Period Porb Research Articles

From Be X-Ray Binaries to Double Neutron Stars: Exploring the Spin and Orbital Evolution

We explore the evolutionary link between Galactic Be X-ray binaries (BeXBs) and Galactic field double neutron stars (DNSs) based on the properties of the NS spin period—P and binary orbital period—Porb. First, both BeXB and DNS sources show positive correlation trends in P versus Porb relation (P−Porb diagram), which may relate to the influence of the accretion evolution. Secondly, the two types of sources exhibit similar bi-modal P/Porb distributions with the gaps of P∼40 s/Porb∼60 days for BeXBs and P∼50 ms/Porb∼1 day for DNSs, respectively. We propose a possibility that Galactic BeXBs may transfer the bi-modal P/Porb classifications to Galactic field DNSs during the evolution process. Furthermore, based on the bi-modal gaps, we infer the contraction factors of P (by ∼1800) and Porb (by ∼160) valuesin the evolution from Galactic BeXBs to Galactic field DNSs. We find that the scaled P or Porb values of Galactic BeXBs share the similar bi-modal distributions to that of Galactic field DNSs, and these results can offer a reference to trace the classification and evolution between the two types of sources.

VELOcities of CEpheids (VELOCE)

Classical Cepheids provide valuable insights into the evolution of stellar multiplicity among intermediate-mass stars. Here, we present a systematic investigation of single-lined spectroscopic binaries (SB1s) based on high-precision velocities measured by the VELOcities of CEpheids (VELOCE) project. We detected 76 (29%) SB1 systems among the 258 Milky Way Cepheids in the first VELOCE data release, 32 (43%) of which were not previously known to be SB1 systems. We determined 30 precise and three tentative orbital solutions, 18 (53%) of which are reported for the first time. This large set of Cepheid orbits provides a detailed view of the eccentricity e and orbital period Porb distribution among evolved intermediate-mass stars, ranging from e ∈ [0.0, 0.8] and Porb ∈ [240, 9000] d. The orbital motion on timescales exceeding the 11 yr VELOCE baseline was investigated using a template-fitting technique applied to literature data. Particularly interesting objects include (a) R Cru, the Cepheid with the shortest orbital period in the Milky Way (∼238 d); (b) ASAS J103158−5814.7, a short-period overtone Cepheid exhibiting time-dependent pulsation amplitudes as well as orbital motion; and (c) 17 triple systems with outer visual companions, among other interesting objects. Most VELOCE Cepheids (21/23) that exhibit evidence of a companion based on a Gaia proper motion anomaly are also spectroscopic binaries, whereas the remaining do not exhibit significant (> 3σ) orbital radial velocity variations. Gaia quality flags, notably the renormalized unit weight error (RUWE), do not allow Cepheid binaries to be identified reliably although statistically the average RUWE of SB1 Cepheids is slightly higher than that of non-SB1 Cepheids. A comparison with Gaia photometric amplitudes in G-, Bp, and Rp also does not allow one to identify spectroscopic binaries among the full VELOCE sample, indicating that the photometric amplitudes in this wavelength range are not sufficiently informative of companion stars.

The Orbit of NGC 5907 ULX-1

We report on the orbit of the binary system powering the most extreme ultraluminous X-ray pulsar known to date: NGC 5907 ULX-1 (hereafter ULX1). ULX1 has been the target of a substantial multi-instrument campaign, mainly in the X-ray band, but no clear counterparts are known in other bands. Although ULX1 is highly variable and pulsations can be transient (regardless of the source flux), the timing data collected so far allow us to investigate the orbit of this system. We find an orbital period Porb=5.7−0.6+0.1days and a projected semi-axis A1=3.1−0.9+0.8lt–s . The most likely ephemeris is P orb = 5.6585(6) days, A 1 = 3.1(4) lt-s, and the epoch of ascending nodes passage is T asc = 57751.37(5) MJD. However, there are six similar solutions acceptable within 3σ. We find further indications that ULX1 is a high-mass X-ray binary. This implies that we are observing its orbit face on, with an inclination <5°.

