arXiv subjects

Shourya Khanna

Publications and source records attributed to Shourya Khanna.

At least 19 recordsLinked to original sources

arXiv2026

Rogue Ones: Orbital census of Galactic Cepheids and their Anomalies

Classical Cepheids (DCEPs) are excellent standard candles expected to trace the spatial and kinematic distribution of the Galaxy's young and dynamically cold stellar disc. Using the most precise mid-infrared DCEP distances to date combined with Gaia-DR3 astrometry and line-of-sight velocities, we perform a comprehensive 6D dynamical census of the Milky Way's DCEP population. While the vast majority exhibit the expected disc-like kinematics, we identify 18 kinematically anomalous Cepheids. These `rogue' stars reside on highly inclined orbits (three at > 70 degrees), including two in retrograde motion and one with a total velocity of ~480 \kms. Despite their extreme trajectories, their optical light curves are consistent with DCEP classifications. We explore whether these anomalies originate from possible classification systematics or physical processes and find only three of our sources are likely misclassified. Assuming a runaway scenario we derive dynamical ages for the kinematic anomalies, which we find highly consistent with their Cepheid ages. Spectroscopic follow-up would be insightful as one source in particular is exceptionally metal poor ([Fe/H] ~ -1.6 dex), which is highly atypical for a DCEP. Integrating photometric classification with 6D kinematics will help fully characterise the Galaxy's variable star populations.

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arXiv2026

High-fidelity stellar extinction with Gaia and APOGEE -- I. The method and a new extinction curve

The scarcity of high-fidelity extinction measurements remains a bottleneck in deriving accurate stellar properties from Gaia parallaxes. In this work, we aim to derive precision extinction estimates for APOGEE DR19 stars, establishing a new benchmark for Galactic stellar population studies. We first determine reddening by comparing observed colorsr, etrieved from photometric surveys or standardized synthetic magnitudes from Gaia BP/RP spectra, to intrinsic colors predicted via an XGBoost model. The model is trained on minimally reddened stars to infer intrinsic colors and their associated uncertainties, using APOGEE stellar parameters (Teff, logg, [Fe/H], and [alpha/Fe]). The derived reddening values are then converted into extinctions using an anchor ratio of A_BP / A_RP = 1.694 +/- 0.004, derived from red-clump-like stars. Here, we provide extinction measurements in 39 filters across 10 photometric systems and introduce a new empirical extinction curve optimized for broadband passbands. Our extinction estimates (Av) outperform existing results (Bayestar19, StarHorse, SEDEX), achieving a typical precision of 0.03 mag in Av. Notably, we identify systematic deviations of up to 30% between monochromatic and passband-integrated extinction ratios at wavelengths greater than 700 nm. This result highlights the necessity of adopting passband-specific coefficients when correcting extinction to derive stellar parameters. The derived extinction and reddening data are available to the community for download through Zenodo.

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arXiv2026

SpiralMap: A Python library of the Milky Way's spiral arms

We present a library of the major spiral arm models (and maps) of the Galaxy. The package is written in Python, and allows the user to both extract the 2D trace and overplot the spiral arms in Cartesian/Polar coordinates in both Heliocentric (HC) and Galactocentric (GC) frames. The current version is a compilation of 9 models (and maps) covering a range of tracers (stars, gas). Other models can also be added upon request.

