Temperature constraints on the coldest brown dwarf known: WISE 0855-0714

¹ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306 Santiago 22, Chile
e-mail: jcbeamin@astro.puc.cl
² European Southern Observatory, Ave. Alonso de Cordoba 3107, Casilla 19001 Santiago, Chile
³ Millennium Institute of Astrophysics, Casilla 36- D Santiago, Chile
⁴ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Instituto de Física y Astronomía, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Casilla 5030 Valparaíso, Chile
⁶ Centre de Recherche Astrophysique de Lyon, UMR 5574, CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 Allée d’Italie, 69364 Lyon Cedex 07, France
⁷ Departamento de Ciencias Fisicas, Universidad Andres Bello, Republica 220, Santiago, Chile
⁸ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁹ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹⁰ Centro de Astrobiologìa (INTA-CSIC), Carretera de Ajalvir km 4, 28550 Torrejón de Ardoz, Madrid, Spain
¹¹ Universidad de Chile, Departamento de Astronomía, Casilla 36-D Santiago, Chile

Received: 30 June 2014
Accepted: 29 September 2014

Abstract

Context. Nearby isolated planetary mass objects are beginning to be discovered, but their individual properties are poorly constrained because their low surface temperatures and strong molecular self-absorption make them extremely faint.

Aims. We aimed to detect the near-infrared emission of the coldest brown dwarf (BD) found so far, WISE0855-0714, located ~2.2 pc away, and to improve its temperature estimate (T_eff = 225−260 K) from a comparison with state-of-the-art models of BD atmospheres.

Methods. We observed the field containing WISE0855-0714 with HAWK-I at the VLT in the Y band. For BDs with T_eff< 500 K theoretical models predict strong signal (or rather less molecular absorption) in this band.

Results. WISE0855-0714 was not detected in our Y-band images, thus placing an upper limit on its brightness to Y> 24.4 mag at 3σ level, leading to Y − [ 4.5 ] > 10.5. Combining this limit with previous detections and upper limits at other wavelengths, WISE0855-0714 is confirmed as the reddest BD detected, further supporting its status as the coldest known brown dwarf. We applied spectral energy distribution fitting with collections of models from two independent groups for extremely cool BD atmospheres leading to an effective temperature of T_eff< 250 K,