Jesper Storm

Context . The central regions of galaxies pose challenges for observers due to their high density of stars, gas, and dust. The Large Magellanic Cloud (LMC) hosts a dense stellar bar, whereas the Small Magellanic Cloud (SMC) hosts a dense core. Both of these dense centres have eluded even the most sensitive large-scale wide-field surveys of individual stars from the ground and from space. Aims . This work describes deep imaging observations obtained with the Visual and Infrared Survey Telescope for Astronomy (VISTA) of the inner regions of the Clouds. They were obtained to resolve stars fainter than the red clump, which due to crowding were missed in previous observations, and to quantify features associated with the star formation, kinematics, and structure of the galaxies. These studies will allow us to establish how the dynamical interactions between the LMC and the SMC, and the interaction between the Clouds and the Milky Way, have influenced the most gravitationally bound regions. Methods . Multiple near-infrared images were obtained in the J and K _s filters during excellent weather conditions (e.g. with a seeing of ∼0.7 arcsec). The data were reduced using the VISTA Data Flow System, which provides single-filter and band-merged calibrated images and catalogues, with associated confidence maps, for individual observations and combined observations for deep tiles. Results . Approximately one million and 920 thousand sources, with only minor quality issues (i.e. by excluding sources with a problematic photometric calibration or location on the detectors) and about 40 epochs per filter, are detected in the central 1.77 deg ² region of the LMC and SMC, respectively. They provide a more contiguous spatial area coverage than sources in the VISTA survey of the Magellanic Clouds system (VMC), which covers the same regions by overlapping two VISTA tiles per galaxy. Two thirds of these sources are likely stars, whereas the other one third could be stellar blends or background galaxies. The catalogues, which are made available with this work, contain parameters to select unique good-quality detections for which we provide examples of ongoing scientific applications. Conclusions . These data represent valuable complementary information to deep imaging observations at other wavelengths and provide additional epochs for sources in common with the VMC survey, for proper motion and photometric variability studies extending the time baseline to ∼11 yr, as well as targets for spectroscopic observations.

Context. The Baade–Wesselink method, while theoretically applicable for calibrating the distance scale via Cepheids, is not currently employed in the cosmic distance ladder. There are various versions of this method based mainly on interferometry and/or surface–brightness color relations. In all these approaches, the projection factor used to convert the radial into the pulsation velocity remains the limiting quantity. Aims. By comparing for the first time simultaneous cross-correlated optical and H-band radial velocities of five Cepheids, we aim to better understand the geometric part of the projection factor that is directly linked to the limb darkening of the stars. The five Cepheids were observed with the HARPS-N and GIANO-B instruments mounted at the Telescopio Nazionale Galileo in the GIARPS configuration. Methods. A two-parameter linear fit was applied to the simultaneous radial velocity measurements of each star in order to extract the amplitude ratio of both curves ( f _VH ) as well as the difference in mean velocities. Results. We find a mean value of f _VH of 1.056 with a root mean square over the five stars of 0.016, which means that the V-band radial velocity amplitudes are around 5% larger than the ones obtained in the H band. In addition, we find that the H-band radial velocity curves have an average (i.e., a γ velocity) of about 1 km s ⁻¹ blueshifted compared to the visible ones, except for one star, X Cyg, which has a blueshift of 0.4 km s ⁻¹ . At first order, the f _VH values are directly linked to the ratio of limb darkening coefficients in both bands (or equivalently the ratio of geometrical projection factors). The f _VH values are found to be consistent with expectations from static stellar atmosphere models. The difference in the γ velocity is attributed to the interplay of convection and the deeper location of H-band line-forming regions. Conclusions. This work shows that the differential limb darkening of Cepheids in visible and H bands, and consequently the geometrical part of the projection factor, follows what is expected from stellar atmosphere models. The current dispersion of about 8% in the projection factors (for a given pulsation period) is most likely due to poorly understood dynamical effects in the atmosphere of Cepheids.

