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

TL;DR

This study identifies and characterizes new very low-mass brown dwarf and planetary-mass candidates in Taurus using deep infrared and optical surveys, including spectral analysis and proper motion data, to explore the low-mass end of the initial mass function.

Contribution

It presents six new low-mass object candidates in Taurus, with detailed spectral and photometric analysis, including the discovery of the latest spectral type object in Taurus and mass estimates near the deuterium burning limit.

Findings

One object has a spectral type of L2+/-0.5, the latest in Taurus.

Mass estimates for the objects are between 5 and 15 Jupiter masses.

Proper motion suggests they are likely Taurus members.

Abstract

The number of low-mass brown dwarfs and even free floating planetary mass objects in young nearby star-forming regions and associations is continuously increasing, offering the possibility to study the low-mass end of the IMF in greater detail. In this paper, we present six new candidates for (very) low-mass objects in the Taurus star-forming region one of which was recently discovered in parallel by Luhman et al. (2009). 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 four magnitudes deeper than the 2MASS survey and covers currently ~1.5 square degree. Complementary optical photometry from SDSS were available for roughly 1.0 square degree. 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 a definite proof of the young, low-mass nature of the object as the expected steep water vapor absorption at 1.33 micron 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 Jupiter masses) as well. A first proper motion analysis for both objects shows that they are good candidates for being Taurus members.