White Dwarf Companion Research Articles

A Chandra Study: Are Dwarf Carbon Stars Spun Up and Rejuvenated by Mass Transfer?

Abstract Carbon stars (with C/O > 1) were long assumed to all be giants, because only asymptotic giant branch (AGB) stars dredge up significant carbon into their atmospheres. The case is nearly ironclad now that the formerly mysterious dwarf carbon (dC) stars are actually far more common than C giants and have accreted carbon-rich material from a former AGB companion, yielding a white dwarf (WD) and a dC star that has gained both significant mass and angular momentum. Some such dC systems have undergone a planetary nebula phase, and some may evolve to become CH, CEMP, or Ba giants. Recent studies indicate that most dCs are likely from older, metal-poor kinematic populations. Given the well-known anticorrelation of age and activity, dCs would not be expected to show significant X-ray emission related to coronal activity. However, accretion spin-up might be expected to rejuvenate magnetic dynamos in these post-mass-transfer binary systems. We describe our Chandra pilot study of six dCs selected from the SDSS for Hα emission and/or a hot WD companion, to test whether their X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo. We detect all six dCs in the sample, which have X-ray luminosities ranging from log –29.7, preliminary evidence that dCs may be active at a level consistent with stars that have short rotation periods of several days or less. More definitive results require a sample of typical dCs with deeper X-ray observations to better constrain their plasma temperatures.

Blue Lurkers: Hidden Blue Stragglers on the M67 Main Sequence Identified from Their Kepler/K2 Rotation Periods

Abstract At an age of 4 Gyr, typical solar-type stars in M67 have rotation rates of 20–30 days. Using K2 Campaign 5 and 16 light curves and the spectral archive of the WIYN Open Cluster Study, we identify 11 3D kinematic members of M67 with anomalously fast rotation periods of 2–8 days, implying ages of less than 1 Gyr. We hypothesize that these anomalously fast rotators have been spun up by mass transfer, mergers, or stellar collisions during dynamical encounters within the 1 Gyr and thus represent lower-luminosity counterparts to the blue straggler stars. These 11 candidate post-interaction stellar systems have much in common with the blue stragglers, including a high binary fraction (73%); a number of long-period, low-eccentricity binary systems; and in at least one case a UV excess consistent with the presence of a hot white dwarf companion. The identification of these 11 systems provides the first picture of the low-luminosity end of the blue straggler distribution, providing new constraints for detailed binary evolution models and cluster population studies. This result also clearly demonstrates the need to properly account for the impact of binaries on stellar evolution, as significant numbers of post-interaction binaries likely exist on cluster main sequences and in the field. These stars are not always easy to identify but make up ∼10% or more of the spectroscopic binary population among the solar-type stars in M67.

The Mass of the White Dwarf Companion in the Self-lensing Binary KOI-3278: Einstein versus Newton

Abstract KOI-3278 is a self-lensing stellar binary consisting of a white dwarf secondary orbiting a Sun-like primary star. Kruse & Agol noticed small periodic brightenings every 88.18 days in the Kepler photometry and interpreted these as the result of microlensing by a white dwarf with about 63% of the mass of the Sun. We obtained two sets of spectra for the primary that allowed us to derive three sets of spectroscopic estimates for its effective temperature, surface gravity, and metallicity for the first time. We used these values to update the Kruse & Agol Einsteinian microlensing model, resulting in a revised mass for the white dwarf of M ⊙. The spectra also allowed us to determine radial velocities and derive orbital solutions, with good agreement between the two independent data sets. An independent Newtonian dynamical MCMC model of the combined velocities yielded a mass for the white dwarf of M ⊙. The nominal uncertainty for the Newtonian mass is about four times better than for the Einsteinian, ±1.1% versus ±4.1%, and the difference between the two mass determinations is 5.2%. We then present a joint Einsteinian microlensing and Newtonian radial velocity model for KOI-3278, which yielded a mass for the white dwarf of M ⊙. This joint model does not rely on any white dwarf evolutionary models or assumptions on the white dwarf mass–radius relation. We discuss the benefits of a joint model of self-lensing binaries, and how future studies of these systems can provide insight into the mass–radius relation of white dwarfs.

