Issue |
A&A
Volume 690, October 2024
|
|
---|---|---|
Article Number | A226 | |
Number of page(s) | 9 | |
Section | Atomic, molecular, and nuclear data | |
DOI | https://doi.org/10.1051/0004-6361/202451255 | |
Published online | 10 October 2024 |
M giants with IGRINS
IV. Identification and characterisation of a NIR line of the s-element barium
1
Division of Astrophysics, Department of Physics, Lund University,
Box 118,
221 00
Lund,
Sweden
2
Materials Science and Applied Mathematics, Malmö University,
205 06
Malmö,
Sweden
3
Department of Astronomy and McDonald Observatory, The University of Texas,
Austin,
TX
78712,
USA
★ Corresponding author; govind.nandakumar@fysik.lu.se
Received:
25
June
2024
Accepted:
22
August
2024
Context. Neutron-capture elements represent an important nucleosynthetic channel in the study of the Galactic chemical evolution of stellar populations. For stellar populations behind significant extinction, such as those in the Galactic centre and along the Galactic plane, abundance analyses based on near-infrared (NIR) spectra are necessary. Previously, spectral lines from the neutron-capture elements, such as copper (Cu), cerium (Ce), neodymium (Nd), and ytterbium (Yb), have been identified in the H band, while yttrium (Y) lines have been identified in the K band.
Aims. Due to the scarcity of spectral lines from neutron-capture elements in the NIR, the addition of useful spectral lines from other neutron-capture elements is highly desirable. The aim of this work is to identify and characterise a spectral line suitable for abundance determination from the most commonly used s-process element, namely barium.
Methods. We observed the NIR spectra of 37 M giants in the solar neighbourhood at high spectral resolution and with a high signal-to-noise ratio using the IGRINS spectrometer on the GEMINI South telescope. The full H- and K-bands were recorded simultaneously at R = 45 000. Using a manual spectral synthesis method, we determined the fundamental stellar parameters for these stars and derived the barium abundance from the Ba line (6s5d 3D2 → 6s6p 3P2o) at λair = 23 253.56 Å in the K band.
Results. We demonstrate that the Ba line in the K band at 2.33 μm (λ23 253.56) is useful for abundance analyses from the spectra of M giants. The line becomes progressively weaker at higher temperatures and is only useful in M giants and the coolest K giants at supersolar metallicities.
Conclusions. We can now add Ba to the trends of the heavy elements Cu, Zn, Y, Ce, Nd, and Yb, which can be retrieved from high-resolution H- and K-band spectra. This opens up the study of nucleosynthetic channels, including the s-process and the r-process, in dust-obscured populations. Thus, these elements can be studied for heavily dust-obscured regions of the Galaxy, such as the Galactic centre.
Key words: techniques: spectroscopic / stars: abundances / stars: late-type / Galaxy: abundances / solar neighborhood
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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1 Introduction
This paper is part of a series based on observations from the Immersion Grating Infrared Spectrometer (IGRINS) to study M giants. M giants are luminous, cool giant stars that have been underexploited in spectroscopic studies of stellar populations, primarily because the analysis of K giants is more straight-forward. However, M giants are the only practical targets for investigating stellar populations in regions of the Milky Way obscured by dust, such as the inner Galaxy. In such studies, near-infrared (NIR) observations are essential because of the significant reddening and extinction. Earlier abundance studies have shown the strength of high-resolution NIR spectra recorded with IGRINS in general (see Afşar et al. 2018; Böcek Topcu et al. 2019, 2020; Montelius et al. 2022; Afşar et al. 2023; Nandakumar et al. 2022; Nandakumar et al. 2024b).
The ubiquitous presence of titanium oxide (TiO) spectral lines in the optical spectra of M giants prevents traditional spectral analyses conducted at optical wavelengths. However, the NIR region of M giant spectra proves to be valuable for determining stellar parameters and abundances through spectroscopy (see e.g. Nandakumar et al. 2023a).
