Main-sequence Binary Systems Research Articles

Constraining the helium-to-metal enrichment ratio deltaY/deltaZ from main-sequence binary stars. Theoretical analysis of the accuracy and precision of the age and helium abundance estimates

We aim to investigate the theoretical possibility of accurately determining the helium-to-metal enrichment ratio $ Y/ Z$ from precise observations of double-lined eclipsing binary systems. Using Monte Carlo simulations, we drew synthetic binary systems with masses between 0.85 and 1.00 $M_ sun $ from a grid of stellar models. Both stars were sampled from a grid with $ Y/ Z = 2.0$, with a primary star at 80<!PCT!> of its main-sequence evolution. Subsequently, a broader grid with $ Y/ Z$ from 1.0 to 3.0 was used in the fitting process. To account for observational uncertainties, two scenarios were explored: S1, with realistic uncertainties of 100 K in temperature and 0.1 dex in Fe/H ; and S2, with halved uncertainties. We repeated the simulation at two baseline metallicities: Fe/H = 0.0 and $-0.3$. The posterior distributions of $ Y/ Z$ revealed significant biases. The distributions were severely biased towards the edge of the allowable range in the S1 error scenario. The situation only marginally improved when considering the S2 scenario. The effect is due to the impact of changing $ Y/ Z$ in the stellar effective temperature and its interplay with Fe/H observational error, and it is therefore not restricted to the specific fitting method. Despite the presence of these systematic discrepancies, the age of the systems were recovered unbiased with 10<!PCT!> precision. Our findings indicate that the observational uncertainty in effective temperature and metallicity significantly hinders the accurate determination of the $ Y/ Z$ parameter from main-sequence binary systems.

Volume-limited sample of low-mass red giant stars, the progenitors of hot subdwarf stars

Context. Current theory predicts that hot subdwarf binaries are produced from evolved low-mass binaries that have undergone mass transfer and drastic mass loss during either a common-envelope phase or a stable Roche-lobe overflow while on the red giant branch (RGB). Aims. We perform a spectroscopic survey to find binary systems that include low-mass red giants near the tip of the RGB, which are predicted to be the direct progenitors of subdwarf B (sdB) stars. We aim to obtain a homogeneous sample to search for the observational evidence of correlations between the key parameters governing the formation of sdB stars and constrain the physics of stable mass transfer. Methods. Based on data from the Gaia mission and several ground-based, multiband photometry surveys, we compiled a sample of low-mass red giant branch (RGB) candidates. The candidates were selected according to their Gaia data release 2 (DR2) color, absolute magnitude, and proper motion cuts. In this work, we concentrated on the southern hemisphere targets and conducted a spectroscopic survey of 88 red giant stars to search for the long-period RGB plus main-sequence binary systems within 200 pc. Combining radial velocity (RV) measurements from ground-based observations with CORALIE and RV measurements from Gaia DR2 and from the early data release 3 (eDR3) as well as the astrometric excess noise and renormalized unit weight error measurements from Gaia DR3, we defined a robust binary classification method. In addition, we searched for known binary systems in the literature and in Gaia DR3. Results. We select a total of 211 RGB candidates in the southern hemisphere within 200 pc based on the Gaia DR2 color-magnitude diagram. Among them, a total of 33 red giants were reported as binary systems with orbital periods between 100 and 900 days, some of which are expected to be the direct progenitors of wide binary sdB stars. In addition, we classified 37 new main-sequence plus RGB binary candidates, whose orbital parameters will be measured with future spectroscopic follow-up. Conclusions. Using high-quality astrometric measurements provided by the Gaia mission coupled with high-resolution spectroscopy from the ground, we provide a powerful method for searching for low-mass red giant stars in long-period binary systems.

A White Dwarf–Main-sequence Binary Unveiled by Time-domain Observations from LAMOST and TESS

We report a single-lined white dwarf–main-sequence binary system, LAMOST J172900.17+652952.8, which is discovered by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST)’s medium-resolution time-domain surveys. The radial-velocity semi-amplitude and orbital period of the optical visible star are measured by using follow-up observations with the Palomar 200 inch telescope and light curves from the Transiting Exoplanet Survey Satellite (TESS). Thus the mass function of the invisible candidate white dwarf is derived, f(M 2) = 0.120 ± 0.003 M ⊙. The mass of the visible star is measured based on a spectral energy distribution fitting, M 1 = . Hence, the mass of its invisible companion is M 2 ≳ 0.63 M ⊙. The companion ought to be a compact object rather than a main-sequence star owing to the mass ratio q = M 2/M 1 ≳ 0.78 and the single-lined spectra. The compact object is likely to be a white dwarf if the inclination angle is not small, i ≳ 40°. By using the Galaxy Evolution Explorer (GALEX) near-UV flux, the effective temperature of the white dwarf candidate is constrained as ≲ 12,000–13,500 K. It is difficult to detect white dwarfs which are outshone by their bright companions via single-epoch optical spectroscopic surveys. Therefore, optical time-domain surveys can play an important role in unveiling invisible white dwarfs and other compact objects in binaries.