Discovery of a White Dwarf Companion to HD 159062

Abstract We report on the discovery of a white dwarf companion to the nearby late G dwarf star, HD 159062. The companion is detected in 14 years of precise radial velocity (RV) data, and in high-resolution imaging observations. RVs of HD 159062 from 2003 to 2018 reveal an acceleration of −13.3 ± 0.12 , indicating that it hosts a companion with a long-period orbit. Subsequent imaging observations with the ShaneAO system on the Lick Observatory 3 m Shane telescope, the PHARO AO system on the Palomar Observatory 5 m telescope, and the NIRC2 AO system at the Keck II 10 m telescope reveal a faint companion 2.″7 from the primary star. We performed relative photometry, finding ΔJ = 10.09 ± 0.38 mag, mag, and mag for the companion from these observations. Analysis of the radial velocities, astrometry, and photometry reveals that the combined data set can only be reconciled for the scenario where HD 159062 B is a white dwarf. A full Bayesian analysis of the RV and imaging data to obtain the cooling age, mass, and orbital parameters of the white dwarf indicates that the companion is an old white dwarf with an orbital period of yr, and a cooling age of Gyr.

Barium and related stars, and their white-dwarf companions

Barium (Ba) dwarfs and CH subgiants are the less evolved analogues of Ba and CH giants. They are F- to G-type main-sequence stars polluted with heavy elements by their binary companions when the companion was on the asymptotic giant branch (AGB). This companion is now a white dwarf that in most cases cannot be directly detected. We present a large systematic study of 60 objects classified as Ba dwarfs or CH subgiants. Combining radial-velocity measurements from HERMES and SALT high-resolution spectra with radial-velocity data from CORAVEL and CORALIE, we determine the orbital parameters of 27 systems. We also derive their masses by comparing their location in the Hertzsprung–Russell diagram with evolutionary models. We confirm that Ba dwarfs and CH subgiants are not at different evolutionary stages, and that they have similar metallicities, despite their different names. Additionally, Ba giants appear significantly more massive than their main-sequence analogues. This is likely due to observational biases against the detection of hotter main-sequence post-mass-transfer objects. Combining our spectroscopic orbits with the HIPPARCOS astrometric data, we derive the orbital inclination and the mass of the WD companion for four systems. Since this cannot be done for all systems in our sample yet (but should be possible with upcoming Gaia data releases), we also analyse the mass-function distribution of our binaries. We can model this distribution with very narrow mass distributions for the two components and random orbital orientations on the sky. Finally, based on BINSTAR evolutionary models, we suggest that the orbital evolution of low-mass Ba systems can be affected by a second phase of interactions along the red giant branch of the Ba star, which impact the eccentricities and periods of the giants.

