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The Red Rectangle: a thin disk with big grains

Javier Alcolea, Valentin Bujarrabal, Arancha Castro-Carrizo, Jacques Kluska, Carmen Sanchez Contreras, Hans van Winckel

TL;DR

This study uses new high-resolution ALMA Band 7 observations to dissect the dust and gas structure of the Red Rectangle around the post-AGB binary HD 44179. The authors resolve a compact ionised wind component and a thin, settled equatorial dust disk with a radius of about 175 au and a height of ≈18 au, containing large grains of ~150 μm and a dust mass ~5×10^{-5}–10^{-4} M_⊙, with most solid material concentrated near the midplane. They also map a much larger molecular disk in quasi-Keplerian rotation, extending several thousand AU, and detect high-excitation vibrationally excited lines that trace the inner, warm PDR-rich region. The results provide strong evidence for long-lived, grain-grown circumbinary disks around post-AGB binaries and highlight significant dust settling and a compact wind that likely originates from an accretion disk around the companion.

Abstract

The Red Rectangle is a nebula surrounding the post-AGB star HD 44179. It is the prototype of a particular class of nebulae associated with post-AGB binaries characterised by the presence of stable circumbinary disks in (quasi-)Keplerian rotation. Here we present the results of new high-resolution (20-50 mas) ALMA observations of continuum and line emissions at 0.9 mm. The continuum maps are analysed through a simple model of dust emission, which can reproduce the observational data. We find that most dust emission in the Red Rectangle is concentrated in the central regions of the rotating disk and that the settlement of dust grains onto the equatorial plane is very significant, particularly in comparison with the much larger scale height displayed by the gas distribution. The diameter of the dust-emitting region is about 250 au, with a total width of about 50 au. This region coincides with the warm PDR where certain molecules (like HCN), CI, and CII are presumably formed, as well as probably PAHs. From the spectral index, we confirm the presence in the disk of large grains, with a typical radius of about 0.150 mm, which supports the long-lived hypothesis for this structure. We also confirm the existence of a compact ionised wind at the centre of the nebula, probably emerging from the accretion disk around the companion, for which we derive an extent of about 10 au and a total flux of 8 mJy. We also briefly present the results on molecular lines of 12CO, 13CO, and other less abundant species.

The Red Rectangle: a thin disk with big grains

TL;DR

This study uses new high-resolution ALMA Band 7 observations to dissect the dust and gas structure of the Red Rectangle around the post-AGB binary HD 44179. The authors resolve a compact ionised wind component and a thin, settled equatorial dust disk with a radius of about 175 au and a height of ≈18 au, containing large grains of ~150 μm and a dust mass ~5×10^{-5}–10^{-4} M_⊙, with most solid material concentrated near the midplane. They also map a much larger molecular disk in quasi-Keplerian rotation, extending several thousand AU, and detect high-excitation vibrationally excited lines that trace the inner, warm PDR-rich region. The results provide strong evidence for long-lived, grain-grown circumbinary disks around post-AGB binaries and highlight significant dust settling and a compact wind that likely originates from an accretion disk around the companion.

Abstract

The Red Rectangle is a nebula surrounding the post-AGB star HD 44179. It is the prototype of a particular class of nebulae associated with post-AGB binaries characterised by the presence of stable circumbinary disks in (quasi-)Keplerian rotation. Here we present the results of new high-resolution (20-50 mas) ALMA observations of continuum and line emissions at 0.9 mm. The continuum maps are analysed through a simple model of dust emission, which can reproduce the observational data. We find that most dust emission in the Red Rectangle is concentrated in the central regions of the rotating disk and that the settlement of dust grains onto the equatorial plane is very significant, particularly in comparison with the much larger scale height displayed by the gas distribution. The diameter of the dust-emitting region is about 250 au, with a total width of about 50 au. This region coincides with the warm PDR where certain molecules (like HCN), CI, and CII are presumably formed, as well as probably PAHs. From the spectral index, we confirm the presence in the disk of large grains, with a typical radius of about 0.150 mm, which supports the long-lived hypothesis for this structure. We also confirm the existence of a compact ionised wind at the centre of the nebula, probably emerging from the accretion disk around the companion, for which we derive an extent of about 10 au and a total flux of 8 mJy. We also briefly present the results on molecular lines of 12CO, 13CO, and other less abundant species.
Paper Structure (5 sections, 4 figures, 1 table)

