An empirical test of the theoretical population corrections to the red clump absolute magnitude

Context. Gaia Data Release 1 allows the recalibration of standard candles such as the red clump stars. To use those stars, they first need to be accurately characterised. In particular, colours are needed to derive interstellar extinction. As no filter is available for the first Gaia data release and to avoid the atmosphere model mismatch, an empirical calibration is unavoidable. Aims. The purpose of this work is to provide the first complete and robust photometric empirical calibration of the Gaia red clump stars of the solar neighbourhood through colour–colour, effective temperature–colour, and absolute magnitude–colour relations from the Gaia, Johnson, 2MASS, Hipparcos, Tycho-2, APASS-SLOAN, and WISE photometric systems, and the APOGEE DR13 spectroscopic temperatures. Methods. We used a 3D extinction map to select low reddening red giants. To calibrate the colour–colour and the effective temperature–colour relations, we developed a MCMC method that accounts for all variable uncertainties and selects the best model for each photometric relation. We estimated the red clump absolute magnitude through the mode of a kernel-based distribution function. Results. We provide 20 colour versus G−Ks relations and the first Teff versus G−Ks calibration. We obtained the red clump absolute magnitudes for 15 photometric bands with, in particular, MKs = (−1.606 ± 0.009) and MG = (0.495 ± 0.009) + (1.121 ± 0.128)(G−Ks−2.1). We present a dereddened Gaia-TGAS HR diagram and use the calibrations to compare its red clump and its red giant branch bump with Padova isochrones.

The extinction law from 0.9 to 8 microns in the inner $3\times 3\deg ^2$ of the Milky Way is measured using data from VISTA Variables in the Via Lactea, GLIMPSE, and WISE. Absolute extinction ratios are found by requiring that the observed red clump density peaks at the GRAVITY collaboration distance to the Galactic centre. When combined with selective extinction ratios measured from the bulge giant colour–colour diagrams, we find an extinction law of $A_Z:A_Y:A_J:A_H:A_{K_s}:A_{W1}:A_{[3.6]}:A_{[4.5]}:A_{W2}:A_{[5.8]}:A_{[8.0]} =7.19(0.30):5.11(0.20):3.23(0.11):1.77(0.04):1:0.54(0.02):0.46(0.03):0.34(0.03):0.32(0.03):0.24(0.04):0.28(0.03)$ valid for low extinctions where non-linearities are unimportant. These results imply an extinction law from the Rayleigh Jeans colour excess method of $A_{K_s}=0.677(H-[4.5]-0.188)$. We find little evidence for significant selective extinction ratio variation over the inspected region (around $5\, \mathrm{per\, cent}$). Assuming the absolute extinction ratios do not vary across the inspected region gives an independent measurement of the absolute Ks magnitude of the red clump at the Galactic Centre of $(-1.61\pm 0.07)\, \mathrm{mag}$. This is very similar to the value measured for solar neighbourhood red clump stars giving confidence in the use of red clump stars as standard candles across the Galaxy. As part of our analysis, we inspect the completeness of PSF photometry from the VVV survey using artificial star tests, finding $90\, \mathrm{per\, cent}$ completeness at $K_s\approx 16 \, (17)$ in high (low) density regions and good agreement with the number counts with respect to the GALACTICNUCLEUS and DECAPS catalogues over small regions of the survey.

We present a detailed analysis of the uncertainty on the theoretical population corrections to the Large Magellanic Cloud (LMC) red clump (RC) absolute magnitude, by employing a population synthesis algorithm to simulatetheoretically the photometric and spectroscopic properties of RC stars, under various assumptions concerning the LMC star formation rate (SFR) and age-metallicity relationship (AMR). A comparison of the outcome of our simulations with observations of evolved low- to intermediate-mass stars in the LMC allows one to select the combinations of SFR and AMR that bracket the real LMC star formation history, and to estimate the systematic error on the associated RC population corrections. The most accurate estimate of the LMC distance modulus from the RC method (adopting the OGLE-II reddening maps for the LMC) is obtained from the K-band magnitude, and provides (m - M) 0 , L M C = 18.47 ′ 0.01(random) + 0 . 0 5 -0.06(systematic). Distances obtained from the I band, or from the multicolour RC technique which determines at the same time reddening and distance, both agree (albeit with a slightly larger error bar) with this value.

