GAPS IN THE HD 169142 PROTOPLANETARY DISK REVEALED BY POLARIMETRIC IMAGING: SIGNS OF ONGOING PLANET FORMATION?^*

To study the physical and chemical conditions for planet formation and to search for morphological evidence of forming planets, the inner few tens of AU of protoplanetary disks have to be investigated. In particular, annular gaps in those disks are considered to be possible signatures of ongoing planet formation. In this Letter, we report the detection of such gaps in the disk around the Herbig Ae/Be star HD 169142 using polarimetric differential imaging (PDI). PDI is a powerful high-contrast technique to study the dusty surface layer of protoplanetary disks very close to the star (e.g., Quanz et al. 2011, 2012; Hashimoto et al. 2011, 2012; Muto et al. 2012; Tanii et al. 2012; Kusakabe et al. 2012; Mayama et al. 2012; Grady et al. 2013).

Our new H-band PDI observations of HD 169142 resolve for the first time distinct structures in the inner disk regions with high signal-to-noise and allow an in-depth analysis of the disk profile and morphology. The basic stellar parameters of HD 169142 are summarized in Table 1.

Table 1. Basic Parameters of HD 169142

Parameter	Value for HD 169142	Referencea
R.A. (J2000)	18h24m2979	(1)
Decl. (J2000)	−29°46'4922	(1)
J (mag)	7.31 ± 0.02	(1)
H (mag)	6.91 ± 0.04	(1)
Ks (mag)	6.41 ± 0.02	(1)
Sp. Type	A9III/IVe/A7V	(2), (3)
v sin i (km s−1)	55 ± 5	(2)
Age (Myr)	1–5/12/3–12	(2), (3), (4)
[Fe/H]	−0.5 ± 0.1/−0.25 to −0.5	(2), (5)
log g	3.7 ± 0.1/4.0–4.1	(2), (5)
Teff (K)	7500 ± 200/6500/7650 ± 150	(2), (3), (5)
Mass (M☉)	∼1.65	(3)
R* (R☉)	∼1.6	(3), (5)
L* (L☉)	∼8.6	(3)
$\dot{M}$ (10−9 M☉yr−1)	∼3.1/⩽1.25 ± 0.55	(3), (4)
Distance (pc)	145b/151	(6), (3)

Notes. ^aReferences. (1) 2MASS point source catalog (Cutri et al. 2003); (2) Guimarães et al. 2006; (3) Blondel & Djie 2006; (4) Grady et al. 2007; (5) Meeus et al. 2010; (6) Sylvester et al. 1996. ^bAdopted value in this Letter.

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HD 169142 was classified as a Herbig group Ib member (Meeus et al. 2001), indicating that its spectral energy distribution (SED) rises in the mid-infrared (MIR), but does not show a 10 μm silicate emission feature (e.g., van Boekel et al. 2005). However, polycyclic aromatic hydrocarbon emission features have been detected in MIR spectroscopy (Meeus et al. 2001; van Boekel et al. 2005) and were spatially resolved at 3.3 μm (Habart et al. 2006). The disk was previously resolved in polarized light in the NIR from the ground (Kuhn et al. 2001; Hales et al. 2006), but those data did not allow an in-depth analysis of the disk structure and profile. Grady et al. (2007) and Fukagawa et al. (2010) detected the disk in scattered light at 1.1 μm with Hubble Space Telescope (HST)/NICMOS coronagraphy and in H band from the ground, respectively, and they derived azimuthally averaged surface brightness profiles. In all direct imaging studies the disk was consistent with being seen face-on and no substructures were detected. In the MIR the disk was resolved at 12, 18, 18.8, and 24.5 μm (Honda et al. 2012; Mariñas et al. 2011) and most of this emission is thought to arise from disk regions between ∼30 and 60 AU. From dust continuum and molecular line observations, the mass and outer radius of the disk are estimated to be ∼0.005–0.04 M_☉ and ∼235 AU (Raman et al. 2006; Dent et al. 2006; Panić et al. 2008; Meeus et al. 2010; Sandell et al. 2011). The kinematic pattern and molecular line profiles are best fitted with a disk inclination of ∼13° (Raman et al. 2006; Panić et al. 2008). The lack of a silicate emission feature at 10 μm can be explained with a lack of small (<3–5 μm) dust grains in the inner ∼10–20 AU of the disk possibly due to grain growth (van Boekel et al. 2005). From SED modeling a disk hole was inferred for the inner disk regions (Grady et al. 2007; Meeus et al. 2010; Honda et al. 2012).

