Calibration of the photometric G passband for Gaia Data Release 1

Context. On September 2016 the first data from Gaia were released (DR1). The first release included photometry for over 10^9 sources in the very broad G system. Aims. To test the correspondence between G magnitudes in DR1 and the synthetic equivalents derived using spectral energy distributions from observed and model spectrophotometry. To correct the G passband curve and to measure the zero point in the Vega system. Methods. I have computed the synthetic G and Tycho-2 BV photometry for a sample of stars using the Next Generation Spectral Library (NGSL) and the Hubble Space Telescope (HST) CALSPEC spectroscopic standards. Results. I have found that the nominal G passband curve is too blue for the DR1 photometry, as shown by the presence of a color term in the comparison between observed and synthetic magnitudes. A correction to the passband applying a power law in lambda with an exponent of 0.783 eliminates the color term. The corrected passband has a Vega zero point of 0.070$\pm$0.004 magnitudes.Comment: Four-page letter, accepted for publication in A&

The range of validity of cluster masses and ages derived from broad-band photometry

I analyze the stochastic effects introduced by the sampling of the stellar initial mass function (SIMF) in the derivation of the individual masses and the cluster mass function (CMF) from broad-band visible-NIR unresolved photometry. The classical method of using unweighted UBV photometry to simultaneously establish ages and extinctions of stellar clusters is found to be unreliable for clusters older than approx. 30 Ma, even for relatively large cluster masses. On the other hand, augmenting the filter set to include longer-wavelength filters and using weights for each filter increases the range of masses and ages that can be accurately measured with unresolved photometry. Nevertheless, a relatively large range of masses and ages is found to be dominated by SIMF sampling effects that render the observed masses useless, even when using UBVRIJHK photometry.Comment: 6 pages, 5 figures, to appear in IAU Symposium 266 on Star Cluster

A reanalysis of the Gaia Data Release 2 photometric sensitivity curves using HST/STIS spectrophotometry

CONTEXT: The second Gaia data release (DR2) took place on April 2018. DR2 included photometry for more than 1.3 10^9 sources in G, BP, and RP. Even though Gaia DR2 photometry is very precise, there are currently three alternative definitions of the sensitivity curves that show significative differences. AIMS: The aim of this paper is to improve the quality of the input calibration data to produce new compatible definitions of the three bands and to identify the reasons for the discrepancies between previous definitions. METHODS: We have searched the HST archive for STIS spectra with G430L+G750L data obtained with wide apertures and combined them with the CALSPEC library to produce a high quality SED library of 122 stars with a broad range of colors, including three very red stars. We have used it to compute new sensitivity curves for G, BP, and RP using a functional analytical formalism. RESULTS: The new curves are significantly better than the two previous attempts, REV and WEI. For G we confirm the existence of a systematic bias in magnitude and correct a color term present in REV. For BP we confirm the need to define two magnitude ranges with different sensitivity curves and measure the cut between them at G = 10.87 with a significant increase in precision. The new curves also fit the data better than either REV or WEI. For RP we obtain a sensitivity curve that better fits the STIS spectra and we find that the differences with previous attempts reside in a systematic effect between ground-based and HST spectral libraries. Additional evidence from color-color diagrams indicate that the new sensitivity curve is more accurate. Nevertheless, there is still room for improvement in the accuracy of the sensitivity curves because of the current dearth of good-quality red calibrators: adding more to the sample should be a priority before Gaia data release 3 takes place.Comment: Accepted for publication in A&A, revised version with minor change