arXiv subjects

Richard I. Anderson

Publications and source records attributed to Richard I. Anderson.

At least 55 records · Page 3Linked to original sources

arXiv2023

Investigating Gaia EDR3 parallax systematics using asteroseismology of Cool Giant Stars observed by Kepler, K2, and TESS I. Asteroseismic distances to 12,500 red-giant stars

Gaia EDR3 has provided unprecedented data that generate a lot of interest in the astrophysical community, despite the fact that systematics affect the reported parallaxes at the level of ~ 10 muas. Independent distance measurements are available from asteroseismology of red-giant stars with measurable parallaxes, whose magnitude and colour ranges more closely reflect those of other stars of interest. In this paper, we determine distances to nearly 12,500 red-giant branch and red clump stars observed by Kepler, K2, and TESS. This is done via a grid-based modelling method, where global asteroseismic observables, constraints on the photospheric chemical composition, and on the unreddened photometry are used as observational inputs. This large catalogue of asteroseismic distances allows us to provide a first comparison with Gaia EDR3 parallaxes. Offset values estimated with asteroseismology show no clear trend with ecliptic latitude or magnitude, and the trend whereby they increase (in absolute terms) as we move towards redder colours is dominated by the brightest stars. The correction model proposed by Lindegren et al. (2021) is not suitable for all the fields considered in this study. We find a good agreement between asteroseismic results and model predictions of the red clump magnitude. We discuss possible trends with the Gaia scan law statistics, and show that two magnitude regimes exist where either asteroseismology or Gaia provides the best precision in parallax.

astro-ph.SR↗

arXiv2023

Young stellar distance indicators and the extragalactic distance scale

The extragalactic distance scale is perhaps the most important application of stellar distance indicators. Among these, classical Cepheids are high-accuracy standard candles that support a $1.4\%$ measurement of Hubble's constant, $H_0$. The accuracy of Cepheid distances is thus directly relevant for understanding the implications of the Hubble tension, the $>5σ$ discord among direct, late-Universe $H_0$ measurements and $H_0$ values inferred from the early Universe observations assuming $Λ$CDM cosmology. This invited review aims to provide an accessible overview of the state of the art distance ladder that has established the Hubble tension, with a focus on Cepheids, their absolute calibration using trigonometric parallaxes from the ESA mission Gaia, and other Cepheid-related systematics. New observational facilities such as JWST and upcoming large surveys will provide exciting avenues to further improve distance estimates based on Cepheids.

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arXiv2023

A 0.9% Calibration of the Galactic Cepheid luminosity scale based on Gaia DR3 data of open clusters and Cepheids

We have conducted a search for open clusters in the vicinity of classical Galactic Cepheids based on high-quality astrometry from the third data release (DR3) of the ESA mission Gaia to improve the calibration of the Leavitt law (LL). Our approach requires no prior knowledge of existing clusters, allowing us to both detect new host clusters and cross-check previously reported associations. Our Gold sample consists of 34 Cepheids residing in 28 open clusters, including 27 fundamental mode and 7 overtone Cepheids. Three new bona fide cluster Cepheids are reported (V0378 Cen, ST Tau, and GH Lup) and the host cluster identifications for three others (VW Cru, IQ Nor, and SX Vel) are corrected. The fraction of Cepheids occurring in open clusters within 2 kpc of the Sun is $f_{CC,2kpc} = 0.088^{+0.029}_{-0.019}$. By combining cluster and field Cepheids, we calibrate the LL for several individual photometric passbands, together with reddening-free Wesenheit magnitudes based on Gaia and HST photometry, while solving for the residual offset applicable to Cepheid parallaxes, $Δ\varpi_{\mathrm{Cep}}$. The most direct comparison of our results with the SH0ES distance ladder yields excellent ($0.3σ$) agreement for both the absolute magnitude of a 10d solar metallicity Cepheid in the near-IR HST Wesenheit magnitudes, $M_{H,1}^W=-5.914\pm 0.017$ mag, and the residual parallax offset, $Δ\varpi_{\mathrm{Cep}}=-13 \pm 5\,μ$as. Using the larger sample of 26 Gold cluster Cepheids and $225$ MW Cepheids with recent Gaia DR3 astrometry and photometry, we determine at solar metallicity $M_{G,1}^W = -6.004 \pm 0.019$\,mag and $Δ\varpi_{\mathrm{Cep}}=-19 \pm 3\,μ$as. These results mark the currently most accurate absolute calibrations of the Cepheid luminosity scale based purely on observations of MW Cepheids.

