Color Excess Research Articles

Digging into the Galactic Bulge: Stellar Population and Structure of the Poorly Studied Cluster NGC 6316

High-resolution Hubble Space Telescope optical observations have been used to analyze the stellar population and the structure of the poorly investigated bulge globular cluster NGC 6316. We constructed the first high-resolution reddening map in the cluster direction, which allowed us to correct the evolutionary sequences in the color–magnitude diagram (CMD) for the effects of differential reddening. A comparison between the CMDs of NGC 6316 and 47 Tucanae revealed strikingly similar stellar populations, with the two systems basically sharing the same turnoff, subgiant branch, and horizontal branch morphologies, indicating comparable ages. The red giant branch in NGC 6316 appears slightly bluer than in 47 Tucanae, suggesting a lower metal content. This has been confirmed by the isochrone fitting of the observed CMD, which provided us with updated values of the cluster age, distance, average color excess, and metallicity. We estimated an absolute age of 13.1 ± 0.5 Gyr, consistent with the age of 47 Tucanae, an average color excess E(B − V) = 0.64 ± 0.01, and a true distance modulus (m − M)0 = 15.27 ± 0.03 that sets the cluster distance at 11.3 kpc from the Sun. In addition, the photometric estimate of the cluster metallicity suggests [Fe/H] ≈ −0.9, which is ∼0.2 dex smaller than that of 47 Tucanae. We also determined the gravitational center and the density profile of the system from resolved stars. The latter is well reproduced by a King model. Our results confirm that NGC 6316 is another extremely old relic of the assembly history of the Galaxy.

Photometric Properties and Stellar Parameters of the Rapidly Rotating Magnetic Early-B Star HD 345439

We first present the multicolor photometry results of the rapidly rotating magnetic star HD 345439 using the Nanshan One-meter Wide-field Telescope. From the photometric observations, we derive a rotational period of 0.7699 ± 0.0014 day. The light curves of HD 345439 are dominated by the double asymmetric S-wave feature that arises from the magnetic clouds. Pulsating behaviors are not observed in Sector 41 of the Transiting Exoplanet Survey Satellite. No evidence is found of the occurrence of centrifugal breakout events neither in the residual flux nor in the systematic variations at the extremum of the light curve. Based on the hypothesis of the Rigidly Rotating Magnetosphere model, we restrict the magnetic obliquity angle β and the rotational inclination angle i so that they satisfy the approximate relation β + i ≈ 105°. The color excess, extinction, and luminosity are determined to be E (B−V) = 0.745 ± 0.016 mag, A V = 2.31 ± 0.05 mag, and dex, respectively. Furthermore, we derive the effective temperature as T eff = 22 ± 1 kK and the surface gravity as log g = 4.00 ± 0.22. The mass , radius , and age Myr are estimated from the Hertzsprung–Russell diagram.

THE PRINCIPAL ASTROPHYSICAL PARAMETERS OF THE OPEN CLUSTERS GULLIVER 18 AND GULLIVER 58 DETERMINED USING GAIA EDR3 DATA

A photometric and astrometric study of the two open star clusters Gulliver18 and Gulliver58 was carried out for the first time using the early third data release of the Gaia space observatory (Gaia-EDR3). By studying the proper motions, parallaxes, and color-magnitude diagrams of the two clusters, we determined their actual cluster membership. Therefore, ages, color excesses, and heliocentric distances of the clusters could be determined. The luminosity function, mass function, total mass, mass segregation, and relaxation time of Gulliver18 and Gulliver58 were estimated as well.

A STUDY OF THE NGC 1193 AND NGC 1798 OPEN CLUSTERS USING CCD UBV PHOTOMETRIC AND GAIA EDR3 DATA

We present photometric, astrometric, and kinematic studies of the old open star clusters NGC 1193 and NGC 1798. Both of the clusters are investigated by combining data sets from Gaia Early Data Release 3 (EDR3) and CCD UBV observational data. E(B − V ) color excesses are derived for NGC 1193 as 0.150 ± 0.037 and for NGC 1798 as 0.505 ± 0.100 mag through the use of two-color diagrams. Photometric metallicities are also determined from two-color diagrams with the results of [Fe/H]=−0.30 ± 0.06 dex for NGC 1193 and [Fe/H]=−0.20 ± 0.07 dex for NGC 1798. The isochrone fitting distance and age of NGC 1193 are 5562 ± 381 pc and 4.6±1 Gyr, respectively. For NGC 1798, these parameters are 4451±728 pc and 1.3±0.2 Gyr. Kinematic and dynamic orbital calculations indicate that NGC 1193 and NGC 1798 belong to the thick-disk and thin-disk populations, respectively.

