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Very Large Telescope Interferometer images young, high-mass stars

Near-IR interferometry has enabled the first aperture-synthesis imaging observations of a dusty circumstellar disk around a massive, young stellar object.
24 September 2010, SPIE Newsroom. DOI: 10.1117/2.1201008.003201

In recent years, the discipline of astronomical IR interferometry has experienced dramatic technological advancements, enabling exciting new science prospects and observations of new classes of astrophysical objects. We recently used near-IR interferometry to investigate the close circumstellar environment of a massive, young stellar object (YSO).1 Although scarce in number, massive stars (>10M, where M refers to the Sun's mass) are of fundamental importance for many areas of astrophysics. Starting soon after their birth, they disperse their natal molecular clouds through strong stellar outflows. After a short hydrogen-burning phase, they explode as supernovae and enrich the interstellar medium with heavy elements. Despite their significance, high-mass star formation is still poorly understood. Early theoretical studies suggested that gradual accretion of substellar clumps, which appears a well-established mass-buildup scenario for stars with masses below 10M, might not work for the formation of massive protostars because of strong radiation pressure.2 An alternative scenario proposed that high-mass stars might form by stellar merging.3 Therefore, clear observational evidence—such as the detection of compact, dusty disks around massive YSOs—is needed to unambiguously identify the formation mode of the most massive stars.

Earlier observations, mostly at radio wavelengths, already provided fascinating insights into the distribution and kinematics of the circumstellar material associated with high-mass protostars. However, with angular resolutions of typically 0.2 to 0.5 arcseconds (at near-IR and submillimeter/centimeter wavelengths, respectively), most earlier studies probed physical scales of several hundreds to thousands of astronomical units (AUs, the distance from the earth to the sun), mainly tracing the extended, rotating stellar envelopes. IR interferometry now enables us to achieve the milliarcsecond (mas) resolution required to separate envelope and disk contributions.

To achieve this high resolution, IR interferometers combine light from multiple telescopes, thus forming a ‘virtual telescope’ with a best resolving power corresponding to the maximum telescope separation. However, the technological challenges associated with IR interferometric observations of massive YSOs are significant, mainly because these objects are still deeply embedded in their natal molecular clouds, resulting in an extremely steep drop of the spectral-energy (intensity) distribution at IR wavelengths. In addition, the inner environments of YSOs are rather complex and potentially include the presence of compact stellar clusters or disks with asymmetric substructure. To deal with this expected complexity, model-independent aperture-synthesis imaging is required.

We observed1 the massive YSO IRAS 13481−6124, located at a distance of approximately 3500pc. The object harbors a central object with a mass of ~20M.4 We observed this star with the near-IR three-telescope Astronomical Multi-Beam Combiner (AMBER)5 at the European Southern Observatory (ESO)'s Very Large Telescope Interferometer (VLTI) and its array of 1.8m auxiliary telescopes (ATs). The VLTI ATs are equipped with STRAP (System for Tip/tilt Removal with Avalanche Photodiodes) units to provide tip-tilt correction based on simultaneous observations of an optical guide star. The guide star can be offset from the science object of interest, which allowed us to observe the embedded YSO while guiding the telescope on a nearby (angular distance ~18 arcseconds) 12th-magnitude foreground star. We obtained our VLTI observations using three different telescope configurations, covering baseline lengths between 12 and 85m. We also obtained speckle-interferometric observations with ESO's New Technology Telescope, yielding precise visibility information for baseline lengths up to 3.5m.

Using our extensive VLTI/speckle data set and a dedicated image-reconstruction algorithm,6 we reconstructed a model-independent interferometric image from the measured visibility amplitudes and closure phases. This technique, which is routinely applied in the field of radio interferometry, has recently been demonstrated for the VLTI. Our study was its first application to image the disk around a young star. Our image is characterized by an angular resolution of λ2B=2.4mas (where λ is the observational wavelength and B the maximum baseline) or ~8.4AU. This is at least one order of magnitude higher than what can be achieved with conventional IR imaging techniques at 10m-class telescopes, while the gain compared to state-of-the-art (sub)millimeter disk studies is roughly two orders of magnitude. The reconstructed image—see Figure 1(c)—clearly resolves the inner environment of IRAS 13481−6124 and reveals a compact, elongated structure with a size of ∼5×8mas2 oriented along position angle 120° (east with respect to north).