Context. Anomalous Cepheids (ACEPs) are radially pulsating stars observed in dwarf galaxies, the Galactic bulge and halo, and globular clusters. Similarly to other radially pulsating stars, they can be used as distance indicators through their period-luminosity relations (PLRs) and the geometrical Baade-Wesselink (BW) method. Aims. We aim to calibrate the zero point of the distance scale of ACEPs using nearby representatives of this class of pulsating stars. Methods. We collected optical/near-infrared photometry and spectra for a sample of nearby ACEPs with two telescopes located at the RolfChini Cerro Murphy Observatory (formerly Observatory Cerro Armazones) and optical telescopes offered by the Las Cumbres Observatory, along with instruments hosted and operated by the European Southern Observatory. Using parallaxes measured by the Gaia space mission and mean magnitudes from our new photometry, we calibrated the zero point of the period-luminosity relations (PLRs) in Johnson B , V , 2MASS J , H , K _S , and Pan-STARRS g , r , i passbands, as well as select Wesenheit indices. The slopes were adopted from the PLRs of the LMC ACEPs from the literature. Using the surface brightness-color relation version of the BW technique, we were also able to determine the projection factors (p-factors) and mean radii of three nearby ACEPs. Results. The precision of the measured zero points is at the level of 0.04–0.05 mag and their systematic uncertainty was estimated to be about 0.1 mag. We used our zero point values and literature photometry related to LMC ACEPs to measure the distance modulus of this galaxy. We obtained a value of 18.454 ± 0.045 (statistical) mag, which is in very good agreement with the most accurate value from eclipsing binaries. The p-factors were 1.38 ± 0.13, 1.59 ± 0.21, and 1.35 ± 0.14 for V716 Oph, XX Vir, and UY Eri, respectively. We suspect that the very high p-factor of XX Vir results from the underestimated parallax of this star in the Gaia catalog, as its radius and absolute brightness are also higher than expected for ACEPs at this period. Finally, the radii measured for V716 Oph and UY Eri are in agreement with the period-radius relation obtained from LMC ACEPs.

Context. The C-MetaLL project has provided homogeneous spectroscopic abundances of 290 Classical Cepheids (DCEPs) for which we have the intensity-averaged magnitudes in multiple optical and NIR bands, periods, pulsation modes, and Gaia parallaxes corrected for individual zero-point (ZP) biases. Aims. Our goal is to derive updated period–Wesenheit–metallicity (PWZ) relations using the largest and most homogeneous metallicity sample ever used for such analyses, covering a range of −1.3 < [Fe/H] < +0.3 dex, and to assess the metallicity dependence of these relations. Methods. We computed several optical and NIR Wesenheit magnitudes adopting both Cardelli et al. (1989, ApJ, 345, 245) and Fitzpatrick (1999, PASP, 111, 63) reddening laws, and transformed Johnson-Cousins Wesenheit magnitudes into their HST equivalents using empirical relations. Using 275 DCEPs with reliable parallaxes, we applied a robust photometric parallax technique, which simultaneously fits all parameters – including the global ZP counter-correction to Gaia parallaxes – and handles outliers via a Cauchy likelihood to account for the sample’s excess variance. Results. We find a stronger metallicity dependence ( γ ≈ −0.5 mag/dex in optical, −0.4 mag/dex in NIR) than recent literature reports. Gaia parallax ZP counter-correction ( ϵ ) varies moderately across bands, with an average value of ∼10 μas, aligning with previous determinations. Applying our PWZ relations to ∼4500 LMC Cepheids yields distances generally consistent within 1 σ with geometric estimates. The choice of reddening law has a small impact, while using only fundamental-mode pulsators significantly increases the uncertainties. Including α element corrections increases | γ | and reduces ϵ . However, we find 1 σ consistency γ values with the literature, particularly for the Wesenheit magnitude in the HST bands, by restricting the sample to brighter (i.e. closer) objects, or by including only pulsators with −0.7 < [Fe/H] < 0.2 dex. Our results hint at a large γ or a non-linear dependence on metallicity of DCEP luminosities at the metal-poor end, which is difficult to quantify with the precision of parallaxes of the present dataset.