Photometric Analysis of Three Potential Red Nova Progenitors

We present photometric analysis of three bright red nova progenitor contact binary systems: ASAS J082151-0612.6, TYC 7281-269-1 and TYC 7275-1968-1. The primary components in all three systems are solar-type low mass stars with radii somewhat larger than their zero age main sequence counterparts. The secondaries, as in most contact binary systems, have radii and luminosities well above their main sequence counterparts. All three have extremely low mass ratios ranging from 0.075 to 0.097 and two have high degrees of contact, in excess of 75%. All three have mass ratios and separations below the theoretical values for orbital stability. Chromospheric activity, a hallmark of magnetic activity and magnetic braking, considered important in mediating angular momentum loss, is also explored. All three systems demonstrate the O’Connell effect, and all systems require the introduction of star spots for a better light curve solution. In addition, we show that ASAS J082151-0612.6 and TYC 7281-269-1 have a UV color excess in the range indicating high chromospheric activity. Another measure of potential significant magnetic activity is X-ray luminosity; TYC 7275-1968-1 and probably also TYC 7281-269-1 have X-ray luminosity well above other contact binary systems. We conclude that it is likely that all three are unstable and hence are potential merger candidates.

Re-examining the Bayesian colour excess estimation for the local star-forming galaxies observed in the HETDEX pilot survey

ABSTRACTIn my previous reanalysis of the local star-forming galaxies observed in the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) pilot survey, I reported that the overestimation of E(B − V), hence the star formation rate (SFR), undermined the claim of new galaxy population discovery in the original study. Herein, I re-examine whether the E(B − V) overestimation problem can be alleviated in the Bayesian parameter estimation framework by adopting scientifically motivated new priors. I modelled the emission-line fluxes of galaxies using the strong-line method and four model parameters – metallicity $12 + \text{log {}(O/H)}$, nebula emission-line colour excess E(B − V), intrinsic [O iii] λ5007 line flux, and intrinsic [N ii] λ6584 line flux. Based on mock data tests, I found that all input values can be recovered within and around the 1σ credible interval by adopting suitable priors for the intrinsic [O iii] λ5007 and [N ii] λ6584 line fluxes: the inverse gamma distribution reflecting the logical constraint that an intrinsic emission-line flux must exceed the observed (reddened) emission-line flux. The mock data tests were performed for two metallicity calibrations, three colour excess input values [E(B − V) = 0.1, 0.3, and 0.5] and two metallicity input values [$12 + \text{log {}(O/H)}$ = 8.0 and 8.5]. I also found that the new prior can diminish the SFR overestimation eightfold. This study demonstrates how the Bayesian parameter estimation can achieve more accurate estimates with no further observations when the likelihood does not constrain the model parameters correctly.

The Stellar Abundances and Galactic Evolution Survey: Photonic Passbands and Extinction Coefficients for the u and v Bands

The Stellar Abundances and Galactic Evolution Survey (SAGES) is a multi-band photometric survey focused on estimation of stellar atmospheric parameters and interstellar extinction. In this paper we have derived photonic passbands for the intermediate-band u and v filters of the SAGES photometric system. The derived photonic passbands have been compared with those of the u and v filters of the Strömgren and SkyMapper systems. Synthetic photometry based on the derived photonic passbands could reproduce the observations very well. We have also derived observed, model-free extinction coefficients for the SAGES u and v bands (as well as the Pan-STARRS grizy bands) using the “standard pair” method. The derived reddening coefficients have been compared with those predicted by the extinction laws. Variations of reddening coefficients with effective temperatures and color excesses of B–V given by Schlegel et al. (E(B−V)SFD) have been investigated. No obvious trends or significant variations with effective temperatures have been found, but reddening coefficients for all the colors exhibit declining trends with increasing E(B−V)SFD, with typical relative variations of twenty-some percent from E(B − V)SFD ∼ 0 to 1.

