H II regions and high-mass starless clump candidates

Abstract

Context. The role of ionization feedback on high-mass (> 8 M ) star formation is still highly debated. Questions remain concerning the presence of nearby H ii regions changes the properties of early high-mass star formation and whether H ii regions promote or inhibit the formation of high-mass stars. Aims. To characterize the role of H ii regions on the formation of high-mass stars, we study the properties of a sample of candidates high-mass starless clumps (HMSCs), of which about 90% have masses larger than 100 M . These high-mass objects probably represent the earliest stages of high-mass star formation; we search if (and how) their properties are modified by the presence of an H ii region. Methods. We took advantage of the recently published catalog of HMSC candidates. By cross matching the HMSCs and H ii regions, we classified HMSCs into three categories: 1) The HMSCs associated with H ii regions both in the position in the projected plane of the sky and in velocity; 2) HMSCs associated in the plane of the sky, but not in velocity; and 3) HMSCs far away from any H ii regions in the projected sky plane. We carried out comparisons between associated and nonassociated HMSCs based on statistical analyses of multiwavelength data from infrared to radio. Results. We show that there are systematic differences of the properties of HMSCs in different environments. Statistical analyses suggest that HMSCs associated with H ii regions are warmer, more luminous, more centrally-peaked and turbulent. We also clearly show, for the first time, that the ratio of bolometric luminosity to envelope mass of HMSCs (L/M) could not be a reliable evolutionary probe for early massive star formation due to the external heating effects of the H ii regions. Conclusions. We show HMSCs associated with H ii regions present statistically significant differences from HMSCs far away from H ii regions, especially for dust temperature and L/M. More centrally peaked and turbulent properties of HMSCs associated with H ii regions may promote the formation of high-mass stars by limiting fragmentation. High-resolution interferometric surveys toward HMSCs are crucial to reveal how H ii regions impact the star formation process inside HMSCs.

The Hi-GAL compact source catalogue – II. The 360° catalogue of clump physical properties

“…15 shows that in the inner Galaxy this threshold falls well inside the region of the pre-stellar/protostellar overlap, whereas in the outer Galaxy only a small fraction of pre-stellar clumps is found above this threshold. As seen by Zhang et al (2020), quiescent massive clumps associated with H regions can reach bol / > 10 / because of significant external heating, suggesting a more advanced evolutionary state than is the case. The higher density of H regions in the inner Galaxy compared to the outer Galaxy (Anderson et al 2014) and, in general, the stronger interstellar radiation field (e.g., Mathis et al 1983) can create the higher degree of overlap in the inner Galaxy.…”

“…This scatter corresponds to the relatively wide range of temperatures found (Section 4.2), which depends, in turn, on different levels of external irradiation (Section 4.6) combined with the absence of a central energy source. For example, recently Zhang et al (2020), focusing on massive starless clumps, showed that those associated with an H region generally exhibit larger bol / values, more typical of protostellar sources.…”

The Hi-GAL compact source catalogue -- II. The 360° catalogue of clump physical properties

“…This scatter corresponds to the relatively wide range of temperatures found (Section 4.2), which depends, in turn, on different levels of external irradiation (Section 4.6) combined with the absence of a central energy source. For example, recently Zhang et al (2020), focusing on massive starless clumps, showed that those associated with an H region generally exhibit larger 𝐿 bol /𝑀 values, more typical of protostellar sources.…”

HII regions and high-mass starless clump candidates II. Fragmentation and induced star formation at ~0.025 pc scale: An ALMA continuum study

“…A unified T dust for all cores embedded in one HMSC, which normally comes from single-dish clump-scale observations, is not solid due to T dust gradients of the clump and different evolutionary stages of the cores. Dust temperature is expected to decline towards the center of starless clump (Guzmán et al 2015;Svoboda et al 2020;Zhang et al 2020). The temperature could drop to 10 K for the starless center (Lin et al 2020).…”

“…Previous studies showed that at least 30% of HMSF in the Galaxy are located at the edges of the H ii regions (Deharveng et al 2010;Kendrew et al 2016;Palmeirim et al 2017). In our first paper (hereafter Paper I, Zhang et al 2020), we investigated the feedback of H ii regions on candidate HMSCs that are quiet from ∼ 1 µm to 70 µm and without any star-formation signposts (Yuan et al 2017). Higher resolution interferometric observations show that many of these candidate HMSCs are not absolutely starless because low-to intermediate-mass protostellar objects usually exist there (Feng et al 2016;Traficante et al 2017;Contreras et al 2018;Sanhueza et al 2019;Svoboda et al 2019;Pillai et al 2019;Li et al 2019).…”