arXiv subjects

P. G. Tuthill

Publications and source records attributed to P. G. Tuthill.

At least 55 records · Page 3Linked to original sources

arXiv2007

The radius and mass of the subgiant star bet Hyi from interferometry and asteroseismology

We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of beta Hydri. This star is a nearby G2 subgiant whose mean density was recently measured with high precision using asteroseismology. We determine the radius and effective temperature of the star to be 1.814+/-0.017 R_sun (0.9%) and 5872+/-44 K (0.7%) respectively. By combining this value with the mean density, as estimated from asteroseismology, we make a direct estimate of the stellar mass. We find a value of 1.07+/-0.03 M_sun (2.8%), which agrees with published estimates based on fitting in the H-R diagram, but has much higher precision. These results place valuable constraints on theoretical models of beta Hyi and its oscillation frequencies.

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arXiv2007

Born Again Protoplanetary Disk Around Mira B

The Mira AB system is a nearby (~107 pc) example of a wind accreting binary star system. In this class of system, the wind from a mass-losing red giant star (Mira A) is accreted onto a companion (Mira B), as indicated by an accretion shock signature in spectra at ultraviolet and X-ray wavelengths. Using novel imaging techniques, we report the detection of emission at mid-infrared wavelengths between 9.7 and 18.3 $μ$m from the vicinity of Mira B but with a peak at a radial position about 10 AU closer to the primary Mira A. We interpret the mid-infrared emission as the edge of an optically-thick accretion disk heated by Mira A. The discovery of this new class of accretion disk fed by M-giant mass loss implies a potential population of young planetary systems in white-dwarf binaries which has been little explored, despite being relatively common in the solar neighborhood.

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arXiv2007

γ^2 Velorum: Orbital Solution and Fundamental Parameter Determination with SUSI

The first complete orbital solution for the double-lined spectroscopic binary system γ^2 Velorum, obtained from measurements with the Sydney University Stellar Interferometer (SUSI), is presented. This system contains the closest example of a Wolf-Rayet star and the promise of full characterisation of the basic properties of this exotic high-mass system has subjected it to intense study as an archetype for its class. In combination with the latest radial-velocity results, our orbital solution produces a distance of 336^{+8}_{-7} pc, significantly more distant than the Hipparcos estimation (Schaerer et al. 1997; van der Hucht 1997). The ability to fully specify the orbital parameters has enabled us to significantly reduce uncertainties and our result is consistent with the VLTI observational point (Millour et al. 2006), but not with their derived distance. Our new distance, which is an order of magnitude more precise than prior work, demands critical reassessment of all distance-dependent fundamental parameters of this important system. In particular, membership of the Vela OB2 association has been reestablished, and the age and distance are also in good accord with the population of young stars reported by Pozzo et al. (2000). We determine the O-star primary component parameters to be M_V(O) = -5.63 \pm 0.10 mag, R(O) = 17 \pm 2 R_{\sun} and {\cal M}(O) = 28.5 \pm 1.1 M_{\sun}. These values are consistent with calibrations found in the literature if a luminosity class of II--III is adopted. The parameters of the Wolf-Rayet component are M_v(WR) = -4.33 \pm 0.17 mag and {\cal M}(WR) = 9.0 \pm 0.6 M_{\sun}.

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arXiv2006

The Keck Aperture Masking Experiment: Near-Infrared Sizes of Dusty Wolf-Rayet Stars

We report the results of a high angular resolution near-infrared survey of dusty Wolf-Rayet stars using the Keck-1 Telescope, including new multi-wavelength images of the pinwheel nebulae WR 98a, WR 104, and WR 112. Angular sizes were measured for an additional 8 dusty WR stars using aperture masking interferometry, allowing us to probe characteristics sizes down to ~20 milliarcseconds (~40 AU for typical sources). With angular sizes and specific fluxes, we can directly measure the wavelength-dependent surface brightness and size relations for our sample. We discovered tight correlations of these properties within our sample which could not be explained by simple spherically-symmetric dust shells or even the more realistic ``pinwheel nebula'' (3-D) radiative transfer model, when using optical constants of Zubko. While the tightly-correlated surface brightness relations we uncovered offer compelling indirect evidence of a shared and distinctive dust shell geometry amongst our sample, long-baseline interferometers should target the marginally-resolved objects in our sample in order to conclusively establish the presence or absence of the putative underyling colliding wind binaries thought to produce the dust shells around WC Wolf-Rayets.

