arXiv subjects

Richard I. Anderson

Publications and source records attributed to Richard I. Anderson.

At least 37 records · Page 2Linked to original sources

arXiv2025

Variable stars in Galactic globular clusters II. Population II Cepheids

We identified a sample of 88 bona fide Population II Cepheids (henceforth referred to as Cepheids) and 44 candidates in Galactic globular clusters (GCs). Seventy-eight of the Cepheids in the sample align within $2\,σ$ of the period-luminosity relation for Milky Way Type II Cepheids (T2CEPs). Nine align with the period-luminosity relation for fundamental-mode anomalous Cepheids (ACEPs), and only one (BL Boötis) follows the relation for first-overtone ACEPs, as determined from observations of ACEPs in the Large Magellanic Cloud. For sources in common between our catalog and the OGLE catalog, the classification agrees in 94% of cases. In comparison, for sources shared between the Gaia Specific Object Study (SOS) and OGLE, the agreement is 74%. In the dense environments of GCs, our analysis shows that the completeness of the Gaia catalogs for Cepheids is 64% for the SOS and 74% for the classifier of variable stars. We determined the red and blue edges of the instability strip for T2CEPs using linear MESA-RSP models. We find that the best-fit models, with $M = 0.6\,{\rm M}_\odot$ and Z = 0.0003, are able to fit $90%$ of the stars in our sample. This percentage is the same for helium abundances $Y = 0.220$ and $0.245$. Higher values of $Y$ lower this percentage, and the same effect is observed with lower values of $Z$. In the future, combining the sample of T2CEPs with the precise parallaxes obtained from GCs will strengthen the geometric calibration of a distance ladder based on Population II stars. This will be useful for determining distances within the Milky Way and for cross-checking distances to Local Group galaxies determined through other methods.

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arXiv2024

Tip of the Red Giant Branch Distances with JWST. II. I-band Measurements in a Sample of Hosts of 10 SN Ia Match HST Cepheids

The Hubble Tension, a >5 sigma discrepancy between direct and indirect measurements of the Hubble constant (H0), has persisted for a decade and motivated intense scrutiny of the paths used to infer H0. Comparing independently-derived distances for a set of galaxies with different standard candles, such as the tip of the red giant branch (TRGB) and Cepheid variables, can test for systematics in the middle rung of the distance ladder. The I band is the preferred filter for measuring the TRGB due to constancy with color, a result of low sensitivity to population differences in age and metallicity supported by stellar models. We use James Webb Space Telescope (JWST) observations with the maser host NGC 4258 as our geometric anchor to measure I-band (F090W vs F090W-F150W) TRGB distances to 8 hosts of 10 Type Ia supernovae (SNe Ia) within 28 Mpc: NGC 1448, NGC 1559, NGC 2525, NGC 3370, NGC 3447, NGC 5584, NGC 5643, and NGC 5861. We compare these with Hubble Space Telescope (HST) Cepheid-based relative distance moduli for the same galaxies and anchor. We find no evidence of a difference between their weighted means, 0.01 +/- 0.04 (stat) +/- 0.04 (sys) mag. We produce fourteen variants of the TRGB analysis, altering the smoothing level and color range used to measure the tips to explore their impact. For some hosts, this changes the identification of the strongest peak, but this causes little change to the sample mean difference producing a full range of 0.01 to 0.03 mag, all consistent at 1 sigma with no difference. The result matches past comparisons of I-band TRGB and Cepheids when both use HST. SNe and anchor samples observed with JWST are too small to yield a measure of H0 that is competitive with the HST sample of 42 SNe Ia and 4 anchors; however, they already provide a vital systematic crosscheck to HST measurements of the distance ladder.

astro-ph.CO↗

arXiv2024

JWST Validates HST Distance Measurements: Selection of Supernova Subsample Explains Differences in JWST Estimates of Local H0

