Statistics – Computation
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aj....108..222n&link_type=abstract
The Astronomical Journal, vol. 108, no. 1, p. 222-246
Statistics
Computation
144
Metallicity, Periodic Variations, Stellar Luminosity, Stellar Magnitude, Variable Stars, Cepheid Variables, Globular Clusters, Lyra Constellation, Tables (Data)
Scientific paper
Period-luminosity-metallicity (P-L-(Fe/H) relations are presented for RR Lyrae stars, Pop. II Cepheids, anomalous Cepheids and SX Phe stars pulsating in the fundamental (F) and first-overtone (H) modes. The relations were derived by fitting regression lines to the observed pulsation periods and mean B, V, and K magnitudes of over 1200 stars in approximately 40 stellar systems. Analysis of covariance methods, which allow the simultaneous computation of more than one P-L-(Fe/H) relation, were used to estimate the slopes and intercepts. Of the 24 possible P-L-(Fe/H) relations for the four kinds of stars, two pulsation modes, and three passbands considered here, 18 relations have been derived-the others could not be derived because of a lack of photometry in one or more of the three passbands. The slopes for the F and H pulsators were tested for departures from equality for all types of stars and passbands; the results suggest that the observations are consistent with the assumption that, for each kind of star (except possibly the Pop. II Cepheids), the P-L-(Fe/h) relations for the F and H pulsation modes are parallel but vertically offset, with a family of lines corresponding to a range of metallicities. Pulsation modes and absolute magnitudes are presented for the non-RR Lyrae variable stars considered in the analysis, and distances are estimated for the program clusters. It is well established from previous studies that the P-L relations for RR Lyrae stars are approximately flat for the B passband, and have a slope delta mk/ delta log P approximately 2.4 for the K passband. We recover these slopes and find that the P-L-(Fe/H) relation in V has an intermediate slope, delta mv/delta log P = -0.52 plus or minus 0.11. A similar dependence of slope on passband is seen for classical Cepheids (see Madore & Freedman, PASP, 103, 933 (1991). The available B, V photometry for approximately 40 of the known globular cluster Cepheids are found to be consistent with Arp's AJ, 60, 1 (1955) assumption that Pop. II Cepheids pulsate in the F and H modes (and not in the F mode only, as has been often assumed). The estimated slopes of the corresponding P-L-(Fe/H) relations are delta mb/delta log P = -1.69 plus or minus 0.05 and delta mv/delta log P = -1.93 plus or minus 0.05, with a separation between the F and H modes of 0.59 mag in B, and 0.45 mag in V. As in the case of the RR Lyrae stars the redder passband has the steeper slope. We find that 15 of the 21 short-period BL Her Cepheids (P less than or equal to 8 days) appear to be H pulsators, while 15 of the 19 W Vir Cepheids (12 less than or equal to P less than or equal to 30 days) appear to be F pulsators. The period-amplitude (P-Ab) diagram suggests that at a given period the first-overtone pulsators have larger amplitudes than the fundamental mode pulsators. For the Anomalous Cepheids the estimated P-L-(Fe/H) relations have slopes delta mb/delta log P = -3.20 plus or minus 0.18, and delta mv/delta log P = -3.13 plus or minus 0.28. The offsets bet ween the F and H mode relations are 0.68 and 0.66 mag in B and V, respectively.
Lutz Thomas E..
Nemec Amanda F. Linnell
Nemec James M.
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