High-Resolution ALMA Observations Reveal Double Ejection Event and Sub-Keplerian Disk around S255IR NIRS3

Core Concepts

High-resolution ALMA observations of the S255IR star-forming region provide evidence for a double ejection event from the protostar NIRS3, suggesting episodic accretion, and confirm the presence of a sub-Keplerian disk, challenging existing models of high-mass star formation.

Abstract

Bibliographic Information:

Zinchenko, I. I., Liu, S.-Y., & Su, Y.-N. (2024). Fine structure and kinematics of the ionized and molecular gas in the jet and disk around S255IR NIRS3 from high resolution ALMA observations. Astronomy & Astrophysics.

Research Objective:

This study aimed to investigate the structure and kinematics of the ionized and molecular gas in the S255IR star-forming region, specifically around the protostar NIRS3, using high-resolution observations from the Atacama Large Millimeter/submillimeter Array (ALMA).

Methodology:

The researchers conducted high-resolution (∼15 mas) observations of the S255IR region using ALMA at a wavelength of 0.9 mm. They analyzed the continuum emission to study the morphology and brightness of the central source and jet features. Additionally, they examined several molecular line emissions, including C34S(7–6), SiO(8–7), CO(3–2), and CH3CN(19–18), to understand the gas kinematics and physical properties within the region.

Key Findings:

• The observations revealed a central bright source, likely a hypercompact H II region, elongated along the jet direction.
• Two pairs of bright knots were detected in the jet lobes, suggesting a double ejection event from NIRS3 with a time interval of approximately 1.5 years.
• The jet's orientation differed significantly (∼20°) from observations at larger scales, indicating strong jet precession.
• A rotating, sub-Keplerian disk-like structure, approximately 400 AU in diameter, was observed around NIRS3.
• Strong absorption features in molecular lines towards the central source suggested infalling material.

Main Conclusions:

• The double ejection event supports the theory of episodic accretion in high-mass star formation.
• The sub-Keplerian nature of the disk challenges existing models of high-mass star formation and requires further investigation.
• The presence of a hypercompact H II region and the detection of infalling material provide valuable insights into the early stages of massive star formation.

Significance:

This research provides crucial observational evidence for episodic accretion and the presence of a sub-Keplerian disk in the S255IR NIRS3 system. These findings contribute significantly to our understanding of the formation processes of high-mass stars and the complex dynamics within their surrounding environments.

Limitations and Future Research:

Further observations at multiple frequencies and higher resolutions are needed to confirm the nature of the central source and refine the models of the disk and jet. Investigating the chemical composition of the region and studying the evolution of the ejection events over time will provide a more comprehensive understanding of the ongoing star formation processes in S255IR.

Stats

The central source has a brightness temperature of ~850 K.
The projected distance between the NE knots in the jet is 138 AU, and between the SW knots, it is 93 AU.
The projected expansion speed is estimated as ~450 km/s for the NE lobe and ~285 km/s for the SW lobe.
The time interval between the two ejection events is estimated to be ~550 days or 1.5 years.
The estimated emission measure of the central source is ~(0.5-2) × 10^10 pc cm^-6, and the electron density is ~(0.5-1) × 10^7 cm^-3.
The diameter of the rotating disk around NIRS3 is about 400 AU.

Quotes

Key Insights Distilled From

Fine structure and kinematics of the ionized and molecular gas in the jet and disk around S255IR NIRS3 from high resolution ALMA observations

by I. I. Zinche... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2411.00116.pdf

Deeper Inquiries

How do the observed characteristics of S255IR NIRS3 compare to other high-mass star-forming regions, and what can this tell us about the diversity of massive star formation processes?

S255IR NIRS3 exhibits several characteristics observed in other high-mass star-forming regions, but also presents unique aspects that highlight the diversity of massive star formation processes:
Similarities:

Disk-mediated accretion: The presence of a rotating disk and evidence of episodic accretion bursts are consistent with observations of other high-mass protostars. This supports the idea that disk-mediated accretion plays a crucial role in the formation of massive stars, similar to low-mass star formation.
Outflows and jets: The collimated jet and associated outflows are common features in high-mass star-forming regions, indicating that these energetic events are crucial for removing angular momentum and regulating accretion onto the central protostar.
Sub-Keplerian rotation: While not universal, sub-Keplerian rotation profiles have been observed in other high-mass protostellar disks. This suggests that factors beyond simple Keplerian motion, such as magnetic fields or interactions with companion stars, might be influencing the disk dynamics.
Differences:

Double ejection event: The observation of a double ejection event with a 1.5-year interval is relatively rare. This suggests that episodic accretion in S255IR NIRS3 might be driven by mechanisms with shorter timescales compared to other high-mass protostars.
Strong jet precession: The significant precession of the jet axis (20°) is not observed in all high-mass star-forming regions. This suggests that the environment or the presence of a companion star in S255IR NIRS3 might be influencing the dynamics of the jet launching mechanism.
Diversity of massive star formation:
The similarities between S255IR NIRS3 and other high-mass star-forming regions suggest that some fundamental processes, like disk-mediated accretion and jet launching, are common. However, the unique characteristics, such as the double ejection event and strong jet precession, highlight the diversity in the specific mechanisms and timescales involved in massive star formation. This diversity could be influenced by factors like initial conditions, environment, binarity, and magnetic fields, leading to a wide range of observed properties in high-mass star-forming regions.

