Nascent chemical complexity in prestellar core IRAS 16293E: complex organics and deuterated methanol

TL;DR

This paper investigates nascent chemical complexity in the prestellar core IRAS 16293E by detecting and quantifying deuterated methanol and several complex organic molecules (COMs) with ARO 12 m and Yebes 40 m single-dish observations. The authors apply non-LTE radiative transfer (RADEX) for CH$_3$OH, its isotopologue, and HCOOCH$_3$, and LTE rotation-diagram analyses for CH$_2$DOH, CHD$_2$OH, CH$_3$CHO, CH$_3$OCH$_3$, and CH$_2$CHCN to derive column densities and excitation temperatures, accounting for beam dilution with an assumed source size. They find CH$_3$OH and isotopologues with $N eq 10^{14}$ cm$^{-2}$ and $T_ ext{ex} ax{around 7 K}$, along with substantial deuteration (CH$_2$DOH and CHD$_2$OH) and detections of CH$_3$CHO, HCOOCH$_3$, and CH$_3$OCH$_3$, while CH$_2$CHCN remains undetected. The results show COM/D-COM abundance ratios in IRAS 16293E are similar to IRAS 16293A/B and to a heterogeneous core/comet sample, supporting a scenario where much of the chemical inventory is inherited from the prestellar stage with limited reprocessing during collapse and protostellar heating. The study highlights the influence of nearby outflows on the observed gas-phase chemistry and suggests that future spatial mapping and solid-phase measurements (e.g., with JWST) will further illuminate formation pathways and environmental effects on complex astrochemistry.

Abstract

Prestellar cores represent early sites of low-mass ($M$ $\leq$ few M$_\odot$) star and planet formation and provide insight into initial chemical conditions of complex organic molecules (COMs). Deuterated COMs trace the degree of molecular inheritance and/or reprocessing, as high deuteration in protostellar systems suggests COMs forming during the prestellar stage when deuteration is enhanced. Within the L1689N molecular cloud, the prestellar core IRAS 16293E sits $90^{"}$ eastward of the chemically-rich IRAS 16293-2422 A and B protostellar system. A unique view of star formation inside a common natal cloud, IRAS 16293A, B, and E all show some of the highest levels of deuteration in the ISM, with a number of D/H ratios $10^{5}$ times higher than Solar. We investigate for the first time the deuteration levels of the simplest COM, methanol (CH$_3$OH), in IRAS 16293E. Using the Arizona Radio Observatory (ARO) 12 m telescope, we target favorable transitions of CH$_2$DOH, CHD$_2$OH, $^{13}$CH$_3$OH, and several higher complexity COMs (including acetaldehyde, CH$_3$CHO, methyl formate, HCOOCH$_3$, and dimethyl ether, CH$_3$OCH$_3$) in the 3 mm band. Follow-up observations with the Yebes 40 m telescope provided additional transitions in the 7 mm (Q-band). We report the first detections of these COMs and deuterated methanol in prestellar core IRAS 16293E and use our observations to calculate excitation temperatures, column densities, and relative abundance ratios. Striking similarities are found between relative molecular ratios and D/H values when comparing IRAS 16293E to the A and B protostars, as well as to a heterogeneous sample of other prestellar cores, protostars, and the comet 67P/Churyumov-Gerasimenko. Our results support the idea that there is a limited amount of chemical reprocessing of COMs when prestellar cores collapse and heat-up during the protostellar phase.

Figures (13)

• Figure 1: Map of the IRAS 16293-2422 region from publicly available APEX data presented in 2023AA...673A.143K, where the colormap is N$_2$H$^{+}$ (3-2) at 279.511 GHz and the cyan contours show the CH$_3$OH-A$^{+}$$1_1 - 0_0$ b-type transition at 350.905 GHz (beam size for N$_2$H$^{+}$ is 23.7$^{"}$ and shown in bottom left). Crosses designate where the IRAS 16293E prestellar core, labeled 'E', and the IRAS 16293A and B protostars labeled as 'A/B', sit. Other emission regions studied in 2023AA...673A.143K are labeled by white squares. For the single-pointing observations presented here, we also show the largest beam size (ARO 12 m beam at 73$^{"}$) and the smallest beam size (Yebes 40 m beam at 37$^{"}$).
• Figure 2: Observed CH$_3$OH spectrum toward IRAS 16293E shown in black and Gaussian fit as the red curve. A vertical red line at v$_{lsr}$ of 3.8 km/s is centered on the strongest line with lowest $E_{u}$. (top) A single-component fit, as reported in Table \ref{['tab:lines']} and (bottom) a three-velocity fit to each transition, separated out into the green, purple and blue curves with a total red composite curve, which is better able to reproduce the full profile; however we note that the centrally peaked profile (i.e., at the v$_{lsr}$ of 3.8 km/s) from our single component fit remains the most dominant.
• Figure 3: Observed ARO 12 m spectra toward IRAS 16293E shown in black and Gaussian fits as red curves. Spectrum with no Gaussian fits are considered non-detections and upper limits are derived. Vertical red lines are centered at v$_{lsr}$ of 3.8 km/s and the gray horizontal lines show the $1\sigma$ noise ($rms$) level. From top left across to bottom right: $^{13}$CH$_3$OH spectra offset by 50 mK, CH$_2$DOH spectra offset by 100 mK, CHD$_2$OH spectra offset by 50 mK, CH$_3$CHO spectra offset by 100 mK, CH$_2$CHCN spectra (non-detections) offset by 50 mK, HCOOCH$_3$ spectra offset by 25 mK, and a single CH$_3$OCH$_3$ spectrum (non-detection).
• Figure 4: Observed Yebes 40 m spectra toward IRAS 16293E shown in black and Gaussian fits as blue curves. Vertical blue lines are centered at v$_{lsr}$ of 3.8 km/s and the gray horizontal lines show the $1\sigma$ noise ($rms$) level. From top left down to bottom right: CH$_3$OH spectra offset by 1000 mK, CH$_2$DOH spectrum, CH$_3$CHO spectra offset by 100 mK, HCOOCH$_3$ spectra offset by 20 mK, and CH$_3$OCH$_3$ spectra offset by 30 mK.
• Figure 5: Rotation diagrams with associated linear best-fits (solid curves) and corresponding uncertainty (dashed curves) for (top) CH$_3$CHO A that utilizes five transitions and for (bottom) CH$_2$DOH that utilizes three transitions.