Ions (e.g., H3+ , H2O+) have been used extensively to quantify the cosmic-ray ionization rate (CRIR) in diffuse sightlines. However, measurements of the CRIR in low-to-intermediate density gas environments are rare, especially when background stars are absent. In this work, we combine molecular line observations of CO, OH, CH, and HCO+ in the star-forming cloud IC 348, and chemical models to constrain the value of the CRIR and study the response of the distribution of chemical abundances. The cloud boundary is found to have an A V of approximately 4 mag. From the interior to the exterior of the cloud, the observed 13CO line intensities drop by an order of magnitude. The calculated average abundance of 12CO (assuming 12C/13C=65) is (1.2 ± 0.9) × 10−4, which decreases by a factor of 6 from the interior to the outside regions. The average abundance of CH (3.3 ± 0.7 × 10−8) is in good agreement with previous findings in diffuse and translucent clouds (A V < 5 mag). However, we did not find a decline in CH abundance in regions of high extinction (A V ≃ 8 mag) as previously reported in Taurus. By comparing the observed molecular abundances and chemical models, we find a decreasing trend of the CRIR as A V increases. The inferred CRIR of ζ cr = (4.7 ± 1.5) × 10−16 s−1 at low A V is consistent with H3+ measurements toward two nearby massive stars.

Dependence of Chemical Abundance on the Cosmic-Ray Ionization Rate in IC 348

We present high-sensitivity CH 9 cm ON/OFF observations toward 18 extra-galactic continuum sources that have been detected with OH 18 cm absorption in the Millennium survey with the Arecibo telescope. CH emission was detected toward six of eighteen sources. The excitation temperature of CH has been derived directly through analyzing all detected ON and OFF velocity components. The excitation temperature of CH 3335 MHz transition ranges from $-54.5$ to $-0.4$ K and roughly follows a log-normal distribution peaking within [$-$5, 0] K, which implies overestimation by 20% to more than ten times during calculating CH column density by assuming the conventional value of $-60$ or $-10$ K. Furthermore, the column density of CH would be underestimated by a factor of $1.32\pm 0.03$ when adopting local thermal equilibrium (LTE) assumption instead of using the CH three hyperfine transitions. We found a correlation between the column density of CH and OH following log$N$(CH) = (1.80$\pm$ 0.49) log$N$(OH) $-11.59 \pm 6.87$. The linear correlation between the column density of CH and H$_2$ is consistent with that derived from visible wavelengths studies, confirming that CH is one of the best tracers of H$_2$ component in diffuse molecular gas.

https://robots.iopscience.iop.org/article/10.3847/1538-4365/ac27a5/pdf

Precise Measurements of CH Maser Emission and Its Abundance in Translucent Clouds

We investigate the infall properties in a sample of 11 infrared dark clouds (IRDCs) showing blue-asymmetry signatures in HCO$^{+}$ J=1--0 line profiles. We used JCMT to conduct mapping observations in HCO$^{+}$ J=4--3 as well as single-pointing observations in HCO$^{+}$ J =3--2, towards 23 clumps in these IRDCs. We applied the HILL model to fit these observations and derived infall velocities in the range of 0.5-2.7 km s$^{-1}$, with a median value of 1.0 km s$^{-1}$, and obtained mass accretion rates of 0.5-14$\times$10$^{-3}$ Msun yr$^{-1}$. These values are comparable to those found in massive star forming clumps in later evolutionary stages. These IRDC clumps are more likely to form star clusters. HCO$^{+}$ J =3--2 and HCO$^{+}$ J =1--0 were shown to trace infall signatures well in these IRDCs with comparable inferred properties. HCO$^{+}$ J=4--3, on the other hand, exhibits infall signatures only in a few very massive clumps, due to smaller opacties. No obvious correlation for these clumps was found between infall velocity and the NH3/CCS ratio.

Studying infall in infrared dark clouds with multiple HCO+ transitions

Ground resonance (GR) is a potentially destructive mechanical instability involving the coupling of the regressive rotor mode to the fuselage motion. Vibration can be disastrous if measures are not taken in time to mitigate vibrations. This paper presents a study on the use of adaptive landing gear to eliminate GR in helicopters. A GR model considering adaptive landing gear is established, and the dynamic characteristics of the system are analyzed. In this paper, the Linear Quadratic Regulator (LQR) controller is applied to suppress the GR. The setting of the weighting matrix coefficients directly determines the effect of the control system. Therefore, a design method of LQR weighting matrix based on multi-objective genetic algorithm is proposed, which enables the control system to meet multiple performance indicators concurrently. Numerical simulations of GR suppression were performed for the rotor speed range where GR is most likely to occur. The results show that even if the rotor system is not equipped with any dampers, the suppression of helicopter GR can be achieved by using the control strategy proposed in this paper to control the adaptive landing gear. Therefore, the application of adaptive leg landing gear to helicopters can provide some ideas to eliminate GR.

Jifeng Xia

Papers

Precise Measurements of CH Maser Emission and Its Abundance in Translucent Clouds

Studying infall in infrared dark clouds with multiple HCO+ transitions

LQR-based Ground Resonance Suppression of Helicopter with Adaptive Landing Gear*

Studying Infall in Infrared Dark Clouds with Multiple HCO+ Transitions.