Context . Classical Cepheids (DCEPs) are among the most important distance calibrators thanks to the correlation between their period and luminosity (PL relation), and play a crucial role in the calibration as the first rung of the extragalactic distance ladder. Given their typical age, they also constitute an optimal tracer of the young population in the Galactic disc. Aims . We aim to increase the number of available DCEPs with high-resolution spectroscopic metallicities, study the galactocentric radial gradients of several chemical elements, and analyse the spatial distribution of the Galactic young population of stars in the Milky Way disc. Methods . We performed a complete spectroscopical analysis of 136 spectra obtained from three different high-resolution spectrographs, for a total of 60 DCEPs. More than half have pulsational periods longer than 15 days, up to 70 days, doubling the number of stars in our sample with P > 15 d . We derived radial velocities, atmospheric parameters, and chemical abundances for up to 33 different species. Results . We present an updated list of trusted spectroscopic lines for the detection and estimation of chemical abundances. We used this new set to revisit the abundances already published in the context of the C-MetaLL (Cepheids-Metallicity in the Leavitt Law) survey and increase the number of available chemical species. For the first time (to our knowledge), we present the estimation of abundances for Cepheids for dysprosium (Dy, Z = 66), as well as a systematic estimation of erbium (Er, Z Z = 68), lutetium (Lu, Z = 71), and thorium (Th, Z = 90) abundances. Conclusions . We calculated a galactic radial gradient for [ Fe/H ] with a slope of −0.064 ± 0.002 dex kpc ⁻¹ , in good agreement with recent literature estimation. The other elements also exhibit a clear negative radial trend, with this effect diminishing and eventually disappearing for heavier neutron-capture elements. Depending on the proposed spiral arms model present in several literature sources, our most external stars agree on tracing either the Perseus, the Norma-Outer, or both the Outer and the association Outer-Scutum-Centaurus arms.

Context . Surface brightness–colour relations (SBCRs) are widely used to estimate the angular diameters of stars. In particular, they are employed in the Baade–Wesselink distance determination method, which relies on the comparison between the linear and angular amplitudes of Cepheids. The SBCR can be calibrated by combining different photometric systems. An SBCR was recently calibrated based on Gaia DR3 magnitudes alone for fundamental-mode classical Cepheids with solar metallicity. This relation appears to be strongly affected by metallicity, however. Aims . We derive SBCRs for classical Cepheids in the Milky Way and in the Magellanic Clouds using the photometric data available in the Gaia database, and we quantify the metallicity effect. Methods . We first selected the data on the basis of a number of quality criteria and chose the best photometric data and the best parallaxes available in Gaia for Milky Way classical Cepheids. Secondly, we compiled an extensive list of period–radius (PR) relations available in the literature, and we also provide a new PR relation based on interferometric data in our previous work. Thirdly, combining the radius of classical Cepheids with distance estimates (based on Gaia parallaxes for the Milky Way and on eclipsing binaries for the Magellanic Clouds), we derived the surface brightness and colour of about 1700 classical Cepheids. Results . We first derived a new PR relation based on interferometric data and distances from the literature of seven classical Cepheids: log(R/R _⊙ ) = 1.133 _±0.019 + 0.688 _±0.016 log(P). The metallicity does not affect the PR relations. Secondly, we calculated three different SBCRs for the Milky Way and Large and Small Magellanic Cloud classical Cepheids based on this new PR relation that clearly show the dependence of the metallicity on the SBCR based on Gaia magnitudes alone. Finally, we derived relations between the slopes, the zero points (ZP), and the metallicity ([Fe/H]) of these three SBCRs: Slope _SBCR = −0.0663 _±0.0121 [Fe/H] − 0.3010 _±0.0030 and ZP _SBCR = −0.1016 _±0.0091 [Fe/H] + 3.9988 _±0.0029 . Conclusions . These new SBCRs, dedicated to classical Cepheids in the Milky Way and Magellanic Clouds, are of particular importance to apply the inverse Baade–Wesselink method to classical Cepheids observed by Gaia in a forthcoming study.

