Clump Stars Research Articles

Finding the unusual red giant remnants of cataclysmic variable mergers

Mergers between helium white dwarfs and main-sequence stars are likely common, producing red giant-like remnants making up roughly a few percent of all low-mass ( ≲2M⊙) red giants. Through detailed modeling, we show that these merger remnants possess distinctive photometric, asteroseismic, and surface abundance signatures through which they may be identified. During hydrogen shell burning, merger remnants reach higher luminosities and possess pulsations which depart from the usual degenerate sequence on the asteroseismic Δν– ΔΠ diagram for red giant branch stars. For sufficiently massive helium white dwarfs, merger remnants undergo especially violent helium flashes which can dredge up a large amount of core material (up to ∼0.1M⊙) into the envelope. Such post-dredge-up remnants are more luminous than normal red clump stars, are surface carbon-, helium-, and possibly lithium-rich, and possess a wider range of asteroseismic g-mode period spacings and mixed-mode couplings. Recent asteroseismically determined low-mass ( ≲0.8M⊙) red clump stars may be core helium-burning remnants of mergers involving lower-mass helium white dwarfs.

Testing the asteroseismic estimates of stellar radii with surface brightness-colour relations and Gaia DR3 parallaxes. Red giants and red clump stars

We compared stellar radii derived from asteroseismic scaling relations with those estimated using two independent surface brightness-colour relations (SBCRs) combined with Gaia DR3 parallaxes. We cross-matched asteroseismic and astrometric data for over 6,400 red giant branch (RGB) and red clump (RC) stars from the APO-K2 catalogue with the TESS Input Catalogue v8.2 to obtain precise $V$ band magnitudes and $E(B-V)$ colour excesses. We then adopted two different SBCRs from the literature to derive stellar radius estimates, denoted as $R^a$ and $R^b$, respectively. We analysed the ratio of these SBCR-derived radii to the asteroseismic radius estimates, $R,$ provided in the APO-K2 catalogue. Both SBCRs exhibited good agreement with asteroseismic radius estimates. On average, $R^a$ was overestimated by 1.2<!PCT!> with respect to $R$, while $R^b$ was underestimated by 2.5<!PCT!>. For stars larger than 20 $R_ sun $, SBCR radii are systematically lower than asteroseismic ones. The dispersion in the radius ratio was similar for the two methods (around 10<!PCT!>). The agreement with asteroseismic radii shows a strong dependence on the parallax. The dispersion is halved for stars with a parallax greater than 2.5 mas. In this subsample, $R^b$ showed perfect agreement with $R$, while $R^a$ remained slightly overestimated, by 3<!PCT!>. A trend with Fe/H was found at a level of 4<!PCT!> to 6<!PCT!> per dex. Additionally, a clear trend with asteroseismic mass is found. For stars less massive than about 0.95 $M_ sun $, SBCR radii were significantly higher than asteroseismic ones, by about 6<!PCT!>. This overestimation correlated with the presence of extended helium cores in these stars' structures relative to their envelopes. Furthermore, radius ratios showed a dichotomous behaviour at higher masses, mainly due to the presence of several RC stars with SBCR radii significantly lower with respect to asteroseismology. This behaviour originates from a different response of asteroseismic scaling relations and SBCR to alpha /Fe abundance ratios for massive stars, both in RGB and RC phases, which is reported here for the first time.

Li Evolution among Stars of Low/Intermediate Mass: the Metal-deficient Open Cluster NGC 2204

