arXiv subjects

Neda Hejazi

Publications and source records attributed to Neda Hejazi.

13 recordsLinked to original sources

arXiv2025

Abundance Measurements of the Metal-poor M subdwarf LHS 174 Using High-resolution Optical Spectroscopy

Metal-poor M subdwarfs are among the oldest stellar populations and carry valuable information about the chemical enrichment history of the Milky Way. The measurements of chemical abundances of these stars therefore provide essential insights into the nucleosynthesis in the early stages of the Galaxy's formation. We present the detailed spectroscopic analysis of a nearby metal-poor M subdwarf, LHS 174 from its high-resolution optical spectrum, and apply our previously developed spectral fitting code, \texttt{AutoSpecFit}, to measure the abundances of five elements:[O/H]=$-$0.519$\pm$0.081, [Ca/H]=$-$0.753$\pm$0.177, [Ti/H]=$-$0.711$\pm$0.144, [V/H]=$-$1.026$\pm$0.077, and [Fe/H]=$-$1.170$\pm$0.135. We compare the abundances of O, Ti, and Fe derived from this work and those from previous studies and demonstrate the observed data is clearly better matched with the synthetic model generated based on our abundances than those from the other analyses. The accuracy of inferred stellar abundances strongly depends on the accuracy of physical parameters, which motivates us to develop a reliable technique to determine the parameters of low-mass M dwarfs more accurately ever than before and infer abundances with smaller uncertainties.

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arXiv2024

Chemical Links between a Young M-type T Tauri Star and its Substellar Companion: Spectral Analysis and C/O Measurement of DH Tau A

The chemical abundance measurements of host stars and their substellar companions provide a powerful tool to trace the formation mechanism of the planetary systems. We present a detailed high-resolution spectroscopic analysis of a young M-type star, DH Tau A, which is located in the Taurus molecular cloud belonging to the Taurus-Auriga star-forming region. This star is host to a low-mass companion, DH Tau b, and both star and the companion are still in their accreting phase. We apply our technique (Hejazi et al. 2024) to measure the abundances of carbon and oxygen using carbon- and oxygen-bearing molecules, such as CO and OH, respectively. We determine a near-solar carbon-to-oxygen abundance ratio of C/O=0.555$\pm$0.063 for the host star DH Tau A. We compare this stellar abundance ratio with that of the companion from our previous study (C/O=0.54$^{+0.06}_{-0.05}$, Xuan et al. 2024), which also has a near-solar value. This confirms the chemical homogeneity in the DH Tau system, which suggests a formation scenario for the companion consistent with a direct and relatively fast gravitational collapse, rather than a slow core accretion process.

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arXiv2024

The Wanderer: Charting WASP-77A b's Formation and Migration Using a System-Wide Inventory of Carbon and Oxygen Abundances

The elemental and isotopic abundances of volatiles like carbon, oxygen, and nitrogen may trace a planet's formation location relative to H$_2$O, CO$_2$, CO, NH$_3$, and N$_2$ "snowlines", or the distance from the star at which these volatile elements sublimate. By comparing the C/O and $^{12}$C/$^{13}$C ratios measured in giant exoplanet atmospheres to complementary measurements of their host stars, we can determine whether the planet inherited stellar abundances from formation inside the volatile snowlines, or non-stellar C/O and $^{13}$C enrichment characteristic of formation beyond the snowlines. To date, there are still only a handful of exoplanet systems where we can make a direct comparison of elemental and isotopic CNO abundances between an exoplanet and its host star. Here, we present a $^{12}$C/$^{13}$C abundance analysis for host star WASP-77A (whose hot Jupiter's $^{12}$C/$^{13}$C abundance was recently measured). We use MARCS stellar atmosphere models and the radiative transfer code TurboSpectrum to generate synthetic stellar spectra for isotopic abundance calculations. We find a $^{12}$C/$^{13}$C ratio of $51\pm 6$ for WASP-77A, which is sub-solar ($\sim 91$) but may still indicate $^{13}$C-enrichment in its companion planet WASP-77A b ($^{12}$C/$^{13}$C = 26 $\pm$ 16, previously reported). Together with the inventory of carbon and oxygen abundances in both the host and companion planet, these chemical constraints point to WASP-77A b's formation beyond the H$_2$O and CO$_2$ snowlines and provide chemical evidence for the planet's migration to its current location $\sim$0.024 AU from its host star.

