Water Snowline in Young Stellar Objects with Various Density Structures Using Radiative Transfer Models

TL;DR

This study investigates how the water snowline in low-mass young stellar objects depends on luminosity across three density structures: envelope-only (E), envelope+disk+cavity (E+D), and protoplanetary disk (PPD). Using 2D dust continuum radiative transfer with RADMC-3D, the authors compute dust temperatures and define the snowline at $T_{\rm dust}=100$ K, finding a robust power-law relation $R_{snow}=a\times (L/L_{\odot})^{p}$ with $p\approx 0.5$ across models, while the scale factor $a$ decreases as the structure becomes more disk-dominated, causing the snowline to move inward. Viscous heating in the disk-dominated PPD model shifts the snowline outward by up to ~15 au, increasing $a$ and lowering $p$, whereas the effect is negligible in E+D at low disk mass. Comparisons to observations (e.g., V883 Ori and B335) show that disk structure and heating mechanisms are essential to reproducing observed snowlines, and the offsets between snowlines inferred from molecular tracers and current luminosity can serve as indicators of recent accretion bursts. Overall, the work provides a framework to interpret snowline locations in diverse YSO environments and to diagnose burst events from radiative-transfer-based snowline tracings.

Abstract

Tracing the water snowline in low-mass young stellar objects (YSOs) is important because dust grain growth is promoted and the chemical composition varies at the water snowline, which influences planet formation and its properties. In protostellar envelopes, the water snowline can be estimated as a function of luminosity using a relation derived from radiative transfer models, and these predictions are consistent with observations. However, accurately estimating the water snowline in protoplanetary disks requires new relations that account for the disk structure. We present the relations between luminosity and water snowline using the dust continuum radiative transfer models with various density structures. We adopt two-dimensional density structures for an envelope-only model (Model E), an envelope+disk+cavity model (Model E+D), and a protoplanetary disk model (Model PPD). The relations between the water snowline, where T_dust = 100 K, and the total luminosity, ranging 0.1-1,000 solar luminosity, are well fitted by a power-law relation, R_snow=a * (L/L_solar)^p au. The factor a decreases with increasing disk density, while the power index p has values around 0.5 in all models. As the disk becomes denser, the water snowline forms at smaller radii even at the same luminosity, since dense dust hinders photon propagation. We also explore the effect of viscous heating on the water snowline. In Model PPD with viscous heating, the water snowline shifts outward by a few au up to 15 au, increasing the factor a and decreasing the power index p. In Model E+D with lower disk mass, the effect of viscous heating is negligible, indicating that the disk mass controls the effect. The discrepancy between our models and direct observations provides insights into the recent outburst event and the presence of a disk structure in low-mass YSOs.

Figures (9)

• Figure 1: The dust mass density distributions of low-mass YSOs. The first column shows an envelope-only model (Model E). The second column shows an envelope + disk + cavity model (Model E+D), and the third column shows a zoomed-in view of Model E+D within 200 au. The fourth column shows a protoplanetary disk model (Model PPD).
• Figure 2: Total dust opacity $\kappa_{\nu}$ with the wavelength. The dust opacity profiles for different structures in Model E and E+D are adopted from Baek2020(left). The dust opacity profiles for two different grain size populations in Model PPD are adopted from DAlessio2006(right).
• Figure 3: The 2-dimensional viscous heat dissipation distribution of Model PPD, where $\dot{M} = 5.32 \times 10^{-5} M_{\odot} yr^{-1}$. The inset in the upper panel zooms in on the inner 80 au$\times$32 au.
• Figure 4: The 2-dimensional dust temperature distributions without viscous heating are shown for 10 $L_{\odot}$ (top) and 100 $L_{\odot}$ (bottom); Model E (left), Model E+D (middle), and Model PPD (right) are presented. The white solid line indicates the isothermal contour of $T_{\rm dust}$ = 100 K. The upper zoom-in panels of Model PPD show the water snowline inside the box of 20au$\times$8au.
• Figure 5: The relation between luminosity and the water snowline in three different density structures. Each symbol represents the location of the water snowline in the model for each luminosity, and the colored solid lines represent the fitting relations of $R_{\rm snow}=a\times (L/L_{\odot})^p$ au for each model. Bluish colors and circle symbols represent Model E. Colors from lightest to darkest blue represent models with envelope masses of 0.5, 1.0, 2.0, 5.0, and 6.5 $M_{\odot}$, respectively. Green and triangle symbols represent Model E+D. Reddish colors represent Model PPD. Red and square symbols represent models without viscous heating, while dark red and diamond symbols represent models with viscous heating. The black dashed line corresponds to the relation from Bisschop2007 and the gray shade area represents a 30% uncertainty range suggested by vant2022.