Ross 458 ABC  
Bertrand Goldman  
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Last modified: 03.03.2010
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Research projects  

I study brown dwarfs, faint and cool compact objects less massive than about 73 Jupiter masses. My first goal is to find new brown dwarfs, including brown dwarfs cooler, or older, or younger,... than currently known. These new and potentially different brown dwarfs will tell us about the physics of their atmospheres, and about the brown dwarf content of the Solar neighbourhood. Below I describe the surveys I'm using or will be using to detect new obects. My second goal is to study the atmospheres of brown dwarfs, which contain dust grains and molecules such as water and methane and ammonia, not found in the warmer atmosphere of main sequence stars. I list below the various methods I'm using.

The Pan-STARRS1 survey
Click on the images below to get more information:

Pan-STARRS1 (or PS1) is going to be the major optical survey of the decade. With the largest optical digital camera ever built, with 1.4 billions of pixels covering 7 square degrees, it will spend 55% of the time surveying the whole sky North of a declinaison of -30degrees, in five filters, four times each every year over 3 years. It is basically a multi-epoch SDSS over three times SDSS's area. The rest of the time will be used to observe 70 sq.deg. to great depths with a 1-day time sampling, search for transiting exoplanets, and study the Andromeda galaxy.

Nearby brown dwarfs: I will combine PS1 with data from 2MASS, UKIDSS and other catalogues, to search for the coolest neighbours of the Sun. Multi-epoch data and careful scheduling will also provide us with precise distances to most brown dwarfs up to 30 pc.

Brown dwarfs in Taurus: In its main component PS1 will survey multiple times the Taurus-Auriga association, and provide us with new faint ~1-Myr brown dwarfs, their spatial and kinematic distribution.

PS1 dome

PS1 camera


The UKIDSS and SDSS surveys

Nearby brown dwarfs: UKIDSS is the largest on-going near-infrared (1–2.5µm) survey. I'm combining one of its components, the Large Area Survey, with the SDSS optical data, to search for extremely cool T-type dwarfs, and possibly cooler ones. Preliminary results are being published in Goldman et al. (2010).

The VISTA surveys

Nearby brown dwarfs: VISTA is the largest near-infrared imager ever built, with 16 2kx2k detectors, and is installed in Cerro Paranal. It will be mostly used to conduct surveys, such as the hemispheric survey VHS and the Variables in the Via Lactea survey VVV (Minniti et al., 2010). Combined with PS1, upcoming optical or mid-infrared surveys, these surveys will discover the coolest brown dwarfs in the Southern hemisphere.

The Calar Alto Omega2000 Taurus survey

Very young and low-mass brown dwarfs: We have conducted a deep near-infrared survey of dark cores in the Taurus-Auriga association (age: ~1Myr, distance: ~140pc), using the wide-field camera Omega2000 in Calar Alto. Combined with SDSS data, we have identified six new brown dwarf candidates based on colour-colour and colour-magnitude diagrams. VLT/ISAAC J-band spectroscopy indicates a ~L2 type for one candidate, making it the latest-type ~1-Myr brown dwarf (Quanz et al., 2010).

Taurus map
Star-forming regions and clusters

Mass function: the substellar mass function gives clues to the formation scenario of brown dwarfs, the brown dwarf content of the Milky Way and the cluster mass segregation and evaporation into the field. We have studied in particular the sigma Orionis (e.g. Bihain et al., 2009) and Praesepe (Boudreault et al., 2010) clusters.

sigma Orionis
Light polarisation

Dust in brown dwarf atmospheres: Light scattering on dust particles produces polarized light. The polarisation integrated over the brown dwarf surface will be null except if there is any asymmetry. Rotation-induced flattening, or dust cloud patterns, will result in non-zero polarisation, which may be in the latter case variable with time, and will depend on the dust grain size distribution. Multi-band multi-epoch observations may bring new constraints on the dust composition of brown dwarf atmosphere. We have used Calar Alto CAFOS and VLT/FORS1 camera to obtain the exquisite measurements required by the small signal (<1%) (Goldman et al., 2009).

Binarity and high-resolution imaging

Resolving the L/T transition puzzle: The L/T transition brown dwarfs are intermediate objects between the L-type dwarfs with dusty atmospheres, and the T-type dwarfs for which the dust has setlled below the photosphere. The processes driving the transition between the two states, at nearly constant effective temperatures, remain unclear, and it has been suggested that L/T transition objects are mostly binaries made of a L dwarfs and a T dwarf. We observed 13 brown dwarfs with the VLT adaptive optics system, the NACO camera and the PARSEC laser guide star system, and resolved no new brown dwarfs (Goldman et al., 2008).

NACO image

Weather on brown dwarfs: Dust is a key component of the brown dwarf photosphere, especially for the L-type dwarfs. As for Jupiter, it is possible that structures of clouds appear on the brown dwarf surface, which may evolve, and rotate in and out of view as the brown dwarf rotates. This may cause variability. Many detections have been claimed but clear detections are scarse. I have used VLT/ISAAC near-infrared spectroscopy to monitor five brown dwarfs, with some low levels of variability (to be confirmed) found in a L/T transition dwarf (Goldman et al., 2008).

correlated spectra

Brown (and white) dwarfs as dark matter candiates? As a massive, possibly dark object nearly crosses the line of sight of a distant star, the gravitational lensing causes multiple images of the background star to appear. They are usually unresolved, which results in an apparent brightening and spatial shift of the background star. In the EROS collaboration we have used this effect to study the dark matter composition of the Milky Way, the disk and bulge structure, and as a side product stellar variability. We have also used the EROS wide-field camera to directly search for nearby brown and white dwarfs (Goldman et al., 1999 and Goldman et al., 2001, resp.)

light curve