Resolving the inner regions of the HD 97048 circumstellar disk with VLT/NACO polarimetric differential imaging

Abstract

Context. Circumstellar disks are the cradles of planetary systems and their physical and chemical properties directly influence the planet formation process. Because most planets supposedly form in the inner disk regions, i.e., within a few tens of AU, it is crucial to study circumstellar disks on these scales to constrain the conditions for planet formation. Aims. Our aims are to characterize the inner regions of the circumstellar disk around the young Herbig Ae/Be star HD 97048 in polarized light. Methods. We used VLT/NACO to observe HD 97048 in polarimetric differential imaging (PDI) mode in the H and K s band. This technique offers high-contrast capabilities at very small inner working angles and probes the dust grains on the surface layer of the disk that act as the scattering surface. Results. We spatially resolve the disk around HD 97048 in polarized flux in both filters on scales between ∼0.1 -1.0 corresponding to the inner ∼16-160 AU. Fitting isophots to the flux calibrated H-band image between 13-14 mag/arcsec 2 and 14-15 mag/arcsec 2 , we derive an apparent disk inclination angle of 34 • ± 5 • and 47 • ± 2 • , respectively. The disk position angle in both brightness regimes is almost identical and roughly 80 • . Along the disk major axis the surface brightness of the polarized flux drops from ∼11 mag/arcsec 2 at ∼0.1 (∼16 AU) to ∼15.3 mag/arcsec 2 at ∼1.0 (∼160 AU). The brightness profiles along the major axis are fitted with power-laws falling off as ∝r −1.78±0.02 in H and ∝r −2.34±0.04 in K s . Because the surface brightness decreases more rapidly in K s compared to H, the disks becomes relatively bluer at larger separations, possibly indicating changing dust grain properties as a function of radius. Conclusions. We imaged for the first time the inner ∼0.1 -1.0 (∼16-160 AU) of the surface layer of the HD 97048 circumstellar disk in scattered light, which demonstrates the power of ground-based imaging polarimetry. Our data fill an important gap in a large collection of existing data that include resolved thermal dust and polycyclic aromatic hydrocarbon (PAH) emission and also resolved gas emission lines. HD 97048 therefore is an ideal test case for sophisticated models of circumstellar disks and a prime target for future high-contrast imaging observations.

Polycyclic aromatic hydrocarbon ionization as a tracer of gas flows through protoplanetary disk gaps

“…The gap of HD 97048 may not be completely empty because no gap is seen in polarized scattered light (Quanz et al 2012). Therefore, we fill the gap of HD 97048 with a gas surface density which of Σ gas = 2.5 × 10 −4 g cm −2 , which is similar to the solutions we find for the other transitional disks in our sample (in the next Sect.…”

“…Thus, the PAHs in the disk gap only contribute 5% to the total PAH luminosity. A small fraction of dust may also be present in the gap, which may explain the scattered light observations of Quanz et al (2012). However, we do not further investigate that scenario since that is not the focus of our paper.…”

Cavity and other radial substructures in the disk around HD 97048

“…The presence of a disk cavity with a radius of 34±4 au is predicted by Maaskant et al (2013) based on simultaneous modeling of the SED and spatially resolved Q band (20 µm) spectrum. In contrast, polarimetric differential imaging did not detect any cavity with a size larger than 16 au in radius (Quanz et al 2012), while spatially resolved 12 CO ro-vibrational emission in the fundamental band at 4.6 µm has been detected as close in as 11 au from the star (van der Plas et al 2009Plas et al , 2015. Such a trend of cavity size that increases for larger grain sizes is in quantitative agreement with filtering of the dust by a pressure maximum induced by e.g.…”

“…In the (sub) mm bands, this disk has only been detected using single dish observations and has never resolved (Henning et al 1998;Phillips 2011;Hales et al 2014). The disk has, however, been resolved by polarimetric differential imaging of polarized scattered light (Quanz et al 2012), showing a bright disk surface between ≈ 0.1 -1.0 (≈16-160 AU), but no evidence for a disk cavity. Maaskant et al (2013) also resolve the disk in the Q band (20 µm) spectrum and find that a large gap should be present in the disk between 2 and 34 +4 −4 au.…”

Shadows cast on the transition disk of HD 135344B

“…Figure 6 shows the difference of the disk polarized surface brightness and the stellar magnitude in the same filter (see Table 1). In this way, each surface brightness profile is normalized to the stellar flux and depends mainly on the scattering properties of the dust grains (e.g., Quanz et al 2012). We note that the IRDIS photometric calibration has a larger uncertainty than the ZIMPOL photometric calibration because we derive a zero-point for the J-band observation from the 2MASS stellar magnitude and the detector integrated total intensity with only applying a correction for the filter response (see Appendix A for more details).…”