[Paper Review] An unbiased search for the signatures of protostars in the rho Ophiuchi Molecular Cloud. II. Millimetre continuum observations

Thomas Stanke, M. D. Smith+2 more|ArXiv.org|Nov 2, 2005

Astrophysics and Star Formation Studies81 citations

TL;DR

This study presents an unbiased 1.2 mm continuum survey of the ρ Ophiuchi Molecular Cloud, detecting 136 dust sources including starless cores and protostars. It finds a core mass function similar to the stellar initial mass function but shifted by a factor of two in mass, with no significant variation across regions—supporting turbulence-driven core formation in a supersonic, hierarchical environment with isotropic core orientations and preferred NW-SE alignment in core pairs.

ABSTRACT

We have performed a 1 square degree 1.2mm dust continuum survey in the rho Oph molecular cloud. We detect a number of previously unknown sources, ranging from extended cores over compact, starless cores to envelopes surrounding young stellar objects of Class 0, Class I, and Class II type. We analyse the mass distribution, spatial distribution and the potential equilibrium of the cores. For the inner regions, the survey results are consistent with the findings of previous narrower surveys. The core mass function resembles the stellar initial mass function, with the core mass function shifted by a factor of two to higher masses (for the chosen opacity and temperature). In addition, we find no statistical variation in the core mass function between the crowded inner regions and those in more isolated fields except for the absence of the most massive cores in the extended cloud. The inner region contains compacter cores. This is interpreted as due to a medium of higher mean pressure although strong pressure variations are evident in each region. The cores display a hierarchical spatial distribution with no preferred separation scale length. However, the frequency distribution of nearest neighbours displays two peaks, one of which at 5000AU can be the result of core fragmentation. The orientations of the major axes of cores are consistent with an isotropic distribution. In contrast, the relative orientations of core pairs are preferentially in the NW-SE direction on all separation scales. These results are consistent with core production and evolution in a turbulent environment. We report a new low-mass Class 0 object and its CO outflow.

Motivation & Objective

• To conduct an unbiased, wide-field survey of dust continuum sources in the ρ Ophiuchi Molecular Cloud at 1.2 mm to avoid selection bias from prior narrow-field studies.
• To characterize the mass distribution, spatial distribution, and dynamical state of dense cores, including their potential equilibrium and virial parameters.
• To investigate whether core properties such as mass function and spatial clustering vary between crowded inner regions and more isolated outer fields.
• To identify new protostellar candidates and study their outflows, particularly focusing on low-mass Class 0 objects.
• To link millimetre continuum properties with infrared and molecular outflow data to constrain star formation models.

Proposed method

• Conducted a wide-field (4,600 arcmin²) 1.2 mm continuum survey using the European Southern Observatory, achieving a resolution of ~24 arcsec (~3,500 AU at 125 pc).
• Used dust emission at 1.2 mm to estimate core masses assuming a dust opacity index β = 2 and a dust temperature of 10 K, with mass derived from flux density and distance.
• Analyzed core spatial distribution using nearest-neighbor statistics and orientation analysis to detect clustering, preferred separation scales, and alignment patterns.
• Compared the core mass function (CMF) to the stellar initial mass function (IMF), testing for shifts and breaks, and evaluated its consistency with log-normal or power-law forms.
• Assessed core stability by comparing observed masses to critical Bonnor-Ebert or Jeans masses, using core size and density estimates.
• Identified a new low-mass Class 0 candidate and its associated CO outflow through spectral line analysis and spatial coincidence with millimetre continuum peaks.

Experimental results

Research questions

• RQ1 Does the core mass function in the ρ Ophiuchi Molecular Cloud resemble the stellar initial mass function, and if so, by what factor is it shifted?
• RQ2 Are there significant differences in the core mass function or spatial distribution between the dense inner region and more isolated outer fields?
• RQ3 What is the role of turbulence in shaping the spatial clustering, orientation, and mass distribution of cores?
• RQ4 Do core orientations and relative pair alignments indicate a preferred direction of formation or dynamical evolution?
• RQ5 Can the observed core properties be explained by a turbulent, gravitationally unstable environment rather than quiescent collapse?

Key findings

• The core mass function (CMF) is consistent with the stellar initial mass function but shifted by a factor of two to higher masses when assuming standard dust opacity and temperature.
• No significant variation in the CMF is observed between the crowded inner region and isolated outer fields, except for the absence of the most massive cores in the extended cloud.
• The inner region contains more compact cores, interpreted as a result of higher mean ambient pressure, despite strong local pressure variations.
• Core orientations are isotropically distributed, but relative orientations of core pairs show a preferred NW-SE alignment across all separation scales, indicating large-scale anisotropy.
• The frequency distribution of nearest-neighbor separations shows two peaks, one at ~5,000 AU, which may reflect core fragmentation processes.
• A new low-mass Class 0 object candidate was discovered, associated with a CO outflow, providing direct evidence of ongoing early-stage star formation.