astro-ph.IM↗

arXiv2025

GaiaUnlimited: The old stellar disc of the Milky Way as traced by the Red Clump

We present an exploration of the Milky Way's structural parameters using an all-sky sample of RC giants to map the stellar density from the Galactic disc beyond 3 kpc. These evolved giants are considered to be standard candles due to their low intrinsic variance in their absolute luminosities, allowing us to estimate their distances with reasonable confidence. We exploit all-sky photometry from the AllWISE mid-infrared survey and the Gaia survey, along with astrometry from Gaia Data Release 3 and recent 3D extinction maps, to develop a probabilistic scheme in order to select with high confidence RC-like stars. Our curated catalogue contains about 10 million sources, for which we estimate photometric distances based on the WISE W1 photometry. We then derive the selection function for our sample, which is the combined selection function of sources with both Gaia and AllWISE photometry. Using the distances and accounting for the full selection function of our observables, we are able to fit a two-disc, multi-parameter model to constrain the scale height (hz), scale-length (rd), flaring, and the relative mass ratios of the two disc components. We illustrate and verify our methodology using mock catalogues of RC stars. We find that the RC population is best described by a flared disc with scale length rd=$4.24\pm0.32$ kpc and scale height at the Sun of hz(at Sun)=$0.18\pm0.01$ kpc, and a shorter and thicker disc with rd=$2.66\pm0.11$ kpc, hz(at Sun)=$0.48\pm0.11$ kpc, with no flare. The thicker disc constitutes 66\% of the RC stellar mass beyond 3 kpc, while the flared disc shows evidence of being warped beyond 9 kpc from the Galactic center. The residuals between the predicted number density of RC stars from our axisymmetric model and the measured counts show possible evidence of a two-armed spiral perturbation in the disc of the Milky Way.

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arXiv2025

The Milky Way as Seen by Classical Cepheids II: Spiral Structure

As a relatively young and bright population and the archetype of standard candles, classical Cepheids are an ideal population on which to trace the non-axisymmetric structure in the young stellar disk to large distances. We used the new distances derived in Paper I based on mid-IR WISE photometry for a selected sample of 2857 dynamically young Cepheids to trace the spiral arms of the Milky Way. The Perseus and Sagittarius-Carina arms are clearly evident in the third and fourth Galactic quadrants, while the Local and Scutum arms are much weaker, and extinction severely limits our view of the latter innermost spiral arm. Pitch angles were derived for each arm over various ranges of Galactic azimuth, each covering at least 90deg in azimuth. Our method of detecting spiral arms and deriving pitch angles does not rely on pre-assigning sources to specific arms. While the spiral structure in the first and second quadrant is not obvious in part because of extinction effects, it is not inconsistent with the structure seen in the third and fourth quadrants. In summary, the Cepheids allow us to map spiral structure in the third and fourth Galactic quadrants where currently few masers have astrometric parallaxes, significantly extending our understanding of the Milky Way at large scales.

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arXiv2025

The Milky Way as Seen by Classical Cepheids I: Distances Based on Mid-infrared Photometry

Classical Cepheids are the archetype of the standard candle, thanks to the period-luminosity relation which allows to measure their intrinsic brightness. They are also relatively young and bright, potentially making them excellent tracers of the young stellar population that is responsible for shaping the visible aspect of our Galaxy, the Milky Way. However, being observers embedded in the dusty interstellar medium of the Galaxy, deriving reliable photometric distances to classical Cepheids of the Milky Way is a challenge. The typical approach is to use "reddening-free" indices, such as Wesenheit magnitudes, to obviate the need for an extinction correction. However, this approach could lead to unknown systematics - especially toward the inner Galaxy - as its assumption of a universal total-to-selective extinction ratio is not satisfied, particularly in lines of sight where the extinction is high and crosses spiral arms. We instead estimate new distances for 3424 Cepheids based on mid-IR photometry from WISE, which suffers minimally from extinction, and by adopting a 3D extinction map to calculate the necessary (albeit small) extinction corrections. We show that our distances are consistent with Gaia's parallaxes for the subset with relative parallax errors smaller than 10%, verifying that our mean distance errors are of the order of 6% and that the mean parallax zero point for this subsample is 7 $μ$as.

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arXiv2025

C3PO IV: co-natal stars depleted in refractories are magnetically more active -- possible imprints of planets

Chemical abundance anomalies in twin stars have recently been considered tell-tale signs of interactions between stars and planets. While such signals are prevalent, their nature remains a subject of debate. On one hand, exoplanet formation may induce chemical depletion in host stars by locking up refractory elements. On the other hand, exoplanet engulfment can result in chemical enrichment, both processes potentially producing similar differential signals. In this study, we aim to observationally disentangle these processes by using the Ca II infrared triplet to measure the magnetic activity of 125 co-moving star pairs with high SNR, high-resolution spectra from the Magellan, Keck, and VLT telescopes. We find that co-natal star pairs in which the two stars exhibit significant chemical abundance differences also show differences in their magnetic activity, with stars depleted in refractories being magnetically more active. Furthermore, the strength of this correlation between differential chemical abundances and differential magnetic activity increases with condensation temperature. One possible explanation is that the chemical anomaly signature may be linked to planet formation, wherein refractory elements are locked into planets, and the host stars become more active due to more efficient contraction during the pre-main-sequence phase or star-planet tidal and magnetic interactions.

