arXiv subjects

Deokkeun An

Publications and source records attributed to Deokkeun An.

At least 19 recordsLinked to original sources

arXiv2026

Backlighting young stellar objects in the Central Molecular Zone: an ensemble-averaged abundance structure of methanol ices

The Central Molecular Zone (CMZ) contains a substantial reservoir of dense molecular gas, where numerous young stellar objects (YSOs) and dense cloud cores have been identified. However, the large distance and severe foreground extinction complicate interpretation of infrared ice absorption features tracing chemical and evolutionary properties of these embedded objects. To better characterise YSOs and dense cores in this region, we combined spectra from multiple YSOs, each likely backlit by a giant star, allowing us to probe their outer layers and derive an ensemble-averaged ice abundance profile. We obtained L-band spectra of 15 point-like sources with extremely red colours using Gemini/GNIRS, enabling measurements of the CH3OH absorption feature at 3.535 micron. To better constrain the foreground extinction and H$_2$O ice column densities, we combined these data with K-band and mid-infrared spectra using NASA/IRTF and Spitzer/IRS. We found that the CH$_3$OH abundance in the CH$_3$OH-CO$_2$ ice mixture is 2 to 5 percent, confirming that it is systematically lower than those typically observed in the Galactic disk. Furthermore, by using the local excess of foreground extinction as a proxy for the projected distance between a backlit source and the centre of a YSO, we found that the CH$_3$OH abundance relative to solid CO$_2$ remains near 10 percent in the inner regions of the envelope, but increases sharply to about 30 percent in the outer regions. The relatively low methanol ice abundance may reflect the unique chemical environment of the CMZ. However, our results offer an alternative interpretation: since our sample is biased towards massive and luminous YSOs, intense heating from the central protostar may have caused substantial sublimation of methanol ice in the inner regions of their envelopes, thereby systematically lowering the observed CH$_3$OH/H$_2$O ice ratios.

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arXiv2025

Deciphering the Milky Way's star formation at cosmic noon with high proper-motion stars: A precursor to the merger-driven starburst

Evidence suggests that the Milky Way (MW) underwent a major collision with the Gaia-Sausage/Enceladus (GSE) dwarf galaxy around cosmic noon. While GSE has since been fully disrupted, it brought in ex situ stars and dynamically heated in situ stars into the halo. In addition, the gas-rich merger may have triggered a burst of in situ star formation, potentially giving rise to a chemically distinct stellar component. We investigated the region of phase space where stars formed during the GSE merger likely reside, and retain distinct chemical and dynamical signatures. Building on our previous investigation of metallicity ([Fe/H]) and vertical angular momentum ($L_Z$) distributions, we analysed spectroscopic samples from GALAH, APOGEE, SDSS, and LAMOST, combined with Gaia kinematics. We focused on high proper-motion stars as effective tracers of the phase-space volume likely influenced by the GSE merger. To correct for selection effects, we incorporated metallicity estimates derived from SDSS and SMSS photometry. Our analysis reveals that low-$α$ stars with GSE-like kinematics exhibit bimodality in [Na/Fe] and [Al/Fe] at $-1.0 \lesssim {\rm [Fe/H]} \lesssim -0.4$. One group follows the low light-element abundances of GSE stars, while another exhibits enhanced values. These low-$α$, high-Na stars have eccentric orbits but are more confined to the inner MW. Eos overlaps with a high-eccentricity subset of these stars, implying that it constitutes a smaller structure nested within the broader population. After correcting for sampling biases, we estimated a population ratio of approximately 1:10 between the low-$α$, high-Na stars and the GSE debris. These results suggest that the low-$α$, high-Na stars formed in a compact region, likely fuelled by gas from the GSE progenitor, analogous to clumpy star-forming clouds seen in high-redshift galaxies.

