CCD UBV(RI)CUBV(RI)_{C} Photometry of Twenty Open Clusters

Fundamental astrophysical parameters have been derived for 20 open clusters (O\!Cs) using CCD~$U\!BV\!(RI)_C$ photometric data observed with the 84~cm telescope at the San Pedro M\'artir National Astronomical Observatory, M\'exico. The interstellar reddenings, metallicities, distances, and ages have been compared to the literature values. Significant differences are usually due to the usage of diverse empirical calibrations and differing assumptions, such as concerning cluster metallicity, as well as distinct isochrones which correspond to differing element-abundance ratios, internal stellar physics, and photometric systems. Different interstellar reddenings, as well as varying reduction and cluster-membership techniques, are also responsible for these kinds of systematic differences and errors. The morphological ages, which are derived from the morphological indices ($\delta V$ and $\delta 1$) in the CM diagrams, are in good agreement with the isochrone ages of 12 O\!Cs, those with good red clump (RC) and red giant (RG) star candidates. No metal abundance gradient is detected for the range $6.82 \leq R_{GC} \leq 15.37$ kpc, nor any correlation between the cluster ages and metal abundances for these 20 O\!Cs. Young, metal-poor O\!Cs, observed here in the third Galactic quadrant, may be associated with stellar over-densities, such as that in Canis Major (Martin et al.) and the Monoceros Ring (Newberg et al.), or signatures of past accretion events, as discussed by Yong et al. and Carraro et al.Comment: 68 pages, 33 figures, 15 tables. Accepted in New Astronom

Optical Counterparts of ULXs and Their Host Environments in NGC 4490/4485

We report the identification of the possible optical counterparts of five out of seven Ultraluminous X-ray Sources (ULXs) in NGC 4490/4485 galaxy pair. Using archival Hubble Space Telescope ({\it HST}) imaging data, we identified a single optical candidate for two ULXs (X-4 and X-7) and multiple optical candidates for the other three ULXs (X-2, X-3 and X-6) within $\sim$ 0\farcs2 error radius at the 90\% confidence level. Of the two remaining ULXs, X-1 has no {\it HST} imaging data and photometry could not be performed due to the position of X-5 in NGC4490. Absolute magnitudes ($M_{V}$) of the optical candidates lie between $-5.7$ and $-3.8$. Color-Magnitude Diagrams (CMDs) have been used to investigate the properties of counterparts and their environments. The locations of the counterparts of X-2, X-4, and X-6 suggest possible association with nearby group of stars while others have no association with a star cluster or group of stars. For comparison purposes, we analyzed previously unused three archival XMM-Newton observations. The long-term X-ray light curves of the sources (except transient X-7) show variability by a factor of three in a time scale more than a decade. The use of disk blackbody model for the mass of the compact objects indicates that these objects might have masses most likely in the range 10$-$15 M_{\sun}.Comment: 20 pages, 13 figures, submitted to Ap

UI CCD photometry of Berkeley 55 open cluster

Fundamental astrophysical parameters have been derived for Berkeley 55 (Be 55) open cluster based on the UBVI charge-coupled device (CCD) photometric data observed using a AZT-22 1.5 m telescope at Maidanak Astronomical Observatory (MAO) in Uzbekistan. The mean reddening is obtained as E(B-V)=1.77 +/- 0.10mag from early-type members. The zero-age main sequence fitting in the QV lambda - Q ' diagrams indicates the distance modulus, (V0-MV)=12.4 +/- 0.20. This photometric distance is consistent with the distances of Gaia EDR3 (d=3.09 +/- 0.16 and period-luminosity relation (d= 2.78 +/- 0.32 kpc) of its Cepheid S5 within the uncertainties. This distance also locates the cluster near the Perseus spiral arm. The Geneva isochrone fittings to the Hertzsprung-Russell diagram and observational colour-magnitude diagrams derive turn-off age, 85 +/- 13 Myr, by taking care of five red supergiants/bright giants. The possible inconsistencies in the locations of the bright giants with the rotating/non-rotating isochrones may be due to both the age spread of stars in young open clusters and the diversity in rotational velocities