Search for very low-mass brown dwarfs and free-floating planetary-mass objects in Taurus

The number of low-mass brown dwarfs and even free floating planetary-mass objects in young nearby star-forming (SF) regions and associations is continuously increasing, offering the possibility to study the low-mass end of the initial mass function in greater detail. In this paper, we present six new candidates for (very) low-mass objects in the Taurus SF region one of which was recently discovered in parallel by Luhman et al. The underlying data we use is part of a new database from a deep near-infrared survey at the Calar Alto observatory. The survey is more than 4 mag deeper than the Two Micron All Sky Survey and covers currently similar to 1.5 deg(2). Complementary optical photometry from Sloan Digital Sky Survey were available for roughly 1.0 deg(2). After selection of the candidates using different color indices, additional photometry from Spitzer/IRAC was included in the analysis. In greater detail, we focus on two very faint objects for which we obtained J-band spectra. Based on comparison with reference spectra, we derive a spectral type of L2 +/- 0.5 for one object, making it the object with the latest spectral type in Taurus known today. From models, we find the effective temperature to be 2080 +/- 140 K and the mass 5-15 Jupiter masses. For the second source, the J-band spectrum does not provide definite proof of the young, low-mass nature of the object, as the expected steep water vapor absorption at 1.33 mu m is not present in the data. We discuss the probability that this object might be a background giant or carbon star. If it were a young Taurus member, however, a comparison to theoretical models suggests that it lies close to or even below the deuterium burning limit (< 13 M-Jup) as well. A first proper motion analysis for both objects shows that they are good candidates for being Taurus members.