JWST provides new opportunities to cross-check the HST Cepheid/SNeIa distance ladder, which yields the most precise local measure of H0. We analyze early JWST subsamples (~1/4 of the HST sample) from the SH0ES and CCHP groups, calibrated by a single anchor (N4258). We find HST Cepheid distances agree well (~1 sigma) with all 8 combinations of methods, samples, and telescopes: JWST Cepheids, TRGB, and JAGB by either group, plus HST TRGB and Miras. The comparisons explicitly include the measurement uncertainty of each method in N4258, an oft-neglected but dominant term. Mean differences are ~0.03 mag, far smaller than the 0.18 mag "Hubble tension." Combining all measures produces the strongest constraint yet on the linearity of HST Cepheid distances, 0.994+-0.010, ruling out distance-dependent bias or offset as the source of the tension at ~7 sigma. Yet, measurements of H0 from current JWST subsamples produce large sampling differences whose size and direction we can directly estimate from the full HST set. We show that Delta(H0)~2.5 km/s/Mpc between the CCHP JWST program and the full HST sample is entirely consistent with differences in sample selection. Combining all JWST samples produces a new, distance-limited set of 16 SNeIa at D<25 Mpc and more closely resembles the full sample thanks to "reversion to the mean" of larger samples. Using JWST Cepheids, JAGB, and TRGB, we find 73.4+-2.1, 72.2+-2.2, and 72.1+-2.2 km/s/Mpc, respectively. Explicitly accounting for SNe in common, the combined-sample three-method result from JWST is H0=72.6+-2.0, similar to H0=72.8 expected from HST Cepheids in the same galaxies. The small JWST sample trivially lowers the Hubble tension significance due to small-sample statistics and is not yet competitive with the HST set (42 SNeIa and 4 anchors), which yields 73.2+-0.9. Still, the joint JWST sample provides important crosschecks which the HST data passes.

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arXiv2024

Calibrating and standardizing the Tip of the Red Giant Branch in the Small Magellanic Cloud using small-amplitude red giants

We investigate the absolute calibration of the Tip of the Red Giant Branch (TRGB) in the Small Magellanic Cloud (SMC) using small amplitude red giant stars (SARGs) classified by the Optical Gravitational Lensing Experiment (OGLE). We show that all stars near the SMC's TRGB are SARGs. Distinguishing older and younger RGs near the Tip according to two period-luminosity sequences labeled A and B, we show many similarities among SARG populations of the LMC and the SMC, along with notable differences. Specifically, SMC SARGs have shorter periods due to lower metallicity and smaller amplitudes due to younger ages than LMC SARGs. We discover two period-color relations near the TRGB that span all A- and B-sequence stars in the OGLE-III footprints of the SMC and LMC, and we investigate using periods instead of color for TRGB standardization. Using variability derived information only, we trace the SMC's age and metallicity gradients and show the core to be populated by younger, more metal rich RGs. The B-sequence yields the brightest and most accurate calibration ($M_{\mathrm{F814W,syn}} = -4.057 \pm 0.019 (\mathrm{stat.}) \pm 0.029 (\mathrm{syst.})$ mag), which we use to measure the distance modulus difference between the Clouds and investigate metallicity effects. Distance measurements not informed by variability should employ the SARGs-based calibration based on all stars near the tip ($M_{\mathrm{F814W,syn}} = -4.024 \pm 0.041 (\mathrm{stat.}) \pm 0.029 (\mathrm{syst.})$ mag). Our work highlights the impact of RG population diversity on TRGB distance measurements. Further study is needed unravel these effects and improve TRGB standardization.

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arXiv2024

The Components of Cepheid Systems: The FN Vel System

Cepheid masses continue to be important tests of evolutionary tracks for intermediate mass stars as well as important predictors of their future fate. For systems where the secondary is a B star, {\it Hubble Space Telescope} ultraviolet spectra have been obtained. From these spectra a temperature can be derived, and from this a mass of the companion M$_2$. Once {\it Gaia} DR4 is available, proper motions can be used to determine the inclination of the orbit. Combining mass of the companion, M$_2$, the mass function from the ground-based orbit of the Cepheid and the inclination produces the mass of the Cepheid, M$_1$. The Cepheid system FN Vel is used here to demonstrate this approach and what limits can be put on the Cepheid mass for inclination between 50 and 130$^o$.

astro-ph.SR↗

arXiv2024

On the dynamical evolution of Cepheid multiplicity in star clusters and its implications for B-star multiplicity at birth

