Searching for planetary-mass T-dwarfs in the core of Serpens

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: lspezzi@eso.org
² Herschel Science Centre, European Space Astronomy Centre (ESA), PO Box, 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
⁴ European Space Agency (ESTEC), PO Box 299, 2200 AG Noordwijk, The Netherlands
⁵ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
⁶ Centro de Astrobiología (INTA-CSIC), LAEFF, PO Box 78, 28691 Villanueva de la Cañada, Spain
⁷ Canada-France-Hawaii Telescope, 65-1238 Mamalahoa Hwy, Kamuela, HI 96743, USA

Received: 8 May 2012
Accepted: 1 August 2012

Abstract

Context. The knowledge of the present-day mass function of young clusters and the mass of their coolest substellar members is essential to clarify the brown dwarf formation mechanism, which still remains a matter of debate.

Aims. We searched for isolated planetary-mass T-dwarfs in the  ~3 Myr old Serpens Core cluster.

Methods. We performed a deep imaging survey of the central part of this cluster using the WIRCam camera at the CFHT. Observations were performed through the narrow-band CH₄off and CH₄on filters, to identify young T-dwarfs from their 1.6 μm methane absorption bands, and the broad-band JHK_S filters, to better characterize the selected candidates. We complemented our WIRCam photometry with optical imaging data from MegaCam at CFHT and Suprime-Cam at the Subaru telescope and mid-infrared flux measurements from the Spitzer “core to disk” (c2d) Legacy Survey.

Results. We report four faint T-dwarf candidates in the direction of the Serpens Core with CH₄on−CH₄off above 0.2 mag, estimated visual extinction in the range 1−9 mag and spectral type in the range T1−T5 based on their dereddened CH₄on−CH₄off colors. Comparisons with T-dwarf spectral models and optical to mid-infrared color–color and color–magnitude diagrams, indicate that two of our candidates (ID 1 and 2) are background contaminants (most likely heavily reddened low-redshift quasars). The properties of the other two candidates (ID 3 and 4) are consistent with them being young members of the Serpens Core cluster, although our analysis can not be considered conclusive. In particular, ID 3 may also be a foreground T-dwarf. It is detected by the Spitzer c2d survey but only flux upper limits are available above 5.8 μm and, hence, we can not assess the presence of a possible disk around this object. However, it presents some similarities with other young T-dwarf candidates (S Ori 70 in the σ Orionis cluster and CFHT_J0344+3206 in the direction of IC 348). If ID 3 and 4 belong to Serpens, they would have a mass of a few Jupiter masses and would be amongst the youngest, lowest mass objects detected in a star-forming region so far.