Nandakumar et al. (2023a, Paper I) presented an iterative method for determining the stellar parameters of M giants with 3400 < Teff < 4000 K from high-resolution H-band spectra and isochrones. This method used temperature-sensitive OH lines in combination with CO and Fe lines. A total of 50 M-giants observed with IGRINS were analysed and benchmarked. Nandakumar et al. (2023b, Paper II) demonstrated the potential and accuracy of this method in the detailed investigation of ten hydrogen fluoride (HF) molecular lines for determining the Galactic chemical evolution of fluorine. Similarly, Nandakumar et al. (2024a, Paper III) derived the abundance trends of 21 elements versus metallicity for these 50 solar-neighbourhood stars, including the α-elements, as well as odd-Z elements, iron-peak elements, and neutron-capture elements. Thse latter authors identified the optimal spectral lines to use and the precision achievable for all 21 elements. This range of elements is crucial for obtaining a comprehensive chemical view of the Milky Way and its components.
In particular, the neutron-capture elements introduce important nucleosynthetic channels with their own evolutionary timescales (see e.g. Manea et al. 2023). Low-mass asymptotic giant branch (AGB) stars are responsible for most of the main s-process, producing elements such as barium (Ba), yttrium (Y), cerium (Ce), and neodymium (Nd). Because of the lower masses of these stars (1.3–3 M⊙; Grisoni et al. 2020) compared to the progenitors of the supernovae type II (SNII)-dominated elements, the s-process elements are formed on a longer timescale. In Paper III the elemental trends versus metallicity were derived for the neutron-capture elements copper (Cu; weak s-process element), Y, Ce, and Nd (main s-process elements), as well as Yb (50/50 r/s-process element). These abundances were determined from H-band lines, except for the yttrium abundances, which were determined from Y lines (from Y I) in the K band at 2.2 μm.
In general, there has been a lack of useful spectral lines in the NIR for determining the abundances of neutron-capture elements. In APOGEE spectra (R ~ 22 500), Cunha et al. (2017) identified and characterised cerium lines (from Ce II) in the H band. Similarly, Hasselquist et al. (2016) identified and analysed neodymium lines (from Nd II) also in APOGEE spectra. By carefully addressing and treating blends, Hayes et al. (2022) were additionally able to determine the chemical evolution for Cu I and Nd II from APOGEE spectra. Using NIR spectra observed at higher spectral resolution with the IGRINS spectrometer (R ~ 45 000), Montelius et al. (2022) succeeded in deriving the abundance trend for ytterbium (Yb) using a Yb II line in H band. Ytterbium is particularly interesting because it has the highest contribution from the r-process among these elements1.
In this paper, we identify and characterise a barium (Ba) line from Ba I in IGRINS spectra in the K-band. This adds the most widely used s-process-dominated element to the list of neutron-capture elements that can be used in NIR spectra. Barium is one of the most s-process-dominated elements, with s/r(Ba)= 90/10 (Prantzos et al. 2020).
2 Observations and data reduction
Among the 50 M giants analysed in Paper I and, we determined the barium (Ba) abundances for 37 M giants (Teff < 4000 K) from high-resolution spectra (spectral resolution of R ~ 45 000) observed with the Immersion GRating INfrared Spectrograph (IGRINS; Yuk et al. 2010; Wang et al. 2010; Gully-Santiago et al. 2012; Moon et al. 2012; Park et al. 2014; Jeong et al. 2014). In addition to six nearby M giants available in the IGRINS spectral library (Park et al. 2018; Sawczynec et al. 2022), observed at the McDonald Observatory in 2015 and 2016 (Mace et al. 2016), the remaining stars were observed with IGRINS mounted on the Gemini South telescope (Mace et al. 2018) under the programs GS-2020B-Q-305 and GS-2021A-Q302. These were observed over a period from January to April 2021. All the stars are located in the solar neighbourhood and serve as a good reference sample.
The IGRINS observations were conducted using one or more ABBA nod sequences along the slit to facilitate sky background subtraction. Exposure times were chosen to achieve an average signal-to-noise ratio (S/N) of at least 1002, resulting in observing times ranging from 30 seconds to 15 minutes. While for three of the stars the observations did not reach an S/N of 100, for 15 stars the S/N was well above this value; see Table 1.
