Mira Wind Research Articles

Benchmark experiments for simulations of a vertical axis wind turbine

Driven by the need to verify a CFD model of a novel vertical axis wind turbine (VAWT) device, a 1.6m diameter prototype was designed, built to a high specification and tolerance and then tested in the industry standard MIRA wind tunnel over the full wind range that might be expected in an urban environment. Although the original intention was simply to provide data to verify a CFD model of the specific device – documented separately – it was realised that given (a) the repeatability and quality of the experimental data, and (b) its utility for the authors in verifying their CFD simulations, the data itself might well form the basis for a requirement for a reliable benchmark for the wind engineering community in assessing CFD simulation technology against a specific VAWT device design with the complexity of a housing. The experiments cover a full range of operational conditions for a specific design including the variation of wind speed, wind direction, tip speed ratio and blade pitch angle, recorded in the experiments as well as the torque output and the pressure levels at a range of specific locations. Over 60 tests were carried out in such a way as to assess the repeatability of the measurements; the results showed an overall remarkable degree of consistency.

THE RADIO LIGHT CURVE OF THE GAMMA-RAY NOVA IN V407 CYG: THERMAL EMISSION FROM THE IONIZED SYMBIOTIC ENVELOPE, DEVOURED FROM WITHIN BY THE NOVA BLAST

We present multi-frequency radio observations of the 2010 nova event in the symbiotic binary V407 Cygni, obtained with the Karl G. Jansky Very Large Array and spanning 1-45 GHz and 17-770 days following discovery. This nova---the first ever detected in gamma rays---shows a radio light curve dominated by the wind of the Mira giant companion, rather than the nova ejecta themselves. The radio luminosity grew as the wind became increasingly ionized by the nova outburst, and faded as the wind was violently heated from within by the nova shock. This study marks the first time that this physical mechanism has been shown to dominate the radio light curve of an astrophysical transient. We do not observe a thermal signature from the nova ejecta or synchrotron emission from the shock, due to the fact that these components were hidden behind the absorbing screen of the Mira wind. We estimate a mass loss rate for the Mira wind of Mdot_w ~ 10^-6 M_sun/yr. We also present the only radio detection of V407 Cyg before the 2010 nova, gleaned from unpublished 1993 archival VLA data, which shows that the radio luminosity of the Mira wind varies by a factor of >~20 even in quiescence. Although V407 Cyg likely hosts a massive accreting white dwarf, making it a candidate progenitor system for a Type Ia supernova, the dense and radially continuous circumbinary material surrounding V407 Cyg is inconsistent with observational constraints on the environments of most Type Ia supernovae.

The X-Ray Evolution of the Symbiotic Star V407 Cyg During its 2010 Outburst

Abstract We present a summary of Swift and Suzaku X-ray observations of the 2010 nova outburst of the symbiotic star, V407 Cyg. the Suzaku spectrum obtained on day 30 indicates the presence of the supersoft component from the white dwarf surface, as well as optically thin component from the shock between the nova ejecta and the Mira wind. the Swift observations then allow us to track the evolution of both components from day 4 to day 150. Most notable is the sudden brightening of the optically thin component around day 20. We identify this as the time when the blast wave reached the immediate vicinity of the photosphere of the Mira. We have developed a simpe model of the blast wave - wind interaction that can reproduce the gross features of the X-ray evolution of V407 Cyg. If the model is correct, the binary separation is likely to be larger than previously suggested and the mass-loss rate of the Mira is likely to be relatively low.

The spectroscopic evolution of the symbiotic-like recurrent nova V407 Cygni during its 2010 outburst

The nova outburst of V407 Cyg in 2010 Mar. 10 was the first observed for this star but its close resemblance to the well known symbiotic-like recurrent nova RS Oph suggests that it is also a member of this rare type of Galactic novae. The nova was the first detected at $\gamma$-ray energies and is the first known nova explosion for this system. The extensive multiwavelength coverage of this outburst makes it an ideal comparison with the few other outbursts known for similar systems. We extend our previous analysis of the Mira and the expanding shock from the explosion to detail the time development of the photoionized Mira wind, circumstellar medium, and shocked circumstellar environment to derive their physical parameters and how they relate to large scale structure of the environment, extending the previous coverage to more than 500 days after outburst. Absorption lines of Fe-peak ions formed in the Mira wind were visible as P Cyg profiles at low velocity before Day 69, around the time of the X-ray peak and we identified many absorption transitions without accompanying emission for metal lines. The H Balmer lines showed strong P Cyg absorption troughs that weakened during the 2010 observing period, through Day 128. We distinguish the components from the shock, the photoionized environment, and the chromosphere and inner Mira wind using spectra taken more than one year after outburst. The multiple shells and radiative excitation phenomenology are similar to those recently cited for GRBs and SNIa (severely truncated)