Stellar spectral-type (mass) dependence of the dearth of close-in planets around fast-rotating stars

In 2013 a dearth of close-in planets around fast-rotating host stars was found using statistical tests on Kepler data. The addition of more Kepler and Transiting Exoplanet Survey Satellite (TESS) systems in 2022 filled this region of the diagram of stellar rotation period (Prot) versus the planet orbital period (Porb). We revisited the Prot extraction of Kepler planet-host stars, we classify the stars by their spectral type, and we studied their Prot–Porb relations. We only used confirmed exoplanet systems to minimize biases. In order to learn about the physical processes at work, we used the star-planet evolution code ESPEM (French acronym for Evolution of Planetary Systems and Magnetism) to compute a realistic population synthesis of exoplanet systems and compared them with observations. Because ESPEM works with a single planet orbiting around a single main-sequence star, we limit our study to this population of Kepler observed systems filtering out binaries, evolved stars, and multi-planets. We find in both, observations and simulations, the existence of a dearth in close-in planets orbiting around fast-rotating stars, with a dependence on the stellar spectral type (F, G, and K), which is a proxy of the mass in our sample of stars. There is a change in the edge of the dearth as a function of the spectral type (and mass). It moves towards shorter Prot as temperature (and mass) increases, making the dearth look smaller. Realistic formation hypotheses included in the model and the proper treatment of tidal and magnetic migration are enough to qualitatively explain the dearth of hot planets around fast-rotating stars and the uncovered trend with spectral type.

Nature of the Eclipsing Polar 1RXS J184542.4+483134

We have carried out a comprehensive study of the poorly investigated eclipsing polar1RXS J184542.4+483134 with a short orbital period Porb ≈ 79 min. An analysis of its long-term lightcurves points to a change in the position and sizes of the accretion spot as the accretion rate changes.Narrow and broad components, which are probably formed on the ballistic segment of the accretion streamand on the magnetic trajectory, respectively, are identified in the emission line profiles. An inversion ofthe line profiles from emission to absorption due to the obscuration of the accretion spot by the accretionstream is observed. Based on the eclipse duration and the radial velocities of the narrow line component,we impose constraints on the white dwarf mass, 0.49 ≤ M1/M ≤ 0.89, and the orbital inclination,79.7◦ ≤ i ≤ 84.3◦. An analysis of the cyclotron spectra points to the presence of two accretion spotswith magnetic field strengths B1 = 28.4+0.1−0.2 MG and B2 = 30 − 36 MG. The main spot has a complexstructure that apparently has a dense core and a less dense periphery emitting a spectrum with cyclotronharmonics. Polarization observations reveal a circular polarization sign reversal during the orbital periodand an anticorrelation of the polarization with the brightness of the polar. Our modeling of polarizationobservations using the simple model of an accreting white dwarf shows that the polarization propertiescan be interpreted in terms of two-pole accretion with different optical depths of the accretion spots(τ1/τ2 ∼ 10). An analysis of the Swift/XRT observations points to a predominance of bremsstrahlungin the X-ray radiation from the system.

Structure of the accretion flow of IX Velorum as revealed by high-resolution spectroscopy

Context. Several high mass-transfer cataclysmic variables show evidence of outflow from the system, which could play an important role in their evolution. We investigate the system IX Vel, which was proposed to show similar characteristics. Aims. We study the structure of the IX Vel system, particularly the structure of its accretion flow and accretion disc. Methods. We used high-resolution time-resolved spectroscopy to construct radial velocity curves of the components in IX Vel. We computed Doppler maps of the system, which we used to estimate the temperature distribution maps. Results. We have improved the spectroscopic ephemeris of the system and its orbital period Porb = 0.19392793(3) d. We constructed Doppler maps of the system based on hydrogen and helium emission lines and the Bowen blend. The maps show features corresponding to the irradiated face of the secondary star, the outer rim of the accretion disc, and low-velocity components located outside the accretion disc and reaching towards L3. We constructed a temperature distribution map of the system using the Doppler maps of Balmer lines. Apart from the features found in the Doppler maps, the temperature distribution map shows a region of high temperature in the accretion disc connecting the expected position of a bright spot and the inner parts of the disc. Conclusions. We interpret the low-velocity emission found in the Doppler map as emission originating in the accretion disc wind and in an outflow region located in the vicinity of the third Lagrangian point L3. This makes IX Vel a member of the RW Sex class of cataclysmic variables.

