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

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 already been 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 Ӎ550-600 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 parallaxes, but the structure of the field they reveal is no longer in discrepancy with that uvbyb found by the photometric method.

The interstellar Ca II distance scale

“…The errors in the Hipparcos parallaxes may be correlated in objects located close (1 • ) on the sky; this is the probable cause of the surprisingly large discrepancy between the distance of the Pleiades estimated from the Hipparcos data and from other sources (Munari et al 2004;Pan et al 2004;Percival et al 2005;Soderblom et al 2005, and references therein). The same effect has been recently reported by Kaltcheva & Makarov (2007) for Collinder 121. As long as there is no systematic, global offset affecting Hipparcos parallaxes, the local correlation of errors should have only limited influence on this work.…”

“…Kaltcheva & Makarov (2007) show that, despite the correction, the combined effect of the correlated parallax errors and the bias resulting from the removal of the negative parallaxes can be significant ( (3) is 2400−5800 pc. The M bol of HD 93250 and HD 303308 are given by Sanchawala et al (2007) as −11.3 and −10.4 respectively, at the distance of 2500 pc.…”

A uvbyβ photometric analysis of the Monoceros star‐forming field

“…outside of the limit of the Balona & Shobbrook (1984) calibration). Note that the procedure outlined here provides photometric distances in excellent agreement with the recalculated Hipparcos parallaxes (Kaltcheva & Makarov 2007).…”

3D maps of the local ISM from inversion of individual color excess measurements

“…This allows checking that those structures have their counterpart in the form of a dense region (for the clouds) or a cavity closely bounded by a dense region (for the OB associations) in the inverted maps. We did not include Collinder 121 whose distance has been the object of several discussions (e.g., Kaltcheva & Makarov 2007;Burningham et al 2003) and maybe much larger than 500 pc. The new H determinations of van Leeuwen (2007) indeed show that there are various distance ranges for the members.…”