A large-scale CO survey of the Rosette Molecular Cloud: assessing the effects of O stars on surrounding molecular gas

Abstract

We present a new large-scale survey of the J = 3-2 12 CO emission covering 4.8 deg 2 around the Rosette Nebula. The results reveal the complex dynamics of the molecular gas in this region. We identify about 2000 compact gas clumps having a mass distribution given by dN/dM ∼ M −1.8 , with no dependence of the power-law index on distance from the central O stars. A detailed study of a number of the clumps in the inner region shows that most exhibit velocity gradients in the range 1-3 km s −1 pc −1 , generally directed away from the exciting nebula. The magnitude of the velocity gradient decreases with distance from the central O stars, and we compare the apparent clump acceleration with a photoionized gas acceleration model. For most clumps outside the central nebula, the model predicts lifetimes of a few 10 5 yr. In one of the most extended of these clumps, however, a near-constant velocity gradient can be measured over 1.7 pc, which is difficult to explain with radiatively driven models of clump acceleration.As well as the individual accelerated clumps, an unresolved limb-brightened rim lies at the interface between the central nebular cavity and the Rosette Molecular Cloud. Extending over 4 pc along the edge of the nebula, this region is thought to be at an earlier phase of disruption than the accelerating compact globules.Blueshifted gas clumps around the nebula are in all cases associated with dark absorbing optical globules, indicating that this material lies in front of the nebula and has been accelerated towards us. Redshifted gas shows little evidence of associated line-of-sight dark clouds, indicating that the dominant bulk molecular gas motion throughout the region is expansion away from the O stars. In addition, we find evidence that many of the clumps lie in a molecular ring, having an expansion velocity of 30 km s −1 and radius 11 pc. The dynamical time-scale derived for this structure (∼10 6 yr) is similar to the age of the nebula as a whole (2 × 10 6 yr).The J = 3-2/1-0 12 CO line ratio in the clumps decreases with radial distance from the exciting O stars, from 1.6 at ∼8 pc distance to 0.8 at 20 pc. This can be explained by a gradient in the surface temperature of the clumps with distance, and we compare the results with a simple model of surface heating by the central luminous stars.We identify seven high-velocity molecular flows in the region, with a close correspondence between these flows and embedded young clusters or known young luminous stars. These

Rosette nebula globules: Seahorse giving birth to a star

“…The velocity map reveals however a clear velocity difference between RN A and the large shell structure west of it. Dent et al (2009) measure a LSR velocity of 2.5 km s −1 for RN A. The shell velocity has been measured as 1.3-1.9 km s −1 in Schneps et al (1980) using CO (1-0) observations which agrees with the APEX observations.…”

“…Earlier attention was drawn to this region by Wang et al (2010) who found an X-ray source at the center of the globule, and by Li (2005) who noted a highly reddened star next to the globule core. Some outflows are detected in the southern RN and the Rosette Molecular Cloud (e.g., Phelps & Ybarra 2005;Dent et al 2009) but so far only one is reported in the NW region of the RN, about 20 west of our observed field (Dent et al 2009). …”

“…The two velocity components seen in the APEX CO (3-2) spectra indicate that the body of RN A and the Seahorse tail are separate structures. The spatial resolution of the large scale CO (3-2) velocity map in Dent et al (2009) is 20 and can be used to study the velocity structure in the area. Because the velocity resolution of the Dent et al (2009) map is only 0.45 km s −1 small discrete jumps in the velocity like that observed between the RN A body and the Seahorse tail will blend together.…”

“…The spatial resolution of the large scale CO (3-2) velocity map in Dent et al (2009) is 20 and can be used to study the velocity structure in the area. Because the velocity resolution of the Dent et al (2009) map is only 0.45 km s −1 small discrete jumps in the velocity like that observed between the RN A body and the Seahorse tail will blend together. The velocity map reveals however a clear velocity difference between RN A and the large shell structure west of it.…”

Rosette globulettes and shells in the infrared

“…The northwestern (NW) part of the Rosette nebula is rich in filaments and trunks connected to the shell, which was noted early from optical images (Minkowski 1949). The shell moves away from the central OB stars at velocities of more than 20 km s −1 as concluded from surveys of associated radio molecular line emission (Schneps et al 1980;Gahm et al 2006;Dent et al 2009). Any star formed in this environment is ejected into interstellar space.…”