# Multiple Populations in NGC 1851: Abundance Variations and UV Photometric Synthesis in the Washington and HST/WFC3 Systems

## TL;DR

This study demonstrates that the Washington C filter effectively detects multiple stellar populations in NGC 1851 by combining photometric and abundance data, revealing distinct RGB and MS populations with specific chemical signatures.

## Contribution

It introduces a new photometric method using the Washington C filter to identify multiple populations and links photometric features to detailed chemical abundances in NGC 1851.

## Key findings

- Washington C filter more efficiently detects MPs than traditional filters.
- Two RGB branches show different abundance patterns in elements like Ba, Na, and O.
- Photometric synthesis reproduces observed populations considering abundance and He variations.

## Abstract

The analysis of multiple populations (MPs) in globular clusters, both spectroscopically and photometrically, is key in understanding their formation and evolution. The relatively narrow Johnson U, F336W, and Stromgren and Sloan u filters have been crucial in exhibiting these MPs photometrically, but in Paper I we showed that the broader Washington C filter can more efficiently detect MPs in the test case globular cluster NGC 1851. In Paper I we also detected a double MS that has not been detected in previous observations of NGC 1851. We now match this photometry to NGC 1851's published RGB abundances and find the two RGB branches observed in C generally exhibit different abundance characteristics in a variety of elements (e.g., Ba, Na, and O) and in CN band strengths, but no single element can define the two RGB branches. However, simultaneously considering [Ba/Fe] or CN strengths with either [Na/Fe], [O/Fe], or CN strengths can separate the two photometric RGB branches into two distinct abundance groups. Matches of NGC 1851's published SGB and HB abundances to the Washington photometry shows consistent characterizations of the MPs, which can be defined as an O-rich/N-normal population and an O-poor/N-rich population. Photometric synthesis for both the Washington C filter and the F336W filter finds that these abundance characteristics, with appropriate variations in He, can reproduce for both filters the photometric observations in both the RGB and the MS. This photometric synthesis also confirms the throughput advantages that the C filter has in detecting MPs.