Was a cloud-cloud collision the trigger of the recent star formation in Serpens?

Abstract

Context. The complexity of the interstellar medium (ISM) is such that it is unlikely that star formation is initiated in the same way in all molecular clouds. While some clouds seem to collapse on their own, others may be triggered by an external event such as a cloud/flow collision forming a gravitationally unstable enhanced density layer. Aims. This work tests cloud-cloud collisions as the triggering mechanism for star formation in the Serpens Main Cluster as has been suggested by previous work. Methods. A set of smoothed particle hydrodynamics (SPH) simulations of the collision between two cylindrical clouds are performed and compared to (sub)millimetre observations of the Serpens Main Cluster. Results. A configuration was found that reproduces many of the observed characteristics of Serpens, including some of the main features of the peculiar velocity field. The evolution of the velocity with position throughout the model is similar to the observed one and the column density and masses within the modelled cloud agree with those measured for the SE sub-cluster. Furthermore, our results also show that an asymmetric collision provides the ingredients to reproduce lower density filaments perpendicular to the main structure, similar to those observed. In this scenario, the formation of the NW sub-cluster of Serpens can be reproduced only if there is a pre-existing marginally gravitationally unstable region at the time the collision occurs. Conclusions. This work supports the interpretation that a collision between two clouds may have been the trigger of the most recent burst of star formation in Serpens. It not only explains the complicated velocity structure seen in the region, but also the temperature differences between the north (in "isolated" collapse) and the south (resulting from the shock between the clouds). In addition it provides an explanation for the sources in the south having a larger spread in age than those in the north.

ISM gas studies towards the TeV PWN HESS J1825−137 and northern region

“…In fact, we note that this cloud shows similarities with the studied molecular clouds next to the Serpens cluster (Duarte-Cabral et al 2011) and RCW120 (Torii et al 2015), whose velocity components are thought to be caused by cloud-cloud collisions. Cloud-cloud collisions, studied using hydrodynamical simulations (Habe & Ohta 1992, Duarte-Cabral et al 2011,Takahira, Tasker & Habe 2014,Torii et al 2015, recently renewed popularity to explain the presence of high mass star-formation inside molecular clouds. Notably, such collisions between molecular clouds are thought to generate OB stars, filamentary clouds, dense cores and complex velocity distribution.…”

A study on subarcsecond scales of the ammonia and continuum emission toward the G16.59−0.05 high-mass star-forming region

“…9c) seems to confirm that the FWHM increase is not due to two velocity components overlapping along the line of sight, but to a real enhancement of the column density. Although hazardous at this stage, one might even speculate that the formation of the mm-core has been triggered by the collision of the two clumps, as it has been observed in other regions (Duarte-Cabral et al 2011;Henshaw et al 2013).…”

How can young massive clusters reach their present-day sizes?

“…Perhaps the most well studied example is the Galactic embedded region W3 Main that is continuing to form stars for at least 3 Myr (Feigelson & Townsley 2008;Bik et al 2014). Another more recently cited example is the W33 complex (Messineo et al 2015).…”

“…However, how they form in the first place still poses a major challenge in astronomy. It is crucial to address this fundamental question because most, if not all, stars appear to form in embedded clusters (Elmegreen 1983;Lada & Lada 2003). A key question in this regard is how the exposed clusters' parsec-scale, smooth, centrally pronounced, near spherical shape, observed at all ages 1 Myr, can be explained.…”

“…From such observations, it can be generally concluded that the individual filamentary overdensities and their junctions in molecular clouds are highly compact; of typical (projected) widths of 0.1-0.3 pc (André et al 2011. Both theoretical and observational studies indicate that groups of (proto-)stars preferentially form within the filaments and at filament junctions Schneider et al 2010Schneider et al , 2012Tafalla & Hacar 2014;Duarte-Cabral et al 2011), giving rise to 0.5 pc dense protostellar clumps (or pre-cluster cloud core; e.g., Lada & Lada 2003;Tapia et al 2011;Traficante et al 2015). Semi-analytic studies, e.g., Marks & Kroupa (2012), also indicate similar gas clump sizes that depend weakly on their total mass.…”