arXiv subjects

E. Pedretti

Publications and source records attributed to E. Pedretti.

36 records · Page 2Linked to original sources

arXiv2009

Imaging and Modeling Rapidly Rotating Stars: Alpha Cephei and Alpha Ophiuchi

We present sub-milliarcseond resolution imaging and modeling of two nearby rapid rotators Alpha Cephei and Alpha Ophiuchi, obtained with the CHARA array - the largest optical/IR interferometer in the world. Incorporating a gravity darkening model, we are able to determine the inclination, the polar and equatorial radius and temperature, as well as the fractional rotation speed of the two stars with unprecedented precision. The polar and equatorial regions of the two stars have ~2000K temperature gradient, causing their apparent temperatures and luminosities to be dependent on their viewing angles. Our modeling allow us to determine the true effective temperatures and luminosities of Alpha Cep and Alpha Oph, permitting us to investigate their true locations on the H-R diagram. These properties in turn give us estimates of the masses and ages of the two stars within a few percent of error using stellar evolution models. Also, based on our gravity darkening modeling, we propose a new method to estimate the masses of single stars in a more direct way through Vsin(i) measurements and precise geometrical constraint. Lastly, we investigate the degeneracy between the inclination and the gravity darkening coefficient, which especially affects the modeling of Alpha Oph. Although incorporating Vsin(i) has lifted the degeneracy to some extent, higher resolution observations are still needed to further constrain the parameters independently.

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arXiv2009

Detection of non-radial pulsation and faint companion in the symbiotic star CH Cyg

We have detected asymmetry in the symbiotic star CH Cyg through the measurement of precision closure-phase with the IONIC beam combiner, at the IOTA interferometer. The position of the asymmetry changes with time and is correlated with the phase of the 2.1-yr period found in the radial velocity measurements for this star. We can model the time-dependent asymmetry either as the orbit of a low-mass companion around the M giant or as an asymmetric, 20% change in brightness across the M giant. We do not detect a change in the size of the star during a 3 year monitoring period neither with respect to time nor with respect to wavelength. We find a spherical dust-shell with an emission size of 2.2+/-0.1 D* FWHM around the M giant star. The star to dust flux ratio is estimated to be 11.63+/-0.3. While the most likely explanation for the 20% change in brightness is non-radial pulsation we argue that a low-mass companion in close orbit could be the physical cause of the pulsation. The combined effect of pulsation and low-mass companion could explain the behaviour revealed by the radial-velocity curves and the time-dependent asymmetry detected in the closure-phase data. If CH Cyg is a typical long secondary period variable then these variations could be explained by the effect of an orbiting low-mass companion on the primary star.

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arXiv2009

Imaging the asymmetric dust shell around CI Cam with long baseline optical interferometry

We present the first high angular resolution observation of the B[e] star/X-ray transient object CI Cam, performed with the two-telescope Infrared Optical Telescope Array (IOTA), its upgraded three-telescope version (IOTA3T) and the Palomar Testbed Interferometer (PTI). Visibilities and closure phases were obtained using the IONIC-3 integrated optics beam combiner. CI Cam was observed in the near-infrared H and K spectral bands, wavelengths well suited to measure the size and study the geometry of the hot dust surrounding CI Cam. The analysis of the visibility data over an 8 year period from soon after the 1998 outburst to 2006 shows that the dust visibility has not changed over the years. The visibility data shows that CI Cam is elongated which confirms the disc-shape of the circumstellar environment and totally rules out the hypothesis of a spherical dust shell. Closure phase measurements show direct evidence of asymmetries in the circumstellar environment of CI Cam and we conclude that the dust surrounding CI Cam lies in an inhomogeneous disc seen at an angle. The near-infrared dust emission appears as an elliptical skewed Gaussian ring with a major axis a = 7.58 +/- 0.24 mas, an axis ratio r = 0.39 +/- 0.03 and a position angle theta = 35 +/- 2 deg.