Barium and related stars, and their white-dwarf companions

Context. Barium and S stars without technetium are red giants and are suspected of being members of binary systems due to their overabundances in heavy elements. These elements are produced by thes-process of nucleosynthesis, despite the stars not being evolved enough to be able to activate thes-process in their interiors. A companion formerly on the asymptotic giant branch (now a white dwarf) is supposed to be responsible for the barium- and S-star enrichment ins-process elements through mass transfer.Aims. This paper provides both long-period and revised orbits for barium and S stars, adding to previously published orbits. The sample of barium stars with strong anomalies (i.e., those classified as Ba3, Ba4, or Ba5 in the Warner scale) comprises all known stars of that kind, and in that sense forms a complete sample that allows us to investigate several orbital properties of these post-mass-transfer binaries in an unbiased way.Methods. Orbital elements are derived from radial velocities collected from a long-term radial-velocity monitoring campaign performed with the HERMES spectrograph mounted on theMercator1.2 m telescope. These new measurements were combined with older, CORAVEL measurements. With the aim of investigating possible correlations between orbital properties and abundances, we also collected a set of abundances for barium stars with orbital elements that is as homogeneous as possible. When unavailable in the literature, abundances were derived from high-resolution HERMES spectra.Results. We find orbital motion for all barium and extrinsic S stars monitored (except for the mild barium star HD 95345). We obtain the longest period known so far for a spectroscopic binary involving an S star, namely 57 Peg with a period of the order of 100−500 yr. We present the mass distribution for the barium stars, which ranges from 1 to 3M⊙, with a tail extending up to 5M⊙in the case of mild barium stars. This high-mass tail is mostly comprised of high-metallicity objects ([Fe/H] ≥ −0.1). The distribution of the companion masses was extracted from the barium-star mass distribution combined with the finding thatQ≡f(MBa,MWD)/sin3i=MWD3/(MBa+MWD)2is peaked at 0.057 ± 0.009 and 0.036 ± 0.027M⊙for strong and mild barium stars, respectively (f(MBa, MWD) is the mass function obtained from the orbital elements of spectroscopic binaries with one observable spectrum). Mass functions are compatible with WD companions whose masses range from 0.5 to 1M⊙. Strong barium stars have a tendency to be found in systems with shorter periods than mild barium stars, although this correlation is rather lose, with metallicity and WD mass also playing a role. Using the initial–final mass relationship established for field WDs, we derived the distribution of the mass ratioq′=MAGB, ini/MBa(whereMAGB, iniis the WD progenitor initial mass, i.e., the mass of the former primary component of the system) which is a proxy for the initial mass ratio (the less mass the barium star has accreted, the better the proxy). It appears that the distribution ofq′ is highly nonuniform, and significantly different for mild and strong barium stars, the latter being characterized by values mostly in excess of 1.4, whereas mild barium stars occupy the range 1−1.4.Conclusions. The orbital properties presented in this paper pave the way for a comparison with binary-evolution and nucleosynthesis models, which should account for the various significant correlations found between abundances and dynamical parameters (e.g. betweenMBaon one hand andMWD, [Fe/H], and [s/Fe] on the other hand, betweenq′ and [s/Fe], betweenPande, and betweenPand [s/Fe] altogether).

Detection of White Dwarf companions to Blue Straggler Stars from UVIT observations of M67

AbstractWe investigate the old open cluster M67 using ultraviolet photometric data of Ultra-Violet Imaging Telescope in multi-filter far-UV bands. M67, well known for the presence of several blue straggler stars (BSS), has been put to detailed tests to understand their formation pathways. Currently, there are three accepted formation channels: mass transfer due to Roche-lobe overflow in binary systems, stellar mergers either due to dynamical collisions or through coalescence of close binaries. So far, there had not been any confirmed detection of a white dwarf (WD) companion to any of the BSSs in this cluster. Here, we present the detection of WD companions to 5 bright BSSs in M67. The multiwavelength spectral energy distributions covering 0.12 -11.5 μm range, were found to require binary spectral fits for 5 BSSs, consisting of a cool (BSS) and a hot companion. The parameters (Luminosity, Temperature, Radius and Mass) of the hot companions suggest them to be WDs with mass in the range 0.2 - 0.35 M⊙ with Teff ~11000 –24000 K.

Detection of a White Dwarf Companion to a Blue Straggler Star in the Outskirts of Globular Cluster NGC 5466 with the Ultraviolet Imaging Telescope (UVIT)

Abstract We report the discovery of a hot white dwarf (WD) companion to a blue straggler star (BSS) in the globular cluster (GC) NGC 5466, based on observations from the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat. The Spectral Energy Distribution (SED) of the Far-UV detected BSS NH 84 was constructed by combining the flux measurements from four filters of UVIT, with GALEX, GAIA, and other ground-based observations. The SED of NH 84 reveals the presence of a hot companion to the BSS. The temperature and radius of the BSS (T K, R/R ) derived from Gemini spectra and SED fitting using Kurucz atmospheric models are consistent with each other. The temperature and radius of the hotter companion of NH 84 (T K, R/R ) derived by fitting Koester WD models to the SED suggest that it is likely to be a hot WD. The radial velocity derived from the spectra along with the proper motion from GAIA DR2 confirms NH 84 to be a kinematic member of the cluster. This is the second detection of a BSS-WD candidate in a GC, and the first in the outskirts of a low-density GC. The location of this BSS in NGC 5466 along with its dynamical age supports the mass-transfer pathway for BSS formation in low-density environments.

