Polarimetric Differential Imaging with VLT/NACO. A comprehensive PDI pipeline for NACO data (PIPPIN)

This dataset consists of the reduced data products generated with PIPPIN (PDI pipeline for NACO data). PIPPIN employs the Polarimetric Differential Imaging (PDI) technique to distinguish the polarised scattered light from the (largely) un-polarised stellar light. As a result, circumstellar dust can be uncovered around young stellar objects. PIPPIN was specifically developed to reduce the archival polarimetric observations made by the VLT/NACO instrument. During its operation, NACO employed different instrument configurations (e.g. half-wave plate or de-rotator usage, Wollaston beam-splitter or wiregrid observations) which are appropriately handled by PIPPIN. As part of the PDI reduction, PIPPIN performs various levels of corrections for instrumental polarisation and crosstalk.

Documentation for the PIPPIN pipeline can be found here. If you use PIPPIN or PIPPIN-reduced data products for your publication, we ask you to cite our paper. 

The dataset is sorted per (potential) young stellar object and sub-divided into the respective nights of observation (if applicable). Within the pipeline_output/ directories, the data are sorted by the utilised observing strategy (<OBS_ID>_<EXPTIME>_<FILTER>/). The _beams.fits files comprise the sky-subtracted, cropped images that are used to generate the PDI data products in the PDI/ directory.

Depending on the observations, the PDI/ directory contains the following files:

Total intensities:

• I.fits: Total intensity \(I = \frac{1}{2}(I_{Q}+I_{U})\).
  [CUBE_I, MEDIAN_I, CUBE_I_CTC, MEDIAN_I_CTC]
• I_Q.fits: Intensity as \(I_Q = \frac{1}{2}(I_{Q^+}+I_{Q^-})\).
  [CUBE_I_Q, MEDIAN_I_Q]
• I_Q+.fits: Intensity of \(Q^+\) measurements.
  [CUBE_I_Q+, MEDIAN_I_Q+]
• I_Q-.fits: Intensity of \(Q^-\) measurements.
  [CUBE_I_Q-, MEDIAN_I_Q-]
• I_U.fits: Intensity as \(I_U = \frac{1}{2}(I_{U^+}+I_{U^-})\).
  [CUBE_I_U, MEDIAN_I_U, CUBE_I_U_CTC, MEDIAN_I_U_CTC]
• I_U+.fits: Intensity of \(U^+\) measurements.
  [CUBE_I_U+, MEDIAN_I_U+, CUBE_I_U+_CTC, MEDIAN_I_U+_CTC]
• I_U-.fits: Intensity of \(U^-\) measurements.
  [CUBE_I_U-, MEDIAN_I_U-, CUBE_I_U+_CTC, MEDIAN_I_U-_CTC]

Stokes parameters:

• Q.fits: Stokes \(Q = \frac{1}{2}(Q^+-Q^-)\). 
  [CUBE_Q, MEDIAN_Q, MEDIAN_Q_IPS, MEDIAN_Q_CTC_IPS]
• Q+.fits: Stokes \(Q^+=I_\mathrm{ord}-I_\mathrm{ext}\).
  [CUBE_Q+, MEDIAN_Q+]
• Q-.fits: Stokes  \(Q^-=I_\mathrm{ord}-I_\mathrm{ext}\). 
  [CUBE_Q-, MEDIAN_Q-]
• U.fits: Stokes \(U = \frac{1}{2}(U^+-U^-)\).
  [CUBE_U, MEDIAN_U, MEDIAN_U_IPS, MEDIAN_U_CTC_IPS]
• U+.fits: Stokes \(U^+=I_\mathrm{ord}-I_\mathrm{ext}\).
  [CUBE_U+, MEDIAN_U+]
• U-.fits: Stokes  \(U^-=I_\mathrm{ord}-I_\mathrm{ext}\). 
  [CUBE_U-, MEDIAN_U-]

Final polarised light products:

• PI.fits: Polarised intensity \(PI = \sqrt{Q^2+U^2}\).
  [PI, PI_R2, PI_IPS, PI_IPS_R2, PI_CTC_IPS, PI_CTC_IPS_R2]
• Q_phi.fits: Azimuthal Stokes parameter \(Q_\phi = -Q\cos 2\phi - U\sin 2\phi\).
  [Q_PHI, Q_PHI_R2, Q_PHI_IPS, Q_PHI_IPS_R2, Q_PHI_CTC_IPS, Q_PHI_CTC_IPS_R2, Q_PHI_UPC_CTC_IPS, Q_PHI_UPC_CTC_IPS_R2]
• U_phi.fits: Azimuthal Stokes parameter \(U_\phi = +Q\sin 2\phi - U\cos 2\phi\).
  [U_PHI, U_PHI_R2, U_PHI_IPS, U_PHI_IPS_R2, U_PHI_CTC_IPS, U_PHI_CTC_IPS_R2, U_PHI_UPC_CTC_IPS, U_PHI_UPC_CTC_IPS_R2]

The values given in brackets [] are the fits extensions. CUBE_ extensions are given for each cycle, while MEDIAN_ indicates the median-combination of the CUBE_ images. The extension _IPS gives the instrumental-polarisation subtracted result, _CTC gives the crosstalk-corrected image, and _UPC gives the \(U_\phi\)-minimised result. _R2-products are scaled by the squared projected radius.

_EXTENDED data products are generated if the position angle was rotated, rather than the half-wave plate, to obtain the linear Stokes parameters. The position-angle rotation permits a broader coverage of the sky, which results in images with the characteristic shape of an eight-pointed star.