Spectral reductions were performed using the IGRINS Pipeline Package (IGRINS PLP; Lee et al. 2017). This process included flat-field correction, and A-B frame subtraction, telluric correction, and wavelength calibration using sky OH emission lines (Han et al. 2012; Oh et al. 2014). To eliminate telluric lines, the science spectra were divided by a telluric standardstar spectrum of a fast-rotating late-B to early-A dwarf observed close in time and at a similar air mass. Subsequently, the spectral orders of the science targets and telluric standards were stitched together after normalising each order and then combining and resampling them in iraf (Tody 1993), excluding low-S/N edges of each order. Finally, the spectra were shifted to laboratory wavelengths in air following stellar radial velocity correction. Figure 1 shows the raw observed science spectra before telluric correction (blue line), reduced science spectra after telluric correction (black line), and telluric standard star spectra (orange line) surrounding the barium line of interest in this work (yellow band) for three target stars in this work.
Considerable effort was invested into defining specific local continua in the spectral segment where the Ba line resides in order to address any residual modulations in the continuum levels. To make sure we select the best continuum points, we went through the spectrum of each star manually within the 30 Å wavelength window that includes the barium line. We then identified continuum points that are least affected by telluric absorption correction or bad noisy pixels. Thus, we used a separate continuum mask file for each star while determining barium abundance. Furthermore, thanks to the higher spectral resolution (R ~45000) of IGRINS, we were able to identify reliable continuum points in between the CO bands in the region. No broad absorption features caused by diffuse interstellar bands (DIBs; see e.g. Geballe 2016; Geballe et al. 2011) are present near the Ba line or other lines used in this spectral analysis. Further details on the observations are provided in Paper I and Paper II, where stellar parameters and abundance trends versus metallicity are presented for 21 elements for these stars.
Fig. 1 Example spectra surrounding the barium line (yellow band) of three stars, showing the telluric correction carried out in this work. The raw observed target spectrum before telluric correction is shown in blue, the telluric corrected reduced spectrum in black, and the telluric star spectrum in orange. The dashed grey line represents the normalised continuum level at 1. |
Observational details of M giant stars.
3 Analysis
The barium feature at 23 253.56 Å was analysed in the spectra of the 37 M giants observed by synthesising spectra for given fundamental stellar parameters. The synthetic spectra were generated using the Spectroscopy Made Easy (SME; Valenti & Piskunov 1996, 2012) tool by calculating the spherical radiative transfer through a relevant stellar atmosphere model defined by these fundamental parameters. We chose the stellar atmosphere model by interpolating in a grid of one-dimensional Model Atmospheres in a Radiative and Convective Scheme (MARCS) stellar atmosphere models (Gustafsson et al. 2008).
The fundamental stellar parameters, namely the effective temperature (Teff), surface gravity (log g), metallicity ([Fe/H]), and microturbulence (ξmicro), are provided in Paper I and were determined using the method devised in that paper. The surface gravity, log g, is therefore constrained based on the Teff and [Fe/H] values from 3–10 Gyr Yonsei-Yale isochrones (Demarque et al. 2004). The stellar parameters and C/O ratios are given in Table 2 and typical uncertainties are found to be ± 100 K in Teff, ±0.2 dex in log g, ±0.1 dex in [Fe/H], and ±0.1 km/s in ξmicro. For majority of stars, C/O ratios are lower than the solar value (~0.5) indicating they are in the red giant branch (RGB) phase of the stellar evolution. For the remaining few stars, slightly higher than solar C/O ratio are found, which might be within uncertainties or indicating higher than scaled solar values of carbon (compare the study of Foster et al. 2024).
In Figure 2, we show the synthetic spectrum (crimson line) fits to the observed spectra (black circles) of four stars with Teff ranging from 3400 K to 3600 K and [Fe/H] ranging from −0.5 dex to 0.25 dex. The yellow bands in each panel represent the line masks defined for the Ba line, wherein SME fits observed spectra by varying the barium abundance and finds the best synthetic spectral fit by χ2 minimisation. The red band denotes the variation in the synthetic spectrum for a difference of ±0.3 dex in [Ba/Fe], indicating the line sensitivity to [Ba/Fe]. The green line shows the synthetic spectrum without Ba, also indicating any possible blends in the line.