We have analyzed high-dispersion spectra in the Li 6708 Å region for 167 stars within the anticenter cluster NGC 2204. From 105 probable members, abundance analysis of 45 evolved stars produces [Fe/H] = −0.40 ± 0.12, [Si/Fe] = 0.14 ± 0.12, [Ca/Fe] = 0.29 ± 0.07, and [Ni/Fe] = −0.12 ± 0.10, where quoted errors are standard deviations. With E(B − V) = 0.07 and (m − M)0 = 13.12, appropriate isochrones provide an excellent match from the main sequence through the tip of the giant branch for an age of 1.85 ± 0.05 Gyr. Li spectrum synthesis produces A(Li) below 1.4 at the base of the red giant branch to a detectable value of −0.4 at the tip. Six probable asymptotic giant branch stars and all but one red clump star have only Li upper limits. A rapidly rotating red giant is identified as a possible Li-rich giant, assuming it is a red clump star. Main-sequence turnoff stars have a well-defined A(Li) = 2.83 ± 0.03 (sem) down to the Li-dip wall at the predicted mass of 1.29 M ☉. Despite having the same isochronal age as the more metal-rich NGC 2506, the luminosity distribution of red giants reflects a younger morphology similar to NGC 7789, possibly indicating a deeper impact of metallicity on stellar structure and A(Li) than previously assumed. As in NGC 2506 and NGC 7789, the NGC 2204 turnoff exhibits a broad range of rotation speeds, making abundance estimation impossible for some stars. The place of the cluster within Galactic A(Li) evolution is discussed.

Detection of a Spatially Extended Stellar Population in M33: A Shallow Stellar Halo?

We analyze the outer regions of M33, beyond 15 kpc in projected distance from its center, using Subaru/Hyper Suprime-Cam multicolor imaging. We identify red giant branch (RGB) stars and red clump (RC) stars using the surface-gravity-sensitive NB515 filter for the RGB sample and a multicolor selection for both samples. We construct the radial surface density profiles of these RGB and RC stars and find that M33 has an extended stellar population with a shallow power-law index of α > −3, depending on the intensity of the contamination. This result represents a flatter profile than the stellar halo that was detected by the previous study focusing on the central region, suggesting that M33 may have a double-structured halo component, i.e., inner/outer halos or a very extended disk. Also, the slope of this extended component is shallower than those typically found for halos in large galaxies, implying intermediate-mass galaxies may have different formation mechanisms (e.g., tidal interaction) from large spirals. We also analyze the radial color profiles of RC/RGB stars and detect a radial gradient, consistent with the presence of an old and/or metal-poor population in the outer region of M33, thereby supporting our proposal that the stellar halo extends beyond 15 kpc. Finally, we estimate that the surface brightness of this extended component is μ V = 35.72 ± 0.08 mag arcsec−2. If our detected component is the stellar halo, this estimated value is consistent with the detection limit of previous observations.

Revealing the Fate of Exoplanet Systems: Asteroseismic Identification of Host Star in the Red Clump or Red Giant Branch

Determining the evolutionary stage of stars is crucial for understanding the evolution of exoplanetary systems. In this context, red giant branch (RGB) and red clump (RC) stars, which formed at stages in the later evolution of stars situated before and after the helium flash, harbor critical clues to unveiling the evolution of planets. The first step in revealing these clues is to confirm the evolutionary stage of the host stars through asteroseismology. However, up to now, host stars confirmed to be RGB or RC stars are extremely rare. In this investigation, we present a comprehensive asteroseismic analysis of two evolved stars, HD 120084 and HD 29399, known to harbor exoplanets, using data from the Transiting Exoplanet Survey Satellite. We have discovered for the first time that HD 120084 is an RC star in the helium-core-burning phase, and confirmed that HD 29399 is an RGB star in the hydrogen-shell burning phase. Through the precise measurement of asteroseismic parameters such as νmax , Δν, and ΔΠ1, we have determined the evolutionary states of these stars and derived their fundamental stellar parameters. The significance of this study lies in the application of automated techniques to measure asymptotic period spacings in red giants, which provides critical insights into the evolutionary outcomes of exoplanet systems. We demonstrate that asteroseismology is a potent tool for probing the internal structures of stars, thereby offering a window into the past and future dynamics of planetary orbits. The presence of a long-period giant planet orbiting HD 120084, in particular, raises intriguing questions about the potential engulfment of inner planets during the host star’s expansion, a hypothesis that warrants further investigation.

