Are infrared dark clouds really quiescent?

Abstract

Context. The dense, cold regions where high-mass stars form are poorly characterized, yet they represent an ideal opportunity to learn more about the initial conditions of high-mass star formation (HMSF) since high-mass starless cores (HMSCs) lack the violent feedback seen at later evolutionary stages. Aims. We investigate the initial conditions of HMSF by studying the dynamics and chemistry of HMSCs. Methods. We present continuum maps obtained from the Submillimeter Array (SMA) interferometry at 1.1 mm for four infrared dark clouds (IRDCs, G28.34S, IRDC 18530, IRDC 18306, and IRDC 18308). For these clouds, we also present line surveys at 1 mm/3 mm obtained from IRAM 30 m single-dish observations. Results.(1) At an angular resolution of 2 (∼ 10 4 AU at an average distance of 4 kpc), the 1.1 mm SMA observations resolve each source into several fragments. The mass of each fragment is on average > 10 M , which exceeds the predicted thermal Jeans mass of the entire clump by a factor of up to 30, indicating that thermal pressure does not dominate the fragmentation process. Our measured velocity dispersions in the lines obtained by 30 m imply that non-thermal motion provides the extra support against gravity in the fragments. (2) Both non-detection of high-J transitions and the hyperfine multiplet fit of N 2 H + (J = 1 → 0), C 2 H (N = 1 → 0), HCN (J = 1 → 0), and H 13 CN (J = 1 → 0) indicate that our sources are cold and young. However, the obvious detection of SiO and the asymmetric line profile of HCO + (J = 1 → 0) in G28.34 S indicate a potential protostellar object and probable infall motion. (3) With a large number of N-bearing species, the existence of carbon rings and molecular ions, and the anti-correlated spatial distributions between N 2 H + /NH 2 D and CO, our large-scale high-mass clumps exhibit similar chemical features to small-scale low-mass prestellar objects.Conclusions. This study of a small sample of IRDCs illustrates that thermal Jeans instability alone cannot explain the fragmentation of the clump into cold (T ∼ 15 K), dense (> 10 5 cm −3 ) cores and that these IRDCs are not completely quiescent.

Long-term Variability of H2CO Masers in Star-forming Regions

“…We note that additional thermal methanol transitions have been recently observed towards NGC 7538 IRS1 in the millimeter range (Beuther et al 2013;Feng et al 2016b). Interestingly, while some of the lower excited transitions were seen in absorption, several other transitions showed emission lines (see Feng et al 2016b, their Table 5).…”

“…But it appeared that transitions with upper-level energies >250 K usually show emission. This tentative trend is not obeyed in our data where the 20.97 GHz transition appears in absorption despite its high upper-level energy of 452 K. One has to keep in mind, however, that our observations were conducted with a single dish telescope (GBT) while Feng et al (2016b) report Plateau de Bure Interferometre data, and that the underlying continuum is dominated in our case by free-free emission, while for the observations listed in Feng et al (2016b), the continuum is a mixture of free-free emission with a steeply rising contribution from thermal dust. The small-scale brightness temperature distribution in the continuum is different between the two regimes.…”

“…Interestingly, while some of the lower excited transitions were seen in absorption, several other transitions showed emission lines (see Feng et al 2016b, their Table 5). No clear trend for absorption or emission could be seen with regard to A-or E-symmetry, or regarding torsionally excited vs. ground state.…”

C2H N = 1 − 0 and N2H+ J = 1 − 0 observations of Planck Galactic cold clumps

“…Using the HFS fitting program in GILDAS/CLASS, we performed hyperfine structure fitting toward spectra of C 2 H N = 1 − 0 and N 2 H + J = 1 − 0. In the HFS fitting, the optical depths of different hyperfine lines are all assumed as Gaussian with the same width, and the excitation temperatures for different HFS components are the same (Feng et al 2016). Hyperfine structure fitting can give the parameters such as line width (∆V) and velocity (V LSR ) very precisely.…”

The TOP-SCOPE Survey of Planck Galactic Cold Clumps: Survey Overview and Results of an Exemplar Source, PGCC G26.53+0.17

“…Feng et al (2016) recently observed a sample of four 70 μm dark IRDC clumps, whose CO seems to be heavily depleted with depletion factors in the range of 14-50, i.e., about 3-10 times larger than that of G26 clump 6. They also derived high [N 2 H + ]/[H 13 CO + ] abundance ratios (∼400-10,000) in the 70 μm dark young IRDC clumps, which are more than two orders of magnitudes larger than that of clump 6 in G26 (∼0.8).…”