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arXiv2024

High-resolution Elemental Abundance Measurements of Cool JWST Planet Hosts Using AutoSpecFit: An Application to the Sub-Neptune K2-18b's Host M dwarf

We present an in-depth, high-resolution spectroscopic analysis of the M dwarf K2-18 that hosts a sub-Neptune exoplanet in its habitable zone. We show our technique to accurately normalize the observed spectrum, which is crucial for a proper spectral fitting. We also introduce a new automatic, line-by-line model-fitting code, AutoSpecFit, that performs an iterative $χ^{2}$ minimization process to measure individual elemental abundances of cool dwarfs. We apply this code to the star K2-18, and measure the abundance of 10 elements - C, O, Na, Mg, Al, K, Ca, Sc, Ti, and Fe. We find these abundances moderately supersolar, except for Fe with a slightly subsolar abundance. The accuracy of the inferred abundances is limited by the systematic errors due to uncertain stellar parameters. We also derive the abundance ratios associated with several planet-building elements such as Al/Mg, Ca/Mg, Fe/Mg, and (a solar-like) C/O=0.568 $\pm$ 0.026, which can be used to constrain the chemical composition and the formation location of the exoplanet. On the other hand, the planet K2-18 b has attracted considerable interest, given the JWST measurements of its atmospheric composition. Early JWST studies reveal an unusual chemistry for the atmosphere of this planet, which is unlikely to be driven by formation in a disk of unusual composition. The comparison between the chemical abundances of K2-18 b from future JWST analyses and those of the host star can provide fundamental insights into the formation of this planetary system.

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arXiv2024

Stellar Characterization and Chemical Abundances of Exoplanet Hosting M dwarfs from APOGEE Spectra: Future JWST Targets

Exoplanets hosting M dwarfs are the best targets to characterize Earth-like or super-Earth planetary atmospheres with the James Webb Space Telescope (JWST). We determine detailed stellar parameters ($T_{\rm eff}$, log$g$, and $ξ$) and individual abundances of twelve elements for four cool M dwarfs hosting exoplanets TOI-1685, GJ 436, GJ 3470, and TOI-2445, scheduled for future observations by the JWST. The analysis utilizes high-resolution near-infrared spectra from the SDSS-IV APOGEE survey between 1.51-1.69$μ$. Based on 1D-LTE plane-parallel models, we find that TOI-2445 is slightly metal-poor ([Fe/H] = -0.16$\pm$0.09), while TOI-1685, GJ 436 and GJ 3470 are more metal-rich ([Fe/H] = 0.06$\pm$0.18, 0.10$\pm$0.20 dex, 0.25$\pm$0.15). The derived C/O ratios for TOI-2445, TOI-1685, GJ 436, and GJ 3470 are 0.526$\pm$0.027, 0.558$\pm$0.097, 0.561$\pm$0.029, and 0.638$\pm$0.015, respectively. From results for 28 M dwarfs analyzed homogeneously from APOGEE spectra, we find exoplanet-hosting M dwarfs exhibit a C/O abundance ratio approximately 0.01 to 0.05 higher than those with non-detected exoplanets, at limits of a statistically significant offset. A linear regression of [Fe/H] \textit{vs.} C/O distribution reveals a noticeable difference in the angular coefficient between FGK dwarfs (0.27) and M dwarfs (0.13). Assuming our abundance ratios of Ca/Mg, Si/Mg, Al/Mg, and Fe/Mg, we determine a mass of 3.276$^{+0.448}_{-0.419}$$M_{\oplus}$ for TOI-2445 b, having density (6.793$^{+0.005}_{-0.099}$ g.cm$^{-3}$) and core mass fraction (0.329$_{-0.049}^{+0.028}$) very similar to Earth's. We also present an atlas of 113 well-defined spectral lines to analyze M dwarfs in the $H$-band and a comprehensive evaluation of uncertainties from variations in the atmospheric parameters, signal-to-noise, and pseudo-continuum.

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arXiv2024

Are these planets or brown dwarfs? Broadly solar compositions from high-resolution atmospheric retrievals of ~10-30 $M_\textrm{Jup}$ companions

