Detection of Irregular, Sub-mm Opaque Structures in the Orion Molecular Clouds: Protostars within 10000 years of formation? (2001.04997v1)

Abstract: We report ALMA and VLA continuum observations that potentially identify the four youngest protostars in the Orion Molecular Clouds taken as part of the Orion VANDAM program. These are distinguished by bright, extended, irregular emission at 0.87 mm and 8 mm and are optically thick at 0.87 mm. These structures are distinct from the disk or point-like morphologies seen toward the other Orion protostars. The 0.87 mm emission implies temperatures of 41-170 K, requiring internal heating. The bright 8 mm emission implies masses of 0.5 to 1.2 M_sun assuming standard dust opacity models. One source has a Class 0 companion, while another exhibits substructure indicating a companion-candidate. Three compact outflows are detected, two of which may be driven by companions, with dynamical times of ~300 to ~400 years. The slowest outflow may be driven by a first hydrostatic core. These protostars appear to trace an early phase when the centers of collapsing fragments become optically thick to their own radiation and compression raises the gas temperature. This phase is thought to accompany the formation of hydrostatic cores. A key question is whether these structures are evolving on free fall times of ~100 years, or whether they are evolving on Kelvin-Helmholtz times of several thousand years. The number of these sources imply a lifetime of ~6000 years, in closer agreement with the Kelvin-Helmholtz time. In this case, rotational and/or magnetic support could be slowing the collapse.

• The paper reveals early protostars with irregular sub-mm and mm emissions, indicating hydrostatic core formation less than 10,000 years after collapse.
• It employs high-resolution ALMA and VLA observations to derive temperatures (41–170 K), mass estimates (0.5–1.2 solar masses), and compact outflow timescales (300–1400 years).
• The study challenges standard formation models by suggesting Kelvin-Helmholtz contraction (~6000 years) in protostellar evolution, implicating rotation and magnetic effects.

Analyzing the Formation of the Youngest Protostars in the Orion Molecular Clouds

This paper investigates the early stages of protostar formation in the Orion Molecular Clouds, utilizing high-resolution observations from the Atacama Large Millimeter Array (ALMA) and the Very Large Array (VLA). The focus is on identifying potential hydrostatic cores (HSCs) amidst the early stages of protostellar evolution, specifically within a timeframe speculated to be as short as 10,000 years post-formation.

Observational Insights

The observations reveal four notable protostars distinguished by irregular, extended emission at sub-millimeter (0.87 mm) and millimeter (8 mm) wavelengths. Characterized by optical thickness at 0.87 mm, these structures diverge from the classical disk- or point-like morphologies typically observed in mature protostars in the Orion region. The derived temperatures from the 0.87 mm emission, ranging from 41-170 K, imply significant internal heating. Mass estimates derived from 8 mm emissions suggest the presence of 0.5 to 1.2 solar masses within these early-stage stellar objects.

Dynamical and Structural Features

The paper details three detected compact outflows, two potentially driven by binary companions, signifying activities typically associated with the earliest phases of stellar formation. Dynamical timescales for these outflows range from approximately 300 to 1400 years, pinpointing their nascent nature. Notably, the slowest outflow is hypothesized to be driven by a first hydrostatic core (FHSC), emphasizing its key role during the initial evolutionary stage.

Implications for Stellar Formation Theory

The findings present a critical question regarding whether the evolution of these protostars aligns more closely with free-fall times (~100 years) or Kelvin-Helmholtz times (>1000 years). The analysis suggests a mean lifetime of ~6000 years for these structures, leaning toward timescales indicative of Kelvin-Helmholtz contraction, implying possible intervening factors such as rotational dynamics or magnetic fields that may retard collapse.

The existence of such nascent protostars underscores a physical state where the centers of collapsing gas fragments become optically thick, prohibiting radiative cooling and allowing internal temperature to rise due to compressional heating. This phase is theorized to mark the formation of hydrostatic cores, a fundamental component in star formation.

Future Prospects and Challenges

The paper propounds key implications on the models of early star formation, notably challenging existing simulations that underpredict the observed sizes and masses of such opaque structures. Future astrophysical inquiries might explore the roles of magnetic fields and rotational forces in influencing early protostellar structures and their evolution. Additionally, consideration of more complex opacity models may refine mass estimates and shed light on the grain growth processes in these environments.

In conclusion, the observations presented significantly enrich the understanding of protostellar evolution, bridging theoretical predictions with empirical data, while also setting the groundwork for further exploration of the early dynamics in star formation. Expanding the observational dataset and improving the resolution of existing simulations will prove crucial in unraveling the nuanced processes governing the inception of stars within molecular clouds.