Keywords

The Kepler Space Telescope observed over 15,000 stars for asteroseismic studies. Of these, 75% of dwarfs (and 8% of giants) were found to show anomalous behavior, such as suppressed oscillations (low amplitude) or no oscillations at all. The lack of solar-like oscillations may be a consequence of multiplicity, due to physical interactions with spectroscopic companions or due to the dilution of oscillation amplitudes from "wide" (AO detected; visual) or spectroscopic companions introducing contaminating flux. We present a search for stellar companions to 327 of the Kepler asteroseismic sample, which were expected to display solar-like oscillations. We used direct imaging with Robo-AO, which can resolve secondary sources at ∼015, and followed up detected companions with Keck AO. Directly imaged companion systems with both separations of ≤05 and amplitude dilutions >10% all have anomalous primaries, suggesting these oscillation signals are diluted by a sufficient amount of excess flux. We also used the high-resolution spectrometer ESPaDOnS at the Canada–France–Hawai'i Telescope to search for spectroscopic binaries. We find tentative evidence for a higher fraction of spectroscopic binaries with high radial velocity scatter in anomalous systems, which would be consistent with previous results suggesting that oscillations are suppressed by tidal interactions in close eclipsing binaries.

We have studied double-detonation explosions in double-degenerate (DD) systems with different companion white dwarfs (WDs) for modeling Type Ia supernovae (SNe Ia) by means of high-resolution smoothed particle hydrodynamics (SPH) simulations. We have found that only the primary WDs explode in some of the DD systems, while the explosions of the primary WDs induce the explosions of the companion WDs in the other DD systems. The former case is a so-called dynamically-driven double-degenerate double-detonation (D⁶) explosion, or helium-ignited violent merger explosion. The SN ejecta of the primary WDs strip materials from the companion WDs, whose mass is ∼10⁻³ M_⊙. The stripped materials contain carbon and oxygen when the companion WDs are carbon–oxygen (CO) WDs with He shells ≲0.04 M_⊙. Since they contribute to low-velocity ejecta components as observationally inferred for iPTF14atg, D⁶ explosions can be counterparts of subluminous SNe Ia. The stripped materials may contribute to low-velocity C seen in several SNe Ia. In the latter case, the companion WDs explode through He detonation if they are He WDs and through the double-detonation mechanism if they are CO WDs with He shells. We name these explosions "triple" and "quadruple" detonation (TD/QD) explosions after the number of detonations. The QD explosion may be counterparts of luminous SNe Ia, such as SN 1991T and SN 1999aa, since they yield a large amount of ⁵⁶Ni, and their He-detonation products contribute to the early emissions accompanying such luminous SNe Ia. On the other hand, the TD explosion may not yield a sufficient amount of ⁵⁶Ni to explain luminous SNe Ia.

Recent studies show that the majority of blue straggler stars (BSSs) in old open clusters are formed through mass transfer from an evolved star onto a main-sequence companion, resulting in a BSS and white dwarf (WD) in a binary system. We present constraints on the mass transfer histories of two BSS–WD binaries in the open cluster NGC 188, using WD temperatures and surface gravities measured with Hubble Space Telescope COS far-ultraviolet spectroscopy. Adopting a Gaia-based cluster distance of 1847 ± 107 pc, we determine that one system, WOCS 4540, formed through Case C mass transfer resulting in a CO-core WD with T_eff = ${{\rm{17,000}}}_{-200}^{+140}$ K and a log g = ${7.80}_{-0.06}^{+0.06}$, corresponding to a mass of ${0.53}_{-0.03}^{+0.03}$ M_⊙ and a cooling age of ${105}_{-5}^{+6}$ Myr. The other system, WOCS 5379, formed through Case B mass transfer resulting in a He-core WD with T_eff = ${{\rm{15,500}}}_{-150}^{+170}$ K and a log g = ${7.50}_{-0.05}^{+0.06}$, corresponding to a mass of ${0.42}_{-0.02}^{+0.02}$ M_⊙ and an age of ${250}_{-20}^{+20}$ Myr. The WD parameters are consistent across four different cluster distance assumptions. We determine possible progenitor binary systems with a grid of accretion models using MESA, and investigate whether these systems would lead to stable or unstable mass transfer. WOCS 4540 likely resulted from stable mass transfer during periastron passage in an eccentric binary system, while WOCS 5379 challenges our current understanding of the expected outcomes for mass transfer from red giant branch stars. Both systems are examples of the value in using detailed analyses to fine-tune our physical understanding of binary evolutionary processes.