Could the observed sub-Keplerian rotation be explained by mechanisms other than episodic accretion, such as magnetic fields or interactions with companion stars?

Yes, the observed sub-Keplerian rotation in S255IR NIRS3 could be explained by mechanisms other than episodic accretion, with magnetic fields and interactions with companion stars being plausible candidates:
Magnetic fields:

Magnetic braking: Strong magnetic fields threading the disk can lead to magnetic braking, where angular momentum is transferred from the disk to the surrounding material. This can slow down the disk rotation, resulting in sub-Keplerian velocities.
Magnetic pressure: Magnetic pressure can also influence the disk structure and dynamics. If the magnetic pressure is significant compared to the thermal pressure, it can alter the radial distribution of mass in the disk, leading to deviations from Keplerian rotation.
Companion stars:

Gravitational torques: The presence of a companion star orbiting the central protostar can exert gravitational torques on the disk. These torques can redistribute angular momentum within the disk, leading to regions of both super-Keplerian and sub-Keplerian rotation.
Disk truncation: A close companion star can truncate the disk, limiting its radial extent. This truncation can affect the overall gravitational potential of the system and result in observed sub-Keplerian rotation in the outer regions of the disk.
Other factors:

Turbulence: Turbulent motions within the disk can also contribute to deviations from Keplerian rotation. However, the exact role of turbulence in high-mass star formation is still under debate.
Disk wind launching: The launching of a disk wind, a less collimated outflow compared to a jet, can also remove angular momentum from the disk and contribute to sub-Keplerian rotation.
It is important to note that these mechanisms are not mutually exclusive, and a combination of factors might be responsible for the observed sub-Keplerian rotation in S255IR NIRS3. Further observations and modeling efforts are needed to disentangle the contributions of these different mechanisms and fully understand the disk dynamics in this system.

What are the long-term implications of the double ejection event on the evolution of the protostar and the surrounding disk, and how might it influence the formation of planets or other stellar companions?

The double ejection event observed in S255IR NIRS3 has significant long-term implications for the protostar and its surrounding environment, potentially influencing planet formation and stellar multiplicity:
Protostar evolution:

Accretion variability: The double ejection event suggests that episodic accretion in S255IR NIRS3 might be characterized by short timescales and multiple bursts. This variable accretion can impact the protostar's luminosity and temperature evolution, leading to a more complex path compared to a steady accretion scenario.
Final mass: The efficiency of accretion during these bursts can influence the final mass of the star. Frequent and powerful bursts might lead to a higher final mass compared to a scenario with less frequent or weaker accretion events.
Disk evolution:

Disk dispersal: Outflows and jets, especially those associated with powerful ejection events, are known to contribute to disk dispersal. The double ejection event might have carved out cavities in the disk or even truncated its outer regions, affecting its lifetime and overall evolution.
Chemical composition: The high-energy radiation from the ejection events can alter the chemical composition of the disk by heating up the gas and triggering chemical reactions. This can influence the building blocks available for planet formation.
Planet formation:

Truncated disk: The potential truncation of the disk due to the ejection events can limit the amount of material available for planet formation, potentially hindering the formation of giant planets in the outer regions.
Planetesimal formation: The variable luminosity of the protostar due to episodic accretion can affect the conditions for planetesimal formation. While bursts might hinder planet formation in the inner disk due to high temperatures, they could promote planetesimal formation in the outer disk by heating the dust and enhancing coagulation.
Stellar companions:

Companion ejection: The strong gravitational perturbations during the ejection events could potentially eject a close companion star from the system, influencing the final stellar multiplicity.
Disk fragmentation: On the other hand, the ejection events might trigger disk fragmentation, leading to the formation of new companion stars within the system.
Overall, the double ejection event observed in S255IR NIRS3 highlights the dynamic and complex nature of high-mass star formation. While it provides valuable insights into the accretion processes and feedback mechanisms at play, it also raises further questions about the long-term implications for the protostar, the disk, and the potential for planet formation and stellar multiplicity in such a dynamic environment. Further observations and modeling efforts are crucial to fully understand the evolution of this system and its implications for our understanding of massive star formation.