astro-ph.EP↗

arXiv2024

The realm of Aurora. Density distribution of metal-poor giants in the heart of the Galaxy

The innermost portions of the Milky Way's stellar halo have avoided scrutiny until recently. The lack of wide-area survey data, made it difficult to reconstruct an uninterrupted view of the density distribution of the metal-poor stars inside the Solar radius. In this study, we utilize red giant branch (RGB) stars from Gaia, with metallicities estimated using spectro-photometry from Gaia Data Release 3. Accounting for Gaia's selection function, we examine the spatial distribution of metal-poor ([M/H]<-1.3) RGB stars, from the Galactic centre (r~1 kpc) out to beyond the Solar radius (r~18 kpc). Our best-fitting single-component cored power-law model shows a vertical flattening of ~0.5 and a slope -3.4, consistent with previous studies. Motivated by the mounting evidence for two distinct stellar populations in the inner halo, we additionally test a range of two-component models. One of the components models the tidal debris from the Gaia Sausage/Enceladus merger, while the other captures the Aurora population -- stars that predate the Galactic disk formation. Our best-fit two-component model suggests that both populations contribute equally around the Solar radius, but Aurora dominates the inner halo with a steeper power-law index of -4.5, in agreement with the nitrogen-rich star distribution measured by Horta et al. (2021).

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arXiv2024

Gaia DR3 detectability of unresolved binary systems

Gaia can not individually resolve very close binary systems, however, the collected data can still be used to identify them. A powerful indicator of stellar multiplicity is the sources reported Renormalized Unit Weight Error (ruwe), which effectively captures the astrometric deviations from single-source solutions. We aim to characterise the imprints left on ruwe caused by binarity. By flagging potential binary systems based on ruwe, we aim to characterise which of their properties will contribute the most to their detectability. We develop a model to estimate ruwe values for observations of Gaia sources, based on the biases to the single-source astrometric track arising from the presence of an unseen companion. Then, using the recipes from previous GaiaUnlimited selection functions, we estimate the selection probability of sources with high ruwe, and discuss what binary properties contribute to increasing the sources ruwe. We compute the maximum ruwe value which is compatible with single-source solutions as a function of their location on-sky. We see that binary systems selected as sources with a ruwe higher than this sky-varying threshold have a strong detectability window in their orbital period distribution, which peaks at periods equal to the Gaia observation time baseline. We demonstrate how our sky-varying ruwe threshold provides a more complete sample of binary systems when compared to single sky-averaged values by studying the unresolved binary population in the Gaia Catalogue of Nearby Stars. We provide the code and tools used in this study, as well as the sky-varying ruwe threshold through the GaiaUnlimited Python package

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arXiv2024

Uniting Gaia and APOGEE to unveil the cosmic chemistry of the Milky Way disc

The spatial distribution of Galactic stars with different chemical abundances encodes information on the processes that drove the formation and evolution of the Milky Way. Survey selection functions are indispensable for analysing astronomical catalogues produced by large-scale surveys. The use of these selection functions in data modelling is more complex when data from different surveys are to be modelled simultaneously. We introduce a procedure for constructing the selection function of a sample of red clump stars that have parallaxes and elemental abundances from the Gaia mission. We separately constructed the selection function of the APOGEE DR17 red clump stars, which depends on very different observables and has a very different spatial coverage. We combined the two surveys and accounted for their joint selection function to provide strong constraints on the radial and vertical density distribution of mono-abundance populations, with Gaia offering a dense coverage of the solar neighbourhood, while APOGEE reaches larger distances near the Galactic plane. We confirm that the radial density profile steepens with increasing metallicity. The combined sample also indicates a metallicity-dependent flaring of the alpha-poor disc. We provide the code for constructing the Gaia selection function we used in this study through the GaiaUnlimited Python package.

