Abstract
Abstract Recent spectroscopic observations by sensitive radio telescopes require accurate molecular spectral line frequencies to identify molecular species in a forest of lines detected. To measure rest frequencies of molecular spectral lines in the laboratory, an emission-type millimeter and submillimeter-wave spectrometer utilizing state-of-the-art radio-astronomical technologies is developed. The spectrometer is equipped with a 200 cm glass cylinder cell, a two-sideband (2SB) superconductor-insulator-superconductor (SIS) receiver in the 230 GHz band, and wide-band auto-correlation digital spectrometers. By using the four 2.5 GHz digital spectrometers, a total instantaneous bandwidth of the 2SB SIS receiver of 8 GHz can be covered with a frequency resolution of 88.5 kHz. Spectroscopic measurements of CH3CN and HDO are carried out in the 230 GHz band so as to examine the frequency accuracy, stability, sensitivity, as well as the intensity calibration accuracy of our system. As for the result of CH3CN, we confirm that the frequency accuracy for lines detected with sufficient signal-to-noise ratio is better than 1 kHz, when the high-resolution spectrometer having a channel resolution of 17.7 kHz is used. In addition, we demonstrate the capability of this system by spectral scan measurement of CH3OH from 216 GHz to 264 GHz. We assign 242 transitions of CH3OH, 51 transitions of 13CH3OH, and 21 unidentified emission lines for 295 detected lines. Consequently, our spectrometer demonstrates sufficient sensitivity, spectral resolution, and frequency accuracy for in-situ experimental-based rest frequency measurements of spectral lines for various molecular species.
Highlights
More than 200 molecular species are known as interstellar molecules, as complied in the Cologne Database for Molecular Spectroscopy (CDMS: Muller et al 2001; Muller et al 2005), most of which are detected through radioastronomical observations of rotational transition lines
Most of them are likely high excitation lines and/or isotopologue lines of known interstellar molecules not listed in the molecular spectral line databases such as CDMS and Microwave Spectral Line Catalog provided by Jet Propulsion Laboratory (JPL) (Pickett et al 1998), some of them could be spectral lines of new interstellar molecules
The Gaussian function was employed for the spectral line shape function because the Doppler effect due to the MaxwellBoltzmann distribution makes a dominant contribution to the line shape in the measured frequency range
Summary
More than 200 molecular species are known as interstellar molecules, as complied in the Cologne Database for Molecular Spectroscopy (CDMS: Muller et al 2001; Muller et al 2005), most of which are detected through radioastronomical observations of rotational transition lines. Thanks to continuous development of sensitive receivers as well as wideband and high frequency-resolution spectrometers for radio astronomy, we can scan a wide range of frequency and observe many spectral lines of various molecular species with reasonable observation time Taking advantage of this progress, many spectral line surveys have been conducted during the last two decades toward various types of astronomical sources such as hot cores (e.g., Schilke et al 1997; Schilke et al 2001; Tercero et al 2010; Watanabe et al 2015), low-mass protostellar cores (e.g., Caux et al 2011; Watanabe et al 2012; Lindberg et al 2015; Jørgensen et al 2016; Yoshida et al 2019), dark clouds (e.g., Kaifu et al 2004), shocked regions (e.g., Sugimura et al 2011; Yamaguchi et al 2012), Asymptotic Giant Branch stars (e.g., Cernicharo et al 2000), nearby galaxies (e.g.,Martın et al 2006; Aladro et al 2015; Takano et al 2019; Watanabe et al 2014; Watanabe et al 2019), and low-metallicity dwarf galaxies (e.g., Nishimura et al 2016a; Nishimura et al 2016b). We report the apparatus and the basic performance of our emission-type spectrometer covering the 210-270 GHz band and some demonstrative measurements for CH3CN, HDO, and CH3OH
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