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arXiv2008

A Tale of Two Herbig Ae stars -MWC275 and AB Aurigae: Comprehensive Models for SED and Interferometry

We present comprehensive models for the Herbig Ae stars MWC275 and AB Aur that aim to explain their spectral energy distribution (from UV to millimeter) and long baseline interferometry (from near-infrared to millimeter) simultaneously. Data from the literature, combined with new mid-infrared (MIR) interferometry from the Keck Segment Tilting Experiment, are modeled using an axisymmetric Monte Carlo radiative transfer code. Models in which most of the near-infrared (NIR) emission arises from a dust rim fail to fit the NIR spectral energy distribution (SED) and sub-milli-arcsecond NIR CHARA interferometry. Following recent work, we include an additional gas emission component with similar size scale to the dust rim, inside the sublimation radius, to fit the NIR SED and long-baseline NIR interferometry on MWC275 and AB Aur. In the absence of shielding of star light by gas, we show that the gas-dust transition region in these YSOs will have to contain highly refractory dust, sublimating at ~1850K. Despite having nearly identical structure in the thermal NIR, the outer disks of MWC275 and AB Aur differ substantially. In contrast to the AB Aur disk, MWC275 lacks small grains in the disk atmosphere capable of producing significant 10-20micron emission beyond ~7AU, forcing the outer regions into the "shadow" of the inner disk

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arXiv2008

First Resolved Images of the Eclipsing and Interacting Binary Beta Lyrae

We present the first resolved images of the eclipsing binary Beta Lyrae, obtained with the CHARA Array interferometer and the MIRC combiner in the H band. The images clearly show the mass donor and the thick disk surrounding the mass gainer at all six epochs of observation. The donor is brighter and generally appears elongated in the images, the first direct detection of photospheric tidal distortion due to Roche-lobe filling. We also confirm expectations that the disk component is more elongated than the donor and is relatively fainter at this wavelength. Image analysis and model fitting for each epoch were used for calculating the first astrometric orbital solution for Beta Lyrae, yielding precise values for the orbital inclination and position angle. The derived semi-ma jor axis also allows us to estimate the distance of Beta Lyrae; however, systematic differences between the models and the images limit the accuracy of our distance estimate to about 15%. To address these issues, we will need a more physical, self-consistent model to account for all epochs as well as the multi-wavelength information from the eclipsing light curves.

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arXiv2008

The limb-darkened Arcturus; Imaging with the IOTA/IONIC interferometer

This paper is an H band interferometric examination of Arcturus, a star frequently used as a spatial and spectral calibrator. Using the IOTA 3 telescope interferometer, we performed spectro-interferometric observation (R~35) of Arcturus. Atmospheric models and prescriptions were fitted to the data to derive the brightness distribution of the photosphere. Image reconstruction was also obtained using two software algorithms: Wisard and Mira. An achromatic power law proved to be a good model of the brightness distribution, with a limb darkening compatible with the one derived from atmospheric model simulations using our Marcs model. A Rosseland diameter of 21.05+/-0.21 was derived, corresponding to an effective temperature of T_eff=4295+/-26 K. No companion was detected from the closure phases, with an upper limit on the brightness ratio of 8e-4 at 1AU. Dynamic range at such distance from the photosphere was established at 1.5e-4 (1sigma rms). An upper limit of 1.7e-3 was also derived for the level of brightness asymmetries present on the photosphere.

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arXiv2008

Strong Near-Infrared Emission Interior to the Dust-Sublimation Radius of Young Stellar Objects MWC275 and AB Aur

Using the longest optical-interferometeric baselines currently available, we have detected strong near-infrared (NIR) emission from inside the dust-destruction radius of Herbig Ae stars MWC275 and AB Aur. Our sub-milli-arcsecond resolution observations unambiguously place the emission between the dust-destruction radius and the magnetospheric co-rotation radius. We argue that this new component corresponds to hot gas inside the dust-sublimation radius, confirming recent claims based on spectrally-resolved interferometry and dust evaporation front modeling.

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arXiv2007

Imaging the Surface of Altair

Spatially resolving the surfaces of nearby stars promises to advance our knowledge of stellar physics. Using optical long-baseline interferometry, we present here a near-infrared image of the rapidly rotating hot star Altair with <1 milliarcsecond resolution. The image clearly reveals the strong effect of gravity darkening on the highly-distorted stellar photosphere. Standard models for a uniformly rotating star can not explain our results, requiring differential rotation, alternative gravity darkening laws, or both.

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arXiv2007

Visual/infrared interferometry of Orion Trapezium stars: Preliminary dynamical orbit and aperture synthesis imaging of the Theta 1 Orionis C system