The K2 M67 Study: A Curiously Young Star in an Eclipsing Binary in an Old Open Cluster*

Abstract We present an analysis of a slightly eccentric (e = 0.05), partially eclipsing, long-period (P = 69.73 days) main-sequence binary system (WOCS 12009, Sanders 1247) in the benchmark old open cluster M67. Using Kepler K2 and ground-based photometry, along with a large set of new and reanalyzed spectra, we derived highly precise masses (1.111 ± 0.015 and 0.748 ± 0.005 M ⊙) and radii (1.071 ± 0.008 ± 0.003 and 0.713 ± 0.019 ± 0.026 R ⊙, with statistical and systematic error estimates) for the stars. The radius of the secondary star is in agreement with theory. The primary, however, is approximately 15% smaller than reasonable isochrones for the cluster predict. Our best explanation is that the primary star was produced from the merger of two stars, as this can also account for the nondetection of photospheric lithium and its higher temperature relative to other cluster main-sequence stars at the same V magnitude. To understand the dynamical characteristics (low measured rotational line broadening of the primary star and low eccentricity of the current binary orbit), we believe that the most probable (but not the only) explanation is the tidal evolution of a close binary within a primordial triple system (possibly after a period of Kozai–Lidov oscillations), leading to merger approximately 1 Gyr ago. This star appears to be a future blue straggler that is being revealed as the cluster ages and the most massive main-sequence stars die out.

First- versus second-generation planet formation in post-common envelope binary (PCEB) planetary systems

We examine planets orbiting post-common envelope binaries from the perspective of angular momentum evolution, and conclude that the planets are more likely to be first-generation (FG) planets than second generation (SG) planets. FG planets were born together with the parent stars, while SG planets form later from an SG protoplanetary disc formed by mass-loss from the evolved primary star during its red giant branch phase or asymptotic giant branch phase. We find that in some systems the SG scenario requires that more than 20 per cent of the SG protoplanetary disc mass ends in planets. Although we cannot rule out SG planet formation in these systems, this fraction of mass that ends in planets is much higher than the value commonly used in planet formation theories. On the other hand, we find that for each of the systems, we can build a progenitor system composed of a main-sequence binary system orbited by the appropriate planets. This can be done if the secondary star was in a resonance with the inner planet. To account for the progenitor properties, we suggest that in cases where the secondary star has a mass of ∼0.1–0.2舁M⊙, it was formed in the same way planets are formed, i.e. from a disc.

Four ultra-short-period eclipsing M-dwarf binaries in the WFCAM Transit Survey

We report on the discovery of four ultra-short-period (P ≤ 0.18 d) eclipsing M-dwarf binaries in the Wide-Field Camera (WFCAM) Transit Survey. Their orbital periods are significantly shorter than that of any other known main-sequence binary system, and are all significantly below the sharp period cut-off at P ∼ 0.22 d as seen in binaries of earlier-type stars. The shortest-period binary consists of two M4-type stars in a P = 0.112 d orbit. The binaries are discovered as part of an extensive search for short-period eclipsing systems in over 260 000 stellar light curves, including over 10 000 M-dwarfs down to J = 18 mag, yielding 25 binaries with P ≤ 0.23 d. In a popular paradigm, the evolution of short-period binaries of cool main-sequence stars is driven by the loss of angular momentum through magnetized winds. In this scheme, the observed P ∼ 0.22 d period cut-off is explained as being due to time-scales that are too long for lower-mass binaries to decay into tighter orbits. Our discovery of low-mass binaries with significantly shorter orbits implies that either these time-scales have been overestimated for M-dwarfs, e.g. due to a higher effective magnetic activity, or the mechanism for forming these tight M-dwarf binaries is different from that of earlier-type main-sequence stars.

Lithium Depletion Boundary in a Pre–Main-Sequence Binary System

A lithium depletion boundary is detected in HIP 112312 (GJ 871.1 A and B), a \~12 Myr old pre-main sequence binary system. A strong (EW 300 mA) Li 6708 A absorption feature is seen at the secondary (~M4.5) while no Li 6708 A feature is detected from the primary (~M4). The physical companionship of the two stars is confirmed from common proper motions. Current theoretical pre-main sequence evolutionary models cannot simultaneously match the observed colors, brightnesses, and Li depletion patterns of this binary system. At the age upper limit of 20 Myr, contemporary theoretical evolutionary models predict too slow Li depletion. If true Li depletion is a faster process than predicted by theoretical models, ages of open clusters (Pleiades, alpha Persei, and IC 2391) estimated from the Li depletion boundary method are all overestimated. Because of the importance of the open cluster age scale, development of self-consistent theoretical models to match the HIP 112312 data is desirable.