Reddening and the shape of the variable component of the continua of active galactic nuclei from the optical to the far-ultraviolet – I

ABSTRACT We analyse the photometric variability of 4611 active galactic nuclei (AGNs) from the Sloan Digital Sky Survey (SDSS) Stripe 82. We recover the spectral energy distribution (SED) of the variable flux as a function of wavelength. For rest wavelengths longer than ∼2200 Å, we find that the SED of the variable component of the bluest AGNs is consistent with the Fν ∝ ν+1/3 spectrum predicted for an externally illuminated accretion disc. We confirm there is some residual variable emission corresponding to the ‘small blue bump’ and other broad-line region variability. We interpret steeper optical spectra of the variable component as being due to intrinsic reddening. This is supported by the correlation of the Balmer decrement with the colour excess of the variable component. We find the median internal reddening of SDSS AGNs in Stripe 82 with z < 0.4 to be E(B − V) ≈ 0.10 in agreement with the reddening derived from the Balmer decrement. Individual AGNs in the sample can have E(B − V) > 0.4.

Effects of Binaries on Open Cluster Age Determination in Bayesian Inference

We investigate the effects of binaries on the cluster age determination for 561 open clusters in the Galactic disk via the Bayesian statistical framework. Stellar properties of these star clusters, including age, metallicity, distance modulus, color excess, binary fraction, and rotating star fraction, are derived from color–magnitude-diagrams (CMDs) via isochrone fitting to high-precision Gaia EDR3 data. Across the simple stellar population of binary and single-star, age differences can be found with the same star cluster. A Bayesian applied regression modeling software, Stan, is employed to explore how much binaries affect the age determination of open clusters. Our results present less statistically significant difference between the binary-star simple populations (bsSSPs) and the single-star simple populations (ssSSPs) for cluster age determination. For all clusters in our sample, the ages estimated using the bsSSPs models are younger than those estimated using the ssSSPs, with a mean value of ∼70 Myr. However, it is found that for 52.5% clusters in our sample, ages are relatively sensitive to the presence of binaries, at least ∼25% younger. This suggests that in studies of open cluster age determinations, the effects of binary interactions on the whole sample are not prominent, but its effects on some clusters should still be included as an essential ingredient.

Sensitivity of the Hubble Constant Determination to Cepheid Calibration

Motivated by the large observed diversity in the properties of extragalactic extinction by dust, we reanalyze the Cepheid calibration used to infer the Hubble constant, H 0, from Type Ia supernovae, using Cepheid data in 19 Type Ia supernova host galaxies from Riess et al. and anchor data from Riess et al. Unlike the SH0ES team, we do not enforce a fixed universal color–luminosity relation to correct the Cepheid magnitudes. Instead, we focus on a data-driven method, where the optical colors and near-infrared magnitudes of the Cepheids are used to derive individual color–luminosity relations for each Type Ia supernova host and anchor galaxy. We present two different analyses, one based on Wesenheit magnitudes, resulting in H 0 = 73.2 ± 1.3 km s−1 Mpc−1, a 4.2σ tension with the value inferred from the cosmic microwave background. In the second approach, we calibrate an individual extinction law for each galaxy, with noninformative priors using color excesses, yielding H 0 = 73.9 ± 1.8 km s−1 Mpc−1, in 3.4σ tension with the Planck value. Although the two methods yield similar results, in the latter approach, the Hubble constants inferred from the individual Cepheid absolute distance calibrator galaxies range from H 0 = 68.1 ± 3.5 km s−1 Mpc−1 to H 0 = 76.7 ± 2.0 km s−1 Mpc−1. Taking the correlated nature of H 0 inferred from individual anchors into account, and allowing for individual extinction laws, the Milky Way anchor is in 2.1–3.1 σ tension with the NGC 4258 and Large Magellanic Cloud anchors, depending on prior assumptions regarding the color–luminosity relations and the method used for quantifying the tension.

The extinction law in the inner 3 × 3 deg2 of the Milky Way and the red clump absolute magnitude in the inner bar-bulge

ABSTRACT 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.

