Star formation, structure, and formation mechanism of cometary globules: near-infrared observations of CG 1 and CG 2^{⋆,⋆⋆,⋆⋆⋆}

¹ Department of Physics, Division of Geophysics and Astronomy, PO Box 64, 00014 University of Helsinki, Finland
e-mail: minja.makela@helsinki.fi
² Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland

Received: 16 July 2012
Accepted: 22 October 2012

Abstract

Context. Cometary globule (CG) 1 and CG 2 are “classic” cometary globules in the Gum Nebula. They have compact heads and long dusty tails that point away from the centre of the Gum Nebula.

Aims. We study the structure of CG 1 and CG 2 and the star formation in them to find clues to the CG formation mechanism. The two possible CG formation mechanisms, radiation-driven implosion (RDI) and a supernova blast wave, produce a characteristic mass distribution where the major part of the mass is situated in either the head (RDI) or the tail (supernova blast).

Methods. CG 1 and CG 2 were imaged in the near infrared (NIR) JsHKs bands. NIR photometry was used to locate NIR excess objects and to create visual extinction maps of the CGs. The A_V maps allow us to analyse the large-scale structure of CG 1 and CG 2. Archival images from the WISE and Spitzer satellites and HIRES-processed IRAS images were used to study the globule’s small-scale structure. Fits were made to the spectral energy distribution plots of the NIR-excess stars to estimate their age and mass.

Results. In addition to the previously known CG 1 IRS 1 we discovered three new NIR-excess objects in IR imaging, two in CG 1 and one in CG 2. CG 2 IRS 1 is the first detection of star formation in CG 2. The objects are young low-mass stars. CG 1 IRS 1 is probably a class I protostar in the head of CG 1. CG 1 IRS 1 drives a bipolar outflow, which is very weak in CO, but the cavity walls are seen in reflected light in our NIR and in the Spitzer 3.6 and 4.5 μm images. Strong emission from excited polycyclic aromatic hydrocarbon particles and very small grains were detected in the CG 1 tail. The total mass of CG 1 in the observed area is 41.9 M_⊙ of which 16.8 M_⊙ lies in the head. For CG 2 these values are 31.0 M_⊙ total and 19.1 M_⊙ in the head. The observed mass distribution does not offer a firm conclusion for the formation mechanism of the two CGs: CG 1 is in too evolved a state, and in CG 2 part of the globule tail was outside the observed area.

Conclusions. Even though the masses of the two CGs are similar, star formation has been more efficient in CG 1. By now, altogether six young low-mass stars have been detected in CG 1 and only one in CG 2. A possible new outflow was discovered to be emanating from CG 1 IRS 1.