VVV catalog of ab-type RR Lyrae in the inner Galactic bulge

Observational evidence has accumulated in recent years, showing that the Galactic bulge includes two populations, a metal-poor one and a metal-rich one, which in addition to having different metallicities show different alpha over iron abundances, spatial distribution, and kinematics. While the metal-rich, barred component has been fairly well characterized, the metal-poor, spheroidal component has been more elusive and harder to describe. RR Lyrae variables are clean tracers of the old bulge component, and they are, on average, more metal-poor than red clump stars. In the present paper, we provide a new catalog of 16488 ab-type RR Lyrae variables in the bulge region within |l|$ and |b|$ extracted from multi-epoch Point Spread Function photometry performed on VISTA Variable in the V\' a L\'actea survey data. We used the catalog to constrain the shape of the old, metal-poor, bulge stellar population. The identification of ab-type RR Lyrae among a large sample of candidate variables of different types has been performed via a combination of a Random Forest classifier and visual inspection. We optimized this process in such a way to extract a clean catalog with high purity, although for this reason its completeness, close to the midplane, is lower compared to a few other near-infrared catalogs covering the same region of the sky. We used the present catalog to derive the shape of their distribution around the Galactic center, resulting in an elongated spheroid with projected axis ratio of b/asim 0.7 and an inclination angle of phi sim 20 degrees. We discuss how observational biases, such as errors on the distances and a nonuniform sampling in longitude, affect both the present measurements and previous ones, especially those based on red clump stars. Because the latter have not been taken into account before, we refrain from a quantitative comparison between these shape parameters and those derived for the main Galactic bar. Nonetheless, qualitatively, taking into account observational biases would lower the estimated ellipticity of the bar derived from red clump stars, and hence reduce the difference with the present results. We publish a high-purity RRab sample for future studies of the oldest Galactic bulge population, close to the midplane. We explore different choices for the period-luminosity-metallicity relation, highlighting how some of them introduce spurious trends of distance with either the period or the metallicity, or both. We provide evidence that they trace a structure that is less elongated than the main bar, though we also highlight some biases of these kind of studies not discussed before.

Enrichment of Chemical Element 7Li in the Rotating Red Clump Star

Abstract About 2 percent of red clump stars are found to be the lithium-rich and thus the surface lithium increases obviously in some red clump stars. The physical mechanism of the lithium enrichment in these stars has not yet been explained satisfactorily by the evolutionary models of single stars. The flash induced internal gravity wave mixing (i.e., IGW) could play a primary role in explaining the red clump star with lithium enrichment and it has a very significant impact on the internal structure and surface compositions of a star. Rotation can significantly increase the mixing efficiency of the internal gravity wave because the timescale for the enrichment event has been enlarged. Thermohaline mixing can explain the observed behavior of lithium on red giant stars that are more luminosity than the RGB bump. However, it has a very small effect on the diffusion of elements because its diffusion coefficient is much smaller than the one of IGW induced mixing after the core helium flash.

UOCS XIV: Study of the Open Cluster NGC 2627 Using UVIT/AstroSat

We study the intermediate-age open cluster NGC 2627, located at a distance of ∼2 kpc, using UVIT/AstroSat and other archival data. Using a machine learning-based algorithm, ML-MOC, on the Gaia DR3 data, we identify 422 cluster members, including four blue straggler stars (BSSs), one yellow straggler star (YSS), one blue lurker (BL), one red clump (RC) star, and two binary candidates with detection in both UVIT/F148W and UVIT/F169M filters. We characterise them using multiwavelength spectral energy distributions (SEDs). Out of the above nine sources, one BSS, the BL, and one binary candidate have a source nearby; hence, we did not fit their SEDs. Of the remaining six sources, we successfully fit two with single-component SEDs and four with binary-component SEDs. The binary-component SED-based parameters indicate that the hot companions of BSSs, the YSS, the RC star, and the binary candidate are extremely low-mass white dwarfs, confirming that at least four out of nine stars (44%) are formed via the mass transfer channel. We fit King’s profile function to the high-probability (p > 0.8) cluster members and estimate the cluster core radius (r C ) to be 3.84′ and the tidal radius (r t ) to be 36.85′. We find that the equal-mass binaries are most concentrated towards the cluster center, followed by the single massive stars, and single low-mass stars. The BSS population of the cluster is also found to be located within a radius r ∼ 10 × r C from the cluster center, suggesting the dynamical evolution of the cluster.

