An Updated Metal-dependent Theoretical Scenario for Classical Cepheids

The superb phase resolution and quality of the Optical Gravitational Lensing Experiment (OGLE) data on the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) Cepheids, together with existing data on Galactic Cepheids, are combined to study the period–colour (PC) and amplitude–colour (AC) relations as a function of pulsation phase. Our results confirm earlier work that the LMC PC relation (at mean light) is more consistent with two lines of differing slopes, separated at a period of 10 d. However, our multiphase PC relations reveal much new structure which can potentially increase our understanding of Cepheid variables. These multiphase PC relations provide insight into why the Galactic PC relation is linear but the LMC PC relation is non-linear. This is because the LMC PC relation is shallower for short (log P < 1) and steeper for long (log P > 1) period Cepheids than the corresponding Galactic PC relation. Both of the short- and long-period Cepheids in all three galaxies exhibit the steepest and shallowest slopes at phases around 0.75–0.85, respectively. A consequence is that the PC relation at phase ∼ 0.8 is highly non-linear. Further, the Galactic and LMC Cepheids with log P > 1 display a flat slope in the PC plane at phases close to the maximum light. When the LMC period–luminosity (PL) relation is studied as a function of phase, we confirm that it changes with the PC relation. The LMC PL relation in V and I band near the phase of 0.8 provides compelling evidence that this relation is also consistent with two lines of differing slopes joined at a period close to 10 d.

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 colour–magnitude diagrams of young stellar systems (from 20 to 250 Myr) are computed in several photometric bands for Y = 0.25 and 0.35, at a fixed metallicity (Z = 0.008). The PLRs appear to be very similar in the two cases, with negligible effects (few per cent) on distances, while PWRs differ somewhat, with systematic uncertainties in deriving distances as high as ∼ 7 per cent 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 multiwavelength data sets 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.

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We present a new grid of convective BL Herculis models using the state-of-the-art 1D non-linear radial stellar pulsation tool mesa-rsp. We investigate the impact of metallicity and four sets of different convection parameters on multiwavelength properties. Non-linear models were computed for periods typical for BL Her stars, i.e. 1 ≤ P(d) ≤ 4 covering a wide range of input parameters – metallicity (−2.0 dex ≤ [Fe/H] ≤ 0.0 dex), stellar mass (0.5–0.8 M⊙), luminosity (50–300 L⊙), and effective temperature (full extent of the instability strip; in steps of 50 K). The total number of BL Her models with full-amplitude stable pulsations used in this study is 10 280 across the four sets of convection parameters. We obtain their multiband (UBVRIJHKLL′M) light curves and derive new theoretical period–luminosity (PL), period–Wesenheit (PW), and period–radius (PR) relations at mean light. We find that the models computed with radiative cooling show statistically similar slopes for PL, PW, and PR relations. Most empirical relations match well with the theoretical PL, PW, and PR relations from the BL Her models computed using the four sets of convection parameters. However, PL slopes of the models with radiative cooling provide a better match to empirical relations for BL Her stars in the Large Magellanic Cloud in the HKS bands. For each set of convection parameters, the effect of metallicity is significant in U and B bands and negligible in infrared bands, which is consistent with empirical results. No significant metallicity effects are seen in the PR relations.

This thesis presents new observations and analysis of the molecular gas in the Large Magellanic Cloud (LMC). The observations were conducted at the Mopra Telescope as part of the Magellanic Mopra Assessment (MAGMA) project, which has obtained high resolution (45′′) maps of the 12CO(J = 1 → 0) emission from 70% by mass of the LMC’s molecular cloud population. We show that CO emission in the LMC arises predominantly in spatially compact structures with high surface brightness, and that the total CO luminosity of the LMC is two orders of magnitude lower than would be predicted by the correlations between CO luminosity, stellar mass and 1.4 GHz radio continuum that are observed for nearby late-type galaxies. We present a catalogue of giant molecular clouds (GMCs) in the LMC using the MAGMA CO data, and investigate whether the catalogued clouds are similar to GMCs in the Milky Way and other nearby galaxies. We find that GMCs in the LMC roughly follow the scaling relations between radius, velocity dispersion, mass and CO luminosity that have been determined for Galactic GMCs, but that LMC clouds have narrow linewidths and faint CO luminosities relative to their size. The physical properties of the observed GMCs are mostly insensitive to variations in the local interstellar conditions, but there are significant positive correlations between the atomic gas column density and the GMC velocity dispersion, and the stellar mass surface density and both the peak CO brightness and CO surface brightness of the GMCs. Our results are difficult to reconcile with models that posit molecular clouds as equilibrium structures that are regulated by either the interstellar radiation field or the ambient interstellar pressure. Finally, we consider whether molecular gas is relevant for the correlation between the 1.4 GHz radio continuum and far-infrared (FIR) emission within the LMC. We find robust correlations between the non-thermal radio continuum and the gas and dust emission in regions covering half the LMC’s gas disk, and we identify star formation and the neutral gas surface density as the key parameters that determine the strength of these correlations. In regions where the star-formation activity is low relative to the availability of dense gas, the non-thermal radio continuum is more tightly correlated with the gas and dust emission. We demonstrate that coupling between the magnetic field strength and the gas volume density can account for the exponent of the local radio-FIR correlation that we observe in the LMC, adopting plausible assumptions for the LMC’s UV opacity, dust-to-gas ratio and cosmic ray distribution.

