Early Planet Formation in Embedded Disks (eDisk). XVIII. Indication of a possible spiral structure in the dust-continuum emission of the protostellar disk around IRAS 16544-1604 in CB 68

Abstract

We performed numerical simulations along with radiative transfer calculations to reproduce an intriguing asymmetric shoulder feature in the dust-continuum emission of the protostellar disk around one of the eDisk targets, the Class 0 protostar IRAS 16544-1604 in CB 68. This is our first attempt to bridge the theoretical works of protostellar disk evolution and the eDisk observations. We found that while our hydrodynamic simulations form spiral structures caused by gravitational instability, they become less discernible after the disk is inclined and convolved with the telescope beam. The widths of the spiral structure as obtained by our numerical simulations are ~0.1-0.8 times the eDisk beam size of 4.5 au. Our modeling effor implies that the apparent absence of spiral features in the eDisk observations does not necessarily indicate the real absence of internal substructures and gravitational instability. We also found that the asymmetric shoulder structure of the continuum profile along the major axis appears when the disk is massive enough with a Toomre parameter Q~1. This mechanism offers a potential explanation for the observed, asymmetric shoulder features in the disks surrounding IRAS 16544-1604 and the other eDisk sources.

Figures (16)

• Figure 1: 1.3 mm dust continuum image in IRAS 16544 obtained by eDisk 2023ApJ...953..190K. Contour levels are 5$\sigma$, 20$\sigma$, 40$\sigma$, 60$\sigma$, 80$\sigma$, 100$\sigma$, 120$\sigma$, and 180$\sigma$ (1$\sigma$ = 21 $\mu$Jy beam$^{-1}$). The beam size is 0$.\!\!{}^{\prime\prime}$036$\times$0$.\!\!{}^{\prime\prime}$027 (P.A = 69 degree), as shown in a filled ellipse at the bottom-left corner. Alt text: Color and contour image.
• Figure 2: Observed brightness temperature profile of the 1.3 mm dust continuum emission along the disk major axis in IRAS 16544. The beam size is shown in the bottom left. A shoulder structure is observed in the left-hand side of this profile. Alt text: Line graph. x axis shows the positional offset from the protostellar position from 0.3 to -0.3 arcsecond. The y axis shows the brightness temperature in Kelvin.
• Figure 3: Radial distribution of initial $Q$ values for Models 1, 2 and 3. Alt text: Line graph. x axis shows the radius from 1 to 60 astronomical unit in log scale. The y axis shows the Toomre $Q$ value from 0.5 to 3 in linear scale.
• Figure 4: Results of the numerical simulation of the self-gravitating disk of Model 1 at $t \sim$$2.4\times 10^{4}$ yr after the start of the simulation. The left and right panels show the surface density and the temperature images, respectively. The white regions at the center of these panels correspond to the inner boundary of the simulation. The spiral structures are formed in both surface density and temperature distributions due to the gravitational instability and the compressional heating. Alt text: Two color images.
• Figure 5: 1.3-mm dust continuum images (left) and intensity profiles along the major axis of the model disk (right) obtained with the radiative transfer calculations. Contour levels in left panels are 1$\sigma$, 3$\sigma$, and 5$\sigma$ of the eDisk observations (1$\sigma$= 0.5 K). The corresponding density and temperature distributions are shown in Figure \ref{['fig:sim_results']}. Dashed lines denote the cuts of the intensity profile of the relevant right panels. The results of the face-on view without beam convolution, the face-on view with convolution, the inclined view without convolution, and the inclined view with convolution, are presented in each row from top to bottom. In the panels of the face-on view without convolution (the first row), the brightness temperature is decreased in the central region because of the inner boundary of the simulation. Alt text: Four color and contour images in the left column, and four line graphs in the right column. In the right column, x axis shows the positional offset from the protostellar position from -50 to 50 astronomical unit. The y axis shows the brightness temperature from 0 Kelvin to 250 Kelvin.