Asteroseismology of evolved stars in six star clusters observed by Kepler/K2

In this study, we have explored the frequency separations, Δν and δν02, the height and width of the oscillation power excess, HGauss and δνenv, as a function of the frequency of maximum power νmax by analyzing 187 evolved stars in six star clusters observed by the Kepler/K2 missions. We have also examined the asteroseismic relation in the Christensen-Dalsgaard diagram. Given the importance of scaling relations, the dependency of asteroseismic scaling relations on physical quantities must be verified to reduce systematic errors through the exploration of observational data obtained from various sources. In this context, the star cluster provides a valuable means to assess the age and metallicity. Focusing on evolved stars with 30μHz<νmax<220μHz, we have exploited the mass effect without the need for deriving the individual stellar mass. We have found that the considered relations appear to be associated with the age of star clusters, thereby the mass of the stars in a given evolutionary status for star clusters with different ages. By separately considering red giant branch stars and red clump stars, we have found that red clump stars appear more sensitive to the cluster age compared with red giant branch stars. It has been suggested that conclusions regarding the dependency of metallicity should be drawn with due care as outcomes are subject to how to treat metallicity. Finally, we conclude by briefly pointing out implications of our findings on asteroseismic inferences.

Unveiling the purely young star formation history of the SMC’s northeastern shell from colour–magnitude diagram fitting

ABSTRACT 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.

The VISTA Variables in the Vía Láctea eXtended (VVVX) ESO public survey: Completion of the observations and legacy

The ESO public survey VISTA Variables in the V\'ia L\'actea (VVV) surveyed the inner Galactic bulge and the adjacent southern Galactic disk from $2009-2015$. Upon its conclusion, the complementary VVV eXtended (VVVX) survey has expanded both the temporal as well as spatial coverage of the original VVV area, widening it from $562$ to $1700$ sq. deg., as well as providing additional epochs in s $ filters from $2016-2023$. With the completion of VVVX observations during the first semester of 2023, we present here the observing strategy, a description of data quality and access, and the legacy of VVVX. VVVX took sim \,$2000$ hours, covering about 4&lt;!PCT!&gt; of the sky in the bulge and southern disk. VVVX covered most of the gaps left between the VVV and the VISTA Hemisphere Survey (VHS) areas and extended the VVV time baseline in the obscured regions affected by high extinction and hence hidden from optical observations. VVVX provides a deep $JHK_ s $ catalogue of $ point sources, as well as a s $ band catalogue of sim \,$10^7$ variable sources. Within the existing VVV area, we produced a $5D$ map of the surveyed region by combining positions, distances, and proper motions of well-understood distance indicators such as red clump stars, RR Lyrae, and Cepheid variables. In March 2023 we successfully finished the VVVX survey observations that started in 2016, an accomplishment for ESO Paranal Observatory upon 4200 hours of observations for VVV+VVVX. The VVV+VVVX catalogues complement those from the Gaia mission at low Galactic latitudes and provide spectroscopic targets for the forthcoming ESO high-multiplex spectrographs MOONS and 4MOST.

Comparing bulge RR Lyrae stars with bulge giants

Context. The structure and kinematics of the old component of the Galactic bulge are still a matter of debate. It is clear that the bulk of the bulge as traced by red clump stars includes two main components, which are usually identified as the metal-rich and metal-poor components. They have different shapes, kinematics, mean metallicities, and alpha-element abundances. It is our current understanding that they are associated with a bar and a spheroid, respectively. On the other hand, RR Lyrae variables trace the oldest population of the bulge. While it would be natural to think that they follow the structure and kinematics of the metal-poor component, the data analysed in the literature show conflicting results. Aims. We aim to derive a rotation curve for bulge RR Lyrae stars in order to determine that the old component traced by these stars is distinct from the two main components observed in the Galactic bulge. Methods. This paper combines APOGEE-2S spectra with OGLE-IV light curves, near-infrared photometry, and proper motions from the VISTA Variables in the Vía Láctea survey for 4193 RR Lyrae stars. Six-dimensional phase-space coordinates were used to calculate orbits within an updated Galactic potential and to isolate the stars. Results. The stars that stay confined within the bulge represent 57% of our sample. Our results show that bulge RR Lyrae variables rotate more slowly than metal-rich red clump stars and have a lower velocity dispersion. Their kinematics is compatible with them being the low-metallicity tail of the metal-poor component. We confirm that a rather large fraction of halo and thick disc RR Lyrae stars pass by the bulge within their orbits, increasing the velocity dispersion. A proper orbital analysis is therefore critical to isolate bona fide bulge variables. Finally, bulge RR Lyrae seem to trace a spheroidal component, although the current data do now allow us to reach a firm conclusion about the spatial distribution.

