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Register a new userNew insights on the AU-scale circumstellar structure of FU Orionis
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Authors
Fabien Malbet
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We report new near-infrared, long-baseline interferometric observations at the AU scale of the pre-main-sequence star FU Orionis with the PTI, IOTA and VLTI interferometers. This young stellar object has been observed on 42 nights over a period of 6 years from 1998 to 2003. We have obtained 287 independent measurements of the fringe visibility with 6 different baselines ranging from 20 to 110 meters in length, in the H and K bands. Our extensive (u,v)-plane coverage, coupled with the published spectral energy distribution data, allows us to test the accretion disk scenario. We find that the most probable explanation for these observations is that FU Ori hosts an active accretion disk whose temperature law is consistent with standard models. We are able to constrain the geometry of the disk, including an inclination of 55 deg and a position angle of 47 deg. In addition, a 10 percent peak-to-peak oscillation is detected in the data (at the two-sigma level) from the longest baselines, which we interpret as a possible disk hot-spot or companion. However, the oscillation in our best data set is best explained with an unresolved spot located at a projected distance of 10 AU at the 130 deg position angle and with a magnitude difference of DeltaK = 3.9 and DeltaH = 3.6 mag moving away from the center at a rate of 1.2 AU/yr. we propose to interpret this spot as the signature of a companion of the central FU Ori system on an extremely eccentric orbit. We speculate that the close encounter of this putative companion and the central star could be the explanation of the initial photometric rise of the luminosity of this object.
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We present Keck Interferometer observations of the three prototypical FU Orionis stars, FU Ori, V1057 Cyg, and V1515 Cyg. With a spatial resolution of a few milli-arcseconds and a spectral resolution of 2000, our near-infrared observations spatially resolve gas and dust emission extending from stellocentric radii of ~0.05 AU to several AU. We fit these data with accretion disk models where each stellocentric radius of the disk is represented by a supergiant-type stellar emission spectrum at the disk temperature. A disk model is consistent with the data for FU Ori, although we require some local asymmetry in the disk. For V1057 Cyg the disk model does not fit our data well, especially compared to the fit quality achieved for FU Ori. We speculate that a disk wind may be contributing substantially to the observed near-IR emission in this source. The data for V1515 Cyg are noisier than the data obtained for the other two objects, and do not strongly constrain the validity of an accretion disk model.
New high-resolution spectra of FU Ori, obtained with the HIRES spectrograph at the Keck I telescope in 2003-2006, make it possible to compare the optical line profiles with those predicted by the self-luminous accretion disk model. A dependence of line width on excitation potential and on wavelength, expected for a Keplerian disk, is definitely not present in the optical region, nor is the line duplicity due to velocity splitting. The absorption lines observed in the optical region of FU Ori must originate in or near the central object, and here their profiles are shown to be those expected of a rigidly rotating object. They can be fitted by a rapidly rotating (v sin i = 70 km/s) high-luminosity G-type star having a large dark polar spot, with axis inclined toward the line of sight. Over these years, the radial velocity of FU Ori has remained constant to within +/-0.3 km/s, so there is no indication that the star is a spectroscopic binary. These results apply to the optical region ($lambda< 8800$ AA); more distant, cooler regions of the disk contribute in the infrared.
We present the first infrared interferometric observations of a young stellar object with a spatial projected resolution better than 2 AU. The observations were obtained with the Palomar Testbed Interferometer. FU Ori exhibits a visibility of V^2 =0.72 +/- 0.07 for a 103 +/- 5 m projected baseline at lambda = 2.2 microns. The data are consistent on the spatial scale probed by PTI both with a binary system scenario (maximum magnitude difference of 2.7 +/- 0.5 mag and smallest separation of 0.35 +/- 0.05 AU) and a standard luminous accretion disk model (approx. accretion rate of 6e-5 Mo/yr) where the thermal emission dominates the stellar scattering, and inconsistent with a single stellar photosphere.
By using the ALFA adaptive optics system at the 3.6m telescope of the Calar Alto Observatory we detected a faint red star in the apparent vicinity of FU Ori, the prototype of the FUor outburst stars. Independent confirmation of the detection is obtained from archival PUEO/CFHT images. The separation between the companion candidate and FU Ori is 0.50 and their brightness contrast is around 4 magnitudes. We discuss the possible nature of the newly detected star based on near-infrared photometry and its proper motion relative to FU Ori. The photometric data are consistent with a nearby late-type main sequence star, a background giant star, and a pre-main sequence star. On the basis of the proper motion and the stellar surface density in the direction towards FU Ori, we argue that the probabilities of the first two options are very low.
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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