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HL Tau: Band 9, high angular resolution test observation taken in the High Frequency Long Baseline campaign 2017

 

Science Target Overview

Located in the Taurus molecular cloud, HL Tau is a protoplanetary disk system surrounding a young star at a distance of 140 pc (Rebull et al. 2004). This system is very young, and its age is estimated to be less than 1 Myr (Beckwith et al. 1990; Robitaille et al. 2007). It is the first of two complex structure sources observed to examine whether the image quality of B2B phase referencing is as expected in Band 9 when compared with the known high-fidelity image from previous ALMA Science Verification (SV) data. The previous SV observations of HL Tau show substructures of bright rings and dark gaps in the disk in Bands 3, 6, and 7, which could indicate the presence of protoplanets (ALMA Partnership et al. 2015).

ALMA Data Overview 

Two band-to-band (B2B) phase referencing EBs were taken on 2017 November 3 with switching cycle times of 37 and 26 s, respectively, as test observations in the High Frequency Long Baseline Campaign (Asaki et al. 2020). The scan lengths obtained for HL Tau were 27 and 16s, while a 6s scan was obtained on a phase calibrator, QSO J0431+1731, 0.7 deg away from HL Tau. The total observing time was 65 and 100 minutes for the first and second executions, respectively. Combined, there is 45 minutes on-source time for HL Tau. The DGC source QSO J0522−3627 is 56 deg away from the target and was repeated twice in the first EB and three times in the second. The LO1 frequencies for the target and the phase calibrator are 671 (Band 9) and 149 (Band 4) GHz, respectively. The correlator was configured to have eight SPWs in Band 9 with a bandwidth of 2 GHz each using the 90 deg phase switching. The PWV was 0.53 mm. Due to instrumental instabilities, a number of flags were applied so that some of the longest baseline antennas were flagged out; thus, we could not achieve the expected ~10 mas angular resolution with the EBs. We evaluated the image fidelity of our Band 9 data by comparison with the long baseline SV in Band 7 taken in 2014 (ADS/JAO.ALMA #2011.0.00015.SV).

HL Tau in Band 9 was imaged after combining the two EBs. The images delivered use Natural weighting, Briggs robust = 0.5, and natural weighting where baselines shorter than 500kl are excluded. Smaller robust weightings form too sharp beams and resolve out almost all of the extended emission. As published in Asaki et al. 2020, the final beam achieved was 20x18 mas for the natural weigthed images and 14×11 mas when using robust=0.5. Assuming that the central peak of emission is completely optically thick and scaling from the Band 7 data (11.56 mJy/beam, ALMA Partnership et al. 2015), the estimated peak flux density at 671 GHz would be expected to be ~44 mJy/beam. The naturally weighted Band 9 image yields a peak of 24.9 mJy/beam. This suggest out image coherence is around 56%, in agreement with the less than ideal phase stability during the tests. We have not performed self-calibration and also note that the band 9 beam is smaller compared to the Band 7 image; hence, this is likely the lower limit of the coherence. Note the images in this test data delivery differ slightly from the aformentioned publication, as the reduction was made with CASA 5.6.1-8 and imaging uses the 'tclean' algorithm with a fixed mask and shallow cleaning. The resulting beams of the natural and robust 0.5 weighted images are 19x16 mas and 12 x 10 mas respectively. See the readme for more information.

In the Band 9 images, a few clear features related to the bright and dark lanes in the disk can be discerned. Measured along the major axis, at a radius of ~0.''09 (13 au), we see a strong depression of emission, while at ~0.''13 (19 au), there is a slight increase in flux. The features are coincident with D1 and B1, dark and bright features reported by ALMA Partnership et al. (2015). The next dark region at ~0.''22 (31 au) related to feature D2 is only partially visible in the naturally weighted image when contrasted against the somewhat brighter feature to the southeast at ~0.''26 (37 au) that is an arc shape representing the incomplete B2 ring. The Briggs-weighted image resolves out any larger scales. Due to the relatively short on-source time and the handful of short baselines, the poor (u, v) sampling of the largely extended disk structure causes a striped side-lobe pattern throughout our Band 9 image at the 50%-60% level with natural weighting. Spatially the side lobe is roughly co-located with the bright large ring B6; however, its structure is far from complete. Natural weighting is not the optimal choice; however, the robust 0.5 Briggs weighting already begins to resolve the disk bright and dark substructures.

Using the data for publication

The following statement should be included in the acknowledgement of papers using the datasets listed above:

"This paper makes use of the following ALMA data:ADS/JAO.ALMA#2011.0.00005.E. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and MOST 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 downloadable files:

Readme
Uncalibrated data
Calibrated data
Data reduction scripts
Reference images