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arXiv2022

Discovery of a binary-origin classical Cepheid in a binary system with a 59-day orbital period

We report the discovery of a surprising binary configuration of the double-mode Cepheid OGLE-LMC-CEP-1347 pulsating in the first (P_1=0.690d) and second overtone (P_2=0.556d) modes. The orbital period (P_orb=59d) of the system is five times shorter than the shortest known to date (310d) for a binary Cepheid. The Cepheid itself is also the shortest-period one ever found in a binary system and the first double-mode Cepheid in a spectroscopically double-lined binary. OGLE-LMC-CEP-1347 is most probably on its first crossing through the instability strip, as inferred from both its short period and fast period increase, consistent with evolutionary models, and from the short orbital period (not expected for binary Cepheids whose components have passed through the red giant phase). Our evolutionary analysis yielded a first-crossing Cepheid with a mass in a range of 2.9-3.4 Msun (lower than any measured Cepheid mass), consistent with observations. The companion is a stable star, at least two times fainter and less massive than the Cepheid (preliminary mass ratio q=0.55), while also redder and thus at the subgiant or more advanced evolutionary stage. To match these characteristics, the Cepheid has to be a product of binary interaction, most likely a merger of two less massive stars, which makes it the second known classical Cepheid of binary origin. Moreover, further evolution of the components may lead to another binary interaction.

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arXiv2022

An Improved Calibration of the Wavelength Dependence of Metallicity on the Cepheid Leavitt law

The Cepheid period-luminosity (PL) relation (or Leavitt law) has served as the first rung of the most widely used extragalactic distance ladder and is central to the determination of the local value of the Hubble constant ($H_0$). We investigate the influence of metallicity on Cepheid brightness, a term that significantly improves the overall fit of the distance ladder, to better define its wavelength dependence. To this aim, we compare the PL relations obtained for three Cepheid samples having distinct chemical composition (in the Milky Way and Magellanic Clouds) and focusing on the use of improved and recent data while covering a metallicity range of about 1 dex. We estimate the metallicity effect (hereafter $γ$) in 15 filters from mid-IR to optical wavelengths, including five Wesenheit indices, and we derive a significant metallicity term in all filters, in agreement with recent empirical studies and models, in the sense of metal-rich Cepheids being brighter than metal-poor ones. We describe the contribution of various systematic effects in the determination of the $γ$ term. We find no evidence of $γ$ changing over the wavelength range $0.5-4.5 \, \rm μm$, indicating that the main influence of metallicity on Cepheids is in their luminosity rather than color. Finally, we identify factors that sharpen the empirical constraints on the metallicity term over past studies, including corrections for the depth of the Magellanic Clouds, better-calibrated Cepheid photometry, improved Milky Way extinction estimates, and revised and expanded metallicity measurements in the LMC.

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arXiv2022

Cluster Cepheids with High Precision Gaia Parallaxes, Low Zeropoint Uncertainties, and Hubble Space Telescope Photometry

We present HST photometry of 17 Cepheids in open clusters and their mean parallaxes from Gaia EDR3. These parallaxes are more precise than those from individual Cepheids (G<8 mag) previously used to measure the Hubble constant because they are derived from an average of >300 stars per cluster. Cluster parallaxes also have smaller systematic uncertainty because their stars lie in the range (G>13 mag) where the Gaia parallax calibration is most comprehensive. Cepheid photometry employed in the period--luminosity relation was measured using the same instrument(WFC3) and filters(F555W,F814W,F160W) as extragalactic Cepheids in SNIa hosts. We find no evidence of residual parallax offset in this magnitude range, zp=-3+/-4 muas, consistent with Lindegren:2021b and most studies. The Cepheid luminosity (P=10d, solar-metallicity) in the HST near-infrared, Wesenheit system derived from the cluster sample is M_{H,1}^W=-5.902+/-0.025 and -5.890+/-0.018 mag with or without simultaneous determination of a parallax offset, respectively. These results are similar to measurements from field Cepheids, confirming the accuracy of the Gaia parallaxes over a broad range of magnitudes. The SH0ES distance ladder calibrated solely from this sample gives H_0=72.8+/-1.3 and H_0=73.2+/-1.1 km/s/Mpc with or without offset marginalization; combined with all anchors we find H_0=73.01+/-0.99 and 73.15+/-0.97, respectively, a 5% or 7% reduction in the uncertainty and a 5.3 sigma Hubble Tension relative to Planck+LambdaCDM. It appears increasingly difficult to reconcile two of the best measured cosmic scales, parallaxes from Gaia and the angular size of the acoustic scale of the CMB, using the simplest form of LambdaCDM to join the two.

