arXiv subjects

Shogo Otsubo

Publications and source records attributed to Shogo Otsubo.

At least 19 recordsLinked to original sources

arXiv2026

High-Precision Near-Infrared Abundances of Solar Analogs in the YJ Bands

We present a near-infrared abundance analysis of 46 solar analogs with known ages, observed with the WINERED WIDE-mode spectrograph at a resolution of $λ/Δλ= 28,000$. Using an empirically calibrated, line-by-line approach in the $YJ$ bands (0.976--1.089 and 1.182--1.319~{$μ$}m), we derive abundances for 16 elements. Despite the intrinsic weakness of near-infrared phosphorus diagnostics, the combination of five P\,{\sc i} lines yields a typical uncertainty half-width of $\sim$0.04~dex, providing an estimate of the internal precision over the solar-analog sample. For other elements, the internal precision ranges from $\sim$0.01~dex for Fe and Si to over 0.05--0.14~dex for elements with only a couple of lines available. The resulting per-object abundances for various elements are consistent with previous measurements using high-precision optical spectra with residuals of 0.03--0.2~dex depending on the element. The inferred age--[X/Fe] relations reproduce known trends for the thin disk, while extending them to elements difficult to access in the optical, including P and K. We find the slope of the age--[P/Fe] relation is steeper than that for $α$ elements, which provides an empirical constraint for future modeling of Galactic phosphorus evolution. In addition, we publish a high signal-to-noise (S/N 500--1000) reference spectrum constructed by combining solar-analog spectra, together with the spectra of individual stars, and an empirically calibrated line list with per-line zero-point corrections, for future near-infrared spectroscopic studies.

astro-ph.SR↗

arXiv2026

Kinematical and dynamical properties of recently discovered bulge and disc star clusters with WINERED

Context. Galactic globular clusters are a very important tool in explaining the characteristics of the Milky Way. Therefore it is essential to determine the kinematical and dynamical properties of the new star cluster candidates, especially at the low-latitude regions that suffer from heavy extinction and crowding. Aims. In this work, we report the first spectroscopic analysis for seven recently identified star cluster candidates: CWNU 4193, FSR 1700, Garro 02, Patchick 98, FSR 1767, Mercer 08, and BH 140. Our aim is to determine the kinematical properties, such as the mean cluster radial velocity, and the dynamical properties, such as the orbital parameters and the global dynamical mass, of these clusters in order to spectroscopically confirm the nature of these seven stellar systems. Methods. We collected the high-resolution infrared spectra of 33 candidate members of these clusters using the WINERED spectrograph mounted on the Magellan Clay 6.5 m telescope. Using the WINERED spectra, we measured the radial velocity of each individual star to confirm its membership in the clusters. From the confirmed members, we derived the mean cluster radial velocity of each cluster. In addition, for these clusters, we computed the orbital elements using the GravPot16 model and estimated their global dynamical masses based on the virial theorem. Results. As a result, we confirmed enough member stars (from three to seven stars per cluster) to reliably derive the mean cluster radial velocity and compute the orbital parameters of the clusters CWNU 4193, FSR 1700, Garro 02, and BH 140. For clusters CWNU 4193, FSR 1700, and BH 140, the number of confirmed members also allowed us to estimate their global dynamical masses. Therefore, we successfully derived key kinematical and dynamical properties for four of the most obscured star clusters in the Milky Way.

astro-ph.GA↗

arXiv2025

Assessing robustness and bias in 1D retrievals of 3D Global Circulation Models at high spectral resolution: a WASP-76 b simulation case study in emission

High-resolution spectroscopy (HRS) of exoplanet atmospheres has successfully detected many chemical species and is quickly moving toward detailed characterization of the chemical abundances and dynamics. HRS is highly sensitive to the line shape and position, thus, it can detect three-dimensional (3D) effects such as winds, rotation, and spatial variation of atmospheric conditions. At the same time, retrieval frameworks are increasingly deployed to constrain chemical abundances, pressure-temperature (P-T) structures, orbital parameters, and rotational broadening. To explore the multidimensional parameter space, they need computationally fast models that are consequently mostly one-dimensional (1D). However, this approach risks introducing interpretation bias since the planet's true nature is 3D. We investigate the robustness of this methodology at high spectral resolution by running 1D retrievals on simulated observations in emission within an observational framework using 3D Global Circulation Models of the quintessential HJ WASP-76 b. We find that the retrieval broadly recovers conditions present in the atmosphere, but that the retrieved P-T and chemical profiles are not a homogeneous average of all spatial and phase-dependent information. Instead, they are most sensitive to spatial regions with large thermal gradients, which do not necessarily coincide with the strongest emitting regions. Our results further suggest that the choice of parameterization for the P-T and chemical profiles, as well as Doppler offsets among opacity sources, impact retrieval results. These factors should be carefully considered in future retrieval analyses.