Classical Cepheid variable stars provide a unique probe to binary evolution in intermediate-mass stars over the course of several tens to hundreds of Myr. We studied the binary and multiple properties of Cepheids, assuming that all mid-B stars form in binaries inside star clusters. The binaries were subjected both to stellar evolution and dynamical encounters with other stars in the cluster. The dynamical cluster environment results in a higher binary fraction among the Cepheids that remain in star clusters ($\approx 60$%) than among the Cepheids which have escaped to the field ($\approx 35$%). In clusters, the binary, triple, and multiple fraction decreases with increasing cluster mass. More massive clusters have binaries of shorter orbital periods than lower mass clusters and field Cepheids. Mergers are very common with $\approx 30$% of mid-B stars not evolving to Cepheids because of the interaction with their companion. Approximately $40$ % of Cepheids have merged with their companion, and the merger event impacts stellar evolution; the age of Cepheids expected from their mass can differ from the age of their host cluster. Our models predict that one in five Cepheids is the result of a merger between stars with mass below the lower mass limit for Cepheids; in clusters, these objects occur substantially later than expected from their mass. Approximately $3$ to $5$ % of all Cepheids have a compact companion ($\approx 0.15$ % of all Cepheids are accompanied by a black hole). The binary fraction derived from our simulations (42%) underestimates the observed binary Cepheid fraction by approximately a factor of 2. This suggests that the true multiplicity fraction of B-stars at birth could be substantially larger than unity and, thus, that mid-B stars may typically form in triple and higher order systems.

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arXiv2024

Relation between the geometric shape and rotation of Galactic globular clusters

We homogeneously measured the elliptical shapes of 163 globular clusters (GCs) using the on-sky distribution of their cluster members and the third data release of the ESA mission Gaia (DR3). The astrometry enables the differentiation of stars within clusters from those in the field. This feature is particularly valuable for clusters located in densely populated areas of the sky, where conventional methods for measuring the geometry of the GCs are not applicable. The median axial ratio of our full sample is $\langle b/a \rangle = 0.935^{+0.033}_{-0.090}$ and $0.986^{+0.009}_{-0.004}$ for the subset of 11 GCs previously studied based on Hubble Space Telescope imaging. We investigated whether the minor axis of the ellipses can be interpreted as a pseudo-rotation axis by comparing it to measurements of cluster rotation. Using the radial velocities from Gaia, we detected rotation for three clusters, NGC 5139, NGC 104, and NGC 6341, and observed an alignment between the pseudo-rotation axis and the 2D projection of the real rotation axis. To expand the set of clusters for which rotation has been detected, we analyzed multiple literature references. Depending on the reference used for comparison, we observed an alignment in between 76% to 100% of the clusters. The lack of an alignment observed in some clusters may be linked to different scales analyzed in various studies. Several studies have demonstrated that the orientation of rotation varies with the distance from the center. We estimate that the next Gaia release will increase the number of stars with radial velocities in GCs from $\sim 10,000$ in Gaia DR3 to $\sim 55,000$ in Gaia DR4. This will enable the measurement of rotation and ellipticities at identical angular scales for additional clusters, which will help us to clarify whether the previously mentioned alignment occurs in all clusters.

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arXiv2024

VELOcities of CEpheids (VELOCE) II. Systematic Search for Spectroscopic Binary Cepheids

Classical Cepheids provide valuable insights into the evolution of stellar multiplicity among intermediate-mass stars. Here, we present a systematic investigation of single-lined spectroscopic binaries (SB1) based on high-precision velocities measured by the VELOcities of CEpheids (VELOCE) project. We detected 76 (29%) SB1 systems among the 258 Milky Way Cepheids in the first VELOCE data release, 32 (43%) of which were not previously known to be SB1 systems. We determined 30 precise and 3 tentative orbital solutions, 18 (53%) of which are reported for the first time. This large set of Cepheid orbits provides a detailed view of the eccentricity e and orbital period Porb distribution among evolved intermediate-mass stars, ranging from e=[0.0, 0.8] and Porb=[240, 9 000] d. Orbital motion on timescales exceeding the 11 yr VELOCE baseline was investigated using a template fitting technique applied to literature data. Particularly interesting objects include a) R Cru, the Cepheid with the shortest orbital period in the Milky Way (240 d), b) ASAS J103158-5814.7, a short-period overtone Cepheid exhibiting time-dependent pulsation amplitudes as well as orbital motion, c) 17 triple systems with outer visual companions, among other interesting objects. Most VELOCE Cepheids (21/23) that exhibit evidence for a companion based on Gaia proper motion anomaly are also spectroscopic binaries, whereas the remaining do not exhibit significant (> 3-sigma) orbital RV variations. Gaia quality flags, notably the Renormalized Unit Weight Error (RUWE), do not allow to reliably identify Cepheid binaries, although statistically the average RUWE of SB1 Cepheids is slightly higher than that of non-SB1 Cepheids. Comparison with Gaia photometric amplitudes in G, Bp, and Rp also does not allow to identify spectroscopic binaries among the full VELOCE sample.