Distances to individual stars in our own Galaxy are critical in order to piece together the nature of its velocity and spatial structure. Core helium burning red clump (RC) stars have similar luminosities, are abundant throughout the Galaxy, and thus constitute good standard candles. We build a hierarchical probabilistic model to quantify the quality of RC stars as standard candles using parallax measurements from the first Gaia data release. A unique aspect of our methodology is to fully account for (and marginalize over) parallax, photometry, and dust corrections uncertainties, which leads to more robust results than standard approaches. We determine the absolute magnitude and intrinsic dispersion of the RC in 2MASS bands J, H, Ks, Gaia G band, and WISE bands W1, W2, W3, and W4. We find that the absolute magnitude of the RC is $-1.61 \pm$ 0.01 (in Ks), $+0.44 \pm$ 0.01 (in G) , $-0.93 \pm$ 0.01 (in J), $-1.46 \pm$ 0.01 (in H), $-1.68 \pm$ 0.02 (in W1), $-1.69\pm$ 0.02 (in W2), $-1.67 \pm$ 0.02 (in W3), $1.76 \pm$ 0.01 mag (in W4). The mean intrinsic dispersion is $\sim 0.17 \pm$ 0.03 mag across all bands (yielding a typical distance precision of $\sim$ 8%). Thus RC stars are reliable and precise standard candles. In addition, we have also re-calibrated the zero point of the absolute magnitude of the RC in each band, which provide a benchmark for future studies to estimate distances to RC stars. Finally, the parallax error shrinkage in the hierarchical model outlined in this work can be used to obtain more precise parallaxes than Gaia for the most distant RC stars across the Galaxy.

First identification and absolute magnitudes of the red clump stars in the Solar neighbourhood for WISE

We present the most extensive and detailed reddening maps of the Magellanic Clouds (MCs) derived from the color properties of Red Clump (RC) stars. The analysis is based on the deep photometric maps from the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV), covering approximately 670 deg2 of the sky in the Magellanic System region. The resulting maps provide reddening information for 180 deg2 in the Large Magellanic Cloud (LMC) and 75 deg2 in the Small Magellanic Cloud (SMC), with a resolution of 1.′7 × 1.′7 in the central parts of the MCs, decreasing to approximately 27′ × 27′ in the outskirts. The mean reddening is E(V − I) = 0.100 ± 0.043 mag in the LMC and E(V − I) = 0.047 ± 0.025 mag in the SMC. We refine methods of calculating the RC color to obtain the highest possible accuracy of reddening maps based on RC stars. Using spectroscopy of red giants, we find the metallicity gradient in both MCs, which causes a slight decrease of the intrinsic RC color with distance from the galaxy center of ∼0.002 mag/deg in the LMC and between 0.003 and 0.009 mag/deg in the SMC. The central values of the intrinsic RC color are 0.886 and 0.877 mag in the LMC and SMC, respectively. The reddening map of the MCs is available both in downloadable form and as an interactive interface.

Red clump (RC) stars form a distinguishable clump on the color-magnitude diagram, making it a good distance indicator. Currently, the two main absolute magnitude estimation methods in the I-band (700–900 nm) conflict: one assumes a constant RC magnitude as supported by empirical data and one relates magnitude with other physical characteristics as supported by theoretical models. Studies attribute these discrepancies to population effects, such as dust, and recommend the K-band (2000–2400 nm) to minimize them. Past studies analyzed relations between the K-band magnitude, color, metallicity, and age, including those stratified by age at 2 Gyr. This study aims to investigate these different relations to clarify trends from past studies, discover new trends, and compare them to similar relations in the I-band. After analyzing cluster RC data from Gaia DR3, 2MASS, and Gaia-ESO DR5, K-band magnitude exhibits insignificant correlation in all relations (p>0.05) but has the greatest dependence and the least root mean square error (RMS) with metallicity in the old RC. Significant correlation was found between I-band magnitude and metallicity for all RC and young RC (p<0.05), especially the latter (R2 = 0.804). This consistency with theoretical trends suggests weaker I-band population effects than previously believed. Overall, these three relations yield more accurate predictions than the mean magnitude. Thus, studies cannot solely rely on the mean K-band or I-band magnitude to estimate distance, and magnitude relations stratified by age can potentially lead to more accurate RC distance estimations and a more accurately calibrated distance ladder.

