G31.41+0.31 in Band 2
Science Target Overview
Unbiased spectral surveys at high spectral resolution are needed to understand the degree of chemical complexity in star-forming regions and identify the precursors of building blocks of life in the ISM. The high-mass star-forming region G31.41+0.31 at a distance of 3.75 kpc is one of the most chemically rich hot molecular cores located outside the Galactic Center, including numerous complex organic molecules, which in turn is one of the main science cases for Band 2.
G31.41+0.31 is quite compact, with a spatial extent of 2”-3” for a majority of lines. ALMA observed the entire Band 3 (32 GHz bandwidth) with an angular resolution of 1.2”x1.2” (4400 AU x 4400 AU) and a spectral resolution of 0.488 MHz (1.3-1.7 km/s) [project code 2017.1.00501.S]. Therefore, this is an excellent target to demonstrate and check the ALMA Band 2 imaging capability for spectral lines and detection of line forests.
ALMA Data Overview
Observations were carried out with three different MOUS each with a total on-source time of about 40 min. The baseline lengths range from 15 m to 2.4 km. The spectral scan mode was used to cover the frequency range between 67 and 92 GHz as follows:
• MOUS *X42: observed on the 20th of November 2025 using 24 antennas. The spectral scan mode covers the frequency range from 75 to 83 GHz. The typical PWV was 1.7-2.2 mm.
• MOUS *X3e: observed on the 20th of November 2025 using 24 antennas. The spectral scan mode covers the frequency range from 67 to 75 GHz. The typical PWV was 2.0-2.5 mm.
• MOUS *X46: observed on the 23rd of November 2025 using 23 antennas. The spectral scan mode covers the frequency range from 83 to 92 GHz. The typical PWV was 0.7-0.9 mm.
Each spectral scan uses 20 spectral windows with a bandwidth of 468.75 MHz and a spectral resolution of 244.14 kHz to cover the above indicated frequency ranges.
The calibration and imaging were performed using the ALMA Pipeline in a standard manner, including the standard mitigation parameter of the pipeline to generate the images. Self-calibration and auto-masking were used for generating the images unless indicated otherwise. Continuum subtraction was performed using Pipeline’s findcont.
Additional flagging and intervention in the pipeline were needed as follows: The two innermost baselines were flagged for all MOUSs due to being sensitive to a large amount of extended emission that was not possible to recover with the current configuration, which was creating ripples and sidelobes in the images. In addition, we manually added a mask of about 21 pixel radius on the central peak on one of the MOUS (*X46) before running the selfcal stage (the auto-masking was used for the other two MOUSs). Finally, for MOUS *X46 we also flagged a few antennas in four spectral windows on the target and phase calibrator because of a concurrent subarray problem creating a bad correlation, and a few channels in two other spectral windows on the phase calibrator due to an amplitude drop. Details on the specific applied flags are available in the README file.
Note: the flux calibration for Band 2 data is still being refined The flux.csv table provided in the calibration folders needs to be used, as it provides the best values retrieved through a curvature fitting of the calibrator source at the other ALMA bands. In addition, some sidelobe residuals are present in the image cubes due to the UV-coverage of these observations being poorer than for standard observations because of the small number of antennas used during science verification. A large number of iterations and fine tuning of the threshold parameter in the cleaning procedure may improve the image quality. The continuum subtraction may also be optimized if images are manually improved.
The continuum emission is clearly detected and the rms achieved in the image is 0.35, 0.18, 0.37 mJy/beam (for MOUS *X42, *X3e, *X46, respectively) in the continuum image, using only 2.63 GHz bandwidth deemed to be line-free. A line forest is detected with a high signal-to-noise, with a typical rms of about 5 mJy/beam per channel (more details per MOUS in the README).
Using the data for publication
The following statement should be included in the acknowledgement of papers using the dataset listed above: “This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00024.SV. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSTC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ."
Obtaining the Data
The data products are contained in the following directory:
Parent directory for download
The above directory contains README, uncalibrated data, calibrated data, data reduction and imaging scripts, and reference images.