astro-ph.EP↗

arXiv2025

Discovery of WINERED-HVS1: A metal-rich hyper-velocity star candidate ejected from the Galactic center

We report the discovery of a metal-rich red giant star, WINERED-HVS1, which is a candidate for a hyper-velocity star (HVS). Its past trajectory suggests that this star may have been ejected by the Galactic supermassive black hole (SMBH; Sgr A*), with a modest ejection velocity of at least $500\; \mathrm{km\;s^{-1}}$. Since WINERED-HVS1 is gravitationally bound to the Milky Way and is not necessarily young, it is not an unambiguous HVS candidate from a kinematic perspective, unlike the previously confirmed A-type main-sequence HVS known as S5-HVS1. Therefore, the chemical characterization of this star is essential for understanding its origin. Through a spectroscopic follow-up observation with Magellan/WINERED, we determined its metallicity $\mathrm{[Fe/H]}=-0.147^{+0.046}_{-0.045}$, as well as the abundances for Na, Mg, Si, S, K, Ca, Ti, Cr, Ni, and Sr. The high metallicity value suggests that this star was ejected from the Galactic center and is unlikely to be a halo star with a radial orbit. The abundance pattern of this star -- including the pattern of [$α$/Fe], [Mg/Fe], [Si/Mg], and [Ca/Mg] -- is consistent with that of the nuclear star cluster surrounding the SMBH, further supporting our view. This discovery opens a new window to look through the Galactic center environment without the hindrance of dust extinction, by using the HVSs moving in the halo region where dust extinction is minimal.

astro-ph.GA↗

arXiv2025

First Result for Dark Matter Search by WINERED

The identity of dark matter has been a mystery in astronomy, cosmology, and particle theory for about a century. Bessho, Ikeda, and Yin (2022), three of the current authors, proposed using the state-of-the-art infrared spectrographs, including WINERED at $6.5$m Magellan Clay telescope and NIRSpec at James Webb Space Telescope, as efficient detectors for the indirect detection of dark matter with the mass around eV by measuring the line photons from the dark matter two body decays. Applying this concept, we have performed spectrographic observations of dwarf spheroidal galaxies (dSphs) Leo V and Tucana II using WINERED by utilizing an object-sky-object nodding observation technique for background subtraction. We present the first result from this dark matter search. Employing zero consistent flux data after the sky subtraction, we have established one of the most stringent limits to date on dark matter decaying into line photons in the mass range of $1.8-2.7\,$eV. Our data can also be applied to constrain other spectra of photons from the dSphs.

astro-ph.CO↗

arXiv2025

MAGIS (Measuring Abundances of red super Giants with Infrared Spectroscopy) project I. Establishment of an abundance analysis procedure for red supergiants and its evaluation with nearby stars

[Abbreviated] Context. Given their high luminosities (L>~10^4Lsun), red supergiants (RSGs) are good tracers of the chemical abundances of the young stellar population in the Milky Way and nearby galaxies. However, previous abundance analyses tailored to RSGs suffer some systematic uncertainties originating in, most notably, the synthesized molecular spectral lines for RSGs. Aims. We establish a new abundance analysis procedure for RSGs that circumvents difficulties faced in previous works, and test the procedure with ten nearby RSGs observed with the near-infrared high-resolution spectrograph WINERED (0.97--1.32 micron, R=28,000). Results. We determined the [X/Fe] of ten elements (Na I, Mg I, Al I, Si I, K I, Ca I, Ti I, Cr I, Ni I, and Y II). We estimated the relative precision in the derived abundances to be 0.04--0.12 dex for elements with more than two lines analyzed (e.g., Fe I and Mg I) and up to 0.18 dex for the other elements (e.g., Y II). We compared the resultant abundances of RSGs with the well-established abundances of another type of young star, namely the Cepheids, in order to evaluate the potential systematic bias in our abundance measurements, assuming that the young stars (i.e., both RSGs and Cepheids) in the solar neighborhood have common chemical abundances. We find that the determined RSG abundances are highly consistent with those of Cepheids within <~0.1 dex for some elements (notably [Fe/H] and [Mg/Fe]), which means the bias in the abundance determination for these elements is likely to be small. In contrast, the consistency is worse for some other elements (e.g., [Si/Fe] and [Y/Fe]). Nevertheless, the dispersion of the chemical abundances among our target RSGs is comparable with the individual statistical errors on the abundances. Hence, the procedure is likely to be useful to evaluate the relative difference in chemical abundances among RSGs.