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arXiv2006

Orbital elements, masses and distance of lambda Scorpii A and B determined with the Sydney University Stellar Interferometer and high resolution spectroscopy

The triple system HD158926 (lambda Sco) has been observed interferometrically with the Sydney University Stellar Interferometer and the elements of the wide orbit have been determined. These are significantly more accurate than the previous elements found spectroscopically. The inclination of the wide orbit is consistent with the inclination previously found for the orbit of the close companion. The wide orbit also has low eccentricity, suggesting that the three stars were formed at the same time. The brightness ratio between the two B stars was also measured at lambda = 442nm and 700nm. The brightness ratio and colour index are consistent with the previous classification of lambda Sco A as B1.5 and lambda Sco B as B2. Evolutionary models show that the two stars lie on the main sequence. Since they have have the same age and luminosity class (IV) the mass-luminosity relation can be used to determine the mass ratio of the two stars: M_B/M_A = 0.76+/-0.04. The spectroscopic data have been reanalyzed using the interferometric values for P, T, e and omega, leading to revised values for a_1sin i and the mass function. The individual masses can be found from the mass ratio, the mass function, spectrum synthesis and the requirement that the age of both components must be the same: M_A = 10.4+/-1.3 Msun and M_B = 8.1+/-1.0 Msun. The masses, angular semimajor axis and the period of the system can be used to determine the dynamical parallax. We find the distance to lambda Sco to be 112+/-5 pc, which is approximately a factor of two closer than the HIPPARCOS value of 216+/-42 pc.

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arXiv2005

Keck Interferometer Observations of FU Orionis Objects

We present new K-band long baseline interferometer observations of three young stellar objects of the FU Orionis class, V1057 Cyg, V1515 Cyg and Z CMa-SE, obtained at the Keck Interferometer during its commissioning science period. The interferometer clearly resolves the source of near-infrared emission in all three objects. Using simple geometrical models we derive size scales (0.5-4.5 AU) for this emission. All three objects appear significantly more resolved than expected from simple models of accretion disks tuned to fit the broadband optical and infrared spectro-photometry. We explore variations in the key parameters that are able to lower the predicted visibility amplitudes to the measured levels, and conclude that accretion disks alone do not reproduce the spectral energy distributions and K-band visibilities simultaneously. We conclude that either disk models are inadequate to describe the near-infrared emission, or additional source components are needed. We hypothesize that large scale emission (10s of AU) in the interferometer field of view is responsible for the surprisingly low visibilities. This emission may arise in scattering by large envelopes believed to surround these objects.

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arXiv2005

Dust Scattering in Miras R Car and RR Sco resolved by optical interferometric polarimetry

We present optical interferometric polarimetry measurements of the Mira-like variables R Car and RR Sco, using the Sydney University Stellar Interferometer. By making visibility measurements in two perpendicular polarisations, the relatively low-surface brightness light scattered by atmospheric dust could be spatially separated from the bright Mira photospheric flux. This is the first reported successful use of long-baseline optical interferometric polarimetry. Observations were able to place constraints on the distribution of circumstellar material in R Car and RR Sco. The inner radius of dust formation for both stars was found to be less than 3 stellar radii: much closer than the expected innermost stable location for commonly-assumed astrophysical ``dirty silicate'' dust in these systems (silicate dust with a significant iron content). A model with the dust distributed over a shell which is geometrically thin compared to the stellar radius was preferred over an outflow. We propose dust components whose chemistry and opacity properties enable survival at these extreme inner radii.

