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

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&

Biases on initial mass function determinations. III. Cluster masses derived from unresolved photometry

It is currently common to use spatially unresolved multi-filter broad-band photometry to determine the masses of individual stellar clusters (and hence the cluster mass function, CMF). 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 CMF and I establish that such effects are the largest contributor to the observational uncertainties. An analytical solution, valid in the limit where uncertainties are small, is provided to establish the range of cluster masses over which the CMF slope can be obtained with a given accuracy. The validity of the analytical solution is extended to higher mass uncertainties using Monte Carlo simulations and the Gamma approximation. The value of the Poisson mass is calculated for a large range of ages and a variety of filters for solar-metallicity clusters measured with single-filter photometry. A method that uses the code CHORIZOS is presented to simultaneously derive masses, ages, and extinctions. 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. A revision of some literature results affected by these effects is presented and possible solutions for future observations and analyses are suggested.Comment: Acepted for publication in ApJ, 47 pages, 8 figue

UV Extinction Towards a Quiescent Molecular Cloud in the SMC

Context: The mean UV extinction law for the Small Magellanic Cloud (SMC) is usually taken as a template for low-metallicity galaxies. However, its current derivation is based on only five stars, thus placing doubts on its universality. An increase in the number of targets with measured extinction laws in the SMC is necessary to determine its possible dependence on parameters such as metallicity and star-forming activity. Aims: To measure the UV extinction law for several stars in the quiescent molecular cloud SMC B1-1. Methods: We obtained HST/STIS slitless UV spectroscopy of a 25"x25" field of view and we combined it with ground-based NIR and visible photometry of the stars in the field. The results were processed using the Bayesian photometric package CHORIZOS to derive the visible-NIR extinction values for each star. The unextinguished Spectral Energy Distributions (SEDs) obtained in this way were then used to derive the UV extinction law for the four most extinguished stars. We also recalculated the visible-NIR extinction for the five SMC stars with preexisting UV extinction laws. Results: The UV extinction law for four SMC B1-1 stars within several pc of each other differs significantly from star to star. The 2175 {\AA} bump is moderately strong in one, weak in two, and absent in the fourth.Comment: 10 pages, 5 figures, accepted for publication in Astronomy & Astrophysic