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arXiv2023

A new resonance-like feature in the outer disc of the Milky Way

Modern astrometric and spectroscopic surveys have revealed a wealth of structure in the phase space of stars in the Milky Way, with evidence of resonance features and non-equilibrium processes. Using Gaia's third data release, we present evidence of a new resonance-like feature in the outer disc of the Milky Way. The feature is most evident in the angular momentum distribution of the young Classical Cepheids, a population for which we can derive accurate distances over much of the Galactic disc. We then search for similar features in the outer disc using a much larger sample of red giant stars, as well as a compiled list of over 31 million stars with spectroscopic line-of-sight velocity measurements. While much less evident in these two older samples, the distribution of stars in action-configuration space suggests that resonance features are present here as well. The position of the feature in action-configuration space suggests that the new feature may be related to the Galactic bar, but other possibilities are discussed.

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arXiv2023

Measuring the Streaming motion in the Milky Way disc with Gaia EDR3 +

We map the 3D kinematics of the Galactic disc out to 3.5 kpc from the Sun, and within 0.75 kpc from the midplane of the Galaxy. To this end, we combine high quality astrometry from \gedrthree{}, with heliocentric line-of-sight velocities from \gdrtwo{}, and spectroscopic surveys including \apogee{}, \galah{}, and \lamost{}. We construct an axisymmetric model for the mean velocity field, and subtract this on a star-by-star basis to obtain the residual velocity field in the Galactocentric components (\vphi{}, \vR, \vz), and \vlos{}. The velocity residuals are quantified using the power spectrum, and we find that the peak power ($A/$[\rm \kms{}]) in the midplane ($|z|<0.25$ kpc) is ($A_ϕ,A_{\rm R},A_{\rm Z},A_{\rm los}$)=($4.2,8.5,2.6,4.6$), at $0.25 < |z|/[{\rm kpc}] < 0.5$, is ($A_ϕ,A_{\rm R},A_{\rm Z},A_{\rm los}$)=($4.0,7.9,3.6,5.3$), and at $0.5 < |z|/[{\rm kpc}] < 0.75$, is ($A_ϕ,A_{\rm R},A_{\rm Z},A_{\rm los}$)=($1.9,6.9,5.2,6.4$). Our results provide a sophisticated measurement of the streaming motion in the disc and in the individual components. We find that streaming is most significant in \vR, and at all heights ($|Z|$) probed, but is also non-negligible in other components. Additionally, we find that patterns in velocity field overlap spatially with models for Spiral arms in the Galaxy. Our simulations show that phase-mixing of disrupting spiral arms can generate such residuals in the velocity field, where the radial component is dominant, just as in real data. We also find that with time evolution both the amplitude and physical scale of the residual motion decrease.

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arXiv2023

Revised extinctions and radii for 1.5 million stars observed by APOGEE, GALAH, and RAVE

Asteroseismology has become widely accepted as a benchmark for accurate and precise fundamental stellar properties. It can therefore be used to validate and calibrate stellar parameters derived from other approaches. Meanwhile, one can leverage large-volume surveys in photometry, spectroscopy, and astrometry to infer stellar parameters over a wide range of evolutionary stages, independently of asteroseismology. Our pipeline, SEDEX, compares the spectral energy distribution predicted by the MARCS and BOSZ model spectra with 32 photometric bandpasses, combining data from 9 major, large-volume photometric surveys. We restrict the analysis to targets with available spectroscopy from the APOGEE, GALAH, and RAVE surveys to lift the temperature-extinction degeneracy. The cross-survey atmospheric parameter and uncertainty estimates are homogenized with artificial neural networks. Validation of our results with CHARA interferometry, HST CALSPEC spectrophotometry, and asteroseismology, shows that we achieve high precision and accuracy. We present a catalog of improved interstellar extinction ($σ_{A_V} \simeq$ 0.14 mag) and stellar radii ($σ_R/R \simeq$ 7.4\%) for $\sim$1.5 million stars in the low- to high-extinction ($A_V \lesssim 6 $ mag) fields observed by the spectroscopic surveys. We derive global extinctions for 184 Gaia DR2 open clusters, and confirm the differential extinction in NGC 6791 and NGC 6819 that have been subject to extensive asteroseismic analysis. Furthermore, we report 36,854 double-lined spectroscopic main-sequence binary candidates. This catalog will be valuable for providing constraints on detailed modelling of stars and for constructing 3D dust maps of the Kepler field, the TESS CVZs, and the PLATO long duration observation fields.