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arXiv2024

A Blueprint for the Milky Way's Stellar Populations. V. 3D Local Dust Extinction

Using a grid of empirically calibrated synthetic spectra developed in our previous study, we construct an all-sky 3D extinction map from the large collection of low-resolution XP spectra in Gaia DR3. Along each line of sight, with an area ranging from $0.2$ to $13.4$ deg$^2$, we determine both the reddening and metallicity of main-sequence stars and model the foreground extinction up to approximately $3$ kpc from the Sun. Furthermore, we explore variations in the total-to-selective extinction ratio in our parameter search and identify its mean systematic change across diverse cloud environments in both hemispheres. In regions outside the densest parts of the clouds, our reddening estimates are validated through comparisons with previous reddening maps. However, a notable discrepancy arises when compared to other independent work based on XP spectra, although our metallicity scale shows reasonable agreement with the high-resolution spectroscopic abundance scale. We also assess the accuracy of the XP spectra by applying our calibrated models, and confirm an increasing trend of flux overestimation at shorter wavelengths below $400$ nm.

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arXiv2023

A Blueprint for the Milky Way's Stellar Populations. IV. A String of Pearls $-$ the Galactic Starburst Sequence

We continue our series of papers on phase-space distributions of stars in the Milky Way based on photometrically derived metallicities and Gaia astrometry, with a focus on the halo-disk interface in the local volume. To exploit various photometric databases, we develop a method of empirically calibrating synthetic stellar spectra based on a comparison with observations of stellar sequences and individual stars in SDSS, SMSS, and PS1, overcoming band-specific corrections employed in our previous work. In addition, photometric zero-point corrections are derived to provide an internally consistent photometric system with a spatially uniform metallicity zero point. Using our phase-space diagrams, we find a remarkably narrow sequence in the rotational velocity ($v_ϕ$) versus metallicity ([Fe/H]) space for a sample of high proper-motion stars ($>25$ mas yr$^{-1}$), which runs along Gaia Sausage/Enceladus (GSE) and the Splash sub-structures, and is linked to the disk, spanning nearly $2$ dex in [Fe/H]. Notably, a rapid increase of $v_ϕ$ from a nearly zero net rotation to $\sim180$ km s$^{-1}$ in a narrow metallicity interval ($-0.6 \leq$ [Fe/H] $\leq -0.4$) suggests that some of these stars emerged quickly on a short gas-depletion time scale. Through measurements of a scale height and length, we argue that these stars are distinct from those heated dynamically by mergers. This chain of high proper-motion stars provides additional support for recent findings that suggest a starburst occurred when the young Milky Way encountered the gas-rich GSE progenitor, which eventually led to the settling of metal-enriched gas onto the disk.

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arXiv2022

Chemodynamical Analysis of Metal-rich High-eccentricity Stars in the Milky Way's Disk

We present a chemodynamical analysis of 11,562 metal-rich, high-eccentricity halo-like main-sequence (MS) stars, which has been referred to as the Splash or Splashed Disk, selected from Sloan Digital Sky Survey (SDSS) and Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). When divided into two groups, a low-[$α$/Fe] population (LAP) and a high-[$α$/Fe] population (HAP), based on kinematics and chemistry, we find that they exhibit very distinct properties, indicative of different origins. From a detailed analysis of their orbital inclinations, we suggest that the HAP arises from a large fraction (~ 90%) of heated disk stars and a small fraction (~ 10%) of in situ stars from a starburst population, likely induced by interaction of the Milky Way with Gaia Sausage/Enceladus (GSE) or other early merger. The LAP comprises about half accreted stars from the GSE and half formed by the GSE-induced starburst. Our findings further imply that the Splash stars in our sample originated from at least three different mechanisms - accretion, disk heating, and a merger induced starburst.

