X-Shooting ULLYSES: Massive stars at low metallicity

II. DR1: Advanced optical data products for the Magellanic Clouds^{★,★★,★★★}

¹ Institute of Astronomy, KU Leuven, Celestijnlaan 200D, 3001 Leuven, Belgium
e-mail: hugues.sana@kuleuven.be
² ESO – European Organisation for Astronomical Research in the Southern Hemisphere, Alonso de Cordova 3107, Vitacura, Santiago de Chile, Chile
³ Instituto de Astrofísica de Canarias, C. Vía Láctea, s/n, 38205 La Laguna, Santa Cruz de Tenerife, Spain
⁴ Universidad de La Laguna, Dpto. Astrofísica, Av. Astrofísico Francisco Sánchez, 38206 La Laguna, Santa Cruz de Tenerife, Spain
⁵ Royal Observatory of Belgium, Avenue Circulaire/Ringlaan 3, 1180 Brussels, Belgium
⁶ Université Libre de Bruxelles, Av. Franklin Roosevelt 50, 1050 Brussels, Belgium
⁷ IAASARS, National Observatory of Athens, 15236 Penteli, Greece
⁸ Institute of Astrophysics FORTH, 71110, Heraklion, Greece
⁹ NAT – Universidade Cidade de São Paulo, Rua Galvão Bueno 868, São Paulo, Brazil
¹⁰ Instituto de Astronomía, Universidad Nacional Autónoma de México, Unidad Académica en Ensenada, Km 103 Carr. Tijuana–Ensenada, Ensenada, B.C., C.P. 22860, Mexico
¹¹ Centro Universitário da FEI, Dept. de Física. Av. Humberto Alencar de Castelo Branco, 3972, São Bernardo do Campo-SP, CEP 09850-901, Brazil
¹² Department of Physics & Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK
¹³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁴ Department of Physics and Astronomy & Pittsburgh Particle Physics, Astrophysics and Cosmology Center (PITT PACC), University of Pittsburgh, 3941 O’Hara Street, Pittsburgh, PA 15260, USA
¹⁵ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
¹⁶ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
¹⁷ Max-Planck-Insitut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
¹⁸ Departamento de Astrofísica, Centro de Astrobiología, (CSIC-INTA), Ctra. Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁹ Max Planck Institute for Astronomy, Königstuhl 17, 69117, Heidelberg, Germany
²⁰ Departamento de Física y Matématicas, Facultad de Ciencias, Universidad de Alcalá, Ctra. Madrid-Barcelona km 33.6, Alcalá de Henares, Madrid, Spain
²¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
²² Armagh Observatory and Planetarium, College Hill, BT61 9DG Armagh, Northern Ireland, UK
²³ Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s.n., 18008 Granada, Spain
²⁴ Faculty of Physics, University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany

Received: 14 July 2023
Accepted: 23 February 2024

Abstract

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 subsolar 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 in 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 artifacts when the SCIENCE and FLUX standard stars are observed on different nights. The improved FLUX standard models and the new grid of anchor points limit artifacts of the response curve correction, for example on 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 are 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 on the order of 2 to 3 km s^-1 is 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, and applying various processing, including absolute flux calibration, telluric removal, normalization 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 the UVB and in 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, and empirical libraries for population synthesis, to the study of the local nebular environment and feedback of massive stars in subsolar metallicity environments.