We have demonstrated the advantage of combining multiwavelength observations, from the ultraviolet (UV) to near-infrared, to study Kron 3, a massive star cluster in the Small Magellanic Cloud. We have estimated the radius of the cluster Kron 3 to be 2.0 arcmin and for the first time, we report the identification of NUV-bright red clump (RC) stars and the extension of the RC in colour and magnitude in the NUVversus (NUV–optical) colour–magnitude diagram (CMD). We found that extension of the RC is an intrinsic property of the cluster and it is not due to contamination of field stars or differential reddening across the field. We studied the spectral energy distribution of the RC stars, and estimated a small range in temperature ∼5000–5500 K, luminosity ∼60–90 L⊙, and radius ∼8.0–11.0 R⊙ supporting their RC nature. The range of UV magnitudes amongst the RC stars (∼23.3–24.8 mag) is likely caused by the combined effects of variable mass-loss, variation in initial helium abundance (Yini = 0.23–0.28), and a small variation in age (6.5–7.5 Gyr) and metallicity ([Fe/H] = −1.5 to −1.3). Spectroscopic follow-up observations of RC stars in Kron 3 are necessary to confirm the cause of the extended RC.

As a part of our ongoing Araucaria Project on the improvement of stellar distance indicators we present deep near-infrared JK imaging of several fields in four Local Group galaxies: LMC, SMC, and the Carina and Fornax dwarf galaxies. These data were obtained under excellent seeing conditions at the European Southern Observatory Very Large Telescope and New Technology Telescope. We determine the mean red clump star magnitudes in the J and K bands in the four galaxies. A comparison of the extinction-corrected K-band red clump star magnitudes with the tip of the red giant branch magnitude, the mean RR Lyrae star V-band magnitude, and the mean K-band magnitude of Cepheid variables at a period of 10 days (for the LMC and SMC) strongly suggests that the red clump star absolute K-band magnitude has a very low (if any) dependence on metallicity over the broad range of metallicities covered by our target galaxies. This finding is in contrast to the mean I- and J-band red clump star magnitudes, which do have a clear metallicity dependence and which we calibrate from our data. Excellent agreement with the former calibration of the red clump I-band magnitude dependence on metallicity of Udalski is found from our new data. We use the Galactic cluster K-band red clump star data of Grocholski & Sarajedini to demonstrate that the K-band red clump star absolute magnitude also has very little (if any) dependence on age over an age range of about 2–8 Gyr. The present study therefore provides clear evidence that the mean K-band magnitude of red clump stars is an excellent distance indicator, with very small (if any) population corrections to be applied over a large range in metallicity and age. Our findings imply that present-day population corrections calculated from models are only accurate at a ±0.15 mag level, which is a great achievement in itself but not accurate enough for high-precision distance scale work. We determine the distances to all our target galaxies from the K-band red clump magnitude, with very small statistical uncertainties. Comparing these distances with those coming from the observed mean I-band magnitudes of the red clump stars, we find evidence that there is likely to be a problem in the photometric calibration of the local, solar neighborhood red clump star K- or I-band magnitudes, which amounts to some 0.2 mag. A redetermination of the absolute photometric calibration of the Hipparcos-observed nearby red clump stars seems necessary to resolve this problem and put the derivation of absolute distances to Local Group galaxies from their red clump stars on a firmer basis.

