Paper II of the XShootU collaboration is finally accepted at A&A! That is a cornerstone paper for the collaboration since it presents the analysis of the observations and provides the final scientific products with which the whole collaboration (and everybody else) can work with.
X-Shooting ULLYSES: Massive Stars at low metallicity II. DR1: Advanced optical data products for the Magellanic Clouds
H. Sana, F. Tramper, M. Abdul-Masih, R. Blomme, K. Dsilva, G. Maravelias, L. Martins, A. Mehner, A.
Wofford, G. Banyard, C.L. Barbosa, J. Bestenlehner, C. Hawcroft, D. John Hillier, H. Todt, C.J.K.
Larkin, L. Mahy, F. Najarro, V. Ramachandran, M.C. Ramírez-Tannus, M.M. Rubio-Díez, A.A.C.
Sander, T. Shenar, J.S. Vink, F. Backs, S. A. Brands, P. Crowther, L. Decin, A. de Koter, W.-R.
Hamann, C. Kehrig, R. Kuiper, L. Oskinova, D. Pauli, J. Sundqvist, O. Verhamme, and the XSHOOT-U
collaboration
Context. The XShootU project aims to obtain ground-based optical to near-infrared spectroscopy of all targets observed by the Hubble Space Telescope (HST) under the Director’s Discretionary program ULLYSES. Using the medium resolution spectrograph X-shooter, spectra of 235 OB and Wolf-Rayet (WR) stars in sub-solar metallicity environments have been secured. The bulk of the targets belong to the Large and Small Magellanic Clouds, with the exception of three stars in NGC 3109 and Sextans A.
Aims. This second paper of the series focuses on the optical observations of Magellanic Clouds targets. It describes the uniform reduction of the UVB (300 − 560 nm) and VIS (550 − 1020 nm) XShootU data as well as the preparation of advanced data products that are suitable for homogeneous scientific analyses.
Methods. The data reduction of the raw data is based on the ESO CPL X-shooter pipeline. We paid particular attention to the determination of the response curves. This required equal flat-fielding of the science and flux standard star data and the derivation of improved flux standard models. The pipeline products were then processed with our own set of routines to produce a series of advanced data products. In particular, we implemented slit-loss correction, absolute flux calibration, (semi-)automatic rectification to the continuum, and a correction for telluric lines. The spectra of individual epochs were further corrected for the barycentric motion, re-sampled and co-added, and the spectra from the two arms were merged into a single flux calibrated spectrum covering the entire optical range with maximum signal-to-noise ratio.
Results. We identify and describe an undocumented recurrent ghost visible on the raw data. We present an improved flat-fielding strategy that limits artefacts when the science and flux standard stars are observed on different nights. The improved flux standard models and the new grid of
anchor points allow to limit artefacts of the response curve correction on, e.g., the shape of the wings of the Balmer lines, from a couple of per cent of the continuum level to less than 0.5%. We confirm the presence of a radial velocity shift of about 3.5 km s−1 between the UVB and the VIS arm of X-shooter and that there is no short term variations impacting the RV measurements. RV precision better than 1 km s−1 can be obtained on sharp telluric lines while RV precision of the order of 2 to 3 km s−1 are obtained on data with the best S /N.
Conclusions. For each target observed by XShootU, we provide three types of data products: (i) two-dimensional spectra for each UVB and VIS exposure before and after correction for the instrument response; (ii) one-dimensional UVB and VIS spectra as produced by the X-shooter pipeline before and after response-correction, as well as after applying various processing, including absolute flux calibration, telluric removal, normalisation and barycentric correction; and (iii) co-added flux-calibrated and rectified spectra over the full optical range, for which all available XShootU exposures were combined. For the large majority of the targets, the final signal-to-noise ratio per resolution element is above 200 in both the UVB and the VIS co-added spectra. The reduced data and advanced scientific data products are made available to the community. Together with the HST UV ULLYSES data, they should enable various science goals, from detailed stellar atmosphere and stellar wind studies, to empirical libraries for population synthesis, to study of the local nebular environment and feedback of massive stars in sub-solar metallicity environments.
arXiv: 2402.16987