Globules and pillars in Cygnus X

Abstract

Globules and pillars, impressively revealed by the Spitzer and Herschel satellites, for example, are pervasive features found in regions of massive star formation. Studying their embedded stellar populations can provide an excellent laboratory to test theories of triggered star formation and the features that it may imprint on the stellar aggregates resulting from it. We studied the globule IRAS 20319+3958 in Cygnus X by means of visible and near-infrared imaging and spectroscopy, complemented with mid-infrared Spitzer/IRAC imaging, in order to obtain a census of its stellar content and the nature of its embedded sources. Our observations show that the globule contains an embedded aggregate of about 30 very young ( 1 Myr) stellar objects, for which we estimate a total mass of ∼90 M . The most massive members are three systems containing early B-type stars. Two of them most likely produced very compact H II regions, one of them being still highly embedded and coinciding with a peak seen in emission lines characterising the photon dominated region (PDR). Two of these three systems are resolved binaries, and one of those contains a visible Herbig Be star. An approximate derivation of the mass function of the members of the aggregate gives hints of a slope at high masses shallower than the classical Salpeter slope, and a peak of the mass distribution at a mass higher than that at which the widely adopted log-normal initial mass function peaks. The emission distribution of H 2 and Brγ, tracing the PDR and the ionised gas phase, respectively, suggests that molecular gas is distributed as a shell around the embedded aggregate, filled with centrally-condensed ionised gas. Both, the morphology and the low excitation of the H II region, indicate that the sources of ionisation are the B stars of the embedded aggregate, rather than the external UV field caused by the O stars of Cygnus OB2. The youth of the embedded cluster, combined with the isolation of the globule, suggests that star formation in the globule was triggered by the passage of the ionisation front.

Globules and pillars in Cygnus X

“…In any case, we confirm the conclusion from Djupvik et al ( 2017) that the emission distribution of H 2 indicates that the sources of ionisation are the B stars of the embedded aggregate, rather than the external UV field caused by the O-stars of Cyg OB2. We note that another line of evidence in Djupvik et al (2017) showed that the visible spectrum of the H II region is that of a soft ionising radiation, typical of an early-B star. In Sect.…”

“…Earlier studies reported nine cluster members within a projected radius of ∼0.5 pc (Kronberger et al 2006;Kumar et al 2006) and two visible stars had been estimated to have mid-B spectral types (Cohen et al 1989). The scenario of embedded stars was further explored in Djupvik et al (2017), where we found that the globule contains an embedded aggregate of about 30-40 young stellar objects within one arcmin (or 0.4 pc with the adopted distance of 1.4 kpc). Based on the high ratio of Class I to Class II objects, the small cluster was estimated to have an age <1 Myr.…”

“…The H II region cavity has a radius of ∼15 -20 , which is consistent with the extent of the area of brightest IR and H 2 and Br γ emission (Figs. 2 and 7 in Djupvik et al 2017). A significantly larger cavity and, therefore, a lower FUV at the clumpy, internal PDR is unlikely.…”

“…External UV-radiation creates photodissociation regions (PDRs) on the surfaces of pillars and globules, often visible as a bright rim, facing the ionising source. A rare example of a globule suggested to be also illuminated internally by massive stars is IRAS 20319+3958 in Cygnus X (Schneider et al 2012(Schneider et al , 2016Djupvik et al 2017). This source (hereafter, 'the globule') is explored in this paper.…”

“…In the first part of this series of papers (Schneider et al 2016), we set up a categorisation based on Herschel 70 µm photometry and we propose that pillars advance into globules, which, in turn, evolve into evaporating gaseous globules (EGGs), dense gas condensations without star-formation, or objects with protoplanetary disks (proplyds); or they only resemble proplyds (proplyd-like), depending on density and incident UV-field. In the second paper (Djupvik et al 2017), we carried out optical and near-IR imaging and spectroscopy of the globule in order to obtain a census of its stellar content and the nature of its embedded sources.…”