Sequential and spontaneous star formation around the mid-infrared halo H ii region KR 140

Abstract

We use Two-Micron All-Sky Survey and Midcourse Space Experiment infrared observations, along with new molecular line (CO) observations, to examine the distribution of young stellar objects (YSOs) in the molecular cloud surrounding the halo H II region KR 140 in order to determine if the ongoing star formation activity in this region is dominated by sequential star formation within the photodissociation region (PDR) surrounding the H II region. We find that KR 140 has an extensive population of YSOs that have 'spontaneously' formed due to processes not related to the expansion of the H II region. Much of the YSO population in the molecular cloud is concentrated along a dense filamentary molecular structure, traced by C 18 O, that has not been erased by the formation of the exciting O star. Some of the previously observed submillimetre clumps surrounding the H II region are shown to be sites of recent intermediateand low-mass star formation while other massive starless clumps clearly associated with the PDR may be the next sites of sequential star formation.Key words: stars: formation -stars: pre-main-sequence -H II regions -infrared: stars. I N T RO D U C T I O NThis study is the fourth in a series of papers exploring the structure of the H II region KR 140 and its associated star-forming activity. In Kerton, Ballantyne & Martin (1999) VES 735, the O star powering the H II region was examined. A second paper, Ballantyne, Kerton & Martin (2000), was a multiwavelength study of the structure, energetics, and kinematics of the H II region. Finally, Kerton et al. (2001) presented an analysis of submillimetre (submm) observations of the region at 450 and 850 μm. The main result of Kerton et al. (2001) was the discovery of numerous submm dust cores located within the molecular gas surrounding the H II region, including a large number of cores that were clearly located at the interface between ionized and molecular gas, a likely location for star formation induced or 'triggered' by the expansion of the H II region (Elmegreen 1998). Two of the more isolated cores were clearly associated with IRAS sources, but the low resolution of IRAS combined with the extensive diffuse emission of dust associated with the H II region made it impossible to determine if any of the other cores were associated with star formation activity.The goal of this study is to determine the distribution of young stellar objects (YSOs) throughout the KR 140 molecular cloud. We focus especially on the embedded stellar content of the submm cores as these are potentially the youngest star-forming regions. E-mail: kerton@iastate.eduWe use the spatial distribution of the YSOs relative to the photodissociation region (PDR) surrounding the H II region as a means of gauging the relative importance of spontaneous and sequential, or triggered, star formation in this region. To achieve this we have analysed a combination of submm data from Kerton et al. (2001) and Moore et al. (2007), Two-Micron All-Sky Survey (2MASS) and Midcourse Space Experiment (MSX)...

The gas properties of the W3 giant molecular cloud: a HARP study

“…The rest of the W3 GMC contains less intense emission and less active star formation. On the south‐eastern corner of the GMC, the cloud associated with the KR 140 H ii bubble (Kerton et al 2008) is easily identifiable.…”

“…The rest of the cloud seems so far to have been largely unaffected by this interaction and the star formation within it should be mainly spontaneous. One notable exception is the KR 140 H ii region, located in the far south‐west corner of the cloud; this may be an example of the spontaneous formation of an isolated massive star which is now triggering new star formation in a surrounding shell (Kerton et al 2008).…”

Unveiling the birth and evolution of the H ii region Sh2-173

“…To look for tracers of stellar formation activity in the 2MASS catalogue, we have adopted the criteria developed by Kerton et al (2008). Based on a sample of YSOs having low (<2 M ⊙ ) and intermediate (2 < M ⊙ < 9) masses extracted from Kenyon & Hartmann (1995) (T Tauri) and Thé, de Winter & Perez (1994) (Herbig/AeBe), respectively, Kerton et al (2008) inferred colour criteria to analyse the presence of stellar formation activity in the surroundings of the H ii region KR 140. They scaled the data of the sample to the distance of KR 140 (2.3 kpc) and the corresponding visual extinction ( A v = 5.5 mag).…”

“…Thus, the relative location of the YSO candidates with respect to the PDR may be considered as an indication of the role that the H ii region is playing in the formation of the new stars. The probability for a source of being triggered by the action of the H ii region is higher when it is located inside the region of direct influence of the H ii region (Kerton et al 2008 and references therein).…”

Observational studies of the structure, content and environment of intermediate and high-mass star-forming regions

“…In addition, these new data will be combined with data on KR 140, another small halo H II region that has been the subject of several studies (Kerton et al, 1999;Ballantyne et al, 2000;Kerton et al, 2001Kerton et al, , 2008. This larger sample will be contrasted with sub-mm observations of a very different environment, the interface between the massive star-forming complex W3 and its surrounding molecular cloud.…”