The main spectral features in Figure 2, apart from the Ba line, are due to the first overtone transitions of 12CO, both Δν = 2 ← 0 and Δν = 3 ← 1 transitions. The 12CO(Δν = 3 − 1) band-head lies just outside the figure at 23 220 Å. The far blue wing of the CO(Δν = 3 − 1) R63 line to the right of the Ba line marginally influences the Ba line. This influence is well taken care of, because this CO line is modelled well, as are the other highly rotational transitions of the CO(Δν = 3 − 1) transition in the figure. These lines are formed deeper in the atmosphere than the lower rotational transitions, that is, the stronger lines in the figure (CO(Δν = 3 − 1) R37), which are not properly modelled. Lines with lower excitation energies form further out where the modelling of the atmosphere is less certain and 3D and nonlocal thermodynamic equillibirum (NLTE) effects may play a significant role. This was indeed also seen in the H-band study of high-resolution spectra by Montelius et al. (2022), where they discuss blending CO lines.
The orange line shows the telluric spectrum of the standard star that is used to correct for telluric contaminations in the observed star spectrum. The telluric spectra are divided out and the telluric lines should have ideally been removed, resulting in a pure stellar spectrum, although with slightly larger noise. However, the telluric division is not always perfect, and so the purpose of showing the orange telluric spectra is to show where we might expect spurious spectral features, which are actually residuals from the telluric reduction procedure.
The [Ba/Fe] value corresponding to the best-fit case for the Ba I line is listed in each panel. We have scaled the barium abundances with respect to the solar abundance value of 2.17 from Grevesse et al. (2007). It is clear from Figure 2 that the Ba I line at 23 253.56 Å is independent of any atomic or molecular line blends, is sensitive to barium abundance, and is strong enough for the temperature and metallicity ranges of the stars in our sample in order to ensure that the barium abundances determined in this work are reliable.
Fig. 2 Wavelength regions centred at the barium line at 23 253.56 Å for four stars with Teff ranging from 3400 K to 3600 K and [Fe/H] ranging from −0.5 dex to 0.25 dex. In each panel, the black circles denote the observed spectrum, the crimson line denotes the best-fit synthetic spectrum, and the red band denotes the variation in the synthetic spectrum for a difference of ±0.3 dex in the [Ba/Fe]. The yellow bands in each panel represent the line masks defined for the Ba line. The green line shows the synthetic spectrum without Ba, and the orange line shows the telluric spectrum of the standard star that is used to correct for telluric contamination in the observed star spectrum. The [Ba/Fe] value corresponding to the best-fit case for the Ba line is listed in each panel. All identified atomic and molecular lines are also denoted at the top of each panel. |
Stellar parameters, C/O ratio, and [Ba/Fe] values for each star determined in this work.
4 Results
Figure 3 shows the barium abundance trend as a function of metallicity for the 37 stars in our sample. We see a general downward trend with metallicity, with a larger scatter between metallicities above [Fe/H] > −0.5 up to solar metallicity, typical for s-process elements. We also find s-element enhanced stars at subsolar metallicities.
We followed the method described in Paper I and Paper III to determine the uncertainties in barium abundances arising from typical uncertainties in the stellar parameters (±100 K in Teff, ±0.2 dex in log g, ±0.1 dex in [Fe/H], and ±0.1 km s−1 in ξmicro). For this, we chose the metal-rich star 2M14261117-6240220 and redetermined 50 values of the barium abundances by setting the stellar parameters to randomly chosen values from normal distributions, with the actual stellar parameter value as the mean and the typical uncertainties as the standard deviation. The uncertainty in the barium abundance was then taken to be the mid-value of the difference between the 84th and 16th percentiles of the abundance distribution, which is estimated to be ± 0.1 dex. We further estimated an uncertainty of ~ 0.1 dex due to uncertainties in fitting the neighbouring CO lines and an uncertainty of ~0.05 dex due to the incorrect selection of continuum points surrounding the barium line. Adding all these uncertainties in quadrature, we estimated a typical uncertainty of 0.15 dex, which is indicated as the error bar in Figure 3. In order to retrieve an accurate barium abundance, the blending CO line wing has to be taken into account and modelled correctly.