A Comprehensive Kinematic Model of the Large Magellanic Cloud Disk from Star Clusters and Field Stars using Gaia DR3: Tracing the Disk Characteristics, Rotation, Bar, and Outliers

The internal kinematics of the Large Magellanic Cloud (LMC) disk have been modeled by several studies using different tracers with varying coverage, resulting in a range of parameters. Here, we model the LMC disk using 1705 star clusters and field stars, based on a robust Markov Chain Monte Carlo method, using Gaia DR3 data. The dependency of the model parameters on the age, coverage, and strength of the clusters are also presented. This is the first comprehensive 2D kinematic study using star clusters. Red clump (RC) stars and young main-sequence stars are also modeled for comparison. The clusters and field stars are found to have distinctly different kinematic centers, disk inclination, position angle of the line of nodes, and scale radius. We also note a significant radial variation of the disk parameters. Clusters and young stars are found to have a large residual proper motion and a relatively large velocity dispersion when compared to the RC field population, which could be due to perturbation from the bar and spiral arms. We trace the presence of the large residual proper motion and noncircular motion among clusters likely to be due to the bar and detect a decrease in the scale radius as a result of the possible evolution of the bar. The kinematically deviant clusters point to a spatiotemporal disturbance in the LMC disk, matching with the expected impact factor and time of the recent collision between the LMC and the Small Magellanic Cloud.

A MUSE view of the core of the giant low-surface-brightness galaxy Malin 1

Aims. The central region of the giant low-surface-brightness galaxy Malin 1 has long been known to have a complex morphology, with evidence of a bulge, disc, and potentially a bar hosting asymmetric star formation. In this work, we use VLT/MUSE data to resolve the central region of Malin 1 in order to determine its structure. Methods. We used careful light profile fitting in every image slice of the datacube to create wavelength-dependent models of each morphological component, from which we were able to cleanly extract their spectra. We then used the kinematics and emission line properties from these spectra to better understand the nature of each component extracted from our model fitting. Results. We report the detection of a pair of distinct sources at the centre of this galaxy with a separation of ∼1.05″, which corresponds to a separation on sky of ∼1.9 kpc. The radial velocity data of each object confirm that they both lie in the kinematic core of the galaxy. An analysis of the emission lines reveals that the central compact source is more consistent with being ionised through star formation and/or a LINER, while the off-centre compact source lies closer to the separation between star-forming galaxies and active galactic nuclei. Conclusions. This evidence suggests that the centre of Malin 1 hosts either a bar with asymmetric star formation or two distinct components. In the latter scenario, we propose two hypotheses for the nature of the off-centre compact source-it could either be a star-forming clump, containing one or more star clusters, that is in the process of falling into the core of the galaxy and eventually merging with the central nuclear star cluster, or it could be a clump of gas falling into the centre of the galaxy from either outside or from the disc and triggering star formation there.

A path towards constraining the evolution of the interstellar medium and outflows in the Milky Way using APOGEE

