Alessi 95 and the short-period Cepheid SU Cassiopeiae

Abstract

The parameters for the newly discovered open cluster Alessi 95 are established on the basis of available photometric and spectroscopic data, in conjunction with new observations. Colour excesses for spectroscopically observed B- and A-type stars near SU Cas follow a reddening relation described by E(U − B)/E(B − V) = 0.83 + 0.02E(B − V), implying a value of R = A_V/E(B − V) ≃ 2.8 for the associated dust. Alessi 95 has a mean reddening of E(B−V)(B0)= 0.35 ± 0.02 s.e., an intrinsic distance modulus of V₀ − M_V = 8.16 ± 0.04 s.e. (±0.21 s.d.), d = 429 ± 8 pc, and an estimated age of 10^8.2 yr from zero-age main sequence (ZAMS) fitting of available UBV, CCD BV, NOMAD, and Two Micron All Sky Survey JHK_s observations of cluster stars. SU Cas is a likely cluster member, with an inferred space reddening of E(B − V) = 0.33 ± 0.02 and a luminosity of ⟨M_V⟩ = −3.15 ± 0.07 s.e., consistent with overtone pulsation (P_FM = 2.75 d), as also implied by the Cepheid's light-curve parameters, rate of period increase and Hipparcos parallaxes for cluster stars. There is excellent agreement of the distance estimates for SU Cas inferred from cluster ZAMS fitting, its pulsation parallax derived from the infrared surface brightness technique and Hipparcos parallaxes, which all agree to within a few per cent.