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arXiv2005

Multi-wavelength diffraction-limited imaging of the evolved carbon star IRC +10216, II

High angular resolution images of IRC +10216 taken at various bandpasses within the near-infrared h, k & l bands are presented. The maps have the highest angular resolution yet recovered, and were reconstructed from interferometric measurements obtained at the Keck 1 telescope in 1997 December and 1998 April, forming a subset of a 7-epoch monitoring program presented earlier [paper 1]. Systematic changes with observing wavelength are found and discussed in context of present geometrical models for the circumstellar envelope. With these new high-resolution, multi-wavelength data and contemporaneous photometry, we also re-visit the hypothesis that the bright compact Core of the nebula (component `A') marks the location of the central carbon star. We find that directly measured properties of the Core (angular size, flux density, color temperature) are consistent with a reddened carbon star photosphere (line-of-sight tau=5.3).

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arXiv2004

Subarcsecond mid-infrared and radio observations of the W3 IRS5 protocluster

Observations at mid-infrared (4.8-17.65 um) and radio (0.7-1.3 cm) wavelengths are used to constrain the structure of the high-mass star-forming region W3 IRS5 on 0.1'' (200 AU) scales. Two bright mid-infrared sources are detected, as well as diffuse emission. The bright sources have associated compact radio emission and probably are young high-mass stars. The measured sizes and estimated temperatures indicate that these sources together can supply the observed far-infrared luminosity. However, an optically thick radio source with a possible mid-infrared counterpart may also contribute significant luminosity; if so, it must be extremely deeply embedded. The infrared colour temperatures of 350-390 K and low radio brightness suggest gravitational confinement of the H II regions and ongoing accretion at a rate of a few 10^-8 M0/yr or more. Variations in the accretion rate would explain the observed radio variability. The low estimated foreground extinction suggests the existence of a cavity around the central stars, perhaps blown by stellar winds. At least three radio sources without mid-infrared counterparts appear to show proper motions of ~100 km/s, and may be deeply embedded young runaway OB stars, but more likely are clumps in the ambient material which are shock-ionized by the OB star winds.

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arXiv2004

Pulsation of M-type Mira variables with moderately different mass: search for observable mass effects

Models of M-type Miras with masses of 1 $M_\odot$ and 1.2 $M_\odot$, i.e. with envelope masses of about 0.4 $M_\odot$ and 0.6 $M_\odot$, have been constructed, and a comparison has been made of their observable properties. Geometric pulsation of continuum-forming layers is found to be little affected by the mass difference. The influence of molecular contamination of near-infrared continuum bandpasses upon interferometrically measured fit diameters ranges from undetectable to quite significant. Some pulsation cycles of the lower-mass model Mira show substantially stronger contamination than that found in any cycle of the higher-mass star. Observations which sample pulsation phase well and continuously are crucial for avoiding misinterpretations, because the assignment of absolute pulsation phases is inherently uncertain by at least 0.1 cycles, diameter changes may be strongly phase-dependent, and cycle-to-cycle variations may be substantial. In accord with expectations, we find that cycle-to-cycle variations that show up in light curves and in near-continuum diameters tend to be larger and more common in the low-mass models, leading to one possible way to discriminate mass. Two other methods, based on high-precision measurements of the pulsation amplitude and on derivation of pre-maximum effective temperatures from diameter measurements, are also discussed. High-layer features that may be strongly affected by mass are not well described by present dust-free models.

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arXiv2004

High Angular Resolution Mid-infrared Imaging of Young Stars in Orion BN/KL

We present Keck LWS images of the Orion BN/KL star forming region obtained in the first multi-wavelength study to have 0.3-0.5" resolution from 4.7 to 22 microns. The young stellar objects designated infrared source-n and radio source-I are believed to dominate the BN/KL region. We have detected extended emission from a probable accretion disk around source-n but infer a stellar luminosity on the order of only 2000 Lsun. Although source-I is believed to be more luminous, we do not detect an infrared counterpart even at the longest wavelengths. However, we resolve the closeby infrared source, IRc2, into an arc of knots ~1000 AU long at all wavelengths. Although the physical relation of source-I to IRc2 remains ambiguous, we suggest these sources mark a high density core (10^7-10^8 pc^-3 over 1000 AU) within the larger BN/KL star forming cluster. The high density may be a consequence of the core being young and heavily embedded. We suggest the energetics of the BN/KL region may be dominated by this cluster core rather than one or two individual sources.

