Pulsating variable stars as tracers of galactic structure and interaction mechanisms

My PhD project has been focused on the study of the pulsating variable stars in two ultra-faint dwarf spheroidal satellites of the Milky Way, namely, Leo IV and Hercules; and in two fields of the Large Magellanic Cloud (namely, the Gaia South Ecliptic Pole calibration field, and the 30 Doradus region) that were repeatedly observed in the KS band by the VISTA Magellanic Cloud (VMC, PI M.R. Cioni) survey of the Magellanic System

Variable stars in the ultra-faint dwarf spheroidal galaxy Ursa Major I

We have performed the first study of the variable star population of Ursa Major I (UMa I), an ultra-faint dwarf satellite recently discovered around the Milky Way by the Sloan Digital Sky Survey. Combining time series observations in the B and V bands from four different telescopes, we have identified seven RR Lyrae stars in UMa I, of which five are fundamental-mode (RRab) and two are first-overtone pulsators (RRc). Our V, B-V color-magnitude diagram of UMa I reaches V~23 mag (at a signal-to-noise ratio of ~ 6) and shows features typical of a single old stellar population. The mean pulsation period of the RRab stars = 0.628, {\sigma} = 0.071 days (or = 0.599, {\sigma} = 0.032 days, if V4, the longest period and brightest variable, is discarded) and the position on the period-amplitude diagram suggest an Oosterhoff-intermediate classification for the galaxy. The RR Lyrae stars trace the galaxy horizontal branch at an average apparent magnitude of = 20.43 +/- 0.02 mag (average on 6 stars and discarding V4), giving in turn a distance modulus for UMa I of (m-M)0 = 19.94 +/- 0.13 mag, distance d= 97.3 +6.0/-5.7 kpc, in the scale where the distance modulus of the Large Magellanic Cloud is 18.5 +/- 0.1 mag. Isodensity contours of UMa I red giants and horizontal branch stars (including the RR Lyrae stars identified in this study) show that the galaxy has an S-shaped structure, which is likely caused by the tidal interaction with the Milky Way. Photometric metallicities were derived for six of the UMa I RR Lyrae stars from the parameters of the Fourier decomposition of the V-band light curves, leading to an average metal abundance of [Fe/H] = -2.29 dex ({\sigma} = 0.06 dex, average on 6 stars) on the Carretta et al. metallicity scale.Comment: Accepted for publication in Ap

New insights into the use of Ultra Long Period Cepheids as cosmological standard candles

Ultra Long Period Cepheids (ULPs) are pulsating variable stars with a period longer than 80d and have been hypothesized to be the extension of the Classical Cepheids (CCs) at higher masses and luminosities. If confirmed as standard candles, their intrinsic luminosities, 1 to 3 mag brighter than typical CCs, would allow to reach the Hubble flow and, in turn, to determine the Hubble constant, H_0, in one step, avoiding the uncertainties associated with the calibration of primary and secondary indicators. To investigate the accuracy of ULPs as cosmological standard candles, we first collect all the ULPs known in the literature. The resulting sample includes 63 objects with a very large metallicity spread with 12 + log([O/H]) ranging from 7.2 to 9.2 dex. The analysis of their properties in the VI period-Wesenheit plane and in the color-magnitude diagram (CMD) supports the hypothesis that the ULPs are the extension of CCs at longer periods, higher masses and luminosities, even if, additional accurate and homogeneous data and a devoted theoretical scenario are needed to get firm conclusions. Finally, the three M31 ULPs, 8-0326, 8-1498 and H42, are investigated in more detail. For 8-1498 and H42, we cannot confirm their nature as ULPs, due to the inconsistency between their position in the CMD and the measured periods. For 8-0326, the light curve model fitting technique applied to the available time-series data allows us to constrain its intrinsic stellar parameters, distance and reddening.Comment: MNRAS - Accepted 2020 November 19. Received 2020 November 19; in original form 2020 July 15 - 9 pages and 8 figure

Studying Stellar Systems with Classical Pulsators

Classical Cepheids and RR Lyrae are very important primary distance indicators and stellar population tracers, thanks to the relations between pulsation properties and intrinsic stellar parameters. Through the theoretical prediction of these relations as well as through the direct comparison between predicted and observed light and radial velocity variations, we are able to constrain the stellar properties of the investigated pulsating stars and their host stellar populations, as well as to study the dependence on chemical composition

New near-infrared period-luminosity-metallicity relations for RR Lyrae stars and the outlook for Gaia

We present results of the analysis of 70 RR Lyrae stars located in the bar of the Large Magellanic Cloud (LMC). Combining the spectroscopically determined metallicity of these stars from the literature with precise periods from the OGLE III catalog and multi-epoch Ks photometry from the VISTA survey of the Magellanic Clouds system, we derive a new near-infrared period-luminosity-metallicity (PLZ Ks ) relation for RR Lyrae variables. In order to fit the relation we use a fitting method developed specifically for this study. The zero-point of the relation is estimated two different ways: by assuming the value of the distance to the LMC and by using Hubble Space Telescope parallaxes of five RR Lyrae stars in the Milky Way (MW). The difference in distance moduli derived by applying these two approaches is ∼0.2mag. To investigate this point further we derive the PLZ Ks relation based on 23 MW RR Lyrae stars that had been analyzed in Baade-Wesselink studies. We compared the derived PLZ Ks relations for RR Lyrae stars in the MW and LMC. Slopes and zero-points are different, but still consistent within the errors. The shallow slope of the metallicity term is confirmed by both LMC and MW variables. The astrometric space mission Gaia is expected to provide a huge contribution to the determination of the RR Lyrae PLZ Ks relation; however, calculating an absolute magnitude from the trigonometric parallax of each star and fitting a PLZ Ks relation directly to period and absolute magnitude leads to biased results. We present a tool to achieve an unbiased solution by modeling the data and inferring the slope and zero-point of the relation via statistical methods