Herschel’sview into Mira’s head

Herschel's PACS instrument observed the environment of the binary system Mira Ceti in the 70 and 160 micron bands. These images reveal bright structures shaped as five broken arcs and fainter filaments in the ejected material of Mira's primary star. The overall shape of the IR emission around Mira deviates significantly from the expected alignment with Mira's exceptionally high space velocity. The observed broken arcs are neither connected to each other nor are they of a circular shape; they stretch over angular ranges of 80 to 100 degrees. By comparing Herschel and GALEX data, we found evidence for the disruption of the IR arcs by the fast outflow visible in both Halpha and the far UV. Radial intensity profiles are derived, which place the arcs at distances of 6-85" (550 - 8000 AU) from the binary. Mira's IR environment appears to be shaped by the complex interaction of Mira's wind with its companion, the bipolar jet, and the ISM.

The spectroscopic evolution of the symbiotic-like recurrent nova V407 Cygni during its 2010 outburst

On 2010 Mar 10, V407 Cyg was discovered in outburst, eventually reaching V< 8 and detected by Fermi. Using medium and high resolution ground-based optical spectra, visual and Swift UV photometry, and Swift X-ray spectrophotometry, we describe the behavior of the high-velocity profile evolution for this nova during its first three months. The peak of the X-ray emission occurred at about day 40 with a broad maximum and decline after day 50. The main changes in the optical spectrum began at around that time. The He II 4686A line first appeared between days 7 and 14 and initially displayed a broad, symmetric profile that is characteristic of all species before day 60. Low-excitation lines remained comparatively narrow, with v(rad,max) of order 200-400 km/s. They were systematically more symmetric than lines such as [Ca V], [Fe VII], [Fe X], and He II, all of which showed a sequence of profile changes going from symmetric to a blue wing similar to that of the low ionization species but with a red wing extended to as high as 600 km/s . The Na I D doublet developed a broad component with similar velocity width to the other low-ionization species. The O VI Raman features were not detected. We interpret these variations as aspherical expansion of the ejecta within the Mira wind. The blue side is from the shock penetrating into the wind while the red wing is from the low-density periphery. The maximum radial velocities obey power laws, v(rad,max) t^{-n} with n ~ 1/3 for red wing and ~0.8 for the blue. (truncated)

The 2010 nova outburst of the symbiotic Mira V407 Cyg

Abstract The nova outburst experienced in 2010 by the symbiotic binary Mira V407 Cyg has been extensively studied at optical and infrared wavelengths with both photometric and spectroscopic observations. This outburst, reminiscent of similar events displayed by RS Oph, can be described as a very fast He/N nova erupting while being deeply embedded in the dense wind of its cool giant companion. The hard radiation from the initial thermonuclear flash ionizes and excites the wind of the Mira over great distances (recombination is observed on a time scale of 4 d). The nova ejecta is found to progressively decelerate with time as it expands into the Mira wind. This is deduced from linewidths which change from a FWHM of 2760 km s−1 on day +2.3 to 200 km s−1 on day +196. The wind of the Mira is massive and extended enough for an outer neutral and unperturbed region to survive at all outburst phases.

Cometary Astropause of Mira Revealed in the Far-Infrared

Evolved mass-losing stars such as Mira enrich the interstellar medium (ISM) significantly by their dust-rich molecular wind. When these stars move fast enough relative to the ISM, the interaction between the wind and ISM generates the structure known as the astropause (a stellar analog of the heliopause), which is a cometary stellar wind cavity bounded by the contact discontinuity surface between the wind and ISM. Far-infrared observations of Mira spatially resolve the structure of its astropause for the first time, distinguishing the contact surface between Mira's wind and the ISM and the termination shock due to Mira's wind colliding with the ISM. The physical size of the astropause and the estimated speed of the termination shock suggest the age of the astropause to be about 40,000 yr, confirming a theoretical prediction of the shock re-establishment time after Mira has entered the Local Bubble.

