Exploring Dust around HD 142527 down to 0025 (4 au) Using SPHERE/ZIMPOL

HD 142527 is an example of a Herbig star surrounded by an optically thick protoplanetary disk, and it is well studied because of its proximity (${156}_{-6}^{+7}$ pc, Gaia Collaboration et al. 2016), the brightness of its disk (${F}_{\mathrm{IR}}$/${F}_{\mathrm{star}}=0.92$, Verhoeff et al. 2011), and the size of its gap (∼140 au given the new Gaia distance). It is only moderately inclined (estimates range from ∼20° to ∼27°, Pontoppidan et al. 2011; Boehler et al. 2017). These factors together have allowed for high-resolution images with high signal-to-noise ratio (S/N) of the disk in optical/near-IR scattered light (Fukagawa et al. 2006; Casassus et al. 2012, 2013; Rameau et al. 2012; Canovas et al. 2013; Avenhaus et al. 2014b; Rodigas et al. 2014), the mid-infrared (Leinert et al. 2004), and at submillimeter wavelengths (Casassus et al. 2013, 2015; Perez et al. 2015; Muto et al. 2015). The disk has also been studied with respect to its submillimeter polarization (Kataoka et al. 2016). These studies have revealed the gap to be highly depleted in micron-sized grains, although optically thick CO gas is present. They have also led to the interpretation of the two prominent local minima in scattered light as shadows cast from an inner tilted disk, in agreement with the ALMA studies of gas close to the star (Marino et al. 2015; Casassus et al. 2015). More recently, Min et al. (2016) have produced an updated model of the inner and outer disk based on Herschel data, focusing on the water ice within the disk. Spirals in the outer disk extend outward from its inner rim and are detected in both submillimeter and scattered light (Fukagawa et al. 2006; Christiaens et al. 2014), although they are displaced with respect to each other at the two wavelengths. The inner disk has so far been revealed through mid-IR imaging and SED modeling, but has not been imaged directly in the near-IR at high resolution (sub-01). The disk still retains significant mass (total disk mass of ∼0.1–0.15 M_⊙, Acke et al. 2004; Fukagawa et al. 2006), and in addition to the fact that there is no good agreement on the extinction toward or the exact luminosity of the star, the star is still accreting at a significant rate ($\dot{M}=6.9\times {10}^{-8}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$, Garcia Lopez et al. 2006), which means that material must be transported in some way from the outer to the inner regions of the disk.

So far, the inner disk surrounding the star has escaped detections in scattered light. Dust close to the star (up to ∼30 au) is consistent with SED modeling and mid-IR imaging (Verhoeff et al. 2011), but using NACO, Avenhaus et al. (2014b) reached an inner working angle (IWA) of ∼01 (15 au) without detecting traces of the inner disk. This is interesting because HD 142527 has a still-accreting M-dwarf companion that has been detected with NACO/SAM and subsequently been followed-up with NACO, GPI, MagAO, and most recently with SPHERE at close separation (77–90 mas). While it does not seem to orbit in the same plane as the outer disk, its orbit could be in agreement with the orientation of an inner disk casting shadows, although it is still not very well determined (Biller et al. 2012; Close et al. 2014; Lacour et al. 2016, S. P. Quanz et al. 2017, in preparation). This companion must necessarily be in causal contact and thus shape the dust and gas present within the inner 10–30 au around the star. Recent observations by Rodigas et al. (2014) performed with the Gemini Planet Imager (Y band, 0.95–1.14 μm) detect the companion in total intensity and find a point source in polarized light slightly offset from the location of the secondary, suggesting the presence of dust close to the location of the M-dwarf companion, possibly in a circumsecondary disk.

Here, we present new SPHERE/ZIMPOL observations of HD 142527 that are specifically designed to study the smallest possible separations at the highest possible detail and S/N, being able to detect and resolve circumstellar dust down to an IWA of ∼25 mas (4 au). In Section 2 we describe the data and data reduction procedures. In Section 3 we present results and analysis of these data, which are then discussed in more detail in Section 4. We conclude in Section 5.

