Perfecting our set of spectrophotometric standard DA white dwarfs

We verified for photometric stability a set of DA white dwarfs with Hubble Space Telescope magnitudes from thenear-ultraviolet to the near-infrared and ground-based spectroscopy by using time-spaced observations from theLas Cumbres Observatory network of telescopes. The initial list of 38 stars was whittled to 32final ones, whichcomprise a high-quality set of spectrophotometric standards. These stars are homogeneously distributed around thesky and are all fainter thanr~16.5 mag. Their distribution is such that at least two of them would be available tobe observed from any observatory on the ground at any time at airmass less than 2. Light curves and differentvariability indices from the Las Cumbres Observatory data were used to determine the stability of the candidatestandards. When available, Pan-STARRS1, Zwicky Transient Facility, and TESS data were also used to confirmthe star classification. Our analysis showed that four DA white dwarfs may exhibit evidence of photometricvariability, while afifth is cooler than our established lower temperature limit, and a sixth star might be a binary. Insome instances, due to the presence of faint nearby red sources, care should be used when observing a few of thespectrophotometric standards with ground-based telescopes. Light curves andfinding charts for all the stars areprovided.
This study was supportedby NASA through grant O1904 from the Space TelescopeScience Institute, which is operated by AURA, Inc., underNASA contract NAS 5-26555 and the Space Telescope ScienceInstitute. The analysis was also supported by the DDRF grantD0001.82481. E.O. was also partially supported by the NSFthrough grant AST-1815767. R.R. received funding from thepostdoctoral fellowship program Beatriu de Pinós, funded bythe Secretary of Universities and Research(Government ofCatalonia)and by the Horizon 2020 program of research andinnovation of the European Union under the Maria Skło-dowska-Curie grant agreement No. 801370. C.S. is supportedby the US DOE through award DE-SC0007881. This work hasmade use of data from the European Space Agency(ESA)mission Gaia(https://www.cosmos.esa.int/gaia), processedby the Gaia Data Processing and Analysis Consortium(DPAC;https://www.cosmos.esa.int/web/gaia/dpac/consortium).Funding for the DPAC has been provided by nationalinstitutions, in particular the institutions participating in theGaia Multilateral Agreement. This publication makes use ofVOSA, developed under the Spanish Virtual Observatoryproject supported by the Spanish MINECO through grantAyA2017-84089. VOSA has been partially updated by usingfunding from the European Union’s Horizon 2020 Research andInnovation Programme, under grant Agreement No. 776403(EXOPLANETS-A). This work includes data from the AsteroidTerrestrial-impact Last Alert System(ATLAS)project. ATLAS isprimarily funded to search for near-earth asteroids through NASAgrants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575;byproducts of the NEO search include images and catalogs fromthe survey area. The ATLAS science products have been madepossible through the contributions of the University of HawaiiInstitute for Astronomy, the Queen’s University Belfast, the SpaceTelescope Science Institute, and the South African AstronomicalObservatory. G.N. and K.M. gratefully acknowledge support fromNASA under grant 80NSSC20K04
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