Binary neutron star populations in the Milky Way

ABSTRACT Galactic binary neutron stars (BNSs) are a unique laboratory to probe the evolution of BNSs and their progenitors. Here, we use a new version of the population synthesis code sevn to evolve the population of Galactic BNSs, by modelling the spin up and down of pulsars self-consistently. We analyse the merger rate $\mathcal {R}_{\rm MW}$, orbital period Porb, eccentricity e, spin period P, and spin period derivative $\dot{P}$ of the BNS population. Values of the common envelope parameter α = 1−3 and an accurate model of the Milky Way star formation history best reproduce the BNS merger rate in our Galaxy ($\mathcal {R}_{\rm MW}\approx {}30$ Myr−1). We apply radio-selection effects to our simulated BNSs and compare them to the observed population. Using a Dirichlet process Gaussian mixture method, we evaluate the four-dimensional likelihood in the $(P_{\rm orb}, e, P, \dot{P})$ space, by comparing our radio-selected simulated pulsars against Galactic BNSs. Our analysis favours an uniform initial distribution for both the magnetic field (1010−13 G) and the spin period (10−100 ms). The implementation of radio selection effects is critical to match not only the spin period and period derivative, but also the orbital period and eccentricity of Galactic BNSs. According to our fiducial model, the Square Kilometre Array will detect ∼20 new BNSs in the Milky Way.

The white dwarf mass–orbital period relation under wind mass-loss

ABSTRACT Helium white dwarfs (HeWDs) are thought to form from low-mass red giant stars experiencing binary interaction. Because the helium core mass of a red giant star is closely related to the stellar radius, there exists well-known relation between the orbital period (Porb) and the mass (MWD) of the HeWDs, which is almost independent of the type of the companion star. Traditional derivation of the MWD–Porb relation generally neglected the effect of wind mass-loss from the red giants, while observations show that wind mass-loss from red giants in binary systems is systematically higher than that from isolated stars. In this work, we calculate binary evolution with tidally enhanced stellar wind (TEW) and find that it causes significantly scatter of the traditional MWD–Porb relation. The TEW can prevent the red giants from overflowing their Roche lobes and slow down the growth of the helium core, leaving a lower mass HeWD for given orbital period. This scenario may account for some of the HeWD binaries that deviate from the traditional MWD–Porb relation. However, we point out that observations of more HeWD binaries in wide orbits are needed to test the TEW model and to constrain the enhanced wind factor.

SDSS J085414.02+390537.3—A New Asynchronous Polar

Based on data from the ZTF photometric survey, we have revealed asynchrony of the polar SDSS J085414.02+390537.3. A beat period Pbead=24,6+0,1 days, during which the system changes its brightness by =3m, is distinguished in the light curves. Power peaks at the white-dwarf rotation period Pspin=113.197+0,001 min and orbital period Porb=113,56+0,001 min are revealed in the periodograms, with the corresponding polar asynchrony being 1-Porb/Pspin=0,3%. The photometric behavior of the polar points to a change of the main accreting pole during the beat period. Based on the Zeeman splitting of the Hb line, we have estimated the mean magnetic field strength of the white dwarf to be B=28,5+1,5 MG. The magnetic field strength near the magnetic pole has been found by modeling the cyclotron spectra to be B=34+2 MG. The Doppler tomograms in the Hb line exhibit a distribution of emission sources typical for polars in velocity space with evidence of the transition of the accretion stream from the ballistic trajectory to the magnetic one.

Evidence for mass-dependent peculiar velocities in compact object binaries: towards better constraints on natal kicks