Discovery of a Compact Companion to a Nearby Star

Abstract Radial velocity (RV) searches for exoplanets have surveyed many of the nearest and brightest stars for long-term velocity variations indicative of a companion body. Such surveys often detect high-amplitude velocity signatures of objects that lie outside the planetary mass regime, most commonly those of a low-mass star. Such stellar companions are frequently discarded as false-alarms to the main science goals of the survey, but high-resolution imaging techniques can be employed to either directly detect or place significant constraints on the nature of the companion object. Here, we present the discovery of a compact companion to the nearby star HD 118475. Our Anglo-Australian Telescope RV data allow the extraction of the full Keplerian orbit of the companion, which is found to have a minimum mass of 0.445 M ⊙. Follow-up speckle imaging observations at the predicted time of maximum angular separation rule out a main-sequence star as the source of the RV signature at the 3.3σ significance level, implying that the companion must be a low-luminosity compact object, most likely a white dwarf. We provide an isochrone analysis combined with our data that constrain the possible inclinations of the binary orbit. We discuss the eccentric orbit of the companion in the context of tidal circularization timescales and show that non-circular orbit was likely inherited from the progenitor. Finally, we emphasize the need for utilizing such an observation method to further understand the demographics of white dwarf companions around nearby stars.

Accurate mass and radius determinations of a cool subdwarf in an eclipsing binary.

Cool subdwarfs are metal-poor low-mass stars that formed during the early stages of the evolution of our Galaxy. Because they are relatively rare in the vicinity of the Sun, we know of few cool subdwarfs in the solar neighbourhood, and none with both the mass and the radius accurately determined. This hampers our understanding of stars at the low-mass end of the main-sequence. Here we report the discovery of SDSS J235524.29+044855.7 as an eclipsing binary containing a cool subdwarf star, with a white dwarf companion. From the light-curve and the radial-velocity curve of the binary we determine the mass and the radius of the cool subdwarf and we derive its effective temperature and luminosity by analysing its spectral energy distribution. Our results validate the theoretical mass-radius-effective temperature-luminosity relations for low-mass low-metallicity stars.

Spectral analysis of the barium central star of the planetary nebula Hen 2−39

Context. Barium stars are peculiar red giants characterized by an overabundance of the elements synthesized in the slow neutron-capture nucleosynthesis (s-process elements) along with an enrichment in carbon. These stars are discovered in binaries with white dwarf companions. The more recently formed of these stars are still surrounded by a planetary nebula. Aims. Precise abundance determinations of the various s-process elements, of further key elements that act as indicators for effectiveness of nucleosynthesis on the asymptotic giant branch and, especially, of the lightest, short-lived radionuclide technetium will establish constraints for the formation of s-process elements in asymptotic giant branch stars as well as mass transfer through, for example, stellar wind, Roche-lobe overflow, and common-envelope evolution. Methods. We performed a detailed spectral analysis of the K-type subgiant central star of the planetary nebula Hen 2−39 based on high-resolution optical spectra obtained with the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope using local thermodynamic equilibrium model atmospheres. Results. We confirm the effective temperature of Teff = (4350 ± 150) K for the central star of the planetary nebula Hen 2−39. It has a photospheric carbon enrichment of [C∕H] = 0.36 ± 0.08 and a barium overabundance of [Ba∕Fe] = 1.8 ± 0.5. We find a deficiency for most of the iron-group elements (calcium to iron) and establish an upper abundance limit for technetium (log ɛTc < 2.5). Conclusions. The quality of the available optical spectra is not sufficient to measure abundances of all s-process elements accurately. Despite large uncertainties on the abundances as well as on the model yields, the derived abundances are most consistent with a progenitor mass in the range 1.75–3.00 M⊙ and a metallicity of [Fe∕H] = −0.3 ± 1.0. This result leads to the conclusion that the formation of such systems requires a relatively large mass transfer that is most easily obtained via wind-Roche lobe overflow.

The ultracool helium-atmosphere white dwarf companion of PSR J0740+6620?