The observations were carried out with the NACO camera at the Very Large Telescope (VLT)/NACO (Lenzen et al. 2003; Rousset et al. 2003) in the H and NB1.64 filter. A description of the camera properties and the PDI observing mode is given in Quanz et al. (2011). In short, a Wollaston prism splits the light in an ordinary and extraordinary beam with orthogonal, linear polarization directions. Both beams are imaged simultaneously on the detector. The polarization direction can be changed by rotating a half-wave plate (HWP). One full polarization cycle includes exposures at four HWP positions (0° and −45° for Stokes Q, −225 and −675 for Stokes U). We summarize the observations for HD 169142 in Table 2. The central few pixels of the point-spread function were saturated in the H filter. The exposures in the NB1.64 filter were unsaturated and used for photometric calibration (see below).

Table 2. Summary of NACO/PDI Observations of HD 169142 on 2012 July 25

Filter	DIT × NDITa	Dither	Average	Average	Average 〈τ0〉d
		Positionsb	Airmass	Seeingc
NB1.64	1 s × 15	3	∼1.02	∼157	∼14 ms
H	1 s × 45	2 × 6	∼1.06	∼104	∼22 ms
NB1.64	1 s × 20	3	∼1.11	∼107	∼20 ms

Notes. ^aDetector integration time (DIT) × number of integrations (NDIT), i.e., total integration time per dither position and per retarder plate position. ^bAt each dither position four different HWP positions (00, −225, −450, −675) were used. ^cDIMM seeing in the optical as measured from seeing monitor. ^dAverage coherence time of the atmosphere calculated by the Real Time Computer of the AO system.

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NACO suffers from instrumental polarization and cross-talk between the different Stokes components (Witzel et al. 2011; Quanz et al. 2011). To obtain reliable results in PDI observations these effects need to be corrected for. Having obtained data sets for five more Herbig Ae/Be stars allowed us to do a systematic analysis and we refer to an upcoming publication for a detailed description of our data reduction and calibration approach (H. Avenhaus et al., in preparation). The key steps are as follows.

All exposures were dark-current-corrected and flat-fielded. Bad pixels were replaced with the mean value of surrounding pixels. Each quadrant of the detector showed a noise pattern affecting every other row, which was eliminated by subtracting the mean value of each row from each pixel in that row. Detector regions close to the star were excluded to compute the mean values. The ordinary and extraordinary images in each exposure were extracted and the stellar position in all individual images determined. We rebinned the images to a factor three higher resolution (bicubic interpolation) and shifted all images to a common center (bilinear interpolation). Assuming that the central star is unpolarized in the H band, we corrected instrumental polarization and polarization cross-talk effects.⁴ For each dither position, the fractional Stokes parameters p_Q and p_U were computed using the double ratio approach (see, e.g., Tinbergen 1996; Schmid et al. 2006):

$$\begin{equation} p_Q=\frac{R_Q-1}{R_Q+1};\quad p_U=\frac{R_U-1}{R_U+1} \end{equation} \tag{ 1 }$$

with

$$\begin{equation} R_Q=\sqrt{\frac{{I^{0^{\circ }}_{\rm ord}}/{I^{0^{\circ }}_{\rm extra}}}{I^{-45^{\circ }}_{\rm ord}/I^{-45^{\circ }}_{\rm extra}}}; \quad R_U=\sqrt{\frac{{I^{-22{.\ssty\!\!^\circ }5}_{\rm ord}}/{I^{-22{.\ssty\!\!^\circ }5}_{\rm extra}}}{I^{-67{.\ssty\!\!^\circ }5}_{\rm ord}/I^{-67{.\ssty\!\!^\circ }5}_{\rm extra}}}. \end{equation} \tag{ 2 }$$