ABSTRACT We compile a catalogue of low-mass and high-mass X-ray binaries, some recently reported binaries that likely host a neutron star (NS) or a black hole (BH), and binary pulsars (a pulsar and a non-degenerated companion) that have measured systemic radial velocities (γ). Using Gaia and radio proper motions together with γ, we integrate their Galactic orbits and infer their post-supernova (post-SN) 3D peculiar velocities ($v_\mathrm{pec }^{z=0}$ at Galactic plane crossing); these velocities bear imprints of the natal kicks that compact objects received at birth. With the sample totalling 85 objects, we model the overall distribution of $v_\mathrm{pec }^{z=0}$ and find a two-component Maxwellian distribution with a low- ($\sigma _v \approx 21\, \mathrm{km s^{-1}}$) and a high-velocity ($\sigma _v \approx 107\, \mathrm{km s^{-1}}$) component. A further comparison between distributions of binary subgroups suggests that binaries hosting high-mass donors/luminous companions mostly have $v_\mathrm{pec }^{z=0}\lesssim 100\, \mathrm{km s^{-1}}$, while binaries with low-mass companions exhibit a broader distribution that extends up to $\sim 400\, \mathrm{km s^{-1}}$. We also find significant anticorrelations of $v_\mathrm{pec }^{z=0}$ with binary total mass (Mtot) and orbital period (Porb), at over 99 per cent confidence. Specifically, our fit suggests $v_\mathrm{pec }^{z=0}\propto M_\mathrm{tot}^{-0.5}$ and $v_\mathrm{pec }^{z=0}\propto P_\mathrm{orb}^{-0.2}$. Discussions are presented on possible interpretation of the correlations in the context of kinematics and possible biases. The sample should enable a range of follow-up studies on compact object binary kinematics and evolution.

Discovery of delta Scuti variables in eclipsing binary systems II. Southern TESS field search

ABSTRACT The presence of pulsating stars in eclipsing binary systems (EBs) makes these objects significant since they allow us to investigate the stellar interior structure and evolution. Different types of pulsating stars could be found in EBs, such as δ Scuti variables. δ Scuti stars in EBs have been known for decades, and the increasing number of such systems is important for understanding pulsational structure. Hence, in this study, research was carried out on the southern TESS field to discover new δ Scuti stars in EBs. We produced an algorithm to search for detached and semidetached EBs considering three steps: the orbital period (Porb)’s harmonics in the Fourier spectrum, skewness of the light curves, and classification of upsilon program. If two of these steps classify a system as an EB, the algorithm also identifies it as an EB. The TESS pixel files of targets were also analysed to see whether the fluxes were contaminated by other systems. No contamination was found. We researched the existence of pulsation through EBs with a visual inspection. To confirm δ Scuti-type oscillations, the binary variation was removed from the light curve, and residuals were analysed. Consequently, we identified 42 δ Scuti candidates in EBs. The Porb, L, and MV of systems were calculated. Their positions on the H–R diagram and the known orbital-pulsation period relationship were analysed. We also examined our targets to see if any of them showed frequency modulation with the orbital period and discovered one candidate of tidally tilted pulsators.

Post-dynamical inspiral phase of common envelope evolution

After the companion dynamically plunges through the primary’s envelope, the two cores remain surrounded by a common envelope and the decrease of the orbital period Porb stalls. The subsequent evolution has never been systematically explored with multidimensional simulations. For this study, we performed 3D hydrodynamical simulations of an envelope evolving under the influence of a central binary star using an adaptively refined spherical grid. We followed the evolution over hundreds of orbits of the central binary to characterize the transport of angular momentum by advection, gravitational torques, turbulence, and viscosity. We find that local advective torques from the mean flow and Reynolds stresses associated with the turbulent flow dominate the angular momentum transport, which occurs outward in a disk-like structure about the orbital plane and inward along the polar axis. Turbulent transport is less efficient, but can locally significantly damp or enhance the net angular momentum radial transport and may even reverse its direction. Short-term variability in the envelope is remarkably similar to circumbinary disks, including the formation and destruction of lump-like overdensities, which enhance mass accretion and contribute to the outward transport of eccentricity generated in the vicinity of the binary. If the accretion onto the binary is allowed, the orbital decay timescale settles to a nearly constant value τb ∼ 103 to 104 Porb, while preventing accretion leads to a slowly increasing τb ∼ 105 Porb at the end of our simulations. Our results suggest that the post-dynamical orbital contraction and envelope ejection will slowly continue while the binary is surrounded by gas and that τb is often much shorter than the thermal timescale of the envelope.

