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New Young Sub-stellar Companion Of the Star GQ Lupi Imaged With the VLT

Garching bei München, Germany – April 7, 2005:   Astronomers using the NACO adaptive optics system on the Very Large Telescope (VLT) have discovered a sub-stellar companion of the T-Tauri star GQ Lupi.
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  Since the discovery in 1995 of the first planet orbiting a normal star other than the Sun, more than 150 candidates of these so-called exoplanets have been found. Most of them have been detected by indirect methods, based either on variations of the radial velocity or the dimming of the star as the planet passes in front of it (e.g. see "ESO HARPS Instrument Discovers Smallest Ever Extra-Solar Planet").

Astronomers would, however, prefer to obtain a direct image of an exoplanet, allowing them to better characterize the object's physical nature. This is an exceedingly difficult task, as the planet is generally hidden in the "glare" of its host star.

To partly overcome this problem, astronomers study very young objects. Sub-stellar objects are much hotter and brighter when young and therefore can be detected more easily than older objects of similar mass.
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Based on this approach, it may well be that last year's detection of a feeble speck of light next to the young brown dwarf 2M1207 by an international team of astronomers using the ESO Very Large Telescope (see news item) is the long-sought bona-fide image of an exoplanet. A recent report based on data from the Hubble Space Telescope seems to confirm this result. The even more recent observations made with the Spitzer Space Telescope of the warm infrared glow of two previously detected "hot Jupiter" planets is another interesting result in this context. This wealth of new results, obtained in the time span of a few months, illustrates perfectly the dynamic nature of this field of research.

Now, a different team of astronomers has possibly made another important breakthrough in this field by finding a tiny companion to a young star. For several years these scientists have been conducting a search for planets and low-mass objects, in particular around stars still in their formation process – so-called T-Tauri stars – using both the direct imaging and radial velocity techniques. One of the objects on their list is GQ Lupi, a young T-Tauri star, located in the Lupus I (the Wolf) cloud, a region of star formation about 400 to 500 light-years away. The star GQ Lupi is apparently a very young object still surrounded by a disc, with an age between 100,000 and 2 million years.
The astronomers observed GQ Lupi on 25 June 2004 with the adaptive optics instrument NACO attached to Yepun, the fourth 8.2-m Unit Telescope of the Very Large Telescope located on top of Cerro Paranal (Chile). The instrument's adaptive optics (AO) overcomes the distortion induced by atmospheric turbulence, producing extremely sharp near-infrared images.

As the image on the right shows, the series of NACO exposures clearly reveal the presence of the tiny companion, located in the close vicinity of the star. This newly found object is only 0.7 arcsecond away, and would have been overlooked without the use of the adaptive optics capabilities of NACO.

At the distance of GQ Lupi, the separation between the star and its feeble companion is about 100 astronomical units (or 100 times the distance between the Sun and the Earth). This is roughly 2.5 times the distance between Pluto and the Sun.

The companion, called GQ Lupi B or GQ Lupi b (depending on whether it turns out to be a brown dwarf or a planet), is roughly 250 times fainter than GQ Lupi A as seen in this series of image. Further images obtained with
GQ Lupi The image shows the VLT NACO image, taken in the Ks-band, of GQ Lupi. The feeble point of light to the right of the star is the newly found cold companion. It is 250 times fainter than the star itself and it located 0.73 arcsecond west. At the distance of GQ Lupi, this corresponds to a distance of roughly 100 astronomical units.
North is up and East is to the left.
 Image:  © ESO
NACO in August and September confirmed the presence and the position of this companion.

The astronomers then uncovered that the star had been observed previously by the Subaru telescope and by the Hubble Space Telescope. They retrieved the corresponding images from the data archives of these facilities for further analysis.
The older images, taken in July 2002 and April 1999, respectively, also showed the presence of the companion, giving the astronomers the opportunity to precisely measure the position of the two objects over a period of several years. This in turn allowed them to determine whether the stars move together in the sky – as should be expected if they are gravitationally bound together – or whether the smaller object is only a background object, just aligned by chance.