A reliable barium abundance has been estimated for only one thick disc star at subsolar metallicity and it is found to be lower than that for thin disc stars at similar metallicities. This fits well in the overall picture of neutron-capture elements in the disk components found in Delgado Mena et al. (2017), Tautvaišienė et al. (2021), and Forsberg (2023), with the thick-disk following the lower envelope of the thin-disk ‘cloud’. This is also found in the trends of other s-process elements, such as cerium and neodymium, in Paper III. Similar observations were made by Liu et al. (2020) in their analysis of high-resolution, high-S/N spectra of 602 stars from the Keck/HIRES. These authors find the mean value of [Ba/Fe] for the high-α (thick disc) stars to be lower than that for the low-α (thin disc) stars. At the same time, Prantzos et al. (2023) used chemical evolution models to show that the thin and thick disc trends overlap and follow a single branch behaviour in the [Ba/Fe] versus [Fe/H] diagram. It is crucial to observe a greater number of thick disc stars and to carry out more accurate analyses in a larger sample of stars to answer this question.
Fig. 3 [Ba/Fe] vs. [Fe/H] for stars in our sample with their respective stellar population represented by different symbols (thin disk – red circle, thick disc – orange diamond). |
5 Discussion
In this section, we discuss the atomic barium transitions available in the NIR wavelengths, and compare these with trends in the abundance ratio of barium to metallicity in the literature and with similar trends in other s-process elements derived for the same stars from the same spectra. Finally, we discuss in which stars the barium line is detectable and useful for abundance analyses.
Atomic line data for the Ba I 6s5d 3D → 6s6p 3Po fine structure transitions.
Fig. 4 Energy level diagram showing the lowest 12 energy levels in neutral barium, Ba I. The line used in this study, 6s5d 3D2−6s6p 3P2, is marked in red. |
5.1 The fine-structure lines of barium in the K-band
The barium line identified is a 6s5d 3D2 − 6s6p transition in Ba I, with a lower excitation level at 1.14 eV; see Table 3. Neutral barium is a two-electron system, and the lower part of the energy level diagram is shown in Figure 4. For NIR studies, transitions from 6s5d to 6s6p levels are expected to be the strongest lines in the spectrum, as the lower 6s5d states have excitation energies of only ~1.1 eV and the transitions from the ground state to these levels are forbidden (same parity).
The 6s5d–6s6p multiplet has six lines, as marked in Figure 4 and in the atomic data detailed in Table 3. Two lines fall in the K-band, where the 6s5d 3D2−6s6p transition at 23 253.56 Å is the strongest one, with a larger log g f by 1.1 dex. The four transitions at longer wavelengths fall outside the K-band.
The wavelengths of the NIR transitions were measured by Karlsson & Litzén (1999) using archive spectra from the Kitt Peak Fourier transform spectrometer. In addition, they provide line intensities, but these are not corrected for instrumental transmission and should therefore be treated with care. These wavelengths and intensities are included in the NIST ADS database (Martin et al. 2000) and are given in Table 3, where we also calculate the relative line strengths with the weak-line formula, where the equivalent width is given by the proportionality
From this, we see that the line at 22311.47 Å is more than ten times weaker than the detected line at 23 253.56 Å, which explains why the 22 311.47 Å line is not detected in our spectra of the K band.
There exist no experimental oscillator strengths for the NIR 6s5d–6s6p transition multiplet. The most recent theoretical values are from semi-empirical fitting by Kurucz (2017), and these are added to Table 3.
With an ionisation potential of only 5.2 eV (Karlsson & Litzén 1999), most Ba is ionised and Ba I is a minority species. The line opacity is therefore sensitive to the electron pressure, as is the continuous opacity, which is due to . In general, the strengths of weak lines depend on the line and continuum opacities, χline and χcont, such that the equivalent width of a line W ∝ χline/χcont. This means that the sensitivity to the electron pressure cancels out in this ratio, and therefore line strength, to a certain degree is unaffected by electron pressure. This explains the very small changes in line strengths for varying log g (related to the electron pressure through the hydrostatic equilibrium equation), as shown below in Section 5.3.