astro-ph.SR↗

arXiv2022

The disturbed outer Milky Way disc

The outer parts of the Milky Way's disc are significantly out of equilibrium. Using only distances and proper motions of stars from Gaia's Early Data Release 3, in the range |b|<10°, 130°<l<230°, we show that for stars in the disc between around 10 and 14 kpc from the Galactic centre, vertical velocity is strongly dependent on the angular momentum, azimuth, and position above or below the Galactic plane. We further show how this behaviour translates into a bimodality in the velocity distribution of stars in the outer Milky Way disc. We use an N-body model of an impulse-like interaction of the Milky Way disc with a perturber similar to the Sagittarius dwarf to demonstrate that this mechanism can generate a similar disturbance. It has already been shown that this interaction can produce a phase spiral similar to that seen in the Solar neighbourhood. We argue that the details of this substructure in the outer galaxy will be highly sensitive to the timing of the perturbation or the gravitational potential of the Galaxy, and therefore may be key to disentangling the history and structure of the Milky Way.

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arXiv2022

An empirical model of the Gaia DR3 selection function

Interpreting and modelling astronomical catalogues requires an understanding of the catalogues' completeness or selection function: objects of what properties had a chance to end up in the catalogue. Here we set out to empirically quantify the completeness of the overall Gaia DR3 catalogue. This task is not straightforward because Gaia is the all-sky optical survey with the highest angular resolution to date and no consistent ``ground truth'' exists to allow direct comparisons. However, well-characterised deeper imaging enables an empirical assessment of Gaia's $G$-band completeness across parts of the sky. On this basis, we devised a simple analytical completeness model of Gaia as a function of the observed $G$ magnitude and position over the sky, which accounts for both the effects of crowding and the complex Gaia scanning law. Our model only depends on a single quantity: the median magnitude $M_{10}$ in a patch of the sky of catalogued sources with $\texttt{astrometric_matched_transits}$ $\leq 10$. $M_{10}$ reflects elementary completeness decisions in the Gaia pipeline and is computable from the Gaia DR3 catalogue itself and therefore applicable across the whole sky. We calibrate our model using the Dark Energy Camera Plane Survey (DECaPS) and test its predictions against Hubble Space Telescope observations of globular clusters. We find that our model predicts Gaia's completeness values to a few per cent across the sky. We make the model available as a part of the $\texttt{gaiasf}$ Python package built and maintained by the GaiaUnlimited project: $\texttt{https://github.com/gaia-unlimited/gaiaunlimited}$

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arXiv2021

The GALAH+ Survey: Third Data Release

The ensemble of chemical element abundance measurements for stars, along with precision distances and orbit properties, provides high-dimensional data to study the evolution of the Milky Way. With this third data release of the Galactic Archaeology with HERMES (GALAH) survey, we publish 678 423 spectra for 588 571 mostly nearby stars (81.2% of stars are within <2 kpc), observed with the HERMES spectrograph at the Anglo-Australian Telescope. This release (hereafter GALAH+ DR3) includes all observations from GALAH Phase 1 (bright, main, and faint survey, 70%), K2-HERMES (17%), TESS-HERMES (5%), and a subset of ancillary observations (8%) including the bulge and >75 stellar clusters. We derive stellar parameters $T_\text{eff}$, $\log g$, [Fe/H], $v_\text{mic}$, $v_\text{broad}$ & $v_\text{rad}$ using our modified version of the spectrum synthesis code Spectroscopy Made Easy (SME) and 1D MARCS model atmospheres. We break spectroscopic degeneracies in our spectrum analysis with astrometry from $Gaia$ DR2 and photometry from 2MASS. We report abundance ratios [X/Fe] for 30 different elements (11 of which are based on non-LTE computations) covering five nucleosynthetic pathways. We describe validations for accuracy and precision, flagging of peculiar stars/measurements and recommendations for using our results. Our catalogue comprises 65% dwarfs, 34% giants, and 1% other/unclassified stars. Based on unflagged chemical composition and age, we find 62% young low-$α$, 9% young high-$α$, 27% old high-$α$, and 2% stars with $\mathrm{[Fe/H]} \leq -1$. Based on kinematics, 4% are halo stars. Several Value-Added-Catalogues, including stellar ages and dynamics, updated after $Gaia$ eDR3, accompany this release and allow chrono-chemodynamic analyses, as we showcase.

