Photometric distances to young stars in the inner galactic disk

TL;DR

This study uses deep CCD photometry and spectroscopy to analyze the distribution of young stars near Trumpler 27, revealing it is not a true cluster but a superposition of stars along the line of sight, with implications for understanding galactic structure.

Contribution

It demonstrates that what was thought to be an open cluster is actually a superposition of stars at various distances, providing new insights into the distribution of young stars and spiral arms in the inner Galaxy.

Findings

Trumpler 27 is not a true cluster but a superposition of stars.

Young stars are distributed over several kiloparsecs, not confined to a single cluster.

Some stellar groups may be linked to inner spiral arms, including the Scutum-Crux and Near 3 kpc arms.

Abstract

We present results of the first extensive and deep CCD photometric survey carried out in the field of the open cluster Trumpler 27, an object in the constellation of Sagittarius not far from the Galaxy center. We look for young stars clumps that might plausibly be associated with spiral structure. Wide-field UBVI photometry combined with infrared information allows us to reconstruct the distribution in reddening and distance of young stars in the field using the Color-Color and Color-Magnitude diagrams. The analysis of our data, combined too with extensive spectroscopy taken from literature shows that the real entity of Trumpler 27 as an open cluster is far from being firmly stated. In fact, instead of finding a relatively compact group of stars confined to a small distance range, we found that stars associated to Trumpler 27 are, indeed, a superposition of early type stars seen along the line of sight extending over several kiloparsecs beyond even the center of the galaxy. We demonstrate that at each distance range it becomes possible to generate a color-magnitude diagram resembling that of an open cluster. This way, our analysis indicates that what was considered an open cluster characterized by a significant age spread is a stellar continuum that reaches its maximum number of stars at approximately 3.5 kpc from the Sun, the distance of the Scutum-Crux arm approximately. At the same time, and after analyzing the way early type stars distribute with distance, we found some of these stellar groups may be linked, within the distance errors, with other inner spiral arms of our galaxy, including the Near 3 kpc arm at approximately 5 kpc from the Sun. However, very young stars by themselves do not seem to trace strongly the inner spiral arms since they are distributed evenly across several kiloparsecs toward the center of the Galaxy.