I will review our recent analysis of the magnetic properties of the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the Canada-France-Hawaii telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. I discuss our most recent determination of the rotational period from longitudinal magnetic field measurements and Halpha variability - the latter obtained from over a decade's worth of professional and amateur spectroscopic observations. Lastly, I will report on our investigation of the magnetic field geometry and the effects of the field on the circumstellar environment.
Discovery of a magnetic field in the O9 sub-giant star HD 57682 by the MiMeS Collaboration
我们报告检测到一个强大的，有组织的磁场在O 9 IV星HD 57682，使用分光偏振观测ESPaDOnS在3.6米加拿大-法国-夏威夷望远镜的背景下，在大质量恒星（MiMeS）的磁性大计划。从我们的光谱拟合使用NLTE模型大气，我们确定HD 57682是一个$17^{+19}_{-9}$ M $_{\odot}$星，半径为$7.0^{+2.4}_{-1.8}$ R $_\odot$，和一个相对较低的质量损失率为$1.4^{+3.1}_{-0.95}\times10^{-9}$ M $_{\odot}$ yr $^{-1}$。光球的吸收谱线很窄，我们用傅里叶变换技术推知了$v\sin i=15\pm3$ km s $^{-1}$ .这$v\sin i$意味着一个最大的旋转周期为31.5天，一个值定性与所观察到的光吸收和发射线的变化，以及斯托克斯$V$配置文件和纵向字段一致。利用速度分辨Stokes $V$剖面的贝叶斯分析来推断磁场特征，我们尝试性地推导出了$1680^{+134}_{-356}$ G.推导出的场强和风的特性意味着风是强烈限制的磁场。
Connecting the evolution of thermally pulsing asymptotic giant branch stars to the chemistry in their circumstellar envelopes -- I. The case of hydrogen cyanide
We report an unbiased radio line survey towards the circumstellar envelopes of evolved stars at the frequency range from 20 to 25 GHz, aiming to obtain a more complete unbiased picture of the chemical evolution in the final stages of stellar evolution. The observation sample includes the asymptotic giant branch (AGB) star IRC+10216, the proto-planetary nebulae (PPNs) CRL\,2688 and CRL\,618, and the young planetary nebula (PN) NGC\,7027, representing an evolutionary sequence spanning about 10,000 years. Rotational transitions from cyanopolyyne chains and inversion lines from ammonia are detecte…
The amount of dust present in circumstellar disks is expected to steadily decrease with age due to the growth from micron-sized particles to planetesimals and planets. Mature circumstellar disks, however, can be observed to contain significant amounts of dust and possess high dust-to-gas ratios. Using HD 163296 as our case study, we explore how the formation of giant planets in disks can create the conditions for collisionally rejuvenating the dust population, halting or reversing the expected trend. We combine N-body simulations with statistical methods and impact scaling laws to estimate the…
HI Observations of the Asymptotic Giant Branch Star X Herculis: Discovery of an Extended Circumstellar Wake Superposed on a Compact High-Velocity Cloud
The Asymptotic Giant Branch (AGB) is the terminal phase of red giant evolution with timescales of millions of years and a total mass lost from the star that is a significant fraction of the initial mass. Investigation of one of these stars, WISEA J173046.10-344455.5, a kpc in the direction of the center of the Galaxy, reveals a cool oxygen rich star with a dust shell of black-body temperature 1305 K.
Pre-outburst 2MASS and WISE photometry of Nova Sco 2014 (V1534 Sco) have suggested the presence of a cool giant at the location of the nova in the sky. The spectral evolution recorded for the nova did not however support a direct partnership because no flash-ionized wind and no deceleration of the ejecta were observed, contrary to the behavior displayed by other novae which erupted within symbiotic binaries like V407 Cyg or RS Oph. We have therefore obtained an 0.8-2.5 micron spectra of the remnant of Nova Sco 2014 in order to ascertain if a cool giant is indeed present and if it is physically…
We report the discovery of the diatomic molecule SiO in the gas phase in the environment of the recurrent nova T Coronae Borealis. While some of the SiO is photospheric, a substantial portion must arise in the wind from the red giant component of T CrB. A simple fit to the SiO feature, assuming local thermodynamic equilibrium, suggests a SiO column density of 2.8x10^17 /cm2 and temperature ~1000K; the SiO column density is similar to that present in the winds of field red giants. A search for SiO maser emission is encouraged both before and after the next anticipated eruption. We find that the…
Circumstellar effects on the Rb abundances in O-rich AGB stars
我们首次探讨了富氧渐近巨星分支（AGB）恒星的Rb（和Zr）丰度测定的拱星效应，考虑存在的气体拱星信封与径向风。谱合成代码Turbospectrum的一个修改版本被用来处理扩展的大气模型和速度场。分别利用共振7800 A Rb I谱线和6474 A ZrO谱线，测定了5颗具有代表性的、具有不同膨胀速度和金属丰度的富氧AGB星的Rb和Zr丰度。通过使用我们的新的动力学模型，Rb I线轮廓（光球和拱星组件）是非常好的再现。有趣的是，导出的Rb丰度低得多（1-2 dex），在那些富O的AGB星显示出较高的星周膨胀速度。然而，Zr丰度仍然接近太阳值。Rb丰度和Rb/Zr比值的导出显着缓解了目前的问题之间的不匹配的中等质量（4-8太阳质量）的富Rb AGB星的观测和AGB核合成的理论预测。
The progenitor systems accounting for explosions of type Ia supernovae (SNe Ia) is still under debate. Symbiotic channel is one of the possible progenitor scenarios, in which the WDs in these systems increase in mass through wind accretion from their red giant companions. The mass-loss processes of the giants in the symbiotic systems could produce amount of circumstellar medium (CSM), and the detections of the interaction signals between SN ejecta and CSM can be used as an ideal way to distinguish the different progenitor models. However, the density distribution and geometric structure of the…