ABSTRACT We report detection of the likely companion of the binary millisecond pulsar (MSP) J0740+6620 with the Gran Telescopio Canarias in the r′ and i′ bands. The position of the detected starlike source coincides with the pulsar coordinates within the 1σ uncertainty of ≈0.2 arcsec. Its magnitudes are r′ = 26.51 ± 0.17 and i′ = 25.49 ± 0.15. Comparing the data with the white dwarf (WD) cooling tracks suggests that it can be an ultracool helium-atmosphere WD with the temperature ≲3500 K and cooling age ≳5 Gyr. The age is consistent with the pulsar characteristic age corrected for kinematic effects. This is the reddest source among known WD companions of MSPs. Detection of the source in other bands would be useful to clarify its properties and nature.

PSR J2234+0611: A New Laboratory for Stellar Evolution

Abstract We report the timing results for PSR J2234+0611, a 3.6 ms pulsar in a 32 day, eccentric (e = 0.13) orbit with a helium white dwarf. The precise timing and eccentric nature of the orbit allow measurements of an unusual number of parameters: (a) a precise proper motion of 27.10(3) mas yr−1 and a parallax of 1.05(4) mas resulting in a pulsar distance of 0.95(4) kpc; enabling an estimate of the transverse velocity, 123(5) km s−1. Together with previously published spectroscopic measurements of the systemic radial velocity, this allows a 3D determination of the system’s velocity; (b) precise measurements of the rate of advance of periastron yields a total system mass of M ⊙; (c) a Shapiro delay measurement, h 3 = 82 ± 14 ns, despite the orbital inclination not being near 90°; combined with the measurement of the total mass yields a pulsar mass of and a companion mass of (d) we measure precisely the secular variation of the projected semimajor axis and detect a significant annual orbital parallax; together these allow a determination of the 3D orbital geometry of the system, including an unambiguous orbital inclination ( ) and a position angle for the line of nodes ( ). We discuss the component masses to investigate the hypotheses previously advanced to explain the origin of eccentric MSPs. The unprecedented determination of the 3D position, motion, and orbital orientation of the system, plus the precise pulsar and WD masses and the latter’s optical detection make this system a unique test of our understanding of white dwarfs and their atmospheres.

The post-common-envelope X-ray binary nucleus of the planetary nebula NGC 2392

Abstract The Chandra X-ray Observatory has detected relatively hard X-ray emission from the central stars of several planetary nebulae (PNe). A subset has no known late-type companions, making it very difficult to isolate which of several competing mechanisms may be producing the X-ray emission. The central star of NGC 2392 is one of the most vexing members, with substantial indirect evidence for a hot white dwarf (WD) companion. Here we report on the results of a radial velocity (RV) monitoring campaign of its central star with the HERMES échelle spectrograph of the Flemish 1.2 m Mercator telescope. We discover a single-lined spectroscopic binary with an orbital period of 1.902208 ± 0.000013 d and an RV semi-amplitude of 9.96 ± 0.13 km s−1. The high degree of nebula ionisation requires a WD companion (M ≳ 0.6M⊙), which the mass-function supports at orbital inclinations ≲ 7°, in agreement with the nebula orientation of 9°. The hard component of the X-ray spectrum may be explained by the companion accreting mass from the wind of the Roche lobe filling primary, while the softer component may be due to colliding winds. A companion with a stronger wind than the primary could produce the latter and would be consistent with models of the observed diffuse X-ray emission detected in the nebula. The diffuse X-rays may also be powered by the jets of up to 180 km s−1, and active accretion would imply that they may be the first active jets of a post-common-envelope PN, potentially making NGC 2392 an invaluable laboratory to study jet formation physics. The 1.9 d orbital period rules out a double-degenerate merger leading to a Type Ia supernova, and the weak wind of the primary likely also precludes a single-degenerate scenario. We suggest that a hard X-ray spectrum, in the absence of a late-type companion, could be a powerful tool to identify accreting WD companions.

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

The recent discovery of long-period eccentric binaries hosting a He-white dwarf has been a challenge for binary-star modelling. 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 in order to avoid mass transfer by Roche-lobe overflow, where tides systematically circularize the orbit.