Core helium-burning red clump (RC) stars are excellent standard candles in the Milky Way. These stars may have more precise distance estimates from spectrophotometry than from Gaia parallaxes beyond 3 kpc. However, RC stars have values of T eff and that are very similar to some red giant branch (RGB) stars. Especially for low-resolution spectroscopic studies where T eff, , and [Fe/H] can only be estimated with limited precision, separating RC stars from RGB through established methods can incur ∼20% contamination. Recently, Hawkins et al. demonstrated that the additional information in single-epoch spectra, such as the C/N ratio, can be exploited to cleanly differentiate RC and RGB stars. In this second paper of the series, we establish a data-driven mapping from spectral flux space to independently determined asteroseismic parameters, the frequency and the period spacing. From this, we identify 210,371 RC stars from the publicly available LAMOST DR3 and APOGEE DR14 data, with ∼9% of contamination. We provide an RC sample of 92249 stars with a contamination of only ∼3%, by restricting the combined analysis to LAMOST stars with S/Npix ≥ 75. This demonstrates that high-signal-to-noise ratio (S/N), low-resolution spectra covering a broad wavelength range can identify RC samples at least as pristine as their high-resolution counterparts. As coming and ongoing surveys such as TESS, DESI, and LAMOST will continue to improve the overlapping training spectroscopic-asteroseismic sample, the method presented in this study provides an efficient and straightforward way to derive a vast yet pristine sample of RC stars to reveal the three-dimensional (3D) structure of the Milky Way.

We use the main-sequence stars in the LMC cluster NGC 1866 and red clump stars in the local field to obtain two independent estimates of the LMC distance. We apply an empirical main-sequence-fitting technique based on a large sample of subdwarfs with accurate Hipparcos parallaxes in order to estimate the cluster distance modulus, and the multicolor red clump method to derive distance and reddening of the LMC field. We find that the main-sequence fitting and the red clump distance moduli are in significant disagreement; the NGC 1866 distance modulus is equal to (m - M)0,NGC 1866 = 18.33 ± 0.08 (consistent with a previous estimate using the same data and theoretical main-sequence isochrones), while the field stars provide (m - M)0,field = 18.53 ± 0.07. This difference reflects the more general dichotomy in the LMC distance estimates found in the literature. Various possible causes for this disagreement are explored, with particular attention paid to the still uncertain metallicity of the cluster and the star formation history of the field stars.

We obtain a quantitative star formation history (SFH) of a shell-like structure (‘shell’) located in the northeastern part of the Small Magellanic Cloud (SMC). We use the Survey of the MAgellanic Stellar History to derive colour–magnitude diagrams (CMDs), reaching below the oldest main-sequence turnoff, from which we compute the SFHs with CMD-fitting techniques. We present, for the first time, a novel technique that uses red clump (RC) stars from the CMDs to assess and account for the SMC’s line-of-sight depth effect present during the SFH derivation. We find that accounting for this effect recovers a more accurate SFH. We quantify an $\sim$7 kpc line-of-sight depth present in the CMDs, in good agreement with depth estimates from RC stars in the northeastern SMC. By isolating the stellar content of the northeastern shell and incorporating the line-of-sight depth into our calculations, we obtain an unprecedentedly detailed SFH. We find that the northeastern shell is primarily composed of stars younger than $\sim$500 Myr, with significant star formation enhancements around $\sim$250 and $\sim$450 Myr. These young stars are the main contributors to the shell’s structure. We show synchronicity between the northeastern shell’s SFH with the Large Magellanic Cloud’s (LMC) northern arm, which we attribute to the interaction history of the SMC with the LMC and the Milky Way (MW) over the past $\sim$500 Myr. Our results highlight the complex interplay of ram pressure stripping and the influence of the MW’s circumgalactic medium in shaping the SMC’s northeastern shell.