astro-ph.CO↗

arXiv2022

Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies

In this paper we will list a few important goals that need to be addressed in the next decade, also taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant $H_0$, the $σ_8$--$S_8$ tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the $5.0\,σ$ tension between the {\it Planck} CMB estimate of the Hubble constant $H_0$ and the SH0ES collaboration measurements. After showing the $H_0$ evaluations made from different teams using different methods and geometric calibrations, we list a few interesting new physics models that could alleviate this tension and discuss how the next decade's experiments will be crucial. Moreover, we focus on the tension of the {\it Planck} CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density $Ω_m$, and the amplitude or rate of the growth of structure ($σ_8,fσ_8$). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the $H_0$--$S_8$ tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals.[Abridged]

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arXiv2022

The eta Aquilae System: Radial Velocities and Astrometry in Search of eta Aql B

The classical Cepheid eta Aql was not included in past Leavitt Law work (Benedict et al. 2007) because of a presumed complicating orbit due to a known B9.8V companion. To determine the orbit of eta Aql B, we analyze a significant number of radial velocity measures (RV) from eight sources. With these we establish the RV variation due to Cepheid pulsation, using a twelve Fourier coefficient model, while solving for velocity offsets required to bring the RV data sets into coincidence. RV residuals provide no evidence of orbital motion, suggesting either nearly face-on orientation or very long period. Reanalysis of Hubble Space Telescope Fine Guidance Sensor astrometry now includes reference star parallax and proper motion priors from Gaia EDR3. As modeling confirmation, we reanalyze zeta Gem in parallel, deriving zeta Gem parallax and proper motion values consistent with Gaia EDR3, and consistent with the Benedict 2007 Leavitt Law. In an effort to further characterize eta Aql B, we hypothesize that eta Aql residuals larger than those of the associated reference stars or a parallax inconsistent with EDR3 and the Benedict 2007 Leavitt Law indicate unmodeled orbital motion. Using the astrometric noise or parallax mismatch with EDR3 we estimate possible periods and mass for eta Aql B. Ascribing photocenter motion to the photometric variation of the Cepheid, eta Aql A, yields a plausible separation, consistent with a long period, explaining the lack of RV variation. None of these approaches yields an unassailable characterization of the eta Aql A-B system

astro-ph.SR↗

arXiv2022

Relativistic corrections for measuring Hubble's constant to 1% using stellar standard candles

Relativistic corrections are estimated for classical Cepheids and the Tip of the Red Giant Branch (TRGB stars), to enable future unbiased 1% measurements of Hubble's constant, $H_0$. We considered four effects: $K-$corrections, time-dilation, the apparent change of host dust extinction due to non-comoving reference frames, and the change of observed color due to redshift. Extinction-dependent $K-$corrections were computed using stellar atmosphere models applicable to giant stars for $0.005 < z < 0.03$ in HST, JWST, and 2MASS filters. The optical-NIR Wesenheit function advantageously combines filters with oppositely signed $K-$corrections and avoids complications due to host extinction. For TRGB stars, the JWST/NIRCAM F277W filter combines insensitivity to reddening with $K-$corrections $<1$% at Coma cluster distances. Missing corrections for host extinction due to circumgalactic or circumstellar material are discussed as potential systematics for TRGB distances although their impacts are insufficient to explain differences between $H_0$ based on Cepheid or TRGB supernova calibrations. All stellar standard candles require relativistic corrections to achieve an unbiased 1% $H_0$ measurement in the future. The combined relativistic correction involving $K$, redshift-Leavitt bias, and the redshift-dependence of the Wesenheit function yield an increase of the Cepheid-based $H_0$ by $0.45 \pm 0.05$ km/s/Mpc to $73.65 \pm 1.30$ km/s/Mpc and raises the tension with the {\it Planck} value from $4.2σ$ to $4.4σ$. For TRGB stars, we estimate a $\sim 0.5\%$ increase of $H_0$ reported by Freedman et al. (to $70.2\pm1.7$km/s/Mpc) and a small decrease by $-0.15\%$ for $H_0$ reported by Anand et al. (to $71.4 \pm 1.8$km/s/Mpc). The opposite sign of these corrections is due to different reddening systematics and reduces the difference between the studies by $\sim 0.46$km/s/Mpc.[abridged]

