The Structure and the Distance of Collinder 121 from Hipparcos and Photometry: Resolving the Discrepancy

TL;DR

This study revises the distance estimates of the star cluster Cr 121 using improved Hipparcos data, resolving previous discrepancies and revealing a more extended and distant structure consistent with photometric measurements.

Contribution

The paper introduces an improved algorithm for Hipparcos astrometric solutions, providing more accurate parallaxes that reconcile Hipparcos data with photometric distances for Cr 121.

Findings

Cr 121 is farther than previously thought, at 750-1000 pc.

The extended OB association has a large depth, possibly beyond 1 kpc.

Revised parallaxes align with photometric measurements, resolving earlier discrepancies.

Abstract

We present further arguments that the Hipparcos parallaxes for some of the clusters and associations represented in the Hipparcos catalog should be used with caution in the study of the Galactic structure. It has been already shown that the discrepancy between the Hipparcos and ground based parallaxes for several clusters including the Pleiades, Coma Ber and NGC 6231 can be resolved by recomputing the Hipparcos astrometric solutions with an improved algorithm diminishing correlated errors in the attitude parameters. Here we present new parallaxes obtained with this algorithm for another group of stars with discrepant data - the galactic cluster Cr 121. The original Hipparcos parallaxes led de Zeeuw et al. to conclude that Cr 121 and the surrounding association of OB stars form a relatively compact and coherent moving group at a distance of 550 -- 600 pc. Our corrected parallaxes reveal a different spatial distribution of young stellar populace in this area. Both the cluster Cr 121 and the extended OB association are considerably more distant (750 -- 1000 pc), and the latter has a large depth probably extending beyond 1 kpc. Therefore, not only are the recalculated parallaxes in complete agreement with the photometric uvbybeta parallaxes, but the structure of the field they reveal is no longer in discrepancy with that found by the photometric method.