Predicted Masses of Galactic Cepheids in the Gaia Data Release 2

On the basis of recently computed nonlinear convective pulsation models of Galactic Cepheids, spanning wide ranges of input stellar parameters, we derive theoretical mass-dependent Period-Wesenheit relations in the Gaia bands, namely, G, G BP, and G BR, that are found to be almost independent of the assumed efficiency of superadiabatic convection. The application to a selected subsample of the Gaia Data Release 2 Galactic Cepheids database allows us to derive mass-dependent estimates of their individual distances. By imposing their match with the astrometric values inferred from Gaia, we are able to evaluate the individual mass of each pulsator. The inferred mass distribution is peaked around 5.6M o˙ and 5.4M o˙ for the F and FO pulsators, respectively. If the estimated Gaia parallax offset «Δϖ» mas is applied to Gaia parallaxes before imposing their coincidence with the theoretical ones, the inferred mass distribution is found to shift toward lower masses, namely, ∼5.2M o˙ and 5.1M o˙ for the F and FO pulsators, respectively. The comparison with independent evaluations of the stellar masses, for a subset of binary Cepheids in our sample, seems to support the predictive capability of the current theoretical scenario. By forcing the coincidence of our mass determinations with these literature values we derive an independent estimate of the mean offset to be applied to Gaia DR2 parallaxes, «Δϖ» = 0.053 ± 0.029 mas, slightly higher but in agreement within the errors with the Riess et al. value

A theoretical scenario for Galactic RR Lyrae in the Gaia data base: constraints on the parallax offset

On the basis of an extended set of non-linear convective RR Lyrae pulsation models we derive the first theoretical light curves in the Gaia bands G, GBP, and GRP and the corresponding intensity-weighted mean magnitudes and pulsation amplitudes. The effects of chemical composition on the derived Bailey diagrams in the Gaia filters are discussed for both Fundamental and first overtone mode pulsators. The inferred mean magnitudes and colours are used to derive the first theoretical Period-Wesenheit relations for RR Lyrae in the Gaia filters. The application of the theoretical Period-Wesenheit relations for both the Fundamental and first overtone mode to Galactic RR Lyrae in the Gaia Data Release 2 data base and complementary information on individual metal abundances allows us to derive theoretical estimates of their individual parallaxes. These results are compared with the astrometric solutions to conclude that a very small offset, consistent with zero, is required in order to reconcile the predicted distances with Gaia results

The VMC Survey -- XXXV. Model fitting of LMC Cepheid light curves

We present the results of the light curve model fitting technique applied to optical and near-infrared photometric data for a sample of 18 Classical Cepheids (11 fundamentals and 7 first overtones) in the Large Magellanic Cloud (LMC). We use optical photometry from the OGLE III database and near--infrared photometry obtained by the European Southern bservatory public survey "VISTA near--infrared survey of the Magellanic Clouds system". Iso--periodic nonlinear convective model sequences have been computed for each selected Cepheid in order to reproduce the multi--filter light curve amplitudes and shape details. The inferred individual distances provide an intrinsic weighted mean value for the LMC distance modulus of $\mu_0=18.56$ mag with a standard deviation of 0.13 mag. We derive also the Period--Radius, the Period--Luminosity and the Period--Wesenheit relations that are consistent with similar relations in the literature. The intrinsic masses and luminosities of the best--fitting models show that all the investigated pulsators are brighter than the redictions of the canonical evolutionary mass--luminosity relation, suggesting a significant efficiency of non--canonical phenomena, such as overshooting, mass loss and/or rotation.Comment: 14 pages, 10 figures, MNRAS accepte

Quantitative experiments to explain the change of seasons

The science education literature shows that students have difficulty understanding what causes the seasons. Incorrect explanations are often due to a lack of knowledge about the physical mechanisms underlying this phenomenon. To address this, we present a module in which the students engage in quantitative measurements with a photovoltaic panel to explain changes to the sunray flow on Earth’s surface over the year. The activities also provide examples of energy transfers between the incoming radiation and the environment to introduce basic features of Earth’s climate. The module was evaluated with 45 secondary school students (aged 17–18) and a pre-/posttest research design. Analysis of students’ learning outcomes supports the effectiveness of the proposed activities

Stellar archaeology in the Milky Way halo. Variable stars and stellar populations in the new Milky Way ultra-faint dwarfs

We summarize results from the photometric survey of a number of ultra-faint dwarf satellites of the Milky Way. We are studying these systems in a systematic way to characterize their stellar populations and structural parameters, as well as their variable star content, with the aim of deriving hints on the formation process of the Galactic halo. <P /