astro-ph.CO↗

arXiv2022

Do the majority of stars form as gravitationally unbound?

Some of the youngest stars (age $\lesssim 10$ Myr) are clustered, while many others are observed scattered throughout star forming regions or in complete isolation. It has been intensively debated whether the scattered or isolated stars originate in star clusters, or if they form truly isolated, which could help constrain the possibilities how massive stars are formed. We adopt the assumption that all stars form in gravitationally bound star clusters embedded in molecular cloud cores ($Γ$-$1\;$ model), which expel their natal gas, and compare the fraction of stars found in clusters with observational data. The star clusters are modelled by the code nbody6, which includes stellar and circumbinary evolution, gas expulsion, and the external gravitational field of their host galaxy. We find that small changes in the assumptions in the current theoretical model estimating the fraction, $Γ$, of stars forming in embedded clusters have a large influence on the results, and we present a counterexample as an illustration. This calls into question theoretical arguments about $Γ$ in embedded clusters, and it suggests that there is no firm theoretical ground for low $Γ$ in galaxies with lower star formation rates (SFRs). Instead, the assumption that all stars form in embedded clusters is in agreement with observational data for the youngest stars (age $\lesssim 10$ Myr). In the $Γ$-$1\;$ scenario, the observed fraction of the youngest stars in clusters increases with the SFR only weakly; the increase is caused by the presence of more massive clusters in galaxies with higher SFRs, which release fewer stars to the field in proportion to their mass. The $Γ$-$1\;$ model yields a higher fraction of stars in clusters for older stars (age between $10$ and $300$ Myr) than what is observed. This discrepancy can be caused by interactions with molecular clouds.

astro-ph.GA↗

arXiv2021

New Periodograms Separating Orbital Radial Velocities and Spectral Shape Variation

We present new periodograms that are effective in distinguishing Doppler shift from spectral shape variability in astronomical spectra. These periodograms, building upon the concept of partial distance correlation, separate the periodic radial velocity modulation induced by orbital motion from that induced by stellar activity. These tools can be used to explore large spectroscopic databases in search of targets in which spectral shape variations obscure the orbital motion; such systems include active planet-hosting stars or binary systems with an intrinsically variable component. We provide a detailed prescription for calculating the periodograms, demonstrate their performance via simulations and real-life case studies, and provide a public Python implementation.

astro-ph.IM↗

arXiv2021

On the dynamical evolution of Cepheids in star clusters

We investigate the occurrence of classical (type-I) Cepheid variable stars (henceforth: Cepheids) in dynamically evolving star clusters from birth to an age of approximately 300 Myr. The clusters are modelled by the Aarseth code nbody6, and they feature a realistic stellar initial mass function and initial binary star population, single star and binary star evolution, expulsion of the primordial gas, and the tidal field of the galaxy. Our simulations provide the first detailed dynamical picture of how frequently Cepheids remain gravitationally bound to their birth clusters versus how frequently they occur in the field. They allow us to quantify the relevance of various cluster escape mechanisms and how they depend on stellar mass. Overall, the simulations agree with the empirical picture that a small fraction ($\approx 10\%$) of Cepheids reside in clusters, that cluster halo membership is relatively common, and that the majority of Cepheid hosting clusters have only a single Cepheid member. Additionally, the simulations predict that a) Cepheid progenitors are much more likely to escape from low-mass than higher-mass clusters; b) higher-mass (long-period) Cepheids are $\approx 30\%$ more likely to be found in clusters than low-mass (short-period) Cepheids; c) the clustered Cepheid fraction increases with galactocentric radius since cluster dispersal is less efficient at greater radii; d) a lower metallicity reduces the overall clustered Cepheid fraction; e) high-mass clusters are much more likely to have more than one Cepheid member at any given time, in particular at a lower metallicity. We interpret the results as outcomes of various aspects of star cluster dynamics. The comparison of predicted and observed clustered Cepheid fractions, $f_{\rm CC}$, highlights the need for additional cluster disruption mechanisms, most likely encounters with giant molecular clouds.