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arXiv2004

Multi-wavelength Diameters of Nearby Miras and Semiregulars

We have used optical interferometry to obtain multi-wavelength visibility curves for eight red giants over the wavelength range 650--1000 nm. The observations consist of wavelength-dispersed fringes recorded with MAPPIT (Masked APerture-Plane Interference Telescope) at the 3.9-m Anglo-Australian Telescope. We present results for four Miras (R Car, $o$ Cet, R Hya, R Leo) and four semi-regular variables (R Dor, W Hya, L$_2$ Pup, $γ$ Cru). All stars except $γ$ Cru show strong variations of angular size with wavelength. A uniform-disk model was found to be a poor fit in most cases, with Gaussian (or other more tapered profiles) preferred. This, together with the fact that most stars showed a systematic increase in apparent size toward the blue and a larger-than-expected linear size, even in the red, all point toward significant scattering by dust in the inner circumstellar environment. Some stars showed evidence for asymmetric brightness profiles, while L$_2$ Pup required a two-component model, indicating an asymmetrical circumstellar dust shell.

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arXiv2004

High-resolution imaging of dust shells using Keck aperture masking and the IOTA Interferometer

We present first results of an experiment to combine data from Keck aperture masking and the Infrared-Optical Telescope Array (IOTA) to image the circumstellar environments of evolved stars with ~20 milliarcsecond resolution. The unique combination of excellent Fourier coverage at short baselines and high-quality long-baseline fringe data allows us to determine the location and clumpiness of the inner-most hot dust in the envelopes, and to measure the diameters of the underlying stars themselves. We find evidence for large-scale inhomogeneities in some dust shells and also significant deviations from uniform brightness for the photospheres of the most evolved M-stars. Deviations from spherically-symmetric mass loss in the red supergiant NML Cyg could be related to recent evidence for dynamically-important magnetic fields and/or stellar rotation. We point out that dust shell asymmetries, like those observed here, can qualitatively explain the difficulty recent workers have had in simultaneously fitting the broad-band spectral energy distributions and high-resolution spatial information, without invoking unusual dust properties or multiple distinct shells (from hypothetical ``superwinds''). This paper is the first to combine optical interferometry data from multiple facilities for imaging, and we discuss the challenges and potential for the future of this method, given current calibration and software limitations.

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arXiv2002

Imaging the disk around the luminous young star LkHalpha 101 with infrared interferometry

The Herbig Ae/Be star LkHalpha 101 has been imaged at high angular resolution at a number of wavelengths in the near-infrared (from 1 to 3 microns) using the Keck 1 Telescope, and also observed in the mid-infrared (11.15 microns) using the U.C. Berkeley Infrared Spatial Interferometer (ISI). The resolved circular disk with a central hole or cavity reported in Tuthill et al. (2001) is confirmed. This is consistent with an almost face-on view (inclination < 35 deg) onto a luminous pre- or early-main sequence object surrounded by a massive circumstellar disk. With a multiple-epoch study spanning almost four years, relative motion of the binary companion has been detected, together with evidence for changes in the brightness distribution of the central disk/star. The resolution of the LkHalpha 101 disk by ISI mid-infrared interferometry constitutes the first such measurement of a young stellar object in this wavelength region. The angular size was found to increase only slowly from 1.6 to 11.15 microns, inconsistent with standard power-law temperature profiles usually encountered in the literature, supporting instead models with a hot inner cavity and relatively rapid transition to a cool or tenuous outer disk. The radius of the dust-free inner cavity is consistent with a model of sublimation of dust in equilibrium with the stellar radiation field. Measurements from interferometry have been combined with published photometry enabling an investigation of the energetics and fundamental properties of this prototypical system.