A previous set of data for Ba I were estimated from laboratory spectra, as given by Kurucz (1993) and included in the VALD database (Piskunov et al. 1995; Ryabchikova et al. 2015). However, these data are not consistent with the more recent data from Kurucz (2017) or with the experimental data by Karlsson & Litzén (1999). A particular example is the two lines in the K-band region, where 22 311.47 and 23 253.56 Å lines are predicted to be of similar strength in the earlier data, whereas the data of Karlsson & Litzén (1999) are consistent with our findings that the 23 253.56 Å line is observed but the 22 311.47 Å line is much weaker.
5.2 Comparison with other s-process elements and the literature
In Paper III, the abundances of other s-process elements were determined for the same sample of stars analysed in the present work. Hence it is only logical to compare the barium abundance trends determined in this work with other s-process element abundances for the same stars. In the left panel of Figure 5, we compare the abundance trends of cerium (blue squares) and neodymium (cyan inverted triangles) with the barium abundance trends (red circles) in the present work. The cerium abundances have been shifted by +0.3 dex to pass through the solar value at solar metallicity. Both the cerium and neodymium abundances exhibit very similar trends, as already seen in the mean s-process abundance plot (Figure 23) in Paper III. The barium abundances in the present work show greater scatter but follow a qualitatively similar trend to those for the cerium and neodymium abundances. We note that the four stars that have high mean s-process abundances in Paper III also show higher barium abundances. This is especially evident in the case of the two subsolar metallicity stars, 2M06520463-0047080 and 2M06574070-1231239, with similar high abundances in cerium and neodymium. This is further evidence that we have been able to determine reliable abundances from the barium line at 23 253.56 Å.
In order to compare our barium abundance trends with metallicity with those from the literature we chose the barium abundances determined from high-resolution (R ~ 115 000) optical spectra (4000–7000 Å) of 1111 FGK dwarf stars from the HARPS GTO planet search program presented by Delgado Mena et al. (2017). We show this comparison in the right panel of Figure 5. Our barium abundances generally lie within the overall trend of Delgado Mena et al. (2017), especially at metallicities above −0.5 dex. Our trend does not exhibit the decrease in barium abundance from supersolar to subsolar values at metallicities below −0.5 dex, as evident in the trend found by Delgado Mena et al. (2017). We have only four stars with reliable barium measurements at metallicities below −0.5 dex, and more stars at lower metallicities are needed to confirm this trend using the abundance measured from the barium line at 23 253.56 Å. At the same time, we note that there are stars with similarly high barium abundance measurements at similarly low metallicities in Delgado Mena et al. (2017) as well.
Fig. 5 Comparison of barium abundance trends in this work with other s-process elements such as cerium ([Ce/Fe]) and neodymium ([Nd/Fe]) for the same stars (left panel) and with the values obtained by Delgado Mena et al. (2017) (right panel). |
5.3 Best stars with which to measure the Ba I line at 23 253.56 Å
In Figure 2, we show that the Ba I line at 23 253.56 Å is devoid of any atomic or molecular blends, making it a reliable line with which to measure abundances. Furthermore, in the previous sections, we demonstrate that the barium abundance trend determined from the Ba I line at 23 253.56 Å is qualitatively consistent with other s-process elemental abundance trends determined for the same stars and also with reliable barium abundance trends in the literature. We also find high barium abundances for the two stars with high cerium and neodymium abundances.
Considering that there is a lack of spectral lines in the NIR that are useful for determining the abundances of neutron-capture elements, we want to investigate which type of stars should be targeted in the future to get reliable barium abundances from this Ba I line. In order to investigate this, we estimated the equivalent widths of the Ba I line for typical red giant stars with Teff from 3100 K to 3900 K (in 200 K intervals), log g from 0.0 to 1.7 dex, and [Fe/H] values of −1.0, −0.5, 0.0, and +0.5 dex. In Figure 6, we show the resulting equivalent width measurements for different combinations of Teff, log g and [Fe/H]. The coloured lines show different metallicities: blue for [Fe/H] = −1.0, orange for [Fe/H] = −0.5, green for [Fe/H] = 0.0, and red for [Fe/H] = +0.5. The sizes of the symbols indicate the surface gravities corresponding to a given Teff and [Fe/H] according to a typical isochrone. The sizes go from the largest being log g = 1.7 to the smallest being log g = 0.0.