Using Keck Planet Imager and Characterizer (KPIC) high-resolution ($R$~35000) spectroscopy from 2.29-2.49 $μ$m, we present uniform atmospheric retrievals for eight young substellar companions with masses of ~10-30 $M_\textrm{Jup}$, orbital separations spanning ~50-360 au, and $T_\textrm{eff}$ between ~1500-2600 K. We find that all companions have solar C/O ratios, and metallicities, to within the 1-2$σ$ level, with the measurements clustered around solar composition. Stars in the same stellar associations as our systems have near-solar abundances, so these results indicate that this population of companions is consistent with formation via direct gravitational collapse. Alternatively, core accretion outside the CO snowline would be compatible with our measurements, though the high mass ratios of most systems would require rapid core assembly and gas accretion in massive disks. On a population level, our findings can be contrasted with abundance measurements for directly imaged planets with m<10 $M_\textrm{Jup}$, which show tentative atmospheric metal enrichment. In addition, the atmospheric compositions of our sample of companions are distinct from those of hot Jupiters, which most likely form via core accretion. For two companions with $T_\textrm{eff}$~1700-2000 K (kap And b and GSC 6214-210 b), our best-fit models prefer a non-gray cloud model with >3$σ$ significance. The cloudy models yield 2-3$σ$ lower $T_\textrm{eff}$ for these companions, though the C/O and [C/H] still agree between cloudy and clear models at the $1σ$ level. Finally, we constrain 12CO/13CO for three companions with the highest S/N data (GQ Lup b, HIP 79098 b, and DH Tau b), and report $v$sin($i$) and radial velocities for all companions.

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arXiv2023

Elemental Abundances of the Super-Neptune WASP-107b's Host Star Using High-resolution, Near-infrared Spectroscopy

We present the first elemental abundance measurements of the K dwarf (K7V) exoplanet-host star WASP-107 using high-resolution (R = 45,000), near-infrared (H- and K-band) spectra taken from Gemini-S/IGRINS. We use the previously determined physical parameters of the star from the literature and infer the abundances of 15 elements: C, N, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, and Ni, all with precision < 0.1 dex, based on model fitting using MARCS model atmospheres and the spectral synthesis code Turbospectrum. Our results show near-solar abundances and a carbon-to-oxygen ratio (C/O) of 0.50 (+/-0.10), consistent with the solar value of 0.54 (+/-0.09). The orbiting planet, WASP-107b, is a super Neptune with a mass in the Neptune regime (= 1.8 M_Nep) and a radius close to Jupiter's (= 0.94 R_Jup). This planet is also being targeted by four JWST Cycle 1 programs in transit and eclipse, which should provide highly precise measurements of atmospheric abundances. This will enable us to properly compare the planetary and stellar chemical abundances, which is essential in understanding the formation mechanisms, internal structure, and chemical composition of exoplanets. Our study is a proof-of-concept that will pave the way for such measurements to be made for all JWST's cooler exoplanet-host stars.

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arXiv2022

M Subdwarf Research III. Spectroscopic Diagnostics for Breaking Parameter Degeneracy

To understand the parameter degeneracy of M subdwarf spectra at low resolution, we assemble a large number of spectral features in the wavelength range of 0.6-2.5 μm with band strength quantified by narrowband indices. Based on the index trends of BT-Settl model sequences, we illustrate how the main atmospheric parameters (Teff, log g, [M/H], and [alpha/Fe]) affect each spectral feature differently. Furthermore, we propose a four-step process to determine the four parameters sequentially, which extends the basic idea proposed by Jao et al. Each step contains several spectral features that break the degeneracy effect when determining a specific stellar parameter. Finally, the feasibility of each spectroscopic diagnostic with different spectral qualities is investigated. The result is resolution-independent down to R~200.

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arXiv2022

Chemical Properties of the Local Disk and Halo. II. Abundances of 3745 M dwarfs and Subdwarfs from Improved Model Fitting of Low-Resolution Spectra

We present a model-fit pipeline to determine the stellar parameters of M-type dwarfs, which is an improvement upon our previous work described in Hejazi et al. 2020. We apply this pipeline to analyze the low-resolution (R~2000) spectra of 3745 M dwarfs/subdwarfs, collected at the MDM Observatory, Lick Observatory, Kitt Peak National Observatory, and Cerro Tololo Interamerican Observatory. We examine the variation of the inferred parameter values in the HR diagram constructed from their Gaia Early Data Release 3 (EDR3) parallaxes and optical magnitudes. We also study the distribution of our stars in the abundance diagram of [alpha/Fe] versus [M/H] and inspect the variation of their metallicity class, effective temperature, and surface gravity as well as their Galactic velocity components U, V, and W in this diagram. In addition, the analyses of the stars' projected motions in the two-dimensional UV, VW, and UW planes and the variation of their chemical parameters in these planes, and also their distribution in the abundance-velocity diagrams are important parts of this study. The precision of our model-fit pipeline is confirmed by the clear stratification of effective temperature and chemical parameters in the HR diagram, the similarity of the stars' distribution in the [alpha/Fe] versus [M/H] diagram and in the metallicity-velocity planes with those from other studies, revealing substructure in the abundance-velocity diagrams, and chemical homogeneity between the components of a set of binary systems.