ABSTRACT In recent years, the study of the Milky Way has significantly advanced due to extensive spectroscopic surveys of its stars, complemented by astroseismic and astrometric data. However, it remains disjoint from recent advancements in understanding the physics of the Galactic interstellar medium (ISM). This paper introduces a new model for the chemical evolution of the Milky Way that can be constrained on stellar data, because it combines a state-of-the-art ISM model with a Milky Way stellar disc model. Utilizing a data set of red clump stars from APOGEE, known for their precise ages and metallicities, we concentrate on the last 6 billion years – a period marked by Milky Way’s secular evolution. We examine the oxygen abundance in the low-$\alpha$ disc stars relative to their ages and birth radii, validating or constraining critical ISM parameters that remain largely unexplored in extragalactic observations. The models that successfully reproduce the radius–metallicity distribution and the age–metallicity distribution of stars without violating existing ISM observations indicate a need for modest differential oxygen enrichment in Galactic outflows, meaning that the oxygen abundance of outflows is higher than the local ISM abundance, irrespective of outflow mass loading. The models also suggest somewhat elevated ISM gas velocity dispersion levels over the past 6 billion years compared to galaxies of similar mass. The extra turbulence necessary could result from energy from gas accretion onto the Galaxy, supernovae clustering in the ISM, or increased star formation efficiency per freefall time. This work provides a novel approach to constraining the Galactic ISM and outflows, leveraging the detailed insights available from contemporary Milky Way surveys.

The Kinematic and Chemical Properties of the Close-in Planet Host Star 8 UMi

A recent study by Hon et al. reported that a close-in planet around the red clump star, 8 UMi, should have been engulfed during the expansion phase of its parent star’s evolution. They explained the survival of this exoplanet through a binary-merger channel for 8 UMi. The key to testing this formation scenario is to derive the true age of this star: is it an old “imposter” resulting from a binary merger, or a genuinely young red clump giant? To accomplish this, we derive kinematic and chemical properties for 8 UMi using astrometric data from Gaia DR3 and the element-abundance pattern measured from a high-resolution (R ∼ 75,000) spectrum taken by SOPHIE. Our analysis shows that 8 UMi is a normal thin-disk star with orbital rotation speed of V ϕ = 244.96 km s−1, and possesses a solar metallicity ([Fe/H] = −0.05 ± 0.07) and α-element-abundance ratio ([α/Fe] = +0.01 ± 0.03). By adopting well-established relationships between age and space velocities/elemental abundances, we estimate a kinematic age of 3.50−2.00+3.00 Gyr, and a chemical age of 3.25−1.50+2.50 Gyr from [C/N] and 3.47 ± 1.96 Gyr from [Y/Mg] for 8 UMi, respectively. These estimates are consistent with the isochrone-fitting age ( 1.90−0.30+1.15 Gyr) of 8 UMi, but are all much younger than the timescale required in a binary-merger scenario. This result challenges the binary-merger model; the existence of such a closely orbiting exoplanet around a giant star remains a mystery yet to be resolved.

Reconnaissance with JWST of the J-region Asymptotic Giant Branch in Distance Ladder Galaxies: From Irregular Luminosity Functions to Approximation of the Hubble Constant

We study stars in the J-regions of the asymptotic giant branch (JAGB) of near-infrared color–magnitude diagrams in the maser host NGC 4258 and four hosts of six Type Ia supernovae (SNe Ia): NGC 1448, NGC 1559, NGC 5584, and NGC 5643. These clumps of stars are readily apparent near 1.0 < F150W − F277W < 1.5 and m F150W = 22–25 mag with James Webb Space Telescope NIRCam photometry. Various methods have been proposed to assign an apparent reference magnitude to this recently proposed standard candle, including the mode, median, sigma-clipped mean, or a modeled luminosity function parameter. We test the consistency of these by measuring intrahost variations, finding differences of up to ∼0.2 mag that significantly exceed statistical uncertainties. Brightness differences appear intrinsic, and are further amplified by the nonuniform shape of the JAGB luminosity function, also apparent in the LMC and SMC. We follow a “many methods” approach to measure consistently JAGB magnitudes and distance moduli to the SN Ia host sample calibrated by NGC 4258. We find broad agreement with distance moduli measured from Cepheids, tip of the red giant branch, and Miras. However, the SN host mean distance modulus estimated via the JAGB method necessary to estimate H 0 differs by ∼0.19 mag among the above definitions, the result of different levels of luminosity function asymmetry. The methods yield a full range of 71−78 km s−1 Mpc−1, i.e., a fiducial result of H 0 = 74.7 ± 2.1(stat) ± 2.3(sys, ±3.1 if combined in quadrature) km s−1 Mpc−1, with systematic errors limited by the differences in methods. Future work may seek to standardize and refine this promising tool further, making it more competitive with established distance indicators.

