We investigate the wind of lambda And, a solar-mass star that has evolved off the main sequence becoming a sub-giant. We present spectropolarimetric observations and use them to reconstruct the surface magnetic field of lambda And. Although much older than our Sun, this star exhibits a stronger (reaching up to 83 G) large-scale magnetic field, which is dominated by the poloidal component. To investigate the wind of lambda And, we use the derived magnetic map to simulate two stellar wind scenarios, namely a polytropic wind (thermally-driven) and an Alfven-wave driven wind with turbulent dissipation. From our 3D magnetohydrodynamics simulations, we calculate the wind thermal emission and compare it to previously published radio observations and more recent VLA observations, which we present here. These observations show a basal sub-mJy quiescent flux level at ~5 GHz and, at epochs, a much larger flux density (>37 mJy), likely due to radio flares. By comparing our model results with the radio observations of lambda And, we can constrain its mass-loss rate Mdot. There are two possible conclusions. 1) Assuming the quiescent radio emission originates from the stellar wind, we conclude that lambda And has Mdot ~ 3e-9 Msun/yr, which agrees with the evolving mass-loss rate trend for evolved solar-mass stars. 2) Alternatively, if the quiescent emission does not originate from the wind, our models can only place an upper limit on mass-loss rates, indicating that Mdot <~ 3e-9 Msun/yr.
Probing the structures of stellar winds is of prime importance for the understanding of massive stars. Based on their optical spectral morphology and variability, the stars of the Oef class have been suggested to feature large-scale structures in their wind. High-resolution X-ray spectroscopy and time-series of X-ray observations of presumably-single O-type stars can help us understand the physics of their stellar winds. We have collected XMM-Newton observations and coordinated optical spectroscopy of the O6Ief star lambda Cep to study its X-ray and optical variability and to analyse its high-…
We present the analysis of a multi-epoch spectroscopic monitoring campaign of the O6Ief star lambda Cep. Previous observations reported the existence of two modes of non-radial pulsations in this star. Our data reveal a much more complex situation. The frequency content of the power spectrum considerably changes from one epoch to the other. We find no stable frequency that can unambiguously be attributed to pulsations. The epoch-dependence of the frequencies and variability patterns are similar to what is seen in the wind emission lines of this and other Oef stars, suggesting that both phenome…
The abundance anomalies in lambda Boo stars are popularly explained by element-specific mass inflows at rates that are much greater than empirically-inferred bounds for interstellar accretion. Therefore, a lambda Boo star's thin outer envelope must derive from a companion star, planet, analogs to Kuiper Belt Objects or a circumstellar disk. Because radiation pressure on gas-phase ions might selectively allow the accretion of carbon, nitrogen, and oxygen and inhibit the inflow of elements such as iron, the source of the acquired matter need not contain dust. We propose that at least some lambda…
We have analysed around 170 000 individual photometric WASP measurements of 15 well established lambda Bootis stars to search for variability. The lambda Bootis stars are a small group of late-B to early-F, Pop. I, stars that show moderate to extreme (surface) underabundances (up to a factor 100) of most Fe-peak elements, but solar abundances of lighter elements (C, N, O and S). They are excellent laboratories for the study of fundamental astrophysical processes such as diffusion, meridional circulation, stellar winds, and accretion in the presence of pulsation. From the 15 targets, eight are …
Binary population synthesis (BPS) study provides a comprehensive way to understand evolutions of binaries and their end products. Close white dwarf (WD) binaries have crucial characteristics in examining in uence of yetunresolved physical parameters on the binary evolution. In this paper, we perform Monte Carlo BPS simulations, investigating the population of WD/main sequence (WD/MS) binaries and double WD binaries, with a publicly available binary star evolution code under 37 different assumptions on key physical processes and binary initial conditions. We considered different combinations of…
We studied the X-ray properties of the young (~1-8 Myr) open cluster around the O8III star lambda Ori using XMM-Newton. We detected 167 X-ray sources, of which 58 are identified with known cluster members, from massive stars down to low-mass stars with spectral types M5.5. Another 23 sources were identified with new possible photometric candidates. Late-type stars have a median log LX/Lbol ~ -3.3, close to the saturation limit. Variability was observed in 35% of late-type members, including six flaring sources. The emission from the central hot star lambda Ori is dominated by plasma at 0.2-0.3…
Many O and B stars show unexplained cyclical variability in their winds, i.e. modulation of absorption features on the rotational timescale, but not strictly periodic over longer timescales. For these stars no dipolar magnetic fields have been detected, with upper limits below 300 G. Similar cyclical variability is also found in many optical lines, which are formed at the base of the wind. We propose that these cyclical variations are caused by the presence of multiple, transient, short-lived, corotating magnetic loops, which we call "stellar prominences". We present a simplified model represe…
We show that suprathermal particles, termed pick-up ions (PUIs), might reduce the postshock temperature of the fast wind and jets in some planetary nebulae (PNs) and in symbiotic systems. The goal is to explain the finding that the temperature of the hot bubble formed by the post-shock gas in some PNs and symbiotic nebulae is lower, sometimes by more than an order of magnitude, than the value expected from simple hydrodynamical calculations. Although various explanations have been proposed, there is as yet no prefered solution for this low tempeature problem. PUIs have been invoked to explain …
