Direct Measurement of Extinction in a Planet-Hosting Gap

TL;DR

By using a background star shining through the AS 209 disk gap, the authors perform transmission spectrophotometry to directly measure extinction within a planet-opening gap. They combine VLT/SPHERE, JWST/NIRCam coronography and imaging, and archival HST data to model the background source’s SED over 19 years, testing multiple extinction models. The results favor a two-component extinction model (ISM-like plus grey) with $A_{4\,\mu\mathrm{m}}\approx 2.7$ mag and $A_{H\alpha}\approx 4.2$ mag, implying substantial attenuation even at near- to mid-IR wavelengths and offering a resolution to tensions between ALMA-based planet-disk interactions and imaging non-detections. Including the 2005 epoch strengthens the ISM+Grey interpretation and demonstrates the potential of using background sources to map dust opacities across disk gaps, with broad implications for interpreting protoplanet searches and constraining disk dust properties.

Abstract

Recent disk observations have revealed multiple indirect signatures of forming gas giant planets, but high-contrast imaging has rarely confirmed the presence of the suspected perturbers. Here, we exploit a unique opportunity provided by the background star AS209bkg, which shines through a wide annular gap in the AS209 disk, to perform transmission spectrophotometry and directly measure the extinction from gap material for the first time. By combining new VLT/SPHERE and JWST/NIRCam observations with archival HST data from 2005, we model the spectral energy distribution (SED) of AS209bkg over a 19-year baseline. We find that the SED and its variability are best explained by increasing extinction along the line of sight as AS209bkg approaches the gap edge in projection. The extinction is best described by a combination of ISM-like extinction component and a grey extinction component. This points to the presence of grains in the disk outer gap that are larger than in the ISM. We find that the extinction in the gap at $λ\sim4.0~μ$m is $A_{4\,μ\mathrm{m}} = 2.7^{+0.7}_{-0.7}$ mag, while at H$α$ ($λ=0.656~μ$m), where most searches for accretion signatures take place, the extinction could be as high as $A_\mathrm{Hα} = 4.2^{+0.9}_{-1.2}$ mag ($A_V=4.6^{+1.0}_{-1.3}$ mag). This suggests that even wide, deep gaps can significantly obscure emission from protoplanets, even those following a hot-start evolutionary model. Our extinction measurements help reconcile the discrepancy between ALMA-based predictions of planet-disk interactions and the non-detections from sensitive optical and near-infrared imaging campaigns.

Figures (10)

• Figure 1: $Y$ and $J$ band residuals of AS 209 obtained with VLT/SPHERE. AS 209bkg (CC1) is marked by a white arrow.
• Figure 2: JWST/NIRCam coronography residuals for F182M, F210M, F300M and F335M. AS 209bkg (CC1) is marked by a white arrow, while for the non-detection in F300M the position of the source is marked with a white circle.
• Figure 3: JWST/NIRCam imaging residuals for F200W, F410M (top row, Cycle 1) and F250M and F323N and F405N (bottom row, Cycle 3). AS 209bkg (CC1) is marked by a white arrow. North is pointing up in the image, East to the left. The central mask has a radius of $1\farcs1$.
• Figure 4: HST/NICMOS F110W residuals obtained from the ALICE survey Hagan2018. The white arrow points to AS 209bkg (CC1).
• Figure 5: Astrometry of the AS 209 system. Left: Proper motion analysis of AS 209bkg. The red square represent the location of the source in 2005. The black line shows the expected trajectory from 2005 through 2024 assuming it is a stationary object, with the expected locations at the SPHERE and NIRCam epochs highlighted with black circles. The NIRCam and SPHERE data agree with the prediction (when considering the HST uncertainties), demonstrating that the source is indeed a background object. Right: Projected location of AS 209bkg in the AS 209 disk. On the right side, the millimeter image is reported, showcasing a series of dust rings Andrews2018. On the left side, the CO gas as measured by the DSHARP program Andrews2018Guzman2018 is reported. AS 209bkg shines through the deep outer CO gap. Grey crosses mark the expected position of AS 209bkg in 2050 and 2075.