EDP Sciences
Free access
Volume 393, Number 2, October II 2002
Page(s) L41 - L44
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20021206

A&A 393, L41-L44 (2002)
DOI: 10.1051/0004-6361:20021206


Small-area molecular structures without shielding

A. Heithausen

Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

(Received 25 June 2002 / Accepted 20 August 2002)

Using the IRAM 30 m telescope two molecular structures have been detected which cover very small areas, $FWHM\le1'$. The clouds have velocities of $v_{\rm lsr}\approx 5$  km s -1 and linewidth of $\Delta v\approx0.8$  km s -1; thus they belong most likely to the Milky Way. Applying standard conversion factors one finds that even at the upper distance limit of 2200 pc the structures are low mass objects ( $M=(1{-}6)\times10^{-4}\,
\large ({d\over100~{\rm pc}}\large )^2$   $M_{\odot}$) which are not gravitationally virialized. $\ion{H}{I}$ 21 cm line data towards the clouds show no prominent HI clouds. The total $\ion{H}{I}$ column densities for both structures are below N(HI $)\le2.1\times10^{20}$ cm -2, corresponding to $A_{\rm
V}\le0.2$ mag, assuming a standard gas-to-dust ratio. IRAS 100  $\mu$m data towards the structures show also only low emission, consistent with low extinction. Unless there is unseen cold dust associated with the structures this shielding is too low for the structures to survive the interstellar radiation field for a long time. The detection of 2 such structures in a rather limited sample of observations suggests that they could be a rather common feature in the interstellar medium, however, so far not recognized as such due to the weakness of their lines and their small extent.

Key words: interstellar medium (ISM): abundances -- ISM: clouds -- ISM: molecules -- dark matter candidates

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