Isochrone fitting of Galactic globular clusters – I. NGC 5904

Abstract

We present new isochrone fits to colour-magnitude diagrams of the Galactic globular cluster NGC 5904 (M5). We utilise 29 photometric bands from the ultraviolet to midinfrared by use of the data from the Hubble Space Telescope, Gaia DR2, Wide-field Infrared Survey Explorer, Sloan Digital Sky Survey (SDSS), and other photometric data. In our isochrone fitting we use the PAdova and TRieste Stellar Evolution Code, the MESA Isochrones and Stellar Tracks, the Dartmouth Stellar Evolution Program, and a Bag of Stellar Tracks and Isochrones both for the solar-scaled and enhanced He and α abundances with a metallicity about [Fe/H]= −1.33 adopted from the literature. All tools provide us with estimates of the distance, age, and extinction law to the cluster. The best-fit distance, true distance modulus, and age are 7.4 ± 0.3 kpc, 14.34 ± 0.09 mag, and 12.15 ± 1.00 Gyr, respectively. The derived distance agrees with the literature, including the Gaia DR2 parallax with its known global zero-point correction. All the data and models, except some UV and SDSS data, agree with the extinction law of Cardelli-Clayton-Mathis with R V = 3.60 ± 0.05 and A V = 0.20 ± 0.02 mag. This extinction is twice as high as generally accepted due to a rather high extinction between 625 and 2000 nm. An offset of the model colours instead of the high extinction in this range is a less likely, yet possible explanation of the discovered large deviations of the isochrones from the data.

Mapping the Interstellar Reddening and Extinction toward Baade’s Window Using Minimum Light Colors of ab-type RR Lyrae Stars: Revelations from the De-reddened Color–Magnitude Diagrams

“…A very recent determination of the distance to M5 by Gontcharov et al (2019) gives (m − M ) 0 = 14.34 ± 0.09 by multi-band isochrone and main-sequence fitting, which, if adopted, would decrease R 0 to 9.04 kpc. in the Gaia era, we should look to astrometric measurements for a more definitive distance determination.…”

Accurate distances to Galactic globular clusters through a combination of Gaia EDR3, HST and literature data

“…Distances to globular clusters are determined either through fits of their color-magnitude diagrams (CMDs) with theoretical isochrones (e.g Ferraro et al 1999;Dotter et al 2010;Gontcharov, Mosenkov & Khovritchev 2019; Valcin E-mail: h.baumgardt@uq.edu.au et al 2020), or by using variable stars that follow known relations between their periods and absolute luminosities like RR Lyrae stars (e.g Bono, Caputo & Di Criscienzo 2007;Hernitschek et al 2019), Type II Cepheids (Matsunaga et al 2006) or Mira type variables (Feast, Whitelock & Menzies 2002). In order to avoid circularity, the absolute luminosities of these stars need to be determined independently from globular cluster distances by using for example theoretical models (e.g Catelan, Pritzl & Smith 2004) or Hipparcos or Gaia parallaxes (Fernley et al 1998;Neeley et al 2019;Ripepi et al 2019).…”

When Does the Onset of Multiple Stellar Populations in Star Clusters Occur? Detection of Enriched Stellar Populations in NGC 2121

“…Recently, Barker & Paust (2018) reported a problem associated with isochrone fitting to HST photometry in UV-optical-IR passbands. This problem has also been recognized by the community at large (e.g., Gontcharov et al 2019;Howes et al 2019). Specifically, model parameters determined through isochrone fitting to the optical-IR CMD cannot be used to adequately describe photometric measurements involving UV passbands.…”