J/A+A/706/A127 CIT 6 and IRC+10216 30-50GHz spectra (Nguyen-Luong+, 2026)
A spectral-line survey of CIT 6 between 30 and 50 GHz with the eQ receiver
at NRO 45m.
Nguyen-Luong Q., Nakamura F., Taniguchi K., Dinh-V-Trung, Liu S.-Y.,
Shimoikura T., Chiong C.-C., Dobashi K., Hirano N., Yonekura Y.,
Nishimura A., Ogawa H., Chien C., Ho C.-T., Hwang Y.-J., Yeh Y.-T.,
Lai S.-P., Fujii Y., Yamasaki Y., Kawabe R., Nolan A., Dung Pham V.
<Astron. Astrophys. 706, A127 (2026)>
=2026A&A...706A.127N 2026A&A...706A.127N (SIMBAD/NED BibCode)
ADC_Keywords: Stars, giant ; Spectra, radio
Keywords: stars: AGB and post-AGB - stars: carbon - ISM: lines and bands -
ISM: molecules - planetary nebulae: general -
planetary nebulae: individual: CIT 6
Abstract:
At the end of the long H-burning phase (main sequence), low-to
intermediate- mass stars evolve into asymptotic giant branch (AGB)
stars that can have carbon-rich envelopes depending on the initial C/O
ratio. In their circumstellar envelopes (CSEs), dust particles and
molecules are formed and shed back into the interstellar medium.
Therefore, these stars significantly contribute to the Galactic
astrochemical evolution. To shed light on the chemical properties of
carbon-rich CSEs, especially carbon- and silicon- bearing molecules,
we performed parallel spectral-line surveys of IRC+10216 and CIT∼6,
the brightest and second-brightest carbon-rich circumstellar envelopes
on the sky. We conducted 30-50GHz observations toward both sources
using the high-sensitivity wide-band extended Q-band receiver (eQ) of
the Nobeyama 45 m telescope. We analysed the data of CIT∼6 and used
the data of IRC+10216 for comparison. We applied the
rotational-diagram method to derive rotational temperatures and column
densities for HC5N and HC7N. For other molecules, we assumed an
excitation temperature to derive their column densities. Molecular
column densities in CIT∼6 are systematically lower than those in
IRC+10216, typically by about one order of magnitude. Silicon- and
sulfur-bearing species such as SiS and CS show the strongest
depletion, whereas carbon-chain molecules (HCnN, C6H) remain
relatively prominent, indicating that carbon-chain formation is still
efficient in CIT 6. Rotational temperatures are higher in CIT 6,
consistent with emission arising from warmer and more compact regions
of its envelope. Both sources show low 12C/13C ratios and
mildly sub-solar 28Si/29Si values, indicating non-solar
isotopic compositions. Both envelopes display canonical carbon-rich
AGB chemistry and comparable isotopic compositions. CIT 6, however,
shows slightly higher excitation temperatures, stronger carbon-chain
growth, and deeper depletion of Si- and S-bearing species. These
signatures point to a more evolved circumstellar environment, where
dust condensation and shock processing further modulate the molecular
composition.
Description:
In March 2024, we observed two carbon-rich stars, CIT 6 and IRC+10216,
using the NRO 45m telescope equipped with the eQ receiver and the
SAM45 FX-type spectrometer. Observations were conducted in
position-switching mode, with the emission-free reference position
located +30' away from the target in right ascension. To cover the
30-50 GHzfrequency range, we used six different frequency settings,
each covering 0.4GHzx8=3.2GHz with a frequency resolution of 120kHz
and a bandwidth of 500 MHz. The velocity resolution ranges from
0.7km/s at 50GHz to 1.2km/s at30 GHz. For the final data product, we
binned two channels, resulting in a spectral resolution of 240kHz.
During the observation season, only eight arrays of the SAM45
spectrometer were available for the eQ receiver. The beam size (HPBW)
is 37arcsec at 43GHz, which corresponds to 0.022pc and 0.056 pc at the
distances of IRC+10216 and CIT 6, respectively. The main-beam
efficiency was 0.73 at 43GHz. The typical system temperature Tsys
during the observations was in the range of 100-200K (at frequencies
above 40GHz) and 50-80K (at frequencies below 40GHz). The telescope
pointing was checked every 1.5 hours by observing the SiO maser line
(J=1-0, nu=1 and nu=2) from R Leo, maintaining a pointing accuracy
within 3-5arcsec. The total observation time for one frequency setting
was around two hours for the position-switch observations. The
on-source integration time of each setting was about 40 min. The rms
noise levels are 3.8-10.7mK in IRC+10216 and 3.6-21.0mK in CIT 6 at
30-38 GHz, respectively, due to higher system noise temperatures at
the upper end of the band.
Objects:
-------------------------------------------------
RA (2000) DE Designation(s)
-------------------------------------------------
10 16 02.27 +30 34 19.0 CIT 6 = IRC +30219
09 47 57.40 +13 16 43.5 IRC+10216 = V* CW Leo
-------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 33 20 List of spectra
sp/* . 20 Individual spectra
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Name Star name
11- 13 A3 --- Inst [LSB/USB] Instrument
15- 23 A9 --- Band Frequency interval
25- 33 A9 --- FileName Name of the spectrum table in subdirectory sp
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Byte-by-byte Description of file: sp/*
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
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1- 4 I4 --- Seq Sequential number
6- 21 F16.13 GHz Freq Frequency
23- 33 E11.7 K T* Antenna temperature
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Acknowledgements:
Quang Nguyen-Luong, luongquangbmt(at)gmail.com
(End) Patricia Vannier [CDS] 14-Jan-2026