The ${Q}_{\phi }$ images reveal the well-known outer disk at high S/N and at higher resolution than in previously available NACO data (Canovas et al. 2013; Avenhaus et al. 2014b). They are furthermore able to detect dust scattering close to the star. This structure is elongated in the ESE-WNW (position angle ∼120° east of north) direction. However, it does not resemble a uniform disk of any inclination, but rather has extensions both on the southeastern and western/northwestern sides (these two lobes are seen in each of the six independent sub-data sets described before). The more prominent extension is seen on the northwestern side. The western side is also special because there is a prominent dip in brightness toward the west, very close to the star (∼25–50 mas). The dust structure as a whole has no apparent relation to the shadows seen in the outer disk (Marino et al. 2015), because an inclined disk explaining these shadows would be elongated in the north-south direction.

Furthermore, the gap that was first revealed to be largely devoid of dust down to ∼15 au (Avenhaus et al. 2014b) can now be seen to be asymmetric. The gap clearly deviates from an elliptic shape in the southwest. The spiral arms in this region seem to cross the inner wall of the outer disk, extending inward into the gap region.

In order to further determine the reliability of the detected signal, we calculate azimuthally averaged surface brightness profiles for the disk. The results can be seen in Figure 2. The polarization signal close to the star is detected at more than 20σ, with σ calculated from the ${U}_{\phi }$ data as described in Avenhaus et al. (2014b). This takes into account the (systematic) errors close to the star discussed above. We can thus clearly state that the inner dust structure is detected.

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3.2.1. Signal Dampening Through PSF Smearing Effects and Brightness of the Inner Disk

It is worth noting that when corrected for the fall-off of the stellar illumination by multiplying the data with r², r being the projected distance from the stellar position (right side of Figure 2), the dust scattering close to the star seems to be significantly fainter than the outer disk. However, this does not take into account the dampening effect of point-spread function (PSF) smearing in PDI. This can reduce the polarimetric flux close to the star when employing the PDI method (see Avenhaus et al. 2014a). The magnitude of this effect depends on both the distribution of scattered light itself and the shape of the PSF. It is weaker for stable high-Strehl PSFs and farther away from the star. The ZIMPOL very broad band filter (590–881 nm) is more strongly affected by this problem than the near-IR IRDIS filters because of the significantly lower Strehl ratios at this wavelength. The inner dust structure is particularly affected because of its proximity to the star.

In principle, the best way to understand these effects is a forward-modeling of scattered-light images produced with a radiative-transfer code, which are then (Stokes Q and U vectors) convolved with the PSF retrieved from the observations. Because developing a radiative-transfer model is beyond the scope of this paper, specifically for the complex asymmetric dust structure we observe, we instead use the derived ${Q}_{\phi }$ image in order to estimate the magnitude of signal suppression within our scattered-light data. The process works as follows:

• 1.  
  Produce an azimuthally averaged image ${Q}_{\phi ,\mathrm{avg}}$ of the ${Q}_{\phi }$ image and smooth it with a small (∼25 mas) Gaussian kernel in order to avoid effects from small-scale structures and noise.
• 2.  
  Split this image into the Stokes Q and U vectors using the inverse of the formulas shown in Section 2.
• 3.  
  Convolve the obtained Stokes vectors with the PSF obtained from the unsaturated science frames.
• 4.  
  Calculate ${Q}_{\phi ,\mathrm{avg},\mathrm{damp}}$ from these convolved Stokes vectors
• 5.  
  Calculate the local damping factor as ${F}_{\mathrm{damp}}=\tfrac{{Q}_{\phi ,\mathrm{avg}}}{{Q}_{\phi ,\mathrm{avg},\mathrm{damp}}}$. We then obtain an approximation of the real (undamped) polarimetric scattered-light signature by multiplying ${Q}_{\phi }$ with ${F}_{\mathrm{damp}}$.

Both ${Q}_{\phi }$ and ${Q}_{\phi }\cdot {F}_{\mathrm{damp}}$ are displayed alongside each other in Figure 3. As can be seen, the inner dust structure brightens up significantly with this processing (factor of ∼5). The outer disk brightens as well (showing that PSF smearing has an effect even at >1''), but by a smaller factor. We then produce averaged radial surface brightness plots again, which show that the inner disk is indeed as bright as the outer disk when corrected for the drop-off in stellar illumination (see Figure 5, left side).

