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Register a new userAlessi 95 and the short period Cepheid SU Cassiopeiae
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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+0.02*E(B-V), implying a value of R=Av/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 Vo-Mv=8.16+-0.04 s.e. (+-0.21 s.d.), d=429+-8 pc, and an estimated age of 10^8.2 yr from ZAMS fitting of available UBV, CCD BV, NOMAD, and 2MASS JHKs 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 <Mv>=-3.15+-0.07 s.e., consistent with overtone pulsation (P_FM=2.75 d), as also implied by the Cepheids 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 percent.
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Cepheid stars play a considerable role as extragalactic distances indicators, thanks to the simple empirical relation between their pulsation period and their luminosity. They overlap with that of secondary distance indicators, such as Type Ia supernovae, whose distance scale is tied to Cepheid luminosities. However, the Period-Luminosity (P-L) relation still lacks a calibration to better than 5%. Using an original combination of interferometric astrometry with optical and ultraviolet spectroscopy, we measured the geometrical distance d = 720.35+/-7.84 pc of the 3.33 d period Cepheid V1334 Cyg with an unprecedented accuracy of +/-1 %, providing the most accurate distance for a Cepheid. Placing this star in the P-L diagram provides an independent test of existing period-luminosity relations. We show that the secondary star has a significant impact on the integrated magnitude, particularly at visible wavelengths. Binarity in future high precision calibrations of the P-L relations is not negligible, at least in the short-period regime. Subtracting the companion flux leaves V1334 Cyg in marginal agreement with existing photometric-based P-L relations, indicating either an overall calibration bias or a significant intrinsic dispersion at a few percent level. Our work also enabled us to determine the dynamical masses of both components, M1 = 4.288 +/- 0.133 Msun (Cepheid) and M2 = 4.040 +/- 0.048 Msun (companion), providing the most accurate masses for a Galactic binary Cepheid system.
V440 Per is a Population I Cepheid with the period of 7.57 day and low amplitude, almost sinusoidal light and radial velocity curves. With no reliable data on the 1st harmonic, its pulsation mode identification remained controversial. We obtained a radial velocity curve of V440 Per with our new high precision and high throughput Poznan Spectroscopic Telescope. Our data reach the accuracy of 130 m/s per individual measurement and yield a secure detection of the 1st harmonic with the amplitude of A_2= 140+/- 15 m/s. The velocity Fourier phase phi_21 of V440 Per is inconsistent at the 7.25 sigma level with those of the fundamental mode Cepheids, implying that the star must be an overtone Cepheid, as originally proposed by Kienzle et al.(1999). Thus, V440 Per becomes the longest period Cepheid with the securely established overtone pulsations. We show, that the convective nonlinear pulsation hydrocode can reproduce the Fourier parameters of V440 Per very well. Requirement to match the observed properties of V440 Per constrains free parameters of the dynamical convection model used in the pulsation calculations, in particular the radiative losses parameter.
A new compilation of UBV data for stars near the Cepheid S Vul incorporates BV observations from APASS and NOMAD to augment UBV observations published previously. A reddening analysis yields mean colour excesses and distance moduli for two main groups of stars in the field: the sparse cluster Turner 1 and an anonymous background group of BA stars. The former appears to be 1.07+-0.12 kpc distant and reddened by E(B-V)=0.45+-0.05, with an age of 10^9 yrs. The previously overlooked latter group is 3.48+-0.19 kpc distant and reddened by E(B-V)=0.78+-0.02, with an age of 1.3x10^7 yrs. Parameters inferred for S Vul under the assumption that it belongs to the distant group, as also argued by 2MASS data, are all consistent with similar results for other cluster Cepheids and Cepheid-like supergiants.
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Most of our knowledge about the structure of the Milky Way has come from the study of variable stars. Among the variables, mimicking the periodic variation of pulsating stars, are the eclipsing binaries. These stars are important in astrophysics because they allow us to directly measure radii and masses of the components, as well as the distance to the system, thus being useful in studies of Galactic structure alongside pulsating RR Lyrae and Cepheids. Using the distinguishing features of their light curves, one can identify them using a semi-automated process. In this work, we present a strategy to search for eclipsing variables in the inner VVV bulge across an area of 13.4 sq. deg. within $1.68^{rm o}<l<7.53^{rm o}$ and $-3.73^{rm o}<b<-1.44^{rm o}$, corresponding to the VVV tiles b293 to b296 and b307 to b310. We accurately classify 212 previously unknown eclipsing binaries, including six very reddened sources. The preliminary analysis suggests these eclipsing binaries are located in the most obscured regions of the foreground disk and bulge of the Galaxy. This search is therefore complementary to other variable stars searches carried out at optical wavelengths.
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