astro-ph.GA↗

arXiv2024

[N I] 10400/10410 Å Lines as Possible Disk Wind Tracers in a Young Intermediate-Mass Star

In this study, we performed high-resolution near-infrared (NIR) spectroscopy (R = 28,000; $λ = 0.90$-1.35 $μ$m) with a high signal-to-noise ratio on HD 200775, a very young (${\sim}$0.1 Myr old) and massive intermediate-mass star (a binary star with a mass of about 10 $M_{\odot}$ each) with a protoplanetary disk. The obtained spectra show eight forbidden lines of three elements: two of [S II] (10289 and 10323 Å), two of [N I] (10400 and 10410 Å), and four of [Fe II] (12570, 12946, 12981, and 13209 Å). This is the first time that the [N I] lines are detected in a young stellar object with a doublet deblended. Gaussian fitting of the spectra indicates that all line profiles have low-velocity components and exhibit blueshifted features, suggesting that all lines originate from the disk winds (magnetohydrodynamic disk wind and/or photoevaporative wind). Based on the fit, the [N I] and [Fe II] lines are categorized into narrow components, while the [S II] lines are at the boundary between broad and narrow components. These forbidden lines are suggested to be very promising disk wind tracers among the existing ones because they are in the NIR-wavelength range, which can be observed from early stages with high sensitivities. Among these lines, [N I] lines would be a rather powerful probe for deriving the basic physical parameters of disk wind gases. However, the study of these lines herein is limited to one object; thus, further studies are needed to examine their properties.

astro-ph.EP↗

arXiv2024

A High-Resolution Non-Detection of Escaping Helium In The Ultra-Hot Neptune LTT 9779b: Evidence for Weakened Evaporation

The recent discovery of ``ultra-hot'' ($P < 1$ day) Neptunes has come as a surprise: some of these planets have managed to retain gaseous envelopes despite being close enough to their host stars to trigger strong photoevaporation and/or Roche lobe overflow. Here, we investigate atmospheric escape in LTT 9779b, an ultra-hot Neptune with a volatile-rich envelope. We observed two transits of this planet using the newly-commissioned WINERED spectrograph ($R\sim68,000$) on the 6.5 m Clay/Magellan II Telescope, aiming to detect an extended upper atmosphere in the He 10830 A triplet. We found no detectable planetary absorption: in a 0.75 A passband centered on the triplet, we set a 2$σ$ upper limit of 0.12% ($δR_p/H < 14$) and a 3$σ$ upper limit of 0.20% ($δR_p/H < 22$). Using a H/He isothermal Parker wind model, we found corresponding 95% and 99.7% upper limits on the planetary mass-loss rate of $\dot{M} < 10^{10.03}$ g s$^{-1}$ and $\dot{M} < 10^{11.11}$ g s$^{-1}$ respectively, smaller than predicted by outflow models even considering the weak stellar XUV emission. The low evaporation rate is plausibly explained by a metal-rich envelope, which would decrease the atmospheric scale height and increase the cooling rate of the outflow. This hypothesis is imminently testable: if metals commonly weaken planetary outflows, then we expect that \textit{JWST} will find high atmospheric metallicities for small planets that have evaded detection in He 10830 A.

astro-ph.EP↗

arXiv2024

WARP: The Data Reduction Pipeline for the WINERED spectrograph

We present a data reduction pipeline written in Python for data obtained with the near-infrared cross-dispersed echelle spectrograph, WINERED, which yields a 0.91$-$1.35 $μ$m spectrum with the resolving power of $R_{\text{max}} \equiv λ/ Δλ= 28,000$ or 70,000 depending on the observing mode. The pipeline was developed to efficiently extract the spectrum from the raw data with high quality. It comprises two modes: the calibration and the science mode. The calibration mode automatically produces the flat-fielding image, bad pixel map, echellogram distortion map and the dispersion solution from the set of the calibration data. Using calibration images and parameters, the science data of astronomical objects can be reduced automatically using the science mode. The science mode is also used for the real-time quick look at the data during observations. An example of the spectra reduced with WARP is presented. The effect of the highly inclined slit image on the spectral resolution is discussed.