astro-ph.GA↗

arXiv2021

The iron and oxygen content of LMC Classical Cepheids and its implications for the Extragalactic Distance Scale and Hubble constant

Classical Cepheids are primary distance indicators and a crucial stepping stone to determining the present-day Hubble constant Ho to the precision and accuracy required to constrain apparent deviations from the LCDM Concordance Cosmological Model. We have measured the iron and oxygen abundances of of 89 Cepheids in the LMC, one of the anchors of the local Distance Scale, quadrupling the prior sample and including 68 of the 70 Cepheids used to constrain Ho by the SH0ES program. The goal is to constrain the extent to which the Cepheid luminosity is influenced by their chemical composition, an important contributor to the uncertainty on the determination of the Ho itself and a critical factor in the internal consistency of the distance ladder. We have derived stellar parameters and abundances from a self-consistent spectroscopic analysis based on Equivalent Width of absorption lines. The [Fe/H] distribution of LMC Cepheids is a single Gaussian with a mean of -0.4079+-0.003 dex (0.1 dex systematic uncertainty) and sigma 0.076+-0.003 dex. The latter is fully compatible with the measurement error and supports the low dispersion of 0.069 mag seen in the NIR HST LMC period-luminosity relation. The uniformity of the abundance has the important consequence that the LMC Cepheids alone cannot provide any meaningful constraint on the dependence of the Cepheid Period-Luminosity relation on chemical composition at any wavelength. This revises a prior claim based on a small sample of 22 LMC Cepheids that there was little dependence (or uncertainty) between composition and NIR luminosity, a conclusion which would produce a conflict between anchors of the distance ladder with different mean abundance. The chemical homogeneity of the LMC Cepheid population makes it an ideal environment to calibrate the metallicity dependence between the more metal poor SMC and metal rich Milky Way and NGC4258.

astro-ph.CO↗

arXiv2021

Maintaining scientific discourse during a global pandemic: ESO's first e-conference #H02020

From 22 to 26 June 2020, we hosted ESO's first live e-conference, #H02020, from within ESO headquarters in Garching, Germany. Every day, between 200 and 320 researchers around the globe tuned in to discuss the nature and implications of the discord between precise determinations of the Universe's expansion rate, H0. Originally planned as an in-person meeting, we moved to the virtual domain to maintain strong scientific discourse despite the SARS-CoV-2 (COVID-19) pandemic. Here, we describe our conference setup, participants feedback gathered before and after the meeting, and lessons learned from this unexpected exercise. As e-conferencing will become increasingly common in the future, we provide our perspective on how e-conferences can make scientific exchange more effective and inclusive, in addition to climate friendly.

astro-ph.IM↗

arXiv2021

Cepheids with giant companions. I. Revealing a numerous population of double-lined binary Cepheids

Masses of classical Cepheids of 3 to 11 M$\odot$ are predicted by theory but those measured, clump between 3.6 and 5 M$\odot$. As a result, their mass-luminosity relation is poorly constrained, impeding our understanding of basic stellar physics and the Leavitt Law. All Cepheid masses come from the analysis of 11 binary systems, including only 5 double-lined and well-suited for accurate dynamical mass determination. We present a project to analyze a new, numerous group of Cepheids in double-lined binary (SB2) systems to provide mass determinations in a wide mass interval and study their evolution. We analyze a sample of 41 candidate binary LMC Cepheids spread along the P-L relation, that are likely accompanied by luminous red giants, and present indirect and direct indicators of their binarity. In a spectroscopic study of a subsample of 18 brightest candidates, for 16 we detected lines of two components in the spectra, already quadrupling the number of Cepheids in SB2 systems. Observations of the whole sample may thus lead to quadrupling all the Cepheid mass estimates available now. For the majority of our candidates, erratic intrinsic period changes dominate over the light travel-time effect due to binarity. However, the latter may explain the periodic phase modulation for 4 Cepheids. Our project paves the way for future accurate dynamical mass determinations of Cepheids in the LMC, Milky Way, and other galaxies, which will potentially increase the number of known Cepheid masses even 10-fold, hugely improving our knowledge about these important stars.