Figure 1. Zoom in on the young stellar object IRAS 13481-6124, covering spatial scales spanning more than five orders of magnitude. (a) In Spitzer/IR Array Camera (IRAC) images, we detect two bow-shock structures, indicating a collimated outflow. NE: Northeast. SW: Southwest. AU: Astronomical unit (the distance from the earth to the sun). (Note that 3′ is an angular scale, representing 3 arcminutes.) (b) The outflow is also detected in carbon monoxide (12CO) molecular-line emission using the Atacama Pathfinder Experiment (APEX)/Swedish Heterodyne Facility Instrument (SHFI), on scales of a few 10,000AU. (30′′ refers to 30 arcseconds.) (c) Applying our image-reconstruction algorithm to Very Large Telescope Interferometer (VLTI)/Astronomical Multi-Beam Combiner (AMBER) data, we reconstructed a model-independent aperture-synthesis image of IRAS 13481-6124. The image reveals an elongated structure that is oriented perpendicular to the outflow direction (successive contours decrease from the peak intensity by factors of ). SED: Spectral-energy distribution. (d) Elongation of the compact-emission component, determined with our temperature-gradient disk model.

To further characterize the structure, we employed model-fitting techniques that show that the disk size increases toward longer wavelengths, thus providing evidence for a temperature gradient within the disk. In addition, the model suggests that the disk is free of dust inside a radius of ~6AU, indicating a dust-free inner hole comparable to the holes observed in irradiated disks around low-mass YSOs.

In addition, in archival Spitzer Space Telescope images—see Figure 1(a)—we discovered two faint bow shocks, revealing the presence of a large-scale bipolar outflow. With a relatively narrow bow-shock opening angle of ~6°, the outflow is remarkably collimated, exhibiting similarities to the jets observed in low-mass star-forming regions. To also search for outflow signatures in molecular-line emission, we obtained a carbon monoxide (12CO) 346GHz map with the Atacama Pathfinder Experiment (APEX) 12m telescope. We detected a bipolar outflow perpendicular to the disk plane, suggesting that the AU-scale disk resolved by VLTI/AMBER is indeed the driving engine of the collimated outflow.

Our study provides direct evidence for the existence of disks around young, high-mass stars and allows, for the first time, to characterize the geometry of the innermost disk regions. Our study also illustrates the importance of a multiwavelength (2–870μm) and multiscale (from parsecs to AUs) approach, and demonstrates that IR interferometry can now address key scientific issues, such as important open questions in the field of high-mass star formation. However, with the currently available near-IR interferometric instrumentation, the number of accessible high-mass YSOs is limited, in particular because of the current sensitivity limits and the need for off-axis telescope guiding. The next generation of instruments will provide considerable improvements. For instance, IR wavefront-sensing units, such as planned for the VLTI/GRAVITY instrument,7 will enable interferometric observations of massive YSOs without suitable visual guide stars. Further progress is also expected from interferometric instruments with extended spectral coverage, such as the second-generation VLTI/Multi-Aperture Mid-IR Spectroscopic Experiment (MATISSE),8 which will cover the L, M, and N passbands between 3 and 13μm.

This work was done in part under contract with the California Institute of Technology, funded by NASA through the Sagan Fellowship Program. We thank the ESO staff for support and their efforts in improving the VLTI.

Stefan Kraus
University of Michigan
Ann Arbor, MI

Stefan Kraus is a NASA Carl Sagan postdoctoral fellow. His research focuses on interferometric studies related to star and planet formation.