astro-ph.IM↗

arXiv2023

Shock Excitation in Narrow Line Regions Powered by AGN Outflows

Outflows in the Active Galactic Nucleus (AGN) are considered to play a key role in the host galaxy evolution through transfer of a large amount of energy. A Narrow Line Region (NLR) in the AGN is composed of ionized gas extending from pc-scales to kpc-scales. It has been suggested that shocks are required for ionization of the NLR gas. If AGN outflows generate such shocks, they will sweep through the NLR and the outflow energy will be transferred into a galaxy-scale region. In order to study contribution of the AGN outflow to the NLR-scale shock, we measure the [\ion{Fe}{2}]$\lambda12570$/[\ion{P}{2}]$\lambda11886$ line ratio, which is a good tracer of shocks, using near-infrared spectroscopic observations with WINERED (Warm INfrared Echelle spectrograph to Realize Extreme Dispersion and sensitivity) mounted on the New Technology Telescope. Among 13 Seyfert galaxies we observed, the [\ion{Fe}{2}] and [\ion{P}{2}] lines were detected in 12 and 6 targets, respectively. The [\ion{Fe}{2}]/[\ion{P}{2}] ratios in 4 targets were found to be higher than 10, which implies the existence of shocks. We also found that the shock is likely to exist where an ionized outflow, i.e., a blue wing in [\ion{S}{3}]$\lambda9533$, is present. Our result implies that the ionized outflow present over a NLR-scale region sweeps through the interstellar medium and generates a shock.

astro-ph.GA↗

arXiv2022

Survey of near-infrared diffuse interstellar bands in $Y$ and $J$ bands. I. Newly identified bands

We searched for diffuse interstellar bands (DIBs) in the 0.91$<λ<$1.33 $μ$m region by analyzing the near-infrared (NIR) high-resolution ($R=20,000$ and 28,000) spectra of 31 reddened early-type stars ($0.04<E(B-V)<4.58$) and an unreddened reference star. The spectra were collected using the WINERED spectrograph, which was mounted on the 1.3 m Araki telescope at Koyama Astronomical Observatory, Japan, in 2012--2016, and on the 3.58 m New Technology Telescope at La Silla Observatory, Chile, in 2017--2018. We detected 54 DIBs -- 25 of which are newly detected by this study -- eight DIB candidates. Using this updated list, the DIB distributions over a wide wavelength range from optical to NIR are investigated. The FWHM values of the NIR DIBs are found to be narrower than those of the optical DIBs on average, which suggests that the DIBs at longer wavelengths tend to be caused by larger molecules. Assuming that the larger carriers are responsible for the DIBs at longer wavelengths and have the larger oscillator strengths, we found that the total column densities of the DIB carriers tend to decrease with increasing DIB wavelength. The candidate molecules and ions for the NIR DIBs are also discussed.

astro-ph.GA↗

arXiv2022

Effective temperatures of red supergiants estimated from line-depth ratios of iron lines in the YJ bands, 0.97--1.32 micron

Determining the effective temperatures (Teff) of red supergiants (RSGs) observationally is important in many fields of stellar physics and galactic astronomy, yet some significant difficulties remain due to model uncertainty originating majorly in the extended atmosphere of RSGs. Here we propose the line-depth ratio (LDR) method in which we use only Fe I lines. As opposed to the conventional LDR method with lines of multiple species involved, the LDR of this kind is insensitive to the surface gravity effects and expected to circumvent the uncertainty originating in the upper atmosphere of RSGs. Therefore, the LDR--Teff relations that we calibrated empirically with red giants may be directly applied to RSGs, though various differences, e.g., caused by the three-dimensional non-LTE effects, between the two groups of objects need to be kept in mind. Using the near-infrared YJ-band spectra of nine well-known solar-metal red giants observed with the WINERED high-resolution spectrograph, we selected 12 pairs of Fe I lines least contaminated with other lines. Applying their LDR--Teff relations to ten nearby RSGs, the resultant Teff with the internal precision of 30--70 K shows good agreement with previous observational results assuming one-dimensional LTE and with Geneva's stellar evolution model. We found no evidence of significant systematic bias caused by various differences, including those in the size of the non-LTE effects, between red giants and RSGs except for one line pair which we rejected because the non-LTE effects may be as large as ~250 K. Nevertheless, it is difficult to evaluate the systematic bias, and further study is required, e.g., with including the three-dimensional non-LTE calculations of all the lines involved.

astro-ph.SR↗

arXiv2022

A Very Metal-poor RR Lyrae Star with a Disk Orbit Found in the Solar Neighborhood