On the Origin of the Distribution of Binary Star Periods

Pre-main-sequence and main-sequence binary systems are observed to have periods, P, ranging from 1 day to 1010 days and eccentricities ranging from 0 to 1. We pose the problem of whether stellar-dynamical interactions in very young and compact star clusters will broaden an initially narrow period distribution to the observed width. N-body computations of extremely compact clusters containing 100 and 1000 stars initially in equilibrium and in cold collapse are performed. In all cases, the assumed initial period distribution is uniform in the narrow range 4.5 ≤ log P ≤ 5.5 (P in days), which straddles the maximum in the observed period distribution of late-type Galactic field dwarf systems. None of the models lead to the necessary broadening of the period distribution, despite our adopted extreme conditions that favor binary-binary interactions. Stellar-dynamical interactions in embedded clusters thus cannot, under any circumstances, widen the period distribution sufficiently. The wide range of orbital periods of very young and old binary systems is therefore a result of cloud fragmentation and immediate subsequent magnetohydrodynamic processes operating within the multiple protostellar system.

Kinematics of the HH 43 Flow: Evidence for a Precessing Jet?

High spectral resolution long-slit spectra of HH 43 obtained in the 1–0 S(1) line of H2 are reported. A direct image of HH 43 obtained in the 1–0 S(1) line of H2 reveals a close correspondence with knots as seen in optical [S II] and Hα emission. We suggest that ambient molecular gas is being entrained and shocked by an atomic flow. The position-velocity diagram along the central axis of HH 43 as seen in H2 reveals a sinusoidal-like structure with a semiamplitude of 8 km s-1 and a spatial wavelength of about 8000 AU. A direct Hα image of HH 43 shows evidence for a morphological wiggle with a semiamplitude of about 2'' that has the same spatial wavelength and phase seen in the 8 km s-1 kinematic oscillation. These data suggest that HH 43 may be produced by a precessing jet in a close pre–main-sequence binary system. Possible system parameters are explored.

Photometric binary stars in Praesepe and the search for globular cluster binaries

A radial velocity study of the stars which are located on a second sequence above the single-star zero-age main sequence at a given color in the color-magnitude diagram of the open cluster Praesepe, (NGC 2632) shows that 10, and possibly 11, of 17 are binary systems. Of the binary systems, five have full amplitudes for their velocity variations that are greater than 50 km/s. To the extent that they can be applied to globular clusters, these results suggests that (1) observations of 'second-sequence' stars in globular clusters would be an efficient way of finding main-sequence binary systems in globulars, and (2) current instrumentation on large telescopes is sufficient for establishing unambiguously the existence of main-sequence binary systems in nearby globular clusters.

On the consequences of low-mass white dwarf mergers

The theory of binary star evolution suggests that about 10 percent of all main-sequence binary systems should evolve into a close pair of light white dwarfs which merge within a Hubble time. This paper explores the consequences of such mergers on the assumption that a merger can be approximated by a mass-transfer event which occurs on a time scale shorter than that given by the Eddington accretion limit. The evolution of He + He mergers and of CO + He and of hybrid + He mergers are discussed. The birthrate of helium degenerate pairs which merge in less than a Hubble time is estimated, and the space density of low-luminosity merger products currently present in the Galaxy is predicted. It is shown that the evolutionary tracks of models of simulated mergers pass through the region in the H-R diagram occupied by subdwarfs, but that the predicted space density of merger products exceeds by over a factor of three the space density of subdwarf estimated form the known sample of such stars.

Theoretical and Observational Tests for the Mass Transfer Scenario of Ba II stars

Ba II stars are red giants showing an enhancement of carbon and s-process elements. The elucidation of their nature seems to require a mass transfer, either by wind or Roche lobe overflow, during their past evolution. Were it really the case, all Ba II stars would be binaries with a white dwarf as companion. To better understand the exact role of their binarity, more orbits are definitely needed. They can be obtained by monitoring the radial velocity variations of those stars. However, a quicker way to find new Ba II stars with orbital elements would be to search for their existence among known spectroscopic binaries. This would also crucially test whether mass transfer is a necessary and sufficient condition to explain Ba II stars. If it is indeed the case, then all spectroscopic binaries, made of a giant and a white dwarf, in a reasonable range of periods, would exhibit the Ba II pecularity. However, the discovery of a peculiar giant+main sequence binary system would imply a revision of our ideas about Ba II stars. To this end have we begun a systematic spectral survey of spectroscopic binaries with orbital periods in the range characteristic of known Ba II stars and containing a red giant. The realization that some stars of the catalogue we compiled were already identified as semibariium stars encourages us to pursue our investigation. Coude spectra were taken with the 152 cm telescope, at a dispersion of 12 Å mm−1 . Until now, 2 stars out of a sample of 31 present a slight enhancement of s-process elements (their anomaly being in the range Ba 0.3 to 0.5), and 2 more appear to be good candidates. The study of a larger sample is currently in progress. A discussion of the nature of the companion to the 2 newly discovered semibarium stars is presented on grounds of their mass function and photometric indices.