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The inner dust structure is in fact very bright. When compared to the outer disk, it scatters more light than the entire outer region of the disk as seen in our images. To show this, we divide our image into three regions: the inner dust structure (0025–02), the gap region (02–055), and the outer disk (055–135). Using the corrected ${Q}_{\phi }$ image and conservative error estimates constructed from the corrected ${U}_{\phi }$ image, we calculate a polarized flux ratio of $\tfrac{{F}_{\mathrm{inner}}}{{F}_{\mathrm{outer}}}=1.43\pm 0.36$. Most of this flux is very close to the star, in the region between 25 and 50 mas. If we furthermore divide the inner disk based on this into a region of 25–50 mas and 50–200 mas, we obtain $\tfrac{{F}_{25\mbox{--}50}}{{F}_{\mathrm{outer}}}=1.04\pm 0.23$ and $\tfrac{{F}_{50\mbox{--}200}}{{F}_{\mathrm{outer}}}=0.37\pm 0.07$. The gap is darker than the outer disk, with $\tfrac{{F}_{\mathrm{gap}}}{{F}_{\mathrm{outer}}}=0.062\pm 0.007$. Figure 4 shows the chosen regions of the disk for reference.

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Comparing the scattered light within 135 to the total flux (star and scattered light, both polarized and unpolarized) in the same region, we obtain a ratio of ${F}_{\mathrm{pol},\mathrm{scattered}}/{F}_{\mathrm{total}}\approx 0.83 \%$. This ratio is affected by both the albedo of the grains and the polarization fraction. The scattered unpolarized light is also seen in the pure intensity image.

Lacking a suitable reference PSF for subtracting the stellar halo, we cannot repeat the analysis performed in Canovas et al. (2013) and Avenhaus et al. (2014b). However, a rough estimate based on subtracting scaled versions of the ${Q}_{\phi }$ image from the intensity image indicates that the polarization fraction shows qualitatively the same behavior as noted in those works—the polarization fraction is higher on the eastern and lower on the western side.

3.2.2. Dust Within the Gap

In addition to the structure within the innermost ∼200 mas, we also detect a signal in ${Q}_{\phi }$ at >8σ throughout the gap, consistent with previous findings in Avenhaus et al. (2014b), where the S/N was much weaker (<3σ), however. There are two possible sources of this signal: it can either arise from light from the outer or inner disk that has been smeared into the gap region by the PSF, or it is scattered light from within the gap region.

In order to distinguish between these two possibilities, we artificially map out a gap between 02 and 04 in our corrected data, setting this region to zero, and fold it (Stokes Q and U individually) with the PSF. The result is displayed in Figure 5 alongside the other surface brightness plots. While some seeping in occurs, this could account for a signal on the order of ∼0.5% of the peak of the outer disk (in r²-scaling), but not for the observed signal of ∼2% of the outer disk peak. We thus conclude that most of the observed signal ($\sim 75 \%$) must come from dust scattering in this region, and only a minor fraction ($\sim 25 \%$) of it results from PSF smearing.

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This means that dust scattering within the gap region is detected at a significance level of $\sim 6\sigma$ after taking into account possible smearing of signal from in- or outside of the gap. The gap is not completely devoid of dust, as already hinted at in Avenhaus et al. (2014b). The dust scattering is weak (the signal is weaker than the signal from the outer/inner disk by a factor of ∼50–70 when corrected for the r² drop-off in illumination), meaning that the dust is either shadowed or optically very thin.

In an attempt to better understand the dust distribution within the gap, we create σ confidence maps by comparing the signal in the Qr image with the noise in the Ur image (cf. Benisty et al. 2015). These images are generated as follows: (1) smooth the image with a Gaussian kernel of 30 mas width (approximately the size of the beam) in order to exclude high spatial frequency noise, (2) calculate the variance in the (smoothed) Ur image over an area five times as wide as the smoothing kernel, and (3) divide the signal in Qr by the square root of the obtained variance. The results are shown in Figure 6. This technique is not perfect because the results depend on the used smoothing kernel for the variance calculation, but it allows us to give an estimate of the signal strength within the gap.

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We note that there is significant scattering close to the outer edge of the gap, potentially from gas streaming into the gap and carrying small dust grains with it. This can also be seen from the spiral-like features at the outer edge of the gap in the southwest, consistent with the fact that the inner disk would not be able to support the accretion rate of the star for extended periods of time and thus material has to accrete from the outer to the inner disk (Verhoeff et al. 2011). This feature could not be clearly seen in previous observations (Avenhaus et al. 2014b). The signal of the outer disk is very strong (up to 150σ). On the other hand, locating the dust within the gap, while it is clearly detected in the azimuthal averages, remains fairly inconclusive given the low S/N and the fact that butterfly patterns can be easily introduced into ${Q}_{\phi }$ and ${U}_{\phi }$ images if the correction for instrumental polarization is not perfect (these do not affect azimuthal averages, as the positive and negative butterfly wings cancel out). What can be seen, however, is that neither in the 2015 nor in the 2016 or in the combined data, any shadow can be seen in the direction of the outer disk shadows.

