To derive the degree of non-thermal gas motions and gas turbulence in G035.39 from the am-monia data, we use the following relations. Firstly, accounting for the channel width broadening effects, we subtract the channel width from the observed full width at half-max maximum of the line:
∆VFW H M = q
∆Vobs2−∆Vchan2,
where∆Vobsis the fitted line width from the observed line profile and ∆Vchan is taken to be the spectral resolution of VLA at 0.2 km s−1. To find the degree of non-thermal gas motions, we then remove the component caused by the thermal broadening of the line, obtaining the non-thermal velocity dispersion from the line width:
σnt =
s∆Vobs2
8 ln 2 − kBTkin
mNH3 ,
wheremNH3 = 17mHis the molecular weight of the ammonia molecule.
A useful way to quantify the degree of the non-thermal motions in the gas is to compare those against the local sonic gas speedcs ≡
rkBTkin
mgas , where mgas, the average mass of the particle in the gas medium, is taken to be 2.33 a.m.u. Following this, a sonic Mach number is computed as the ratio of the two quantities: M ≡σnt/cs.
−200 −150 −100 −50 0 50 100 RAJ2000,off(arcsec)
42 43 44 45 46 47 48
Velocity(kms−1)
0 20
DecJ2000,off(arcsec) 42.00
42.25 42.50 42.75 43.00
Figure 4.13 A PPV-diagram of the fitted velocity components within IRDC G035.39 along the Right Ascension projection. The coordinates are given in arcsecond offset relative to the α(J2000)= 18h57m08s, δ(J2000)= +2◦1003000 coordinate. All the data are plotted in black, similarly to Fig. 4.2, and individual velocity components are marked in different colors. The data not found to be associated with any clusters are plotted in gray. The figure shows the clus-tering obtained with the relxed masking criteria (introduced in§4.3.2). In addition to the R.A.
projection, a projeciton along Dec. is shown in the inset axis for the F1 filament.
In order to enable a statistically sound discussion on the Mach numbers, one needs to first estimate the uncertainty on their values. In our analyses, we assume the uncertainty to come from two factors: the temperature of the gas, used to calculate the thermal contribution to line width and the sonic sound speed in the medium, and the error on the line width itself. Both errors are estimated withinpyspeckitpackage, which provides one-sigma uncertainties onTkin andσ.
Assuming these uncertainties to be uncorrelated, we propagate the errors as follows:
σM= s
σσobs
2 ∂M
∂σobs
2+σTkin2 ∂M
∂Tkin
2, (4.1)
where the notations σTkin and σσobs are the uncertainties on the gas kinetic temperatures and observed velocity dispersion, respectively.
Substituting appropriate terms into Equation4.1, we arrive at the following expression:
σM =
√mgasmNH3 2
sσTkin2(σobs2−σchan2)2+4σσobs
2σobs2Tkin2
kBTkin3(mNH3(σobs2−σchan2)−kBTkin) , (4.2) where the termsσchan= ∆Vchan/√
8 ln 2 andσobs= ∆Vobs/√
8 ln 2 are introduced for clarity.
The expression above is used to quantify the uncertainties on theM values throughout this work (where applicable), as well as inSokolov et al.(2018).
Future Work and Outlook
5.1 Summary and Future Work
This work presented analyses of observations towards IRDC G035.39–00.33, previously found to be an excellent target to probe the initial conditions for high-mass star formation. Below the summary of preceding chapters is given, and interesting future prospects for IRDC and high-mass star formation are outlined. With the large-scale GBT observations of ammonia inversion transitions and the complementary archival Herschel photometric data, a quantitative compar-ison between the two temperature estimating methods is made. Without careful modelling of the background and foreground emission, the conventional method of deriving dust temperature is shown not only to systematically overestimate the IRDC temperatures by 2− 3 K, but also conceals signs of protostellar heating of the core envelopes. What does this entail for the future studies of IRDCs? Because the spectral line surveys that can reliably measure the gas temper-ature are time-consuming, far-infrared surveys are commonly used in their place for large-scale temperature measurements of cold clouds. Given the richness of the availableHerschelGalactic plane surveys, any systematic search for starless core candidates through far-infrared emission would benefit from tighter constraints of the spatial temperature distribution, which should take the line-of-sight contamination into account. Additionally, any chemical modelling of the in-frared dark clouds has to take the systematics above into account, because the chemical reaction rates are highly dependant on the temperature, making the 2−3 K bias found significant enough to account for.
On smaller scales, the combined single-dish and interferometric observations reveal a sur-prising insight. Compared to other surveys of IRDCs conducted at the same facilities, G035.39 exhibits levels of turbulence lower than other infrared dark clouds. Is G035.39 an oddball in an otherwise established and unshaken picture of massive star formation that always occurs in a highly turbulent medium, or is it one of the first pieces of evidence signalling that massive star formation undergoes through a quiescent stage, more similar to the low-mass star forming clouds than previously thought? The follow-up surveys of IRDCs, conducted with high frequency reso-lution and sensitivity, should be able to give an answer to that question. However, an argument for the lower non-thermal motions prevailing in IRDCs can be found in other recent studies as
well as recent ALMA observations towards the Orion Integral Filament, which resolve it into col-lections of subsonic fiber-like structures. Within G035.39 itself, more observations are needed to investigate the boundary over which the gas kinematics becomes subsonic, and to determine if a phenomenon similar to transition to coherence, found in low-mass dense cores, plays a role in the IRDC cores.
Finally, the similarity of IRDC G035.39–00.33 to the low-mass star forming regimes does not apply only to the non-thermal motion magnitude. Although most of the dense extinction cores can be associated with the mid-infrared emission (likely of protostellar origin), the temper-ature of the core with the highest extinction and no mid-infrared emission closely resembles that of the low-mass starless cores. Similarly, the kinematics of the IRDC resembles the low-mass dense cores closely, with similar velocity gradient magnitudes and specific angular momenta of the cores if solid-body core rotation is assumed. Observations of high-mass protostellar objects which aim to probe the initial conditions of massive star formation are naturally biased by ener-getic feedback processes from the embedded massive YSOs. Dedicated future surveys of dense gas kinematics of sources not yet exhibiting signs of disruptive feedback, conducted with high spectral and angular resolution, are needed to further evaluate the extent to which degree the low-and high-mass star-forming conditions are similar.