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1. Introduction
HD 34540 is a southern, seventh-magnitude G-type star at a distance of 35 pc. As such, it has not received much attention in the literature, however, the Hipparcos experiment (Perryman et al. 1997) found HD 34540 to be an astrometric binary with an orbital period P = 685.1827 ± 8.5280 days, photocentric semimajor axis α = 19.71 ± 1.08 mas, orbital inclination i = 50
33 ± 408, and a low eccentricity e = 0.1088 ± 0.1444, later also confirmed as a spectroscopic binary by Nordström et al. (2004). In this research note, we present high signal-to-noise, high-resolution spectroscopic observations that disclose HD 34540 as a barium dwarf at [Ba/Fe] = +0.50, likely enriched as such from its companion, that is, from a former asymptotic giant branch (AGB) primary, now a white dwarf.
2. Evidence for a Population I Field Blue Straggler
We observed HD 34540 with the FEROS échelle spectrograph (Kaufer et al. 1999) on 2017 December 5 at the La Silla Observatory in Chile. Our 20 minutes exposure of HD 34540 has a signal-to-noise ratio S/N ≃ 250 and we compare our observations of HD 34540 to a similar G-type dwarf, HD 34101, secured with the same spectrograph and under similar conditions on 2017 November 30.
Our model atmosphere analysis of HD 34540 results in a single-lined solar-type dwarf at Teff,Aa ≃ 5608 K, log gAa = 4.49, [Fe/H] = −0.05, [Fe/Mg] = −0.03, and mass MAa ≃ 0.93 M⊙, that is, with typical abundances for a Population I star (see Figure 3 in Fuhrmann et al. 2017). For HD 34101, which is also an astrometric and single-lined spectroscopic binary with a similar orbital period P = 803.51 days (Nidever et al. 2002; Jancart et al. 2005; Ren & Fu 2013) we get a slightly cooler, slightly more metal-rich primary at Teff,Aa ≃ 5538 K, log gAa = 4.46, [Fe/H] = −0.01, [Fe/Mg] = −0.03, and mass MAa ≃ 0.91 M⊙. Both Population I stars display similar kinematics, distinct from regions in velocity space usually occupied by nearby young stars.
A major difference between HD 34540 and HD 34101 arises from their companion masses at MAb ≃ 0.60 M⊙ and MAb ≃ 0.19 M⊙, respectively, and in Figure 1, in particular, we observe a considerable [Ba/Fe] = +0.50 barium enrichment for HD 34540 Aa, compared to a solar [Ba/Fe] = −0.02 for HD 34101 Aa.
Figure 1. High signal-to-noise, high-resolution spectra of the G-type stars HD 34540 and HD 34101. Both sources are astrometric and single-lined spectroscopic binaries with orbital periods P = 685.18 days (HD 34540) and P = 803.51 days (HD 34101) and companion masses MAb ≃ 0.60 M⊙ (HD 34540) and MAb ≃ 0.19 M⊙ (HD 34101). The [Ba/Fe] = +0.50 barium-enriched HD 34540 Aa is strong evidence for a blue straggler star with a degenerate companion. The inset highlights the λ5853 Ba ii line.
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The barium enrichment of HD 34540 Aa, along with the MAb ≃ 0.60 M⊙ companion mass, are both strong evidence for a mass transfer from a former AGB primary. In other words, we expect HD 34540 Ab to be a white dwarf, and the current primary, HD 34540 Aa, is a blue straggler. The P = 685 days short orbital period of HD 34540, which in principle would allow for Roche-lobe overflow in the first-ascent red giant phase (and hence prevent the formation of a barium dwarf), is no real concern, for orbital shrinkage is to be expected upon mass transfer, at least for high initial mass ratios.
Based on observations with the MPG 2.2 m telescope at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 0100.A-9022(A). K.F. acknowledges support from the DFG grant FU 198/11-2.