The 2015 and 2016 epochs have similar S/N across the disk and thus can be compared well. The appearance of the disk is very similar, as is to be expected, and combining the data increases the S/N of both the inner dust structure and the outer disk (see Figure 6). However, there are minor differences worth pointing out.

First, the inner dust structure, while appearing elongated in the SE-NW direction in both epochs, appears broader in the 2016 images in the N-S direction. This can be seen in both the SE and NW extension (Figure 6) and is independent of reduction parameters. Second, the structure of the gap in the σ maps is different. While there seems to be a bridge-like structure extending from the SE to the NW in the 2015 data set, the opposite is true for the 2016 epoch—the bridge extends from the NE to the SW. However, as pointed out in the previous section, butterfly patterns can easily be accidentally introduced into these images. The exact structure depends on the reduction parameters, specifically the inner and outer radius used for the instrumental polarization correction. While for any specific set of parameters used the dust appears differently in the two epochs, we are not convinced that these differences are significant.

Marino et al. (2015) linked the two prominent local minima in the north and south of the outer disk to a shadowing from a highly inclined inner disk. Within the context of our observations, this presents two challenges: (1) the inner dust structure we observe does not resemble a disk that could conceivably be responsible for that shadowing, and (2) no shadow is observed within the gap.

Verhoeff et al. (2011) find that to reproduce their data, they need to invoke not only an inner disk (which is not inclined in their model due to the knowledge at the time), but also a dust halo close to the star. Min et al. (2016) show that in order to fit the SED using a two-component (inclined inner and outer disk) model without a halo, they need to extend the scale height of the inner disk beyond the expected hydrostatic equilibrium in order to fit the near-IR flux. They do not discuss but neither do they exclude the possibility of a halo that would enhance the near-IR flux. We furthermore know from Lacour et al. (2016) that the secondary M-dwarf HD 142527B is likely to be in an orbit that is coplanar with the suggested highly inclined inner disk. This means that what we observe could in fact be either the inner disk or an extended not necessarily spherical halo. We note at this point that Verhoeff et al. (2011) pointed out that a halo could be replenished by a highly excited debris disk. The stellar companion of HD 142527 was not known at the time, but would excite any debris material in the inner disk. A dust halo producing gray extinction also helps to explain the unusually high infrared flux ratio of ${F}_{\mathrm{IR}}$/${F}_{\mathrm{star}}=0.92.$

In this interpretation, the inner disk would then still be within or very close to our IWA, and the east-west extended structure seen would not be part of the inner disk, but part of the (dynamically excited) halo. This does not explain the "missing" shadow in the gap, but we have to remember that the scattering signal we observe, while a clear detection when azimuthally averaged, is locally still of very low S/N.

While the dust scattering seems to be faint at first sight, our ad hoc correction for PSF smearing effects reveals that the polarimetric flux from the inner disk region is in fact greater than that of the outer disk. Inclination effects could play a role here, and we do not know the line-of-sight position of the inner dust, but we do know that the scattering angles in the outer disk are close to 90°, the ideal scattering angle for strong polarization because of the low inclination of the disk. Dust grain properties (e.g., a higher albedo) could also play a role, but to first order we have to assume that as much or more light is reprocessed in the inner dust structure than in the outer disk. Thus, the inner dust structure must be close to optically thick—at least very close to the star, where most of the polarized flux stems from.

Combining these findings with our results, we deem the following scenario conceivable: (1) a highly inclined inner disk close to the star, potentially at least partly responsible for the high polarimetric flux very close to the star (25–50 mas); (2) a dust halo that is responsible for the weaker polarimetric signal, mostly in the ∼50–200 mas regime and where the unexpected east-west extension is explained by the fact that it is strongly distorted by interactions with the secondary M-dwarf and material accreting from the outer disk; and (3) the well-known outer disk that reprocesses large portions of light beyond 100 au. The proposed size of the inner disk is consistent with recent submillimeter observations, in which an inclined disk with a radius of only 2–3 au is sufficient to explain the submillimeter measurements (Boehler et al. 2017).

