Short time scale period variations of the RRc star V468 Hya

We find large period changes of the first-overtone RR Lyrae type star V468 Hya

O-C diagrams for 33 RR Lyrae-type stars

In this paper we study period variations of 33 RR Lyrae type stars using O-C analysis, and calculate their mean light elements

Young and Intermediate-age Distance Indicators

Distance measurements beyond geometrical and semi-geometrical methods, rely mainly on standard candles. As the name suggests, these objects have known luminosities by virtue of their intrinsic proprieties and play a major role in our understanding of modern cosmology. The main caveats associated with standard candles are their absolute calibration, contamination of the sample from other sources and systematic uncertainties. The absolute calibration mainly depends on their chemical composition and age. To understand the impact of these effects on the distance scale, it is essential to develop methods based on different sample of standard candles. Here we review the fundamental properties of young and intermediate-age distance indicators such as Cepheids, Mira variables and Red Clump stars and the recent developments in their application as distance indicators.Comment: Review article, 63 pages (28 figures), Accepted for publication in Space Science Reviews (Chapter 3 of a special collection resulting from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space Age

Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy

We review HB stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters (GCs) in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the second-parameter problem is presented. A technique is proposed to estimate the HB types of extragalactic GCs on the basis of integrated far-UV photometry. The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyrae, is also revisited, giving a distance modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are studied. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and Space Scienc

Age Dependence of the Vertical Distribution of Young Open Clusters

The ages of 203 open clusters from the list of Dambis (1999) are computed in terms of Cambridge evolutionary tracks with and without the allowance for convective overshooting (Pols et al. 1998). The vertical scaleheight of the cluster layer for 123 objects at Galactocentric distances $R_{\rm g} = R_0 \pm 1$ kpc is found to vary non-monotonically with age exhibiting a wavelike pattern similar to the one earlier found for the Cepheid population (Joeveer 1974). The period of these variations is equal to $P_{Z} = 74 \pm 2$ Myr and $P_{Z} = 92 \pm 2$ Myr if cluster ages are computed in terms of evolutionary models of Pols et al. (1998) without and with overshooting, respectively. If interpreted as a manifestation of vertical virial oscillations, the implications of the pattern found are threefold: (1) the period of vertical oscillations can be reconciled with the known local density of visible matter only if cluster ages are computed with no or mild overshooting ($P_{Z} = 74 \pm 2$ Myr), which implies a maximum local density of $\rho = 0.118 \pm 0.006~M_{\odot}$ pc-3 compared to $\rho = 0.102~M_{\odot}$ pc-3 recently inferred from Hipparcos data (Holmberg & Flynn 2000), whereas the period derived from ages computed using models with overshooting ($P_{Z} = 92 \pm 2$ Myr) implies a maximum local density of only $\rho = 0.075 \pm 0.003~M_{\odot}$ pc-3 and is thus totally incompatible with recent estimates; (2) there is not much room left for the dark matter ($\rho_{\rm DM} \le 0.027~M_{\odot}$ pc-3) in the Galactic disk near the solar Galactocentric distance, and (3) at the time of their formation open clusters have, on the average, excess kinetic energy (in the vertical direction) and as a population are not in virial equilibrium; moreover, the initial vertical coordinates of open clusters (at the time of their birth) are strongly and positively correlated with initial vertical velocities (the correlation coefficient is $r(Z_0,V_{Z(0)}) = 0.81 \pm 0.08$), thus favoring a scenario where star formation in the disk is triggered by some massive objects falling to the Galactic plane

The log P -

The kinematical parameters of the local field RR Lyrae population and the zero-point of the log $P-\langle M_K\rangle$ relation for these variables are inferred by applying the statistical parallax (maximum-likeli-hood) technique to a sample of 182 RR Lyraes with known periods, radial-velocities, metallicities, K-band photometry, and absolute proper motions on the ICRS system. Hipparcos, Tycho-2, SPM, UCAC, NPM1, and the Four-Million Star Catalog (Volchkov et al. 1992) were used as the sources of proper motions; the proper motions of the last two catalogs are reduced to the Hipparcos (ICRS) system (Dambis & Rastorguev 2001). The K-band magnitudes were adopted from the list of Fernley et al. (1998) and supplemented by the data of the 2MASS Second Incremental Data Release. The parameters of the velocity distribution are found to be: ($U_0 = -10 \pm 10$, $V_0 = -51 \pm 8$, $W_0 = -14 \pm 5)$ km s-1; ($\sigma_U = 62 \pm 10$, $\sigma_V = 45 \pm 8$, $\sigma_W = 28 \pm 6$) km s-1: and ($U_0 = -23 \pm 13$, $V_0 = -213 \pm 12$, $W_0 = -5 \pm 8$) km s-1; ($\sigma_U = 157 \pm 12$, $\sigma_V = 98 \pm 8$, $\sigma_W = 91 \pm 7$) km s-1 for the thick-disk (41 stars) and halo (141 stars) objects, respectively. The zero-point of the infrared PL relation of Jones et al. (1992) (based on the results obtained using the Baade-Wesselink method) is confirmed: we find $\langle M_K\rangle = -2.33 \cdot \log P_{\rm F}-0.82 \pm 0.12$ compared to $\langle M_K\rangle = -2.33 \cdot \log P_{\rm F}-0.88$ as inferred by Jones et al. (1992). A conversion of the resulting log $P-\langle M_K\rangle$ relation to V-band luminosities yields the metallicity-luminosity relation $\langle M_V\rangle = +1.04 + 0.14 \cdot [$Fe/H$] \pm 0.11$. Our results imply a solar Galactocentric distance of $R_0 = 7.6 \pm 0.4$ kpc and an LMC distance modulus of $DM_{\rm LMC} = 18.18 \pm 0.12$ (cluster RR Lyraes) or $DM_{\rm LMC} = 18.11 \pm 0.12$ (field RR Lyraes), thereby favoring the so-called short distance scale

Rotation Curve and Kinematic Properties of Young Disk Populations

We use the method of statistical parallaxes to construct the rotation curve of the Milky Way spanning the Galactocentric distance interval from 2 to 14 kpc based on observational data for young disk populations and refine the distance scales of the objects belonging to young disk populations (classical Cepheids, young open clusters, blue supergiants, HII regions, and HI)