Three-dimensional geometry and dust/gas ratios in massive star-forming regions over the entire LMC as revealed by the IRSF/SIRIUS survey

Abstract We derive the entire dust extinction (AV) map for the Large Magellanic Cloud (LMC) estimated from the color excess at near-infrared wavelengths. Using the percentile method we recently adopted to evaluate AV distribution along the line of sight, we derive three-dimensional (3D) AV maps of the three massive star-forming regions of N44, N79, and N11 based on the IRSF/SIRIUS point source catalog. The 3D AV maps are compared with the hydrogen column densities N(H) of three different velocity components where one is of the LMC disk velocity and the other two are of velocities lower than the disk velocity. As a result, we obtain a 3D dust geometry suggesting that gas collision is ongoing between the different velocity components. We also find differences in the timing of the gas collision between the massive star-forming regions, which indicates that the gas collision in N44, N79, and N11 occurred later than that in 30 Doradus. In addition, a difference of a factor of two in AV/N(H) is found between the velocity components for N44, while a significant difference is not found for N79 and N11. From the 3D geometry and AV/N(H) in each star-forming region, we suggest that the massive star formation in N44 was induced by an external trigger of tidal interaction between the LMC and the Small Magellanic Cloud, while that in N79 and N11 is likely to have been induced by internal triggers such as gas converging from the galactic spiral arm and expansion of a supershell, although the possibility of tidal interaction cannot be ruled out.

The MaNGA firefly Value-Added-Catalogue: resolved stellar populations of 10,010 nearby galaxies

Abstract We present the MaNGAfireflyValue-Added-Catalogue (VAC) – a catalogue of ∼3.7 million spatially resolved stellar population properties across 10,010 nearby galaxies from the final data release of the MaNGA survey. The full spectral fitting code firefly is employed to derive parameters such as stellar ages, metallicities, stellar and remnant masses, star formation histories, star formation rates and dust attenuation. In addition to Voronoi-binned measurements, our VAC also provides global properties, such as central values and radial gradients. Two variants of the VAC are available: presenting the results from fits using the M11-MILES and the novel MaStar stellar population models. MaStar allows to constrain the fit over the whole MaNGA wavelength range, extends the age-metallicity parameter space, and uses empirical spectra from the same instrument as MaNGA. The fits employing MaStar models find on average slightly younger ages, higher mass-weighted metallicities and smaller colour excesses. These differences are reduced when matching wavelength range and converging template grids. We further report that firefly stellar masses are systematically lower by ∼0.3 dex than masses from the MaNGA PCA and Pipe3D VACs, but match masses from the NSA best with only ∼0.1 dex difference. Finally, we show that firefly stellar ages correlate with spectral index age indicators HδA and Dn(4000), though with a clear additional metallicity dependence.

The young Galactic cluster NGC 225: binary stars’ content and total mass estimate

Abstract Galactic star clusters are known to harbour a significant amount of binary stars, yet their role in the dynamical evolution of the cluster as a whole is not comprehensively understood. We investigated the influence of binary stars on the total mass estimate for the case of the moderately populated Galactic star cluster NGC 225. The analysis of multi-epoch radial velocities of the 29 brightest cluster members, obtained over two observational campaigns, in 1990-1991 and in 2019-2020, yields a value of binary fraction of α = 0.52 (15 stars out of 29). Using theoretical isochrones and Monte Carlo simulations we found that the cluster mass increases at least 1.23 times when binaries are properly taken into account. By combining Gaia EDR3 photometric data with our spectroscopic observations, we derived estimates of NGC 225 fundamental parameters foras follows: mean radial velocity <Vr > = − 9.8 ± 0.7 km s−1, log (τ)=8.0-8.2 dex, distance D = 676 ± 22 pc, and colour excess E(B − V) = 0.29 ± 0.01 mag.

Infrared Spectroscopic Survey of the Quiescent Medium of Nearby Clouds. II. Ice Formation and Grain Growth in Perseus and Serpens

