A study of high quality, near-infrared spectra of eight spectral type of K stars: precise radial velocities, chromospheric emission, and fundamental stellar parameters

Date

2018-07-17

Authors

Larson, Ana Marie

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Abstract

We examine the precise radial velocities and chromospheric emission and derive the fundamental parameters of eight K stars - 36 Ophiuchi A, B (K0 V, K1 V), 61 Cygni A, B (K5 V, K7 V), β Geminorum (K0 IIIb ), δ Sagittarii (K2.5 IIIa), α Tauri (K5 III), and ϵ Pegasi (K2 Ib) - through analyses of high quality (S/N > 1000) near-infrared (864-878 nm) spectra. The spectra were obtained as part of the hydrogen fluoride precise (≈ 15 — 30 m s-1) radial velocity (RV) program at the Canada-France-Hawaii 3.6-m telescope (1981- 1992) and the Dominion Astrophysical Observatory 1.22-m telescope (1991- 1995). We define the ∆EW 866.2 index used to quantify changes in the core flux of the Ca II 866.214 nm line and show the index is a sensitive measure of changes in chromospheric emission. We compare the “reference” spectrum for each star with synthetic spectra of the 864.7-867.7 nm region and derive the fundamental parameters: effective temperature [special characters omitted] surface gravity [special characters omitted], metallicity ([M/H]), and microturbulence (ξ). We describe an efficient, time-saving method which identifies and eliminates insignificant lines. Through our comparisons of the narrow spectral region for these “well-known” stars and through our development of a rapid synthesis method, we demonstrate a solid foundation for a broader, more comprehensive study of this region of the H-R diagram. The nearly identical stars 36 Oph A and B have dissimilar chromospheric activity. For these stars, we derived [special characters omitted] = 5125 K, log g = 4.67, [M/H] = -0.25, and ξ = 1.4 km s-1, in excellent agreement with relationships predicted by stellar interior models for [special characters omitted] ≈ 0.75 and [Fe/H] = -0.3. For 61 Cyg A, we detect a rotation period of 36.2 days in the ∆EW 866.2 index and of 37.8 days in the radial velocities, implying that active regions are spatially and temporally coherent over long time scales for this star. For 61 Cyg A, we derived [special characters omitted] = 4545 K, log g = 4.55, [M/H] = -0.40, and ξ = 1.5 km s-1; for 61 Cyg B, [special characters omitted] = 4150 K, log g = 4.55, [M/H] = -0.40, and ξ = 0.7 km s-1. These parameters are in excellent agreement with relationships predicted by stellar interior models for [Fe/H] = -0.4 and [special characters omitted] ≈ 0.65 for 61 Cyg A, and [special characters omitted] ≈ 0.55 for 61 Cyg B. Low-amplitude RV variability is a ubiquitous characteristic of the K giants. For β Gem, we find similar RV {K = 46.23 ± 3.9 m s-1, P = 584.65 ± 3.3 dy) and ∆EW 866.2 index (K = 0.583 ± 1.9 pm, P = 587.7 ± 12 dy) periods. If due to rotation modulation of some surface feature, this period is inconsistent with the most reliable ν sin i value for this star. We detect a long-term (> 12 yr) change in the ∆EW 866.2 index for this star, reminiscent of a solar-type magnetic cycle. For δ Sgr, we find significant long-term trends in the radial velocities and ∆EW 866.2 index, and significant, but aliased, RV periods at 1.98 days (K = 82.1 ± 9.1 m s-1) and 293 days {K = 68.8 ± 9.8 m s-1). α Tauri has a 647.93-dy period (K = 114.9±10.6 m s-1) in the radial velocities, but no corresponding period in the ∆EW 866.2 index. From 1.22- m telescope observations, we find a 1.8358-dy RV period (K = 32.0 ± 5.0 m s-1) consistent with theoretical granulation-driven acoustic modes or a fundamental overtone (n ≈ 4). The supergiant ϵ Peg resembles a semi-regular RV variable. We find multiple RV periods (not aliases) of 65.2 days (K = 415.8 ± 59.0 m s-1), 46.3 days (K = 559.1 ± 57.0 m s-1), perhaps both fundamental overtones, and 10.7 days (K = 410.3 ± 66.0 m s-1), perhaps related to solar-type spicules. The 10.7-dy period is present in the ∆EW866.2 index for this star.

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Keywords

Stars, Spectra, Astronomical spectroscopy

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