RESEARCH
Research Highlights:
Turbulence measurements in the neutral ISM from HI 21-cm emission-absorption spectra:
The study of turbulence in astrophysical systems is an exciting area of research. Most of the astrophysical system is magnetized. Therefore, the power law does not follow the standard incompressible hydrodynamic law of turbulence (Kolmogorov scaling). The nature of incompressible magneto-hydrodynamic turbulence has been well established, however, the nature of compressible magneto-hydrodynamic turbulence is still poorly understood. There are many astrophysical systems that fall under this category, including the interstellar medium or more specifically, the cold neutral medium (CNM) and the molecular cloud.
In my recent study, using a large number of HI gas components taken from various published surveys and with the help of some numerical results (which are based on various empirical laws) I have given a simple analytical method through which one can study the nature of turbulence in the CNM phase of the neutral interstellar medium (ISM). The power-law index (p) obtained from this study is close to the Burger law of turbulence (0.50). In my study, I used three major large scale surveys, and a similar trend is observed in all of them. It is necessary to study the turbulence using this method in the future. Additionally If, similar kind of result is obtained then this method can be an alternative method for studying the nature of turbulence in the neutral ISM in addition to the other standard methods, like power spectrum analysis, velocity channel analysis (VCA), position-position-velocity (PPV) cube analysis, etc. ( Reference: (A. Koley 2023 ) 2023, PASA, 40, e046).
Measurement of magnetic field using CCS (2_1 -1_0) Zeeman splitting :
Most of the astronomical system is magnetized. It is therefore necessary to measure magnetic fields in order to better understand the various physical processes taking place within these systems. Depending on the medium, magnetic fields can be measured in different ways. In fully ionized medium synchroton, Faraday rotation are an appropriate mechanism for measuring the magnetic field. Additionally, in partially ionized media (low ionization fraction), dust polarization and Zeeman splitting are both viable mechanisms.
By using dust polarization, we can estimate the magnetic field, whereas by using Zeeman splitting, we can measure it directly. There are limited suitable tracers are available through which one can measure the magnetic field using the Zeeman splitting method. One of those is CCS radical, whose critical density(n_cr) is ~ 10^4.5 cm ^{-3} and thus this radical is a suitable tracer for measuring the magnetic field in the prestellar core (density ~ 10^{4} - 10^{5} cm ^{-3}). Despite the fact that this is a suitable tracer, only a single-dish telescope measurements was there in the literature where researchers have successfully measured the magnetic field in the TMC-1 core. Prior to our study, no interferometric observations had been made. Through our study, for the first time, we have been able to successfully measure the magnetic field inside the highly compact prestellar core TMC-1C and obtain the magnetic field ~2 mG.
Magnetic field not only create pressure. If the magnetic field is high it will also help to collapse. For example, in the partially ionized medium, when the effective magnetic Reynolds number (R_m) is ~ 1, neutral particles are not able to couple to the ion and electron through collision and neutral particles drift through the magnetic field and core becomes magnetically supercritical. This diffusion time scale (t_AD) is inversely proportional to the magnetic field (see the book conversations of electric and magnetic fields in the cosmos by E. N. Parker). In our study, we have also calculated the diffusion time scale (t_AD) and compared it with the lifetime of the core nucleus.
The finding of this research opens up the possibility of studying the magnetic field using deep interferometric observations in the prestellar core using the CCS radical, thereby leading to an improved understanding of the role of magnetic field in the prestellar cores (Reference: (Koley et al. 2022) 2022, MNRAS, 516, L48).
It is quite obvious from various studies of prestellar cores, that the outer part is static or expanding and the inner central part is collapsing. For example in L1517B core, outer part (beyond ~0.02 pc) is expanding and inner central part is collapsing. Therefore, there is no need to consider that whole prestellar core is collapsing, if one obtains the double peak profile towards the central region of the prestellar core. CCS is such a radical whose spatial distribution is very interesting obtained from various high resolution interferometric observations. It doesn't trace the diffuse outer part (like C120, C130) and the extremely dense central part (like N2H+, NH3, DCN, etc.), rather it traces the intermediate part in between the two environments. That is why through this radical one can get the actual transition phase between sub-critical phase and the super-critical phase.