astro-ph.SR↗

arXiv2024

Tip of the Red Giant Branch Distances with JWST: An Absolute Calibration in NGC 4258 and First Applications to Type Ia Supernova Hosts

The tip of the red giant branch (TRGB) allows for the measurement of precise and accurate distances to nearby galaxies, based on the brightest ascent of low-mass red giant branch stars before they undergo the helium flash. With the advent of JWST, there is great promise to utilize the technique to measure galaxy distances out to at least 50 Mpc, significantly further than HST's reach of 20 Mpc. However, with any standard candle, it is first necessary to provide an absolute reference. Here we use Cycle 1 data to provide an absolute calibration in the F090W filter. F090W is most similar to the F814W filter commonly used for TRGB measurements with HST, which had been adopted by the community due to minimal dependence from the underlying metallicities and ages of stars. The imaging we use was taken in the outskirts of NGC 4258, which has a direct geometrical distance measurement from the Keplerian motion of its water megamaser. Utilizing several measurement techniques, we find $M_{TRGB}^{F090W}$ = -4.362 $\pm$ 0.033 (stat) $\pm$ 0.045 (sys) mag (Vega) for the metal-poor TRGB. We also perform measurements of the TRGB in two Type Ia supernova hosts, NGC 1559, and NGC 5584. We find good agreement between our TRGB distances and previous distance determinations to these galaxies from Cepheids ($Δ$ = 0.01 $\pm$ 0.06 mag), with these differences being too small to explain the Hubble tension ($\sim$0.17 mag). As a final bonus, we showcase the serendipitous discovery of a faint dwarf galaxy near NGC 5584.

astro-ph.CO↗

arXiv2024

VELOcities of CEpheids (VELOCE) I. High-precision radial velocities of Cepheids

This first VELOCE data release comprises 18,225 high-precision RV measurements of 258 bona fide classical Cepheids on both hemispheres collected mainly between 2010 and 2022, alongside 1161 additional observations of 164 other stars. The median per-observation RV uncertainty is 0.037 km/s, and some reach 0.002 km/s. Non-variable standard stars characterize RV zero-point stability and provide a base for future cross-calibrations. We determined zero-point differences between VELOCE and 31 literature data sets using template fitting and measured linear period changes of 146 Cepheids. Seventy six spectroscopic binary Cepheids and 14 candidates are identified using VELOCE data alone and are investigated in detail in a companion paper (VELOCE II). Several new insights into Cepheid pulsations were obtained, including: a) the most detailed description of the Hertzsprung progression by RVs; b) the identification of double-peaked bumps in the RV curve; c) clear evidence that virtually all Cepheids feature spectroscopic variability signals that lead to modulated RV variability. We identified 36 such stars, of which 4 also exhibit orbital motion. Linear radius variations depend strongly on pulsation period and a steep increase in slope of the $Δ$R/p versus logP-relation is found near 10d, challenging the existence of a tight relation between Baade-Wesselink projection factors and pulsation periods. We investigated the accuracy of RV time series measurements, v$_γ$, and RV amplitudes published in Gaia's DR3 and determined an average offset of 0.65 \pm 0.11 km/s relative to VELOCE. We recommend adopting a single set of template correlation parameters for distinct classes of large-amplitude variable stars to avoid systematic offsets in v$_γ$ among stars belonging to the same class. Peak-to-peak amplitudes of Gaia RVs exhibit significant (16%) dispersion compared to VELOCE. [abridged]