Magnetic braking saturates: evidence from the orbital period distribution of low-mass detached eclipsing binaries from ZTF

ABSTRACT We constrain the orbital period (Porb) distribution of low-mass detached main-sequence eclipsing binaries (EBs) with light-curves from the Zwicky Transient Facility (ZTF), which provides a well-understood selection function and sensitivity to faint stars. At short periods (Porb ≲ 2 d), binaries are predicted to evolve significantly due to magnetic braking (MB), which shrinks orbits and ultimately brings detached binaries into contact. The period distribution is thus a sensitive probe of MB. We find that the intrinsic period distribution of low-mass (0.1 ≲ M1/M⊙ &amp;lt; 0.9) binaries is basically flat (${\rm d}N/{\rm d}P_{\rm orb} \propto P_{\rm orb}^0$) from Porb = 10 d down to the contact limit. This is strongly inconsistent with predictions of classical MB models based on the Skumanich relation, which are widely used in binary evolution calculations and predict ${\rm d}N/{\rm d}P_{\rm orb} \propto P_{\rm orb}^{7/3}$ at short periods. The observed distributions are best reproduced by models in which the magnetic field saturates at short periods with a MB torque that scales roughly as $\dot{J}\propto P_{\rm orb}^{-1}$, as opposed to $\dot{J} \propto P_{\rm orb}^{-3}$ in the standard Skumanich law. We also find no significant difference between the period distributions of binaries containing fully and partially convective stars. Our results confirm that a saturated MB law, which was previously found to describe the spin-down of rapidly rotating isolated M dwarfs, also operates in tidally locked binaries. We advocate using saturated MB models in binary evolution calculations. Our work supports previous suggestions that MB in cataclysmic variables (CVs) is much weaker than assumed in the standard evolutionary model, unless mass transfer leads to significant additional angular momentum loss in CVs.

On the peculiar long-term orbital evolution of the eclipsing accreting millisecond X-ray pulsar SWIFT J1749.4 − 2807

ABSTRACT We present the pulsar timing analysis of the accreting millisecond X-ray pulsar SWIFT J1749.4 − 2807 monitored by NICER and XMM–Newton during its latest outburst after almost 11 yr of quiescence. From the coherent timing analysis of the pulse profiles, we updated the orbital ephemerides of the system. Large phase jumps of the fundamental frequency phase of the signal are visible during the outburst, consistent with what was observed during the previous outburst. Moreover, we report on the marginally significant evidence for non-zero eccentricity (e ≃ 4 × 10−5) obtained independently from the analysis of both the 2021 and 2010 outbursts and we discuss possible compatible scenarios. Long-term orbital evolution of SWIFT J1749.4 − 2807 suggests a fast expansion of both the NS projected semimajor axis (x), and the orbital period (Porb), at a rate of $\dot{x}\simeq 2.6\times 10^{-13}\, \text{lt-s}\, \text{s}^{-1}$ and $\dot{P}_{\rm orb}\simeq 4 \times 10^{-10}\, \text{s}\, \text{s}^{-1}$, respectively. SWIFT J1749.4 − 2807 is the only accreting millisecond X-ray pulsar, so far, from which the orbital period derivative has been directly measured from appreciable changes on the observed orbital period. Finally, no significant secular deceleration of the spin frequency of the compact object is detected, which allowed us to set a constraint on the magnetic field strength at the polar caps of BPC &amp;lt; 1.3 × 108 G, in line with typical values reported for AMXPs.

Activity of the post-nova V1363 Cygni on long timescales

Abstract V1363 Cyg is a cataclysmic variable (CV) and a post-nova. Our analysis of its long-term optical activity used the archival data from the AAVSO database and literature. We showed that the accretion disk of V1363 Cyg is exposed to the thermal-viscous instability (TVI) for at least part of the time. The time fraction spent in the high state or the outbursts dramatically changed on the timescale of decades. The highly variable brightness of V1363 Cyg displayed several episodes of a strong brightening (bumps in the light curve) from a cool disk in the TVI zone. This can be interpreted to mean their vastly discrepant decay rates show that only some of these bumps can be attributed to the dwarf nova outbursts without strong irradiation of the disk by the hot white dwarf. The Bailey relation of the decay rate, if ascribed to a DN outburst of V1363 Cyg, speaks in favor of its orbital period Porb being very long for a CV, about 20–40 h. A dominant cycle length of about 435 d was present in the brightness changes all the time, even when the disk was well inside the TVI zone. We interpret it as modulation of the companion’s mass outflow by differential rotation of the active region(s).

