IPA. Accretion rate of a low-mass Class 0 protostar, measured via mid-infrared fluorescent OH emission

TL;DR

This work addresses how to infer disk–star accretion in the earliest protostars by exploiting UV-driven OH fluorescence resulting from water photodissociation in the water SAB (114–145 nm). JWST/MIRI observations of the low-mass Class 0 protostar IRAS 16253-2429 reveal a spectrum dominated by OH and CO2 lines with no H2O emission, enabling an indirect measurement of the protostar’s accretion-generated UV and, hence, the accretion rate. By modeling extinction toward the emitting gas and the CO2 population (as a proxy for H2O parent abundance), the authors derive an adopted accretion rate of $\dot{M}_a=(3.3\pm2.2)\times10^{-10}\,M_\odot\,\mathrm{yr}^{-1}$, with luminosity mainly from the photosphere rather than accretion, consistent with a quiescent phase between higher-rate episodes. This result supports episodic accretion scenarios and demonstrates a novel, UV-based method to probe protostellar accretion in deeply embedded (Class 0) systems, linking accretion activity to the observed outflow rate and early stellar evolution.

Abstract

The earliest stages of star formation are highlighted by complex interactions between accretion, outflow, and radiative processes, which shape the chemical and physical environment of the emerging protostar. James Webb Space Telescope observations of the low-mass, low-luminosity Class 0 protostar IRAS 16253-2429 reveal a central compact source. This object exhibits a rich mid-infrared emission spectrum of OH pure rotational lines and $\rm CO_2$ ro-vibrational lines. Unusually for a young stellar object, it has no mid-infrared line emission from $\rm H_2O$ to match the other molecules. We demonstrate that the emitting OH molecules arise from UV photodissociation of $\rm H_2O$ in its second absorption band at $λ= 114-145$ nm, and that the OH emission is a fluorescent cascade starting with highest-excitation rotational states. This situation offers the opportunity of using the infrared OH spectrum to measure the UV flux from the central protostar. Thereby we determine the disk-star accretion rate to be $3 \times 10^{-10} \ M_\sun \ {\rm year^{-1}}$, and demonstrate that the system luminosity arises mostly from the protostar's photosphere rather than from accretion luminosity. The result is in accord with the measured outflow rate of IRAS 16253-2429 and lies within the outflow/accretion-flow rate trend often inferred for protostars; and with episodic accretion as the dominant mechanism by which this protostar has grown.

Figures (8)

• Figure 1: Images of the central object in IRAS 16253-2429. From left: $\lambda~=~14~\micron$ continuum, arcsinh stretch; $\rm{CO_2}\ 0 1^1 0 - 0 0^0 0$ Q branch, $\lambda = 14.98~\micron$, linear stretch; and OH X1.5 $N = 16~A'$, $\lambda = 16.84~\micron$, linear stretch. Brightness limits are $-1\sigma$ (black) to peak intensity (white) in each case. The yellow circle is the 1 arcsec radius extraction aperture for the measurements listed in Table \ref{['list_of_fluxes']}, and is centered on ICRS coordinates $\alpha = 16:28:21.60$, $\delta = -24:36:23.4$Hsieh+2017.
• Figure 2: Representative sections of the JWST-MIRI spectrum of I16253, shown as gray points and errorbars. The latter include propagated noise and flux-calibration uncertainty. The continuum fit is overlaid in black, along with fits of the MIRI spectral point-spread function to emission lines of OH (pink), CO$_2$ (blue), and assorted H$_2$ and atomic or ionic lines associated with the protostar’s outflow: collimated jet or wider-angle wind, or cavity wall (green). Marked along the bottom of each frame are lines not detected: H$_2$O in red, HCN in cyan, and C$_2$H$_2$ in orange. They are plotted with notional flux densities, in the vertical scale of their frame and measured from the wavelength axis. These flux densities are calculated from the relative abundances and excitation temperatures found for these molecules in the Class II YSO GW Lup by Grant+2023, which we use only as an example of a spectrum containing the complete cast of strong mid-infrared lines from YSOs. The number of $\rm CO_2$ molecules we detect is an order of magnitude larger than detected in GW Lup, but the temperature we find for them $(T = 110~\rm K)$ is substantially smaller than GW Lup's $T$ = 400 K (Section \ref{['extinction']}).
• Figure 3: Energy level diagram of the lowest-energy v = 0 states of OH. Left: rotational states of the ground electronic configuration, with fine structure and $\Lambda$-doubling shown, and detected transitions indicated with pink lines. Inset: example MIRI spectrum of the $N$ = 16-15 transitions. Right: zoom in to the states involved in the $N$ = 16-15 transitions. For clarity, the $\Lambda$-doublet $A"$ and $A'$ states are each plotted with symmetrical displacement about their total angular momentum quantum number $J$. Comparison of the inset spectrum and the zoomed view illustrates the faintness of the $A"$ lines relative to the $A'$ transitions, a common feature of the OH lines in Figure \ref{['Spectra']}.
• Figure 4: Optical depth spectrum of the $\rm{CO_2}$ ice absorption feature. Overlaid on the JWST data (gray points and errorbars) is the best fit found for small ($\ll 15~\micron$) grains of CDE shape distribution, coated with cold $(T = 10~\rm K)$ ice analogues: $\rm{CO_2:CO}$ from Pontoppidan+2008 and Rocha+2025, $\rm{H_2O:CO_2}$ and pure $\rm{CO_2}$ from Ehrenfreund+1997. This ice mixture has column density ratio $\mathcal{N}(\rm{H_2O})/\mathcal{N}(\rm{CO_2}) = 1.9$.
• Figure 5: OH rotational-state population ratios: X1.5 $A^{\prime\prime}/A^{\prime}$ (blue squares), X0.5 $A^{\prime\prime}/A^{\prime}$ (blue diamonds), and $X0.5~A^\prime /X1.5~A^\prime$ (green circles), all as functions of the lower-state orbital angular momentum quantum number $N$. As these ratios are extremely insensitive to extinction, they are plotted without extinction correction. The open green circles at $N$ = 27-28 and 30-33 represent spectrally-unresolved combinations of $X0.5~A^\prime /X1.5~A^\prime$ lines, plotted with notional ratios of unity merely to show their propagated uncertainties. The upper limits at $N = 27-28$ and $30-33$ account for both $A^{\prime\prime}/A^{\prime}$ ratios and the unresolved $A^\prime -A^\prime$ pair. The dashed green line marks the mean value of the solid green points, $1.04\pm 0.21$.