Molecular gas and stars in the translucent cloud MBM 18 (LDN 1569)^{⋆,⋆⋆,⋆⋆⋆}

¹ INAF – Istituto di Radioastronomia & Italian ALMA Regional Centre, via P. Gobetti 101, 40129 Bologna, Italy
e-mail: brand@ira.inaf.it
² Joint Astronomy Centre, 660 N. A’Ohoku Place, University Park, Hilo, HI 96720, USA
e-mail: j.wouterloot@jach.hawaii.edu
³ Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA
e-mail: loris@physast.athens.edu

Received: 15 June 2012
Accepted: 5 August 2012

Abstract

Context. We investigate star formation in translucent, high-latitude clouds.

Aims. Our aim is to understand the star-formation history and rate in the solar neighbourhood.

Methods. We used spectroscopic observations of newly found candidate Hα emission-line stars to establish their pre-main-sequence nature. The environment was studied through molecular line observations of the cloud (MBM 18/LDN 1569) in which the stars are presumably embedded.

Results. Ten candidate Hα emission-line stars were found in an objective grism survey of a  ~1 square degree region in MBM 18, of which seven have been observed spectroscopically in this study. Four of these have weak (| W(Hα)| ≲ 5 Å) Hα emission, and six out of seven have spectral types M1−M4 V. One star is of type F7-G1 V, and has Hα in absorption. The spectra of three of the M-stars may show an absorption line of LiI, although none of these is an unambiguous detection. The M-stars lie at distances between  ~60 pc and 250 pc, while most distance determinations of MBM 18 found in the literature agree on 120–150 pc. For the six M-stars a good fit is obtained with pre-main-sequence isochrones indicating ages between 7.5 and 15 Myr. The mass of the molecular material, derived from the integrated ¹²CO(1–0) emission, is  ~160 M_⊙ (for a distance of 120 pc). This is much smaller than the virial mass (~10³ M_⊙), and the cloud is not gravitationally bound. Using a clump-finding routine, we identify 12 clumps from the CO-data, with masses between 2.2 and 22 M_⊙. All clumps have a virial mass at least six times higher than their CO-mass, and thus none are in gravitational equilibrium. A similar situation is found from higher-resolution CO-observations of the northern part of the cloud.

Conclusions. Considering the relative weakness or absence of the Hα emission, the absence of other emission lines, and the lack of clear LiI absorption, the targets are not T Tauri stars. With ages between 7.5 and 15 Myr they are old enough to explain the lack of lithium in their spectra. Based on the derived distances, some of the stars may lie inside the molecular cloud. From the fact that the cloud as a whole, as well as the individual clumps, are not gravitationally bound, in combination with the ages of the stars we conclude that it is not likely that (these) stars were formed in MBM 18.