The properties of dust change during the transition from diffuse to dense clouds as a result of ice formation and dust coagulation, but much is still unclear about this transformation. We present 2–20 μm spectra of 49 field stars behind the Perseus and Serpens Molecular Clouds and establish relationships between the near-infrared continuum extinction (A K) and the depths of the 9.7 μm silicate (τ 9.7) and 3.0 μm H2O ice (τ 3.0) absorption bands. The τ 9.7/A K ratio varies from large, diffuse interstellar medium-like values (∼0.55), to much lower ratios (∼0.26). Above extinctions of A K ∼ 1.2 (A V ∼ 10; Perseus, Lupus, dense cores) and ∼2.0 (A V ∼ 17; Serpens), the τ 9.7/A K ratio is lowest. The τ 9.7/A K reduction from diffuse to dense clouds is consistent with a moderate degree of grain growth (sizes up to ∼0.5 μm), increasing the near-infrared color excess (and thus A K), but not affecting the ice and silicate band profiles. This grain growth process seems to be related to the ice column densities and dense core formation thresholds, highlighting the importance of density. After correction for Serpens foreground extinction, the H2O ice formation threshold is in the range of A K = 0.31–0.40 (A V = 2.6–3.4) for all clouds, and thus grain growth takes place after the ices are formed. Finally, abundant CH3OH ice (∼21% relative to H2O) is reported for 2MASSJ18285266+0028242 (Serpens), a factor of >4 larger than for the other targets.

Strong [O iii] λ5007 Emission-line Compact Galaxies in LAMOST DR9: Blueberries, Green Peas, and Purple Grapes

Abstract Green Pea and Blueberry galaxies are well known for their compact size, low mass, strong emission lines, and analogs to high-z Lyα-emitting galaxies. In this study, 1547 strong [O iii] λ5007 emission-line compact galaxies with 1694 spectra are selected from LAMOST DR9 at the redshift range from 0.0 to 0.59. According to the redshift distribution, these samples can be separated into three groups: Blueberries, Green Peas, and Purple Grapes. Optical [Mg ii] λ2800 line feature, BPT diagram, multiwavelength spectral energy distribution (SED) fitting, mid-IR (MIR) color, and MIR variability are deployed to identify 23 active galactic nucleus candidates from these samples, which are excluded for the following star formation rate (SFR) discussions. We perform the multiwavelength SED fitting with GALEX UV and WISE MIR data. Color excess from the Balmer decrement shows that these strong [O iii] λ5007 emission-line compact galaxies are not highly reddened. The stellar mass of the galaxies is obtained by fitting LAMOST calibrated spectra with the emission lines masked. We find that the SFR is increasing with the increase of redshift, while for the sources within the same redshift bin the SFR increases with mass with a similar slope to the star-forming main sequence. These samples have a median metallicity of 12 + log(O/H) of 8.10. The metallicity increases with mass, and all the sources are below the mass–metallicity relation. The direct-derived T e -based metallicity from the [O iii] λ4363 line agrees with the empirical N2-based empirical gas-phase metallicity. Moreover, these compact strong [O iii] λ5007 lines are mostly in a less dense environment.

LISC Catalog of Star Clusters. I. Galactic Disk Clusters in Gaia EDR3

Abstract This work searches for the candidates of Galactic disk star clusters in Gaia Early Data Release 3 (Gaia EDR3) and determines their basic parameters from color–magnitude diagrams (CMDs). A friends-of-friends method for membership determination and stellar population models including binary stars (ASPS) and rotating stars are adopted. As a result, 868 new star cluster candidates are found, besides 2729 known ones. When checking the CMD of each candidate, 61 new candidates show main sequences including a turnoff, which suggests that they are real star clusters. The basic parameters, including distance modulus, color excess, metallicity, age (or age range), primordial binary fraction, and rotating star fraction, are determined carefully by fitting the morphologies of CMDs of 61 newly identified star clusters and 594 known star clusters, which have relatively clear main sequences. The CMDs are fitted in considerable detail to ensure the reliability of property parameters of clusters. All final results are included in a new star cluster catalog, which is named LI team’s Star Cluster (LISC), and the catalog is available in the Zenodo repository.

A detailed study of stellar populations of three star clusters with extended main-sequence turnoffs

