The Flying Saucer edge-on disc's Near Infrared silhouette revealed by the JWST JEDIce program

Abstract

Edge-on discs offer a unique opportunity to probe radial and vertical dust and gas distributions in the protoplanetary phase. This study aims to investigate the distribution of micron-sized dust particles in the Flying Saucer (BKLT J162813-243139) in Rho Ophiuchi, leveraging the unique observational conditions of a bright infrared background that enables the edge-on disc to be seen in both silhouette and scattered light at certain, specific wavelengths. As part of the JWST Edge-on Disc Ice program ('JEDIce'), we use NIRSpec IFU observations of the Flying Saucer, serendipitously observed against a PAH-emitting background, to constrain the dust distribution and grain sizes through radiative transfer modelling. Observation of the Flying Saucer in silhouette at 3.29 microns reveals that the midplane radial extent of small dust grains is ~235 au, larger than the large-grain disc extent previously determined to be 190 au from millimetre data. The scattered light observed in emission probes micron sized icy grains at large vertical distances above the midplane. The vertical extent of the disc silhouette is similar at visible, near-IR, and mid-IR wavelengths, corroborating the conclusion that dust settling is inefficient for grains as large as tens of microns, vertically and radially.

Figures (10)

• Figure 1: NIRSpec spectroscopic imaging data at 3.29$\pm$0.015 $\mu$m (i.e. stacking channels at the peak of the PAH band). The disc is observed both in emission (lobes, red-yellow) and in silhouette (midplane, black) against the ambient field emission arising from the CH stretching mode of UV-excited astro-PAHs (blue).
• Figure 2: Benchmark model of the Flying Saucer from near-IR to mm, compared to observations on a $\rm 5^{\prime\prime} \times5^{\prime\prime}$ field of view. The upper row shows the model images, convolved to observed spatial resolutions, and the bottom row displays the observations. Selected wavelengths across the NIRSpec range are shown along with archival images at 7.639$\mu$m (MIRI/Imager F770W filter pivot wavelength), 17.035$\mu$m (MIRI/MRS) and millimetre (ALMA) wavelengths. The contribution of the H$_2$ disc wind at 3.004 and 17.035$\mu$m was not included in the model. The components dominating the ambient emission field at each wavelength are labelled astro-PAH and H$_2$ on the corresponding images. Images were rotated by 3$^\circ$ to account for the position angle.
• Figure 3: Observations and models of disc emission/silhouette. Left and centre columns show the emission at 3.2$\mu$m ($\rm{I_{3.2}}$, no astro-PAH background) and 3.29$\mu$m ($\rm{I_{3.29}}$, peak astro-PAH background), respectively. The right column shows the extincted optical depth $\tau$=$\rm{-ln(T)}$=$\rm{-ln(I_{3.29}/(I_{3.2}+I_{3.29}^{ambient}))}$. The bottom row shows profile cuts along the midplane comparing observations (solid lines) and models (dashed lines). Blue lines (left) correspond to the 3.2 $\mu$m cut. Red lines (centre) are the 3.29 $\mu$m cut. Blue cuts include an offset corresponding to the astro-PAH background contribution, i.e. the expected cut if no silhouette extinction were present. Black lines (right) show T=$\rm I_{3.29}/(I_{3.2}+I_{3.29}^{ambient}$) along the midplane cut in both model and observations.
• Figure 4: Silhouette of the Flying Saucer disc seen against the ambient field at different wavelengths (green contours) plotted over the ALMA Band 7 millimetre dust disc image (colormap). From top to bottom, the disc silhouette is shown at 0.47 (HST F475W), 3.29 (NIRSpec), 7.7 (MIRI F770W) and 17.035 (MIRI/MRS) microns. Image and contours are rotated by 3$\rm^o$ and the image is 5" wide. In each panel, two sets of vertical tick marks denote the outer radii determined in the model for the small grain (235 a.u., green) and large grain (190 a.u., orange) populations, assuming a distance of 120 pc. The FWHM of the PSFs are shown as circles. At each wavelength, normalised vertical profiles are plotted to the right of the image. These are the projection across the full disc diameter for the ALMA image (dot-dashed, black) and cuts taken through the thickest point of the left lobe of the silhouette (red) and right lobe of the silhouette (blue). In both the radial and vertical directions, the disc silhouette clearly has a greater extent than the continuum image.
• Figure 5: Archival data showing the disc in broadband imaging from HST (visible, extinction), JWST MIRI/Imager (MIR, extinction), and ALMA (Band 7, emission), and a narrowband filter derived from spectroimaging with JWST MIRI/MRS (MIR, extinction).