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arXiv2002

Proper motions of new dust in the colliding-wind binary WR 140

The eccentric WR+O binary system WR 140 produces dust for a few months at intervals of 7.94 yrs coincident with periastron passage. We present the first resolved images of this dust shell, at binary phases ~0.039 and ~0.055, using aperture masking techniques on the Keck-I telescope to achieve diffraction-limited resolution. Proper motions of approximately 1.1 milliarcsecond per day were detected, implying a distance ~1.5 kpc from the known wind speed. The dust plume observed is not as simple as the ``pinwheel'' nebulae seen around other WR colliding wind binaries, indicating the orbital plane is highly inclined to our line-of-sight and/or the dust formation is very clumpy. Follow-up imaging in the mid-infrared and with adaptive optics is urgently required to track the dust motion further, necessary for unambiguously determining the orbital geometry which we only partially constrain here. With full knowledge of the orbital elements, these infrared images can be used to reconstruct the dust distribution along the colliding wind interface, providing a unique tool for probing the post-shock physical conditions of violent astrophysical flows.

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arXiv2001

Radio Properties of Pinwheel Nebulae

A small number of dusty Wolf-Rayet stars have been resolved into pinwheel nebulae, defined by their ``rotating'' spiral dust shells observed in the infrared. This morphology is naturally explained by dust formation associated with colliding winds in a binary system. In order to confirm and further explore this hypothesis, we have observed the known pinwheel nebulae (WR 104 and WR 98a) as well as the suspected binary WR 112 at multiple radio wavelengths with the Very Large Array to search for non-thermal radio emission from colliding winds. The spectrum of each target is nearly flat between 5 and 22 GHz, consistent with the presence of non-thermal emission that is reduced at low frequencies by free-free absorption. This emission must lie outside the radio ``photosphere,'' leading us to estimate a lower limit to the physical size of the non-thermal emitting region that is larger than expected from current theory. Based on a radio and infrared comparison to WR 104 and 98a, we conclude that WR 112 is a likely candidate pinwheel nebula, but its temporal variability indicates an eccentric binary orbit or a pinwheel viewed nearly edge-on. A sensitive radio survey of IR-bright WRs would stringently test the hypothesis that colliding winds lie at the heart of_all_ dusty WR systems. We also discuss the effects of dust obscuration in the ultra-violet and how radio-determined mass-loss rates of pinwheel nebulae (and dusty WR stars in general) may be underestimated due to shadowing effects.

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arXiv2001

Radio and Infrared Properties of Dust-Enshrouded Wolf-Rayet Stars

This paper discusses our ongoing efforts to characterize dust-enshrouded Wolf-Rayet (WR) stars in the radio and infrared. We have used the Very Large Array to measure the broadband radio spectrum of WR stars in suspected binary systems and discovered non-thermal emission, which is usually attributed to colliding winds. In addition, infrared imaging using aperture masking interferometry on the Keck-I telescope has resolved the dust shells around a number of WR stars with K-magnitudes brighter than ~6. Although this admittedly small study suffers from selection bias, we note that all the dust-enshrouded WR stars with radio detections show evidence for colliding winds, supporting the theory that wind compression in a binary system is necessary for efficient dust production. A consequence of this hypothesis is that virtually all WC8-10 stars must be in binaries, since most are dusty. Single-star and binary stellar evolution models will have to be modified to accommodate this observational result if confirmed.

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arXiv2000

Diffraction-limited near-IR imaging at Keck reveals asymmetric, time-variable nebula around carbon star CIT 6

We present multi-epoch, diffraction-limited images of the nebula around the carbon star CIT 6 at 2.2 microns and 3.1 microns from aperture masking on the Keck-I telescope. The near-IR nebula is resolved into two main components, an elongated, bright feature showing time-variable asymmetry and a fainter component about 60 milliarcseconds away with a cooler color temperature. These images were precisely registered (~35 milliarcseconds) with respect to recent visible images from the Hubble Space Telescope (Trammell et al. 2000), which showed a bipolar structure in scattered light. The dominant near-IR feature is associated with the northern lobe of this scattering nebula, and the multi-wavelength dataset can be understood in terms of a bipolar dust shell around CIT 6. Variability of the near-IR morphology is qualitatively consistent with previously observed changes in red polarization, caused by varying illumination geometry due to non-uniform dust production. The blue emission morphology and polarization properties can not be explained by the above model alone, but require the presence of a wide binary companion in the vicinity of the southern polar lobe. The physical mechanisms responsible for the breaking of spherical symmetry around extreme carbon stars, such as CIT 6 and IRC+10216, remain uncertain.

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