Kinematic and thermal structure at the onset of high-mass star formation

Abstract

Context. Even though high-mass stars are crucial for understanding a diversity of processes within our galaxy and beyond, their formation and initial conditions are still poorly constrained. Aims. We want to understand the kinematic and thermal properties of young massive gas clumps prior to and at the earliest evolutionary stages of high-mass star formation. Do we find signatures of gravitational collapse? Do we find temperature gradients in the vicinity or absence of infrared emission sources? Do we find coherent velocity structures toward the center of the dense and cold gas clumps? Methods. To determine kinematics and gas temperatures, we used ammonia, because it is known to be a good tracer and thermometer of dense gas. We observed the NH 3 (1, 1) and (2, 2) lines within six very young high-mass star-forming regions comprised of infrared dark clouds (IRDCs), along with ISO-selected far-infrared emission sources (ISOSS) with the Karl G. Jansky Very Large Array (VLA) and the Effelsberg 100 m Telescope.Results. The molecular line data allows us to study velocity structures, linewidths, and gas temperatures at high spatial resolution of 3−5 , corresponding to ∼0.05 pc at a typical source distance of 2.5 kpc. We find on average cold gas clumps with temperatures in the range between 10 K and 30 K. The observations do not reveal a clear correlation between infrared emission peaks and ammonia temperature peaks. Several infrared emission sources show ammonia temperature peaks up to 30 K, whereas other infrared emission sources show no enhanced kinetic gas temperature in their surrounding. We report an upper limit for the linewidth of ∼1.3 km s −1 , at the spectral resolution limit of our VLA observation. This indicates a relatively low level of turbulence on the scale of the observations. Velocity gradients are present in almost all regions with typical velocity differences of 1 to 2 km s −1 and gradients of 5 to 10 km s −1 pc −1 . These velocity gradients are smooth in most cases, but there is one exceptional source (ISOSS23053), for which we find several velocity components with a steep velocity gradient toward the clump centers that is larger than 30 km s −1 pc −1 . This steep velocity gradient is consistent with recent models of cloud collapse. Furthermore, we report a spatial correlation of ammonia and cold dust, but we also find decreasing ammonia emission close to infrared emission sources.

Subsonic islands within a high-mass star-forming infrared dark cloud

“…A similar VLA survey of IRDC kinematics by Ragan et al (2012) found no values of M below 2, favouring an even higher degree of non-thermal motions present inside the massive dense cores. Three other VLA surveys of ammonia (Lu et al 2014; upper limit of 1.3 km s −1 on line widths in Bihr et al 2015; typical M from 3 to 5 reported in Dirienzo et al 2015) report highly supersonic gas motions within IRDCs. We suggest that the coarser spectral resolution of the previous VLA ammonia studies in similar IRDCs might have inhibited the detection of the subsonic regime, as our spectral resolution of 0.2 km s −1 is higher than that of the previous studies (0.6 km s −1 in Ragan et al 2012;Lu et al 2014;Dirienzo et al 2015; 0.6 km s −1 for most of the data used by Sánchez-Monge et al 2013; 0.8 km s −1 in Bihr et al 2015).…”

“…Three other VLA surveys of ammonia (Lu et al 2014; upper limit of 1.3 km s −1 on line widths in Bihr et al 2015; typical M from 3 to 5 reported in Dirienzo et al 2015) report highly supersonic gas motions within IRDCs. We suggest that the coarser spectral resolution of the previous VLA ammonia studies in similar IRDCs might have inhibited the detection of the subsonic regime, as our spectral resolution of 0.2 km s −1 is higher than that of the previous studies (0.6 km s −1 in Ragan et al 2012;Lu et al 2014;Dirienzo et al 2015; 0.6 km s −1 for most of the data used by Sánchez-Monge et al 2013; 0.8 km s −1 in Bihr et al 2015). Our results thus point to consistently lower line widths and temperatures than those found in other IRDCs, and closer to the typical values in nearby Gould Belt low-mass star-forming regions (e.g.…”

Gravitational Contraction Versus Supernova Driving and the Origin of the Velocity Dispersion–size Relation in Molecular Clouds

“…Although the clouds are constantly being deformed by SN explosions, these do not drive substantial internal turbulence. While SN continue to explode in the diffuse ISM, the clouds maintain low velocity dispersions, which appears (Gibson et al 2009); and (yellow stars) infrared dark clouds observed with NH 3 (Bihr et al 2015). The black triangle shows the constant value of the column density reported by Solomon et al (1987) in their size-density relation.…”

Are infrared dark clouds really quiescent?

“…Since a 2-D Gaussian is not suitable for modeling a filamentary structure, we use the same algorithm that measures the total flux per unit projection size to measure the linear masses in rectangles, which are centered on the mapping center of 30 m observations and individually have lengths that are one, two, or three times the local FWHM width, and then derive the mean.17 Even if the velocity distribution of our source is biased by our field of view, rotation energy is in general not comparable to the gravitational energy(Goodman et al 1993;Bihr et al 2015).…”