IP Eridani: A surprising long-period binary system hosting a He white dwarf

Eck

Winckel

et al. 2016

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The HERMES spectrograph installed on the 1.2-m Mercator telescope has been used to monitor the radial velocity of 13 lowmetallicity carbon stars, among which seven carbon-enhanced metal-poor (CEMP) stars and six CH stars (including HIP 53522, a new member of the family, as revealed by a detailed abundance study). All stars but one show clear evidence for binarity. New orbits are obtained for eight systems. The sample covers an extended range in orbital periods, extending from 3.4 d (for the dwarf carbon star HE 0024-2523) to about 54 yr (for the CH star HD 26, the longest known among barium, CH, and extrinsic S stars). Three systems exhibit low-amplitude velocity variations with periods close to 1 yr superimposed on a long-term trend. In the absence of an accurate photometric monitoring of these systems, it is not clear yet whether these variations are the signature of a very lowmass companion or of regular envelope pulsations. The period -eccentricity (P − e) diagram for the 40 low-metallicity carbon stars with orbits now available shows no difference between CH and CEMP-s stars (the latter corresponding to those CEMP stars enriched in s-process elements, as are CH stars). We suggest that they must be considered as one and the same family and that their different names only stem from historical reasons. Indeed, these two families have as well very similar mass-function distributions, corresponding to companions with masses in the range 0.5-0.7 M , indicative of white-dwarf companions, adopting 0.8-0.9 M for the primary component. This result confirms that CH and CEMP-s stars obey the same mass-transfer scenario as their highermetallicity analogues, barium stars. The P − e diagrams of barium, CH, and CEMP-s stars are indeed very similar. They reveal two different groups of systems: one with short orbital periods (P < 1000 d) and mostly circular or almost circular orbits, and another with longer period and eccentric (e > 0.1) orbits. These two groups either trace different evolutionary channels during the mass-transfer episode responsible for the chemical peculiarities of the Ba/CH/CEMP-s stars, or result from the operation of tidal circularisation in a more recent past, when the current giant star was ascending the first giant branch.

Section: Discussionmentioning

confidence: 99%

Binary properties of CH and carbon-enhanced metal-poor stars

Eck

Winckel

et al. 2016

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“…For the fast rotators 14 Aur C (V sin i = 16 km s −1 ) and V1379 Aql (V sin i = 18 km s −1 ) we convolved the synthetic spectra with a rotational profile. Merle et al (2014) performed an abundance analysis of IP Eri and obtained the following atmospheric parameters: T eff = 4960 ± 100 K, log g = 3.3 ± 0.3, and [Fe/H]= +0.09 ± 0.08, which are used here as well.…”

Section: Atmospheric Parametersmentioning

confidence: 99%

“…References. (1) Fekel et al (1993); (2) Landsman et al (1993); (3) Vennes et al (1998) (4) Burleigh et al (1997); (5) Merle et al (2014); (6) Jorissen et al (1996); (7) ; (8) Stefanik et al (2011); (9) Dominy & Lambert (1983); (10) Griffin & Keenan (1992); (11) Schindler et al (1982); (12) Griffin (2006); (13) Frankowski & Jorissen (2006); (14) Böhm-Vitense (1980); (15) Jorissen et al (1998); (16) Jorissen et al (1995); (17) Fekel & Simon (1985).…”

Section: Introductionmentioning

confidence: 99%

To Ba or not to Ba: Enrichment ins-process elements in binary systems with WD companions of various masses

Merle

Eck

et al. 2016

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Context. The enrichment in s-process elements of barium stars is known to be due to pollution by mass transfer from a companion formerly on the thermally pulsing asymptotic giant branch (AGB), now a carbon-oxygen white-dwarf (WD). Aims. We are investigating the relationship between the s-process enrichment in the barium star and the mass of its WD companion. It is expected that helium WDs, which have masses lower than about 0.5 M and never reached the AGB phase, should not pollute their giant companion with s-process elements. Therefore the companion should never turn into a barium star. Methods. Spectra with a resolution of R ∼ 86 000 were obtained with the HERMES spectrograph on the 1.2 m Mercator telescope for a sample of 11 binary systems involving WD companions of various masses. We used standard 1D local thermodynamical equilibrium MARCS model atmospheres coupled with the Turbospectrum radiative-transfer code that is embedded in the BACCHUS pipeline to derive the atmospheric parameters through equivalent widths of Fe i and Fe ii lines. Least-squares minimization between the observed and synthetic line shape was used to derive the detailed chemical abundances of CNO and s-process elements.Results. The abundances of s-process elements for the entire sample of 11 binary stars were derived homogeneously. The sample encompasses all levels of overabundances: from solar [s/Fe] = 0 to 1.5 dex in the two binary systems with S-star primaries (for which dedicated MARCS model atmospheres were used). The primary components of binary systems with a WD more massive than 0.5 M are enriched in s-process elements. We also found a trend of increasing [s/Fe] with [C/Fe] or [(C+N)/Fe]. Conclusions. Our results confirm the expectation that binary systems with WD companions less massive than 0.5 M do not host barium stars.

“…The recent discovery of several long-period binaries (P ∼ 10 3 d) hosting a subdwarf (sdB) star (Østensen & Van Winckel 2011Vos et al 2012;Deca et al 2012;Barlow et al 2012Barlow et al , 2013 or a He white dwarf (WD) like HR 1608 = 63 Eri (Landsman et al 1993;Vennes et al 1998) or IP Eri studied here (Merle et al 2014) have challenged our understanding of their formation. These two classes of stars are closely related, since sdB stars consist of He-burning cores surrounded by extremely thin H envelopes (Heber 1986), while He WDs are degenerate, nuclearly extinct He cores.…”

Section: Introductionmentioning

confidence: 98%

“…Observational and evolutionary constraints Merle et al (2014) derive the first orbital solution for IP Eri (=HD 18131 = EUVE J0254-053), a K0IV + DA WD system with P = 1071.00 ± 0.07 d and e = 0.25 ± 0.01. An analysis of the WD atmospheric lines by Burleigh et al (1997) and Vennes et al (1998) yielded an effective temperature of ∼30 000 K and gravity log g ∼ 7.5.…”

Section: Introductionmentioning

confidence: 99%

The formation of long-period eccentric binaries with a helium white dwarf

Siess

Davis

2014

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The recent discovery of long-period eccentric binaries hosting a He-white dwarf or a subdwarf star has been challenging binary-star modeling. Based on accurate determinations of the stellar and orbital parameters for IP Eri, a K0 + He-WD system, we propose an evolutionary path that is able to explain the observational properties of this system and, in particular, to account for its high eccentricity (0.25). Our scenario invokes an enhanced-wind mass loss on the first red giant branch (RGB) in order to avoid mass transfer by Roche-lobe overflow, where tides systematically circularize the orbit. We explore how the evolution of the orbital parameters depends on the initial conditions and show that eccentricity can be preserved and even increased if the initial separation is large enough. The low spin velocity of the K0 giant implies that accretion of angular momentum from a (tidally-enhanced) RGB wind should not be efficient.