Title: Line profiles of cores within clusters: I. The anatomy of a filament
Authors: Rowan J. Smith, Rahul Shetty, Amelia M. Stutz, Ralf S. Klessen
First Author’s Institution: Centre for Astronomy at the University of Heidelberg, Germany
Stars are a fundamental component of the universe, building blocks of larger structures like galaxies and galaxy clusters. The cycle of star formation, evolution, death and rebirth, recycling on cosmic scales, is interesting but difficult to observe at all stages. Within our lifetimes, we can’t observe one star as it progresses through a complete life-cycle, but we can observe a variety of stars at a variety of evolutionary stages in order to tell the whole story.
However, it remains a challenge to observe stars in the earliest stages as they are forming. Stars form in dense environments obscured by cold dusty material. One tool for identifying star-forming cores (this is the terminology used when a cloud of molecular gas has collapsed and begins to form a star) is the observed blue infall asymmetry. Let me explain.
When you (the observer) stare straight into a (spherical) core as it collapses, you will see some gas on the near side of the cloud moving away from you and some gas on the far side of the cloud moving towards you. Remember, you only see a 2-D picture, and using the doppler shifting of light, you will only measure the velocity component projected along the line of sight. Along any line of sight, there will be two elements of gas with similar line-of-sight velocity components. However, considering that the gas is dense enough and becomes opaque at the center (called optically thick), then along any line of sight you should see only the gas with the velocity component that is nearest to you (the other background gas becomes obscured by the foreground thick gas).
This means that you should preferentially see the blue-shifted (moving towards you) gas coming from the more-dense central regions, and with more intensity. The red-shifted (moving away you) gas that you see comes from less-dense outer regions, with less intensity. Also, the low-velocity surrounding envelope obscures the core, and this results in a dip in the profile between blue- and red-shifted gas. Stare at Figure 1 for a while, and this tricky concept should begin to make sense.
This ideal case, however, is not always seen in actual molecular clouds, likely because star-forming regions are composed of multiple substructures, and the asymmetric collapse can be complicated by the environment. Observational campaigns of star-forming regions nonetheless search for blue-asymmetric line profiles to identify cores and tally star-formation. This paper asks the questions “Could a greater fraction of cores be collapsing than what is inferred by blue asymmetric line profiles?”
To answer this question, Smith et al. use a giant molecular cloud (GMCs, see other astrobites here and here) simulation and specifically investigate properties of three typical regions containing a core embedded within a larger filament (it’s thought that most stars form within dense filaments).
The simulations include models of emission, scattering and absorption from dust and molecules commonly found in star-forming regions, including N2H+, CS, and HCN. Then, the authors measure the line profiles (intensity of emission at different wavelengths) as if they were making real observations of cores. The advantage of the simulation, rather than real observations, is that the authors can “look” from whichever angle they want, and they also know what they should expect to see. They find that the core line profiles vary depending on viewing angle, and even when looking at the same angle but from the opposite direction. The profiles also depend on which molecules are measured. Sometimes the expected blue asymmetry is seen, but not the majority of the time.
It has also been determined in other observational studies of cores that often collapsing cores do not reveal the theoretically “expected” signature profile. In surveys of cores, and when determining at which stage a core is forming, this effect could lead to an under-estimation of the actual number of collapsing cores. Objects are not always as they appear!