Summary: Below are the posters presented at the NAOC/ISM Spring Science Forum 2020, in the order they were presented.
From Carl Sagan to Kardashev
I’d like to present our recent testing results from FAST SETI backend. I quantify FAST’s reach in SETI based on the equivalent isotropic radiation power (EIRP). Equipped with a dedicated pipeline and its world-leading instantaneous gain, FAST will be able to detect human-like transmitters on TESS planets and any Kardashev type II or more advanced civilizations in the Andromeda galaxy.
A Joint Study with FAST and Parkes for PSR J1926-0652
PSR J1926−0652, a pulsar recently discovered with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Using sensitive single-pulse detections from FAST and long-term timing observations from the Parkes 64 m radio telescope.
Complex Organic Molecules Formation on Stochastically Heated Grains in Prestellar Cores
In recent years, complex organic molecules (COMs) have been detected in various astronomical sources including prestellar cores, such as CH3OCH3, CH3CHO and HCOOCH3 in L1689B, B1-b and L1544. Their origin have been arouse many interesting questions for astrochemist. There is a suggestion that COMs are formed on dust grains and sublime into gas phase as the temperature increases. However, the detection of COMs in prestellar cores where the gas temperature T ~10 K suggested that COMs in the cold cores may be synthesized by a different mechanism because radicals cannot diffuse to recombine to form COMs on dust grains at a temperature as low as 10 K. In our recent models which considering the small dust grains temperature fluctuations on the influence of gas-grain chemistry, we found that amount of COMs can be synthesized on those small grains. Meanwhile gaseous COMs can be produced with a reasonable agreement if the reactive desorption mechanism is presented in the cold cores.
A Study of the Co-existence of an Outflow and a Bubble
We have found a new molecular bubble in the Taurus B18 cloud in our CO map. A previous identified outflow is located at the center of the bubble (Li et al. 2015) and they are coincident in velocity. Also, Herbig–Haro 319 is located at the same center produced by the outflow, holding the important clues of the origin of the bubble. The strong CO emission, the extremely high energy injection, the peculiarity of this structure and the location of the outflow cloud make a great interest to study the origin and evolution of the stellar feedback activity.
Radio Frequency Interference Mitigation using Pseudoinverse Learning AutoEncoders
Radio frequency interference (RFI) is an important challenge in radio astronomy. It comes from various sources and increasingly impact astronomical observation as telescope getting more sensitive. In this study, we propose a fast and effective method for removing RFI in pulsar data. We use pseudo-inverse learning to train a single hidden layer auto-encoder (AE). We demonstrate that the AE can quickly learn the RFI signatures and remove them from fast-sampled spectra, leaving real pulsar signals. This method has the advantage over traditional threshold-based filter method in that it does not completely remove contaminated channels, which could also contain useful astronomical information.
A RECIPE FOR DERIVING KINETIC TEMPERATURE FROM AMMONIA INVERSION LINES
Ammonia is a classical interstellar thermometer. The previous methods for estimating the rotational and kinetic temperatures are largely affected by blended hyperfine components (HFCs). We developed a new reliable recipe, referred to as the hyperfine group ratio (HFGR), which utilizes only direct observables, namely the intensity ratios between the grouped HFCs. From the simulated spectra, we show that the rotational temperature Trot can be unambiguously derived from the HFC intensity ratios using a set of empirical formulae. Compared to classical methods, the current recipe has two major advantages. Firstly, the Trot calculation is based on empirical formulae instead of fitting for optimized physical parameters of Trot, line width ∆V, and the NH3 optical depth. Secondly, the calculations only involve the integrated intensities of the HFC groups thus does not require modeling the HFC and fitting the line profiles. And the blended HFCs would no longer undermine the accuracy of Trot. Consequently, the computational load can be reduced to a very small amount (∼ 10−4)) compared to the complete spectral fitting. In the meantime, HFGR maintains the accuracy of better than 0.15 K over a broad temperature range of 10 to 70 K. The simplicity of HFGR makes it orders of magnitude faster than the classical methods and more robust against noise and HFC blending.
The Filament Intersections and Cold Dense Cores in Orion A North
We studied the filament structures in OMC-2/3 dense molecular cloud using a high-resolution N2H+ (1-0) spectral cube observed with the Atacama Large Millimeter/Submillimeter Array (ALMA). The filament network over a total length 2.0-pc is found to contain about 170 intersections and 128 candidate dense cores. The cores exhibit a concentration around the intersections, that 101 ones are near the intersections while only 27 ones are on the long single- path filaments or isolated from the filament. The column density (Ntot) probability distribution function (N-PDF) shows that the power-law component in the range of Ntot > 2.3 × 1023 cm−2 is also more intense around the intersections. The dense cores are also displaced from the infrared point sources (young stars), suggesting them to be prestellar. Most cores have virial parameter of αvir = Mvir/Mgas < 1.0, suggesting they are bounded by the self gravity. In the mean time, only one third of the cores have critical mass ratio of αcrit = Mcrit/Mgas < 1.0 thus would be unstable to the self-gravity. The result suggests that the major fraction of the cold starless cores in OMC-2,3 would be able to stably exist, but more extensive core collapse and star formation would require continuous core mass growth or additional perturbations.