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arXiv2022

Massive Young Stellar Objects in the Galactic Center. II. Seeing Through the Ice-rich Envelopes

To study the demographics of interstellar ices in the Central Molecular Zone (CMZ) of the Milky Way, we obtain near-infrared spectra of $109$ red point sources using NASA IRTF/SpeX at Maunakea. We select the sample from near- and mid-infrared photometry, including $12$ objects in the previous paper of this series, to ensure that these sources trace a large amount of absorption through clouds in each line of sight. We find that most of the sample ($100$ objects) show CO band-head absorption at $2.3\ μ$m, tagging them as red (super-) giants. Despite the photospheric signature, however, a fraction of the sample with $L$-band spectra ($9/82=0.11$) exhibit large H$_2$O ice column densities ($N > 2\times10^{18}\ {\rm cm}^{-2}$), and six of them also reveal CH$_3$OH ice absorption. As one of such objects is identified as a young stellar object (YSO) in our previous work, these ice-rich sight lines are likely associated with background stars in projection to an extended envelope of a YSO or a dense cloud core. The low frequency of such objects in the early stage of stellar evolution implies a low star-formation rate ($<0.02\ M_\odot$ yr$^{-1}$), reinforcing the previous claim on the suppressed star-formation activity in the CMZ. Our data also indicate that the strong "shoulder" CO$_2$ ice absorption at $15.4\ μ$m observed in YSO candidates in the previous paper arises from CH$_3$OH-rich ice grains having a large CO$_2$ concentration [$N {\rm (CO_2)} / N {\rm (CH_3OH)} \approx 1/3$].

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arXiv2021

A Blueprint for the Milky Way's Stellar Populations. III. Spatial Distributions and Population Fractions of Local Halo Stars

We analyze the observed spatial, chemical and dynamical distributions of local metal-poor stars, based on photometrically derived metallicity and distance estimates along with proper motions from the Gaia mission. Along the Galactic prime meridian, we identify stellar populations with distinct properties in the metallicity versus rotational velocity space, including Gaia Sausage/Enceladus (GSE), the metal-weak thick disk (MWTD), and the Splash (sometimes referred to as the "in situ" halo). We model the observed phase-space distributions using Gaussian mixtures and refine their positions and fractional contributions as a function of distances from the Galactic plane ($|Z|$) and the Galactic center ($R_{\rm GC}$), providing a global perspective of the major stellar populations in the local halo. Within the sample volume ($|Z|<6$ kpc), stars associated with GSE exhibit a larger proportion of metal-poor stars at greater $R_{\rm GC}$ ($Δ\langle{\rm[Fe/H]}\rangle /ΔR_{\rm GC} =-0.05\pm0.02$ dex kpc$^{-1}$). This observed trend, along with a mild anticorrelation of the mean rotational velocity with metallicity ($Δ\langle v_ϕ\rangle / Δ\langle{\rm[Fe/H]} \rangle \sim -10$ km s$^{-1}$ dex$^{-1}$), implies that more metal-rich stars in the inner region of the GSE progenitor were gradually stripped away, while the prograde orbit of the merger at infall became radialized by dynamical friction. The metal-rich GSE stars are causally disconnected from the Splash structure, whose stars are mostly found on prograde orbits ($>94\%$) and exhibit a more centrally concentrated distribution than GSE. The MWTD exhibits a similar spatial distribution to the Splash, suggesting earlier dynamical heating of stars in the primordial disk of the Milky Way, possibly before the GSE merger.

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arXiv2021

Hunting for Planetary Nebulae toward the Galactic Center

We present near-infrared (IR) spectra of two planetary nebula (PN) candidates in close lines of sight toward the Galactic center (GC) using the Gemini Near-Infrared Spectrograph (GNIRS) at Gemini North. High-resolution images from radio continuum and narrow-band IR observations reveal ringlike or barrel-shaped morphologies of these objects, and their mid-IR spectra from the Spitzer Space Telescope exhibit rich emission lines from highly-excited species such as [S IV], [Ne III], [Ne V], and [O IV]. We also derive elemental abundances using the Cloudy synthetic models, and find an excess amount of the $s$-process element Krypton in both targets, which supports their nature as PN. We estimate foreground extinction toward each object using near-IR hydrogen recombination lines, and find significant visual extinctions ($A_V > 20$). The distances inferred from the size versus surface brightness relation of other PNe are $9.0\pm1.6$ kpc and $7.6\pm1.6$ kpc for SSTGC 580183 and SSTGC 588220, respectively. These observed properties along with abundance patterns and their close proximity to Sgr A$^*$ (projected distances $<20$ pc) make it highly probable that these objects are the first confirmed PN objects in the nuclear stellar disk. The apparent scarcity of such objects resembles the extremely low rate of PN formation in old stellar systems, but is in line with the current rate of the sustained star formation activity in the Central Molecular Zone.