Subdwarf A-type stars (sdAs) are objects that have hydrogen-rich spectra with surface gravity similar to that of hot subdwarf stars but effective temperature below the zero-age horizontal branch. They are considered to be metal-poor main-sequence (MS) stars or extremely low-mass white dwarfs (ELM WDs). In this work, using the stellar evolution code Modules for Experiments in Stellar Astrophysics, we investigate the sdAs formed both by the evolution of (pre-)ELM WDs in double-degenerate systems and metal-poor MS stars with single evolution models. We find that both of the evolutionary tracks of ELM WDs and metal-poor MS stars can explain the observation properties of sdAs. However, the proportions between these two populations are uncertain. In this work, we adopt the method of binary population synthesis of both ELM WDs in the disk and metal-poor MS stars in the halo to obtain their populations at different stellar population ages and calculate their proportions. We find that the proportion of metal-poor MS stars to sdAs for a stellar population of 10 Gyr is ∼98.5%, which is consistent with the conclusion that most sdAs (>95%) are metal-poor MS stars. And the proportion of ELM WDs (metal-poor MS stars) to sdAs increases (decreases) from 0.1% (99.9%) to 20% (80%) with stellar population ages from 5 to 13.7 Gyr.

We present new CCD photometric light curves about two eclipsing binaries of V582 Lyr and V1016 Oph. Our observations were carried out by the SARA 91.4 cm telescope of America in 2016 and the 60 cm telescope of Chile in 2018. V582 Lyr's spectra type was classified as K5, and its radial velocity was determined using the LAMOST spectral survey. There are absorptions in the observed H_α line and excess emissions in the subtracted H_α line, which show weak chromospheric activity. We obtained the updated ephemeris information for V582 Lr and V1016 Oph, and found that their orbital periods are both decreasing. We concluded that the decreased rate is −0.474 (±0.011) × 10⁻⁷ days yr⁻¹ for V582 Lyr and 3.460 (±0.014) × 10⁻⁷ days yr⁻¹ for V1016 Oph. For V582 Lyr, the period variation was interpreted as a mass transfer from the secondary component to the primary one, and the corresponding rate is dM₂/dt = −1.10 (±0.03) × 10⁻⁷ M_⊙ yr⁻¹. For V1016 Oph, we explain it by transferring from the primary component to the secondary one, and the corresponding rate is dM₁/dt = −2.69 (±0.04) × 10⁻⁷ M_⊙ yr⁻¹. The photometric solution of V1016 Oph was obtained by analyzing the CCD photometry with the Wilson–Devinney program. We also obtained the orbital parameters of V1016 Oph by simultaneously analyzing our BVRI light curves and radial-velocity curve from the LAMOST low-resolution spectral survey. Finally, our orbital solution shows that they are contact eclipsing binaries with contact factors of 3.35 (±0.08)% for V582 Lyr and 41.0 (±0.1)% for V1016 Oph.

We present a volume-limited, spectroscopically verified sample of M7−L5 ultracool dwarfs (UCDs) within 25 pc. The sample contains 410 sources, of which 93% have trigonometric distance measurements (80% from Gaia DR2) and 81% have low-resolution (R ∼ 120), near-infrared (NIR) spectroscopy. We also present an additional list of 60 sources that may be M7−L5 dwarfs within 25 pc when distance or spectral-type uncertainties are taken into account. The spectra provide NIR spectral and gravity classifications, and we use these to identify young sources, red and blue J − K_S color outliers, and spectral binaries. We measure very low gravity and intermediate-gravity fractions of ${2.1}_{-0.8 \% }^{+0.9 \% }$ and ${7.8}_{-1.5 \% }^{+1.7 \% }$, respectively; fractions of red and blue color outliers of ${1.4}_{-0.5 \% }^{+0.6 \% }$ and ${3.6}_{-0.9 \% }^{+1.0 \% }$, respectively; and a spectral binary fraction of ${1.6}_{-0.5 \% }^{+0.5 \% }$. We present an updated luminosity function for M7−L5 dwarfs continuous across the hydrogen-burning limit that agrees with previous studies. We estimate our completeness to range between 69% and 80% when compared to an isotropic model. However, we find that the literature late-M sample is severely incomplete compared to L dwarfs, with completeness of ${62}_{-7 \% }^{+8 \% }$ and ${83}_{-9 \% }^{+10 \% }$, respectively. This incompleteness can be addressed with astrometric-based searches of UCDs with Gaia to identify objects previously missed by color- and magnitude-limited surveys.