Here, the subscripts refer to either the ordinary or extraordinary beam and the superscripts refer to the angular position of the HWP. The final p_Q and p_U images were obtained by averaging over all dither positions. From p_Q and p_U and the intensity images I, we obtained the Stokes Q and U images, from which we computed the radial Stokes parameters Q_r and U_r:

$$\begin{equation} Q_{\rm r}=+Q\,{\rm cos}\,2\phi +U\,{\rm sin}\,2\phi \end{equation} \tag{ 3 }$$

$$\begin{equation} U_{\rm r}=-Q\,{\rm sin}\,2\phi +U\,{\rm cos}\,2\phi \end{equation} \tag{ 4 }$$

with

$$\begin{equation} \phi ={\rm arctan} \frac{x-x_0}{y-y_0}+\theta \end{equation} \tag{ 5 }$$

being the polar angle of a given position (x, y) on the detector, (x₀, y₀) denoting the central position of the star (see, e.g., Schmid et al. 2006) and θ being an offset due to polarization cross-talk effects (H. Avenhaus et al., in preparation), which amounts to 7° in our case. Q_r is equivalent to the polarized flux P under the assumption that the polarized flux has only a tangential component, but, in contrast to P, Q_r is free of any biases introduced from computing the squares of the Q and U components (Schmid et al. 2006). Under the same assumption, and assuming the correction for instrumental effects was done properly, U_r only contains noise and can be used to estimate the error in Q_r via

$$\begin{equation} \Delta Q_{\rm r} = \sqrt{\sigma ^2_{U_{\rm r}}} / \sqrt{n_{\rm res}}. \end{equation} \tag{ 6 }$$

Here, ΔQ_r refers to the 1σ uncertainty in Q_r, $\sigma ^2_{U_{\rm r}}$ is the variance in the final U_r image, and n_res the number of resolution elements in the region of interest.

The calibration of the surface brightness level in the final Q_r image was done using the non-saturated images in the NB1.64 filter following the description in Quanz et al. (2011). We estimate that the absolute flux calibration is good to ∼30%.

Figure 1 shows the final Q_r and U_r images, an intensity image, and a polar coordinate mapping of the Q_r image. While the Q_r image shows extended polarized flux which we interpret as the disk around HD 169142, the U_r image does not reveal any significant signal as expected for scattering of dust particles without any preferred alignment.

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Figure 1(a) reveals a protoplanetary disk with a complex radial morphology. Very close to the star, but outside the innermost saturated pixels, the detected polarized flux is low and it increases going outward to reach a ring-like maximum at ∼017 (∼25 AU). Outside of the narrow ring the surface brightness decreases and an annular gap stretches from ∼028 to 048 (∼40 to 70 AU) with a local minimum at ∼038 (∼55 AU). Outside of ∼055 (∼80 AU) the disk surface brightness drops off smoothly.

The bright inner ring does not appear fully circular (Figure 1, panels (a) and (d)) and the radius of the peak brightness varies between ∼016 and ∼019 with a mean value of ∼017. A dip in the brightness of the ring seems to be present at position angle (P.A.) ≈ 80° (east of north). On the opposite side of the annular gap at P.A. ≈ 90°, next to an adaptive optics (AO) artifact, a region of enhanced brightness is seen stretching a bit into the gap. The FWHM of the ring varies slightly with P.A. but the ring is not spatially resolved. Using the ring we estimated an upper limit in disk inclination of i < 20°, so that i is still in agreement with the fluctuations in ring radius. However, our image is consistent with a face-on disk. Analyzing the mean surface brightness as a function of azimuth for different radii also supports a small inclination angle, as no significant variation is seen (Muto et al. (2012) used this method for the SAO206462 disk).