Mapping the Chemodynamics of the Galactic Disk Using the LAMOST and APOGEE Red Clump Stars

A detailed measurement is made of the metallicity distributions, kinematics, and dynamics of the thin and thick disks across a large disk volume (5.0 ≤ R ≤ 15.0 kpc and ∣Z∣ ≤ 3.0 kpc) by using the LAMOST–APOGEE red clump stars. The metallicity distribution results show that the radial metallicity gradient Δ[Fe/H]/ΔR of the thin disk weakens with ∣Z∣ from −0.06 dex kpc−1 at around ∣Z∣ < 0.25 kpc to −0.02 dex kpc−1 at around ∣Z∣ > 2.75 kpc, while the thick disk displays a global weak positive Δ[Fe/H]/ΔR that is generally weaker than 0.01 dex kpc−1. The vertical metallicity gradient Δ[Fe/H]/Δ∣Z∣ steadily weakened from −0.36 dex kpc−1 at R ∼ 5.5 kpc to −0.05 dex kpc−1 at around R > 11.5 kpc for the thin disk, while the thick disk presents an almost constant value (nearly −0.06∼−0.08 dex kpc−1) for all the R bins. These results indicate the contribution of the radial migration to the disk evolution, and the obvious north–south asymmetry in [Fe/H] may be linked to disk warp and/or disk perturbation events. The oscillations in the corrected Δ[Fe/H]/Δ∣Z∣ with R likely arise from the resonances with the Galactic bar. Our detailed measurements of ΔV ϕ /Δ[Fe/H] indicate an inside-out and upside-down star formation scenario for the thick disk. The results of eccentricity distributions and [α/Fe]–velocity dispersion relations are likely to suggest that thick-disk stars require an obvious contribution from other heating mechanisms, such as mergers and accretion, or are born in the chaotic mergers of gas-rich systems and/or the turbulent interstellar medium.

Verification of Hypervelocity Bulge Red Clump Stars

We verify candidate hypervelocity red clump stars located in the Galactic bulge that were selected based on the VVV and the Gaia DR2 data by Luna et al. To do so, we analyze data from the OGLE-IV survey: difference images and astrometric time series. We have data for 30 stars out of 34 hypervelocity candidates. We confirmed the high proper motion of only one of these stars and find out that it is a nearby one, hence, not a hypervelocity star. To sum up, we do not confirm the candidate stars as hypervelocity ones. Hence, we disprove the production rate of hypervelocity red clump stars by the central supermassive black hole provided by Luna et al.

Data-driven Discovery of Diffuse Interstellar Bands with APOGEE Spectra

Data-driven models of stellar spectra are useful tools to study nonstellar information, such as the diffuse interstellar bands (DIBs) caused by intervening interstellar material. Using ∼55,000 spectra of ∼17,000 red clump stars from the APOGEE DR16 data set, we create second-order polynomial models of the continuum-normalized flux as a function of stellar parameters (T eff, logg , [Fe/H], [α/Fe], and age). The model and data show good agreement within uncertainties across the APOGEE wavelength range, although many regions reveal residuals that are not in the stellar rest-frame. We show that many of these residual features—having average extrema at the level of ∼3% in stellar flux on average—can be attributed to incompletely removed spectral lines from the Earth’s atmosphere and DIBs from the interstellar medium (ISM). After removing most of the remaining contamination from Earth’s sky, we identify 84 absorption features not seen in unreddened sightlights that have <50% probability of being noise artifacts—with 25 of these features having <5% probability of being noise artifacts—including all 10 previously known DIBs in the APOGEE wavelength range. Because many of these features occur in the wavelength windows that APOGEE uses to measure chemical abundances, note that characterization and removal of this nonstellar contamination establish an important step in reaching the precision required for chemical tagging experiments. Proper characterization of these features will benefit Galactic ISM science and the currently ongoing Milky Way Mapper program of Sloan Digital Sky Survey V, which relies on the APOGEE spectrograph.