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arXiv2021

The Photometric Metallicity and Carbon Distributions of the Milky Way's Halo and Solar Neighborhood from S-PLUS Observations of SDSS Stripe 82

We report photometric estimates of effective temperature, $T_{\rm eff}$, metallicity, [Fe/H], carbonicity, [C/Fe], and absolute carbon abundances, $A{\rm (C)}$, for over 700,000 stars from the Southern Photometric Local Universe Survey (S-PLUS) Data Release 2, covering a substantial fraction of the equatorial Sloan Digital Sky Survey Stripe 82. We present an analysis for two stellar populations: 1) halo main-sequence turnoff stars and 2) K-dwarf stars of mass $0.58 < M/M_{\odot} <0.75$ in the Solar Neighborhood. Application of the Stellar Photometric Index Network Explorer (SPHINX) to the mixed-bandwidth (narrow- plus wide-band) filter photometry from S-PLUS produces robust estimates of the metallicities and carbon abundances in stellar atmospheres over a wide range of temperature, $4250 < T_{\rm eff} \textrm{(K)} < 7000$. The use of multiple narrow-band S-PLUS filters enables SPHINX to achieve substantially lower levels of "catastrophic failures" (large offsets in metallicity estimates relative to spectroscopic determinations) than previous efforts using a single metallicity-sensitive narrow-band filter. We constrain the exponential slope of the Milky Way's K-dwarf halo metallicity distribution function (MDF), $λ_{10, \textrm{[Fe/H]}} = 0.85 \pm 0.21$, over the metallicity range $-2.5 < \textrm{[Fe/H]} < -1.0$; the MDF of our local-volume K-dwarf sample is well-represented by a gamma distribution with parameters $α=2.8$ and $β=4.2$. S-PLUS photometry obtains absolute carbon abundances with a precision of $\sim 0.35$dex for stars with $T_{\rm eff} < 6500$K. We identify 364 candidate carbon-enhanced metal-poor stars, obtain assignments of these stars into the Yoon-Beers morphological groups in the $A$(C)-[Fe/H] space, and derive the CEMP frequencies.

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arXiv2020

A Blueprint for the Milky Way's Stellar Populations. II. Improved Isochrone Calibration in the SDSS and Pan-STARRS Photometric Systems

We improve the identification and isolation of individual stellar populations in the Galactic halo based on an updated set of empirically calibrated stellar isochrones in the Sloan Digital Sky Survey (SDSS) and Pan-STARRS 1 (PS1) photometric systems. Along the Galactic prime meridian ($l=0^{\circ}$ and $180^{\circ}$), where proper motions and parallaxes from Gaia DR2 can be used to compute rotational velocities of stars in the rest frame of the Milky Way, we use the observed double color-magnitude sequences of stars having large transverse motions, which are attributed to groups of stars in the metal-poor halo and the thick disk with halo-like kinematics, respectively. The Gaia sequences directly constrain color-magnitude relations of model colors, and help to improve our previous calibration using Galactic star clusters. Based on these updated sets of stellar isochrones, we confirm earlier results on the presence of distinct groups of stars in the metallicity versus rotational-velocity plane, and find that the distribution of the most metal-poor ([Fe/H] $<-2$) stars in our sample can be modeled using two separate groups on prograde and retrograde orbits, respectively. At $4$-$6$ kpc from the Galactic plane, we find approximately equal proportions of the Splashed Disk, and the metal-rich ($\langle {\rm [Fe/H]} \rangle\sim-1.6$) and metal-poor ($\langle {\rm [Fe/H]} \rangle\sim-2.5$) halos on prograde orbits. The Gaia-Sausage-Enceladus, the metal-weak thick disk, and the retrograde halo structure(s) ($\langle {\rm [Fe/H]} \rangle\sim-2.2$) constitute approximately $10\%$ of the rest of the stellar populations at these distances.