Continuous and burst-like accretion on to substellar companions in Mira winds

ABSTRACT We present numerical hydrodynamical modeling of the effects of a giant planetor brown dwarf companion orbiting within the extended atmosphere and wind for-mation zone of an approximately solar-mass Mira variable star. The large-scale, time-dependent accretion flows within the radially oscillating and outflowing circumstellargas around Miras are related to Bondi-Hoyle-Lyttleton flows, but have not, to ourknowledge, been previously modelled. The new models presented in this paper illus-trate the changes in accretion and wake dynamics as the companion mass is variedover a range from 10 to 50 Jupiter masses (M J ), and generalize the results of thesingle model we presented in an earlier paper.The character of the accretion onto the companion changes greatly as the com-panion mass is increased. At the lowest companion masses considered here, a lowcontinuous rate of mass accretion is punctuated by large, nearly periodic bursts of ac-cretion. When the companion mass is large, the mass accretion has both a continuouspart, and a rapidly varying, nearly stochastic part. Surprisingly, the angular momen-tum of the accreted gas shows an opposite trend with mass, varying nearly periodicallyat large companion masses, and stochastically at low masses. These trends can be un-derstood as the result of the interplay between the shocks and radial oscillations inthe circumstellar gas, and the wake flow behind the companion. Boundary conditionsalso affect the character of the accretion. The equation of state, however, is found tohave little effect, at least for gamma-law gases, with gamma in the range from 1 to5/3.Models with accretion bursts may produce observable optical brightenings, andmay affect SiO maser emission, as we suggested in previous papers. Interruptions ofcontinuous accretion, or shadowing effects, could give rise to bursts of dimming inthe optical. Such dimming effects are likely to be correlated with bursts and opticalflashes, helping to explain some rather mysterious Hipparcos observations.Key words:stars: variables: stars – AGB and Post-AGB – accretion, accretion discs – masers –circumstellar matter – planetary systems: stars – winds.

Models of Planets and Brown Dwarfs in Mira Winds

We present numerical hydrodynamical models of the effects of planets or brown dwarfs orbiting within the extended atmosphere and wind formation zone of Mira variables. We find time-dependent wake dynamics and episodic accretion phenomena which may give rise to observable optical events and affect SiO maser emission.

Detection of H[TINF]2[/TINF] Emission from Mira B in Ultraviolet Spectra from the [ITAL]Hubble Space Telescope[/ITAL

We present ultraviolet spectra of Mira's companion star from the Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope (HST). The companion is generally assumed to be a white dwarf surrounded by an accretion disk fed by Mira's wind, which dominates the UV emission from the system. The STIS UV spectrum is dominated by numerous, narrow H2 lines fluoresced by H I Lyα, which were not detected in any of the numerous observations of Mira B by the International Ultraviolet Explorer (IUE). The high-temperature lines detected by IUE (e.g., C IV λ1550) still exist in the STIS spectrum but with dramatically lower fluxes. The continuum fluxes in the STIS spectra are also much lower, being more than an order of magnitude lower than ever observed by IUE and also an order of magnitude lower than fluxes observed in more recent HST Faint Object Camera objective prism spectra from 1995. Thus, the accretion rate onto Mira B was apparently much lower when STIS observed the star, and this change altered the character of Mira B's UV spectrum.

The Southern Crab from a New Perspective

New Hubble Space Telescope images and Cerro Tololo Inter-American Observatory echelle spectra of the nebula around the symbiotic Mira Henize 2-104 are presented. The nebula consists of two nested hourglass-shaped structures and a collimated polar jet. Spatiokinematical modeling shows that all three outflows were ejected during approximately the same epoch, namely, about 5700 years ago, with the adopted distance of 4.4 kpc, in a mass-loss event unrelated to the present outburst. We present a tentative model in which the outer pair of lobes and jets are produced by a high-velocity outflow from the white dwarf companion to the Mira, while the inner, slowly expanding lobes are produced by the Mira wind itself.