astro-ph.SR↗

arXiv2021

A Synoptic View of the Magellanic Clouds: VMC, Gaia, and Beyond

The year 2019 marked the quincentenary of the arrival in the southern hemisphere of Ferdinand Magellan, the namesake of the Magellanic Clouds, our nearest example of dwarf galaxies in the early stages of a minor merging event. These galaxies have been firmly established as laboratories for the study of variable stars, stellar evolution, and galaxy interaction, as well as being anchors for the extragalactic distance scale. The goal of this conference was to provide fertile ground for shaping future research related to the Magellanic Clouds by combining state-of-the-art results based on advanced observational programmes with discussions of the highly multiplexed wide-field spectroscopic surveys that will come online in the 2020s.

astro-ph.GA↗

arXiv2020

Hubble Space Telescope Snapshot Survey for Resolved Companions of Galactic Cepheids: Final Results

Cepheids in multiple systems provide information on the outcome of the formation of massive stars. They can also lead to exotic end-stage objects. This study concludes our survey of 70 galactic Cepheids using the {\it Hubble Space Telescope\} (\HST) Wide Field Camera~3 (WFC3) with images at two wavelengths to identify companions closer than $5\arcsec$. In the entire WFC3 survey we identify 16 probable companions for 13 Cepheids. The seven Cepheids having resolved candidate companions within $2"$ all have the surprising property of themselves being spectroscopic binaries (as compared with a 29\% incidence of spectroscopic binaries in the general Cepheid population). That is a strong suggestion that an inner binary is linked to the scenario of a third companion within a few hundred~AU\null. This characteristic is continued for more widely separated companions. Under a model where the outer companion is formed first, it is unlikely that it can anticipate a subsequent inner binary. Rather it is more likely that a triple system has undergone dynamical interaction, resulting in one star moving outward to its current location. {\it Chandra\} and {\it Gaia\} data as well as radial velocities and \HSTSTIS and {\it IUE\} spectra are used to derive properties of the components of the Cepheid systems. The colors of the companion candidates show a change in distribution at approximately 2000~AU separations, from a range including both hot and cool colors for closer companions, to only low-mass companions for wider separations.

astro-ph.SR↗

arXiv2020

The Milky Way Cepheid Leavitt law based on Gaia DR2 parallaxes of companion stars and host open cluster populations

Classical Cepheids provide the foundation for the empirical extragalactic distance ladder. Milky Way Cepheids are the only stars in this class accessible to trigonometric parallax measurements. However, the parallaxes of Cepheids from the second Gaia data release (GDR2) are affected by systematics because of the absence of chromaticity correction, and occasionally by saturation. As a proxy for the parallaxes of 36 Galactic Cepheids, we adopt either the GDR2 parallaxes of their spatially resolved companions or the GDR2 parallax of their host open cluster. This novel approach allows us to bypass the systematics on the GDR2 Cepheids parallaxes that is induced by saturation and variability. We adopt a GDR2 parallax zero-point (ZP) of -0.046 mas with an uncertainty of 0.015 mas that covers most of the recent estimates. We present new Galactic calibrations of the Leavitt law in the V, J, H, K_S , and Wesenheit W_H bands. We compare our results with previous calibrations based on non-Gaia measurements and compute a revised value for the Hubble constant anchored to Milky Way Cepheids. From an initial Hubble constant of 76.18 +/- 2.37 km/s/Mpc based on parallax measurements without Gaia, we derive a revised value by adopting companion and average cluster parallaxes in place of direct Cepheid parallaxes, and we find H_0 = 72.8 +/- 1.9 (statistical + systematics) +/- 1.9 (ZP) km/s/Mpc when all Cepheids are considered and H0 = 73.0 +/- 1.9 (statistical + systematics) +/- 1.9 (ZP) km/s/Mpc for fundamental mode pulsators only.

astro-ph.SR↗