Time-domain spectroscopy of the classical accreting T Tauri star, TW Hya, covering a decade and spanning the far UV to the near-infrared spectral regions can identify the radiation sources, the atmospheric structure produced by accretion,and properties of the stellar wind. On time scales from days to years, substantial changes occur in emission line profiles and line strengths. Our extensive time-domain spectroscopy suggests that the broad near-IR, optical, and far-uv emission lines, centered on the star, originate in a turbulent post-shock region and can undergo scattering by the overlying st…
In most astrophysical situations, the radioactive decay of 56Ni to 56Co occurs via electron capture with a fixed half-life of 6.1 days. However, this decay rate is significantly slowed when the nuclei are fully ionized because K-shell electrons are unavailable for capture. In this paper, we explore the effect of these delayed decays on white dwarfs (WDs) that may survive Type Ia and Type Iax supernovae (SNe Ia and SNe Iax). The energy released by the delayed radioactive decays of 56Ni and 56Co drives a persistent wind from the surviving WD's surface that contributes to the late-time appearance…
We use recent observations of the He I $\lambda10830 Å$ absorption line and 3D hydrodynamical numerical simulations of the winds collision, to strengthen the case for an orientation of the semimajor axis of the massive binary system Eta Carinae where the secondary star is toward us at periastron passage. Those observations show that the fast blue absorption component exists for only several weeks prior to the periastron passage. We show that the transient nature of the fast blue absorption component supports a geometry where the fast secondary wind, both pre and post-shock material, passes in …
Aims. To explore the chemical pattern of early-type stars with planets, searching for a possible signature of planet formation. In particular, we study a likely relation between the lambda Bootis chemical pattern and the presence of giant planets. Methods. We performed a detailed abundance determination in a sample of early-type stars with and without planets via spectral synthesis. Results. We compared the chemical pattern of the stars in our sample (13 stars with planets and 24 stars without detected planets) with those of lambda Bootis and other chemically peculiar stars. We have found four…
Most O and many B stars show unexplained cyclical variability in their spectral lines, i.e. modulation on the rotational timescale, but not strictly periodic. The variability occurs in the so-called discrete absorption components (DACs) that accelerate through the UV-wind line profiles and also in many optical lines. For such OB stars no dipolar magnetic fields have been detected with upper limits of ~300G. We investigate whether multiple magnetic loops on the surface rather than non-radial pulsations or a dipolar magnetic field can explain the observed cyclical UV and optical spectral line va…
Eta Carinae shows broad peaks in near-infrared (IR) JHKL photometry, roughly correlated with times of periastron passage in the eccentric binary system. After correcting for secular changes attributed to reduced extinction from the thinning Homunculus Nebula, these peaks have IR spectral energy distributions (SEDs) consistent with emission from hot dust at 1400-1700 K. The excess SEDs are clearly inconsistent, however, with the excess being entirely due to free-free wind or photospheric emission. One must conclude, therefore, that the broad near-IR peaks associated with Eta Carinae's 5.5 yr va…
The state of the space environment plays a significant role for forecasting of geomagnetic storms produced by disturbances of the solar wind (SW). Coronal mass ejections (CMEs) passing through the heliosphere often have a prolonged (up to several days) trail with declining speed, which affects propagation of the subsequent SW streams. We studied the CME and the post-eruption plasma flows behind the CME rear in the event on 2010 August 18 observed in quadrature by several space-based instruments. Observations of the eruption in the corona with EUV telescopes and coronagraphs revealed several di…
Neutron stars are extremely relativistic objects which abound in our universe and yet are poorly understood, due to the high uncertainty on how matter behaves in the extreme conditions which prevail in the stellar core. It has recently been pointed out that the moment of inertia I, the Love number lambda and the spin-induced quadrupole moment Q of an isolated neutron star, are related through functions which are practically independent of the equation of state. These surprising universal I-lambda-Q relations pave the way for a better understanding of neutron stars, most notably via gravitation…
Post-processing Probabilistic Forecasts of the Solar Wind by Data Mining Similar Scenarios
NASA Goddard Spaceflight Center（美国宇航局戈达德太空飞行中心）；Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County（马里兰大学巴尔的摩分校戈达德行星日球物理研究所）；Institute for Space Weather Sciences, New Jersey Institute of Technology（新泽西理工学院空间天气科学研究所）；Department of Physics, Catholic University of America（美利坚天主教大学物理系）
We present the first quantitative spectroscopic modeling of an early-time supernova that interacts with its progenitor wind. Using the radiative transfer code CMFGEN, we investigate the recently-reported 15.5 h post-explosion spectrum of the type IIb SN 2013cu. For the first time, we are able to directly measure the chemical abundances of a SN progenitor and find a relatively H-rich wind, with H and He abundances (by mass) of X=0.46 +- 0.2 and Y=0.52 +- 0.2, respectively. The wind is enhanced in N and depleted in C relative to solar values (mass fractions of 8.2e-3 and 1e-5). We obtain that a …
Our knowledge of the chemical properties of the circumstellar ejecta of the most massive evolved stars is particularly poor. We aim to study the chemical characteristics of the prototypical yellow hypergiant star, IRC +10420. For this purpose, we obtained full line surveys at 1 and 3mm atmospheric windows. We have identified 106 molecular emission lines from 22 molecular species. Approximately half of the molecules detected are N-bearing species, in particular HCN, HNC, CN, NO, NS, PN, and N2H+. We used rotational diagrams to derive the density and rotational temperature of the different molec…