Precise Ages of Field Stars from White Dwarf Companions

Abstract Observational tests of stellar and Galactic chemical evolution call for the joint knowledge of a star’s physical parameters, detailed element abundances, and precise age. For cool main-sequence (MS) stars the abundances of many elements can be measured from spectroscopy, but ages are very hard to determine. The situation is different if the MS star has a white dwarf (WD) companion and a known distance, as the age of such a binary system can then be determined precisely from the photometric properties of the cooling WD. As a pilot study for obtaining precise age determinations of field MS stars, we identify nearly 100 candidates for such wide binary systems: a faint WD whose GPS1 proper motion matches that of a brighter MS star in Gaia/TGAS with a good parallax (σ ϖ /ϖ ≤ 0.05). We model the WD’s multi-band photometry with the BASE-9 code using this precise distance (assumed to be common for the pair) and infer ages for each binary system. The resulting age estimates are precise to ≤10% (≤20%) for 42 (67) MS–WD systems. Our analysis more than doubles the number of MS–WD systems with precise distances known to date, and it boosts the number of such systems with precise age determination by an order of magnitude. With the advent of the Gaia DR2 data, this approach will be applicable to a far larger sample, providing ages for many MS stars (that can yield detailed abundances for over 20 elements), especially in the age range of 2–8 , where there are only few known star clusters.

Observations of Spin-down in Post-mass-transfer Stars and the Possibility for Blue Straggler Gyrochronology

Abstract Blue stragglers and other mass transfer/collision products are likely born with rapid rotation rates due to angular momentum transfer during mass-transfer, merger, or collisional formation. However, less is known about the angular momentum evolution of these stars as they age. Here we compare rotation rates and post-formation ages of mass-transfer products to models of angular momentum evolution for normal main-sequence (MS) stars and collisionally formed blue stragglers. In our sample, we include both F- and G-type blue stragglers in the cluster NGC 188 and post-mass-transfer GK MS stars in the field, all binaries with white dwarf (WD) companions. We compare ages derived from WD cooling models to photometric rotation periods and/or spectral v sin i measurements. We demonstrate that these systems have rapid rotation rates soon after formation. They then spin down as they age, much like standard solar-type MS stars do. We discuss the physical implications of this result, which suggests that the spin-down of post-mass transfer stars can be described by standard magnetic-braking prescriptions. This opens up the possibility of using gyrochronology as a method to determine the time since formation of blue straggler stars and other post-mass-transfer binaries.

Three-dimensional Simulation of Double Detonations in the Double-degenerate Model for Type Ia Supernovae and Interaction of Ejecta with a Surviving White Dwarf Companion

Abstract We study the hydrodynamics and nucleosynthesis in the double-detonation model of Type Ia supernovae (SNe Ia) and the interaction between the ejecta and a surviving white dwarf (WD) companion in the double-degenerate scenario. We set up a binary star system with 1.0 and 0.6 M ⊙ carbon–oxygen (CO) WDs, where the primary WD consists of a CO core and helium (He) shell with 0.95 and 0.05 M ⊙, respectively. We follow the evolution of the binary star system from the initiation of an He detonation, ignition and propagation of a CO detonation, and the interaction of SN ejecta with the companion WD. The companion (or surviving) WD gets a flung-away velocity of ∼1700 km s−1 and captures 56Ni of ∼0.03 M ⊙ and He of . Such He can be detected on the surface of surviving WDs. The SN ejecta contains a “companion-origin stream” and unburned materials stripped from the companion WD ( ), although the stream compositions would depend on the He shell mass of the companion WD. The ejecta has also a velocity shift of ∼1000 km s−1 due to the binary motion of the exploding primary WD. These features would be prominent in nebular-phase spectra of oxygen emission lines from the unburned materials like SN 2010lp and iPTF14atg and of blue- or redshifted Fe-group emission lines from the velocity shift like a part of subluminous SNe Ia. We expect that SN Ia counterparts to the D6 model would leave these fingerprints for SN Ia observations.