ABSTRACT This paper investigates the extended main-sequence turnoffs (eMSTOs) and stellar populations of three clusters with different ages using Gaia Data Release 2 (DR2) data in detail. The colour–magnitude diagrams (CMDs) of three Galactic clusters show broad main sequences, eMSTOs, blue stragglers, and red giant clumps. We explore the effects of age spread, binaries, and rotating stars on the morphology of the entire CMD, paying particular attention to the effects of binary stars and rotating stars on eMSTOs. Some synthetic CMDs are built on the basis of the advanced stellar population synthesis (ASPS) model to study the observed CMDs and search for the best-fitting stellar population models. The age of the youngest stars, age spread, metallicity, colour excess, distance modulus, binary fraction, and rotating star fraction of clusters are determined. The results show that both binaries and rotating stars are responsible for the eMSTO of intermediate-age cluster LP 585, but binary stars affect more than rotating stars on the eMSTO morphologies of the youngest (M11) and oldest (NGC 6819) clusters in our samples. It suggests that M11 (∼0.3 Gyr) is possibly a composite stellar population of rotating and binary stars, while clusters LP 585 (∼1.1 Gyr) and NGC 6819 (∼2.0 Gyr) are simple stellar populations of rotating and binary stars.

Inspecting the Cepheid parallax of pulsation using Gaia EDR3 parallaxes

Context. As primary anchors of the distance scale, Cepheid stars play a crucial role in our understanding of the distance scale of the Universe because of their period-luminosity relation. Determining precise and consistent parameters (radius, temperature, color excess, and projection factor) of Cepheid pulsating stars is therefore very important. Aims. With the high-precision parallaxes delivered by the early third Gaia data release (EDR3), we aim to derive various parameters of Cepheid stars in order to calibrate the period-luminosity and period-radius relations and to investigate the relation of period to p-factor. Methods. We applied an implementation of the parallax-of-pulsation method through the algorithm called spectro-photo-interferometry of pulsating stars (SPIPS), which combines all types of available data for a variable star (multiband and multicolor photometry, radial velocity, effective temperature, and interferometry measurements) in a global modeling of its pulsation. Results. We present the SPIPS modeling of a sample of 63 Galactic Cepheids. Adopting Gaia EDR3 parallaxes as an input associated with the best available dataset, we derive consistent values of parameters for these stars such as the radius, multiband apparent magnitudes, effective temperatures, color excesses, period changes, Fourier parameters, and the projection factor. Conclusions. Using the best set of data and the most precise distances for Milky Way Cepheids, we derive new calibrations of the period-luminosity and period-radius relations: MKS = −5.529±0.015 − 3.141±0.050(log P − 0.9) and log R = 1.763±0.003 + 0.653±0.012(log P − 0.9). After investigating the dependences of the projection factor on the parameters of the stars, we find a high dispersion of its values and no evidence of its correlation with the period or with any other parameters such as radial velocity, temperature, or metallicity. Statistically, the p-factor has an average value of p = 1.26 ± 0.07, but with an unsatisfactory agreement (σ = 0.15). In absence of any clear correlation between the p-factor and other quantities, the best agreement is obtained under the assumption that the p-factor can take any value in a band with a width of 0.15. This result highlights the need for a further examination of the physics behind the p-factor.

Two c’s in a pod: cosmology-independent measurement of the Type Ia supernova colour–luminosity relation with a sibling pair

ABSTRACT Using Zwicky Transient Facility (ZTF) observations, we identify a pair of ‘sibling’ Type Ia supernovae (SNe Ia), i.e. hosted by the same galaxy at z = 0.0541. They exploded within 200 d from each other at a separation of $0.6\,\mathrm{ arcsec}$ corresponding to a projected distance of only 0.6 kpc. Performing salt2 light-curve fits to the gri ZTF photometry, we show that for these equally distant ‘standardizable candles’, there is a difference of 2 mag in their rest-frame B-band peaks, and the fainter supernova (SN) has a significantly red salt2 colour c = 0.57 ± 0.04, while the stretch values x1 of the two SNe are similar, suggesting that the fainter SN is attenuated by dust in the interstellar medium of the host galaxy. We use these measurements to infer the salt2 colour standardization parameter, β = 3.5 ± 0.3, independent of the underlying cosmology and Malmquist bias. Assuming the colour excess is entirely due to dust, the result differs by 2σ from the average Milky Way total-to-selective extinction ratio, but is in good agreement with the colour–brightness corrections empirically derived from the most recent SN Ia Hubble–Lemaitre diagram fits. Thus we suggest that SN ‘siblings’, which will increasingly be discovered in the coming years, can be used to probe the validity of the colour and light-curve shape corrections using in SN Ia cosmology while avoiding important systematic effects in their inference from global multiparameter fits to inhomogeneous data sets, and also help constrain the role of interstellar dust in SN Ia cosmology.