IRAS 16293E: Prestellar Core Properties

• Prestellar core IRAS 16293E is a dense, cold molecular region with a central H₂ density of ~3×10⁶ cm⁻³ and a flat inner density profile transitioning to a power-law decline.
• It exhibits advanced deuterium fractionation and a rich inventory of complex organic molecules, including a notably high N₂D⁺/N₂H⁺ ratio of 0.44 and deuterated methanol species.
• Observations reveal dynamic interactions with nearby outflows, evidenced by velocity gradients and chemical stratification that may trigger localized collapse.

IRAS 16293E is a cold, dense prestellar core located within the L1689N region of the Ophiuchus molecular cloud complex, approximately 90″ east of the low-mass Class 0 protostellar binary IRAS 16293-2422 A/B. It serves as a site for the investigation of initial physical conditions, chemical complexity, and evolutionary processes leading to the formation of stars and planetary systems. IRAS 16293E demonstrates pronounced deuterium fractionation, rich reservoirs of complex organic molecules, and evidence for dynamic interactions with the circumstellar environment, including molecular outflows.

1. Physical Structure and Density Profile

The central density of IRAS 16293E is determined to be on the order of several $10^6$ cm$^{-3}$, with a best-fit value near $3 \times 10^6$ cm$^{-3}$ based on Herschel far-infrared surface brightness maps and inverse Abel transformation techniques (Spezzano et al., 18 Dec 2024). The dust continuum maps display a centrally concentrated, nearly flat density profile within an inner region up to $\sim$3000 au, transitioning to a power-law decline ($n \propto r^{-2.45}$) at larger radii. The dust temperature in the central core ranges between 10.5–15 K, somewhat warmer than the coldest known cores, but supporting efficient deuteration and freeze-out processes.

The volumetric H$_2$ gas density determined from non-LTE radiative transfer modeling of molecular line emission (notably H$_2$CO and N$_2$H$^+$) is consistent with these Herschel-inferred profiles, requiring high central densities to reproduce the observed intensities, particularly of high-$J$ transitions (Kahle et al., 2022, Spezzano et al., 18 Dec 2024).

2. Chemical Inventory and Deuterium Fractionation

Observational programs using APEX, Yebes 40 m, and ARO 12 m telescopes have catalogued a wide array of molecular species in IRAS 16293E, including diatomics, triatomics, and numerous complex organic molecules (COMs). A notable fraction (∼50%) of the detected inventory comprises deuterated isotopologues, such as:

• D$_2$H$^+$, H$_2$D$^+$, N$_2$D$^+$, NH$_2$D, ND$_3$, DNC, HDCO, D$_2$CO, CH$_2$DOH, CHD$_2$OH
• Complex molecules: acetaldehyde (CH$_3$CHO), methyl formate (HCOOCH$_3$), dimethyl ether (CH$_3$OCH$_3$)
• Methanol and its isotopologues: CH$_3$OH, $^{13}$CH$_3$OH, CH$_2$DOH, CHD$_2$OH

The N$_2$D$^+$/N$_2$H$^+$ abundance ratio reaches 0.44 (Spezzano et al., 18 Dec 2024), one of the largest measured in any pre-stellar core, indicative of advanced chemical evolution and highly efficient deuterium enrichment at cold temperatures ($T_\mathrm{gas} \simeq 10$–12 K). The detection and quantification of both singly and doubly deuterated methanol (CH$_2$DOH/CH$_3$OH $\simeq$ 2.1%, CHD$_2$OH/CH$_3$OH $\simeq$ 8.5%) confirm extreme deuteration levels, similar to those found in nearby protostars and cometary material (Scibelli et al., 6 Aug 2025).

3. Molecular Tracers of Evolution and Core Structure

D$_2$H$^+$ and H$_2$D$^+$ serve as sensitive tracers of the inner, chemically evolved and depleted regions of the core. Mapping with APEX and JCMT reveals that para-D$_2$H$^+$ emission demarcates the true core center, where both dust and traditional molecular tracers fail due to local temperature stratification induced by the impact of outflows (Pagani et al., 16 Sep 2024). The para-D$_2$H$^+$/ortho-H$_2$D$^+$ abundance ratio ranges from a conservative lower limit of 3.9 (at 12 K) to 8.3 (at 8 K), exceeding predictions from state-of-the-art chemical models. This anti-correlation between the two species highlights localized chemical maturity and potentially necessitates revision of trihydrogen cation isotopologue collisional rate coefficients and associated ‘deuteration clock’ predictions.

High-excitation lines of N$_2$D$^+$ and N$_2$H$^+$, with critical densities $>10^6$ cm$^{-3}$, detect the densest central region without evidence for significant depletion, a feature that may be unique to IRAS 16293E or representative of rapid dynamical evolution under external perturbation (Spezzano et al., 18 Dec 2024).

Stratification is further demonstrated for nitrogen chemistry: ND emission arises from the dense core center, whereas NH absorption is detected against dust continuum in the outer layers. This spatial differentiation is observed in radial velocity offsets ($v_\mathrm{ND} \sim 3.53$ km\,s$^{-1}$, $v_\mathrm{NH} \sim 4.00$ km\,s$^{-1}$) and confirmed by non-LTE radiative transfer modeling (Bacmann et al., 2015).

