UV-irradiated outflows from low-mass protostars in Ophiuchus with JWST/MIRI

TL;DR

This paper uses JWST/MIRI/MRS to spatially resolve H$_2$ and ionic emission in outflows from five Class I protostars in Ophiuchus, aiming to understand how external UV irradiation shapes shock excitation. The data reveal two H$_2$ temperature components ($T_ ext{warm} \sim 500$–$600$ K and $T_ ext{hot} \sim 1000$–$3000$ K) and mixed shock signatures (C-type at high density and J-type at low density) that require UV irradiation in the models. UV fields of order $G_0 \sim 10$–$100$ (and up to $\sim 10^3$ in some cases) are needed to reproduce observed line ratios, suggesting a significant internal UV contribution from accretion shocks in addition to external irradiation. The study shows four outflows with diverse morphologies (collimated jets and wide cavities) and indicates that, despite environmental UV variability, excitation within the outflows is not strongly different across the region, underscoring the role of local shocks and UV processing in shaping protostellar feedback.

Abstract

The main accretion phase of protostars is characterized by the ejection of material in the form of jets/outflows. External UV irradiation can potentially have a significant impact on the excitation conditions within these outflows. High-resolution observations in the mid-infrared allow us to investigate the details of those energetic processes through the emission of shock-excited H$_2$ . Our aim is to spatially resolve H$_2$ and ionic/atomic emission within the outflows of low-mass protostars, and investigate its origin in connection to shocks influenced by external ultraviolet irradiation. We analyze spectral maps of 5 Class I protostars in the Ophiuchus molecular cloud from the James Webb Space Telescope (JWST) Medium Resolution Spectrometer (MIRI/MRS). Four out of five protostars show strong H$_2$, [\ion{Ne}{II}], and [\ion{Fe}{II}] emission associated with outflows/jets. Pure rotational H$_2$ transitions from S(1) to S(8) are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of $\sim$500-600 K and $\sim$1000-3000 K respectively. Both $C$-type shocks propagating at high pre-shock densities (n$_\text{H} \ge$10$^4$ cm$^{-3}$) and $J$-type shocks at low pre-shock densities (n$_\text{H} \le$10$^3$ cm$^{-3}$) reproduce the observed line ratios. However, only $C$-type shocks produce sufficiently high column densities of H$_2$, whereas predictions from a single $J$-type shock reproduce the observed rotational temperatures of the gas better. A combination of various types of shocks could play a role in protostellar outflows as long as UV irradiation is included in the models. The origin of this radiation is likely internal, since no significant differences in the excitation conditions of outflows are seen at various locations in the cloud.

Figures (32)

• Figure 1: Overview of the studied region. H$_2$ column density map from lad20 derived from Herschel data. Orange circles mark the location of the targeted sources and red stars mark the nearby massive stars HD147899 and S1. The red arrows point to the direction of the surrounding massive stars $\sigma$-Sco, $\alpha$-Sco and $\rho$-Oph located at a distance of $\sim4.2$-4.3, $\sim$4.3-5.2 and $\sim$2.3-3.1 pc from the protostars, respectively.
• Figure 2: Integrated emission of H$_2$ S(5) line at 6.91 $\mu$m toward GSS30-IRS 1, ISO-Oph 137, [GY92] 197, and WL 17. The positions of the protostars, measured from the line-free region at 6.9 $\mu$m, are shown with black stars. The red arrows shows the apparent outflow direction based on the H$_2$ emission. The black filled circle marks the spatial resolution of MIRI at $\lambda = 6.9$$\mu$m ($\sim0.2\arcsec$).
• Figure 3: Integrated emission of selected emission lines of ionized species: the [FeII] line at 25.9 $\mu$m toward GSS30-IRS 1, the [FeII] line at 5.34 $\mu$m toward ISO-Oph 137, and [GY92] 197, and -- since no extended [FeII] emission was detected -- the [NeII] line at 12.8 $\mu$m toward WL 17. The labels are the same as in Fig. \ref{['fig:h2_maps']}.
• Figure 4: Continuum-subtracted spectra of H$_2$ S(5) (left) and [NeII] 12.8 $\mu$m (right) toward WL 17. An offset is added to the spectra at various positions on the maps for better visualization. The dashed lines show the laboratory wavelengths of the respective transitions.
• Figure 5: H$_2$ rotational diagrams for the outflow positions in WL 17, [GY92] 197, ISO-Oph 137, and GSS30-IRS 1 with the largest number of line detections. The natural logarithm of the column density from a level $u$, $N_\mathrm{u}$, divided by the degeneracy of the level, $g_\mathrm{u}$, is written on the Y-axis. The extinction-corrected values are shown in red, and the ones before the correction in blue. The two-component fits cover the transitions below and above $E_\mathrm{u}$$\sim$4000 K for the ' warm' (dashed line) and ' hot' (dotted line) component (see text). The combined fit is shown as a black line. Finally, letters next to source name mark the aperture where the H$_2$ were measured.