[Paper Review] L483: A Protostar In Transition From Class 0 to Class I

M. Tafalla, P. Myers+2 more|Americanae (AECID Library)|May 25, 2000

Astrophysics and Star Formation Studies3 citations

TL;DR

This paper proposes that IRAS 18148-0440 in the L483 core is a protostar in transition from Class 0 to Class I, based on its hybrid outflow characteristics: moderately collimated and heated like Class 0 outflows, but with low velocity, weak CH3OH outflow wings, and a NIR scattering nebula typical of Class I sources. Observations of CO, CH3OH, and H2CO reveal a centrally concentrated core with infall motions (infall rate ≈ 2×10⁻⁶ M⊙ yr⁻¹) and a 20× H2CO abundance enhancement in the outflow, indicating ongoing chemical and dynamical evolution during early protostellar evolution.

ABSTRACT

We present molecular-line observations toward the dense core in L483 and its bipolar outflow powered by the Class 0 object IRAS 18148-0440. CO (carbon monoxide) maps show that the outflow is well collimated and asymmetric, and that its gas is warmer than the surrounding cloud by at least a factor of 2. H2CO (formaldehyde) lines toward the outflow show prominent high-velocity wings and evidence for an H2CO abundance enhancement of a factor of 20. At ambient velocities, these lines show strong self-absorption and a brighter blue peak, a characteristic signature of inward motions. Finally, and in contrast with the outflows from other Class 0 objects, the CH3OH (methanol) lines in L483 do not show high velocity wings, and no evidence for abundance enhancement is found in this molecule or in SiO (silicon monoxide). Comparing the physical and chemical properties of the outflow in L483 with those of other outflows from Class 0 and Class I sources, we find that the L483 outflow is somewhat intermediate between these types. This suggests that the L483 central source has already started its transition between Class 0 and Class I, and that its mixed properties illustrate how this transition occurs.

Motivation & Objective

• To determine the evolutionary status of IRAS 18148-0440, a Class 0 source with ambiguous characteristics.
• To investigate the kinematics and chemistry of the L483 outflow using molecular line observations of CO, CH3OH, and H2CO.
• To assess whether the source exhibits features typical of Class 0 or Class I protostars, or a transitional state.
• To quantify infall rates and abundance enhancements in the core and outflow to understand early protostellar evolution.

Proposed method

• Conducted 12 CO(2–1) mapping to trace the bipolar outflow and assess its collimation, temperature, and velocity structure.
• Observed CH3OH(2k–1k) lines to search for high-velocity outflow wings and assess abundance enhancements in the outflow.
• Analyzed H2CO(212–111) line profiles to detect self-absorption and high-velocity wings, indicating infall and outflow motions.
• Used background emission subtraction from ambient spectra (e.g., at (−40′′, 40′′)) to isolate pure outflow emission and correct for extended foreground/background contamination.
• Modeled the CH3OH emission as optically thin with a radial density gradient between r⁻¹ and r⁻¹.⁵ over 15′′–100′′ radii.
• Estimated infall rate via a simple analysis of H2CO line profiles, focusing on the self-absorbed component with a brighter blue peak.

Experimental results

Research questions

• RQ1 Is IRAS 18148-0440 truly a Class 0 object, or does it show signs of evolving into a Class I source?
• RQ2 What are the kinematic and chemical characteristics of the L483 outflow, and how do they compare to typical Class 0 and Class I outflows?
• RQ3 Does the H2CO line profile indicate infall motions, and what is the inferred infall rate?
• RQ4 Are there chemical anomalies in the outflow, such as enhanced abundances of H2CO or CH3OH, and what do they imply about shock processing?
• RQ5 How can background and ambient emission be effectively subtracted to isolate true outflow emission in low-velocity, extended systems?

Key findings

• The 12 CO(2–1) outflow is well-collimated and asymmetric, with gas temperatures elevated by at least a factor of 2 compared to the ambient cloud.
• CH3OH(2k–1k) lines show no strong high-velocity wings, only a small (~0.3 km s⁻¹) velocity shift along the outflow axis, and no significant abundance enhancement in the flow.
• H2CO(212–111) lines exhibit strong high-velocity wings aligned with the outflow and a 20× abundance enhancement in the outflow relative to the ambient cloud.
• The H2CO line profile shows self-absorption with a brighter blue peak over the central 40′′, indicating inward motions, and a simple model yields an infall rate of 2×10⁻⁶ M⊙ yr⁻¹.
• The outflow's low velocity, lack of bright SiO or CH3OH wings, and association with a NIR scattering nebula suggest Class I-like properties, despite the source being classified as Class 0.
• The combination of Class 0 and Class I characteristics leads the authors to conclude that IRAS 18148-0440 is in transition from Class 0 to Class I, making it a key object for studying early protostellar evolution.