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arXiv2024

The Wide-field Spectroscopic Telescope (WST) Science White Paper

The Wide-field Spectroscopic Telescope (WST) is proposed as a new facility dedicated to the efficient delivery of spectroscopic surveys. This white paper summarises the initial concept as well as the corresponding science cases. WST will feature simultaneous operation of a large field-of-view (3 sq. degree), a high multiplex (20,000) multi-object spectrograph (MOS) and a giant 3x3 sq. arcmin integral field spectrograph (IFS). In scientific capability these requirements place WST far ahead of existing and planned facilities. Given the current investment in deep imaging surveys and noting the diagnostic power of spectroscopy, WST will fill a crucial gap in astronomical capability and work synergistically with future ground and space-based facilities. This white paper shows that WST can address outstanding scientific questions in the areas of cosmology; galaxy assembly, evolution, and enrichment, including our own Milky Way; origin of stars and planets; time domain and multi-messenger astrophysics. WST's uniquely rich dataset will deliver unforeseen discoveries in many of these areas. The WST Science Team (already including more than 500 scientists worldwide) is open to the all astronomical community. To register in the WST Science Team please visit https://www.wstelescope.com/for-scientists/participate

astro-ph.IM↗

arXiv2024

On Cepheid distances in the $H_0$ measurement

Classical Cepheids were the first stellar standard candles and have played a crucial role for astronomical distance measurements ever since the discovery of the Leavitt law (period-luminosity relation). Enormous improvements in distance accuracy have been achieved since Hertzsprung's first application of Leavitt's law to measure the distance to the Small Magellanic Cloud in 1913, notably in very recent years thanks to a large data set of highly accurate space astrometry from the ESA mission Gaia. Complemented by homogeneous space photometry, Cepheids enable the most accurate distance estimates to galaxies hosting type-Ia supernovae up to approximately 70 Mpc distant. Here, I review the history of Cepheid distance measurements, open questions on the side of stellar astrophysics, and recent studies seeking to quantify and mitigate systematics with a view to further improve the accuracy on the Hubble constant. For example, the recently launched James Webb Space Telescope will enhance precision due to 4x lower sensitivity to source blending in crowded regions and greater sensitivity in dust-insensitive infrared bands. Future 30m-class telescopes could in principle further improve Cepheid distance measurements towards the Hubble flow, if technical challenges related to a continuously evolving instrument can be overcome.

astro-ph.SR↗

arXiv2024

Small amplitude red giants elucidate the nature of the Tip of the Red Giant Branch as a standard candle

The tip of the red giant branch (TRGB) is an important standard candle for determining luminosity distances. Although several $10^5$ small amplitude red giant stars (SARGs) have been discovered, variability was previously considered irrelevant for the TRGB as a standard candle. Here, we show that all stars near the TRGB are SARGs that follow several period-luminosity sequences, of which sequence A is younger than sequence B as predicted by stellar evolution. We measure apparent TRGB magnitudes, m$_{\mathrm{TRGB}}$, in the Large Magellanic Cloud (LMC), using Sobel filters applied to photometry from the Optical Gravitational Lensing Experiment and the ESA Gaia mission, and we identify several weaknesses in a recent LMC-based TRGB calibration used to measure the Hubble constant. We consider four samples: all Red Giants (RGs), SARGs, and sequences A & B. The B-sequence is best suited for measuring distances to old RG populations, with M$_{\mathrm{F814W,0}}$ = -4.025 $\pm$ 0.014(stat.) $\pm$ 0.033(syst.) mag assuming the LMC's geometric distance. Control of systematics is demonstrated using detailed simulations. Population diversity affects m$_{\mathrm{TRGB}}$ at a level exceeding the stated precision: the SARG and A-sequence samples yield 0.039 mag and 0.085 mag fainter (at 5σ significance) m$_{\mathrm{TRGB}}$ values, respectively. Ensuring equivalent RG populations is crucial to measuring accurate TRGB distances. Additionally, luminosity function smoothing ($\sim$ 0.02 mag) and edge detection response weighting (as much as -0.06 mag) can further bias TRGB measurements, with the latter introducing a tip-contrast relation. We are optimistic that variable red giants will enable further improvements to the TRGB as a standard candle.