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arXiv2021

linemake: An Atomic and Molecular Line List Generator

In this research note, we present linemake, an open-source atomic and molecular line list generator. Rather than a replacement for a number of well-established atomic and molecular spectral databases, linemake aims to be a lightweight, easy-to-use tool to generate formatted and curated lists suitable for spectral synthesis work. We encourage users of linemake to understand the sources of their transition data and cite them as appropriate in published work. We provide the code, line database, and an extensive list of literature references in a GitHub repository (https://github.com/vmplacco/linemake), which will be updated regularly as new data become available.

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arXiv2019

Chemical Properties of the Local Galactic Disk and Halo. I. Fundamental Properties of 1,544 Nearby, High Proper-Motion M dwarfs and subdwarfs

Large numbers of low-to-medium resolution spectra of M-type dwarf stars from both the local Galactic disk and halo are available from various surveys. In order to fully exploit these data, we develop a template-fit method using a set of empirically assembled M dwarf/subdwarf classification templates, based on the measurements of the TiO and CaH molecular bands, to classify M dwarfs/subdwarfs by spectral type and metallicity class. We further present a pipeline to automatically determine the effective temperature, metallicity ([M/H]), alpha-element to iron abundance ratio ([alpha/Fe]), and surface gravity of M dwarfs/subdwarfs using the latest version of BT-Settl model atmospheres. We apply these methods to a set of low-to-medium resolution optical spectra of 1,544 high proper-motion (> 0.4"/yr) M dwarfs/subdwarfs. Our metallicity estimates appear to be consistent with the expected color-magnitude variation of stars relative to atmospheric composition, as our sample shows a clear stratification with respect to metallicity in the Gaia H-R diagram. Furthermore, the measured chemical parameters of the two components in 48 binary systems are in a good agreement with each other, which suggest a precision of 0.22 dex in [M/H], 0.08 dex in [alpha/Fe], and 0.16 dex in the combined index [alpha/Fe]+[M/H]. There is also a good consistency between metallicity class, obtained from the empirical classification templates, and the index [alpha/Fe]+[M/H] (~ [alpha/H]), obtained from model-fitting, which means that the more easily measured metallicity class can be used as a relatively reliable indicator of absolute alpha-element abundance, [alpha/H], in low-mass stars. Finally, we examine the distribution of our stars in the [alpha/Fe] vs. [M/H] diagram, which shows evidence of clustering in chemical abundance makeup, suggestive of discrete populations among the local disk and halo stars.

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arXiv2015

Optical - Near Infrared Photometric Calibration of M-dwarf Metallicity and Its Application

Based on a carefully constructed sample of dwarf stars, a new optical-near infrared photometric calibration to estimate the metallicity of late-type K and early-to-mid-type M dwarfs is presented. The calibration sample has two parts; the first part includes 18 M dwarfs with metallicities determined by high-resolution spectroscopy and the second part contains 49 dwarfs with metallicities obtained through moderate-resolution spectra. By applying this calibration to a large sample of around 1.3 million M dwarfs from the Sloan Digital Sky Survey and the Two-Micron All Sky Survey, the metallicity distribution of this sample is determined and compared with those of previous studies. Using photometric parallaxes, the Galactic heights of M dwarfs in the large sample are also estimated. Our results show that stars farther from the Galactic plane, on average, have lower metallicity, which can be attributed to the age-metallicity relation. A scarcity of metal-poor dwarf stars in the metallicity distribution relative to the Simple Closed Box Model indicates the existence of the "M dwarf problem," similar to the previously known G and K dwarf problems. Several more complicated Galactic chemical evolution models which have been proposed to resolve the G and K dwarf problems are tested and it is shown that these models could, to some extent, mitigate the M dwarf problem as well.

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arXiv2014

Touchstone Stars: Highlights from the Cool Stars 18 Splinter Session

We present a summary of the splinter session on "touchstone stars" -- stars with directly measured parameters -- that was organized as part of the Cool Stars 18 conference. We discuss several methods to precisely determine cool star properties such as masses and radii from eclipsing binaries, and radii and effective temperatures from interferometry. We highlight recent results in identifying and measuring parameters for touchstone stars, and ongoing efforts to use touchstone stars to determine parameters for other stars. We conclude by comparing the results of touchstone stars with cool star models, noting some unusual patterns in the differences.

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