X-Shooting ULLYSES: Massive stars at low metallicity.
II. DR1: Advanced optical data products for the Magellanic Clouds
Sana, H.; Tramper, F.; Abdul-Masih, M.; Blomme, R.; Dsilva, K.; Maravelias, G.; Martins, L.; Mehner, A.; Wofford, A.; Banyard, G.; Barbosa, C. L.; Bestenlehner, J.; Hawcroft, C.; John Hillier, D.; Todt, H.; Larkin, C. J. K.; Mahy, L.; Najarro, F.; Ramachandran, V.; Ramírez-Tannus, M. C.; Rubio-Díez, M. M.; Sander, A. A. C.; Shenar, T.; Vink, J. S.; Backs, F.; Brands, S. A.; Crowther, P.; Decin, L.; de Koter, A.; Hamann, W. -R.; Kehrig, C.; Kuiper, R.; Oskinova, L.; Pauli, D.; Sundqvist, J.; Verhamme, O.
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.
This paper in ADS