The VISTA near-infrared $YJK_\mathrm{s}$ survey of the Magellanic Clouds System (VMC, PI M.-R. L. Cioni) is collecting deep $K_\mathrm{s}$-band time-series photometry of the pulsating variable stars hosted in the system formed by the two Magellanic Clouds and the Bridge connecting them. In this paper we present for the first time $K_\mathrm{s}$-band light curves for Anomalous Cepheid (AC) variables. In particular, we have analysed a sample of 48 Large Magellanic Cloud ACs, for which identification and optical magnitudes were obtained from the OGLE III and IV catalogues. The VMC $K_\mathrm{s}$-band light curves for ACs are well sampled, with the number of epochs ranging from 8 to 16, and allowing us to obtain very precise mean $K_\mathrm{s}$ magnitudes with errors on average of the order of 0.01 mag. The $\langle K_\mathrm{s} \rangle$ values were used to build the first Period-Luminosity and Period-Wesenheit relations in the near-infrared for fundamental-mode and first overtone ACs. At the same time we exploited the optical ($V,I$) OGLE data to build accurate Period-Luminosity, Period-Luminosity-Colour and Period-Wesenheit relations both for fundamental-mode and first overtone ACs. For the first time these relations were derived from a sample of pulsators which uniformly cover the whole AC instability strip. The application of the optical Period-Wesenheit relation to a sample of dwarf galaxies hosting a significant population of ACs revealed that this relation is a valuable tool for deriving distances within the Local Group. Due to its lower dispersion, we expect the $K_\mathrm{s}$ Period-Wesenheit relations first derived in this paper to represent a valuable tool for measuring accurate distances to galaxies hosting ACs when more data in near-infrared filters become available.

The Wesenheit function is widely used to reduce the effects of interstellar reddening in distance measurements. Its construction, however, relies on the assumption of a universal extinction curve and on fixed values of the total-to-selective extinction ratio, R V . Recent studies have shown that R V varies significantly across the Milky Way and between different galaxies, raising concerns about systematic biases in Wesenheit magnitudes and period–Wesenheit relations. In this work, we discuss the impact of nonuniversal extinction on Wesenheit indices by combining the R V -dependent extinction curve with a grid of stellar atmosphere models. We compute the integrated extinction in optical and near-infrared passbands, derive R V -dependent R coefficients for multiple Wesenheit indices, and examine how changes in R V propagate into Wesenheit magnitudes and Cepheid distances in our Galaxy. We find that the R coefficients in the Wesenheit functions vary strongly with R V . For classical Cepheids in the Milky Way disk, variations of R V within the typical observed range (2.6–3.6) can lead to substantial differences in the Wesenheit function, reaching ± 0.7 mag from the mean for the Gaia-based Wesenheit index W G and resulting in distance errors of almost 40%. Near-infrared Wesenheit indices are much less sensitive to R V changes. Our results clearly show that accounting for variable R V is essential when applying period–Wesenheit relations, particularly in the optical regime, or that near- or mid-infrared-based distances should be used. While we present this effect for classical Cepheids, it applies to all pulsating stars for which period–Wesenheit relations are used to infer distances.