We present the results of spectroscopic observations and Hα Doppler tomography of the massive close binary system UU Cas where a giant star fills its Roche lobe (donor) and transfers material through the vicinity of the L₁ point onto a massive but rather compact stellar companion (accretor). The system has been observed at the 1.2 m telescope of the Kourovka Astronomical Observatory. By analyzing the obtained spectra and computed Doppler tomogram, we suggest that at least three elements of the gas dynamic pattern in the system contribute to the Balmer Hα emission line profiles. These elements are: the stream from the L₁ point, the stellar wind from the accretor, and the diffuse disk, surrounding the accretor. In combination with the estimates of the companions' masses, our findings indicate that the system is at the late stage of the first mass transfer phase of its evolution.

Using archival Chandra observations with a total effective exposure of 734 ks, we derive an updated catalog of point sources in the massive globular cluster (GC) Terzan 5. Our catalog covers an area of 58.1 arcmin² (R ≤ 43) with 489 X-ray sources, and more than 75% of these sources are first detected in this cluster. We find significant dips in the radial distribution profiles of X-ray sources in Terzan 5, with the projected distance and width of the distribution dips for bright (L_X ≳ 9.5 × 10³⁰ erg s⁻¹) X-ray sources larger than those of the faint (L_X ≲ 9.5 × 10³⁰ erg s⁻¹) sources. By fitting the radial distribution of the X-ray sources with a "generalized King model," we estimated an average mass of 1.48 ± 0.11 and 1.27 ± 0.13 M_⊙ for the bright and faint X-ray sources, respectively. These results are in agreement with that observed in 47 Tuc, which may suggest a universal mass segregation effect for X-ray sources in GCs. Compared with 47 Tuc, we show that the two-body relaxation timescale of Terzan 5 is much smaller, but its dynamical age is significantly younger than 47 Tuc. These features suggest that the evolution of Terzan 5 is not purely driven by two-body relaxation, and the tidal stripping effect also plays an important role in accelerating the dynamical evolution of this cluster.

We present the discovery from Transiting Exoplanet Survey Satellite (TESS) data of LTT 1445Ab. At a distance of 6.9 pc, it is the second nearest transiting exoplanet system found to date, and the closest one known for which the primary is an M dwarf. The host stellar system consists of three mid-to-late M dwarfs in a hierarchical configuration, which are blended in one TESS pixel. We use MEarth data and results from the Science Processing Operations Center data validation report to determine that the planet transits the primary star in the system. The planet has a radius of ${1.38}_{-0.12}^{+0.13}$ ${R}_{\oplus }$, an orbital period of ${5.35882}_{-0.00031}^{+0.00030}$ days, and an equilibrium temperature of ${433}_{-27}^{+28}$ K. With radial velocities from the High Accuracy Radial Velocity Planet Searcher, we place a 3σ upper mass limit of 8.4 ${M}_{\oplus }$ on the planet. LTT 1445Ab provides one of the best opportunities to date for the spectroscopic study of the atmosphere of a terrestrial world. We also present a detailed characterization of the host stellar system. We use high-resolution spectroscopy and imaging to rule out the presence of any other close stellar or brown dwarf companions. Nineteen years of photometric monitoring of A and BC indicate a moderate amount of variability, in agreement with that observed in the TESS light-curve data. We derive a preliminary astrometric orbit for the BC pair that reveals an edge-on and eccentric configuration. The presence of a transiting planet in this system hints that the entire system may be co-planar, implying that the system may have formed from the early fragmentation of an individual protostellar core.

All four circumbinary (CB) protoplanetary disks orbiting short-period (P < 20 days) double-lined spectroscopic binaries (SB2s)—a group that includes UZ Tau E, for which we present new Atacama Large Millimeter/Submillimeter Array data—exhibit sky-plane inclinations i_disk that match, to within a few degrees, the sky-plane inclinations i_⋆ of their stellar hosts. Although for these systems the true mutual inclinations θ between disk and binary cannot be directly measured because relative nodal angles are unknown, the near coincidence of i_disk and i_⋆ suggests that θ is small for these most compact of systems. We confirm this hypothesis using a hierarchical Bayesian analysis, showing that 68% of CB disks around short-period SB2s have θ < 30. Near coplanarity of CB disks implies near coplanarity of CB planets discovered by Kepler, which in turn implies that the occurrence rate of close-in CB planets is similar to that around single stars. By contrast, at longer periods ranging from 30 to 10⁵ days (where the nodal degeneracy can be broken via, e.g., binary astrometry), CB disks exhibit a wide range of mutual inclinations, from coplanar to polar. Many of these long-period binaries are eccentric, as their component stars are too far separated to be tidally circularized. We discuss how theories of binary formation and disk–binary gravitational interactions can accommodate all these observations.