ALMA detection of ethyl cyanide and methyl formate in the hot molecular
  core IRAS 18566+0408
  In millimeter/submillimeter wavelength, the studies of the complex molecular
emission lines towards the hot molecular cores demonstrate valuable details
about the chemical composition and massive star formation processes. We present
the interferometric detection of rotational molecular emission lines of complex
organic species ethyl cyanide (C$_{2}$H$_{5}$CN) and methyl formate
(CH$_{3}$OCHO) towards the hot molecular core IRAS 18566+0408 using data of
Atacama Large Millimeter/Submillimeter Array (ALMA). We detected a total of
three unblended strong rotational emission lines of ethyl cyanide and eight
strong rotational emission lines of methyl formate between the frequency range
of 85.64$-$100.42 GHz with ALMA band 3 observation. We applied the rotational
diagram method under the Local Thermodynamic Equilibrium (LTE) conditions to
determine the molecular column density ($N$) in cm$^{-2}$ and rotational
temperature ($T_{rot}$) in K. The estimated statistical column density of ethyl
cyanide was $N$(C$_{2}$H$_{5}$CN) = (3.50$\pm$0.68)$\times$10$^{15}$ cm$^{-2}$
with rotational temperature $T_{rot}$ = 150$\pm$2.5 K and methyl formate was
$N$(CH$_{3}$OCHO) = (1.55$\pm$0.31)$\times$10$^{16}$ cm$^{-2}$ with rotational
temperature $T_{rot}$ = 150$\pm$2.8 K. We also fitted the LTE model spectrum
over the original transitions of detected C$_{2}$H$_{5}$CN and CH$_{3}$OCHO
spectral lines using the Markov Chain Monte Carlo (MCMC) algorithm to extract
the line parameters. We estimated the fractional abundance of ethyl cyanide and
methyl formate towards IRAS 18566+0408 relative to H$_{2}$ as
1.66$\times$10$^{-9}$ and 7.38$\times$10$^{-9}$ respectively where $N$(H$_{2}$)
= 2.1$\times$10$^{24}$ cm$^{-2}$. Except C$_{2}$H$_{5}$CN and CH$_{3}$OCHO, we
also detected simple organic molecules SO, CH$_{2}$CO, HC$_{3}$N,
H$_{2}$$^{34}$CS, HC$^{13}$CCN, HCC$^{13}$CN, and CH$_{3}$CHO towards IRAS
18566+0408.