astro-ph.CO↗

arXiv2024

Variable stars in galactic globular Clusters I. The population of RR Lyrae stars

We present a comprehensive catalog of 2824 RR Lyrae stars (RRLs) residing in 115 Galactic globular clusters (GCs). Our catalog includes 1594 fundamental-mode (RRab), 824 first-overtone (RRc), and 28 double-mode (RRd) RRLs, as well as 378 RRLs of an unknown pulsation mode. We cross-matched 481349 RRLs reported in the third data release (DR3) of the ESA mission Gaia and the literature to 170 known GCs. Membership probabilities were computed as the products of a position and shape-dependent prior and a likelihood was computed using parallaxes, proper motions, and, where available, radial velocities from Gaia. Membership likelihoods of RRLs were computed by comparing cluster average parameters based on known member stars and the cross-matched RRLs. We determined empirical RRL instability strip (IS) boundaries based on our catalog and detected three new cluster RRLs inside this region via their excess Gaia G-band photometric uncertainties. We find that 77% of RRLs in GCs are included in the Gaia DR3 Specific Object Study, and 82% were classified as RRLs by the Gaia DR3 classifier, with the majority of the missing sources being located at the crowded GC centers. Surprisingly, we find that 25% of cluster member stars located within the empirical IS are not RRLs and appear to be non-variable. Additionally, we find that 80% of RRab, 84% of RRc, and 100% of the RRd stars are located within theoretical IS boundaries predicted using MESA models with Z = 0.0003, M = 0.7 (M_\odot), and Y = 0.290. Unexpectedly, a higher Y = 0.357 is required to fully match the location of RRc stars, and lower Y = 0.220 is needed to match the location of RRab stars. Lastly, our catalog does not exhibit an Oosterhoff dichotomy, with at least 22 GCs located inside the Oosterhoff "gap," which is close to the mode of the distribution of mean RRL periods in GCs.

astro-ph.SR↗

arXiv2024

JWST Observations Reject Unrecognized Crowding of Cepheid Photometry as an Explanation for the Hubble Tension at 8 sigma Confidence

We present high-definition observations with the James Webb Space Telescope of >1000 Cepheids in a geometric anchor of the distance ladder, NGC4258, and in 5 hosts of 8 SNe~Ia, a far greater sample than previous studies with JWST. These galaxies individually contain the largest samples of Cepheids, an average of >150 each, producing the strongest statistical comparison to those previously measured with the Hubble Space Telescope in the NIR. They also span the distance range of those used to determine the Hubble constant with HST, allowing us to search for a distance-dependent bias in HST measurements. The superior resolution of JWST negates crowding noise, the largest source of variance in the NIR Cepheid Period-Luminosity relations (Leavitt laws) measured with HST. Together with the use of two-epochs to constrain Cepheid phases and three filters to remove reddening, we reduce the dispersion in the Cepheid PL relations by a factor of 2.5. We find no significant difference in the mean distance measurements determined from HST and JWST, with a formal difference of -0.01+/-0.03 mag. This result is independent of zeropoints and analysis variants including metallicity dependence, local crowding, choice of filters, and relation slope. We can reject the hypothesis of unrecognized crowding of Cepheid photometry from HST that grows with distance as the cause of the ``Hubble Tension'' at 8.2 sigma, i.e., greater confidence than that of the Hubble Tension itself. We conclude that errors in photometric measurements of Cepheids across the distance ladder do not significantly contribute to the Tension.

astro-ph.CO↗

arXiv2023

Investigating Gaia EDR3 parallax systematics using asteroseismology of Cool Giant Stars observed by Kepler, K2, and TESS II. Deciphering Gaia parallax systematics using red clump stars

We analyse Gaia EDR3 parallax systematics as a function of magnitude and sky location using a recently published catalogue of 12,500 asteroseismic red-giant star distances. We selected ~ 3500 red clump (RC) stars of similar chemical composition as the optimal subsample for this purpose. We perform a detailed assessment of systematic uncertainties relevant for parallax offset estimation based on the asteroseismic distances. Following this assessment, we adopt for our baseline analysis the asteroseismic parameters measured as in Elsworth et al. (2020), spectroscopy from APOGEE (DR17), and we further restrict the sample to low-extinction RC stars with quality astrometric solutions from Gaia EDR3. We then investigated both the parallax offset relative to the published Gaia EDR3 parallaxes and the residual parallax offset after correcting Gaia EDR3 parallaxes following Lindegren et al. (2021). We find residual parallax offsets very close to zero (-1.6 +/- 0.5 (stat.) +/- 10 (syst.) muas) for stars fainter than G > 11 mag in the initial Kepler field. For 17 K2 campaigns in the same magnitude range, the residual parallax offset is +16.5 +/- 1.7 (stat.) +/- 10 (syst.) muas. At brighter magnitudes (G <= 11 mag), we find inconsistent residual parallax offsets between the Kepler field, 17 K2 campaigns, and the TESS southern continuous viewing zone, with differences of up to 60 muas. This suggests a significant dependence on sky location at bright magnitudes due to the lack of bright physical pairs available for determining the parallax offset corrections. Finally, we estimate the absolute magnitude of the RC and obtain M_Ks^RC = -1.650 +/- 0.025 mag in the 2MASS Ks-band and M_G^RC = (0.432 +/- 0.004) - (0.821 +/- 0.033) (Teff [K] - 4800K)/1000K [mag] in the Gaia G-band.