Modern spectroscopic surveys output large data volumes. Theoretical models provide a means to transform the information encoded in these data to measurements of physical stellar properties. However, in detail the models are incomplete and simplified, and prohibit interpretation of the fine details in spectra. Instead, the available data provide an opportunity to use data-driven, differential analysis techniques, as a means towards understanding spectral signatures. We deploy such an analysis to examine core helium-fusing red clump (RC) and shell hydrogen-fusing red giant branch (RGB) stars, to uncover signatures of evolutionary state imprinted in optical stellar spectra. We exploit 786 pairs of RC and RGB stars from the GALAH survey, chosen to minimise spectral differences, with evolutionary state classifications from TESS and K2 asteroseismology. We report sub-percent residual, systematic spectral differences between the two classes of stars, and show that these residuals are significant compared to a reference sample of RC−RC and RGB−RGB pairs selected using the same criteria. First, we report systematic differences in the Swan ($\rm {C}_2$) band and CN bands caused by stellar evolution and a difference in mass, where RGB stars at similar stellar parameters have higher masses than RC stars. Secondly, we observe systematic differences in the line-width of the Hα and Hβ lines caused by a difference in microturbulence, as measured by GALAH, where we measure higher microturbulence in RC stars than RGB stars. This work demonstrates the ability of large surveys to uncover the subtle spectroscopic signatures of stellar evolution using model-free, data-driven methods.

Although red clump (RC) stars are easy to identify due to their stability of luminosity and colour, about 20–50 per cent are actually red giant branch (RGB) stars in the same location on the HR diagram. In this paper, a sample of 210 504 spectra for 184 318 primary RC (PRC) stars from the LAMOST DR7 is identified, which has a purity of higher than 90 per cent. The RC and the RGB stars are successfully distinguished through LAMOST spectra (R ∼ 1800 and signal-to-noise ratio >10) by adopting the XGBoost ensemble learning algorithm, and the secondary RC stars are also removed. The SHapley Additive exPlanations (SHAP) value is used to explain the top features that the XGBoost model selected. The features are around Fe5270, MgH & Mg Ib, Fe4957, Fe4207, Cr5208, and CN, which can successfully distinguish RGB and RC stars. The XGBoost is also used to estimate the ages and masses of PRC stars by training their spectra with Kepler labelled asteroseismic parameters. The uncertainties of mass and age are 13 and 31 per cent, respectively. Verifying the feature attribution model, we find that the age-sensitive element XGBoost is consistent with the literature. Distance of the PRC stars is derived by KS absolute magnitude calibrated by Gaia EDR3, which has an uncertainty of about 6 per cent and shows the stars mainly located at the Galactic disc. We also test the XGBoost with R ∼ 250, which is the resolution of the Chinese Space Station Telescope under construction; it is still capable of finding sensitive features to distinguish RC and RGB.

This study presents the multiwavelength investigation of the absolute magnitudes and colors of the red clump (RC) stars selected from APOGEE and GALAH DR2 combined catalog which is complemented with Gaia DR2 astrometric data and multiwavelength photometric data of GALEX GR6/7, SDSS DR7, Gaia DR2, 2MASS, and WISE sky surveys. The analyses are centered on the different distance estimation methods using Gaia trigonometric parallaxes, (1/ϖ) and Bayes statistics, and chemically defined Galactic disk populations on [α/Fe] × [Fe/H] plane. This investigation questions the long-studied problem of the population effects on RC luminosity. Using two different distance estimation approaches, (i) chemical thin and chemical thick disk RC stars are shown to have different absolute magnitudes, while colors remain the same in all photometric bands. Absolute magnitudes vary between −0.12 and +0.13 mag for the 1/ϖ with the change of the Galactic population. This variation in absolute magnitudes is found to be larger for the other method, (ii) the Besançon population synthesis model of Galaxy for 2MASS photometry, in which the absolute magnitude difference between chemical populations were found between −0.35 and −0.40 mag from thin disk to thick disk. When results compared with each other, differences of absolute magnitudes are about three times larger in the model than in the observations. We confirm that the RC absolute magnitudes depend on α-element abundances of Galactic populations.