Context . The infrared surface brightness (IRSB) technique is a specific application of the Baade-Wesselink method. Given proper calibration, well-covered optical and near-infrared photometry, as well as radial velocity curves, it allows the estimation of distances to individual pulsating stars and the determination of their mean radii. Even though it is observationally demanding, it offers a way of distance determination to systems of pulsating stars that is independent of period-luminosity relations. The technique is fully empirical and does not depend on stellar atmosphere models. Aims . The goal of the work is to test the precision of distance determinations to individual RR Lyrae stars and to their host system as a whole using the IRSB technique for a relatively distant globular cluster M 3 (NGC 5272). We also determined the mean radii and period-radius relations for these stars in order to compare them with the existing theoretical predictions and empirical estimations for the field stars from the solar neighborhood. Methods . We used the data available in the literature and the calibration of the IRSB technique based on the RR Lyrae stars from the solar neighborhood we published previously in order to determine distances to 14 RR Lyrae stars in the globular cluster M 3. We applied the IRSB technique, as previously presented in our work for RR Lyraes, from the solar vicinity with an extra determination and implementation of phase shifts between observables due to the phase incoherence of data gathered at different epochs. We studied the impact of the selection of the fitting procedure (bisector vs. the least-squares fit) on the results. We applied five different empirical surface brightness-color relations from the literature in the analysis. Results . We obtained a mean distance to M 3 of r _M3 = (10.07 ± 0.19 ± 0.29) kpc, which corresponds to a distance modulus μ _M3 = (15.015 ± 0.041 ± 0.063) mag and a 7% scatter of individual stellar distances for 14 RR Lyrae stars in M 3. We obtained a very good agreement between the two fitting techniques; the bisector proved to be a biased estimator, while the least-squares fit yielded slightly larger uncertainties. We also determined mean stellar radii for pulsators in the sample with a precision of 0.5% and obtained excellent agreement with a theoretical prediction of the period-radius relation for RRab stars available in the literature.

With the 4-metre Multi-Object Spectroscopic Telescope (4MOST) expected to provide an influx of transient spectra when it begins observations in early 2026 we consider the potential for real-time classification of these spectra. We investigate three extant spectroscopic transient classifiers: the Deep Automated Supernova and Host classifier (dash), Next Generation SuperFit (ngsf), and SuperNova IDentification (snid), with a focus on comparing the completeness and purity of the transient samples they produce. We manually simulate fibre losses critical for accurately determining host contamination and use the 4MOST Exposure Time Calculator to produce realistic, 4MOST-like, host-galaxy contaminated spectra. We investigate the three classifiers individually and in all possible combinations. We find that a combination of dash and ngsf can produce a supernova (SN) Ia sample with a purity of 99.9 per cent, while successfully classifying 70 per cent of SNe Ia. However, it struggles to classify non-SN Ia transients. We investigate photometric cuts to transient magnitude and the transient’s fraction of total fibre flux, finding that both can be used to improve non-SN Ia transient classification completeness by 8–44 per cent with SNe Ibc benefitting the most and superluminous (SL) SNe the least. Finally, we present an example classification plan for live classification and the predicted purities and completeness across five transient classes: Ia, Ibc, II, SL, and non-SN transients. We find that it is possible to classify 75 per cent of input spectra with $>$70 per cent purity in all classes except non-SN transients. Precise values can be varied using different classifiers and photometric cuts to suit the needs of a given study.