Birth of the ELMs: a ZTF survey for evolved cataclysmic variables turning into extremely low-mass white dwarfs

ABSTRACT We present a systematic survey for mass-transferring and recently detached cataclysmic variables (CVs) with evolved secondaries, which are progenitors of extremely low mass white dwarfs (ELM WDs), AM CVn systems, and detached ultracompact binaries. We select targets below the main sequence in the Gaia colour–magnitude diagram with ZTF light curves showing large-amplitude ellipsoidal variability and orbital period Porb &amp;lt; 6 h. This yields 51 candidates brighter than G = 18, of which we have obtained many-epoch spectra for 21. We confirm all 21 to be completely or nearly Roche lobe filling close binaries. Thirteen show evidence of ongoing mass transfer, which has likely just ceased in the other eight. Most of the secondaries are hotter than any previously known CV donors, with temperatures 4700 &amp;lt; Teff/K &amp;lt; 8000. Remarkably, all secondaries with $T_{\rm eff} \gtrsim 7000\, \rm K$ appear to be detached, while all cooler secondaries are still mass-transferring. This transition likely marks the temperature where magnetic braking becomes inefficient due to loss of the donor’s convective envelope. Most of the proto-WD secondaries have masses near $0.15\, \rm M_{\odot }$; their companions have masses near $0.8\, \rm M_{\odot }$. We infer a space density of $\sim 60\, \rm kpc^{-3}$, roughly 80 times lower than that of normal CVs and three times lower than that of ELM WDs. The implied Galactic birth rate, $\mathcal {R}\sim 60\, \rm Myr^{-1}$, is half that of AM CVn binaries. Most systems are well-described by mesa models for CVs in which mass transfer begins only as the donor leaves the main sequence. All are predicted to reach minimum periods 5 ≲ Porb min−1 ≲ 30 within a Hubble time, where they will become AM CVn binaries or merge. This sample triples the known evolved CV population and offers broad opportunities for improving understanding of the compact binary population.

Multiwavelength observations reveal a faint candidate black hole X-ray binary in IGR J17285−2922

ABSTRACT IGR J17285−2922 is a known X-ray binary with a low peak 2–10 keV X-ray luminosity of ∼ 1036 erg s−1 during outburst. IGR J17285−2922 exhibited two outbursts in 2003 and 2010 and went into outburst again in 2019. We have monitored this ∼ 4-month long 2019 outburst with Swift in X-ray and the Very Large Array in radio. We have also obtained four optical spectra with the Gran Telescopio Canarias and Southern Astrophysical Research Telescope, three optical photometry measurements with the Las Cumbres Observatory, and one near-infrared spectrum with the Gemini South telescope. The ratio between its X-ray and radio luminosity is consistent with both samples of neutron star and black hole (BH) X-ray binaries, while the ratio between the X-ray and optical luminosity is consistent with BH X-ray binaries. Studying the evolution of its X-ray power-law index throughout the outburst, we find additional evidence for a BH as compact object. The four optical spectra show no H α emission and the nIR spectrum shows no Brγ emission, suggesting that the donor star could be hydrogen-poor and hence that IGR J17285−2922 might have an ultracompact binary orbit. The shape of the X-ray light curve is well described by an exponential, followed by a linear decay, from which we obtain a relation between the orbital period Porb and the binary mass ratio. We discuss how this relation is consistent with theoretical predictions and known ultracompact X-ray binaries. Lastly, we discuss how the observed properties are reminiscent of short-Porb BH X-ray binaries.

Orbital evolution of potentially habitable planets of tidally interacting binary stars

We simulate the coupled stellar and tidal evolution of short-period binary stars (orbital period Porb ≲ 8 days) to investigate the orbital oscillations, instellation cycles, and orbital stability of circumbinary planets (CBPs). We consider two tidal models and show that both predict an outward-then-inward evolution of the binary’s semi-major axis abin and eccentricity ebin. This orbital evolution drives a similar evolution of the minimum CBP semi-major axis for orbital stability. By expanding on previous models to include the evolution of the mass concentration, we show that the maximum in the CBP orbital stability limit tends to occur 100 Myr after the planets form, a factor of 100 longer than previous investigations. This result provides further support for the hypothesis that the early stellar-tidal evolution of binary stars has removed CBPs from short-period binaries. We then apply the models to Kepler-47 b, a CBP orbiting close to its host stars’ stability limit, to show that if the binary’s initial ebin ≳ 0.24, the planet would have been orbiting within the instability zone in the past and probably wouldn’t have survived. For stable, hypothetical cases in which the stability limit does not reach a planet’s orbit, we find that the amplitudes of abin and ebin oscillations can damp by up to 10% and 50%, respectively. Finally, we consider equal-mass stars with Porb = 7.5 days and compare the HZ to the stability limit. We find that for stellar masses ≲0.12 M⊙, the HZ is completely unstable, even if the binary orbit is circular. For ebin ≲ 0.5, that limit increases to 0.17 M⊙, and the HZ is partially destabilized for stellar masses up to 0.45 M⊙. These results may help guide searches for potentially habitable CBPs, as well as characterize their evolution and likelihood to support life after they are found.