Environmental effects on the ionisation of brown dwarf atmospheres

Context. Brown dwarfs emit bursts of Hα, white-light flares, and show radio flares and quiescent radio emission. They are suggested to form aurorae, similar to planets in the solar system, but much more energetic. All these processes require a source gas with an appropriate degree of ionisation, which, so far, is mostly postulated to be sufficient. Aims. We aim to demonstrate that the Galactic environment influences atmospheric ionisation, and that it hence amplifies or enables the magnetic coupling of the atmospheres of ultra-cool objects, like brown dwarfs and free-floating planets. Methods. We build on our previous work on thermal ionisation of ultra-cool atmospheres and explore the effect of environmental high-energy radiation on the degree of ionisation in the atmosphere. We consider the effect of photoionisation by Lyman-continuum radiation in three different environments: in the interstellar radiation field (ISRF), O and B stars in star-forming regions, and in white dwarf companions in binary systems. We apply our Monte Carlo radiation transfer code to investigate the effect of Lyman-continuum photoionisation for prescribed atmosphere structures for very low-mass objects. Results. The external radiation environment plays an important role for the atmospheric ionisation of very low-mass, ultra-cool objects. Lyman-continuum irradiation greatly increases the level of ionisation in the uppermost atmospheric regions. Our results suggest that a shell of an almost fully ionised atmospheric gas emerges for brown dwarfs in star-forming regions and brown dwarfs in white dwarf binary systems. As a consequence, brown dwarf atmospheres can be magnetically coupled, which is the presumption for chromospheric heating to occur and for aurorae to emerge. First tests for assumed chromosphere-like temperature values suggest that the resulting free-free X-ray luminosities are comparable with those observed from non-accreting brown dwarfs in star-forming regions.

Formation of hot subdwarf B stars with neutron star components

Context. Binary population synthesis predicts the existence of subdwarf B stars (sdBs) with neutron star (NS) or black hole (BH) companions. Several works have been dedicated to finding such systems, but none has been confirmed yet. Theoretically, the formation of sdBs with white dwarf (WD) and main sequence (MS) companions has been well investigated, while those with NS or BH companions remain to be explored further. Aims. We systematically investigate the formation of sdB+NS binaries from binary evolution and aim to obtain some clues for a search for such systems. Methods. We started from a series of MS+NS systems and determined the parameter spaces for producing sdB+NS binaries from the stable Roche-lobe overflow (RLOF) channel and from the common envelope (CE) ejection channel. The parameters for sdB+NS binaries were obtained from detailed binary evolution calculation with the code called modules for experiments in stellar astrophysics (MESA), and the CE parameters were given by the standard energy budget for CE evolution. The MS star had an initial mass ranging from 0.8 to 5 M⊙. Various NS accretion efficiencies and NS masses were examined to investigate the effects they have. We show the characteristics of the produced sdB+NS systems, such as the mass of components, orbital period, the semi-amplitude of the radial velocity (K), and the spin of the NS component. Results. sdB+NS binaries can be produced either from stable RLOF or from CE ejection. In the stable RLOF channel, sdBs can be formed when the donor starts mass transfer close to the tip of the giant branch if the donor has an initial mass ≤2.0 M⊙. For more massive donors, sdBs can be formed when the donor starts mass transfer during the Hertzsprung gap or near the end of the MS. The orbital period of sdB+NS binaries produced in this way ranges from several days to more than 1000 days and moves toward the short-period (∼hr) side with increasing initial MS mass. The highest K is about 150 km s−1 for an MS star of initially 5 M⊙. However, the sdB+NS systems that result from CE ejection have very short orbital periods and then high values of K (up to 800 km s−1). Such systems are born in very young populations (younger than 0.3 Gyr) and are potential gravitational wave sources that might be resolved by the Laser Interferometer Space Antenna (LISA) in the future. Gravitational wave radiation may again bring them into contact on a timescale of only ∼Myr. As a consequence, they are rare and hard to discover. The pulsar signal is likely a feature of sdB+NS systems caused by stable RLOF, and some NS components in sdB binaries may be millisecond pulsars. Various NS accretion efficiencies and NS masses change some properties of sdB+NS binaries, but not our general results.