Distances to dense cores that contain very low luminosity objects

¹ Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263 129, India
e-mail: maheswar@aries.res.in
² Korea Astronomy & Space Science Institute, 61-1 Hwaam-dong, Yusung-gu, Daejeon 305-348, Korea
e-mail: cwl@kasi.re.kr
³ Astrophysics Group, Blackett Laboratory, Imperial College London, SW7 2AZ, UK
e-mail: s.dib@imperial.ac.uk

Received: 5 January 2011
Accepted: 3 October 2011

Abstract

Aims. We estimate the distances to dense molecular cores that harbour very low luminosity objects (VeLLOs) detected by the Spitzer Space Telescope and attempt to confirm their VeLLO nature.

Methods. The cloud distances are estimated using a near-IR photometric method. We use a technique that performs a spectral classification of stars lying towards the fields containing the clouds as either main-sequence stars or giants. In this technique, the observed (J − H) and (H − K_s) colours are dereddened simultaneously using trial values of A_V and a normal interstellar extinction law. The best fit of the dereddened colours to the intrinsic colours giving a minimum value of χ² then yields the corresponding spectral type and A_V for the star. The main-sequence stars, thus classified, are then utilized in an A_V versus distance plot to bracket the cloud distances. The typical error in the estimation of distances to the clouds are found to be  ~18%.

Results. We estimate distances to seven cloud cores, IRAM 04191, L1521F, BHR 111, L328, L673-7, L1014, and L1148 using the above method. These clouds contain VeLLO candidates. The estimated distances to the cores are found to be 127 ± 25 pc (IRAM 04191), 136 ± 36 pc (L1521F), 355 ± 65 pc (BHR 111), 217 ± 30 pc (L328), 240 ± 45 pc (L673-7), 258 ± 50 pc (L1014), and 301 ± 55 pc (L1148). We re-evaluated the internal luminosities of the VeLLOs discovered in these seven clouds using the distances estimated from this work. Except for L1014 − IRS (L_int = 0.15   L_⊙), all other VeLLO candidates are found to be consistent with the definition of a VeLLO (L_int ≤ 0.1   L_⊙). In addition to the cores that harbour VeLLO candidates, we also obtained distances to the clouds L323, L675, L676, CB 188, L1122, L1152, L1155, L1157, and L1158, which are located in the directions of the above seven cores. Towards L1521F and L1148, we found evidence of multiple dust layers.