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arXiv2020

A Blueprint for the Milky Way's Stellar Populations: The Power of Large Photometric and Astrometric Surveys

Recent advances from astronomical surveys have revealed spatial, chemical, and kinematical inhomogeneities in the inner region of the stellar halo of the Milky Way Galaxy. In particular, large spectroscopic surveys, combined with Gaia astrometric data, have provided powerful tools for analyzing the detailed abundances and accurate kinematics for individual stars. Despite these noteworthy efforts, however, spectroscopic samples are typically limited by the numbers of stars considered; their analysis and interpretation are also hampered by the complex selection functions that are often employed. Here we present a powerful alternative approach $-$ a synoptic view of the spatial, chemical, and kinematical distributions of stars in the Milky Way based on large photometric survey databases, enabled by a well-calibrated technique for obtaining individual stellar metal abundances from broad-band photometry. We combine metallicities with accurate proper motions from the Gaia mission along the Prime Meridian of the Galaxy, and find that various stellar components are clearly separated from each other in the metallicity versus rotation-velocity space. The observed metallicity distribution of the inner-halo stars deviates from the traditional single-peaked distribution, and exhibits complex substructures comprising varying contributions from individual stellar populations, sometimes with striking double peaks at low metallicities. The substructures revealed from our less-biased, comprehensive maps demonstrate the clear advantages of this approach, which can be built upon by future mixed-band and broad-band photometric surveys, and used as a blueprint for identifying the stars of greatest interest for upcoming spectroscopic studies.

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arXiv2019

Radial Dependence of the Proto-Globular Cluster Contribution to the Milky Way Formation

Recent interpretation of the color$-$magnitude diagrams of the Milky Way (MW) bulge has suggested that the observed double red-clump feature can be a natural consequence of He-enhanced stellar populations in the MW bulge. This implies that globular clusters (GCs), where the He-enhanced second-generation (SG) stars can be efficiently created, are the most likely candidate contributors of He-rich stars to the MW bulge. We extend this idea to the Galactic inner halo and investigate the fraction of the SG stars as a function of the Galactocentric distance. We use bluer blue-horizontal branch (bBHB) stars, which are assumed to be originated from He-rich SG populations, as proxies of SG stars, and find that the fraction of bBHB stars increases with decreasing Galactocentric distance. Simulations of the GC evolution in the MW tidal field qualitatively support the observed trend of bBHB enhancement in the inner halo. In these simulations, the increasing tidal force with decreasing Galactocentric distance leads to stripping of stars not only from the outskirts but also from the central regions of GCs, where SG stars are more abundant. We discuss the implication and prospect of our findings concerning the formation history of the bulge and inner halo of the MW.

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arXiv2019

Comparison of the Asteroseismic Mass Scale of Red Clump Giants with Photometric Mass Estimates

Asteroseismology can provide joint constraints on masses and radii of individual stars. While this approach has been extensively tested for red giant branch (RGB) stars, it has been more difficult to test for helium core-burning red-clump (RC) giants because of the lack of fundamental calibrators. To provide independent mass estimates, we utilize a number of widely used horizontal-branch (HB) models in the literature, and derive photometric masses from a comparison with $griBVI_CJHK_s$ photometry. Our selected models disagree with each other on the predicted mass-luminosity-temperature relation. We adopt first-order corrections on colors and magnitudes to minimize the dispersion between different models by forcing models to match the observed location in the solar-metallicity cluster M67. Even for these calibrated models, however, the internal consistency between models deteriorates at higher metallicities, and photometric masses become smaller than asteroseismic masses, as seen from metal-rich field RC stars with Gaia parallaxes. Similarly, the average photometric mass for metal-rich NGC 6791 stars ranges from $0.7\ M_\odot$ to $1.1\ M_\odot$, depending on the specific set of models employed. An ensemble average of the photometric masses ($0.88\pm0.16\ M_\odot$) in NGC 6791 is marginally consistent with the asteroseismic mass ($1.16\pm0.04\ M_\odot$). There is a clear tension between the masses that one would predict from photometry for metal-rich field RC stars, asteroseismic masses, and those that would be expected from the ages of stars in the Galactic disk populations and canonical RGB mass loss. We conclude that standard RC models need to be re-examined in light of these powerful new data sets.