Metal-deficient stars are important tracers for understanding the early formation of the Galaxy. Recent large-scale surveys with both photometric and spectroscopic data have reported an increasing number of metal-deficient stars whose kinematic features are consistent with those of the disk stellar populations. We report the discovery of an RR~Lyrae variable (hereafter RRL) that is located within the thick disk and has an orbit consistent with the thick-disk kinematics. Our target RRL (HD 331986) is located at around 1 kpc from the Sun and, with V=11.3, is among the 130 brightest RRLs known so far. However, this object was scarcely studied because it is in the midplane of the Galaxy, the Galactic latitude around -1 deg. Its near-infrared spectrum (0.91-1.32 micron) shows no absorption line except hydrogen lines of the Paschen series, suggesting [Fe/H] less than -2.5. It is the most metal-deficient RRL, at least, among the RRLs whose orbits are consistent with the disk kinematics, although we cannot determine to which of the disk and the halo it belongs. This unique RRL would provide us with essential clues for studying the early formation of stars in the inner Galaxy with further investigations, including high-resolution optical spectroscopy.

astro-ph.SR↗

arXiv2020

Mg II and Fe II fluxes of luminous quasars at z ~ 2.7 and evaluation of the Baldwin effect in the flux-to-abundance conversion method for quasars

To investigate the chemical abundance of broad-line region clouds in quasars at high redshifts, we performed near-infrared spectroscopy of six luminous quasars at z ~ 2.7 with the WINERED spectrograph mounted on the New Technology Telescope (NTT) at the La Silla Observatory, Chile. The measured Fe II/Mg II flux ratios nearly matched with the published data for 0.7 < z < 1.6, suggesting that there is no evolution over a long period of cosmic time, which is consistent with previous studies. To derive the chemical abundances from the measured equivalent widths (EWs), their dependence on nonabundance factors must be corrected. In our previous paper, we proposed a method to derive the [Mg/Fe] abundance ratio and the [Fe/H] abundance by correcting the dependence of EW(Mg II) and EW(Fe II) on the Eddington ratio. To the best of our knowledge, that was the first report to discuss the star-formation history through a direct comparison with chemical evolution models. In the present study, we further investigated the dependence of EWs on luminosity, which is known as the Baldwin effect (BEff). Additional correction for the BEff significantly affects the derived chemical abundances for the six luminous quasars at z ~ 2.7, and the resultant abundances agree well with the prediction of chemical evolution models. Given that most distant quasars found thus far are biased toward luminous ones, the correction of the measured EWs for the BEff is crucial for extending the chemical evolution study to higher redshifts.

astro-ph.GA↗

arXiv2020

The effect of surface gravity on line-depth ratios in the wavelength range 0.97-1.32 μm

A line-depth ratio (LDR) of two spectral lines with different excitation potentials is expected to be correlated with the effective temperature ($T_\mathrm{eff}$). It is possible to determine $T_\mathrm{eff}$ of a star with a precision of tens of Kelvin if dozens or hundreds of tight LDR-$T_\mathrm{eff}$ relations can be used. Most of the previous studies on the LDR method were limited to optical wavelengths, but Taniguchi and collaborators reported 81 LDR relations in the $YJ$ band, 0.97-1.32 $μ\mathrm{m}$, in 2018. However, with their sample of only 10 giants, it was impossible to account for the effects of surface gravity and metallicity on the LDRs well. Here we investigate the gravity effect based on $YJ$-band spectra of 63 stars including dwarfs, giants, and supergiants observed with the WINERED spectrograph. We found that some LDR-$T_\mathrm{eff}$ relations show clear offsets between the sequence of dwarfs and those of giants/supergiants. The difference between the ionization potentials of the elements considered in each line pair and the corresponding difference in the depths can, at least partly, explain the dependency of the LDR on the surface gravity. In order to expand the stellar parameter ranges that the LDR method can cover with high precision, we obtained new sets of LDR-$T_\mathrm{eff}$ relations for solar-metal G0-K4 dwarfs and F7-K5 supergiants, respectively. The typical precision that can be achieved with our relations is 10-30 K for both dwarfs and supergiants.

astro-ph.SR↗

arXiv2019

Identification of Absorption Lines of Heavy Metals in the Wavelength Range 0.97--1.32 Micron