ABSTRACTWe revisit the problem of the positive correlation between age and Galactocentric distance seen in Galactic Classical Cepheids, which at first sight may seem counter‐intuitive in the context of inside‐out galaxy formation. To explain it, we use the Besançon Galaxy Model and a simulation of star particles in the Galactic disc coupled with stellar evolutionary models. We then select Classical Cepheids from this simulation and test in qualitative terms which ingredients are necessary to find agreement with the observational data. We show that the interplay of the Galactic disc's metallicity gradient and the metallicity dependence of the Cepheids' life‐time in the instability strip results in a pronounced positive age‐Galactocentric distance relation. This renders a reconstruction of the recent star‐formation history based on Classical Cepheids unrealistic. It also has important consequences on our interpretation of the observed scatter about the radial metallicity gradient measured with Galactic Classical Cepheids.

On the basis of observational data for the absolute ΔR and relative ΔR/R amplitudes of variations in radius of galactic classical cepheids (55 stars from Balona and Stobie (1979) and 30 stars from Sollazzoet al. (1981)), four kinds of empirical linear relations are obtained: log(P ΔV)−logΔR, logP−logΔR, log(P ΔV)−log(ΔR/R), and logP−log(ΔR/R);P, R, and ΔV are the pulsation periods, the mean stellar radii, and the amplitudes of light variations, respectively. Three groups of stars are considered: short-period cepheids (SPC)-with logP≤1.1; long-period cepheids (LPC)-with logP>1.1; and s-cepheids (sC). Both the ΔR values and the ΔR/R values increase withP andP ΔV, for a given group of variables. A comparison is performed with our results obtained from data in other sources (Kurochkin, 1966; Gieren, 1982; etc.). The investigated relations can be applied for determining ΔR and ΔR/R of galactic classical cepheids, by using their observedP and ΔV. All studied galactic classical cepheids have ΔR/R<0.35, ΔR<10R ⊙ for SPC and 10R ⊙<ΔR≲60R ⊙ for LPC. The sC have smaller ΔR and ΔR/R values than other classical cepheids, at the same periods (the difference is about 2 times for ΔR and 1.4–2.8 times for ΔR/R); the studied sC have ΔR/R in the range 0.025–0.075 and ΔR in the range 1–3R ⊙ (only Y Oph has ΔR≃8R ⊙).

Virial mass and kinematics of the Virgo cluster

Classical Cepheids are fundamental primary distance indicators and crucial tracers of the young stellar population in the Milky Way and nearby galaxies. While most chemical abundance studies of Cepheids have been carried out in the optical domain, near-infrared (NIR) spectroscopy offers unique advantages in terms of reduced extinction and access to new elemental tracers. Our goal is to validate NIR abundance determinations against well-established optical results and to explore the diagnostic power of previously unexplored NIR lines. NIR spectroscopy is far less hampered by interstellar extinction than optical observations, which allows us to probe Cepheids at larger distances and in highly obscured regions of the Galaxy. Moreover, the H and K bands provide access to diagnostic lines of elements (e.g., P, K, and Yb) that are not available in the optical domain. We acquired high-resolution (R≈45,000) spectra of 21 Galactic and 2 Large Magellanic Cloud (LMC) classical Cepheids with the high-resolution Immersion Grating Infrared Spectrometer (IGRINS) in the H and K bands. Effective temperatures were derived from a photometric approach and line-depth ratios, and the gravities and microturbulent velocities were estimated using empirical calibrations and statistical constraints. The abundances of 16 elements were determined through a full spectral synthesis in local thermodynamic equilibrium. We performed an extensive error analysis and compared our results with previous optical studies of the same stars. Our NIR abundances and the optical literature values agree very well (Δ Fe/H ,łe,0.02 dex and σ ≈,0.07 dex), which confirms the reliability of IGRINS-based measurements. The derived abundance gradients in the Galactic disk are fully consistent with previous optical determinations, with slopes of –0.06, –0.05, and –0.05 dex -1 for Fe, Mg, and Si, respectively. We provide homogeneous determinations of P, K, and Yb abundances from NIR lines for classical Cepheids for the first time, and we report trends that are consistent with Galactic chemical evolution models. Moreover, the two LMC Cepheids included in our sample that were previously analyzed in the optical provide a direct benchmark that confirms the accuracy of NIR abundance determinations in extragalactic metal-poor environments. Our study demonstrates that high-resolution NIR spectroscopy of Cepheids yields robust abundances that are fully compatible with optical results and provides access to additional elements of nucleosynthetic interest. These results pave the way for future large-scale NIR surveys of Cepheids with facilities such as MOONS, ELT, and JWST, which are crucial for tracing the chemical evolution of the Milky Way and nearby galaxies in heavily obscured regions.

