Abstract
Aims. We demonstrate that reliable photometric distances of stellar clusters, and more generally of stars, can be obtained using pseudomagnitudes and rough spectral type without having to correct for visual absorption. Methods. We determine the mean absolute pseudomagnitude of all spectral (sub)types between B and K. Distances are computed from the difference between the star’s observed pseudomagnitude and its spectral type’s absolute pseudomagnitude. We compare the distances of 30 open clusters thus derived against the distances derived from TGAS parallaxes. Results. Our computed distances, up to distance modulus 12, agree within 0.1 mag rms with those obtained from TGAS parallaxes, proving excellent distance estimates. We show additionally that there are actually two markedly different distances in the cluster NGC 2264. Conclusions. We suggest that the pseudomagnitude distance estimation method, which is easy to perform, can be routinely used in all large-scale surveys where statistical distances on a set of stars, such as an open cluster, are required.
Highlights
Accurate measurements of distances in the universe are arguably a pillar of Astrophysics, and astronomers are all conscious of the fundamental importance of accuracy for the first steps of the cosmic distance ladder: distances to the nearby stars and star clusters of our Galaxy
We further used the parallaxes of the stars surveyed by the HIPPARCOS satellite (ESA 1997; van Leeuwen 2007) and their pseudomagnitudes to estimate the mean absolute pseudomagnitudes (APMs) of main sequence (MS) stars as a function of spectral type
Select all the TGAS MS stars with a parallax error of less than 10%, with or without luminosity class selection depending on the degree of confusion; 2. for each spectral type, keep only the stars located at a distance smaller than that at which the APMs start to decrease; 3. fit the resulting APM distributions with a set of three Gaussian functions of the same variance; 4. of the three Gaussian positions, keep that of the single stars, i.e., the highest value, as the APM’s value
Summary
Accurate measurements of distances in the universe are arguably a pillar of Astrophysics, and astronomers are all conscious of the fundamental importance of accuracy for the first steps of the cosmic distance ladder: distances to the nearby stars and star clusters of our Galaxy. We further used the parallaxes of the stars surveyed by the HIPPARCOS satellite (ESA 1997; van Leeuwen 2007) and their pseudomagnitudes to estimate the mean absolute pseudomagnitudes (APMs) of main sequence (MS) stars as a function of spectral type This served us to derive, as an example, the distance of the Pleiades open cluster: 139 ± 1.2 pc (Chelli & Duvert 2016) from 360 stars, in good agreement with the recent values of 136.2±1.2 (Melis et al 2014) and 133.7 ± 4.5 pc (Gaia Collaboration 2017; hereafter fvl17) based on the first results of the Gaia satellite, TGAS (Gaia Collaboration 2016a,b).
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