyon et al. 1994).
We first investigated the grains in the nebulosity by means of one-dimensional
single scattering modeling (Murakawa et al. 2008c,b). We assumed the
following model geometry parameters: an inner radius of 0.05 AU, a luminosity
of 10
(e.g. Robitaille et al. 2007), an outer radius of 1300 AU
(Hayashi et al. 1993), a radial dust density distribution of
,
and an optical depth of 1 at V band.
For dust grains, we assumed the DL-chemistry (Draine & Lee 1984) and a power-law size
distribution of
with a size range of
.
Because the lower limit of the grain size is
insensitive in our modeling, we determined only the maximum value
.
We examined
values from 0.2 to 1.0
m
with 0.1
m increments and calculated the degree of polarization.
We compared these model results with our observations and found that
m is a reasonable solution. Although the accuracy of
the estimation from our polarization fit is actually quite good (about
m for submicron sizes), one should consider that:
(1) the real chemical composition of dust in HL Tau nebula could be somewhat
different from the DL-chemistry. For instance, a water ice mantle is omitted in
our modeling; (2) our single scattering modeling fits the polarization in a
region where a centro-symmetric polarization vector pattern is seen. However,
this sampling region does not have a large enough separation from the central
star due to the data quality. Thus, the data could be diluted somewhat by
multiple scattering components.
We estimated the grain size in the region close to the central star using the
color excess ratio presented in Sect. 2.2. This value can be
determined with the extinction cross section
as
.
We calculated this
for the above grain model with
values of 0.2 to 10.0
m.
Because the extinction efficient
converges to 1 for size
parameter
,
the
EJ-H/EH-K also
becomes a constant value, which is found to be 1.1 in our calculation and is
consistent with our observational result. The corresponding grain size for this
result is
m. Lucas et al. (2004) applied a single grain
model of spheroidal shape in the entire nebula and found sizes slightly larger
than 1
m and a grain axis ratio of 1.015 based on their polarization data,
which shows higher values than ours. If our high-angular-resolution data and
multiple dust models are applied, more spherical shapes and/or larger sizes
are expected.
The main interests of these studies are grain alignment mechanisms and magnetic
field interaction with star formation. We propose adding another point of the
grain size in data which has spatial information. The above infrared polarimetry
has been obtained applying large apertures, which acquire almost entire flux
from the source. Thus, the results are affected by the polarized flux from the
nebula, where small grains (i.e. sub-micron-size) are expected to exist, and
show similar polarization properties to the interstellar polarization, i.e.
a strong wavelength dependence. We compare some results of HL Tau and other
object of high-resolution data. In HL Tau, the
is less than 3%
at the central star using a 0
3 diameter aperture and does not follow
Serkowski's law. Because of this result, the polarization efficiency is also
as low as 0.23, 0.55, and 0.27 in the
bands, respectively, applying the
optical depth estimated by Lucas et al. (2004). These results are explained by the
presence of large grains in the disk. Murakawa et al. (2008c) presented the
-band imaging polarimetry of the Herbig Be star R Mon. Their
results also show low polarizations of PJ=7%, PH=2%, and PK=1%.
Radio observation has shown evidence of large grains from a millimeter flux
excess (Fuente et al. 2006). Thus, interpreting infrared polarization of this
object is similar to doing it for HL Tau. On the other hand, other objects such
as AFGL 2519 (Minchin et al. 1991), AFGL 437 (Meakin et al. 2005),
and CRL 2136 (Murakawa et al. 2008b), which are probably higher mass than
R Mon, show very different results. Although, in these data, the polarization
disk is spatially resolved and the central star feature is seen, the
polarization is still high (
%) and the polarization vectors towards
the central star are systematically aligned. For CRL 2136, Murakawa et al. (2008b)
estimated the grain size (
)
to be 0.45
m from their
radiative transfer modeling. They also discussed the possible mechanisms of
polarization vector alignment and concluded that the high polarization is
probably produced by dichroic absorption by small non-spherical grains.
The aforementioned results suggest that many protostars of low- to intermediate-counterparts show low infrared polarization in the polarization disk and evidence of large grains, and the opposed results are seen in many massive stars. High-resolution near-infrared imaging polarimetry is a potential technique for investigating grain growth in the disk. Further studies of more samples and detailed modeling will provide us with new insights into the disk environment to determine grain growth and the magnetic field interaction in the disk and envelope.
Acknowledgements
Based on data collected at the Subaru telescope, which is operated by the National Astronomical Observatory of Japan.