Located in the Orion Trapezium cluster, Theta 1 Orionis C is one of the youngest and nearest high-mass stars (O5-O7) and also known to be a close binary system. Using new multi-epoch visual and near-infrared bispectrum speckle interferometric observations obtained at the BTA 6 m telescope, and IOTA near-infrared long-baseline interferometry, we trace the orbital motion of the Theta 1 Ori C components over the interval 1997.8 to 2005.9, covering a significant arc of the orbit. Besides fitting the relative position and the flux ratio, we apply aperture synthesis techniques to our IOTA data to reconstruct a model-independent image of the Theta 1 Ori C binary system. The orbital solutions suggest a high eccentricity (e approx. 0.91) and short-period (P approx. 10.9 yrs) orbit. As the current astrometric data only allows rather weak constraints on the total dynamical mass, we present the two best-fit orbits. From these orbital solutions one can be favoured, implying a system mass of 48 M_sun and a distance to the Trapezium cluster of 434 pc. When also taking the measured flux ratio and the derived location in the HR-diagram into account, we find good agreement for all observables, assuming a spectral type of O5.5 for Theta 1 Ori C1 (M=34.0 M_sun) and O9.5 for C2 (M=15.5 M_sun). We find indications that the companion C2 is massive itself, which makes it likely that its contribution to the intense UV radiation field of the Trapezium cluster is non-negligible. Furthermore, the high eccentricity of the preliminary orbit solution predicts a very small physical separation during periastron passage (approx. 1.5 AU, next passage around 2007.5), suggesting strong wind-wind interaction between the two O stars.

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arXiv2006

Physical Orbit for Lambda Virginis and a Test of Stellar Evolution Models

Lambda Virginis (LamVir) is a well-known double-lined spectroscopic Am binary with the interesting property that both stars are very similar in abundance but one is sharp-lined and the other is broad-lined. We present combined interferometric and spectroscopic studies of LamVir. The small scale of the LamVir orbit (~20 mas) is well resolved by the Infrared Optical Telescope Array (IOTA), allowing us to determine its elements as well as the physical properties of the components to high accuracy. The masses of the two stars are determined to be 1.897 Msun and 1.721 Msun, with 0.7% and 1.5% errors respectively, and the two stars are found to have the same temperature of 8280 +/- 200 K. The accurately determined properties of LamVir allow comparisons between observations and current stellar evolution models, and reasonable matches are found. The best-fit stellar model gives LamVir a subsolar metallicity of Z=0.0097, and an age of 935 Myr. The orbital and physical parameters of LamVir also allow us to study its tidal evolution time scales and status. Although currently atomic diffusion is considered to be the most plausible cause of the Am phenomenon, the issue is still being actively debated in the literature. With the present study of the properties and evolutionary status of LamVir, this system is an ideal candidate for further detailed abundance analyses that might shed more light on the source of the chemical anomalies in these A stars.

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arXiv2006

No Expanding Fireball: Resolving the Recurrent Nova RS Ophiuchi with Infrared Interferometry

Following the recent outburst of the recurrent nova RS Oph on 2006 Feb 12, we measured its near-infrared size using the IOTA, Keck, and PTI Interferometers at multiple epochs. The characteristic size of ~3 milliarcseconds hardly changed over the first 60 days of the outburst, ruling out currently-popular models whereby the near-infrared emission arises from hot gas in the expanding shock. The emission was also found to be significantly asymmetric, evidenced by non-zero closure phases detected by IOTA. The physical interpretation of these data depend strongly on the adopted distance to RS Oph. Our data can be interpreted as the first direct detection of the underlying RS Oph binary, lending support to the recent ``reborn red giant'' models of Hachisu & Kato. However, this result hinges on an RS Oph distance of ~< 540 pc, in strong disagreement with the widely-adopted distance of ~1.6 kpc. At the farther distance, our observations imply instead the existence of a non-expanding, dense and ionized circumbinary gaseous disk or reservoir responsible for the bulk of the near-infrared emission. Longer-baseline infrared interferometry is uniquely suited to distinguish between these models and to ultimately determine the distance, binary orbit, and component masses for RS Oph, one of the closest-known (candidate) SNIa progenitor systems.

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arXiv2006

First Surface-resolved Results with the IOTA Imaging Interferometer: Detection of Asymmetries in AGB stars

We have measured non-zero closure phases for about 29% of our sample of 56 nearby Asymptotic Giant Branch (AGB) stars, using the 3-telescope Infrared Optical Telescope Array (IOTA) interferometer at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 milliarcseconds. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained, and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.