In Figure 2 we show the azimuthally averaged surface brightness profile and the signal-to-noise profile for the Q_r and U_r images. Fitting a power law to the brightness profile between 85 and 250 AU yields an exponent of −3.31 ± 0.11, where the error denotes the 95% confidence level (black dashed line in top and middle panel of Figure 2). However, fitting the regions between 85–120 AU and 120–250 AU separately with power laws yields a mathematically better fit in a χ² sense (black solid line in top and middle panel of Figure 2). In this case the power-law exponents are $-2.64^{+0.15}_{-0.17}$ and $-3.90^{+0.10}_{-0.11}$ for the inner and outer regions, respectively. The power-law fit to the inner region connects well with the bright inner ring at ∼25 AU. A 015 × 01 region centered on the AO feature seen in Figure 1 has been masked out before the power-law fits were computed.

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The maximum surface brightness in polarized flux of S ≈ 11.3 mag arcsec⁻² is reached at the location of the bright ring. Close to the inner edge of the outer disk (∼75 AU), we find ≈14.3 mag arcsec⁻² and at ∼220 AU ≈18.1 mag arcsec⁻². Inside the gap (∼55 AU) the surface brightness is reduced by a factor of ∼3 compared to the corresponding value of the power-law fit. Integrating the polarized surface brightness between ∼14.5 and 250 AU and comparing it to the H-band flux of the central star yields a flux ratio of $F_{\rm Disk}^{\rm polarized}/F_{\rm Star}\approx 4.1\times 10^{-3}$. Here, the regions between ∼14.5–40 AU (the ring), ∼40–70 AU (the gap), and ∼70–250 AU (the outer disk) contribute ∼49%, ∼14%, and ∼37% to the integrated disk flux, respectively.

The face-on orientation and general extent of the disk have been reported in earlier scattered or polarized light studies (Kuhn et al. 2001; Hales et al. 2006; Grady et al. 2007; Fukagawa et al. 2010). However, no substructures were revealed so far.

4.1. The Bright Inner Ring

4.2. The Annular Gap

4.3. The Disk Surface Brightness

Our PDI images of the protoplanetary disk around HD 169142 reveal previously undetected features and emphasize the power of this observing technique. With PDI on ground-based 8 m class telescopes the inner few tens of AU of the dusty surface layer of protoplanetary disks can be reliably resolved opening up a new era of disk imaging. Several PDI studies in the last few years have shown that substructures in disks (e.g., holes, gaps, spiral arms) appear to be the rule rather than the exception (e.g., Quanz et al. 2011; Hashimoto et al. 2011, 2012; Muto et al. 2012; Mayama et al. 2012; Grady et al. 2013).

In the case of HD 169142, our images support previous model results suggesting that the inner ≲20 AU are largely devoid of scattering dust particles. In addition, our images reveal an annular gap roughly between ∼40 and 70 AU, localized brightness asymmetries on opposite sides of this gap, and a discontinuity in the radial surface brightness profile at ∼120 AU. Our data alone do not allow us to unambiguously reveal the physical nature of the annular gap, i.e., whether it is a gap in surface density possibly induced by one (or several) forming planet(s), a shadowing effect, or combination of radially changing dust properties and dust settling. However, our results make HD 169142 an excellent target for follow-up studies aiming at a better understanding of disk physics and potentially ongoing planet formation.

This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. We thank the staff at VLT for their excellent support during the observations and M.R. Meyer, F. Meru, and O. Panić for useful discussions. This work is supported by the Swiss National Science Foundation.

Facilities: VLT:Yepun (NACO) - Very Large Telescope (Yepun)