Reanalysis of nearby open clusters using Gaia DR1/TGAS and HSOY (1802.04234v1)

Abstract: Open clusters have long been used to gain insights into the structure, composition, and evolution of the Galaxy. With the large amount of stellar data available for many clusters in the Gaia era, new techniques must be developed for analyzing open clusters, as visual inspection of cluster color-magnitude diagrams is no longer feasible. An automatic tool will be required to analyze large samples of open clusters. We seek to develop an automatic isochrone-fitting procedure to consistently determine cluster membership and the fundamental cluster parameters. Our cluster characterization pipeline first determined cluster membership with precise astrometry, primarily from TGAS and HSOY. With initial cluster members established, isochrones were fitted, using a chi-squared minimization, to the cluster photometry in order to determine cluster mean distances, ages, and reddening. Cluster membership was also refined based on the stellar photometry. We used multiband photometry, which includes ASCC-2.5 BV, 2MASS JHK_s, Gaia G band. We present parameter estimates for all 24 clusters closer than 333 pc as determined by the Catalogue of Open Cluster Data and the Milky Way Star Clusters catalog. We find that our parameters are consistent to those in the Milky Way Star Clusters catalog. We demonstrate that it is feasible to develop an automated pipeline that determines cluster parameters and membership reliably. After additional modifications, our pipeline will be able to use Gaia DR2 as input, leading to better cluster memberships and more accurate cluster parameters for a much larger number of clusters.

• The paper presents an automated isochrone-fitting pipeline that determines cluster membership and key parameters using Gaia DR1/TGAS and HSOY astrometry.
• It employs a Levenberg–Marquardt fitting method with proper motion, parallax, and multiband photometry to refine initial zero-age main sequence estimates.
• Results for 24 clusters show strong consistency with catalog data, highlighting the pipeline’s efficacy and potential for future Gaia DR2 studies.

Reanalysis of Nearby Open Clusters using Gaia DR1/TGAS and HSOY

The research paper "Reanalysis of nearby open clusters using Gaia DR1/TGAS and HSOY" investigates the methodology and outcomes of an automated tool designed to analyze open star clusters amid the influx of data available in the Gaia era. Open clusters are pivotal in understanding stellar evolution and the structure of the Galactic disk; hence, precise methodologies to determine their properties are essential.

Objectives and Methodology

The primary objective of this research was to develop an automated isochrone-fitting procedure that consistently determines cluster membership and fundamental parameters—distance, age, and reddening—for nearby open clusters. The authors employed Gaia DR1/TGAS and HSOY data, which provided high-quality astrometry, combined with multiband photometry from ASCC-2.5, 2MASS, and the Gaia G band, to refine cluster parameters.

The proposed methodology consists of a cluster characterization pipeline with two major segments: determination of cluster membership and subsequent isochrone fitting, accompanied by membership refinement. A Levenberg-Marquardt fitting method was utilized to perform the isochrone fitting. The approach emphasizes the use of proper motion and parallax selections followed by photometric selections for non-member rejection. The initial fitting focuses on deriving a Zero-Age Main Sequence (ZAMS) fit for preliminary parameter estimates, setting the stage for precise isochrone fitting. The pipeline iteratively refines cluster membership, considering unresolved binaries, photometric errors, and statistical analysis to ensure robustness in parameter estimation.

Results and Comparison

The paper produced parameter estimates for 24 nearby clusters located within 333 pc, comparing them with entries from the Milky Way Star Clusters catalog and deriving agreement on various cluster parameters. Statistical comparisons revealed good consistency with the catalog data, albeit with noted deviations in clusters like Ruprecht 147 due to additional early-type stars influencing age estimation. The pipeline demonstrated applicability in different cluster environments, with the degree of the agreement substantiating the automated approach's efficacy.

The results bear particular significance on parallax measurements. Although discrepancies were identified between photometric distances derived from isochrone fitting and TGAS mean parallaxes, these differences were within acceptable limits. The methodology assumes solar metallicity, introducing minor biases, but future enhancements to this approach will incorporate more refined treatments of metallicity variability.

Implications and Future Developments

This paper lays the groundwork for leveraging Gaia data in open cluster analysis, highlighting the importance of meticulous data integration and parameter optimization. The implications extend to enhanced models of the Galactic disk's dynamical and morphological understanding based on precisely characterized open clusters.

Future directions for this research include pipeline adaptations for distant cluster analysis, with particular emphasis on incorporating Gaia DR2 data for improved parallax constraints and photometric precision. Improvements in parameter determination will potentially encompass spectroscopic data where available. There is also an ongoing exploration into using maximum likelihood methods for isochrone fitting as an alternative to least squares, offering potentially enhanced robustness against data outliers.

The paper represents a crucial step towards achieving homogeneous cluster parameter databases, contributing significantly to the collective endeavor of astronomical research in the Gaia era. As methodologies continue to evolve, the enhanced precision in stellar cluster characterization will inevitably unfold new dimensions in understanding Galactic evolution and dynamics.