Some like it cold: molecular emission and effective dust temperatures of dense cores in the Pipe Nebula

Author(s)

Jan Forbrich, Karin Öberg, Charles J. Lada, Marco Lombardi, Alvaro Hacar Gonzalez, João Alves, Jill M. Rathborne

Abstract

Aims: The Pipe Nebula is characterized by a low star-formation

rate and is therefore an ideal environment to explore how initial

conditions, including core characteristics, affect star-formation

efficiencies. Methods: In a continued study of the molecular core

population of the Pipe Nebula, we present a molecular-line survey of 52

cores. Previous research has shown a variety of different chemical

evolutionary stages among the cores. Using the Mopra Radio Telescope, we

observed the ground rotational transitions of HCO+,

H13CO+, HCN, H13CN, HNC, and

N2H+. These data are complemented with

near-infrared extinction maps to constrain the column densities,

effective dust temperatures derived from Herschel data, and

NH3-based gas kinetic temperatures. Results: The

target cores are located across the nebula, span visual extinctions

between 5 and 67 mag, and effective dust temperatures (averaged along

the lines of sight) between 13 and 19 K. The extinction-normalized

integrated line intensities, a proxy for the abundance in constant

excitation conditions of optically thin lines, vary within an order of

magnitude for a given molecule. The effective dust temperatures and gas

kinetic temperatures are correlated, but the effective dust temperatures

are consistently higher than the gas kinetic temperatures. Combining the

molecular line and temperature data, we find that

N2H+ is only detected toward the coldest and

densest cores, while other lines show no correlation with these core

properties. Conclusions: Within this large sample,

N2H+ is the only species to exclusively trace the

coldest and densest cores, in agreement with chemical considerations. In

contrast, the common high-density tracers HCN and HNC are present in a

majority of the cores, demonstrating the utility of these molecules for

characterizing cores over a wide range of extinctions. The correlation

between the effective dust temperatures and the gas kinetic temperatures

suggests that the former are dominated by dust that is both dense and

thermodynamically coupled to the dense gas traced by NH3. A

direct use of the effective dust temperatures in a determination of dust

column densities from dust emission measurements would, however, result

in an underestimate of the dust column densities.

Table 1 is available in electronic form at www.aanda.org

Organisation(s)

Department of Astrophysics

External organisation(s)

Università degli Studi di Milano-Bicocca, Harvard-Smithsonian Center for Astrophysics, Commonwealth Scientific and Industrial Research Organisation (CSIRO)

Journal

Astronomy & Astrophysics

Volume

568

No. of pages

ISSN

0004-6361

DOI

https://doi.org/10.1051/0004-6361/201423913

Publication date

08-2014

Peer reviewed

Yes

Austrian Fields of Science 2012

103003 Astronomy, 103004 Astrophysics

Keywords

ASJC Scopus subject areas

Astronomy and Astrophysics, Space and Planetary Science

Portal url

https://ucrisportal.univie.ac.at/en/publications/f5bfa823-c37e-4359-b84a-a9e189d55584