Globules and pillars in Cygnus X

Abstract

The radiative feedback of massive stars on molecular clouds creates pillars, globules and other features at the interface between the H II region and molecular cloud. Optical and near-infrared observations from the ground as well as with the Hubble or Spitzer satellites have revealed numerous examples of such cloud structures. We present here Herschel far-infrared observations between 70 µm and 500 µm of the immediate environment of the rich Cygnus OB2 association, performed within the Herschel imaging survey of OB Young Stellar objects (HOBYS) program. All of the observed irradiated structures were detected based on their appearance at 70 µm, and have been classified as pillars, globules, evaporating gasous globules (EGGs), proplyd-like objects, and condensations. From the 70 µm and 160 µm flux maps, we derive the local far-ultraviolet (FUV) field on the photon dominated surfaces. In parallel, we use a census of the O-stars to estimate the overall FUV-field, that is 10 3 -10 4 G 0 (Habing field) close to the central OB cluster (within 10 pc) and decreases down to a few tens G 0 , in a distance of 50 pc. From a spectral energy distribution (SED) fit to the four longest Herschel wavelengths, we determine column density and temperature maps and derive masses, volume densities and surface densities for these structures. We find that the morphological classification corresponds to distinct physical properties. Pillars and globules are massive (∼500 M ) and large (equivalent radius r ∼ 0.6 pc) structures, corresponding to what is defined as "clumps" for molecular clouds. EGGs and proplyd-like objects are smaller (r ∼ 0.1 and 0.2 pc) and less massive (∼10 and ∼30 M ). Cloud condensations are small (∼0.1 pc), have an average mass of 35 M , are dense (∼6 × 10 4 cm −3 ), and can thus be described as molecular cloud "cores". All pillars and globules are oriented toward the Cyg OB2 association center and have the longest estimated photoevaporation lifetimes, a few million years, while all other features should survive less than a million years. These lifetimes are consistent with that found in simulations of turbulent, UV-illuminated clouds. We propose a tentative evolutionary scheme in which pillars can evolve into globules, which in turn then evolve into EGGs, condensations and proplyd-like objects.

Theoretical ISM Pressure and Electron Density Diagnostics for Local and High-redshift Galaxies

“…This heating amplifies overdensities created by the turbulent ISM, which can then collapse and form stars (Gritschneder et al 2009;Dale et al 2012). These processes can create globules of dense gas (e.g., Tremblin et al 2013;Walch et al 2015;Schneider et al 2016), while stellar winds can drive outflows that create horse-shoe or other complex geometries (Park et al 2010). Ionized gas or radio continuum density measurements within H II regions show many complex radial gradients (Franco et al 2000;Pérez et al 2001;Binette et al 2002;Phillips 2007;Herrera-Camus et al 2016) Franco et al 2000;Kurtz 2002;Johnson & Kobulnicky 2003;Phillips 2007), while larger H II regions typically have more shallow or flat density gradients (Phillips 2008).…”

KFPA Examinations of Young STellar Object Natal Environments (KEYSTONE): Hierarchical Ammonia Structures in Galactic Giant Molecular Clouds

“…The OB associations range in age and size from the young proto-globular cluster Cyg OB2 (Knödlseder 2000;Wright et al 2014), harboring nearly one hundred O-stars, to the slightly older and smaller Cyg OB1, OB3, and OB9 (Uyanıker et al 2001). It has been mapped extensively in a variety of molecular gas tracers (Schneider et al 2006(Schneider et al , 2010bCsengeri et al 2011a,b;Duarte-Cabral et al 2013Dobashi et al 2014;Schneider et al 2016;Pillai et al 2012), dust continuum (Motte et al 2007;Bontemps et al 2010;Hennemann et al 2012), and dust polarization (e.g., Ching et al 2017).…”

Carina’s pillars of destruction: the view from ALMA

“…Out of the chaos involved in the formation of high-mass stars come the beautiful pillar structures seen at the rims of the ionised bubbles produced when the stars feed radiation back into their environment (e.g., Hester et al 1996;White et al 1999;Westmoquette et al 2013;Hartigan et al 2015;Schneider et al 2016). The structure and dynamics of these pillars can tell us about the past, present and future star-formation in the region, and are naturally produced in simulations of ionisation feedback (e.g., Gritschneder et al 2010;Mackey & Lim 2010;Tremblin et al 2012).…”