Determining Electron Temperature and Density in a H ii Region Using the Relative Strengths of Hydrogen Radio Recombination Lines

Abstract

We have introduced a new method of estimating the electron temperature and density of H II regions by using single dish observations. In this method, multiple hydrogen radio recombination lines of different bands are computed under the assumption of low optical depth. We use evolutionary hydrodynamical models of H II regions to model hydrogen recombination line emission from a variety of H II regions and assess the reliability of the method. According to the simulated results, the error of the estimated temperature is commonly < 13%, and that of the estimated density is < 25% for a < 1% uncertainty of the observed line fluxes. A reasonable estimated value of electron density can be achieved if the uncertainty of the line fluxes are lower than 3%. In addition, the estimated values are more representative of the properties in the relatively high-density region if the gas density gradient is present in the H II region. Our method can be independent of the radio continuum observations. But the accuracy will be improved if a line-to-continuum ratio at millimeter wavelengths is added to the estimation. Our method provides a way to measure the temperature and density in ionized regions without interferometers.

ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions – I. Survey description and a first look at G9.62+0.19

“…Higher electron temperature will lead to higher free-free emission contributions. The electron temperature could be better constrained by observing multiple RRLs in future (Zhu et al 2019). We found that the 3 mm continuum emission in "B", "MM1/C" and "MM11/D" is dominated by free-free emission, as suggested in previous works (Hofner et al 1996).…”

A Q-band Line Survey toward Orion KL Using the Tianma Radio Telescope

“…The oscillator strength can be calculated more accurately from the method described in Brocklehurst (1971) and Regemorter & Prudhomme (1979). The accurate values have been used in recent models of Prozesky & Smits (2018) and Zhu et al (2019Zhu et al ( , 2022. The modeled spectrum in this work will not change obviously when the more accurate values of f n n , 1 2 are adopted.…”

“…If the RRL emitters are in LTE state, the departure coefficients should be unity for all levels. For n e = 10 4 cm −3 and T = 8000 K, we calculated the population of hydrogen using the method by Zhu et al (2019Zhu et al ( , 2022. The departure coefficients b n increase from 0.85-0.99 as n increases from 50-100.…”

Recombination lines and maser effects

“…Storey & Hummer (1995) included later the angular momentum l, and finally the contribution of the continuum radiation was introduced (Prozesky A. & Smits 2018, Zhu et al 2019).…”