From Figure 6, it is evident that the Ba I line at 23 253.56 Å is the strongest for cool (Teff < 3500 K) solar and supersolar metallicity red giant stars. Even though the line gets weaker with increasing Teff and decreasing [Fe/H], we demonstrate that it is possible to determine reliable abundances from this line in high-resolution, high-S/N spectra acquired with a capable instrument such as IGRINS. However, at very low metallicities ([Fe/H]~ −1.0), this barium line will be very weak, making it impossible to determine abundances. We also note that the equivalent width of the Ba line is quite insensitive to the surface gravity (a change of 1 dex in log g changes the equivalent width by less than 3%.), which is indeed expected as described in Section 5.1.
Fig. 6 Equivalent widths in mÅ of the barium line for typical red giants as a function of temperature. The coloured lines show equal metallicities (blue for [Fe/H] = −1.0, orange for [Fe/H] = −0.5, green for [Fe/H] = 0.0, and red for [Fe/H] = +0.5). The size of the symbols indicates the surface gravities corresponding to a given Teff and [Fe/H] according to a typical isochrone. The sizes go from largest being log g = 1.7 to the smallest being log g = 0.0. The equivalent width of the Ba line is relatively insensitive to the surface gravity (a change of 1 dex in log g changes the equivalent width by less than 3%.). |
6 Conclusions
In this work, we identified and characterised a Ba I line at 23 253.56 Å in high-resolution (R ~ 45 000) IGRINS spectra of M giants (Teff <4000 K) in the solar neighbourhood. While there are five other Ba I lines within the same fine-structure multiplet, the line at 23 253.56 Å is found to be the strongest and to have the highest transition probability in the K-band (see Section 5.1).
We show in Figure 2 that the Ba I line at 23 253.56 Å is devoid of any atomic or molecular line blends, is sensitive to barium abundances, and is sufficiently strong in the spectra of the stars in our sample to allow the reliable determination of barium abundances. We determine [Ba/Fe] for 37 M giants and find a general downward trend with metallicity, with a larger scatter between metallicities above [Fe/H]> −0.5 up to solar metallicity, typical for s-process elements (Figure 3).
We show that our [Ba/Fe] versus [Fe/H] trend is very similar to the other s-process element trends (cerium and neodymium) determined for the same stars and from the same spectra. The two barium-enhanced stars with similar high cerium and neodymium abundances further confirms this similarity. We further demonstrate that our [Ba/Fe] versus [Fe/H] trend, within uncertainties, is consistent with barium abundance trends in the literature. Our investigations into the equivalent width measurements of synthetic spectra of stars with different combinations of Teff, log g, and [Fe/H] indicate that the Ba I line at 23 253.56 Å is the strongest for cool (Teff < 3500 K) solar and supersolar metallicity giant stars and becomes gradually weaker with decreasing Teff and [Fe/H].
Thus, in this work, we have successfully identified and characterised the Ba I line at 23 253.56 Å and determined reliable barium abundances for 37 M giants from their high-resolution, high-S/N IGRINS spectra. We also emphasise the importance of systematic and consistent analysis in the determination of stellar parameters and abundances in order to avoid systematic effects. This brings forth the importance of capable NIR instruments such as IGRINS in the identification and characterisation of unidentified atomic and molecular lines of rare and interesting chemical species in the NIR wavelength regime (see also Cunha et al. 2017; Montelius et al. 2022; Nandakumar et al. 2022, 2023b). This work is therefore a prime example of the immense potential of the existing (GIANO, CRIRES+), near-future (MOONS), and future high-resolution NIR spectrometers (proposed for the extremely large telescopes such as the extremely large telescope (ELT) and thirty meter telescope (TMT)) for studying the Galactic chemical evolution of stellar populations, as they will enable the observation of more distant M giants, including those in other galaxies.