An upper limit for the growth of inner planets?

ABSTRACT The exotic range of known planetary systems has provoked an equally exotic range of physical explanations for their diverse architectures. However, constraining formation processes requires mapping the observed exoplanet population to that which initially formed in the protoplanetary disc. Numerous results suggest that (internal or external) dynamical perturbation alters the architectures of some exoplanetary systems. Isolating planets that have evolved without any perturbation can help constrain formation processes. We consider the Kepler multiples, which have low mutual inclinations and are unlikely to have been dynamically perturbed. We apply an adaption of previous modelling efforts, accounting for the two-dimensionality of the radius ($R_\mathrm{pl} =0.3\!-\!20\, R_\oplus$) and period (Porb = 0.5–730 d) distribution. We find that an upper limit in planet mass of the form $M_\mathrm{lim} \propto a_\mathrm{pl}^{\beta } \exp (-a_\mathrm{in}/a_\mathrm{pl})$, for semimajor axis apl and a broad range of ain and β, can reproduce a distribution of Porb, Rpl that is indistinguishable from the observed distribution by our comparison metric. The index is consistent with β = 1.5, expected if growth is limited by accretion within the Hill radius. This model is favoured over models assuming a separable PDF in Porb, Rpl. The limit, extrapolated to longer periods, is coincident with the orbits of RV-discovered planets (apl &amp;gt; 0.2 au, $M_\mathrm{pl}\gt 1\, M_\mathrm{J}$) around recently identified low density host stars, hinting at isolation mass limited growth. We discuss the necessary circumstances for a coincidental age-related bias as the origin of this result; such a bias is possible but unlikely. We conclude that, in light of the evidence suggesting that some planetary systems have been dynamically perturbed, simple models for planet growth during the formation stage are worth revisiting.

ASASSN-18aan: An eclipsing SU UMa-type cataclysmic variable with a 3.6-hr orbital period and a late G-type secondary star

Abstract We report photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova ASASSN-18aan. We observed the 2018 superoutburst with 2.3 mag brightening and found the orbital period (Porb) to be 0.149454(3) d, or 3.59 hr. This is longward of the period gap, establishing ASASSN-18aan as one of a small number of long-Porb SU UMa-type dwarf novae. The estimated mass ratio, [q = M2/M1 = 0.278(1)], is almost identical to the upper limit of tidal instability by the 3 : 1 resonance. From eclipses, we found that the accretion disk at the onset of the superoutburst may reach the 3 : 1 resonance radius, suggesting that the superoutburst of ASASSN-18aan results from the tidal instability. Considering the case of long-Porb WZ Sge-type dwarf novae, we suggest that the tidal dissipation at the tidal truncation radius is enough to induce SU UMa-like behavior in relatively high-q systems such as SU UMa-type dwarf novae, but that this is no longer effective in low-q systems such as WZ Sge-type dwarf novae. The unusual nature of the system extends to the secondary star, for which we find a spectral type of G9, much earlier than typical for the orbital period, and a secondary mass M2 of around 0.18 M⊙, smaller than expected for the orbital period and the secondary’s spectral type. We also see indications of enhanced sodium abundance in the secondary’s spectrum. Anomalously hot secondaries are seen in a modest number of other CVs and related objects. These systems evidently underwent significant nuclear evolution before the onset of mass transfer. In the case of ASASSN-18aan, this apparently resulted in a mass ratio lower than typically found at the system’s Porb, which may account for the occurrence of a superoutburst at this relatively long period.