Brown Dwarf Star Joins The Jet-set

The brown dwarf with the name 2MASS1207-3932 is full of surprises [1]. Its companion, a 5 Jupiter-mass giant, was the first confirmed exoplanet for which astronomers could obtain an image (see ESO 23/04 and 12/05), thereby opening a new field of research - the direct detection of alien worlds. It was then later found (see ESO 19/06) that the brown dwarf has a disc surrounding it, not unlike very young stars.

Now, astronomers using ESO's Very Large Telescope (VLT) have found that the young brown dwarf is also spewing jets, a behaviour again quite similar to young stars.

The mass of the brown dwarf is only 24 Jupiter-masses. Hence, it is by far the smallest object known to drive an outflow. "This leads us to the tantalizing prospect that young giant planets could also be associated with outflows," says Emma Whelan, the lead-author of the paper reporting the results.

The outflows were discovered using an amazing technique known as spectro-astrometry, based on high resolution spectra taken with UVES on the VLT. Such a technique was required due to the difficulty of the task. While in normal young stars - known as T-Tauri stars for the prototype of their class - the jets are large and bright enough to be seen directly, this is not the case around brown dwarfs: the length scale of the jets, recovered with spectro-astrometry is only about 0.1 arcsecond long, that is, the size of a two Euro coin seen from 40 km away.

The jets stretch about 1 billion kilometres and the material is rushing away from the brown dwarf with a speed of a few kilometres per second.

The astronomers had to rely on the power of the VLT because the observed emission is extremely faint and only UVES on the VLT could provide both the sensitivity and the spectral resolution they required.

"Discoveries like these are purely reliant on excellent telescopes and instruments, such as the VLT," says Whelan. "Our result also highlights the incredible level of quality which is available today to astronomers: the first telescopes built by Galileo were used to observe the moons of Jupiter. Today, the largest ground-based telescopes can be used to observe a Jupiter size object at a distance of 200 light-years and find it has outflows!"

Using the same technique and the same telescope, the team had previously discovered outflows in another young brown dwarf. The new discovery sets a record for the lowest mass object in which jets are seen [2].

Outflows are ubiquitous in the Universe, as they are observed rushing away from the active nuclei of galaxies - AGNs - but also emerging from young stars. The present observations show they even arise in still lower mass objects. The outflow mechanism is thus very robust over an enormous range of masses, from several tens of millions of solar mass (for AGNs) down to a few tens of Jupiter masses (for brown dwarfs).

These results were reported in a Letter to the Editor in the Astrophysical Journal (vol. 659, p. L45): "Discovery of a Bipolar Outflow from 2MASSW J1207334-393254 a 24 MJup Brown Dwarf", by E.T. Whelan et al.

The team is composed of Emma Whelan and Tom Ray (Dublin Institute for Advanced Studies, Ireland), Sofia Randich and Ray Jayawardhana (University of Toronto, Canada), Francesca Bacciotti and Antonella Natta (Osservatorio Astrofisico di Arcetri, Italy), Leonardo Testi (ESO), and Subu Mohanty (Harvard-Smithsonian CfA, USA).

Notes

1. Brown dwarfs are objects whose masses are below those of normal stars - the borderline is believed to be about 8% of the mass of our Sun - but larger than those of planets. Unlike normal stars, brown dwarfs are unable to sustain stable nuclear fusion of hydrogen.

2. The brown dwarf 2MASS1207-3932 belongs to the TW Hydrae Association and is therefore about 8 million years old. Albeit this is relatively young, this also implies that this brown dwarf is one of the oldest Galactic objects with a resolved jet, highlighting the fact that outflows can persist for a relatively long time.

Technical information

Spectro-astrometry is simply Gaussian fitting of the spatial profile of the continuum and emission line regions of a spectrum in order to very accurately measure positions. In this way spatial information is recovered beyond the limitations of the seeing of an observation. For example spectro-astrometry has been primarily used to investigate binarity in sources where the binary separation is far less than the seeing and to confirm outflow activity where the line emission tracing the outflow originates at a distance again much smaller than the seeing and therefore appears confined to the source. The first step is to measure the continuum centroid i.e. the source position.

The spatial profile of the continuum is extracted at many positions along the dispersion axis. Each extracted profile is fitted with a Gaussian to measure the centroid position of the continuum emission and the result is a position spectrum of the continuum. This map of the continuum position is easily corrected for curvature or tilting in the spectrum. Next the continuum is removed and the position of a pure emission line region is measured (again with Gaussian fitting) with respect to the continuum position.

The presence of the continuum will tend to drag the position of an emission line region back towards the source so it must be removed. The accuracy with which one can measure positions with spectro-astrometry is strongly dependent on the signal to noise of the observation and is given by sigma=seeing/[2.3548(sqrt{Np})] where Np is the number of detected photons. For example, for a seeing of 1 arcsecond and a value of Np of 10,000, positions can be recovered to an accuracy of less than 5 milliarcseconds.

The forbidden emission lines found in the spectra of some young brown dwarfs were a strong indication of outflow activity. However the regions were not extended and therefore that they originated in an outflow could not be directly confirmed. Using spectro-astrometry the astronomers were able to show that the line regions were shifted by small amounts with respect to the brown dwarf continuum (shifts were small relative to the seeing) and therefore were indeed tracing an outflow.