A search for OB associations near southern long-period Cepheids. V. AQ Puppis and V620 Puppis

Abstract

A photometric UBV survey is presented for 610 stars in a region surrounding the Cepheid AQ Puppis and centered southwest of the variable, based upon photoelectric measures for 14 stars and calibrated iris photometry of photographic plates of the field for 596 stars. An analysis of reddening and distance for program stars indicates that the major dust complex in this direction is ∼1.8 kpc distant, producing differential extinction described by a ratio of total-to-selective extinction of R = A_V /E_B−V = 3.10±0.20. Zero-age main-sequence fitting for the main group of B-type stars along the line of sight yields a distance of 3.21 ± 0.19 kpc (V₀ − M_V = 12.53 ± 0.13 s.e.). The 29^d.97 Cepheid AQ Pup, of field reddening E_B−V = 0.47 ± 0.07 (E_B−V (B0) = 0.51 ± 0.07), appears to be associated with B-type stars lying within 5′ of it as well as with a sparse group of stars, designated Turner 14, centered south of it at J2000.0 = 07:58:37, −29:25:00, with a mean reddening of E_B−V = 0.81 ± 0.01. AQ Pup has an inferred luminosity as a cluster member of ⟨M_V⟩ = −5.40 ± 0.25 and an evolutionary age of 3 × 10⁷ yr. Its observed rate of period increase of +300.1 ± 1.2 s yr^−1 is an order of magnitude larger than what is observed for Cepheids of comparable period in the third crossing of the instability strip, and may be indicative of a high rate of mass loss or a putative fifth crossing. Another sparse cluster, designated Turner 13, surrounds the newly recognized 2^d.59 Cepheid V620 Pup, of space reddening E_B−V = 0.64 ± 0.02 (E_B−V (B0) = 0.68 ± 0.02), distance 2.88 ± 0.11 kpc (V₀ − M_V = 12.30 ± 0.08 s.e.), evolutionary age 10⁸ yr, and an inferred luminosity as a likely cluster member of ⟨M_V⟩ = −2.74 ± 0.11. V620 Pup is tentatively identified as a first crosser, pending additional observations.