Imaging studies with the VLA have revealed HI emission associated with the extended circumstellar shells of red giants. We analyse the spectral map obtained on Y CVn, a J-type carbon star on the AGB. The HI line profiles can be interpreted with a model of a detached shell resulting from the interaction of a stellar outflow with the local interstellar medium. We reproduce the spectral map by introducing a distortion along a direction corresponding to the star's motion in space. We then use this fitting to simulate observations expected from the FAST radiotelescope, and discuss its potential for…
Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. II. Abundances and spatial distributions of CS
Chalmers University of Technology（查尔姆斯理工大学）；Uppsala University（乌普萨拉大学）；Joint ALMA Observatory（ALMA联合天文台）；European Southern Observatory（欧洲南方天文台）；Leiden University（莱顿大学）；Monash University（蒙纳士大学）；KU Leuven（荷语鲁汶大学）；Queen’s University Belfast（贝尔法斯特女王大学）；NASA Goddard Space Flight Center（NASA戈达德太空飞行中心）；Gemini Observatory（双子座天文台）
(abridged) The role of magnetic field in the shaping of Planetary Nebulae (PNe), either directly or indirectly after being enhanced by binary interaction, has long been a topic of debate. Large scale magnetic fields around pre-PNe have been inferred from polarisation observations of masers. However, because masers probe very specific regions, it is still unclear if the maser results are representative of the intrinsic magnetic field in the circumstellar envelope (CSE). Molecular line polarisation can provide important information about the magnetic field. A comparison between the field morphol…
One of the major puzzles in the study of stellar evolution is the formation process of bipolar and multi-polar planetary nebulae. There is growing consensus that collimated jets create cavities with dense walls in the slowly-expanding (10--20 ~km~s$^{-1}$) envelope ejected in previous evolutionary phases, leading to the observed morphologies. However, the launching of the jet and the way it interacts with the circumstellar material to create such asymmetric morphologies have remained poorly known. Here we present for the first time CO emission from the asymptotic giant branch star W43A that tr…
Carbon monoxide is the most abundant molecule after H$_2$ and is important for chemistry in circumstellar envelopes around late-type stars. The size of the envelope is important when modelling low-J transition lines and deriving mass-loss rates from such lines. Now that ALMA is coming to full power the extent of the CO emitting region can be measured directly for nearby asymptotic giant branch (AGB) stars. In parallel, it has become obvious in the past few years that the strength of the interstellar radiation field (ISRF) can have a significant impact on the interpretation of the emission line…
The dusty envelopes of asymptotic giant branch stars with ultraviolet excesses
Centro de Astrobiología, CSIC-INTA（天体生物学中心，西班牙国家研究委员会-国家航空航天技术研究所）；Escuela de Doctorado UAM, Centro de Estudios de Posgrado, Universidad Autónoma de Madrid（马德里自治大学博士学校，研究生教育中心）；Jet Propulsion Laboratory, California Institute of Technology（喷气推进实验室，加州理工学院）
Recent spectropolarimetric surveys (MiMeS, BOB) have revealed that approximately 7% of massive stars host stable, surface dipolar magnetic fields with strengths on the order of kG. These fields channel the dense radiatively driven stellar wind into a circumstellar magnetosphere. Wind-sensitive UV spectral lines can probe the density and velocity structure of massive star magnetospheres, providing insight into wind-field interactions. To date, large-scale magnetohydrodynamic modeling of this phenomenon has been limited by the associated computational cost. Our analysis, using the Analytic Dynam…
We consider whether the subset of carbon-rich asymptotic giant branch (AGB) stars that exhibit detached, expanding circumstellar shells may reveal the past histories of these stars as having undergone helium shell flashes (thermal pulses) on the AGB. We exploit newly available Gaia parallaxes and photometry, along with archival infrared photometry, to obtain refined estimates of the luminosities of all (12) known detached shell carbon stars. We examine the relationship between these luminosities and the estimated dynamical ages (ejection times) of the detached shells associated with the 12 sta…
Circumstellar disks of planetary debris are now known or suspected to closely orbit hundreds of white dwarf stars. To date, both data and theory support disks that are entirely contained within the preceding giant stellar radii, and hence must have been produced during the white dwarf phase. This picture is strengthened by the signature of material falling onto the pristine stellar surfaces; disks are always detected together with atmospheric heavy elements. The physical link between this debris and the white dwarf host abundances enables unique insight into the bulk chemistry of extrasolar pl…
Asymptotic Giant Branch (AGB) stars are major contributors of cosmic dust to the universe. Typically, dust around AGB stars is investigated via radiative transfer (RT) modeling, or via simple deconstruction of observed spectra. However, methodologies applied vary. Using archival spectroscopic, photometric, and temporal data for the archetypal dusty star, Mira, we identify its circumstellar silicate dust grains. This is achieved by matching the positions and widths of observed spectral features with laboratory data. To do this comparison properly, it is necessary to account for the continuum em…
