Molecular gas and stars in the translucent cloud MBM 18 (LDN 1569)

TL;DR

This study investigates the properties of stars and molecular gas in the MBM 18 cloud, revealing that the stars are older than typical star-forming regions and likely did not form within this cloud.

Contribution

The paper provides new spectroscopic data on candidate stars in MBM 18, showing they are older and not gravitationally bound to the cloud, challenging previous assumptions about star formation in this region.

Findings

Stars are 7.5-15 Myr old, not typical T Tauri stars.

The cloud's mass is much less than its virial mass, indicating it is not gravitationally bound.

Stars likely did not form in MBM 18.

Abstract

Seven of ten candidate H-alpha emission-line stars found in an objective grism survey of a 1 square degree region in MBM 18, were observed spectroscopically. Four of these have weak H-alpha emission, and 6 out of 7 have spectral types M1-M4V. One star is of type F7-G1V, and has H-alpha 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. For the six M-stars a good fit is obtained with pre-main-sequence isochrones indicating ages between 7.5 and 15Myr. The molecular cloud mass, derived from the integrated 12CO(1-0) emission, is 160Mo (for a distance of 120pc), much smaller than the virial mass (10^3Mo), and the cloud is not gravitationally bound. Nor are the individual clumps we identified through a clump-finding routine. Considering the relative weakness or absence of the H-alpha 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 15Myr they are old enough to explain the lack of lithium in their spectra. Based on the derived distances (60-250pc), some of the stars may lie inside the molecular cloud (120-150pc). 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.