From their measurements, the astronomers found that the separation between the two objects did not change over the five-year period covered by the observations (see the graph on the right). For the scientists this is clear proof that both objects are moving in the same direction in the sky. "If the faint object would be a background object", says Ralph Neuhäuser of the University of Jena (Germany) and leader of the team, "we would see a change in separation as GQ Lup would be moving in the sky. From 1999 to 2004, the separation would have changed by 0.15 arcsec, while we are confident that the change is a least 20 times smaller."
Proper Motion of GQ Lupi: click to enlarge The graph presents the observed separations between the primary star GQ Lupi and its companion, as deduced from the images taken with HST in 1999 (left), Subaru in 2002 (middle) and NACO on the VLT in 2004 (right). All the observed separations are consistent with no changes in separation, implying the two objects move in the same direction (red line).  
The curved line shows the change in separation expected if the faint object was a background star, due to the proper motion of GQ Lup.
 Image:   © ESO
To further probe the physical nature of the newly discovered object, the astronomers used NACO on the VLT to take a series of spectra. These showed the typical signature of a very cool object, in particular the presence of water and CO bands. Taking into account the infrared colours and the spectral data available, atmospheric model calculations point to a temperature between 1,600 and 2,500 degrees and a radius that is twice as large as Jupiter (see graph on right). According to this, GQ Lupi B is thus a cold and rather small object.

But what is the nature of this faint object? Is it a bona-fide exoplanet or is it a brown dwarf, those "failed" stars that are not massive enough to centrally produce major nuclear reactions? Although the borderline between the two is still a matter of debate, one way to distinguish between the two is by their mass (as this is also done between brown dwarfs and stars): (giant) planets are lighter than about 13 Jupiter-masses (the critical mass needed to ignite deuterium fusion), brown dwarfs are heavier.
IR Spectrum of GQ Lupi: click to enlarge The graph shows the NACO spectrum of the companion of GQ Lupi (thick line, bottom) in the near-infrared (around the Ks-band at 2.2 µm).  
For comparison, the spectrum of a young M8 brown dwarf (top, in red) and of a L2 brown dwarf (second line, in brown) are shown. Also presented is the spectrum calculated using theoretical models for an object having a temperature of 2,000 degrees. This theoretical spectrum compares well with the observed one.
 Image:   © ESO
Unfortunately, the new observations do not provide a direct estimate of the mass of the object. Therefore the astronomers must rely on a comparison with theoretical models of such objects. But this is not as easy as it sounds. If, as astronomers generally accept, GQ Lupi A and B formed simultaneously, the newly found object is very young. The problem is that for such very young objects, traditional theoretical models are probably not applicable. If they are used, however, they provide an estimate of the mass of the object that lies somewhere between 3 to 42 Jupiter-masses, that is, encompassing both the planet and the brown dwarf domains.

These early phases in brown dwarf and planet formation are essentially unknown territory for models. It is very difficult to model the early collapse of the gas clouds given the conditions around the forming parent star. One set of models, specifically tailored to model the very young objects, provide masses as low as one to two Jupiter-masses. But as Ralph Neuhäuser points out "these new models still need to be calibrated, before the mass of such companions can be determined confidently".

The astronomers also stress that from the comparison between their VLT/NACO spectra and the theoretical models of co-author Peter Hauschildt from Hamburg University (Germany), they arrive at the conclusion that the best fit is obtained for an object having roughly 2 Jupiter radii and 2 Jupiter masses. If this result holds, GQ Lupi b would thus be the youngest and lightest exoplanet to have been imaged.

Further observations are still required to precisely determine the nature of GQ Lupi B. If the two objects are indeed bound, then the smallest object will need more than 1,000 years to complete an orbit around its host star. This is of course too long to wait, but the effect of the orbital motion might possibly be detectable – as a tiny change in the separation between the two objects – in a few years. The team therefore plans to perform regular observations of this object using NACO on the VLT, in order to detect this motion. No doubt that in the mean time, further progress on the theoretical side will be achieved and that many sensational discoveries in this field will be made.

Note on nomenclature:  
In the astronomical literature, the convention is to put capitals for stars member of multiple systems, but small letters for planets. If the companion to GQ Lupi A turns out to be a planet, it would be called GQ Lupi b, while if it is a brown dwarf, it would be identified as GQ Lupi B. Given the present uncertainty, this press release uses both denominations, as do the authors in the original scientific paper.

R. Neuhäuser, E.W.Guenther, G. Wuchterl, M. Mugrauer, A. Bedalov and P.H. Hauschildt, "Evidence for a co-moving sub-stellar companion of GQ Lup," Astron. & Astrophys. 435, L13–L16 (2005)     (ArXiv e-print)

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