This paper contains 5 sections, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. The new ALMA observations
  3. Continuum emission results
  4. Continuum modelling
  5. Molecular line results

Figures (4)

  • Figure 1: To the left, the new ALMA 0.9 mm continuum map of the Red Rectangle. The size of the circular restoring beam is 20 mas (14 au at 710 pc). To the right, results from our best-fit model of the continuum emission of the Red Rectangle at 0.9 mm as observed by ALMA (see Sect. \ref{['modelling']}). The model includes the contribution of the dust grain emission (DDC) and of the free-free of the central ionised wind (CCC). Intensity units are mJy·beam$^{-1}$. Colour-coded scale is on the right side. Plotted contours are 1 to 12 by 1 mJy·beam$^{-1}$ (36 to 432 by 36 K in Rayleigh-Jeans equivalent brightness temperature). Colour-code scale and contours are the same in both real observed data and the results of the radiative transfer code.
  • Figure 2: To the left, total integrated continuum fluxes in the Red Rectangle for the CCC in cyan, and the more extended DDC in red: we are assuming that the DDC flux is the total flux minus that from the CCC. cm-wave data (triangles) are from jura1997, while sub-mm measurements (circles) are from valentin2013bvalentin2016, and this work valentin2023. The spectral indexes (and the corresponding formal errors), computed from regression lines, are also indicated. To the right, a sketch of the geometry, sizes, density distribution (grey scale), and viewing angle used for the model of the best fit of the data. The cyan dot at the centre represents the CCC, the compact ionised wind discussed in Sect. \ref{['continuum']}. See Sect. \ref{['modelling']} and valentin2023 for additional details.
  • Figure 3: Results from the new ALMA band 7 observations of the Red Rectangle for the $J$=3--2 line of $^{12}$CO. Here the restoring beam is $\sim$ 55 mas and the velocity resolution is 0.5 km·s$^{-1}$. On the left panel, maps of the $^{12}$CO emission for selected velocities (indicated in red figures at the top right corner). To the right, the velocity-position (PV) diagram along the equatorial direction (at PA=103$^\circ$, i.e., positive offsets are in the east by south-east direction). The red cross marks the assumed central position and systemic velocity of the source (--0.25 km·s$^{-1}$). The yellow line comprises the locations of the gas in the PV diagram according to the velocity law described in Sect. \ref{['sectlines']}. In both panels, the contours are drawn every 0.005 mJy·beam$^{-1}$ ($\sim$ 20 K); negative levels are drawn in dashed blue.
  • Figure 4: Results from the new ALMA band 7 observations of the Red Rectangle for species other than $^{12}$CO. Velocity-position (PV) diagrams along the equatorial direction (PA=103$^\circ$) for $^{13}$CO (top left), ground and vibrationally excited H$^{13}$CN (top middle and top right respectively), and for vibrationally excited H$_2$O and SiO (bottom middle and bottom right respectively). The restoring beam is $\sim$ 55 mas and the velocity resolution is 0.5 km·s$^{-1}$ for $^{13}$CO and 1.0 km·s$^{-1}$ in the rest of the cases. The red cross marks the assumed central position and systemic velocity of the source (--0.25 km·s$^{-1}$). The plot zooms into the central region of the disk where species other than CO are detected. On the top left panel, $^{13}$CO clearly shows the signature of the quasi-Keplerian rotation. Here positive/negative contours are drawn in black/red. The negative values at the centre of the $^{13}$CO panel (cyan and dark blue pixels) are due to the subtraction of the continuum emission and the presence of cold molecular gas in front of the dust disk. The rotation signature is barely hinted at in the H$^{13}$CN emission, which extends about $\pm$0.$"$35. For the vibrationally excited emissions, just the inner rim of the molecule-rich rotating disk, extending just $\pm$0.$"$15, is detected. Except for $^{13}$CO, contours are drawn every 0.0025 mJy·beam$^{-1}$ (equivalent to $\sim$ 10 K) and negative levels are drawn in dashed blue: the colour-code scale is shown below the SiO emission panel.