The symbiotic binary system RX Puppis: a possible recurrent nova with a Mira companion

We present an analysis of photometric and spectroscopic observations of the symbiotic binary system RX Pup with the aims of developing a reliable binary for the system and identifying mechanisms responsible for its spectacular activity. The binary is composed of a long-perod Mira variable surrounded by a thick dust shell and a hot white dwarf companion. The hot component produces practically all activity observed in the UV, optical and radio range, while variable obscuration of the Mira by circumstellar dust is responsible for long-term changes in the near-IR magnitudes. The observations show RX Pup underwent a nova-like eruption during the last three decades. The hot component contracted in radius at roughly constant luminosity from 1975 to 1986, and was the source of a strong stellar wind which prevented it from accreting material lost in the Mira wind. Around 1988/9 the hot component turned over in the HR diagram and by 1991 its luminosity had faded by a factor of about 30 with respect to the maximum plateau value and the hot wind had practically ceased. By 1995 the nova remnant started to accrete material from the Mira wind, as indicated by a general increase in intensity of the optical continuum and HI emission. The quiescent spectrum resembles the quiescent spectra of symbiotic recurrent novae, and its intensity indicates the hot component must accrete as much as about 1 per cent of the Mira wind, which is more or less the amount predicted by Bondi-Hoyle theory. The earliest observational records from the 1890s suggest that another nova-like eruption of RX Pup occurred around 1894.

High-resolution radio observations of HM Sge — II. Two decades after outburst

HM Sge is a symbiotic binary star that underwent a nova-like outburst in 1975. Its radio emission has been monitored using MERLIN and the VLA, and maps from 1992 to 1997 are presented. Thermal emission within 0.4 arcsec of the optical peak is elongated north--south and hotspots appear to be moving around an anticlockwise ellipse. If this is due to motion in an inclined disc, the period is ∼ 90 yr. Eyres et al. predicted that if this is due to Mach shocks in the post-nova wind, proper motions should be seen in expansion. This has not been detected since 1992, so an alternative model is adopted, based on the work of Eyres et al. and Kenny et al., of colliding winds in a binary system following the nova-like outburst. The emission peaks appear to be corotating with the binary orbit as the ionization front and the hot wind from the white dwarf interact with the Mira wind. The positions of the stars are estimated, at a separation of ∼ 25 au. A distance of ∼ 1 kpc is most consistent with the observations reported here. On arcsecond scales the emission is extended east--west, consistent with a biconical outflow arising from the collimation of the nova outburst through interaction with the pre-existing cool wind. The presence of non-thermal emission at a separation of ∼700 mas from the stars is confirmed. This is very unusual at such a distance from low-mass stars, and could arise from synchrotron emission in a μT magnetic field. A model for this is developed, which shows that non-thermal emission is expected to decline within decades as the nova wind decelerates. This could explain the non-detection of non-thermal emission in other symbiotic stars.

Speckle observations of Mira's companion

We present the first speckle interferometric observations of Mira's hot companion. The companion accretes mass from Mira's wind and provides an excellent opportunity for studying the accretion processes in detached binaries. The separation between the components of the system is crucial for accurate determination of the accretion rate onto the companion. Speckle measurements of the separation made at three epochs since 1983 with much higher precision than previous visual observations show that the companion's distance from Mira is substantially larger than expected

A Model for R Aqr from HST

High spatial resolution observations R Aqr have been carried out with the HST/FOC. R Aqr is the nearest symbiotic star, i.e. an interacting system consisting of a cool giant star and a hot ionizing source embedded in a larger and complex nebula. We suggest, here, that the binary system is composed of a Mira and a hot stellar source with a radiation temperature of T* ≈ 40 000 K, a luminosity of L ≈ 10L⊙ and a radius R ≲ 0.1 R⊙. Mass transfer between the two components of the system occurs via capture of the Mira wind. Of the plausible mechanisms that may produce the R Aqr jet, an accretion/ejection scenario or the collision of winds seem to be unlikely, but spectral observations of the core are needed to conclude. Another possible origin is based on a spherically symmetric stellar wind that sweeps out a cavity in an ambient medium. This bubble elongates in the directions of least resistance, and matter eventually flows out through two symmetric nozzles. Comparison with models shows that the emission in the NE and SW outer features of the jets is due to a radiative shock but the pre-shock gas must be partially photoionized by the central hot stellar source.