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arXiv2020

Asteroseismology of luminous red giants with Kepler. II. Dependence of mass loss on pulsations and radiation

Mass loss by red giants is an important process to understand the final stages of stellar evolution and the chemical enrichment of the interstellar medium. Mass-loss rates are thought to be controlled by pulsation-enhanced dust-driven outflows. Here we investigate the relationships between mass loss, pulsations, and radiation, using 3213 luminous Kepler red giants and 135000 ASAS-SN semiregulars and Miras. Mass-loss rates are traced by infrared colours using 2MASS and WISE and by observed-to-model WISE fluxes, and are also estimated using dust mass-loss rates from literature assuming a typical gas-to-dust mass ratio of 400. To specify the pulsations, we extract the period and height of the highest peak in the power spectrum of oscillation. Absolute magnitudes are obtained from the 2MASS Ks band and the Gaia DR2 parallaxes. Our results follow. (i) Substantial mass loss sets in at pulsation periods above ~60 and ~100 days, corresponding to Asymptotic-Giant-Branch stars at the base of the period-luminosity sequences C' and C. (ii) The mass-loss rate starts to rapidly increase in semiregulars for which the luminosity is just above the Red-Giant-Branch tip and gradually plateaus to a level similar to that of Miras. (iii) The mass-loss rates in Miras do not depend on luminosity, consistent with pulsation-enhanced dust-driven winds. (iv) The accumulated mass loss on the Red Giant Branch consistent with asteroseismic predictions reduces the masses of red-clump stars by 6.3%, less than the typical uncertainty on their asteroseismic masses. Thus mass loss is currently not a limitation of stellar age estimates for galactic archaeology studies.

astro-ph.SR↗

arXiv2020

The GALAH Survey: Dependence of elemental abundances on age and metallicity for stars in the Galactic disc

Using data from the GALAH survey, we explore the dependence of elemental abundances on stellar age and metallicity among Galactic disc stars. We find that the abundance of most elements can be predicted from age and [Fe/H] with an intrinsic scatter of about 0.03 dex. We discuss the possible causes for the existence of the abundance-age-metallicity relations. Using a stochastic chemical enrichment scheme based on the size of Supernovae remnants, we show the intrinsic scatter is expected to be small, about 0.05 dex or even smaller if there is additional mixing in the ISM. Elemental abundances show trends with both age and metallicity and the relationship is well described by a simple model in which the dependence of abundance ([X/Fe]) on age and [Fe/H] are additively separable. Elements can be grouped based on the direction of their abundance gradient in the (age,[Fe/H]) plane and different groups can be roughly associated with three distinct nucleosynthetic production sites, the exploding massive stars, the exploding white dwarfs and the AGB stars. However, the abundances of some elements, like Co, La, and Li, show large scatter for a given age and metallicity, suggesting processes other than simple Galactic chemical evolution are at play. We also compare the abundance trends of main-sequence turn-off stars against that of giants, whose ages were estimated using asteroseismic information from the K2 mission. For most elements, the trends of main-sequence turn-off stars are similar to that of giants. The existence of abundance relations implies that we can estimate the age and birth radius of disc stars, which is important for studying the dynamic and chemical evolution of the Galaxy.

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