On the impact of Helium abundance on the Cepheid Period-Luminosity and Wesenheit relations and the Distance Ladder

This work analyses the effect of the Helium content on synthetic Period-Luminosity Relations (PLRs) and Period-Wesenheit Relations (PWRs) of Cepheids and the systematic uncertainties on the derived distances that a hidden population of He-enhanced Cepheids may generate. We use new stellar and pulsation models to build a homogeneous and consistent framework to derive the Cepheid features. The Cepheid populations expected in synthetic color-magnitude diagrams of young stellar systems (from 20 Myr to 250 Myr) are computed in several photometric bands for Y = 0.25 and Y = 0.35, at a fixed metallicity (Z = 0.008). The PLRs appear to be very similar in the two cases, with negligible effects (few %) on distances, while PWRs differ somewhat, with systematic uncertainties in deriving distances as high as about 7% at log P < 1.5. Statistical effects due to the number of variables used to determine the relations contribute to a distance systematic error of the order of few percent, with values decreasing from optical to near-infrared bands. The empirical PWRs derived from multi-wavelength datasets for the Large Magellanic Cloud (LMC) is in a very good agreement with our theoretical PWRs obtained with a standard He content, supporting the evidence that LMC Cepheids do not show any He effect

Multipopulation aftereffects on the color-magnitude diagram and Cepheid variables of young stellar systems

Context: The evidence of a multipopulation scenario in Galactic globular clusters raises several questions about the formation and evolution of the two (or more) generations of stars. These populations show differences in their age and chemical composition. These differences are found in old- and intermediate- age stellar clusters in the Local Group. The observations of young stellar systems are expected to present footprints of multiple stellar populations. Aims: This theoretical work intends to be a specific step in exploring the space of the observational indicators of multipopulations, without covering all the combinations of parameters that may contribute to the formation of multiple generations of stars in a cluster or in galaxy. The goal is to shed light on the possible observational features expected by core He-burning stars that belong to two stellar populations with different original He content and ages. Methods: The tool adopted was the stellar population synthesis. We used new stellar and pulsation models to construct a homogeneous and consistent framework. Synthetic color-magnitude diagrams (CMDs) of young- and intermediate-age stellar systems (from 20 Myr up to 1 Gyr) were computed in several photometric bands to derive possible indicators of double populations both in the observed CMDs and in the pulsation properties of the Cepheids. Results: We predict that the morphology of the red/blue clump in VIK bands can be used to photometrically indicate the two stellar populations in a rich assembly of stars if there is a significant difference in their original He content. Moreover, the period distribution of the Cepheids appears to be widely affected by the coeval multiple generations of stars within stellar systems. We show that the Wesenheit relations may be affected by the helium content of the Cepheids.Comment: in press on A&

A Raman scattering study of permanently densified vitreous B2O3

Inelastic light scattering spectra of normal and permanently densified B2O3 glasses were investigated over the frequency range between 6 and 1600 cm−1. Densification from 1825 kg/m3 up to 2082 kg/m3 was obtained by loading B2O3 glasses in a multi-anvil apparatus for the synthesis at 2GPa; they were fused at 1500K and then quenched at that pressure. The low-frequency Raman scattering includes the Boson peak, which dominates the spectra between 10 and 100 cm−1. Densification significantly decreases the intensity of the Boson peak and shifts its position from about 26 cm−1 up to 39 cm−1. At higher frequencies, the spectral features of the bands associated to the vibrational motion of boroxol rings and BO3 units building up the network indicate that these molecular groups keep the main characteristics of their conformation, even if the connectivity of these groups is varied as a consequence of a decrease of the ring concentration. These observations imply that densification drives the system toward a structure having a more efficient packing of molecular units

Tales from Dubna’s Oakwood: Bogoliubov, Pontecorvo, and the JINR Seminars

Not much has been said about the scientific milieu in which Bru- no Pontecorvo worked after his emigration to the USSR (1950): the Joint Institute for Nuclear Research (JINR) in Dubna. In this paper, we begin to fill this gap by focusing on some of the distinguished scientists he interacted with, besides direct collaborations, paying particular attention to Nikolai N. Bogoliubov around 1957

Specific-heat Study of Low-energy Vibrational State m Vitreous Samarium Phosphates

Measurements of specific heat in the temperature range 1.5K to 30K were made for two samples of vitreous samarium phosphates. A large contribution to the specific heat, well above what is expected from acoustic phonons (in the Debye approximation), is discussed in terms of the theoretical predictions of a phonon-fracton cross-over approach. The phonon-fracton density of states used to fit the excess specific heat gives rise to model parameters with the same magnitudes as those found previously for a wide range of glasses