Stellar absorption lines of heavy elements can give us various insights into the chemical evolution of the Galaxy and nearby galaxies. Recently developed spectrographs for the near-infrared wavelengths are becoming more and more powerful for producing a large number of high-quality spectra, but identification and characterization of the absorption lines in the infrared range remain to be fulfilled. We searched for lines of the elements heavier than the iron group, i.e., those heavier than Ni, in the Y (9760--11100 AA) and J (11600--13200 AA) bands. We considered the lines in three catalogs, i.e., Vienna Atomic Line Database (VALD), the compilation by R. Kurucz, and the list published in 1999 by Melendez and Barbuy. Candidate lines were selected based on synthetic spectra and the confirmation was done by using WINERED spectra of 13 supergiants and giants within FGK spectral types (spanning 4000--7200 K in the effective temperature). We have detected lines of Zn I, Sr II, Y II, Zr I, Ba II, Sm II, Eu II, and Dy II, in the order of atomic number. Although the number of the lines is small, 23 in total, they are potentially useful diagnostic lines of the Galactic chemical evolution, especially in the regions for which interstellar extinction hampers detailed chemical analyses with spectra in shorter wavelengths. We also report the detection of lines whose presence was not predicted by the synthetic spectra created with the above three line lists.

astro-ph.SR↗

arXiv2019

Possible progression of mass flow processes around young intermediate-mass stars based on high-resolution near-infrared spectroscopy. I. Taurus

We used the WINERED spectrograph to perform near-infrared high-resolution spectroscopy (resolving power $R = 28$,000) of 13 young intermediate-mass stars in the Taurus star-forming region. Based on the presence of near- and mid-infrared continuum emission, young intermediate-mass stars can be classified into three different evolutionary stages: Phases I, II, and III in the order of evolution. Our obtained spectra ($λ= 0.91$--1.35\,$μ$m) depict \HeI $λ$10830 and P$β$ lines that are sensitive to magnetospheric accretion and winds. We also investigate five sources each for P$β$ and \ion{He}{1} lines that were obtained from previous studies along with our targets. We observe that the P$β$ profile morphologies in Phases I and II corresponded to an extensive variety of emission features; however, these features are not detected in Phase III. We also observe that the He I profile morphologies are mostly broad subcontinuum absorption lines in Phase I, narrow subcontinuum absorption lines in Phase II, and centered subcontinuum absorption features in Phase III. Our results indicate that the profile morphologies exhibit a progression of the dominant mass flow processes: stellar wind and probably magnetospheric accretion in the very early stage, magnetospheric accretion and disk wind in the subsequent stage, and no activities in the final stage. These interpretations further suggest that opacity in protoplanetary disks plays an important role in mass flow processes. Results also indicate that \ion{He}{1} absorption features in Phase III sources, associated with chromospheric activities even in such young phases, are characteristics of intermediate-mass stars.

astro-ph.SR↗

arXiv2019

First detection of $A$--$X$ (0,0) bands of interstellar C$_2$ and CN

We report the first detection of C$_2$ $A^1Π_u$--$X^1Σ_g^+$ (0,0) and CN $A^2Π_u$--$X^2Σ^+$ (0,0) absorption bands in the interstellar medium. The detection was made using the near-infrared (0.91--1.35 $μ$m) high-resolution ($R=20,000$ and 68,000) spectra of Cygnus OB2 No.\,12 collected with the WINERED spectrograph mounted on the 1.3 m Araki telescope. The $A$--$X$ (1,0) bands of C$_2$ and CN were detected simultaneously. These near-infrared bands have larger oscillator strengths, compared with the $A$--$X$ (2,0) bands of C$_2$ and CN in the optical. In the spectrum of the C$_2$ (0,0) band with $R=68,000$, three velocity components in the line of sight could be resolved and the lines were detected up to high rotational levels ($J''\sim20$). By analyzing the rotational distribution of C$_2$, we could estimate the kinetic temperature and gas density of the clouds with high accuracy. Furthermore, we marginally detected weak lines of $^{12}$C$^{13}$C for the first time in the interstellar medium. Assuming that the rotational distribution and the oscillator strengths of the relevant transitions of $^{12}$C$_2$ and $^{12}$C$^{13}$C are the same, the carbon isotope ratio was estimated to be $^{12}\text{C}/^{13}\text{C}=50$--100, which is consistent with the ratio in the local interstellar medium. We also calculated the oscillator strength ratio of the C$_2$ (0,0) and (1,0) bands from the observed band strengths. Unfortunately, our result could not discern theoretical and experimental results because of the uncertainties. High-resolution data to resolve the velocity components will be necessary for both bands in order to put stronger constraints on the oscillator strength ratios.

astro-ph.GA↗