astro-ph.SR↗

arXiv2023

Crowded No More: The Accuracy of the Hubble Constant Tested with High Resolution Observations of Cepheids by JWST

High-resolution JWST observations can test confusion-limited HST observations for a photometric bias that could affect extragalactic Cepheids and the determination of the Hubble constant. We present JWST NIRCAM observations in two epochs and three filters of >330 Cepheids in NGC4258 (which has a 1.5% maser-based geometric distance) and in NGC5584 (host of SNIa 2007af), near the median distance of the SH0ES HST SNIa host sample and with the best leverage among them to detect such a bias. JWST provides far superior source separation from line-of-sight companions than HST in the NIR to largely negate confusion or crowding noise at these wavelengths, where extinction is minimal. The result is a remarkable >2.5x reduction in the dispersion of the Cepheid P-L relations, from 0.45 to 0.17 mag, improving individual Cepheid precision from 20% to 7%. Two-epoch photometry confirmed identifications, tested JWST photometric stability, and constrained Cepheid phases. The P-L relation intercepts are in very good agreement, with differences (JWST-HST) of 0.00+/-0.03 and 0.02+/-0.03 mag for NGC4258 and NGC5584, respectively. The difference in the determination of H_0 between HST and JWST from these intercepts is 0.02+/-0.04 mag, insensitive to JWST zeropoints or count-rate non-linearity thanks to error cancellation between rungs. We explore a broad range of analysis variants (including passband combinations, phase corrections, measured detector offsets, and crowding levels) indicating robust baseline results. These observations provide the strongest evidence yet that systematic errors in HST Cepheid photometry do not play a significant role in the present Hubble Tension. Upcoming JWST observations of >12 SNIa hosts should further refine the local measurement of the Hubble constant.

astro-ph.CO↗

arXiv2023

Standardized Luminosity of the Tip of the Red Giant Branch utilizing Multiple Fields in NGC 4258 and the CATs Algorithm

The Tip of the Red Giant Branch provides a luminous standard candle for calibrating distance ladders that reach Type Ia supernova (SN Ia) hosts. However, recent work reveals that tip measurements vary at the $\sim$ 0.1 mag level for different stellar populations and locations within a host, which may lead to inconsistencies along the distance ladder. We pursue a calibration of the tip using 11 Hubble Space Telescope fields around the maser host, NGC 4258, that is consistent with SN Ia hosts by standardizing tip measurements via their contrast ratios. We find $F814W$-band tips that exhibit a full 0.3 mag range and 0.1 mag dispersion. We do not find any correlation between HI column density and the apparent tip to 0.04 $\pm$ 0.03 mag/cm$^{-2}$. We search for a tip-contrast relation (TCR) and measure the TCR within the fields of NGC 4258 of $-0.015\pm0.008$ mag/$R$, where $R$ is the contrast ratio. This value is consistent with the TCR originally discovered in the GHOSTS sample (Wu et al. 2022) of $-0.023\pm0.005$ mag/R. Combining these measurements, we find a global TCR of $-0.021\pm0.004$ mag/R and a calibration of $M_I^{TRGB} = -4.025 \pm 0.035 - (R-4)\times0.021$ mag. We also use stellar models to simulate single age and metallicity stellar populations with [Fe/H] from $-2.0$ to $-0.7$ and ages from 3 Gyr to 12 Gyr and reconstruct the global TCR found here to a factor of $\sim$ 2. This work is combined in a companion analysis with tip measurements of nearby SN Ia hosts to measure $H_0$.

astro-ph.GA↗