Context. Anomalous Cepheids (ACs) are pulsating variable stars, and are less studied compared to the well-known Classical Cepheids (CCs) and RR Lyrae stars. The ACs are metal poor ([Fe/H] < 1.5) and follow distinct period-luminosity (PL) and period-Wesenheit (PW) relations that can be used for distance measurements, and they can pulsate in the fundamental (F) and first overtone (1O) modes. Aims. Our goal is to evaluate the precision and accuracy of distances obtained via PL and PW relations of ACs and thus to assess if they could be used to establish a cosmic distance scale independent from CCs. To this aim, we derived new, precise PL and PW relations for the F mode, the 1O mode, and, for the first time, the combined F+1O mode ACs in the Magellanic Clouds. We investigated the wavelength dependence of these relations and applied them to calculate the distances of various stellar systems in the Local Group hosting ACs, as well as to confirm the classification of these variable stars. Methods. We analyzed near-infrared (NIR) time series photometry in the Y, J, and K_s bands for about 200 ACs in the Magellanic Clouds acquired during 2009–2018 in the context of the VISTA survey of the Magellanic Clouds system (VMC), a European Southern Observatory public survey. The VMC NIR photometry was complemented with optical data from Gaia DR3 and the Optical Gravitational Lensing Experiment IV survey, which also provided the identification, periods, and pulsation mode for the investigated ACs. Custom templates generated from our best light curves were used to derive precise intensity-averaged mean magnitudes for 118 and 75 ACs in the Large (LMC) and Small Magellanic Clouds (SMC), respectively. Results. Optical and NIR mean magnitudes were used to derive multiband PL and PW relations, which were calibrated with the geometric distance modulus to the LMC based on eclipsing binaries. We investigated the dependence of PL relations on wavelength, finding that slopes increase and dispersion decreases when going from optical to NIR bands. We calculated the LMC distance modulus through calibrated AC PW relations in the Milky Way using Gaia parallaxes, the LMC-SMC relative distance modulus, and we confirmed the AC nature of a few new pulsators in Galactic globular clusters. We derived a distance modulus for the Draco dwarf spheroidal galaxy of 19.425 ± 0.048 mag, which is in agreement with recent literature determinations, but a discrepancy of 0.1 mag with RR Lyrae-based distance hints at possible metallicity effects on the AC PL and PW relations. Future spectroscopic surveys and Gaia DR4 will refine the AC distance scale and assess metallicity effects on PLRs and PWRs.

Context. Surface brightness–colour relations (SBCRs) are widely used to determine the angular diameters of stars. They are in particular used in the Baade-Wesselink (BW) method of distance determination of Cepheids. However, the impact of the SBCR on the BW distance of Cepheids is about 8%, depending on the choice of SBCR considered in the literature. Aims. We aim to calibrate a precise SBCR dedicated to Cepheids using the best quality interferometric measurements available as well as different photometric bands, including the Gaia bands. Methods. We selected interferometric and photometric data in the literature for seven Cepheids covering different pulsation periods. From the phased photometry in the different bands (VJHKGG_BP G_RP) corrected from extinction and the interferometric limb-darkened angular diameters, we calculated the SBCR associated with each combination of colours. Results. We first find that the seven Cepheids have consistent SBCRs as long as the two magnitudes considered are not too close in wavelengths. For the SBCR (F_V, V − K): F_V = −0.1336_±0.0009(V − K)₀ + 3.9572_±0.0015, we obtain a root mean square (RMS) of 0.0040 mag, which is three times lower than the latest estimate from 2004. Also, for the first time, we present an SBCR dedicated to Cepheids based on Gaia bands only: FG_BP = −0.3001_±0.0030(G_BP − G_RP)₀ + 3.9977_±0.0029, with an excellent RMS of 0.0061 mag. However, using theoretical models, we show that this SBCR is highly sensitive to metallicity. From this empirical multi-wavelength approach, we also show that the impact of the CircumStellar Environment (CSE) of Cepheids emission is not negligible and should be taken into account in the future. Conclusions. With this study, we improve the calibration and our understanding of the SBCR of Cepheids. The overall goal of this project is to provide a purely empirical SBCR version of the BW method that takes into account the metallicity and the CSE emission of Cepheids and that could be applied to individual Cepheids in the local group in the context of JWST and ELT.