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arXiv2006

Few Skewed Disks Found in First Closure-Phase Survey of Herbig Ae/Be stars

Using the 3-telescope IOTA interferometer on Mt. Hopkins, we report results from the first near-infrared (lambda=1.65 mu) closure-phase survey of Young Stellar Objects (YSOs). These closure phases allow us to unambiguously detect departures from centrosymmetry (i.e., skew) in the emission pattern from YSO disks on the scale of ~4 milliarcseconds, expected from generic ``flared disk'' models. Six of fourteen targets showed small, yet statistically-significant, non-zero closure phases, with largest values from the young binary system MWC 361-A and the (pre-main sequence?) Be star HD 45677. Our observations are quite sensitive to the vertical structure of the inner disk and we confront the predictions of the ``puffed-up inner wall'' models of Dullemond, Dominik, and Natta (DDN). Our data support disks models with curved inner rims because the expected emission appear symmetrically-distributed around the star over a wide range of inclination angles. In contrast, our results are incompatible with the models possessing vertical inner walls because they predict extreme skewness (i.e., large closure phases) from the near-IR disk emission that is not seen in our data. In addition, we also present the discovery of mysterious H-band ``halos'' (~5-10% of light on scales 0.01-0.50 arcsec) around a few objects, a preliminary ``parametric imaging'' study for HD 45677, and the first astrometric orbit for the young binary MWC 361-A.

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arXiv2006

Bright Localized Near-Infrared Emission at 1-4 AU in the AB Aurigae Disk Revealed by IOTA Closure Phases

We report on the detection of localized off-center emission at 1-4 AU in the circumstellar environment of the young stellar object AB Aurigae. We used closure phase measurements in the near-infrared made at the long baseline interferometer IOTA, the first obtained on a young stellar object using this technique. When probing sub-AU scales, all closure phases are close to zero degrees, as expected given the previously-determined size of the AB Aurigae inner dust disk. However, a clear closure phase signal of -3.5 +/- 0.5 degrees is detected on one triangle containing relatively short baselines, requiring a high degree of non-point symmetry from emission at larger (AU-sized) scales in the disk. We have not identified any alternative explanation for these closure phase results and demonstrate that a ``disk hot spot'' model can fit our data. We speculate that such asymmetric near-infrared emission detected might arise as a result of localized viscous heating due to a gravitational instability in the AB Aurigae disk, or to the presence of a close stellar companion or accreting sub-stellar object.

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arXiv2004

Diameters of Mira Stars Measured Simultenously in the J,H,K' Near-Infrared Bands

We present the first spatially resolved observations of a sample of 23 Mira stars simultaneously measured in the near-infrared J, H and K' bands. The technique used was optical long baseline interferometry, and we present for each star visibility amplitude measurements as a function of wavelength. We also present characteristic sizes at each spectral band, obtained by fitting the measured visibilities to a simple uniform disk model. This approach reveals the general relation J diameter < H diameter < K' diameter.

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arXiv2004

LLiST - a new star tracker camera for tip-tilt correction at IOTA

The tip-tilt correction system at the Infrared Optical Telescope Array (IOTA) has been upgraded with a new star tracker camera. The camera features a backside-illuminated CCD chip offering doubled overall quantum efficiency and a four times higher system gain compared to the previous system. Tests carried out to characterize the new system showed a higher system gain with a lower read-out noise electron level. Shorter read-out cycle times now allow to compensate tip-tilt fluctuations so that their error imposed on visibility measurements becomes comparable to, and even smaller than, that of higher-order aberrations.

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arXiv2004

First results with the IOTA3 imaging interferometer: The spectroscopic binaries lambda Vir and WR 140

We report the first spatially-resolved observations of the spectroscopic binaries lambda Vir and WR 140, which includes the debut of aperture-synthesis imaging with the upgraded three-telescope IOTA interferometer. Using IONIC-3, a new integrated optics beam combiner capable of precise closure phase measurement, short observations were sufficient to extract the angular separation and orientation of each binary system and the component brightness ratio. Most notably, the underlying binary in the prototypical colliding-wind source WR 140 (WC7 + O4/5) was found to have a separation of ~13 milli-arcseconds with a position angle consistent with the images of the 2001 dust shell ejection only if the Wolf-Rayet star is fainter than the O star at 1.65 microns. We also highlight lambda Vir whose peculiar stellar properties of the Am star components will permit direct testing of current theories of tidal evolution when the full orbit is determined.

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arXiv2000

First images on the sky from a hyper telescope

We show star images obtained with a miniature ``densified pupil imaging interferometer'' also called a hyper-telescope. The formation of such images violates a ``golden rule of imaging interferometers'' which appeared to forbid the use of interferometric arrangements differing from a Fizeau interferometer. These produce useless images when the sub-apertures spacing is much wider than their size, owing to diffraction through the sub-apertures. The hyper-telescope arrangement solves these problems opening the way towards multi-kilometer imaging arrays in space. We experimentally obtain an intensity gain of 24 +- 3X when a densified-pupil interferometer is compared to an equivalent Fizeau-type interferometer and show images of the double star alpha Gem. The initial results presented confirm the possibility of directly obtaining high resolution and high dynamic range images in the recombined focal plane of a large interferometer if enough elements are used.

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