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arXiv2019

Asymmetric Mean Metallicity Distribution of the Milky Way's Disk

I present the mean metallicity distribution of stars in the Milky Way Galaxy based on photometry from the Sloan Digital Sky Survey. I utilize an empirically calibrated set of stellar isochrones developed in previous work to estimate the metallicities of individual stars to a precision of $0.2$ dex for reasonably bright stars across the survey area. I also obtain more precise metallicity estimates using priors from the $Gaia$ parallaxes for relatively nearby stars. Close to the Galactic mid-plane ($|Z|<2$ kpc), a mean metallicity map reveals deviations from the mirror symmetry between the northern and southern hemispheres, displaying wave-like oscillations. The observed metallicity asymmetry structure is almost parallel to the Galactic mid-plane, and coincides with the previously known asymmetry in the stellar number density distribution. This result reinforces the previous notion of the plane-parallel vertical waves propagating through the disk, in which a local metallicity perturbation from the mean vertical metallicity gradient is induced by the phase-space wrapping of stars in the $Z$-$V_Z$ plane. The maximum amplitude of the metallicity asymmetry ($Δ$[Fe/H]$\sim0.05$) implies that these stars have been pulled away from the Galactic mid-plane by an order of $Δ|Z|\sim80$ pc as a massive halo substructure such as the Sagittarius dwarf galaxy plunged through the Milky Way. This work provides evidence that the $Gaia$ phase-space spiral may continue out to $|Z|\sim1.5$ kpc.

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arXiv2018

The Second APOKASC Catalog: The Empirical Approach

We present a catalog of stellar properties for a large sample of 6676 evolved stars with APOGEE spectroscopic parameters and \textit{Kepler} asteroseismic data analyzed using five independent techniques. Our data includes evolutionary state, surface gravity, mean density, mass, radius, age, and the spectroscopic and asteroseismic measurements used to derive them. We employ a new empirical approach for combining asteroseismic measurements from different methods, calibrating the inferred stellar parameters, and estimating uncertainties. With high statistical significance, we find that asteroseismic parameters inferred from the different pipelines have systematic offsets that are not removed by accounting for differences in their solar reference values. We include theoretically motivated corrections to the large frequency spacing ($Δν$) scaling relation, and we calibrate the zero point of the frequency of maximum power ($ν_{\rm max}$) relation to be consistent with masses and radii for members of star clusters. For most targets, the parameters returned by different pipelines are in much better agreement than would be expected from the pipeline-predicted random errors, but 22\% of them had at least one method not return a result and a much larger measurement dispersion. This supports the usage of multiple analysis techniques for asteroseismic stellar population studies. The measured dispersion in mass estimates for fundamental calibrators is consistent with our error model, which yields median random and systematic mass uncertainties for RGB stars of order 4\%. Median random and systematic mass uncertainties are at the 9\% and 8\% level respectively for RC stars.