We cross-correlated the OGLE-II database with the SIRIUS JHK survey data in the Large Magellanic Cloud (LMC). After eliminating obvious spurious variables, we determined the pulsation periods for 9 681 stars by applying the Phase Dispersion Minimization (PDM) technique to the OGLE-II data. Based on these data, we studied the period-K magnitude (PK) relations of variable stars in the LMC and found a new sequence. Comparison between the theoretical pulsation model (Wood &amp; Sebo 1996) and observational data suggests that the variable stars on the new sequence are Mira variables pulsating in the first overtone mode.

Context. The upcoming Rubin-LSST is expected to revolutionize the field of classical pulsators by offering well-sampled multi-epoch photometric data in multiple wavelengths. Type II Cepheids (T2Cs) exhibit weak or negligible metallicity dependence on period-luminosity (PL) relations. Thus, they may potentially be used as an alternative to classical Cepheids for extragalactic distance estimations, when used together with RR Lyraes and the tip of the red giant branch. It is therefore crucial to study an updated theoretical pulsation scenario of BL Herculis stars (BL Her; the shortest period T2Cs) in the corresponding Rubin-LSST photometric system. Aims. We present new theoretical light curves in the Rubin-LSST filters for a fine grid of BL Her models computed using MESA-RSP. We have also derived new theoretical PL and period-Wesenheit (PW) relations in the Rubin-LSST filters with the aim to study the effect of convection parameters and metallicity on these relations. Methods. The grid of BL Her models was computed using the non-linear radial stellar pulsation tool MESA-RSP with the input stellar parameters: metallicity (−2.0 dex ≤ [Fe/H] ≤ 0.0 dex), stellar mass (0.5 M⊙ − 0.8 M⊙), stellar luminosity (50 L⊙ − 300 L⊙), and effective temperature (across the full extent of the instability strip; in steps of 50 K) and using four sets of convection parameters. Bolometric correction tables from MIST were used to transform the theoretical bolometric light curves of the BL Her models into the Rubin–LSST ugrizy filters. Results. The PL relations of the BL Her models exhibit steeper slopes but smaller dispersion with increasing wavelengths in the Rubin-LSST filters. The PL and PW slopes for the complete set of BL Her models computed with radiative cooling (sets B and D) are statistically similar across the grizy filters. The BL Her models exhibit weak or negligible effect of metallicity on the PL relations for wavelengths longer than the g filter for the case of the complete set of models as well as for the low-mass models. However, we find a significant effect of the metallicity on the PL relation in the u filter. Strong metallicity effects are observed in the PWZ relations involving the u filter and are found to have significant contribution from the high-metallicity BL Her models. Due to a negligible metallicity effect for relations involving the Wesenheit indices W(i, g − i), W(z, i − z), and W(y, g − y), we recommend these filter combinations for BL Her stars during observations with Rubin–LSST for use as reliable standard candles.

A sample of 111 Fermi blazars each with a well-established radio core luminosity, broad-line luminosity, bolometric luminosity and black hole mass has been compiled from the literatures. We present a significant correlation between radio core and broad-line emission luminosities that supports a close link between accretion processes and relativistic jets. Analysis reveals a relationship of \(\mathrm{Log}\mathrm{L}_{\mathrm{BLR}}\sim(0.81\pm0.06)\mathrm{Log} \mathrm{L}_{\mathrm{R}}^{\mathrm{C}}\) which is consistant with theoretical predicted coefficient and supports that blazar jets are powered by energy extraction from a rapidly spinning Kerr black hole through the magnetic field provided by the accretion disk. Through studying the correlation between the intrinsic bolometric luminosity and the black hole mass, we find a relationship of \(\mathrm{Log}\frac{\mathrm{L}_{\mathrm{in}}}{\mathrm{L}_{\odot}}=(0.95\pm0.26)\mathrm{Log} \frac{\mathrm{M}}{\mathrm{M}_{\odot}}+(3.53\pm2.24)\) which supports mass-luminosity relation for Fermi blazars derived in this work is a powerlaw relation similar to that for main-sequence stars. Finally, EVOLUTIONARY SEQUENCE OF BLAZARS is discussed.

The second survey of the molecular clouds in 12CO (J = 1-0) was carried out in the Large Magellanic Cloud by NANTEN. The sensitivity of this survey is twice as high as that of the previous NANTEN survey, leading to a detection of molecular clouds with M_C