4. Dynamical Processes and Environmental Interactions

IRAS 16293E resides within a filament structure subject to dynamical interaction with at least one molecular outflow from the neighboring IRAS 16293-2422 A/B protostellar system (Kahle et al., 2022, Lis et al., 2016). Multiple lines of evidence support this view:

• Maps of CO (3–2), SiO, and water in the region overlap into the prestellar core, revealing outflow-driven perturbations.
• Systematic velocity gradients are detected across the core in deuterated species (e.g., H$_2$D$^+$, N$_2$D$^+$, DNC, NH$_2$D) with $v_\mathrm{LSR}$ increasing NE–SW.
• Line width broadening and velocity shifts in NH$_2$D correspond to zones of ongoing outflow–core interaction.
• Spatial offsets ($\sim$ 5″) between compact continuum peaks and peaks of molecular emission (N$_2$D$^+$, ND$_3$) indicate internal structural differentiation likely driven by shock compression and dynamic evolution.

The core is composed of a static inner zone ($<3000$ au) and an infalling envelope with velocities up to –0.5 km\,s$^{-1}$ (Spezzano et al., 18 Dec 2024), with the dust continuum source size measured to be $\sim$ 1100 au and central density $(1-2)\times10^7$ cm$^{-3}$ in compact substructures (Lis et al., 2016). The northern portion is more extended and quiescent, whereas the southern region is denser and possibly approaching collapse, a scenario possibly seeded by outflow impact.

5. Isotopic Composition and Chemical Inheritance

Nitrogen isotopic analysis yields N$_2$H$^+$/N$^{15}$NH$^+$ ratios of $\sim$ 330 (Daniel et al., 2016)—comparable to the elemental value of the local ISM (270–290)—with minimal evidence for $^{15}$N enrichment or fractionation within this core, in contrast to other low-mass regions (e.g., L1544, ratios up to $\sim$1000).

Relative abundance ratios and D/H values for COMs (e.g., CH$_2$DOH/CH$_3$OH, CHD$_2$OH/CH$_3$OH) are notably similar between IRAS 16293E, IRAS 16293A/B, other prestellar cores, protostellar systems, and comet 67P/Churyumov–Gerasimenko (Scibelli et al., 6 Aug 2025). This suggests limited reprocessing of complex organics during collapse and warm-up, implying a strong inheritance of chemical complexity from the prestellar phase into the formation of planetary systems.

6. Methodologies and Observational Strategies

Recent advances in multi-frequency, multi-band mapping with high-sensitivity single-dish telescopes (APEX, Yebes, ARO, JCMT) and far-infrared Herschel imaging have enabled the detailed paper of IRAS 16293E's structure and chemical evolution. Radiative transfer modeling, both LTE and non-LTE (LOC, RADEX, CASSIS, CLASS), is applied to derive temperatures, column densities, density profiles, and abundance ratios. Step function models in velocity and abundance distribution are convolved with observational beams for direct comparison.

Data reduction and spectral analysis utilize extensive frequency coverage (277–375 GHz), high spectral and spatial resolution (channel widths $\lesssim$0.08 km\,s$^{-1}$, beam FWHM 37″–62″), and robust calibration standards, allowing identification of over 144 transitions from 36 species and their isotopologues (Kahle et al., 2022, Scibelli et al., 6 Aug 2025, Spezzano et al., 18 Dec 2024).

7. Implications for Star and Planet Formation

IRAS 16293E typifies a prestellar core immediately preceding the protostellar collapse, providing a benchmark for densities, temperature gradients, velocity structure, chemical complexity, and interaction-induced feedback. The high D/H ratios in both simple and complex molecules offer a stringent test of chemical models and stress the necessity of accurate reaction networks and collisional rates. The extreme level of deuteration, density profile, and core stratification observed in IRAS 16293E—likely influenced by external outflow perturbations—provide essential initial conditions for the formation of stars and planet-forming disks.

Significant discrepancies between observed abundance ratios (e.g., para-D$_2$H$^+$/ortho-H$_2$D$^+$ $\gtrsim$4) and chemical model predictions highlight the need for improved theoretical understanding and potentially recalibration of astrochemical chronometers. Future work is recommended to employ full 3D radiative transfer and chemical/dynamical modeling to resolve remaining ambiguities in core evolution, outflow interaction, and inheritance of complex organics (Pagani et al., 16 Sep 2024, Spezzano et al., 18 Dec 2024).

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Table: Key Physical and Chemical Properties of IRAS 16293E

Quantity	Value/Range	Reference
Central H$_2$ density	$3 \times 10^6$ cm$^{-3}$	(Spezzano et al., 18 Dec 2024)
Gas kinetic temperature	10–12 K	(Kahle et al., 2022)
N$_2$D$^+$/N$_2$H$^+$ ratio	0.44	(Spezzano et al., 18 Dec 2024)
CH$_2$DOH/CH$_3$OH ratio	$\sim$2.1%	(Scibelli et al., 6 Aug 2025)
CHD$_2$OH/CH$_3$OH ratio	$\sim$8.5%	(Scibelli et al., 6 Aug 2025)
para-D$_2$H$^+$ / ortho-H$_2$D$^+$ ratio	3.9–8.3+	(Pagani et al., 16 Sep 2024)

This collection of physical and chemical diagnostics underscores IRAS 16293E’s utility as a reference object for elucidating the earliest stages of star and planet formation, the origin and inheritance of deuterated and complex organic species, and the dynamic impact of feedback mechanisms within molecular clouds.