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arXiv2017

Medium-resolution Spectroscopy of Red Giant Branch Stars in $ω$ Centauri

We present [Fe/H] and [Ca/Fe] of $\sim600$ red giant branch (RGB) members of the globular cluster $ω$ Centauri. We collect medium-resolution ($R\sim2000$) spectra using the Blanco 4 m telescope at the Cerro Tololo Inter-American Observatory equipped with Hydra, the fiber-fed multi-object spectrograph. We demonstrate that blending of stellar light in optical fibers severely limits the accuracy of spectroscopic parameters in the crowded central region of the cluster. When photometric temperatures are taken in the spectroscopic analysis, our kinematically selected cluster members, excluding those that are strongly affected by flux from neighboring stars, include relatively fewer stars at intermediate metallicity ([Fe/H]$\sim-1.5$) than seen in the previous high-resolution survey for brighter giants in Johnson & Pilachowski. As opposed to the trend of increasing [Ca/Fe] with [Fe/H] found by those authors, our [Ca/Fe] estimates, based on Ca II H & K measurements, show essentially the same mean [Ca/Fe] for most of the metal-poor and metal-intermediate populations in this cluster, suggesting that mass- or metallicity-dependent SN II yields may not be necessary in their proposed chemical evolution scenario. Metal-rich cluster members in our sample show a large spread in [Ca/Fe], and do not exhibit a clear bimodal distribution in [Ca/Fe]. We also do not find convincing evidence for a radial metallicity gradient among RGB stars in $ω$ Centauri.

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arXiv2017

Abundant Methanol Ice toward a Massive Young Stellar Object in the Central Molecular Zone

Previous radio observations revealed widespread gas-phase methanol (CH$_3$OH) in the Central Molecular Zone (CMZ) at the Galactic center (GC), but its origin remains unclear. Here, we report the discovery of CH$_3$OH ice toward a star in the CMZ, based on a Subaru $3.4$-$4.0\ μ$m spectrum, aided by NASA/IRTF $L'$ imaging and $2$-$4\ μ$m spectra. The star lies $\sim8000$ au away in projection from a massive young stellar object (MYSO). Its observed high CH$_3$OH ice abundance ($17\%\pm3\%$ relative to H$_2$O ice) suggests that the $3.535\ μ$m CH$_3$OH ice absorption likely arises in the MYSO's extended envelope. However, it is also possible that CH$_3$OH ice forms with a higher abundance in dense clouds within the CMZ, compared to within the disk. Either way, our result implies that gas-phase CH$_3$OH in the CMZ can be largely produced by desorption from icy grains. The high solid CH$_3$OH abundance confirms the prominent $15.4\ μ$m shoulder absorption observed toward GC MYSOs arises from CO$_2$ ice mixed with CH$_3$OH.

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arXiv2016

Spectroscopic Survey of G and K Dwarfs in the Hipparcos Catalog. I. Comparison between the Hipparcos and Photometric Parallaxes

The tension between the Hipparcos parallax of the Pleiades and other independent distance estimates continues even after the new reduction of the Hipparcos astrometric data and the development of a new geometric distance measurement for the cluster. A short Pleiades distance from the Hipparcos parallax predicts a number of stars in the solar neighborhood that are sub-luminous at a given photospheric abundance. We test this hypothesis using spectroscopic abundances for a subset of stars in the Hipparcos catalog, which occupy the same region as the Pleiades in the color-magnitude diagram. We derive stellar parameters for 170 nearby G and K type field dwarfs in the Hipparcos catalog based on high-resolution spectra obtained using KPNO 4-m echelle spectrograph. Our analysis shows that, when the Hipparcos parallaxes are adopted, most of our sample stars follow empirical color-magnitude relations. A small fraction of stars are too faint compared to main-sequence fitting relations by $ΔM_V \geq 0.3$ mag, but the differences are marginal at a $2σ$ level partly due to relatively large parallax errors. On the other hand, we find that photometric distances of stars showing signatures of youth as determined from lithium absorption line strengths and $R'_{\rm HK}$ chromospheric activity indices are consistent with the Hipparcos parallaxes. Our result is contradictory to a suggestion that the Pleiades distance from main-sequence fitting is significantly altered by stellar activity and/or the young age of its stars, and provides an additional supporting evidence for the long distance scale of the Pleiades.

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