Squeezed between shells? The origin of the Lupus I molecular cloud

Abstract

Context. The Lupus I cloud is found between the Upper Scorpius (USco) and the Upper Centaurus-Lupus (UCL) subgroups of the Scorpius-Centaurus OB association, where the expanding USco H I shell appears to interact with a bubble currently driven by the winds of the remaining B-stars of UCL. Aims. We want to study how collisions of large-scale interstellar gas flows form and influence new dense clouds in the ISM. Methods. We performed LABOCA continuum sub-mm observations of Lupus I that provide for the first time a direct view of the densest, coldest cloud clumps and cores at high angular resolution. We complemented these data with Herschel and Planck data from which we constructed column density and temperature maps. From the Herschel and LABOCA column density maps we calculated probability density functions (PDFs) to characterize the density structure of the cloud. Results. The northern part of Lupus I is found to have, on average, lower densities, higher temperatures, and no active star formation. The center-south part harbors dozens of pre-stellar cores where density and temperature reach their maximum and minimum, respectively. Our analysis of the column density PDFs from the Herschel data show double-peak profiles for all parts of the cloud, which we attribute to an external compression. In those parts with active star formation, the PDF shows a power-law tail at high densities. The PDFs we calculated from our LABOCA data trace the denser parts of the cloud showing one peak and a power-law tail. With LABOCA we find 15 cores with masses between 0.07 and 1.71 M and a total mass of ≈8 M . The total gas and dust mass of the cloud is ≈164 M and hence ∼5% of the mass is in cores. From the Herschel and Planck data we find a total mass of ≈174 M and ≈171 M , respectively. Conclusions. The position, orientation, and elongated shape of Lupus I, the double-peak PDFs and the population of pre-stellar and protostellar cores could be explained by the large-scale compression from the advancing USco H I shell and the UCL wind bubble.

The JCMT Gould Belt Survey: A First Look at SCUBA-2 Observations of the Lupus I Molecular Cloud

“…The ratio of Class 0/I to Class III sources in the SCUBA-2 map of Lupus I is 1:1, compared to 0.275 in the Dunham et al (2015) catalogue. The apparent recent trigger for star formation is consistent with the hypothesis of Gaczkowski et al (2015) that expanding shells from the Sco-Cen OB association have recently shocked Lupus I into a star-forming event. The number of YSOs in Lupus I (19) is low, so caution should be exercised with this result.…”

“…These higher temperatures, particularly for L1YSO 1, could be caused by the influence of the nearby Sco-Cen OB association, although Pattle et al (2015) do not find such high temperatures in the nearby region of Ophiuchus. Gaczkowski et al (2015) argue that Lupus I is affected by the interaction between an H I shell from upper Scorpius (de Geus 1992) and a wind bubble from upper Centaurus-Lupus. Such an interaction would have the potential to send shocks through the Lupus I molecular cloud.…”

“…Rumble et al 2015) can be determined. 870 μm observations of Lupus I by Gaczkowski et al (2015) have already enabled the detection of dense cores and the estimation of their masses. This work uses the SCUBA-2 continuum maps of Lupus I to study the population of young stellar objects (YSOs), largely identified from Spitzer Space Telescope observations (Merín et al 2008), as well as dense cores and their stability.…”

The role of magnetic fields in the structure and interaction of supershells

“…The same is true of galactic shell collisions. Gaczkowski et al (2015) argue that the star formation properties of the Lupus I cloud are consistent with it being squeezed by stellar feedback from the Upper Scorpius and Upper Centaurus-Lupus regions. These shells have expansion velocities of about 20-30 km/sec (Heiles 1979), which are supersonic with respect to the WNM of the Galaxy.…”

“…Both the size and color of the arrows indicate the strength of the projected field in µG. It is instructive to observe the relative orientation of the projected field with respect to the dense filamentary structures that appear in column density because this is a possible observable feature of such a system (for example, the results of Clark et al 2014 for HI filaments, Gaczkowski et al 2015 for the Lupus molecular cloud, or statistically, the dust polarization results from Planck Collaboration XXXV 2016 and Planck Collaboration XXXII 2016). When the magnetic field is perpendicular to the shell collision axis (run mhd1t), there is a noticeable change both in strength and in orientation of the projected field on the shock surfaces, even at early times.…”