Comments on the cluster main-sequence fitting method. I. The distance of Ruprecht 44.
Abstract
A new zero-age main sequence (ZAMS) fit is presented for the young cluster Ruprecht 44 which is based upon: (i) a combination of all photoelectric UBV observations for cluster members using the data of Miller and McCarthy (1974) as a reference system; (ii) the adoption of a reddening relation which fits the color excess data for spectroscopically observed cluster stars and (iii) the adoption of Turner's (1976a) ZAMS. The derived cluster distance is 4.66 ± 0.11 kpc, which agrees with the distance inferred from Hβ and/or Hγ measures for bright cluster members (mean distance = 4.61±0.11 kpc from these three estimates) and confirms the membership of Ruprecht 44 in Pup OB2 (as suggested by Havlen 1976), but which is ∼50% smaller than the value derived by FitzGerald and Moffat (1976). This discrepancy is attributed to FitzGerald and Moffat's use of Blaauw's (1963) ZAMS and the somewhat bluer intrinsic colors found by them for cluster members, which for Ruprecht 44 are found to result in overestimates in distance by factors of ∼15% and ∼30%, respectively. On the basis of these findings, suggestions are made for improvements to the cluster main-sequence fitting method. The cases for possible membership of the WN 4 star HD 65865 and the 30d Cepheid AQ Pup in the Ruprecht OB2 complex are reexamined, and it is concluded that HD 65865 is a likely member of the complex, in which case its luminosity is found to be Mυ ≅ -3.5. In contradiction with previous investigations, however, AQ Pup is suggested to belong to neither Pup OB2 nor Pup OB1 on the basis of available observational data.
- Publication:
-
The Astronomical Journal
- Pub Date:
- February 1981
- DOI:
- 10.1086/112877
- Bibcode:
- 1981AJ.....86..222T
- Keywords:
-
- Curve Fitting;
- Distance;
- Main Sequence Stars;
- Star Clusters;
- Stellar Spectrophotometry;
- Ultraviolet Spectroscopy;
- B Stars;
- Early Stars;
- Electrophotometers;
- Giant Stars;
- H Beta Line;
- Hertzsprung-Russell Diagram;
- O Stars;
- Stellar Evolution;
- Stellar Luminosity;
- Astronomy