This scenario does not explain the scattered light seen within the gap, however, nor does it explain the lack of shadows therein. We thus could imagine as component (4) a larger-scale dust halo with dust not only located in the plane of the outer disk, but also high above the mid-plane. This would dilute the shadows from the inner disk and thus be compatible with polarized flux from the entire gap region. The scenario would be similar to the scenario described by Verhoeff et al. (2011), but with a smaller inner disk and an additional large diluted halo of unexplained origin (it could, for example, stem from radiation pressure blow-out of small dust grains from the inner halo).

We caution, however, that our data within the gap are of low S/N and might be affected by systematic effects from the unstable PSF of SPHERE in the optical (relatively, compared to the near-IR). Thus, the faint regions in the disk and specifically the gap are difficult to interpret conclusively, and the explanation for the "missing" shadows in the gap might simply be caused by the too low S/N. Ignoring the "missing" shadows, another explanation for faint, scattered light within the disk gap would be secondary scattering, i.e., light scattered in the inner region of the disk toward the mid-plane and then rescattered by dust along the mid-plane within the gap region. However, Boehler et al. (2017) find a contrast ratio in the submillimeter between the outer disk peak and the gap region in excess of 1200, meaning that if there is any dust at the mid-plane in the gap region, it must be very thin. We show sketches of the two proposed possible explanations in Figure 7.

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While this explanation is compatible with our data, as far as we can see without producing a detailed radiative-transfer model, there could be other explanations for this complex circumstellar environment. The exact geometrical shape of the dust we detect remains unknown, also because of possible projection and complicated shadowing effects. It has to tie in, however, with the known kinematic signatures of the gas as studied by ALMA (Casassus et al. 2015; Perez et al. 2015). To fully understand this, a coupling of a hydrodynamical model that takes into account both the gas and dust dynamics as well as the orbit of HD 142527B with a radiative-transfer code (beyond current state-of-the-art capabilities), or at least either adequate hydrodynamical simulations that can handle both gas and dust or radiative-transfer modeling would be necessary.

Contrary to the results of Rodigas et al. (2014), we do not see an enhancement of dust close to the location of the M-dwarf companion detected by Biller et al. (2012) and Lacour et al. (2016). Thus, we cannot confirm their findings. We note that the contrast limits we reach are better (Rodigas et al. (2014) did not detect the faint dust scattering we report here) and that quick tests with inserting their detection into our images show that we should have detected such a bright point-like source. A disappearance of a circumbinary disk on such short timescales is unlikely. We therefore cannot confirm circumsecondary dust scattering, even though a circumsecondary disk is also needed to fit the SED of the secondary (Lacour et al. 2016). As a side note, we would like to mention that a detectable signal would stem from light from the primary scattered off the disk of the secondary. The secondary itself is too faint compared to the primary, and the butterfly pattern it would produce in Stokes Q and U would be so small that it would be washed out by convolution with the PSF.

It is worth mentioning that Boehler et al. (2017) detect an additional point source of submillimeter emission within the gap, toward the north of the star. A possible explanation for this is dust surrounding a third object within the system, but we do not detect any additional scattered light in this region.

In addition to being able to show unambiguously that scattered light exists within the gap of HD 142527, which could also stem from a larger halo as described above, we are also able to tentatively localize the polarized flux. The strongest signal is found close to the outer edge of the gap, which could be caused by gas that is dragged into the gap to cross the gap and eventually accrete onto the star (given the accretion rate of HD 142527), and dragging micron-sized dust along.

However, Lacour et al. (2016) found that the M-dwarf companion is unlikely to be responsible for truncating the outer disk (due to inclination), and thus also unlikely to be responsible for dragging dust into the gap via companion-disk interactions, despite the line-of-sight proximity of its possible apocenter and the dust signature entering the gap. Unseen planets within the gap would offer a potential explanation.

The difference in the appearance of the inner dust and the potential difference in the appearance of dust in the gap between the 2015 and 2016 epoch points to a possible astrophysical difference between the two epochs. A time baseline of 11 months, while short, is still significant compared to the orbital timescale at the location of the IWA (∼4 au, ∼5 yr). It is very short compared to the orbital timescale at 01 (∼36 yr), however.

It is possible that variations are caused by differential shadowing from dust inside our IWA, closer to the star than the dust structure we observe. Dust at that location would evolve on its own shorter orbital timescale. However, a variable shadowing would not only influence the gap region, but also the outer disk. Variable shadowing on the outer disk is not observed. Thus, future observations are required to confirm variations of the observed dust structure and understand their origins.