Study of the molecular clump associated with the high-energy source HESS J1858+020

¹ Instituto de Astronomía y Física del Espacio (CONICET-UBA), CC 67, Suc. 28, 1428 Buenos Aires, Argentina
e-mail: sparon@iafe.uba.ar
² FADU – Universidad de Buenos Aires, Argentina
³ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile

Received: 22 December 2010
Accepted: 20 March 2011

Abstract

Aims. HESS J1858+020 is a weak γ-ray source lying near the southern border of the SNR G35.6-0.4. A molecular cloud, composed of two clumps, shows signs of interaction with both the SNR and a nearby extended HII region. In particular, the southernmost clump coincides with the center of the H.E.S.S. source. We study this clump in detail to help us identify the nature of the very-high energy emission.

Methods. We observed the aforementioned molecular clump using the Atacama Submillimeter Telescope Experiment (ASTE) in the ¹²CO J = 3−2, ¹³CO J = 3−2, HCO⁺J = 4−3, and CS J = 7−6 lines with an angular resolution of 22′′. To complement these observations, we analyzed IR and submillimeter continuum archival data.

Results. From the ¹²CO and ¹³CO J = 3−2 lines and the 1.1 mm continuum emission, we derive a density of between 10³ and ⁴ cm^-3 for the clump. We discover a young stellar object (YSO), probably a high mass protostar, embedded in the molecular clump. However, we do not find any evidence of molecular outflows from this YSO that would represent a thermal jet capable of generating the observed γ-rays. We conclude that the most probable origin of the TeV γ-ray emission is a hadronic interaction between the molecular gas and the cosmic rays accelerated by the shock front of the SNR G35.6-0.4.