Acknowledgements
We thank the anonymous referee for the constructive comments and suggestions that improved the quality of the paper. G.N. acknowledges the support from the Wenner-Gren Foundations (UPD2020-0191 and UPD2022-0059), the Royal Physiographic Society in Lund through the Stiftelsen Walter Gyllenbergs fond and the Royal Swedish Academy of Sciences “Vetenskapsakademiens stiftelser”. N.R. acknowledge support from the Swedish Research Council (grant 2023-04744) and the Royal Physiographic Society in Lund through the Stiftelsen Walter Gyllenbergs and Märta och Erik Holmbergs donations. H.H. acknowledges support from the Swedish Research Concil VR (grant 2023-05367). This work used The Immersion Grating Infrared Spectrometer (IGRINS) was developed under a collaboration between the University of Texas at Austin and the Korea Astronomy and Space Science Institute (KASI) with the financial support of the US National Science Foundation under grants AST-1229522, AST-1702267 and AST-1908892, McDonald Observatory of the University of Texas at Austin, the Korean GMT Project of KASI, the Mt. Cuba Astronomical Foundation and Gemini Observatory. This work is based on observations obtained at the international Gemini Observatory, a program of NSF’s NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation on behalf of the Gemini Observatory partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). The following software and programming languages made this research possible: TOPCAT (version 4.6; Taylor 2005); Python (version 3.8) and its packages ASTROPY (version 5.0; Astropy Collaboration 2022), SCIPY (Virtanen et al. 2020), MATPLOTLIB (Hunter 2007) and NUMPY (van der Walt et al. 2011).
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The ratio of the s- and r-process contributions in the solar isotopic mixture are s/r(Y)=70/30 [in %] and s/r(Ce)=83/17, s/r(Nd)=60/40, and s/r(Yb)=50/50 (Bisterzo et al. 2014; Prantzos et al. 2020; Kobayashi et al. 2020).
The S/Ns were provided by RRISA (The Raw & Reduced IGRINS Spectral Archive; Sawczynec et al. 2022) and are the average S/Ns for the H- and K-bands per resolution element. The S/N varies over the orders and is lowest at the ends of the orders.
All Tables
Stellar parameters, C/O ratio, and [Ba/Fe] values for each star determined in this work.
All Figures
Fig. 1 Example spectra surrounding the barium line (yellow band) of three stars, showing the telluric correction carried out in this work. The raw observed target spectrum before telluric correction is shown in blue, the telluric corrected reduced spectrum in black, and the telluric star spectrum in orange. The dashed grey line represents the normalised continuum level at 1. |
|
In the text |
Fig. 2 Wavelength regions centred at the barium line at 23 253.56 Å for four stars with Teff ranging from 3400 K to 3600 K and [Fe/H] ranging from −0.5 dex to 0.25 dex. In each panel, the black circles denote the observed spectrum, the crimson line denotes the best-fit synthetic spectrum, and the red band denotes the variation in the synthetic spectrum for a difference of ±0.3 dex in the [Ba/Fe]. The yellow bands in each panel represent the line masks defined for the Ba line. The green line shows the synthetic spectrum without Ba, and the orange line shows the telluric spectrum of the standard star that is used to correct for telluric contamination in the observed star spectrum. The [Ba/Fe] value corresponding to the best-fit case for the Ba line is listed in each panel. All identified atomic and molecular lines are also denoted at the top of each panel. |
|
In the text |
Fig. 3 [Ba/Fe] vs. [Fe/H] for stars in our sample with their respective stellar population represented by different symbols (thin disk – red circle, thick disc – orange diamond). |
|
In the text |
Fig. 4 Energy level diagram showing the lowest 12 energy levels in neutral barium, Ba I. The line used in this study, 6s5d 3D2−6s6p 3P2, is marked in red. |
|
In the text |
Fig. 5 Comparison of barium abundance trends in this work with other s-process elements such as cerium ([Ce/Fe]) and neodymium ([Nd/Fe]) for the same stars (left panel) and with the values obtained by Delgado Mena et al. (2017) (right panel). |
|
In the text |
Fig. 6 Equivalent widths in mÅ of the barium line for typical red giants as a function of temperature. The coloured lines show equal metallicities (blue for [Fe/H] = −1.0, orange for [Fe/H] = −0.5, green for [Fe/H] = 0.0, and red for [Fe/H] = +0.5). The size of the symbols indicates the surface gravities corresponding to a given Teff and [Fe/H] according to a typical isochrone. The sizes go from largest being log g = 1.7 to the smallest being log g = 0.0. The equivalent width of the Ba line is relatively insensitive to the surface gravity (a change of 1 dex in log g changes the equivalent width by less than 3%.). |
|
In the text |
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