Speaker
Mrs
Karla Peña Ramírez
(Instituto de Astrofísica de Canarias)
Description
The results of our research are focused in the search and characterization of brown dwarfs (objects with ~13-75 times the mass of Jupiter; Chabrier, G. & Baraffe, I. 2000, ARA&A, 38, 337) and isolated planetary mass objects (with masses below the deuterium burning-mass limit at ~13MJup; Saumon, D., et al. 1996, ApJ, 460, 993) in the star forming regions sigma Orionis(~3Myr) and Upper Scorpio (~5Myr). In the sigma Orionis cluster these discoveries double the number of sigma Orionis planetary-mass candidates known so far and in Upper Scorpius it has been identified the less massive planetary mass candidates to date in the entire association. The advances in this field are the keystone to constrain the substellar mass function. To determine the minimum mass value that would have a body formed in a similar way than low mass stars is a critical parameter to explain the stellar and substellar formation processes.
The search in the sigma Orionis cluster combines an homogeneous survey depth, broad spatial coverage (>75%), and high percentage of sources with confirmed membership (70%) which has allowed to study the cluster spatial distribution, disk frequencies and cluster mass function. The survey is focused on a circular area of 2798.4arcmin^2 around the multiple star sigma Ori. The deepest VISTA observations reach completeness magnitudes of 22.6 and 21 in the $Z$- and $J$-filters, respectively. To derive the results presented here, it has been combined VISTA photometry (ZYJHK_s) with optical (I) and mid-infrared data (WISE filters W1, W3 and Spitzer filters [3.6], [4.5], [8.0]). It has been found 210 candidates with magnitudes J = 13-20 mag that according to 3Myr theoretical isochrones correspond to the mass interval 0.25-0.004 Msun, 23 of them are new candidates with masses ranging 0.011-0.004Msun, i.e., belonging to the planetary-mass domain of the cluster. One of the new candidates presents colors compatible with a T-type dwarf. The confirmation of methane absorption in its atmosphere and therefore its spectral type is presented by means of methane photometry in the H band obtained with LIRIS/WHT. Using the Mayrit catalog, it has been presented the cluster mass function covering the wide mass range ~19-0.004Msun. The study indicates that the sigma Orionis mass function is a rising function that extends well into the planetary-mass regime. However, the number of T-type sources found in the VISTA sigma Orionis survey appears to be smaller than what is predicted by the extrapolation of the cluster mass function down to the survey $J$-band completeness. This seems to indicate that either the cluster substellar mass function may has a turnover at around 0.004 Msun (associated to the restricted formation of these cool objects in open clusters or to strong dynamical interactions that eject the faintest objects after their formation) or cluster T-type objects are fainter than predicted by theoretical models.
Previous to this study in the sigma Orionis cluster it has been identified two T-type member candidates: SOri70 (Zapatero Osorio, M. R., et al. 2002, ApJ, 578, 536) and SOri73 (Bihain, G., et al. 2009, A&A, 506, 1169). The former object has a spectral type T5.5+/-1 measured from its near-infrared low resolution spectrum covering the H- and K- bands. Using HAWK-I/VLT methane imaging it was performed a deep search (J=19--21.7 mag within completeness) to identify additional T-type candidates. None of the selected candidates using proper photometric cuts appears as a conclusive candidate. It has been analyzed the atmospheric properties of SOri73, confirming for the first time its methane absorption in the H-band and estimating its spectral type as a T4+/-1. In the same work, it has been studied SOri70 and SOri73 cluster membership via photometric colors and a proper motion analysis. The giving proper motion measurements are larger than the motion of sigma Orionis, rendering the cluster membership uncertain for the two sources. SOri73 has similar colors to those of T3-T5 field dwarfs, which in addition to its high proper motion suggests that it is probably a field dwarf located at 170-200pc. The origin of SOri70 remains unclear: it can be a field, foreground mid- to late-T free-floating dwarf with peculiar colors, or a planet ejected through strong dynamical interactions from $\sigma$ Orionis or from a nearby star-forming region in Orion.
In the Upper Scorpius association it was pursued a survey of 1.17 deg^2 using VIMOS/VLT data (Iz) combined with UKIDSS DR8 photometry (ZYJHK). This survey is sensitive within completeness to the late M dwarfs through mid T type sources. The explored area is the only one in the entire Upper Scorpio where has been covered the planetary mass population down to 0.004 Msun. As a result it has been identified 4 candidates that follow the association sequence from low mass brown dwarfs (~0.025 Msun) to isolated planetary mass objects (~0.004 Msun). Three of the sources are new candidates and show colors consistent with late L dwarfs. Their proper motion measurements confirm their association membership. These candidates would be the least massive members (~5-7 MJup) known to date in the Upper Scorpius association.
Primary author
Mrs
Karla Peña Ramírez
(Instituto de Astrofísica de Canarias)
